2 General Description
Service Manual – Osmetech OPTI™ Analyzer
2-7
Principles of Operation
Luminescence is the emission of light energy resulting from "excited" molecules
returning to a resting state. When luminescence is initiated by light, it is
commonly referred to as fluorescence. When a fluorescent chemical is exposed
to light energy of an appropriate "color", electrons in the molecules of the
fluorescent chemical are "excited". A very short time later, the electrons return
to a resting state and in this process sometimes emit a small amount of light
energy. This energy is less than the excitation energy and thus has a different
color. That is, the emitted light (fluorescence emission), is red-shifted from the
excitation light and is much less intense.
Fluorescent optodes (from
opt
ical electr
odes
) essentially measure the intensity
of light emitted from fluorescent dyes. The emitted light is distinguished from
excitation light by means of optical filters. Because the excitation light energy
is kept constant, the small amount of light that results is changed only by the
concentration of the analyte. The concentration of the analyte is determined by
the calculation of the difference in fluorescence measured at a known calibration
point and fluorescence measured with the unknown concentration of analyte.
The
P
O
2
optode measurement principle is based upon luminescence, first
documented in the 1930's, and commercially utilized to measure blood
P
O
2
in
1983. The relationship of luminescence to
P
O
2
is quantified by the Stern-
Volmer equation which describes how the fluorescence emission intensity "I" is
reduced as the
P
O
2
"P" is increased. Unlike conventional electrochemical
"Clark"
P
O
2
electrodes, the oxygen optode does not consume oxygen molecules
during the measurement.
The pH optode measurement principle is based upon pH-dependent changes of
the luminescence of a dye molecule immobilized in the optode. Chemists have
used such pH indicator dyes for many years to perform acid-base titrations in
turbid media. The relationship of luminescence to pH is quantified by a variant
of the Mass-Action Law of chemistry that describes how the fluorescence
emission intensity increases as the blood pH is increased above the dye's
characteristic pKa. pH optodes do not need a reference electrode to measure pH;
however, they exhibit a small sensitivity to the ionic strength of the sample
being measured.
The
P
C
O
2
optode measurement principle is based upon placing a pH optode
behind an ion-impermeable membrane, just as conventional
P
C
O
2
blood gas
electrodes employ the Severinghaus C
O
2
electrode construction. As such,
P
C
O
2
optodes suffer from non-selective interference from volatile acids and bases in
blood just as conventional
P
C
O
2
electrodes.
