RESUMO
We report on the peak capacity that can be produced by operating a state-of-the-art core-shell particle type (d(p)=2.6 µm) at its kinetic optimum at ultra-high pressures of 600 and 1200 bar. The column-length optimization needed to arrive at this kinetic optimum was realized using column coupling. Whereas the traditional operating mode (using a single 15 cm column operated at its optimum flow rate of 0.4 mL/min) offered a peak capacity of 162 in 10.8 min, a fully optimized train of 60 cm (4×15 cm) columns offered a peak capacity of 325 in 61 min when operated at 1200 bar. Even though the particles have a reputed low flow resistance and a relatively large size (>2 µm), it was found that the increase in performance that can be generated when switching from a fully optimized 600 bar operation to a fully optimized 1200 bar operation is significant (roughly 50% reduction of the analysis time for the same peak capacity and approximately a 20% increase in peak capacity if compared for the same analysis time). This has been quantified in a generic way using the kinetic plot method and is illustrated by showing the chromatograms corresponding to some of the data points of the kinetic plot curve.
Assuntos
Cromatografia Líquida/métodos , CinéticaRESUMO
The present contribution addresses the problem of the accurate determination of the extra-column band-broadening dispersion by applying the method of moments (MOM) on pulse response experiments. The MOM provides the only mathematically rigorous way to determine the variance of a pulse response, but suffers from the fact that the obtained variance values usually depend very strongly and unpredictably on the selected integration boundaries. In the present study, it is investigated whether the MOM cannot be made more accurate by repeating the pulse response experiment a number of times, then add all measured peaks (after retention time alignment) and subsequently perform the integration on this summed peak. Testing this approach for a number of different integration boundary detection methods consistently more accurate results are obtained than when the different repeat response pulses are integrated individually and the variances are averaged afterwards. It was also found that adding five repeats already leads to a significant improvement of the variance estimate, whereas the addition of 10-20 repeats is needed before the variance estimate converges to a steady value.
RESUMO
We report on a series of computer simulations that have been made to explore the operation and performance limits of a periodically heated and cooled trapping segment (liquid analog of the modulator device used in GC×GC) interfacing the separation column and the detector. The initial peak width and the retention of the molecules on the trapping segment have the most important influence on the trapping efficiency. Higher retention of the trapping segment and smaller peaks will lead to higher signal enhancements. However when the resulting concentration gradients become too large, as is the case for very small peaks and/or for very high retention factors on the trapping segment, the dispersion will strongly decrease the focussing efficiency. The molar diffusion coefficient and the linear velocity mainly have an impact on the dispersive behaviour, which can be directly calculated from the associated plate height values. General design rules for the trapping segment, validated with the computer simulations, give a good estimate of the required trapping time and the length of the trapping segment. Equations for the estimation of the trapped peak width and signal enhancement are also given.
RESUMO
We report on a proof-of-principle experiment with a novel thermal modulation device with potential use in two-dimensional liquid chromatography (LC × LC) systems. It is based on the thermal desorption concept used in two-dimensional gas chromatography (GC × GC) systems. Preconcentration of neutral analytes eluting from the first dimension column is performed in a capillary "trap" column packed with highly retentive porous graphitic carbon particles, placed in an aluminum low-thermal-mass LC heating sleeve. Remobilization of the trapped analytes is achieved by rapidly heating the trap column, by applying temperature ramps up to +1200 °C/min. Compared to the nonmodulated signal, the presented thermal modulator yielded narrow peaks, and a concentration enhancement factor up to 18 was achieved. With a thermally modulated LC separation of an epoxy resin, it is shown that when the thermal modulation is applied periodically, the trapped and concentrated molecules can be released periodically and that the modulating interface can both serve as a preconcentration device and as an injector for the second dimension column of an LC × LC setup. Because of the thermal modulation, a high-molecular-weight epoxy resin could be adequately separated and the different fractions were identified with a GPC analysis, as well as an offline second dimension LC analysis.
RESUMO
We report upon the experimental investigation of the heat transfer in low thermal mass LC (LTMLC) systems, used under temperature gradient conditions. The influence of the temperature ramp, the capillary dimensions, the material selection and the chromatographic conditions on the radial temperature gradients formed when applying a temperature ramp were investigated by a numerical model and verified with experimental temperature measurements. It was found that the radial temperature gradients scale linearly with the heating rate, quadratically with the radius of the capillary and inversely to the thermal diffusivity. Because of the thermal radial gradients in the liquid zone inside the capillary lead to radial viscosity and velocity gradients, they form an additional source of dispersion for the solutes. For a temperature ramp of 1 K/s and a strong temperature dependence of the retention of small molecules, the model predicts that narrow-bore columns (i.d. 2.1 mm) can be used. For a temperature ramp of 10 K/s, the maximal inner diameter is of the order of 1 mm before a substantial increase in dispersion occurs.
