Unknown substances

Determine the melting point of pure cinnamic acid (133°) and pure urea (133°). Intimately mix approximately equal weights (ca. 01 g.) of the two finely-powdered compounds and determine the melting point a considerable depression of melting point will be observed. Obtain an unknown substance from the demonstrator and, by means of a mixed melting point determination, discover whether it is identical with urea or cinnamic acid. 

Present day techniques for structure determination in carbohydrate chemistry are sub stantially the same as those for any other type of compound The full range of modern instrumental methods including mass spectrometry and infrared and nuclear magnetic resonance spectroscopy is brought to bear on the problem If the unknown substance is crystalline X ray diffraction can provide precise structural information that m the best cases IS equivalent to taking a three dimensional photograph of the molecule... 

Before the widespread availability of instrumental methods the major approach to structure determination relied on a battery of chemical reactions and tests The response of an unknown substance to various reagents and procedures provided a body of data from which the structure could be deduced Some of these procedures are still used to supple ment the information obtained by instrumental methods To better understand the scope and limitations of these tests a brief survey of the chemical reactions of carbohydrates is m order In many cases these reactions are simply applications of chemistry you have already learned Certain of the transformations however are unique to carbohydrates... 

Three years later Robert F Furchgott discov ered that the relaxing of smooth muscles such as blood vessel walls was stimulated by an unknown substance produced in the lining of the blood vessels (the endothelium) He called this substance the endothelium-dependent relaxing factor or EDRF and in 1986 showed that EDRF was NO Louis J Ignarro reached the same conclusion at about the same time Further support was provided by Salvador Moncada who showed that endothelial cells did in deed produce NO and that the l arginine to l citrulline conversion was responsible... 

There is a more important use. Suppose a mass spectrometer has accurately measured the molecular mass of an unknown substance as 58.04189. Reference to tables of molecular mass vs. elemental composition will reveal that the molecular formula is CjH O (see Table 38.2). The molecular formula for an unknown substance can be determined which is enormously helpful in identifying it. 

Assuming that the mass spectrometer has sufficient mass resolution, the computer can prepare accurate ma.ss data on the m/z values from an unknown substance. To prepare that data, the system must acquire the mass spectrum of a known reference substance for which accurate masses for its ions are already known, and the computer must have a stored table of these reference masses. The computer is programmed first to inspect the newly acquired data from the reference compound in comparison with its stored reference spectrum if all is well, the system then acquires data from the unknown substance. By comparison and interpolation techniques using the known reference... 

Metastable ions are useful for determining the paths by which molecular ions of an unknown substance have decomposed to give fragment ions. By retracing these fragmentation routes, it is often possible to deduce some or all of the molecular structure of the unknown. 

The molecular ions yield molecular mass information and molecular formulae. These in themselves are very useful but are usually insufficient to fully elucidate the structure of an unknown substance. 

Along with the normal, routine mass spectrum, the resulting metastable ion connections often supply enough information to allow a structure of an unknown substance to be deduced or to confirm the identity of one substance with another by comparison with their metastable ion behavior. 

Automated linked scanning of metastable ions is valuable for deducing a whole or partial molecular structure of an unknown substance. 

When the gas chromatograph is attached to a mass spectrometer, a very powerful analytical tool (gas chromatography-mass spectrometry, GC-MS) is produced. Vapour gas chromatography allows the analyses of mixtures but does not allow the definitive identification of unknown substances whereas mass spectrometry is good for the identification of a single compound but is less than ideal for the identification of mixtures of... 

Fig. 1 Fluorescence scan of a blank track (A) and of the testosterone derivative (B, 2 jig). Start (1), testosterone-INH derivative hRf 39) (2), unknown substances (3, 4).

Synthesis of Quinine. This required a parallel series of operations jn which the still unknown substance Aomomeroquinenine (3-vinylpiperi-dine-4-propionic acid) replaced its dihydro-derivative, Aomocincholoipon, for the final condensation with ethyl quininate. The first step was taken by ProStenik and Prelog,who converted cinchonine to cinchqnjcjn ... 

Every pure substance has its own unique set of properties that serve to distinguish it from all other substances. A chemist most often identifies an unknown substance by measuring its properties and comparing them with the properties recorded in the chemical literature for known substances. 

The half-wave potential is also independent of the electrode characteristics, and can, therefore, serve for the qualitative identification of an unknown substance. Owing to the proximity of many different half-wave potentials, its use for qualitative analysis is of limited application unless the number of... 

In many of the normal modes of vibration of a molecule the main participants in the vibration will be two atoms held together by a chemical bond. These vibrations have frequencies which depend primarily on the masses of the two vibrating atoms and on the force constant of the bond between them. The frequencies are also slightly affected by other atoms attached to the two atoms concerned. These vibrational modes are characteristic of the groups in the molecule and are useful in the identification of a compound, particularly in establishing the structure of an unknown substance. 

The preparations of luciferin (Ln, an electron acceptor) and soluble enzyme used were crude or only partially purified. The luciferase was an insoluble particulate material, possibly composed of many substances having various functions. Moreover, the luciferin-luciferase reaction was negative when both luciferin and luciferase were prepared from certain species of luminous fungus. It appears that the light production reported was the result of a complex mechanism involving unknown substances in the test mixture, and probably the crucial step of the light-emitting reaction is not represented by the above schemes. 

The subject of this chapter is the replacement of protons in primary and secondary aromatic and aliphatic amines by arenediazonium ions. In these reaction (Scheme 13-1) derivatives of the unknown substance triazene, HN = N-NH2, are formed. ... 

This relation can be used to estimate the molar mass of a substance by comparing the time required for a given amount of the unknown substance to effuse with that required for the same amount of a substance with a known molar mass. 

Fig. 1 Reflectance scan of a chromatogram track of a Colchicum autumnale extract (A) and of a reference track with 1 g colchicine (1) per chromatogram zone (B) unknown substance (2).

C is obtained for mesitylene. Yet the tables give 165°C. So the method has satisfactory reliability. It can be used with the unknown substance by adding increment 27 or 26.5 to the boiling point of o-xylene ie fib = 171°C. 

Read the entire laboratory activity. Form a hypothesis about the expected ratios of reacting volumes. Form a second hypothesis about how these ratios can be used to determine the cation to anion ratio in an unknown substance. Record your hypotheses in the next column. 

Table 6.6 presents a list of some of the most commonly encountered atoms in polymer/additive analysis, together with their monoisotopic and average masses. For the same nominal mass, different exact masses (elemental compositions) do exist. Knowledge of the exact mass of an unknown substance allows its atomic composition to be established. The exact mass of an ion proves the presence of a particular species (compound in a mixture). 

The different ways of species analysis - qualitative and quantitative - are well known. However, in structure analysis, they can also be differentiated between qualitative and quantitative ways according to the type and amount of information obtained (Eckschlager and Danzer [1994]). Identification of a sample or a given constituent may have an intermediate position between species and structure analysis. In any case, identification is not the same as qualitative analysis. The latter is the process of determining if a particular analyte is present in a sample (Prichard et al. [2001]). Qualitative analysis seeks to answer the question of whether certain components are present in a sample or not. On the other hand, identification is the process of finding out what unknown substance(s) is or are present (Eckschlager and Danzer [1994]). In Sects 9.1 and 9.3 it will be shown that there is a... 

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