The Field Test That Disagreed
A traffic stop yields a crystalline substance in a folded paper. The officer's field presumptive test (cobalt thiocyanate) turns blue, indicating cocaine. The substance is submitted to your lab. GC-MS shows a single major peak whose mass spectrum matches methamphetamine, not cocaine. The crystals are white and dissolve readily in water. The officer's report already states 'cocaine' and the arrest was made on that basis. Your lab result will be issued as a formal report. The prosecutor asks whether the field test was 'wrong' and whether the arrest was invalid.
1. Explain the discrepancy between the field test and GC-MS in a way that is scientifically defensible.
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The cobalt thiocyanate test is a presumptive color test with known cross-reactivity; a blue color is consistent with, but not proof of, cocaine. Other compounds, including some amines and adulterants, can produce similar color changes. The field test's role is screening, not identification. GC-MS with a library match is the confirmatory method, and its result governs. The correct framing is 'presumptive positive, not confirmed,' not 'the field test was wrong' - though in this case the presumptive result was a false positive for cocaine.
2. Does the confirmatory result invalidate the arrest? Separate the analytical question from the legal one.
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Analytically, the lab's job is to report what the substance is: methamphetamine, not cocaine. Whether the arrest remains valid is a legal question (probable cause at the time of arrest, which may rest on the field test plus other observations) and is outside the chemist's scope. The report should state the confirmed identity, note the field test result and its presumptive nature, and avoid opining on arrest validity. Coordinate with the prosecutor rather than editorializing in the report.