A small analytical quadrupole mass spectrometer designed for operation in high and ultra high vacuum systems

The characteristics of a small quadrupole mass spectrometer are described in detail. These show that the requirements of an analytical instrument for use in high and ultra high vacuum systems have been met. The performance is adequate for the measurement of partial pressures down to and below 10^?11 torr. This is achieved with constant sensitivity over the range 0–200 amu with a resolution adequate to separate completely individual peaks up to 50 amu. The instrument does not have an electron multiplier. The consequent disadvantages of a relatively slow response and a limitation to the minimum detectable pressure are compensated by a higher stability and a smaller, simpler, and reliable instrument. Observations with a number of units over a period of many months indicate a maximum change in sensitivity of any one and a difference between individual instruments to be of the order of ±20%. This is therefore a gauge head of approximately the same size as a modulated BA gauge with the same sensitivity, stability and baking characteristics.