Validation of liquid chromatography mass spectrometry (LC-MS) methods

Self-test 5.4C

1. Glyphosate was analyzed in the lake water with high organic content. The analyses contained sample purification and concentration with solid phase extraction (SPE). However there was a strong suspicion that a strong matrix effect may occur. To test this suspicion a blank sample extract was treated with SPE and the obtained extract was deviated into small parts. Each part was spiked with analyte on a known concentration level (similar to matrix matched calibration). Also the standard solutions at the same concentration levels were prepared. After the LC-MS analyses following results were obtained:

c (µg/L)	Signal, standard	Signal, spiked sample
2	15931	6002
5	35198	15171
10	73501	25734
15	115041	37313
20	129941	55175
50	361007	124671

Calculate the matrix effect based on the calibration graph slopes.

Please give the answer in percents with one decimal place.

Question 1 of 3

2. Standard addition is one possibility to account for matrix effect. However it assumes that matrix effect is a proportional effect independent of the analyte concentration. In order to control this assumption analyst prepared both standards and post-extraction spiked samples at analyte concentration of 10 and 2.5 mg/kg.

Following results were observed:

Standard 10 mg/kg	Sample 10 mg/kg	Standard 2.5 mg/kg	Sample 2.5 mg/kg
7832180	6964080	2406174	827948
7718368	6722779	2434535	854663
7822817	7387993	2586852	764868

Question 1: Estimate matrix effect for 10 mg/kg and 2.5 mg/kg from the measured 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%).

Matrix effect (10 mg/kg)

Matrix effect (2.5 mg/kg)

Question 2: Can standard addition be used in this case in order to account for matrix effect?

Question 2 of 3

3. Sometimes calibration graph is used for estimating matrix effect. This approach additionally gives possibility to estimate if matrix effect is concentration dependent.

Standard		Sample
C (mg/kg)	Signal	C (mg/kg)	Signal
0.045514	4729453	0.054981	2479148
0.09502	10239100	0.094095	2829679
0.125402	14069331	0.131234	5879648
0.175325	16628949	0.179036	6189080
0.225785	20635051	0.232482	8489240
0.281598	23964984	0.28458	11626711
0.335968	29837123	0.327439	11796479
0.386379	30453933	0.371094	15791685
0.481088	37769230	0.487316	21413804

Question 1: Estimate matrix effect based on calibration graph slopes!

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%).

Answer:

Question 2: Is the matrix effect independent of analyte concentration?

Question 3 of 3