Sample preparation hydrolysis

For the higher alkoxy groups, standard carbon and hydrogen analysis may be used, although careful sample preparation is required because of the ease of hydrolysis. Quantitative vapor-phase chromatography of alcohol Hberated during hydrolysis may also be used, but care must be taken in this case to ensure that hydrolysis is complete before the estimation is carried out. 

A general feature of optimum sample preparation is that maximum recovery of the analyte is observed. Consider a graph of recovery vs. variation in one experimental condition. Figure 5 shows such a graph, with temperature as the experimental variable. The curve exhibits a maximum and a decline on either side of the maximum. The assay will be most reproducible at the point of zero slope, i.e., at the maximum recovery, because small variations in conditions will not affect the result. In hydrolysis of a protein to its constituent amino acids, for example, it will be found that at very high temperatures or long hydrolysis times, degradation of the product amino acids occurs, while at low temperatures or short hydrolysis times, the protein... 

The use of internal standards is somewhat controversial.115 There is agreement that an internal standard may be used as a correction for injection volume or to correct for pipetting errors. If an internal standard is included before sample hydrolysis or derivatization, it must be verified that the recovery of the internal standard peak is highly predictable. Ideally, the internal standard is unaffected by sample handling. Using an internal standard to correct for adsorptive or chemical losses is not generally approved, since the concentration of the standard may be altered by the conditions of sample preparation. An example of internal vs. external standards is given in Chapter 4. 

A 20 g sample, prepared and stored in a dry box for several months, developed a thin crust of oxidation/hydrolysis products. When the crust was disturbed, a violent explosion occurred, later estimated as equivalent to 230 g TNT. A weaker explosion was observed with potassium tetrahydroaluminate. The effect was attributed to superoxidation of traces of metallic potassium, and subsequent interaction of the hexahydroaluminate and superoxide after frictional initiation. Precautions advised include use of freshly prepared material, minimal storage in a dry diluent under an inert atmosphere and destruction of solid residues. Potassium hydrides and caesium hexahydroaluminate may behave similarly, as caesium also superoxidises in air. 

It is worth noting that the partide sizes of samples prepared by the urea hydrolysis method are larger than other LDHs. Such a finding can be rationalized by considering the slow hydrolysis of urea [61], since it can be classified as a very weak Bronsted base (pfCb= 13.8). Its hydrolysis mechanism consists of the formation of ammonium cyanate as the rate-determining step, followed by fast hydrolysis of the cyanate to ammonium carbonate ... 

RP-HPLC found application in the monitoring of the alkali hydrolysis kinetics of alkali-clearable azo disperse dyes containing a fluorosulphonyl group. The chemical structures of dyes included in the experiments are shown in Fig. 3.85. Samples for RP-HPLC analysis were neutralized to pH 4.0 - 4.5 with diluted HC1 mixed with five volumes of ACN and injected without any other sample preparation step. Separation was carried out in an ODS column at ambient temperature. The isocratic mobile phase consisted of ACN-water (80 20, v/v) and dyes were detected at their absorption maxima. HPLC measurements indicated that dyes are easily hydrolysed under relatively mild alkaline conditions, and the hydrolysis follows a pseudo first-order kinetics [148],... 

Even when hydrolysis and epimerization can be avoided during sample preparation and handling, it is not possible to conclude definitively whether the compounds found in plasma and urine are true metabolites or simply degradation products. Indeed, chemical degradation can also occur within the body since urine and plasma contain a wide variety of potential catalysts, including metal ions, phosphate ions, proteins, and sugars (see Sect. 5.2.6). Whereas the existence of mammalian enzymes that act on penicillins and cephalosporins is considered possible [155], no such mammalian enzyme appears to have been identified to date. 

In an assay that offers acceptable HOPi/ri ratios for the carbamates, potential HOP can be estimated by preparing the sample under conditions that insure hydrolysis of the sulfamates. Unfortunately, the conditions specified for sample preparation in the standard mouse bioassay are not sufficiently acidic to insure complete hydrolysis ( ). As currently employed in state monitoring laboratories, the mouse assay may substantially underestimate the potential HOP of samples containing the sulfamate toxins. 

In order to accelerate sample preparation, new extraction methodologies such as accelerated solvent extraction (ASE) and MAE, based on the use of elevated temperature and pressure to heat the mixture sample-solvent, have been recently developed and applied for PAH extraction from meat [695] and vegetables [696-698]. Garda Falcon et al. [699] used microwave treatment with hexane to accelerate PAH extraction from freeze-dried foods. The fat extracted in this way underwent microwave assisted saponification with ethanolic KOH. Hernandez-Borges et al. [700] combined microwave-assisted hydrolysis and extraction to isolate organic pollutants from mussels, while... 

Recently, Yekkala et al. [144,145] have proposed an HPLC method with fluorescence detection. The method involves a rather laborious sample preparation due to the peculiar nature of the substrate involved (teeth), including pulverization, demineralization, hydrolysis of dentin and derivati-zation with o-phthaldialdehyde and A-acetyl-L-cysteine in order to obtain the enantioseparation of aspartic acid. Using a similar procedure, Benesova et al. [146] found that, in comparison with GC, HPLC provides shorter analysis time and higher sensitivity. 

Analytical sample preparation Storage conditions prior to analysis Nature of sample analyzed Sample extraction and hydrolysis... 

The hydrolysis of these model precursors was studied at 37 , with catalysis by hog liver esterase. The major product, isolated in 60-70% yield from the hydrolysis of a-acetoxyNPy, was 2-hyd oxy-tetrahydrofuran. This compound was identified by comparison to a reference sample,prepared by lead tetraacetate oxidation of 1,2,5-pentanetriol (53). Additional evidence was obtained by lithium aluminum hydride reduction of the product to 1,4-butanediol. Minor amounts of butenals were also identified as products of the hydrolysis of a-acetoxy IPy. 

If the factor chosen for testing is unlikely to show a linear response to changes then three levels are required. Such factors include UV wavelength, column manufacturers for HPLC, acids for use in acid hydrolysis as a sample preparation procedure for ICP, etc. 

A number of hydrolysis products of chloroaluminafe salfs have been defecfed using FAB MS. The problem here is that under some circumstances FAB MS can provide only a restricted view on the surface of fhe sample that is more likely to be oxidized or hydrolyzed rather than the average sample [13]. Nevertheless, this problem is not directly related to MS itself, but rather to a problem of proper sample preparation. Inert conditions during sample preparation and construction of special sample inlet chambers for MS can help avoid fhese reacfions. The addition of phosgene was suggested to reduce problems encountered with such side reactions [22], but here the problems encountered in working with an extremely toxic chemical have to be taken into account. 

DL-Dihydrostreptose and its ribo isomer were similarly obtained. Birch reduction of 2-methyl-3-furoic acid, followed by addition of methanol, bromination, and dehydrobromination, gave 402 as a mixture of the isomers. Hydroxylation of 402 with osmium tetraoxide-so-dium chlorate, and subsequent treatment with acetone-sulfuric acid afforded three isomeric acetals (403-405). The structures of these compounds were assigned on the basis of their H-n.m.r. spectra. In addition, the relationship between 403 and 404 was established by hydrolysis and reglycosidation. The methyl esters 403-405 were quantitatively reduced to the corresponding alcohols. The mixture of alcohols obtained from 403 and 404 was converted into crystalline 5-deoxy-3-C-(hydroxymethyl)-l,2-0-isopropylidene-a-DL-ribofuran-ose (406), which was compared directly with a sample prepared from D-xylose. Methyl 5-deox y-3-C-(hydroxy methyl)-2,3-O-isopropy lidene-/3-DL-lyxofuranoside (407), obtained by reduction of 405 with lithium aluminum hydride, was hydrolyzed with dilute hydrochloric acid, to give a,/3-DL-dihydrostreptose.2,ifi... 

Equal volumes (1. iL) of standard and sample solutions were injected into the system and the amount present in the sample solution was calculated according to the HPLC method. Equivalent results were obtained from the HPLC and GC methods. If a product contains a lower concentration of Polysorbate 80 (e.g., lower than 0.05 g/kg), the GC method may be superior over the HPLC method. In such a case, derivatization and extraction steps may be required during sample preparation. The yield of oleic acid from hydrolysis of Polysorbate 80 was examined by analyzing a Polysorbate 80 spiked sample against an oleic acid standard. The calculation was based on the fact that 1 mol of polysorbate 80 releases 1 mol of oleic acid. 

Hydrolysis of Alkoxides - A different thermal evolution of the precursor phase was observed for a BaAli20i9 sample prepared via hydrolysis of alkoxides.8,9 For this sample no XRD diffraction lines were detected after heating at 1000°C. At 1200°C Ba-hexaaluminate formed, and it was the only phase detected in the XRD spectra. Calcination at 1450 °C resulted in higher sharpness and intensity of the XRD reflection of Ba-hexaaluminate. No further changes were observed upon calcination at 1600°C. Also in this case, the appearance of Ba-(3-Al203 was accompanied by a marked drop of surface area. However, the... 

A synthesis of the marine antibiotic from Pseudomonas bromoutilis has been achieved by 1,3-dipolar cycloaddition of the oxazolone (388) with diethyl acetylenedicarboxylate (73CI(L)275). Hydrolysis and decarboxylation gave the pyrrole (390), identical with a sample prepared previously by other workers, and converted by them to pentabromopseudilin (391 Scheme 85) (66JA4509). 

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