A partial pressure measurement unit is characterized essentially by the following properties (DIN 28 410):
The line width is a measure of the degree to which differentiation can be made between two adjacent lines of the same height. The resolution is normally indicated. It is defined as: R = M / ΔM and is constant for the quadrupole spectrometer across the entire mass range, slightly greater than 1 or ΔM < 1.
Often an expression such as “unit resolution with 15% valley” is used. This means that the “bottom of the valley” between two adjacent peaks of identical height comes to 15 % of the height of the peak or, put another way, at 7.5 % of its peak height the line width DM measured across an individual peak equals 1 amu (atomic mass unit); see in this context the schematic drawing in Fig. 4.10.
The mass range is characterized by the atomic numbers for the lightest and heaviest ions with a single charge which are detected with the unit.
Sensitivity E is the quotient of the measured ion flow and the associated partial pressure; it is normally specified for argon or nitrogen:
The smallest detectable partial pressure is defined as a ratio of noise amplitude to sensitivity:
The definition is:
SDPPR = pmin / pΣ (ppm)
This definition, which is somewhat “clumsy” for practical use, is to be explained using the detection of argon36 in the air as the example: Air contains 0.93 % argon by volume; the relative isotope frequency of Ar40 to Ar36 is 99.6 % to 0.337 %. Thus, the share of Ar36 in the air can be calculated as follows:
Figure 4.11 shows the screen print-out for the measurement. The peak height for Ar36 in the illustration is determined to be 1.5 · 10-13 A and noise amplitude Δ · i+R to be 4 · 10-14 A. The minimum detectable concentration is that concentration at which the height of the peak is equal to the noise amplitude. This results in the smallest measurable peak height being 1.5 · 10-13 A/2.4 · 10-14 A = 1.875. The smallest detectable concentration is then derived from this by calculation to arrive at:
The linearity range is that pressure range for the reference gas (N2, Ar) in which sensitivity remains constant within limits which are to be specified (± 10 % for partial pressure measurement devices).
In the range below 1 · 10-6 mbar the relationship between the ion flow and partial pressure is strictly linear. Between 1 · 10-6 mbar and 1 · 10-4 mbar there are minor deviations from linear characteristics. Above 1 · 10-4 mbar these deviations grow until, ultimately, in a range above 10-2 mbar the ions for the dense gas atmosphere will no longer be able to reach the ion trap. The emergency shutdown for the cathode (at excessive pressure) is almost always set for 5 · 10-4 mbar. Depending on the information required, there will be differing upper limits for use.
In analytical applications, 1 · 10-6 mbar should not be exceeded if at all possible. The range from 1 · 10-6 mbar to 1 · 10-4 mbar is still suitable for clear depictions of the gas composition and partial pressure regulation (see Fig. 4.12).
Additional information required to evaluate a sensor includes specifications on the bake-out temperature (during measurement or with the cathode or SEMP switched off), materials used and surface areas of the metal, glass and ceramic components and the material and dimensions for the cathode; data is also needed on the electron impact energy at the ion source (and on whether it is adjustable). These values are critical to uninterrupted operation and to any influence on the gas composition by the sensor itself.