Online Preconcentration in Capillaries by Multiple Large-Volume Sample Stacking: An Alternative to Immunoassays for Quantification of Amyloid Beta Peptides Biomarkers in Cerebrospinal Fluid.

A novel electrokinetic preconcentration approach, so-called multiple pressure-assisted large-volume sample stacking with an electroosmotic flow pump (M-PA-LVSEP), was developed to allow in-capillary enrichment and separation of analytes from unlimited sample volumes. With this approach, the inherent limitation of in-capillary electrokinetic preconcentrations to the separation capillary volume can be overcome. The M-PA-LVSEP protocol relies on repeated cycles of pressure-assisted electroosmotic pumping and injection of extremely large sample volumes for analyte stacking and sample matrix removal. This technique was developed to address the challenge of sensitive and simultaneous determination of several amyloid β (Aβ) peptides, which are biomarkers for the molecular diagnosis of Alzheimer's disease (AD). For the first time, reliable quantification of different species of fluorescently derivatized Aβ peptides, that is, Aβ 1-42, Aβ 1-40, and Aβ 1-38 down to subnanomolar ranges in cerebrospinal fluids (CSF) from AD and non-demented patients (healthy controls) was made possible without recourse to immunoassay, immunoprecipitation, or mass spectrometry approaches. Based on the stacking from a sample plug representing up to 400% of the total capillary volume, sensitive enhancement factors up to 170 could be achieved with this "antibody free" approach. Quantification limits for these Aβ peptides down to 0.05 nM with capillary electrophoresis coupled with laser-induced fluorescent detection could be obtained. Excellent agreement between results from M-PA-LVSEP and the gold standard ELISA method was achieved for measurements of Aβ 1-42 in CSF, with a determination correlation (r2 ) better than 0.993.