Increased sensitivity and decreased background noise are possible due to the ionizer design. An enclosed ion source operates with sample pressures as high as 10-3 torr with no need for futher pressure reduction of this differentially pumped inlet system. Higher sample pressures contain greater numbers of neutrals and more sample to ionize. With each order of magnitude increase in pressure, signal strength varies proportionally with sample density. This is accomplished by confining the sample within a (relatively) small ionization volume. The sample is constrained within the ion volume that that has a two small slits on either side of the ion volume. Bombarding electrons enter through these slits, ionizing the sample. Ions formed near the small exit hole of the ion volume are immediately extracted into the mass filter, and have little opportunity to react with neutrals. Ionization efficiency is high, since virtually the entire sample passes through the small ionization zone. The main transport mechanism from the source to the downstream mass filter is a pressure gradient, resulting from the higher pressure in the ionizer versus the lower pressure in the downstream mass filter. Hence, the source is conductance limited by the pressure difference.