A closed electron impact ion source with overall opening area of less than 30 mm2 is employed for direct and pulsed extraction into a time-flight mass spectrometer in order to enhance sensitivity and immunity to chemical noise of oil and fumes of the vacuum system. For compatibility with dual stage GCxGC separation, the source may contain an inert liner surrounded by an isothermal cage of thermally conductive material. The source inner surface may be reduced to under 100 mm2. A portion of carrier gas may be pumped down before admitting the sample and carrier gas into the source. A cooled surface may be used to condense fumes at the analysis time.
