Appendix G Operating Principles - Siemens RAPIDLAB 248 Operator's Manual

Appendix G
The 248 measurement technology is based on electrochemistry.
Electrochemistry is the measurement of current, or voltage, occurring in an
electrochemical cell, between a chemical and an electrical system.
Each electrode, or sensor, is designed to selectively measure the activity of
a specific substance. Many elements in a sample may interact with a
sensor, but the sensor is highly selective for one substance over others.
The potential generated at the sensor is converted into an electronic signal
by a transducer mechanism. The 248 uses potentiometry and
amperometry. Potentiometry measures the potential that develops at the
sensor. Amperometry involves applying a voltage to the sensor and then
measuring the current generated.
The electronic signal is filtered and smoothed, and converted into a
concentration measurement expressed in standard units.
Potentiometry
During sample analysis, a potential develops at the sensor as a result of the
interaction with the analyte (ion). The potential is related to the amount of
analyte in the sample.
The reference sensor provides a fixed potential, which is independent of
analyte activity, and is used to compare the measured potential.
The sensor potential corresponds to the analyte activity, and is directly
related of the concentration of the analyte in solution. The potential is
expressed by the Nernst equation:
where: E
This equation shows that the potential is logarithmically related to the
activity of the analyte in the sample.
However, the sensor actually measures the activity of the analyte in
solution. In clinical chemistry results are typically expressed as
concentration rather than activity. The activity of an ion is equivalent to
the concentration (mol/L) multiplied by the activity coefficient (the degree
with which the ion interacts with other ions in solution). The activity
coefficient depends on the ionic strength of the solution, and generally
decreases with increasing ionic strength
Operating Principles
E = K + (2.3RT/ZF) log a
cell
= electrochemical cell potential
cell
K = a constant (produced by various sources such as the liquid
junction)
R = gas constant
T = absolute temperature
Z = ionic charge
F = Faraday's constant
a = activity of the ion in the sample
i
i
10
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