Accuracy under stationary conditions

RI values do not depend on column dimension or flow rate. They must be obtained in isothermal mode, and they are less dependent on temperature than other retention parameters (fc, K). Because there is a linear relationship between the logarithm of the adjusted retention times, obtained under isothermal conditions, of the components of a homologous series and their number of carbon atoms, n-alkanes that differ in two carbon atoms (z and z + 2) can be used as standards in Equation (3) with only a small loss in accuracy. Other homologous series can be used instead of n-alkanes when these compounds are not appropriate — for instance, in columns with high-polarity stationary phases. 

For the equilibrium measurements the dyestuffs were precipitated on the surface of selected kieselguhrs as stationary phases and filled into a chromatographic column. Then temperature and pressure were adjusted. Before each measurement series a reference spectrum was recorded. Firstly, purification from better soluble impurities of the dyestuffs was performed in a continuous flow. In the following, the dependence of the saturation concentration on the flow-rate and the time which is necessary for equilibration was determined. Within the experimental accuracy no dependence on these parameters was found. Nevertheless, for equilibration the substance was treated more than 10 min under isothermal-isobaric conditions. 

Type of data used in the estimation. The minimum requirement is the availability (from experimental measures or from published data) of the retention time of the analytes in GC columns having the same stationary phase used in the 2D GC xGC set object of the estimation, assuming a variation of retention with temperature similar for all the compounds. In order to obtain higher prediction accuracy, retention times should be measured at least at three different temperatures in the isothermal mode. If RI values are used, reference n-alkanes should be run under the same conditions. If the objective is optimisation, several runs are also required in the specific GCxGC set to be optimised, in order to correct errors related to column geometry or to the use of stationary phases from different batches. 

Quitasol and Krastins [504] used a polyhydroxyethylmethacrylate (Universal Anion, Grace Alltech, USA) as a stationary phase 5 mmol/L potassium nitrate adjusted to pH 3.5 with nitric acid serving as an eluent. Under these chromatographic conditions, the organic phosphate can be separated to baseline from orthophosphate and orthophosphite within 10 min. Because Quitasol and Krastins determined only the drug component in various pharmaceutical formulations, they used a refractive index detector for the sake of simplicity. This procedure can be validated in terms of precision, accuracy, and linearity without any difficulties. 

Some optimization system models rely on predetermined libraries of compounds already calibrated on several common stationary phases. If a peak of interest is in such a library and the analyst is using one of the characterized phases, then no additional calibration may be necessary as long as (1) the calibration column and the experimental column dimensions are known with good accuracy, (2) the calibration gas chromatograph s oven temperature and experimental oven temperatures are standardized, and (3) the pressure drops and ambient pressures for the calibration and experimental systems are known accurately. If not, then the simulations will be less accurate. However, small errors in these areas will not distort simulated results so much that peak elution order and relative retention will be meaningless. Even when not exact down to the second, simulations provide a wealth of useful information about peak retention behavior under a range of test conditions. 

---