Hamid Badiei

Plasma-Assisted Reaction Chemical Ionization for Elemental Mass Spectrometry of Organohalogens

AbstractWe present plasma-assisted reaction chemical ionization (PARCI) for elemental analysis of halogens in organic compounds. Organohalogens are broken down to simple halogen-containing molecules (e.g., HBr) in a helium microwave-induced plasma followed by negative mode chemical ionization (CI) in the afterglow region. The reagent ions for CI originate from penning ionization of gases (e.g., N2) introduced into the afterglow region. The performance of PARCI-mass spectrometry (MS) is evaluated using flow injection analyses of organobromines, demonstrating 5–8 times better sensitivities compared with inductively coupled plasma MS. We show that compound-dependent sensitivities in PARCI-MS mainly arise from sample introduction biases.n Figureᅟ

Gas Chromatography Plasma-Assisted Reaction Chemical Ionization Mass Spectrometry for Quantitative Detection of Bromine in Organic Compounds

We have recently introduced plasma-assisted reaction chemical ionization mass spectrometry (PARCI-MS) for elemental analysis of halogens in organic compounds. Here, we utilize gas chromatography (GC) coupled to PARCI-MS to investigate the mechanism of Br(-) ion generation from organobromines and to evaluate analytical performance of PARCI for organobromine analysis. Bromine atoms in compounds eluting from GC are converted to HBr in a low-pressure microwave induced helium plasma with trace amounts of hydrogen added as a reaction gas. Ionization is achieved by introducing nitrogen into the afterglow region of the plasma, liberating electrons via penning ionization and leading to formation of negative ions. We demonstrate that N2 largely affects the ionization process, whereas H2 affects both the ionization process and in-plasma reactions. Our investigations also suggest that dissociative electron capture is the main ionization route for formation of Br(-) ions. Importantly, GC-PARCI-MS shows a uniform response factor for bromine across brominated compounds of drastically different chemical structures, confirming PARCIs ability to quantify organobromines in the absence of compound-specific standards. Over 3 orders of magnitude linear dynamic range is demonstrated for bromine quantification. We report a detection limit of 29 fg of bromine on-column, ~4-fold better than inductively coupled plasma-MS.

Atmospheric pressure plasma assisted reaction chemical ionization for analysis of chlorinated compounds separated by liquid chromatography

We report development of an atmospheric pressure plasma assisted reaction chemical ionization (PARCI) source with liquid sample introduction, enabling high sensitivity detection of chlorine in LC-separated compounds. In this novel approach, analytes are introduced into an argon inductively coupled plasma for high-temperature reactions including atomization. The plasma reaction products are then guided into an atmospheric pressure reaction tube where formation of negative ions is enhanced at lower temperatures. Cl− ions emerging from the reaction tube are then detected by an atmospheric sampling single quadrupole mass spectrometer without any modifications. We demonstrate that Cl− ions are generated in PARCI with similar efficiencies between inorganic and organic chlorine-containing compounds, confirming the elemental nature of the ionization for quantitative measurements. We further demonstrate that the ionization reactions within the reaction tube can be controlled by addition of ionization reagents to the plasma. Specifically, we find that addition of low ionization potential elements such as sodium in conjunction with methanol leads to significant enhancements in sensitivity. Chlorine sensitivity and detection limits using LC-PARCI-MS compare favorably with those obtained using state-of-the-art LC-ICP-MS/MS. Operation of PARCI in negative mode alleviates the isobaric interferences for chlorine detection and obviates the need for complex mass analyzers. Notably, the simplicity of PARCI and compatibility with atmospheric sampling mass spectrometers make this approach readily adoptable for integration of elemental quantification with molecular characterization using other ion sources.

Atmospheric-Pressure Dielectric Barrier Discharge as an Elemental Ion Source for Gas Chromatographic Analysis of Organochlorines

Atmospheric-pressure dielectric barrier discharge (AP-DBD) plasma has emerged in recent years as a versatile plasma for molecular ionization and elemental spectroscopy. However, its capabilities as an elemental ion source have been less explored, partly because of difficulties in the detection of positive elemental ions from this low-gas-temperature plasma. In this work, we investigate the detection of negative elemental ions to enable elemental mass spectrometry (MS) using AP-DBD. A gas chromatograph is coupled to a helium AP-DBD apparatus and positioned in front of an atmospheric-pressure-sampling mass spectrometer with no modifications to the ion sampling interface. We demonstrate that Cl- ions are detected with a compound-independent efficiency, enabling elemental quantification of organochlorines. Further, addition of oxygen at low concentration (11 ppm, v/v) to the helium plasma improves the analytical performance by reducing postcolumn peak broadening, whereas high oxygen concentrations (>110 ppm, v/v) lead to loss of the compound-independent response. The optimized GC-AP-DBD-MS setup shows close to 2 orders of magnitude of linearity for its compound-independent Cl response and offers detection limits of 0.5-1 pg of Cl on column (0.6 pg/s), suitable for analysis of organochlorines in food samples. We demonstrate this capability by analyzing orange juice spiked with pesticides at 9 μg/L and a single internal standard. Importantly, we demonstrate that a quick, easy, cheap, effective, rugged, and safe (QuEChERS) extraction followed by GC-AP-DBD-MS quantification using the single standard provides acceptable recoveries (80-120%). These results highlight uniform QuEChERS extraction of a range of compounds and the compound-independent response of AP-DBD for Cl, making the combination of the two methods desirable for the rapid quantification of organochlorines. Furthermore, we discuss ionization matrix effects in AP-DBD for chlorine detection and offer strategies to flag matrix-impacted analytes. These results suggest that AP-DBD has the potential to become a unified ion source for both elemental quantification and molecular identification of GC eluents on a single MS platform.