Fundamentals of Vacuum Technology (1248463), страница 44

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4.15 Model spectrum4.6.2 Partial pressure measurementThus+igasThe number of ionsproduced from a gas in the ion source isproportional to the emission current iÐ, to the specific ionization Sgas, to ageometry factor f representing the ionization path inside the ionizationsource, to the relative ionization probability RIPgas, and to the partial pressure pgas. This number of ions produced is, by definition, made equal to thesensitivity Egas times the partial pressure pgas:+igasi-(produced) = á Sgas á f á RIPgas á pgas= Egas á pgasdue toRIPN2 = 1EN2 is equal to iÐ á SN2 á fand Egas is equal to EN2 á RIPgasAlmost all gases form fragments during ionization. To achieve quantitativeevaluation one must either add the ion flows at the appropriate peaks ormeasure (with a known fragment factor [FF]) one peak and calculate theoverall ion flow on that basis:+igas,m1+++igas(produced )= igas+i+....=,m 1 gas ,m2BFgas,m 1+igas,m2==....

= Egas ⋅ pgasBFgas,m2In order to maintain the number of ions arriving at the ion trap, it isnecessary to multiply the number above with the transmission factor TF(m),which will be dependent on mass, in order to take into account thepermeability of the separation system for atomic number m (analogous tothis, there is the detection factor for the SEMP; it, however, is often alreadycontained in TF). The transmission factor (also: ion-optical transmission) isthus the quotient of the ions measured and the ions produced.pgas =⇒ pgas =+igas, m2(produced)BFgas , m2 ⋅ Egas+igas , m (measured )2BFgas , m ⋅ E gas ⋅TF (m)2and withaEgas = EN2 á RIPgasthe ultimate result is:+pgas = igas, m 2 (measured ) ⋅11⋅E N2 BFgas , m2 ⋅ RIPgas ⋅TF (m)(4.3)The partial pressure is calculated from the ion flow measured for a certainfragment by multiplication with two factors. The first factor will depend onlyon the nitrogen sensitivity of the detector and thus is a constant for thedevice.

The second will depend only on the specific ion properties.These factors will have to be entered separately for units with direct partialpressure indication (at least for less common types of ions).4.6.3 Qualitative gas analysisThe analysis of spectra assumes certain working hypotheses:1. Every type of molecule produces a certain, constant mass spectrum orfragment 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 found106HomeMass spectrometrythrough 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 eachcomponent responsible for the peak. Since the ion flow is proportional tothe partial pressure, the constant of proportionality (sensitivity) variesfrom one gas to the next.Although these assumptions are not always correct (see Robertson: MassSpectrometry) they do represent a useful working hypothesis.In qualitative analysis, the unknown spectrum is compared with a knownspectrum in a library. Each gas is Òdefinitively determinedÓ 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 anumber ancillary aids. So, for example, in accordance with the position,size and sequence of the five or ten highest peaks.

Naturally, comparison ispossible only after the spectrum has been standardized, by setting theheight of the highest line equal to 100 or 1000 (see Table 4.5 as anexample).many gas components; here a qualitative analysis will have to be madebefore attempting quantitative analysis. The degree of difficulty encounteredwill depend on the number of superimpositions (individual / a few / many).In the case of individual superimpositions, mutual, balancing of the ionflows during measurement of one and the same type of gas for severalatomic numbers can often be productive.Where there is a larger number of superimpositions and a limited number ofgases overall, tabular evaluation using correction factors vis ˆ vis thespectrum of a calibration gas of known composition can often be helpful.Parent spectrumAThe comparison can be made manually on the basis of collections of tables(for example, A.

Cornu & R. Massot: Compilation of Mass Spectral Data) ormay 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 payattention to whether identical ion sources or at least identical electronimpact energies were used.Parent spectrum21All these capabilities are, however, generally too elaborate for the problemsencountered in vacuum technology.

Many commercial mass spectrometerscan show a number of library spectra in the screen so that the user can seeimmediately whether the Òlibrary substanceÓ might be contained in the substance measured. Usually the measured spectrum was the result of a mixof gases and it is particularly convenient if the screen offers the capacity forsubtracting (by way of trial) the spectra of individual (or several) gases fromthe measured spectrum. The gas can be present only when the subtractiondoes not yield any negative values for the major peaks. Figure 4.16 showssuch a step-by-step subtraction procedure using the Transpector-Waresoftware.Parent spectrum without kryptonRegardless of how the qualitative analysis is prepared, the result is alwaysjust a ÒsuggestionÓ, i.e. an assumption as to which gases the mixture mightcontain. This suggestion will have still to be examined, e.g. by consideringthe likelihood that a certain substance would be contained in the spectrum.In addition, recording a new spectrum for this substance can help toachieve clarity.Parent spectrum without argon4.6.4 Quantitative gas analysisParticular difficulties are encountered when interpreting the spectrum of anunknown mixture of gases.

The proportions of ion flow from various sourcescan be offset one against the other only after all the sources have beenidentified. In many applications in vacuum technology one will be dealingwith mixtures of a few simple gases of known identity, with atomic numbersless of than 50 (whereby the process-related gases can representexceptions). In the normal, more complicated case there will be a spectrumwith a multitude of superimpositions in a completely unknown mixture ofA = ParentrangeAssumption:Groups1 = Kr ypton +2 = Krypton ++Library spectrum:Krypton43Assumption:3Argon +4Argon ++Library spectrum:Argon5Assumption:5Neon +Library spectrum:NeonParent spectrum after detectionof krypton, argon and neonFig. 4.16 Subtracting spectra contained in libraries107HomeMass spectrometryIn the most general case a plurality of gases will make a greater or lessercontribution to the ion flow for all the masses.

The share of a gas g in eachcase for the atomic number m will be expressed by the fragment factorFfm,g. In order to simplify calculation, the fragment factor Ffm,g will alsocontain the transmission factor TF and the detection factor DF. Then the ioncurrent to mass m, as a function of the overall ion currents of all the gasesinvolved, in matrix notation, is:i j+   BF j, k  ⋅⋅ ⋅  ⋅i +   ⋅= m ⋅  ⋅⋅  ⋅ + iu   BFu, k⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅FFm, g ⋅⋅⋅⋅⋅⋅⋅⋅ BF j,o   I k+  ⋅⋅  ⋅  ⋅⋅  ⋅⋅⋅  ·  I g+   ⋅⋅  ⋅⋅⋅  ⋅  ⋅ BFu,o  I o+ The ion current vector for the atomic numbers m (resulting from the contributions by the fragments of the individual gases) is equal to the fragmentmatrix times the vector of the sum of the flows for the individual gases.0im+ = ∑ BFm, g · I g+or:g=k(in simplified notation: i = FF á I)+where im = ion flow vector for the atomic numbers, resulting fromcontributions of fragments of various individual gases0∑ BFm, g= fragment matrixg=kI+g = Vector of the sum of the flows for the individual gases or:Ff m, g6444474448im = ∑ pg ⋅ E N2 ⋅ RIPg ⋅ FFm ⋅ TFm+Transmission factorfor the mass mFragment factorfor the gas to mass mRelative ionization probabilityfor the gasFFT á i = FFT á FF á Iwhich can be evaluated direct by the computer.

The ion current vector forthe individual gases is then:–1I=4.7[FF T ⋅ i] ⋅ [FF T ⋅ BF ]det[FF T ⋅ BF ]Software4.7.1 Standard SQX software (DOS) forstand-alone operation (1 MS plus 1PC, RS 232)The conventional software package (SQX) contains the standard routinesfor the operation of the mass spectrometer (MS)Ð various spectradepictions, queries of individual channels with the corresponding screendisplays as tables or bar charts, partial pressure conversion, trend displays,comparison with spectra libraries (with the capability for trial subtraction oflibrary spectra), leak testing mode etc.

Ð and for sensor balancing, as well.Using PCs as the computer and display unit naturally makes available allthe usual functions including storing and retrieving data, printing, etc.Characteristic of the conventional software package is that specificindividual spectra will be measured, even though the measurement is fullyautomated and takes place at a point in time which is specified in advance.A spectrum of this type can thus be only a ÒsnapshotÓ of a process inprogress.4.7.2 Multiplex/DOS software MQX(1 to 8 MS plus 1 PC, RS 485)The first step toward process-oriented software by LEYBOLD is the MQX.

Itmakes possible simultaneous monitoring of a maximum of eight sensorsand you can apply all the SQX functions at each sensor.Nitrogen sensitivity (equipment constant)Partial pressure of the gasIon current for atomic number mOne sees that the ion flow caused by a gas is proportional to the partialpressure. The linear equation system can be solved only for the specialinstance where m = g (square matrix); it is over-identified for m > g. Due tounavoidable measurement error (noise, etc.) there is no set of overall ionflow I+g (partial pressures or concentrations) which satisfies the equationsystem exactly.

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