Electrochemical Measurement Systems

Types of Electrodes

• Glass Electrodes
  • H+, Na+, K+, Li+, Ag+ NH4+
• pH Combination Electrodes
• Solid State Electrodes
  • Cl-
• Liquid Ion-Exchange
  • K+ (valinomycin binds K+ in centre ring)
  • Ca2+ (organo-phosphate salt in PVC)
• Compound Gas Electrodes
  • pCO2 (Severinghaus CO2 electrode)
    • CO2 diffuses past first membrane and alters pH of internal bicarbonate buffer → lowers pH
  • pO2 (Clark electrode)
    • o2 reduced at platinum cathode, releasing electrons
• Enzyme Electrodes
  • Non-ionic compounds
  • Glucose, urea, etc.
  • Measure ions generated from enzymatic reactions

Indirect vs Direct Ion-Selective Electrodes (ISEs)

	Indirect ISE	Direct ISE
Principle	Sample is diluted (generally between 1:16 to 1:34) before the ion concentration is measured A fixed volume of sample is diluted, of which the water volume (which contains electrolytes) is assumed to remain constant at 93% of the total volume The change in potential is measured, which is converted to a concentration by comparison to a calibration curve Measured ion concentration is proportional to the concentration in all phases of plasma	Sample is directly measured without a dilution step Measured ion concentration is proportional to the concentration in the water phase Results are standardized with indirect ISEs by multiplying result by aqueous portion (93%)
Benefits	Smaller sample volume is required due to dilution Increases electrode lifespan Cheaper, suitable for high-volume testing	Whole blood samples can be used Unaffected by changes in solid phase of plasma
Limitations	Subject to electrolyte exclusion effect Abnormal levels of solid phase products in plasma (proteins, lipids, etc.) affect results Increase in solid phase results in false decrease in measured electrolytes Decrease in solid phase results in false increase in measured electrolytes

Electrolyte Exclusion Effect

• Blood plasma normally consists of 93% water/aqueous portion (electrolytes, etc.) and 7% solids (includes proteins, lipids, etc.)
• Indirect ISEs have a dilution step, which is based upon the assumption of that of a given volume of plasma, 93% of it will be in the water phase (e.g., for 100 μL of sample, 93 μL will contain electrolytes)
• If there are more solid phase products, then less of the sample will contain electrolytes. However, the sample will be diluted with the same amount of diluent as normal. This results in an over-dilution of the sample, causing falsely low measured ion concentration.
• Conversely, if there are less solid phase products, then more of the sample contains electrolytes. When the sample is diluted with the diluent, there will be a greater volume of aqueous phase electrolytes in the sample than expected for the dilution. This will result in a falsely increased measured ion concentration.

Conditions Associated with the Electrolyte Exclusion Effect

Conditions associated with increased plasma solids (and thus falsely decreased electrolyte measurements) include:

• Hyperlipidemia
  • Lipemic samples
• Hyperproteinemia

Pseudohyponatremia is caused by erroneous sodium measurements by indirect ISE. It is associated with a low sodium measurement in the presence of normal plasma osmolarity.

Conditions associated with decrease plasma solids (and thus falsely increased electrolyte measurements) include: