Multiple-column systems

The above circumstances may frequently lead to undesirable consequences that are well known to numerous scientists in biology and medicine. These primarily include decreased column lifetimes, formation of artifact peaks, sample decomposition, and impaired detection capabilities. As pointed out by Homing et al. [31], such difficulties lead to two divergent opinions about GC methods (a) that GC is primarily a way to introduce a very limited number of purified components into the detector for a high-sensitivity measurement and (b) that the sample should ideally be used in a relatively non-fractionated form, while the column should separate as many components as possible. 

Both views have their own individual problems. Extensive sample purifications may result in uncontrolled sample losses together with unreasonably tedious and time-consuming procedures for routine analysis. In the second case, chromatography of relatively crude mixtures often leads to a decreased reliability of multicomponent analyses on a repetitive basis. If there is a generally acceptable middle course , GC will become considerably more popular in biochemical investigations than it has been thus far. In order to systematically approach the problems, the contemporary goals and uses of biochemical GC should briefly be re-examined. 

Multiple-column systems were previously explored in the petroleum industry and some process-control situations. In the former case, typical petrochemical samples share some similarities with biochemical samples in terms of complexity while the GC column typically receives a total sample, only certain portions of it may be of interest. Thus, selected parts of a column effluent can be pneumatically switched over to a second column for an optimum analysis, while the residual uninteresting substances (heavy ends) are being rapidly removed through backflushing. In the case of process GC analysis, such backflushing is essential to the speed of analysis required from these industrial analyzers indeed, a similar situation is often found in a clinical laboratory. 

An interesting approach to the simplification of chromatographic compound profiles is the use of a subtraction column. Such a column, containing a highly 

The current techniques in multidimensional chromatography have recently been reviewed [37,38]. 

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The many examples given in this book have illustrated that dynamic simulations of distillation columns can be used to develop effective control structures for a wide variety of individual columns and multiple-column systems. However, there is another use of dynamic simulations that is very important for the safe operation when process and equipment emergencies occur. The most common example is a cooling water failure, which can lead to very rapid increases in column pressure. 

PERIODIC COUNTERCURRENT SORPTION IN MULTIPLE-COLUMN SYSTEMS... 

Fixed-bed sorption eolumns can be operated as single units or multiple units. The columns can operate by upflow or downflow, and multiple-column systems can operate in series or parallel. Figure 15.23 shows eolumns in series operating downflow. Each bed can be replaced as a eomplete, separate unit. When breakthrough occurs in the last column, the first column will be in equUibrium with the influent, thus achieving maximum adsorbent sorption capacity and utilization. This may be as a result of economic necessity. This first column will then beeome the last eolumn in the system. 

Examination of possible systems for boron isotope separation resulted in the selection of the multistage exchange-distillation of boron trifluoride—dimethyl ether complex, BF3 -0(CH3 )2, as a method for B production (21,22). Isotope fractionation in this process is achieved by the distillation of the complex at reduced pressure, ie, 20 kPa (150 torr), in a tapered cascade of multiplate columns. Although the process involves reflux by evaporation and condensation, the isotope separation is a result of exchange between the Hquid and gaseous phases. 

Figures 13.25-13.28 show the ultrahigh resolution separations in chloroform of polystyrene standards, polytetramethylene glycol, urethanes and isocyanates, and epoxy resins, respectively. Multiple column sets of anywhere from two to six columns in series have been used for well over a year with no apparent loss of efficiency. The 500- and 10 -A gels can easily tolerate 15,000 psi or more. In fact, the limiting factor in the number of columns that can be used in series is generally the pump or injector in the FIPLC system. A pump capable of 10,000 psi operation should allow the use of a column bank of 10-12 50-cm columns with a total plate count of 500,000 or more.

The first reported case of timesharing for a mass spectrometer9 involved the design of an Ionspray interface with multiple sprayers to support the analysis of effluents from multiple columns. This approach led to the development of a multiplexed electrospray interface (MUX)10 using an LC/MS interface and multiple (identical) sprayers linked to a HPLC system and a spinning screen to allow the output of only a single sprayer to enter the MS (Figure 4.5). The injections of the HPLC systems... 

The LC/MS throughput enhancement approach developed in our laboratory in 1997 and 1998 used multiple HPLC systems, each of which had dedicated HPLC pumps, autosamplers, and columns. 

Other techniques to improve throughput are instrumentation based and may involve multiple HPLC systems. The simplest method involves the automated use of solid phase extraction cartridges for sample cleanup followed by direct injection into the mass spectrometer [114], Coupling of multiple HPLC systems to one mass spectrometer allows one column to equilibrate and separate while another column to flow into the mass spectrometer. Multiple HPLC systems may be configured such that the mass spectrometer is only exposed to each serial HPLC eluent as the analyte of interest is eluted [115,116]. Although multiple H P LC-based methods may increase throughput, they also typically decrease sensitivity and may confound data workup and interpretation. 

Once it was realized that multiple columns in the first SEC really did not offer cmy advantage in terms of greater injection amounts because of increased dilution in the columns, smaller injections and less columns reduced emalysis times 50% with no loss in sensitivity. For the analyses shown in Figure 8, only three columns were used in the first SEC and three in the second. With this system the first analysis by both SBC instruments required a total of 30 minutes and subsequent analyses of the same sample eibout 15 minutes each. However, despite these significant reductions in analysis times in comparison to the initial work, complete analysis of even one complex polymer required many cross fractionations and generated much data. 

As discussed above for low-permeability systems, the analysis of spatial contaminant distributions is an appealing strategy for shortening the duration of column experiments. Again, the use of multiple columns operated over... 

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