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What is qualitative gas analysis?

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The analysis of spectra assumes certain working hypotheses:

1. Every type of molecule produces a certain, constant mass spectrum or fragment spectrum which is characteristic for this type of molecule (fingerprint, cracking pattern).
2. The spectrum of every mixture of gases is the same as would be found through linear superimposition of the spectra of the individual gases. The height of the peaks will depend on the gas pressure.
3. The ion flow for each peak is proportional to the partial pressure of each component responsible for the peak. Since the ion flow is proportional to the partial pressure, the constant of proportionality (sensitivity) varies from one gas to the next.

Although these assumptions are not always correct, they do represent a useful working hypothesis.

How does qualitative gas analysis work?

In qualitative analysis, the unknown spectrum is compared with a known spectrum in a library. Each gas is “definitively determined” by its spectrum. The comparison with library data is a simple pattern recognition process. Depending on the availability, the comparison may be made using any of a number ancillary aids. So, for example, in accordance with the position, size and sequence of the five or ten highest peaks. Naturally, comparison is possible only after the spectrum has been standardized, by setting the height of the highest line equal to 100 or 1000 (see Table 4.5 as an example).

Table 4.5 Spectrum library of the 6 highest peaks for the TRANSPECTOR

The comparison can be made manually on the basis of collections of tables (for example, A. Cornu & R. Massot: Compilation of Mass Spectral Data) or may be effected with computer assistance; large databases can be used (e.g. Mass Spectral Data Base, Royal Society of Chemistry, Cambridge).

When making comparisons with library information, it is necessary to pay attention to whether identical ion sources or at least identical electron impact energies were used.

All these capabilities are, however, generally too elaborate for the problems encountered in vacuum technology. Many commercial mass spectrometers can show a number of library spectra in the screen so that the user can see immediately whether the “library substance” might be contained in the substance measured. Usually the measured spectrum was the result of a mix of gases and it is particularly convenient if the screen offers the capacity for subtracting (by way of trial) the spectra of individual (or several) gases from the measured spectrum. The gas can be present only when the subtraction does not yield any negative values for the major peaks. Figure 4.16 shows such a step-by-step subtraction procedure using the Transpector-Ware software.

Fig 4.16 Subtracting spectra contained in libraries

Regardless of how the qualitative analysis is prepared, the result is always just a “suggestion”, i.e. an assumption as to which gases the mixture might contain. This suggestion will have still to be examined, e.g. by considering the likelihood that a certain substance would be contained in the spectrum. In addition, recording a new spectrum for this substance can help to achieve clarity.

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