Plant extracts were quantified utilizing LC-MS/MS in conjunction with SIM (selected ion monitoring). I anticipated obtaining extremely low values since the LOQ represents the lowest detectable concentration. However, the values I obtained are excessively high; what might the issue be?
Thank you
Firstly, LOQ is the lowest quantificable concentration. And it depends on how you determined it, it might be due to a poor signal-to-noise ratio, which can be caused by a too intense background signal caused by a failing column, dirty detector...
the LOQ is the ratio of signal over noise. To see better LOQ, you need to increase signal and lower the noise level in the chromatogram.
1 in increase sample sample amount
2 has a better sample cleanup
3 choose the appropriate mass that give you lower noise (may not be the mass gives the max signal)
4 optimize detection parameters i.e. all the voltages of lenses
LOD is defined by the S/N ratio (for example S/N = 3 is considered as a significant signal).
LOQ is the minimum signal for quantification (for example S/N = 10is considered as a measurable signal).
"Zahnit Wafa", LOD and LOQ values are not just determined through math using experimental data, but the values obtained are entirely dependent on the actual LC-MS method choice and instrument settings used. Use a well developed, optimized method/technique to find the highest sensitivity. No single "universal' analysis method exists for all samples and no universal settings for the instrument either. Contact someone local to help you develop a high quality method of analysis for your sample set. Troubleshoot the system to find out what can be changed to improve detection. Review the LC-MS method and settings used to find the reason for the loss of signal over time. It could be something simple such as column fouling, source contamination, sample degradation etc. Optimization and review of the method being used should be performed to insure that the method is: appropriate for the sample(s); the method is selective for the sample; the column and flow path of the system (that includes the source) are being washed properly after each analysis (to reduce fouling) and the system is properly maintained. Please have an experienced chromatographer assist you with the analysis. Perform these steps and you will find the reason for the changes observed.
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