BME 543 Exam 2

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dispersion of a grating means: a. ability to distinguish two closely spaced peaks b. bending of light by the grating c. ability to produce angular separation of adjacent wavelengths d. reflection from a flat reflective surface e. loss of energy due to the interaction with the particles of the medium

c. ability to produce angular separation of adjacent wavelengths

The basic spectrophotometer is composed of four components. Which components listed below is NOT one of the four? a. sample b. monochromator c. beam splitter d. light source e. light detector

c. beam splitter

ionization interference

can be a problem in the analysis of alkali metals because they have low ionization potentials

how can overlap between lines of different elements in the sample

choose another wavelength for analysis

resolving power

ability of mass spectrometer to separate two peaks with similar mass

photoconductivity transducers

absorption of radiation by a semiconductor produces electrons and holes, thus leading to enhanced conductivity

the kinetic energy added to the ion in electron ionization is due to the

acceleration potential

effective bandwidth

an inverse measure of the quality of the device, a narrower bandwidth representing better performance; full width at half peak height

solvent qualifications

analyte should not dissociate, associate, or react with a solvent to produce a substance with different qualities than the analyte

x-ray spectrometry

atomization is not required because x-ray spectra for most elements are largely independent of their chemical composition in a sample quantitative results can therefore be based on the direct measurement of the fluorescence, absorption, or emission spectrum of the sample

atomic absorption spectra

atoms absorb radiation of wavelengths characteristic of electronic transitions from the 3s state to higher excited states

atomic fluorescence spectra

atoms or ions in a flame fluoresce when they are irradiated with an intense source containing wavelengths that are absorbed by the element; observed radiation is often the result of resonance fluorescence resulting from absorption of radiation by ground-state atoms and emission by excited-state atoms returning to the ground state

to select narrower bandwidth of wavelengths from the monochromator, the exit slit width is

decreased

most scanning UV-Vis spectrometers measure the light over wavelengths from 190 to 900 nm. What are the likely sources that would be used over this wavelength range?

deuterium lamps, xenon lamps, tungsten lamps

which of the following is not related to decreasing the exit slit width? a. sharper, better resolved peaks are recorded b. it decreases the signal-to-noise ratio c. the radiant power decreases significantly d. it decreases the effective bandwidth e. it reduces the stray light

e. it reduces the stray light (stray light is independent of system design)

ionization energy

energy required to remove the single 3s electron to produce an ion

cuvettes are typically made of

fused silica (SiO2), quartz, or glass for visible spectra

why are prisms preferred over gratings in modern instrumentation

gratings are cheaper to fabricate, provide better wavelength separation for the same size dispersing elements, and disperse radiation linearly along the focal plane

absorption filter

greater bandwidth than interference filters which make them inferior; restricted to the visible region of the spectrum

typical state of atoms at room temperature

ground state

sources of atom excitation

heat from a flame, a plasma, or an electric arc or spark

the greater the number of grating grooves (smaller spacing), the _______ the higher the resolution

higher

chemical ionization

ionization source is filled with a reagent gas such as methane, isobutane, or ammonia, resulting in MH+

dark current

signal present even when light is absent

light source for infrared radiation

silicon carbide glowbar

resolution

smallest difference in m/z values that can be detected as separate peaks

desorption source

solid or liquid state sample is converted directly into gaseous ions

charge-transfer transducer

the charges developed in a silicon crystal as a result of absorption of photons are collected and measured

optical spectrometry

the elements present in a sample are converted to gaseous atoms or elementary ions by a process called atomization the ultra-visible absorption, emission, or fluorescence of the atomic species in the vapor is then measured

When using the Matrix Assisted Laser Desorption Ionization (MALDI) source, the analyte _________ has a much greater influence on volatilization and ionization than EI, CI or other sources

matrix

time-of-flight mass analyzers

measure the time required for positive ions to travel from an ionization source to a detector is measured; velocities of ions vary with inversely with their masses

molecular luminescence spectrometry

measurement of emission of light from an excited molecular state; inherently more sensitive enough to observe single molecules

which are broader: molecular or atomic spectra?

molecular

what occurs upon an excited atoms return to the ground state in atomic emission spectra

photo emission

how does the emission spectrum compare to the absorbance spectrum

the emission spectrum is roughly the mirror image of the absorbance spectra

detector response is a function of

wavelength of incident light

at what angle is fluorescence spectrum usually measured

90 degrees

fluorescence and phosphorescence are measured at what angle inside of the instrument and why

90 degrees; right angle geometry minimizes the contributions from scattering and the intense source radiation

direct analysis in real time (DART)

A DART source produces excited He or N2, which is directed at the surface of an object to be sampled in ambient atmosphere. The excited species react with ambient moisture to create protonated water clusters that react with analyte M to make MH+. The MH+ is measured by mass spectrometry.

interference filter

a specific wavelength is reinforced and most other wavelengths (which are out of phase) undergo destructive interference; available for the UV, visible, and well into the IR region

advantages of a photodiode array

allow faster spectral acquisition than dispersive instruments, which require several minutes; have almost no moving parts

Potassium bromide (KBr) is widely used as a cell material for infrared spectroscopy, but rarely used for UV-Vis. It is not used often for UV-Vis because many samples for UV-Vis analysis are in ________ solution.

aqueous

true or false: spectrophotometric photodiode array detector is based on the photoelectric effect

false - no actual production of current occurs in a photodiode array detector

3 types of ion sources

gas phase source, desorption source, ambient source

using a lean flame

gives a hotter flame

why can't glass cuvettes be used for ultraviolet radiation

glass absorbs UV radiation

radiation source in atomic spectroscopy

hollow cathode lamp

hard ionization sources

impart enough energy to analyte molecules to leave them in a highly excited energy state; provide structural information about analytes

blank subtraction

in most spectrophotometric analysis, it is important to prepare a reagent blank containing all reagents but with analyte replaced by distilled water; any absorbance of the blank is due to the color of uncomplexed ferrozine plus the color caused by the iron impurities in the reagents and glassware; subtract the blank absorbance from the absorbance of samples and standards before doing any calculations

using a rich flame

increases sensitivity because excess carbon can reduce metal oxides and hydroxides

spectrophotometers

instruments used to quantitatively measure the reflection or transmission properties of a material as a function of wavelength (a spectrum)

interference elimination

interference is eliminated if neocuproine or thiorea is added; these reagents mask Cu+ by forming strong complexes that prevent Cu+ from reacting with ferrozine

are LEDs more or less stable than hydrogen, deuterium, xenon, and tungsten lamps

less stable

what is a laser

light amplification by stimulated emission of radiation; monochromatic bright collimated polarized and coherent

Beer's law describes the absorption behavior of media containing relatively ________ analyte concentrations

low (<0.01 M)

disadvantages of photovoltaic cells

low internal resistance of the cell makes the amplification of its output less convenient, exhibits fatigue in which its current output decrease gradually during continued illumination

mass spectrum shows detector response as a function of

m/z (mass to charge ratio)

mass spectrometry

samples are atomized, but in this case, the gaseous atoms are converted to positive ions (usually singly charged) and separated according to their mass-to-charge ratios the separated ions are then counted to provide quantitative data

mass selector function

separate ions based on mass to charge ratio

magnetic sector mass spectrometer

separates gaseous ions by accelerating in an electric field and deflecting ions of different mass-to-charge ratio through different arcs in a magnetic field; ions are detected by an electron multiplier, which works like a photomultiplier tube

photovoltaic cells

simple device used for detecting and measuring radiation in the visible region; the radiant energy generates a current at the interface of a semiconductor layer and a metal whose magnitude is proportional to the number of photons that strike the semiconductor

advantages of furnaces over flames

smaller sample required; furnace confines the sample in the light path for several seconds; multiple aliquots of sample can be injected onto the furnace platform and evaporated; no nebulization is required in a furnace

disadvantages of photodiode arrays

the resolution of 0.1 nm attainable with dispersive instrument and the wavelength accuracy are better than those of a photodiode array (~0.5-1.5 nm); stray light is less in a dispersive instrument than a diode array instrument

gas phase source

the sample is first vaporized then ionized

spectroscopy

the theoretical approach to the science of studying the interaction between matter and radiated energy

to produce narrow lines of the correct frequency, hollow cathode lamps must contain

the vapor of the same element being analyzed

doppler broadening (thermal broadening)

the wavelength of radiation emitted or absorbed by a rapidly moving atom decreases if the motion is toward a transducer and increases if the atom is receding from the transducer; doppler broadening causes spectral lines to be about two orders of magnitude wider than the natural line width

problem with concentric and cross-flow nebulizers

they only produce approximately 1% of droplets of the correct size to attain the plasma

what is the function of a nebulizer

to convert the sample to gaseous atoms or ions

what is MALDI (matrix-assisted laser desorption/ionization) used for?

to obtain accurate molecular mass information about polar biopolymers

true or false: in a magnetic field mass spectrometer, the spectrum of masses is obtained by varying the magnetic field strength

true

true or false: sensitivity with an inductively coupled plasma is enhanced by using a piezoelectric crystal as an ultrasonic nebulizer

true

most common light source in continuous visible and near-IR analyses

tungsten lamps

charge transfer devices and photodiodes are often used in instruments where there is a simultaneous detection of a range of wavelengths of light. For this type of detection, individual transducers are arranged in _______ which detect light after it has been separated by a polychromator.

two dimensional arrays

spectral interference

unwanted signals overlapping analyte signals

the wider the exit slit, the ______ the band of wavelength selected by the monochromator

wider

photomultiplier tube

contains a photo emissive surface as well as several additional surfaces that emit a cascade of electrons when struck by electrons from the photosensitive area; 10^6 - 10^7 electrons reach the anode for each incident photon

most common light sources in a molecular spectrometer

continuum sources including deuterium lamp, tungsten lamp, LED

5 components of atomic spectroscopy instruments

1. source (hollow cathode lam, electrodeless discharge lamp) 2. nebulizers 3. sample atomizers (flame, electrochemical vaporization, hydride, ICP) 4. wavelength selection (monochromator) 5. detector (PMT, CID)

3 major components of mass spectrometers

1. source of ions 2. mass separator 3. detector

5 molecular spectroscopy instrument components

1. stable light sources (deuterium arc lamps, tungsten filament, LED, Xe arc lamp) 2. wavelength selectors 3. sample containers (transparent sample holder made or quartz/glass/plastic) 4. radiation transducers (PMT, photodiode, PDA detector) 5. signal processors and read out devices

5 components of typical spectroscopic instruments

1. stable source of radiant energy 2. a transparent container for holding the sample 3. a device that isolates a restricted region of the spectrum for measurement 4. a radiation detector which converts radiant energy to a usable electrical signal 5. a signal processor and readout which displays the transduced signal on a digital display, a computer screen, or another recording device

wavelength range of deuterium arc lamp

110 to 400 nm

wavelength range of silicon carbide glowbar

4000 to 200 cm-1

max wavelength of ferrozine

560 nm

wavelength range of tungsten lamp

320 to 2500 nm

maximum sensitivity of photovoltaic cells

550 nm; range similar to human eye

nebulization

the formation of small droplets

electrospray ionization (ESI) process

1. A solution of the sample is pumped through a stainless steel capillary needle at a rate of a few microliters per minute. 2. The needle is maintained at several kilovolts with respect to a cylindrical electrode that surrounds the needle. 3. The resulting charged spray of fine droplets then passes through a desolvating capillary, where evaporation of the solvent and attachment of charge to the analyte molecules take place. 4. As the droplets become smaller as a consequence of evaporation of the solvent, their charge density becomes grater until, at a point caller the Rayleigh limit, the surface tension can no longer support the charge. 5. Here Coulombic explosion occurs and the droplet is torn apart into smaller droplets. 6. These small droplets can repeat the process until all the solvent is removed from the analyte, leaving a multiply charged analyte molecule.

6 spectroscopic phenomena

1. absorption 2. fluorescence 3. phosphorescence 4. scattering 5. emission 6. chemiluminescence

how does a photodiode array work?

1. at the beginning of a measurement, each diode is fully charged 2. photons absorbed in the semiconductor create mobile electron-hole pairs 3. the more radiation that strikes each diode, the less charge remains at the end of the measurement 4. the state of each diode is determined at the end of the cycle by measuring the current needed from the instrument to recharge the diode.

3 types of atomic spectroscopy

1. emission from a thermally populated excited state 2. absorption of sharp lines from hollow-cathode lamp 3. fluorescence following absorption of laser radiation

5 components of monochromators

1. entrance slit to provide rectangular optical image 2. collimating lens or mirror produces a parallel beam of radiation 3. prism or grating to disperse the radiation into its component wavelengths 4. focusing element that reforms the image of the entrance slit and focuses it on a planar surface called a focal plane 5. exit slit in the focal plane isolates the desired spectral band

7 characteristics of ideal detectors

1. high sensitivity 2. high signal to noise ratio 3. constant detector response as a function of wavelength 4. fast response time 5. no dark current 6. signal proportional to radiant power 7. rugged, cheap, simple

what does a wide grating slit imply

1. increased energy reaching the detector 2. high signal to noise ratio 3. good precision in measuring absorbance

3 main components of spectrophotometers

1. light source (lamps and lasers) 2. wavelength selector (gratings and monochromators) 3. detectors (photomultiplier tubes, photodiodes, thermocouples, ferroelectric materials, etc.)

7 steps of molecular absorption spectrometry

1. sample prep 2. selection of wavelength 3. variables that influence absorbance 4. cleaning and handling of cells 5. determining the relationship between absorbance and concentration (calibration curve) 6. analyze sample absorbance

how does atomic spectroscopy work

1. samples are vaporized and decompose into atoms and ions 2. their concentrations are measured and based on their absorbance or emission of light 3. gives the ability to distinguish one element from another in a complex mixture 4. analyte can be measured at ppm or ppt levels

3 spectral line broadening effects

1. uncertainty effect 2. doppler broadening (thermal broadening) 3. pressure broadening

Bunsen prism monochromator vs Czerney-Turner grating monochromator

Bunsen prism monochromator uses prism and collimating lens Czerney-Turner grating monochromator used mirrors and reflection grating

which statements are true for the various ion selection techniques? I. Time-of-flight MS - Ions expelled from the source all have the same kinetic energy. For ions of different masses but equivalent kinetic energies, the lighter ions travel faster than heavier ions, separating the ions. II. Quadrupole MS - A constant voltage and a radio-frequency oscillating voltage is applied to the four metal rods. The electric field deflects ions in complex trajectories as the ions migrate from source to detector, allowing only one particular m/z to reach the detector. III. Three-dimensional quadrupole ion-trap MS - A constant-frequency radio-frequency voltage is applied to the central ring electrode causes ions to circulate in stable, three-dimensional orbits in the cavity, with the lowest m/z in the outermost orbits. Increasing the amplitude of the radio-frequency voltage destabilizes m/z ion orbits one value at a time. IV. Magnetic sector MS - Ions are accelerated into the magnetic sector with equivalent kinetic energies. Ions are selected by passing through a magnetic field; lighter ions are deflected the most and the heavier ions are deflected the least. The magnetic field is varied to select individual m/z.

I, II, III, IV

which statement(s) is/are true for photomultiplier tubes? I. photomultiplier tubes are very sensitive detectors, amplifying the photoelectron current by approximately 1 million II. emitted photoelectrons are accelerated toward positively charged electrode called dynodes III. electrons strike the dynode with a kinetic energy less than the original emitted kinetic energy IV. for each electron striking a dynode, more than one electron is knocked free. The additional electrons are accelerated toward the next dynode in the detector where amplification is repeated

I, II, IV

which of the following statements are true of atomic emission spectroscopy (AES)? I. atoms are promoted to an excited state with a laser II. atoms are promoted to an excited state by gaining energy from collisions with other atoms or from the high thermal energy of the flame III. the intensity of emitted light is proportional to analyte concentration IV. plasma often replaces flame in AES

II, III, IV

Which of the following statement(s) are true when comparing electron impact to chemical ionization sources for mass spectrometry? I. EI sources never result in a molecular ion peak (M+) peak in the mass spectrum II. CI sources often result in a molecular ion (M+) peak in the mass spectrum III. EI sources use reagent gases such as ammonia or methane to ionize the analyte IV. it is impossible to obtain useful structural information about an analyte using a mass spectrometer with a CI source V. ejection of an electron from the analyte due to electrostatic repulsion from source generated electrons is what causes ionization in the EI source

II, V

Analytes must be ionized prior to entering the mass analyzer in a mass spectrometer. Electron ionization and chemical ionization are two ionization techniques. Which of the following statements are INCORRECT regarding either technique? I. Electron ionization uses an electron beam to create molecular ions, M+. II. Chemical ionization uses an electron beam to ionize reagent gas III. Electron ionization gives very little fragmentation of the molecular ion IV. The ionized reagent gas in a chemical ionization undergoes a complex set of chemical reactions before protonating the analyte to create MH+ V. Both ionization techniques give identical mass spectra

III, V

In IR, cells are constructed of

NaCl or KBr (rarely disposable plastic)

ICP-MS (inductive coupled plasma mass spectrometry

a plasma can be directed into a mass spectrometer, which separates and measure ions according to their mass-to-charge ratio

desorption electrospray ionization (DESI)

a solvent is electrosprayed onto a surface to dissolve analyte from the surface into aerosol microdroplets, which can be analyzed with a mass spectrometer

how does a grating work

a grating has closely ruled lines, each behaving as a separate source of radiation: 1. different wavelengths of light are diffracted at different angles from the grating 2. rotation of the grating allows different wavelengths to reach the sample

how does UV-Vis work?

a change between deuterium and tungsten lamps when passing through 360 nm so that the lamp of highest intensity is always used; lamp switch causes noise

which of the following is true for emission spectra? a. excitation wavelength is fixed, and the emission wavelengths are scanned b. excitation wavelength is scanned, and the emission wavelength is fixed c. excitation wavelength is fixed, and the emission wavelength is fixed d. excitation wavelength is scanned, and the emission wavelength is scanned e. non of these answers are true

a. excitation wavelength is fixed, and the emission wavelengths are scanned

the most intense peak in the mass spectrum is called: a. the base peak b. the molecular ion c. the adduct d. the protonated molecule e. the fragment

a. the base peak

ambient source

allow desorption ionization with minimal sample pretreatment and without the enclosures of typical ionization sources

which interferences can be minimized by using releasing agents? a. spectral b. chemical c. isobaric d. ionization e. none of the above

b. chemical

which is not an appropriate detector for a spectrophotometer? a. photomultiplier tube b. thermal conductivity c. photodiode array d. charged coupled device e. phototube

b. thermal conductivity

slit width is usually measured in terms of

bandwidth of radiation selected by the slit

how to choose monochromator bandwidth

bandwidth should be as wide as the spectrum permits to allow the most possible light to reach the detector

in the schematic shown here, the excitation beam pathway is orthogonal (at a right angle) to the emission beam pathway. Why is this? a. source light can be absorbed by the emission wavelength selector b. source light that is passed to the transducer would not be detected c. fluorescence measures the light that is emitted by atoms or molecules in the sample, which happen to be excited by light from the source d. you are interested in measuring the light absorbed by the sample and not the source e. any other arrangement would lead to light being detected by the reference transducer

c. fluorescence measures the light that is emitted by atoms or molecules on the sample, which happen to be excited by light from the source

The doppler effect is one of the reasons for line broadening in the atomic absorption and emission. Which of the following factors increases magnitude of Doppler broadening? a. higher atomic mass b. decreasing temperature c. higher velocity of the absorbing or emitting species d. higher number of atoms moving perpendicular to the transducer's path e. higher concentration of atoms in the plasma

c. higher velocity of the absorbing or emitting species

soft ionization sources

cause little fragmentation; supply information about molecular mass of the analyte molecule(s)

chemical interference

caused by a component of the sample that decreases the extent of atomization of the analyte

releasing agent

chemicals that are added to a sample to decrease chemical interference

sample preparation for molecular absorption spectrometry

collect the serum sample by centrifuging human blood; reduce Fe3+ in transferrin to Fe2+ which is released from the protein; add trichloroacetic acid to precipitate proteins, leaving Fe2+ in solution; centrifuge the mixture to remove the precipitate; transfer a measured volume of supernatant liquid from step c to a fresh vessel and add buffer plus excess ferrozine to form a purple complex

advantages of double beam instruments

compensate for all but the most short term fluctuations in the radiant output of the source as well as for drift in the transducer and amplifier; they compensate for wide variations in source intensity with wavelength; the double beam design lends itself well to the continuous recording of transmittance or absorbance spectra

charge coupled device (CCD)

extremely sensitive detector that stores photo-generated charge in a two dimensional array

all of the following are true for luminescence, except: a. luminescence is emission of light from an excited state of a molecule b. luminescence is inherently more sensitive than absorption c. at low concentration, emission intensity is proportional to analyte concentration d. at high concentration, emission intensity decreases due to dilution e. light emitted from a chemical reaction is called chemiluminescence

d. at high concentration, emission intensity decreases due to dilution

The atomic absorption and emission spectra consist of sharp lines while the molecular spectra exhibit broad absorption bands. This is because a. the atomic absorption and emission are faster than the corresponding molecular processes b. high temperature increases the population of the atomic excited states c. atomic spectra are recorded in the gaseous phase only d. atomic energy levels do not have vibrational and rotational sublevels e. molecular peaks are broadened by the solvent

d. atomic energy levels do not have vibrational and rotational sublevels

The most common source in atomic absorption is the hollow-cathode lamp. Which of the following statements is not true about the hollow-cathode lamp? a. Doppler broadening and pressure broadening are reduced b. the anode and the cathode are sealed in a glass tube c. the ionized inert gas is sputtering off atoms from the cathode d. it produces single-wavelength monochromatic radiation e. the performance of the lamp depends on the ionization potential of the carrier gas

d. it produces single-wavelength monochromatic radiation

the function of the reflection part of a time-of-flight mass spectrometer is to: a. accelerate the ions towards the detector b. increase the resolving power of the instrument c. collide with ions of lower energy d. turn around ions toward the grid in front of the detector e. separate ions of the same mass by kinetic energy

d. turn around ions towards the grid in front of the detector

ionization supressor

decreases the extent of ionization of the analyte

pros and cons of inductive coupled plasma

detection limit is typically two orders of magnitude lower than that observed with a flame; twice as hot as flame, stable, inert air environment, simultaneous multi-element analysis, costs more to purchase and operate than the flame

light source of UV radiation

deuterium arc lamp

monochromator

device that uses an entrance to define the area of the source viewed and exit slit to isolate a single wavelength band; disperses light into its component wavelengths and selects a narrow band to pass on to the sample or detector

which of the following applies to mass spectra from chemical ionization? a. the nitrogen rule does not apply b. abundance of fragment ions is observed c. small or absent molecular ion peak is observed d. no ions higher than m/z = M+ are observed e. adduct ions with m/z higher than MH+ can be observed

e. adduct ions with m/z higher than MH+ can be observed

what happens at the electron multiplier detector

each arriving ion starts a cascade of electrons that reach the anode where current is measured

which results in more fragmentation: chemical or electron ionization

electron ionization

2 methods of converting neutral molecules to ions

electron ionization and chemical ionization

electron ionization

electrons emitted from a hot filament are accelerated through 70V before interacting with incoming molecules M, resulting in M++

line sources

emit a limited number of lines, or bands of radiation, each of which spans a limited range of wavelength widely used in atomic absorption spectroscopy, atomic and molecular fluorescence spectroscopy, and Raman spectroscopy

continuum source

emit radiation that changes in intensity smoothly as a function of wavelength useful for absorption and fluorescence

rich flame

excess fuel

lean flame

excess oxidant

Beer's law is strictly applied only when measurements are made with what type of radiation?

monochromatic source radiation

the part of a spectrophotometer that selects the wavelength to irradiate the sample is called the

monochromator

what monochromator bandwidth generally gives acceptably small distortion or peak shape

monochromator bandwidth should be one fifth as wide as the absorption peak

quadrupole mass spectromter

most common mass analyzers in use today; offer low scan time; 4 parallel rods serve as electrodes and opposite rods are connected electronically with two + and two -, only ions with limited range of m/z values reach the transducer

which does not describe the limitations to Beer's law? I. Beer's law describes absorption behavior of media containing low analyte concentrations (<0.01 M), exhibiting minimal solute-solvent and solute-solute interactions II. Absorptivity depends on the refractive index of the medium III. Beer's law strictly applied only when measurements are made with monochromatic source radiation IV. It fails of the analyte undergoes a chemical change and the product of this change has a different spectrum than the original analyte V. an analyte should not dissociate, associate, or react with a solvent.

none

piezoelectric crystal

one whose dimensions changed in an applied electric field; a sinusoidal voltage applied between two faces of the crystal causes it to oscillate

emission and absorption spectra

opposite of each other

silicon photodiodes

photons cause the formation of electron-hole pairs and a current across a reverse-biased p-n junction

pressure broadening

pressure, or collisional, broadening is caused by collisions of the emitting or absorbing species with other atoms or ions in the heated medium broadening in the hollow cathode lamps and discharge lamps results mainly from collisions

detector

produces an electrical signal when it is struck by photons

Basically, single beam spectrophotometers use simple phototubes for detecting electromagnetic radiation in the visible range. These are useful as transducers because at saturation potential the current is __________ to the radiant power of light striking the cathode. At the same time, one must consider that such detectors have a measurable _________ that must be accounted for.

proportional; dark current

phototubes

radiation causes emission of electrons from a photosensitive solid surface

Unlike single beam spectrophotometers, a fluorometer is also a single beam instrument but it uses a _______ transducer to adjust for source lamp fluctuations

reference

absorptivity depends on what quality of the medium

refractive index

increasing the bandwidth of a wavelength selector may increase the intensity of radiation reaching the sample or the transducer. However, this generally results in a decrease in

resolution

photodiode array

rows of p-type silicon on a substrate of the n-type silicon create a series of pn junction diodes; essential for rapid spectroscopy as it records the entire spectrum at once in a fraction of a second

uncertainty effect

spectral lines always have finite width because the lifetimes of the upper and lower states of the transition are finite, which leads to uncertainties in the energies of the states and to line broadening; although the lifetime of a ground-state electron is long, the lifetimes of excited states are generally short, typically 10^-7 to 10^-8 s; line width due to uncertainty broadening are sometimes called natural line width and are generally about 10^-5 nm

the nitrogen rule

states that if m/z for M is odd, then the molecular formula must have an odd number of nitrogens. if m/z for M is even, then the molecular formula must have an even number of nitrogens (includes 0)

the hydrogen rule

states that the maximum number of hydrogens in the molecular formula is 2C+N+2. in the formula, C is the number of carbons and N is the number of nitrogens

what is mass spectrometry used for

studying the masses of atoms or molecules or fragments of molecules; provides information about the elemental composition of samples of matter, structures of inorganic and biological molecules, qualitative and qualitative composition of solid surfaces, isotopic ratios of atoms in samples

self-absorption

the absorption of excitation or emission energy by analyte molecules in the solution

complete resolution of two lines is feasible only if the slit width is adjusted so that

the effective bandwidth of the monochromator is equal to one half the wavelength difference of the lines

spectrophotometry

the method used to measure how much a chemical substance absorbs light as a beam of light passes through a sample solution

spectrometry

the practical application of spectroscopy; uses instruments called spectrometers


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