Preparative chromatography detectors

The dimensions of the exit tube from the detector are not critical for analytical separations but they can be for preparative chromatography if fractions are to be collected for subsequent tests or examination. The dispersion that occurs in the detector exit tube is more difficult to measure. Another sample valve can be connected to the detector exit and the mobile phase passed backwards through the detecting system. The same experiment is performed, the same measurements made and the same calculations carried out. The dispersion that occurs in the exit tube is normally considerably greater than that between the column and the detector. However, providing the dispersion is known, the preparative separation can be adjusted to accommodate the exit tube dispersion and allow an accurate collection of each solute band. 

Elimination of wet chemical sample preparation enables a complete analysis to be performed and data to be quickly analyzed. The detection limits are in the low part-per-million range using mass spectrometric detection. Alternatively, detection of compounds can be achieved by all common gas chromatography detectors (flame ionization detector, electron capture detector and flame photometric detector), and detection limits are determined by the method of detection employed. 

Isolation and Identification of Hydroperoxide. A solution produced by oxidizing trans-2-butene was concentrated at reduced pressure without heating from 0.085M to 1.5M. The hydroperoxide was isolated from the concentrate by preparative chromatography under the following conditions a 5-foot 3/8-inch column of aluminum containing 10% diisodecyl phthalate on Fluoropak 80 Autoprep 705 with flame ionization detector carrier, 200 ml. per minute helium split 8 to 1 between trap and detector ... 

With the bewildering choice of parameters in gas chromatography, is there a rational way to choose a procedure for a particular problem In general, there are many satisfactory solutions. We now discuss some broad guidelines for selecting a method to use.18 The order in which decisions should be made is to consider (1) the goal of the analysis, (2) sample preparation, (3) detector. (41 column, and (5) injection. 

The detector converts a change in the column effluent into an electrical signal that is recorded by the data system. Detectors are classified as selective or universal depending on the property measured. Selective (solute property) detectors, such as fluorescence detectors, measure a physical or chemical property that is characteristic of the solute(s) in the mixture only those components which possess that characteristic will be detected. Universal (bulk property) detectors measure a physical property of the eluent. Thus, with refractive index (RI) detectors, for example, all the solutes which possess a refractive index different from that of the eluent will be detected. Selective detectors tend to be more sensitive than universal detectors, and they are much more widely used. Universal detectors are more commonly used in preparative chromatography, where a universal response is desired and sample size is large. 

Minimum Variance Purity Control of Preparative Chromatography with Simultaneous Optimization of Yield An On-Line Species-Specific Detector... 

The volatility of perfluorinated IPR was also beneficial when an evaporative light scattering detector was employed [5,17,24,50]. Perfluorinated carboxylic adds proved suitable for preparative chromatography [51]. Interestingly, since they prevent strong IPR build-up in the column, they are particularly useful when gradient elution is performed to reduce analysis time without compromising resolution [28,52-54]. 

An interesting example of a very simple fixed wavelength detector suitable for use in preparative chromatography is shown in Figure 4. This detector was invented by Miller and Strusz [7] and originally manufactured by GOW-MAC Instruments. 

Finding an online detection method is an important step in carrying out any analytical or preparative chromatography operation. In a preparative operation, a reliable online detector can be used to determine at which time the desired product is eluting from a chromatography column. 

An ideal detector for preparative chromatography has the following properties ... 

Table 5.2 lists the different detectors and their operation principles that are used in (analytical as well as preparative) chromatography. The optimal detection system only detects the target components and all impurities but is not affected by the solvent. Detectors can be divided into two groups (1) those that detect any change in composition of the solvent and can, therefore, obviously only be used under isocratic... 

If the components to be detected fluoresce, a fluorescence detector can be employed. A mercury or xenon lamp with a monochromator is used as the source for the exitation wavelength. Modern systems use lasers as light source, but such systems are mainly used in trace analysis and not in preparative systems. The main advantage of fluorescent detectors is their high sensitivity. Their reduced robustness and limitation to fluorescent compounds makes them not widely used in preparative chromatography. 

Polarimetry detectors are applied to detect optically active components. The emitted linearly polarized light is rotated by optically active components in the eluent stream and the angle of rotation is detected. Since the introduction of these detectors, which use laser light as the light source, the drawback of low sensitivity has been overcome. Similar to the DAD detectors for the UV range, circular dichroism (CD) detectors are available to detect the CD spectrum of substances. Such detectors are, so far, not widely used in preparative chromatography. 

Analytical and preparative HPLC were done using a JASCO Gulliver HPLC system with a MD-910 detector. A Cosmosil 5C18-MS column (4.6 nun x 150 mm) was used for the analytical HPLC. HPLC preparative chromatography was carried out with a Cosmosil 5C18-MS column (20 nun x 250 mm). Preparative open chromatography was conducted with a Fuji Silysia Chromatorex DM1020T ODS... 

RI detectors are highly useful for preparative chromatography, where they can be operated at low sensitivity, and for polymer or macromolecular separations, where the change in refractive index from that of the mobile phase will be great, even for low concentrations of solute. 

After the mobile phase exits the column, possibly carrying analytes, it will almost always go through one or more detectors (the only exception likely would be that the mobile phase might be collected in a fraction collector as might be done in preparative chromatography). Detection is the topic of the next section. 

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