Methods for impurity profiling
15
and, infrequently, even a significant amount of codeine. However, the analyst
should be careful when making this assessment as a dark brown (black) tar sam-
ple containing high O6MAM can arise from causes other than post-processing
hydrolysis. For instance, if a dark brown, tar-like sample contains high levels of
O6MAM along with significant quantities of O3MAM and little or no codeine, then
the sample could well have originated from a “homebake” process. Also the tar-
like dark brown heroin samples that originate in Mexico typically have a process-
ing-related O6MAM content higher than 6%, with 12% or greater (relative to
heroin) not being at all uncommon. It is thus not always a simple matter to dis-
tinguish between heroin samples that have undergone significant post-processing
hydrolysis and those samples where there was significant hydrolysis during pro-
cessing. Not surprisingly, the task of comparing two samples where one has under-
gone significant post-process hydrolysis versus one that has not undergone any
significant hydrolysis, although not impossible, is very much more difficult. There
are several publications included in the list of references [29-33] that will enhance
the reader’s understanding of the conditions necessary to produce O3MAM and
O6MAM.
Over time a properly prepared and stored heroin hydrochloride (or for that
matter cocaine hydrochloride) will not degrade in any significant manner. In this
context “properly prepared and stored” means fully hydrated +99.5% pure
hydrochloride salt containing no unbound water or acid and stored in the dark at
ambient temperature in a tightly sealed container. Obviously there are few prop-
erly prepared and stored drug samples in the illicit marketplace. As a result, some
degradation over time is common for illicitly produced heroin, in particular when
it is the free base, as the base is less stable than the hydrochloride salt. However,
for high-purity illicit heroin samples the rate of degradation (hydrolysis) is so slow
that it can be difficult to measure year-to-year.
The analyst does need to exercise care in order to avoid hydrolysis of the
heroin when performing impurity profiling analyses. For instance, the hydrolysis
rate for heroin is markedly increased at extremes of pH (e.g. pH < 3 and > 10)
and as a result such routine tasks as liquid-liquid extractions need to be performed
carefully. Gas chromatography (GC) is an analytical tool frequently used in impu-
rity profiling and it can also result in hydrolysis of heroin and/or the trans-esteri-
fication of co-injection compounds. These problems are not limited to heroin,
since most esters are more or less subject to these reactions, as is evidenced by
the well known formation of O6MAM when morphine and aspirin are dissolved
in methanol and co-injected into a gas chromatograph. It is for these reasons that
GC methods utilizing direct dissolution into an injection solvent may not provide
as rigorous a result as do impurity profiling techniques that incorporate a deriva-
tization step. The previous two paragraphs apply equally to illicit cocaine.
GC analysis of heroin without the use of a derivatization step will result in
the formation of three injection port artefacts. One of these compounds has not
been identified (MW = 381), while the others are 15,16-didehydroheroin [34] and
O
6
-monoacetylmorphine. An injection of a heroin sample, in the absence of a
derivatization step, will nearly always result in the
production of some quantity
16
Methods for impurity profiling of heroin and cocaine
of O
6
-monoacetylmorphine in the injection port, where the amount so produced
is a function of injection port temperature and the quantity of activation sites with-
in the injection port. The unknown compound and 15,16-didehydroheroin* are
usually observed at trace levels (< 0.2% relative to heroin) and chromatograph
immediately after heroin on a 100% methylsilicone column. The presence of either
of these two compounds at a level greater than 0.2% relative to heroin may sug-
gest the need for injection port maintenance.
2.
Methods for the determination of major components
Methods for the determination of major impurities frequently incorporate quan-
tification of the primary analyte, that is, heroin. The remaining components in the
sample may or may not be quantified and, if not, the response of these remain-
ing components is typically set up as a ratio relative to the primary analyte. These
methods are often referred to as screening methods, or ratio methods. In actual
fact nearly all impurity profiling methods are ratio methods. A ratio method is
one where the various components of the sample are separated, generally using a
chromatographic technique, concomitant with the tabulation of response measure-
ments followed by determining the ratios of the response measurements against
either an added internal standard or a common sample component. Major impu-
rity analyses are typically used to eliminate samples from comparison that are
clearly different from other samples under examination or to obtain an indication
of the heroin sample origin, that is, South-East Asia, South-West Asia, Mexico
or South America.**
All of the following methods are equally applicable to heroin base and heroin
hydrochloride, although sample preparation may be different for base and
hydrochloride samples. All methods should be regarded as guidance. In general,
minor modification to suit local circumstances will not normally change the valid-
ity of the results. However, any modification must be carefully validated to ensure
that the results have not been compromised. The analytical chemist should also
be aware that not all methods described below are suitable for all types of heroin
sample and that the probability that the method used provides a correct conclu-
sion can vary significantly depending on both the method employed and the exact
nature of the sample.
*Produced in the injection port from the precursor
ͬ
16,17
-dehydroheroinium hydrochloride.
**For a description of chemical characteristics of heroin samples from different source regions,
see the United Nations manual Recommended Methods for Testing Opium, Morphine and Heroin
([6], pp. 8-11).