A sheathless interface making use of a porous tip has been used for coupling capillary electrophoresis and electrospray ionization mass spectrometry. First, effective flow rates using the interface have been characterized. It was found that the interface is capable of generating a stable spray with flow rates ranging from below 10 nL/min to >340 nL/min, enabling its use in either the mass or concentration-sensitive region of the electrospray process. Subsequently, by analyzing peptide mixtures of increasing complexity, we have demonstrated that this platform provides exquisite sensitivity enabling the detection of very low amounts of materials with very high resolving power. Transient isotachophoresis (t-ITP) can also be integrated with this setup to increase the mass loading of the system while maintaining peak efficiency and resolution. Concentration limits of detection in the subnanomolar or nanomolar range can be achieved with or without t-ITP, respectively. The application of a vacuum at the inlet of the separation capillary further allowed the peak capacity of the system to be improved while also enhancing its efficiency. As a final step in this study, it was demonstrated that the intrinsic properties of the interface allows the use of coated noncharged surfaces so that very high peak capacities can be achieved.

An overview of the use of CE-MS in the field of metabolomics is provided. Metabolomics is concerned with the comprehensive analysis of endogenous low-molecular-weight compounds in biological samples. CE-MS has demonstrated to be a powerful technique for the profiling of polar metabolites in biological samples. This review covers the use of various CE separation modes, capillary coatings, MS analyzers, sample preparation techniques, and data analysis methods used in CE-MS for metabolomics. The applicability of CE-MS in metabolomics research is illustrated by giving examples of the analysis of bacterial extracts, plant extracts, urine, plasma, and cerebrospinal fluid samples. The relevant CE-MS metabolomics studies published between 2000 and 2008 are presented in tabular form, including information on sample type and pretreatment and MS detection mode. Future developments with regard to the use of alternative ionization techniques, the use of coupled separation systems and the potential of microchip CE systems for metabolomics are discussed.

Over the last two decades, coupled capillary electrophoresis (CE)-mass spectrometry (MS) has developed into a generally accepted technique with a wide applicability. A growing number of CE-MS applications make use of capillaries where the internal wall is modified with surface coating agents. In CE-MS, capillary coatings are used to prevent analyte adsorption and to provide appropriate conditions for CE-MS interfacing. This paper gives an overview of the various capillary coating strategies used in CE-MS. The main attention is devoted to the way coatings can contribute to a proper CE-MS operation. The foremost capillary coating methods are discussed with emphasis on their compatibility with MS detection. The role of capillary coatings in the control of the electroosmotic flow and the consequences for CE-MS coupling are treated. Subsequently, an overview of reported applications of CE-MS employing different coating principles is presented. Selected examples are given to illustrate the usefulness of the coatings and the overall applicability of the CE-MS systems. It is concluded that capillary coatings can enhance the performance and stability of CE-MS systems, yielding a highly valuable and reproducible analytical tool.

Sheathless capillary electrophoresis-mass spectrometry (CE-MS), using a porous tip sprayer, is proposed for the first time for highly sensitive metabolic profiling of human urine. A representative metabolite mixture and human urine were used for evaluation of the sheathless CE-MS platform. For test compounds, relative standard deviations (RSDs) for migration times and peak areas were below 2% and 12%, respectively, and an injection volume of only ∼8 nL resulted in detection limits between 11 and 120 nM. Approximately 900 molecular features were detected in human urine by sheathless CE-MS whereas about 300 molecular features were found with classical sheath-liquid CE-MS. This difference can probably be attributed to an improved ionization efficiency and increased sensitivity at low flow-rate conditions. The integration of transient-isotachophoresis (t-ITP) as an in-capillary preconcentration procedure in sheathless CE-MS further resulted in subnanomolar limits of detection for compounds of the metabolite mixture, and more than 1300 molecular features were observed in urine. Compared to the classical CE-MS approaches, the integration of t-ITP combined with the use of a sheathless interface provides up to 2 orders of magnitude sensitivity improvement. Hence, sheathless CE-MS can be used for in-depth metabolic profiling of biological samples, and we anticipate that this approach will yield unique information in the field of metabolomics.

Metabolomics provides a direct functional read-out of the physiological status of an organism and is in principle ideally suited to describe someone's health status. Whereas only a limited number of small metabolites are used in the clinics, in inborn errors of metabolism an extensive repertoire of metabolites are used as biomarkers. We discuss that the proper clinical phenotyping is crucial to find biomarkers and obtain biological insights for multifactorial diseases. This requires to study the phenotype dynamics including the concepts of homeostasis and allostasis, that is, the ability to adapt and cope with a challenge. We also elaborate that biology-driven metabolomics platforms (i.e. development of metabolomics technology driven by the need of studying and answering important biomedical questions) addressing clinically relevant pathways and at the same time providing absolute concentrations are key to allow discovery and validation of biomarkers across studies and labs. Following individuals over years will require high throughput metabolomics approaches, which are emerging for nuclear magnetic resonance spectroscopy and direct-infusion mass spectrometry, but should also include the biochemical networks needed for personalized health monitoring.

In the field of metabolomics, CE-MS is now regarded as a useful complementary analytical technique for the profiling of (highly) polar ionogenic metabolites in biological samples. Over the past few years, significant advancements have been made in CE-MS approaches for metabolic profiling studies. This paper, which is a follow-up of three previous review papers covering the years 2000-2012 [Electrophoresis 2009, 30, 276-291; Electrophoresis 2011, 32, 52-65; Electrophoresis 2013, 34, 86-98], provides an update of these developments covering the scientific literature from July 2012 to June 2014. Attention will be paid to novel interfacing techniques for coupling CE to MS and their implications for metabolomics studies. The potential of CEC-MS and MEKC-MS are also considered, and CE-MS systems for high-throughput metabolic profiling are discussed. The applicability of CE-MS for metabolomics studies is demonstrated by representative examples in the fields of biomedical, clinical, microbial, plant, environmental, and food metabolomics. An overview of recent CE-MS-based metabolomics studies is given in a table, which provides information on sample type and pretreatment, capillary coatings, and MS detection mode. Finally, general conclusions and perspectives are given.

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