Analysis of multiple quaternary ammonium compounds in the brain using tandem capillary column separation and high resolution mass spectrometric detection

Simultaneous Analysis of Hydrophobic Atractylenolides, Atractylon and Hydrophilic Sugars in Bai-Zhu Using a High-Performance Liquid Chromatography Column Tandem Technique

An analytical method was established using high-performance liquid chromatography coupled with diode array and evaporative light scattering detectors (HPLC-DAD-ELSD) with -C18 and -NH2 column tandem for the simultaneous determination of hydrophobic atractylenolide I, II, III, atractylone and hydrophilic compounds glucose, fructose and sucrose in the dried rhizome of Atractylodes macrocephala Koidz (a natural raw material for health foods, Bai-Zhu aka. in Chinese). The method combines the different separation capabilities of reversed-phase liquid chromatography and hydrophilic interaction liquid chromatography. It can provides a new choice for the simultaneous determination of hydrophilic and hydrophobic compounds in traditional Chinese medicines and health foods. It provided a reference method for the quality control of Bai-Zhu. The results showed that the linear correlation coefficients of the established column tandem chromatographic method were all greater than 0.9990, the relative standard deviation was 0.1-2.8%, and the average recovery was 96.7-103.1%. The contents of atractylenolide I, II, III, atractylone, fructose, glucose, and sucrose in 17 batches of Baizhu were 172.3-759.8 μg/g, 201.4-612.8 μg/g, 160.3-534.2 μg/g, 541.4-8723.1 μg/g, 6.9-89.7 mg/g, 0.7-7.9 mg/g, and 1.2-21.0 mg/g, respectively.

Serially coupled column liquid chromatography: An alternative separation tool

Despite the rapid development of liquid chromatography (LC) in recent decades, it remains a challenge to achieve the desired chromatographic separation of complex matrices using a single column. Multi-column LC techniques, particularly serially coupled column LC (SCC-LC), have emerged as a promising solution to overcome this challenge. While more attention has been focused on heart-cutting or comprehensive two-dimensional LC, reviews specifically focusing on SCC-LC, which offers advantages in terms of precision and facile instrumentation, are scarce. Here, our concerns are devoted to the progress summary regarding the instrumentation and applications of SCC-LC. Emphasis is placed on column selection aiming to enlarge peak capacity, selectivity, or both through the optimization of combination types (e.g. RPLC-RPLC, -RPLC-HILIC, and achiral-chiral LC), connection devices (e.g. zero dead volume connector, tubing, and T-type connector), elution program (i.e. isocratic or gradient) and detectors (e.g. mass spectrometer, ultraviolet detector, and fluorescence detector). The application of SCC-LC in pharmaceutical, biological, environmental, and food fields is also reviewed, and future perspectives and potential directions for SCC-LC are discussed. We envision that the review can give meaningful information to analytical scientists when facing heavy chromatographic separation tasks for complicated matrices.

A sensitive approach for screening acetylcholinesterase inhibition of water samples using ultra-performance liquid chromatography-tandem mass spectrometry

A sensitive assay was developed to evaluate inhibitory effects of aqueous solution on acetylcholinesterase (AChE) activity via measuring hydrolysis rates of acetylcholine (ACh) based on ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). Upon having identified precursor ions and product ions of the ACh and its hydrolysis products choline (Ch), the separation chromatogram for these two analytes has been established using a 50 mm reverse-phase BEH Shield RP18 column. The total chromatographic separation time is 7 min; limits of detection (LODs) for ACh and Ch are 0.14 µg L^-1 and 0.12 µg L^-1, respectively. A simple method for inactivation of AChE and optimization of operational parameters were then sequentially performed. It was found that adjusting solution pH to 2.5 not only can terminate the enzymatic reaction but also solve band shifting and broadening caused by aqueous matrices in chromatographic separation during UPLC-MS/MS detection. Under conditions of 0.00075 U mL^-1 AChE, initial concentration of ACh at 100 µg L^-1 and 20 min observation time, IC₅₀ values of the proposed assay for chlorpyrifos-oxon, diazoxon, malaoxon, methidathion oxon, omethoate and paraoxon were 3.5 nM, 16.8 nM, 2.4 nM, 6.8 nM, 270 nM and 36.9 nM, respectively. They are 4.5-51.9 times smaller than those reported in a LC-MS based method, and >120 times lower than those obtained by the traditional Ellman method. The results suggested that, the proposed assay significantly increases the sensitivity of commercial AChE. In addition, inhibition efficiencies of three surface waters, a groundwater and four commercial brands of bottled drinking water samples on AChE activity were firstly measured using this UPLC-MS/MS based method. These water samples were proved to have different inhibitory effects on AChE activity, and the inhibition efficiencies dependent on concentrations of dissolved organic carbon (DOC) but are independent of UV absorbance at 254 nm (UV₂₅₄) values. These results indicate that the proposed method has advantages of high sensitivity over all other conventional methods. It may become a promising AChE inhibition assay for assessing toxicity of aqueous solution containing neurotoxicity contaminants such as organophosphorus pesticides (OPPs) at low levels, or used to evaluate potential inhibition effects of natural waters on AChE activity.

Analysis of Choline, Carnitine, Acetylcarnitine and Acetylcholine in Animal Feeds, Blood and Urine Using Ion Chromatography Coupled with Tandem Mass Spectrometry

A simple and sensitive method was proposed for choline, carnitine, acetylcarnitine (ACa) and acetylcholine (ACh) separation in feed, blood and urine of animals by using ion chromatography (IC) and detection by tandem mass spectrometry (MS/MS). Analytes were extracted using a mixture of acetonitrile and water, purified by C18 solid-phase extraction columns, separated via IC with an IonPac SCS-1 column and detected by an MS/MS detector by using isotopic internal standards for quantification. The effects of different chromatographic parameters on the separation were also investigated. Under optimal conditions, the recovery was >90%, with the relative standard deviations of <15%. The proposed method was highly reliable for the simultaneous determination of choline, carnitine, ACa and ACh in feed, blood and urine.

Automated Sequential Analysis of Hydrophilic and Lipophilic Fractions of Biological Samples: Increasing Single-Injection Chemical Coverage in Untargeted Metabolomics

In order to increase metabolite coverage in LC–MS-based untargeted metabolomics, HILIC- and RPLC-mode separations are often combined. Unfortunately, these two techniques pose opposite requirements on sample composition, necessitating either dual sample preparations, increasing needed sample volume, or manipulation of the samples after the first analysis, potentially leading to loss of analytes. When sample material is precious, the number of analyses that can be performed is limited. To that end, an automated single-injection LC–MS method for sequential analysis of both the hydrophilic and lipophilic fractions of biological samples is described. Early eluting compounds in a HILIC separation are collected on a trap column and subsequently analyzed in the RPLC mode. The instrument configuration, composed of commercially available components, allows easy modulation of the dilution ratio of the collected effluent, with sufficient dilution to obtain peak compression in the RPLC column. Furthermore, the method is validated and shown to be fit for purpose for application in untargeted metabolomics. Repeatability in both retention times and peak areas was excellent across over 140 injections of protein-precipitated blood plasma. Finally, the method has been applied to the analysis of real perilymph samples collected in a guinea pig model. The QC sample injections clustered tightly in the PCA scores plot and showed a high repeatability in both retention times and peak areas for selected compounds.

Simultaneous determination of eight biogenic amines in the traditional Chinese condiment Pixian Douban using UHPLC-MS/MS

A UHPLC-MS/MS method was developed to simultaneously determine eight biogenic amines (BAs) in Pixian Douban. Under optimal conditions, the linear ranges of determination were 5-1000 μg/L (that of spermine was 8-1000 μg/L). Correlation coefficients ranged from 0.9955 to 0.9987. The limits of detection were 0.11-5.5 μg/L. The matrix effect and analytical performance of the present method were evaluated, and the eight BAs were analyzed by this method in 19 samples, indicating the potential pollution of BAs in chili oil Pixian Douban.

In situ derivatization of Au nanoclusters via aurophilic interactions of a triphenylphosphine gold(i) salt with neurotransmitters and their rapid MALDI-TOF-MS detection in mice brain tissue extracts

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) has attracted much attention for the detection of small molecules such as neurotransmitters due to its softness, high sensitivity, extensive compatibility and diverse mass analyzers. However, it has been really a difficult challenge to develop a highly specific organic compound as a matrix for the rapid, sensitive and selective detection of neurotransmitters. Herein, we report tris(triphenylphosphine)gold oxonium tetrafluoroborate ([Ph₃PAu]₃O⁺BF₄^-) for the first time as an efficient matrix for the rapid and simultaneous MALDI-MS detection of neurotransmitters. [Ph₃PAu]₃O⁺BF₄^- facilitates the in situ derivatization of gold nanoclusters (Au NCLs) during the interaction with neurotransmitters, which increases their ionization energy by absorbing more ultra-violet (UV) radiation during MALDI-TOF-MS detection. The results show that this [Ph₃PAu]₃O⁺BF₄^- matrix can exhibit a 10-fold faster response time compared to previously reported pyrylium matrices. In addition, [Ph₃PAu]₃O⁺BF₄^- can also provide the simultaneous derivatization of various neurotransmitters, including dopamine (DA), noradrenaline (NAd), serotonin (5-HT), γ-aminobutyric acid (GABA), histamine (H) and tyramine (TY), in mice brain tissue extracts, which can be detected in the MALDI-TOF-MS spectra.

Validated HPLC-MS/MS method for quantification of ethylmethylhydroxypyridine succinate in rat brain and its application to a pharmacokinetic study

2-ethyl-6-methyl-3-hydroxypyridine (EMHP) succinate is the original antioxidant and antihypoxic drug commonly prescribed in Russia. The objective of this study was to develop a rapid, simple and sensitive high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) method for EMHP quantitation in rat brain tissue with the use of a bead beating homogenizer. The comparison between two approaches to brain tissue preparation was performed, when spiking the blank brain tissue with EMHP reference standard and internal standard (IS) before and after homogenization step. Chromatographic separation was achieved using Zorbax Eclipse Plus C18 column (1.8 μm, 2.1 × 50 mm) and elution was performed with the mobile phase, consisting of 10 mM of ammonium formate aqueous solution with 0.1% formic acid as solvent A and 0.1% formic acid in methanol as solvent B [44%(А):56%(В), v/v]. Flow rate was 0.4 mL/min and the total run time for each sample analysis was 2.0 min. EMHP and amantadine, IS of this study, were analyzed in positive ionization mode. Ion transitions of m/z 138.0 → 123.0 for EMHP and m/z 152.0 → 135.0 for amantadine were selected in multiple reaction monitoring mode. The developed method for EMHP determination in rat brain samples was validated for selectivity, linearity, accuracy, precision, matrix effects, and stability. The lower and upper limits of quantification were determined to be 1 and 1500 ng/g, respectively. The developed and validated HPLC-MS/MS method was successfully applied to determine EMHP concentrations in rat brain tissue following the intraperitoneal administration at a dose of 3.4 mg/kg.

Analysis of amino acids using serially coupled columns

Single conventional columns in reversed-phase liquid chromatography are insufficient for analysing the isoindoles of primary amino acids due to their limited functionality. An interesting possibility for increasing the separation power is the combination of several columns of different nature, where the length is modified by coupling small segments. This approach may require a considerable investment to have multiple lengths for each stationary phase. However, the combination of only two columns of fixed length can be enough to resolve satisfactorily relatively complex mixtures, provided that an optimised gradient program is applied. In this work, a mixture of 19 primary amino acid isoindoles found in proteins was analysed. Four stationary phases were assayed: C18, pentafluorophenyl-C18, C4 and cyano. The mixture of isoindoles was successfully resolved in practical times using a pentafluorophenyl-C18 column coupled to a C4 column, in spite of the extremely poor performance obtained when each column is used isolatedly, independently of the length. The extreme diversity in the polarities of the isoindoles and the need of extrapolating the retention behaviour in certain regions of the solvent content domain makes the modelling of the retention behaviour of the isoindoles particularly difficult. Nevertheless, the predicted optimal separations were very satisfactory.

Advantages and Pitfalls of Mass Spectrometry Based Metabolome Profiling in Systems Biology

Mass spectrometry-based metabolome profiling became the method of choice in systems biology approaches and aims to enhance biological understanding of complex biological systems. Genomics, transcriptomics, and proteomics are well established technologies and are commonly used by many scientists. In comparison, metabolomics is an emerging field and has not reached such high-throughput, routine and coverage than other omics technologies. Nevertheless, substantial improvements were achieved during the last years. Integrated data derived from multi-omics approaches will provide a deeper understanding of entire biological systems. Metabolome profiling is mainly hampered by its diversity, variation of metabolite concentration by several orders of magnitude and biological data interpretation. Thus, multiple approaches are required to cover most of the metabolites. No software tool is capable of comprehensively translating all the data into a biologically meaningful context yet. In this review, we discuss the advantages of metabolome profiling and main obstacles limiting progress in systems biology.

A novel route to recognizing quaternary ammonium cations using electrospray mass spectrometry

Characterizing and elucidating structures is a commonplace and necessary activity in the pharmaceutical industry with mass spectrometry and NMR being the primary tools for analysis. Although many functional groups are readily identifiable, quaternary ammonium cations have proven to be difficult to unequivocally identify using these techniques. Due to the lack of an N-H bond, quaternary ammonium groups can only be detected in the (1)H NMR spectra by weak signals generated from long-range (14)N-H coupling, which by themselves are inconclusive evidence of a quaternary ammonium functional group. Due to their low intensity, these signals are frequently not detected. Additionally, ions cannot be differentiated in a mass spectrum as an M(+) or [M + H](+) ion without prior knowledge of the compound's structure. In order to utilize mass spectrometry as a tool for determining this functionality, ion cluster formation of quaternary ammonium cations and non-quaternary amines was studied using electrospray ionization. Several mobile phase modifiers were compared; however, the addition of small amounts of trifluoroacetic acid proved superior in producing characteristic and intense [M +2TFA](-) clusters for compounds containing quaternary ammonium cations when using negative electrospray. By fragmenting this characteristic ion using CID, nearly all compounds studied could be unambiguously identified as containing a quaternary ammonium cation or a non-quaternary amine attributable to the presence (non-quaternary amine) or absence (quaternary ammonium cation) of the resulting [2TFA + H](-) ion in the product spectra. This method of analysis provides a rapid, novel, and reliable technique for indicating the presence of quaternary ammonium cations in order to aid in structural elucidation.

Towards a generic variable column length method development strategy for samples with a large variety in polarity

The development of a novel set-up for the sequential analysis of compounds with a large variety in polarity on HILIC and reversed-phase columns, coupled in series, is discussed. For this purpose, a commercially available ultra-high performance LC system, equipped with two switching valves is employed. The switching valves allow connecting the HILIC and reversed-phase columns either in series or in parallel to the system. An interface to couple the HILIC and reversed-phase columns is developed and optimized. The sample is first injected onto a HILIC column. Apolar compounds in the sample are not retained and will elute close to or within the void volume of the HILIC column. Accurate switching of the valves allows redirecting these compounds towards a trap loop while more polar compounds are retained and separated on the HILIC column. After separation and detection of the polar compounds, the configuration of the valves is switched again to direct the apolar compounds from the trap loop towards a reversed-phase column for separation. To deal with the incompatibility of the mobile phases of HILIC and reversed-phase column separations, commercially available Jet weaver mixers are included in the set-up to allow for an intermediate solvent exchange. The proof-of-concept is demonstrated for the analysis of pharmaceuticals that can be found in waste water and surface water. It is demonstrated that the set-up provides robust analyses with peak capacities that are intermediate to one-dimensional and two-dimensional separations.

Altered neurochemical levels in the rat brain following chronic nicotine treatment

Converging evidence shows that neurochemical systems are crucial mediators of nicotine dependence. Our present study evaluates the effect of 3-month chronic nicotine treatment on the levels of multiple quaternary ammonium compounds as well as glutamate and gamma aminobutyric acid in the rat prefrontal cortex, dorsal striatum and hypothalamus. We observed a marked decrease of acetylcholine levels in the dorsal striatum (22.88%, p<0.01), reflecting the impact of chronic nicotine in local interneuron circuits. We found decreases of carnitine in the dorsal striatum and prefrontal cortex (19.44%, p<0.01; 13.58%, p<0.01, respectively), but robust enhancements of carnitine in the hypothalamus (26.59%, p<0.01), which may reflect the alterations in food and water intake during chronic nicotine treatment. Finally, we identified an increase of prefrontal cortex glutamate levels (8.05%, p<0.05), supporting previous studies suggesting enhanced prefrontal activity during chronic drug use. Our study shows that quaternary ammonium compounds are regulated in a highly brain region specific manner during chronic nicotine treatment, and provides novel insights into neurochemical regulation during nicotine use.

Separation of carnitine and acylcarnitines in biological samples: a review

Carnitine and its acylesters are a family of compounds that can be used in the early diagnosis of many diseases. Carnitine and acylcarnitines have a crucial role in fatty acid transportation. The increased level of free carnitine, total carnitine, or the acylesters can act as biomarkers for many metabolic disorders, including diabetes, encephalopathy and cardiomyopathy. The determination of these compounds is difficult owing to the simple aliphatic structure, the chiral center and the permanent positive charge. Although MS detection can be enough to differentiate between some carnitine derivatives, closely related structural isomers of the acylcarnitines must be separated before detection because they form the same base peak and second most abundant ion peak. Different separation methods are discussed in this review, including reversed-phase, hydrophilic interaction, ion exchange, ion pairing, mixed mode liquid chromatography, gas chromatography and electrophoresis. Representative example chromatograms are shown. The sample preparation and the different derivatization reactions are also covered. A table that summarizes the most important analytical methods by detailing the analyte mixture, the sample matrix, the separation mode and the detection method is provided.

Serial coupling of RP and zwitterionic hydrophilic interaction LC-MS: suspects screening of diclofenac transformation products by oxidation with a boron-doped diamond electrode

The presence of pollutants and their transformation products (TPs) in the water system is a big concern because of possible adverse effects on the aquatic environment. Their identification is still a challenge that requires the combination of different chromatographic techniques. In the current research, serial coupling of RPLC and zwitterionic hydrophilic interaction LC with TOF-MS was investigated as a single separation technique for the screening of suspected TPs from electrochemical oxidation of diclofenac using a boron-doped diamond electrode. Diclofenac oxidation was performed in three water matrices in order to study its transformation in different chemical contexts. 47 TPs resulting from similar oxidation methods were selected from the literature. As in most cases standards were not available, an identification procedure based on accurate mass data and chromatographic behavior was proposed. According to this procedure, 11 suspected TPs, previously analyzed by LC, GC, or ion chromatography, were detected in a single injection. The method was proved to be reliable and versatile and it could be efficiently employed as a comprehensive analytical tool for the simultaneous analysis of compounds in a wide polarity range.

Serial coupling of reversed-phase and zwitterionic hydrophilic interaction LC/MS for the analysis of polar and nonpolar phenols in wine

In the present study, an easy and efficient method based on the serial coupling of analytical reversed-phase and zwitterionic hydrophilic interaction liquid chromatography was developed for the simultaneous separation of polar and nonpolar phenols occurring in wine. The zwitterionic hydrophilic column was connected in series to the reversed-phase one via a T-piece, with which the ACN content in eluent of the second dimension was increased, in order to cope the solvent strength incompatibility between the two columns. The final mobile phase at low-flow rate (≤0.5 mL/min), high-ACN content (90%), and low-salt concentration was directed to an ESI-TOF-MS , for high accurate mass detections. The developed method was applied for the identification of target phenols in several wines. Retention time and peak width intra- and interday repeatability studies proved the reliability of the method for the simultaneous analysis of all the polar and nonpolar analytes in wine. The serial reversed-phase/zwitterionic hydrophilic interaction liquid chromatography coupling offered the possibility to enlarge the number of identified compounds and it represents a valid approach for nontarget analysis of complex samples by a single injection.