Retention relative

A limit ofk 2 is considered acceptable for most forensic applications. Relative Retention 

The relative retention time is a measure of the retention time of two independent peaks relative to one another and is represented by the following equation  

Resolution is a measure of how well two peaks are separated and it is a necessary parameter when quantitative measurement is conducted to ensiue that accurate assessment of peak area is achieved. It is also a useful parameter to measure where there may be interfering peaks eluting close to the peaks of interest. This is of particular interest in forensic applications where the exact sample composition is unknown. Resolution is measured using the following equation  

A limit o/R 1.5 between peaks of interest is considered acceptable for most 

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A comparison of the molar volumes of 2-, 4-, and 5-alkylthiazoles with their relative retention volumes shows that these values also vary in the same direction (see Fig. III-2). 

The accurate determination of relative retention volumes and Kovats indices is of great utility to the analyst, for besides being tools of identification, they can also be related to thermodynamic properties of solutions (measurements of vapor pressure and heats of vaporization on nonpolar columns) and activity coefficients on polar columns by simple relationships (179). 

Recovery factor Reduced column length Reduced plate height Reduced velocity Relative retention ratio Retardation factor d Retention time Retention volume Selectivity coefficient Separation factor... 

Relative Retention. The relative retention a of two solutes, where solute 1 elutes before solute 2, is given variously by... 

The relative retention is dependent on (1) the nature of the stationary and mobile phases and (2) the column operating temperature. 

To accomplish any separation of two cations (or two anions), one of these ions must be taken up by the resin in distinct preference to the other. This preference is expressed by the separation factor (or relative retention), using K+ and Na+ as the example ... 

By comparing the ratio of the distribution coefficients for a pair of ions, a separation factor (or relative retention) is obtained for a specific experimental condition. 

Two gas chromatograms showing the effect of polarity of the stationary phase on the separation efficiency for three substances of increasing polarity toluene, pyridine, and benzaldehyde. (a) Separation on silicone SE-30, a nonpolar phase, and (b) separation on elastomer OV-351, a more polar phase. Note the greatly changed absolute and relative retention times the more polar pyridine and benzaldehyde are affected most by the move to a more polar stationary phase. 

The simplest mode of IGC is the infinite dilution mode , effected when the adsorbing species is present at very low concentration in a non-adsorbing carrier gas. Under such conditions, the adsorption may be assumed to be sub-monolayer, and if one assumes in addition that the surface is energetically homogeneous with respect to the adsorption (often an acceptable assumption for dispersion-force-only adsorbates), the isotherm will be linear (Henry s Law), i.e. the amount adsorbed will be linearly dependent on the partial saturation of the gas. The proportionality factor is the adsorption equilibrium constant, which is the ratio of the volume of gas adsorbed per unit area of solid to its relative saturation in the carrier. The quantity measured experimentally is the relative retention volume, Vn, for a gas sample injected into the column. It is the volume of carrier gas required to completely elute the sample, relative to the amount required to elute a non-adsorbing probe, i.e. 

Fig. 17. A schematic of the alkane line obtained by inverse gas chromatography (IGC) measurements. The relative retention volume of carrier gas required to elute a series of alkane probe gases is plotted against the molar area of the probe times the. square root of its surface tension. The slope of the plot is yielding the dispersion component of the surface energy of...

The Cl 8 reverse phase exhibits the maximum dispersive interactions with the solutes and is thus, chosen when the difference in dispersive character of the solutes is small or subtle. Employing a Cl 8 reverse phase accentuates the dispersive interactions with the solutes and consequently improves their relative retention. Cl 8 columns also exhibit a somewhat higher loading capacity and so large charges can be placed on the column before overload occurs. This can be useful in trace analysis, where large charges are often necessary to detect the minor components at a level where they can quantitatively evaluated. 

In general, the majority of separations are achieved by exploiting dispersive interactions in the stationary phase and modifying and controlling the absolute and relative retention of the solutes by adjusting the composition of the mobile phase. It is far easier to adjust the mobile phase by selecting different mixtures of water and the solvents methanol, acetonitrile and/or tetrahydrofuran than change from column to column. 

RRT - Relative retention time for elution in HPLC TGA - Thermogravimetric analysis... 

Alexander, G. V., Nusbaum, R.E. and MaeDonald, N.S. 1956 The relative retention of strontium and calcium in bone Tissae. Journal of Biological Chemistry 218 911-919. 

Prinsloo SM, De Beer P143R. 1985. Gas chromatographic relative retention data for pesticides on nine packed columns I. Organophosphorus pesticides, using flame photometric detection. J Assoc Off Anal Chem 68 1100-1108. 

Parameter setup Example 6.5. Relative retention of two solutes... 

From Table II, we can reach the following Important conclusions. First, for simple separations of only a few components and not great demands on analysis time (e.g., 15-30 minutes), PLB are the logical choice. As the number of components increase, as their relative retentions become smaller, or as the time demands increase, one ought to use small particle PB. With respect to clinical chemistry, many problems can now be successfully solved with PLB. Ultimately because of the faster analysis potential, small particle PB will be column of choice. However extra care must be excerised in using such colimns. As experience grows with small particle PB, it is clear that they will become simpler to work with. 

Because a chemical step is imposed on top of the physical distribution process of partition, there is a great potential for selectivity, as noted by Schill et al, (49>50), Such factors as pH, type and composition of the organic phase, and ionic strength of the aqueous phase can be used to control relative retention. The concentration and type of counterion mainly control the absolute retention. 

On the other hand, the analyst might not be interested in global retention indices. Indeed, by increasing the temperature for SF3, he would obtain similar retention indices as for the other two. He will then observe that the relative retention time, i.e. the retention times of the substances compared with each other, are the same for SF, and SF3 and different from SFj. Chemically, this means that SF3 has different polarity from SFj, but the same specific interactions. This is best expressed by using the correlation coefficient as the similarity measure. Indeed, rj3 = 1, indicating complete similarity, while r 2 23 much lower. Since both... 

Alternatively, LC is used for the separation and quantification of PAHs using both UV and fluorescence detection. The analytes are identified based on their relative retention times and UV and/or fluorescence emission spectra. For UV detection an efficient cleanup is a prerequisite since this detection method is not very selective (almost universal for PAHs), and hence it also responds to many coeluting compounds. Due to the high specificity of fluorescence detection for most PAHs, this LC detection method is less susceptible to potential interferences. As in the case of GC the apphcation of internal standard(s) is mandatory since solvents have to be evaporated during the cleanup, which may result in partial losses of some of the more volatile analytes. 

Polar or thermally labile compounds - many of the more modern pesticides fall into one or other of these categories - are not amenable to GC and therefore LC becomes the separation technique of choice. HPLC columns may be linked to a diode-array detector (DAD) or fluorescence detector if the target analyte(s) contain chromophores or fluorophores. When using a DAD, identification of the analyte(s) is based on the relative retention time and absorption wavelengths. Similarly, with fluorescence detection, retention time and emission and absorption wavelengths are used for identification purposes. Both can be subject to interference caused by co-extractives present in the sample extract(s) and therefore unequivocal confirmation of identity is seldom possible. 

In multi-residue analysis, an analyte is identified by its relative retention time, e.g., relative to aldrin when using ECD or relative to parathion or chlorpyrifos when using a flame photometric detection (FPD) and NPD. Such relative retention times are taken from corresponding lists for the columns used. Further evidence for the identity of an analyte is provided by the selectivity of the different detectors (Modules D1 to D3), by its elution behavior during column chromatography (Modules Cl and C2) and in some cases even by the peak form in a gas chromatogram. In a specific analysis for only some individual analytes, their retention times are compared directly with the corresponding retention times of the analytes from standard solutions. 

The variables that control the extent of a chromatographic separation are conveniently divided into kinetic and thermodynamic factors. The thermodynamic variables control relative retention and are embodied in the selectivity factor in the resolution equation. For any optimization strategy the selectivity factor should be maximized (see section 1.6). Since this depends on an understandino of the appropriate retention mechanism further discussion. .Jll be deferred to the appropriate sections of Chapters 2 and 4. 

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