Multi-solvent pumps

A simple system is comprised of an isocratic pump, a manual injector, a UV detector, and a strip-chart recorder. A schematic diagram of an HPLC instrument is shown in Fig. 15.4. This simple configuration is rarely used in most modern laboratories. A typical HPLC system is likely to consist of a multi-solvent pump, an autosampler, an on-line degasser, a column oven, and a UV/Vis or photodiode array detector all connected to and controlled by a data-handling workstation. Examples of modular and integrated systems are shown in Fig. 15.5. Some of the important instrumental requirements are summarized in Table 15.2. 

Multi-solvent pump Low-pressure mixing system, online degasser... 

Multi-solvent pumps, photodiode array detectors (PDA), modeling software, automated development systems, and multi-column selector valves... 

As in GC temperature programming, the gradient can be stepwise or continually ramped. The pumps are programmed to keep the total flow rate constant. The starting solvent should have a polarity that rapidly elutes and resolves the first components, and the solvent strength (polarity) is increased to a value that resolves the last peaks in a reasonable time. The multi-solvent method described above can help determine the starting and finishing polarities. 

PBLG molecular weights were verified by gel permeation chromatography (GPC). Instrumentation (Waters Associates, Inc.) included a dual pump Multi-Solvent Delivery system (Waters 600/600E), 712 WISP Autosampler, a Waters 486 tunable ultraviolet/visible detector, and a Waters 410 differential refractometer detector. Data collection and analysis are facilitated by a Maxima 825 software program. 

In place of the diluent (or in addition to it, using more channels) one can add a buffer to obtain the optimum pH for the most efficient extraction. Or, one can add a reagent (or two, or more) and perform a pre-column derivatization, if that is required to improve separation or sensitivity. Multi-channel proportioning pumps are available with up to 28 channels, with pump tubes which can handle acids, bases, and most solvents at flow rates ranging from about 50 microliters per minute up to nearly four milliliters per minute. 

Among the best well-known examples of photostability after UV radiation, the ultrafast nonradiative decay observed in DNA/RNA nucleobases, has attracted most of the attention both from experimental and theoretical viewpoints [30], Since the quenched DNA fluorescence in nucleobase monomers at the room temperature was first reported [31] new advances have improved our knowledge on the dynamics of photoexcited DNA. Femtosecond pump-probe experiments in molecular beams have detected multi-exponential decay channels in the femtosecond (fs) and picosecond (ps) timescales for the isolated nucleobases [30, 32-34], The lack of strong solvent effects and similar ultrafast decays obtained for nucleosides and nucleotides suggest that ultrashort lifetimes of nucleobases are intrinsic molecular properties, intimately... 

An ideal propulsion system should ensure reproducible flow rates on a short-term (hours) and long-term (days) basis, multi-channel capability (at least four parallel pumping channels to provide system versatility), resistance to aggressive reagents and solvents, readily adjustable flow rates and low initial investment and running costs [7]. The maintenance of a consistent flow is essential to obtain good analytical reproducibility. Flow rates are typically in the 0.2—5.0 mL min-1 range so that the system operates under low pressure, normally lower than 10 psi [0.689 bar]. The number of channels used depends on the manifold complexity. 

Hitherto, most FI liquid-liquid extractions are performed in a multi-segmented flow comprised of the two immiscible phases. Since the extraction coil is merely an extension of the outlet of the segmentor, the two components may be viewed upon as a single pan in the FI manifold. This pan of the manifold is closely connected with the propulsion system which should meet the demands for the delivery of organic solvents. Although these may be satisfactorily propelled by solvent resistant piston pumps, peristaltic pumps are used more frequently, often making the displacement bottle (cf. Sec. 2.1.1) an indispensable component of this pan of the manifold. 

Figure 9.16 shows a schematic of the dry spinning process. The spinning solution is dehvered into the multi-hole spinneret by a metering pump at an accurately controlled rate. The spirmeret often is placed in an enclosed drying tower for solvent recovery. Upon exiting the spirmeret, the polymer solution comes into contact with a stream of hot inert gas (typically, air), and the solvent evaporates. The hot gas carries most, if not all, of the solvent evaporated from the polymer... 

---