Highly sensitive and selective separation of intact parathyroid hormone and variants by sheathless CE‐ESI‐MS/MS

Parathyroid hormone (PTH) is a common clinical marker whose quantification relies on immunoassays, giving variable results as batch, brand, or target epitope changes. Sheathless CE-ESI-MS, combining CE resolution power and low-flow ESI sensitivity, was applied to the analysis of PTH in its native conformation in the presence of related forms. Fused silica and neutral-coated capillaries were investigated, as well as preconcentration methods such as transient isotachophoresis, field-amplified sample injection (FASI), and electrokinetic supercharging (EKS). The method for the separation of PTH and its variants was first developed using fused-silica capillary with UV detection. An acidic BGE was used to separate 1-84 PTH (full length), 7-84 PTH, and 1-34 PTH. Acetonitrile was added to the BGE to reduce peptide adsorption onto the capillary wall and transient isotachophoresis was used as analyte preconcentration method. The method was then transferred to a sheathless CE-ESI-MS instrument. When using a fused silica capillary, CE-MS was limited to μg/mL levels. The use of a neutral coating combined with FASI or EKS allowed a significant increase in sensitivity. Under these conditions, 1-84 PTH, 7-84 PTH, and 1-34 PTH were detected at concentrations in the low ng/mL (FASI) or pg/mL (EKS) range.