Effects of sample size, dry ashing temperature and duration on determination of ash content in algae and other biomass

Many reported methods on ash analysis by dry ashing vary greatly in ashing conditions, making results incomparable. The present study investigated effects of sample size (1, 4 g), ashing temperature (550, 600 °C) and duration (6, 16 h) on ash measurement of 13 algae and 4 non-algae (grains, soymeal, forage) samples under a factorial model. Results show that for most biomass both temperature and duration affected ash measurement significantly (p < .05). Ashing at 600 °C for 6 h or 550 °C overnight (about 16 h) gave values similar to ashing at 600 °C overnight, but ashing 6 h at 550 °C produced higher ash content than other combinations. Furthermore, for algae having higher ash content, sample size was also a determining factor when ashing at lower temperature (550 °C) and/or shorter duration (6 h), but the effect could be alleviated by ashing at 600 °C overnight. For comparable results, a standardized method of ashing 1 to 4 g samples at 600 °C overnight is proposed for all types of biomass.