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Fast, Simple Sample Cleanup

Using QuEChERS SPE Tubes


By Julie Kowalski, Innovations Chemist, Lydia Nolan, Innovations Chemist, Jack Cochran, Director of New Business and Technology, and Irene DeGraff, Product Marketing Manager.
  • Achieve a four-fold increase in sample throughput.
  • Significantly reduce material costs.
  • Convenient, ready to use centrifuge tubes with ultra pure, pre-weighed adsorbent mixtures.
QuEChERS SPE Tubes

Quick, Easy, Cheap, Effective, Rugged, and Safe, the QuEChERS (“catchers”) method for extracting pesticides from food is based on research by the US Department of Agriculture.[1] In addition to using less solvent and materials versus conventional SPE methods, QuEChERS employs a novel and much quicker dispersive solid phase extraction cleanup (dSPE). QuEChERS methods, including an AOAC Official Method [2] and modifications to the methods, have been posted on the Internet.[3] These methods have several basic steps in common:

Step 1: Sample preparation and extraction – Commodities are uniformly comminuted. Acetonitrile solvent is added for a shake extraction. Salts, acids, and buffers may be added to enhance extraction efficiency and protect sensitive analytes. Surrogate standards can be added to monitor extraction efficiencies.

Step 2: Extract cleanup – A subsample of solvent extract is cleaned up using dSPE, a key improvement incorporated in the QuEChERS technique. Small polypropylene centrifuge tubes are prefilled with precise weights of MgSO4 and SPE adsorbents to remove excess water and unwanted contaminants from the extracted samples. After agitation and centrifugation, the cleaned extracts are ready for analysis.

Step 3: Sample analysis – Samples may be pH adjusted to protect sensitive pesticides and/or solvent-exchanged to improve analysis by either GC/MS or LC/MS. Internal standards can be added.

QuEChERS methods are convenient, rugged methods that simplify extract cleanup, reduce material costs, and improve sample throughput. Here we demonstrate the effectiveness of QuEChERS sample cleanup using a multiresidue analysis of pesticides on strawberries.

Experimental

Strawberry extracts were prepared, spiked, and dSPE treated according to Table I. Analytical conditions are presented in Table II.

One microliter splitless injections of the extracts were performed by a Shimadzu AOC-20i autosampler using “mid” injection speed into a Shimadzu QP-2010 Plus GC/MS system operated under the conditions in Table II.

Table I  Modified mini-multiresidue QuEChERS for pesticides from strawberries.

Sample preparation and extraction

Sample:

10g of strawberries were homogenized and placed in a 50mL PTFE centrifuge tube

Solvent:

10mL of acetonitrile were added to homogenate
Shake for 1 minute, until uniform

Salts:

4.0g MgSO4 (powder or granular)
1.0g NaCl
1.0g trisodium citrate dihydrate
0.5g disodium hydrogencitrate sesquihydrate

Salts were added and vigorously shaken for 1 minute. Sample was centrifuged and the supernatant removed for cleanup. Pesticides standards (200ng/mL) were spiked in at this point

Sample extract cleanup

QuEChERS tubes:

1mL of supernatant from the previous step was placed into several 2mL polypropylene centrifuge tubes, each containing one of the following adsorbent mixes:

A. 50mg PSA + 150mg MgSO4 (cat.# 26124)
B. 50mg PSA + 150mg MgSO4 + 50mg C18 (cat.# 26125)
C. 50mg PSA + 150mg MgSO4 + 50mg GCB (cat.# 26123)

Cleanup:

Samples were shaken with the adsorbents for 30 seconds (carbon for 2 minutes), then centrifuged to produce a clear supernatant for GC/MS analysis.

Internal standard:

Pentachloronitrobenzene in a formic acid solution, pH 5.

PSA = primary-secondary amine, GCB = graphitized carbon black

One microliter splitless injections of the extracts were performed by a Shimadzu AOC-20i autosampler using “mid” injection speed into a Shimadzu QP-2010 Plus GC/MS system operated under the conditions below.

Table II  Instrument conditions

Column:

Rtx®-CLPesticides2 20m, 0.18mm ID, 0.14µm (cat.# 42302)

Sample:

custom pesticide mix 200µg/mL each pesticide,
internal standards:
8140-8141 ISTD, 1000µg/mL (cat.# 32279),
508.1 ISTD 100µg/mL (cat.# 32091),
triphenylphosphate 1000µg/mL (cat.# 32281)

Inj.:

1.0µL splitless (hold 1 min.)

Inj. temp.:

250°C

Carrier gas:

helium

Flow rate:

constant linear velocity @ 40cm/sec

Oven temp.:

40°C (hold 1 min.) to 320°C @ 12°C/min.

Det:

Shimadzu GCMS-QP2010 Plus

Transfer line temp.:

300°C

Ionization:

Electron ionization

Mode:

Selected ion monitoring

PSA = primary-secondary amine, GCB = graphitized carbon black

Results and Discussion

Primary and secondary amine exchange material (PSA) is the base sorbent used for dSPE cleanup of QuEChERS fruit and vegetable extracts because it removes many organic acids and sugars that might act as instrumental interferences. A pesticide-spiked strawberry extract (200ng/mL) subjected to dSPE with PSA was used to generate one-point calibration curves. Spiked strawberry extracts subjected to additional dSPE sorbents were analyzed and the results versus PSA dSPE are shown as percent recoveries in Table III. C18 is suggested for use when samples might contain fats, not an issue for a strawberry extract, but it was important to verify that gross losses of more hydrophobic pesticides (e.g. Endrin and DDT) would not occur. GCB is used to remove pigments, and when treated, the pink/red strawberry extract became clear. However, GCB can also have a negative effect on certain pesticides, especially those that can assume a planar shape like chlorothalonil and thiabendazole.

Restek dSPE products in a variety of standard sizes and formats make QuEChERS even simpler. The centrifuge tube format, available in 2mL and 15mL sizes, contains magnesium sulfate (to partition water from organic solvent) and a choice of SPE sorbents, including PSA (to remove sugars and fatty acids), C18 (to remove nonpolar interferences such as fats), and GCB (to remove pigments and sterols). Custom products also are available by request. If you are frustrated by the time and cost involved with your current approach to pesticide sample cleanup, we suggest you try this simple and economical new method.

Table III  Pesticide percent recoveries in strawberry extracts treated with C18 or GCB dSPE, relative to PSA only.

Rt (min.)

Pesticide

CAS Number

Action/Use

Classification

C181

GCB2

9.50Dichlorvos62-73-7InsecticideOrganophosphorus111116
9.67Methamidophos10265-92-6InsecticideOrganophosphorus105107
11.75Mevinphos7786-34-7InsecticideOrganophosphorus112130
12.02o-Phenylphenol90-43-7FungicideUnclassified10697
12.14Acephate30560-19-1InsecticideOrganophosphorus128147
13.89Omethoate1113-02-6InsecticideOrganophosphorus120119
14.74Diazinon333-41-5InsecticideOrganophosphorus108127
14.98Dimethoate60-51-5InsecticideOrganophosphorus124151
15.69Chlorothalonil1897-45-6FungicideOrganochlorine12513
15.86Vinclozolin50471-44-8FungicideOrganochlorine10298
16.21Metalaxyl57837-19-1FungicideOrganonitrogen105117
16.28Carbaryl63-25-2InsecticideCarbamate114111
16.60Malathion121-75-5InsecticideOrganophosphorus124160
16.67Dichlofluanid1085-98-9FungicideOrganohalogen122103
17.51Thiabendazole148-79-8FungicideOrganonitrogen8814
17.70Captan133-06-2FungicideOrganochlorine8891
17.76Folpet133-07-3FungicideOrganochlorine10863
18.23Imazalil35554-44-0FungicideOrganonitrogen11595
18.39Endrin72-20-8InsecticideOrganochlorine104101
18.62Myclobutanil88671-89-0FungicideOrganonitrogen119114
19.074,4-DDT50-29-3InsecticideOrganochlorine10295
19.22Fenhexamid126833-17-8FungicideOrganochlorine11877
19.40Propargite 12312-35-8AcaricideOrganosulfur11095
19.43Propargite 22312-35-8AcaricideOrganosulfur121114
19.75Bifenthrin82657-04-3InsecticidePyrethroid10681
20.04Dicofol115-32-2AcaricideOrganochlorine9854
20.05Iprodione36734-19-7FungicideOrganonitrogen11890
20.21Fenpropathrin39515-41-8InsecticidePyrethroid11396
21.32cis-Permethrin52645-53-1InsecticidePyrethroid10665
21.47trans-Permethrin51877-74-8InsecticidePyrethroid10971
23.74Deltamethrin52918-63-5InsecticidePyrethroid9752

1. 50mg PSA, 50mg C18, 2. 50mg PSA, 50mg GCB

% recovery = RRF C18 or GCB/RRF PSA X 100


References

[1] Michelangelo Anastassiades, Steven J. Lehotay, Darinka Štajnbaher, Frank J. Schenck, Fast and Easy Multiresidue Method Employing Acetonitrile Extraction/Partitioning and Dispersive Solid-Phase Extraction for the Determination of Pesticide Residues in Produce, J AOAC International, 2003, vol. 86(22), pp.412-431.
[2] AOAC Official Method 2007.01, Pesticide Residues in Foods by Acetonitrile Extraction and Partitioning with Magnesium Sulfate.
[3] http://www.quechers.com/

References not available from Restek.

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