Organic acids detectors

Recently, multidimensional GC has been employed in enantioselective analysis by placing a chiral stationary phase such as a cyclodextrin in the second column. Typically, switching valves are used to heart-cut the appropriate portion of the separation from a non-chiral column into a chiral column. Heil et al. used a dual column system consisting of a non-chiral pre-column (30 m X 0.25 mm X 0.38 p.m, PS-268) and a chiral (30 m X 0.32 mm X 0.64 p.m, heptakis(2,3-di-(9-methyl-6-(9-tert-butyldimethylsilyl)-(3-cyclodextrin) (TBDM-CD) analytical column to separate derivatized urinary organic acids that are indicative of metabolic diseases such as short bowel syndrome, phenylketonuria, tyrosinaemia, and others. They used a FID following the pre-column and an ion trap mass-selective detector following the... 

It is appropriate to refer here to the development of non-suppressed ion chromatography. A simple chromatographic system for anions which uses a conductivity detector but requires no suppressor column has been described by Fritz and co-workers.28 The anions are separated on a column of macroporous anion exchange resin which has a very low capacity, so that only a very dilute solution (ca 10 4M) of an aromatic organic acid salt (e.g. sodium phthalate) is required as the eluant. The low conductance of the eluant eliminates the need for a suppressor column and the separated anions can be detected by electrical conductance. In general, however, non-suppressed ion chromatography is an order of magnitude less sensitive than the suppressed mode. 

In general, CE is simple, rapid, and low cost because it needs neither laborious treatment of the samples nor long times of analysis. However, its high detection limit is a major limitation of CE. CE is often poorly reproducible. Enzymatic assay is more suitable for quantifying one organic acid in honey samples because it is specific, precise, and accurate. GC is more suitable for analyzing volatile or semivolatile chemicals. HPLC is versatile and reproducible. However, common HPLC detectors such as UV-VIS are not very sensitive for organic aliphatic acids. 

A conductivity detector measures the electrical conductivity of the HPLC eluent stream and is amenable to low-level determination (ppm and ppb levels) of ionic components such as anions, metals, organic acids, and surfactants. It is the primary detection mode for ion chromatography. Manufacturers include Dionex, Alltech, Shimadzu, and Waters. 

At the start of the twenty-first century, 250,000 million litres of wine were being consumed annually, valued at about 100 billion. The market is growing by about 5% annutilly. Wine is a chemically complex mixture but it is not the major constituents (water, alcohol, sugar and organic acids) that give a wine its value but the minor ones— the plant-derived NPs or compormds derived from them. It is the NPs that give a wine a rmique flavour or odour hence wine experts are simply well-trained, capable NP detectors. 

QUANTITATIVE ASPECTS. Normally the presence of high quantities of the organic acid is sufficient for diagnosis. However, quantification is sometimes required. To do this, serial amounts of the individual acid must be carried through the derivatization, and a calibration curve set up. The various acids show widely different responses to the argon detector. From the standard curves the amounts can be derived by interpolation. Calculation on the basis of urine dilution must be included as well as the aliquot of the final solution that was taken for injection. The procedure for this is similar to that described for estriol in pregnancy urine. 

In most cases, the separation of alcohols, usually methanol, ethanol, and glycerol, is carried out contemporaneously with the separation of sugars and organic acids, and almost always the desire is to quantify all these analytes. It is seen, therefore, that the mobile phase is often an aqueous acid solution, even though only water may be used (5,9). Sulphuric acid is the one most frequently used, although phosphoric acid is preferred by some, since it is less corrosive on the components of the HPLC system (10). The concentration of sulphuric acid normally varies between 0.004 N and 0.01 N or more. The choice of acid may, however, be dictated by other considerations. This is the case, for example, with the use of a conductivity detector, which requires an appropriate conductivity suppressor system. If such a device is not available for a particular... 

Since, as has already been said, the separation of alcohols is performed very often together with other analytes, such as organic acids and sugars, the choice of the type of detector also takes into account their chemical-physical properties. Usually, the choice is the ultraviolet detector (UV), the refractive index detector (RI), or the electrochemical detector (EC). Of the last, various types exist, which we shall describe briefly. 

Organic acids can also be quantified using HPLC linked to a conductivity detector. This has one advantage over UV detection in that only charged species are measured, which means that the method is liable to fewer interferences. Depending on the actual approach chosen, it is sometimes possible to detect other anions, such as Cl-, S04 and P04, in the same ran. If the conditions given in Dionex application no. 21 are used, this allows the organic acids to be separated without interferences from the fully ionised anions such as Cl-, S04 (Anon, n.d.d). 

The oxidation detector for the fluorimetric analysis of carbohydrates in effluents from liquid chromatography columns provides a sensitive method of analysis in blood and urine [110]. The principle involves the reduction of cerium(IV) to cerium(III) by oxidizable compounds such as organic acids and many carbohydrates. The fluorescence... 

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