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Food Safety
So long, troublemakers I
The QuEChERS method may be a recent arrival on the scene, but it is conquering the world. QuEChERS provides
fast and reasonably priced extraction, enabling efficient determination of pesticide levels in agricultural samples.
GERSTEL application chemists have used automated Disposable Pipette Extraction (DPX), a miniaturized dispersive
SPE technique, combined with GC/MS to optimize the determination of pesticides in QuEChERS extracts. Interfering
matrix compounds can be chased off quite easily - without laborious sample preparation.
Pesticides have long helped provide ample,
affordable, and safe food supplies for billions
of people across the globe. Even so, constant
vigilance is needed in order to protect
the environment and consumers from the
consequences of improper application of pesticides
to plants and crops. When it comes to
food safety, the necessary first step is always to
find efficient ways of controlling the quality
of our food on a large scale in our globalized
economy. Just the pesticides that can legally
be used represent a list of hundreds of compounds
with widely different characteristics.
Multimethods are required,
spanning both liquid chromatography
(HPLC) and
gas chromatography (GC)
combined with Mass Spectrometry
(MS). In theory, the
analytical chemist can track
down pretty much every pesticide
known to man. In practice,
the right sample preparation
is critical if one wants to
achieve accurate results; and it
had better be automated, we’re
talking about a big job.
In just a few years, the
QuEChERS method (Quick,
Easy, Cheap, Efficient, Rugged,
and Safe) has become the
method of choice for extracting
toxins from a variety of foods.
Initially, QuEChERS was
developed as a fast and inexpensive method
to extract pesticides from various, mainly
plant-based matrices. Validation studies have
proven that the QuEChERS method results
in good recovery and low standard deviation
for a wide range of pesticides. Furthermore,
the QuEChERS method is much less laborintensive
and requires much less solvent than
previously used methods. A wide range of pesticides
can be extracted. In many laboratories,
QuEChERS has caused a veritable productivity
boom.
QuEChERS extracts the
analytes – and more...
Every solution results in new, unexpected
and interesting challenges and there is a
price to pay for going QuEChERS: Fruit
and vegetable extracts produced using the
QuEChERS method contain large amounts
of matrix residue. But „quick and dirty“
is fine if we can get rid of the dirt afterwards.
There are two ways to accomplish
this: Clean up the extract or use analytical
instruments that can handle samples with
matrix residue. If we take a separate
look at each of these alternatives, further
clean-up of QuEChERS extracts is
typically performed using
manual dispersive solid
phase extraction (SPE) followed
by centrifugation to
remove solids. These steps
are not easily automated,
but the same clean-up effect
can be accomplished
using Disposable Pipette
Extraction (DPX), a dispersive
SPE technique that
is fully automated. In this
article, examples are presented,
which show that
The GERSTEL MultiPurpose Sampler (MPS)
PrepStation enables combined DPX extraction
and sample introduction to the GC.
GERSTEL Solutions Worldwide – No. 11 19

Spinach extract
Orange extract
without
clean-up
clean-up
with
DPX-Qg
without
clean-up
clean-up
with
DPX-Q
clean-up
with
DPX-Qg
Spinach and orange extracts
before and after DPX clean-up.
Schematic diagram of the DPX clean-up.
DPX is an attractive and efficient alternative
for clean-up of spinach and orange extracts.
Should the analyst not want to perform further
clean-up of such extracts prior to GC/
MS analysis, the only remedy is to replace
the GC inlet liner at regular intervals. Liner
exchange is normally a time-consuming
Full scan chromatograms of the spinach extract before (A) and after
(B) DPX clean-up.
SIM chromatogram after DPX clean-up of spinach extract spiked to a
concentration of 200 ppb with a standard pesticide mixture.
task, but GERSTEL‘s Automated Liner
EXchange (ALEX) performs Liner EXchange
automatically, enabling the analysis
of samples with undissolved sample matrix.
Matrix build-up has negative consequences
especially for the GC/MS system: If raw
QuEChERS extracts are injected directly
into the GC, residue will accumulate in the
inlet. This build-up will
lead to compound loss
by adsorption on active
surfaces and increased
variability, negatively
impacting the results.
In combination
with the MultiPurpose
Sampler (MPS),
ALEX performs automated
liner exchange at
user-defined intervals.
Disposable Pipette
Extraction (DPX
DPX is an SPE technique
that relies not
on packed adsorbents
in standard cartridges,
but on adsorbent powder
placed inside disposable
pipette tips.
In the case of spinach
and orange extracts,
graphitized Carbon
Black was used among
other adsorbents (DPX
Qg-tips, as specified
in DIN EN 15662).
Plant colorants, such
as chlorophyll, and free
acids were successfully
removed. The transport
adapter at the top and a
frit at the bottom help
contain adsorbent and
sample inside the pipette
tip while enabling
highly efficient airbubble
induced mixing.
The transport adapter also serves the dual
purpose of allowing the MPS to get a grip
on the cartridge in order to transport it and
to introduce the syringe needle into the cartridge
for liquid transfer. The DPX process is
clever yet simple: The MPS picks up a DPX
tip from the tray. Depending on the method,
the adsorbent can be washed with a suitable
solvent, which is taken from a solvent reservoir.
The solvent can either be aspirated into
the tip from below or added to the top using
the autosampler syringe. A 500 µL sample of
the extract in question was aspirated into the
tip. Extracts had been spiked with organochlorine-
and organophosphorous pesticide
standard mixtures at different concentration
levels. Samples were aspirated into the
DPX tip from below, which means they were
never in contact with the syringe needle or
piston. „There is no sample-to-sample cross
contamination or carry-over“ said Carlos
Gil, Manager, Analytical Services at GER-
STEL Headquarters, while adding: „Since
DPX is a dispersive SPE technique, the extraction
efficiency is not influenced by the
flow path or the flow rate through the adsorbent,
making the technique highly rugged
and reliable“. Once the sample has entered
the DPX tip, the syringe pulls air through
the tip and the sample from below. The liquid
suspension undergoes highly efficient
turbulent mixing leading to optimal contact
between the phases and highly efficient and
fast extraction. „The efficiency of the cleanup
is clearly demonstrated by the fact that
the final spinach extract is almost completely
colorless“, says Carlos Gil. The extraction
takes place in less than two minutes. Then
the cleaned QuEChERS extract is then
transferred directly to a clean autosampler
vial for analysis or to undergo further liquid
sample preparation steps prior to the analysis
as needed. The used pipette tip is discarded.
As soon as the prepared sample has been introduced
to the GC system, a clean pipette
tip is picked up and the next sample prepared.
„Analysis and sample preparation are
performed in parallel, ensuring best possible
utilization and return on investment for the
entire instrument set-up“, says Carlos Gil.
20 GERSTEL Solutions Worldwide – No. 11

The power of DPX - conclusion
Apart from the visible removal of spinach and orange matrix, the
analysis results bear testament to the efficiency of the DPX process.
Carlos Gil: „The results were convincing, we had excellent
recovery of the organochlorine- and organophosphorous pesticides
that were determined in the study“. The relative standard
deviation (n=3) was under 10 % both for the extract spiked at 20
ppb and for the extract spiked at 200 ppb. Average recoveries were
119 % for the orange sample and 91 % for the spinach. The study
proved that automated DPX is useful for second stage clean-up
of QuEChERS extracts prior to GC/MS analysis. The DPX tips
efficiently removed interfering matrix material, improving overall
system reliability and productivity, and reducing the need for
maintenance since there was less residue build-up in the GC/MS
system.
Method parameters
The analysis was performed on a GC 7890 / MSD 5975C
GC/MS system (Agilent Technologies) configured with a
GERSTEL Cooled Injection System (CIS) PTV-type inlet and a
GERSTEL MultiPurpose Sampler (MPS) sample preparation
robot fitted with a 10 µL syringe for liquid injection.
Analysis parameters
CIS 4: splitless
25 °C; 12 °C/s; 280 °C
(3 min)
Column: 30 m DB5-MS (Agilent)
di = 0.25 mm; df = 0.25 µm
Carrier gas: He, constant flow
1.0 mL/min
GC oven: 60 °C (1 min); 10 °C/min;
300 °C (3 min)
Standards
A standard mixture of organochlorine- and organophosphorous
pesticides at a concentration of 1000 µg/L was prepared
in Acetonitrile.
Sample preparation
For quantification purposes, fruit and vegetable extracts were
spiked with diluted pesticide standards. (Concentrations: 20
µg/L and 200 µg/L in 500 µL extract).
DPX extraction
1 mL QuEChERS tips from DPX-Labs, LLC. 500 µL of the
fruit- or vegetable extract was automatically transferred to a
vial by the MPS fitted with a 2.5 mL syringe. A 1 µL aliquot
of the extract was introduced to the GC.
More information
http://www.gerstel.de/pdf/p-gc-an-2009-01.pdf
The sequence table for automated DPX sample preparation is easily and quickly
set up by mouse-click in the MAESTRO.
Orange
Spinach
Analyte % Recovery % RSD % Recovery % RSD
20 ppb 200 ppb 20 ppb 200 ppb 20 ppb 200 ppb 20 ppb 200 ppb
Dichlorvos 139 80 14 15 92 51 8.3 15
Mevinphos 89 68 15 10 60 34 8.3 15
Phorate 68 122 16 3.7 16 93 32 5.2
α-BHC 100 113 5.9 4.4 47 76 16 6.2
δ-BHC 126 89 6.3 8.2 105 51 17 24
Diazinon 151 116 5.1 4.4 117 96 2.5 3.9
Methyl Parathion 263 104 7.9 12 165 54 20 14
Ronnel 97 75 5.5 5.8 95 63 11 9.8
Aldrin 173 138 5.2 3.4 148 124 1.9 3.3
Trichloronate 119 74 26 3.6 152 87 8.3 6.8
Heptachlor Epoxide 142 135 4.3 3.0 120 102 4.2 3.4
t-Chlordane 147 140 4.9 2.7 135 116 3.7 3.5
Prothiofos 131 98 1.9 4.0 162 104 4.7 6.3
Dieldrin 168 137 5.2 2.4 128 123 9.0 3.2
Endrin 167 149 6.5 4.0 142 118 5.4 3.9
β-Endosulfan 156 134 4.7 2.9 138 102 12 7.4
Fensulfothion 121 142 6.9 5.1 63 88 4.6 11
Sulprofos 196 136 3.6 4.3 200 122 4.3 6.4
DDT 213 179 4.5 11 208 117 6.9 7.3
Endrin Keton 174 144 2.6 3.8 138 97 2.1 7.7
Average 147 119 7.6 5.7 122 91 9.1 8.2
Percent recovery and relative standard deviation for the pesticides.
Analyte Orange Spinach
No DPX DPX No DPX DPX
Dichlorvos 128 80 120 51
Mevinphos 179 68 145 34
Phorate 180 122 170 93
α-BHC 158 113 150 76
δ-BHC 213 89 170 51
Diazinon 162 116 160 96
Methyl Parathion 483 104 300 54
Ronnel 196 75 195 63
Aldrin 204 138 210 124
Trichloronate 242 74 245 87
Heptachlor Epoxide 198 135 155 102
t-Chlordane 167 140 175 116
Prothiofos 257 98 265 104
Dieldrin 198 137 260 123
Endrin 197 149 195 118
β-Endosulfan 192 134 180 102
Fensulfothion 197 142 165 88
Sulprofos 246 136 250 122
DDT 224 179 195 117
Endrin Ketone 168 144 155 97
Average 209 119 193 91
Recovery of the pesticides with and without DPX clean-up
(spiked to a concentration level of 200 ppb).
GERSTEL Solutions Worldwide – No. 11 21