Loss on ignition as a method for estimating organic and carbonate content in sediments: reproducibility and comparability of results

Five test runs were performed to assess possible bias when performing the l oss on ignition (LOI) method to estimate organic matter and carbonate conte nt of lake sediments. An accurate and stable weight loss was achieved after 2 h of burning pure CaCO3 at 950 degreesC, whereas LOI of pure graphite at 530 degreesC showed a direct relation to sample size and exposure time, wi th only 40-70% of the possible weight loss reached after 2 h of exposure an d smaller samples losing weight faster than larger ones. Experiments with a standardised lake sediment revealed a strong initial weight loss at 550 de greesC, but samples continued to lose weight at a slow rate at exposure of up to 64 h, which was likely the effect of loss of volatile salts, structur al water of clay minerals or metal oxides, or of inorganic carbon after the initial burning of organic matter. A further test-run revealed that at 550 degreesC samples in the centre of the furnace lost more weight than margin al samples. At 950 degreesC this pattern was still apparent but the differe nces became negligible. Again, LOI was dependent on sample size. An analytical LOI quality control experiment including ten different labora tories was carried out using each laboratory's own LOI procedure as well as a standardised LOI procedure to analyse three different sediments. The ran ge of LOI values between laboratories measured at 550 degreesC was generall y larger when each laboratory used its own method than when using the stand ard method. This was similar for 950 degreesC, although the range of values tended to be smaller. The within-laboratory range of LOI measurements for a given sediment was generally small. Comparisons of the results of the ind ividual and the standardised method suggest that there is a laboratory-spec ific pattern in the results, probably due to differences in laboratory equi pment and/or handling that could not be eliminated by standardising the LOI procedure. Factors such as sample size, exposure time, position of samples in the furn ace and the laboratory measuring affected LOI result?;, with LOI at 550 deg reesC being more susceptible to these factors than LOI at 950 OC. We, there fore, recommend analysts to be consistent in the LOI method used in relatio n to the ignition temperatures, exposure times, and the sample size and to include information on these three parameters when referring to the method.