Fundamentals of FTIR
Dr. Brian C. Smith
Spectros Associates
bcsmith@spectros1.com
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Terms and Definitions
 Spectroscopy – The study of the interaction of light with matter
 Infrared Spectroscopy – The study of the interaction of infrared light with
matter
 Electromagnetic Radiation – a wave that consists of an electric vector and
magnetic vector undulating in perpendicular planes, propagating in a third
plane. Translation – light, heat, radio waves, microwaves, X-rays, gamma rays
etc.
 Cycle – When a wave traverses zero three times
 Wavelength (l) – length/cycle
– in FTIR, typically use light of wavelengths of 2.5 to 25 microns
 Frequency (n) – cycles/second
 Wavenumber - cycles/cm
 Photon Energy = hcW
– Where h = Planck’s constant, c = light speed, W = Wavenumber
 Mid-IR Radiation – 4000 to 400 cm-1. Same thing as heat.
– Most molecules in the universe have strong absorbances in this wavenumber range 2
Wavelength and Cycles
l
1 Cycle
 Figure 1.1. A plot of the amplitude of the electric vector part of a light wave
versus time. The arrow denotes the distance between adjacent crests, and is
called the wavelength, l.
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Properties of Light: Wavenumbers
 Wavenumber (W) is 1/Wavelength or
– W = 1/l
– If l is in units of length, W is
in units of (length)-1
– e.g. l is in cm, W is in cm-1
 W Measures the # of Cycles per Unit
Length
– e.g. # of cycles/cm
 Light Energy  Wavenumber
 In FTIR, Typically Use Mid-IR Light
– 4000 to 400 cm-1 light
If a peak is at 3000 wavenumbers,
it is absorbing light of 3000
cycles/cm.
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The Electromagnetic Spectrum
> 14,000
cm-1
Visible
UV & Xrays
14,000 to
4000
cm-1
Near
Infrared
4000 to 400
cm-1
Mid
Infrared
Higher Wavenumber
Higher Frequency
Higher Energy
Shorter Wavelength
400 to 4
cm-1
Far
Infrared
<4
cm-1
Microwaves
Radio
Waves
Lower Wavenumber
Lower Frequency
Lower Energy
Longer Wavelength
 Figure 1.2. The electromagnetic spectrum, showing the wavenumber ranges for
different types of light.
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An Infrared Spectrum
 A Plot of Light Intensity vs. Some Property of Light
 Typically Wavenumber on the X-Axis
 Normally high W is to the LEFT
 Figure 1.3. The infrared spectrum of polystyrene plotted in absorbance units
on the Y-axis.
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Transmittance Units
 Figure 1.4. The transmittance spectrum of polystyrene.
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Y Axis Units
 Y-Axis May be in Absorbance or Transmittance
 Absorbance: A = log(I0/I) where I0 = Bkgd. Spectrum & I = Sample Spectrum
–
–
–
–
Measures amount of light absorbed by sample
Peaks Point UP. Peak intensity should be < 2
A (pathlength)(concentration) or A  lc.
Absorbance MUST be used for quantitative measurements
 Transmittance: T = (I/I0) the fraction of light transmitted
 Percent Transmittance: %T = 100x (I/I0)
– The percentage of light transmitted
– Peak point down, should be > 10%
– Okay for qualitative analysis, but not for quant. since has a non-linear relationship
with concentration
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Which Y Axis Unit Should You Use?
 Which Unit Should You Use, Absorbance or Transmittance?
 Quantitative Analysis: MUST Use Absorbance
– The relationship between absorbance and concentration is linear. The relationship
between %T and concentration is NON-linear.
 Qualitative Analysis: Either unit, may be determined by boss, tradition or SOP
– Qualitative analysis means trying to identify chemical structures from the spectrum
 Absorbance units are required for spectral subtraction, library searching and
quantitation.
– Rather than having to convert to perform these operations, may be simpler to
always measure absorbance spectra.
 Get used to reading both
– Spectra in the book are plotted both ways
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Peak Size Limit
 Peaks Can Be “Too Big”
– If Abs. > 2 or %T < 10, almost all light is absorbed.
– Mineral Oil Peaks in Bottom Spectrum are Distorted
– Change pathlength or concentration to get peaks the “right” size
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What are Spectra Used For?
 Determine Unknowns
– “What molecule(s) are in this sample?
– Use peak positions of known functional groups
 Identities (Spectral Comparisons)
– “Are these two samples the same?”
– Must have a reference spectrum for comparison
– Easier than unknowns, but still must be done properly
 Quantitation
– What is(are) the concentration(s) of molecule(s) in this sample
– Must first obtain a calibration, lots of experimental work involved
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