Analysis of Pesticides in Spaghetti Sauce by Direct Immersion SPME Using an Overcoated Fiber

By: Klaus Buckendahl1, Katherine K. Stenerson1, Tyler Young (Summer Intern)2, Robert Shirey2, and Yong Chen2,
1Sigma-Aldrich Chemie GmbH, Taufkirchen, Germany
2MilliporeSigma, Bellefonte, PA, USA

Introduction

In 2014, overcoated (OC) SPME fibers were introduced for use in immersion extractions. Studies using these fibers have shown them to be more amenable to different matrices than standard non-overcoated (non-OC) fibers. Some advantages exhibited by OC fibers are increased method precision, fiber life, and durability.

In this study, an SPME method using an overcoated version of a PDMS/DVB fiber was developed for extraction of pesticides from spaghetti sauce. The optimized method using the OC fiber was then compared to a standard, nonovercoated fiber of the same chemistry for accuracy, precision and durability. Finally, the SPME method using the OC fiber was compared to recovery of the same pesticides out of spaghetti sauce using QuEChERS for extraction and cleanup.

SPME Method
 

sample : 10 mL vial containing 4 g spaghetti sauce and 4 mL 0.1 M phosphate
buffer at pH 7 containing 25% NaCl
fiber : 65 μm PDMS/DVB-OC fiber and standard PDMS/DVB fiber
incubation : 50 °C, 6 min, with agitation at 600 rpm
extraction : 30 min at 50 °C, with agitation at 250 rpm; fiber penetration set to 30 mm
post extraction wash : 30 sec. in deionized water with agitation
desorb : 250 °C, 3 min splitless; injection penetration set to 45 mm
postbake : 260 °C, 5 min, split 10:1 (done in second GC inlet)
  • Final analysis was done by GC/MS-SIM
  • Quantitation was done against a calibration curve from 1-20 ng/g prepared in blank spaghetti sauce matrix. No
    internal standard was used.
  • Spaghetti sauce samples were spiked at 10 ng/g and allowed to equilibrate for a minimum of 3 hours prior to
    extraction. This time was to allow for binding to occur between the matrix and the pesticide analytes.

Results and Discussion

OC vs. non-OC Fiber

Figure 1 shows a comparison of method accuracy between the OC and standard, non-OC fibers for extraction of n=5
spiked spaghetti sauce replicates. Accuracy was determined as amount measured vs. amount spiked, and was
similar between both fiber types. Several pesticides could not be quantitated due to low response or matrix
interference. 4,4’- DDT did not exhibit linear response using the OC fiber and showed erratic and decreasing
response with the non-OC fiber as the runs progressed.

SPME Method Accuracy for Extraction of Pesticides from Spaghetti Sauce; OC vs. non-OC Fiber

Figure 1. SPME Method Accuracy for Extraction of Pesticides from Spaghetti Sauce; OC vs. non-OC Fiber

 

Figure 2 shows a comparison of method precision, measured as %RSD for n=5 replicates, between the OC and non-
OC fibers. As indicated by the lower % RSD values (y-axis), precision was significantly better using the OC fiber.
Reproducibilities for extraction of methoxychlor and several other heavier pesticides were poor using both fiber
types due to low responses.

Comparison of Method Precision between OC and non-OC PDMS/DVB SPME Fibers for Extraction of Pesticides from Spaghetti Sauce

Figure 2. Comparison of Method Precision between OC and non-OC PDMS/DVB SPME Fibers for Extraction
of Pesticides from Spaghetti Sauce

 

Figure 3 shows an example of pesticide response over repeated extractions of spaghetti sauce. For all pesticides
analytes, the non-OC fiber showed a total loss in response after 19 extractions. This was due to contamination of the
fiber and GC inlet liner. This did not occur with the OC fiber. The over-coating made the post-extraction wash much
more effective, thus preventing contamination of the fiber and GC inlet with residual sample. This is evident in the
amount of matrix remaining on the surface of the standard fiber after repeated use. Figure 4 shows the appearance of
the OC and non-OC fibers after 38-40 extractions. The non-OC fiber shows caramelized sample residue and partial separation of the SPME coating from the crimp portion of the assembly.

Pesticide Response from OC and non-OC Fibers over Repeated Extractions of Spiked Spaghetti Sauce

Figure 3. Pesticide Response from OC and non-OC Fibers over Repeated Extractions of Spiked
Spaghetti Sauce

 

SPME Fibers after 38-40 Extractions of Spaghetti Sauce

Figure 4. SPME Fibers after 38-40 Extractions of Spaghetti Sauce

 

Comparison of SPME Method to QuEChERS

Samples of spiked spaghetti sauce were extracted using QuEChERS as described in AOAC Official Method 2007.01.
Extracts were subjected to QuEChERS cleanup using PSA/C18 and concentrated 10X prior to GC/MS-SIM analysis.

Accuracy and reproducibility results for the SPME method with the OC fiber and QuEChERS are compared in Figure 5.
With a few exceptions, SPME yielded results that were comparable to QuEChERS, with accuracies for most pesticides
falling in the range of 80-120%. Several pesticides could not be analyzed due to matrix interference; and this was more
of a problem with the QuEChERS samples. SPME showed a specific advantage for several very hydrophobic
organochlorine pesticides. These analytes showed reduced recoveries by QuEChERS, possibly due to inefficient
extraction, and/or retention on the C18 sorbent used for cleanup.

Comparison of Method Accuracy and Reproducibility; QuEChERS vs SPME. Spiking Level of 10 ng/g in Spaghetti Sauce

Pesticides not analyzed:
4,4’-DDT by SPME - nonlinear response
Mevinphos by SPME - matrix interference
Endosulfan-α by SPME and QuEChERS - matrix interference
Diazinon, malathion, azinphos-methyl by QuEChERS - matrix interference

Figure 5. Comparison of Method Accuracy and Reproducibility; QuEChERS vs SPME. Spiking Level of
10 ng/g in Spaghetti Sauce

 

Conclusions

  • An SPME method using a new overcoated PDMS/DVB fiber was developed for immersion extraction of pesticide
    residues from spaghetti sauce.
  • The overcoated PDMS/DVB fiber showed advantages over a standard, non-overcoated fiber for method reproducibility,
    ruggedness, and physical stability.
  • In comparison to QuEChERS, SPME showed accuracy and reproducibility that was comparable. The SPME method was
    also easier to perform, produced less waste, and was more sensitive. The enhanced sensitivity of the SPME method
    was evidenced by the need to concentrate the QuEChERS extracts prior to analysis by GC/MS-SIM.

Materials

     
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