A multivariate study of the acid effect and the selection of internal standards for inductively coupled plasma mass spectrometry

The effects of nitric acid concentration and liquid flow rate on the signals obtained with an inductively coupled plasma mass spectrometer, operated with a microconcentric nebulizer (MCN-100), have been studied by the use of partial least squares regression. Thirteen elements, covering the mass range 51-238, were studied. The nitric acid concentration was varied in the range 0.14-2.8 mol l(-1) and the acid effect was found to vary significantly between the elements - depending mainly on the value of the first ionisation potential of the element. The effect of the liquid flow rate was studied within the range 34.5-172.5 mu l min(-1) and it was only significant (P = 0.05) for Ag-107 and Ag-109. It is possible to compensate completely for the acid matrix effect by the use of a proper internal standard. The use of multivariate analysis methods, e.g, partial least squares regression is recommended in order to choose the most suitable internal standards. (C) 1999 Elsevier Science B.V. All rights reserved.