IT is frequently desirable to be able to determine the

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Bernard Cummings
5 years ago
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1 NEW INVENTIONS A PORTABLE OXYGEN ANALYSER By A. BRACKEN Research and Development Department, The British Oxygen Company, Limited IT is frequently desirable to be able to determine the oxygen concentration in a gas mixture using a rapid, simple method of analysis not dependent on caustic or corrosive absorbent solutions. Investigations involving oxygen tents of various kinds, certain physiological experiments and some problems in anaesthesia are typical cases. A slight loss of accuracy compared with the standard Haldane apparatus could be tolerated, provided the other advantages of convenience and rapidity were present. A few prototypes of a suitable instrument, which is also compact and portable, have now been produced and a recent article by Burns (Burns and Hall, 1953) and his colleagues at Guy's Hospital illustrates some of the uses to which one of the prototypes has been put in a large London Hospital. The analyser used was based on one developed in America during the war (Wood and Shoemaker, 1944), but the actual instrument was made in Britain. GENERAL DESCRIPTION The instrument (fig. 1) is housed in a small, light box, part of the front of which is in the form of a translucent calibrated scale. This is illuminated by a spot of light on depressing a switch on the top of the case. A rubber bulb and drying tube are mounted on the back of the box, and 383

2 384 British Journal of Anzesthesia they are connected to the inside of the instrument by smallbore rubber tubing. A second, longer length of small-bore rubber tubing leads from the inside of the instrument to the test atmosphere. The rubber bulb is fitted with a small uni-directional valve, so arranged that on compression of the bulb the air in it is passed out to waste. When die bulb expands the atmosphere inside the instrument is drawn into the bulb. On repeating these compressions and expansions several times during about one minute, the test atmosphere is drawn through die instrument and finally replaces completely the air or other gas mixture originally present. On depressing die switch the spot of light on die scale indicates at once the percentage of oxygen in die sample. The instrument used covers the range 0 per cent to 100 per cent oxygen with an accuracy of ± 2 per cent absolute, at normal temperatures and pressures. A suitable correction would be required for readings taken at widely different temperatures or pressures. SCIENTIFIC BASIS OF INSTRUMENT The instrument depends upon what has come to be known as the Pauling principle (Pauling, 1941) which utilizes the fact that oxygen is the only common gas widi pronounced magnetic properties. Its volume susceptibility is +151 X 10"' c.g.s. electromagnetic units. As a contrast, die figure for nitrogen is only X 10"* c.g.s. electromagnetic units. No gases or vapours normally used in anajsthesia or physiological work (nitrous oxide, carbon dioxide, carbon monoxide, helium, cyclopropane, ether vapour and trichlorediylene vapour) have magnetic properties comparable with those of oxygen. The only gases which are paramagnetic like oxygen and might conceivably interfere widi

3 FIG. 1 Oxygen Analyser

5 New Inventions 385 an analytical procedure based on this property are nitric oxide and chlorine dioxide. If a test body, such as a glass sphere, is placed in the magnetic field lying between two large flat pole pieces of a magnet, it will not be subjected to any force because the magnetic field is uniform. If the same body is placed in a non-uniform field, for example to one side of the centre line between two wedge-shaped pole pieces, then it will be acted upon by a force whose magnitude depends upon two factors. These are: (a) that dependent upon the magnetic properties of the test body and the magnetic system, producing forces which would be exerted even in a vacuum; and (b) that which results from the displacement of the medium by the test body. The effect of the first forces is a constant for the system; that of the second depends upon the magnetic susceptibility of the gases present. If oxygen forms a significant part of these gases, then the resultant force acting on the test body will vary with the oxygen content. Conversely, the magnitude of the force may be measured, and this measurement used as the analytical method for determining the partial pressure of oxygen in the gas mixture. In the instrument used here, two hollow glass, nitrogenfilled spheres are joined by a glass rod and the resulting dumb-bell, which weighs only eight milligrams, is cemented at right angles across a vertical thin quartz fibre. A small diamond-shaped mirror is also fixed to the fibre. Two magnets are used, so that there are four pole pieces, and they are arranged so that the forces acting upon the spheres reinforce each other. A twisting force is thus exerted upon the fibre and the dumb-bell rotates into an equilibrium position. This position depends upon the partial pressure of the oxygen in the surrounding medium, and the restoring force exerted by the quartz fibre.

DURING the course of certain investigations it became

VOLUMETRIC DETERMINATION OF ETHER OR CYCLOPROPANE, CARBON DIOXIDE, NITROUS OXIDE AND OXYGEN IN ANESTHETIC MIXTURES By F. J. PRIME DURING the course of certain investigations it became necessary to be able