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11
the tailing of this compound is inherent with the liquid phase and does not
appear to affect linearity for the limited ranges used in pesticide calibrations.
However, when the tailing peaks are caused by nonvolatile contaminants de-
posited from sample extracts, the poor chromatography does affect linearity.
Thenonvolatile compounds areusually locatedat the front end of the column
or guard column. The contaminated section of column can be removed by
cutting off a piece of the inlet end of the capillary column or rinsing the col-
umn with solvent. If dirty samples are being analyzed and tailing of com-
pounds is a problem, remove one loop of the guard column. This is usually
enough to eliminate the tailing. If the analytical column is affected, the col-
umn can be rinsed with solvent to remove nonvolatile compounds. Using
methylene chloride, rinse the column from back to front.
The linearity of ECDs for a16- to100-fold concentration range is sufficient to
pass linearity requirements. Linearity for ECDs is affected by the flow rate of
themake-upgas,nitrogenorargon/methane. Toset the flow rateof themake-
up gas, run a calibration curve including
α
-BHC and methoxychlor. Using
response factors, calculate the percent relative standard deviation (RSD) of
each compound. Set the make-up gas flow rate so the percent RSD of these
two compounds is the same. An increase inmake-upgas flowwill improve the
linearity of
α
-BHC butmake linearityworse formethoxychlor. The remaining
pesticideswill exhibit linear curves once themake-up gashasbeen set to give
good linearity for
α
-BHC andmethoxychlor.
Because several of the pesticide compounds, most notably endrin, react with
hot metal surfaces, cold on-column or direct injections are suggested. With
certain GCs this becomes even more important if the sample is exposed to
metal seals.
Cold On-Column Injections
In cold on-column injections, the needle is inserted directly into the column
and the sample extract is deposited. On-column injections work extremely
well for relatively clean samples. If contamination levels are low, and not too
muchnonvolatile residue ispresent (lipids, hydrocarbons, sulfurs, etc.) in the
sample extracts, then on-column injections provide the best detectability and
linearity, and narrowest peak width.
On-column injections are best suited for the analysis of water sample ex-
tracts, where analyte concentration levels are usually low and the amount of
non-volatilematerial is relatively small. Both small and large volumes can be
injected on-column, with the large-volume injections being evenmore sensi-
tive to non-volatile residue. Conventional on-column injections are typically
less than1µL, and require theuse of 0.53mm ID columns. Large-volume, on-
column injections are typically 10µL to 100µL and require the use of a pre-
column to eliminate the solvent. Several suppliers now offer autosamplers
that permit both types of on-column injections. These systems areworth con-
sidering if you analyze relatively clean sample extracts. However, they gener-
ally only provide acceptable results for the drinking water methods (US EPA
Method 500 series). If used for solid and biota extracts, the systems would
require frequent maintenance.
Direct Injections
Direct injections aremade by injecting the sample extract into ahot injection
port liner. The extract vaporizes and the carrier gas transfers the analytes to
the GC column, where they are refocused. In conventional direct injection
ports using aUniliner
®
glass liner, the column is connected to it bymeans of
a press-tight seal at the bottom of the liner. This type of injection port set-up
eliminates contact of analytes with the active metal surfaces below the bot-
The injectionport iswhere a
majorityof analytical prob-
lems occur in the analysis of
pesticides. Themainproblem
is the cleanliness and inert-
ness of the injectionportwith
which the sample extract
comes intocontact.
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✶
For tech support, call
800-356-1688, ext. 4
(814-353-1300, ext. 4)
On-columnor direct injections
are suggestedbecause several
of thepesticide compounds,
most notably endrin, react
withhotmetal surfaces.
With certainGCs, this be-
comes evenmore important
if the sample is exposed
tometal seals.
