Make-up Gas Flow Rate Research Articles

Doubly Charged Molecular Ions in GC-MS with Cold EI.

We report the finding of doubly charged molecular ions in a range of relatively large molecules including hydrocarbons upon their electron ionization as vibrationally cold molecules in supersonic molecular beams (SMB) (also named as Cold EI). Furthermore, we also report the detection by mass spectrometry of triply charged molecular ions in large PAHs such as decacyclene and ovalene upon their cooling in SMB. We found that the relative abundance of doubly charged molecular ions strongly depends on the internal vibrational cooling. While after some vibrational cooling the fragmentation pattern became cooling independent, the relative abundance of the doubly charged molecular ions was noticeably increased upon further cooling via increasing of the cooling make-up gas flow rate. In addition, the relative abundance of the doubly charged molecular ions was strongly increased with the compounds' size, and its electron energy threshold was lower than expected. These observations indicate a new mechanism that involves two separate electron ionization processes in the same compound, most likely with the same electron but at two separate atoms (places) in large molecules, to reduce Coulombic repulsion energy that can lead to fragmentation into two singly charged ions. These findings are shedding new light on electron ionization mass spectra. Accordingly, electron ionization mass spectra are the result of three separate mechanisms with relative magnitudes that depend on the compound size: (a) single electron ionization; (b) double electron ionization; and (c) single electron ionization with subsequent internal excitation by the same ionizing electron in another place.

Establishment of Gas Chromatography Analytical Method for 4-Amino-2-Methylpentane

The aim of this paper was to establish a gas chromatography (GC) method for the determination of 4-amino-2-methylpentane and validate the methodology. The gas chromatography conditions such as column temperature, carrier gas flow rate, split ratio and the make-up gas flow rate were investigated in detail. The chromatographic column used in this method was capillary column of DA-1. The oven temperature was initially held at 40 °C for 15 min and was then programmed temperature to 140 °C at the rate of 75 °C/min and held for 10 min. The injection temperature was 180 °C and the flame ionization detector temperature was 200 °C. The carrier gas was high pure nitrogen with a flow rate of 0.8 mL/min. The split ratio was 130:1 and the make-up gas flow rate was 30 mL/min. Good linearity of 4-amino-2-methylpentane was obtained within the concentration range of 25.44 mg/mL to 84.81 mg/mL and the correlation coefficient was 0.9990. Good accuracy and precision were obtained and the average recovery was 101.76%. The quantification limit of 4-amino-2-methylpentane was 7.21 μg/mL and the detection limit of that was 3.72 μg/mL. The method was well-validated and proved to be used for quality test of 4-amino-2-methylpentane.

Signal enhancement in laser ablation inductively coupled plasma-mass spectrometry using water and/or ethanol vapor in combination with a shielded torch

To improve the precision and accuracy of in situ trace element and isotope analysis and to apply LA-ICP-MS to small or isotopically heterogeneous samples such as minerals, greater analytical sensitivity is required. In this study, we investigate the effects of the addition of water and/or ethanol vapor to the central gas flow (Ar + He) of an Ar plasma in combination with a shielded torch on the signal intensity of laser-generation using an 193 nm ArF laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) setup. A custom-built sample introduction system was developed to add water and/or ethanol vapor to dry aerosol at different and constant rates. The optimum makeup gas flow rate is negatively correlated with the water and/or ethanol load into the ICP. The effect of the shielded torch on the LA-ICP-MS sensitivity is minimal for dry plasma conditions but becomes significant in the presence of water and/or 1–4% (v/v) ethanol vapor. For most of the 60 investigated elements reported in this study, the addition of small amounts of water and/or 1–4% (v/v) ethanol vapor into the carrier gas increases the sensitivity by a factor of 2.5–3.0 or 1.2–1.5 with or without the shielded torch, respectively. The suppression of the ion kinetic energy distribution by the shield in the presence of water and/or 1–4% ethanol vapor may account for the increase in sensitivity. With the shielded torch, the oxide ratio increases from 0.47% in dry plasma to 6.41% in water plasma and decreases to 0.81% in 4% ethanol plasma. Hydride formation increases by a factor of 27 in the presence of 4% ethanol vapor relative to dry plasma. The yield of doubly charged ions decreases from 1.2% in dry plasma to 0.8% and 0.7% in water and 4% ethanol plasmas with the shielded torch, respectively. Relative to the spatial profiles of the ion distributions in dry plasma, the addition of water and/or 2% (v/v) ethanol vapor in combination with a shielded torch shows several effects: (1) axial profiles are significantly narrower; (2) the maximum ion density zone shifts away from the load coil; and (3) there are more uniform ion distributions for elements with different physical and chemical properties. The combination of changes in ICP geometry, enhanced vaporization, atomization and ionization of analyte, and efficient ion transmission with a shielded torch contributes to the sensitivity enhancement observed in this study.

An improved GC-ECD method for measuring atmospheric N2O

Gas chromatography equipped with an electron capture detector (GC-ECD) has been widely used for measuring atmospheric N2O, but nonlinear response and the influence of atmospheric CO2 have been recognized as defects for quantification. An original GC-ECD method using N2 as carrier gas was improved by introducing a small flow rate of CO2 makeup gas into the ECD, which could well remedy the above defects. The N2O signal of the improved method was 4-fold higher than that of the original method and the relative standard deviation was reduced from > 1% to 0.31%. N2O concentrations with different CO2 concentrations (172.2 × 10−6–1722 × 10−6 mol/mol) measured by the improved GC-ECD method were in line with the actual N2O concentrations. However, the N2O concentrations detected by the original method were largely biased with a variation range of –4.5%∼7%. The N2O fluxes between an agricultural field and the atmosphere measured by the original method were greatly overestimated in comparison with those measured by the improved method. Good linear correlation (R2 = 0.9996) between the response of the improved ECD and N2O concentrations (93 × 10−9–1966 × 10−9 mol/mol) indicated that atmospheric N2O could be accurately quantified via a single standard gas. Atmospheric N−2O concentrations comparatively measured by the improved method and a high precision GC-ECD method were in good agreement.

Mass Cytometry: Protocol for Daily Tuning and Running Cell Samples on a CyTOF Mass Cytometer

In recent years, the rapid analysis of single cells has commonly been performed using flow cytometry and fluorescently-labeled antibodies. However, the issue of spectral overlap of fluorophore emissions has limited the number of simultaneous probes. In contrast, the new CyTOF mass cytometer by DVS Sciences couples a liquid single-cell introduction system to an ICP-MS. Rather than fluorophores, chelating polymers containing highly-enriched metal isotopes are coupled to antibodies or other specific probes. Because of the metal purity and mass resolution of the mass cytometer, there is no "spectral overlap" from neighboring isotopes, and therefore no need for compensation matrices. Additionally, due to the use of lanthanide metals, there is no biological background and therefore no equivalent of autofluorescence. With a mass window spanning atomic mass 103-203, theoretically up to 100 labels could be distinguished simultaneously. Currently, more than 35 channels are available using the chelating reagents available from DVS Sciences, allowing unprecedented dissection of the immunological profile of samples. Disadvantages to mass cytometry include the strict requirement for a separate metal isotope per probe (no equivalent of forward or side scatter), and the fact that it is a destructive technique (no possibility of sorting recovery). The current configuration of the mass cytometer also has a cell transmission rate of only ~25%, thus requiring a higher input number of cells. Optimal daily performance of the mass cytometer requires several steps. The basic goal of the optimization is to maximize the measured signal intensity of the desired metal isotopes (M) while minimizing the formation of oxides (M+16) that will decrease the M signal intensity and interfere with any desired signal at M+16. The first step is to warm up the machine so a hot, stable ICP plasma has been established. Second, the settings for current and make-up gas flow rate must be optimized on a daily basis. During sample collection, the maximum cell event rate is limited by detector efficiency and processing speed to 1000 cells/sec. However, depending on the sample quality, a slower cell event rate (300-500 cells/sec) is usually desirable to allow better resolution between cells events and thus maximize intact singlets over doublets and debris. Finally, adequate cleaning of the machine at the end of the day helps minimize background signal due to free metal.

Improved in situ Hf isotope ratio analysis of zircon using newly designed X skimmer cone and jet sample cone in combination with the addition of nitrogen by laser ablation multiple collector ICP-MS

The effect of three different cone combinations on the performance of laser ablation MC-ICP-MS (Neptune plus) for the in situ Hf isotope analysis of zircon were investigated. The signal sensitivities of Hf, Yb and Lu were improved by a factor of 1.4 and 2.5, respectively, with using the X skimmer cone + standard sampler cone and the X skimmer cone + Jet sample cone compared to the standard arrangement (H skimmer cone + standard sample cone). However, when using the high-sensitivity Jet sample cone, the instrumental mass fractionation for hafnium displayed a large non-linear component that could not be corrected using the normal mass fractionation laws. The magnitude of this non-linear mass fractionation was strongly related to the central gas flow rate. The in situ Hf isotope analysis of zircon standards 91500 and Mud Tank using the Jet cone displayed large deviations (410–470 ppm) at the optimum central gas flow rate for Hf, which seriously deteriorated the performance of the Jet cone. The addition of 4 ml min−1 nitrogen to the central gas flow in laser ablation MC-ICP-MS was found to not only increase the sensitivity of Hf by a factor of 2.1, but also suppress this non-linear mass fractionation. The determined Yb/Hf and Lu/Hf ratios at their corresponding optimum makeup gas flow rates for Hf intensity were found to be reduced by factors of 2 and 1.3 in the presence of nitrogen, respectively, which would benefit the accurate in situ determination of Hf isotopes in high-content Yb and Lu samples. Compared to the standard arrangement, the corresponding precision (2σ) of 176Hf/177Hf for single spot analysis of zircon standard 91500 was improved from 224 ppm to 50 ppm by using the newly designed X-skimmer cone and Jet sample cone in combination with the nitrogen addition technique. The determined 176Hf/177Hf ratios are in excellent agreement with published values in five reference zircon standards (91500, GJ-1, Mud Tank, Penglai and Plesovice). Our first Hf isotopic results from zircon standard M257 (0.281544 ± 0.000018; 2SD, n = 151) showed that it was fairly homogeneous in Hf isotopes. These results clearly demonstrate that the present analytical method has the potential to become an important tool for the pursuit of high-quality in situ Hf isotope data for zircons.

Determination of the volatiles from tobacco by capillary gas chromatography with atomic emission detection and mass spectrometry

A new gas chromatograph-atomic emission detector (GC-AED) coupled with Deans switching technique for analyzing volatiles from tobaccos were developed. The detector operating parameters (reagent gas pressure and make-up gas flow rate) were optimized. The detection limits for the elements carbon (193 nm), hydrogen (486 nm) and oxygen (171 nm) ranged 0.05-0.2, 0.05-0.3 and 1-11 ng, respectively, depending on the compound. The sensitivity and linearity for the elements carbon (193 nm), hydrogen (486 nm) and oxygen (171 nm) decreased in the order O>H>C. Calibration curves were obtained by plotting peak area versus concentration, and the correlation coefficients relating to linearity were at least 0.9359. Elemental response factors measured on these channels, relative to the carbon 193-nm channel, were hydrogen, 0.38-0.48 (mean %RSD=5.64), and oxygen, 0.085-0.128 (mean %RSD=14.9). The evaluation was also done for the new technique and for an established GC-MS technique for the same real samples. The results of GC-AED and GC-MS showed that there was a relatively good agreement between the two sets of data.

Total Mercury, Inorganic Mercury and Methyl Mercury Determination in Red Wine

This study deals with the development of a method for total Hg, inorganic Hg and methylmercury (MeHg) determination in red wine by using flow injection-cold vapour generation–inductively coupled plasma mass spectrometry (FI-CVG-ICP-MS) and gas chromatography-ICP-MS (GC-ICP-MS). For Hg speciation analysis, a derivatization step was carried out using a 1% (m/v) sodium tetraphenylborate (NaBPh4) solution, followed by extraction of Hg species and their quantification by GC-ICP-MS. The main parameters evaluated were the make-up gas flow rate, volume of the NaBPh4 solution, time for derivatization reaction/analyte extraction and solvent used for Hg species extraction. Accuracy was evaluated by analyte recovery, whereas recoveries ranged from 99% to 104% for Hg(II) and MeHg. The limits of detection (LODs) for Hg(II) and MeHg were 0.77 and 0.80 μg L−1, respectively. Wine from Argentina, Brazil, Chile and Uruguay were analysed. The wine samples were also acid digested for total Hg determination by FI-CVG-ICP-MS. The LOD of the method used for total Hg determination was 0.01 μg L−1. The concentrations of Hg species in red wine measured by GC-ICP-MS were lower than the respective LODs. Only total Hg was detected in the analysed samples, where the highest concentration of Hg found was 0.55 ± 0.02 μg L−1.

Simultaneous determination of methyl- and ethyl-mercury by solid-phase microextraction followed by gas chromatography atomic fluorescence detection

A method for trace level determination of organomercury species in different biota matrixes by using aqueous-phase propylation followed by headspace solid-phase microextraction (HS-SPME) and gas chromatography (GC) coupled to pyrolysis-atomic fluorescence spectrometry (Py-AFS) detection has been optimized. To maximize peak area and symmetry factors of methylmercury (MeHg) and ethylmercury (EtHg) analyzed as propyl derivatives, carrier and make-up flow rates were optimized by a user-defined experimental design. A multiple response simultaneous optimization was applied using the desirability function to achieve global optimal operating conditions. They were attained at 2 and 6 mL min(-1) as carrier and make-up gas flow rates, respectively. In addition, pyrolyser temperature was also optimized, yielding the best value at 750 degrees C. Limits of detection and quantification at the optimum conditions were 0.04 ng g(-1) and 0.13 ng g(-1) for both, MeHg and EtHg. The developed analytical procedure was validated with a certified reference material (DORM-2) and applied to the determination of organomercury incurred in waterfowl egg and fish samples.

Classical electron ionization mass spectra in gas chromatography/mass spectrometry with supersonic molecular beams

A major benefit of gas chromatography/mass spectrometry (GC/MS) with a supersonic molecular beam (SMB) interface and its fly-through ion source is the ability to obtain electron ionization of vibrationally cold molecules (cold EI), which show enhanced molecular ions. However, GC/MS with an SMB also has the flexibility to perform 'classical EI' mode of operation which provides mass spectra to mimic those in commercial 70 eV electron ionization MS libraries. Classical EI in SMB is obtained through simple reduction of the helium make-up gas flow rate, which reduces the SMB cooling efficiency; hence the vibrational temperatures of the molecules are similar to those in traditional EI ion sources. In classical EI-SMB mode, the relative abundance of the molecular ion can be tuned and, as a result, excellent identification probabilities and very good matching factors to the NIST MS library are obtained. Classical EI-SMB with the fly-through dual cage ion source has analyte sensitivity similar to that of the standard EI ion source of a basic GC/MS system. The fly-through EI ion source in combination with the SMB interface can serve for cold EI, classical EI-SMB, and cluster chemical ionization (CCI) modes of operation, all easily exchangeable through a simple and quick change (not involving hardware). Furthermore, the fly-through ion source eliminates sample scattering from the walls of the ion source, and thus it offers full sample inertness, tailing-free operation, and no ion-molecule reaction interferences. It is also robust and enables increased column flow rate capability without affecting the sensitivity.

Signal enhancement in laser ablation ICP-MS by addition of nitrogen in the central channel gas

The effects of adding nitrogen to the central gas flow (Ar + He) of an Ar plasma in laser ablation inductively coupled plasma mass spectrometry are presented. The optimum central gas flow rate was found to be negatively correlated with the N2 gas flow rate. The addition of 5–10 ml min−1nitrogen to the central channel gas in LA-ICP-MS increases the sensitivity for most of the 65 investigated elements by a factor of 2–3. The degree of enhancement depends, to some extent, on the 1st ionization energy. Another important advantage of N2 mixed gas plasma for LA-ICP-MS is that the oxide ratios (ThO+/Th+) are significantly reduced (one order of magnitude). The hydride ratio (ArH+/Ar+) is also reduced up to a factor of 3, whereas the doubly charged ion ratio (Ca2+/Ca+) is increased. The background signals at masses 29, 31, 42, 51, 52 and 55 are significantly increased due to the nitrogen based polyatomic interferences. Compared to the spatial profiles of the ion distributions in the normal mode (without nitrogen), the addition of 5 ml min−1nitrogen leads to significant wider axial profiles and more uniform distribution of ions with different physical and chemical properties. Our results also show that the makeup gas flow (central channel gas) rate has a significant effect on the ion distribution of elements with different physical and chemical properties. A very consistent increase of argon signal by the addition of nitrogen (5 ml min−1) corroborates better energy transfer effect of nitrogen in the plasma.

Carrier gas flow optimization for an end-on type of electrothermal vaporizer used for sample introduction in plasma source spectrometry

In case of end-on type of graphite tube electrothermal vaporizers (ETVs) the vaporization product is streamed towards the plasma through one of the ends of the graphite tube, which is represented in the present work by the commercial unit of Perkin–Elmer HGA-600MS. The properties of this ETV unit are compared under two sets of gas-flow conditions: (A) using a high flow rate of carrier gas within the graphite tube and a low flow rate of make-up gas that is introduced into the interface close to the outlet end of the graphite tube, and (B) using a decreased carrier gas flow and a relatively high flow rate of make-up gas, while the total transport gas flow rate was kept constant. The ETV unit was coupled to a multi-channel optical emission spectrometer and time integrated line intensities for 26 elements were measured under (A) and (B) conditions and also with the use of pneumatic nebulization of the same multi-element standard solution. A relatively low heating rate was applied to the ETV to minimize plasma loading and also for a better estimation of the appearance temperatures (volatility) of the elements. The intensity ratios with low and high flow rate of the carrier gas (B/A) were between 1.2 and 0.8 for 20 elements. Significantly higher ratios (B/A) were found for volatile elements (Cd and Zn), while the ratios were significantly lower for heavy volatile elements (V, Ti, B). This means that apart from the latter group of elements, the use of a relatively low flow rate of the carrier gas does not result in an essential decrease of analytical sensitivity. This finding can be utilized, e.g. to the direct analysis of finely powdered samples which is handicapped under high carrier gas flow conditions, due to possible sample loss. Comparing the sensitivities with ETV(A) conditions and pneumatic nebulization sample introduction methods there was an unfavorable tendency for the ETV(A) method with decreasing volatility of the analytes. This observation suggests an increased vapor condensation of low volatile elements close to the graphite tube end that is characteristic for this type of ETV units.

Optimization of the coupling of multicapillary GC with ICP-MS for mercury speciation analysis in biological materials

S. Slaets, F. Adams, I. Rodriguez Pereiro and R. Łobiński, J. Anal. At. Spectrom., 1999, 14, 851 DOI: 10.1039/A809296F

Development, Validation, and Application of a Method for Quantification of Methylmercury in Biological Marine Materials Using Gas Chromatography Atomic Emission Detection

A gas chromatographic method was developed for the quantification of alkylmercury species using microwave-induced plasma atomic emission detection (GC-AED). The column conditioning and analyte derivatization required for previous methods were found to be unnecessary for stable, accurate, and sensitive element-specific detection using GC-AED. Chromatographic and detection parameters such as stationary phase type, stationary phase film thickness, GC column dimensions, helium mobile phase column head pressure, detector makeup gas flow rate, and detector reagent gas type and flow rate were found to significantly affect analyte response. The detection limit for the optimized GC-AED conditions was 0.8 pg (0.1 pg/s) of methylmercury chloride (as mercury). A solid-liquid extraction procedure with preparative gel permeation chromatography cleanup and GC-AED analysis was used to quantify methylmercury in a variety of complex matrix marine materials. The methylmercury quantification method was validated with four marine certified reference materials (CRMs). The method was then applied to 13 standard reference materials, CRMs, and control materials for which no certified reference values for methylmercury have been determined. Four National Institute of Standards and Technology Standard Reference Materials and one control material, which were analyzed using the GC-AED method, were also analyzed by two other laboratories using independent methods to further validate the method.

Evaporative Light Scattering Detection for Supercritical Fluid Chromatography

A high-performance liquid chromatograph-evaporative light scattering detector is modified and interfaced with a supercritical fluid chromatograph. The detector performance is evaluated by monitoring the response of several steroids. Specifically the effects of nitrogen makeup gas flow rate, carbon dioxide modifier type, modifier concentration, evaporative light scattering detector orifice size, and detector temperature are determined. As the nitrogen gas flow rate increases, the response of the analyte decreases, but the increased flow improves peak shape to mimic that obtained with ultraviolet detection. Furthermore, increasing the detector temperature causes the response of the analytes to decrease. A detection limit of 10 ng or less is determined for progesterone and testosterone with 2% and 20% (v/v) methanol-modified carbon dioxide on a Deltabond cyano column (4.6 mm × 15 cm, 5 µm) at 150 mL/min and 1000 mL/min decompressed carbon dioxide. The separations of polyethylene glycols and ginko biloba leaf extract with the optimized conditions

Optimizing electron-capture detector performance for gas chromatographic analysis of polychlorinated biphenyls

A modified simplex technique has been applied to optimize the performance of an electron-capture detector under constant-current and constant-frequency pulse modes for the gas chromatographic analysis of polychlorinated biphenyls. Optimal performance was obtained by adjusting the four experimental parameters of pulse voltage, detector temperature, make-up gas flow rate, and reference current (or pulse frequency) using the signal-to-noise ratio of the eluting peak as the criterion. The results show that constant-current pulse mode has better sensitivity and dynamic range whereas the constant-frequency pulse mode has the lower detection limit. Subsequent principal component factor analysis indicated that similar performance is achievable using only three parameters.

Serially interfaced gas chromatography/Fourier transform infrared spectrometer/ion trap mass spectrometer system

A serial gas chromatography/Fourier transform infrared spectrometer/mass spectrometer (GC/FTIR/MS) system has been developed with an ion trap detector or mass analyzer that is interfaced to the light pipe in the FTIR spectrometer. A modification of the manufacturer-supplied open-split interface to the ion trap was required to obtain chromatographic results free of discrimination and activation effects. The flow rate of the helium make-up gas in the light pipe was used to control the amount of material that enters the ion trap. Hydrogen carrier gas was used for the chromatographic separations with no adverse effects on the mass spectra obtained. The analyses of three samples are described to demonstrate the capabilities of the system. These include the Grob test mixture, a steam distillate from a gasification water treatment plant, and a sample of methyl esters of coal oxidation products.