Transcript Part_IV-Instrumentation

Vibrational Spectroscopy for
Pharmaceutical Analysis
Part IV. Fourier Transform Infrared (FT-IR) Spectroscopy
Rodolfo J. Romañach, Ph.D.
ENGINEERING RESEARCH CENTER FOR
STRUCTURED ORGANIC PARTICULATE SYSTEMS
RUTGERS UNIVERSITY
PURDUE UNIVERSITY
NEW JERSEY INSTITUTE OF TECHNOLOGY
UNIVERSITY OF PUERTO RICO AT MAYAGÜEZ
10/11/2005
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LAB INSTRUMENTS
Spectrum One – Perkin Elmer
Tensor 27 – Bruker Optics
Scimitar-Varian
Thermo Nicolet 6700
ABB - 100
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PROCESS INSTRUMENTS
ABB-200
Hamilton Sundstrand
RefinIR
ABB-400
Mettler Toledo
ReactIR™4000
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Instrumentation
• The vast majority of modern infrared
spectrometers are Fourier Transform Infrared
Spectrometers.
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Advantages of FT-IR Spectroscopy
Fellgett or Multiplex Advantage
• Obtains the information on all the frequencies at
the same time (Multiplex or Fellgett Advantage)
and as a result:
• Signal to noise ratio may be improved by
increasing number of scans by N1/2.
• Fast, may be used for dynamic processes:
GC/FT-IR.
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Advantages of FT-IR Spectroscopy
• Radiation may be moved away from optical bench
to interface instrument to IR microscope or
GC/FT-IR.
• Provides precise and accurate determination of
absorption wavelength. Frequency calibration
with He-Ne laser.
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FIXED POSITION MIRROR
MOVABLE MIRROR
SINGLE
FREQUENCY
SOURCE ()
BEAMSPLITTER
 = 0  =/2  =
 =3/2
SAMPLE
POSITION
DETECTOR
THE MICHELSON INTERFEROMETER
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Schematic of a Michelson Interferometer. Figure 8, page 87
Chalmers and Dent.
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Schematic showing summation of two cosine function interferogram.
Chalmers and Dent page 88.
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An IR spectrum can be obtained, after applying the Fourier Transform.
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Essential Components of FT-IR
Spectrometer
• Interferometer (KBr/Ge beamsplitter used for
mid-IR, CaF2/Fe2O3 for NIR).
• Computer.
• He:Ne laser.
• Source.
• Mirrors.
• Detectors.
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Detectors
• DTGS (Deuterium Trygliceride Sulfate) – most commonly
used, can be used at room temperature without cooling.
Detector by default, DTGS will be included in new FT-IR
spectrometer, unless MCT is requested.
• Mercury Cadmium Telluride (HgCdTe, or MCT) – have higher
sensitivity and higher response speed than DTGS and are
used in IR microscopes and in GC/FT-IR experiments. Liquid
nitrogen cooling required. Non-linear at higher radiation
intensities.
H. Günzler, H. U. Gremlich, IR Spectroscopy An Introduction, Wiley-VCH, 2002,
pages 60 – 61, 70
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Fourier transform from space domain to frequency
domain. B(ν) is the single beam spectrum, δ is the
optical retardation.
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For a polychromatic source, the interferogram
represents the summation of all the individual
cosine functions corresponding to each of the
wavelengths (wavenumbers in the source).
They are only in phase at the centerburst
(position of zero path difference.
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Sequence for Obtaining Spectrum
• Interferogram of Background is obtained (without sample)
• System uses Fourier Transform to create single beam
background spectrum.
• Interferogram of Sample is obtained.
• System uses Fourier Transform to create single beam
spectrum of sample.
• System calculates the transmittance or absorbance
spectrum.
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The CO2 Band
• If the CO2 concentration is the same for the background and
sample spectra it will be ratioed out.
• Should have a stream of dry air going through the sample
compartment to keep CO2 constant. If air stream is too
high, it could cause disturbance in sample compartment.
• When sample compartment is open, CO2 comes in, wait a
few seconds to re-establish purge and take spectrum.
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Methyl Heptanoate
Problem with CO2 Band.
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More CO2 in sample spectrum than in background.
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100% line test
• The efficiency of the spectrometer may be checked with
successive single beam spectra of the empty sample
compartment.
• The spectrum should be a straight line, but some spectral
noise will always be observed.
• This test also serves to document the spectral noise.
H. Günzler, H. U. Gremlich, IR Spectroscopy An Introduction, Wiley-VCH,
2002, pages 67-69
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Strong CO2 and water bands but the baseline is flat, instrument is working well.
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Wavelength Accuracy
• The He:Ne laser monitors the position of the
moving and triggers data acquisition.
• The He:Ne laser serves as an internal reference
for every interferogram obtained.
• “The abscissa precision of a data point in a FT
spectrum recorded on a commercial spectrometer
is usually quoted as better than 0.01 cm-1” page
97 Chalmers and Dent.
• You will see a yellow-red beam, this is the source
and He:Ne laser together.
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Accuracy in wavelength determination, because of He:Ne laser.
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Source of Radiation
• Laser is not the source of radiation.
• Globar is most commonly used (silicon carbide
rods). At a temperature of 1500 K, it provides
substantial energy.
• Other instruments use chromium nickel alloy
wires, or nerst rod (zirconium oxide) that when
heated emit radiation.
• Sources may last several years, not necessary to
turn them off.
H. Günzler, H. U. Gremlich, IR Spectroscopy An Introduction, Wiley-VCH, 2002,
page 40.
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Choice of Parameters for Spectral
Acquisition
The following parameters must be taken into
consideration when taking spectra:
1. Apodization & Number of Points
2. Resolution
3. Number of Spectra
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The moving mirror does not travel to plus or
minus infinity, so the interferograms is truncated
(cut) by a boxcar function.
Schematic of Boxcar Truncation of the interferogram for a
single frequency source. Chalmers and Dent figure 11, page 90.
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Software will have a command requesting how many points you want to
keep away from centerburst. Most of the information is in first 100
points, but most users keep 300 – 500 points. System has ADC with
dynamic range of 1016 or better.
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Effect of Boxcar Truncation
• The Fourier
Transform is now a
sinc function and
positive and negative
lobes are introduced
to each band.
Effect of Boxcar Truncation
You might choose double sided interferogram and 300 points, you are
specifying the truncation of the spectrum.
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Apodization
• The lobes may be eliminated by replacing the boxcar
function with another function.
• However, this process always increases the spectral band
width and decreases band intensity.
• The functions are called apodization functions.
• The IR spectrometer software will give you a wide range of
choices.
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•There is a tradeoff
between eliminating the
lobes and having band
widening with reduction in
intensity.
•However, apodization is
used in most experiments.
•The weak Norton-Beer
function is probably the
most commonly used
apodization function.
Effect of Triangular
Apodization.
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Spectral Resolution
• The spectral resolution is related to the travel of the moving
mirror.
• To resolve two bands in the spectrum, the mirror must travel
to complete one beat pattern generated between the two
cosine waves in the interferogram that represent the
wavenumber portions of the spectrum.
• Most FT-IR spectrum can provide resolution of at least 2 cm1.
• For liquids usually 8 cm-1 resolution is sufficient, some solids
may require 4 cm-1 resolution.
• The spectral bandwidths could be much greater than the
resolution.
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Spectral Resolution
• At higher resolutions (2 cm-1) you will see more noise in
the spectrum than at a lower resolution (8 cm-1).
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Number of Spectra
• Use multiplex advantage to improve SNR (signal
to noise ratio) of spectra.
• Take at least 16 spectra for a sample to improve
SNR by a factor of 4.
• If you take 64 spectra at a spectral resolution of 8
cm-1, you can improve SNR by a factor of 8, and
the total spectral collection time is about 1
minute.
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