Capillary electrophoresis sample volume/concentration

Source Adapted from Baker, D. R. Capillary Electrophoresis. Wiley-Interscience New York, 1995. "Concentration depends on the volume of sample injected. 

Microdialysis is a sampling technique that must be coupled with an analytical method to identify and quantify chemical components of the dialysate. The samples can be analyzed immediately upon collection (i.e., online), or they can be stored (—80°C) for future analysis. Only analytical techniques sensitive enough to measure both small sample volumes and low concentrations of substances can be used to measure compounds in dialysate samples. High-performance liquid chromatography (HPLC) or capillary electrophoresis (CE) combined with ultraviolet (UV), electrochemical (EC), or laser-induced fluorescence (LIF)... 

EC detection is a promising alternative for capillary electrophoresis microchips due to its inherent characteristics, allowing a proper miniaturisation of the devices and compatibility with the fabrication processes, in case of an integrated detection. Moreover, the low cost associated permit the employment of disposable elements. As the EC event occurs on the surface of electrodes and the decrease in size usually results in new advantages (see Chapter 32), the possibilities of incorporating EC detectors are broad. The simplicity of the required instrumentation, portable in many cases, suit well with the scaling-down trend. Moreover, as the sample volume in conventional micro-channel devices is less than 1 nL, a very highly sensitive detector should be constructed to analyse even modest concentrations of sample solutions. Since sensitivity is one of the accepted characteristics of EC detection EC-CE microchips approach to the ideal analytical devices. 

Capillary electrophoresis has been demonstrated to be useful in monitoring the identity and purity of hGH. CZE is capable of discrimination between hGH and several closely related impurities and degradation products, either as the intact species (previous work) or as the trypsin digests (this work). Thus, it is an important adjunct to conventional methods, such as RP-HPLC of digests or conventional electrophoresis of intact proteins, for the identification of hGH. CZE possesses adequate sensitivity to monitor minor impurities it is comparable to RP-HPLC with respect to linearity and precision. Samples with a volume of at least 10 nanoliters will provide acceptable precision those that contain an internal standard will provide the best precision. The peak area response is linear it is possible to extend linearity by increasing concentration as long as the contribution of the sample and its matrix to field inhomogeneities is minimized. The work shows that CZE has potential to be useful in the quality control of proteins such as hGH. 

The preponderance of analytical (and preparative) separations that precede mass spectrometric analyses are carried out using HPLC. Despite its potential for extremely high efficiencies, capillary electrophoresis (CE) remains something of a specialized application, particularly with mass spectrometric detection. The technique is hampered by its reliance on small sample volumes (typically nL injections) in order to avoid loss of separation efficiency. The consequence of these small volumes is a concentration detection limit that is substantially higher than that of techniques such as HPLC. Despite the exquisite sensitivity of mass spectrometers, characterization of related substances present at low femtomole levels (assuming a hypothetical related substance compound with molecular weight 500 daltons present at 0.1% w/w in an equally hypothetical drug substance sample... 

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