Solid-phase microextraction low temperature plasma mass spectrometry for the direct and rapid analysis of chemical warfare simulants in complex mixtures

Abstract

Solid-phase microextraction (SPME) is directly integrated with low temperature plasma ionisation mass spectrometry to rapidly detect organophosphate chemical warfare agent simulants and their hydrolysis products in chemical mixtures, including urine. In this sampling and ionization method, the fibre serves: (i) to extract molecules from their native environment, and (ii) as the ionization electrode that is used to desorb and ionize molecules directly from the SPME surface. By use of a custom fabricated SPME fibre consisting of a stainless steel needle coated with a Linde Type A (LTA) zeolitic microporous material and low temperature plasma mass spectrometry, protonated dimethyl methylphosphonate (DMMP), diethyl ethylphosphonate (DEEP) and pinacolyl methylphosphonic acid (PinMPA) can be detected at less than 100 ppb directly in water and urine. Organophosphates were not readily detected by this approach using an uncoated needle in negative control experiments. The use of the LTA coating significantly outperformed the use of a high alumina Zeolite Socony Mobil-5 (ZSM-5) coating of comparable thickness that is significantly less polar than LTA. By conditioning the LTA probe by immersion in an aqueous CuSO4 solution, the ion abundance for protonated DMMP increased by more than 300% compared to that obtained without any conditioning. Sample recovery values were between 96 and 100% for each analyte. The detection of chemical warfare agent analogues and hydrolysis products required less than 2 min per sample. A key advantage of this sampling and ionization method is that analyte ions can be directly and rapidly sampled from chemical mixtures, such as urine and seawater, without sample preparation or chromatography for sensitive detection by mass spectrometry. This ion source should prove beneficial for portable mass spectrometry applications because relatively low detection limits can be obtained without the use of compressed gases, fluid pumps, and lasers. Moreover, the ion source is compact, can be powered with a 10 V battery, and is tolerant of complex mixtures.

Internal Energy Deposition in Dielectric Barrier Discharge Ionization is Significantly Lower than in Direct Analysis in Real-Time Mass Spectrometry

“…[24,25] For more complex mixtures, active capillary DBDI can be directly integrated with solid-phase microextraction (SPME) to reduce ion suppression effects. [17,35] For example, we have demonstrated chemical warfare agent simulants can be detected directly from urine and waterway samples in less than 2 min at trace levels (,100 ppb) using SPME active capillary DBDI MS. [17] An important characteristic of an ion source is the extent of energy that is deposited into ions during formation. Thermometer ions can be used to determine the extent of ion activation on formation [20,23,28,36,37] and storage, [38] and during ion activation.…”

“…[1] Methods for forming ions under ambient conditions can be classified into three primary groups that are based on liquid solution sprays, [2,3] laser desorption from surfaces, [4][5][6] and plasma ionization. [7] Such ionization methods can be used to analyse samples in the open atmosphere without the need for ion source enclosure, which facilitates high-throughput screening applications, including in the analysis of pharmaceutical tablets, [8] forensic samples, [9][10][11] explosives, [12,13] narcotics, [14] chemical warfare agents, [15][16][17] and persistent organic pollutants, [18][19][20] and bacterial profiling. [21] Plasma-based ionization methods have the advantages that (i) solvents, fluid pumps, and lasers are not required, and (ii) such ion sources tend to be significantly less susceptible to ion suppression, in which the sample matrix can detrimentally affect analyte ionization efficiency, than spray-and laser-based ion sources.…”

“…Two of the most widely used types of plasma ionization sources are direct analysis in real time (DART) [9,14,22] and dielectric barrier discharge ionization (DBDI), [15,17,20,23] which can both be used to sample analytes from air, [24,25] liquids, [17] and surfaces. [15] DART and DBDI are mechanistically similar to atmospheric pressure chemical ionization and atmospheric pressure photoionization.…”

“…In dielectric barrier discharge, the resulting plasma can be classified as a 'low-temperature plasma' in cases where the ion and electron effective temperatures are near ambient temperature and the electron density in the plasma is relatively low (, 10 19 m À3 versus 10 21 to 10 26 m À3 for arc plasmas). [29] In DBDI MS, [15,17,20,23] a low-temperature plasma plume is used to desorb and ionize molecules from surfaces and volatile molecules in the ambient air. The DBDI plasma is maintained by a positive pressure of a gas (e.g.…”

“…[27,28] In dielectric barrier discharge ionization, [15,17,20,23] a gas flow is ionized by applying a high voltage and high-frequency alternating current between two electrodes that are separated by a dielectric barrier. In dielectric barrier discharge, the resulting plasma can be classified as a 'low-temperature plasma' in cases where the ion and electron effective temperatures are near ambient temperature and the electron density in the plasma is relatively low (, 10 19 m À3 versus 10 21 to 10 26 m À3 for arc plasmas).…”