Fourier Transform Infrared Spectroscopy

1. Introduction
2. Sample Preparation
3. Infrared Window Materials
4. Characteristic IR Band Position
5. IR Unit Conversions
6. Instrument at MCPF

1. Introduction

What information can IR spectroscopy provide?
  • Molecular vibrate with frequency in IR range

  • Identify unknown chemicals by matching spectra with known database

  • Structural ideas: can determine chemical groups in a specific compound

  • Optical or electronic properties in IR range

What does FT-IR measure?
AMolecule may absorb infrared radiation of the appropriate frequency to excite it from one vibrational or rotational level to another.
FT-IR measures the absorption spectra which characterize the particular molecular and its molecular motions

2. Sample Preparation

nSolid samples
pPowders: You can press powder into a pellet. In this method, you can select different diluents (matrixes) for various applications. For mid-IR frequency range, KBr, KCl  can be used. For far-infrared testing, high-density polyethylene (HDPE). For near-infrared analysis, CsI or KBr can be selected.
qSmall size or nonuniform : IR Microscopy
qThin Film: Direct transmission or Attenuated Total Reflection (ATR)
qThin Film on Substrate: Specular or diffuse reflection
nLiquid samples
qDrop of liquid on salt disc (no water)
qLiquid cell

3. Infrared Window Materials

For all non-self-supported samples, an IR transparent substrate or container (IR window)  is required for transmission measurement. The selection of window materials is based on your interested wavenumber range and the properties of the materials. The followed table can help you to chose the window material for your experiment. MCPF only can provide with limited NaCl and KBr powder for your sample preparation. 

Window Selection Guide
Material Range (cm-1) Refractive Index
(at 2 Ám)
%T Graph
AgBr 22,000-286 2.30 A soft crystal; insoluble in water; darkens upon exposure to UV radiation; will cold flow.
AgCl 10,000-360 2.07 Soft crystal that is insoluble in water; darkens upon exposure to UV radiation; will cold flow.
Al2O3 (Sapphire) 50,000-1,650 1.5   Glass-like
(GeAsSe Glass)
11,000-625 2.5   AMTIR (Amorphous Material Transmitting IR) is a glass; insoluble in water, resistant to corrosion.
BaF2 67,000-740 1.46 A hard, brittle crystal; insoluble in water; good resistance to fluorine and fluorides; no fog.
CaF2 77,000-1,110 1.42 A strong crystal; resists most acids and alkalis; withstands high pressure; insoluble in water; no fog.
CdTe 20,000-400 2.67   Lower thermal conductity than ZnSe (used with CO2 lasers). Attacked by oxidizers. Also known as Irtran-6.
(AsSeTe glass)
4,000-900 2.8   Good for Mid-IR fiber optics; chemically inert.
CsI 40,000-200 1.74   Soft crystal; soluble in water; hydroscopic; offers an extended transmission range.
Diamond 25,000-33 2.37   Phonon bands around 1900-2600, except in Type IIa diamonds. Very useful for high-pressure or corrosive work.
GaAs 7,000-650 3.33   Hard crystal, can be made amorphous
Ge 5,500-600 4.00 A hard, brittle crystal; insoluble in water; well suited for ATR.
Irtran-1 (see MgF2)
Irtran-2 (see ZnS)
Irtran-3 (see CaF2)
Irtran-4 (see ZnSe)
Irtran-5 (see MgO)
Irtran-6 (see CdTe)
(Irtran is a registered trademark of the Eastman Kodak Company)
KBr 40,000-400 1.53 Very soft, water soluble crystal; low cost and good transmission range; fogs.
(Thallium Bromide-Iodide)
20,000-250 2.37 A soft crystal, deforms under pressure; good ATR material. Soluble in bases and insoluble in acids. Toxic.
LiF 95,000-1,700 1.4   Best VUV transmitter available
MgF2 87,000-1,250 1.35  
NaCl 40,000-625 1.52 Very soft, water soluble crystal; low cost and good transmission range; fogs.
(high density)
600-30 1.54   Excellent for Far-IR, very cheap, attacked by few solvents, difficult to clean
Pyrex 30,000-4,000 1.473 Labware glass
Si 83,300-660 300-0 (depends on doping) 3.40 A hard and brittle crystal; inert; ideal material for far-IR.
SiO2 (Quartz) 65,000-2,700 1.4 A hard crystal, clear in the visible
17,000-833 2.2 A water-free form of ZnS. Insoluble in water. Also known as Irtran-2
ZnSe 17,000-720 2.2 A hard and brittle crystal; inert; ideal material for ATR. Also known as Irtran-1.

4. Characteristic IR Band Positions

The followed table list some common characteristic IR band in polymers. MCPF suggests the users to do reference search before submitting your samples for IR analysis.

 Group Frequency Range (cm-1)
OH stretching vibrations  
  Free OH 3610-3645 (sharp)
  Intramolecular H bonds 3450-3600 (sharp)
  Intermolecular H Bonds 3200-3550 (broad)
  Chelate Compounds 2500-3200 (very broad)
NH Stretching vibrations  
  Free NH 3300-3500
  H bonded NH 3070-3350
CH Stretching vibrations  
  =-C-H 3280-3340
  =C-H 3000-3100
  C-CH3 2862-2882, 2652-2972
  O-CH3 2815-2832
  N-CH3 (aromatic) 2810-2820
  N-CH3 (aliphatic) 2780-2805
  CH2 2843-2863,2916-2936
  CH 2880-2900
SH Stretching Vibrations  
  Free SH 2550-2600
C=-N Stretching Vibrations  
  Non-conjugated 2240-2260
  Conjugated 2215-2240
C=-C Stretching Vibrations  
  C=-CH (terminal) 2100-2140
  C-C=-C-C 2190-2260
  C-C=-C-C=-CH 2040-2200
C=O Stretching Vibrations  
  Nonconjugated 1700-1900
  Conjugated 1590-1750
  Amides ~1650
C=C Stretching Vibrations  
  Non-conjugated 1620-1680
  Conjugated 1585-1625
CH Bending Vibrations  
  CH2 1405-1465
  CH3 1355-1395, 1430-1470
C-O-C Vibrations in Esters  
  Formates ~1175
  Acetates ~1240, 1010-1040
  Benzoates ~1275
C-OH Stretching Vibrations  
  Secondary Cyclic Alcohols 990-1060
CH out-of-plane bending vibrations in substituted ethylenic systems  
  -CH=CH2 905-915, 985-995
  -CH=CH-(cis) 650-750
  -CH=CH-(trans) 960-970
  C=CH2 885-895

5. IR Unit Conversions

Here is some constants for unit conversion for FT-IR data processing:  

Energy unit conversions:

1 cm-1 =   0.124 meV 0.030 THz 1.44 Kelvin
1 meV = 8.1 cm-1   0.242 THz 11.6 Kelvin
1 THz = 33 cm-1 4.14 meV   48 Kelvin
1 Kelvin = 0.70 cm-1 0.086 meV 0.021 THz  

Conversions for microns (wavelength) to and from wavenumbers (cm-1):

microns = 104 / x cm-1
cm-1 = 104 / x microns

Absorbance units:

A = log ( 1/T )
Where A is Absorbance and T is Transmittance

6. Instrument at MCPF

The optical lab of MCPF is equipped with a FT-IR reserach system: Bio Rad FTS 6000. The FTS 6000 Spectrometer is a high performance research grade multi-range FT-IR spectrometer capable of both rapid scan and step scan operation and covering the spectral range 50,000-10 cm-1. Its high quality 60o Michelson air bearing piezoscan interferometer features piezoelectric-based continuous dynamic alignment and an interchangeable beamsplitter design. Other features include operation into the UV/Visible range and patented in-scan co-adding (providing effective 22 bit A/D operation). The piezoscan interferometer is capable of spectral resolution to 0.10 cm-1

6.1 Specification

Note: The typical setting of FTIR is : 

Transmission: Mid IR 400-4000cm-1, 4 cm-1 resolution.
ATR ZnSe Crystal: Mid IR 650-4000cm-1, 4 cm-1 resolution.
Microscopy: Mid IR 700-4000cm-1, 4 cm-1 resolution.

Spectrometer optical bench
Interferometer Stable air-bearing type 60 degree Michelson interferometer
Automatic beamsplitter alignment
Real time dynamic alignment during scan with three piezoelectric actuators at the back of the fixed mirror.
Source Mid-IR: High intensity water cooled ceramic source
Far-IR: Mercury arc lamp
Near IR: Tungsten halogen lamp
UV-Vis: Xenon lamp
Beamsplitter Mid-IR: Germanium on KBr beamsplitter
Far-IR: A set of 5 Mylar beamsplitter
Near IR: Quartz beamsplitter.
UV-Vis: UV Quartz beamsplitter
Range Mid-IR: 6000-400 cm-1
Far IR: 600-10 cm-1
Near IR: 15800-3000 cm-1
UV-Vis: 30,000-15800 cm -1
Detector Mid-IR: DTGS with CsI window
Far-IR: DTGS with PE window in dual detector plate
Near IR: PdSe detector & Si detector in dual detector plate
UV-Vis: UV detector assembly with R166, R466 & IP28 PMT detctors
Resolution Better than 0.1cm-1 (FWHM=0.08 cm-1)
Rapid-Scan Better than 80 scans/second at 32 cm-1 resolution
Step-Sacn Step rate from less than 1 Hz to 800 Hz (in Far, Mid & Near IR)
TRS Time-resolved spectroscopy, with time resolution of 5 Ás
Phase modulation: Phase and amplitude modulation
Operation mode Transmission, Reflection & Side Reflection
Objective 15X Schwartzchild objective
Condenser Two 15X Schwartzchild condensers.
Eyepieces 10X eyepiece.
Viewer Trinocular viewer
Sampling area Less than 10x10 Ám up to 250x250 Ám (Transmission & Reflection)
Stage 2"X3" manual stage
Polarzier IR Polarizer

Last updated on 07/08/2017