Influence of frictional heating on temperature gradients in ultra-high-pressure liquid chromatography on 2.1mm I.D. columns

Some practical comparisons of the efficiency and overloading behaviour of sub-2 μm porous and sub-3 μm shell particles in reversed-phase liquid chromatography

At their optimum flow, sub-3 μm superficially porous or "shell" particles demonstrate similar efficiency to sub-2 μm totally porous particles. The performance of 0.21 cm i.d shell columns is however inferior to those of 0.46 cm i.d., presumably due to packing difficulties. At high flow, shell columns can give flatter Knox curves due to lower operating pressure (half or less of that of the totally porous particles) producing less frictional heating, which combined with the increased thermal conductivity of their non-porous core, gives more efficient heat dissipation. However, the effects of frictional heating for sub-2 μm columns are considerably exaggerated when using pure ACN as mobile phase, as it has a thermal conductivity 3 times less than that of pure water, leading to poorer heat dissipation. Overloading is already problematic for ionised solutes, a group which contains many pharmaceuticals and compounds of clinical relevance, on conventional columns (5 μm porous particles). However, it becomes a more serious issue for both new column types, partially as a result of their very high efficiency, which concentrates the sample as a very narrow band. The sample capacity of one type of shell particle was estimated to be 60% of that of the small totally porous particles, in line with the fraction of the particle volume that is porous. Due to overloading, it is barely possible to achieve perfect peak symmetry for ionised acids or bases with either of these new column types, even by injecting the lowest amounts of sample detectable by UV. While ammonium formate and potassium phosphate buffers gave similar results in overloading studies, use of formic acid as sole mobile phase additive is not recommended for these solutes, as its ionic strength is too low, leading to a catastrophic deterioration in efficiency when sample concentrations of even a few mg/L are injected.

Comparison of columns packed with porous sub-2 microm particles and superficially porous sub-3 microm particles for peptide analysis at ambient and high temperature

The objective of this study was to evaluate various chromatographic approaches for peptide analysis. Initially, the ultra-HPLC (UHPLC) strategy, which consists of using columns packed with sub-2 microm particles at a maximal pressure of 1000 bar, was tested. To limit the backpressure generated by small particles, columns packed with superficially porous sub-3 microm particles (fused-core technology) that should theoretically improve mass transfer, particularly beneficial for large biomolecules, were investigated. To evaluate these claims, kinetic plots were constructed in both isocratic and gradient modes at ambient and elevated temperature (up to 90 degrees C). For peptide analysis, both UHPLC and fused-core technologies showed a significant gain in peak capacity when compared with conventional HPLC using 5 mum particles and monolithic supports. Additionally, it has been shown that high temperature was of utmost interest to further improve kinetic performance and peak shape due to the improvement of secondary interaction kinetics. Finally, the best conditions developed for UHPLC using the gradient kinetic plot methodology were applied to the analysis of a complex tryptic digest of various proteins. The expected and experimental peak capacity values obtained were similar. In addition, the resolving power of UHPLC at 60 degrees C was appropriate for resolving complex mixtures of peptides.

Chromatography with higher pressure, smaller particles and higher temperature: a bioanalytical perspective

Recent years have seen widespread use of ultra-high-pressure liquid chromatography (UHPLC) in biological fluids. Most commonly the emphasis is on developing high throughput assay methods to reduce the analysis time and cost. Particle size and temperature are chromatographic parameters that can be changed to improve efficiency and obtain rapid separations. UHPLC and high-temperature liquid chromatography (HTLC) are two techniques that reduce the analysis time by decreasing the size of the column packing material and increasing the column temperature, respectively. Both of these techniques have advantages and limitations. In this article we have summarized the history, theoretical background of UHPLC and HTLC and the various advantages and limitations of sample preparation techniques and the detection systems (mass spectrometry and ultraviolet) used for the bioanalytical assays. In addition, selected bioanalytical applications of the two techniques have been reviewed and tabulated.

High performance liquid chromatography analysis of wine anthocyanins revisited: effect of particle size and temperature

The complex anthocyanin fraction of red wines poses a demanding analytical challenge. We have found that anthocyanins are characterised by extremely low optimal chromatographic velocities, and as a consequence generic HPLC methods suffer from limited resolving power. Slow on-column inter-conversion reactions, particularly between carbinol and flavylium species, are shown to occur on the same time scale as chromatographic separation, leading to increased plate heights at normal chromatographic velocities. In order to improve current routine HPLC separations, the use of small (1.7 microm) particles and high temperature liquid chromatography (HTLC) were investigated. 1.7 microm particles provide better efficiency and higher optimal linear velocities, although column lengths of approximately 20 cm should be used to avoid the detrimental effects of conversion reactions. More importantly, operation at temperatures up to 50 degrees C increases the kinetics of inter-conversion reactions, and implies significantly improved efficiency under relatively mild analysis conditions. It is further demonstrated using relevant kinetic data that no on-column thermal degradation of these thermally labile compounds is observed at 50 degrees C and analysis times of <2h.

Chromatography in industry

This review focuses on the chromatography research that has been carried out within industry or in close cooperation with industry and that has been reported in the scientific literature between 2006 and mid-2008. Companies in the health care sector, such as pharmaceutical and biotechnology companies, are the largest contributors. Industrial research seems to take place in an open environment in cooperation with academia, peer companies, and institutions. Industry appears ready to embrace new technologies as they emerge, but they focus strongly on making chromatography work robustly, reliably, rapidly, and automatically. "Hyphenated" systems that incorporate on-line sample-preparation techniques and mass-spectrometric detection are the rule rather than the exception. Various multidimensional separation methods are finding numerous applications. Strategies aimed at speeding up the development of new chromatographic methods remain the focus of attention. Also, there is a clear trend toward exploring chromatographic methods for parallel processing along with other strategies for high-throughput analysis.

High throughput liquid chromatography with sub-2μm particles at high pressure and high temperature

In this study, ultra performance liquid chromatography (UPLC) using pressures up to 1,000 bar and columns packed with sub-2 microm particles has been combined with high temperature mobile phase conditions (up to 90 degrees C). By using high temperature ultra performance liquid chromatography (HT-UPLC), it is possible to drastically decrease the analysis time without loss in efficiency. The stability and chromatographic behavior of sub-2 microm particles were evaluated at high temperature and high pressure. The chromatographic support remained stable after 500 injections (equivalent to 7,500 column volumes) and plate height curves demonstrated the capability of HT-UPLC to obtain fast separations. For example, a separation of nine doping agents was performed in less than 1 min with sub-2 microm particles at 90 degrees C. Furthermore, a shorter column (30 mm length) was used and allowed a separation of eight pharmaceutical compounds in only 40s.

The application of small porous particles, high temperatures, and high pressures to generate very high resolution LC and LC/MS separations

The effect of combining sub-2 microm porous particles with elevated operating temperatures on chromatographic performance has been investigated in terms of chromatographic efficiency, productivity, peak elution order, and observed operating pressure. The use of elevated temperature in LC does not increase the obtainable performance but allows the same performance to be obtained in less time. Increasing the column temperature did allow the use of longer columns, generating column efficiencies in excess of 100,000 plates and gradient peak capacities approaching 1000. Raising the temperature increased the optimal mobile phase linear velocity, negating somewhat the pressure benefits observed by reducing the solvent viscosity. When operating at higher temperature the analyte retention is not only reduced, but the order of elution will also often change. High temperature separations allowed exotic organic modifiers such as isopropanol to be exploited for alternative selectivity and faster analysis. Finally, care must be taken when using high temperature separations to ensure that the narrow peak widths produced do not compromise the quality of data obtained from detectors such as high resolution mass spectrometers.

Consequences of the radial heterogeneity of the column temperature at high mobile phase velocity

When a high velocity stream of mobile phase percolates through a chromatographic column, the bed cannot remain isothermal. Due to the mobile phase decompression, heat is generated along the column. Longitudinal and radial temperature gradients take place along and across its bed. The various consequences of this thermal heterogeneity are calculated and their effects on the column efficiency investigated for a 0.46 cm x 25 cm stainless steel column packed with 5 microm particles. The maximum pressure drop applied was varied from 0.1 to 2 kbar. The amplitude of the longitudinal temperature gradient can be estimated on the basis of the integral heat balance equation applied to the whole column and of measurements of the eluent temperature at the column exit. Assuming that the radial gradient is parabolic and the longitudinal gradient linear, the amplitude of the radial gradient can be determined on the basis of the energy balance across the column and of direct measurements of the radial gradient at high inlet pressures. A radial temperature gradient causes a radial distribution of the eluent viscosity, hence of its local velocity. The result is that bands move faster in their center than along the wall, become warped, hence a radial concentration gradient, similar in origin to the one observed in open cylindrical tubes. Diffusion relaxes this gradient. If there is only a longitudinal temperature gradient, the column efficiency would be 30% smaller for a 2 kbar pressure drop than if there is no longitudinal temperature gradient. However, when both a longitudinal and a radial temperature gradient coexist, there is a large loss of efficiency. If the influence of the diffusive relaxation of the radial concentration gradient is neglected, the peak shape would be broad and exhibit a marked shoulder.

Characterisation of RPLC columns packed with porous sub-2 μm particles

Eight commercially available sub-2 microm octadecyl silane columns (C18 columns) have been characterised by the Tanaka protocol. The columns can be grouped into two groups that display large differences in selectivity and peak shape due to differences in hydrophobicity, degree of surface coverage and silanol activity. Measurements of particle size distributions were made using automated microscopy and electrical sensing zone measurements. Only a weak correlation could be found between efficiency and particle size. Large differences in column backpressure were observed. These differences are not related to particle size distribution. A more likely explanation is differences in packing density. In order to take full advantage of 100-150 mm columns packed with sub-2 microm particles, it is often necessary to employ not only an elevated pressure but also an elevated temperature. A comparison between columns packed with sub-2, 3 and 5 microm versions of the same packing indicates potential method transferability problems for several of the columns due to selectivity differences. Currently, the best alternative for fast high-resolution LC is the use of sub-2 microm particles in combination with elevated pressure and temperature. However, as shown in this study additional efforts are needed to improve transferability as well as column performance.

Fundamental and practical aspects of ultrahigh pressure liquid chromatography for fast separations

The ongoing development of HPLC has been focused on increasing the speed and efficiency of separations over the past decade. The advances in separation speed have been primarily related to the development of column technology and instrumentation. Relatively short columns packed with sub-2 microm particles provide high-speed separations while maintaining or increasing resolution. Ultrahigh pressure pump systems have been developed to overcome the high-pressure drop generated by such sub-2 microm packings. In this review, fundamental and practical aspects of ultrahigh pressure or ultrahigh performance liquid chromatography (U-HPLC) are discussed. Applications of fast U-HPLC separations are also presented.

Mass transport in sub-2-μm high-performance liquid chromatography

Ultra-high pressure liquid chromatography enables increased separation speed and efficiency. The quantitative improvement in efficiency is lower than that predicted by theory, and the reasons are not known. In this work, slow mass transport due to analyte desorption from the stationary phase is discussed as a possible contribution to the lower than expected efficiency. Data in the literature for the reversed phase elution of acetophenone, for which particle size was varied with constant particle composition and mobile phase, were used to test this possibility. The mass transport terms for the three particles sizes (1.7, 3.5 and 5.0 microm) fit well to a model that includes desorption from the stationary phase as a contribution, and this analysis yields an apparent desorption time constant of 2.0(+/-0.2)ms for acetophenone in a reversed-phase separation. The results indicate that it is reasonable to consider slow desorption as a possible contribution to the reduced plate height for sub-2-microm particles.

Use of the kinetic plot method to analyze commercial high-temperature liquid chromatography systems

Using a set of experimentally determined plate height data obtained on three commercial high-temperature HPLC supports, and evaluating their isocratic separation speed potential under the application of a set of instrumental constraints, a qualitative map of the practically achievable critical pair separation speed potential of high-temperature HPLC has been established. The obtained data show that the gain in separation speed is more strongly affected by the instrumental limitations in the high-temperature range than it is for the low temperatures. For the presently considered case of alkylbenzene separations, the potential gain in analysis time that can be obtained by going from T=30 to 120 degrees C in the presence of a typical set of instrumental limitations nevertheless remains of the order of a factor of 2-4. The study also shows that improvements on the instrumentation side (increased detector frequency, pumping flow rate, smaller extra-column volumes, ...) are indispensable to fully benefit from the high temperature advantages for all separations requiring less than 10,000 effective theoretical plates.

Use of the kinetic plot method to analyze commercial high-temperature liquid chromatography systems

The present study aimed at mapping the separation speed potential of a critical pair on commercial high-temperature HPLC (HT-HPLC) supports at elevated temperatures. For this purpose, band broadening and pressure drop measurements were conducted on three different commercial HT-HPLC columns operated at various elevated temperatures but by keeping the same retention factor. The plate height data were subsequently transformed into a plot showing the minimal required analysis time needed to yield a given required effective plate number. For the considered RPLC alkylbenzene separations, it was found that the maximal gain in separation speed of the critical pair that can be obtained by varying the operating temperature from T=30 to 120 degrees C can be expected to be of the order of a factor of 3-4, if using an individually optimized column length for each considered temperature and if no secondary adsorption effects occur at the lower temperature. This gain factor, remaining more or less constant over the most relevant range of plate numbers, largely paralleled the reduction of the mobile phase viscosity accompanying the temperature increase.