Subsurface Laboratory Methods 323 



For unaided-eye observations, one part of oil in 100,000 parts of carbon 

 tetrachloride can readily be seen. With electronic or photographic meth- 

 ods, the detection threshold may range to the order of parts per hundreds 

 of millions. Heavy oils can be detected at a dilution of one part in 200,000 

 to 500,000 parts of water, and gasoline only at one part in 20,000 for the 

 same solvent.-^ Dispersed in soils, oil concentrations of a few parts in tens 

 to hundreds of millions can be detected by photographic methods. 



Methods of Fluoroanalysis 



Every * luminescent substance exhibits certain distinguishing fluoro- 

 chemical characteristics. Since luminescence is usually visible light, often 

 with varying admixtures of ultraviolet and infrared wave lengths, the 

 methods and means of optics serve for an analytic appraisal of the lumin- 

 escence. 



This means that luminescent light can be subjected to photometric, 

 spectrometric, and spectrophotometric determinations. Likewise, since the 

 luminescent system must absorb light, i.e., exciting energy, before an 

 emission can take place, the absorption characteristics of that system can 

 be studied. With the phenomenologic distinction between fluorescence 

 and phosphorescence, it is evident that the lifetime of the luminescence 

 may be a distinguishing property of light-emitting systems. 



These purely physical techniques, directed to the luminescence per se, 

 reinforce a number of chemical and physiochemical methods of analysis 

 which have proved valuable in petroleum work. It is not possible to discuss 

 here in detail the relative merits or limitations of each of the fluoroanalytic 

 methods, since considerable literature is devoted to that subject, but a 

 brief resume is in order so that their application to the specialized field 

 of petroleum detection and assay can be more adequately appraised. 



Fluorometry 



The photometry, i.e., the determination of intensity, of fluorescence 

 is called "fluorometry." The intensity or brightness to the eye, being de- 

 pendent in many instances upon physical and chemical conditions and 

 upon the nature of the exciting radiation and its wave length, provides 

 valuable information about the kind and concentration of a luminescent 

 substance in solution. Fluorescence intensity depends upon several fac- 

 tors, including (1) the concentration of the fluorescent substance in solu- 

 tion, (2) the intensity and wave length of the exciting light, (3) the pH 

 and temperature of the solution, (4) the nature of the solvent, and (5) 

 the effect of interfering materials that may be present. 



Electronic fluorometry relies upon the response of a photoelectric 

 cell or, in some cases, upon a photon counter tube, under standardized 

 conditions, for the measurement of fluorescence intensity. A number of 

 these instruments have been designed and constructed for chemical and 



' Halstrik, J., Archiv fiir Hygiene und Bakteriologie. Band 128, pp. 155-168, 1942. 



