A modification of the MGA 200 mass spectrometer to enable measurement of anaesthetic gas mixtures.

The measurement of anaesthetic gas mixtures with a mass spectrometer is complicated by the overlap of fragmentation products of the anaesthetics with the normal respiratory gas. For the MGA 200 quadrupole instrument the problem has been overcome by incorporating simple circuitry which, for an appropriate mass number, sums the concentrations of the unwanted fragments and subtracts them from the total output for the particular channel; this provides an accurate and stable system (within +/- 1% f.s.d.) over at least 24 h.