Automation of a Residual Gas Analyzer on a Time-shared Computer

Abstract

The automation of a quadrupole mass spectrometer -residual gas analyzer (RGA) on a time-shared IBM 1800 Data Acquisition and Control Computer is described. The RGA. which is used to determine the partial pressures of various gases in a vacuum system. may be operated up to a maximum data rate of about 20.000 points/sec (pps) in an interleaved manner with multiple slow-scanning (:::20 pps) instruments. We review in detail the hardware and software considerations regarding the design and subsequent interfacing of the instrument to the computer. Methods for initiating data collection. entering experimental parameters. and analyzing the experimental data. such as spectrum plotting. smoothing. peak location. mass identification. and calculation of the partial pressures by using a least-squares approximation to fit mass peaks. are discussed with examples.

Analysis of multicomponent mass spectra applying Bayesian probability theory

“…One explanation for this trend is provided by the threshold energies needed to produce the respective ions. These are 14.3 eV, 15.1 eV, 22.2 eV, and 25.0 eV for CH 3 C , CH 2 C , CH C , and C C respectively. With an electron energy of 70 eV in the ion source we see that this corresponds to 4.9 times the threshold energy for production of CH 3 C , whereas it means only 2.8 times the threshold energy for C C production.…”

“…The tables do not provide data on CH 3 , nor do we know estimates of the CH 3 cracking pattern from other sources. The values that we assign to CH 3 are derived from CH 4 assuming that the probability of producing CH 4 x from CH 4 is similar to CH 3 x from CH 3 . This assumption will turn out to be very poor.…”

“…The surface loss probability of CH 3 radicals is very small (10 3 ), 18 so that particle losses in the entrance of the mass spectrometer and the ionizer can be neglected. Since the cross-sections for direct ionization of CH 4 and CH 3 are similar, 25 a flux of CH 3 radicals can be inferred from the absolute flux of CH 4 molecules measured in the calibration experiment, assuming the mass transmission in the quadrupole analyzer is the same for CH 3 C and CH 4 C .…”

“…1 shows a mass spectrum of (CH 3 ) 2 N 2 with the capillary held at room temperature. The parent molecular ion (CH 3 In summary, we analyze the spectrum in panel (b) assuming that the contributing species are N 2 , CH 3 , CH 4 , acetylene (C 2 H 2 ), ethane (C 2 H 4 ), and C 2 H 6 . Calibration measurements were therefore performed on each of these components except CH 3 .…”

“…In favorable cases, such as Methyl (CH 3 ) detection in the presence of Methane (CH 4 ), ionization threshold mass spectrometry provides a way out of the difficulty. 9 Ionization thresholds for the production of CH 3 C from CH 3 and CH 4 are 9.5 eV and 12.6 eV respectively. An electron impact energy chosen within this window guarantees that the CH 3 C ions detected originate from the CH 3 radical only.…”