Hi, I am a PhD student working at Jagiellonian University, Faculty of Chemistry. My work is related to the development of new analytical methods for the quantification of short-chain fatty acids in biological samples. In my research, I used a gas chromatograph (Shimadzu GC-2030 Nexis) coupled with a triple quadrupole mass spectrometer (Shimadzu TQ-8040). I developed a new GC-MS/MS method for the quantification of short-chain fatty acids, but I have a problem with the carry-over effect. After injection of high concentrations of analytes, such as 10 ug/ml, I observed peaks from the analytes in the blank sample. The problem is a little bit reduced when I frequently change the solvents (water and ethanol) and utilize a long washing procedure after injection, but it still exists. The current washing procedure includes 4x8µl H2O, 4x8µl EtOH, and 4x8µl EtOH. In addition, sequence – 20x8µl H2O, 20x8µl EtOH, and 20x8µl EtOH were utilized for extra washing following the analytes with the highest concentration.
Do you have some experience with the carry-over effect in the GC-MS/MS system and some suggestions to solve my problem?
Dear Joanna,
Can you please clarify:
- by washing do you mean syringe wash?
- what is the purpose of using water in your procedure?
- have you tried to use split mode?
Dear Albert,
To answer your questions:
- yes, washing means syringe wash
- water was used because of the good solubility of analytes in this solvent (many times higher than in ethanol)
- so far, we have used splitless mode, but thanks for this suggestion.
Syringe wash is not the only approach to clean Your system. If You inject "dirty" samples the whole injector (including tubing) might get dirty. A solvent injection between the samples should will help in most cases....... If there are ghost peaks it might help to prolong the GC program.
Dear Joanna,
Alright, so as Ludwig said, a solvent injection between the samples should help. It obviously brings some disadvantages, but in some cases it is the way to do.
Also, I would like to suggest you another syringe washing procedure. Even if water is good for your analytes I would replace it with methanol, and ethanol would replace with less polar solvents, maybe ethyl acetate or TBME.
Please feel free to update if this will not work.
I would isolate the source of carryover. If it is the syringe issue or dirty sample. 1) inject sample with high concentration and follow without using the syringe (hit the start button on GC to star the run) If you see the ghost peak or analyte in the second run, it doesn’t come from the syringe. If yo don’t see, then you need to focus on cleaning the syringe. My favorite cleaning solvent is DMSO.
Dear Joanna,
Based on your question, I understand that the issue may be due to column overloading with analytes (10 µg/mL). If carryover is occurring, it suggests that the analytes might not be completely desorbing in the given temperature program. You may need to optimize the temperature program for fatty acid elution, select an appropriate GC column, implement a high post-run temperature hold, clean or replace the septa, utilize a bake-out protocol, adjust the injection volume, or consider split injection along with syringe rinsing with an appropriate solvent that dissolves fatty acids and matrix.