CHEMICAL MICKOCRYSTAL IDENTIFICATIONS 



to a residue that has been separated as satis- may be put at about .000-4 microgram of 



factorily as possible from a mixture. atropine. 



"Selective" is a term used in two different In general, however, the writer agrees with 

 senses: a reagent may demonstrate that a Chamot and Mason: "It is perhaps unfor- 

 compound belongs to a particular group, or tunate that so much stress has been laid 

 it may actually pick out a reactive com- upon the sensitivity of microscopical crystal 

 pound among others that are unreactive tests, both because their advantages of ease 

 or that do not react as strongly or in the same and directness are thereby obscured, and 

 way. Thus one kind of identity test may because the impression is given that they 

 require an absolutely pure substance and be are appropriate only in case extremely mi- 

 specific, or characteristic, or merely selective nute amounts of material or low concentra- 

 (in the first sense); while another type of test tions are to be dealt with." Sensitivities are 

 may not require great pmity, and therefore important — -sometimes very important, even 

 when specific or characteristic for a com- absolutely essential in such a science as 

 pound may also be quite selective in the sec- toxicology — -but it does seem that when a 

 ond sense, as well. The latter is very true of microcrystal test is described in any detail, 

 the chemical microciystal tests. sensitivity is likely to be the one point 



These tests will work on mixtures with seized on, in abstracts for example, to the 

 nonreactive material, with little or no inter- exclusion of everything else. This degrades 

 ference, and can generally be applied directly the microscopical crystal tests to the level 

 to medicinal tablets, for example, in which of mere detection tests, which are often sen- 

 a drug is mixed or diluted with material that sitive but very general, and worthless for 

 is inert in the chemical reaction concerned, identification whereas the purpose of the 

 The reagent itself seeks out the particles or microcrystal tests is identification, whatever 

 molecules of the substance that interests us, the amount of substance available. Their ad- 

 among particles and molecules of all sorts vantages are not only in ease and directness, 

 of other (nonreactive) substances. In the as mentioned, but in providing highly char- 

 same way, these tests may be just the kind acteristic and even specific identification 

 needed for extracts or isolates when it is not tests, not mere detection. To the extent that 

 possible to get extremely high purity without sensitivity statements obscure this, they 

 risk of losing the little bit of material that should be played down; at the same time it 

 is all that is available. should be realized that the better micro- 



When procedures on a very small scale are crystal tests are exceedingly sensitive, and 

 combined with the use of the microscope, as thus quite adequate for identification on al- 

 is done by Clarke, a quite common sensitiv- most any minute scale, 

 ity for a highly characteristic crystal test The statement is rather frequently en- 

 will be 2 or 3 hundredths of a microgram, countered, that in general color tests are 

 Atropine gives tiny characteristic crystals more sensitive than precipitation (or micro- 

 with the right variety of iodine-KI reagent crystal) tests. This is both incorrect and 

 down to an aqueous dilution of 1 to 1,000,- fallacious. Some compounds give excellent 

 000. Using a fair-sized ordinary drop of 0.04 color tests and few and poor precipitation 

 ml the crystals can therefore be obtained tests, or none; others are just the reverse, 

 with 0.04 microgram; by using a microdrop Some compounds, e.g., morphine, are re- 

 of 0.1 microliter (.0001 ml), the extreme markable for both kinds, and in such cases 

 sensitivity is .0001 microgram; or, as the they are hkely to be of the same order of sen- 

 crystals are hard to find at the extreme limit, sitii-ity, as a consultation of published state- 

 the working sensitivity, using microdrops, ments in regard to morphine tests will show. 



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