Abstract

A variable frequency, switched, direct-current gas sampling glow discharge ionization source was interfaced to an orthogonally extracted time-of-flight mass spectrometer for the purpose of dynamically generating atomic and molecular mass spectra of halogenated hydrocarbons. The plasma duty cycle is 50% in each mode of operation. The operating mode is selected simply by switching the polarity of voltage applied to the sample-introduction plate. The self-igniting helium discharge is contained within the first vacuum stage of the differentially pumped spectrometric interface. Both atomic and molecular mass spectra could be obtained with similar ion-optical lens settings, with the exception of the steering plate potential, which had to be alternated between two values (at the plasma switching frequency). A range of discharge switching rates can be used, although a non-linear drop in the modulated signal occurs with frequency. Switching rates above 100 Hz appears to be impractical with current experimental apparatus. Long-term temporal stability of the atomic (⁷⁹Br⁺) and molecular (¹⁷²CHBr₂⁺) signals was measured over a period greater than 1 h, with an overall precision of 13.3 and 6.1% RSD, respectively. The ⁷⁹Br⁺/⁸¹Br⁺ isotope ratio precision was 1.1% RSD over this period, while the ¹⁷²CHBr₂⁺/¹⁷³CHBr₂⁺ molecular ratio was determined to be 1.6% RSD. The precision of the isotope and ionic ratios are counting statistics limited. The atomic detection limit is in the range 3–110 pg s⁻¹ (as the halogen) and 15–250 pg s⁻¹ (molecular) for analytes manually introduced into an exponential-dilution sample-introduction device, with boxcar averagers being employed for data collection. A variety of chlorinated hydrocarbons were introduced into the discharge via the flow cell, and it was possible to differentiate (or speciate) the compounds based on their ³⁵Cl⁺/¹²C⁺ elemental ratios with a correlation coefficient (R) of 0.995.