Chromatography online

P. Kilz, High speed SEC methods, In J. Cazes (Ed.), Encyclopedia of Chromatography, online edition, Marcel Dekker, New York, 2002. 

I. N. Papadoyannis and V. F. Samanidou, Validation of HPLC, Encyclopedia of Chromatography, online, Marcel-Dekker, Inc., 2003, http //www.dekker.com. 

Wu S., Yang B., Xi L. Zhu Y. Determination of phenols with ion chromatography-online electrochemical derivatization based on porous electrode-fluorescence detection, Journal of Cftromatography A 2012 1229,288-292. 

Asche S, Beck T, Hener U, Mosandl A (2000) Multidimensional gas chromatography, online coupled with isotope ratio mass spectrometry (MDGC-IRMS) a new technique for analytical authentication of genuine flavour components. In Frontiers of Flavour Science. DFA, Garching... 

Tire aqueous or organic extract obtained at this point may be a very dilute solution containing interfering compounds and making it difficult to determine trace level concentrations of the analyte(s) of interest. To reduce interferences and concentrate the analyte(s), the primary sample extract is furiher subjected to various types of sample cleanup procedures such as conventional liquid-liquid partitioning, solid-phase extraction, matrix solid-phase dispersion, online trace enrichment, liquid chromatography, online dialysis and subsequent trace enrichment, and supercritical fluid extraction. In most cases some of Urese procedures are used in combination to obtain highly purified extracts. 

Du, L., Musson, D. G., and Wang, A. Q. (2005). High turbulence liquid chromatography online extraction and tandem mass spectrometry for the simultaneous determination of suberoyla-nilide hydroxamic acid and its two metabolites in human serum. Rapid Commun. Mass Spectrom. 19 1779-1787. 

Frank et al. (1995), by adopting capillary gas chromatography online coupled with isotope ratio mass spectrometry (cGC-IRMS), compared compounds of coriander with those of commercially available spices and essential oils. They found that isotopic effects among genuine monoterpenes are determined exclusively by the influence of... 

Wu, S.-L., Wang, Y. J., Hu, J. and Leung, D. The detection of the organic extractables in a biotech product by liquid chromatography online with electrospray mass spectrometry. PDA J. Pharm. Sci. Technol. 51 229-237, 1997. 

Cuyckens, F. et al., Improved liquid chromatography—Online radioactivity detection for metabolite profiling, J. Chromatogr. A, 1209(1-2), 128, 2008. 

Gas Chromatography from on-line Chrom-Ed Series hy Scott RPW www.chromatography-online.org/3/contents.html... 

Cuyckens F, Koppen V, Kembuegler R, Leclercq L. Improved liquid chromatography— Online radioactivity detection for metabolite profiling. J Chromatogr A 2008 1209 128-135. 

Figure 3.2 The elution curve of a single component, plotted as the analyte concentration at the column exit (proportional to the detector response Rj,) as a function of V, the total volume flow of mobile phase that has passed through the column since injection of the analytical sample onto the column. (V is readily converted to time via the volume flow rate U of the mobile phase.) The objective of theories of chromatography is to predict some or all of the features of this elution curve in terms of fundamental physico-chemical properties of the analyte and of the stationary and mobile phases. Note that the Plate Theory addresses the position of the elution peak but does not attempt to account for the peak shape (width etc.). The inflection points occur at 0.6069 of the peak height, where the slope of the curve stops increasing and starts decreasing (to zero at the peak maximum) on the rising portion of the peak, and vice versa for the falling side the distance between these points is double the Gaussian parameter O. Modified from Scott, www.chromatography-online.org, with permission.

There are many levels at which chromatography theory can be discussed. A very rigorous and complete discussion is fieely available (Scott, www.chromatography-online.org). However, here a more intuitive and less rigorous treatment, which should be sufficient to permit informed design and use of chromatography in conjunction with mass spectrometry for trace analysis, is presented. 

Equation [3.18] assumes that the extra-column volume Vg is negligible. However, there are two definitions of void volume, and thus also of the capacity ratio of a solute. The two void volumes are called the thermodynamic and the dynamic void volumes and they are not equal (Scott, www.chromatography-online.org) the two void volumes and capacity ratios are used for different purposes. Equations [3.16-3.18] incorporate the thermodynamic dead volume and all further discussion in this chapter assumes this definition. 

Figure 3.4 Graphs of logio(NA/B), calculated from Equation [3.23b] for peak resolution R a/b = 2, as a function of the capacity factor k of the first-eluting solute A, for various values of the separation ratio oia/b- R a/b implies separation of peak maxima by a single peak width (measured between the inflection points), not a very stringent criterion for chromatographic separation of analytes A and B, more usually R a/b is used. Adapted from Scott, http //www.chromatography-online.org/, with permission.

The proof of the relationship between Ng and N is given elsewhere (Scott http //www.chromatography-online.org/). The correction factor (based on k ) applied to N has a limiting value of 0.25 when k is close to unity (solute is barely retained), but for large k it approaches unity thus Ng as defined in Equation [3.24] does account to some extent for the variation of N with k, discussed above. 

It is possible to correct this ultra-simple approach with a Plate Theory model for the variation of peak width with retention time (volume), conveniently expressed via the capacity factor k. The derivation and final result are complex (Scott, http //www.chromatography-online.org/) and are not reproduced here. Instead, Figure 3.5 shows representative plots of Cp vs k for several values of N, calculated from this more realistic model. The values of N and k are of course those for the last-eluting peak, but this last peak will be different for different chromatographic detectors with different sensitivities. It is clear from Figure 3.5 that any chromatographic conditions that limit the k value for the last-detected peak will thus limit the peak capacity, particularly at lower values of k. ... 

There are four independent dispersion processes operating in a packed column that contribute to the total band broadening multipath dispersion dispersion from longitudinal diffusion and dispersion from resistance to mass transfer in each of the mobile and stationary phases. These are now be discussed separately in a somewhat qualitative fashion a rigorous discussion can be found elsewhere (Scott, http //www.chromatography-online.orgZ). It is important to note that, in the following derivations... 

Figure 3.6 Illustration of the effect on analyte band dispersion of multiple paths through stationary phase particles, resulting in a path length difference of 8L. Adapted from Scott, http //www.chromatography-online.org/, with permission.

Clearly these dispersive processes will increase in relative importance as the mobile phase flow rate increases and as the diffusion rates decrease (the opposite to the dependence for longitudinal diffusion discussed above). This is because the faster the mobile phase (and slower the inter-phase diffusion) the greater the extent to which the molecules originally far from the stationary phase interface will be swept ahead of the molecules that made it into the stationary phase and were retained there for some time. A detailed discussion (Scott http //www.chromatography-online.org/), based on that originally derived by van Deemter, of the form of this dependence of band dispersion on resistance to mass transfer, gives ... 

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