Improved sample preparation and HPLC/MS determination of pesticides in fat-rich vegetables

Improved sample preparation and HPLC/MS
determination of pesticides in fat-rich vegetables
Frank Michel1, E. Barrey2, K. Stenerson2, O.Shimelis2, M.Ye2
1Sigma-Aldrich,
Taufkirchen, GER, 2Supelco/Sigma-Aldrich, Bellefonte/PA, USA
Dispersive Solid Phase Extraction (dSPE, known as QuEChERS) is the mostly used clean-up for the analysis of
pesticides in food prior to LC/MS [1-3]. Fruits and vegetables with a high content of fat usually still exhibit a challenge,
because the fat can interfere with analytes in the HPLC/MS determination.
In this work we present a new adsorbent (zirconia-coated silica) for QuEChERS method. This new particle has got a
higher capacity to retain lipids and pigments than the usually used C18 adsorbent.
• Supel™ QuE Z-Sep/C18
• Supel QuE Z-Sep+
• Mixture of of Discovery® DSC-18 and
Z-Sep (zirconia-coated silica)
Results (continued)
Using the zirconia-coated silica higher recovery of several
pesticides was achieved and ion-suppression by lipids
was reduced (Fig. 4). HPLC conditions on request.
160
Recoveries in Olives
140
Z-Sep/C18
C18/PSA
120
PSA
100
%
Introduction
The usual approach to apply QuEChERS to fat-rich
samples is either to add C18 silica to PSA adsorbent in
the purification step or to freeze out the fat. Fat interferes
by either co-extracting non-polar pesticides compromising
recovery or by co-elution in chromatogram.
Zirconia-coated silica is an interesting approach for lipid
removal, because the d-electron deficient Zirconium acts
a Lewis acid that interacts strongly with Lewis bases such
as phospholipids (Fig. 1).
80
60
40
20
• Dual Bonded C18 and Z-Sep
0
• Use for cleanup of LC-MS samples
ZrO
ZrO
C18
ZrO
ZrO
ZrO
ZrO
C18
C18
ZrO
C18
+
ZrO
ZrO
C18
ZrO
C18
C18
ZrO
C18
ZrO
Fig. 4: Recoveries of different pesticides from olives at 50 ppb (n=3). Quantification against
calibration curve standards made in solvent.
C18
C18
C18
C18
Fig. 1: Graphic of Z-Sep/C18 and Z-Sep+ Particles
Results
Zirconia-coated silica works well for the removal of oleins
from in acetonitrile solution (Fig. 2).
140
100%
120
90%
100
80%
C18/PSA
Z‐Sep(+)/PSA
Z‐Sep+
80
70%
60%
60
50%
40
40%
The mixture of zirconia-coated silica and C18 was applied
in the QuEChERS method for sample prep of olives. The
work-up using this adsorbent was compared to PSA and
PSA/C18. Details of the method can be found in Fig. 3.
Weigh 10 g olive sample into 50 mL centrifuge tube
Add 10 mL acetonitrile, mix
Add citrate buffer, MgSO4 and NaCl, shake for 1 min
Centrifuge at 3200 rpm for 5 min
Add aliquot 0.7 mL of supernatant to adsorbent tubes:
PSA , PSA/C18, Z-Sep/C18
Shake for 1 min and centrifuge at 5000 rpm for 5 min
Dilute (if appropriate), analysis by LC/MS
Fig. 3.: Conditions for extraction and clean-up of olive samples.
Deltamethrin
Cypermethrin
Coumaphos
4,4'‐DDT
Cyfluthrin isomers
Fig. 2: 25 mg of sorbent was mixed with 1 mL solution of oleins (400 mg/mL total concentration)
in acetonitrile. The remaining mono-, di- and trioleins were quantified by LC-ELSD and removal
was calculated.
Methoxychlor
Silica
Endosulfan I
LRA
Chloropyrifos
alumina
Malathion
NH2
Metolachlor
PSA
Heptachlor
C18
Qunitozene
ZSep/C18
gamma‐BHC
Z-Sep+
Simazine
0%
Dimethoate
10%
Hexachlorobenzene
0
20%
Trifluralin
20
30%
alpha‐BHC
Removal
A similar work-up using Z-Sep+, PSA/Z-Sep+ and
PSA/C18 was applied for the sample preparation of
avocado (details on the method on request). The dualbonded Z-Sep+ resulted in the best recoveries of several
pesticides (Fig. 5). HPLC conditions on request.
Fig 5.: Comparison of recoveries of pesticides from avocado using different adsorbents in
QuEChERS clean-up.
Conclusion
• For fatty vegetables using zirconia-coated silica in
QuEChERS method improved clean-up compared to
other adsorbents.
• In olives, Z-Sep/C18 cleaned extracts exhibited better
pesticide recoveries than PSA and PSA/C18. This is
possibly due to ion supression effects from lipids in the
PSA and PSA/C18 cleaned extracts.
• Z-Sep+ provided higher recoveries for the extraction of
pesticides from avocado.
• Zirconia-based adsorbents retain more oleins than
traditional cleanup sorbents.
• More matrix was removed by Z-Sep+ than other cleanup sorbents (data not shown).
References:
1. M. Anastassiades, S.J.Lehotay. et al., J. AOAC International 86(22) (2003) 412-431
2. EN15662:2008
3. AOAC Official Method 2007.01