Blood Gas Instrumentation Clinical Chemistry Review 126
DEVICE DESCRIPTION MEASURES CALIBRATION
pH electrode
PCO 2 electrode
(Severinghaus
electrode)
PO 2 electrode
(Clark electrode)
Co-oximeter
H+-sensitive glass electrode containing Ag/AgCl wire
in electrolyte of known pH & reference (calomel)
electrode (Hg/Hg 2 Cl 2 ). Measurement is potentiomet-
ric (change in voltage indicates activity of analyte).
pH electrode covered with membrane permeable to
CO 2 , with bicarbonate buffer between membrane &
electrode. Measurement is potentiometric.
Platinum cathode & Ag/AgCl anode covered with
semipermeable membrane. Measurement is
amperometric (amount of current flow is indication
of O 2 present).
Spectrophotometer that reads absorbance or
reflectance at isobestic point (wavelength where
reduced & oxyhemoglobin have same absorbance
or reflectance, e.g., 805 nm) & differential point
(wavelength where reduced & oxyhemoglobin have
different absorbance or reflectance, e.g., 650 nm).
[H+]
Dissolved CO 2
Dissolved O 2
Oxygen saturation. Some
also measure carboxyhe-
moglobin, methemoglobin,
& sulfhemoglobin by using
additional wavelengths.
2 phosphate buffers
of known pH. (Store
at RT; don’t expose
to air)
2 gases of known
PCO 2
2 gases of known PO 2
Calibration curve pre-
pared from specimens
with 0% & 100%
O 2 saturation
