Document 6533160

Determination of Pesticides in Strawberries: A Comparison of Sample
Preparation Techniques SPME and QuEChERS
Blair Surridgex, Brenna Stanfordx, Rene Bilodeaup
xApplied
pMB
Chemistry and Biotechnology Camosun College,Victoria BC
Laboratories Ltd,, Sidney BC
Project Goals
1) To evaluate conditions for SPME-GC/MS for a diverse set of pesticide
compounds.
2) To compare extraction methods of SPME to QuEChERS for select pesticides.
3) To evaluate DRS as an option to SIM for the identification of pesticides.
Results - Calibration
Introduction
Calibration solutions were prepared using organic strawberries (Andrew and Williamson) . Example
calibration curves for γ-chlordane is provided below for QuEChERS- left and SPME-right. A linear regression
model was used with 1/x weighting in each case. For QuEChERS the internal standards cis-permethrin –C6
and chlorpyrifos-D10 were added before GC/MS analysis to correct for instrument variation. These internals
standards were evaluated for SPME with limited suggest however more work is required.
Ensuring pesticide residues in agricultural products are below acceptable limits
is important to both growers and producers as well as the general public.
Significant advances have been made to laboratory analysis1,2, however, the
cost of analysis remains a barrier for extensive testing of commodities. Also,
most tests do not cover an extensive list of pesticides. We have been
investigating two methods of extracting of 73 pesticides including
organonitrogens,
organphosphates,
organochlorines,
and
common
contaminants such as PCBs and phthalates. These pesticides were selected as
they were most frequently detected in a recently published CFIA report3. The
automated solvent less extraction technique headspace SPME (solid-phase
micro-extraction) was compared to QuEChERs (Quick, Easy, Cheap, Effective,
Rugged and Safe). Analysis was carried out by GC/MS using both SIM and open
scan acquisitions with retention time locking. Open scan acquisitions will allow
for mass spectral analysis and identification using DRS (Deconvolution
Reporting Software). The acetonitrile extracts from QuEChERS were also
analyzed by LC-MS/MS, however this data is not reported here.
.
g-Chlordane
g-Chlordane
Response
Response Ratio
r2-0.998
4.00e+004
r2-0.984
Discussion
0.5
The HS SPME automated technique has been shown to be a simple and fast
method of measuring a diverse set of organic pesticides and contaminants
at low concentrations.
2.00e+004
0
0
1
2
Concentration Ratio
0
3
0
50
100
Concentration
The detection limits were estimated as an amount equivalent to a signal-to-noise ratio of 3 to 1. This was
calculated from a spiked sample at a low concentration prepared in organic strawberries.
In order to calculate the % recovery homogenized strawberries were spiked at 90ng/g before being taken
through the procedure. Quantitation was achieved using standards spiked into the final extract before
GC/MS analysis.
Select
Compounds
RT (min)
Ion
(m/z)
Headspace SPME Method
SPME
LOD
ng/g)
QuEChERS
LOD
(ng/g)
% Recovery
QuEChERS
SPME for 20minutes with a 65µm PDME fiber at 60oC
Desorption in GC inlet at 270oC for 2 minutes
Other preliminary extraction tests were carried out on the following
vegetables and fruits: Green Beans, Radish, Tomatoes, and Grapes. Only
radishes were purchased as an organic product.
12.7
197
0.06
5.4
108
Ethion
15.7
231
3.73
2.8
113
166
177
Fenthion
12.9
278
0.10
1.8
98
155
Malathion
12.7
125
13.24
NA
106
103
Organochlorines
QuEChERS Method
15.0 grams of sample + 15mL of 1% AcOH in ACN I and mix
Aldrin
12.9
66
0.04
3.3
79
147
a-BHC
9.9
183
0.02
2.9
131
117
b-BHC
10.4
219
0.04
15.4
95
92
14.2
375
0.01
0.5
92
138
141
a-Chlordane
14.4
375
10.9
266
2,4’-DDT
16.4
235
Dieldrin
11.9
263
Buprofezin
15.0
105
0.09
12.8
98
135
Cyprodinil
13.5
224
8.45
2.6
Na
130
Diphenylamine
9.1
169
0.13
2.6
101
93.7
GC : Agilent 7890 with CTC CombiPal including SPME/Headspace
Triazine
18.5
239
1.26
3.6
74
132
Column: ZB-5MS, 30m x 0.25mm x 250µm (Phenomenex)
Injection volume: 1µL
Inlet Conditions: Pulsed Split less at 270oC
400
Retention Time Locking:
Methyl chlorpyrifos , 12.7 min at 22.5 psi
Permethrin
19.4
183
NA
1.4
105
g-Chlordane
Add MgSO4/NaOAc mix then spin at 1500 rcf for 1minute
Chlorothalonil
Aliquot 1.0mL into tube containing PSA, MgSO4, and C18
Mix and spin at 1500 rcf for 1minute
Add internal standards
0.03
1.1
95
1.95
18.8
100
88
0.06
0.7
93
165
3.02
3.1
103
107
Organonitrogens
LC-MS/MS Analysis
GC/MS Analysis
Temperature (oC)
The 30µm PDMS fiber gave reasonable results as a first fiber tested. A
mixed phase fiber of DVB/PDMS was also evaluated. Most of the
compounds dropped in peak area response and some remained constant.
% Recovery
SPME
Chlorpyrifos
Analysis by GC/MS (SIM & open scan acquisition)
Ionization mode: EI (+) ion
Source Temp: 250oC
Acquisition: SIM (5 segments)
mass scan: 50-400 m/z
The reported column oven temperature was found to be optimal for most
compounds. The initial column temperature was found to have a
significant effect on peak shape and 70 oC was determined to be optimal.
Organophosphates
1.0 grams of sample + 3mL of 1% MeOH in NaCl sat’d water
Mass Spec : Agilent 5975C
The detection limits obtained by SPME were generally 80x lower than
QuEChERS. Detection limits ranged from 0.5 to 18.8 ng/g for QuEChERS
and 0.01 to 3.7 ng/g for SPME. Internal standards were applied to the
QuEChERS method of analysis. The internal standards used for QuEChERS
were not suitable for SPME extraction therefore external standard
quantitation was used for SPME. The %recovery for QuEChERS was
generally better (mean-95%, RSD-9.2%) than SPME (mean -128%, RSD-21%).
Reasonable calibration curves were obtained (r2>0.98) for SPME however
QuEChERS calibration curves were typically better (r2>0.99).
150
Results – Detection Limits and % Recovery
Experimental Methods
Approximately 1kg of fresh strawberries were chopped in a Osterizer food
processor. A 0.2kg portion was then homogenized thoroughly using a with a
Braun hand blender. This material was then frozen before analysis. The initial
sample extraction procedure and the aqueous solvent for headspace SPME was
reported elsewhere4. A DVB/PDMS fiber was also evaluated under the conditions
shown below. The procedure for QuEChERS has been reported elsewhere5.
GC/MS Analysis
GC/MS Chromatogram of Cyprodinil in a strawberry sample
Concentrations:
QuEChERS: 582 ppb
HS SPME: 700 ppb
Column Oven Temperature
Program
Future Work
1. Different SPME fibers types/sizes and extraction temperatures will be
investigated to improve extraction recovery of organonitrogens and
organophoshate compounds. One such fiber is the 85µm polyacrylate fiber.
2. Direct immersion SPME technique will also be investigated to improve
extraction recovery. To accomplish this the extract containing the extracted
material will filtered or spun using a centrifuge.
3. Triphenyl phosphate will be evaluated as an internal standard for SPME and
QuEChERs.
4. Application of deconvolution reporting software (DRS) will be investigated
further as a method of detection .
5. To apply the developed methods to other fruits and also to vegetables.
Acknowledgements
Funding for this project was obtained through NSERC’s College and Community
Innovation (CCI) Applied Research and Innovation (ARTI) fund. We would like to
thank Wendy Riggs and Dr. Harry Hartman from MB Labs Ltd for time taken to set
up the project.
References
1.
2.
3.
4.
5.
Lehoytay, S.,Kok A., Hiemstra M., Bodegraven P. (2005) J. AOAC Int.88, 595
Navalon A., Prieto A., Araujo L., Vilchez J. (2002) J.Chrom. A. 975, 355
CFIA Food Safety Action Plan Report, 2009-2010 Targeted Surveys Chemistry
Thermo Scientific Application Note: 51943
AOAC Official 2007.01 method
Contaminants
Anthracene
11.1
178
0.08
2.8
101
122
Phenanthene
10.9
178
0.04
2.7
92
121
PCB 28
11.9
256
0.03
1.8
85
131
PCB 101
14.3
326
0.04
1.3
86
117
300
200
100
0
0
10
Time (mins)
20
30
LOD – limit of detection RT – retention time m/z – mass to charge
This poster was presented at the
Western Canada Trace Organics
Conference