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Instrumental Analysis > FTIR (4) > Flashcards

FTIR (4) Flashcards

1
Q

How do entrance and exit slits on a monochromator control the amount of radiation reaching the detector?

A

amount energy allowed in or out

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2
Q

What is spectral resolution?

A

amount energy exiting

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3
Q

How much incident energy reaches the detector at any one time in a dispersive IR experiment of a specified resolution?

A

.11%

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4
Q

What is a beamsplitter?

A

50% reflection, 50% transmission

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5
Q

How are beamsplitters used in interferometers?

A

divides source radiation and redirects it to two mirrors

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6
Q

What are some typical beamsplitter materials used in the IR spectral regions?

A

near = Fe2O3 on quartz; mid = Ge on KBr; Far = mylar

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7
Q

what is optical retardation?

A

path difference between radiation

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8
Q

At what values of optical retardation is the monochromatic radiation in both arms of the interferometer in phase?

A

at 0 and when equal to wavelength

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9
Q

At what values of optical retardation in both arms of the interferogram out of phase?

A

wavelength/2

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10
Q

how does the constructive/destructive interference of radiation from the two arms of the beamsplitter resulting a modulated signal?

A

Two parts of recombined beam precisely in phase means signal power is at maximum, moving mirror 1/4 wavelength changes corresponding reflected beam path length by 1/2 wavelength. Here destructive interference reduces radiant power to 0. Cycle repeats

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11
Q

Which one (AC or DC) corresponds to the measured component of the interferogram?

A

AC

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12
Q

How does the interferogram for a polychromatic source differ from that for a monochromatic source?

A

integral over all frequencies

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13
Q

What is the zero path difference?

A

OM=OF

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14
Q

Centerburst/Zero path difference in a polychromatic source interferogram?

A

one point where all wavelengths constructively interfere; poly has max intensity here

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15
Q

How are line shapes reflected in their fourier transforms?

A

decreasing width of band increases width of decay; broad bands cause rapid decay

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16
Q

What is the mathematical relationship for the S/N in dispersive spectrometers

A

sqrt(t/n)

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17
Q

What is the mathematical relationship for the S/N in an interferometric spectrometer?

A

sqrt t

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18
Q

What is a resolution element?

A

number of subdivisions contained in spectral range

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19
Q

What is the mathematical form for the improvement in s/n of an interferometer as compared to dispersive?

A

sqrt n

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20
Q

How does improvement in S/N for interferometer change with increasing resolution

A

multiplex most pronounced in high resoltuion

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21
Q

What is Fellgett’s/multiplex advantage?

A

all radiation passes all wavelengths at all times

22
Q

how is fellgett/multiplex stated in terms of time rather than S/N?

A

scan only 1/n as long; dispersive is sqrt(t/n), interfero is sqrt(t)

23
Q

What is Jacquinot’s/throughput advantage?

A

increasing s/n results from increasing observed signal strength

24
Q

What is the wavenumber dependence of the throughput advantage?

A

wavenumber squared

25
Q

Is wavenumber dependence in throughput advantage significant for mid-ir spectroscopy?

A

not as significant as it is in fellget’s’

26
Q

What is Conne’s/precision advantage?

A

increased inherent precision in wavenumber scale of fourier transform versus dispersive

27
Q

What is the inherent precision of a dispersive IR spectrometer?

A

.5 to 1 cm^-1

28
Q

What is the inherent precision of an FTIR spectrometer?

A

.01 cm^-1

29
Q

How is resolution controlled in FTIR

A

further moving mirror is scanned, higher resolution of the final spectrum

30
Q

What is the mathematical relationship between movement of mirror scan in an interferometer and the resolution of the spectrum?

A

change in wavenumber = 1/optical retardation (cm-1); optical retardation = 1/A(wvnmbr) cm

31
Q

Why does an unapodized IR spectrum look severely distorted

A

halting mirror scan imposes truncation so upon fourier transform the sinc function distorts

32
Q

What is a boxcar function?

A

simplest truncation

33
Q

What is a triangular apodization function?

A

One of the most common apodization functions to minimize distortions in interferogram

34
Q

What are some other types of apodization functions?

A

boxcar, trapezoidal, triangular, triangular squared, bessel, cos, sinc squared, gaussian

35
Q

How does the degree of side lobe depression relate to the intensity of the resulting spectrum?

A

Smaller half widths = less suppression of side lobes

36
Q

How does the degree of side lobe depression relate to the width of the resulting spectrum?

A

successful suppression of side lobes results in larger half widths

37
Q

What is the Nyquist sampling theorm

A

must digitize at 2x max frequency (any waveform that is sinusoidal function of time or distance can be digitized using frequency 2x the bandwidth of the system)

38
Q

What is the effect of digitizing a sine wave at less than 2x frequency?

A

observed at wrong frequency

39
Q

What is aliasing

A

digitized frequency less than true frequency

40
Q

What is the wagon wheel effect?

A

media still frames 24 Hz, Nyquist says only 12 digitized correctly

41
Q

How do aliasing and wagon wheel affect spectroscopy?

A

Aliased see frequencies masquerading as 1-16Hz frequencies instead of their true value; = sample - cosine; rotation not properly digitized so they lower in displayed range

42
Q

How does the number of points in the interferogram depend on the spectral resolution?

A

smaller resolution = bigger N

43
Q

Why is it very important to precisely determine the displacement of the moving mirror in an interferometer?

A

conne’s advantage; leads directly to precision in wavenumber scale in resulting spectrum

44
Q

How is the interferometer displacement related to the precision n the wavenumber scale?

A

directly

45
Q

How is the interferometer itself used to generate its own time scale?

A

line from internal laser source to produce discrete signal time locked to mirror position and interferograms.

46
Q

What is the most common laser used in this fringe reference system?

A

monochromatic

47
Q

what is a zero crossing?

A

each time output signal level crosses OV level

48
Q

How are zero crossings related to the retardation interval

A

occurs at equal retardation intervals

49
Q

How can zero crossings be used to precisely determine the displacement of the interferometers moving mirror?

A

Electronics count each time output signal crossing 0V, which can be used to digitize the signal from the main interferometer since they occur at equal retardation intervals

50
Q

What are the steps involved in obtaining a single beam FTIR spectrum?

A
  1. collect background spectrum
  2. collect sample spectrum
  3. determine final ratio
51
Q

How is the final transmittance or absorbance spectrum obtained from the background and sample interferograms?

A

computer ratios single beam sample spectrum to single beam background spectrum to calculate final transmittance or absorption

52
Q

Advantages FTIR compared to dispersive

A

Superior s/n; speed; higher resolution; highly accurate and reproducible frequency axis; freedom from stray radiation effects

Instrumental Analysis (45 decks)

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