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The injection of analytes into a gas chromatography-mass spectrometry (GC-MS) system using dichloromethane (DCM) as solvent led to gradual deterioration of chromatographic signals, with significant tailing and loss of sensitivity for C17+ hydrocarbons. The injector, gas chromatograph and transfer line were excluded as causes. Normal peak shape could only be restored by the insertion of a cleaned MS ion source. To elucidate potential surficial contaminants, the ion source was heated from 260 to 320°C, leading to the release of increasing concentrations of ferrous chloride [FeCl2(g)]. The ferrous chloride probably formed through the decomposition of DCM on metal surfaces in the ion source. We posit that the tailing was caused by the adsorption of analytes to sub-µm layers of FeCl2 at crystal defect sites in the metal, followed by the slow release of molecules back into the gas phase. There are at least two other cases in the literature in which tailing is specifically associated with the use of halogenated solvents. However, it is possible that the problem is relatively common, albeit rarely diagnosed and reported. The tailing of chromatographic peaks caused by the formation of ferrous chloride in the mass spectrometer can be diagnosed by scanning the MS background signal for the diagnostic isotopic pattern of FeCl2(+). The problem is easily solved by cleaning the MS ion source and avoiding halogenated solvents.

Tailing of Chromatographic Peaks in GC-MS Caused by Interaction of Halogenated Solvents with the Ion Source