An efficient liquid chromatography--mass spectrometry interface for the generation of electron ionization spectra

The use of a new LC-MS interface (cap-EI), part of a Waters Integrity system, capable of generating EI spectra at micro flow rates is presented. The cap-EI interface relies on the production of a fine aerosol by means of a nebulizer and supported by a nitrogen jet. Sensitivity, response linearity, reproducibility, and LC compatibility of the interface were thoroughly examined using testosterone, caffeine, a mixture of antiinflammatory drugs, and 3,4-dihydroxybenzoic acid as test compounds. The interface is fully compatible with LC requirements such as high-water- and/or -buffer-content mobile phases. Reproducibility, high sensitivity in scan mode, as well, to produce library-searchable EI spectra, 2 orders of magnitude linearity, together with an intrinsic simplicity of the entire system are the key features of cap-EI interface.     

Analysis of alkenone unsaturation indices with fast gas chromatography/time-of-flight mass spectrometry

Extensively purified C37 alkenone references and mixtures thereof were analyzed by gas chromatography/flame ionization detection (GC/FID) and fast gas chromatography/time-of-flight mass spectrometry (GC/TOF-MS), to establish the latter as an alternative, fast, and reliable analysis method for alkenone unsaturation indices (U(k')(37)). This index is a tool for past sea surface temperature reconstructions with extensive use in paleoclimate and paleoceanographic research. TOF-MS was chosen because of its unique capability to acquire full-range spectra at high data rates (up to 500 spectra s(-1)) and to produce homogeneous spectra across a gaschromatographic peak, allowing faster separations than conventional GC/MS and the employment of enhanced peak deconvolution algorithms. Analysis time per sample could be reduced to run times of <10 min, i.e., by a factor of approximately 10 compared to conventional GC/FID (90-100 min) methods. However, %@mt;sys@%%@ital@%%@bold@%U%@reset@%%@rsf@%%@sx@%37%@be@%%@ital@%k%@rsf@%'%@sxx@%%@mx@% values from GC/TOF-MS showed deviations from those obtained by GC/FID, resulting from sensitivity differences between the C37:2 and C37:3 alkenone when analyzed by GC/TOF-MS. A solution to this bias is presented by determining compound-specific linear response factor equations to derive sensitivity ratios (SR) that allow conversion of GC/TOF-MS values into calibrated GC/FID data. Using alkenone mixtures of known composition and a variety of samples from natural environments, the applicability of this approach is demonstrated.     

A gas chromatograph/resonant electron capture-TOF mass spectrometer for four dimensions of negative ion analytical information

A prototype gas chromatograph (GC) electron monochromator (EM) reflectron time-of-flight (TOF) mass spectrometer has been constructed and demonstrated to simultaneously record four-dimensional resonant electron capture (REC) mass spectra (m/z, ion-intensity, electron-energy, and retention time) of electron-capturing compounds in real time. Specifically, complete REC mass spectra of all of the components in a mixture of perfluorocarboxylic acids and in a sample of pentafluorobenzyl alcohol were recorded in the GC mode. For each compound, the data enable one to distinguish different electronic states of the molecular ion and different possible decomposition pathways for each state. This new instrument can be used to obtain analytical information unrecognizable by any other mass spectrometric technique from the isomeric species of a variety of electron-capturing structures.     

Study of the Theophylline Content of Single Coated Particles by Gas Chromatography/Mass Spectrometry

The aim of the present study was to determine by gas chromatography/mass spectrometry (GC/MS) the content uniformity of single theophylline microcapsules of different particle size ranges. Microencapsulation was carried out in a laboratory fluidized bed system using Eudragit L30D aqueous dispersion. Scanning electron microscopy was applied for the characterization of the surface morphology of the prepared theophylline microcapsules of two different particle size ranges. The theophylline content of single particles was determined by GC/MS analysis. It was found that the particle size of microcapsules greatly influenced their theophylline content. The GC/MS analysis was successfully applied to indicate the changes in the content uniformity and thus the interparticular coating distribution of single theophylline microcapsules in the presence of several excipients.     

Study of the Theophylline Content of Single Coated Particles by Gas Chromatography/Mass Spectrometry

The aim of the present study was to determine by gas chromatography/mass spectrometry (GC/MS) the content uniformity of single theophylline microcapsules of different particle size ranges. Microencapsulation was carried out in a laboratory fluidized bed system using Eudragit L30D aqueous dispersion. Scanning electron microscopy was applied for the characterization of the surface morphology of the prepared theophylline microcapsules of two different particle size ranges. The theophylline content of single particles was determined by GC/MS analysis. It was found that the particle size of microcapsules greatly influenced their theophylline content. The GC/MS analysis was successfully applied to indicate the changes in the content uniformity and thus the interparticular coating distribution of single theophylline microcapsules in the presence of several excipients.     

裂解气相色谱鉴别PES和PSF材料

红外光谱鉴别聚合物材料是行业内通常采用的简便,快速的检测手段,针对结构相近的材料如PES和PSF,虽然谱图比较相似,但是还能作出判定。利用热裂解气相色谱,不仅色谱图存在明显的差异,还能通过特征裂解碎片,清晰地鉴别出它们的不同。     

Reduction of detection limits of the direct deposition GC/FT-IR interface by surface-enhanced infrared absorption

Even though the interface of gas chromatography (GC) and mass spectrometry (MS) is by far the most popular way of identifying molecules eluting from a GC in real time, the identification of compositional isomers by MS is equivocal at best. Much better results would be found by infrared spectrometry (IR) if the sensitivity of the GC/IR interface could be improved. In this paper, we show how the smallest quantity of molecules injected into a GC for which an identifiable infrared spectrum can be measured on-line has been reduced by a factor of 10 below the detection limit of the most sensitive current technique. A commercial direct deposition interface between a GC and a Fourier transform infrared spectrometer was modified by vapor-depositing an island film of silver on the surface of the zinc selenide substrate. Band intensities in the spectra of molecules located within approximately 4 nm of the surface of the silver islands were increased by at least 1 order of magnitude through surface-enhanced infrared absorption (SEIRA). The effectiveness of this approach was illustrated by comparing the limits of identification of butylbenzene isomers measured with and without the silver film. Comparison with the spectra of the same molecules measured by mass spectrometry showed the increased sensitivity and specificity of the GC/SEIRA interface.     

Unambiguous identification of volatile organic compounds by proton-transfer reaction mass spectrometry coupled with GC/MS

Interest in on-line measurements of volatile organic compounds (VOCs) is increasing, as sensitive, compact, and affordable direct inlet mass spectrometers are becoming available. Proton-transfer reaction mass spectrometry (PTR-MS) distinguishes itself by its high sensitivity (low ppt range), high time resolution (200 ms), little ionization-induced fragmentation, and ionization efficiency independent of the compound to be analyzed. Yet, PTR-MS has a shortcoming. It is a one-dimensional technique that characterizes compounds only via their mass, which is not sufficient for positive identification. Here, we introduce a technical and analytical extension of PTR-MS, which removes this shortcoming, while preserving its salient and unique features. Combining separation of VOCs by gas chromatography (GC) with simultaneous and parallel detection of the GC effluent by PTR-MS and electron impact MS, an unambiguous interpretation of complex PTR-MS spectra becomes feasible. This novel development is discussed on the basis of characteristic performance parameters, such as resolution, linear range, and detection limit. The recently developed drift tube with a reduced reaction volume is crucial to exploit the full potential of the setup. We illustrate the performance of the novel setup by analyzing a complex food system.     

Baseline correction method using an orthogonal basis for gas chromatography/mass spectrometry data

A baseline correction method that uses basis set projection to estimate spectral backgrounds has been developed and applied to gas chromatography/mass spectrometry (GC/MS) data. An orthogonal basis was constructed using singular value decomposition (SVD) for each GC/MS two-way data object from a set of baseline mass spectra. A novel aspect of this baseline correction method is the regularization parameter that prevents overfitting that may produce negative peaks in the corrected mass spectra or ion chromatograms. The number of components in the basis, the regularization parameter, and the mass spectral range from which the spectra were sampled to construct the basis were optimized so that the projected difference resolution (PDR) or signal-to-noise ratio (SNR) was maximized. PDR is a metric similar to chromatographic resolution that indicates the separation of classes in a multivariate data space. This new baseline correction method was evaluated with two synthetic data sets and a real GC/MS data set. The prediction accuracies obtained by using the fuzzy rule-building expert system (FuRES) and partial least-squares-discriminant analysis (PLS-DA) as classifiers were compared and validated through bootstrapped Latin partition (BLP) between data before and after baseline correction. The results indicate that baseline correction of the two-way GC/MS data using the proposed methods resulted in a significant increase in average PDR values and prediction accuracies.     

Gas chromatography/electron ionization-mass spectrometry-selected ion monitoring screening method for a thorough investigation of polyhalogenated compounds in passive sampler extracts with quadrupole systems

Nontarget analysis and identification of unknown polyhalogenated compounds is important in acquiring a thorough picture of the present pollution status as well as for identifying emerging environmental problems. Such analyses usually require the application of electron ionization mass spectrometry because the resulting mass spectra frequently allow for compound identification. When quadrupoles are used as mass separators, the full scan technique often suffers from low sensitivity along with nonspecificity for polyhalogenated trace compounds which often result in interference by matrix compounds. We have developed a novel nontarget gas chromatography/electron ionization-mass spectrometry-selected ion monitoring (GC/EI-MS-SIM) method that overcomes these sensitivity and selectivity issues. Our method is based on the fact that the molecular ions and isotope patterns of polyhalogenated compounds involve the most relevant primary information with regard to the structure of polyhalogenated compounds. Additionally, the retention times of polyhalogenated compounds generally increase with increasing molecular weight. The retention time range of polyhalogenated compounds was divided in three partly overlapping segments of 112 u (segment A: m/z 300-412; segment B: m/z 350-462; segment C: m/z 450-562) that were screened in eight GC runs consisting of 15 consecutive SIM ions. This method was tested with a passive water sampler extract known to contain over 30 polyhalogenated compounds according to the sensitive analysis by GC/electron capture negative ion (ECNI)-MS. While none of these polyhalogenated compounds could be detected by GC/EI-MS in full scan mode, our nontarget GC/EI-MS-SIM method allowed for the detection of 38 polyhalogenated compounds. Only seven could be identified by means of reference standards while more than 15 of the unknowns could be traced back to at least the class of compounds based on the mass spectrometric data from the nontarget SIM runs. All compounds identified originated from halogenated natural products. The nontarget GC/EI-MS-SIM method combines the high sensitivity obtainable with quadrupole systems for trace analysis with the structural information essential for the identification of unknown pollutants.     

GC/MS with post-column switching for large volume injection of headspace samples: sensitive determination of volatile organic compounds in human whole blood and urine

When volatile or semivolatile compounds are measured by headspace (HS) gas chromatography (GC)/mass spectrometry (MS), the maximum gas volume to be injected is usually 0.5-1.0 mL; over the volume, the MS detector automatically shuts down due to impairment of the vacuum rate of the MS ionization chamber. To overcome the problem, we modified the gas flow routes of a new type of GC/MS instrument to create a postcolumn switching system, which can eliminate the large volume of gas before introduction of target compounds into the MS ionization chamber. Our HS-GC/MS system enabled injection of as large as 5 mL of HS gas without any disturbance. As the first example analysis, we tried to establish the analysis of naphthalene and p-dichlorobenzene in human whole blood and urine by this method with large volume injection. The limits of detection for both compounds in whole blood and urine were as low as about 10 and 5 pg/mL, respectively. The validation data and actual measurements were also demonstrated. The new GC/MS system has great potential to analyze any type of volatile or semivolatile organic compounds in biological matrixes with very high sensitivity and full automation.     

Picogram determination of "earthly-musty" odorous compounds in water using modified closed loop stripping analysis and large volume injection GC/MS

"Earthy-musty" off-flavor problems in water samples are due to organic compounds present at the sub-part-per-trillion level. Numerous analytical methods such as purge and trap, liquid/liquid extraction, and closed-loop stripping analysis (CLSA) followed by GC/MS analysis have been used to determine these compounds. However, these methods offer poor sensitivity (detection limits of approximately 1 to 10 ng/L) when compared to the 20-30 pg/L of sensorial sensitivity. The purpose of this study was to develop a new method involving a modified CLSA preconcentration technique together with large volume injection GC/MS in order to attain analytical sensitivity equal to or better than olfactory sensitivity. For eight target compounds that cause taste and odor problems in water at trace levels, the method developed was linear in the 0.05-10 ng/L range and provided recoveries greater than 70% together with satisfactory repeatability. Detection limits as low as 15-30 pg/L were achieved, representing a 50-fold improvement in sensitivity as compared to current methods. The accuracy and sensitivity of the method were demonstrated in different aqueous matrixes, including raw surface water. The method was successfully applied to earthy-musty water samples that had remained unsolved by conventional techniques, thus proving its effectiveness.     

Potential analytical applications of interfacing a GC to an FT-ICR MS: fingerprinting complex sample matrixes

Details of interfacing a high-pressure gas chromatograph to the internal ion source of a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) are described. We present our preliminary results and potential analytical applications of GC/FT-ICR for analyzing complex biological and environmental sample matrixes, such as petroleum mixtures. Based on GC/FT-ICR data, rapid characterization of various automobile gasoline samples is possible. Comparison between acquired data from the GC/FT-ICR MS (in broadband mode) and a commercial GC quadrupole mass spectrometer (QMS) (over a wide mass range) indicates that sensitivity of the GC/FT-ICR MS is an order of magnitude lower. High mass resolution and mass measurement accuracy of FT-ICR MS can be utilized for unambiguous molecular formula identification of unknown analytes.     

Characterization of archaeological beeswax by electron ionization and electrospray ionization mass spectrometry

To better detect and identify beeswax in ancient organic residues from archaeological remains, we developed a new analytical methodology consisting of the analysis of (i) the trimethylsilylated organic extract by GC/MS and (ii) the crude extract by ESI-MS. Selective scanning modes, such as SIM or MRM, permit separate quantification of each chemical family (fatty acids, monoesters, monohydroxyesters, and diesters) and allow an improvement in sensitivity and selectivity, allowing the crude extract to be treated without further purification. GC/MS (SIM) was revealed to be a powerful method for the detection of components, with a detection limit down to a total lipid extract in the range of approximately 50 ng in a complex matix, such as archaeological degraded material, whereas ESI-MS/MS is instead used for the detection of nonvolatile biomarkers. Identification by GC/MS (SIM) and ESI-MS/ MS (MRM) of more than 50 biomarkers of beeswax in an Etruscan cup at the parts-per-million level provides the first evidence for the use of this material by the Etruscans as fuel or as a waterproof coating for ceramics.     

Broad spectrum analysis of 109 priority compounds listed in the 76/464/CEE Council Directive using solid-phase extraction and GC/EI/MS

A single multiresidue method was developed to determine 109 priority organic compounds included in the 76/464/EEC Council Directive on Pollution of the European Union. Such Directive includes 132 priority pollutants with a broad spectrum of polarities to be analyzed in drinking and surface waters, with the aim to protect water quality. From this list, the compounds analyzed included benzidines, chloroanilines, chloronitrobenzenes, chloronitrotoluenes, chlorophenols, chloronitrotoluidines, PAHs, PCBs, pesticides, phenylurea, and triazine herbicides. The method was developed in four steps. First, automated off-line solid-phase extraction using polymeric sorbent Oasis 60 mg cartridges was optimized to trap 109 compounds. Second, gas chromatography coupled to mass spectrometry with electron impact ionization (GC/EI/MS) was used in selected ion monitoring (SIM) mode for tentative identification of target analytes. Third, GC/EI/MS under full scan conditions was used for spectrum identification and analyte confirmation. Last, quantification was performed from SIM chromatogram using surrogates and internal standard. This method offered excellent sensitivity and selectivity, and the preconcentration of 200 mL permitted the achievement of limits of detection at the low nanogram/liter level and recoveries between 70 and 120%. Such methodology was applied to determine 109 organic compounds in French surface waters, and several pollutants were detected at levels from ppt to ppb. This multiresidue method developed was highly reproducible and robust and permitted a high sample throughput.     

Liquid chromatography/mass spectrometry for timely response in regulatory analyses: identification of pentobarbital in dog food

A limited liquid chromatography/mass spectrometry (LC/ MS) data set was acquired under conditions which called for timely response without benefit of a fully developed method. Quality assurance elements verified that an LC/ MS procedure developed in a short-time was sufficiently under control to meet its purpose. LC/MS was used to rule out a potential problem with a gas chromatography (GC)/MS method that had been developed for regulatory purposes. The LC/MS data set showed that signals identified by GC/MS as diagnostic of pentobarbital (PB) were not artifacts of derivatization or GC analysis. Samples of dry dog food identified by GC/MS as containing PB were also shown by LC/MS to contain PB. The LC/MS method would not be recommended as a substitute for GC/MS, primarily because of poorer sensitivity. Although the data set is limited, and justifiably represents only the starting point for conventional method development, the purpose at hand was served adequately. This work demonstrates the utility of LC/MS for rapid regulatory response, provided there is a framework of quality assurance checks.     

Two trace analytical methods for determination of hydroxylated PCBs and other halogenated phenolic compounds in eggs from Norwegian birds of prey

Two new trace analytical methods are presented for identification and quantification of phenolic compounds in complex biological matrixes such as bird of prey eggs. One method is based on derivatization with methyl chloroformate prior to GC/high-resolution MS (HRMS) analysis in electron impact ionization mode. Alternatively, the underivatized phenolic analytes were separated and detected by HPLC coupled to time-of-flight MS (TOF-MS) in the negative ion electrospray ionization mode. For both methods, the egg samples were homogenized and dried with acidified sodium sulfate, cold column-extracted, and cleaned up by gel permeation chromatography and subsequently a Florisil column. Recovery rates for pentachlorophenol (PCP), tetrabromobisphenol A (TBBPA), and selected hydroxylated PCBs (HO-PCBs) from spiked hen's eggs (spiking level 1 ng/g of wet weight (ww)) were in the range of 56-98% for the HPLC/MS method and 57-108% for GC/MS including derivatization. Typical detection limits of the HPLC/TOF-MS method were 5 pg/g ww (1-2 pg injected) for HO-PCBs and PCP and 20 pg/g ww (3 pg injected) for TBBPA. The GC/HRMS method achieved detection limits of approximately 1 pg/g ww in predatory bird eggs for all analytes (0.2 pg injected for derivatized TBBPA and 0.05 pg injected for derivatized HO-PCBs and PCP). Eight eggs from four different Norwegian predatory bird species were analyzed. The concentrations determined with the two different quantification methods corresponded well with each other. PCP and TBBPA were found in all samples at concentrations up to 1350 and 13 pg/g ww, respectively (GC/HRMS values). A total of 55 penta- to nonachloro-HO-PCB congeners were detected in the eight eggs, 10 of those could be structurally identified. The maximum HO-PCB congener concentration was found for 4-HO-CB 187 in a peregrine falcon egg with estimated 388 pg/g ww. Another peregrine falcon egg was highest contaminated with sum HO-PCBs (estimated 2.1 ng/g ww). This level was 1.2 per thousand of the sum PCBs value for the same egg. Furthermore, indications were found that the HO-PCB congener distribution pattern could be species specific for predatory birds.     

Gas chromatography/mass spectroscopy for plastics failure analysis

Gas chromatography/mass spectroscopy is particularly useful as an analytical method for plastics failure analysis in cases where detection of an unknown contaminant or other compositional factor may be the cause or a contributor to failure. It takes advantage of the fact that GC is a method of separating compounds in a mixture, permitting their identification and possibly quantification. MS is not only a very sensitive detector but also gives mass spectra of GC peaks, permitting their identification in many cases. In thermal desorption GC/MS compounds are transferred from the sample to the GC with heat. Completely nonvolatile materials are not detected. Using desorption temperatures up to 300–350 °C, many components of plastics can be analyzed. In pyrolysis GC/MS the sample is decomposed at temperatures up to 900 °C; GC/MS analyzes the pyrolyzate. Examples are given of causes of plastics failures that have been determined by GC/MS.     

Matrix-assisted laser desorption/ionization high-energy collision-induced dissociation of steroids: analysis of oxysterols in rat brain

Neutral steroids have traditionally been analyzed by gas chromatography/mass spectrometry (GC/MS) after necessary derivatization reactions. However, GC/MS is unsuitable for the analysis of many conjugated steroids and those with unsuspected functional groups. Here we describe an alternative analytical method specifically designed for the analysis of oxosteroids and those with a 3beta-hydroxy-delta5 or 5alpha-hydrogen-3beta-hydroxy structure. Steroids were derivatized with Girard P (GP) hydrazine to give GP hydrazones, which are charged species and readily analyzed by matrix-assisted laser desorption/ionization mass spectrometry. The resulting [M]+ ions were then subjected to high-energy collision-induced dissociation on a tandem time-of-flight instrument. The product ion spectra give structurally informative fragment ion patterns. The sensitivity of the analytical method is such that steroid structures can be determined from low-picogram (low-femtomole) amounts of sample. The utility of the method has been demonstrated by the analysis of oxysterols extracted from rat brain.     

PTR—MS与GC／MS在VOCs分析中的比较

挥发性有机物（VOCs）分析通常使用气相色谱／质谱（GC／MS）联用技术,但该技术对VOCs痕量分析仍有待改进。质子传递反应质谱（PTR-MS）技术弥补了GC／MS技术检测VOCs种类受限及耗时、费力等不足,本文通过测试室内低浓度VOCs,对比讨论了两种技术的性能及优缺点。