Short gradient analyses

Figure 4.21. Effect of instrumental dispersion on gradient analysis on a short 2-mm column, (a) Analysis on a standard HPLC system, (b) Analysis of the same sample on a low-dispersion system (Waters Acquity). Diagram courtesy of Waters Corporation.

For the analysis of nonvolatile compounds, on-line coupled microcolumn SEC-PyGC has been described [979]. Alternatively, on-line p,SEC coupled to a conventional-size LC system can be used for separation and quantitative determination of compounds, in which volatility may not allow analysis via capillary GC [976]. An automated SEC-gradient HPLC flow system for polymer analysis has been developed [980]. The high sample loading capacity available in SEC makes it an attractive technique for intermediate sample cleanup [981] prior to a more sensitive RPLC technique. Hence, this intermediate step is especially interesting for experimental purposes whenever polymer matrix interference cannot be separated from the peak of interest. Coupling of SEC to RPLC is expected to benefit from the miniaturised approach in the first dimension (no broadening). Development of the first separation step in SEC-HPLC is usually quite short, unless problems are encountered with sample/column compatibility. 

There will be velocity gradients in the radial direction so all fluid elements will not have the same residence time in the reactor. Under turbulent flow conditions in reactors with large length to diameter ratios, any disparities between observed values and model predictions arising from this factor should be small. For short reactors and/or laminar flow conditions the disparities can be appreciable. Some of the techniques used in the analysis of isothermal tubular reactors that deviate from plug flow are treated in Chapter 11. 

Well-to-wheel analysis needs to be applied for all relevant time steps to understand the evolution of environmental effects and possibly costs in the short to long term. This is of specific importance when innovative processes are considered, as these are characterised by technology development and cost curves with high gradients. 

Romanyshyn L. Tiller P. Ultra-short columns and ballistic gradients considerations for ultra-fast chromatographic liquid chromatographic-tandem mass spectrometric analysis. Journal of Chromatography A, 2001,928,41-51. 

At the other end of curve 1 in Fig. 10, we see that we ould generate 50 plates per second using 1.9-p.m particles at a pressure gradient of 31.5 atm/cm. The analysis time is now 100 sec, the column length 2.85 cm, and the pressure 90 atm. Such column inlet pressure is fairly easy to use and the analysis time is quite attractive. On the other hapd the column is somewhat short and it is difficult to make a column With a length of 2.83 0.05 cm. The most difficult task, however, wolttd be to prepare... 

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