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This paper describes a novel two-site peptide immunoassay using the isotope 14C as the label and accelerator mass spectrometry as the detection system. A mouse monoclonal antibody (1A5) against the amino terminal region of human parathyroid hormone-related protein (PTHrP) was labeled with 14C by growing the hybridoma cells in a miniPERM bioreactor in the presence of [U-14C]L-leucine and [U-14C]D-glucose. The antibody was purified from the culture media using protein G affinity chromatography. The purified 14C-labeled antibody (14C-1A5) fractions showed excellent correlation between the levels of radioactivity and binding activity for PTHrP. Using 14C-1A5 as the detection antibody in a two-site immunoassay format for PTHrP1-141, a 16-kDa polypeptide, an analytic sensitivity of 10 pmol/L was achieved with a linear measurement range up to 1.3 nmol/L. Only approximately 17 pCi/ well (or 1.6 nCi/96-well microtiter plate) 14C-1A5 was used, which is far below the limit (50 nCi/g) for disposal as nonradioactive waste. This study may serve as a model for the development of sensitive and "nonradioactive" immunoassays for peptides, including polypeptide tumor markers.

An electrospray ionization ion source coupled to a time-of-flight mass analyzer incorporating a multi-anode time-to-digital converter is described. High-speed data acquisition (kHz mass spectral acquisition) rates are achieved. The four-anode detector produces a significant increase in detection/counting efficiency over that for a single-anode detector. In this work a 2.5 times increase in detection efficiency is demonstrated. The multi-anode detector is also used as a diagnostic tool to optimize transmission of the ion optics.

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