CHEMICAL MICROSCOPY 



essentially the same, and these tests can well 

 be used along with others, especially color 

 tests for the spot-plate or designed for mi- 

 nute residues, or "spot tests" as developed by 

 Feigl. However, the microcrystal tests are 

 usually so highly characteristic, that two or 

 three of them, even just using the ordinary 

 microscope, and making comparisons, as 

 necessary, with a known sample, will often 

 make an identification certain, without nec- 

 essarily having or obtaining any other type 

 of information. 



With the polarizing microscope the results 

 are especially definite since a number of fur- 

 ther observations can be made, even without 

 removing the crystals from the solution in 

 which they form. They do not have to be 

 separated, washed, dried, recrystallized, and 

 so forth, as for melting-point determinations 

 and most other types of further tests, includ- 

 ing refractive indices. All of the following 

 are simply a matter of direct observation: 

 not only the shape of the crystals, their 

 grouping, color, size, and so on, but also bire- 

 fringence, whether high, medium, low, or 

 absent; whether extinction is inclined, and 

 if so, the angle of extinction; in the case of 

 colored crystals whether dichroism is pres- 

 ent, and if so, the two different colors shown, 

 and if the crystals are regularly elongated, 

 the direction of dichroism (sign of absorp- 

 tion), and in the case of colorless elongate 

 crystals, the sign of elongation. All these 

 things can be observed without having to 

 treat or prepare the crystals in any way, 

 once they form in a solution. 



Microcrystal tests are especially useful for 

 distinguishing among closely related reactive 

 compounds. Usually suitable tests can be 

 found even to distinguish an isomer from 

 the corresponding racemate, by completely 

 different crystals. In fact, such tests have 

 been used to identify (/-amphetamine even 

 in the presence of d/-amphet amine, and vice 

 versa. 



E. G. C. Clarke has shown how micro- 

 crystal tests can sometimes be used to dis- 



tinguish microgram amounts of a d-isomer 

 from the Z-isomer, and without using an op- 

 tically active reagent, which is another possi- 

 bility. The question of identifying a d- or 

 Z-isomer is important in some cases now be- 

 cause the ^isomer of certain new synthetics 

 is restricted as a narcotic while the d-isomer 

 is also offered commercially as a different 

 drug not subject to narcotic restrictions. The 

 distinction is based on the fact that the race- 

 mate will give crystals in certain cases where 

 a separate isomer will not. Besides knowing 

 and having a suitable reagent, the analyst 

 needs one known isomer, let us say the d-. 

 If he mixes some of this with the suspected 

 substance and it is 1-, then he has the race- 

 mate and can get the appropriate crystals, 

 but if the suspected substance is d- and he 

 mixes d- with it, then of course he still can- 

 not get crystals due to the racemate com- 

 pound. If he has both known isomers for a 

 cross-check, this kind of test can be com- 

 pletely certain. 



The question about microcrystal tests so 

 often asked, "what will they do on mix- 

 tures?" — reveals fundamental misunder- 

 standing. 



Most identification tests require that a 

 substance to be identified be fairly pure. This 

 is perhaps especially true of most physical 

 and physicochemical tests, because they 

 are so very general — melting-point tests, for 

 example. A strictly chemical test or reaction, 

 on the other hand, is bound to be selective 

 in some degree, and will pick out a substance 

 in the presence of all kinds of impurities ex- 

 cept those that react with the same reagent, 

 and occasionally even in the presence of 

 compounds that react, but in a different way. 

 A microcrystal test, where not merely the 

 fact of a reaction, but crystals of a particular 

 kind are looked for, cannot in general be 

 expected to work on a complex of closely 

 related, reactive compounds, although some 

 tests that are remarkably searching, in this 

 sense, are known. The application is often 



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