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Final Exam Flashcards

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1
Q

What is spectrophotometry?

A

The use of electromagnetic radiation to measure chemical concentrations

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2
Q

What is wavelength?

A

the crest-to-crest distance between waves

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3
Q

What is frequency?

A

the number of oscillations the wave makes each second

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4
Q

What is wavenumber?

A

inverse of wavelength

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5
Q

What 3 properties does energy increase with?

A

increasing frequency, decreasing wavelength, increasing wavenumber

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6
Q

What is the relationship between energy and wavelength and wavenumber?

A

Inversely proportional to wavelength and directly proportional to wavenumber

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7
Q

What is the electromagnetic spectrum?

A

the range of wavelengths or frequencies over which electromagnetic radiation extends

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8
Q

What is the ground state?

A

lowest energy state of a molecule

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9
Q

What is an excited state?

A

when a molecule absorbs a photon and its energy increases

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10
Q

What is a spectrophotometer?

A

an instrument that measures transmission of light

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11
Q

What is a monochromator?

A

a device that selects a narrow band of wavelengths

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12
Q

What is monochromatic?

A

light with a narrow range of wavelengths

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13
Q

Draw a block diagram for a single-beam spectrophotometer

A

Light source ~~> Wavelength selector (monochromator) ~> sample ~> light detector —> computer to display results

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14
Q

What is transmittance?

A

the fraction of incident light that passes through a sample

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15
Q

What is absorbance proportional to?

A

the concentration of light-absorbing molecules in the sample and the pathlength of substance through which light travels

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16
Q

What is T defined as? (equation)

A

P/P0

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17
Q

What is molar absorbtivity?

A

how much light is absorbed at a particular wavelength by a particular substance

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18
Q

What is an absorption spectrum?

A

a graph showing how A or E varies with wavelength

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19
Q

What is a cuvet?

A

A cell that has flat, fused-silica faces that can transmit visible and UV radiation

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20
Q

Draw a block diagram for a double-beam spectrophotometer

A

Light source ~~> scanning monochromator ~~~|rotating mirror| path A: ~~> sample cuvet ~~|semitransparent mirror| ~~> detector —> display
path b: ~~|mirror| ~~> reference cuvet ~~|mirror| ~~> |semitransparent mirror| ~~~detector —-> display

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21
Q

What are the 3 different light sources in a spectrophotometer?

A

deuterium arc lamp, tungsten lamp, globar (silicon carbide)

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22
Q

What is a grating?

A

a series of closely ruled lines that disperses light

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23
Q

What is diffraction?

A

The bending of light rays by grating

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24
Q

What is the trade off between resolution and signal?

A

the narrower the exit slit, the greater the ability to resolve closely spaced peaks and the noisier the spectrum

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25
What is a detector?
something that produces an electric signal when it is struck by photons
26
What is a photomultiplier tube?
a very sensitive detector that includes light striking a photosensitive cathode which emits electrons, which then strike a positive dynode. Those electrons then get struck off and the cycle repeats over until 10^6
27
What is a photodiode array?
Records entire spectrum at once * White light passes through the sample * Polychromator disperses light into its component wavelengths & directs light to the diode array
28
What are the pros and cons of a photodiode array?
Pros: faster, wavelength repeatability, simultaneous measurement, low errors Cons: poor resolution
29
What is an isosbestic point?
a wavelength at which the absorption of light by a mixed solution remains constant as the equilibrium between the components in the solution change
30
What is the relationship of absorption in mixtures?
Absorbance at a wavelength is the sum of all of the species' absorbances at that wavelength
31
What is a spectrophotometric titration?
titration where you monitor changes in absorption or emission to detect the end point
32
What are molecular oribitals?
the distribution of electrons in a molecule
33
What is corrected absorbance?
Corrected absorbance = (total volume ÷ initial volume) x (observed absorbance)
34
What is an electronic transition?
When an electron moves from one orbital to another
35
What is a singlet state?
the state in which spins are opposed
36
What is a triplet state?
The state in which spins are parallel
37
What are the relative energies of vibrational and rotational states?
rotation of a molecule is less energy than vibration
38
Are singlet or triplet states lower energy?
triplet
39
What is fluoresence?
transition from S1-S0
40
What is phosphoresence?
transition from T1-S0
41
Where do fluorescence and phosphorescence occur relatively?
phosphorescence is at a lower energy and longer wavelength
42
What is photochemistry?
a chemical reaction initiated by absorption of light
43
What is chemiluminescence?
chemical reactions that release light
44
Draw a block diagram of an emission monochromator
Light source ~~~> excitation monochromator ~~~> sample cell ~~> emission monochromator ~~~> detector
45
Why is luminescence more sensitive than absorption?
luminescence measurements typically involve detecting light emitted from a sample, whereas absorption measurements detect light that has passed through a sample and has been partially absorbed
46
What does luminescence utilize?
90 degree detection
47
Why is there an upper limit to the range of useful concentrations of analyte in fluorescence?
the upper limit to the range of useful concentrations of analyte in fluorescence primarily arises due to a phenomenon known as "concentration quenching" or "self-quenching." This effect reduces the fluorescence intensity and efficiency when the concentration of the fluorescent species becomes too high
48
What is atomic absorption spectroscopy?
when radiation is passed through a flame and the intensity of transmitted radiation is measured (lamp)
49
What is atomic emission spectroscopy?
radiation is emitted by hot atoms whose electrons have been promoted to excited states in the flame (no lamp)
50
Draw a block diagram for atomic absorption
Hollow-cathode lamp --> Flame --> monochromator --> detector --> amplifier ---> computer
51
What is the difference between atomic absorption and emission?
Absorption = light source beams atom through flame to the monochromator & detector Emission = atom in the flame gives a signal to the monochromator and detector
52
What is atomization?
the process of breaking analyte into gaseous atoms which are then measured by their absorption or emission
53
What is the benefit of a furnace over a flame?
furnaces offer increased sensitivity and require less sample
54
Why does a graphite furnace have high sensitivity?
it confines atoms in the optical path for several seconds
55
What are the pros and cons to inductively coupled plasma?
Pros: high temp and stability eliminate problems flames have Cons: expensive
56
What are the relative detection limits of a flame and furnace? Why?
Furnace = 100x lower than flame because the sample is in a small volume for a long time in a furnace
57
Why is a hollow cathode lamp used?
to produce narrow lines of correct frequency
58
Compare and contrast flame atomic absorption, furnace atomic absorption, plasma emission, and inductively couples plasma
Flame atomic absorption: lowest cost, different lamp required per element, poor sensitivity Furnace: expensive, different lamp required per element, high background signals, high sensitivity Plasma: expensive, no lamp, low background and low interference, moderate sensitivity ICP: very expensive, no lamp, lease background and interference, highest sensitivity
59
What are the types of interference?
spectral, chemical, ionization
60
What is spectral interference?
unwanted signals overlap analyte signal
61
What is chemical interference?
chemical reactions decrease the concentration of analyte atoms
62
What is ionization interference?
ionization of analyte atoms decreases the concentration of neutral atoms
63
What is the ICP method most sensitive for?
trace analysis
64
What is chromatography?
a process in which we separate compounds from one another
65
What is the mobile phase?
the solvent moving through the column
66
What is the stationary phase?
the substance that stays fixed inside the column
67
What is an eluent?
fluid entering the column
68
What is the eluate?
fluid exiting the column
69
What are the different types of chromatography?
adsorption, partition, ion-exchange, molecular exclusion, affinity
70
What is adsorption chromatography?
uses a solid stationary phase and a liquid or gas mobile phase, solute is adsorbed on the surface of solids
71
What is partition chromatography?
thin liquid stationary phase, solid equilibrates between stationary liquid and mobile phase
72
What is ion-exchange chromatography?
features ionic groups covalently attached to the stationary solid phase. solute ions are attracted to the stationary phase
73
What is molecular exclusion chromatography?
separates molecules by size (large molecules pass first)
74
What is affinity chromatography?
specific interactions between one kind of solute molecule and a second molecule that is covalently attached to the stationary phase
75
What is retention time?
the time needed after injection for an individual solute to reach the detector
76
What do more theoretical plates mean?
narrower bandwidth
77
What does smaller plate height mean?
narrower peaks
78
What are the factors that lead to band broadening?
longitudinal diffusion, multiple paths, equilibration time
79
Why doesn't an open tubular column experience band broadening?
it doesn't have multiple paths like packed columns and it is longer
80
What is the downside to open tubular column?
it cannot handle as much solute since there is less stationary phase so it is useful only for analytical separations
81
What happens in a mass spectrometer?
* Molecules are ionized, separated based on mass to charge ratio, & detected * Molecular ion peak gives the mass of the molecule * Fragments provide clues about structure of the molecule
82
What are the components of a mass spec?
4 parallel metal rods and an electron multiplier ion detector
83
What are the pros and cons of electron ionization vs chemical ionization?
Electron ionization: pros: High Sensitivity, Fragmentation Patterns, which can be used to deduce the structure of the molecule, Widely Available and Standardized cons: too much fragmentation, limited to volatile compounds Chemical ionization: pros: softer ionization, flexibility cons: Complex Spectra, Sensitivity to Experimental Conditions, Less Fragmentation Information
84
What is the nominal mass?
the integer mass of species with the most abundant isotope of each constituent
85
What are the components of a gas chromatograph?
rubber septum, injector oven, column oven, column, detector oven, detector, computer
86
What is the difference between WCOT, SCOT, and PLOT
WCOT: stationary liquid phase SCOT: solid support coated with stationary liquid phase PLOT: stationary solid-phase particles
87
What is temperature programming?
raising column temperature during the separation to separate compounds with a wide range of boiling points
88
What is the function of a guard column?
a silanized column that collect nonvolatile components that would otherwise be injected into the column and never be eluted and ruin the column
89
What are the components of an HPLC?
solvent delivery system, injection valve, a detector, and a computer
90
What is the difference between a normal-phase and reverse phase HPLC?
normal-phase HPLC uses a polar stationary phase & less polar mobile phase reverse-phase HPLC uses a nonpolar stationary phase & polar mobile phase
91
What is isocratic gradient elution?
elution with a single solvent or mixture
92
What is gradient elution?
when solvent is changed from weak to stronger eluent during chromatography

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