GC Carrier Gas Research Articles

Comparison of nitrogen and helium as carrier gases for determination of pesticide residues in foods via gas chromatography–tandem mass spectrometry with atmospheric pressure chemical ionization

The applicability of nitrogen as an alternative carrier gas to helium for the multiresidue analysis of pesticides using gas chromatography–tandem mass spectrometry with atmospheric pressure chemical ionization (GC–(APCI)MS/MS) was examined. The methods using both carrier gases were validated for 151 pesticides in foods at a spiking level of 0.01 ppm, maintaining identical conditions for both gases, including the GC column and carrier gas flow rate. The use of nitrogen resulted in slightly broader peaks and lower peak intensities that those observed upon using helium. However, sufficient sensitivity was achieved for all target compounds because of the high sensitivity of the GC–(APCI)MS/MS method. Further, target-compound separation was not significantly affected by the carrier gas type, and no interfering peaks were observed, demonstrating the high selectivity of the nitrogen-based method, even beyond the optimum linear velocity. Finally, the analytical performance of the nitrogen-based method was comparable to those of the helium-based method. Considering cost-effectiveness of nitrogen, this method is highly valuable in the event of helium shortages and for routine pesticide residue monitoring.

Enhancing Sensitivity for High-Selectivity Gas Chromatography-Molecular Rotational Resonance Spectroscopy.

A next-generation gas chromatograph-molecular rotational resonance (MRR) spectrometer (GC-MRR) with instrumental improvements and higher sensitivity is described. MRR serves as a structural information-rich detector for GC with extremely narrow linewidths and capabilities surpassing 1H nuclear magnetic resonance/Fourier transform infrared spectroscopy/mass spectrometry (MS) while offering unparalleled specificity in regard to a molecule's three-dimensional structure. With a Fabry-Pérot cavity and a supersonic jet incorporated into a GC-MRR, dramatic improvements in sensitivity for molecules up to 244 Da were achieved in the microwave region compared to the only prior work, which demonstrated the GC-MRR idea for the first time with millimeter waves. The supersonic jet cools the analytes to ∼2 K, resulting in a limited number of molecular rotational and vibrational levels and enabling us to obtain stronger GC-MRR signals. This has allowed the limits of detection of the GC-MRR to be comparable to a GC thermal conductivity detector with an optimized choice of gases. The performance of this GC-MRR system is reported for a range of molecules with permanent dipole moments, including alcohols, nitrogen heterocyclics, halogenated compounds, dioxins, and nitro compounds in the molecular mass range of 46-244 Da. The lowest amount of any substance yet detected by MRR in terms of mass is reported in this work. A theoretically unexpected finding is reported for the first time about the effect of the GC carrier gas (He, Ne, and N2) on the sensitivity of the analysis in the presence of the gas driving the supersonic jet (He, Ne, and N2) in the GC-MRR. Finally, the idea of total molecule monitoring in the GC-MRR analogous to selected ion monitoring in GC-MS is illustrated. Structural isomers and isotopologues of bromobutanes and bromonitrobenzenes are used to demonstrate this concept.

Optimization of gas chromatography/vacuum ultraviolet (GC/VUV) spectroscopy for explosive compounds and application to post-blast debris

A statistical optimization of gas chromatography/vacuum ultraviolet spectroscopy (GC/VUV) for the analysis of explosives has yet to be presented. In this work, a central composite design of experiments was utilized to optimize GC/VUV parameters for explosive and explosive related compounds such as triacetone triperoxide (TATP), dimethyldinitrobutane (DMNB), nitroglycerine (NG), diphenylamine (DPA), 2,4,6-trinitrotoluene (TNT), pentaerythritol tetranitrate (PETN), and cyclonite (RDX). Parameters optimized include the final temperature of a ramped multimode inlet program (200 °C), GC carrier gas flow rate (1.9 mL/min), and VUV make-up gas pressure (0.00 psi). The impact of transfer line/flow cell temperature was determined not to be statistically significant. Post-blast debris was successfully analyzed to illustrate applicability to forensic analysis of explosives. Relevant compounds in single- and double-base smokeless powders were successfully identified in post-blast fragments originating from PVC and steel pipes.

Thermal and spectroscopic analysis of nitrated compounds and their break-down products using gas chromatography/vacuum UV spectroscopy (GC/VUV)

Gas chromatography/vacuum UV spectroscopy (GC/VUV) was utilized to study various explosives and pharmaceuticals in the nitrate ester and nitramine structural classes. In addition to generating specific VUV spectra for each compound, VUV was used to indicate the onset of thermal decomposition based upon the appearance of break-down products such as nitric oxide, carbon monoxide, formaldehyde, water, and molecular oxygen. The effect of temperature on decomposition could be fit to a logistical function where the fraction of intact compound remaining decreased as the transfer line/flow cell temperature was increased from 200 °C to 300 °C. Utilizing this relationship, the decomposition temperatures for the nitrate ester and nitramine compounds were determined to range between 244 °C and 277 °C. It was also discovered that the decomposition temperature was dependent on the GC carrier gas flow rate and, therefore, the residence time of the compounds in the transfer line/flow cell. For example, the measured decomposition temperature of nitroglycerine ranged from 222 °C to 253 °C across four flow rates. Tracking the appearance/disappearance of decomposition products across this temperature range indicated that NO, CO, and H2CO are final decomposition products while O2 and H2O are intermediate products. The decomposition temperatures for all explosives were highly correlated to similar decomposition measurements taken by differential scanning calorimetry (DSC) (r = 0.91) and thermal gravimetric analysis (TGA) (r = 0.90–0.98). In addition, the decomposition temperatures for all explosives were negatively correlated to the heat of explosion at constant volume (r = −0.68) and strongly positively correlated to the oxygen balance (r = 0.92).

Atomic fluorescence spectrometric detection of methylmercury in seawater at sub ng L−1 level by UV-induced atomization of gaseous methylethylmercury after NaBEt4 derivatization with purge and trap preconcentration and gas chromatography separation

A sensitive method for methylmercury (MeHg) analysis in seawater by UV-induced atomization of methylethylmercury (MeHgEt) based on NaBEt4 derivatization with purge and trap preconcentration and gas chromatography- atomic fluorescence spectrometry (GC-AFS) detection was developed. The UV atomizer was demonstrated successfully to atomize MeHgEt to Hg0 with low temperature (<45 °C) and low consumption (11 W). The effects of UV lamp power, the quartz tube length, the GC carrier gas flow rate and the NaBEt4 concentration on MeHg determination were optimized. The relative standard deviations obtained from 0.25 ng L−1 MeHg was 3.5%. The detection limits was calculated as 0.0002 ng L−1 for MeHg, which not only reduced two orders of magnitude compared to the solid-phase extraction and cation or anion exchange column preconcentration methods, but also was comparable or even better than the thermal decomposition and dielectric barrier discharge (DBD) atomization methods after NaBEt4 or NaBPr4 derivatization with purge and trap preconcentration. In addition, compared with KBH4 as reductant without preconcentration, the detection limit of this method was nearly reduced six orders of magnitude for MeHg. The accuracy of the proposed method was also validated by analyzing a certified reference sample (GBW08675) with a satisfactory result and successfully applied to the determination of MeHg in sub ng L−1 level in different seawater samples.

The Use of a Gas Chromatography/Milli-whistle Technique for the On-line Monitoring of Ethanol Production Using Microtube Array Membrane Immobilized Yeast Cells.

Hollow, poly(L-lactic acid) microtube array membranes (MTAM) were used in preparing membranes that contained immobilized yeast cells. To evaluate the performance of the developed system for continuous and fed-batch fermentation, a gas chromatography/milli-whistle device was used to on-line monitor the production of ethanol. The milli-whistle was connected to the outlet of a GC capillary, and when the fermentation gases and the GC carrier gas passed through it, a sound with a fundamental frequency was produced. The online data obtained for frequency-change vs. retention time can be recorded after a fast Fourier transform. In typical bioethanol fermentation, the yeast cells cannot be recycled, whereas the artificial yeast-MTAMs can be. The hollow-MTAM containing immobilized yeast cells significantly enhanced to bioethanol productivity, and represent a novel, promising technology for bioethanol fermentation. Our data indicate that the gas chromatography/milli-whistle device, which is economical and stable, is a very useful detector for long-term monitoring.

Effects of GC temperature and carrier gas flow rate on on-line oxygen isotope measurement as studied by on-column CO injection.

Although deemed important to δ18 O measurement by on-line high-temperature conversion techniques, how the GC conditions affect δ18 O measurement is rarely examined adequately. We therefore directly injected different volumes of CO or CO-N2 mix onto the GC column by a six-port valve and examined the CO yield, CO peak shape, CO-N2 separation, and δ18 O value under different GC temperatures and carrier gas flow rates. The results show the CO peak area decreases when the carrier gas flow rate increases. The GC temperature has no effect on peak area. The peak width increases with the increase of CO injection volume but decreases with the increase of GC temperature and carrier gas flow rate. The peak intensity increases with the increase of GC temperature and CO injection volume but decreases with the increase of carrier gas flow rate. The peak separation time between N2 and CO decreases with an increase of GC temperature and carrier gas flow rate. δ18 O value decreases with the increase of CO injection volume (when half m/z 28 intensity is <3 V) and GC temperature but is insensitive to carrier gas flow rate. On average, the δ18 O value of the injected CO is about 1‰ higher than that of identical reference CO. The δ18 O distribution pattern of the injected CO is probably a combined result of ion source nonlinearity and preferential loss of C16 O or oxygen isotopic exchange between zeolite and CO. For practical application, a lower carrier gas flow rate is therefore recommended as it has the combined advantages of higher CO yield, better N2 -CO separation, lower He consumption, and insignificant effect on δ18 O value, while a higher-than-60 °C GC temperature and a larger-than-100 µl CO volume is also recommended. When no N2 peak is expected, a higher GC temperature is recommended, and vice versa. Copyright © 2015 John Wiley & Sons, Ltd.

Selective determination of 2,4-xylenol by gas chromatography/supersonic jet/resonance-enhanced multiphoton ionization/time-of-flight mass spectrometry

Gas chromatography/supersonic jet/resonance-enhanced multiphoton ionization/time-of-flight mass spectrometry (GC/SSJ/REMPI/TOF-MS) was employed for isomer-selective determination of 2,4-xylenol in river and seawater samples. The sample containing 2,4-xylenol was measured using argon, rather than helium, as the GC carrier gas to cool the analyte molecule sufficiently. The instrumental detection limit (IDL) achieved at a flow rate of 1 mL min −1 was 14 pg. Although this value was comparable to the value (ca. 10 pg) obtained by gas chromatography/electron impact/quadrupole mass spectrometry (GC/EI/QMS). When the flow rate was increased to 8 mL min −1, interference from the 2,5-xylenol isomer was completely suppressed. The IDL was degraded to 83 or 160 pg at a flow rate of 5 or 8 mL min −1, respectively. The recovery of 2,4-xylenol from the river and the seawater samples was 85 and 93%, respectively. The time for analysis was only 10 min per one sample in GC/SSJ/REMPI/TOF-MS. These results suggest that GC/SSJ/REMPI/TOF-MS is useful for the selective measurement of 2,4-xylenol, which has been designated a Class I chemical substance in the Pollutant Release and Transfer Register (PRTR).

On-line analysis of complex hydrocarbon mixtures using comprehensive two-dimensional gas chromatography

This paper discusses the first setup for on-line qualitative and quantitative comprehensive two-dimensional gas chromatography (GC × GC) of complex hydrocarbon mixtures. A built-in 4-port 2-way valve allows switching between flame ionization detection (FID) and time-of-flight mass spectrometry (TOF-MS) between runs, without the need to cool down and vent the MS. Proper selection of GC carrier gas flow rates enables maximal agreement between the obtained chromatograms in both configurations. For on-line analysis of reactor effluents, a dedicated sampling system allows automatic sampling of the hot reactor effluent gases and immediate injection of the sample on the GC × GC. To determine a complete effluent composition in a single run of the GC × GC, a subzero oven starting temperature was employed. Modulation is started when the oven temperature reaches 40 °C, thus dividing the chromatogram in a conventional 1D and a comprehensive 2D part. This work illustrates the mature and robust character of GC × GC, extending its capabilities from mere laboratory use to on-line routine analysis for industrial processes in the (petro-)chemical industry.

Simple sample transfer technique by internally expanded desorptive flow for needle trap devices

Needle trap devices (NTDs) are improving in simplicity and usefulness for sampling volatile organic compounds (VOCs) since their first introduction in early 2000s. Three different sample transfer methods have been reported for NTDs to date. All methods use thermal desorption and simultaneously provide desorptive flow to transfer desorbed VOCs into a GC separation column. For NTDs having 'side holes', GC carrier gas enters a 'side hole' and passes through sorbent particles to carry desorbed VOCs, while for NTD not having a 'side hole', clean air as desorptive flow can be provided through a needle head by a air tight syringe to sweep out desorbed VOCs or water vapor has been reported recently to be used as desorptive flow. We report here a new simple sample transfer technique for NTDs, in which no side holes and an external desorptive flow are required. When an NTD enriched by a mixture of benzene, toluene, ethylbenzene, and xylene (BTEX) or n-alkane mixture (C6-C15) is exposed to the hot zone of GC injector, the expanding air above the packed sorbent transfers the desorbed compounds from the sorbent to the GC column. This internal air expansion results in clean and sharp desorption profiles for BTEX and n-alkane mixture with no carryover. The effect of desorption temperature, desorption time, and overhead volumes was studied. Decane having vapor pressure of approximately 1 Torr at 20 degrees C showed approximately 1% carryover at the moderate thermal desorption condition (0.5 min at 250 degrees C).

Fast gas chromatography–negative‐ion chemical ionization mass spectrometry with microscale volume sample preparation for the determination of benzodiazepines and α‐hydroxy metabolites, zaleplon and zopiclone in whole blood

Fast gas chromatography/negative-ion chemical ionization mass spectrometric (GC/NICI-MS) assay combined with rapid and nonlaborious sample preparation is presented for the simultaneous determination of benzodiazepines and alpha-hydroxy metabolites, zaleplon and zopiclone in whole blood. The compounds were extracted from 100 microl of whole blood by simultaneous multitube, microscale liquid-liquid extraction (LLE) and derivatized by N-methyl-N-(tert-butyldimethylsilyl)trifluoroacetamide (MTBSTFA), without the need for the time-consuming concentration stage. In the analytical separation, various parameters of fast GC/NICI-MS were applied, e.g. the use of hydrogen as a GC carrier gas, a high carrier gas velocity, a small film thickness of the analytical column, fast MS data acquisition, fast temperature ramping, and high initial and final temperatures of GC column. Sensitive identification, screening and quantitation of 18 compounds of interest were achieved in chromatographic separation in only 4.40 min. Accurate and reproducible results were obtained by using five different and carefully selected deuterated analogues on the basis of the chemical properties of the target analytes. Nevertheless, for alpha-OH-midazolam, and for bromazepam and flunitrazepam at low concentrations, the results can be considered only semiquantitative on the basis of the validation data. The extraction efficiencies ranged from 74.3 to 105.7% and the limits of quantitation (LOQ) from 1 to 100 ng ml(-1). Rapid sample preparation and fast chromatographic separation allowed cost-efficient, reliable and high sample-throughput analyses with a low amount of manual work. The method was fully validated and accredited according to EN ISO/IEC 17025 standards and is applicable for sensitive, reliable and quantitative determination of benzodiazepines, zaleplon and zopiclone, e.g. in clinical and forensic toxicology.

Isotope ratio determination of antimony from the transient signal of trimethylstibine by GC-MC-ICP-MS and GC-ICP-TOF-MS

Isotope ratio measurements of organometal(loid) species are of interest in order to determine isotope fractionation of the heavy metal(loid)s which can elucidate their pathways in the environment. We describe isotope ratio measurements of organoantimony using gas chromatography coupled to ICP-MS. We studied the influence of various parameters on the isotope ratio determination of the transient signal from trimethylstibine, using capillary gas chromatography coupled to both multicollector and time of flight ICP-MS with a simultaneous aspiration of aqueous samples creating wet plasma conditions. The influence of different mathematical models on the precision of the isotope ratio determination of transient signals was studied. The trapezium integration of full or half peak gave precisions of 0.08% and 0.02% using the MC-ICP-MS, whereas the ICP-TOF-MS gave values of around 0.2%. The multicollector ICP-MS used as a detector showed a chromatographic isotope fractionation of trimethylstibine. (CH3)3123Sb eluted earlier than (CH3)3121Sb, with a shift of the 123/121 ratio of about 2%. Two different approaches were attempted to correct instrumental mass bias effects in order to descriminate natural isotope fractionation. First, we introduced Cd and Sb in aqueous solution simultaneously with the GC carrier and ICP Ar gas flow. Secondly, we used a standard-sample bracketing approach using a gas standard introduced via the GC. Real samples from a laboratory sewage sludge fermentor were measured and isotope fractionation of the biologically produced trimethylstibine was observed (δ 123Sb +10 and +19‰). The gas samples were analysed for timethylstibine co-eluting with volatile organic compounds using GC-MS. Chloroform was found to co-elute, but had no significant influence on the isotope ratio of antimony in timethylstibine analysed by GC-ICP-MS. Hence, there would be a strong indication that isotope fractionation takes place when antimony is methylated by anaerobic bacteria.

Collisional stabilization of negative ions produced by a monoenergetic electron beam.

The effect of different buffer gases on the intensity of negative ions was studied using a gas chromatography/electron monochromator mass spectrometer (GC/EM-MS). The buffer gas was introduced into the ion source not to moderate the electron energies, but specifically to investigate the process of collisional stabilization of negative ions. Three different designs of ion source were tested to study this phenomenon. It was found that collisional stabilization has a profound effect on the intensity of molecular radical anions and begins to play an important role at buffer gas pressures in the order of 10 mTorr. By using a partitioned ion-forming chamber, it was shown that for optimum stabilization to take place the buffer gas should be present in the region of negative ion formation. The gases possessing internal degrees of freedom which are capable of accommodating the excess energy of short-lived excited molecular anion states showed the largest increase in molecular radical anion intensities. At the same time helium, a widely used GC carrier gas, showed sufficient stabilization properties to allow detection of the molecular radical anions of typical electron-capturing molecules with positive electron affinities.

Simultaneous Element-Selective Detection of C, F, Cl, Br, and I by Capillary Gas Chromatography Coupled with Microplasma Mass Spectrometry

Simultaneous element-selective detection of the halogens and carbon was accomplished with capillary gas chromatography coupled with microplasma mass spectrometry. The microplasma ion source was a radio frequency plasma contained inside the last 4–5 cm of the 0.32 mm i.d. fused silica capillary column. The ion source was located inside the high vacuum housing of the MS, and only the GC carrier gas (2.3 mL min−1 of helium) was used for plasma generation. Atomic ions were detected in the positive mode. Detection limits were in the low picogram area, and the selectivity to carbon ranged from 8×102 for fluorine to higher than 104 for the other halogens. By introduction of both hydrogen and oxygen as reagent gases, peak tailing was avoided by suppression of analyte reactions with the silica walls of the ion source. Special attention was given to the fluorine-selective detection due to an interfering background species at m/z 19, assumed to be H3O+ originating from the reagent gases. The background signal was minimized by careful control of the power level.

Simultaneous Element-Selective Detection of C, F, Cl, Br, and I by Capillary Gas Chromatography Coupled with Microplasma Mass Spectrometry

Simultaneous element-selective detection of the halogens and carbon was accomplished with capillary gas chromatography coupled with microplasma mass spectrometry. The microplasma ion source was a radio frequency plasma contained inside the last 4–5 cm of the 0.32 mm i.d. fused silica capillary column. The ion source was located inside the high vacuum housing of the MS, and only the GC carrier gas (2.3 mL min−1 of helium) was used for plasma generation. Atomic ions were detected in the positive mode. Detection limits were in the low picogram area, and the selectivity to carbon ranged from 8×102 for fluorine to higher than 104 for the other halogens. By introduction of both hydrogen and oxygen as reagent gases, peak tailing was avoided by suppression of analyte reactions with the silica walls of the ion source. Special attention was given to the fluorine-selective detection due to an interfering background species at m/z 19, assumed to be H3O+ originating from the reagent gases. The background signal was minimized by careful control of the power level.

Capillary Gas Chromatography Coupled with Microplasma Mass Spectrometry - Improved Ion Source Design Compatible with Bench-Top Mass Spectrometric Instrumentation

Capillary gas chromatography was performed with mass spectrometric detection using a novel microplasma ion source for operation in an element-selective mode. The ion source was a 350 kHz radio frequency helium plasma, which was sustained inside the 4 cm end of a 0.32 mm i.d. fused silica capillary column, and located inside the high vacuum chamber of the quadrupole mass spectrometer. Due to the low volume of the ion source, a stable low pressure discharge was produced utilizing only the 2.25 mL min−1 of GC carrier gas (helium) for plasma support. Small amounts of oxygen (0.1–0.2% v/v) were added to the plasma gas in order to prevent carbon deposits and to enhance signal-to-noise ratios. Chlorine and bromine were selectively detected at the 5–20 pg s−1 level (S/N = 2), and both produced a response that was linear within 3 orders of magnitude.

Capillary gas chromatography-ICP mass spectrometry: a powerful hyphenated technique for the determination of organometallic compounds

The development and improvement of a gas chromatography inductively coupled plasma mass spectrometry system, GC-ICP-MS, is described. The GC and ICP-MS are coupled with a heated stainless steel transfer line. Xe, present in the GC carrier gas, is used to facilitate the nebuliser gas flow rate setting and the positioning of the torch. Alkyltin compounds are separated by GC using a 30 m capillary column within 9 min. The necessity of applying double internal standardisation (use of Bu3PeSn and Xe gas as internal standards) is shown. The repeatabilities at 50 μg/l concentration for both retention time and peak are better than 0.25% and 5%, respectively. The detection limits for alkyltin compounds are better than those of existing methods and range between 15 and 35 fg Sn. Finally, GC-ICP-MS is applied to the determination of mono-, di- and tributyltin in some harbour waters, after extraction and Grignard derivation with PrMgCl. Concentrations between 1 and 20 ng/l are found.

Calculation of elemental ratios by on-column radiofrequency plasma atomic emission detection coupled with capillary gas chromatography

Atomic emission from a 350-kHz helium plasma was evaluated for the calculation of C/Cl and C/Br ratios in compounds separated by capillary gas chromatography. When the plasma was sustained inside the end of a 0.32 mm I.D. fused-silica GC column in 2 ml/min of GC carrier gas + 18 ml/min of make-up gas (helium), 94% of the elemental ratios obtained deviated by less than 20% from the theoretical values. These results with the on-column r.f. discharge were comparable to data obtained with a commercial GC-atomic emission detection system based on a microwave-induced plasma.

On-column bromine- and chlorine-selected detection for capillary gas chromatography using a radio frequency plasma

A 350-kHz rf plasma sustained inside the end of a 0.25- or 0.32-mm-i.d. fused-silica GC column was evaluated for element-selective detection of brominated and chlorinated compounds separated by gas chromatography. Due to the small volume of the plasma cell, dilution of the GC efluent with make-up gas was unnecessary and a stable 25-W plasma was maintained in only 1.5-5 mL/min GC carrier gas. With the plasma sustained in pure helium, carbon formed from eluting material was deposited in the plasma region causing serious peak tailing and loss of sample

Complication of using hydrogen as the GC carrier gas in chlorinated dibenzofuran and dibenzodioxin GC/MS analysis

In the presence of hydrogen and under normal electron impact conditions, ions corresponding to lower congeners were produced between chlorinated dibenzofuran or dibenzodioxin molecules and the hydrogen carrier gas by a series of chlorine-hydrogen exchange reactions. The exchange reactions present an interference problem in dioxin/furan isomer specific GC/MS/SIM analysis in which polar columns are usually employed and retention time windows of adjacent congeners overlap with each other.