I used pesticides mix standard for repeatability study, and observed the area of peaks increasing with sequence injections while injecting from same vial. The %RSD is between 8 and 14 for different pesticides when analyzing pure standards (having no matrix effect). I fear that it will further increase (due to matrix effect) when i will analyze spiked pesticides in sample.
I suspect that after injection, few amount of anlyte went in column and some retains in liner and it (retained analyte) also transfered to column in next injection. (a possibility for increasing area with each injection).
The injector i using is PTV with temperature programed as (70C for 0.5 min > 280C @ 200/min for 10min > 70C @ 200C/min at the end). liner is in new condition. Injection volume = 1ul.
Can anyone has idea how i can achieve maximum percision at ppb or ppt level?
hi Muhammad Ayub
Increasing the inlet liner temperature might help.
best of luck.
Although I fully support Fabio's input and running both solvent and instrument blanks is highly advised, I suspect the reason may be more practical - simply a result of too many injections from the same vial.
If I may ask, what solvent are you using? I assume something semi-volatile? And what vial caps? Some are i no ways suitable for multiple injections.
The more times you inject from the same vial, the more you permeate and damage the cap septum. Ultimately, as the needle does not go in the exact same spot every time, the vial content becomes exposed and evaporation slowly progresses. It this is the case, then you may also see a likely trend of increasing peak area as a function of retention time. Have you tried injections from separate vials? Based on the number of repeated injections you have done (and over how long), you could try to emulate the conditions. For instance, if your situation has occurred after 10 injections from the vial, try to inject 10 times from one vial and once from another. Leave them both overnight in the autosampler and check the solvent level the next day. I would imagine the level is significantly lower in the 10x injection vial. An easy and very illustrative test.
Please share the wash solution ingredient, the number of post and pre-injection wash cycles, liner format (packed, drilled, baffle...etc)
Additionally, aspirate air before the sample introducing to the needle as a sandwich injection option. If any improvement occurs you can easily identify the problem of the source.
Emir
"Sandwich" the sample between air bubbles. Make sure your syringe is good, new?, and does not leak. Repeat wit 2x and 4x the standard sample size to see if you are performing accurate injections. If test vials are used several (Many?) times, the solvent will escape through the holes.
Thanks everyone for your valuable suggestions, It may solve my problem.
Please look at the following below links which may help you in your analysis:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5316259/pdf/ijpr-15-809.pdf
Thanks
There are many factors are responsible to get more precise results using GC-MS/MS for the determination of pesticide residues. The main responsible factors are outlined below:
1. Use external standard
2. Use Matrix matched calibration standard to minimize the matrix effect
3. The method used for analysis should be validated following the EC document: SANTE/12682/2019 as it is well accepted and nicely described for the method validation guidelines using GC-MS/MS.
4. Prepare the working standard solutions correctly.
5. To extract the desired pesticides using QuEChERS extraction method.
6. To cleanup the analytes appropriately by adding the proper amount of solvent, sorbent and reagents, incase of cleanup try to avoid the use of GCB or use a minimum amount.
Hello, this might be a simple Occam's razor issue.
Just fill the vial with your sample up to its brim. This way, there will be no extra space for the sample to evaporate while waiting for the next injection, therefore no increase in the concentration.
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