Microanalytical study of sub-nanoliter samples by capillary electrophoresis – mass spectrometry with 100 % injection efficiency

Abstract

A method is presented for conducting fast capillary electrophoresis (CE) with ultrasmall sample volumes (<10 nL) and subsequent determination of target analytes by time-of-flight mass spectrometry (TOF-MS). Furthermore, an approach towards preconcentration by solvent evaporation of samples as small as 500 nL is shown. Ultrasmall samples of biologically important cyclic nucleotides (cyclic guanosine monophosphate, cyclic adenosine monophosphate, cyclic cytidine monophosphate) served as model compounds. Sample volumes as small as 500 pL were used to perform sample uptake and CE injections by capillary batch injection (CBI). This resulted in detection limits between 2 and 5 fmol. The fast rate of sample evaporation prevents the operation of nano-/subnano-liter samples under room temperature conditions. A CBI-CE-TOF-MS setup incorporating a Peltier element and a thermostat was used for the handling of very small samples. In combination with tapered fused silica injection capillaries and the high-precision guiding system of the CBI setup, nearly 100 % efficient injections of ultrasmall samples into funnelled separation capillaries could be managed. The overall system allowed for high-throughput CE separations from densely arranged sample droplets or vials with RSD of peak heights ranging from 6 to 8 % (n = 5). CE-TOF-MS separations of mixtures of cyclic nucleotides were carried out in capillaries as short as 20 cm (25 μm ID) in less than 55 s with high separation efficiency. We report details of the setup and optimizations with respect to etching of capillaries (tapering and funnelling), sample uptake and injection as well as separation of the model compounds. Moreover, the capabilities of the experimental setup for preconcentration of target analytes by solvent evaporation are described. Preconcentration factors of up to 100 could be achieved. This approach was successfully used for the determination of the cyclic nucleotide cGMP in (spiked) human urine samples.