1. Please read the following description of experiments for determining bias. What should the analyst have done differently?
An analyst needed to determine bias in determination of the residues of some sulfonamide drugs in sewage sludge. The analyst had certified reference material (CRM) of these drugs as an acetonitrile solution containing accurately known concentrations of these drugs. He decided to determine bias by spiking. He weighed two approximately equal (around 10 g) aliquots of the homogenized sample, denoted 1 and 2. To aliquot 2 he added 100 μl of the CRM solution. This amount was picked with the aim of adding approximately the same amount of analyte that was initially in the aliquot. To aliquot 1 he added 100 μl of pure water. Both aliquots were homogenized again and sample preparation was carried out. The analytes were first extracted from sewage sludge samples using pressurized liquid extraction (PLE) and acidified water: acetonitrile in ratio 1:1 as solvent. The obtained extracts were purified using solid phase extraction (SPE) and thereafter concentrated. The concentrated extracts were injected into the LC system. Solvent calibration was used with solutions made in the same solvent.
| Suggestion for doing differently |
Is it a good suggestion? |
| The analyst should have determined bias by analyzing directly the CRM that he had. |
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| More replicates should have been made |
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| The analyst should have taken significantly more CRM solution for spiking because it is useful if the spiked amount is several times higher than the amount of native analyte in the sample |
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| Instead of adding 100 μl of water to the non-spiked aliquot the analyst should have added 100 μl of acetonitrile so that the same solvent is added as to the spiked aliquot. |
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| Instead of adding the CRM solution before the PLE step it should have been added between PLE and SPE steps. |
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