Gradient instruments

INSTRUMENTAL ASPECTS OF GRADIENT ELUTION 5.5.1 Types of Gradient Instruments... 

Micro-HPLC operation sets special demands on the gradient instrumentation. As the internal column diameter, d, decreases, lower flow rates should be used at comparable mean linear mobile phase velocities, u = 0.2-0.3 mm/s. At a constant operating pressure, the flow rate decreases proportionally to the second power of the column inner diameter, so that narrow-bore LC columns with 1mm i.d. require flow rates in the range of 30-100pL/min, micro-columns with i.d. 0.3-0.5mm, flow rates in between 1 and lOpL/min, and columns with 0.075-0.1 mm i.d. flow rates in the range of hundreds nL/min. Special miniaturized pump systems are required to deliver accurately mobile phase at very low flow rates in isocratic LC. 

HPLC is useful as an analytical tool in several applications in addition to its role in the purification of an activity. For example, HPLC can be useful in the establishment of gradients. Instruments have been manufactured for use with HPLC that can control the flow and mixing of solvents and thereby generate gradients with a variety of concentrations and shapes. In addition, since it is possible to carry out separations on the HPLC column in a comparatively short time, a number of these gradients can be applied to an analytical scale column, and the one best suited to the separation established fairly rapidly. Armed with this information, it is a relatively simple matter to carry over these gradient conditions to a non-HPLC ion-exchange column. 

The instrument needed to perform automated preparative chromatography (which will hereafter be referred to as autoprep) in the desired format is based upon a standard high-pressure binary gradient instrument. The most important feature of the instrument for our particular requirements was the ability to operate from the microtitre plate sample format. This is important for combinatorially derived samples as it appears to be the standard format. It also becomes the most sensible arrangement for collecting large numbers of fractions within a reasonable size collection tray footprint . 

ATS FAAR supplies equipment for the determination of density according to ASTM and DIN specifications (Table 18.2). Martin Instrument Company supplies one- and three-column density gradient instruments for the measurement of density. 

Fig.3 Corrosion calibration standard for eddy curent instrument using the gradient method.

The concentric cylinder viscometer described in Sec. 2.3, as well as numerous other possible instruments, can also be used to measure solution viscosity. The apparatus shown in Fig. 9.6 and its variations are the most widely used for this purpose, however. One limitation of this method is the fact that the velocity gradient is not constant, but varies with r in this type of instrument, as noted in connection with Eq. (9.26). Since we are not considering shear-dependent viscosity in this chapter, we shall ignore this limitation. 

The previous methods are mainly used to measure duct flow. When measuring flows on supply or exhaust terminals, different methods are used. The measurement on exhaust terminals is simple to carry out, as the velocity field near the terminal is relatively constant, with no steep gradients or swirls. In the case of a grill, traversing across the terminal surface using a suitable velocity instrument is a good alternative. A suitable instrument for most cases is the vane anemometer. 

In the magnetic-sector instrument (Figure 1.1), gas phase ions produced in the ion source by one of several different methods are accelerated from near rest (thermal energy) through a potential gradient (commonly kV). These ions travel through a vacuum chamber into a magnetic field at a... 

Repetitive routine analysis of a specific sample (e.g., for Quality Control) will usually require a dedicated instrument. Therefore, the chromatograph and, in particular, the detector will be chosen for that specific analysis. Consequently, only one detector will be necessary and the purchase of an armory of detectors on the basis that they might be needed in the nebulous future is not advised. An alternative detector can always be obtained if and when the demand arises. The same argument applies to multi-solvent reservoirs and multi-solvent gradient programmers and other accessories that are not immediately required for the specific analysis in mind. 

Saarinen, TR Woodward, WS, Computer-Controlled Pulsed Magnetic Field Gradient NMR System for Electrophoretic Mobility Measurements, Reviews of Scientific Instruments 59, 761, 1988. 

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