Online capillary liquid-liquid electroextraction of peptides as fast pre-concentration prior to LC-MS.

In this research paper, we show that capillary electroextraction (cEE) is capable of fast online peptide concentration and that it can be coupled online to LC-MS to result in a fast and sensitive method. Electroextraction takes place when an electrical field is applied in a two-phase liquid-liquid system. Sample molecules in the organic phase migrate very fast into the aqueous phase and are concentrated in a small zone. In this work, cEE of peptides is developed and coupled online to LC-MS via a switching valve. Comparison of 10 min of cEE-LC-MS with a normal LC-MS injection showed more than 100-fold increased peak heights. Of five model peptides, good calibration curves in the range of 0.05-5 μmol/L were obtained. The linearity was good (R(2) values between 0.984 and 0.996) and RSD between 5% at the highest to 25% at the lowest concentration (n=3). The LOD of bradykinin, angiotensin I-converting enzyme inhibitor and angiotensin I was in the low nmol/L range. Analysis of a tryptic digest of eight model proteins resulted in more than 170 peptides, without bias for pI or hydrophilicity. Urine analysis is demonstrated, resulting in an LOD around 0.04 μmol/L urine for tryptic cytochrome C peptides spiked to urine and an increase of 42% in the number of chromatographic peaks compared with the conventional LC-MS. In summary, cEE-LC-MS is a fast electrophoresis-driven sample preconcentration technique that is quantitative, able to extract a wide peptide range and applicable to bioanalysis.