Fundamentals of FT-IR Spectroscopy

A Fourier Transform InfraRed (FT-IR) spectrometer is a device that collects broadband NIR to FIR spectra. Contrary to dispersive instruments such as spectrograph or grating monochromator, FT-IR spectrometers perform simultaneous acquisition of all wavelengths using a feature called the Multiplex or Felgett Advantage. The FT-IR technique involves acquiring an interferogram of a sample signal with an interferometer, followed by Fourier transforming the interferogram to obtain infrared spectra.

Reasons for Selection of an FT-IR Spectrometer

FT-IR spectrometers are recommended over dispersive instruments during the following situations:

  • If the work involves infrared
  • If high spectral resolution is required
  • If the work involves weak signals
  • If the spectra need to be acquired rapidly with high S/N ratio
  • If high spectral accuracy is required

Working Principle of FT-IR Spectrometers

FT-IR Spectrometers are based on a Michelson Interferometer (Figure 1). The interferometer comprises a fixed mirror, a beam splitter, and a mirror that has an accurate back and forth movement. The beam splitter is made from a special material that allows half of the incident radiation to transmit and the remaining to reflect off.

A Schematic of a generic Michelson interferometer.

Figure 1. A Schematic of a generic Michelson interferometer.

Radiation from the source hits the beam splitter and splits into two beams, of which one beam reaches the fixed mirror through the beam splitter and the second is reflected off the beam splitter towards the moving mirror.

The radiation is reflected back to the beam splitter by the fixed and moving mirrors, causing the transmission of only half of the reflected radiation and reflecting off of the remaining radiation at the beam splitter. This results in the transmission of one beam towards the detector and the second back to the source.

Optical Path Difference (OPD) is the variation between the two beams passing through the two arms of an interferometer is equivalent to the product of the physical distance traveled by the moving mirror and the refractive index of the medium filling the interferometer arms (n). FT-IR has a natural reference point when the fixed and moving mirrors are at the same distance from the beam splitter.

This condition is called zero path difference (ZPD). The moving mirror displacement (Δ) is quantified from the ZPD. As shown in Figure 2, the beam reflected off the moving mirror travels 2 Δ more than the beam reflected off the fixed mirror.

Schematic representation of waves and their phases, input, output, and the two arms of the interferometer as the scan goes from ZPD condition to

Figure 2. Schematic representation of waves and their phases, input, output, and the two arms of the interferometer as the scan goes from ZPD condition to OPD=λ . (a) OPD=0 case. (b) λ/4 OPD case. (c) λ/2 OPD case. (d) 3 λ/4 OPD case. (e) 1 λ OPD case.

The OPD can be related to the mirror displacement (Δ) as follows:

    OPD = 2Δn

The Interferogram

Interferogram is the name of the signal format collected by an FT-IR spectrometer (Figure 3). The large spike in the center of the broadband source interferogram shown in Figure 3 is called the center burst. The X-axis of the interferogram represents the OPD. Each individual spectral component contributes a single sinusoid with a frequency inversely proportional to its wavelength to this signal. The wavenumber (cm-1), denoted as ν is the unit of spectral measurement.

The Fourier Transform Algorithm

The Fast Fourier Transform algorithm is applied to translate the acquired interferogram into a spectrum (emission, absorption, transmission, etc.). The spectrum calculation involves a number of steps. Instrumental imperfections and basic scan limitations must be taken into account by performing phase correction and apodization steps. Apodization is employed to correct for spectral leakage and artificial creation of spectral features caused by the truncation of the scan at its limits.

Advantages of FT-IR Instruments over Dispersive Instruments

The following are the advantages of FT-IR instruments over Dispersive instruments:

  • Multiplex (Fellgett) Advantage – which allows simultaneous observation of all the wavenumbers of light in an FT-IR spectrometer, resulting in improved signal-to-noise ratio by a factor of √M, which is count of the resolution elements.
  • The Throughput Advantage – FT-IR instruments provide higher throughput as they can achieve higher resolution without slits. This is known as the Jacquinot Advantage.
  • High Resolution – In FT-IR instruments, the spectral resolution is determined by the maximum achievable value of OPD. The interferograms of light at 2000 cm-1 and 2002 cm-1 can be differentiated from each other at values of 0.5cm or longer.

Wavelength Limit and External Optics for FT-IR Instruments

FT-IR instruments have a short wavelength limit. The limiting wavelength is 1.4µm without oversampling and 700nm with oversampling. Like internal components, external optics are equally important for FT-IR instruments. Figure 3 shows a simplified scanning Michelson interferometer along with a source and detector in a bigger scale. The detector will record an interferogram when the scanning mirror moves. The length of the interferogram increases with the distance traveled by the scanning mirror. This leads to the achievement of higher spectral resolution.

Scanning Michelson Interferometer.

Figure 3. Scanning Michelson Interferometer.

Detector Optics, Source Optics, and Parabolic Mirrors

It is not viable to use a different detector for each resolution. Typically, one detector with a reasonably high resolution, not the highest is selected. The resolution of 4cm-1 is the popular choice as it is adaptable for condensed phase work. The requirement for higher resolution can be addressed by increasing the focal length of the fore optics of the detector. Another method is using an aperture (Jacquinot Stop) to improve the F/# by reducing the effective source size. This leads to the reduction of the spot size on the detector.

The source optics will typically produce a beam with étendue higher than the neded étendue of the interferometer. The étendue of the instrument is generally limited by the detector size and optics or desired resolution. Most FT-IR instruments employ off-axis parabolic mirrors for collimation and focus of light external to the interferometer. Parabolae are instruments suitable for collimation of light from small sources and conversely for the tight focus of collimated beams of radiation. However, they are not suitable for the imaging of larger objects.

Light from a point source positioned in the focus of a parabola (Figure 4) will be transformed subsequent to reflection into an ideally parallel beam. Consequently, a parallel beam will be focused into a small focal spot. This is valid for any portion of the parabola. Hence, it is possible to cut out an off-axis section of the paraboloidal mirror for convenience (Figure 5).

Light from a point source placed at the focus of a parabola.

Figure 4. Light from a point source placed at the focus of a parabola.

Section of off-axis parabolic mirror.

Figure 5. Section of off-axis parabolic mirror.

F/numbers of off-axis parabolic mirrors can achieve very low values. If a finite size source is positioned in the focal point of the parabola, the reflected beam will no longer ideally parallel. It will have some angular divergence in proportion to the angular size of the source. Additionally, it will experience substantial aberrations. Consequently, a parallel incoming beam will be focused into a blurred spot. The energy distribution in the focal plane of the off axis reflector for beams of different divergence is illustrated in Figure 6, showing the increasing impact of aberrations with increasing ‘field of view’ of the parabola.

Energy distribution in the focal plane of an off-axis reflector.

Figure 6. Energy distribution in the focal plane of an off-axis reflector.

Lenses

Despite universality and wide adoption of off-axis parabolic mirrors in FT-IR spectroscopy, they have their own drawbacks. It is a complex process to align off-axis parabolic mirrors as each reflection turns the beam through 90°. This may lead to a bulkier system. At low F/#, they experience substantial aberrations. In most applications, particularly in the Near IR, the use of lenses could be a good option. Figure 7 depicts the energy distribution in the focal spot of a CaF2 lens that has about the same focal length and F/# as the parabolic mirror considered earlier.

Energy distribution in the focal plane of a CaF2 lens.

Figure 7. Energy distribution in the focal plane of a CaF2 lens.

When using lenses, care must be given in the selection of the lens material. CaF2 lenses are recommended in the whole range wherein the CaF2 beam splitter is applicable. Fused silica lenses are good in the very Near IR, up to 3µm, although there can be some loss with lenses that are not ‘IR grade’ due to water absorption bands.

A wide choice of lens materials is available for use in the Mid IR. Some materials are hygroscopic in nature, which can be a big problem. The dispersion of the lens material is another issue. Lenses are absolutely fine for limited wavelength range applications. For a wider wavelength range, the detector needs to be positioned at the focal length position because the system.

This information has been sourced, reviewed and adapted from materials provided by Oriel Instruments.

For more information on this source, please visit Oriel Instruments.

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