A 42-year-old woman presents with severe headache, dizziness, and nausea 6 hours after consuming homemade liquor at a wedding. She has a wide anion gap metabolic acidosis (pH 7.15, anion gap 18). Initial serum osmolality is 320 mOsm/kg (normal 280–295). Which investigation is the most appropriate next step to differentiate between methanol and ethylene glycol poisoning?