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Swiss researchers have demonstrated how conductivity detection can benefit medical monitoring, by developing a quick and sensitive method for measuring levels of a neurological drug in biological fluids. Valproic acid (2-propylvaleric acid) is a small ionic molecule that can raise levels of the neurotransmitter gamma-aminobutyric acid (GABA) in the human brain. As such, it is widely used to treat a variety of neurological disorders, including epilepsy, bipolar disorder (manic-depression) and migraine headaches. In addition, medical researchers have recently discovered that it can help prevent the human immunodeficiency virus (HIV) from infecting human cells and so it may also find use as a treatment for AIDS. Like most drugs, valproic acid can cause a variety of side effects, such as weight gain, fatigue, nausea and even hair loss, and the severity of these side effects depends on the size of the dose. It's therefore important to monitor levels of valproic acid in patients' biological fluid on a regular basis. This monitoring is made difficult by the fact that valproic acid doesn't fluoresce or absorb ultra-violet light. So most of the various chromatographic and electrophoretic techniques that have been developed to detect valproic acid have had to chemically modify the molecule, in order to make it detectable. This led Gamze Belin from the University of Geneva to wonder whether a newly developed conductivity detection technique for use with capillary electrophoresis (CE), known as capacitively coupled contactless conductivity detection (C4D), offered a better alternative. 'The C4D setup is composed of two cylindrical electrodes outside the capillary,' explains Belin. 'When an analyte with a different conductivity from that of the background passes through the detector, a change in the [electrical] signal is measured. The higher the conductivity differences between the analyte and the background co-ions, the larger the detector response.' To adapt this CE-C4D technique to detect valproic acid, the researchers first had to reverse the direction of the electroosmotic flow (EOF) by adding hexadecyltrimethylammonium bromide (HTAB) to the separation buffer. This was necessary because valproic acid naturally migrates towards the positive electrode, which means it would normally be battling against the EOF. They also added the solvent acetonitrile to the samples of biological fluid, in order to ensure that valproic acid could be easily distinguished from the high levels of protein in the fluid. They then tested the technique by spiking samples of urine, blood plasma (the liquid portion of the blood) and blood serum (the liquid portion of the blood with clotting factors removed) with valproic acid. They found that valproic acid could be separated from the proteins in the biological fluids within three minutes and that its peak could clearly be identified in the subsequent electropherogram. The researchers were also able to generate a linear calibration curve for different concentrations of valproic acid (from 2µg/mL to 50µg/mL) in blood plasma. This concentration range covers the levels usually measured in human patients (50-100µg/mL). Furthermore, the researchers were able to detect and measure valproic acid in biological fluids containing a range of other drug compounds that patients might be using at the same time (such as lidocaine and acetaminophen). Overall, this CE-C4D method is faster and more sensitive than any other previously developed method for measuring valproic acid levels. Following this success, Belin is moving on from small ions and is now experimenting with using C4D to detect larger organic molecules, such as peptides and human immunoglobulin. Related links:
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd. |
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