The gas evolved on cleavage of alkali halide single crystals in a high vacuum has been analyzed with an ultrasensitive mass spectrometer. The gas is primarily H2O in the case of NaCl, and primarily CO in the case of KCl, KI, NaF, and KBr. The evolved gas appears to come at least in part from the interior of the crystal, rather than from only the surface and from sliding parts in a vacuum. The amount of gas evolved from NaCl was found to depend on the source of the crystal, with less gas coming from the purer crystals. The total contamination of the freshly cleaved surfaces as a result of the gas is probably less than onehyphen;hundredth of a monolayer coverage. The sensitivity of this method of detecting certain impurities in solids is shown to be high.
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