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General measuring principles
Introduction
There are four different measuring principles employed in the sensors in the ABL90 FLEX analyzer.
• Potentiometry: The potential of a sensor chain is recorded using a voltmeter, and related to the
concentration of the sample (the Nernst equation). The potentiometric measuring principle is
applied in the pH,
p
CO
2
, K
+
, Na
+
, Ca
2+
and Cl
–
sensors.
• Amperometry: The magnitude of an electrical current flowing through a sensor chain is
proportional to the concentration of the substance being oxidized or reduced at an electrode in
the chain. The amperometric measuring principle is applied in the
c
Glu and
c
Lac sensors.
• Optical
p
O
2
: The optical system for
p
O
2
is based on the ability of O
2
to reduce the intensity and
time constant of the phosphorescence from a phosphorescent dye that is in contact with the
sample. This measuring principle is applied in the
p
O
2
sensor.
• Spectrophotometry: Light passes through a cuvette containing a hemolyzed blood sample.
The specific wavelengths absorbed and their intensity generate an absorption spectrum used
to calculate oximetry parameters. This measuring principle is used for measuring
c
tHb, sO
2
,
F
O
2
Hb,
F
COHb,
F
HHb,
F
MetHb,
F
HbF and
c
tBil.
Activity vs. concentration
Strictly speaking, in potentiometry the potential of a sensor chain is related to the activity of a
substance, and not its concentration.
The activity of a substance can be considered the "effective concentration" of a species, taking
non-ideality of the medium into account.
Activity and concentration are related by the following equation:
a
x
=
γ
c
x
where:
a
x
= the activity of the species x
γ
= the activity coefficient of species x under the measurement conditions (for ideal systems
γ
= 1)
c
x
= the concentration of species x (mol/L)
Note:
To be exact, activity is related to the molality of species x, i.e. the number of mol/kg of
solvent. However, molality is converted to concentration (molarity).
Theoretical aspects of the blood gas analyzer
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EXT-VEND-MAN-0023-1 Owner:Julie Monnox Approved:27 Mar 2019
