Direct Chromatographic Methods

If plasticizer analysis presents difficulties by direct chromatographic methods there is always the possibility of chemically converting the plasticizers into reaction products which are easier to separate. Since, chemically, esters predominate among the technical plasticizers, analysis of their saponification products must be dealt with first. 

On mixtures, except for one review [50], little information is available and measures are required both indirect (physical absorption) and direct (chromatographic) methods are possible. In UMR45, by high pressure adjustment of a chromatographic method, we proved that water in organic media, even in trace amount, has a dramatic negative influence on H2 solubility. 

The dynamic methods are based on direct chromatography and are popular because they are faster and easier to automate. Four direct chromatographic methods that are available for determination of adsorption isotherms are frontal analysis (FA) [13, 109] frontal analysis by characteristic points (FACP) [109], elution by characteristic points (ECP) [109] and the perturbation peak (PP) method [118-121], The FACP and ECP methods have... 

The primary use of isotherm data measurements carried out on single-component elution profiles or breakthrough curves is the determination of the single-component adsorption isotherms. This could also be done directly, by conventional static methods. However, these methods are slow and less accurate than chromatographic methods, which, for these reasons, have become very popular. Five direct chromatographic methods are available for this purpose frontal analysis (FA) [132,133], frontal analysis by characteristic point (FACP) [134], elution by characteristic point (ECP) [134,135], pulse methods e.g., elution on a plateau or step and pulse method) [136], and the retention time method (RTM) [137]. 

Scanu, R., Spano, N., Panzanelli, A., Pilo M.I., Piu, P.C., Sanna, G., and Tapparo, A., 2005. Direct chromatographic methods for the rapid determination of homogentisic acid in strawberry tree Arbutus unedo L.) honey. Journal of Chromatography A. 1090 76-80. 

The direct chromatographic method involves the use of a chiral selector either in the stationary phase (CSP) or in the mobile phase (CMPA) in normal and reversed-phase chromatography. In the first case, the chiral selector is chemically bonded, coated, or otherwise attached to the solid support and in the second case the chiral selector is added to the mobile phase [11,49,50,54,56]. 

Integration of the peaks for the two diastereomers accurately quantifies the relative amounts of each enantiomer within the mixture. Such diastereometic derivatives may also be analy2ed by more accurate methods such as gc or hplc. One drawback to diastereometic detivatization is that it requites at least 15 mg of material, which is likely to be material painstakingly synthesized, isolated, and purified. The use of analytical chiral chromatographic methods allows for the direct quantification of enantiomeric purity, is highly accurate to above 99.8% ee, and requites less than one milligram of material. 

Chromatographic methods including thin-layer, hplc, and gc methods have been developed. In addition to developments ia the types of columns and eluents for hplc appHcations, a significant amount of work has been done ia the kiads of detectioa methods for the vitamin. These detectioa methods iaclude direct detectioa by uv, fluoresceace after post-column reduction of the quiaone to the hydroquinone, and electrochemical detection. Quantitative gc methods have been developed for the vitamin but have found limited appHcations. However, gc methods coupled with highly sensitive detection methods such as gc/ms do represent a powerful analytical tool (20). 

The first (direct reading) method is fairly simple and results are available immediately. However, the instruments have limited sensitivity and must be recalibrated periodically. The second (absorption in a liquid or adsorption on a medium) and third (gas container) methods are generally considered more sensitive and more accurate method for trace analysis by gas chromatographs, infrared... 

Current interest is, however, mainly in the coupling of HPLC and TLC, to which considerable attention has been devoted for the solution of difficult separation problems. Since Boshoff et al. (39) first described the direct coupling of HPLC and TLC, several papers (40-43) have been published describing the on-line coupling of liquid chromatographic methods and PC, usually with different interfaces, depending on the first technique applied. If PC is used as the second method, all the MD methods discussed above can be applied to increase the separating power. 

One of the most frequent techniques for analyzing 1,4-dioxane is gas chromatography. Birkel et al. [319] proposed in 1979 a gas chromatographic method based on partial vacuum distillation of the sample, analyzing polysorbate 60 and 80 with sensitivity to the 0.5 ppm. Stafford et al. [320] proposed a direct injection GC method which meant an improvement to the Birkel s technique. Robinson and Ciurczak [321] described a direct GC method for the analysis of... 

A gas chromatographic method is described in this work for the analysis of tetradecane-l,4-sultone (C14 5-sultone) and the combination of 2-chloro-tetradecane-l,3-sultone (C14 2-chloro-y-sultone) and l-tetradecene-l,3-sultone (C14 unsaturated y-sultone) in neutral oils isolated from alkenesulfonate. Samples of the neutral oil are diluted in hexane and injected directly into the gas chromatograph. Quantitative data are obtained by comparison to known amounts of the respective sultones. Through the use of silica gel column chromatography followed by GC of collected fractions, separation and individual quantitation of the 2-chlorotetradecane-l,3-sultone and l-tetradecene-l,3-sultone can be obtained. 

Besides the calculation of the different sulfonated species, it is also possible to determine them directly by chromatographic methods. Separation of the ester sulfonate and the disodium salt is achieved by thin-layer chromatography on silica gel plates. With a solvent mixture of acetone and tetrahydrofuran (90 10 v/v) the disodium salt stays at the start whereas the ester sulfonate has an R value of 0.2. With the more polar solvent 0.1 N H2S04 + methanol + chloroform the ester sulfonate and the disalt have Rf values of 0.36 and 0.14. For visualization, the plate is sprayed with pinacryptol yellow. In UV light (254 and... 

With notable exceptions, the application of HPLC to clinical chemistry has not as yet been extensive. This is somewhat surprising in view of the potential the method has for this area. This potential arises, in part, from the fact that HPLC is well suited to the types of substances that must be analyzed in the biomedical field. Ionic, relatively polar species can be directly chromatographed, without the need to make volatile derivatives as in gas chromatography. Typically, columns are operated at room temperature so that thermally labile substances can be separated. Finally, certain modes of HPLC allow fractionation of high molecular weight species, such as biopolymers. 

HPLC is often reported to be the technique of best choice for the quantification of food colorants. According to European Directive 94/36/EC, the quantities of synthetic colorants to be added to foods are restricted and thus reliable methods for their quantification must be established. Approved colorants, defined by E-coded numbers (Table 6.6.2), are permitted for non-alcoholic beverages, confectionery products, and even for caviar (dying fish roe). For example, a specific HPLC chromatographic method for the quantization of 14 synthetic food colorants belonging to azo dye, triphenyhnethane, or quinophthalone classes (E 102,104, 110, 122,123, 124, 127, 128, 129, 131, 132, 133, 142, 151) was reported to check their contents in caviar. ... 

Gas and liquid chromatography directly coupled with atomic spectrometry have been reviewed [178,179], as well as the determination of trace elements by chromatographic methods employing atomic plasma emission spectrometric detection [180]. Sutton et al. [181] have reviewed the use and applications of ICP-MS as a chromatographic and capillary electrophoretic detector, whereas Niessen [182] has briefly reviewed the applications of mass spectrometry to hyphenated techniques. 

Of course, to be able to use the direct injection method of sample introduction, the analyte or the polymer system must be soluble in a solvent. Other methods of sample introduction need to be considered in order to eliminate the involatile material from the chromatographic separation. These have become extremely effective in the analysis of matrices such as polymers. 

Parkhurst et al. [79] described a high performance liquid chromatographic method for the simultaneous determination of primaquine and its metabolites from plasma and urine samples, utilizing acetonitrile deproteinization, and direct injection onto a cyano column. Levels of 100 ng/mL per 20 pL injection could be quantitated. Preliminary pharmacokinetic analysis is reported for two human subjects after oral doses of 60 90 mg primaquine diphosphate. Two apparent plasma metabolites and two possible urinary metabolites are also reported. 

The most studied of the water-soluble metallophthalocyanines (Zn, Al, Ga) are the sulfonic acids. These are prepared by cyclotetramerization of 4-sulfophthalic acid or its derivatives (Figure 6),246,247 or by the direct sulfonation (oleum) of the metallophthalocyanine.248 The latter method gives complex mixtures, since direct sulfonation occurs at both exo and endo positions. These mixtures may be separated by chromatographic methods (usually reverse-phase HPLC). Thus... 

One of the most powerful methods for determining enantiomer composition is gas or liquid chromatography, as it allows direct separation of the enantiomers of a chiral substance. Early chromatographic methods required the conversion of an enantiomeric mixture to a diastereomeric mixture, followed by analysis of the mixture by either GC or HPLC. A more convenient chromatographic approach for determining enantiomer compositions involves the application of a chiral environment without derivatization of the enantiomer mixture. Such a separation may be achieved using a chiral solvent as the mobile phase, but applications are limited because the method consumes large quantities of costly chiral solvents. The direct separation of enantiomers on a chiral stationary phase has been used extensively for the determination of enantiomer composition. Materials for the chiral stationary phase are commercially available for both GC and HPLC. 

Sample preparation for analysis by hyphenated methods requires some additional planning when compared to nonhyphenated methods. All steps, extraction, concentration, and final solvent selection must take into consideration and be compatible with all the components of the hyphenated instrumentation. For gas chromatographic methods, all the components in the mixture must be in the gaseous state. For liquid chromatography (LC) or high-performance liquid chromatography (HPLC), the samples of the analytes of interest can be solids or liquids, neutral or charged molecules, or ions, but they must be in solution. If the follow-on analysis is by MS, then each of the analytes may require a different method of introduction into the MS. Metals and metal ions may be introduced by HPLC if they are in solution but commonly are introduced via AAS or inductively coupled plasma (ICP). Other analytes may be directly introduced from HPLC to MS [2],... 

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