Flame photometry

Principle

• Flame emission photometry: a diluted sample is sprayed (nebulised) into a flame; heat excites some atoms, which emit light at characteristic wavelengths as they return to ground state.
• Emission intensity is proportional to the concentration of the element.
• Sodium emits at 589 nm (yellow), potassium at 766–767 nm, lithium at 671 nm (red).

Reagents and equipment

• Burner with propane (or natural gas) and compressed air, nebuliser, optical filters, photodetector and readout.
• Lithium is added to all samples as an internal standard for Na and K; it corrects for variations in aspiration rate and flame temperature. Caesium is used when lithium itself is measured.
• Standards of known Na and K concentration, deionised water.

Procedure

• Dilute sample and standards in the lithium diluent (e.g. 1:100 or 1:200 per instrument).
• Light the flame and allow warm-up; aspirate diluent blank to set zero.
• Calibrate with standards, then aspirate samples, flushing with diluent between them.
• Read concentration from the ratio of analyte to internal standard emission.

Results

• Typical serum Na 135–145 mmol/L, K 3.5–5.1 mmol/L; local reference ranges apply.

Quality control and pitfalls

• Like indirect ISE, affected by the electrolyte exclusion effect: severe lipaemia or hyperproteinaemia causes pseudohyponatraemia.
• Blocked nebuliser, unstable gas pressure and dirty burner cause drift.
• Haemolysis raises potassium. Chloride cannot be measured.
• Fire hazard: follow gas safety procedures.

Clinical use

• Formerly the standard for Na and K; now largely replaced by ISE. Still used in some labs for lithium therapeutic monitoring (typical target 0.6–1.2 mmol/L, sample 12 hours post-dose).