Posted in
Alternate GC Carrier Gases
,
Medical Marijuana
,
Optimizing Applications
|
1 Comment »
Screening for Bifenazate (Floramite) in Medical Marijuana Using QuEChERS and GC-FID – Is it
Possible?
Sunday, May 20th, 2012 by
Jack Cochran
Bifenazate (CAS# 149877-41-8) is an acaricide made by Uniroyal Chemical and sold under the trade name
Floramite
. Floramite
is registered in the US for control of mites on a wide variety of plants, and is widely employed in greenhouses and other indoor
growing environments. Its effective control of spider mites leads to its application by medical marijuana growers too, even
though bifenazate is not registered for use on cannabis.
Although the
Pesticide Action Network (PAN) Pesticide Database
indicates that
bifenazate
has only slight acute toxicity and is
an unlikely carcinogen, its intake by medical marijuana users who already have health issues is not preferred. Therefore,
analytical screening methods for bifenazate in marijuana are necessary to assure that patients are receiving a high quality
medicine.
Given that many medical marijuana patients are often dosed with “edibles”, foods that contain marijuana with pre-determined
cannabinoid content, it makes sense to consider the maximum residue limits (MRLs) for bifenazate on plant product
commodities in lieu of the absent MRL for cannabis (for analytical method development purposes NOT health reasons, as the
tolerance level for an unregistered pesticide is ZERO in the US). The USDA Foreign Agricultural Services
Pesticide MRL
Database
indicates that MRLs for bifenazate on plant product commodities range from 0.1 to 7 ppm, depending on the
commodity.
We recently showed a QuEChERS sample preparation approach with cartridge SPE cleanup and GCxGC-TOFMS for
determining pesticides in marijuana in our
Advantage newsletter
. The complexity of the sample extracts and the desire to work
at the ppb range for over 80 pesticides is what dictated the sophisticated analysis approach, but is that approach always
necessary? Maybe not for screening purposes for bifenazate, but let’s see.
I snagged a marijuana QuEChERS extract and my bifenazate standard and went to work. Previously… Finely ground
marijuana, approximately 2 g, was wet by shaking with 10 mL acetonitrile and 10 mL organic-free water in a 50 mL centrifuge
tube and then allowed to soak for one hour. After vortexing vigorously for 30 min,
QuEChERS EN salts
were added and the
sample was shaken by hand for 1 min, followed by phase separation with a
Q-Sep 3000 centrifuge
for 5 min. The supernatant
was removed and stored in a refrigerator. Back to the present… To make it as easy as possible, no further cleanup was done on
the QuEChERS marijuana extract prior to its division into different sample vials that were spiked with bifenazate at various
concentration levels to be analyzed using simple GC with flame ionization detection (FID), the equipment available to most
medical marijuana laboratories. The goal here was to determine when the response for bifenazate became non-linear due to
matrix interference from the complex extract.
Prior to analysis of the bifenazate-spiked cannabis extracts, bifenazate in acetonitrile standards were analyzed on an Agilent
6890 GC with a
15m x 0.25mm x 0.25µm Rxi-5Sil MS
GC column, by 1µL split injection into a
Sky 4mm Precision liner with
wool
. The inlet was at 275°C and the split ratio was 10:1. Helium carrier was used at 1 mL/min constant flow and the GC oven
was programmed as follows: 80°C (1.5 min), 12.5°C/min to 330°C (0.50 min). The FID used nitrogen makeup gas and was at
350°C.
The calibration curve for bifenazate in acetonitrile standards showed an excellent correlation coefficient of 0.9997 from 0.5 to
200 ppm (in solution). But things got more difficult when moving to the spiked extract and you can see why upon inspection of
the 50 ppm spike chromatogram. Bifenazate elutes in a chromatographically-cluttered cannabinoid region, relatively near to the
large THC peak to its right. The bifenazate calibration curve turned out surprisingly good in spite of this sample complexity, but
closer inspection reveals issues with the lower points in the curve as noted by the response factors creeping up (due to
Medical Marijuana « ChromaBLOGraphy: Restek's Chromatography Blog
15 of 23
02/08/2014 5:38 P
