на мой взгляд слова "стандарт" и "эталон" в какой-то мере синонимы)))
может быть имеется в виду разница по классу чистоты реактива? Например здесь:
1.5 What Kinds of Reference Substances Are Used in TLC?
Because of their great importance to TLC, the various types of reference substances are described in the following Section. These are often known as “standards” and must only be used if they are of suitable quality for the intended application. These levels of quality are of especial importance in the field of pharmacy. All the relevant requirements must therefore be controlled in an SOP (standard operating procedure, see Chapter 9 “GMP/GLP-Сonforming Operations in TLC”).1. Pharmacopoeia Substance (PS)
This term indicates a commercially available substance that meets the requirements of the relevant pharmacopoeia. For example, the American pharmacopoeia is indicated by the suffix USP, the British by BP, and the European by CRS. The possible use of a PS is specified by the relevant institutions, and is terminated by a change in the LOT number in the suppliers’ catalogs. The Commission of the USP lists so-called “official distributors”, of which the company LGC Promochem is a member (see Section 12.5 “Market Overview”). Care must be taken when ordering a substance listed in a pharmacopoeia to use the precise term for the substance. Although it is extremely rare, it does happen that the related compounds (rel. c.) of a substance have different names in the DAB and CRS lists.
It is especially confusing if, for example, the “rel. c. A” of a substance (e.g. ranitidine HCl) in the USP list appears as “rel. c. B” in the BP list, “rel. c. A” in the BP list bears the name “rel. c. C” in the USP list, and the “rel. c. B” in the USP list does not appear at all in the BP list. 2. Primary Reference Substance (PRS)
This term denotes a substance referred to as a Class 1 Standard by suppliers (analysis certificate with, e.g., at least two assays performed by different methods) or defined by the user’s own tests without reference to other substances. 3. Secondary Reference Substance (SRS)
A tested and accepted batch of a substance which, after comparison with a PS or PRS, is declared as a “house standard”. Can also be termed a working standard. 4. Related Compound (rel. c.)
This is usually a substance obtained from a supplier, but may be a substance produced by the user for purity testing, which is not a PS or a PRS and does not require information about its concentration. Such substances are in most cases decomposition products or intermediates in the synthesis process, and can be linked to a particular active substance.
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