Method development for the digestion and analysis of four common sedimentary lithologies using ICP-OES and ICP-MS

M.Sc.The understanding of the classification and origin of geological systems is facilitated by the acquisition of accurate and precise analytical data. New instrumentation is rapidly developed for the preparation and quantitative analysis of geological materials using smaller amounts of sample, with lower limits of detection and faster analysis times. The development of new methodologies is crucial for the effective utilisation of new instrumentation. This study was conceptualised because of the relationship between the Department of Geology and the Central Analytical Facility of the Faculty of Science, University of Johannesburg. There is a high demand for accurate and precise chemical data for various lithologies and the availability of high-end analytical equipment, but little practical expertise to utilise this equipment to its full potential. The study is centred round the analysis of four common sedimentary lithological groups that are routinely studied by the Department of Geology namely carbonate rocks, shales, iron ore and manganese ore. A large literature base exists for the decomposition and analysis of geological materials. The bulk of this literature is centred round more established and conventional methods of sample preparation and analysis. The use of microwave digestion instrumentation and methodologies in recent times has revolutionised sample decomposition with shorter decomposition times, smaller sample masses required for decomposition, lower loss of analytes to volatilisation and creating a safer laboratory environment for analysts to work in. Inductively coupled plasma optical emission spectroscopy (ICP-OES) is a more mature method of sample analysis, being commercialised in the mid 1970’s, while inductively coupled plasma mass spectroscopy (ICP-MS) is a newer technique, v being commercialised as of the mid 1980’s. These two techniques are multi-element techniques, with low limits of detection and fast analysis times for a plethora of analytes, over seventy elements for ICP-OES and over eighty elements for ICP-MS. Samples from all four lithological groups were successfully digested in a microwave digestion unit with varying combinations of nitric acid (HNO3), hydrochloric acid (HCl), perchloric acid (HClO4) and hydrofluoric acid (HF) and various digestion programs. Accurate and repeatable methods of analysis were developed for iron, manganese, calcium and potassium for all four lithologies with ICP-OES; aluminium was successfully determined for shale, iron and manganese ore with ICP-OES. Titanium, sodium, arsenic, barium, bismuth, chromium, copper, molybdenum, scandium, strontium and vanadium were determined for all four lithotypes with acceptable accuracy and repeatability using ICP-MS