BME 543 Exam 2
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
