Mass spectrometric analysis of sulfur in microgram quantities of biological macromolecules using a reaction interface

Submicrogram to microgram samples of a variety of proteins have been analyzed using a reaction interface and a mass spectrometer. The interface atomizes the macromolecule in a microwave discharge and produces simple polyatomic molecules from the elements contained in the analyte by reaction with a scavenger gas. The measurement of m/z 64 for SO2 in a CO2-scavenged discharge can be used to determine the amount of sulfur originally introduced into the interface; carbon is quantified as HCN (m/z 27) in a nitrogen-scavenged discharge. Because of the highly energetic, destructive nature of the reaction interface, the character of the original molecule, i.e. size, polarity, etc., does not affect the analysis, only its elemental content. Samples (1 microgram) of a series of 12 amino acid polymers ranging in size from mol. wt 470 (des-TYR-MET-enkephalin) to mol. wt 65,400 (bovine serum albumin) and with a sulfur content of 1 S per amino acid (poly-1-methionine) to 2 S per 153 amino acids (myoglobin) were subjected to elemental analyses of sulfur and carbon. The observed S/C ratio was linear with elemental formula calculation, and produced a log-log regression coefficient of 0.80. With poly-1-methionine (mol. wt 30 000) as a model compound, the detection of sulfur was linear for sample sizes between 20 and 10 000 ng. We conclude that the reaction interface is a promising addition to a mass spectrometer for detection of certain elements from small samples of biological macromolecules.