1. An analyst has a pesticide multi-method for fruits and vegetables. With three of the pesticides – Thiamethoxam, Carbendazim and Pyrimethanil – the method has quite high bias. The analyst wanted to minimize/avoid bias in his analysis and since isotope-labeled compounds were available for these analytes he decided to use the ILIS approach. He did it, but it did not work well. Below is a description of how he tried to do that. What should he have done differently to make the method work better?
The method involves sample preparation using the QuEChERS procedure and subsequent LC-MSMS analysis of the extracts. The following isotope-labeled compounds were purchased for use as isotope-labeled internal standards (ILIS): Thiamethoxam-d3, Carbendazim-d4 and Pyrimethanil-d5. He added the ILIS to the extracts obtained from the QuEChERS treatment and to calibration solutions. Every ILIS was added in such a way that its concentration was the same in sample extracts and in calibration solutions. The analyst chose the concentrations in such a way that the concentration of every ILIS roughly corresponded to the concentration of the respective analyte in the middle of the calibration curve. Ordinary linear regression was used.
| Suggestion for doing differently |
Is it a good suggestion? |
| If possible, the ILIS should not be deuterated compounds but compounds in which some heavier element is labeled. |
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| The ILIS should be added before sample preparation. |
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| Weighted linear regression should be used for calibration instead of ordinary linear regression. |
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| The ILIS should be added at much higher concentration. |
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| The ILIS should be added to each calibration solutions at the same concentration as the analyte in that particular solution. |
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