Cambios en las formas fluviales

The Phase 1-3 investigations (section 3.2) provided progressively more rigorous insight into the methodological compatibility of analyzing acetonitrile-based QuEChERS extracts of crop tissues and growing matrices using proprietary ELISA kits. In Phase 1 (section 3.2.1), analytically large spikes of the target analytes were applied to homogenate of grocery store sweet potato, allowing for large dilutions of the final crop extracts, thereby minimizing the effect of any potentially confounding matrix interferences on the assay. The results from Phase 1

demonstrated that a) the QuEChERS method was capable of extracting the target compounds and b) the ELISA kits for all compounds tested were capable of detecting their respective compounds within the highly dilute extracts. The former was confirmed by both ELISA and GC analysis. Practical detection limits (the lowest analyte concentration in the homogenate from which a linear calibration can be obtained) for using the GC-Ion-Trap-MS to analyze the QuEChERS extracts as prepared, without any additional clean up, were calculated for each compound as shown in Table 33. It was determined in later investigations that caffeine and triclosan were recovered from sweet potato homogenate with better than 90% efficiency, and thus additional extract clean-up and/or concentration would be required to improve upon these values.

Compound Practical Detection Limit (µg/g)

Caffeine 1.026

Triclosan 0.063 Estradiol 0.118 Table 33: Practical Detection Limits for GC-Ion-Trap-MS

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Comparing the practical detection limits calculated for the GC-MS to those using the ELISA kits (Table 34) it was clear that the ELISA approach is far more sensitive with less sample preparation. Even though the values shown in Table 34 assume 100% analyte recovery from the homogenate, these values are 5-6 orders of magnitude lower and

demonstrate the viable application of ELISAs to measuring trace chemical uptake in crops. A demonstration of this was the finding of 6-8ng/g caffeine in store-purchased sweet potato. This would not have been detected using the GC/MS method.

Compound ELISA Quantitation Range Potential Detection Limit Caffeine 0.175-5.0 µg/L 1.75 ng/g Triclosan 0.05-2.5 µg/L 0.5 ng/g Estradiol 2.7-25 ng/L 27 pg/g

Table 34: Potential Detection Limits of ELISA kits

The Phase 2 analysis (section 3.2.2) considered sweet potato leaf extracts at a more environmentally relevant dilution level (i.e. one that would assure that the extracted analytes would be detectable by the assay). Results from this analysis demonstrated the benefit of using dispersive solid phase extraction (dSPE) for preparing extracts for ELISA analysis. Extracts from identically spiked homogenates were prepared with and without dSPE, with similar recoveries observed. This demonstrated that the compounds were not appreciably lost during dSPE. Direct comparison of the extracts prepared with and without dSPE, however, consistently showed that the samples prepared without dSPE reported a

significantly higher concentration. As compounds are not lost during dSPE, it was clear that the elevated responses in the extracts prepared without dSPE were the result of confounding matrix components within the uncleaned extract.

During the Phase 2 investigation analytes were spiked into extracts of an unspiked homogenate prepared without dSPE and clearly demonstrated that the caffeine and triclosan

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ELISA kits were capable of detecting and quantifying the added compound concentration within the complex matrix. Estradiol concentrations were outside the detection range and due to limited availability of the kits the methodology could not be re-evaluated during this phase of the study.

The Phase 3 analysis (section 3.2.3) was the most rigorous method compatibility analysis and was performed using standard addition, spike recovery, and serial dilution of extracts from all matrices of interest alongside those of the various standards employed in the study. The caffeine and triclosan ELISA kits were found to be highly compatible with 10 fold and 20 fold diluted QuEChERS extracts from all matrices tested. As a concluding example of this, consider a final standard addition caffeine ELISA analysis, this time for extracts of the virgin (non-irrigated) soil.

Fit Upper Lower

Unspiked Homogenate Extract 10 0 No Detect No Detect No Detect

Unspiked Homogenate Extract 20 0 No Detect No Detect No Detect

Extract Spiked 10 24.9 +/-7.0 23.2 32.0 16.7

Extract Spiked 20 24.9 +/-7.0 21.6 29.3 16.3

Extract Spiked 40 24.9 +/-7.0 26.3 35.3 18.0

Extract Spiked 80 24.9 +/-7.0 37.3 53.5 24.1

Matrix and sample preparation

Dilution

(x-fold)

Expected caffeine concentration (µg/L) from 10µL spike into

2mL extract

Concentration (µg/L) and 99% CI

range of original extract

Table 35: Caffeine ELISA analysis of extract-spiked and unspiked homogenate extracts of virgin (non- irrigated) soil (VSO) compared to expected responses from spike delivered

Table 35 shows the ELISA analysis of the extract of unspiked homogenate,

unmodified (white) and with 10µL of working solution (~4.99mg/L) spiked into 2mL of the finished extract (grey). These two samples were then serially diluted as shown in Table 35, resulting in a total of six independent ELISA outputs. The resulting concentration for each dilute extract (not shown) was multiplied by the sample dilution factor (column 2) in order to arrive at the best fit concentration and 99% confidence interval (Upper/Lower), within the

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original extract. As seen in Table 35 no caffeine was detected within the extract of the unspiked soil. The concentration observed within the spiked extract was consistent with the expected concentration due to the spike delivered. Indeed, though the number of samples available for comparative analysis is small, a t-test comparing the four (fit) responses in Table 35 to the 3 (fit) responses from the analysis of the working solution (Appendix D.3) returns a p-value of 0.60 (Appendix D.4), indicating that the expected and observed responses for the soil extracts are indistinguishable.

During the Phase 3 investigations, analysis of spiked extracts from crop tissue using the estradiol ELISA kit proved confounded at extract dilutions between 10 and 80 fold (section 3.2.3.4). While the concentrations reported for the sweet potato spiked extracts were elevated compared to those unspiked, the increase observed was only 10-20% of the expected value. For extracts of lettuce leaves, the spiked extracts were completely indistinguishable from those unspiked. It was determined that further investigation would be required for the estradiol ELISA kit and that interpretation would have to be suspended for all 10-80 fold dilutions of crop tissue extracts (such as seen in Phase 1). Analysis of soils using the estradiol ELISA kit proved more successful.

Extraction efficiency of the compounds was found to be variable depending on the matrix, and future work would benefit from an optimization of the matrix homogenization and QuEChERS extraction parameters. In particular, the homogenization of leaf tissue was inconsistently executed during this experiment as unforeseen complications challenged the anticipated homogenization technique and improvisations had to be made in real time. Complications encountered included a) the effects of storage on the leaf tissue prior to sample homogenization (wilting and drying) and b) the limited aggregate leaf mass available

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for some treatment groups. Standardizing and optimizing the homogenization of crop tissues should be thoroughly vetted with these considerations in mind prior to processing

experimental samples. The best extraction efficiencies (greater than 90%) were observed for both caffeine and triclosan from sweet potato tissue homogenates, and this may a function of the more satisfactorily complete homogenization obtained with this matrix.