Instrumental chp 9

Define Spectral Interference

1. Arise when the absorption or emission of an interfering species either overlaps or lies too close to the analyte absorption or emission such that the resolution by the monochromator becomes impossible two things happen

Define Atominization

1. In order to obtain spectra the constituents of a sample must be converted to gaseous atoms (or in some cases ionized atoms) 2. The process by which the sample is converted into an atomic vapor is called atomization 3. Types of atomizers used for atomic spectroscopy A. Flame B. Electrothermal C. Inductively Coupled Argon Plasma (ICP)

Define Releasing Agent

A releasing agent is a cation that preferntially reacts with a species that would otherwise react with the analyte to cause a chemical interference.

Define Ionization Supressor

An ionization suppressor is more easily ionized than the analyte and provides a high concentration of electrons in the flame or plasma.

Chemical Interference alteration B

Dissociation equilibrium i. Can often be corrected by use of fuel-rich or fuel-lean flames dependent on analyte and type of equilibrium interference

How do electrode less discharge lamps work?

Energized by an intense field of radio frequency or microwave radiation. Ionization of the Ar occurs to give ions that are accelerated by the high frequency component of the field until they gain sufficient energy to excite the atoms of the metal whose spectrum is sought

Chemical Interference alteration A

Formation of compounds of low volatility i. Use of higher temperature flames ii. Releasing agents - cations that interact preferentially with the interferant and prevent its with the analyte iii. Protecting agents - prevents interference by forming stable but volatile species with the analyte

Describe the processes that occur during flame atomization. (Use Figure 9.1 as your guide)

Heat evaporate solvent, we want it to be atoms. (Can create ions).

Chemical Interference alteration C

Ionization equilibrium i. Ionization suppressors

Define Protective agent

Protective agents prevent interference by forming stable and volatile products with the analyte.

What does Chemical Interference result from?

Result from various chemical processes occurring during atomization that alter the absorption characteristics of the analyte

Why is source modulation used in atomic absorption spectroscopy?

Source modulation is employed to distinguish between atomic absorption (an ac signal) and flame emission (a dc signal).

Describe how a deuterium lamp can be used to provide a background correction for an atomic absorption spectrum.

The continuum radiation from the D2 (subscript) lamp is passed through the flame alternately with the hollow-cathode beam. Since the atomic lines are very narrow, the D2 (subscript) lamp is mostly absorbed by the background, whereas the hollow-cathode radiation is absorbed by the atoms. By comparing the radiant power of the two beams, the atomic absorption can be corrected for any background absorption.

Why is an electrochemical atomizer more sensitive than a flame atomizer?

The electro thermal atomizer is a more efficient atomizer. It requires much less sample and keeps the atomic vapor in the beam for a longer time than does a flame.

What is the purpose of the flame in flame atomic absorption spectroscopy?

The flame is used to atomize the sample, which breaks the molecular bonds to create atoms.

Why is atomic emission more sensitive to flame instability than atomic absorption or fluorescence?

The population of excited atoms from which emission arises is very sensitive to the flame temperature and other conditions. The population of ground state atoms, from which absorption and fluorescence originate, is not as sensitive to these conditions since it is a much larger fraction of the total population.

What are some of the common interferences encountered in AA and what measures are used to minimize these interferences? (Section 9C)

The position of the equipment, you adjust your burner and your fuel source to optimize the error of interest. Different molecules break apart in different ranges, so it needs to be adjusted.

Why is it so critically important to control flame temperature in flame atomic absorption spectroscopy?

To focus the same part of the flame of the entrance slit for all calibrations and analytical measurements.

What is the purpose of an internal standard in flame emission methods?

When an internal standard is used, the ratio of intensity of the analyte line to that of the internal standard is plotted as a function of the analyte concentration (see Fig. 1.12). If the internal standard and the analyte species are influenced in the same way by variation in the aspiration rate and the flame temperature, and if the internal standard is present at approximately the same concentration in the standards and the unknown, the intensity ratio should be independent of these variables.

Define Radiation Buffer

a substance added in excess to both sample and standards, which swamps the effect of the sample matrix on the analyte emission or absorption.

Why do electrothermal atomizers generally provide higher sensitivities than flame atomizers?

a. Because the entire sample is atomized in a short period and the average residence time of the atoms in the optical path is a second or more. b. In flame atomizers a large portion of the sample flows down the drain and the residence time of individual atoms in the optical path is the flame is brief. c. Because in the electrothermal atomizers it stays in optical path for longer period of time so it is able to gather more information

How do hollow cathode tubes work?

a. Cathode is negatively charged (composed of element of interest) apply voltage. b. Anode is positively charged c. Inside the tube you have an inert gas (like argon or neon) with electrical components, the anode excites the gas making it very energetic and excited gas hits the cathode, which releases photons. d. Sputtering when excited gas hits the cathode and dislodges some of the metal.

Which technique is more sensitive, flame AA or atomic emission spectroscopy and why? (Hint- this is a trick question)

a. Depends on what compound, the condition of the compound, and how much. But overall flame AA because you get a higher response the emission. b. LOOK AT TABLE

What are four atomization techniques?

a. Electro thermal b. Flame c. Hydride generation and Cold-vapor d. ICP = inductively couples plasma

Compare and contrast hollow cathode lamps and electrode less discharge lamps.

a. Filled with the inert gas like argon or neon, when a potential is applied it caused the gas to be excited causing the gas to move toward the cathode, this is repeated; the gas ultimately relaxes and producing specific atomic emission lines. b. A salt containing the metal of interest is sealed in a quartz tube with inert gas; radio frequency is used to excite the gas which causes the metal to be ionized. c. For certain elements electrode less discharge lamps has greater sensitivity

Describe the processes involved in sample introduction and atomization in flame AA. (Use Figure 9.1 as your guide)

a. In a flame atomizer, a solution of a sample is nebulized by a flow of gaseous oxidant, mixed with a gaseous fuel and carried into a flame where atomization occurs. The first is desolvation, in which the solvent evaporates to produce finely divided solid molecular aerosol. Then aerosol is the volatilized to form gaseous molecules. Dissociation of most of these molecules produces an atomic gas. Some of the atoms in the gas ionize to form cations and electrons. Other molecules and atom are produced in the flame as a result of interactions of the fuel with the oxidant and with the various species in the sample.

What two things happen with Spectral Interference?

a. Interference due to overlapping lines can most often be avoided by using an alternate wavelength b. Also result from the presence of combustion products that exhibit broadband absorption spectra or particulates that scatter radiation

Be familiar with the differences and similarities of single beam and double beam spectrometers used in atomic spectroscopy.

a. Spec 20 is a single beam. b. Flame AA is a double beam in time (for the one we viewed in class - in little back room) c. Double beam in time (one detector; half the time behind flame, half the time through flame) some type of chopper d. Double beam in space... has two detectors, light is split off by two mirrors, going off in two different mirrors, going to sample and another to reference (flame aa in air) then to the detector.

Why must the height of the burner head be adjusted each time a new analyte is examined with flame AA?

a. The height of the burner has different regions such as the low flame as the primary combustion zone; the blue color flame that is hydrocarbon, then the internal region; which is in the middle relatively narrow and free atoms are prevalent, than the highest is the secondary combustion zone; converting to stable molecules oxides then disappears into surroundings. The flame must be adjust up and down with respects to the beam until the region of max absorption is located. b. Look at Figure 9-4 p.232 slideshow ch.8-10 c. Basically, to optimize the absorbance.

21. Discuss the temperature profile of a flame and how this profile affects the processes that go on in that flame relevant to flame atomic absorption and emission spectroscopy.

a. The lowest range the atoms are being atomized, then the next range the atoms are stable the largest range, then the atoms are excited, then the last range they are ionized. b. too hot it can form ions, too much of atomic species it can form molecules. For molecular species it's reversible. c. Figure 9.1, pg 231, chp. 8-10pp - slide 22

The effective bandwidths of the lines emitted by AA lamps are significantly narrower than the corresponding bandwidths of the absorption peaks in flame AA. Why is this fact so critically important to the success of flame AA?

a. Use a very narrow wavelength you only get your anon light of interest. b. They need to be narrower to be detected...

22. Describe the processes that occur in electrothermal atomization.

a. We use a graphite cylindrical tube that is open on both sides and a central hole for introduction of sample, then we have interchangeable graphite tubes fit snugly into a pair of cylindrical graphite electrical contact on each side, where that electrical which will pass through electrical current through the thrums and the anon light, but at the same time you are pumping gas and it will be ionized. The internal stream flows into the two ends and out the central sample port excludes air and carries out vapors. The sample is evaporated and ashed onto a platform. When temp is not changing rapidly and atomization is occurring.

Describe the various interferences in atomic absorption spectroscopy and what is done to minimize these interferences.

a. When an internal standard is used, the ratio of intensity of the analyte line to that of the internal standard is plotted as a function of the analyte concentration (fig 1-12). if the internal std and the analyte species are influenced in the same way by variation in the aspiration rate and the flame temp, and if the internal std BB9, slide 45 b. if the analyte and the standard should be the same concentration, otherwise it should be compensated. c. Spectral interferences i. -Arise when the absorption or emission of an interfering species either overlaps or lies too close to the analyte absorption or emission such that the resolution by the monochromator becomes impossible d. Chemical interferences i. -Result from various chemical processes occurring during atomization that alter the absorption characteristics of the analyte

20. Describe the hydride generation process for atomic absorption spectroscopy. Why is this technique used rather than simply introducing the liquid sample into a flame?

a. You want the flame to atomize anon light no your solvent. Heating up the sample at a lower temperature than the actual flame. You always use a liquid sample for AA. b. Look at slide 17 in Ch. 8-10

Define Pressure Broadening

dew to collision, the ground state changes for the collided atoms, so the amount of energy it takes to get to a different ground state is different for the collided atom than the ground atoms.

Elaborate on the presence of combustion products

i. Continuum source correction ii. Zeeman background correction

Explain Flame atomization techniques?

i. Most reproducible of all liquid sample introduction methods for atomic absorption and fluorescence methods ii. However, the sampling efficiency and thus the sensitivity are in general much poorer than other atomization methods 1. A large portion of the sample flows down the drain 2. The residence time of the individual atoms in the optical path is brief (~10-4 seconds)

Explain Inductively Couples Plasma atomization techniques?

i. Plasma= ions more than atoms. ii. RF excites Ar, sample is then sucked up into plasma. The analyte do not ionize because the plasma acts as an ion suppressor since the plasma is in ion state. This is an emission technique iii. Much higher temperatures than AA iv. Thus higher number of atoms in excited state v. Thus stronger signal

Explain Hydride generation and Cold-vapor atomization techniques?

i. Provides a method to introduce arsenic, antimony, tin, selenium, bismuth, and lead into an atomizer as a gas. This often enhances the detection limits by a factor of 10 to 100. 1. Generally brought about by the addition of an acidified aqueous solution of the sample to a small volume of 1% aqueous solution of sodium borohydride. 2. The volatile hydride is then swept into a heated atomization chamber where decomposition of the hydride takes place leading to the formation of atoms of the analyte

Explain Electro thermal atomization techniques?

i. Samples are placed in a carbon tube which is heated electrically ii. The tube is heated in steps 1. Drying (5-200 °C) (remove solvent) 2. Char (200-800 °C) (remove/decompose matrix) with Argon gas. 3. Atomization (2000-3000 °C) (atomize analyte) no gas iii. Sample residence time is much greater compared to flame atomizers thus significantly increasing detection limits and sensitivity iv. Solid samples can also to assayed

Define Hollow-cathode lamp

repeated bombardment of the metal atom by the gas causes it to be excited, it ultimately relaxes, producing specific atomic emission lines.

Define Sputtering

the dislodging of metals by the ionized gas slamming against the cathode surface.

Define Self-absorption

the higher the concentration the higher the probability of self-absorption, a photon leaving a molecule colliding with another (same) molecule before it gets detected.

Define Doppler broadening

the wavelength of the photons changes to the detector based on the direction they are moving.