Urinary organic acid screening by solid-phase microextraction of the methyl esters

Versatile derivatization for GC-MS and LC-MS: alkylation with trialkyloxonium tetrafluoroborates for inorganic anions, chemical warfare agent degradation products, organic acids, and proteomic analysis

Analytical chemists resort to derivatization for improving the detection performance of certain categories of analytes. Within this context, alkylation reactions are regarded as an important asset for many methods based on GC-MS and LC-MS. Trialkyloxonium tetrafluoroborates (R[Formula: see text][BF₄]^-) are powerful alkylating agents with ionic liquid properties: they are nonvolatile salts soluble in water which are easier and safer to handle with respect to common alkylating agents like diazomethane. R[Formula: see text][BF₄]^- can perform the alkylation in both organic and aqueous media at pH conditions ranging from acidic to alkaline. Recent analytical applications of trialkyloxonium derivatizations include the high-precision determination of inorganic anions in complex matrices, the qualitative confirmation of chemical warfare agent degradation products in soils, the profiling of carboxylic acids in urine, and the detection of protein post-translational modifications induced by carbon dioxide. The common denominator for all methods presented can be found in the simplicity of the alkylation protocol which, in most of the cases, requires a single step addition of the reagent directly to the sample. Graphical Abstract Alkylation with trialkyloxonium salts for GC-MS and LC-MS analysis.

Efficient derivatization of methylphosphonic and aminoethylsulfonic acids related to nerve agents simultaneously in soils using trimethyloxonium tetrafluoroborate for their enhanced, qualitative detection and identification by EI-GC-MS and GC-FPD

Trimethyloxonium tetrafluoroborate (TMO·BF₄) has been used in the simultaneous derivatization of phosphonic and 2-aminoethylsulfonic acids related to nerve agents in different soils for their enhanced detection and identification by electron ionization gas chromatography-mass spectrometry (EI-GC-MS). The panel of acids consisted of five Schedule 2 phosphonic acids (methyl methylphosphonic acid, ethyl methylphosphonic acid, isopropyl methylphosphonic acid, pinacolyl methylphosphonic acid and cyclohexyl methylphosphonic acid) along with two sulfonic acids, N,N-diethyl-2-aminoethylsulfonic acid and N,N-diisopropyl-2-aminoethylsulfonic acid. The acids were converted to their corresponding methyl esters at ambient temperature when present at a 10μgg^-1 concentration in three separate soils: Virginia type A soil, Ottawa sand and Nebraska EPA soil. The concentration of the acids reflects values typically encountered during proficiency tests (PTs) administered annually by the Organisation for the Prohibition of Chemical Weapons (OPCW). Derivatization times to yield detectable signals for the methyl ester products for all the acids was found to vary among all three soil samples, however, it was found that generally the most optimal time across all the matrices involved was 3h after the addition of TMO·BF₄. Concomitantly, the analysis of the samples was complemented using GC coupled to flame photometric detection (GC-FPD). The inclusion of GC-FPD in the analysis yielded stronger signals for all seven methylated analytes making their detection after merely 3h possible relative to the ones initially obtained with EI-GC-MS. Regarding the three soils employed in our study, a greater methylating efficiency was found in the Virginia type A soil and Ottawa sand yielding results that were significantly larger in magnitude to those found during the same time points for the Nebraska EPA soil sample. Prolonged reaction times (up to 72h) were explored to find the time for the highest yield of methyl ester production were found instead to be deleterious to the process showcasing the importance of the fast yielding nature of the process specifically in situations where time-sensitive analysis is crucial (e.g. OPCW-PT).

Effective methylation of phosphonic acids related to chemical warfare agents mediated by trimethyloxonium tetrafluoroborate for their qualitative detection and identification by gas chromatography-mass spectrometry

The effective methylation of phosphonic acids related to chemical warfare agents (CWAs) employing trimethyloxonium tetrafluoroborate (TMO·BF4) for their qualitative detection and identification by gas chromatography-mass spectrometry (GC-MS) is presented. The methylation occurs in rapid fashion (1 h) and can be conveniently carried out at ambient temperature, thus providing a safer alternative to the universally employed diazomethane-based methylation protocols. Optimization of the methylation parameters led us to conclude that methylene chloride was the ideal solvent to carry out the derivatization, and that even though methylated products can be observed surfacing after only 1 h, additional time was not found to be detrimental but beneficial to the process particularly when dealing with analytes at low concentrations (∼10 μg mL(-1)). Due to its insolubility in methylene chloride, TMO·BF4 conveniently settles to the bottom during the reaction and does not produce additional interfering by-products that may further complicate the GC-MS analysis. The method was demonstrated to successfully methylate a variety of Schedule 2 phosphonic acids, including their half esters, resulting in derivatives that were readily detected and identified using the instrument's spectral library. Most importantly, the method was shown to simultaneously methylate a mixture of the organophosphorus-based nerve agent hydrolysis products: pinacolyl methylphosphonate (PMPA), cyclohexyl methylphosphonate (CyMPA) and ethyl methylphosphonate (EMPA) (at a 10 μg mL(-1) concentration each) in a fatty acid ester-rich organic matrix (OPCW-PT-O3) featured in the 38th Organisation for the Prohibition of Chemical Weapons (OPCW) Proficiency Test. In addition, the protocol was found to effectively methylate N,N-diethylamino ethanesulfonic acid and N,N-diisopropylamino ethanesulfonic acid that are products arising from the oxidative degradation of the V-series agents VR and VX respectively. The work described herein represents the first report on the use of TMO·BF4 as a viable, stable and safe agent for the methylation of phosphonic acids and their half esters and within the context of an OPCW Proficiency Test sample analysis.

Characterization and analysis of structural isomers of dimethyl methoxypyrazines in cork stoppers and ladybugs (Harmonia axyridis and Coccinella septempunctata)

The three constitutional isomers of dimethyl-substituted methoxypyrazines: 3,5-dimethyl-2-methoxypyrazine 1; 2,5-dimethyl-3-methoxypyrazine 2; and 2,3-dimethyl-5-methoxypyrazine 3 are potent flavor compounds with similar mass spectrometric, gas chromatographic, and nuclear magnetic resonance spectroscopic behavior. Therefore, unambiguous analytical determination is critical, particularly in complex matrices. The unequivocal identification of 1-3 could be achieved by homo- and heteronuclear NMR correlation experiments. The observed mass fragmentation for 1-3 is proposed and discussed, benefitting from synthesized partially deuterated 1 and 2. On common polar and apolar stationary phases used in gas chromatography (GC) 1 and 2 show similar behavior whereas 3 can be separated. In our focus on off-flavor analysis with respect to wine aroma, 1 has been described as a "moldy" off-flavor compound in cork and 2 as a constituent in Harmonia axyridis contributing to the so-called "ladybug taint," whereas 3 has not yet been described as a constituent of wine aroma. A successful separation of 1 and 2 could be achieved on octakis-(2,3-di-O-pentyl-6-O-methyl)-γ-cyclodextrin as stationary phase in GC. Applying heart-cut multidimensional GC analysis with tandem mass spectrometric detection we could confirm the presence of 1 as a "moldy" off-flavor compound in cork. However, in the case of Harmonia axyridis, a previous identification of 2 has to be reconsidered. In our experiments we identified the constitutional isomer 1, which was also found in Coccinella septempunctata, another species discussed with respect to the "ladybug taint." The analysis of such structurally related compounds is a demonstrative example for the importance of a chromatographic separation, as mass spectrometric data by itself could not guarantee the unequivocal identification.

Gas Chromatographic Techniques in Metabolomics

High chemical diversity and abundances ranging from trace to millimolar levels still constitute at times insurmountable challenges in the comprehensive analysis of metabolites in biomedical specimens. Nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS) hyphenated with separation techniques such as liquid chromatography (LC), gas chromatography (GC) and capillary electrophoresis (CE) are the most frequently used techniques for both targeted and discovery‐driven metabolomics. Of the separation techniques, comprehensive two‐dimensional gas chromatography (GC×GC) offers the highest peak resolution and capacity, and in combination with MS lower quantification limits in the submicromolar concentration range are realized. Moreover, electron ionization (EI), the most prominent ionization technique for GC‐MS, is highly reproducible, facilitating the generation of mass spectral libraries for routine metabolite identification. However, GC analysis often requires a derivatization prior to analysis and not all metabolite derivatives are recorded in the libraries available. Consequently, metabolite identification is still a major challenge. To identify unknown metabolite signals, soft ionization techniques in combination with high‐resolution MS are employed to determine the accurate mass of the quasi‐molecular ion. The latter is used to calculate elemental formulae that can be fed into metabolite databases for a putative identification or used for the interpretation of EI spectra.

A method for comprehensive analysis of urinary acylglycines by using ultra-performance liquid chromatography quadrupole linear ion trap mass spectrometry

Acylglycines are an important class of metabolites that have been used in the diagnosis of several inborn errors of metabolism (IEM). However, current analytical methods detect only a few acylglycines. There is a need to profile these metabolites in a comprehensive manner for studying their functions and improving their diagnostic values for different IEM and potentially other diseases. We describe a sensitive method that combines the chromatographic resolving power of ultra-performance liquid chromatography (UPLC) to separate closely related metabolites including isomers with tandem mass spectrometry (MS/MS). Acylglycines were extracted from urine using an anion exchange solid-phase extraction (SPE) cartridge. After UPLC separation, the acylglycines were detected on a hybrid triple quadrupole linear ion trap mass spectrometer. A set of standards were used for the development of an optimal MS acquisition method. Several acquisition modes using information derived from collision-induced dissociation breakdown curves were used to detect acylglycines. Using this method, 18 acylglycines were detected in the urine of healthy individuals and confirmed using standards, while 47 additional acylglycines were detected and tentatively identified, based on their retention and fragmentation pattern. Among the 65 acylglycines detected, only 18 of them have been previously reported in biofluids of healthy individuals. These results will be deposited in a public human metabolome database. This example illustrates that by developing a method tailored to the analysis of a class of metabolites sharing similar structural moieties, we can potentially identify many more new metabolites, thereby expanding the overall metabolome coverage.

Evaluation of exudates by solid phase microextraction-gas chromatography

Head-space solid phase microextraction combined with gas chromatography (SPME-GC) was used for the determination of bacterial volatile fatty acid (VFA) patterns. The method was validated with cultures of reference bacterial strains. It was confirmed that VFA production depends on the composition of the cultivation medium, which limits accurate characterisation of particular bacterial species. A set of 195 clinical exudates of various origin and consistence was analysed using SPME-GC and compared with 73 samples extracted using tert-butyl methyl ether. Approximate agreement of VFA profiles with cultivation findings was found in most cases. However, 20.5% of clinical exudates with distinct VFA profiles appeared to be false-negative by cultivation. Using SPME-GC of exudates, the frequency of false-negative cultivations was higher than that of solvent extraction of exudates or blood cultures found previously. The described method is suitable for preliminary detection of bacteria, particularly non-sporulating anaerobes, in clinical samples. It can reveal false-negative findings due to cultivation. Analysis can be performed in 30 min without the need for cultivation.

Analysis of acidic pesticides using in situ derivatization with alkylchloroformate and solid-phase microextraction (SPME) for GC-MS

A solid-phase microextraction (SPME) method was developed for the analysis of acidic pesticide residues in water. The method utilizes in situ derivatization with butylchloroformate (BuCF), followed by on-line SPME extraction using a PDMS fibre, and analysis by GC-MS. Derivatives of the phenoxy acids mechlorprop (MCPP), dichlorprop (DCPP), MCPA and 2,4-D and their phenol degradation products 4-chloro-2-methylphenol and 2,4-dichlorophenol (DCP) were identified. Detection limits at 0.16-2.3 microg/l were achieved. Optimization of derivatization, ion strength, extraction time, SPME-fibre, desorption time and temperature are described. Standard curves in the range 0.5-10.0 microg/l were fitted to a second-degree polynomial. Standard deviation (n = 5) was below 10% for the phenol derivatives, but 20-50% for the phenoxy acids. For method verification groundwater samples from a field experiment were screened for content of MCPP and compared to the results from the HPLC analysis. A good agreement was obtained with respect to identification of positive samples, even though concentrations measured by the SPME were lower than with HPLC. Even if the precision and accuracy do not meet the demands for a strictly quantitative analysis, the SPME method is suitable for screening, because it is cheap, it can be automated, and uses smaller amounts of potential harmful solvents. Also, the method is less labour-intensive, as it requires a minimum of sample preparation when compared to traditional analyses. The acidic pesticides bentazon, dicamba, bromoxynil, ioxynil, dinoseb and DNOC were included in the study but could not be analysed by the current method.

Headspace solid-phase microextraction profiling of volatile compounds in urine: application to metabolic investigations

Volatile compounds contribute substantially to the metabolic pool in man. Their analysis in body fluids is problematic. We investigated headspace solid-phase microextraction (HS-SPME) with Carboxen-polydimethylsiloxane fibres and gas chromatography-mass spectrometry for profiling urinary volatile components. These fibres were more sensitive for very volatile and sulfur compounds than three other phases tested. We detected a wide range of compounds in normal urine at acid and alkaline pH. Profiles presented for five individuals with metabolic disturbances demonstrate abnormal accumulation of sulfur compounds, fatty acids and plasticisers. HS-SPME can complement profiling of non-volatile compounds in metabolic investigations and could be a useful extension of the diagnostic repertoire.

Solid-phase microextraction for the assay of levomepromazine in human plasma

Solid-phase microextraction (SPME) was investigated as sample preparation for the assay of the neuroleptic drug levomepromazine in human plasma. A mixture of human plasma, water, chloramitriptyline as internal standard, and aqueous NaOH was extracted with a 100-microm polydimethylsiloxane (PDMS) fiber (Supelco). The desorption of the fiber was performed in the injection port of a gas chromatograph at 260 degrees C [HP 5890; BPX-5 (SGE): 30 m x 0.53 mm ID, 1-microm film capillary; nitrogen-phosphorus selective detection]. As repeatedly found for SPME analysis of drugs in plasma, the recovery was low (i.e., 7% for levomepromazine). However, the analyte and internal standard were well separated and the calibration was linear from 5 to 180 ng/mL. The within-day precision was 2%, 4%, and 19% at concentrations of 160 ng/mL, 80 ng/mL, and 5 ng/mL, respectively. The between-day precision was 3%, 7%, and 19%, respectively. The limit of determination was 5 ng/mL. The comparison with an established liquid-liquid extraction gas-liquid chromatography method revealed good agreement for spiked samples and patient samples. No interfering peaks of drugs coadministered with levomepromazine or of endogenous substances were found. It is concluded that the method can be used in the therapeutic drug monitoring and clinical toxicology of levomepromazine.

Solid-phase microextraction in biomedical analysis

Chromatographic methods are preferred in the analysis of organic molecules with lower molecular mass (<500 g/mol) in body fluids, i.e., the assay of drugs, metabolites, endogenous substances and poisons as well as of environmental exposure by gas chromatography (GC) and liquid chromatography (LC), for example. Sample preparation in biomedical analysis is mainly performed by liquid-liquid extraction and solid-phase extraction. However, new methods are investigated with the aim to increase the sample throughput and to improve the quality of analytical methods. Solid-phase microextraction (SPME) was introduced about a decade ago and it was mainly applied to environmental and food analysis. All steps of sample preparation, i.e., extraction, concentration, derivatization and transfer to the chromatograph, are integrated in one step and in one device. This is accomplished by the intelligent combination of an immobilized extraction solvent (a polymer) with a special geometry (a fiber within a syringe). It was a challenge to test this novel principle in biomedical analysis. Thus, an introduction is provided to the theory of SPME in the present paper. A critical review of the first applications to biomedical analyses is presented in the main paragraph. The optimization of SPME as well as advantages and disadvantages are discussed. It is concluded that, because of some unique characteristics, SPME can be introduced with benefit into several areas of biomedical analysis. In particular, the application of headspace SPME-GC-MS in forensic toxicology and environmental medicine appears to be promising. However, it seems that SPME will not become a universal method. Thus, on-line SPE-LC coupling with column-switching technique may be a good alternative if an analytical problem cannot be sufficiently dealt with by SPME.

Headspace solid-phase microextraction procedures for gas chromatographic analysis of biological fluids and materials

Solid-phase microextraction (SPME) is a new solventless sample preparation technique that is finding wide usage. This review provides updated information on headspace SPME with gas chromatographic separation for the extraction and measurement of volatile and semivolatile analytes in biological fluids and materials. Firstly the background to the technique is given in terms of apparatus, fibres used, extraction conditions and derivatisation procedures. Then the different matrices, urine, blood, faeces, breast milk, hair, breath and saliva are considered separately. For each, methods appropriate for the analysis of drugs and metabolites, solvents and chemicals, anaesthetics, pesticides, organometallics and endogenous compounds are reviewed and the main experimental conditions outlined with specific examples. Then finally, the future potential of SPME for the analysis of biological samples in terms of the development of new devices and fibre chemistries and its coupling with high-performance liquid chromatography is discussed.

Solid-phase micro-extraction of drugs from biological matrices

Solid phase micro-extraction was originally designed as a technique for the solvent-free analysis of volatile organic contaminants in environmental samples. However, a wide variety of applications are now being pursued, including the analysis of drugs from a variety of matrices. In this review, the analysis of drugs by SPME from biological and related matrices, including water, urine, blood, hair and saliva, is discussed. A general overview of the special problems and techniques involved in SPME from biological matrices is presented, along with specific references and discussion of the analysis of many types of drugs and metabolites. It is seen that SPME is a highly versatile and flexible technique for these analyses.

Fishing for a drug: solid-phase microextraction for the assay of clozapine in human plasma

Solid-phase microextraction (SPME) was investigated as a sample preparation method for assaying the neuroleptic drug clozapine in human plasma. A mixture of human plasma, water, loxapine (as internal standard) and aqueous NaOH was extracted with a 100-micron polydimethylsiloxane (PDMS) fiber (Supelco). Desorption of the fiber was performed in the injection port of a gas chromatograph at 260 degrees C (HP 5890; 30 m x 0.53 mm I.D., 1 micron film capillary; nitrogen-phosphorous selective detection). Fibers were used repeatedly in up to about 75 analyses. The recovery was found to be 3% for clozapine from plasma after 30 min of extraction. However, in spite of the low recovery, the analyte was well separated and the calibration was linear between 100 and 1000 ng/ml. The within-day and between-day precision was consistently about 8 to 15% at concentrations of 200 ng/ml to 1000 ng/ml. No interfering drug was found. The limit of detection was 30 ng/ml. The sample volume was 250 microliters. The influence of the concentration of proteins, triglycerides and salt, i.e., changes in the matrix on the peak areas and peak-area ratios was studied. The method is not impaired by physiological changes in the composition of the matrix. Good agreement was found with a liquid-liquid extraction-gas-liquid chromatography (LLE-GLC) standard method and an on-line column-switching high-performance liquid chromatography (HPLC) method for patients' samples and spiked samples, respectively. It is concluded that the method can be used in the therapeutic drug monitoring of clozapine because the therapeutic window of clozapine is from 350 to 600 ng/ml.

Profiling of organic acids by capillary gas chromatography-mass spectrometry after direct methylation in urine using trimethyloxonium tetrafluoroborate

Trimethyloxonium tetrafluoroborate (TMO) is applied as derivatising reagent to transform urinary organic acids into their methyl esters. The method is suggested as an alternative to the use of diazomethane which is carcinogenic and explosive. In contrast to other methods avoiding diazomethane, such as derivatizations with acetyl chloride-methanol and boron trifluoride-methanol, which require an organic reaction medium and therefore an extraction of the organic acids from the urine, TMO efficiently reacts with the acids in an aqueous solution and can therefore be directly applied to native urine. The use of TMO simplifies and improves the sample preparation in the profile analysis of urinary organic acids by capillary GC-MS and hereby increases the speed of analysis. The method gives reproducible results which are comparable with the data obtained using conventional solid-phase extraction with strong anion-exchange cartridges prior to derivatisation.