ST-200 aQua Operator’s Manual
PRINCIPLES OF OPERATION
6. Principles of Operation
A. ISE Theory
Electrolyte measurements in blood products were traditionally performed
using flame photometry, in which a sample, diluted with a known
concentration of a reference ion (usually lithium or cesium), is aerosolized
and passed through a flame which excites the caution. They re-emit the
energy as light of different frequencies; the amplitude of this emission is
proportional to the ion concentration in the sample. The development of
sodium and pH-selective glass, and selective organic compounds for
potassium and chloride, has permitted the development of sensors capable
of measuring biological fluids directly, throughout the physiological range.
These sensors are known as Ion selective electrodes.
The ST-200 aQua electrolyte analyzer measures sodium, potassium,
ionized calcium, lithium and chloride in biological fluids, using ion
selective electrode technology. The flow-through sodium electrodes contain
plasma tube, specially formulated to be sensitive to sodium ions. The flow-
through potassium electrodes employ a plastic tube, incorporating neutral
carrier ionophores. The flow-through Ionized Calcium electrodes includes a
plastic tube, specially formulated to be selective to Calcium ions. The flow-
through Lithium electrodes includes a plastic tube, specially formulated to
be selective to Lithium ions. The flow-through Chloride electrodes includes
a plastic tube, specially formulated to be selective to Chloride ions. The
flow-through pH electrode includes a plastic tube, specially formulated to
be selective to Hydrogen ions. The potential of each electrode is measured
relative to a fixed, stable voltage established by the silver/silver chloride
reference electrode. An ion selective electrode develops a voltage that varies
with the concentration of the ion to which it responds. The relationship
between the voltage developed and the concentration of the sensed ion is
logarithmic, as expressed by the Nernst equation:
E = Eº + RT ln (aC)
nF
Where:
E = the potential of the electrode in sample solution
Eº = the potential developed under standard conditions
RT/nF = A temperature dependent “constant” termed the slope (s)
n = 1 for sodium, potassium, chloride, lithium and pH
n = 2 for calcium
ln = Natural log
a = Activity coefficient of the measured ion in the solution
C = Concentration of the measured ion in the solution
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