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. 

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  • 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. 

  • You can also select more robust plasma settings. Depending on what plasma mode you currently have selected, you may just need to use HMI or increase your HMI. If you are running in Low matrix, you can try general purpose. This should improve the robustness of the plasma and help minimize matrix effects. 

  • Thank you. I just run some samples but the IST still unstable after I changed the HMI. Please see the picture. Thank you. 

  • How are your samples being prepared (digested)? 

    I suspect this slight increase is due to the carbon effect in the samples which results in an enhancement of signal. Depending on your digestion, you may have residual carbon in the samples. Since your samples are a biological material that's the likely reason for the small enhancement effects.

    If you are using the blue/orange peripump tubing, you can try to adding 10% IPA to your I.S. and see if this helps since it will better matrix match your samples to your standards and blanks in terms of the carbon effect.

    You can also try using a 1ppm I.S instead (this will deliver ~60ppb to each sample instead of 30ppb from the 500ppb I.S.). You will see less of an effect with the 1ppm solution. 

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