Detection of ultratrace phosphorus and sulfur by quadrupole ICPMS with dynamic reaction cell.
A method of detection of ultratrace phosphorus and sulfur that uses reaction with O2 in a dynamic reaction cell (DRC) to oxidize S+ and P+ to allow their detection as SO+ and PO+ is described. The method reduces the effect of polyatomic isobaric interferences at m/z = 31 and 32 by detecting P+ and S+ as the product oxide ions that are less interfered. Use of an axial field in the DRC improves transmission of the product oxide ions 4-6 times. With no axial field, detection limits (3sigma, 5-s integration) of 0.20 and 0.52 ng/mL, with background equivalent concentrations of 0.53 and 4.8 ng/mL, respectively, are achieved. At an optimum axial field potential (200 V), the detection limits are 0.06 ng/mL for P and 0.2 ng/mL for S, respectively. The method is used for determining the degree of phosphorylation of beta-casein, and regular and dephosphorylated alpha-caseins at 10-1000 fmol/microL concentration, with 5-10% v/v organic sample matrix (acetonitrile, formic acid, ammonium bicarbonate). The measured degree of phosphorylation for beta-casein (4.9 phosphorus atoms/molecule) and regular alpha-casein (8.8 phoshorus atoms/molecule) are in good agreement with the structural data for the proteins. The P/S ratio for regular alpha-casein (1.58) is in good agreement with the ratio of the number of phosphorylation sites to the number of sulfur-containing amino acid residues cysteine and methionine. The P/S ratio for commercially available dephosphorylated alpha-casein is measured at 0.41 (approximately 26% residual phosphate).
Bandura
Dmitry R
DR
PerkinElmer-SCIEX, Concord, Ontario, Canada.
Baranov
Vladimir I
VI
Tanner
Scott D
SD
D002364:Caseins / Q000737:chemistry; D013058:Mass Spectrometry / Q000379:methods*; D010100:Oxygen / Q000737:chemistry; D010750:Phosphoproteins / Q000032:analysis; D010758:Phosphorus / Q000032:analysis*; D010766:Phosphorylation; D013455:Sulfur / Q000032:analysis*
D002364:Caseins; D010750:Phosphoproteins; D010758:Phosphorus; D013455:Sulfur; D010100:Oxygen