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The Impact of a Dirty Marijuana Extract on Pesticide Recoveries from a Splitless GC Inlet
Saturday, July 30th, 2011 by
Jack Cochran
In a previous post I told you about our experimentation with marijuana (that didn’t sound right…) and
pesticide extraction via
QuEChERS
. An important part of that work was cartridge solid phase extraction (cSPE) cleanup with
500mg CarboPrep 90 /
500 mg PSA cartridges
. To minimize losses of planar pesticides (e.g
hexachlorobenzene
), which we know can occur on carbon,
as
mentioned in another blog
, we eluted the cartridge with acetone:toluene,
a tip we picked up from Jon Wong and Frank
Schenck
(retired) from the US FDA. To evaluate the success of this approach, we quantified spiked marijuana extracts that were
QuEChERS extracted and either cleaned up or not cleaned up with cSPE. Or should I say, we
attempted
to quantify the unclean
extract?
Let me go back a bit and give you our Agilent 6890 Split/Splitless GC inlet conditions to set the stage for the recovery table I
want you to review. For a 1µL splitless injection we used a
Sky™ 4mm ID single taper inlet liner with wool
at 250°C and a
purge valve time of 60 sec. The GC column flow was 2 mL/min, which gave a GC inlet flow of 1.6 mL/min. Now some of you
are probably saying, “Waitaminute, how did he know that inlet flow?” It’s because I use a handy, dandy
Pressure/Flow
Calculator
that you can download free from Agilent. Others are wondering, why does the inlet flow matter? Well, it’s because a
general rule of thumb for a splitless injection purge valve time is to set it so the inlet has been swept approximately 1.5 to 2 times
before opening the split valve. This
should
ensure a quantitative transfer of analytes from GC inlet to GC column. My sweep
with the 0.9 mL volume single taper liner was 1.7 times. Almost perfect, right?
However
, if the sample extract is particularly dirty, and especially if it contains a high concentration of non-volatile material like
chlorophyll
, the analytes have to “chromatograph” out of the co-injected non-volatile material to get to the GC column. And
that may be more difficult if the liner contains wool, and it does, in this case. This leads to seriously reduced recoveries (via poor
transfer) of less volatile analytes of interest, which can be seen in the table below. Hopefully you astute blog readers out there
(do we have any other kind?!) have noticed another effect, and that is degradation of
DDT
, which inflates the
DDD
result, since
DDD is a degradation compound of DDT. Also note the complete loss of
Dicofol
in the unclean extract, a DDT-like pesticide
that is even more sensitive to “dirt” in the inlet. THESE RESULTS ARE FROM THE FIRST INJECTION OF THIS DIRTY
EXTRACT! YES, I AM SCREAMING AT YOU! But only in print…
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