{"id":1171,"date":"2024-04-11T14:33:56","date_gmt":"2024-04-11T11:33:56","guid":{"rendered":"https:\/\/sisu.ut.ee\/lcms_method_validation\/self-test-6-3\/"},"modified":"2024-04-11T14:33:56","modified_gmt":"2024-04-11T11:33:56","slug":"self-test-6-3","status":"publish","type":"page","link":"https:\/\/sisu.ut.ee\/lcms_method_validation\/self-test-6-3\/","title":{"rendered":"Self test 6.3"},"content":{"rendered":"
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1. <\/span>An analyst has a pesticide multi-method for fruits and vegetables. With three of the pesticides \u2013 Thiamethoxam, Carbendazim and Pyrimethanil \u2013 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?<\/p>\n

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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, <\/sub>Carbendazim-d4<\/sub> and Pyrimethanil-d5<\/sub>. 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.<\/em><\/p>\n

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Suggestion for doing differently<\/strong><\/td>\nIs it a good suggestion?<\/strong><\/td>\n<\/tr>\n
If possible, the ILIS should not be deuterated compounds but compounds in which some heavier element is labeled.<\/td>\n
The ILIS should be added before sample preparation.<\/td>\n
Weighted linear regression should be used for calibration instead of ordinary linear regression.<\/td>\n
The ILIS should be added at much higher concentration.<\/td>\n
The ILIS should be added to each calibration solutions at the same concentration as the analyte in that particular solution.<\/td>\n<\/div><\/div>\t\t\t\t\t
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