Abbott i-STAT Alinity - Page 130

Where E is the potenal, 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 (posive or
negave charge) for the ion being measured, and (a) is the acvity of that ion.
The Nernst equaon can be rewrien as:
E = E° + S log a
Where S replaces the constant term which denes the slope of the sensor. The slope is the
change in millivolts per tenfold change in the acvity of the analyte. For a posively-charged
monovalent ion, the theorecal slope would be 59.1 mV at 25°C.
The second concept is Acvity versus Concentraon where ion-selecve electrodes measure
acvity rather than concentraon. Acvity (a) is related to concentraon (c) through the acvity
coecient (γ). It is wrien as:
a = γc
Note: While ion acvies, which reect free rather than total ion concentraons,
are the physiologically relevant quanty, acvity values are converted to
convenonal concentraon units so that values obtained by direct ISE
measurements can be compared to values obtained from methods that measure
total ion concentraons. The laer includes the indirect methods, which have
acvity coecients close to unity or one, and ame photometric, atomic
absorpon and traon methods.
Amperometric sensors
In amperometric measurements, a potenal is applied to the measuring electrode while current
generated by the resulng oxidaon or reducon reacons in the test system is measured. The
current generated is directly proporonal to the concentraon of the analyte. An enzyme can be
added to a layer on or near an amperometric sensor to produce electroacve species from
analytes of interest that cannot themselves be oxidized or reduced.
Conductometric sensors
In conductometric measurements, an alternang current is applied between two electrodes in
contact with the test soluon and the resulng voltage dierence is measured. The conducvity
of the soluon is proporonal to the magnitude of the voltage dierence. In aqueous soluons,
conducvity is dependent upon the concentraon of electrolytes; an increase in the electrolyte
concentraon causes an increase in conducvity.
Determinaon of analyte concentraon
Potenometric and amperometric sensors are used for the determinaon of analyte concentraon. For
both sensors, the concentraon of the analyte can be calculated using:
1. The known value of the analyte concentraon in the calibrant soluon
2. The measured voltage (potenometric) or current (amperometric) signal generated by the analyte in
the calibrant
3. The measured signal generated by the analyte in the test soluon
For potenometric sensors, the analyte acvity in the sample is calculated from the Nernst equaon
according to:
E
sample
- E
calibrant
= S log (a
sample
/a
calibrant
)
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i-STAT Alinity — System Operaons Manual Art: 745524-01 Rev. I Rev. Date: 02-Nov-2022