NOVELTY - Determination of toxic halogen levels in halogen-containing compounds (HCC) comprises use of thermal electrons formed inside a mass spectrometer operated under electron capture conditions to give negative ions.    USE - For the specification and quantification of various halogenated species, including halogen atom ions and clusters, especially in rubbers.    ADVANTAGE - The method accurately indicates the types and levels of different toxic halogen in rubbers, specifically but not exclusively bromobutyl rubbers to ensure that rubber handlers are not exposed to unsafe levels of such toxins. It enables halogens of concern, i.e. toxic halogens, to be detected when ''buried'' under many more abundant non-halogenated compounds, which would be difficult to detect by any other method. It enables monohalogen compounds to be distinguished from dihalogen compounds, and Cl2, Br2, and ClBr halogens and their mixtures to be distinguished. The method avoids some drawbacks of previous methods, e.g. failure to distinguish between Br and Cl and inability to determine the number of halogen atoms in a molecule.    DETAILED DESCRIPTION - A reagent gas is ionized which is capable of producing thermal electrons of energy 0-10 eV with a mass spectrometer polychromatic ion source. The thermal electrons are captured by the halogen-containing compound (HCC), and form halogen atom anions or halogen atom clusters. The HCC is passed through a gas chromatograph prior to thermal electron capture, and after capture a mass spectrogram is obtained. The halogen atom ions or clusters are specified and quantified by examination of the chromatogram and mass spectrogram.    An INDEPENDENT CLAIM is included for on-line production of a product containing an HCC, in which the quantification and specification results are fed back to the halogen introduction or formation stage and not less than one process parameter is modified in response to this feedback, so as to give the desired halogen level in the finished product.    DESCRIPTION OF DRAWING(S) - Figure 1 shows a negative ion mass capture spectrum of 1,2-dibromo-3-chloro-2-methylpropane (I), which shows the presence of 35Cl, 37Cl, 79Br, 81Br, 79Br2(m/z 158), and other species.