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analytical chemistry > advanced molecular spectroscopy > Flashcards

advanced molecular spectroscopy Flashcards

1
Q

What kind of interferometer is used in FTIR?

A

Michelson interferometer

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2
Q

What is the output called?

A

Interferogram

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3
Q

Give 3 advantages to FTIR over dispersive spectroscopy?

A

Provides precise measurement method which requires no external calibration, can increase speed and sensitivity

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4
Q

What 3 things contribute to the increased sensitivity?

A

One second scans can be co-added together to ratio out random noise, greater optical throughput, mechanically simple - 1 moving part

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5
Q

what limitation of dispersive techniques was FTIR created to overcome?

A

slow scanning

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6
Q

How long does it take for a interferometer signal to be read?

A

1 second

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7
Q

What does a beam splitter do

A

Takes oncoming IR beam and divides it into 2 optical paths

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8
Q

What do the 2 resulting beams reflect off of?

A

One beam reflects off a flat mirror which is fixed in place, the other off a mirror on a mechanism which allows mirror to move a short distance from beamsplitter

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9
Q

Where are the 2 beams recombined?

A

when they meet back at the beamsplitter

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10
Q

What mathematical technique is used to decode the individual frequencies produced?

A

Fourier transform

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11
Q

What is fellgetts multiplex advantage?

A

info on all frequencies collected similtaniously, saves time by averaging number of interferometer scans

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12
Q

What is Jacquinots throughput advantage?

A

better sensitivity due to no monochromator, and light throughput not restricted

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13
Q

What is the Connes laser calibration advantage?

A

internally calibrated, gives precise frequency calibration without secondary spectral standards

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14
Q

What are the 2 disadvantages of FTIR over dispersive instruments?

A

single beam with long optical path, whole spectrum must be obtained from each transform

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15
Q

What problems might the single beam device lead to?

A

Cancelling atmospheric H2O and CO2

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16
Q

The high resolution work encountered with gaseous mixtures give complex spectras due to what?

A

superposition of vibrational and rotational bands

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17
Q

name 2 applications of FTIR

A

Study of samples with weak absorbancies, IR emission studies

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18
Q

What is evidence of bond between coupling agent and glass fibre surface vital in the development of?

A

optical fibres

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19
Q

Why are normal lenses not suitable for FTIR microscopy?

A

they absorb IR radiation

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20
Q

What is used instead?

A

metal coated relfecting optics instead

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21
Q

What 3 modes can samples be measured in?

A

transmittion, reflectance, or ATR modes

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22
Q

what is the range of Near IR?

A

0.75-2.5um

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23
Q

describe the vibrations in polyatomic molecules?

A

anharmonic

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24
Q

what are overtones?

A

vibrations due to energy transfer from ground level to second or higher levels

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25
Q

why do they occur at smaller wavelengths

A

require more energy

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26
Q

What do combination bands arise as?

A

summation fundamental bands (2 or more fundamental vibrations occur similtaneously)

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27
Q

how much weaker are Near IR absorption bands than corresponding fundamental mid IR absorption bands?

A

10-100x

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28
Q

How can this be used as an advantage

A

Allows direct analysis of strongly absorbing, highly light-scattering matrices

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29
Q

Give 3 advantages of Near IR?

A

Rapid, accurate and non-destructive

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30
Q

is it qualitative or quantitative

A

both

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31
Q

What is the benefit of it having a large pathlength

A

can analyse more sample

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32
Q

Name 3 disadvantages to near IR?

A

not suitable for trace analysis, complex spectra for interpretation, physical conditions of sample and environment add complexity to spectra

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33
Q

What are 2 applications of NIR in pharmaceutical industry

A

identification, information on crystalline state, powder size etc

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34
Q

What does UV-VIS stand for?

A

Ultravoilet and visible spectroscopy

35
Q

What regions of radiation are UV and VIS?

A

200-400nm UV, 400-800nm VIS

36
Q

What does absorption of this energy cause an excitation of?

A

an electron from a bonding to an antibonding molecular orbital

37
Q

what does fluorimetry rely on?

A

the emission of electromagnetic energy by molecules

38
Q

Which regions does the sample absorb and emit light?

A

absorbs in UV region, emits in visible region

39
Q

The light emitted by the sample is always of …… wavelength (……. energy) than the light absorbed by the molecule

A

longer, lower

40
Q

What is this called?

A

Stokes shift

41
Q

what vibrational level does absorption always start at?

A

lowest (ground state)

42
Q

what is required to excite molecules from the ground state?

A

high temperature

43
Q

what is the most versatile spectrofluorimeter?

A

dual monochromator spectrofluorometer

44
Q

What kind of lamp is used? Why?

A

Xenon lamp, as fluorescence intensity is proportional to I0

45
Q

due to fluoresence signals being generally weak, what is used as a detector?

A

PMT

46
Q

Why are all sides of a cell in fluoresence polished? (unlike in absorption)

A

fluoresence is emitted in all directions

47
Q

What makes fluorimetry more specific than ordinary UV spec?

A

2 monochromators + not all molecules with a chromophore fluoresence

48
Q

How much lower is fluorimetrys detection limit than UV/VIS>

A

100 x

49
Q

What kind of analysis is it ideal for?

A

very small amounts of potent drugs

50
Q

comment on the sensitivity of the analysis

A

among the most sensitive

51
Q

What 3 factors affect fluorescence intensity?

A

source intensity, fluoresence efficiency, concentration

52
Q

Name 3 more important factors?

A

quenching, self absorption, matrix effects

53
Q

define quenching

A

a reduction in intensity of light emitted during fluoresence

54
Q

what are the 2 types?

A

self and chemical quenching

55
Q

in what environment does self quenching occur?

A

at high conc

56
Q

What causes this?

A

Due to reabsorption and scattering of emitted light by other molecules/ions of sample

57
Q

how can self quenching be solved?

A

by further dilution of sample

58
Q

the detection system collects fluorescence emitted from which part of the exciting beam?

A

central

59
Q

in large concentrations, what is absorbed significantly before reaching the central part of the cuvette?

A

incident light

60
Q

How does the emitted light concentrated at the face of the cell rather than through the body of the solution affect the calibration?

A

causes it to curve

61
Q

Chemical quenching incolves the ……. of the energy from an …….. molecule by another molecule usually as the result of a ………?

A

removal, excited, collision

62
Q

why is this important in an analysis?

A

since fluoresence of the analyte might be quenched by molecules of some compound present in sample

63
Q

what is this effect known as?

A

matrix effects

64
Q

what are the following quenching mechanisms known as? A+Q=>A+Q and A+Q=>AQ=>AQ

A

collision, and complex formation

65
Q

In the equations, what is A and what is Q?

A

A = fluorescent analyte molecule Q= quenching species

66
Q

Regarding electrons, what type of compound act as efficient quenching agents? give example

A

compounds with unpaired electrons, eg. O2

67
Q

how can this be removed from sample if necessary?

A

bubbling oxygen free N2 through the sample

68
Q

How might halide ions affect fluoresence of quinine? for example in hydrochloric acid vs sulphuric acid?

A

70% reduction in fluorescence - referred to as heavy atom effect

69
Q

what is this due to?

A

increased probablility of intersystem crossing. this leads to non-radioactive deactivation of excited state = no fluorescence

70
Q

give 2 examples of ionsof inorganic species which can naturally fluoresce in solution?

A

lanthanides and actinides

71
Q

what are the general requirements for organic compounds to be able to fluoresce? 3

A

a large molecule containing a conjugated system which has a rigid structure due to ring formation

72
Q

what is the simplest example of a fluorescent derivative?

A

the complex formed between a metal ion and a donor organic molecule

73
Q

ideally, both of these components should be …………. and form a ……….. complex?

A

non-fluorescent, fluorescent

74
Q

What is the name of the most effective type of complex which can be formed by the metal forming bonds in 2 positions?

A

chelate ring

75
Q

name 2 applications of fluorescence based assays in forensic laboratories?

A

quantitation of DNA and RNA yields in solutions, and DNA typing.

76
Q

what are novel fluorescent reagents designed to enhance?

A

sensitivity while using simple methods and conserving precious samples.

77
Q

Name the 2 derivative approaches available?

A

precolumn and post-column derivitisation

78
Q

in precolumn, what are 3 requirements for the derivitisaing agent to be added to the sample before it is injected onto column?

A

useful where the reaction requires several reagents, heat treatment or time

79
Q

What might this technique reduce?

A

efficiency of separation

80
Q

When is the derivitising agent injected into the column in post-column?

A

injected continiously into eluent from column

81
Q

in what instances is this method used?

A

when precolumn seriously affects the separation

82
Q

What might need to be inserted to ensure the reaction is complete before components reach the detector?

A

delay coils

83
Q

without them, what may occur?

A

diffusion and loss of resolution

84
Q
A

analytical chemistry (8 decks)

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