DAVID L. DRABKIN 



globin, if present in normal blood, is of the order of 0-5 per cent or less, 

 an amount too low to be identified in the spectrophotometry of the 

 mixture. That the concentration of ferrihaemoglobin in normal blood 

 does not exceed 0-5 per cent also in carefully conducted gasometric 

 analyses has recently been reported 30 . (The present status of the question 

 of the existence of methaemoglobin in blood is ably presented in the 

 paper by W. N. M. Ramsay, p. 231 .) Since the amount of the oxidized 

 form of haemoglobin was negligible, it permitted the assumption that 

 ' total pigment ' was comprised of Hb0 2 + Hb, thus allowing a simple 

 treatment of our spectrophotometric data as representing a two com- 

 ponent mixture. The presence of a measurable amount of a third 

 component would not have precluded the use of this technique, but 

 would have made the analysis of the measurements more troublesome. 



The appropriateness of the spectrophotometric technique — The ex- 

 tension of spectrophotometry to the direct measurement of the oxygen 

 saturation of arterial blood was made possible by the earlier introduc- 

 tion of the Drabkin and Austin special cuvette of 0-007 cm depth 7 , 

 described in Figure 1 . Within several minutes after its collection, the 

 measurements upon the undiluted blood sample, introduced into the 

 cuvette without exposure to the atmosphere, can be completed. 

 Electively the measurements are performed, as in the present work, 

 on samples haemolyzed with saponin, but accurate results may also 

 be obtained on non-haemolyzed whole blood 8 . At this stage, however, 

 the precision is greater on haemolyzed samples. The 0-007 cm depth 

 cuvette has also been successfully introduced in the vascular system, 

 thus permitting the continuous observation of circulating blood in vivo 

 in the completely anaesthetized dog (Figure 2). 



Barcroft was correctly concerned with what he regarded as one of 

 the least accurate of the analytical steps in such studies of the blood 

 and haemoglobin — the delivery of a specified blood volume by means 

 of a pipette (Memorial Address by A. V. Hill, p. 18). An essential 

 and important feature of our spectrophotometric cuvette (of cali- 

 brated depth) is that the volumetric measurement of the blood sample 

 is circumvented. 



Figure 3 and its accompanying legend should suffice as a description 

 of the spectrophotometric procedure for the direct determination of 

 the percentage HbO a (oxygen saturation), and provide an example of 

 the simple calculations involved in handling data obtained on a two 

 component system. The possibility of accurately deducing the con- 

 centration of individual components (of known spectroscopic character) 

 in a mixture, without the need for their separation, is one of the peculiar 

 advantages of the spectrophotometric technique. Under the best 

 conditions two components in a mixture can be measured with an 



40 



