Criteria of purity

The experimental operations in organic chemistry which occur with greatest frequency are those which are concerned, directly or indirectly, with the isolation and purification of organic compounds. It is necessary therefore to describe in detail the chief methods of purification. Before doing so, however, the criteria of purity (and their observation) must first be discussed, so that when the purification has been attempted, its success can at once be checked and confirmed. 

Application of these tests at successive steps will give a good indication of whether or not the purification is satisfactory and will also show when adequate purification has been achieved. Finally elemental analyses, e.g. of carbon, hydrogen, nitrogen, sulfur, metals etc. are very sensitive to impurities (other than with isomers), and are good criteria of purity. 

EC 1995, Commission Directive 95/15/EC, of July, 1995 laying down specific criteria of purity concerning colours for use in foodstuffs. Official Journal of the European Communities, L226 of September 22, 1995, pp. 1 44. 

Laufberger had tried to obtain the protein from horse liver, but it did not crystallize, and as he described to me when I met him in Prague some years ago, in those days everyone wanted to have protein crystals as a criteria of purity. Although James Sumner had crystallized jack bean urease in 1926, his preparations were somewhat impure, and it was only in the mid-1930s, when John Northrop and Moses Kubnitz showed that there is a direct correlation between the enzymatic activities of crystalline pepsin, trypsin and chymotrypsin that the protein nature of enzymes was generally accepted. 

As required by Directive 89/107/EEC, criteria of purity have been drawn up for all the listed food additives (with a couple of exceptions). Purity criteria for all the permitted sweeteners have been prescribed in Directive 95/31/EC,6 as amended, and criteria for all the permitted colours are contained in Directive 95/ 45/EC,7 as amended. Directives that prescribe purity criteria for all the additives authorised under Directive 95/2/EC have been drawn up in stages. Directive 96/ 77/EC8 containing purity criteria for antioxidants and preservatives is amended by Directives 98/86/EC which lays down purity criteria for emulsifiers, stabilisers and thickeners and 2000/63/EC which contains purity criteria for most additives numbered E 500 and above, and for certain other additives not covered in the earlier directives. Purity criteria for most of the few remaining permitted miscellaneous additives are contained in Directive 2001/30/EC however, purity criteria for E 1201 polyvinylpyrrolidone and E 1202 polyvinylpolypyrrolidone are still being considered by the Scientific Committee on Food. Some methods of analysis for verifying prescribed purity criteria have been developed at EU level these are contained in Directive 81/712/EEC.9... 

The directive has been amended three times to take account of the recommendations of the Scientific Committee for Food and the need for certain solvents by the food industry. General criteria of purity are prescribed for the listed extraction solvents. Extraction solvents need not be listed in the ingredients hst of food products but the directive includes labelling requirements for extraction solvents sold as such for business purposes. 

First Commission Directive 81/712/EEC of 18 July 1981 laying down Community methods of analysis for verifying that certain additives used in foodstuffs satisfy criteria of purity (Official Journal of the European Communities 1981 (L257), pp. 1-27). 

Purity criteria are published by JECFA as well as by national authorities. The JECFA specifications33 may be endorsed by the Codex Alimentarius as advisory specifications which means that they are equivalent to a Codex Alimentarius standard. In the EU a directive laying down special criteria of purity for sweeteners for use in foodstuffs with subsequent amendments sets the purity standards,31 while in the USA criteria may be listed in conjunction with the approval. Normally a monograph of the Food Chemicals Codex34 is considered the applicable basis for assessment of purity. 

Sale (2 ) investigated the characteristic ultraviolet absorption of lemon oil and concluded that adulteration of lemon oil with distilled lemon oil could be detected. He recommended other chemical and physical criteria of purity also be considered. 

Most solvents contain a small amount of impurities, and often stabilizing chemicals have been added. The HPLC-grade solvents are supposed to be especially pure, although the criteria of purity for these solvents is their interference with the UV adsorption of the solute molecules. NMR detection is much more sensitive to smaller amounts of additional chemicals, especially since the concentration of the sample molecules is often of the order of 0.001 % (m/v). 

Apparently, the CIS max must be equal to or greater than the CIS W i.e., CIs.m5x>C S min to simultaneously satisfy the criteria of purity for both the analyte and its internal standard. Accordingly, the following formula is obtained. 

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