Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Chem 315 > Quiz 2 > Flashcards

Quiz 2 Flashcards

You may prefer our related Brainscape-certified flashcards:
Periodic Table flashcards Chemistry 101 flashcards AP® Chemistry flashcards Organic Chemistry 101 flashcards
1
Q

What is spectrophotometry?

A

is any technique that uses light to measure chemical concentrations

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What is frequency as a property of light?

A

Is the number of complete oscillations that a wave of light makes per second. Denoted as u=oscillations/s

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What do light waves consist of?

A

Perpendicular and oscillating electric and magnetic fields

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is wavelength as a property of light?

A

denoted as lambda, and is the crest to crest distance between waves.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What is a hertz?

A

Is one oscillation per second.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What is plane polarized light?

A

Light exists in two dimension that are perpendicular to each other (the magnetic and the electric) and therefore exist in the xy and xz axis. Plane polarized light is when light goes through an object that polarizes it and becomes 2D, as in light only travels in two axis (xy) for example rather than (xyz)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What is the mathematical relationship between frequency and wavelength?

A

frequency (u) times wavelength (l) equals c (speed of light) (3x10^8 m/s)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What changes when light moves between mediums with different refractive indexes?

A

The lights wavelength, but not it’s frequnecy

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What is the equation for light in terms of energy?

A

Energy = Frequency (u) * Plancks constant (h)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is Plancks constant?

A

it is h= 6.626 * 10^-34 J/s

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What is the equation that relates frequnecy, energy, and wavelength?

A

E=hc/l or E=hc*ubar

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What is ubar?

A

Is 1/lamda, and is called wavenumber, units in cm^-1

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What happens when molecules absorb light in the x ray range?

A

Molecules break bonds and ionize

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What happens when molecules absorb light in the ultra violet range?

A

electronic excitation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What happens when molecules absorb light in the infrared range?

A

They vibrate

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What happens when molecules absorb light in the microwave range?

A

rotation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

look at electromagnetic spectrum diagram and name the wavelengths for each type of light.

A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

What is the ground state?

A

The lowest energy state of a molecules

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

What happens when light is absorbed by a molecule?

A

The energy of the molecule increases and the molecule is promoted to an excited state.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

When light is absorbed by a sample what happen to the irradiance of the beam of light?

A

The irradiance of the beam of light is decreased.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

What is irradiance?

A

Irradiance (denoted as I) is the energy per second per unit area of the light beam

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

Why does the irradience of a light beam decrease after it hits a sample? Explain with an example

A

For example if we have a light source which goes through a monochromator and then narrows the range of wavelengths to select one wavelength (Ltr), and then that wavelength of light hits our sample with a certain length, our sample will have absorbed some of the light so the irradiance of our light beam after it hits the sample (L0) is lower.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

What is transmittance?

A

Transmittance (T) is defined as the fraction of the original light that passes through the sample, ie how much light passed through the sample from the original amount that hit it.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

What is the eqn for transmittance?

A

T=Ltr/L0

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

What is absorbance?

A

A= log(L0/Ltr) = -log T

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

What is beers law?

A

It’s A =ebc, C is the concentration of the sample (in mol/l M), b is the pathlength of the sample 9in cm), and e is the molar absorptivity (M^-1 cm^-1). A is absorbance here.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

What is molar absorptivity?

A

It tells us how much light is absorbed at a particular wavelength by a particular substance

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

How can beers law change if we measure how Absorbance or molar absorptivity depends on wavelength?

A

A=ebc becomes A(lambda)= e(lambda)bc

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

What is a chromophore?

A

The part of a molecule that absorbs light

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
30
Q

How do we see colors in objects?

A

When white light (which contains all wavelengths of light in the visible spectrum) hits an object and is transmitted or reflected through it, the object absorbs certain wavelengths of the white light and our eyes detect the wavelengths that are not absorbed which is what make sit appear to have color

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
31
Q

In what situations does beers law work?

A

Only works with monochromatic radiation and in dilute solutions (<0.01 M), by monochromatic radiation the bandwidth of the light must be smaller than the absorption band of the chromophore, and it also does not work samples in which the concentrations are dependent on an equilibrium as absorption differences between the two compounds can cause errors in beers law.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
32
Q

What is a cuvet?

A

Is a sample cell that has flat fused silica surfaces and contains liquid samples for spectroscopy, it has a common pathlength of 1.0 cm.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
33
Q

How do we use a cuvet?

A

We first measure the irradiance of light passing through a cuvet containing a pure solvent or reagent blank, and define that as L0 (so light before hitting sample). Then we measure the irradiance of light passing through through a cuvet with our sample in it and define that as LTr, knowing both L0 and LTR, we can get transmittance and absorbance values.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
34
Q

Why don’t we measure the incident irradiance (L0) directly? Why is it measured as the Ltr of our blank cuvet?

A

This is to account for any absorption, reflection, and scattering that can be done by the cuvet and solvent, so we get the exact absorbance and transmittance of our sample.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
35
Q

Are glass cuvets suitable for visible light? Uv?

A

yes they’re suitable for visible light but not uv because it absorbs uv,

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
36
Q

What kind of cuvets are suitable for uv light?

A

UV light needs cuvets that are made of fused quartz

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
37
Q

How do we measure the IR absorbance/transmittance of an unknown solid sample?

A

We take 1% wt mixture of the solid sample with KBr and grind it to a fine powder, then apply pressure (600 bar) which converts it into a translucent pellet, we do the IR spectroscopy and measure the absorbance of IR light (y axis) as a function of wavenumber.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
38
Q

Can you touch the clear faces of a cuvet?

A

No! This skews results of irradience from the sample and alters absorbance.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
39
Q

Why do we need longer sample cells (cuvets) for gasses? What size do we need?

A

We need longer pathlengths for cuvets (10 cm) when dealing with gasses because gasses are more dilute than liquids and therefore need a large surface area from the light to hit it.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
40
Q

Name 8 types of cuvets?

A

Standard 1 cm path
cylindrical
Micro cells
5 mm path
1 mm path
20 mm path
Flow
Thermal

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
41
Q

Where should we measure absorbance from?

A

Measure absorbance at the wavelength of maximum absorbance, this is because the sensitivity of the analysis is the greatest at maximum absorbance, we get the maximum response for the analyte, and also because there’s less noise at the maximum absorbance so there’s little variation here if the monochromator drifts or if the width of the transmitted band changes slightly

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
42
Q

What absorbance is most precise/reproducible in spectrometry? What should we do to our sample if we do not meet this abosrbance?

A

Absorbance values ranging from 0.3 to 2 are the most reproducible, as anything higher leads to too little light getting through the sample and anything lower leads to too much light getting through sample and therefore hard to differentiate between transmittance from the sample and that of the reference, you should try to adjust the sample conc so that it falls in this range.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
43
Q

What two things are needed for a compound to be analyzed by spectrophotometry?

A

The compound must absorb light, and this absorption should be distinguishable from that due to other substances in the sample.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
44
Q

Why do we usually have the visible region be where we do analysis by spectrophotometry?

A

This is because most compounds absorb UV radiation but specific compounds absorb visible light, if most compounds absorb UV radiation our results become inconclusive and we cannot tell which compounds absorption is which.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
45
Q

What is the first step in serum iron determination (measuring the iron content of transferrin in blood)?

A

Transferrin is protein in blood and has a binding site where Fe3+ binds.

You reduce Fe3+ to Fe2+ which allows the iron to be released from the protein, you do this by adding acid such as thioglycolic acid

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
46
Q

What is the second step in measuring iron content of transferrin in blood?

A

You precipitate proteins by adding trichloroacetic acid, you then centerfuge the mixture to leave the iron in solution but remove the ppt proteins. You do this so ppt does not interfere with absorbance as light scattered by it could be mistaken for absorbance.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
47
Q

What is the third step in serum iron determination?

A

Transfer a measured volume of the supernatant liquid from step 2 to a fresh vessel, add buffer (to create a ph in which a complex can fully form) and add excess ferrozine to form a purple complex. Now measure the absorbance at the 562 nm peak.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
48
Q

Why should you measure a reagent blank before doing analysis of iron in serum?

A

Because you want to make sure all the absorbance from the sample is coming from the iron in the sample, you do this finding the absorbance of any iron impurities or uncomplexed ferrozine from the reagent blank and then subtracting it from your standard.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
49
Q

When creating calibration curves, what types of iron should we use to make our standard solutions?

A

Should use irons dissolved in acid like ferrous ammonium sulfate.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
50
Q

What is a problem in our iron serum determination procedure?

A

That results for iron will be around 10% too high because the blood serum also contains copper which also forms a coloured complex with ferroine, we address this by masking the copper through adding neocuprine/thiourea which form strong complexes with copper making them not want to interact w ferrozine.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
51
Q

What is spectrophotometric titration

A

Is where we monitor changes in absorbance during a titration to tell when the equivalence point has been reached.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
52
Q

Explain how spectrophotometric titration works with a solution of the iron transport protein transferrin?

A

Apotransferrin is transferrin without the iron molecules bonded to it and is colorless, each molecule of apotransferrin binds two Fe3+ ions, when fe3+ binds to it the protein turns into a red colour with an absorbance maximum at a wavelength of 465 nm. This absorbance is proportional to the concentration of Fe3+ bound to transferrin. As apo transferrin can only bind two fe3+ atoms and reaches its equivalence point here, we can use the maximum absorbance to determine the amount of iron it takes to reach the equivalence point and then use the stoich ratio of apo transferrin to two iron molecules to find he original amount of apotransferrin in the sample.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
53
Q

When titrating apo transferrin do we take the absorbances as is or do we correct them, if we correct them, why and how do we do that?

A

We correct them, this is because we have to take into account that when we add titrate to the sample we are also diluting the sample of apotransferrin and the volume is different at each absorbance. We correct absorbance so that it shows what the absorbance would be if the sample was never diluted in order to get accurate absorbance values, corrected absorbance = (tot volume/initial volume)(observed absorbance)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
54
Q

What happens when a molecule absorb light?

A

If a molecule absorbs light that has sufficient energy to cause electronic transition (ie vibrational and rotational transitions), then changes in the vibrational and rotational states of the molecule will occur.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
55
Q

Why are electronic absorption bands broad?

A

Because many different vibrational and rotational levels are available at slightly different energies (wavelengths)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
56
Q

What actually happens when a molecule absorbs energy?

A

When a molecule absorbs energy (like a photon) that promotes the molecule from it’s ground state (S0) to a vibrationally and rotationally excited electronic state (S1). Then from this S1 state a nonradiative transition occurs (R1) where the molecule goes through vibrational relaxation- giving it’s vibrational energy to other molecules through collision and it lowers down to it’s lowest vibrational level of S1. Converting some of the photons energy to heat.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
57
Q

What can happen once the molecule has gone through non radiative transition and reached the lowest vibrational level of S1?

A
  • The molecule could go through internal conversion (IC) where is enters a high excited vibrational level of S0 which is at the same energy level as the lowest vibrational level of S1.
    From this excited state the molecule can relax back to the ground vibrational state (S0) and transfer the energy lost to other particles through collision
    (this process is R2). If the molecule follows this path (A-R1-IC-R2), the entire energy of the photon will have been converted into heat.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
58
Q

What other path (not A-R1-IC-R2) could the molecule go through?

A

After nonradiative transition (R1) the molecule can go through (ICS- intersystem crossing), where the molecule goes from S1 to T1 (triplet electronic state) go through another non radiative transition (R3) and then go through another ICS to go back to S0 followed by another non radiative transition (R4) to reach the lowest energy state in the ground electronic state. This process converts light to heat.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
59
Q

What is luminescence?

A

Is emission of light from an excited state of a molecule.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
60
Q

What path could a molecule go through if it is emitting a photon?

A

A molecule could relax from an excited state 9S1 or T1) to a ground state (S0) by emitting a photon.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
61
Q

What is fluorescence?

A

The radial transition from S1 to S0

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
62
Q

What is phosphorescence?

A

The radial transition from T1 to S0

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
63
Q

Whats the difference between a singlet sate and an triplet state?

A

In a single state the electron are still facing opp directions and are still paired, in a triplet stage electrons are facing opposite directions (and are unpaired).

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
64
Q

What is SBS? How is it set up?

A

SBS is a single beam spectrophotometer, and has one light source, that hits one monochromator, and then one sample, and then hits the light detector.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
65
Q

Why is SBS inconvenient?

A

You have to place a reference blank and a sample alternatively, if measuring at different wavelengths you have to measure the reference at each wavelength, not suitable for kinetic experiments as the detector response and light source intensity (L0) can drift leading to inaccurate results.

66
Q

What is a double beam spectrophotometer?

A

This a is a spectrophotometer where light passes alternately through the reference blank (l0) and the sample (Ltr) through the use of a rotating mirror which acts as a beam chopper directing light towards both the reference and sample.

67
Q

How is a double beam spectrophotometer better than a single beam spectrophotometer?

A

A double beam spectrophotometer has a beam chopper which chops light source several times per second getting multiple Lo’s and Ltrs that give absorbance and transmittance values. Because the power emerging from both samples is compared so frequently this procedure automatically corrects for detector response drift and and drift in the initial light source.

68
Q

What is UV-Vis double beam spectrophotometer?

A

Involves two lamps, an ultraviolet one (is a deuterium arc lamp that emits light in the ranges 200-400 nm) and a visible lamp (which is a quartz halogen lamp).
- Only one lamp is used at a time
-Involves a grating which selects a narrow band of wavelengths to enter the monochromator, which then selects an even narrower band of wavelengths to enter the sample and reference.
-light thats gone through sample and reference is detected by photomultiplier tube which creates current proportional to lights irradiance.

69
Q

How is a light source picked in a UV-Vis double beam spectrophotometer?

A

The mirror selects whether deuterium or tungsten lamp will send light source through sample.

70
Q

What is a stepper motor in a UV-Vis double beam spectrophotometer?

A

The stepper motor is what adjusts the size of exit slit (where light leaves the monochromator)

71
Q

What is a tungsten lamp?

A

Is a lamp that has it’s filament made of tungsten, is an excellent source of visible and near IR radiation and produces radiation in the range (320-2500 nm). The filament operates at a temperature near 3000 K.

72
Q

What does a deuterium arc lamp do?

A

Is used in UV spectroscopy, in this lamp a controlled electric discharge causes D2 (deuterium) to dissociate and through mlclr emission emit UV radiation in the range 110-400 nm.

73
Q

How do we get IR radiation?

A

If we need Ir radiation in the range of (4000-200 cm^-1 (wavenumber)) we get it from a silicon carbide globar which is electrically heated to 1500K.

74
Q

What type of wavelengths do lasers provide?

A

Lasers provide a single wavelength of light and these can be 10^13 times brighter than the sun

75
Q

What does it mean when light is coherent? Give an example of coherent light?

A

This means that in time and space all wavelengths of light are in phase (ie lined up with eachother nad oscillate with eachother), an example of coherent light is laser light.

76
Q

Is laser light plane polarized? What does that mean?

A

Yes this means that the wavelengths of light only oscillates in one plane which is the electric field.

77
Q

What does the acronym laser stand for?

A

light amplification by stimulated emission of radiation

78
Q

Say a laser emits a wavelength of light at 3 um, what is the bandwidth it might have? What does the bandwidth points indicate?

A

The laser might have a bandwidth (range of wavelengths) of 3x10^-14 to 3x10^-8 um. These points are where the radiant power (power of radiation emitted) falls to half of it’s maximum value.

79
Q

What property of lasers allows it to illuminate small targets?

A

The fact that the angular divergence of a laser beam is less than 0.05 degrees, meaning that from where the path of light travels to where it hits it does not increase much.

80
Q

How does laser ablation- ICP-MS work?

A

Laser ablation works by vaporizing (ablating) a material by a laser pulse and sweeping it into plasma, then the ions in plasma enter a mass spectrometer which identifies and quantifies them

81
Q

Describe a practical application for laser ablation-ICP-MS?

A

Can use it to quantify elements in teeth of person in Scandinavia around AD 1800, this gives insight into how elemental composition of teeth ha changes and lets us know about the diet back in those times, old persons teeth had higher concentrations of tin, bismuth, lead, and antimony as well as rare earth metals such as gold. The high concs of lead, bismuth, tin, and antimony are probably because these materials were used to make their cooking utensils.

82
Q

What is population inversion?

A

This when a higher energy state has a greater population of mlcls then a lower energy state in the lasing material(glass etc), this is a necessary condition for lasing

83
Q

What happens to molecules in lasing?

A

Molecules in ground state E0 of the lasing medium are pumped to excited state E3 by broadband radiation (from a powerful lamp or electric discharge), these molecules then relax to a lower energy state (E2), then mlcl decays from E2 to E1 and then rapidly relaxes to E0, gives off light equal to the energy diff.

84
Q

What can photons do if they span exactly two diff energy states?

A

They can be absorbed by mlcls to raise them to an excited state, or the photon can stimulate an excited mlcl to emit a photon and return to the lower state.

85
Q

Define stimulated emission

A

When a photon can stimulate an excited molecule to emit a photon and return to a lower state.

86
Q

Can one photon emission stimulate others?

A

Yes, when a photon emitted from a molecule strikes another mlcl, a second photon with the same phase and polarization can be emitted

87
Q

Say there is a population inversion, how does one photon stimulate the emission of many photons?

A

As the photon stimulates a mlcl to release another photon, and then as that mlcl falls from e2 to e1, if there’s more mlcls in e2 then many photons with the same phase and polarization can be emitted from mlcls in E2 as the incident photon hits them.

88
Q

What is a monochromator?

A

Is a device that disperses light into it’s component wavelengths and selects narrow bands of wavelengths to send to sample

89
Q

Define refraction

A

Light bending in prism

90
Q

Define diffraction

A

Light bending by grating

91
Q

Define grating

A

A reflective or transmissive optical component with a series of closely spaced lines/grooves

92
Q

What is an example of a grating found in nature?

A

Peacock feathers

93
Q

Describe how light travels in a grating monochromator

A

Polychromatic light from the entrance slit is collimated (made into parallel rays) by a concave mirror
this light then hits the reflection grating which then directs only one wavelength of light to hit a second concave mirror and go through the exit slit where it hits the sample

94
Q

Does the orientation of the grating matter?

A

Yes, as this is what causes light to diffract at different angles and allows only certain wavelengths of light to hit our sample.

95
Q

When light is reflected from a grating what dies each groove in it become?

A

Each groove becomes a source of radiation

96
Q

What is constructive interference?

A

When two light waves overlap in such a way that they combine to create a larger wave.

97
Q

When does fully constructive interference occur?

A

When the difference in length of the two paths equals n times labda. So nL= a-b

98
Q

What is diffraction order?

A

Is when n=+/-1, +/- 2 etc

99
Q

What is first order diffraction?

A

When n=+/- 1

100
Q

What does it mean when adjacent light rays are in phase?

A

It means they reinforce eachother (if fully this is constructive interference)

101
Q

What does it mean when adjacent light rays are not in phase?

A

They partially/completely cancel eachother out

102
Q

Define resolution

A

Is a measure of the ability to separate two closely spaced wavelengths.

103
Q

If the resolution is high, what happens to the difference between two wavelengths that can still be separated from eachother?

A

becomes smaller, so the higher the resolution the smaller the difference between wavelengths can be to still be distinguishable.

104
Q

What is the eqn for the resolution of grating? Define all variables.

A

lambda/change in lamda= nN
N= number of grooves
n= diffraction order
L= wavelength

105
Q

What allows us to have better resolution between closely spaced wavelengths?

A

More grooves in our grating

106
Q

If you widen the exit slit what does that do to the bandwidth of the monochromator?

A

It widens it as you get more wavelengths going through the monochromator.

107
Q

What is a detector?

A

Is something that produces an electric signal when struck by photons, the response is a function of the light that hits it (incident light)

108
Q

If a detectors sensitivity increases what happens?

A

The current/voltage produced by the incident irradiance increases.

109
Q

What is a photomultiplier tube?

A

It’s a type of detector that can take extremely low light intensities and translate it into measurable electrical signals

110
Q

How does a PMT work?

A

When light hits it, it takes photons and send it to the cathode which emits electrons and then these electrons hit the positive dynodes again and again releasing more electrons to get a measurable signal. More than 10^6 electrons are collected for each photon that enters the device.

111
Q

What is an error in photodetectors, how do we address this error?

A

All photodetectors produce some small response in the absence of light this is called dark current

112
Q

Why is there a small response due to dark current in photo detectors?

A

This is because there’s spontaneous emission of electrons from cathode in PMT.

113
Q

How can we eliminate dark current?

A

We can get around this by cooling the detector as high temps lead to higher dark current.

114
Q

What is a photodiode array spectrophotometer?

A

It is a device that measures all wavelengths of a compound at once in a fraction of a second.

115
Q

What is photoiodide array spectrophotometer useful for?

A

Useful as a detector in liquid chromtography

116
Q

How does a photoiodide array spectrophotometer work?

A

A light source hits a mirror which directs it through our sample (no monochromator here as we want all wavelengths of light to be absorbed by sample so we can say something abt the types of mlcls in it), then it goes through a slit, gets directed by a mirror to go to a grating which separates light into it’s diff wavelengths and then hits the photoiodide array detector, with each diode getting a sperate wave.

117
Q

What is a charged couple device? (CCD)

A

is an extremely sensitive solid state detector that stores photo generated charge in a two dimensional array (pixels) and has a higher signal to noise ration than PMT

118
Q

How do we read the results of a charged couple device?

A

We see lights that come from emission light, there can be various emission lights due to numerous excited states, can use light to find element in sample and the concentration of that element as intensity of light is proportional to concentration

119
Q

How do we deal with noise in our detectors?

A

Through signal averaging, this is where we measure the signal in unit and the noise, divide them both to find the ratio of signal to noise and then decrease it by averaging a bunch of scans through doing more trials of the experiment.

120
Q

How can we deal with white noise in our detectors (random fluctuation of e?)

A

We can reduce temp of device

121
Q

What can we do to deal with drift (1/f noise)?

A

We can wait for electronics to warm up before using them, also periodically
measure standards and correct the instrument reading for any observed change.

122
Q

How do we deal with interference noise?

A

We put instruments in basement to avoid vibrations from outside that mess with data.

123
Q

What three sources cause noise in a spectrometer?

A

Sources independent of light source, sources proportional to photo generated current, variation in the intensity of the light source

124
Q

How do we address noise due to variation in the intensity of the light source?

A

Through a double beam spectrophotometer, this is where the light source is split and then goes through the ref cell and sample cell instanaously and the signal is then converted into voltages and the difference between both voltages (Vdif), as variation affects them bod the same Vdiff should equal 0 when you remove the factor of absorbance

125
Q

Define spectroscopy?

A

Is the science of studying interactions between matter and radiated energy (is theoretical)

126
Q

Define spectrometry?

A

Method used to acquire a quantitative measurement of the spectrum (practical application)

127
Q

What experiment of Newtons was crucial in our understanding of light?

A

Newton did an experiment where he took white light and put it through a prism, this showed light of 7 different colors letting us know white light is composed of different wavelengths (and therefore colors) of light. He deduced it was not the prism creating this light but the light itself because when you put light through two prisms the refraction of the light is the same.

128
Q

Any material than can emit light of a certain wavelength can also?

A

Absorb light of that wavelength

129
Q

What experiment did kirchoff and bunsen do?

A

They produced a flame by burning gas and this flame produced a cont spectrum of light, they then added sodium chloride and saw a resulting yellow orange color in the flame this is because when the salt mlcls got excited it had an increased energy state and then those electrons fell back to their original state and the wavelength of light emitted is what produced that color. However the sodium atoms also absorbed this type of wavelength causing absorption lines in the spectrum at 589 nm.

130
Q

How did sodium both absorb light at 589 nm and emit it?

A

This is because the electron in it reached an excited state and then fell back down emitting light, but in the experiment when they shined whit light through the flame the energy difference between the excited state and ground state was found in photons in that light causing the electrons to absorb it.

131
Q

What is spectroscopy?

A

Is absorption or emission of light by analyte

132
Q

How many forms of atomic spectroscopy are there?

A

three

133
Q

What is FAAS? Hows it work?

A

It’s Flare atomic absorption spectrometer, here the analyte sample is turned into a mist when enters a flame and analyte is atomized in the flame, the hollow cathode lamp send light through the flame which then goes to monochromator and then detector to measure the intensity of the light.

134
Q

What are the three forms of atomic spectroscopy?

A

emission, absorption, and fluorescence (atoms get excited and then fall to a lower stage and emit radiation- diff then emission as the change is a nonradiative transition

135
Q

Why do we need a hollow cathode lamp in FAAS?

A

Because the light intensity of the signal increases when we use the lamp, without it we would need much more sample and get less sensitive results.

136
Q

Why do we need a monochromator?

A

Need a monochromator as it removes all wavelengths we don’t need.

137
Q

If L0 = Ltr in a FAAS experiment, what does this mean?

A

It means that there were no molecules in the flame

138
Q

If Lt<L0 what does that mean?

A

It means some mlcls were in the flame

139
Q

If Lt=0 what does that mean?

A

It means there were alot of mlcls in the flame so all light from cathode was absorbed

140
Q

What is AFS? Why is it more sensitive than atomic absorption spectroscopy?

A

Is atomic fluorescence spectroscopy and is much more sensitive than atomic absorption spectroscopy because it can observe weak fluorescence signals above a dark background.

141
Q

Describe a real life application of AFS?

A

Can be used to measure mercury in water, do this by reducing mercury to mercury0, then putting it through a sample trap (where it binds to gold) and an analytical trap to remove impurities, it then enters a fluorescence cell.

142
Q

What is GFAAS?

A

Graphite furnace atomic absorption spectroscopy, this technique is used the most today.

143
Q

What is ICP?

A

An atomic spectroscopy technique, called inductively coupled plasma ionizes atoms in a sample.

144
Q

What is a heated quartz cell?

A

A heated quartz cell is a type of spectroscopic cell used in analytical chemistry to perform measurements at high temperatures. The cell is typically made of high-purity quartz and has a cylindrical shape with two optical windows, which allows light to pass through the sample inside the cell.

145
Q

What is the hydride technique?

A

The hydride technique involves the conversion of certain elements in a sample into their corresponding hydrides through a chemical reaction with a reducing agent, these are then vaporized and put in cells for analysis.

146
Q

How does the flame in flame atomic absorption spectroscopy work?

A

The technique uses a premix burner ( in which fuel,
oxidant, and sample are mixed before introduction into the flame).

147
Q

How does a premix burner operate?

A

Has a pneumatic nebulizer which creates an aerosol from the liquid sample and then pushes it into the flame

148
Q

Around how much of the sample enters the flame from the pneumatic nebulizer?

A

only about 5% the rest goes down the drain.

149
Q

What is mass transfer?

A

How much sample actually reaches the flame in FAAS.

150
Q

What is the minimum amount of sample needed in FAAS?

A

1-2 mL

151
Q

How are atoms generated?

A

First aerosol vapour is evaporated, then aerosol solid/gas will melt forming atoms in the ground state, light will then travel through the flame exciting atoms but you have to adjust the light as certain element reach different heights when excited

152
Q

How can we easily determine the Ca content in atomic absorption spectroscopy?

A

We add HCL which allows us to easily volatilize CaCl2 from any calcium containing compounds and therefore easily detectable.

153
Q

When determining the Calcium content in atomic absorption spectroscopy, what happens if the solution contains sulfate or phosphate?

A

Then calcium oxide and pyrophosphate can form which increases them temperature of the flame and causes non spectral inteferences.

154
Q

What plays a key role in non-spectral interferences in AAS?

A

temperature of the flame.

155
Q

What is the most common fuel oxidizer in AAS?

A

Acetylene and air, but this can be changed to increase or decrease fuel temp

156
Q

What are refractory elements?

A

Are elements that form stable oxides, they are bad as they interfere with the signal response of the analyte.

157
Q

How we stop refractory elements from forming stable oxides?

A

We increase temperature of flame, for example we can use acetylene and N2O/O2.

158
Q

What does the height in flame at which maximum atomic absorption/emission observed depend on?

A

Depends on the element, the fuel/oxidizer flow rate, and the nebulization speed.

159
Q

Do we need to correct the signal in AAS? Why?

A

Yes as flames emit light that can increase the signal we get, so that signal must be subtracted from the total signal.

160
Q

Why do we need specific flame types for specific wavelengths in AAS?

A

Because some air types for ex: air/acetylene flame for a 193.7 nm are not good as they don’t absorb at that wavelength well.

Chem 315 (7 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions