Application of inductively coupled plasma sector field mass spectrometry for the fast and sensitive determination and isotope ratio measurement of non-metals in high-purity process chemicals
The determination of non-metals (e.g., Si, P, S, Cl, As, Se and Br) by convenient quadrupole ICP-MS systems is extremely difficult owing to the presence of molecular spectral interferences from H, C, N, O and Ar and the high ionisation energies of these elements (8–13 eV). Therefore, the determination of anion forming elements is the domain of ion chromatography. Since this is a very time-consuming technique giving no isotopic information, the use of inductively coupled plasma sector field mass spectrometry (ICP-SFMS) was studied. By applying medium or high resolution from 3000 for Si and P to 12 000 for Se, the background can be lowered by up to five orders of magnitude, resulting in limits of detection between 1 pg g–1 for As (high purity water without dilution) and 2 µg g–1 for Cl (e.g., in H2SO4 after 25-fold dilution) in routine analysis and multi-element mode. Ionisation rates were found to be between 0.5% for Cl and 50% for Si. These rates can be improved by using a dried aerosol obtainable by an ultrasonic or microconcentric nebuliser. However, this does not always lead to higher sensitivity because these elements are lost in the desolvation stage at a rate between 50 and 90%. Nevertheless, the spectral interferences are lowered. Isotope ratios for Si, S, Cl, Se and Br are obtainable with a precision of 0.5–3%, allowing quantification by isotope dilution.