Columns indirect absorbance

Alcohol sulfates can be characterized by HPLC. Their molecular structure does not permit a direct UV detection but an indirect UV detection of the alcohol sulfate is possible [285]. The detection can be made by the change in UV absorbance of the mobile phase, either by using an ion exchange column contain-... 

Perhaps the most revolutionary development has been the application of on-line mass spectroscopic detection for compositional analysis. Polymer composition can be inferred from column retention time or from viscometric and other indirect detection methods, but mass spectroscopy has reduced much of the ambiguity associated with that process. Quantitation of end groups and of co-polymer composition can now be accomplished directly through mass spectroscopy. Mass spectroscopy is particularly well suited as an on-line GPC technique, since common GPC solvents interfere with other on-line detectors, including UV-VIS absorbance, nuclear magnetic resonance and infrared spectroscopic detectors. By contrast, common GPC solvents are readily adaptable to mass spectroscopic interfaces. No detection technique offers a combination of universality of analyte detection, specificity of information, and ease of use comparable to that of mass spectroscopy. 

For the cationic surfactants, the available HPLC detection methods involve direct UV (for cationics with chromophores, such as benzylalkyl-dimethyl ammonium salts) or for compounds that lack UV absorbance, indirect photometry in conjunction with a post-column addition of bromophenol blue or other anionic dye [49], refractive index [50,51], conductivity detection [47,52] and fluorescence combined with postcolumn addition of the ion-pair [53] were used. These modes of detection, limited to isocratic elution, are not totally satisfactory for the separation of quaternary compounds with a wide range of molecular weights. Thus, to overcome the limitation of other detection systems, the ELS detector has been introduced as a universal detector compatible with gradient elution [45]. 

In suppressed-ion chromatography, a separator column separates ions of interest, and a suppressor membrane converts eluent into a nonionic form so that analytes can be detected by their conductivity. Alternatively, nonsuppressed ion chromatography uses an ion-exchange column and low-concentration eluent. If the eluent absorbs light, indirect spectrophotometric detection is convenient and sensitive. Ion-pair chromatography utilizes an ionic surfactant in the eluent to make a reversed-phase column function as an ion-exchange column. 

Figure 3.20. Analysis of carboxylic acids and alcohols by reversed phase HPLC, with indirect UV detection, (a) Carboxylic acids. Chromatography conditions mobile phase, 3 X 10 4 M l-phenethyl-2-picolinium in acetate buffer (pH 4.6) column, ju-Bondapak phenyl detection, indirect UV absorbance at 254 nm. Peaks 1, acetic acid 2, propionic acid 3, butyric acid 4, valeric acid 5, caproic acid S, system peak, (b) Aliphatic alcohols. Chromatography conditions mobile phase, 4 x 10 4 M nicotinamide in water column. Ultrasphere ODS detection, indirect UV absorbance at 268 nm. Peaks 1, methanol 2, propylene glycol 3, ethanol 4, 2-propanol 5, 1-propanol 6, system peak 7, 2-butanol 8, 2-methyl-l-propanol 9, 1-butanol. (Redrawn from Refs. 23 and 24 with permission.)...

The first report demonstrating the feasibility of indirect detection in CE was published in 1987 by Hjerten et al.45 who employed indirect UV absorbance detection for the analysis of both inorganic ions and organic acids. The UV-background-providing electrolyte was 25 mM sodium veronal, pH 8.6, and detection was monitored on-column at 225 nm. In 1990, the first separation of alkali, alkaline earth, and lanthanide metals was reported by Foret et al46 Indirect UV detection at 220 nm was employed to detect 14 metals in 5 min, with baseline resolution achieved between all but two of the components. The baseline showed a reproducible upward drift between 1 and 3 min. The UV-absorbing component of the electrolyte was creatinine, with a-hydroxyisobutyric acid introduced to complex with the lanthanides and improve resolution. 

Some of the remaining studies did not necessarily observe incomplete catalyst wetting, but included this concept either directly as an adjustable parameter in the model to fit the observed conversion versus liquid mass velocity data,(7,9,13, 16-18), or indirectly via use of a correlation for liquid-solid contacting established for non-porous absorber column packings (11,19-20). 

The general theory of indirect detection was recently put forward [14] and its mechanism stimulated pioneering model makers to provide a realistic, practical and rational description of IPC [15,16]. For example, a UV-absorbing mobile phase containing potassium hydrogen phthalate and triethanolamine allowed the indirect detection of sulfur and nitrogen anions in atmospheric liquids separated on a cetylpyridinium-coated C18 column [17]. Crystal violet was used as the IPR in the... 

Absorbance detection has been applied to ion analysis through two different approaches direct detection of the sample ion and indirect detection. In some cases, a post-column, color-forming reagent can be added to the column eluate to detect sample ions. 

Direct Interfacing of LC-FT-IR Is performed via a flow cell technique. To avoid the Interferences from mobile phase absorbance In the mld-lR region, a compatible (transparent) mobile phase should be used. A "Buffer Memory" technique for Indirect combinations of microbore column and FT-IR was also reported by Jlnno et. al. The buffer memory technique was reported to have advantages over conventional direct LC-FT-IR In achieving a continuous chromatogram and In preventing mobile phase Interference. 

Post-column derivatization Is widely used in liquid chromatography [30, 31], the potential of which is thereby Increased. It affords a number of objectives (a) indirect Improvement of the analyte s intrinsic sensitivity In a given detector (the resultant reaction product absorbs or emits more intense fluorescence than the parent analyte isolated in the column, even after preconcentration) (2) facilitating detection through removal of the excess of derl-vatizing reagent or thanks to the detector s blindness to the separated analytes (c) providing an identification test for one or several of the analytes in a complex mixture. 

High detection sensitivity by absorbance, fluorencence or electrochemical response can be obtained in liquid chromatography provided that the solutes contain structural elements with such activity. If this is not so, detectability can be achieved by pre- or post-column derivatization or, as a third possibility, by ion-pair-mediated indirect detection. In this case, a counter-ion with detector response is added to the chromatographic system or supplied in a post-column extraction system (see Chapter 17). 

---