  Amphiphilic peptide conjugation affords a significant increase in sensitivity
with protein quantification by electrospray-ionization mass spectrometry. This
has been demonstrated here for human growth hormone in serum using
N-(3-iodopropyl)-N,N,N-dimethyloctylammonium iodide (IPDOA-iodide) as
derivatizing reagent. The signal enhancement achieved in comparison to the
method without derivatization enables extension of the applicable concentration
range down to the very low concentrations as encountered with clinical glucose
suppression tests for patients with acromegaly. The method has been validated
using a set of serum samples spiked with known amounts of recombinant 22 kDa
growth hormone in the range of 0.48 to 7.65 \mug/L. The coefficient of
variation (CV) calculated, based on the deviation of results from the expected
concentrations, was 3.5% and the limit of quantification (LoQ) was determined
as 0.4 \mug/L. The potential of the method as a tool in clinical practice has
been demonstrated with patient samples of about 1 \mug/L.
