How to determine, how many times to dilute a sample?

Question

Dear all, 
 We have measured some organic samples (fuels) and used kerosene to dilute the samples. When I changed the dilution times, from 1:5 to 1:10, the results are different. For example, for the content of zinc, it is 25 ppm by 1:5 dilution, 27 ppm by 1:10. Is this a big difference? Could you please expain, how can I determine, which is more accurate? I think from sample to sample, the dilution time could be also different. But I have no idea, how to determine it. 
 Thank you! 
 Best regards 
 Sophia

Agilent · Accepted Answer

Hello sophia , here I am again 
 I believe what you are experiencing is sample transportation issues. As you dilute progressively your sample, your final solution will have physical properties closer to the solvent. This said, when the sample is less diluted, you have physical properties that differ from the calibration curve. There are two ways to solve this - dilute at least 10x your sample (there is an equation that proves that 10x is a number where your final solution comes closer in terms of viscosity as your solvent - I don't remember it now) and do the matrix-matching with mineral oil if necessary; or use internal standardization with Y or Co (if they are not your analytes) around 5-10 ppm to correct the final result. You can purchase the internal standard introduction kit with us (PN 9910124100, using 3710068100 as the tubing) and the internal standard solutions in organic media. In your ICP, choose carefully the Y or Co lines and assign them as internal standard, plus assigning which elements will be corrected by them. It is a simple procedure that you can find out using the Help resource center. 
 I would trust in the 27 ppm result (if you have enough signal and %RSD) since it has been more diluted and closer to viscosity comparing to the analytical curve. 
 Hope this helps! 
 Rodolfo

How to determine, how many times to dilute a sample?

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