The detection of ionic species in a polymeric planar electrophoresis device by contactless conductivity measurement is described. To our knowledge this is the first report of such measurements carried out with external electrodes which are part of the holder rather than the separation chip itself. The approach allows the use of bare devices as used for optical measurements, which greatly simplifies the method. The use of a sine wave of 100 kHz of a high amplitude of 500 V for cell excitation assures high sensitivity which is demonstrated with electropherograms for alkali and heavy metal ions as well as inorganic anions and carboxylates at concentrations between 10 and 50 µM. The determination of underivatized amino acids was also possible by using a buffer in the alkaline region where these species are present in anionic form. Detection limits were found to be in the order of 1-5 µM for the inorganic ions and between about 5 and 50 µM for the organic species.

The detection of human immunoglobulin M (IgM) was performed using capacitively coupled contactless conductivity detection (CCD) in electrophoresis carried out in conventional capillaries as well as on glass and poly(meth-yl methacrylate) (PMMA) microdevices. Also achieved was the analyses of IgG (an anti-human IgM) and the complex formed in the reaction between the two immunoreagents. It is demonstrated that CCD is a powerful tool suitable not only for the detection of antibodies but also for monitoring an immunological interaction. Conductivity measurements allow the direct determination of immunoreagents, and it is advantageous, since no labels are required. The immunoglobulin IgM has been taken as model analyte. The reproducibility of the analytical signal (RSD = 1%), sensitivity and limits of detection obtained for IgM (0.15 ng/mL in conventional capillaries and 34 ng/mL in microchips) are comparable to those previously obtained with amperometric detection. The immunological reaction was performed either in conventional microtiter plates as used in ELISA or in situ on the glass chip.

The detection of underivatized anionic sulfonates, carboxylates, amino acids, sugars, and artificial sweeteners, and of cationic dopamine, ephedrine, and metanephrine in microfabricated electrophoresis devices is demonstrated. This was achieved by high-voltage contactless conductivity measurements with external electrodes. Poly(methyl methacrylate) chips with thin covers to enable sensitive contactless detection were used for most determinations but glass microchips had to be employed for amino acids and sugars. The plastic chips were found not be stable in the alkaline media required to render those two classes of species in the ionic form amenable for separation and detection. The reproducibility of peak area measurements was about 1% or better and the detection limits ranged between 1 and 30 microM for the different compounds examined.

Contactless conductivity measurements were found to be suitable for the direct detection, i.e., without needing any labels, of a range of biochemically relevant species, namely amino acids, peptides, proteins, immunoglobulin, and DNA. It was also possible to monitor the products of the enzymatic digestion of HSA with pepsin. Detection was carried out on bare electrophoresis chips made from poly(methyl methacrylate) by probing the conductivity in the channel with a pair of external electrodes, which are fixed on the chip holder. Separation efficiencies up to 15,000 plates could be obtained and LODs are in the low muM-range, except for immunoglobulin G (IgG) which could be determined down to 0.4 nM. Linear dynamic ranges of two to three orders of magnitude were obtained for the peptides as examples.