No abstract

Abstract. As a reagent gas for positive-and negative-mode chemical ionization mass spectrometry (CI-MS), isobutane (i-C 4 H 10 ) produces superior analyte signal abundance to methane. Isobutane has never been widely adopted for CI-MS because it fouls the ion source more rapidly and produces positive CI spectra that are more strongly dependent on reagent gas pressure compared with methane. Isobutane was diluted to various concentrations in argon for use as a reagent gas with an unmodified commercial gas chromatograph-mass spectrometer. Analyte spectra were directly compared using methane, isobutane, and isobutane/argon mixtures. A mixture of 10% i-C 4 H 10 in argon produced twice the positive-mode analyte signal of methane, equal to pure isobutane, and reduced spectral dependence on reagent gas pressure. Electron capture negative chemical ionization using 1% i-C 4 H 10 in argon tripled analyte signal compared with methane and was reproducible, unlike pure isobutane. The operative lifetime of the ion source using isobutane/ argon mixtures was extended exponentially compared with pure isobutane, producing stable and reproducible CI signal throughout. By diluting the reagent gas in an inert buffer gas, isobutane CI-MS experiments were made as practical to use as methane CI-MS experiments but with superior analytical performance.

1IntroductionThe physical properties of low-volatility military explosives, e.g.,T NT and RDX, have been well characterized,g uiding trace explosives detection efforts worldwide. Due to their simple accessibility and formidable potency,t he explosives comprising improvisede xplosive devices (IEDs) have garnered significant attention in recent years. While the physical properties for some of these explosives, including TATP, are generally well understood, others, such as ammonium nitrate (AN), are not as well characterized.Extensived ata regardingA Na erosols formed in the atmosphere exists as scientists have studied the presence of ammonia and nitric acid in the atmosphere, stemming from multiple anthropogenic sources [1].A tmosphericd ata collected in variousr egions of California [2-5] as well as Japan [6] show thata mmonium nitratea erosol concentrations are in the low to mid mgm À3 range and vary considerably within geographical regions.E ven in regions where relatively low levels are found, it is reasonable to expect, at am inimum, low parts per billion by volume (ppbv) of ammonia and nitric acid as background contamination [4]. Clearly,d etecting an AN-based IED will always be frustrated by the presence of ammonium nitrate aerosols in equilibrium with ammonia and nitric acid vapors.F urthermore, when attempting to generate aq uantitative level of AN vapor in the laboratory,avery careful characterization of the backgroundl evels of ammonia and nitric acid is necessary.