Self-test 5.4B

1.

Aldicarb is a well-known pesticide on fruits and vegetables that is often determine with LC-MS. As a sample preparation method often QuEChERS method is used.

In QuEChERS method 15.00 g of the homogenized sample was weighted into a 50 ml polyethylene centrifuge tube. 15 ml of 1% acetic acid in acetonitrile (v/v)|6 g of anhydrous magnesium sulphate and 1.5 g of anhydrous sodium acetate were added and the tube was vigorously shaken by hand for 1 min. It was necessary to ensure that solvent interacts well with the entire sample and that crystalline agglomerates are broken down sufficiently. The tube was centrifuged at 3000 rpm (900×g) for 1 min. The upper layer of the extract was introduced into a glass centrifuge tube|which contained 50 mg of PSA and 150 mg of anhydrous magnesium sulphate per 1 ml of extract. The tube was sealed and shaken vigorously for 30 s and then centrifuged at 3000 rpm for 1 min. The supernatant was collected for spiking. The pre-concentration factor is 1.

In order to study if matrix matched calibration can be used within the same commodity group (in order to account for matrix effects) the matrix effect was determined in two apple sorts. The extracts of blank apples were spiked at 0.5 mg/kg level in five replica.

As a result of LC-MS analyses following peak areas were obtained:

Question 1: Estimate the matrix effect for both apples! 

Please give the answer in percents with one decimal place and please use dot as decimals separator (i.e. please give 91.7% and not 0.917; 91|7% or 91.765%). Please do not add % in the gap as the % mark is already given after the gap.

Question 2: Can matrix matched calibration be used in this case to account for matrix effect? NoYes

2.

Matrix effect can be calculated both in the signal scale and in the concentration scale. Below signals for post-extraction spiked apple extract and a calibration graph is given. The apple extract was fortified with thiabendazole at 0.05 mg/kg level.

Q1. Calculate the matrix effect from the signals.

Please give the answer in percents with one decimal place and please use dot as decimals separator (i.e. please give 91.7% and not 0.917; 91|7% or 91.765%). Please do not add % in the gap as the % mark is already given after the gap.
%

Q2. Calculate the matrix effect from the concentrations. 

Please give the answer in percents with one decimal place and please use dot as decimals separator (i.e. please give 91.7% and not 0.917; 91|7% or 91.765%). Please do not add % in the gap as the % mark is already given after the gap.
%

Q3. Compare the results of Q1 and Q2 and explain why the results are somewhat different? Open the correct answer by clicking [accordion][acctitle]here[/acctitle]Because the intercept of the calibration graph is significant and therefore concentration and signal are not exactly proportional.[/accordion]

Was your answer correct? almost correctyesno