This review presents an overview of the dynamically developing field of mass spectrometry-based metabolomics. Metabolomics aims at the comprehensive and quantitative analysis of wide arrays of metabolites in biological samples. These numerous analytes have very diverse physico-chemical properties and occur at different abundance levels. Consequently, comprehensive metabolomics investigations are primarily a challenge for analytical chemistry and specifically mass spectrometry has vast potential as a tool for this type of investigation. Metabolomics require special approaches for sample preparation, separation, and mass spectrometric analysis. Current examples of those approaches are described in this review. It primarily focuses on metabolic fingerprinting, a technique that analyzes all detectable analytes in a given sample with subsequent classification of samples and identification of differentially expressed metabolites, which define the sample classes. To perform this complex task, data analysis tools, metabolite libraries, and databases are required. Therefore, recent advances in metabolomics bioinformatics are also discussed.

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Mass spectrometry-based metabolomics.

Validation of new methods.

Separation of polar compounds on polar stationary phases with partly aqueous eluents is by no means a new separation mode in LC. The first HPLC applications were published more than 30 years ago, and were for a long time mostly confined to carbohydrate analysis. In the early 1990s new phases started to emerge, and the practice was given a name, hydrophilic interaction chromatography (HILIC). Although the use of this separation mode has been relatively limited, we have seen a sudden increase in popularity over the last few years, promoted by the need to analyze polar compounds in increasingly complex mixtures. Another reason for the increase in popularity is the widespread use of MS coupled to LC. The partly aqueous eluents high in ACN with a limited need of adding salt is almost ideal for ESI. The applications now encompass most categories of polar compounds, charged as well as uncharged, although HILIC is particularly well suited for solutes lacking charge where coulombic interactions cannot be used to mediate retention. The review attempts to summarize the ongoing discussion on the separation mechanism and gives an overview of the stationary phases used and the applications addressed with this separation mode in LC.

Hydrophilic interaction chromatography.

Hydrophilic interaction liquid chromatography (HILIC) provides an alternative approach to effectively separate small polar compounds on polar stationary phases. The purpose of this work was to review the options for the characterization of HILIC stationary phases and their applications for separations of polar compounds in complex matrices. The characteristics of the hydrophilic stationary phase may affect and in some cases limit the choices of mobile phase composition, ion strength or buffer pH value available, since mechanisms other than hydrophilic partitioning could potentially occur. Enhancing our understanding of retention behavior in HILIC increases the scope of possible applications of liquid chromatography. One interesting option may also be to use HILIC in orthogonal and/or two-dimensional separations. Bioapplications of HILIC systems are also presented.

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Hydrophilic interaction liquid chromatography (HILIC)—a powerful separation technique

There is an advantage for users of electrospray and nanospray mass spectrometry to have an understanding of the processes involved in the conversion of the ions present in the solution to ions in the gas phase. The following processes are considered: Creation of charge droplets at the capillary tip; Electrical potentials required and possibility of gas discharges; Evolution of charged droplets, due to solvent evaporation and Coulomb explosions, to very small droplets that are the precursors of the gas phase ions; Production of gas phase ions from these droplets via the Ion Evaporation and Charge residue models; Analytical uses of ESIMS of small ions, qualitative and quantitative analysis; Effects of the ESI mechanism on the analysis of proteins and protein complexes; Determination of stability constants of protein complexes; Role of additives such as ammonium acetate on the observed mass spectra.

Electrospray: From ions in solution to ions in the gas phase, what we know now

Bioanalytical liquid chromatography tandem mass spectrometry methods on underivatized silica columns with aqueous/organic mobile phases.

Simple means to alleviate sensitivity loss by trifluoroacetic acid (TFA) mobile phases in the hydrophilic interaction chromatography–electrospray tandem mass spectrometric (HILIC–ESI/MS/MS) bioanalysis of basic compounds

Novel liquid chromatographic–tandem mass spectrometric methods using silica columns and aqueous–organic mobile phases for quantitative analysis of polar ionic analytes in biological fluids

Simultaneous assay of morphine, morphine-3-glucuronide and morphine-6-glucuronide in human plasma using normal-phase liquid chromatography-tandem mass spectrometry with a silica column and an aqueous organic mobile phase

Swift growth in the use of LC-MS/MS for the analysis of drugs in biological matrices has been compelled by the need for timely and high-quality data at many stages in drug discovery and development process: from high throughput screening of drug candidates and rapid data generation for pre-clinical studies to almost 'real-time' analysis of clinical samples. Prompt and rational method development, validation, and transfer play a pivotal role in achieving the goals of "faster, better, and cheaper" for pharmacokinetic studies since this could easily account for more than 50% of the time and labor resources for a moderate-sized project. Strategy for rational method development, validation and transfer has been largely kept as institutional knowledge but rarely appeared in literature. In this review article, strategies for developing and validating robust high throughput LC-MS/MS methods will be critically reviewed and discussed. Automated sample preparation, fast chromatography, minimization of matrix effects, and strategy of narrowing the gap between validation and incurred sample analysis are just a few topics covered in this review. Other interesting approaches for improving method efficiency and ruggedness such as direct injection SPE and liquid/liquid extracts as well as multiplexing of LC columns will also be discussed. Potential pitfalls during method development and validation are pointed out. At the end, the question "how fast is fast enough and how fast is too fast?" will be answered after considering all aspects of the method development and validation.

Weng Naidong

Papers

Bioanalytical liquid chromatography tandem mass spectrometry methods on underivatized silica columns with aqueous/organic mobile phases.

Simple means to alleviate sensitivity loss by trifluoroacetic acid (TFA) mobile phases in the hydrophilic interaction chromatography–electrospray tandem mass spectrometric (HILIC–ESI/MS/MS) bioanalysis of basic compounds

Novel liquid chromatographic–tandem mass spectrometric methods using silica columns and aqueous–organic mobile phases for quantitative analysis of polar ionic analytes in biological fluids

Simultaneous assay of morphine, morphine-3-glucuronide and morphine-6-glucuronide in human plasma using normal-phase liquid chromatography-tandem mass spectrometry with a silica column and an aqueous organic mobile phase

Critical Review of Development, Validation, and Transfer for High Throughput Bioanalytical LC-MS/MS Methods