atomic absorption troublshouting, Chemistry

Dear sir,
We need your consultancy in a technical problem that we are confronting, and we would really appreciate your help
We are in the process of implementing the heavy metal analysis in fish and water by atomic atomic absorption spectroscopy,
I’m still in the sample preparation optimization phase, and I’m confronting some problems,

Il explain in brief the problem in my last results obtained:
For arsenic analysis, 0.2 g sample is digested with Nitric acid At 90?C for 45 min ,then placed in oven for 2 hours at 130 ?C, then diluted to desired volume

1- 0.2 g Unknown fish sample spiked with (2.5ml) 1000ppb arsenic standard, after dilution in 25 ml H2O,the theoretical value must be 100 ppb, but the practical value came out 75 ppb
2- 0.2 g Fish muscle certified reference material ,containing 12.7 mg/kg of arsenic, and since the dilution is in 50 ml, then the theoretical value must be 50.8 ppb, the practical value came out 0.57ppb

Knowing that both samples were treated in the same condition and in the same time, the difference in the end results is very weird, please advise concerning any note that can improve our work,
or detect any mistake has been made,
waiting for your reply,many thanks in advance

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