Gas Analysis by Photo-Ionization Mass Spectrometry

Abstract

A photo‐ionization mass spectrometer featuring a special 180‐deg magnetic analyzer with inclined pole faces is described and its usefulness as a gas analytical tool explored. Ion‐source pressures up to 20 μ can be utilized for gas analysis. Higher pressures can be reached but ion‐molecule reactions and increasing light absorption cause a nonlinear ion current‐pressure relationship. The major advantage of photo‐ionization when compared to electron‐impact ionization is the simplicity of fragmentation patterns; the major dis‐advantage is the lower sensitivity. At 20‐μ pressure, spark or resonance light sources used in conjunction with a ½‐m monochromator produced peak ion intensities around 5×10⁻¹³ A. Although these ion intensities are higher than those reported previously, they are still considerably below the ion intensities commonly produced by electron‐impact ion sources.