REDUCING THE VARIABILITY OF LABORATORY TEST RESULTS OF ENVIRONMENTAL SOIL AND ROCK MATERIALS THROUGH PROPER LABORATORY SUBSAMPLING

“The seemingly simple task of taking a small amount of material out of a laboratory sample bottle could possibly be the largest source of error in the whole measurement process.” – “Guidance for Obtaining Representative Laboratory Analytical Samples from Particulate Laboratory Samples”, EPA/600/R-03/027. It is a well known fact that lack of reproducibility of environmental laboratory test results for compounds found within soil and rock samples is a major problem. Most samples collected in the field exhibit considerable heterogeneity of size, shape, density, and composition/rock type. These samples must typically be reduced in mass and volume on the order of 10^-3 to 10^-4 for most analytical techniques (metals - AA, ICP, GC, etc., or asbestos – polarized light microscopy) and 10^-10 for transmission electron microscopy asbestos analysis, respectively. Sample reduction is usually performed through a series of crushing, pulverizing and subsampling steps. Significant errors can occur at every phase of the sample reduction process if proper care is not taken to insure representative subsampling techniques are followed. Unfortunately, little, if any, guidance is given in most soil testing methods on proper sampling methods. Geochemists/geostatisticians, Dr. Pierre Gy and Dr. Francis Pitard, whose work has focused primarily on the mining industry, have developed a theory of ‘proper’ soil and rock sampling and sample handling methods, which can be applied to environmental soil and rock samples to obtain representative subsamples. Their theories are the basis for EPA/600/R-03/027, and have been developed to the point where, with only a modicum of extra effort, the relative variance of the total subsampling error during sample preparation can be reduced by orders of magnitude. A summary of Dr. Gy’s & Dr. Pitard’s theories and approaches to proper sampling techniques will be presented along with laboratory test data on samples prepared by both ‘proper’ and ‘improper’ methods.