Signal enhancement and reduction of interferences in inductively coupled plasma mass spectrometry with an argon–trifluoromethane mixed aerosol carrier gas

Abstract

The addition of trifluoromethane (CHF₃) to the aerosol carrier gas in inductively coupled plasma mass spectrometry was assessed as a method of improving detection limits (DL) for elements such as As, Se, Cu and Zn in matrices containing interfering species. It was observed that in each case the analyte response was significantly increased with a coincident decrease in the blank signal. The improved DL for As in Cl^– and Ca²⁺ matrices (interferences ⁴⁰Ar³⁵Cl⁺ and ⁴³Ca¹⁶O⁺₂ were 0.02 and 0.04 ng ml^–1 compared with 0.65 and 0.28 ng ml^–1, respectively, without CHF₃; for ⁷⁸Se (interference ⁴⁰Ar³⁸Ar⁺) 0.032 compared with 0.88 ng ml^–1; for ⁶³Cu in Na₂SO₄ or Na₂HPO₄ matrices (interferences ⁴⁰Ar²³Na⁺ and ³¹P¹⁶O₂⁺) 0.022 and 0.089 compared with 0.35 and 0.53 ng ml^–1, respectively; and for ⁶⁴Zn in an Na₂HPO₄ matrix (interferences H³¹P¹⁶O₂⁺ and ³¹P¹⁶O¹⁷O⁺) 0.011 compared with 0.42 ng ml^–1. The reduction of the interference is attributed to competitive reactions between the matrix species and the CHF₃ or species derived from it in the plasma. The analyte enhancement effect is not yet clear. It has been suggested that this effect is related to elements with ionization potential (IP) in the 9–11 eV region and is affected by organic compounds added to the aerosol carrier gas stream. Copper (IP 7.73 eV), Al (5.99 eV), Br (11.30 eV) and l (10.44 eV) are exceptions to this assumption. Analytical curves of the studied elements at low ppb and sub-ppb levels (in the interfering matrices) further demonstrated the advantage of adding CHF₃ in trace elemental analysis.