User's Manual

i15 Blood Gas and Chemistry Analysis System User Manual Theory
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 Potentiometric Sensor
 Amperometric Sensor
 Conductometric Sensor
Potentiometry: A potential is recorded using a voltmeter, which relates to the concentration of
the sample. A reference electrode is used to provide a stable, fixed potential against which other
potential differences can be measured. This measurement technique is used for pH, pCO
2
and
electrolytes.
Amperometry: The magnitude of an electrical flow of current is proportional to the
concentration of the substance being oxidized or reduced at an electrode. This measurement
technique is used for pO
2
, glucose and lactic acid.
Conductivity: The specific impedance of a sample as measured by two conducting electrodes
held at a constant voltage is directly proportional to the conductive properties of the sample. This
technique is used for Hct.
11.2 Determination of Test Results
11.2.1 Determination of the Analyte Concentration
The concentration of the analyte is determined with the potentiometric and amperometric sensors.
For both sensors, the analyte concentration is calculated using:
1) The known analyte concentration of calibrant solution;
2) The measured voltage or current of the calibrant solution.
3) The measured voltage or current of the sample.
For potentiometric sensors, the analyte activity in the sample is calculated from the Nerst
equation:
E
sample
– E
calibrant
= S log (α
sample
/α
calibrant
)
Where E denotes the potential, α denotes the activity of an ion, and S denotes the slope of the
sensor.