Unretained marker

There has been an attempt to measure the peak capacity in 1DLC and 2DLC by assigning a range of useful retention time between the unretained marker that elutes at ti and some stated value of the retention factor k leading to a zone at tf and plugging in a value for the peak width W. This number is useful but will never be equal to the number... 

Figure iJ, Comparison of column pressures between apparent and corrected with the extra-column volume effect. Conditions Premixed 50/50 (v/v) water to acetonitrile as the mobile phase 0.2 mg/mL uracil as unretained marker. From Wu, N., et al. J. Sep. Sci. 2012, 35, 2018-2025, with permission. 

Other Experimental Methods. It is probably suitable to discuss here column porous structure. Porous space of a conventional packed column consists of the interparticle volume (Vip—space around particles of packing) and pore volume (Vp— space inside porous particles). The sum of those two constitutes the column void volume. The void volume marker ( unretained ) should be able to evenly distribute itself in these volumes while moving through the column. Only in this case the statistical center mass of its peak will represent the true volume of the Uquid phase in the column. In other words, its chromatographic behavior should be similar to that of the eluent molecules in a monocomponent eluent. If a chosen void volume marker compound has some preferential interaction with the stationary phase compared to that of the eluent molecules, it will show positive retention and could not be used as void marker. If on the other hand it has weaker interaction, it will be excluded from the adsorbent surface and will elute faster than the real void time, meaning that it also could not be used. For any analytical applications (when no thermodynamic dependences are not extracted from experimental data), 10% or 15% error in the determination of the void volume are acceptable. It is generally recommended to avoid elution of the component of interest with a retention factor lower than 1.5. Accurate methods for the determination of the column void volume are discussed in Chapter 2. 

A second set of test compounds, along with the original fo markers, was then put together and a further set of experiments conducted. The results from these experiments are presented in Table 8.2. The main requirement for success was to maximise the number of compounds with k greater than 1 (i.e. the compound was retained at least twice as well as something unretained). The Daisopak column achieved this for seven out of the ten compounds tested, the Aquasil and Platinum EPS achieved it with six compounds and the Alkyl HB with five compounds. 

M where and ii are the retention times of the first- and second-eluted fractions, respectively, and to is the retention time of an unretained compound void volume marker), d) Resolution factor Rs = 2(t2 - ti)/ W] + Wi) where Wj and Are the band-widths of the first- and second-eluted fractions at the base-line level, respectively, e) Mono-MaNP ester. 

Controversial opinions exist among scientists regarding the meaning of column dead time in liquid chromatography (LC). " In its broad sense, the term column dead time refers to the elution volume of an unretained and unexcluded solute, but it is not easy to establish which solute (if any) can be treated as both unretained and unexcluded. In LC practice, quite a large number of solutes (e.g., mobile-phase components, isotopically labeled mobile-phase components, ionic and nonionic species) had been used as void time markers. The most popular... 

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