INDEX relative retention times

Radioactive label, 330 Raman diffusion, 184 Raman scattering, 227 Ratio fluorimeter, 228 Rayleigh scattering, 226 Real mean, 385 Red-shift, 196 Reference electrode, 347 Reflectron, 298 Refractive index detector, 59 Relative response factor, 78 Relative standard deviation, 387 Reliability, 389 Resolving power, 282 Response factor, 77 Restrictor, 98 Retardation factor, 88 Retention factor, 14 Retention index, 41 Retention time, 7 Retention volume, 14 RP-18, 53 RSD, 387 Ruhemann, 112... 

Chromatographic System (See Chromatography, Appendix IIA.) Use a liquid chromatograph equipped with a refractive index detector that can be maintained at a constant temperature of 25°, a 25-cm x 4.6-mm (id) column packed with 10- im porous silica gel bonded with aminopropylsilane (Alltech 35643, or equivalent), and a guard column that contains the same packing. Maintain the column at a constant temperature of 25° 2°, and the flow rate at about 2.0 mL/min. Inject 20 pL of System Suitability Preparation into the chromatograph, and record the peak responses as directed under Procedure. The relative standard deviation for replicate injections is not more than 2.0%, and the alpha-Cyclodextrin and beta-Cyclodextrin peaks exhibit baseline separation, the relative retention times being about 0.8 and 1.0, respectively. 

Identification is based on retention index alternatively retention time (relative to a stmid ) may be used. Details of retention indices or relative retention times of compounds in the systems described below are given in the index to Gas Chromatographic Data in Part 3. 

Acquisition of qualitative data may be obtained by using specific detectors such as GC-MS, FTIR-GC, and FTIR-GC-MS, as well as from relative retention times or by the concept of retention index (vide infra). 

In contrast to the Kovats regression line, the retention index depends only on the stationary phase and not on the column dimensions or the flow rate of the carrier gas. Due to this the retention time is converted into a relative retention time independent of experimental conditions, but normalized to a series of paraffins. 

The Kovats Retention Index for a specific organic compound (i.e., the analyte of interest on a specific GC column) is nothing more than a relative retention time The index is in relation to aliphatic hydrocarbons. For example, the hydrocarbon iso-octane, or 2,2,4-trimethylpentane if gas chromatographed, would probably have a retention time between that of the straight-chained alkanes, -octane Cg and n-nonane C9. The Kovats Index for n-octane is 800 and that for n-nonane is 900. Iso-octane would then have a Kovats Index somewhere in between, such as 860. The Kovats Index is calculated from a GC chromatogram using the retention volume for the analyte whose index is sought, and the retention... 

The separation of bile acids by gas-liquid chromatography is determined by the choice of stationary phase and bile acid derivative. Data that permit the selection of stationary phase and bile acid derivative to suit most separation problems are shown in Tables XII-XIV. As in the original papers relative retention times have been used in the tables instead of the more acceptable retention index (112) which permits better interlaboratory comparisons. Relative retention times are subject to variation, mainly due to temperature differences and, to a lesser extent, to differences in column preparation. In our experience the temperature-dependence is most pronounced with trimethylsilyl ether derivatives on Hi-Eff-8B (cyclohexanedimethanol succinate) columns. Temperature differences do not affect relative retention times on QF-1 (a trifluoropropyl, methyl siloxane) to the same extent. 

Splitless Injection (OTQ. The term applied to a flash vaporization technique wherein the solvent is evaporated in the injection port and condenses on the head of the column. After a suitable time (usually 0.5 min), the splitter is opened and any of the remaining material in the flash vaporizer is vented. The solvent that will have condensed at the head of the column is then slowly vaporized through column temperature programming. Splitless injection is used to concentrate small quantifies of solute in a large injection (2-3 rL) onto a capillary column. The solute should have a higher boiling point than the condensed solvent so that its relative retention time is at least 1.5 and its retention index is greater than 600. 

FIGURE 8.4 Plot of logarithm of adjusted retention time versus Kovats index. TABLE 8.1 Pesticide Relative Retention Times... 

Anal)de identification relies on the comparison of a UV spectrum and a retention parameter to that of corresponding data stored in a reference library. Standardized retention-index (RI) scales have previously been used. These allow interlaboratory exchange of LC—DAD databases, enabling the use of comprehensive spectral libraries. For these to be used between laboratories, however, identical LC conditions are required, so that retention indices can be correctly applied as an identification parameter. Both isocratic and gradient programs have been used in compiling RI scales, which further complicates the sharing of UV libraries. A more common approach is to use the retention time (RT) of an analyte relative to a reference marker. The reference marker is selected in-house and tailored to the LC conditions of the laboratory. This results in a compilation of an in-house relative retention time (RRT) and UV library customized to the selected separation conditions [17,18]. 

The retention index is calculated by logarithmic interpolation between consecutive alkanes and the data acquired under isothermal analysis conditions. Under linear temperature programming, however, an almost identical system of expressing retention data is the methylene unit concept in which methylene unit (MU) values are determined by linear interpolation between the /i-alkanes eluted before and after the compound (Dalgliesh etal, 1966). For example, a peak eluted midway between C19 and C20 under these conditions would have a MU value of 19.50 and an equivalent retention index calculated from this of approximately 1950. The interrelation of isothermal and temperature-programmed data for a particular type of phase, even under widely different analytical conditions, is possible and both concepts permit very useful comparisons of different sets of available literature data. Relative retention times, on the other hand, can show fairly wide variations in values, especially with regard to temperature, and are less suited for use as literature reference data. 

Many factors can influence the Kovats index which can make it unreliable at times for the characterization of gas chromatographic behavior, although it generally varies less than the relative retention with temperature, flow, and column variation. However, for many it is the preferred method of reporting retention data. 

PTFE polytetrafluoroethylene PUFA polyunsaturated fatty acid PV peroxide value PVDF polyvinylidene difluoride PVP polyvinylpyrrolidone PVPP polyvinylpolypyrolidone RAS retronasal aroma stimulator RDA recommended dietary allowance RF radio frequency RFI relative fluorescence intensity RI retention index RNU relative nitrogen utilization ROESY rotational nuclear Overhauser enhancement spectroscopy RP-HPLC reversed-phase HPLC RPER relative protein efficiency ratio RS resistant starch RT retention time RVP relative vapor pressure S sieman (unit of conductance)... 

Reduced parameters, 66-69 Refractive index (RI) detector, 206-207 Regular solution, 49 Relative retention, 20-21, 22, 77 Repeatability, see Precision Reproducibility, see Precision Resolution, 17-19, 55 Response factors (detector), 104, 125 Response time, 94 Retardation factor, Rf, 71 Retention index of Kovats, 78 Retention ratio, 11, 12, 71 Retention time, 6, 9 Retention volume, 9, 75 adjusted, 10, 75 corrected, 62-63, 75 net, 63, 75 specific, 110 Reverse phase LC, 158 Rohrschneider/McReynolds constants, 137-140... 

Chapter 3 ended by noting that an alternative way of estimating the polarity of a stationary phase was to use probes. Basically, the extent of interaction between the stationary phase and a given sample will be reflected in the adjusted retention time (or partition ratio or retention index, /) of the solute. Thus, by choosing as solute probes those chemicals that are thought to have particularly strong selective interactions, one can get a measure of the relative magnitude of that interaction from its retention index. 

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