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• 4 • 800-356-1688 • 2005 vol. 1
By using application-specific capillary GC
columns, such as our Rtx®-CLPesticides and
Rtx®-CLPesticides2 columns, many laboratories
analyzing organochlorine pesticides can sepa-
rate all of the target compounds. But, with com-
plex matrices, there still can be difficulties in
resolving the target compounds from co-extract-
ed interfering matrix components. Especially dif-
ficult are samples contaminated with chlorinat-
ed organic compounds, such as PCBs. Like the
target analytes, these contaminants produce a
signal on the electron capture detectors (ECDs)
commonly used for this application. In order to
separate the target compounds from the co-
extracted contaminants in many sample
extracts, Restek chemists, in collaboration with
colleagues at LECO Corporation, have investigat-
ed GCxGC technology.
Comprehensive GCxGC is a relatively new, excit-
ing technique that increases chromatographic
peak capacity by enabling the analyst to use two
columns of differing selectivity in a single analy-
sis. By coupling two columns in series, and
incorporating a modulation technique at the
junction between the two columns (e.g., valving
or cryomodulation), it is possible to get the ben-
efit of each column, as in independent separa-
tions. This technique has been reviewed in depth
by Professor John Dimandja1, and the reader is
urged to consult this reference for details. There
are several manufacturers of commercial
GCxGC instruments, and the technique can be
adapted to conventional instrumentation.
In determining which column pair to use for a
GCxGC application, it is important to choose sta-
tionary phases that differ in selectivity. For this
application, we choose an Rtx®-5 column for a
volatility-based separation, in series with an
Rtx®-200 column which is selective for halo-
genated compounds. The second-dimension sep-
aration from this column ensemble is focused
on retention of halogenated compounds, and
separates the target compounds from some of
the possible interferences in the sample matrix.
Figure 1 is a GCxGC chromatogram of 22 com-
mon organochlorine pesticides, obtained from
the Rtx®-5 column/Rtx® -200 column ensemble in
a LECO GCxGC/ECD instrument. Table I lists the
compounds and the independent retention times
observed in the two dimensions of separation.
By having two independent retention times, from
two different columns, we obtain a primary col-
umn separation and a secondary column confir-
mation for the target compounds, so this tech-
nique should be in compliance with any method-
ology requiring a primary column/confirmation
column approach.
Additionally, an analysis of a spiked extracted
food sample (tomato) shows we can separate
the target compounds from many co-extracted
interferences (Figure 2). Recovery values for the
spiked sample, listed in Table II, are in agree-
ment with “known” values, indicating little to no
matrix interference with target compound quan-
tification, even for a difficult matrix like a food.
A secondary benefit of using cryomodulation at
the column junction is peak sharpening prior to
“injection” of an analyte onto the second col-
umn. This has the effect of increasing sensitivity.
Due to this analyte refocusing effect, we
obtained linear calibration for these compounds
over a 25-fold wider range of concentration than
by conventional GC. The compounds for which
detection is most sensitive (e.g., the hexachloro-
cyclohexanes, or BHCs) normally are calibrated
from 5 to 80 pg/µL. We were able to calibrate
from 0.2 to 80 pg/µL, thus greatly extending the
reporting limit. We also were able to employ
split injection, which typically reduces injector-
related problems, such as analyte adsorption
and breakdown.
Analysis of Organochlorine Pesticides
Using 2D-GC with Rtx® -5 and Rtx® -200 Capillary GC Columns
by Frank Dorman, Ph.D., Director of Technical Development
•GCxGC analysis combines primary column and confirmation column results.
•Separate target compounds from co-extracted contaminants in sample extracts.
•Analyte refocusing effect increases sensitivity.
•Combination of Rtx®-5 and Rtx®-200 columns resolves all target pesticides.
Figure 1 GCxGC analysis of organochlorine pesticides combines primary column and confirmation
column results.
Figure 2 Organochlorine pesticides separated from interferences in tomato extract.
Columns: Rtx®-5 9m, 0.18mm ID,
0.20µm (10m column, cat.#
40201, with 1m removed)
Rtx®-200 1m, 0.18mm ID,
0.20µm (1m of 10m column,
cat.# 45001)
Inj.: 1µL, split, 250ºC, split ratio
50:1
Oven: Primary: 50ºC (0.2 min.),
30ºC/min. to 140º (no hold),
5ºC/min. to 250ºC (no hold)
Secondary: 50ºC offset from
primary oven
Instrument: LECO GCxGC/ECD
Modulator: Temperature offset: 30ºC
Modulation time: 6 sec
Det.: ECD, 325ºC, 150mL/min.
nitrogen makeup gas, 50Hz
Peak identifications listed inTable I and
Table II.
Conditions: see Figure 1
Pittcon® presentation
Jack Cochran, LECO Corporation, will present this information at the 2005 Pittsburgh Conference in
Orlando, Florida. (1000-7)