Marijuana and the Cannabinoids

MS for Detection of Cannabinoids 181
tion generated qualifying ions at m/z 327 and 299, but their ion intensities were relatively
low and thereby increased the lower limit of detection to 15 ng/mL. Significantly
better sensitivity has been achieved by monitoring the (M – H) – ions for THCA
(m/z 343) and THCA-d 3 (m/z 346) formed by ESI (23).
Weinmann and co-investigators (21) developed a very rapid LC/MS/MS assay
for THCA in urine using negative ion atmospheric pressure chemical ionization (APCI)
in combination with selected-reaction monitoring. When subjected to collision-induced
dissociation, the (M – H) – ion at m/z 343 fragmented to abundant ions at m/z 325, 299,
and 245. The runtime took 6 minutes, and the lower limit of quantitation was 5 ng/mL.
Investigators in the same laboratory reported using positive-ion turboionspray to
determine THCA and THCA glucuronide in urine by LC/MS/MS (31).
Skopp and Potsch used LC/MS/MS to study the stability of THCA and THCAglucuronide
in urine and plasma stored at temperatures of –20, 4, 20, and 40°C (32).
The analytes and their deuterated internal standards were ionized by turboionspray,
and the protonated molecule ions collisionally dissociated to abundant product ions.
Unfortunately, THCA and other cannabinoids are not as efficiently ionized by
either ESI or APCI as most other drugs. Nevertheless, the advantage of not having to
derivatize an analyte prior to analysis is an inducement to utilize LC/MS rather than
GC/MS.
Potential problems that can occur in determination of THCA in urine include
interferences (27,33), adsorptive losses during storage and extraction (12,29,34–36),
and degradation of THCA as a result of adulteration of a urine sample (37).
3. DETERMINATION OF OTHER CANNABINOIDS IN URINE
Although detection of THCA in urine continues to be the primary method for
identifying recent use of marijuana, Manno and Manno and their co-investigators have
shown that THC and other metabolites of THC are also excreted in urine as glucuronide
conjugates that are not, however, as easily hydrolyzed as THCA glucuronide
(38,39). THC and its hydroxylated metabolites are excreted in urine primarily as etherlinked
glucuronide conjugates that do not undergo hydrolysis under alkaline conditions.
Enzymatic hydrolysis using β-glucuronidase from Escherichia coli at a pH of
6.8 is highly effective in cleaving ether-linked glucuronide conjugates. Manno et al.
have used this method for quantitative analysis of cannabidiol, cannabinol, THC, and
six THC metabolites in plasma and urine. After enzymatic hydrolysis, they extracted
the cannabinoids with hexane:ethyl acetate (7:1), derivatized them with BSTFA, and
analyzed the products by electron ionization GC/MS. Analysis of urine samples by
this method proved useful for estimating the time of marijuana use (14).
GC/MS analysis for 11-nor-∆ 9 -tetrahydrocannabivarin-9-carboxylic acid
(THCVA) has been used to determine whether the presence of THCA in a subject’s
urine indicates the use of marijuana or is solely the result of the use of the prescription
drug Marinol ® (synthetic THC; ref. 40). ∆ 9 -Tetrahydrocannabivarin, a homolog of THC,
is present in most marijuana and is metabolized in the body to THCVA (41). Because
THCVA is a homolog of THCA, the two compounds behave very similarly during
extraction and derivatization but have different retention times and form abundant
ions that differ by 28 amu (40).