DETECTION OF A 10(-4) HELIUM PEAK IN A DEUTERIUM ATMOSPHERE USING A MODIFIED HIGH-RESOLUTION QUADRUPOLE MASS-SPECTROMETER

This paper describes how a small helium (He-4) peak can be separated f rom a large adjacent deuterium (D2) peak using a modified quadrupole m ass spectrometer (QMS). This QMS utilized a condition of the second st ability zone (zone II) in the Mathieu diagram. The minimum detectable peak ratio of He-4 to D2 was 10(-4) when a mass scan line was set clos e to the upper tip, and was 10(-3) when the scan line was adjusted to the lower tip. The ion transmission rates simulating the peaks were ca lculated for the upper and lower zone conditions. It was found that a longer peak tail occurs when an unstable orbit has mainly a cosh funct ion. From the experimental and theoretical data, it is concluded that the upper zone condition is the most suitable for realizing a high-res olution peak.