WPI APOLLO 4000, Instruction Manual

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APOLLO 4000
OXYGEN DETECTION
World Precision instruments d-8
since an oxygen detector only responds to the difference in activity across the
membrane rather than the concentration difference. so in samples containing an
electrolyte, while the oxygen concentration falls with increasing salt concentration
the probe current remains constant.
thus, if it is necessary to have a measure of dissolved oxygen in terms of
concentration, then the calibration is somewhat more complicated since the
relationship between activity and concentration may change with the change of
salt concentration in the samples. the activity coefficient, a ratio of the activity to
the concentration, generally cannot be predicted and one must rely on empirical
determinations since the compositions of living fluids such as blood are extremly
complicated. one may directly use the fluid to be tested as a “solvent” to prepare a
calibration stantard. Alternatively, one may use the Bunsen absorption coefficient,
α
, to calculate oxygen concentration in blood in terms of the results with the
oxygen electrode. the equation is:
where K is a conversion factor depending on the unit of pressure chosen ( for
atm), pt and pH
2
o are the total pressure of gas and the partial pressures of water,
respectively. po
2
is the partial pressure of oxygen in blood obtained from the
measurements with the oxygen electrode. Bunsen coefficients for solubility of
oxygen in plasma and blood can be found in
Table 4 . However, it is very important
to calibrate at the same temperature as that of the measurement site.
Probe Structure and Assembly
Fig. D5 shows the principal components of an iso-oXY-2. A gas permeable
membrane
(3)
is cemented to the tip of the outer stainless steel sleeve
(1)
. the
interior of the probe is a slender wand containing the platinum cathode and silver
counter / reference electrode inside the sleeve. the wand is permanently mounted
in the probe’s plastic handle
(2)
. After electrolyte has been deposited inside the
sleeve (see below), the wand is slowly inserted into the sleeve and secured by
screwing the sleeve cap
(4)
gently into the probe handle. if the membrane on the
stainless steel sleeve becomes damaged, the entire sleeve must be replaced. two
spare sleeves are provided with the iso2 kit, and the user may purchase more.
should the user wish to sterilize the probe this should be done with alchohol or
some other chemical for an assembled probe. the membrane sleeves can be