The use of inorganic elemental standards in the quantification of proteins and biomolecular compounds by inductively coupled plasma spectrometry

Abstract

An inductively coupled plasma atomic emission spectrometer (ICP-AES) and an ICP-time-of-flight mass spectrometer (ICP-TOF-MS) have been investigated in terms of their generic behaviour, i.e. if their responses can be considered as structure independent or not in the simple matrices prevailing during, e.g., a liquid chromatographic (LC) run. The signal in ICP-AES and ICP-TOF-MS given by elements bound or associated to intact bovine serum albumin and cyanocobalamin was compared with the signal obtained from inorganic reference material solutions. The sample and standard solutions were directly aspirated into the nebuliser or run through flow injection analysis (FIA) or LC before the ICP spectrometers. The total concentrations of sulfur and cobalt were determined from ICP determinations of nitric acid digests of the two compounds. The quantification was based on external calibration by simple linear regression without internal standard correction. As long as the molecule concentration is sufficiently low, the matrix interference from the surrounding molecule is almost negligible and inorganic elemental standards can be used for quantification with an accuracy of 10% or better. The LC-ICP measurements generally resulted in less accurate concentrations than the corresponding measurements with direct aspiration and FIA, probably due to the inherent susceptibility of separation techniques to drift in the signal level over time. The ICP-TOF-MS could not handle as high absolute concentrations of organometallic or macromolecular compounds as could the ICP-AES, without showing a significant interference from the matrix for some of the measured elements.