On-line Degassing

Chapter 3 Instrumentation for High-Performance Liquid Chromatography 

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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. 

Figure 4. A Schematic Diagram of an On-line Degassing System.

Mobile phases should be degassed if an on-line degasser is not available on the HPLC system. 

Flush the pump with open drain valve at high flow rate. Degas the mobile phase or use an on-line degasser. (This is absolutely necessary with low-pressure gradient systems.) Clogged inlet filter of the pump ... 

Another approach for on-line degassing of solutions is to use a standard sandwich type gas-diffusion separator. One of the ports of the acceptor channel is blocked and the other connected to a vacuum source or to a pump which evacuates the gas from the channel. Such an arrangement was used by Hinkamp and Schwedt [13] in the determination of total phosphorus in waters with amperometric detection to remove gas bubbles generated in the reaction stream after an on-line continuous digestion in a microwave oven. 

The mobile phase reservoirs are generally screw-top plastic-coated glass bottles with inlet lines to allow for degassing of the solvents for 10-15 min either by vacuum or sparging with helium or both. A positive pressure of helium should be maintained in the solvent bottles. Degassing is important to prevent gas bubbles from lodging in the pump, column, or the detector. On-line degassing... 

One way to get a representative product distribution for a specific period is to remove all FT products in the reactor system and replace them with a substance that will not influence selectivity determination. The FT reaction is then run for a specific period, after which a full analysis can be done that will represent only the products produced during that specific period. In Figure 13.8, data are presented for a run started with the catalyst suspended in a highly paraffinic wax (FT HI wax, C30-C90). After a certain time of synthesis, the FT run was stopped and the catalyst placed under inert conditions (argon). The reactor content was then displaced with degassed and dried polyalphaolefin oil (Durasyn). After restarting the FT synthesis, the total product spectrum was determined (HI run after displacement). It was found that the value of a2 was much lower than before the displacement of the HI wax. In fact, the a2 values were quite comparable to those measured when the FT synthesis was started up with Durasyn (compare with Durasyn runs 1, 2, and 3). This clearly illustrates the impact that the reactor medium used to start the FT reaction can have on the determination of the a-value. The results further show that there was no change in the value of a2 of the iron catalyst up to 500 h on-line. 

This seemingly simple series of events does not address all the requirements. If the device is preparing media does that mean it prepares a buffer to be diluted or only degasses the premixed media When media is dispensed, is there a need to perform a preliminary dispense to assure removal of the previous media If samples are to be read on-line is dilution required prior to reading Systems intended for method development (MD) will have many different requirements than one intended for QA. The value of the automation to the user may be very different for each of these two areas. In fact the MD user may not appreciate the need to automate more than one run at a time and will prefer a semiautomated system, since the MD user may have many different experiments to perform that may be labor intensive. Just a few... 

FIGURE 8 A schematic of a 4-solvent on-line vacuum degasser. The preferred device for solvent degassing. 

Turn on the on-line vacuum degasser and the seal wash pump system (if available). 

On-line vacuum degasser and seal wash system—inspect and check functions. 

Figure 16.4 Experimental set-up of an on-line FIA system ( ) bioreactor, (2) glycerol standard solution, (3) water for dilution, (4) buffer solution containing NAD, (5) buffer solution containing ferricyanide, (6-1) switch valve, (6-2 and 6-3) injection valves, (7-1 and 7-2) peristaltic pumps, (8) mixing chamber, (9) degasser, (10) flow sensor containing biosensor, and (11) waste. (Reprinted from Sefcovicova et al/ with permission from Elsevier.)...

The sample preparation vacuum line (often called spray-on line ) should allow for (a) controlled mixing of the host gas with the substrate (or with other components that are added to the matrix) by manometric techniques and (b) the controlled release of the gas (mixture) toward the inlet system of the cryostat. These conditions are met by a vacuum line that incoporates a storage bulb for the gas (mixture), inlets for attachment of evacuable containers that allow degassing of the substrate prior to its mixing with the host gas, pressirre gauges that cover suitable ranges, a needle valve that allows the controlled release of the gas, possibly via a flowmeter, and interfaces to the bottles that contain the host gas(es), and to the inlet system that is attached to the vacuum shroud of the cryostat. 

Temperature programmed desorption (TPD) of C02 (5 °/min, flow of He, 15 ml/min) was carried out on a conventional flow apparatus. In a typical experiment, 0.29 g of the catalyst were activated as above reported, then the system was cooled to 25°C and approximately 2 10 5 mol of Co2 were injected by means of a gas sampling valve. After degassing in flow of helium for 60 min the amount of the irreversibly adsorbed C02 was determined with an on-line g.l.c. equipped with a thermal conductivity detector,... 

Fig. 20. Schematic design of linking a chromatograph on-line to bioprocesses. In principle, the design is almost identical to an FIA system. This is why FIA is often characterized as chromatography without a column. However, degassing of the sample is essential, in particular, when no internal standard is added (as in this sketch). In addition, the technical designs of injection valves differ and the injector to a gas chromatograph is heated to 200 or 250°C which means it needs, therefore, a special construction...

Mobile-phase degassing is an important step in the LC/MS experiment and can be accomplished via on-line membrane or vacuum devices, sonication, helium sparging or as part of the mobile-phase filtration step. Degassing will eliminate pump cavitation, ensure reproducible retention times and minimize possible sputtering from the ion source. 

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