Low-Dispersion Instruments

Low-dispersion HPLC systems are necessitated by the increasing trend of using shorter and narrower HPLC columns, which are more susceptible to the deleterious effects of extra-column band-broadening. HPLC manufacturers are designing newer analytical HPLC systems with improved instrumental bandwidths compatible with 2-mm i.d. columns by using micro injectors, smaller i.d. connection tubing, and detector flow cells. A new generation of ultra-low dispersion systems dedicated for micro and nano LC is also available. 

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The IBW of a standard system can be reduced to 30-40 jL by using shorter lengths of 0.005-0.007" i.d. tubing and a semi-micro flow cell (2-3 tL).i° Further reduction might involve a low dispersion micro-injector or a redesign of the autosampler. Table 4 summarizes the typical IBW and other instrumental requirements of various column types from conventional (4.6 mm), Fast TC, minibore (3 mm), narrowbore (2 mm) and microbore (1mm) to micro TC (<0.5mm) columns. Note that the dispersion... 

Instrumental band broadening or axial dispersion can cause calibration errors when employing polydisperse standards. Correction of the polydisperse standard calibration data for instrumental band broadening will minimize the effect on molecular weight analyses of polymer samples. However, as previously demonstrated in this report, when low dispersion SEC columns are employed instrumental band broadening is minimized and the effect on use of linear calibration methodology is negligible. 

The combination of Fellgett s and Jacquinot s advantage coupled with the inherent speed differential should lead to an enormous difference between FT-IR and dispersive instruments. However, in practice, part of this advantage is offset by the difference in the performance of the triglycine sulfate (TGS) and thermocouple detectors. At low modulation frequencies, the thermocouple detector is about an order of magnitude more sensitive than TGS. 

Energy dispersive instruments are used for qualitative analysis and routine quantitation (Fig. 13.5) and represent the first category of instruments. They are generally equipped with a low power X-ray tube instead of a radioactive source in order to eliminate constraints caused by legislation. 

Figure 13.7—An energy dispersive instrument and an example of a low resolution spectrum. Spectrometer with a low-power X-ray tube, representative of a number of instruments of that type (model Minimate, reproduced by permission of Philips). This model is controlled by a microcomputer (not shown).

Thorium oxide has a high refractive index and low dispersion and thus finds use in high-quality camera and scientific instrument lenses. Thonum oxide also is used as a catalyst in the conversion of ammonia to nitnc acid, in petroleum cracking, and in sulfuric acid production. 

To acquire this information, the two displaced continuum beams are imaged with a cylindrical and a spherical lens onto different positions along the length of the entrance slit of a low dispersion spectrograph (Instruments SA, model UFS-200). The two resulting parallel dispersed spectra are fully separated from each other at the focal plane, where they are detected by the model 1254 SIT detector head of an EG + G Princeton Applied Research Corporation optical multichannel analyzer system. In conjunction with a model 1216 detector controller and model 1215 console, this detector is programmed with a two dimensional... 

There are two types of Mastersizer instruments the Mastersizer Micro and the Mastersizer E, which are low cost instruments for repetitive analyses and the modular series of Mastersizer S and Mastersizer X, the ultimate in resolution and dynamic size range, which are required when samples in the form of aerosols, suspensions and dry powders need to be measured. Mastersizer X provides a selection of small size ranges using a variety of interchangeable lenses whereas the Mastersizer S provides a wider dynamic size range covered in a single measurement. For powders which are to be suspended in a solvent, emulsions, suspensions and particles in liquids there are small volume cells which require as little as 15 ml of dispersant. Where a material is either valuable or toxic the Malvern Small Volume Flow Cell, with a sample volume of 50-80 ml and full sample recovery, can be used. The X-Y sampler is a 40-sample accessory... 

Current micro LC applications are primarily in micropurifications of pro-teins/peptides and in proteomics research. Chapter 4 discusses the stringent instrumentation requirements for micro and nano LC, which must have instruments with very low dispersion and dwell volumes. 

Figure 4.21. Effect of instrumental dispersion on gradient analysis on a short 2-mm column, (a) Analysis on a standard HPLC system, (b) Analysis of the same sample on a low-dispersion system (Waters Acquity). Diagram courtesy of Waters Corporation.

Energy dispersive instruments of small dimensions are reserved for both qualitative analysis and routine measurements (Figures 12.10 and 12.11). The spectrum is obtained by use of a detector installed quite close to the sample, which distinguishes the energy of each of the fluorescence photons captured. These instruments are equipped with either a small size and low power X-ray tube (around 10 W) or a radioactive source for field instruments. 

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