Analyzing gases by FTIR

How can transmission spectroscopy analyze a gas sample?

Like many other samples, a gas (or vapour) can be analyzed using the transmission of an infrared beam through the sample, reading the strength of the various wavenumbers upon exiting the sample and arriving at a detector.

How to analyze a gas using infrared transmission spectroscopy

The user places the sample inside a cell, responsible for holding the gas. Then the user places this gas cell into a compatible spectrometer, which is responsible for producing the infrared beam and registering data (to be analyzed on a computer). The spectrometer produces and casts a beam, containing thousands of different wavelengths of infrared light.

As the infrared beam is passed through the gas cell containing the gas sample, some of the energy is partially absorbed by certain molecules within the sample, while other wavelengths of the energy are transmitted all the way through undisturbed. The infrared beam then arrives at the detector and the strength of the various wavelengths of infrared light are measured individually and plotted on a graph, and in this way a spectrum is generated.

The analyst (often referred to as the ‘Spectroscopist’) must then investigate this spectrum and, using both their experience and libraries of pre-recorded spectra as a reference, make a judgement on what the gas sample does or does not contain by knowing what types of molecules tend to absorb/transmit certain wavelengths of infrared light. So, for example, what pollutants are contained within a sample of air taken from above a busy road, or does a particular paint release harmful vapours when left undisturbed?

In a solid or liquid, molecules are unable to freely rotate, due to intermolecular and intramolecular bonding and collisions with other molecules. However, in a gas, molecules can freely rotate many times before colliding with another molecule. These rotations are quantized and when a gas absorbs an IR photon, rotational fine structure is observed to be centred about the vibrational mode.

The rotational energy level can either:

increase, giving rise to the R branch, higher in energy than the vibrational mode
decrease, giving rise to the P branch lower in energy than the vibrational mode
remain constant, giving rise the Q branch at the same energy as the vibrational transition.

The latter mode is only observed when the vibrational mode is non-linear due to selection rules.

Why does the infrared beam bounce in a gas cell?

Because gas molecules are further apart than those in a liquid or a solid, the beam is deliberately bounced within the gas cell, to maximize how many molecules the energy collides with. This means the pathlength is actually very long when compared to that of a typical liquid or solid transmission cell. The typical pathlength of the Specac Pearl Liquid Transmission Cell is betwen 25 µm – 1000 µm, 1 µm being 1 millionth of a meter. Meanwhile, the Atmos Gas Transmission Cell’s pathlength measures in the meters.

Pathlength	Bounces
2.5 m	24
5 m	36
10 m	40
20 m	44

How gas FTIR compares to liquid and solid analysis

This process is, in many ways, the same as that for analyzing solid, paste, gel or liquid samples using transmission infrared spectroscopy. Except that, the sample is not prepared for analysis, nor inserted into the cell for analysis, in the same way. There are certain complications and challenges that present themselves when analyzing a gas sample, which are not prevalent when investigating other sample types.

What are common challenges of analyzing gas using infrared transmission spectroscopy?

There are some special considerations to make when analyzing gas using infrared transmission spectroscopy.

Gas requires a much larger pathlength

Gases have considerably lower densities than a solid or a liquid. As a result in order to obtain a good spectrum with low signal to noise pathlengths are typically many orders of magnitude greater than for a liquid or solid sample. Specac offers a range of gas cells with pathlengths from 10 cm up to 20 m depending on the exact application.

Spectral Resolution needs to be higher for gas transmission

In order to fully resolve the spectrum the full width at half maximum of the rotational fine structure must be significantly greater than the spectral resolution (typically the FWHM should be at least five times larger than the spectral resolution to fully resolve the fine structure). Otherwise line-broadening and peak height reduction occurs.

The band area remains constant for the same gas at different spectral resolutions. When the spectrum is poorly resolved the troughs will not return to the baseline and as the resolution is decreased further this fine structure will vanish leaving the P and R branches as two broad envelope peaks. This effect is shown in Figure 1, where the same spectrum of water vapour is presented at increasing resolution.

Figure 1: Water vapour spectrum from low (top) to high (bottom) spectral resolution

Effect of Pressure

To a first approximation peak areas increase linearly with partial pressure at a fixed temperature.

Rotational fine structure is sensitive to intermolecular collisions and the FWHM becomes broader with increasing pressure, typically on the order of 0.1 cm^-1 per atmosphere. Some gases have a greater effect on line broadening than others. As a result of this, when dealing with mixtures of gases, the spectral features for a particular gas at a fixed partial and total pressure at the same temperature will not be identical if the other components of the gas mixture are changed.