Oven Temperature Program Research Articles

TRACE LEVEL QUANTIFICATION OF C1-C3 ALKYL CHLORIDE GENOTOXIC IMPURITIES IN TRIMETAZIDINE DI HYDROCHLORIDE DRUG SUBSTANCE USING STATIC HEADSPACE GAS CHROMATOGRAPHY

Objectives: The objective of this study was to develop and validate a gas chromatography-headspace (GC-HS) method for the quantification of genotoxic alkyl chloride impurities (chloromethane, ethyl chloride, and isopropyl chloride) in trimetazidine dihydrochloride, ensuring compliance with International Council for Harmonization M7 guidelines. Methods: A GC-HS method was optimized using a DB-1 column (60 m×0.32 mm, 3.0 μm) with nitrogen as the carrier gas. Key parameters included HS conditions with an oven temperature of 95°C, sample line temperature of 105°C, and transfer line temperature of 115°C, along with a split ratio of 1:10 and a flow rate of 10.2 psi. The oven temperature program was set to start at 40°C for 15 min, followed by an increase of 30°C/min to 250°C, held for 15 min. Method validation assessed linearity, detection limits, quantification limits, accuracy, precision, and solution stability. Results: The method exhibited excellent linearity (r²>0.999), low limits of detection (0.6 ppm) and quantification (1.8 ppm), and high accuracy (91.0–114.0% recovery). Precision was confirmed with relative standard deviations below 5%. Sample solutions remained stable for up to 48 h, demonstrating the method’s robustness and reliability for routine analysis. Conclusion: The developed GC-HS method is a robust, accurate, and regulatory-compliant approach for the trace-level quantification of genotoxic alkyl chloride impurities in trimetazidine dihydrochloride, ensuring the safety and quality of the pharmaceutical product.

A Validated Headspace Gas Chromatographic Method for the Determination of Ethyl Alcohol in Food Supplements.

Food supplements are defined as products containing nutritional elements such as vitamins, minerals, amino acids, and herbal components that cannot be taken in adequate amounts with normal nutrition. Ethyl alcohol (EtOH) is used in the production and standardization of some food supplements. Even a little MeOH can cause serious health problems in EtOH. Authorized institutions issuing halal product certificates consider foods containing more than 1% alcohol to be haram. In this study, new, reliable, sensitive, and fast GC-FID and HS GC-FID methods were developed, validated, and compared to determine the amount of EtOH and MeOH that can be found in food supplements. Chromatographic separation was performed with DB-WAX (30 m × 0.32 mm × 0.5 μm) and TC-WAX (30 m × 0.32 mm × 0.5 μm) in the GC-FID and HS GCFID methods, respectively. Oven temperature program that increased 5°C/min from 80°C to 100°C in 40:1 split condition was used. The calibration range is 0.002-0.1 g/dL. LOD and LOQ values are found as 0.00036 and 0.00050 g/dL and 0.0011 and 0.0015 g/dL for EtOH and 0.00050 and 0.00060 g/dL and 0.0015 and 0.0019 g/dL for MeOH by GC-FID and HS GC-FID methods, respectively. The developed and validated methods were applied to different food supplements commercially sold in the market with the label containing alcohol but not containing alcohol percentage and one food supplement with the label alcohol-free. The developed and validated GC-FID and HS GC-FID methods can be used in the determination of EtOH and MeOH in food supplements.

Development and Application of a Cost–Effective Analytical Method for Hydrofluorocarbons Using Preconcentrator–Gas Chromatograph–Mass Spectrometer

The expansion of atmospheric observation networks for hydrofluorocarbons (HFCs), which are closely related to climate change and contribute to global warming, significantly impacts our society and daily life. Their emissions are estimated to increase in the future, which is a major challenge. Observations of HFCs globally were performed using expensive GHG–specific equipment installed at AGAGE and other sites, but many research institutions find it difficult to install such equipment. Therefore, we successfully developed a measurement method for six components of HFCs (HFC–23, HFC–32, HFC–125, HFC–134a, HFC–143a, and HFC–152a) with high atmospheric concentrations in various parts of the world by optimizing measurement parameters such as the sample transfer volume and rate, module cooling temperatures, the injection time, the GC oven temperature program, and the monitored ions of a commercially available preconcentrator–GC–MS. Because this developed measurement method is cost–effective and simpler to operate than those of GHG–specific equipment, it is expected to provide an opportunity for many research institutes to measure HFCs. Furthermore, in addition to HFCs, we confirmed that simultaneous measurements can be performed for 97 volatile organic compounds (VOCs), including hazardous components. This research can contribute to the observation of HFCs in countries and regions where the actual status of emissions is unclear or where no or few atmospheric observations were conducted. The results of those observations can be used to formulate more detailed global warming countermeasures.

Pentafluoropropionic Anhydride Derivatization and GC-MS Analysis of Histamine, Agmatine, Putrescine, and Spermidine: Effects of Solvents and Starting Column Temperature.

Gas chromatography-mass spectrometry (GC-MS) is useful for the quantitative determination of the polyamines spermidine (SPD) and putrescine (PUT) and of the biogenic amine agmatine (AGM) in biological samples after derivatization. This GC-MS method involves a two-step extraction with n-butanol and hydrochloric acid, derivatization with pentafluoropropionic anhydride (PFPA) in ethyl acetate, and extraction of the pentafluoropropionic (PFP) derivatives by toluene of SPD, PUT, and AGM. We wanted to extend this GC-MS method for the biogenic amine histamine (HA), but we faced serious problems that did not allow reliable quantitative analysis of HA. In the present work, we addressed this issue and investigated the derivatization of HA and the effects of toluene and ethyl acetate, two commonly used water-insoluble organic solvents in GC-MS, and oven temperature program. Derivatization of unlabelled HA (d0-HA) and deuterium-labelled HA (d4-HA) with PFPA in ethyl acetate (PFPA-EA, 1:4, v/v; 30 min, 65 °C) resulted in the formation of d0-HA-(PFP)2 and d4-HA-(PFP)2 derivatives. d4-HA and 13C4-SPD were used as internal standards for the amines after standardization. Considerable quantitative effects of toluene and ethyl acetate were observed. The starting GC column temperature was also found to influence considerably the GC-MS analysis of HA. Our study shows the simultaneous quantitative analysis of HA as HA-(PFP)2, AGM as AGM-(PFP)3, PUT as PUT-(PFP)2, and SPD as SPD-(PFP)3 derivatives requires the use of ethyl acetate for their extraction and injection into the GC-MS apparatus and a starting GC column temperature of 40 °C instead of 70 °C. The PFP derivatives of HA, AGM, PUT, and SPD were found to be stable in ethyl acetate for several hours at room temperature. Analytically satisfactory linearity, precision, and accuracy were observed for HA, AGM, PUT, and SPD in biologically relevant ranges (0 to 700 pmol). The limits of detection of AGM, PUT, and SPD were about two times lower in ethyl acetate compared to toluene (range, 1-22 fmol). The limits of detection were 1670 fmol for d0-HA and 557 fmol for d4-HA. Despite the improvements achieved in the study for HA, its analysis by GC-MS as a PFP derivative is challenging and less efficient than that of PUT, AGM, and SPD.

Determination of 25 polybrominated diphenyl ethers in Chinese mitten crab ecosystems by gas chromatography-mass spectrometry.

A sensitive and reliable method for determining 25 polybrominated diphenyl ethers (PBDEs) in Chinese mitten crabs and their ecosystems ranging from the growing environment to edible feed by gas chromatography coupled to triple quadrupole mass spectrometry with advanced electron ionization (GC-AEI-MS/MS) was developed and validated. Accelerated solvent extraction (ASE) and liquid-liquid extraction were used to extract solid and water samples, respectively. On the basis of a traditional acid-base silica column, deactivated silica was added and n-hexane elution was used to increase the effect of separation and purification. Two oven temperature programs were applied to achieve good separation of low brominated congeners and increase the sensitivity of high brominated congeners. The method provided good linearity (>0.9996). The recoveries of four matrices were in the range of 82-115% and the method quantification limits (MQLs) in crabs, feed, sediment and water ranged from 0.36-6 pg per g wet weight, 0.69-22.29 pg per g dry weight, 1.02-25.26 pg per g dry weight, and 2.43-40.14 pg L-1, respectively. The proposed method was used for ten samples from two aquatic sites and PBDEs were detected in Chinese mitten crabs, commercial feed and sediment, with the highest in crabs. This analytical technique can be used to monitor the content and the accumulation behavior of PBDEs in Chinese mitten crab ecosystems or other aquaculture systems.

Determination of polychlorinated naphthalenes in ambient air by isotope dilution gas chromatography-triple quadrupole mass spectrometry

环境空气中的多氯萘(PCNs)一般为痕量水平(pg/m3),要实现其准确定量必然对分析方法的提取、净化和仪器分析提出较高要求。研究通过考察提取溶剂种类、净化流程和色谱-质谱参数,建立了加速溶剂萃取(ASE)-多层硅胶复合中性氧化铝柱的净化方法,并利用同位素稀释气相色谱-三重四极杆质谱(GC-MS/MS)对环境空气中的多氯萘进行测定。同时,通过在采样、提取和进样分析前分别添加同位素内标,开展质量控制和保证。结果表明,在2~100 ng/mL范围内3~8氯萘的平均相对响应因子(RRF)的相对标准偏差(RSD)均小于16%。PCNs同类物的方法检出限为1~3 pg/m3(以样品体积为288 m3计算)。采用基质加标法评价了方法对环境空气样品中PCNs测定的精密度和准确度,低、中、高加标水平下3 ~8氯萘的平均加标回收率分别为89.0%~119.4%、98.6%~122.5%和93.7%~124.5%,测定结果的平均相对标准偏差分别为1.9%~7.0%、1.6%~6.6%和1.0%~4.8%。整个分析过程中,采样内标和提取内标的平均回收率分别为136.2%~146.0%和42.4%~78.1%, RSD分别为5.6%~7.5%和2.7%~17.5%,满足痕量分析的要求且平行性较好。方法的灵敏度和准确度高,精密度良好,适用于环境空气中3~8氯萘的准确定量测定,可在一定程度上缓解多氯萘监测对高分辨气相色谱-高分辨质谱的依赖,为实现多氯萘的国际履约提供方法支持。

Development and validation of GCHSmethod used for trace level determination of carcinogenic Epichlorohydrin impurity in Rivaroxaban

A simple, sensitive, and selective Headspace-Gas chromatography (GCHS) method was developed and validated for the trace level determination of Epichlorohydrin, which is a probable carcinogenic compound and may be present as a process impurity in Rivaroxaban drug substances. Identification and separation of analytes were achieved on a DB-Wax column having dimensions of 30 m in length and a 0.53 mm internal diameter with a 1.0 μm stationary film thickness. Nitrogen is used as a carrier gas with of flow rate of 6.0 mL/min. The run time of the method is 25 minutes. The gradient column oven-temperature programming method was used for analyte elution. The developed method was validated using ICH guidelines and found a linear in the range between 9.64 ppm and 80.31 ppm. The correlation coefficient and y-intercept observed for the linearity curve are 0.9993 and 1.75%, respectively. The limit of detection and quantitation were 3.03 ppm and 9.19 ppm, respectively. Recovery was observed for Epichlorohydrin between 94.66 % and 99.22 %. The RSD for system precision, precision at LOQ, and repeatability was less than 5.0 %. The developed method can be successfully applied to determine Epichlorohydrin in Rivaroxaban drug substances up to a very low trace level concentration. The method was found to be specific, selective, linear, precise, and accurate.

Determining the double-bond positions of monounsaturated compounds in the alcohol fraction in seep carbonate

Determining the double-bond positions of monounsaturated compounds in the alcohol fraction in seep carbonate

Gas chromatography method to quantify α-monobenzoate glycerol in methanolic solutions using methyl benzoate as internal standard and a Rxi-1ms column

The 2,3-dihydroxypropyl benzoate, also known as α-monobenzoate glycerol (α-MBG), is an important building block used in the organic chemistry industry and its synthesis can be performed by the direct esterification of glycerol and benzoic acid. On the other hand, gas chromatography is a powerful laboratory tool applied for the quantification of known and unknown molar concentrations of chemical compounds. Under those conditions, I developed a method and a calibration curve for the analysis of unknown concentrations of α-MBG in methanolic solutions using a gas chromatograph equipped with a Rxi-1ms column and methyl benzoate (MBe) as an internal standard. Thus, by a suitable chromatograph setup, such as injector temperature and column oven temperature program, it was achieved a good resolution of the chromatographic peaks of each individual compound (MBe and α-MBG). Additionally, it was possible to generate an appropriated calibration curve with a corresponding equation for the calculation of unknow molar concentrations of α-monobenzoate glycerol in methanolic solutions. The highlights of the method described are:•There were no derivatization steps in this method, only dilution of the α-monobenzoate glycerol in methanol.•The calibration curve was made using only four calibration mixtures.•This method also promotes the appropriate separation of glycerol, benzoic acid, and methyl benzoate, which can be useful for the analysis of the progress of an esterification reaction of glycerol and benzoic acid for the synthesis of α-MBG.

Capsaicin determination from pain patch for the calculation of Scoville heat units by gas chromatography-mass spectrometry

Background and Aims: A sensitive, accurate and precise method has been developed for the determination of Capsaicin from pain patches by Gas chromatography Mass Spectrometry (GC-MS). Capsaicin has irritant effects in high concentrations, so these effects can be minimized by knowing the amount present in pain patches for the efficacy and safety of patches. Methods: Capsaicin was extracted by using liquid-liquid extraction from patches. The Gas Chromatographic separation was performed by using 5% diphenyl 95% dimethylpolysiloxane column with high a purity 2 mL/min flow rate helium gas. The separation was made with a gradient oven temperature program. The oven temperature started at 250°C and was increased to 275°C at 10°C.min-1 ramp rate and held at 275°C for 2.5 min. The injection port was adjusted at 300°C and a split injection mode was used. The analysis was carried out in a split mode of 5:1. MS ionization potential was determined at 70 eV. Results: The calibration curve was found to be linear in the range 5 - 50 μg/mL. The limits of detection and quantification for capsaicin was found to be 3.46 μg/mL and 5 μg/mL, respectively. The method developed was validated and successfully applied to the patch analysis. Conclusion: This method is simple, reproducible, and can be used safely for the routine analysis of Capsaicin without derivatization. This study has the potential how to calculate the Scoville Heat Units (SHU) of pain patches that contain Capsaicin. The amount of Capsaicin in the pain patch, its irritant effects, and its efficacy and safety appear to be low when evaluated by the SHU.

Validation of an improved European standard method for the determination of PCBs in oil samples

Two European standard methods (EN 12766 and EN 61619) are currently used for the determination of PCBs in specific oil matrix. However, apolar matrix compounds (e.g. hydrocarbons) elute from the adsorbent together with the PCBs and are injected into the analytical system where their presence contaminates the inlet, detectors and columns; and decreases system performances. Insufficient cleanup causes delay of elution of PCBs from GC columns. By using new sulfoxide-bonded silica, PCBs are better separated from aliphatic hydrocarbons because the specificity of the stationary phase for these compounds is much higher that that used in both standard methods. A gas chromatograph AT6890 with two capillary columns of different polarities (HP-5MS and DB-1701) coupled to two μECDs is used. Oven temperature program is as followed: 90°C (1 min), 70°C/min to 180°C, 5°C/min to 230°C (0.1 min), 1.5°C/min to 280°C. Run time is 46 min. The procedure was validated through regular analysis of blanks, fortified samples (transformer oil, motor used and unused oil) and certified materials (BCR-449 and BCR-420, waste mineral oils, high and low PCB levels). Two internal standards were used (PCB 30 and PCB 209). An average recovery ± RSD of 82.8 ± 5.4 % was achieved for all six PCBs in different matrices. The LOQ per single PCB congener is 0.2 mg kg-1. The average recovery ± RSD for the BCR-420 is 92.0 ± 4.6 % and for the BCR-449 is 105 ± 2.5 % for all certified PCBs in waste oils.

2-Phenoxyethanol derivatization in ink dating determination

Introduction 2-Phenoxyethanol (PE) is a volatile compound present in the composition of inks. After the deposition in a document, it starts to evaporate over time. This ageing process potentially allow to estimate the date of an ink in a document [1], however this is a complex system where the PE derivatization with different derivatization agents and methods can contribute to improve ink dating validation [2]. The aim of this work is to determine if PE derivatization with MSTFA:TMCS, will increase the sensitivity of the method contributing to the estimation of the ink age for a longer period of time. Materials and methods In order to compare peak resolution between PE and derivatized PE, a solution of 50 µg/ml of PE was prepared in hexane. The derivatized sample was prepared using chemical derivatization with MSTFA:TMCS (95:5) at a 80ºC during 30 min. The samples were analysed using GC/MS with an Agilent Technologies 5973 − 6890 N. GC MEGA-5 MS; 0.25µm, 0.32mm, 30 m capillary column was used. Chromatographic analysis was carried out under the following conditions: injection volume 2 µl, split ratio of 2:1 and injection temperature at 280 ºC. The oven temperature program starting at 90 ºC during 8 min, then ramped at 10 ºC min−1 to 100 ºC, and increased to 240 ºC at a rate of 30 ºC min−1 during 4.67 min. The MS analysis was carried out in SIM mode, which m/z of PE was 77, 94, 138 and PE-TMS was 151, 195, 210. Results The chromatogram results (Figure 1) showed that PE-TMS has a longer retention time (23.2 min) and a better resolution than PE (retention time of 13.2 min). Discussion and conclusions: This preliminary study showed an increase of the sensitivity of the PE-TMS. Our results revealed that PE derivatization with MSTFA:TMCS, might be useful in ink dating determination and can contribute to the decreasing of LOQ. However, more work has to be done. In conclusion, derivatization proves to be promising in the field of ink ageing, allowing getting sensitivity to estimate the age of an ink, in a long period of time. Figure 1. GC-MS chromatogram of PE (tr = 13.2) and PE-TMS (tr = 23.2) carried out in SIM mode using a GC MEGA-5 MS; 0.25µm, 0.32mm, 30 m capillary column.

Determination of residual solvents in paclitaxel by headspace gas chromatography

BackgroundA simple and sensitive gas chromatographic method was developed and validated for the simultaneous determination of methanol, ethanol, acetone, isopropyl alcohol, dichloromethane, N-hexane, ethyl acetate, tetrahydrofuran, and N,N-diisopropyl ethyl amine in Paclitaxel. A chromatographic separation was done on DB-624 column, 30 m length × 0.53 mm ID, and film thickness 3 μm, using a flame ionization detector (FID) with gradient column oven temperature program. The injection was carried out in split mode, with a split ratio of 5:1. A mixture of N-methyl-2-pyrrolidinone (contains 1% piperazine) and water in the ratio of 80:20 (v/v) was selected as a diluent to obtain good sensitivity along with the recovery.ResultsThe developed gas chromatographic method offers symmetric peak shape, good resolution of more than 2.0 between the solvent peaks, and the relative standard deviation for replicate injections of all the solvents were found to be not more than 15.0% with reasonable retention time for all the solvents. The limit of detection for methanol, ethanol, acetone, isopropyl alcohol, dichloromethane, N-hexane, ethyl acetate, tetrahydrofuran, and N,N-diisopropyl ethyl amine was found to be 304.69 ppm, 497.98 ppm, 498.99 ppm, 504.49 ppm, 61.81 ppm, 30.07 ppm, 505 ppm, 73.05 ppm, and 2.09 ppm, respectively. Limit of quantitation of methanol, ethanol, acetone, isopropyl alcohol, dichloromethane, N-hexane, ethyl acetate, tetrahydrofuran, and N,N-diisopropyl ethyl amine was found to be 89.62 ppm, 146.47 ppm, 146.76 ppm, 148.38 ppm, 18.18 ppm, 8.84 ppm, 148.53 ppm, 21.49 ppm, and 0.62 ppm, respectively. Precision was found to be satisfactory. Linear in the range of LOQ to 150% level for all the solvents, and accuracy along with robustness, is performed, and acceptable results were obtained.ConclusionThe proposed method was demonstrated to be simple, sensitive, specific, linear, precise, accurate, and robust, hence can be used to determine the residual organic solvents in Paclitaxel drug substance and drug product.

A RAPID SENSITIVE AND SELECTIVE GC-MS/MS METHOD FOR MULTI RESIDUE ANALYSIS OF A LARGE NUMBER OF PESTICIDES IN CHAMOMILE

Multi-residue methods for pesticide residues analysis in herbal plants, using gas chromatography tandem mass spectrometry (GC–MS/MS), usually carried out by a long run time. These longer run times of analyses are performed to avoid the overlapping of the co-extracted natural components with the analyzed pesticides in herbal samples. In the current study, a rapid sensitive and selective GC-MS/MS method, of run time 25 m, has been developed for the analysis of 212 pesticides in chamomile. The oven temperature program has been optimized to give a better distribution for the eluted pesticides along all the run time. Besides, the back flush technique has been used to remove the less volatile compounds and to decrease the analysis runtime. In addition, the sample preparation was a modification of the known QuEChERS extraction method by applying a freezing step at -20 °C for 20 m before adding the salting out mixture. The developed method enables food safety laboratories to increase the number of analyzed samples per day with a reduction of the total analysis cost. The developed method has been fully validated on chamomile according to SANTE/11813, 2018. The obtained results showed that, more than 206 pesticides have average recoveries between 70-120 % at a concentration level of 50 µg kg-1. Limit of quantitation for 160 of the studied pesticides equal 10 µg kg -1. The developed method was also employed for the analysis of real chamomile samples collected from Al-Fayoum governorate, Egypt. The obtained results showed that most of the collected samples were positive by different pesticide residues. There were three samples contaminated by five to six different pesticides, indicating the presence of uncontrolled pesticide practices.

Ionic Liquid Stationary Phase for Improving Comprehensive Two-dimensional Gas Chromatographic Separation of Polychlorinated Naphthalenes

Ionic Liquid Stationary Phase for Improving Comprehensive Two-dimensional Gas Chromatographic Separation of Polychlorinated Naphthalenes

An analytical method for the analysis of trihalomethanes in ambient air using solid-phase microextraction gas chromatography-mass spectrometry: An application to indoor swimming pool complexes.

A simple method for the collection and analysis of the four brominated and chlorinated trihalomethanes (THMs) in air samples is described. Ambient air samples were collected in pre-prepared glass vials, with THM analysis performed using solid-phase microextraction gas chromatography-mass spectrometry, where the need for chemical reagents is minimized. Analytical parameters, including oven temperature program, solvent volume, incubation time, vial agitation, extraction time and temperature, as well as desorption time and temperature, were evaluated to ensure optimal method performance. The developed method allows for point-in-time quantification (compared to an average concentration measured over extended periods of time), with detection limits between 0.7to2.6µg/m3 . Excellent linearity (r2 >0.99), repeatability (3%to11% RSD), and reproducibility (3%to 16% RSD) were demonstrated over a concentration range from 2 to 5000µg/m3 . The method was validated for the analysis of THMs in indoor swimming pool air and was used to investigate the occurrence of THMs in the air above 15 indoor swimming pools. This is the first study to report the occurrence of THMs in swimming pool air in Australia, and concentrations higher than those previously reported in other countries were measured.

Further improvements in pesticide residue analysis in food by applying gas chromatography triple quadrupole mass spectrometry (GC-QqQ-MS/MS) technologies.

Nowadays, the control of pesticide residues in food is well established. The capacity of triple quadrupole technology to satisfy the current food regulations has been demonstrated. However, the permanent high demand of consumers for more sensitive and faster testing is driving the development of improved analytical methodologies that increase the performances of sensitivity and robustness and reduce the analysis time. In this work, the feasibility of decreasing the run time to 12.4min by modifying the oven temperature program, for a multiresidue method covering 203 pesticides, was evaluated. Satisfactory sensitivity results were achieved by reaching a limit of quantitation of 2μgkg-1 for a great variety of fruits and vegetables. The validated method based on updated GC-QqQ-MS/MS has confirmed the abovementioned challenges with adequate robustness by its application to routine analyses for 69 real samples. The proposed method can represent great benefit for laboratories as it allows increasing samples throughput. It is also very useful for risk assessment studies, where the needs of low reporting limits and very wide analytical scope are necessary.

Analysis of underivatised low volatility compounds by comprehensive two-dimensional gas chromatography with a short primary column

Analysis of underivatised low volatility compounds by comprehensive two-dimensional gas chromatography with a short primary column

Optimization of milk odd and branched-chain fatty acids analysis by gas chromatography using an extremely polar stationary phase

Optimization of milk odd and branched-chain fatty acids analysis by gas chromatography using an extremely polar stationary phase

Simultaneous analysis of polychlorinated biphenyls and polychlorinated naphthalenes by isotope dilution comprehensive two-dimensional gas chromatography high-resolution time-of-flight mass spectrometry

Simultaneous analysis of polychlorinated biphenyls and polychlorinated naphthalenes by isotope dilution comprehensive two-dimensional gas chromatography high-resolution time-of-flight mass spectrometry