First you calculate LoD of your method. You cant measure below that value. Other option you can use suitable free concentration method (eg-SPE)

Although heteropoly blue method is well-established for the determination of arsenic in the solution but its optimization requires two factors:

1. Based on multi-components solutions, each requires optimization of its components’ concentrations.

2. The composition of the reagents also depends on the concentration of As.

These factors should be optimized for each particular concentration range. Hence for the lower concentrations there might be different sets of optimizations needed.

Also, For Further Information you can refer to DOI: 10.1016/j.talanta.2006.07.043.

If you want to measure arsenic in water please go through these articles. These might help.

Thank you all for your guidance.

Please look at the following below links which may help you in your analysis:

https://pdfs.semanticscholar.org/337c/3ed06ee6f282c7692e14ddb8ec5346b76e72.pdf

https://www.scholarsresearchlibrary.com/articles/a-simple-spectrophotometric-method-for-the-determination-ofarsenic-in-industrial-and-environmental-samples-using-vanilli.pdf

https://www.academia.edu/16903296/Spectrophotometric_Determination_Of_Arsenic_in_water

Thanks