Arsenic is present in blood only during acute poisoning. In chronic poisoning arsenic gets deposited in the bone, keratin tissues, hair, nail and skin for many years. In the bone arsenic replaces phosphorous and remains for many years. Arsenic appear in the hair and nails within hours of ingestion. Neutron activation analysis and atomic absorption spectroscopy helps to estimate concentration of arsenic in hair, nails, bone etc. Ref:The Essentials of Forensic Medicine and Toxicology By Dr KS Narayan Reddy, Page 475