Capillary electrophoresis–mass spectrometry
Since its introduction in 1987, new developments and application has made CEMS powerful separation and identification technique. Use of CEMS has increased for protein and peptides analysis and other biomolecules. Understanding of CE, the interface setup, ionization technique and mass detection system is important to tackle problems while coupling capillary electrophoresis to mass spectrometry.
The original interface between capillary zone electrophoresis and mass spectrometry was developed in 1987 by Richard D. Smith and coworkers at Pacific Northwest National Laboratory, and who also later were involved in development of interfaces with other CE variants, including capillary isotachophoresis and capillary isoelectric focusing.
Interfacing CE with MS
Capillary electrophoresis is a separation technique which uses high electric field to produce electroosmotic flow for separation of ions. Analytes migrate from one end of capillary to other based on their charge, viscosity and size. Higher the electric field, greater is the mobility. Mass spectrometry is an analytical technique that identifies chemical species depending on their mass-to-charge ratio. During the process, an ion source will convert molecules coming from CE to ions that can then be manipulated using electric and magnetic field. The separated ions are then measured using a detector. The major problem faced when coupling CE to MS arises due to insufficient understanding of fundamental processes when two techniques are interfaced. The separation and detection of analytes can be improved with better interface. CE has been coupled to MS using various ionization techniques like FAB, ESI, MALDI, APCI and DESI. The most used ionization technique is ESI.
Electrospray ionization interface
The first CE-MS interface had cathode end of CE capillary terminated within a stainless steel capillary. An electrical contact was made at that point completing the circuit and initiating the electrospray. This interface system had few drawbacks like mismatch in the flow rates of two systems. Since then, interface system has been improved to have continuous flow rate and good electrical contact. At present, three types of interface system exist for CE/ESI-MS which are discussed briefly.
CE capillary is coupled directly to ionization source in sheathless interface system. The capillary is coated with conducting metal or gold or platinum wire is inserted into CE capillary to establish suitable electrical connection. Since no sheath liquid is used, the system has high sensitivity, low flow rates and minimum background. However, right choice of buffer solution has to be made which is suitable for both CE separation and ESI operation.
Sheath flow interface
Liquid junction interface
This technique uses a stainless steel tee to mix separation electrolyte from CE capillary with make up liquid. The CE capillary and ESI needle are inserted through opposite sides of the tee and a narrow gap is maintained. The electrical contact is established by makeup liquid surrounding the junction between two capillaries. This system is easy to operate. However, the sensitivity is reduced and the mixing of two liquids could degrade separation.
Continuous- flow fast atom bombardment
Miniard et al. first coupled FAB to CE in 1988. The main problem of this interface is the flow rate between the two systems. The CF-FAB needs very high flow rate but CE need very minimal flow rate for better separation. So a make up flow is introduced using the designs discussed above: sheath flow or liquid junction. CF/FAB-MS is not commonly used as CE/ESI-MS, as FAB is rarely used anymore.
Coupling CE with MALDI-MScitation needed] The sample from CE is mixed with matrix coming though another capillary. As the ball rotates the sample is dried before it reaches ionization region. This technique has high sensitivity since no makeup fluid is used.
CE-MS ability to separate analytes present in extremely low concentration with high efficiency at high speed has made it applicable in all fields of science. CE-MS has been used for bioanalytical, pharmaceuticals, environmental and forensic application. The major application of CEMS has been for biological studies, mostly for protein and peptide analysis. Along with that, it is used often for routine analysis of pharmaceutical drugs. There are number of studies reporting characterization of mixtures of peptides and proteins.CE-MS can be used for routine clinical checkup. Body fluids like blood and urine have been analyzed with CE-MS to identify biomarkers for renal diseases and cancer.
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