Gas Flow in the Mass Spectrometer

Abstract

If a mass spectrometer is to be used successfully for gas analyses, the gas flow through the system must meet a number of requirements. The three modes of gas flow—molecular, intermediate, and viscous—are reviewed, and their working ranges are discussed in terms of the pressure of the system. The upper limit for molecular flow, through capillaries or through small openings, is found to lie at a pressure where the mean free path of the molecules is between 15 and 40 times the diameter of such a gas leak. Expressions are developed to show that, for molecular flow, the ion beam intensity is independent of molecular weight but is a linear function of the sample pressure, and that the principle of linear superposition applies to mixture peaks. Since the composition of a gas mixture in a reservoir is found to change with time (a numerical example is worked out), the necessity for analyses of short duration becomes evident. The practical aspects of designing the gas system for molecular flow and for efficient ion production are discussed in detail. The various flow requirements are best met by a gas leak consisting of a small hole in a diaphragm, or several holes in parallel. A relatively high gas pressure within the electron beam boundaries and resulting large ion currents are produced by introducing the gas as a molecular beam into a gas‐tight ionizing region.