ADV06-15_1-412

ick,

asy,

eap,

ffective,

ugged, and

afe, the QuEChERS (“catchers”) method for extracting pesticides from food is based on

research by the US Department of Agriculture.

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

and modifications to the methods, have been posted on the Internet.

These methods have several basic steps

in common:

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 con-

ditions 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.

•

2008 vol. 1

Rtx®-CLPesticides2 Columns (fused silica)

df (µm)

temp. limits

length

cat. #

price

0.18mm

0.14

-60 to 310/330°C

20-Meter

42302

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.

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)

1.1.0g 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 MgSO

(cat.# 26124)

B. 50mg PSA + 150mg MgSO

+ 50mg C18 (cat.# 26125)

C. 50mg PSA + 150mg MgSO

+ 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 and secondary amine exchange material.

GCB—graphitized carbon black

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

Table I

Modified mini-multiresidue QuEChERS for pesticides from

strawberries.

Table II

Instrument conditions.

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 sol-

vent extract is cleaned up using dSPE, a key

improvement incorporated in the QuEChERS

technique. Small polypropylene centrifuge

tubes are prefilled with precise weights of

MgSO

and SPE adsorbents to remove excess

water and unwanted contaminants from the

extracted samples. After agitation and cen-

trifugation, 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.

Sample Preparation