oxides of nitrogen in fact responds to nitrite ions and so may be calibrated with sodium nitrite solutions. However, this is an inferior method for primary calibration as it ignores the collection efficiency and stoicheiometry. Clearly, where analytical methods are involved with legislation, the validity of the methods must be beyond doubt.The production of synthetic mixtures of gases and of vapours in gases involves the use of procedures that may be less accurate than those for liquid mixtures because gases cannot be weighed easily, volumes may change during handling and temperature and pressure effects must be considered. Many methods have been used for preparing mixtures with reasonable accuracy, and these methods are broadly classified as static or dynamic. Static methods involve preparing and storing the mixture in a closed vessel such as a cylinder or a plastic bag, which of course limits the volume of the mixture available. Static systems are preferred when comparatively small volumes of mixture are required at moderately high concentration levels, but losses of components of the mixture to the walls of the vessel may occur. Dynamic systems generate a continuous flow df mixture and can produce large volumes, with losses to surfaces being lower owing to an equilibriun jetween the walls of the system and the hwing gas stream.Some of the more important static and dynamic systems are summarised in Fig. 1. Clearly, many systems are available and any classification is somewhat arbitrary as many methods combine various principles. Nevertheless, such a scheme provides a framework for a survey of the various systems. Nelson1 gave a comprehensive review of many basic techniques, although a number of the methods that are currently popular, such as permeation, diffusion and gas phase titration, were described only briefly. General reviews have appeared,2-1° and also some publications that have referred specifically to gas mixtures containing such components as sulphur dioxide,l1Pl2 oxides of nitrogen13 and hydrocarbons.l4 Multiple rigid G ravi metric Partial pressure Exponential dilution chambers Permeation Volumetric Flexible chambers Diffusion Electrolytic Chemicai reaction Fig, 1. Production of standard gas mixtures.A ugu st, 1981This review is selective, and some systems shown in Fig. 1 will be mentioned only briefly. Particular attention is given to the more popular methods and especially those associated with preparing standard gas mixtures for use in the measurement of air quality. Fundamental PrinciplesI t is important to consider certain fundamental principles before concentrating on specific methods for preparing mixtures. Nelson1 reviewed various aspects regarding components such as flow measurement, evaporation and certain injection devices. Important topics deserving consideration are the major component of the mixture, storage problems, mixing and reaction chambers and units for expressing concentrations. UnitsTo introduce the general discussion it is appropriate to make brief references to the units use...

No abstract

No abstract

A new approach for the study of the pollution of operating theatre atmospheres by volatile anaesthetic agents is described. Disadvantages of earlier work are discussed together with ways in which they may be overcome. Some pitfalls in analysis, sampling and standardization are indicated and some hazards of incorrect data interpretation are shown. The need for a cautious and critical attitude is emphasized.

No abstract