CHEM 2001 Exam 2
irradiance
P or P0, is the energy per unit time per unit area is a light beam
What charcteristics go with Phosphorescence
T1 to S0, lower energy, longer wavelength
detector
Typically a photomultiplier tube that generates an electrical current proportional to the photon flux.
Rank wavelengths from shortest to longest: Visible Light, Infrared, Microwaves, and X-rays
X-rays, Visible Light, Infared, Microwaves
Wavenumber
a measure of spatial frequency
charged couple device
a two- dimensional array of pn junctions that stores electrons until they are read sequentially with very high quantum efficiency
photovoltaic cell
absorption of infared radiation creates electrons and holes, which are attracted to oppsoite sides if the junction
Xenon Arc Lamp
an electrical discharge in Xe gas provides intense continum emission from 175-1000
Spectrophotometry
any technique that uses light to measure chemical concentration
Absorption spectrophotometry
any technique that uses the absorption of light to measure chemical concentrations
Which functional group is common among most organic molecules that fluoresce?
aromatic ring
atomizing
atomizes the sample
In Beer's law we want all radiation not absorbed by the sample to:
be transmitted to the detector
Advantages of monochromator instruments
better wavelength accuracy, less stray light
Identify the highest energy molecular process that occurs when a molecule absorbs an ultraviolet photon.
bond breakage
Monochromator
breaks light into its colors (wavelengths), blocks all but one color, and send the one selected color to the sample
As temperature of blackbody radiation increases:
brightness increases, peal emission wavelength shifts to shorter wavelengths
ideal characteristics of a continuum light source
broad emission over all wavelengths, intense, stable intensity over time
Releasing agents
chemicals that are added to a sample to decrease chemical interference
cleaning
cleans out remaining residue
Emission spectrum
collected at a constant excitation wavelength while varying the emission wavelength
deterium lamp
common arc source of continuous ultraviolet radiation
Photodiode
compact, inexpensive, and low power consumption photodetector in which loght striking semiconductor generates and electrical current
method of standard additions
compensates for many types of interference by adding known quantities of standard to the unkown in its complex matrix
monochromatic light
consists if "one cololor", the better the monochromator, the narrower is the range of wavelengths in the emerging beam
charring
decomposes the sample
Does absorbance increase, decrease, or no change? The stock solution is diluted two times instead of just once
decrease
Beer- Lambert Law
describes the relationship between the sample concentartion and absorbance
Identify an appropriate combination of source and detector for UV absorbance spectroscopy.
deuterium arc lamp and photodiode array
Which of the sources is commonly used as a continuum source in ultraviolet (UV) spectroscopy?
deuterium lamp
diffraction grating
disperses the light into its component colors, seperates a beam of white light into multiple beams called orders
Wavelength
distance between wave crests
When the frequency of the light is doubled, its energy will ________.
double
Drying
dries the sample
a radio uses ____________ radiation
electromagnetic
When a molecule absorbs an ultraviolet photon, which of the molecular transitions is observed?
electronic
Identify the highest energy molecular process that occurs when a molecule absorbs a visible photon.
electronic excitaion
An ultraviolet-visible (UV‑Vis) spectrometer is used to measure
electronic transitions in a molecule from the ground state to the excited state
at a higher concentration:
emission intensity curves due to seld- absorption of the excited beam
at very low concentration:
emission intensity is proportional to analyte concentration
Double- Beam spectrophotometer:
enables synchronus measurment of sample and reference, splits light into two beams, irradiance of sample and reference are measured at the detector and used to calculate transmittance and absorbance
__________ and __________ are directly proportional
energy and frequency
Identify the statement that correctly describes the relationship between wavelength and energy.
energy and wavelength are inversely proportional
polychromatic light
eneters through the entrance slit
Photodiode array v. monochromator instruments
- photodiode arrays allow faster spectral acquisition, more rugged, light not cubstantially increased when the sample compartment is open - monochromator instruments better wavelength accuracy, less stray light,
Which detectors are used in ultraviolet (UV)‑visible spectroscopy?
- phototube - photomultiplier tube
Which detectors are used in infrared (IR) spectroscopy?
- pyroelectric cell - thermocouple
Which conditions can result in deviations from Beer's law when the pathlength is constant?
- the absorbing species undergoes dissociation or association -stray light reaching the detector - the use of polychromatic radiation
When molecular absorption is followed by molecular fluorescence:
- the emission wavelengths are longer than the absorption wavelengths - the emission spectrum is roughly the mirror image of the absorbance spectrum - vibrational structure appears in both the absorption and flourescnce emission spectra
Atomic emission
Atoms are thermally promoted to the excited state through collisions in the plasma. As they return to a lower energy state, they emit photons. No excitation source outside of the plasma is required.
Explain how the plasma is produced by the inductively coupled plasma (ICP) torch by placing the statements in the order in which they occur. - the ions and lectrons flow in a closed annular path - the AR ions and free electrons are acelerated by the flcutuating magnetic field (H) produced by the radio- frequency induction coils -A stream of Ar gas is fed into the quartz tuber at a rate of 11 L/min to 17 L/min - A spark from the tesla coil ionizes the Ar - Resistance by the ions and electron to the flow of charge causes ohmic heatinf of the plasma
- A stream of Ar gas is fed into thr quartz tube at a rate of 11 L/min to 12 L/min -A spark from a Tesla coil ionizes the Ar - The Ar ions and free electrons are accelerated by the fluctuating magnetic field (H) produced by the radio- frequency induction coils - The ions and electrons flow in a closed annular path - Resistance by the ions and electrons to the flow of charge causes ohmic heating of the plasma
Why must a cuvette be completely dry before use?
- Residual liquid inside or outside the cuvette could cause interference with spectra. - Excess solvent in a cuvette can affect the concentration of the substance being studied.
Beer's law is accurate when
- The irradiances are measured using monochromatic light - The concentration of the absorbing analyte is ≤ 0.01 M -The analyte does not undergo any chemical changes that alter the absorption spectrum of the original analy
What are advantages to using a furnace instead of a flame in atomic absorption?
- a smaller amount is required - higher sensitivity because the atomized sample is in the optical path longer
Negative deviations from Beer's Law
- chemical effects - polychromatic light - stray light
Identify the advantages to using a photodiode array spectrophotometer for spectral acquisition instead of a dispersive spectrophotometer? -better resolution and wavelength accuracy -faster spectral acquisition times -higher signal‑to‑noise ratio -greater dynamic range
- faster spectral acquistion times - higher signal-to-noise ratio
Identify the advantages to using a photodiode array spectrophotometer for spectral acquisition instead of a dispersive spectrophotometer? -better resolution and wavelength accuracy - faster spectral acquisition times - higher signal- to- noise ratio - greater dynamic range
- faster spectral aquistion times - higher signal- to- noise ratio
graphite furnace
- maximum recommended temperature is 2550°C - requires only tens of microliters of sample - must be programmed to dry, char, then atomize the sample
Combustion flame
- most common fuel- oxidant combination is acetylene air - requires at least 10 mL of sample
excitation spectrum
Collected at a constant emission wavelength while varying the excitation wavelength
Fourier Transform spectroscopy advantages
entire interferogram is recorded in a few seconds and stored in a compute, signal to noise ratio can by improved by collected more interferograms and averaging them
Charged Coupled Device (CCD)
extremely sensitive detector that store photo- generates charge in two- dimensions array
Advantages of phototide array
faster spectral acquisition, more rugged, light not substantially increased when the sample compartment is opened
Atomic Absorption Spectroscopy
flame atomizes sammple and produces atoms in ground state, illuminated by charcteristic wavelengths of a hollow- cathode lamp, atoms absorbs light, concentration calculated from absorbance
Atomic Fluorescence
flame atomizes sample to produce atoms in ground state, line source irradiates atoms in flame, flourescence and excitation state is measured
Anode
for every one photon that striked the PMT, 10^6 electrons are collected at its surface
Stoichimetric Flame
fuel= oxidant
Visible spectroscopy
glass
Which of the sources is commonly used as a continuum source in infrared (IR) spectroscopy?
globar
Interferogram
graph of output light versus retardation, if light from source is monochromatic the interferogram is a cosine wave
Flame advantages over Graphite furnace
graphite furnace requires more operator skill to find proper conditions for each sample, have limited lifetime, graphite furnace has more interference from prior runs
An effective way to test the amount of red dye in an aqueous solution is to measure the amount of ________________ absorbed by the solution
green light (495-570 nm)
If you _____________ the wavelength, the electromagnetic radiation energy will double.
halve
The energy of the electromagnetic radiation will _____________ if you halve the wavenumber.
halve
Which source would be the best source for 632.8 nm light?
helium neon laser
Ideal characteristics of a detector
high sensitivity over a wide wavelength range, low background, low noise
Suppose a student is preparing a set of standard solutions for use in a spectrophotometric study. If percent transmittance, %T were measured in the standard solutions before the color had a chance to fully develop, the measured value would be __________ compared with solutions that have allowed the color to fully develop
higher
visible light has a _______ frequency than infared raditaion
higher
visible light has a ___________ frequency than infared radiation
higher
Infared Photons
include transitions between vibrational states of motion of a molecule
Does absorbance increase, decrease, or no change? More stock solution is used than was specified during the dilution
increase
Does absorbance increase, decrease, or no change? The dilute sample solution is left uncapped allowing half the solvent to evaporate
increase
Does absorbance increase, decrease, or no change? The solution was diluted to a smaller total volume than was specified
increase
Advantages of wide slits
increases energy reaching the detector and gives high signal- to - noise ratio, good precision in measuring absorbance
Microwave photons
induce transitions between rotational states of a molecule
Arrange the different types of electromagnetic radiation by lowest to highest frequency. Only include the items that are forms of electromagnetic radiation. - short wavelength X- rays - long wavelength X-rays - a waving flag - infrared radiation - gamma ray radiation - blue light
infrared radiations, blue light, long- wavelength, X- rays, short- wavelength, X-rays, gamma ray radiation
Isobaric Interference
interference by ions with a similar mass-to-charge ratio
spectral interference
interference from the overlap of analyte signal with signals from other atoms or molecules, the flame, or the furnace
ionization interference
interference of analyte atoms which decreases the concentration of neutral atoms
chemical interference
interference relates to how some component in the analysis might reduce atomization of a sample, thus preventing complete analysis.
The frequency of a wave is _______________ to its wavelength
inversely proportional
flame
is used to atomize the sample
lasing medium
it contains the active species. Some examples of different active species are semiconductors such as GaAs or gases such as He-Ne
reflection grating
it diffracts radiation into different wavelengths and disperses it at different angles
photoemissive cathode
it emits electrons when exposed to radiation
Anode
it is made of tungsten
emission monochromator
it is positioned 90 degrees to the incident light and selects a narrow wavelength band of radiation to pass to the detector
excitation monochromator
it is typically a grating, prism, or filter that selects a narrow wavelength band of radiation and passes it to the sample
transparent envelope
it is typically made of glass or quartz and allows photons to pass through uninterrupted
exit slit
it is where monochromatic radiation leaves the monochromator
Entrance slit
it is where polychromatic radiation from the source enters the monochromator
dynode
it produces several secondary electrons for every photoelectron that strikes its surface
collimating mirror
it splits the radiation into parallel rays
nebulizer
it uses the rapid flow of the oxidant to break the sample into a fine mist
blackbody radiation
light emitted from a heated object
Infrared radiation has _________ wavelengths than gamma rays
longer
What is the hollow- cathode lamp filled with?
low- preassure Ne or Ar
Radio waves are ___________ in energy than microwaves
lower
Rank energy from lowest to highest: Visible light, Infrared, Microwaves, and X-rays
microwaves, infared, visible light, X-rays
beam chopper
mirror that rotates to direct light to either the sample or the reference cuvette
phosphorescence
molecular light emission that occurs during a transition between states of different spin (triplet or singlet)
Identify the highest energy molecular process that occurs when a molecule absorbs a microwave photon.
molecular rotation
Identify the highest energy molecular process that occurs when a molecule absorbs an infrared photon.
molecular vibration
Properties of Lasers:
monochromatic, extremely bright, collimated, polzarized, coherent
Flourescne instrument cost __________ than absorption instruments.
more
Fourier transform spectroscopy
most important method for observing the entire spectrum at once
what must happen for a molecule to be IR active?
must posses a dipole moment which changes upon vibration
Why should you choose the wavelength located at the peak for Beer's Law?
near the peak the absorbance is relatively uniform
Does absorbance increase, decrease, or no change? A student spills some of the diluted solution prior to measurement
no change
Does absorbance increase, decrease, or no change? More diluted sample solution is poured into the cuvette than neccasary
no change
atomic emmission spectroscopy
no flame, emission intensity proprtional to element concentration, most common form
Line noise (interference or whistle noise)
noise concentrated at discrete frequencies that come from sources external to an intened measuring system
Interystem crossing
nonradiative transition between states with different spin (T1 to S0)
our eyes see wavelengths that are ___________.
not absorbed
Frequency
number of oscillations per second of the wave
triplet state
one in which unpaired electrons have identical spin, in a magnetic field a triplet state splits into three states
singlet state
one in which unpaored electrons have opposite spon, in a magnetic field a singlet state remains a single state
mirror
passes the beam to the detector
Disadvantages of wide slits
peak shape is distorted; and absorbance loses its linear relationship with concentration
flourescence
photonic emission from a molecule that has a lifetime of 10E-8 to 10E-4s
atomization
process in which a compound is decomposed into its atoms at high temperature
light source
provides electromagnetic radiation in the ultraviolet (UV), visible, or infared (IR) region of the spectrum
Ultraviolet (UV) spectroscopy
quartz
White Noise (Gaussian Noise)
random noise that is always present due to random movement of charge carriers in an electric circuit
Infrared (IR) spectroscopy
KBr
Graphite furnace advantages over flames
Lower minimum sample volume, longer sampling time in optical path, easier preconcentration, no nebulization step required in a graphite furnace
intersystem crossing
Nonradiative transition between states with different quantum numbers (T1 to S0)
internal conversion
Nonradiative transition between states with the same quantum number (S1 to S0)
internal conversion
Nonradiative transition between states with the same quantum number and spin (S1 to S0)
inductively coupled plasma
-temperatures reach up 10 10,000 K - very expensive - can be coupled with mass spectrometry
Which conditions can result in deviations from Beer's law when the pathlength is constant?
-the absorbance species undergoes dissociation or association - solute concentrations exceed 0.01 M -the use of polychromatic radiation
What happens in atomic spectroscopy?
1) heat decomposes sample into atoms and ions 2) atoms absorb and emite light at charcteristic wavelengths 3) analyst determs which elements based on charcteristic wavelengths 4) element concentration determined by amount of light absorbed or emitted
molar absorptivity
A characteristic of a molecule that describes how much light it absorbs at a given wavelength in a particular solvent
Quartz-tungsten halogen lamp
A common blackbody source of continuous visible and near-infrared radiation
A spectrophotometer measures the transmittance, the absorbance, or both of a particular wavelength of light after it has passed through a liquid sample. The liquid sample holder is commonly known as a cuvette.
A cuvette has entirely clear walls so that light can pass easily through it.
Which statement correctly describes how a laser works?
A gain medium (lasing material) is pumped with energy to an excited state with a high population by means of light or electrical discharge. When the excited‑state molecule relaxes, a photon is emitted that can perturb another excited‑state molecule, causing the emission of a photon with the same energy, phase, and polarization in a process called stimulated emission. The lasing material is placed between two mirrors, one that is highly reflective and one that allows some light to leave. The stimulated emission reflects back and forth between the mirrors causing more stimulated emission, and a small fraction of light is leaked out as the functional laser light.
photodiode array
A reverse bias is applied to a pn junction, which acts as a capacitor. Light strikes the surface, and holes and electrons are created which move towards the opposite charge.
A flame can be used in both atomic absorption and atomic emission spectroscopy to atomize the sample. In which technique is a stable flame temperature more critical? Why?
A stable flame temperature is MORE critical in atomic EMISSION spectroscopy because a small fluctuation in the temperature can greatly affect the EXCITED- SATE POPULATION of the atoms in the sample.
photomultiplier tube
Accelerated electrons are emitted from a photosensitive surface and strike a second surface, knocking more than one electron from that surface and towards another surface. This process is repeated until there are more than 106 electrons collected per photon that strikes the first surface.
Types of atomic spectroscopy
Emission, absorption, fluorescence
Name two radiative processes
Fluorescence and Phosphorescence
In comparing fluorescence spectroscopy to UV absorption spectroscopy, which of the statements is true?
Fluorescence measurements have better detection limits because at low concentrations a low intensity emission signal is being measured against a low intensity background signal.
Select the meaning of the letters FT in the acronym FTIR.
Fourier transform
E=
H (6.262 x 10E-34) x V (frequency)
sample cuvet
Holds the sample with a defined pathlength.
reference cuvet
Houses the reagent blank.
amplifier
Increases the detector signal for ease of display and quantitation.
Rank from longest to shortest wavelength - an X-ray - gamma rays - infared radiation -high frequency ultraviolet light - low frequency ultraviolet light - a water wave
Infrared radiation, low frequency ultraviolet light, high frequency ultraviolet light, an X-ray, gamma rays
Name two nonradiative processes
Internal conversion and Vibrational relaxation
sample cell
It holds the sample and has a defined pathlength
Hollow cathode lamp
It is composed of the same element that is being analyzed in the sample.
Hollow Cathode
It is cylindrical and made of the element whose emission lines are desired
Ne or Ar gas
It is ionized when a potential of 500 V is applied between the electrodes
Transparent window
It is made of glass or quartz and allows radiation to pass through uninterupted
1% T mirror
It is partially transparent and allows some light to leak out
detector
It is typically a photomultiplier tube that generates an electric signal when struck by photons.
pumping source
It is used to excite the active species and create a population inversion in the laser
Monochromator
It is used to select one line of radiation from the source and remove as much emission for the atomizer as possible.
focal plane
It is what the refocused radiation of different wavelengths is focused onto
spray chamber
It is where the fuel, oxidant, and aerosol are mixed before entering the flame.
Baffles
It promotes further mixing of the mist, oxidant, and fuel; prevents large droplets from reaching the flame.
light source
It provides electromagnetic radiation in the ultraviolet or visible region of the spectrum
0% T mirror
It reflects all light and is used to pass radiation through the active species numerous times
focusing mirror
It refocuses the collimated radiation
What characteristics go with Flourescnece?
S1 to S0, higher energy, shorter wavelength
photoconductive detector
Semiconductor whose conductivity increases when infrared radiation excites electrons from the valence band to the conduction band
What can improve quality if data when dealing with noise?
Signal averaging
atomic absorption
The atomized sample in the flame absorbs a portion of the radiation emitted from the source, which is composed of the same element as that which is being analyzed. The amount of radiation that passes through the flame unabsorbed by the analyte is measured.
Atomic flourescence
The sample is atomized in the flame. A laser is used to promote the atoms in the flame to an excited state. As the atoms return to the ground state, they fluoresce.
glass bead
The spray is directed against it and breaks the droplets into smaller particles.
Why is a single beam spectrophotometer inconvenient?
sample and reference must be placed alternately in the beam, if measurment is at multiple wavelnegths the reference must be measured at each, poorly suited to measuring absorbace as a function of time
1/ f noise (drift)
slow change in the response of an instrument due to various causes such as flow changes in instrument components with temperature and with age
Photon
smallest amount of light that can be generated by a light source; a particle of light
When N spectras are averaged, how much does the signal to noise ratio improve?
sqrt of n
The speed of electromagnteic radiation __________ with frequency
stays the same
Ideal characteristics of a scanning monochromator:
takes in light at all wavelengths, outputs light at a single wavelength, can scan through different output wavelengths
luminescence
the emission of light rom an excited state molecule
change in energy of the molecule =
the energy of the absorbed photon
spectrophotometer
the instruments used to quantitatively measure the reflection or transmission properties of a meterial as a function of wavelength (a spectrum)
Spectrophotomerty
the method used to measure how much a chemical substance absorbs light as a beam of light passes through a sample solution
what happens when a molecule absorbs a photon?
the molecule is promoted to a more energetic state
Complement of Absorbed Color
the observed color
chromophore
the part of a molecule responsible for light absorption
analytical signal
the peak area mesured during the atomizing step
Spectrometry
the practical application of spectroscopy. Uses instruments called spectrometers
speed of light
the product of its wavelength and its frequency
Fluorescence
the release of a photon by radiative relaxation from a singlet state to another singlet state
phosphorescence
the release of a photon by the radiative relaxation from a tripley state to a singlet state
Spectroscopy
the theoretical approach to the science of studying the interaction between matter and radiated energy
Which is NOT a variable in the Beer-Lambert law?
the wavelength of light
Which of the sources is commonly used as a continuum source in visible spectroscopy?
tungsten lamp
Comparison of inductively coupled plasma to flame:
twice as hot, fewer interferences, costs more to purchase and operate
Fuel rich Flame
uses excess fuel to reduce metal oxide and hydroxide formation; Fuel> Oxidant
Fuel- Lean Flame
uses excess oxidant to obtain better heating and atomization; Fuel< Oxidant
Photomultiplier Tube (PMT)
very sensitive detector for measuring the iradiance of light
Does yellow or violet light have a greater frequency?
violet light
Does yellow or violet light have greater energy?
violet light
Identify the statement that correctly describes the relationship between wavelength and frequency.
wavelength and frequency are inversely proportional
scanning monochromator
wavelength selector that passes a narrow band of wavelengths to the sample by scanning through a user-defined range of selected wavelengths
Does yellow or violet light have a longer wavelength?
yellow light
In Beer's law do we want the sample to be homogenous?
yes