A comparative study of acid-extractable and total digestion methods for the determination of inorganic elements in peat material by inductively coupled plasma-optical emission spectrometry

In the absence of a standard wet preparation method for the determination of inorganic elements in peat, four acid digestion methods (nitric acid (HNO3), HNO3/hydrochloric acid (HCl), aqua regia and HNO3/hydrofluoric acid (HF)) were compared using a Canadian fen peat (OGS 1878 P-6), previously proposed as a reference material, and inductively coupled plasma-optical emission spectrometry (ICP-OES) as the instrumental analytical technique. Two microwave-assisted methods, HNO3 and HNO3/HF, representing ‘acid-extractable’ and total digestion, respectively, were then selected for optimisation and application to a newly developed ombrotrophic peat reference material (NIMT/UOE/FM/001) and to the individual sections of a 43 cm peat core, also from Flanders Moss, Scotland, for a range of elements, including Al, Co, Cr, Cu, Fe, Mn, P, Pb, S, Ti, V and Zn. While the results suggested the general efficacy of HNO3 in investigations of the greatly elevated levels of trace elements, such as Cu, Pb and Zn, resulting from the severe anthropogenic contamination of the environment during the industrial era, the need for addition of HF for the determination of major lithogenic elements, such as Al and Ti, present in aluminosilicates and other resistant minerals, and some trace elements (Co, Cr and V) was clearly demonstrated. The inclusion of HF in a total digestion method, such as HNO3/HF, is, therefore, necessary when normalisation of trace element concentrations to those of a conservative lithogenic reference element, such as Ti, is performed for the purpose of determining elemental enrichment factors. For elements other than Hg, the dry ashing of peat samples at 450 °C prior to acid digestion is recommended for the destruction of the predominantly organic matrix, especially relevant where inductively coupled plasma-mass spectrometry (e.g. quadrupole inductively coupled plasma-mass spectrometry (ICP-MS)) is to be used in the determination of the much lower concentrations of trace elements prevalent in pre-industrial and ancient times.