Measurement of serum electrolytes

1. Potentiometric

a) Electrolyte concentration is measured as a function of the potential difference between a sample and a reference solution in a half cell

2. Flame Photometry

a) Measurement of the wavelength of light rays emitted by excited metallic electrons exposed to the heat energy of a flame

b) Used to identify characteristics in clinical specimens of body fluids

c) The intensity of the emitted light is proportional to the concentration of atoms in the fluid

i) A quantitative analysis can be made on this basis

d) Used to measure sodium, potassium & lithium

e) Technique

i) The sample is dispersed into a flame from which the metal ions draw sufficient energy to become excited

ii) On returning to the ground state, energy is emitted as electromagnetic radiation in the visible part of the spectrum, usually as a very narrow wavelength band

iii) Eg: sodium (orange); potassium (lilac); calcium (red)

iv) The radiation is filtered to remove unwanted wavelengths and the resultant intensity measured

3. Atomic absorption spectrophotometry

a) Measures the amount of light that a substance absorbs when applied heat makes its atoms receptive

b) Cathode lamp

i) The cathode is made out of the test element (eg manganese, zinc)

ii) This lamp emits a monochromatic light beam in the wavelength of the test element

iii) The test sample (eg serum containing zinc) is subjected to a flame

a) The heat releases the test element’s atoms from their normal chemical bonds

b) In this free state the test element atoms absorb energy of their own wavelength; the same wavelength as the light beam from the cathode

iv) Light from the cathode that is not absorbed by the test molecules is diffracted so that the very narrow desired wavelength is measured in light intensity by a photocell [detector]

a) The net decrease in intensity of the beam from the cathode is an inverse measure of light absorbed in the flame by the test element and hence the amount of test element (eg zinc) present in the (serum) sample

b) The greater amount of element in the sample, the more light absorbed in the flame, and the less light transmitted to the detector