AVS 63rd International Symposium & Exhibition
    Vacuum Technology Monday Sessions
       Session VT-MoA

Paper VT-MoA11
Calibration of Quadrupole Mass Spectrometers with a Molecular Flow Gas Source

Monday, November 7, 2016, 5:00 pm, Room 104C

Session: Gas Dynamics, Simulation and Partial Pressure Analysis
Presenter: Robert Ellefson, REVac Consulting
Correspondent: Click to Email

Insitu calibration of quadrupole mass spectrometers (QMS) after initial calibration is not regularly done. If no calibration gas is available, it requires removal of the QMS from the vacuum system to a calibration test stand where gases and a reference gauge is available. Providing a low-cost calibration gas source dedicated to the QMS enables local calibration checks to qualify the performance of the QMS. This paper describes a molecular flow calibration device that presents known flow rates of gas species to the ion source. The known flow rate produces partial pressures that can be measured with an ion gauge or calculated knowing the conductance of the QMS molecular flow pumping system. The molecular flow in and molecular flow out preserves the gas composition of even a flowing gas mixture prepared in a volume from attached gas sources. The device prepares a known volume of gas (300 cm3 ) at low pressure (PCDG <10 Torr) to assure molecular flow through an orifice with flow proportional to CN2(28/M)1/2 for each species. With this small volume, the partial pressure depletes for each species in a predictable manner related to the mass of the gas species. By noting the time elapsed since the valve to the molecular leak is opened, the time-dependent partial flow rate, qi(t) of each species is known and a sensitivity Si for that species can be calculated as Ii(t)/[qi(t)/Cout] from the measured ion current, Ii(t). Data showing sensitivity of a QMS as a function of ion source pressures provides information to show stability of sensitivity over a range of pressures. The ability to introduce pure gases and blend or introduce gas mixtures gives conditions to measure QMS accuracy for mixture analysis. The simplicity of the system lends itself to automation of the sensitivity measurement process as the basis for archiving Si values over extended periods of time as quality assurance performance data.