Pyrolytically coated graphite direct sample insertion probe for inductively coupled plasma spectrometry

Abstract

The pyrolytic coating of a graphite cup direct sample insertion (DSI) probe for inductively coupled plasma spectrometry is described. Deposition of a pyrolytic coating on the inside walls of the DSI cup was achieved by heating the device in a 2 kW argon plasma while directing a gaseous mixture of 10% methane in argon (v/v) to the graphite cup interior. A narrow channel hollowed into the stem of the graphite cup facilitated introduction of the methane–argon mixture. A factorial study revealed that the procedure generated optimum results when the coating was applied for at least 10 min at a flow rate of 500 ml min^–1. Peak area and height reproducibility improvements for Cd, Cu, Mg, Pb and Zn ranged from 1.7–3.4 and 2.1–6.7% RSD, respectively, when a pyrolytically coated DSI cup was used, versus 3–7 and 5–12% RSD when no coating was used. The abatement of multiple peaking and an increase in transient signal intensity was observed for all elements except Cu, translating into roughly a two-fold improvement in peak height limit of detection when a pyrolytically coated DSI probe is used. Scanning electron microscopy and Raman spectroscopy of the DSI cup interior confirm the deposition of a uniform pyrolytic phase of high crystalline order on the DSI cup.