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JOSEPH L. McCarthy 



respectively, were found In these samples by PMR Laboratory M 

 using procedures similar to those of the standarized method 

 (lO), thus illustrating the influence of the presence of 

 interfering substances." 



Thus the Pearl-Benson nitroso method is 

 based on a common reaction for certain ortho-, meta- and 

 para- substituted phenols and other substances, as has 

 been discussed in some detail by E. P. Mohler and L. N. 

 Jacob in Analytical Chemistry magazine 29, 1369, 1957, 

 and by others. Interference may arise from such sub- 

 stances as certain phenols as those from or in tannins, 

 analine, xylidlne, indole, urine, unhydrolyzed and 

 hydrolzyed fish meal, and other substances. 



From these observations I conclude that 

 the Pearl-Benson method is a moderately sensitive and re- 

 producible procedure, but one which suffers from some short- 

 comings of non-specificity; that is, the Pearl-Benson reaction 

 takes place not only with lignin sulfonates but also with 

 other substances, which contain a molecular structure or 

 a molecular configuration similar to the configuration or 

 configurations present in lignin sulfonates which give 

 rise to the Pearl-Benson reactions. 



In view of this imperfect specificity of 

 the Pearl-Benson method, several years ago as part of the 



