Ultrasensitive on-column detection of capillary electrophoresis using laser-induced capillary vibration method

Abstract

Capillary electrophoresis (CE), with its advantage of high resolution power, has attracted much attention. But CE has another advantage of needing only a small sample volume, which is expected to make it a useful tool for ultramicroscale analysis as well. However, conventional absorbance detectors are not sensitive enough because the optical light-path is that of the diameter of a capillary, which is several tens of microns. A highly sensitive detector is indispensable to application of CE to ultramicroscale analysis. Several sensitive detection methods have been developed, such as laser-induced fluorometry (LIF), electrochemical detection, mass spectrometry, etc. To make the best use of the high separation efficiency of CE, an on-column detection scheme, which can easily reduce the detection volume needed by using a focused laser beam as a probing beam, seems to be more preferable than other detection schemes including off-column or end-column detection schemes, and LIF seems to be more suitable than electrochemical detection and mass spectrometric detection. However, LIF cannot be applied to nonfluorescent analytes, and fluorescent derivatization is usually needed, which is not suitable for ultramicroscale analysis. Laser-induced photothermal spectrometry is another highly sensitive on-column detection approach, and differing from LIF, nonfluorescent species are also detectable. As a kind of photothermal effect, we found the capillary vibration induced by laser (CVL) effect [1], and proposed it as a highly sensitive on-column detection method of CE [2]. Like other photothermal detection methods, the CVL method is based on light absorption by a sample and successive heat generation due to nonradiative relaxation. This heat changes the capillary tension. Because the excitation laser beam is periodically irradiated onto the capillary, the tension fluctuation occurs periodically, resulting in vibration of the capillary. The amplitude of the capillary vibration has been found to be proportional to sample concentration. Therefore, quantitative analysis can be performed by detecting the amplitude of the capillary vibration.