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A group of instruments have been developed to enable the measurement of certain compound
concentrations within samples by using light reactions. These include:
o 'spectrophotometers', mentioned above, which measure how light is absorbed by the
specimen with the light being generated by the meter itself.
o 'Fluorometers' which measure light emitted by the specimen after excitation of the sample
generated by the meter.
o 'Luminometers' which measure light generated with no light or excitation input - this
makes them comparatively simple compared to the other devices as they only require a
reaction chamber, a light detector and a recorder (and for the measurement of NO, a
photomultipler).
The chemiluminscence analysers (see Figure 4.2) were originally developed to measure NO
as an atmospheric pollutant as discussed in Chapter 2.2. The measurement is based on the
observation that the reaction of NO with ozone produces light (see Figure 4.1).
Figure 4.1: Chemical reaction between nitric oxide and ozone
NO + Os + NO2' 1'. 9,
NO"+NOz+hf
(. = unstable electron)
(hf = light)
The interaction of ozone with gases such as NO, NOz, CO and SOz was described in the early
1960s (AIHA 1966), although ozone reacts most readily with NO. In the excited state the
electrons are unstable and they dissipate energy as they regain their original state. This light is
sufficient to make chemiluminescence one of the most sensitive NO assays available. The
chemiluminescence reaction of NO and Or is very fast and has a low activation energy of 10.5
joules (Johnston and Crosby 1954). At room temperature the rate constant of the reaction
(both steps) is l0-7 I mol-r s-t lclyne, Thrush et al. 1964). This high speed means that the
chemiluminscence assay is able to detect rapid changes in NO concentration and therefore can
be adapted for on-line measurement. It was, for example, used early to monitor NO
concentrations when this was delivered as a treatment trial in intensive care for pulmonary
hypertension @epke-Zaba, Higenbottam et al. 1991; Kinsella, Neish et al. 1992; Roberts,
Polaner et al. 1992: Gerlach, Rossaint et al. 1993). NOz reacts with ozone more slowly and
the reaction requires higher activation energy so the chemiluminscence levels of NO are not
affected by NO2, even if the latter is present in high concentrations (Johnston and Crosby
I9l4;Fontijn, Sabadell et al. 1970).
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