by Katherine K. Stenerson, Michael Ye, Olga Shimelis, Emily Barrey and Michael Halpenny
AL
Sample Cleanup for the Analysis of Pesticide Residues and Polynuclear Aromatic Hydrocarbons in Fatty Food Matrices
Concern about pesticide residues and polynuclear aromatic hydrocarbons (PAHs) in food has led to the regulation of maximum contaminant levels,1,2 and highlighted the need for more sensitive and selective testing meth- odologies that use gas chromatography/mass spectrometry (GC/MS) and liquid chromatography/mass spectrometry (LC/MS). Since samples can- not be injected directly into these instruments, sample preparation and cleanup methods are needed.
Fat-containing samples are a special challenge in the analysis of PAHs and pesticides. These compounds that are often found in the fatty por- tion of samples, and methods to extract them, usually coextract fat. If not removed from the final sample extract, fatty materials can interfere with the chromatographic analysis and/or contaminate instrumentation.
Fatty background can cause interferences in GC/MS and LC/MS analyses, and contaminate GC inlets and columns, resulting in active sites, which reduce the responses of some compounds. In LC, analytical columns with a buildup of fatty background must be washed extensively. Increased background can foul GC/MS and LC/MS detectors, resulting in significant downtime for maintenance and cleaning.
Cleanup approaches include the use of C18 sorbent as part of the QuEChERS (quick, easy, cheap, effective, rugged and safe) method, gel permeation chromatography (GPC) and solid-phase extraction (SPE) with normal-phase sorbents such as silica or alumina gel. While GPC is effec- tive, it requires special instrumentation; normal-phase SPE often uses very large columns and substantial amounts of solvent.
An alternative cleanup method employs zirconia-based sorbents. Zirconia acts as a Lewis acid, interacting with compounds that are electron donors (i.e., Lewis bases). In fats, this includes phospholipids and the hydroxyl groups present on fatty acids, monoglycerides and diglycerides. Z-Sep sorbents (MilliporeSigma/Supelco, Bellefonte, Penn.) are offered in three formulations: a zirconia-coated silica (Z-Sep), zirconia-coated silica blended with a C18 functionalized silica (Z-Sep/C18) and single silica- based material functionalized with both C18 and zirconia (Z-Sep+). The C18 offers additional selectivity for fat removal by providing retention through hydrophobic interactions, interacting with triglycerides through
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the alkyl chains in their structures. In the Z-Sep/C18 and Z-Sep+ sorbents, the zirconia and C18 chemistries together produce a synergistic effect to retain the fatty constituents present in food samples.
Applications using zirconia-based sorbents
for QuEChERS QuEChERS extraction was originally developed to analyze pesticide residues in fruits and vegetables3 and foods.4
as well as other classes of compounds Acetonitrile extraction is followed by a salting-out step. The
resulting extract can be subjected to cleanup using different sorbents. As shown below, the zirconia sorbents were compared to common non- zirconia-based sorbents as part of the QuEChERS method: pesticides from olives, pesticides from avocado and PAHs from grilled hamburger.
Pesticides from olives. Canned olives containing approximately 15% fat were spiked with pesticides at 50 ng/g and extracted using QuEChERS. The extract was then cleaned with different sorbents: primary secondary amine (PSA), PSA/C18 and Z-Sep/C18. PSA removes acidic interferences and some sugars and is often used with C18 for cleanup. Final analysis was performed by LC/MS/MS. Figure 1 shows a comparison of the aver- age pesticide recoveries obtained for spiked replicates after cleanup with the different sorbents. Average recoveries after Z-Sep/C18 cleanup were
Figure 1 – Comparison of average recoveries of pesticides from olives after QuEChERS cleanup with different sorbents; spiking level of 50 ng/g.
APRIL 2016
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