Inficon Transpector SPS, Operating Manual

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5 – 11
Transpector SPS Operating Manual
5.3 How to Interpret The Result
 Qualitative Interpretation Of Mass Spectra explains how to determine which
substances are present in the gas sample being analyzed. (See section 5.3.1 .)
 Quantitative Interpretation of Mass Spectra (Calculating Partial Pressures)
explains how to estimate how much of each substance is present. (See section
5.3.2 on page 5-26 .)
 Additional Information For Interpreting Mass Spectra provides additional
information regarding interpreting mass spectra. (See section 5.3.3 on page
5-32 .)
Software packages for Transpector instruments include routines which serve as
aids in the interpretation of spectra and the calculation of partial pressures and
relative concentrations.
For a discussion of how the Transpector produces its measurements, see Chapter
4, How the Transpector SPS System Works .
5.3.1 Qualitative Interpretation Of Mass Spectra
The graphical output of a partial pressure analyzer is a mass spectrum. A mass
spectrum is a pattern of peaks on a plot of ion intensity as a function of ion
mass-to-charge ratio. Each chemical substance has a characteristic mass
spectrum. Different instruments will give slightly different spectra for the same
substance. The particular characteristics of the ionizer, mass filter, detector, and
the manner in which the sample is introduced into the mass spectrometer all
influence the spectrum that is produced.
Rarely will a mass spectrum be obtained for a pure substance. Most of the time
(especially for residual gas analyzers), the spectrum obtained will be a composite
of the individual substances which together comprise the actual sample present.
Figure 5-6 shows the mass spectrum of air. The top graph shows the mass
spectrum of a single scan during the data run with the raw signal data on the y-axis
and the mass data on the x-axis. The prominent peaks for air are mass 28 (N
2
),
mass 32 (O
2
), mass 40 (Ar) and mass 18 (H
2
0). Also notice the N and O peaks at
masses 14 and 16 respectively. These are created when N
2
and O
2
are broken
apart in the ion source. The bottom graph is a trend analysis showing a few
selected mass signals versus time.