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. 

Parents Reply
  • 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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