Gas Chromatography/electron Capture Negative Ion Mass Spectrometry Research Articles

A simplified screening method for short- and medium-chain chlorinated paraffins in food by gas chromatography-low resolution mass spectrometry

This study evaluates the performance of a simplified screening method for short- and medium-chain chlorinated paraffins (SCCPs and MCCPs, respectively) based on gas chromatography-electron capture negative ionization/mass spectrometry (GC-ECNI/MS) analysis and chlorine content quantification. The response from different combinations of ‘indicator’ congener groups present in technical mixture standards were used within calibration calculations to test the hypothesis that ∑SCCPs and ∑MCCPs could be quantified with acceptable accuracy using only a subset of the commonly analysed C10 to C17 and Cl5 to Cl10 groups. Potential combinations were assessed with respect to calibration curve performance and accuracy of SCCP and MCCP analysis of spiked food samples (olive oil, salmon, pork sausage, breakfast cereal, cow's milk and lard). Based on these trials, a screening method which quantifies ∑SCCPs and ∑MCCPs using only congener groups with 6 and 8 chlorine atoms for each carbon chain length was proposed. Concentrations of SCCPs and MCCPs in triplicate analyses of spiked food samples calculated using the proposed screening method deviated by ≤ 25% for the vast majority of samples (maximum deviation 37%) from levels determined using all analysed congener groups. The mean trueness of the screening method as applied to each of the spiked food samples and lard samples from a previous European Union Reference Laboratory (EURL) interlaboratory study ranged from 65 to 110% for ∑SCCPs and 102 to 175% for ∑MCCPs. Relative standard deviations (RSDs) were ≤ 25% for all triplicate analyses and matrix specific LOQs ranged from 0.7 to 6 ng/g ww for ∑SCCPs and from 1.3 to 12 ng/g ww for ∑MCCPs. The proposed screening method has the potential to deliver substantial time savings in instrumental analysis and manual labour without greatly reducing the overall accuracy and sensitivity of SCCP and MCCP quantification.

Optimization and validation of an analytical method for the quantification of short- and medium-chained chlorinated paraffins in food by gas chromatography-mass spectrometry

This work describes the optimization and validation of an analytical method for the quantification of short- and medium-chained chlorinated paraffins (SCCPs and MCCPs, respectively) in a range of food matrices using gas chromatography-electron capture negative ionization-mass spectrometry (GC-ECNI/MS). A dispersive solid phase extraction (dSPE) method was optimized for fish, meat, oil, milk and whole-grain cereal followed by clean-up with concentrated sulfuric acid and acid silica. Fractionation using silica cartridges efficiently removed a number of potentially interfering halogenated compound classes from sample extracts while retaining 96% of ∑SCCPs and 99% of ∑MCCPs. Limits of quantification (LOQs) estimated for food samples ranged from 0.7 to 6.0 ng/g wet weight (ww) for ∑SCCPs and 1.3–12 ng/g ww for ∑MCCPs. The applicability of the optimized protocol was assessed in each of the described food matrices via repeated analysis (n = 3) of samples fortified with SCCP 55.5%Cl and MCCP 57%Cl technical mixtures at two concentration levels and spiked lard samples from a recent European Union Reference Laboratory (EURL) interlaboratory study on CPs in food. The EURL's accuracy criteria was met for both homologue groups in all food matrices with overall accuracy in the range of 76–130% for in-house spiked samples and 57–150% for the EURL lard analysis. Excellent precision was observed for most samples with relative standard deviation (RSD) between replicates (n = 3) ≤ 12% for ∑SCCPs and ≤17% for ∑MCCPs in all food matrices analysed. The selection of the internal standard was a significant factor in the accuracy of the method and highlights the strong need for more appropriate isotopically labelled CP standards.

Vortex-homogenized matrix solid-phase dispersion for the extraction of short chain chlorinated paraffins from indoor dust samples

A simple and effective method for determining short chain chlorinated paraffins (SCCPs) in indoor dust is presented. The method employed a modified vortex-homogenized matrix solid-phase dispersion (VH-MSPD) prior to its detection by gas chromatography − electron-capture negative-ion mass spectrometry (GC-ECNI-MS) operating in the selected-ion-monitoring (SIM) mode. Under the best extraction conditions, 0.1-g of dust sample was dispersed with 0.1-g of silica gel by using vortex (2min) instead of using a mortar and pestle (3min). After that step, the blend was transferred to a glass column containing 3-g acidic silica gel, 2-g basic silica gel, and 2-g of deactivated silica gel, used as clean-up co-sorbents. Then, target analytes were eluted with 5mL of n-hexane/dichloromethane (2:1, v/v) mixture. The extract was evaporated to dryness under a gentle stream of nitrogen. The residue was then re-dissolved in n-hexane (10μL), and subjected to GC-ECNI-MS analysis. The limits of quantitation (LOQs) ranged from 0.06 to 0.25μg/g for each SCCP congener. Precision was less than 7% for both intra- and inter-day analysis. Trueness was above 89%, which was calculated by mean extraction recovery. The VH-MSPD combined with GC-ECNI-MS was successfully applied to quantitatively detect SCCPs from various indoor dust samples, and the concentrations ranged from 1.2 to 31.2μg/g.

Determination of Dechlorane compounds in aqueous samples using ultrasound-assisted dispersive liquid–liquid microextraction and gas chromatography-electron-capture negative ion-mass spectrometry

A simple and solvent-minimized procedure for the rapid determination of chlorinated flame retardants, Dechlorane compounds: syn- and anti-Dechlorane Plus (syn- and anti-DP), Dechlorane (Dec) 602, Dec 603 and Dec 604, in aqueous samples is described. The method involves the use of ultrasound-assisted dispersive liquid–liquid microextraction (UA-DLLME) prior to their determination by gas chromatography-electron-capture negative ion-mass spectrometry (GC-ECNI-MS) operating in the selected-ion-monitoring (SIM) mode. The parameters affecting the extraction efficiency of analytes from water samples were systematically investigated. The best extraction conditions involved the rapid injection of a mixture of 1.0 mL of acetonitrile (as a dispersant) and 60 μL of carbon tetrachloride (as an extractant) into 10 mL of water sample containing 0.5 g of sodium chloride in a conical bottom glass tube. After ultrasonication for 1.0 min and centrifugation at 5000 rpm (5 min), the volume of the sedimented phase was adjusted to 20 μL and the analytes were then determined by GC-ECNI-MS analysis. The limits of quantitation (LOQs) ranged from 0.08 to 0.3 ng L−1. The precision for these analytes, as indicated by relative standard deviations (RSDs), was less than 9% for both intra- and inter-day analyses. Accuracy, expressed as the mean extraction recovery, was between 75 and 92%. A preliminary result revealed that trace levels of Dechlorane compounds were detected in various water samples with total concentrations ranging from 0.6 to 0.9 ng L−1.

Levels of Chlorinated Paraffins in Water

AbstractWastewater treatment plant influents, effluents, and river water samples were analyzed for chlorinated paraffins (CPs). CPs were not detected in river water samples, in effluents, and in filtered influents by using gas chromatography electron capture negative ion mass spectrometry (GC‐ECNI‐MS) (LOQ = 100 ng/L). However, middle chain chlorinated paraffins (MCCPs) were frequently present in suspended solid matter from influents while short chain chlorinated paraffins (SCCPs) were detected only in 2 of 15 samples. Concentrations of SCCPs and MCCPs measured by GC‐ECNI‐MS varied between n.d. and 4600 ng/L adsorbed on suspended solid matter.

Accumulation and DNA damage in fathead minnows (Pimephales promelas) exposed to 2 brominated flame‐retardant mixtures, Firemaster® 550 and Firemaster® BZ‐54

Firemaster 550 and Firemaster BZ-54 are two brominated formulations that are in use as replacements for polybrominated diphenyl ether (PBDE) flame retardants. Two major components of these mixtures are 2,3,4,5-tetrabromo-ethylhexylbenzoate (TBB) and 2,3,4,5-tetrabromo-bis(2-ethylhexyl) phthalate (TBPH). Both have been measured in environmental matrices; however, scant toxicological information exists. The present study aimed to determine if these brominated flame-retardant formulations are bioavailable and adversely affect DNA integrity in fish. Fathead minnows (Pimephales promelas) were orally exposed to either FM 550, FM BZ54, or the nonbrominated form of TBPH, di-(2-ethylhexyl) phthalate (DEHP) for 56 d and depurated (e.g., fed clean food) for 22 d. At several time points, liver and blood cells were collected and assessed for DNA damage. Homogenized fish tissues were extracted and analyzed on day 0 and day 56 to determine the residue of TBB and TBPH and the appearance of any metabolites using gas chromatography-electron-capture negative ion mass spectrometry (GC/ECNI-MS). Significant increases (p < 0.05) in DNA strand breaks from liver cells (but not blood cells) were observed during the exposure period compared with controls, although during depuration these levels returned to control. Both parent compounds, TBB and TBPH, were detected in tissues at approximately 1% of daily dosage along with brominated metabolites. The present study provides evidence for accumulation, metabolism, and genotoxicity of these new formulation flame retardants in fish and highlights the potential adverse effects of TBB- and TBPH-formulated fire retardants to aquatic species.

Comparative evaluation of liquid chromatography–mass spectrometry versus gas chromatography–mass spectrometry for the determination of hexabromocyclododecanes and their degradation products in indoor dust

Domestic and office dust samples ( n = 37) were analyzed for hexabromocyclododecanes (HBCDs) using gas chromatography–electron-capture negative ionization–mass spectrometry (GC–ECNI/MS) and liquid chromatography–electrospray tandem mass spectrometry (LC–ESI/MS/MS). To determine the best method to quantify HBCDs using GC–ECNI/MS, BDE 128 was used as internal standard (I.S.) in all samples, while 13C-labeled α-HBCD was used as I.S. in some samples. Total HBCD concentrations (sum of α-, β-, and γ-HBCD diastereomers) were calculated using response factors (RFs) for α- and γ-HBCD as individual diastereomers and using an average RF for both diastereomers. Statistical comparison showed that concentrations obtained via GC–ECNI/MS were statistically indistinguishable ( p > 0.05) from those obtained using LC–ESI/MS/MS. The closest match between the two techniques was obtained using [ 13C]α-HBCD as I.S. and the average RF for α- and γ-HBCDs. Excellent linear correlations (Pearson coefficient values r > 0.9) were obtained between the GC–ECNI/MS and LC–ESI/MS/MS results, with slopes ranging from 0.76 to 1.36. Pentabromocyclododecenes (four isomers) and tetrabromocyclododecadienes (two isomers) were detected in the studied samples and were identified as degradation products of HBCDs after separation from the parent compound on the basis of both retention time and mass spectrum. This finding suggests that the elimination of HBr is the major degradation pathway for HBCDs in dust.

Expanding the range of halogenated 1′-methyl-1,2′-bipyrroles (MBPs) using GC/ECNI-MS and GC × GC/TOF-MS

Halogenated 1′methyl-1,2′-bipyrroles (MBPs) have been identified worldwide in marine mammals. Here we present the tentative identification of previously undetected MBP congeners in Delpinus delphis blubber using gas chromatography/electron capture negative ion mass spectrometry (GC/ECNI-MS) and comprehensive two-dimensional gas chromatography/time of flight mass spectrometry (GC × GC/TOF-MS). This is the first report of 26 congeners. The presence of numerous partially halogenated congeners suggests that they are either biosynthesized concomitantly with their perhalogenated counterparts or that their dehalogenation products can also bioaccumulate. The newly found compounds fit the geographic trend that has been previously noted. That is, samples from the Atlantic Ocean are dominated by the more brominated congeners while those from the Pacific are dominated by the more chlorinated congeners.

Highly selective and sensitive gas chromatography–electron-capture negative-ion mass spectrometry method for the indirect enantioselective identification of 2- and 3-hydroxy fatty acids in food and biological samples

A gas chromatographic (GC) method is described for the indirect enantioresolution of 2- and 3-hydroxy fatty acids (OH-FAs). It combines the derivatization of each alkylated enantiomer and the subsequent transfer with ( R)-(−)- α-methoxy- α-trifluoromethylphenylacetyl chloride [( R)-(−)-MTPA-Cl, Mosher's reagent] into a diastereomeric ( S)-MTPA derivative. The enantiomers of each derivatized OH-FA were well separated on three non-chiral GC-columns (CP-Sil 2, CP-Sil 8/20% C18 and VF-5 ms). The derivatives were detected with high sensitivity by GC with electron-capture detection (GC/ECD) and electron-capture negative-ion mass spectrometry (GC/ECNI-MS) because of their enhanced electron-capturing properties. When applied to sunflower oil spiked with a small amount of a racemic 2-OH-FA, the present method allowed for a highly selective identification without influence from the sample matrix. For more complex samples such as wool wax, GC/ECNI-MS was superior to GC/ECD, since the high sensitivity of this method was linked with high selectivity. Using GC/ECNI-MS in the selected ion monitoring (SIM) mode, 16 enantiopure 2-OH-FAs were detected in a wool wax sample.

Structure Elucidation of an Artifact Discharging from Rubber-Based Vial Closures by Means of Gas Chromatography/Tandem Mass Spectrometry

The use of vial closures equipped with butyl rubber septa may lead to sample contamination by rubber additives discharging from the septum material. In this study, the structure elucidation of an artifact causing intense signals in gas chromatography/electron capture negative ion mass spectrometry (GC/ECNI-MS) and gas chromatographic analyses with electron capture detection is described. Tentative identification of the leached compound was achieved by employing tandem mass spectrometric techniques both in electron capture negative ion and in electron ionization modes. The artifact could thus be characterized as 2-benzothiazolyl-N,N-dimethyl dithiocarbamate, which is a known vulcanization accelerator for rubber. It is conceivable that the identified compound or related substances are also used in other applications. Therefore, two food-related matrixes were investigated for a possible migration of this compound into foods. During these analyses, the tentatively identified rubber additive was detected in an aqueous extract of a rubber seal ring for canning jars. GC/ECNI-MS provided better sensitivity and selectivity than GC/EI-MS for the determination of the rubber additive and other mercaptobenzothiazole-derived substances.

Detection and quantitation of fatty acid acyl conjugates of triamcinolone acetonide via gas chromatography–electron-capture negative-ion mass spectrometry

The inherent electron-capture properties of triamcinolone acetonide (TAA) fatty acid conjugates were exploited for development of a GC–MS technique for quantitation of C21 long-chain fatty esters of TAA synthesized in BEAS-2B cells, an immortalized airway epithelium cell line. TAA esters extracted from BEAS-2B cells were purified and detected via selected ion monitoring of the molecular anions generated from the TAA esters under electron-capture negative-ion mass spectrometric conditions. Standard curves were linear over a range of 0.0 to >4.5 ng/mg protein with r 2 values = 1. Levels of TAA conjugates extracted from BEAS-2B treated with 10 −5 M TAA for 24 h ranged from 0.024 to 0.301 ng/mg protein. Further evidence for confirmation of the identity of TAA fatty esters formed in BEAS-2B cells was obtained via selected reaction monitoring. The transition monitored was formation of the carboxy anion generated from each of the respective molecular anions of the TAA esters during collision-induced decomposition. These findings indicate that the GC–MS analysis is suitable for studies of the kinetics of the TAA fatty acid conjugates formation in vitro and may be directly applicable to determination of the kinetics of TAA fatty acid conjugation in vivo.

Quantitative Determination of Plasma C8–C26 Total Fatty Acids for the Biochemical Diagnosis of Nutritional and Metabolic Disorders

We have developed a capillary gas chromatography–electron-capture negative-ion mass spectrometry (GC/MS) method for the quantitative determination of C8–C26 total fatty acids in plasma. Following hydrolysis, hexane extraction, and derivatization with pentafluorobenzyl bromide, fatty acid esters are analyzed in two steps: a splitless injection and a second, split injection (1:100) for the quantitation of the more abundant long-chain species. Fourteen saturated and 25 unsaturated fatty acids are quantified by selected ion monitoring in ratio to 13 stable-isotope-labeled internal standards. Calibrations exhibit consistent linearity and reproducibility. Intraassay (n = 17) and interassay (n = 12) CVs ranged from 2.5 to 13.2% and from 4.6 to 22.9%, respectively. Recoveries ranged from 76 to 106%. Reference ranges were established for four age groups (<1 month, 1 month to 1 year, 1–17 years, >18 years) and compared to specimens from patients with nutritional deficiency of ω-3 and ω-6 polyunsaturated fatty acids, inborn errors of mitochondrial fatty acid oxidation, and peroxisomal disorders. Retrospective evaluation of the concentration of linoleic acid in 35 cases with a diagnosis of essential fatty acid deficiency previously made by gas chromatographic analysis with flame ionization detection (GC/FID) found a specificity and sensitivity of only 55 and 50%, respectively, for the GC/FID method when compared to GC/MS.

Ultratrace determination of highly hydrophilic compounds by 2,2,3,3,4,4,5,5-octafluoropentyl chloroformate-mediated derivatization directly in water

Highly hydrophilic compounds, with multiple carboxylic, hydroxylic, or aminic groups, have been determined at ultratrace level (3–30 fmol injected) in aqueous solution by direct derivatization with 2,2,3,3,4,4,5,5-octafluoropentyl chloroformate, followed by n-hexane extraction and analysis by gas chromatography-electron capture negative ion mass spectrometry (GC-ECNI-MS). The products have high molecular weights, but also high volatility, making their elution from the GC column efficient. The derivatizing agent was synthesized from the corresponding alcohol and phosgene. The derivatization reaction is catalyzed by a pyridine solution of N,N-dicyclohexylcarbodiimide. At 200 °C ion-source temperature, most negative ion mass spectra showed only a weak or no molecular ion, but a controlled and interpretable fragmentation, allowing the derivatives to be easily monitored by high-mass ion chromatograms. Typical distinctive fragments are located in the m/z 500–800 mass range. Optimization of the operating conditions for the derivatization was performed. This was done in order to minimize number and intensity of peaks due to the hydrolysis of chloroformate in the total ion chromatogram. Calibration curves proved linear over two orders of magnitude concentration.

An Improved Method for the Measurement of Urinary and Plasma F2-Isoprostanes Using Gas Chromatography–Mass Spectrometry

We have developed an improved method for the measurement of F2-isoprostanes using stable isotope dilution capillary gas chromatography/electron capture negative ionization mass spectrometry (GC-ECNI-MS). The F2-isoprostane family consists of a series of chemically stable prostaglandin F2(PGF2)-like compounds generated during peroxidation of arachidonic acid in phospholipids. There is evidence that measurement of F2-isoprostanes represents a reliable and useful index of lipid peroxidation and oxidant stressin vivo.Furthermore, 8-epi-PGF2α, which is one of the more abundant F2-isoprostanes, is biologically active, being a potent mitogen and vasoconstrictor of rat and rabbit lung and kidney, as well as a partial agonist of platelet aggregation. Measurement of F2-isoprostanes in biological samples is complex and has involved methods which utilize multiple chromatographic steps, including separation by thin-layer chromatography, leading to poor sample recovery. We now present an improved method for the measurement of plasma and urinary F2-isoprostanes using a combination of silica and reverse-phase extraction cartridges, high-performance liquid chromatography (HPLC), and GC-ECNI-MS. Different approaches to the derivatization of the F2-isoprostanes prior to GC-ECNI-MS are also addressed. The overall recovery of F2-isoprostanes is improved (approx 70% for urine) and the within and between assay reproducibility is 6.7% (n= 23) and 3.7% (n= 3), respectively. The mean urinary excretion of F2-isoprostanes in eight healthy males was 365 ± 5 pmol/mmol creatinine and in three smokers 981 ± 138 pmol/mmol creatinine. The mean total (free + esterified) plasma F2-isoprostane concentration was 952 ± 38 pmol/liter, with a within and between assay reproducibility of 8% (n= 13) and 5.6% (n= 3), respectively. This improved method for the measurement of F2-isoprostanes represents a significant advance in terms of the rapidity and yield in the purification of biological samples. The inclusion of HPLC separation enables improved analysis of F2-isoprostanes by GC-MS. This methodology will assist in defining the role of F2-isoprostanes asin vivomarkers of oxidant stress in clinical and experimental settings.

Amino Acid Analysis of Peptides and Proteins on the Femtomole Scale by Gas Chromatography/Mass Spectrometry

Amino acid analysis (AAA) is a useful aid in protein chemistry, but its routine application is limited by a modest limit of detection. Typically, 10 pmol of material is required, but even at this level the reproducibility can be poor. We have employed isotope dilution gas chromatography electron capture negative ionization mass spectrometry (GC/ECNI/MS) to provide accurate and reliable data on less than 100 fmol of material. Precision and accuracy are good, and all 20 non-hydrolyzed amino acids can be determined in this manner. The protein is hydrolyzed (HCl), and then a cocktail, composed of all 20 amino acids as stable isotope-labeled forms (i.e., (13)C and (2)H), is added. The mixture of protein-derived and stable isotope-labeled amino acids is then converted to volatile electron-capturing derivatives with a multistep approach employing heptafluorobutyric anhydride (HFBA), pentafluorobenzyl bromide (PFBBr), and N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA). These derivatives are then injected directly into the GC/MS system. Groups of selected ions, characteristic of each derivatized amino acid, are thereafter monitored at appropriate time intervals. The ratios of the ion current for the selected ions for the native amino acid and its labeled form are determined and converted to absolute amounts of the native amino acids in the protein hydrolyzate by reference to standard samples prepared at the time of the analysis.

Electron CaptureNegative Ionization Mass Spectrometric Characteristics of Bioaccumulating Methyl SulfoneSubstituted Polychlorinated Biphenyls

Gas chromatography/electron-capture negative ionization mass spectrometry (GC/ECNI-MS) was used to separate and detect a mixture of 24, 3-and 4-methyl sulfone-substituted polychlorinated biphenyls (MeSO2-PCBs). The dominant fragment ions were [M-CH3]- and [M-Cl+H]-, and to a lesser extent [M-2Cl]-, [M-2Cl+2H]-, [M-CH3-Cl+H]- and [M-SO2CH3+H]-). The structures of the 3-and 4-MeSO2-PCBs varied in the degree (i.e. tetrachloro-to heptachloro-) and position of chlorine substitution. With the assistance of multivariate statistical analysis, the abundance of several isotopic clusters of fragment ions relative to that of the molecular ion (M-) was found to vary as a function of MeSO2-PCB structure. Compared with other congeners, the relative abundance of [M-CH3]- was significantly lower for MeSO2-PCBs containing the 4-MeSO2-2,5-dichloro moiety, and the hexachloro-and heptachloro-MeSO2-PCBs had higher relative abundances of [M-Cl+H]-. Subtle ECNI mass spectral fragmentation rules therefore exist for 3-and 4-MeSO2-PCBs with respect to congener structure; however, both ion source temperature and pressure must be controlled. Increasing the ion source temperature from 100 to 260°C resulted in an large increase in the abundance of [M-CH3]- relative to M- from <20% to >100%, for all the MeSO2-PCBs studied. For the set of ten 3-/4-MeSO2-PCB pairs, the [M-CH3]- relative abundance ratio was independent of ion source temperature, and was ∽4.5 and ∽1.5 for congeners possessing 2,5-dichloro and 2,5,6-trichloro substitution on the MeSO2-containing phenyl ring, respectively. The effect of an increase in ion source temperature, over the same temperature range, on the relative abundance of hydrogen inclusion-type fragment ions was much less dramatic. [M-CH3-Cl+H]- and [M-Cl+H]- abundance often increased, but never exceeded ∽40%. Decreasing the methane reagent gas pressure from 0.5 to 0.05 mbar did not significantly alter the ion source temperature effect on the relative abundance of the fragment ions. However, the ECNI response sensitivity decreased by an order of magnitude. © 1997 by John Wiley & Sons, Ltd.

Electron Capture/Negative Ionization Mass Spectrometric Characteristics of Bioaccumulating Methyl Sulfone-Substituted Polychlorinated Biphenyls

Gas chromatography/electron-capture negative ionization mass spectrometry (GC/ECNI-MS) was used to separate and detect a mixture of 24, land 4-methyl sulfone-substituted polychlorinated biphenyls (MeSO 2 PCBs). The dominant fragment ions were [M-CH 3 ] - and [M - Cl + H] - , and to a lesser extent [M - 2Cl] - , [M - 2Cl + 2H] , [M - CH 3 -Cl + H] - and [M -SO 2 CH 3 + H] - ). The structures of the 3-and 4-MeSO 2 -PCBs varied in the degree (i.e. tetrachloro-to heptachloro-) and position of chlorine substitution. With the assistance of multivariate statistical analysis, the abundance of several isotopic clusters of fragment ions relative to that of the molecular ion (M - ) was found to vary as a function of MeSO 2 -PCB structure. Compared with other congeners, the relative abundance of (M-CH 3 ] - was significantly lower for MeSO 2 -PCB 8 containing the 4-MeSO 2 -2,5-dichloro moiety, and the hexachloro-and heptachloro-MeSO2-PCBs had higher relative abundances of [M - Cl + H] - . Subtle ECNI mass spectral fragmentation rules therefore exist for land 4-MeSO2-PCBs with respect to congener structure; however, both ion source temperature and pressure must be controlled. Increasing the ion source temperature from 100 to 260°C resulted in an large increase in the abundance of [M - CH 3 ] - relative to M - from 100%, for all the MeSO 2 -PCBs studied. For the set of ten 3-/4-MeSO 2 -PCB pain, the [M - CH 3 ] - relative abundance ratio was independent of ion source temperature, and was ∼4.5 and ∼1.5 for congeners possessing 2,5-dichloro and 2,5,6-trichloro substitution on the MeSO 2 -containing phenyl ring, respectively. The effect of an increase in ion source temperature, over the same temperature range, on the relative abundance of hydrogen inclusion-type fragment ions was much less dramatic. [M-CH 3 -Cl+ H] - and [M - Cl + H] - abundance often increased, but never exceeded 40%. Decreasing the methane reagent gas pressure from 0.5 to 0.05 mbar did not significantly alter the ion source temperature effect on the relative abundance of the fragment ions. However, the ECNI response sensitivity decreased by an order of magnitude.

Pentafluorobenzylation method for quantification of acidic tryptophan metabolites using electron capture negative ion mass spectrometry.

An improved pentafluorobenzylation method was developed for derivatization of L-tryptophan and its acidic metabolites (L-kynurenine, kynurenic acid, anthranilic acid, xanthurenic acid, 3-hydroxyanthranilic acid, picolinic acid, quinolinic acid) present at trace levels in aqueous samples. This method employs lyophilization of aqueous samples in the presence of excess tetrabutylammonium hydrogen sulfate, followed by base-catalyzed anhydrous pentafluorobenzylation. A comparison with other published methods shows the advantage of this modification for the derivatization of kynurenine metabolites. The derivatives were analyzed by gas chromatography/electron capture negative ion mass spectrometry (GC/ECNI-MS) or liquid chromatography/particle beam/ECNI-MS (LC/ECNI-MS). The detection limits for injected standards are in the femtogram range by GC/ECNI-MS and in the low picogram range by LC/ECNI-MS. GC/ECNIMS is 3.6 (xanthurenic acid) to 66 (quinolinic acid) times more sensitive than LC/ECNI-MS. The simultaneous determination of two neuroactive metabolites, quinolinic and kynurenic acids, in culture medium is presented. The minimum measurable concentrations of these metabolites in 100 microL of culture medium are 0.11 nM for quinolinic acid and 0.21 nM for kynurenic acid.

4-(Trifluoromethyl)-2,3,5,6-tetrafluorobenzyl bromide as a new electrophoric derivatizing reagent

4-(Trifluoromethyl)-2,3,5,6-tetrafluorobenzyl bromide (TMB) was synthesized in a single step from α,α,α,2,3,5,6- heptafluoro- p-xylene. The purpose of TTBB is to function as an analogue of pentafluorobenzyl bromide (PFBB) in electrophoric derivatization reactions prior to detection by gas chromatography-electron-capture negative ion mass spectrometry (GC-ECNI-MS). In more detail, it was anticipated that TTBB could be used along with, or as a substitute for, PFBB to help control some interferences and confirm results. This is because a TTBB-product (of an analyte) would have different retention and sometimes m/z characteristics than a corresponding PFBB product in GC-ECNI-MS, while the two products should be similar in their ease of formation and yields. Results demonstrating these expectations were achieved by derivatizing and detecting two analytes with these reagents: N7-(2-hydroxyethyl)xanthine, and 2,3-pyrenedicarboxylic acid.

Primary contribution of the injector to carryover of a trace analyte in high-performance liquid chromatography

When a low-nanogram amount of N1,N3-bis-(pentafluorobenzyl)-N7-(2-[pentafluorobenzyloxy]ethyl) xanthine was subjected to HPLC, low-picogram amounts of the compound could be detected subsequently (off-line by gas chromatography—electron-capture negative-ion mass spectrometry) after injection of pure mobile phase. This was in spite of significant, intermediate washing of the injector and column. It was determined that essentially 99.9% of this analyte contamination came from the injector. Use of two injectors is a practical remedy for this problem.