Capillary isoelectric focusing-electrospray ionization time-of-flight mass spectrometry for protein analysis

Capillary electrophoresis–mass spectrometry for the analysis of intact proteins

Developments in the fields of protein chemistry, proteomics and biotechnology have increased the demand for suitable analytical techniques for the analysis of intact proteins. In 1989, capillary electrophoresis (CE) was combined with mass spectrometry (MS) for the first time and its potential usefulness for the analysis of intact (i.e. non-digested) proteins was shown. This article provides an overview of the applications of CE-MS within the field of intact protein analysis. The principles of the applied CE modes and ionization techniques used for CE-MS of intact proteins are shortly described. It is shown that separations are predominantly carried out by capillary zone electrophoresis and capillary isoelectric focusing, whereas electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) are the most popular ionization techniques used for interfacing. The combination of CE with inductively coupled plasma (ICP) MS for the analysis of metalloproteins is also discussed. The various CE-MS combinations are systematically outlined and tables provide extensive overviews of the applications of each technique for intact protein analysis. Selected examples are given to illustrate the usefulness of the CE-MS techniques. Examples include protein isoform assignment, single cell analysis, metalloprotein characterization, proteomics and biomarker screening. Finally, chip-based electrophoresis combined with MS is shortly treated and some of its applications are described. It is concluded that CE-MS represents a powerful tool for the analysis of intact proteins yielding unique separations and information.

CE-based analysis of hemoglobin and its applications in clinical analysis

This review focuses on the developments and trends in CE including CIEF, CZE, MEKC, two-dimensional conjunction of CIEF-capillary gel electrophoresis, and MEKC-CZE on microfluidic devices coupled to different detection approaches, such as UV absorbance, LIF, MS, and chemiluminescence etc. for performing analysis of hemoglobin (Hb), also with an emphasis on its applications in clinical analysis. Analysis of human Hb is of important clinical sense for numerous hemoglobinopathies associated with the congenital defects and abnormal contents of Hb. The diversiform modes render CE a comprehensive primary clinical tool for Hb analysis, which is rapid, sensitive, high-resolution, and not labor-intensive.

Advantages and limitations of coupling isotachophoresis and comprehensive isotachophoresis-capillary electrophoresis to time-of-flight mass spectrometry

Capillary isotachophoresis (ITP) and comprehensive isotachophoresis-capillary electrophoresis (ITP-CE) were successfully coupled to electrospray ionization (ESI) orthogonal acceleration time-of-flight mass spectrometry (TOF-MS) using angiotensin peptides as model analytes. The utility of ITP-TOF-MS and ITP-CE-TOF-MS for the analysis of samples containing analyte amounts sufficient to form flat-top ITP zones (30 microM) as well as for samples with trace analyte amounts (0.3 microM) was studied. Separations were performed in 150 microm internal diameter (I.D.) capillaries for the ITP experiments, and in 200 microm I.D. (ITP) and 50 microm I.D. (CE) capillaries for ITP-CE experiments. The fused-silica columns were coated with poly(vinyl alcohol) to suppress electroosmotic flow that can disrupt ITP zone profiles. The sample loading capacity in both ITP and comprehensive ITP-CE was greatly enhanced (up to 10 microl) compared with typical nanoliter-sized injection volumes in CE. It was concluded that ITP-TOF-MS alone was adequate for the separation and detection of high concentration samples. The outcome was different at lower analyte concentrations where mixed zones or very sharp peaks formed. With formation of mixed zones, ion suppression and discrimination could occur, complicating quantitative determination of the analytes. This problem was effectively overcome by inserting a CE capillary between the ITP and TOF-MS. In such an arrangement, samples were preconcentrated in the high load WTP capillary and then injected into a CE capillary where they were separated into non-overlapping peaks prior to their detection by TOF-MS. The advantage of this comprehensive arrangement, which we have described previously, is that there is no need to discard portions of the sample in order to avoid overloading of the CE capillary. The whole sample is analyzed by multiple injections from ITP to CE. Thus, this method can be used for the analysis of complex samples with wide ranges of component concentrations.

Stepwise mobilization of focused proteins in capillary isoelectric focusing mass spectrometry

A stepwise mobilization strategy has been developed for the elution of complex protein mixtures, separated by capillary isoelectric focusing (CIEF) for detection using on-line electrospray ionization mass spectrometry (ESI-MS). Carrier polyampholytes are used to establish a pH gradient as well as to control the electroosmotic flow arising from the use of uncoated fused-silica capillaries. Elution of focused protein zones is achieved by controlling the mobilization pressure and voltage, leaving the remaining protein zones focused inside the capillary. Protein zones are stepwise eluted from the capillary by changing the mobilization conditions. Stepwise mobilization improves separation resolution and simplifies coupling with multistage MS (i.e., MSn) analysis since it allows more effective temporal control of protein elution from the CIEF capillary. We also describe a modified configuration for coupling CIEF with ESI-MS using a coaxial sheath flow interface that facilitate the automation of on-line CIEF-ESI-MS analyses. The stepwise mobilization strategy is demonstrated for the analysis of standard protein mixtures and soluble E. coli lysate proteins using CIEF-ESI-MS. These results indicate that inlet pressure or voltage programming to control the elution of the protein zones from the capillary (i.e., gradient mobilization) may allow for the optimization of the mobilization conditions and provide higher resolution for CIEF separation of complex mixtures with on-line MS.