Abbott i-STAT Alinity Operation Manual page 130

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Where E is the potential, E° is a constant dependent on the electrode/sensor system, R is the gas
constant, T is the absolute temperature, F is Faraday's constant, (n) is the valence (positive or
negative charge) for the ion being measured, and (a) is the activity of that ion.
The Nernst equation can be rewritten as:
E = E° + S log a
Where S replaces the constant term which defines the slope of the sensor. The slope is the
change in millivolts per tenfold change in the activity of the analyte. For a positively-charged
monovalent ion, the theoretical slope would be 59.1 mV at 25°C.
The second concept is Activity versus Concentration where ion-selective electrodes measure
activity rather than concentration. Activity (a) is related to concentration (c) through the activity
coefficient (γ). It is written as:
a = γc
Amperometric sensors
In amperometric measurements, a potential is applied to the measuring electrode while current
generated by the resulting oxidation or reduction reactions in the test system is measured. The
current generated is directly proportional to the concentration of the analyte. An enzyme can be
added to a layer on or near an amperometric sensor to produce electroactive species from
analytes of interest that cannot themselves be oxidized or reduced.
Conductometric sensors
In conductometric measurements, an alternating current is applied between two electrodes in
contact with the test solution and the resulting voltage difference is measured. The conductivity
of the solution is proportional to the magnitude of the voltage difference. In aqueous solutions,
conductivity is dependent upon the concentration of electrolytes; an increase in the electrolyte
concentration causes an increase in conductivity.
Determination of analyte concentration
Potentiometric and amperometric sensors are used for the determination of analyte concentration. For
both sensors, the concentration of the analyte can be calculated using:
1. The known value of the analyte concentration in the calibrant solution
2. The measured voltage (potentiometric) or current (amperometric) signal generated by the analyte in
the calibrant
3. The measured signal generated by the analyte in the test solution
For potentiometric sensors, the analyte activity in the sample is calculated from the Nernst equation
according to:
E - E
sample calibrant
i-STAT Alinity — System Operations Manual
Note: While ion activities, which reflect free rather than total ion concentrations,
are the physiologically relevant quantity, activity values are converted to
conventional concentration units so that values obtained by direct ISE
measurements can be compared to values obtained from methods that measure
total ion concentrations. The latter includes the indirect methods, which have
activity coefficients close to unity or one, and flame photometric, atomic
absorption and titration methods.
= S log (a /a )
sample calibrant
Art: 745524-01 Rev. I
Rev. Date: 02-Nov-2022