FTIR (4) Flashcards
How do entrance and exit slits on a monochromator control the amount of radiation reaching the detector?
amount energy allowed in or out
What is spectral resolution?
amount energy exiting
How much incident energy reaches the detector at any one time in a dispersive IR experiment of a specified resolution?
.11%
What is a beamsplitter?
50% reflection, 50% transmission
How are beamsplitters used in interferometers?
divides source radiation and redirects it to two mirrors
What are some typical beamsplitter materials used in the IR spectral regions?
near = Fe2O3 on quartz; mid = Ge on KBr; Far = mylar
what is optical retardation?
path difference between radiation
At what values of optical retardation is the monochromatic radiation in both arms of the interferometer in phase?
at 0 and when equal to wavelength
At what values of optical retardation in both arms of the interferogram out of phase?
wavelength/2
how does the constructive/destructive interference of radiation from the two arms of the beamsplitter resulting a modulated signal?
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
Which one (AC or DC) corresponds to the measured component of the interferogram?
AC
How does the interferogram for a polychromatic source differ from that for a monochromatic source?
integral over all frequencies
What is the zero path difference?
OM=OF
Centerburst/Zero path difference in a polychromatic source interferogram?
one point where all wavelengths constructively interfere; poly has max intensity here
How are line shapes reflected in their fourier transforms?
decreasing width of band increases width of decay; broad bands cause rapid decay
What is the mathematical relationship for the S/N in dispersive spectrometers
sqrt(t/n)
What is the mathematical relationship for the S/N in an interferometric spectrometer?
sqrt t
What is a resolution element?
number of subdivisions contained in spectral range
What is the mathematical form for the improvement in s/n of an interferometer as compared to dispersive?
sqrt n
How does improvement in S/N for interferometer change with increasing resolution
multiplex most pronounced in high resoltuion
What is Fellgett’s/multiplex advantage?
all radiation passes all wavelengths at all times
how is fellgett/multiplex stated in terms of time rather than S/N?
scan only 1/n as long; dispersive is sqrt(t/n), interfero is sqrt(t)
What is Jacquinot’s/throughput advantage?
increasing s/n results from increasing observed signal strength
What is the wavenumber dependence of the throughput advantage?
wavenumber squared
Is wavenumber dependence in throughput advantage significant for mid-ir spectroscopy?
not as significant as it is in fellget’s’
What is Conne’s/precision advantage?
increased inherent precision in wavenumber scale of fourier transform versus dispersive
What is the inherent precision of a dispersive IR spectrometer?
.5 to 1 cm^-1
What is the inherent precision of an FTIR spectrometer?
.01 cm^-1
How is resolution controlled in FTIR
further moving mirror is scanned, higher resolution of the final spectrum
What is the mathematical relationship between movement of mirror scan in an interferometer and the resolution of the spectrum?
change in wavenumber = 1/optical retardation (cm-1); optical retardation = 1/A(wvnmbr) cm
Why does an unapodized IR spectrum look severely distorted
halting mirror scan imposes truncation so upon fourier transform the sinc function distorts
What is a boxcar function?
simplest truncation
What is a triangular apodization function?
One of the most common apodization functions to minimize distortions in interferogram
What are some other types of apodization functions?
boxcar, trapezoidal, triangular, triangular squared, bessel, cos, sinc squared, gaussian
How does the degree of side lobe depression relate to the intensity of the resulting spectrum?
Smaller half widths = less suppression of side lobes
How does the degree of side lobe depression relate to the width of the resulting spectrum?
successful suppression of side lobes results in larger half widths
What is the Nyquist sampling theorm
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)
What is the effect of digitizing a sine wave at less than 2x frequency?
observed at wrong frequency
What is aliasing
digitized frequency less than true frequency
What is the wagon wheel effect?
media still frames 24 Hz, Nyquist says only 12 digitized correctly
How do aliasing and wagon wheel affect spectroscopy?
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
How does the number of points in the interferogram depend on the spectral resolution?
smaller resolution = bigger N
Why is it very important to precisely determine the displacement of the moving mirror in an interferometer?
conne’s advantage; leads directly to precision in wavenumber scale in resulting spectrum
How is the interferometer displacement related to the precision n the wavenumber scale?
directly
How is the interferometer itself used to generate its own time scale?
line from internal laser source to produce discrete signal time locked to mirror position and interferograms.
What is the most common laser used in this fringe reference system?
monochromatic
what is a zero crossing?
each time output signal level crosses OV level
How are zero crossings related to the retardation interval
occurs at equal retardation intervals
How can zero crossings be used to precisely determine the displacement of the interferometers moving mirror?
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
What are the steps involved in obtaining a single beam FTIR spectrum?
- collect background spectrum
- collect sample spectrum
- determine final ratio
How is the final transmittance or absorbance spectrum obtained from the background and sample interferograms?
computer ratios single beam sample spectrum to single beam background spectrum to calculate final transmittance or absorption
Advantages FTIR compared to dispersive
Superior s/n; speed; higher resolution; highly accurate and reproducible frequency axis; freedom from stray radiation effects
