Based in your methods LoQ and the final prepared sample's volume, calculate the minimum amount of citric acid you need to use to reach the LoQ you need for the citric acid sample. Then, take the amount of sample you determined and digest with nitric acid (adding water might be advisable). Evaporate the digestion to a volume that will allow you to adjust the content of nitric acid in the sample to the one on the calibration standards (typically 1 - 10% volume). Proceed to analysis. This should do it.

Dear Sir. Concerning your issue about determination of the concentration of each As, Pb and Iron in a sample of citric acid 99% . I think the following below links may help you in your analysis:

https://www.sciencedirect.com/science/article/pii/0003267094803501

https://pubs.acs.org/doi/abs/10.1021/acsreagents.4096

Thanks

Thank you for your responses