What is ionization and how is partial pressure measured?
Ionization and fundamental problems in gas analysis
Continuous change in the voltages applied to the electrodes in the separation system (“scanning”) gives rise to a relationship between the ion flow I+ and the “atomic number” which is proportional to the m/e ratio and expressed as:
(Mr = relative molar mass, ne = number of elementary charges e)
This is the so-called mass spectrum, i+ = i+(M). The spectrum thus shows the peaks i+ as ordinates, plotted against the atomic number M along the abscissa. One of the difficulties in interpreting a mass spectrum such as this is due to the fact that one and the same mass as per the equation (4.2) may be associated with various ions. Typical examples, among many others, are: The atomic number M = 16 corresponds to CH4+ and O2++; M = 28 for CO+, N2+ and C2H+! Particular attention must therefore be paid to the following points when evaluating spectra:
1) In the case of isotopes we are dealing with differing positron counts in the nucleus (mass) of the ion at identical nuclear charge numbers (gas type). Some values for relative isotope frequency are compiled in Table 4.2.
2) Depending on the energy of the impacting electrons (equaling the potential differential, cathode – anode), ions may be either singly or multiply ionized. For example, one finds Ar+ at mass of 40, Ar++ at mass of 20 and Ar+++ at mass of 13.3. At mass of 20 one will, however, also find neon, Ne+. There are threshold energy levels for the impacting electrons for all ionization states for every type of gas, i.e., each type of ion can be formed only above the associated energy threshold. This is shown for Ar in Fig. 4.13.
3) Specific ionization of the various gases Sgas, this being the number of ions formed, per cm and mbar, by collisions with electrons; this will vary from one type of gas to the next. For most gases the ion yield is greatest at an electron energy level between about 80 and 110 eV; see Fig. 4.14.
In practice the differing ionization rates for the individual gases will be taken into account by standardization against nitrogen; relative ionization probabilities (RIP) in relationship to nitrogen will be indicated (Table 4.3).
4) Finally, gas molecules are often broken down into fragments by ionization. The fragment distribution patterns thus created are the so-called characteristic spectra (fingerprint, cracking pattern). Important: In the tables the individual fragments specified are standardized either against the maximum peak (in % or ‰ of the highest peak) or against the total of all peaks (see the examples in Table 4.4).
Both the nature of the fragments created and the possibility for multiple ionization will depend on the geometry (differing ion number, depending on the length of the ionization path) and on the energy of the impacting electrons (threshold energy for certain types of ions). Table values are always referenced to a certain ion source with a certain electron energy level. This is why it is difficult to compare the results obtained using devices made by different manufacturers.
Often the probable partial pressure for one of the masses involved will be estimated through critical analysis of the spectrum. Thus, the presence of air in the vacuum vessel (which may indicate a leak) is manifested by the detection of a quantity of O2+ (with mass of 32) which is about one-quarter of the share of N2+ with its mass of 28. If, on the other hand, no oxygen is detected in the spectrum, then the peak at atomic number 28 would indicate carbon monoxide. In so far as the peak at atomic number 28 reflects the CO+ fragment of CO2 (atomic number 44), this share is 11 % of the value measured for atomic number 44 (Table 4.5). On the other hand, in all cases where nitrogen is present, atomic number 14 (N2++) will always be found in the spectrum in addition to the atomic number 28 (N2+); in the case of carbon monoxide, on the other hand, there will always appear – in addition to CO+ – the fragmentary masses of 12 (C+) and 16 (O2++)).
Figure 4.15 uses a simplified example of a “model spectrum” with superimpositions of hydrogen, nitrogen, oxygen, water vapor, carbon monoxide, carbon dioxide, neon and argon to demonstrate the difficulties involved in evaluating spectra.
Evaluation problems: The peak at atomic number 16 may, for example, be due to oxygen fragments resulting from O2, H2O, CO2 and CO; the peak at atomic number 28 from contributions by N2 as well as by CO and CO as a fragment of CO2; the peak at atomic number 20 could result from singly ionized Ne and double-ionized Ar.
Partial pressure measurement
The number of ions i+gas produced from a gas in the ion source is proportional to the emission current i–, to the specific ionization Sgas, to a geometry factor f representing the ionization path inside the ionization source, to the relative ionization probability RIPgas, and to the partial pressure pgas. This number of ions produced is, by definition, made equal to the sensitivity Egas times the partial pressure pgas:
Almost all gases form fragments during ionization. To achieve quantitative evaluation one must either add the ion flows at the appropriate peaks or measure (with a known fragment factor [FF]) one peak and calculate the overall ion flow on that basis:
In order to maintain the number of ions arriving at the ion trap, it is necessary to multiply the number above with the transmission factor TF(m), which will be dependent on mass, in order to take into account the permeability of the separation system for atomic number m (analogous to this, there is the detection factor for the SEMP; it, however, is often already contained in TF). The transmission factor (also: ion-optical transmission) is thus the quotient of the ions measured and the ions produced.
The partial pressure is calculated from the ion flow measured for a certain fragment by multiplication with two factors. The first factor will depend only on the nitrogen sensitivity of the detector and thus is a constant for the device. The second will depend only on the specific ion properties.
These factors will have to be entered separately for units with direct partial pressure indication (at least for less common types of ions).