Illustrated Guide to Home Chemistry Experiments

Chapter 22 Laboratory: Forensic Chemistry 389

LABORATORY 22.2:

pERfoRm A pRESUmpTIvE TEST foR ILLICIT dRUGS

In making arrests, police officers and drug

enforcement agents are often faced with the

problem of determining whether an unknown

material is a controlled substance. Agents may

strongly suspect, for example, that a confiscated

vial of white powder contains cocaine, heroin,

or another illegal drug, but that’s not sufficient

reason to make an arrest. After all, it’s

conceivable that the guy was carrying around a

vial of baking soda. Not likely, but conceivable.

RIREEqU d EqUIpmENT ANd SUppLIES

£ goggles, gloves, and protective clothing

£ balance and weighing paper

£ reaction plate

£ Barnes bottles or dropping bottles (as required)

£ 1 0 mL graduated cylinder (1)

£ timer (optional)

£ samples to be tested (see note)

£ chloroform (a few mL)

£ methanol (a few mL)

£ sulfuric acid, concentrated (a few mL)

£ formaldehyde, 40% (a few drops)

£ nitric acid, concentrated (a few mL)

£ ferric chloride, anhydrous (0.10 g)

£ distilled or deionized water

In an ideal world, the police would deliver the suspect substance
by courier to a forensics lab, which would immediately test the
substance and report the results. In the real world, forensics labs
are understaffed and overburdened, and it sometimes takes
days or weeks for them to determine definitively whether the
confiscated substance is an illegal drug.

As a workaround for this problem, forensic chemists developed
presumptive drug tests, which can be performed by a police
officer in the field. These tests are fast, easy, and inexpensive,
and are sufficiently sensitive and selective that courts consider
a positive test as sufficient cause for making an arrest. Most
presumptive drug tests are available as packaged kits that include
sample containers, small dropper bottles of various reagents,
Munsell Color Charts for comparison, and so on. Sales of these
kits are generally restricted to law enforcement personnel,
presumably to prevent them being used by buyers of illegal drugs.
As is often true of such restrictions, they’re easy enough to get
around, because the test procedures and the required reagents
are readily available to anyone.

False positives are a limitation of most presumptive drug tests.
For example, a presumptive test for cocaine may provide a
positive result even if no cocaine is present because the test is
“triggered” by a different, completely legal substance. Police
eliminate this uncertainty by running two or more different
presumptive drug tests against the suspect sample. If the results
are negative for one or more of the tests, the sample is unlikely
to contain the suspected substance. If the results are positive
for all of the tests, the sample is likely to contain the suspected
substance, because it’s very unlikely that an innocuous substance
triggered a false positive for all of the tests.

In “Color Test Reagents/Kits for Preliminary Identification of
Drugs of Abuse” (http://www.ncjrs.gov/pdffiles1/nij/183258.pdf),

dR. mARy CHERvENAk CommENTS:

The vial could also contain artificial sweetener. Paul

carries this around. I, of course, carry vials of baking soda.

which I call 183258 for short, the National Institute of Justice

(NIJ) specifies a dozen different reagents/kits that can be used

to perform presumptive drug tests. Many of these reagents/kits

use chemicals that are difficult to obtain or extremely hazardous.

For the purposes of this lab, we’ll use three of those presumptive

tests, all of which use chemicals that are relatively easy to obtain

(although two of them use concentrated acids that must be

handled with extreme caution).

marquis reagent

Marquis reagent, which the NIJ designates A.5, is one of the

most frequently used presumptive drug tests. The Marquis test

is very sensitive and detects a very wide range of illicit drugs,

including amphetamines, mescaline, and opioids like morphine

and codeine. Marquis reagent uses concentrated sulfuric acid