Low-Temperature Decomposition of Botanical and Biological Samples for Multielement Analysis by High-Frequency Induced Oxygen–Argon–Fluorine Plasma

The cool plasma asher (CPA) consists of a high-frequency generator and a quartz sample vessel equipped with a cooling finger that prevents loss of volatile elements. After sample decomposition within an O2–Ar–F plasma, the ashing residues and the elements condensed on the surface of the vessel or cooling finger are dissolved by refluxing in 1–5 mL of double-distilled acid. The sample solutions are analyzed for elemental content by inductively coupled plasma-atomic emission spectrometry (ICP–AES). The recovery values for 42 elements (Ag, Al, As, B, Ba, Be, Bi, Ca, Cd, Co, Cr, Cu, Eu, Fe, Hg, K, Li, Mg, Mn, Mo, Na, Ni, P, Pb, Pd, Pt, Rb, S, Sb, Se, Sn, Sr, Te, Ti, V, Y, Yb, Zn, Zr, La, Au, and Sc) are documented after cool plasma ashing of elemental spectrometric standards. In addition, NIST Standard Reference Materials consisting of botanical and biological samples are ashed by CPA, and results are reported for 23 elements (Al, As, Ba, Be, Ca, Cd, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Na, Ni, P, Pb, S, Se, Sr, V, and Zn) analyzed by ICP-AES. This method achieves good recoveries for many elements while allowing decomposition of difficult sample matrixes without acid, at a temperature slightly above 100°C. We investigated several ashing facilitators to improve ashing efficiency. This paper describes improved ashing conditions due to sample agitation, gas mixtures, Teflon balls, and a Teflon vessel. The time required to ash 1.0 g of botanical sample in the CPA was reduced from 80 h with no ashing aids to 3 h with maximum ashing aids. The optimum plasma ashing conditions for 1.0 g of sample was 6 h at a high-frequency power of 30 W with a 1 h acid reflux to dissolve sample ash. Because reflux acid in the final sample volume was minimal, trace elemerits were concentrated and blank contamination was extremely low.