Volatile detection techniques compared

Comparisons of CE and HPLC on similar analytical problems have been reported [879-883]. In contrast to CE and HPLC, GC is best suited for analysis of nonpolar, lower MW, volatile compounds. HPLC and GC have detection limits roughly 100-1000 times lower than CE, while traditional electrophoresis has detection limits comparable to CE. As already mentioned, the inferior detection sensitivity and precision of CE when compared with HPLC are caused by the technique s nanoscale. 

The degree of purity of a sugar sample can be ascertained or the identification and assay of mixtures of sugars can normally be achieved by any one or a combination of chromatographic methods, of which GC of volatile derivatives or HPLC with electrochemical detection of the sugars or derivatives prepared in order to confer suitable spectroscopic properties, are comparable in their efficacy. Supercritical fluid chromatography (SFC) is also a flexible, rapid, and efficient technique comparable with HPLC but by no means as widely used. 

Supercritical fluid extraction combined with high-resolution GC MS proved to be a powerful tool for the analysis of the virgin olive oil aroma [15]. The volatiles identified were compared with those obtained by using the DHS method. Different aromatic profiles were obtained by applying the two extraction procedures. The profiles obtained by DHS-GC-MS corresponded to a genuine extra-virgin olive oil sample in accordance with previous findings [18]. The presence of off-flavors was not detected. In the SFE extracts, however, markers of oxidation processes were identified, since this technique is also suitable for the extraction of semivolatile compounds. These were volatile compounds related to oxidation of linoleic, linolenic, and oleic acids, and in particular aldehydes and acids, which had been previously found in oxidized olive oil samples [19],... 

Ethanol and a long list of carbonyl compounds and aliphatic acids occur in fresh milk (Table 1.5). Some of them have been detected in only a few of the samples in which they were sought. Techniques for detecting such compounds include derivatization with 2,4-dinitrophe-nylhydrazine and various methods of volatilization, extraction, and chromatography (Harper and Huber 1956 Morr et al. 1957 Harper et al. 1961 Wong and Patton 1962 Scanlan et al. 1968 Marsili et al. 1981). The sum of the concentrations of acids listed in Table 1.5 is only 1-3 mmol/liter, compared to the citrate concentration of 10 mmol/liter. Oxalate has been reported to occur in milk (Zarembski and Hodgkin-son 1962) on the basis of a certain colorimetric reaction, but positive identification has not been made. 

An HPLC technique has been developed [59] for volatile carcinogenic N-nitrosamines which shortens the chromatographic analysis time by a factor of five compared with GLC methods. The nitrosamines are converted into their amine reduction products which are then treated with 2,4-dinitrofluorobenzene (DNFB) in order to form the amines. The amines are then separated and detected at 340-360 nm. The reactions involved are shown in Fig.4.31. 

I don t want to leave you with the impression that I feel that HPLC is the perfect analytical system. The basic system is rather expensive compared with some analytical tools columns are expensive with a relatively short operating life, solvents are expensive and disposal of used solvent is becoming a real headache. Other techniques offer more sensitivity of detection or improved separation for certain types of compounds (i.e., volatiles by GLC, large charged molecules by electrophoresis). Nothing else that I know of, however, offers the laboratory the wide range of separating modes, the combination of qualitative and quantitative separation, and the basic versatility of the HPLC system. 

A number of techniques have been used for the speciation of arsenic compounds. The most important has been the formation of volatile hydrides of several species, separation by gas chromatography and detection by AAS. HPLC has been used to separate arsenic species. Several types of detectors have been studied for the determination of arsenic species in the column effluent. These have included AAS both off- and on-line, ICPAES and ICP-MS. An important comparative study of coupled chromatography-atomic spectrometry methods for the determination of arsenic was published (Ebdon et al., 1988). Both GC and HPLC were used as separative methods, and the detectors were FAAS, flame atomic fluorescence spectrometry (FAFS) and ICPAES. The conclusions were (1) that hydride generation and cryogenic trapping with GC-FAAS was the most... 

Since the use of techniques to measure IAA, bioassays have been important to discover PGR activity of many other compounds. Several bioassays for the PGR gibberellic acid (GA3) have been developed. One bioassay was based on reduction of amaranthin levels in Amaranthus caudatus (tassel flower) seedlings.63 This method was sensitive to GA3 from 0.01 to 1 mg L"1. GA3 was also bioassayed, based on anthocyanin reduction in tomato (Lycopersicon esculentum L.).62 Reduction of anthocyanin in tomato seedlings was linear from 10"5 to 10 mg L 1, and thus, this latter plant bioassay method was more sensitive. A multitude of bioassays for nonvolatile and volatile plant growth regulator compounds have been developed and their uses and limitations have been discussed.129 Since many allelochemicals have been shown to have relatively weak phytotoxicity (especially compared to herbicides), some of these bioassays that have been developed for detecting and quantitatively measuring PGR activity may be useful in allelopathy. 

The examples presented in this chapter demonstrate that a combination of various analytical approaches and the selection of suitable model systems can add valuable information to our knowledge about pathways and enzymes involved in the biosynthesis of chiral volatiles. Some of the techniques need further improvement, e.g. by use of radioactively labeled precursors the detection threshold of metabolites can be lowered significantly addition of precursors in concentrations comparable to those in natural plant or microbial systems would be possible. The investigation of the enantioselectivity of enzymes has to be emphasized, eventually not only enzymes commercially available or easily accessible in microorganisms but also those active in plant systems have to be studied. 

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