Derivatization, chemical

There are ill-defined limits on EI/CI usage, based mostly on these issues of volatility and thermal stability. Sometimes these limits can be extended by preparation of a suitable chemical derivative. For example, polar carboxylic acids generally give either no or only a poor yield of molecular ions, but their conversion into methyl esters affords less polar, more volatile materials that can be examined easily by EL In the absence of an alternative method of ionization, EI/CI can still be used with clever manipulation of chemical derivatization techniques. 

The isotope molybdenum-99 is produced in large quantity as the precursor to technetium-99y, a radionucleide used in numerous medical imaging procedures such as those of bone and the heart (see Medical imaging technology). The molybdenum-99 is either recovered from the fission of uranium or made from lighter Mo isotopes by neutron capture. Typically, a Mo-99 cow consists of MoO adsorbed on a lead-shielded alumina column. The TcO formed upon the decay of Mo-99 by P-decay, = 66 h, has less affinity for the column and is eluted or milked and either used directly or appropriately chemically derivatized for the particular diagnostic test (100). 

The crystalline mineral silicates have been well characterized and their diversity of stmcture thoroughly presented (2). The stmctures of siHcate glasses and solutions can be investigated through potentiometric and dye adsorption studies, chemical derivatization and gas chromatography, and laser Raman, infrared (ftir), and Si Fourier transform nuclear magnetic resonance ( Si ft-nmr) spectroscopy. References 3—6 contain reviews of the general chemical and physical properties of siHcate materials. 

Other spectroscopic methods such as infrared (ir), and nuclear magnetic resonance (nmr), circular dichroism (cd), and mass spectrometry (ms) are invaluable tools for identification and stmcture elucidation. Nmr spectroscopy allows for geometric assignment of the carbon—carbon double bonds, as well as relative stereochemistry of ring substituents. These spectroscopic methods coupled with traditional chemical derivatization techniques provide the framework by which new carotenoids are identified and characterized (16,17). 

The reaction conditions can be selected so as to be able to separate substances with the same or similar chromatographic properties (critical substance pairs) by exploiting their differing chemical behavior, thus, making it easier to identify them. Specific chemical derivatization allows, for example, the esterification of... 

Over the years the literature is filled with examples where the initial characterization was incorrect. One example is illustrated below. In 1940, Sethna and Shah presumed that they synthesized coumarins 42 and 43 from a reaction between P-orcacetophenone (44) and its 4-0-methyl ether 45 under standard Kostanecki-Robinson conditions, respectively. Three decades later Bose and Shah synthesized coumarin 43 via another route and concluded that the initial assignment made by Sethna and Shah was incorrect. After the Bose and Shah findings were published, Ahluwalia and Kumar concluded that the Sethna and Shah products were actually chromones 46 and 47 based on proton NMR data and chemical derivatization. Despite these shortcomings, the Kostanecki-Robinson reaction remains an effective method for formation of both coumarins and chromones. 

Once the internal standard has been added to the unknown matrix and is thoroughly mixed, the solution can be concentrated, if necessary, for GCl MS analysis. Chemical derivatization is also performed if needed. Normally, samples and reference standards are evaporated to dryness and then redissolved in a carefully measured quantity of solvent. For trace samples, se-... 

Summary If the molecular weight is odd, then the compound contains an odd number of nitrogens. Fragment ions observed at even-mass numbers suggests the presence of nitrogen. The loss of ammonia is fairly common in nitrogen compounds and may not indicate exclusively that an amine is present. Chemical derivatization will easily determine if the unknown is a tertiary amine. 

Mizutani and coworkers57a confirmed the presence of polychloro(methylsulfonyl)biphenyls (159-170) as sulfur-containing metabolites of chlorobiphenyls (Cl-BP) in the feces of mice based on both GLC-mass spectrometry and chemical derivatization. In some cases comparison with authentic samples (161 and 162) was also made. When preparing 161 and 162,2,5-dichloro-3-(methylsulfonyl)aniline, 2,5-dichloro-l-iodo-3-(methylsulfonyl)benzene and 2,2, 5,5 -tetrachloro-3,3 -bis(methyl-sulfonyl)biphenyl were also obtained and their four peak El mass spectra reported572. Similar data were given for the corresponding 4-substituted intermediates, which were involved in the preparation of 162. Also 2,4, 5-trichloro-2 -(methylsulfonyl)-biphenyl was prepared and its four peak mass spectra given. Metabolites 163 and 164 were also identified by comparison with the authentic standards. 

In analytical LC there are two primary reasons why chemical derivatization of the sample constituents would be necessary, and they are 1) to enhance the separation and 2) to increase the sensitivity of detection. Under certain circumstances, derivatization can also be used to reduce peak asymmetry, i.e. to reduce tailing, or to improve the stability of labile components so that they do not re-arrange or decompose during the chromatographic process. However, sensitivity enhancement is the most common goal of derivatization. For example, aliphatic alcohols that contain no UV chromaphore can be reacted with benzoyl chloride to form a benzoic ester. 

The aromatic nucleus adsorbs in the UV and thus, the derivative can be detected by a UV detector. This is the most common type of chemical derivatization but the derivative may be chosen to be appropriate for different types of detector. For example, the solute can be reacted with a fluorescing reagent, producing a fluorescent derivative and thus be detectable by the fluorescence detector. Alternatively, a derivative can be made that is easily oxidized and, consequently, would be detectable by an electrochemical detector. 

Chemical derivatization can be carried out before the separation (pre-column derivatization) or after the separation and before detection (post-column derivatization). If derivatization is carried out prior to separation, then a phase system must now be selected to separate the... 

Chemical Derivatization in Analytical Chemistry" (Eds R.W.Frei and J.F.Lawrence), Plenum Press,New York (1981). 

Pfeffer, W. D. Yeung, E. S. Anal. Chem. 1986, 58, 2103. Lawrence, J. F. Frei, R. W. Chemical Derivatization in Liquid Chromatography, Elsevier Scientific New York, 1976. 

Bolts JM, Wrighton MS (1978) Chemically derivatized n-type semiconducting germanium photoelectrodes. Persistent attachment and photoelectrochemical activity of ferrocene derivatives. J Am Chem Soc 100 5257-5262... 

Bocarsly AB, Walton EG, Wrighton MS (1980) Use of chemically derivatized n-type sU-icon photoelectrodes in aqueous media. Photooxidation of iodide, hexacyanoiron(II), and hexaammineruthenium(II) at ferrocene-derivatized photoanodes. J Am Chem Soc 102 ... 

More recently, liquid chromatography/mass spectrometry (LC/MS) and liquid chromatography/tandem mass spectrometry (LC/MS/MS) have been evaluated as possible alternative methods for carfentrazone-ethyl compounds in crop matrices. The LC/MS methods allow the chemical derivatization step for the acid metabolites to be avoided, reducing the analysis time. These new methods provide excellent sensitivity and method recovery for carfentrazone-ethyl. However, the final sample extracts, after being cleaned up extensively using three SPE cartridges, still exhibited ionization suppression due to the matrix background for the acid metabolites. Acceptable method recoveries (70-120%) of carfentrazone-ethyl metabolites have not yet been obtained. 

Mo, W., Sakamoto, H., Nishikawa, A., Kagi, N., Langridge, J.I., Shimonishi, Y., and Takao, T., Structural characterization of chemically derivatized oligosaccharides by nanoflow electrospray ionozation mass spectrometry, Anal. Chem., 71, 4100, 1999. 

Ahuja, S., Chemical derivatization for the liquid chromatography of compounds of pharmaceutical interest, /. Chromatogr. Sci., 17, 168, 1979. 

N. A. Parris, Instrumental Liquid Chromatography, a Practical Manual on High-Performance Liquid Chromatographic Methods (Journal of Chromatography Library, Vol. 27), Elsevier, Amsterdam, 2nd revised ed., 1984 J. Drozd, Chemical Derivatization in Gas Chromatography (Journal of Chromatography Library, Vol. 19), Elsevier, Amsterdam, 1981 J. F. Lawrence and... 

R. W. Frei, Chemical Derivatization in Liquid Chromatography (Journal of Chromatography Library, Vol. 7), 1976, 1st reprint 1983, Elsevier, Amsterdam, 1983. 

Gilbart, J. Fox, A. Morgan, S. L. Carbohydrate profiling of bacteria by gas chromatography-mass spectrometry Chemical derivatization and analytical pyrolysis. Eur. I. Clin. Microbiol. 1987, 6,715-723. 

Phenyl-functionalized SWNTs HiPCO ([Pg.199]

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