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Blumberg L.M.,Advachrom | Desmet G.,Vrije Universiteit Brussel
Journal of Chromatography A | Year: 2015

The separation performance metrics defined in Part 1 of this series are applied to the evaluation of general separation performance of linear solvent strength (LSS) gradient LC. Among the evaluated metrics was the peak capacity of an arbitrary segment of a chromatogram. Also evaluated were the peak width, the separability of two solutes, the utilization of separability, and the speed of analysis-all at an arbitrary point of a chromatogram. The means are provided to express all these metrics as functions of an arbitrary time during LC analysis, as functions of an arbitrary outlet solvent strength changing during the analysis, as functions of parameters of the solutes eluting during the analysis, and as functions of several other factors. The separation performance of gradient LC is compared with the separation performance of temperature-programmed GC evaluated in Part 2. © 2015 Elsevier B.V. Source

Klee M.S.,XO Associates LLC | Cochran J.,Restek | Merrick M.,LECO | Blumberg L.M.,Advachrom
Journal of Chromatography A | Year: 2015

The peak capacity gain (Gn) of a GC×GC system is the ratio of the system peak capacity to that of an optimized one-dimensional GC analysis lasting the same time and providing the same detection limit. A near-theoretical maximum in Gn has been experimentally demonstrated in GC×GC-TOF based on a 60m×0.25mm primary column. It was found that Gn was close to 9 compared to the theoretical maximum of about 11 for this system. A six-sigma peak capacity of 4500 was obtained during an 80min heating ramp from 50°C to 320°C. Using peak deconvolution, 2242 individual peaks were determined in a Las Vegas runoff water sample. This is the first definitive experimental demonstration known to us of an order-of-magnitude Gn. The key factors enabling this gain were: relatively sharp (about 20ms at half height) reinjection pulses into the secondary column, relatively long (60m) primary column, the same diameters in primary and secondary columns, relatively low retention factor at the end of the secondary analysis (k≅5 instead of 15, optimal for ideal conditions), optimum flow rate in both columns, and helium (rather than hydrogen) used as the carrier gas. The latter, while making the analysis 65% longer than if using H2, was a better match to the reinjection bandwidth and cycle time. © 2015 The Authors. Source

Blumberg L.M.,Advachrom | Desmet G.,Vrije Universiteit Brussel
Analytical Chemistry | Year: 2016

The mixing rate (Rφ) is the temporal rate of increase in the solvent strength in gradient LC. The optimal Rφ (Rφ,Opt) for a gradient analysis is the one at which a required separation capacity and peak capacity of the analysis are obtained in the shortest time. The Rφ,Opt of LSS (linear solvent strength) gradient LC is found in dimensionless form (rφ,Opt) expressing Rφ,Opt in units of hold-up time (t0) and characteristic strength-constant (Φchar). Previously unknown effect of the gradient band compression on the peak capacity is taken into account. The rφ,Opt depends on the solvent composition range covered by the mixing ramp and on the available pressure. A default rφ at which the analysis time is contained within 30% margin of its minimum at rφ,Opt for a broad range of conditions is proposed. As an example, the recommended default for small-molecule samples is 5% increase in the solvent strength per each t0-long increment in time. At this rate, approximately 0.2√N units of peak capacity are generated per each 10% solvent strength increment. The effect of a column kinetic optimization is also evaluated. © 2016 American Chemical Society. Source

Bicchi C.,University of Turin | Blumberg L.M.,Advachrom | Rubiolo P.,University of Turin | Cagliero C.,University of Turin
Journal of Chromatography A | Year: 2014

Two thermodynamic parameters - entropy (δS) and enthalpy (δH) - ideally describe the thermodynamics of how the retention of an analyte in a stationary phase depends on the temperature. The paper examines the conversion of an analyte's entropy and enthalpy into chromatographically more meaningful equivalents: its characteristic temperature and thermal constant. Thermodynamic and characteristic parameters of 29 enantiomer pairs of chiral analytes, analysed with four cyclodextrin stationary phases, were measured, tabulated, and investigated. The distribution of all newly-measured characteristic parameters was found to be similar to the known distribution of these parameters for some 12,000 pairs of analytes, analysed with several stationary phases. This similarity suggests that the peak widths of the investigated analytes in temperature-programmed analyses should be generally the same as the peak widths of other similarly retained analytes. It also suggests that the previously-known optimum general heating rate (about 10 oC/tM, i.e. 10°C per hold-up time) is also the general optimum for temperature-programmed enantioselective GC analyses with cyclodextrins as stationary phases.The optimum general heating rate corresponds to the shortest analysis time for a predetermined peak capacity. It can substantially differ from specific optima corresponding to the best separation of particular peak pairs. Theoretical prediction of these specific optima requires more complex non-ideal thermodynamic models, and more accurate measurement of the parameters involved-these topics that are outside the scope of this report. © 2014 Elsevier B.V. Source

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