7800 internal standard ratio keep going up

I am very frustrated about the ISTD recovery ratio keeps going up, I run total around 150 animal tissue samples, I treated samples so matrix all match, but after 70 samples the ISTD keeps going up to 120% and data shows abnormal from ref QC. Machine performance good, Tune good, everything looks good at beginning but all of sudden it goes crazy. Could you give me some idea what is going on? Sometime it it goes down to 75%. Thank you. 

  • What concentration is your internal standard solution? Are you using online internal standard addition using the mixing T or adding the internal standard to all the samples? How old are the cones and have you conditioned them for this matrix? 

  • Thanks Chelsea, I use 500ppb IST and  mixing T, I  just replaced with new cone but conditioned with "dirty" sample for 2 hours and washed 1 hour.  The problem might from new tubing and forgot to adjust the volume of the sample. Thank you for your reply. 

  • Another thing you want to check too are the I.S. RSDs. This can help pinpoint tubing and mixing issues. Another culprit for erratic I.S recovery can also be plasma conditions not being robust enough for the sample type. Best of luck and if you have more issues after checking the tubing just let me know. 

  • Thank you. There are a lot of knowledge for me since my background is biology not engineering, kinda hard for me. BTW how can I tell the plasma robust enough? Thanks. 

  • The easiest way to tell is to determine if there are extreme matrix effects when running a sample. This is often seen with significant enhancement or suppression of the I.S on samples but not during calibration or on check samples like QC. When there are too many ions introduced you experience plasma loading (too many ions so poor ionization) and space charge effects (too many positive charges crowded together during focusing).

    That or if you experience significant downwards drift during an analysis as this is indicative there is too much buildup occurring in the sample intro. This occurs on the nebulizer and cones as they become more clogged with buildup so sensitivity decreases.

    HMI is designed to improve plasma robustness for higher matrix samples so choosing the proper settings for a sample matrix is important to achieve stability. 

  • Yes this is what I often see the drifting of the IST during the assay, usually the IST very beautiful during the standard solution and the rinse solutions but when begin to assay samples the IST begins to change, either down or goes up. I usually 5% NA matrix in all samples including the standard and rinse. Maybe the sample tissues are too much then more dilution X will improve this I think. Thank you. 

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