A 32-year-old woman from rural Maharashtra presents to the emergency department with acute onset of severe abdominal pain, profuse watery diarrhea, and vomiting for 6 hours. Her husband reports she consumed food prepared by a neighbour. On examination, she is hypotensive and has signs of severe dehydration. Serum electrolytes show hypokalaemia and hyponatraemia. Clinical suspicion of acute arsenic poisoning is high. Which investigation is most appropriate for confirming arsenic poisoning?