Potentiometric Sensors; The Nernst Equation; Electrochemical Measurements - Abbott i-STAT 1 System Manual

ELECTROCHEMICAL MEASUREMENTS

Method Measurements are performed on undiluted specimens. Undiluted methods are
also called direct methods, while methods requiring dilution of the sample are
called indirect methods.
Indirect methods measure the total molar concentration of analyte per
unit volume of plasma. Direct methods measure the total molar activity of
analyte (apparent or free ion activity) per unit volume of plasma water. It is
understood that the direct method result is the clinically significant result for
electrolytes. When there is disagreement between the methods, such as when
the patient has abnormal total protein or lipid levels, it is due to interference
on the indirect method.
At normal levels of protein and lipids the systematic offset between methods
is often corrected for in commercial direct measuring instruments so that the
normal ranges for all instruments are in agreement. Sensor outputs have been
set so that normal ranges are in agreement with indirect reference methods at
normal levels of total protein and lipids.
Sensors The general term "sensor" is used to refer to the three types of electrodes
incorporated into the cartridges:
Sensors are thin film electrodes microfabricated onto silicon chips. Sensing
functionality is imparted to each electrode by a number of chemically sensitive
films coated over the active region of the electrodes.
Potentiometry is the measurement of the difference in potential that exists
Potentiometric
Sensors
between an indicator electrode and a reference electrode. Ion-selective
electrodes (ISE) are examples of potentiometric sensors. The indicator electrode
is designed to be sensitive to a particular ion in a solution. In cases where other
ions are sensed by the system, selectivity coefficients can be used to correct for
this interference. An enzyme can be added to an ISE to produce ions from
analytes of interest that are not themselves ions.

The Nernst Equation

The Nernst equation relates the measured potential to the activity of the ion
being measured.
E = E° + RT/nF ln a
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 valance (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
Art: 714382-00D
• Potentiometric
• Amperometric
• Conductometric
Rev. Date: 02/20/06
20-3