Test 2

Crystal Diffraction: Instrumental Design Concepts/Sequential Instrument Setup

--> A goniometer has to be used. --> You have a stage with a crystal on top. A stage underneath has the detector. Both the crystal and detector move. When the crystal moves the angle of incidence changes, which changes the constructive interference and what photons can go through. The detector detects whatever fluorescence that was diffracted. --> Has collimators that make the photons go in the same direction. It allows us to measure and direct the photons, as well as decrease intensity. --> Because of the collimator, you are introducing additional components, thus reducing the efficiency (losing light to scattering).

High Energy Photon/Particle Detectors: Geiger Counter

--> A potential of 800-2500V is applied to the central wire anode, which is surrounded by a cylindrical cathode. This voltage is so high that is causes ionization of every single atom in the chamber. --> Filler gas is 80mm Ar and 20 mm Methane or Ethanol or 1% Cl2. --> Ionizing radiation forms ion pairs, but the ions are accelerated under high potential for neutralization at each electrode. --> The x-ray comes in, hits the gas particle (e.g: Argon) and ionizes it (get a cation and electron). Because the voltage between the electrode and chamber is so high, the electron accelerates toward the electrode hitting other particles of gas (Argon) and ionizing them = cascading effect. --> The electrons can also hit the anode (particles of gas) to create a photon. This photon then collides with another atom of argon causing saturated ionization of the gas within the tube along with more electrons. --> Diagram: The purple line is the x-ray photon. On its path through the chamber it can ionize many atoms and impart energy on them causing ionization. One single atom that is ionized produces an electron which can collide with more gas particles creating a cascade of more electrons. The green is a photon and the blue is the original ionizing path. --> Because every single atom causes ionization and gives a high signal, you cannot differentiate between the wavelength or energy of the photon. Will only give one signal. Thus, you cannot use a Geiger counter to make a signal, only to inform you that there is radiation. --> Not good for detection purposes.

High Energy Photon/Particle Detectors: Scintillation Counter

--> A scintillator is a substance that glows when strict by high-energy particles/photons. --> It takes x-rays and converts them into UV-vis. --> The incident high energy photon will collide with the crystal lattice and transfer energy into the crystal lattice. The energy is transferred to impurities within the crystal and because of these impurities, electronic excitation takes place and releases energy in the form of photons. Photons emerging from the crystal lattice are then of longer wavelength and can be detected. --> A PMT is used to transduce/measure and amplify the photons emitted. --> Very sensitive and can give us info about the photons.

Emission Spectroscopy (with instrument)

--> A technique used to measure concentration of analyte of interest in a given sample based on quantitative measurement of emission. --> The atoms of the analyte are excited to higher energy levels using the energy provided by either the atomization source or an external source (ICP torch does not need an external source because it has enough energy to provide excitation). --> The atoms that are excited to a higher energy level relic back down by emitting fluorescence.

Wavelength Selector: Monochromator

--> A wavelength selector for atomic emission that disperses light into different wavelengths. --> It is the main component of a holographic grating. --> Has a low path length, so the light the goes in has to travel a long distance before it hits one mirror, another mirror, and so on. --> Diagram: This setup allows us to have good separation in wavelength of 0.1 nm (resolution), but this is not good enough for atomic emission because the coolidge of each wavelength is in picketers (not enough for good separation but people still use it).

Resonance Conditions Required for X-Ray Fluorescence

--> All you need is a high energy photon that is energetic enough to be able to remove the electron. --> Unlike molecular fluorescence you do not need any special resonance conditions.

Potential Energy & Binding Energy

--> An n shell has a high potential energy because it is away from an electron (the higher the object with reference to some reference point, the higher the potential energy). --> The n shell is further away from the nucleus so it has a high potential energy. BUT it has a low binding energy. --> Close to the nucleus = low potential energy but high binding energy.

Blazed aka Echellette Gratings PART 2

--> Because it has the edges, it does not have the problem of m = 0. --> The grating allows us to separate wavelengths of light into different colours. --> Mλ = d (polychromatic light hits the grating = certain wavelengths light will satisfy the equation = ones that do will undergo constructive interference = able to see this light. If the light does not satisfy the equation = no light will be seen. --> When we move the grating, you are changing the angle of incidence, which is why different wavelengths of light are seen.

X-Ray Fluorescence Instrumentation/Sources: Coolidge Tube PART 1 (what is it & process)

--> Bombarding a metal target with high energy electrons to create X-rays that are then used to measure our sample. --> Evacuated tube with a tungsten filament cathode and large target anode made of an appropriately selected metal to provide the X-ray wavelength of interest. --> Filament is heated by means of resistive heating and a large potential is applied between the filament and target (kV). --> Electrons stream from the heated filament and accelerate toward the target. --> The accelerated electrons loose their K.E on striking the target and cause ionization and formation of X-rays. --> The process efficiency for formation of X-rays is around 1%, with the remaining 99% of the energy given off as heat. So, the target needs to be cooled so it does not melt. --> Filament heating circuit controls the intensity (by adjustment of the filament's Fermi level). --> Potential applied between the filament and target controls the energy of emission.

High Energy Photon/Particle Detectors: Film

--> Can be used to identify spatial distances of radioactive substances in a thin section of material. --> Film badges are worn when doing x-ray work in order to determine the Toal exposure experienced by lab personnel based on degree of film darkening.

Sources for AE: Electrical Discharge (types)

--> Can excite most elements. --> Coat the electrode with the sample or, if analyzing ores, make the ore one of the electrodes. 1. Arcs: --> Standard electron beam (potential difference) between two electrodes under an applied field = creates the arc (electrons moving from one side to the other). --> The sample has to be converted to an electrode and attached to a system with another electrode to create an arc between the two = the energy created is used to ionize and excite the sample. --> Electrons collide with gas phase atoms as they traverse the electrode gap, creating heat, ionization of the gas and plasma sputter of elements from the surface of the electrode. --> Longer lasting "lightening" than sparks. 2. Sparks. --> A single short-lived arc. --> The entire sample is atomized at once. --> Poor precision.

Echelle Grating

--> Comprises of two dispersion elements, either two gratins or the combination of a grating and a prism. --> The first grating, with a very low blaze number disperses the emitted light from the sample in one direction. --> It has a higher angle of incidence which is achieved by increasing the blazing angle. --> Uses shorter side of the blades. --> Light is incident on this first grating at very high incidence angles and so high diffraction orders are sent to the second dispersive element. --> The second dispersive element (prism or grating) further disperses the light exiting the first grating in the other direction. This provides separation of the orders along the vertical axis of the detector to provide for high levels of dispersion overall. --> So much diffraction occurs that we can measure it in pico-meters, which is better for atomic emission.

Band Theory

--> Conduction processes (energy levels/electronic structures) in semiconductors is explained by the band theory. --> The lowest energy level (bonding orbitals) get together to make the valence band. --> The anti-bonding orbitals form the conduction band. Conduction band electrons are attracted to and move towards holes (when an electron combines with a hole, a stationary atom forms). --> Between the valence band and conduction band is an energy gap. --> We call it a band because there are so many molecules and energy levels together that they overlap to form on continuous band.

Doping of Semiconductors

--> Conductivity of semiconductors can be enhanced by doping. --> Involves the controlled introduction of an impurity into the semiconductor crystal matrix. this is done by heating the semiconductor in the presence of the doping agent, which diffuses into the crystal. --> Dopants with 5 valence electrons produce n-type semiconductors by contributing extra electrons to the matrix (e.g: Antimony, Arsenic, Phosphorus). --> Dopants with 3 valence electrons produce p-type semiconductors by producing a hole or electron deficiency in the matrix (e.g: Boron, Aluminum, Gallium).

X-Ray Fluorescence Detector: Photon Counter

--> Count photons. --> Individual pulses of charged are produced as quanta of radiation are absorbed by the transducer. --> These pulses are counted and recorded as number of counts per unit time. --> Works best for beam of low intensity because at high intensity the photon pulses begin to overlap and you won't be able to separate them. --> Provides with more accurate intensity data because the signal pulses are substantially larger than the pulses obtained from the background noise. --> The signal is separated from noise using the pulse-height discriminator. --> Process: You have an x-ray coming in causing a pulse of charge or current spike = you are counting these spikes per unit time. The background noise is low and these detectors are very sensitive (Can measure low concentrations) because regular light cannot cause these spikes/pulses.

Instruments to Measure Emission: Photographic Plates/Films aka Diode Arrays

--> Darkness of line on film (graphic plates) is proportional to the number of photons received by the film. --> The total current passing through a photodiode over a given exposure time is proportional to the total number of photons incident on that detection element. --> Film strips can be placed across a focal plane to record an entire spectrum simultaneously. After, the film can be scanned into a computer software and the density/intensity of each line can be determined. --> A simultaneous spectrographic measurement. --> Precision of +/- 5-10%.

P-Type Semiconductor

--> Dopants with 3 valence electrons that produce a hole or electron deficiency in the matrix (e.g: Boron, Aluminum, Gallium). --> Using an element that has less electrons than your semiconductor. --> Electrons can be elevated from the valence band to the holes in the band gap with the energy provided by an applied voltage. The holes are mobile since electrons can hop from hole to hole. --> When you connect it to a circuit the electrons from the valence band will be attracted to the positive holes and hop from hole to hole to join the circuit. Because all the positive holes are on top of the valence band, once we connect it to a battery the electrons will continue to hop.

N-Type Semiconductor

--> Dopants with 5 valence electrons that contribute extra electrons to the matrix (e.g: Antimony, Arsenic, Phosphorus). --> Can create it by using an element that has more electrons than your semiconductor. --> Electrons can be elevated to the conduction and with the energy provided by an applied voltage and move through the material. --> The electrons are the majority carriers for current flow. --> When you connect this to a circuit/current, the electrons will have no problem joining it because they are extra. --> Electrons in this are in the conduction band which is separated from the valence band by an energy gap.

Instruments to Measure Emission: Charge Coupled Devices

--> E.g: Used in cell phone cameras. --> Diagram: Raindrops represent photons that are collected into the buckets and move to secondary buckets, and so on. When the raindrops (photons) reach the final container they are measured.

X-Ray Fluorescence Instrumentation/Sources: Radio-Isotopes

--> Eliminates requirement for expensive equipment and high power sources to create x-rays. --> Very monochromatic (suitable for a limited range of elements). --> Hazardous to handle (don't want to use because they are very toxic/carcinogenic).

Types of X-Ray Methodologies: Energy Dispersive

--> Energies of different x-rays are measured directly. --> The XRF spectrum is generated by counting and plotting the relative numbers of X-rays at each energy. --> This type of measurement relies on semiconductor based detectors (have to use a detector made up of a semiconductor). --> When the X-ray (photon) from your samples shines on and is absorbed by the semi-conductor, it can cause formation of an electron-hole pair. --> But to make the electron-hole pair, the energy of the photon has to be greater than the binding energy in the semiconductor. --> The energy required to form the electron-hole pair depends on the type of semiconductor used. --> The X-ray will form as many electron-hole pairs as its energy will allow: number of electron-hole pairs = (energy of the photon)/(energy required to form an electron-hole pair). --> The number of electron-hole pairs that you create is proportional to the energy of the photon. So when the instrument measures the current/voltage created, the amount of it is proportional to the energy of the photon and this is how the energy is measured. --> You do not need a monochromator; don't need to split the light into different wavelengths.

X-Ray Absorption

--> Generally an inefficient process (as X-rays are penetrating radiation). --> Absorption spectra consist of a series of well defined absorption peaks which are related to the core shell from which electron ejection occurs. --> The energy of X-Ray photon (hv) absorption is distributed as the energy required to remove an electron from the atom, with any residual energy provided to the photoelectron as K.E. (hv = IP + KE). --> The absorption edge (where hv = IP) is where the absorption probability is greatest (where the quantum of energy required to ionize the atom is matched by that of the incoming photon). The sharp edge exists since at photon energies below IP there is insufficient energy for ionization and this resonance absorption cannot occur. --> In order to remove an electron, the energy of the photon has to equal the binding energy. If energy is less than the binding energy it will not happen. --> Diagram: The absorption spectrum we get when we shine X-Rays on our sample. K represents the absorption edge. As you reach this point, the amount of absorption increases but stops because you could absorb all the wavelengths of light up until this point = up until this point the energy of the photon was sufficient enough to remove an electron and absorption could occur (photon = binding energy). Past this point there is an exponential increase because if the energy of the photon is too high, the efficiency absorption is low.

Wavelength Selector: Rowland Circle

--> Has conclave grating so you don't need to move it around to change the angle of incidence. --> You know the precise location of where the wavelengths are going to come out as you know the size/radius of the circle. Thus, you know where to put the detectors. --> Light comes and hits against the grating = The different colours of light disperse (diffraction) = Reflect off at different points of the circle = knowing these points you know put the detectors here = wavelengths of light are measured. --> Get the signal from the torch. --> Because there are no other components (like mirrors or slits), you don't lose as much light in the process and so the S/N ratio is good (not losing light to scattering).

Sources for AE: Inductively Coupled Plasma (ICP) Torch PART 1 (overall what does it do)

--> ICP Torch atomizes the sample and promotes atomic and ionic transitions which are observable at UV and visible wavelengths. --> Excited atoms and ions emit their characteristic radiation, which are collected by a device that sorts the radiation by wavelength. --> Intensity of the emission is detected and turned into a signal that is plotted against concentration.

Metal

--> In a metal, the valence band and conduction band are practically overlapping, because the energy difference between both bands is practically non-existent. --> Because in a conductor, because both bands are overlapping, the orbitals can be used to conduct electricity = the electrons can be liberated from the valence band and sent over to the conduction band to join the rest of the circuit.

Rowland Circle vs. Monochromator

--> In a rowland circle, you don't need to move it around to change the angle of incidence. You know the precise location of where the wavelengths are going to come out and can put your detector there. --> A monochromator needs to be moved around to change the angle of incidence and disperse light.

Insulator

--> In an insulator, the conduction band and valence band are really far away from each other because the energy difference between the bands is very high. --> For insulators, because of the energy gap, they cannot be used to conduct electricity. The energy being provided to get the electron to move will not be sufficient (not enough energy to cause an electron to jump from the valence band to conduction band = can damage the insulator).

Improving Precision and Accuracy of Atomic Emission Spectroscopy: Homologous Pair

--> Internal standard method - corrects for drift, but doesn't inform the user that instrumental parameters may be changing. --> The standard you are using has to be the same as the sample you are measuring (same emission, spectra, etc.). --> This is problematic because there are not that many elements that are identical to each other. --> Picture: You measure the intensity of the sample of interest and the intensity of the standard of interest and use the formula to find the concentration of the sample. --> Homologous pair element (or internal standard) is added to the sample in a known and sufficiently high concentration for good S/N. --> Emission lines of the standard must have similar characteristics to those of the element under study: 1. Must be in close λ. 2. Sensitivity to excitation/ionization must be similar. 3. Any change in the method or the instrument must affect both lines in a similar manner.

Other soft ionization sources.

--> MALDI. --> ESI. --> Field desorption.

Holographic Gratings

--> Made by exposing photosensitive material to interfering laser beams. --> The interference pattern creates sinusoidal surface pattern. --> The resulting grooves are then covered with aluminum or other reflective material to create a holographic grating. --> The dispersion is based on the number of grooves per mm and not the shape of the grooves. --> The grooves act like a grating and reduce scattering of light. --> Two types: 1. Positive photo resistive material: --> Get destroyed by the light and the sensitized parts are washed off. --> Constructive interference = The polymer getting destroyed (little grooves). --> Destructive interference = No light (peaks). 2. Negative photo resistive material: --> The polymer crosslinks upon exposure to light (makes polymers stronger) and the unaffected parts are washed off (anything that does not cross-link). --> Constructive interference = cross-linking. --> Destructive interference = polymer stays the same = no cross-linking.

What is the meaning of sensitivity?

--> Meaning we want to have an instrument that can detect the lowest possible concentrations (e.g: one single photon). --> X-ray fluorescence produces a small amount of photons/low intensity signal, so if we do not have an instrument that will amplify this signal, we will not able to detect something with a low concentration.

Why is an arc a poor ionization technique?

--> Not all samples can be converted to an electrode. --> E.g: Aqueous samples like water.

P-N Junction

--> P-N junction: Intrinsic layer of the negative and positive where they switch spots = creates a voltage of its own (0.6V). --> As ions are formed at the interface, the junction enters equilibrium. At this point no current flow between the regions. --> At equilibrium, the electrons on the N-side are content to stay near the cations and are repelled by the anions on the P-side. The same is true for the holes. --> The charge stored at equilibrium forms a potential. --> SUMMARY: An equilibrium state, no one wants to move.

High Energy Photon/Particle Detectors: Ionization Chamber

--> Potential applied between two electrodes separated by a gap occupied by a filler gas. --> Ionizing particle or radiation collides with filler gas producing an ion pair. --> Migration of ions under applied field causes electrical current to flow between electrodes. --> The voltage here is only high enough to prevent the cation and ion from getting back together. --> The applied voltage is in the range of 100-250V, which provides sufficient K.E to minimize recombination of the ion pair while not providing sufficient energy to cause further ionization as each ion of the ion pair accelerates towards an electrode. --> Multiple ionization events can occur, depending on the energy of the photon (but typically used to measure counts per second not photon energies). --> Very quick response, even for weaker X-ray radiation.

Which High Energy Photon/Particle Detectors is best?

--> Proportional counter because you will know the energy and wavelength of a photon. --> The voltage of the Geiger counter is too much = ionizes everything. --> The ionization chamber has no amplification of the signal.

High Energy Photon/Particle Detectors: Semiconductors (Semiconductor Theory)

--> Provide high resolution, particularly for g-ray spectroscopy. --> High energy photon striking the semiconductor material liberates an electron from the core level, into the conduction band. Electrons from higher energy level orbital cascade down to fill the positive hole left by the core, ultimately rendering a positive hole near the valence band energy level. --> Charge carriers in the form of electron-hole pairs that result from photoionisation within the semiconductor material that, under applied potential, lead to the flow of electrical current --> The Lithium Drifted Germanium Detector is commonly employed.

X-Ray Fluorescence Detector: Gas Ionization

--> Radiation (e.g: alpha, gamma, beta, x-ray) ionizes an atom or molecule of filler gas creating an ion pair (cation and electron). --> Under the applied potential, these ions migrate to the cathode and anode, and are neutralized. --> An external circuit detects the amount of charge created via electrical current monitoring. --> A metallic chamber wit ha rod inside. Applying a voltage between the rod and the walls of the chamber and filling it with gas. When the x-ray comes in, it interacts with the atoms of the gas an ionizes it. --> Say you have argon and an x-ray coming in. The x-ray interacts with argon causing ionization, so now you have an electron. The electron gets attracted to the rod and moves. Because you are introducing an electron to the rod, you are creating current flow (prevents the cation and argon from getting back together). The cation (argon) gets attracted to the walls. When it reached the wall, the cation absorbs the electron and you get your argon atom back, which is able to be ionized again (diagram on the right).

Improving Precision and Accuracy of Atomic Emission Spectroscopy: Fixation Pair

--> Select two lines where response varies as a function of instrument parameters. --> If variation occurs in operating conditions, then a large change in the ratio of fixation pair response lines is observed. --> E.g: Good for silver and chromium because they have a big difference = A large difference in ratio tells you about malfunctioning in the instrument.

Semiconductor Theory PART 1

--> Semiconductor is a substance with a conductivity between that of a conductor and an insulator. --> Sufficient thermal agitation occurs at room temperature to liberate electrons from bonded states. The liberated electron can move freely through the lattice to conduct electricity. --> Conduction processes (energy levels/electronic structures) in semiconductors is explained by the band theory. --> The fermi level is the top of the collection of electron energy levels at absolute zero temperature.

Discuss the operating principles of a Quadrupole Analyser. Provide well- labelled diagrams to aid your explanation.

--> Spectrometer employs four short (~19 cm length), parallel metal rods arranged symmetrically about the ion beam. --> Rods must be precisely positioned and straight, extremely well aligned (parallel) and of well defined radius. --> Opposed rods are connected together electronically, one pair to positive DC source, the other pair to the negative terminal. --> Superimposed on this is an radio frequency AC potential applied to opposing pairs. --> In the absence of the DC voltage ions travelling between the rods will converge during the positive cycle and diverge during the negative cycle. --> Positive DC: Momentum of ions of equal KE is directly proportional to the square root of the mass. As a result, Heavy ions will not be affected by the AC voltage and remain in the center due to the influence of the DC voltage. The lighter ions will be affected by the AC voltage and will strike the rods. The xz plane is a high-pass mass filter. --> Negative DC: The ions are attracted to the negative rods. The lighter ones can be deflected by the AC voltage, whereas the heavier ones are not. As a result the heavy one can strike the rods and get eliminated. The yz is a low-pass mass filter. --> Ions accelerate and decelerate within the quadrupole region such that they describe a sinusoidal path through the mass analyser. Only ions with m/z must in there region where the high-pass and low pass filter overlap can pass through. --> Scan through a range of DC and AC voltages to get a spectrum.

Semiconductor Theory PART 2

--> Sufficient thermal agitation occurs at room temperature to liberate electrons from bonded states, across the band gap, and into the conduction band. The liberated electron can move freely through the lattice to conduct electricity. --> A positively charged centre (hole) is left from the point where the thermal electron originated. The hole is also mobile. Electrons involved in bonding on neighbouring atoms may jump into the electron deficient region, filling one hole and creating another (electrons jump from hole to hole creating conduction). --> Conduction involves the movement of thermal electrons in one direction and the positive centre in the other under an applied field.

X-Ray Fluorescence PART 1

--> Technique for elemental analysis and chemical analysis of samples involving metals, glass, ceramics and building material. --> Used as a research tool in geochemistry, forensic science, archaeology and art objects (e.g: paintings, murals - can see the painting they started to make and what they covered over it). --> Looks at core level electron processes. --> The sample does not need to be in its atomic form. We do not care about what bond our element is in because core electrons do not participate in bonding (the bond will not affect analysis). The point of elemental analysis is that regardless of the form of the sample we can detect it.

Voltage for an ion chamber, proportional counter, and Geiger counter.

--> The diagram shows you the different voltages used for different detectors. --> Ion chamber = the voltage is low and is only there to prevent the ion and cation from getting back together. Not high enough to cause amplification. --> Proportional counter = The voltage is high enough to prevent the cation and ion from getting back together but also high enough for amplification of the signal. Amplification is important because the intensity coming from the sample with X-ray fluorescence is low. --> Geiger counter = The voltage is way too high. So high that it causes ionization of every atom of gas in the chamber.

The following diagram represents the x-ray absorption spectra for lead and silver. Explain what is meant by "absorption edge".

--> The energy of X-Ray photon (hv) absorption is distributed as first, the energy required to remove the electron from the atom (i.e. the ionisation potential), with any residual energy provided to the photoelectron as Kinetic Energy. --> hv = IP + KE. --> Absorption edges: Where hv = IP. This is where the probability is greatest (where the quantum of energy required to ionise the atom is exactly matched by that of the incoming photon). Sharp edge exists since at photon energies below IP there is insufficient energy for ionisation and therefore resonance absorption cannot occur.

X-Ray Absorption: Beer Lambert Law

--> The mass absorption coefficient is the same idea as the epsilon. It tells you how well you absorb the X-ray photon. --> When you are taking a measurement and have many elements in your sample, the u that you get is a sum of all the other u in the sample. From here, you can look up the u of each individual element in a database and determine the mass of each element present in the sample.

Energy produced from a Coolidge tube

--> The maximum energy produced by source can be determined by the use of the Duane-Hunt law. --> The equation allows us to determine the minimum wavelength we can get from the Coolidge tube. --> The wavelength we get depends on the voltage we apply between the target and the tungsten filament. This is because an electron is flying at a high speed and when it interacts with the nucleus, it slows down and loses photons. The faster the electrons travel, the more they slow down and the more energy will be produced, creating a smaller wavelength. --> A higher voltage means a greater amount of energy and a smaller wavelength. This is because we are making the electron fly much faster, so when it slows down, there will be a greater difference between the initial and final kinetic energy.

Describe the conditions required for X-ray absorption. Use appropriate equation(s) to aid your explanation making sure define each term of the equation(s). What is meant by an "absorption edge"?

--> The only resonance condition available for the core level electrons is to be promoted from the core orbital to the outside world where E = 0, since all other levels are filled. Which means the energy of the incoming X-ray should be at least equal to the binding energy of the electron based on the photoelectric effect equation. --> hv = IP+KE. Where hv is the energy of the incoming photon, IP is the ionization potential (or binding energy) and KE is the kinetic energy of the released electron. --> Absorption Edges - where hv = IP. This is where the absorption probability is greatest. Sharp edge exists since at photon energies below IP there is insufficient energy for ionisation and therefore resonance absorption cannot occur.

How energy shells in X-Ray Fluorescence are labelled

--> The principle quantum numbers like n = 1, n= 2, and n = 3 are given letters K, L, and M. --> Decided to give them separate letters so when labelling their absorbance emission their origin of where they came from is known. --> When labelling a photo that's coming out, you are labeling it based on which shell it came from (what energy level/orbital we lost the electron from). E.g: If an electron got ejected from the K shell, you label it with K. --> The energy scale is logarithmic. The energy difference between K and L is much bigger than M and L.

Diffraction (how a regular grating works) PART 2

--> The reflection happens from the wider surfaces and the narrower surfaces are mostly unused. This geometry provides with efficient diffraction of radiation and allows to concentrate the radiation in the preferred direction. --> Each of the broad faces allows for interference to occur between reflected beams. --> Constructive interference occurs when the path length differ by an integral multiple of n of the wavelength. --> Diffraction results in angular dispersion of radiation. Diagram: The grating has tiny holes that separates purple light into its components: Blue and red. --> When m = 0, there is mostly constructive interference = light particles stick together = no separation = light remain purple = bottom light is brighter. --> This technique is rarely used because there is no separation in the middle.

Difference between X-Ray absorption and regular absorption

--> The regular absorption spectrum is more broad and curved (one hump) --> because of our molecules there are many different rotational and vibrational energy states --> can absorb energy as long as there is a difference between these energy states = photoelectric effect. --> The X-ray absorption spectrum has sharpness not broad humps. The sharp edge is caused by no more absorption because low energy cannot remove an electron from the shell. --> In regards to the photoelectric effect, hv = IP + KE, where hv is the energy of the photon, IP is the ionization potential/binding energy, and KE is the kinetic energy.

Sources for AE: Inductively Coupled Plasma (ICP) Torch PART 2

--> The sample enters through the central tube (1 L/min) as an aerosol (vapour, fine powder, or solid insertion). --> The first component is a nebulizer. The sample we nebulize is mixed with argon (three concentric quartz tubes carry Ar gas at 11-17 L/min). --> The inner tube contains the sample and Argon (tube with black dots). --> Argon flows around the outside tube. --> The tangential flow (10-15 L/min) of Argon in the outer tube is used to cool down the tubing inside to make sure it does not melt and contain the plasma. --> The auxiliary flow of Argon is used to get the plasma going and maintain it. --> Water has to be evaporated from the sample because if water gets into the torch it will decrease the temperature. --> A condenser is present to ensure water gets condensed and does not get into the instrument. --> Coils that coil around the tech pass current though them creating a magnetic field (motion of electrons). This magnetic field is what generates plasma.

R.U. Lines (Raies Ultimes/Ultimate Rays)

--> The three most prominent lines (emission wavelengths) in the atomic emission spectrum of a given element. --> Each element can have multiple lines, but the R.U. lines are the most prominent and have the highest intensity. --> Useful for identification and quantification determinations. --> Take a spectrum and reduce the intensity until only 3 lines stand above the noise (3 lines = R.U. lines). You can reduce the intensity of the overall measurement because the other background noise will go down but not the R.U. lines as they are high intensity.

Geiger counter Dead Time

--> The time during which we cannot measure any photons (when cations are trying to get to the walls of the chamber). --> The photon hits the Geiger counter ionizing everything in the chamber. --> From here, the electrons accelerate toward the electrode. It takes a faction of a microsecond for the electrons to reach the wire. --> The cations are heavy (200 usec for the average travel time through the tube to the cathode) and take their time getting to the walls of the chamber to be neutralized. --> The cations (positive charges) surrounding the electrode affect the electrical field inside the chamber. The sheath of positive ions forms a shield around the wire which reduces the strength of the electric field required for the Geiger action. --> Thus, when an x-ray comes in you cannot measure it (nothing to ionize and affects electrical field). The tube is unresponsive until the positive ions make it to the wall and are neutralized (get reduced and go back to their atomic state). --> The sheath (positive charges) travels slowly away from the site and at some point is far enough that the electric field can increase to a value that is sufficient to support the geiger action. However, since the sheath has not been neutralized the pulses are of reduced size. --> Recovery time: The time it takes for the counter to produce full-size pulses after the initial pulse. --> Top diagram: The inside circle represents the electrode and the positive charges of the cations. --> This dead time is the reason why you get pulses and not a continuous current.

Sources for AE: Inductively Coupled Plasma (ICP) Torch PART 3

--> The top of the tube is surrounded by a water cooled induction coil that is powered by an RF generator (because the torch is so hot you need water running around the coils and inside the tubes to prevent melting). --> Before we get the plasma going, we need the spark source to produce an initial ionization of Argon. --> The induced magnetic field, generated by the induction coil, accelerates electrons (causes them to move around in circles) within the flowing gas to sufficient velocities to collide with Ar atoms and cause ionization. The electrons released in the ionization process are accelerated to cause further collisions/ionization. Under the influence of the induced field, charged species flow in closed annular paths. The net result is a self sustaining white hot fireball (6,000K-10,000K). --> When the magnetic field is applied, causing electrons to move around in circles, plasma is created. Electrons don't want to be moving in circles, thus a resistance is created. This resistance to the flow is what creates the heat and energy inside the plasma.

X-Ray Fluorescence Instrumentation/Sources: Coolidge Tube PART 2 (components)

--> The tungsten filament is what controls the intensity of the Coolidge tube. The purpose of the filament is to eject electrons; the more you heat it, the more electrons will get ejected and the higher the intensity you will get. --> The target can be any metal, depending on what you are trying to study. --> Ground water potential is applied between the tungsten filament and the target to ensure the electrons fly toward the target. --> The beryllium window is there to allow X-rays to exit. As well, it acts as a filter to select what kind of wavelengths of light you want to come out. --> When electrons are ejected from the tungsten filament toward the metal target, there is very high energy so water is present to cool the target and ensure it doesn't melt.

Semiconductor

--> There is an energy gap between both bands but not as significant as an insulator. --> At room temperature, the electrons will have sufficient energy to be able to move from the valence band to the conduction band. Thus, you will be able to conduct electricity (not as good as a metal though).

Instruments to Measure Emission: Photomultiplier Tubes

--> They are used at the exit of a monochromator or positioned at different distances on a focal curve for fixed element observation. --> Output from PMT current is integrated over a given amount of time to provide an output signal as an exposure value. --> High sensitivity method as small signals can be significantly amplified. --> Provides for +/- 1-2% precision.

Draw a schematic representation of an Inductively Coupled Plasma Torch and briefly describe the operating principle of the device.

--> Three concentric quartz tubes that carry Ar gas at 11 - 17 l/min - central tube carries atomized sample in an Ar stream, other channels are for cooling. --> Top of tube surrounded by water cooled induction coil that is powered by an RF generator (~2kW @ 27MHz). --> Tesla Coil spark produces an initial ionisation of Ar. Ions and electrons interact with fluctuating magnetic field. Back and forth motion of ions occurs within a closed annular path in which there is tremendous local heating brought on by collisions. --> Temperatures range from 6,000 K to 10,000 K (depending on distance relative to the plasma centre) with good stability.

X-Ray Fluorescence Instrumentation/Sources: Crystal Diffraction

--> Use of a grating for separation of x-rays is not technically possible because of the small d-value required (focus on wavelength dispersive XRF because the wavelength of x-rays are too small to use a grating). --> Use a crystal based monochromator where the lattice spacing of the crystal is on the order of the wavelengths to be diffracted. --> Diffraction is based on Bragg's law where scattering occurs in successive crystal planes forming constructive and destructive interference depending on the path length through the crystal and angle of incidence. --> Constructive interference will be observed when the conditions shown in the equation (diagram) are met (can see the light when the equation is satisfied). --> The distance between the ions in the crystal have to be equal to the wavelengths of the x-ray you are trying to diffract = so only certain crystals can be used. --> In order to conduct analysis of x-ray fluorescence emission: 1. Separate the emitted x-rays as a function of energy. 2. Relate the observed fluorescence lines back to the element of interest.

Sources for AE: Flame Sources

--> Used for easily excited elements (alkali and alkali earth metals). --> Generally a low temperature (e.g: Natural gas/air flames burn at 1100℃; good for Li, Na, K, Ca). --> High temperature is also possible (e.g: Cyanogen flames (CN)2/O2 burn at 4500℃). --> Poor precision. --> Does not require an excitation source; flame has enough energy to excite the elements.

Detection Motifs: Multichannel Analyser

--> Used for energy dispersive elements. --> Looking at different pulses (height) and different pulse peaks = decides which ones to keep and which ones to reject. --> More complicated, more electronics associated with it.

Solution for Geiger Dead Time

--> Using an organic filler gas (quenching agent) that has a lower IP than Ar. The Ar+ can be neutralized via collisions and charge transfer with the organic filler gas (makes the argon cations and electrons go back to their original atomic state). --> It interacts with the argon cations before it gets to the wall, causing faster reduction. --> Only the organic filler gas migrates and is neutralized at the cathode. They do not produce photons on collision with the cathode because of their size and nature, but they do dissociate into fragments. It is therefore consumed introducing a lifetime to the Geiger Counter. --> The filler gas serves to decrease the dead time of the tube by neutralizing the Ar+ and preventing the emission of photons by the Ar+ on impact with the cathode. --> Chlorine can be used in place of an organic quenching agent. 1. It has a lower IP than Ar. 2. It is larger than Ar and does not provide secondary ionization/photoemission on impacting the electrodes. 3. It does not foul the electrodes and is not readily consumed, thus extending the tube lifetime by 1000-fold. --> This does not solve dead time, but it does reduce it.

X-Ray Fluorescence Process PART 2

--> When an electron gets removed, another electron from a higher energy level/shell drops down to replace it. When it relaxes back down from the higher energy shell it releases an electron (known as a photoelectron) = fluorescence. --> OR the empty space is not replaced, instead another electron is lost or ejected = AUDER process --> Energy of emitted X-rays is equal to the difference between the binding energies of the two electrons involved in the ionization and relaxation process. --> You can use nay photon with high energy to eject the photon, but the photon being produced is always equal to the difference in energy levels (both photons are not equal in energy, only the difference in energy levels).

High Energy Photon/Particle Detectors: Proportional Counter

--> When applied potential is set between that used for ionization chamber and a Geiger counter tube, pulse heights are proportional to the applied potential. --> Signal amplification factor is 100-1000 fold less than the Geiger counter tube. --> The total number of secondary electrons emitted are proportional to the total number of primary ion pairs (the additional ionization we get is proportional to the energy of the photon that came in). --> Short dead time (around 0.25 usec) provides for high counting rates of 50,000-200,000 counts per second. --> Good for detection purposes because this one tells you what the energy and wavelength of the photon was. --> The height of each pulse tells you about the energy of each photon. --> The photon comes in, interacts with the gas and cterm-74auses ionization, the electron produced flies to the electrode and in the process ionizes other atoms. You get a current surge and the magnitude of this surge is proportional to the energy of the photon you introduced.

Crystal Diffraction: Instrumental Design Concepts

--> When using a crystal for wavelength dispersion, a goniometer based system is employed which can rotate the crystal and the detector accurately and simultaneously so as to achieve λ discrimination (use this instead of a monochromator for diffraction of x-ray photons). --> Depending on the crystal chosen, wavelengths in the range of 100-0.2 Å can be investigated (have to choose an appropriate crustal for the experiment). --> Samples are usually powders or liquids (need a reasonable density for a good S/n). --> LODs are on the order of ppm for this technique.

Diffraction PART 1

--> When we talk about fluorescence, light has a particle wave duality = has particle and wavelength properties. --> Diffraction is one of the wave properties light has. --> The idea of diffraction was used to create gratings in order to separate polychromatic light into different gratings. --> When light or sound meets a barrier, instead of going through it, it bends around it. --> Or, when light or sound meets an opening, it goes through it but still bends coming out. --> Say you have two-point sources with two openings. When light comes through the slits, it begins to bend. The two different wavelengths interact and undergo destructive and constructive interferences. During destructive interference, there is darkness. During constructive interference, there is brightness. --> Diagram: Pink = destructive and blue = constructive (brighter).

Types of X-Ray Methodologies: Wavelength Dispersive

--> X-rays are separated using a crystal. --> The crystal diffracts the X-rays in different directions according to their wavelength. --> A detector is placed at a fixed position, and the crystal is rotated so that different wavelengths are picked up by the detector. --> The XRF spectrum is built up point by point. In some instruments, a number of crystal/detector units are used in order to detect a number of elements simultaneously. --> You need additional instrumentation like a monochromator to be able to split the photons that you get from your sample into different wavelengths (measure each one individually).

When you combine an N-Type Semiconductor and P-Type Semiconductor

--> You can produce something that produces light and acts as a detector. --> Because the P-type has extra holes and the N-type has extra electrons, the electrons fro the conduction band of the N-type join the holes of the P-type. --> When you combine the, all the electrons of the N-type jump to the holes of the P-type creating anions and cations at the interface between the P-type and N-type. --> You can use this to make a lamp or detector.

Crystal Diffraction: Instrumental Design Concepts/Simultaneous Instrument Setup

--> You have a few crystals and a few detectors to measure all the different kinds of fluorescence that come out at the same time. --> In order to check all the different kind of wavelengths you have to change the angle of incidence of the grating to get different kinds of constructive interferences to see what waves you can look at. --> Have a curved crystal to allow us not to use collimators. The curvature directs the photons itself; does not need a collimator to direct them.

Compare and contrast Ionization Chamber and Geiger Counter. Provide a labelled diagram of the general set up to aid your explanation

1. Basic principle: --> Radiation (e.g. alpha, beta, gamma, x-ray) ionizes an atom or molecule of filler gas creating an ion pair (cation & electron). --> Under the applied potential, these ions migrate to the cathode and anode, and are neutralized. --> An external circuit detects the amount of charge created via electrical current monitoring. 2. Ionization chamber --> Applied voltage provides sufficient KE to minimise recombination of the ion pair while not providing sufficient energy to cause further ionisation as each ion of the ion pair accelerates towards an electrode. --> Multiple ionization events can occur, dependent on the energy of the photon. However, this is not typically used to determine photon energies owing to poor precision. Typically used to measure counts per second. 3. Geiger Counter. --> Ionising radiation forms ion pairs as previously, however, the ions are accelerated under high potential for neutralisation at each electrode. Mobility of electrons is high and collisions involving electrons and neutral gas species causes further ionization, which in turn causes further electron release and collisions in a cascading effect. (i.e. primary ionization event leads to many more secondary and subsequent ionization events - until all of the gas in the tube has been ionised). --> Geiger Tube Dead Time - the heavier positive ions move to the cathode for neutralisation, however, owing to their much larger size, ~200 μsec is required as an average travel time through the tube to the cathode. The tube then unresponsive until the positive ions are neutralised.

X-Ray Fluorescence Instrumentation/Sources

1. Bombarding a metal target with a beam of high energy electrons (Coolidge tube). 2. Exposing a substance to a primary beam of X-rays to generate a secondary beam of X-rays of lower energy (using an X-ray to create another source of X-ray; e.g: radioisotope). 3. Using a radioactive source emitting X-rays during its decay process. 4. Synchrotron.

Disadvantages of an ICP Torch

1. Complex spectra - hundreds of thousands of lines. 2. Need expensive optical components. 3. Highly trained personnel required (it is such complex information that someones highly trained is needed to interpret it).

Types of emission observed from a Coolidge tube

1. Continuous type emission. --> Means breaking radiation. --> You have electrons flying towards the target, but they end up interacting with the electric field of the nucleus of the target. Because of this interaction, the electron slows down and loses K.E. Based on the law of conservation of energy, when the K.E slows down it doesn't just disappear but instead produces a photon to conserve the energy. Whatever energy difference was there between the initial and final K.E is released as a photon (the lost K.E is turned into a photon). --> Because the electrons can slow down to different extents, you can have different photons of different wavelengths being produced, which is why you have a continuous source. 2. Line type emission. --> From X-ray fluorescence emission from the target. --> Instead of a Coolidge tube you have a metal target. You are using electrons to collide with this target. --> When the collision takes place, the target gets ionized. When this happens, another electrons from a higher level will go down to replace the electron that got ejected. In the process it creates fluorescence (peak in the diagram is the photon/fluorescence produced).

Types of X-Ray Methodologies

1. Energy dispersive. 2. Wavelength dispersive.

Soft Ionization Sources

1. Field Ionization. 2. Field Desorption. 3. Chemical Ionization.

High Energy Photon/Particle Detectors

1. Film. 2. Ionization chamber. 3. Geiger counter. 4. Proportional counter. 5. Scintillation counter. 6. Semiconductors.

Atomization Sources

1. Flame. 2. Electric spark or arc. 3. DC plasmas. 4. Microwave induced plasmas. 5. Inductively coupled plasmas (gaseous mixture formed at high temperatures and containing positively and negatively charged particles).

Advantages of a Rowland Circle

1. Has self-focusing property, since the concave grating can both disperse and focus the light. --> This allows for simpler optical path with fewer elements to align. --> Produces a brighter image, since there are fewer optical elements to absorb or deflect the light. --> It cuts down on the amount of scattered light or noise (less light lost). 2. You don't need prisms or mirrors to reflect the light (no additional elements to cause scattering).

Advantages of an ICP Torch

1. Higher temperature and residence time of sample in the sampling zone, which provides greater atomization and better sensitivity. 2. No need for a radiation buffer: Buffer is required to make sure ionization takes place. The plasma itself acts as a radiation buffer. High population of electrons in plasma shifts equilibrium against ion formation (greater percentage remains in the non-ionized state). 3. Atomization is done in a chemically inert environment because of Argon. No need for a flame so no burning of the sample = no formation of oxides/hydroxides and greater atomic lifetime. 4. Temperature cross section is very uniform --> provides linear calibration curves over several orders of magnitude.

Improving Precision and Accuracy of Atomic Emission Spectroscopy: Two Ways

1. Homologous Pair. 2. Fixation Pair.

Atomic Emission Spectroscopy Error Sources

1. Instrumental: --> Flame or source stability (flickering of the flame/plasma, source intensity fluctuations, reproducibility of the fame and sampling height). --> Clogging of nebulizer jets or burner orifices. 2. Sample: --> Super-positioning of elemental lines (require high resolution). --> Reproducibility of aerosols. --> Enhancement of atomic emission due to radiation buffer effect (not for ICP). --> Anion depression, formation of molecular species. But can be overcome by adding lanthanides nitrate to complex out PO43-. --> Problems are always there due to sample composition = matrix effect = things interfering with the signal (to avoid it use standard addition).

Why is atomic emission better (most applied technique) than atomic absorption?

1. It is very specific: It can measure metals and non-metals like Cl, S, P, whereas atomic absorption cannot measure non-metals = Their absorption is in the UV range (190nm-200nm) along with particles in the atmosphere (e.g: air). This interferes with the signal, causing background noise, making the detection of the sample impossible. 2. It is very sensitive (measures ppm and ppb). 3. It can set up a calibration curve/standard curve to determine the concentration of your sample. 4. Can look at many elements at once vs. atomic absorption requires a specific source for each element (e.g: many hollow cathode lamps). Thus, atomic emission is more cost effective for analysis of multiple elements.

Molecular Fluorescence Instrumentation/Sources

1. Laser. 2. Xenon arc lamp.

Briefly explain how X-ray Fluorescence is different from the other types of elemental analysis discussed in class.

1. Molecular fluorescence techniques measure the fluorescence that originates form the relaxation of valence electrons. In X-ray fluorescence, the fluorescence originates from relaxation of a core shell electron. 2. Atomic emission and atomic absorption analysis also involve excitation and emission of valence shell electrons and as a result they require and atomic population. X-ray fluorescence on the other hand does not require atomic population since chemical composition does not affect the emission from the core shell electrons which are not involved in bonding.

Types of Wavelength Selectors

1. Monochromator. 2. Rowland circle. 3. Blazed aka Echellette Gratings. 4. Echelle grating.

Criteria for Atomic Emission Spectroscopy

1. Must be possible to volatilize the sample (sample should still be broken down into atoms). 2. Must be possible to electronically excite the sample atoms.

Instruments to Measure Emission

1. Photomultiplier tubes. 2. Photographic plates/films aka diode arrays. 3. Charge coupled devices (CCD).

Detection Motifs

1. Pulse height analyser. 2. Multichannel analyser.

Wavelength dispersive X-ray fluorescence requires the use of:

A crystal mounted on a goniometer.

An x-ray spectrum can be collected using:

A proportional counter.

Semiconductor Detector: Lithium Drifted Germanium Detector.

ADV --> Energy resolution is good for semiconductor detectors as a large number of electron-hole pairs are formed relative to ion-pairs formed in Scintillation crystals. --> Semiconductor detectors provide for 14 fold better resolution than scintillation detectors. --> Almost 10x more sensitive. DISV --> Meed liquid nitrogen to use it.

Advantages and disadvantages of wavelength dispersive XRF

Advantage --> Better resolution because you are splitting the wavelengths to be able to measure them individually. Disadvantage --> Because you have to use a monochromator, the efficiency of the measurement is lower. Every time you are using instrumentation with additional components (e.g: mirrors or gratings), scattering of light can occur.

Advantages and disadvantages of energy dispersive XRF

Advantage --> You don't have any additional components so it is more efficient. Disadvantage --> Does not have as good as resolution because you are not splitting your wavelengths first before measuring them.

Ions entering a cyclotron can be excited using:

An alternate electric field.

What technique is most applied for elemental analysis?

Atomic emission spectroscopy.

Provide a description of a "soft" ionization source. Explain the advantages and disadvantages PART 2

Chemical Ionisation: --> Employs a modified E.I. source with an inlet port for introduction of a reagent gas at relatively high pressure (ca. 1 torr). --> Gasses such as methane or isobutane are commonly employed. --> For example, when methane is placed in the electron impact source, the interaction between the electrons from the source and methane produce many ions, such as: CH5 , CH4 , CH2 , H2 , C2H5. --> A number of these are strong proton donors (i.e. acids), such as CH5+. --> Sample is introduced into ionized reagent gas at concentrations that are ~1000- fold lower than that of the reagent gas. --> Sample ionization then occurs primarily by proton acceptance and not electron impact. --> Spectra are very simple (perhaps to the point where not enough information is provided for meaningful analysis). Useful definition of molecular weight is provided, however, very limited structural information is provided (since there is little fragmentation).

What is an image current and what generates it?

Current generated by ions in a cyclotron.

Fourier transform can be used to analyze data collected from:

Cyclotron mass analyser.

Rowland circle uses:

Diodes to detect photons.

What is an example of a hard ionization source?

Electron Ionization.

T/F Emission lines of a fixation pair must have similar characteristics.

False.

T/F In an ICP torch, an external excitation source such as a xenon lamp is required to excite atoms.

False.

T/F The induction coil that is powered by an RF generator is cooled by tangential argon flow.

False.

T/F The initial ionization of argon in the ICP torch is caused by β particles.

False.

The ICP torch uses water to cool the inner tube.

False.

The continuous source of x-rays is generated by the collision of electrons released by a filament that is subjected to a magnetic field.

False.

The quadrupole mass analyzer relies on generation of a magnetic field to separate ions based on their mass to charge ratio.

False.

With regards to atomic emission, the wavelength of the emitted photon is greater than the wavelength of the absorbed photon.

False.

A Coolidge tube provides online a continuous source of excitation.

False. Also a line source of excitation.

Semiconductor LEDs operate in reverse bias.

False. They operate in forward bias.

Provide a description of a "soft" ionization source. Explain the advantages and disadvantages PART 1

Field Ionization: --> Two electrodes are placed in close proximity to one another. --> A high potential is applied between the electrodes to provide high electric field intensity, but not so high that arcing/sparking occurs. --> One electrode (typically the anode) is in the form of a sharp blade or tiny points (carbon whiskers / micro-needles). --> Radius of curvature of the blade end or needle points serves to concentrate the field to 10^8. --> Under these high field conditions, quantum level distortion occurs such that the conduction band energy of the metal/carbon electrode is brought to be on par with the valence band electrons of the sample so that tunnelling can occur to provide ionization of the gas phase sample interacting with the anode. --> Cathode is in the form of a slit so that cations are accelerated out of the ionization chamber in a collimated fashion. --> The sample is in gaseous form. ADV: This is considered a very gentle source of ionisation (i.e. not much fragmentation, good abundance of molecular ions, which is useful for molecular mass determinations). DISV: Does not provide with structural information. ***Field Desorption (same explanation required. The only difference is that the sample is adsorbed on the surface of the electrode).

Quantum tunnelling allows for ionization of molecules in the what ionization source:

Field ionization.

All of the argon gas us completely ionized in the Geiger counter because:

High voltage applied between the cathode and anode.

Atomic Absorption Instrumentation/Sources

Hollow cathode lamp.

Atomic Emission Instrumentation/Sources

ICP torch so no source needed.

The source of excitation in atomic emission is a(an):

ICP torch.

Explain why is it necessary to use a narrow line source (e.g. hollow cathode lamp or an electrodeless discharge lamp) in atomic absorption spectrometry. For clarity, this question is not asking for the operating principles of these sources, rather, the rationale for their implementation.

If a continuum source in conjunction with a monochromator was used, a a band of wavelengths (typically 1 nm bandwidth) from the continuum would be delivered to the sample. As atomic absorption linewidths are typically on the order of a few tenths of an angstrom in width at best, this would result in the reduction in source intensity of only a fraction of a percent, even if the sample concentration was quite high. As such, the signal-to-noise associated with these readings would not be optimal owing to the fact that this motif would result in a small signal change on a bright background. In the case where a line source is used, where the source bandwidth is similar to the absorbance bandwidth of the sample, then, in principle, all of the source intensity could be effectively attenuated for sufficiently high sample concentrations and and the signal to noise ratio would be much improved as excess background radiation would no longer be reaching the detector (i.e. wavelengths outside of what the sample can absorb).

Type of Semiconductor Detector

Lithium Drifted Germanium Detector.

Molecular Fluorescence

Looks at valence electron processes.

Can a soft ionization technique be used to gather structural information about the analyte of interest.

No.

Does molecular fluorescence require atomization.

No.

Does x-ray fluorescence require atomization.

No.

Is the plasma from the ICP torch generated from the RF impedance coil.

No.

Explain the principles of operation of X-ray Source Coolidge Tube. Discuss the spectrum produced by this source of radiation. Provide a well-labelled diagram and the spectrum to aid your explanation.

PART 1 --> Evacuated tube with a tungsten filament cathode and large 'Target' anode made of an appropriately selected metal (e.g. Cu, Ag, Ni) to provide the X-ray wavelength of interest. --> Filament is heated by means of resistive heating and a large potential is applied between the filament and target (kV). --> Electrons stream from the heated filament and accelerate toward the target. --> The accelerated electrons loose their KE on striking the target and cause ionization and formation of x-rays. --> The process efficiency for formation of X- rays is ~1%, with the remaining 99% of the energy given off as heat. As such, the target needs to be cooled so that it doesn't melt. --> Filament heating circuit controls the intensity. --> Potential applied between the filament and target controls the energy of emission. PART 2 --> Two types of emission observed from Coolidge Tubes: continuous and line type emission. 1. Continuous emission originates from collisions between electrons and atoms where the collision causes deceleration of an electron and a photon is released. Broad range of photon energies produced. 2. Line type emission is from X-Ray fluorescence emission from the target.

Problem/Solution for atomic absorption/emission

PROBLEM --> For atomic emission, when an element is excited and absorbs light, it relaxes back down and releases the same wavelength of light (no vibrational/rotational states = releases the exact same energy that is absorbed). --> This means the same wavelength of light being absorbed is also emitted. So, how is there a difference in light that can measured? SOLUTION --> The light we shine on our sample goes in one direction but when the sample emits light, it goes in all different directions (will not be straight). Only a small portion of light that is emitted will be in the same direction as the light shined on the sample. BUT 1. When measuring the emitted light, the amount of emitted light going in the same direction is so small that it does not affect the measurements. 2. Also, the intensity of the light being shined on the sample to excite the elements is not high enough/significant enough to be measured.

With regards to gas ionization detectors, voltage is applied between the chamber and the electrode to:

Prevent the recombination of the cation and electron.

What is the general principle of operation of all three detectors? Provide a well labeled diagram to aid your explanation.

Radiation (e.g. alpha, beta, gamma, x-ray) ionizes an atom or molecule of filler gas creating an ion pair (cation & electron). Under the applied potential, these ions migrate to the cathode and anode, and are neutralized. An external circuit detects the amount of charge created via electrical current monitoring.

How to Make a Holographic Grating

Take a film (photo polymer), put your positive resist on it. Take a laser and shine light on it that passes through the filter/lens and hits against the mirror. The bean splitter allows some wavelengths to hit against the photoresist and others to hit against the tray at the bottom. Some go through, some go down and reflect, causing destructive/constructive interference between the photons of the laser.

Degree of fragmentation in electron ionization cab be controlled to a certain extent by controlling the voltage applied to generate electrons in the electron gun.

True.

For an ion to get through the magnetic sector unit and reach the detector, Fc and Fm must be balanced such that the radius of the ion path is made to match the radius of the ion channel leading to the detector.

True.

T/F Argon ions, once formed in a plasma, are capable of maintaining the temperature at a level which allows foe further ionization and sustains the plasma for a significant period of time.

True.

T/F Atomic emission spectroscopy can be used to simultaneously analyze up to 70 elements.

True.

T/F High population of electrons in the plasma of an ICP torch shifts equilibrium against ion formation which means a greater percentage of atoms remain in the non-ionised state.

True.

T/F The Echelle grating comprises two dispersion elements, either two gratings or the combination of a grating and a prism.

True.

The ionization of the analyte molecules by the electron gun is not caused by the collision of molecules with electrons.

True.

Basic Semiconductor Theory: Diodes

Two ways to make a diode: 1. Reverse bias: --> Attaching the negative side (negative terminal) to the P-type with positive holes. --> When this happens a bunch of positive holes are next to a negative terminal and they won't move anywhere because they are happy. Vice versa. --> So no flow of electrons because everyone is happy where they are = no current. --> Point of this = when you shine light (a photon) and it hits this, the energy coming from the photon will make the electron jump (liberate the electron/electron absorbs energy) towards the positive hole/positive terminal à pushes an electron to join the current/circuit = get a signal/get detected. --> Every photon will produce electron hole pair and depending on the energy of the photon you'll have multiple electron hole pairs. --> In this form the semiconductor can act as a detector = diode. 2. Forward bias: --> What makes a lamp. --> When you attach the positive terminal next to the positive holes they will not like that because same charges repel. Same thing for negative terminal. --> So electrons try to run away. --> There is an energy difference between the conduction band and valence band = so all the electrons running away towards the positive holes are going to jump from the conduction band to valence band = this makes a photon which is used to make lights (e.g.: LED lights). --> Depending on the energy gap, you get different colors of light.

What is meant by elemental analysis?

Type of analysis done to determine the elemental composition of a given sample.

Is atomic emission a form of elemental analysis?

Yes

Does the sample need to be in its atomic form for atomic absorption/emission?

Yes, it requires atomization.

Does X-ray fluorescence require a source of excitation?

Yes.

Is there a source of excitation for atomic fluorescence spectroscopy?

Yes.

Detection Motifs: Pulse Height Analyser

You have a detector to detect the energy, but from here we still need some more electronics to be able to continue. --> Use a pulse height analyzer used for wavelength dispersive instruments. --> When you set your monochromator (to select wavelength) to a certain angle/position, this instrument knows at this angle what kind of radiation you are going to get and for that radiation what kind of signal you should expect = sets a signal limit (determines the height of the pulse that you should expect, and based on this it puts a lower and upper limit). --> The photon you detect is above the upper limit or below the lower limit, counted and the instrument will keep that signal. Anything lower than this signal will be counted as garbage = background noise. --> If the signal it receives is between the lower and upper limit it will keep it and store it for you. --> Also helps with resolution.