The Overweight Net Weight
A patrol officer submits a single knotted plastic bag containing a white powder. The evidence log records a gross weight of 3.42 g. Your lab's SOP requires that gross weight be reported for field submissions but that net weight be established before any quantitative result is released. You remove the powder, weigh the empty bag (0.28 g), and obtain a net powder mass of 3.14 g. You then take a 50.0 mg aliquot for GC-MS and a 25.0 mg aliquot for FTIR, discarding neither. The GC-MS confirms cocaine base, and the quantitative method returns 71.4% cocaine by mass. The prosecutor's charging threshold is 2.0 g of cocaine (not total powder). The defense requests the raw data and challenges the sampling.
1. Compute the reported cocaine mass and explain whether the charge threshold is met.
Read the worked answer
Net powder = 3.42 − 0.28 = 3.14 g. Cocaine mass = 3.14 g × 0.714 = 2.24 g. This exceeds the 2.0 g threshold by 0.24 g (~12% margin). Note the aliquots: 75 mg removed from 3.14 g is ~2.4% of the sample, so the quantitative result applies to the original net mass, not the post-aliquot mass. Report net weight, purity, and calculated cocaine mass separately, and state the analytical uncertainty; a 12% margin is within typical method uncertainty, so the number should not be presented as if exact.
2. The defense argues a single 50 mg aliquot cannot represent 3.14 g of powder. How do you respond analytically?
Read the worked answer
Acknowledge the point is legitimate and address it with sampling evidence, not assertion. Options: (1) report that the powder was visually homogeneous and that a second aliquot from a different location gave a consistent result (if you ran one); (2) if heterogeneity is plausible (e.g., chunks vs. fines), grind and homogenize before sub-sampling and document it; (3) state the sampling plan and its limitations in the report. Do not claim statistical representativeness from one aliquot. If no homogeneity check was done, say so and characterize the result as an estimate with stated uncertainty.