Light scattering detector problems

Thin-layer chromatography (TLC) is used both for characterization of alcohol sulfates and alcohol ether sulfates and for their analysis in mixtures. This technique, combined with the use of scanning densitometers, is a quantitative analytical method. TLC is preferred to HPLC in this case as anionic surfactants do not contain strong chromophores and the refractive index detector is of low sensitivity and not suitable for gradient elution. A recent development in HPLC detector technology, the evaporative light-scattering detector, will probably overcome these sensitivity problems. 

Detection is also frequently a key issue in polymer analysis, so much so that a section below is devoted to detectors. Only two detectors, the ultra-violet-visible spectrophotometer (UV-VIS) and the differential refractive index (DRI), are commonly in use as concentration-sensitive detectors in GPC. Many of the common polymer solvents absorb in the UV, so UV detection is the exception rather than the rule. Refractive index detectors have improved markedly in the last decade, but the limit of detection remains a common problem. Also, it is quite common that one component may have a positive RI response, while a second has a zero or negative response. This can be particularly problematic in co-polymer analysis. Although such problems can often be solved by changing or blending solvents, a third detector, the evaporative light-scattering detector, has found some favor. 

Stolyhwo et al. (106) described a different analytical methodology, one that circumvents most of the problems encountered so far and permits the analysis of TGs up to and including tribehenin and the quantitative analysis of these complex mixtures with few calibrations, using a laser-light-scattering detector. For the achievement of most separations, a 250 X 4.0-mm ID HIBAR LICHROSPHER1000 CH-18/3 (5-/an particles) was used. For the achievement of some difficult separations, a second, similar column was connected online to the first one. 

The advent of the use of mass spectrometers as detectors and new mass detectors such as the charged aerosol detectors (CAD) and evaporative light scattering detectors (ELSD) should provide high-sensitivity detection of compounds that do not absorb UV light. The only problem with most of these is that they are expensive and, therefore, not readily available. When prices come down, they should finally eliminate the use of derivatives in HPLC analysis. 

The detection problems of the LC LC procedmes are similar to LC CC (see section 11.8.3). The zone of interacting tnacromolectrles is partially overlapped with the barrier and this fact complicates quantitative sample monitoring. This is why the evaporative light scattering detectors with all their drawbacks (see section 11.6.1.4) are so far mostly employed in the LC LC measurements. 

The use of diode-array detection is limited because many of the terpenoids do not have chromophores for the UV region. Refractive index, evaporative light scattering detectors (ELSD), or low-UV detectors can overcome, to a certain extent, this problem. However, the refractive index detector... 

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