A 3-day-old neonate born to consanguineous parents presents with poor feeding, lethargy, and a characteristic 'musty' or 'mousy' odour on the breath. Serum phenylalanine is markedly elevated at 1200 µmol/L (normal <120 µmol/L). Which investigation is most appropriate to confirm the diagnosis and differentiate classical phenylketonuria from variant forms?