AL


similar or better than the other two sorbents. Fenhexamide, anilazine and sethoxydim showed significantly better recoveries using Z-Sep/C18 cleanup. This was due to lower background and subsequently less ion suppression in the LC/MS analysis.


Pesticides in avocados. Avocado contains 10–15% fat; in this application, several different classes of pesticides, including organochlorine, organo- phosphorus and pyrethoid, were extracted from spiked samples and analyzed on a single-quadrupole GC/MS operated in selected ion mode (SIM). Homogenized avocado was spiked at 20 ng/g with pesticides and extracted by QuEChERS. For cleanup, three sorbents were evaluated: 1) PSA/C18, 2) Z-Sep+ and 3) Z-Sep+/PSA. Z-Sep+ differs from the Z-Sep/ C18 blend in that the zirconia and C18 are bonded on the same silica particle. The third sorbent mixture was created for this application to determine if addition of PSA offered further background reduction over Z-Sep+ alone. Average recoveries from spiked replicates (n = 3) are shown in Figure 2. Recoveries of many pesticides were highest after Z-Sep+ only cleanup, and the addition of PSA to the Z-Sep+ actually reduced recov- eries. The lowest recoveries were obtained after cleanup with PSA/C18, and these extracts also showed higher background. In the case of the late-eluting pyrethroid pesticides cypermethrin and deltamethrin, high background obscured detection altogether. In addition, PSA/C18 reduced recoveries of some of the more hydrophobic organochlorine pesticides such as 4,4’-DDT and methoxychlor.


Performance of the sorbents for removing background was compared by gravimetric determination of the amount of residue remaining after cleanup of extracts prepared from equal weights of avocado (Figure 3). Compared to no cleanup, the extract cleaned with Z-Sep+ had the lowest weight of residue remaining, indicating that this sorbent retained the greatest amount of background.


PAHs in grilled hamburger. PAHs are formed when fat contacts high- temperature sources such as hot coals. Hamburger that contained 25% fat prior to cooking was grilled to well-done over an open flame. It was then homogenized and spiked at 100 ng/g with a variety of PAHs with


Figure 3 – Comparison of residue remaining from avocado extracts after QuEChERS cleanup with different sorbents.


Figure 4 – Comparison of average recoveries of PAHs from grilled ham- burger after QuEChERS cleanup with different sorbents; spiking level of 100 ng/g.


2–6 rings in their structures. Sample was also reserved for testing with- out spiking, and was used for blank subtraction to determine recoveries. After QuEChERS extraction, the samples were cleaned with four sorbents: 1) Z-Sep (zirconia-coated silica without C18), 2) Z-Sep+, 3) Z-Sep+/PSA and 4) PSA/C18. Due to the hydrophobicity of PAHs, Z-Sep alone was included to evaluate if better recoveries could be obtained with a sorbent that does not contain C18. Samples were analyzed by GC/MS-SIM; the average re- coveries for n = 3 spiked replicates (after blank subtraction) are shown in Figure 4. PAHs are listed on the x-axis in order of increasing size and hydrophobicity. Reproducibility for the set of spikes was very good for all cleanups, with %RSD values less than 10% for all but one compound. The difference in the performance of the sorbents is evident when compar- ing the heavier PAHs. Recoveries of the five- and six-ring PAHs were the highest after Z-Sep-only cleanup. Z-Sep is the only sorbent that did not contain C18, indicating that the presence of C18 decreased recoveries for the heavier compounds. In addition, although not shown here, the Z-Sep cleaned extracts showed the lowest background by GC-MS.


Use of zirconia-based sorbent in SPE


Figure 2 – Comparison of average recoveries of pesticides from avocado after QuEChERS cleanup with different sorbents; spiking level of 20 ng/g.


AMERICAN LABORATORY 33


Analysis of PAHs from olive oil. Olive oil can become contaminated with PAHs through environmental exposure of the fruit and manufacturing


APRIL 2016


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