A new calibration technique for quantitative gas analyses of fluid inclusions using a quadrupole mass spectrometer

Quantitative gas analysis by quadrupole mass spectrometry is used widely in the study of fluid inclusion volatiles, but the currently available instrument calibration techniques have a number of limitations. We describe a new method to overcome these by employing custom-machined stainless-steel calibration volumes that fit into modified bellows valves and can be filled on the high-vacuum prep line of the instrument. This allows the calibration gas to be introduced in a pulse-like fashion, replicating a burst of sample gas from a ruptured fluid inclusion.

The modified bellows valves equipped with calibration volumes are loaded onto the high-vacuum inlet of a quadrupole mass spectrometer. Cobalt-based alloy stem tips are then used to seal calibration gases (dry air, in this case) at known pressures into the calibration volumes for analysis.

Two volumes of different dimensions were used to produce a relationship between moles introduced and integrated beam area for m/z 14, 32, 40, and 44. A linear fit along with a 95% confidence band for the relationship of moles introduced and integrated beam area was determined for the purpose of quantitative gas analysis in fluid inclusions containing atmospheric air. The calibration curves for each m/z have R-squared (COD) values of 0.94 or better. For validation of this calibration technique, measurements were made using outside air as an unknown.

A new calibration technique has been developed for measuring femto- to pico-mole quantities of atmospheric air that greatly reduces contamination and introduces calibration gas in a pulse similar to sample gas. This technique has reduced the error in quantifying these small aliquots of air to an uncertainty of ±1.5%.