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esults (Tracey, Linden et al. 1990), but may not be sensitive or specific enough to measure
NO alone (Schmidt and Kelm t996),
NO will rapidly convert to nitrite and nitrate in biological samples and by adding acids or
reducing substances these can be transformed back to release the NO molecule (Verdon,
Burton et al. 1995; Kelm, Dahmann et al. 1997). However acidification and/or reduction will
release NO from all nitroso compounds, alkyl or inorganic nitrites, the nitrosamines and
nitrosothiols. If the sample contains compounds other than nitrite and nitrate, then there may
be overestimation of the NO that existed as a biologically active or inflammatory mediator.
4.5 Nitric oxide
It has become possible to measure NO directly down to the cellular level by using
microelectrodes. This cornmenced with the modification of a miniature 02 electrode, and
sealing it so that only low molecular weight gases could enter. By introducing a positive
voltage, NO was oxidised on the surface of the electrode and could be measured over l-
3pmol range (Shibuki 1990). The probe was approximately 2mm in diameter. Modification of
this then allowed a much smaller microsensor to be developed using a semi conductor
polymeric porphyrin and a cationic exchanger on a sharpened carbon tip which reduced the
size to 5 microns (Malinski and Taha 1992; Taha, Kiechle et al.1992) and could measure NO
as an electrical current. This has allowed NO measurement in cardiac cells (Xian ,Zhang et al.
2000; Kanai, Pearce et al. 2001; Katrlik and Zalesakova2002),brain and nerve cells (Shibuki
1990; Taha, Kiechle et al. 1992: Malinski, Bailey et al. 1993; Kumar, Porterfield et al. 2001),
osteoclasts (Silverton, Adebanjo et al. 1999), trachea and main bronchi from an animal model
(Ricciardolo, Vergnani et al. 2000) and in suspensions of mitochondria and cells such as
platelets, leukocytes and in cell cultures (Wadsworth, Stankevicius et aL.2006). The detection
rate has been as low as 10-20 Mol, with a linear response between 8.0 to 4.8 x 10-6 mol/L. This
low detection limit is also matched by high sensitivity and selectivity although it does also
detect catecholamine activity (Tu, Xue et al. 2000). It has also been achieved as an online
measurement so release of NO could be followed in activated macrophages, which correlated
well with the nitrite concentration determined by the Greiss assay (Cserey and Gratzl 2001).
However a recent review suggested that further methodological development of these
microsensors was needed in order to avoid the influence of changes in temperature, pH, and
oxygen on the measurements (Wadsworth, Stankevicius et al. 2006).
My interest was to look at measurement in exhaled breath. A number of very early methods of
detection depended on colorimetric analysis but for exhalate these were too slow and too
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