Sodium and potassium are determined by flame photometry using a Model FP640, dual channel photometer, running LPG/compressed air.
- scale each of the Na and K channels to its midpoint using 5mM NaCl and 50uM KCl respectively
- measure the samples, noting the readings for both Na and K
- immediately following the measurement of the samples, measure a range of at least 6 standards for each of Na and K with the concentration range chosen to encompass the measurements obtained in 2 above
- construct a calibration curve for each of Na and K. Where the concentration range of the samples is large, construct two calibration curves as necessary.
- the optical filters for each channel have bandwidths of <= 15nm and stray-light specifications of <= 0.5%, thus the instrument reading is relatively immune to interference from other elements and non-specific matrix effects.
Typical calibration Curves: