Microwave-induced plasma optical emission spectrometry (MIP-OES) analysis of the major elements in woody biomass ash following microwave acid digestion

The increasing use of woody biomass for sustainable heat and power is generating large amounts of woody biomass ash (WBA), which needs to be safely disposed of or recycled. In this study, the utility of Microwave-induced plasma optical emission spectrometry (MIP-OES) with microwave-assisted acid digestion for simplifying analyses of the major elements of WBA was investigated. Calibration curves exhibited good linearity (R² > 0.999). Internal standardization using three emissions lines (324.754 nm of Cu, 451.131 nm of In and 371.029 nm of Y) showed that calibration with Cu (324.754 nm) correction resulted in the best linearity. Physical effects, even the presence of hydrofluoric acid or boric acid in mixed acids used for digestion, were of trivial influence with internal standard recoveries were within 0.95 to 1.05 times those of the blank sample. The limit of detection of MIP-OES for seven measured elements was 0.85–89 μg/L, 0.23–2.8 times higher than that of inductively coupled plasma optical emission spectrometry (ICP-OES). The recovery of elements in a certified fly ash reference material as determined by MIP-OES was 0.80–1.08. Elemental quantification of WBA samples by MIP-OES showed a strong correlation with that by ICP-OES. However, in a Welch's t-test, the MIP-OES values were significantly higher for K and significantly lower for Al and Ca in a majority of the samples. Nonetheless, considering the good correlation between their results, MIP-OES can replace ICP-OES for the analysis of WBA.