Medica EasyStat, Operator'S Manual

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6. Principles of Operation/Theory
E = E° + RT Log ( γ C)
where:
E = The potential of the sensor in sample solution
E° = The potential developed under standard conditions
RT/nF = A temperature-dependent “constant” termed the slope, S
n = 1 for sodium and potassium; 2 for calcium
Log = Base ten logarithm function
γ = Activity coefficient of the measured ion in the solution
C = Concentration of the measured ion in the solution
The EasyStat analyzer measures pH, PCO
2
, PO
2
, Na+, K+, and Ca++
using selective electrode technology and Hct using conductivity. The
sodium, potassium, calcium, pH and PCO2 ion-selective electrodes
measure changes in voltage (a potentiometric measurement). The PO2
electrode measures a change in current (an amperometric measure-
ment). These changes are then compared with established chemical
measurements to produce final results. During sample analysis, blood
samples are held in the electrodes where they are maintained at a
temperature of 37.0°C and protected from air contamination. (See
Electrode Measurement/Diagrams in this chapter.)
pH, Na
+
, K
+
, Ca++, Cl- Electrodes 1
Na+ and K+ electrolyte measurements in blood products were tradition-
ally performed using flame photometry, in which a sample, diluted with
a known concentration of a reference ion (usually cesium) is aerosol-
ized and passed through a flame which excites the cations. The cation
re-emits the energy as light of different frequencies; the amplitude of
this emission is proportional to the ion concentration in the sample. The
development of selective organic compounds for sodium, potassium,
calcium, chloride or lithium, and other electrolytes has permitted the
development of electrodes capable of directly measuring biological fluids
throughout the physiological range. These electrodes are known as ion
selective electrodes.
The EasyStat analyzer measures sodium, potassium, calcium, chloride
and pH in biological fluids, using ion selective electrode technology.
Refer to the Electrode Measurement Diagram in this chapter. The flow-
through sodium electrode uses a selective membrane, specially formu-
lated to be sensitive to sodium ions. The potassium, calcium, chloride
and pH electrodes employ similar designs with appropriate selective
membrane materials.