IS LASER DIFFRACTION GRAIN-SIZE ANALYSIS ON MULTIMODAL DEPOSITS ACCURATE?

The introduction of laser diffraction particle-size analyzers has made it convenient to measure the size of sediments from submicron to about 2000 μm. However, the performance of the laser diffraction techniques on samples containing a multimodal mixture of mud and sand is rarely tested. This is investigated here using three experiments on a CILAS 1190 instrument. The first experiment tests the accuracy, reproducibility, and repeatability of the instrument using multiple reference materials. The second experiment investigates the sensitivity of the instrument on sand in a muddy matrix. The third experiment explores representability of pipetting subsampling of a designed wet mixture of sand and mud. The results showed that the CILAS 1990 has great reproducibility and repeatability and can accurately measure silt-sized particles though it overestimates the sizes of coarse-grained particles that is well-known for the laser diffraction technique. Surprisingly, the CILAS 1190 instrument requires a minimum of >38% sand by weight to identify the sand distribution in a multimodal mixture, demonstrating its low sensitivity to sand in a muddy matrix. Finally, the pipetting method was not able to representatively subsample the wet mixture that was stirred vigorously using a magnetic stirrer. The results suggest that separating sand from finer sediments would be necessary to accurately measured a multimodal mixture of these materials using CILAS 1190.