Effects of the thermal heterogeneity of the column on chromatographic results

Different Stationary Phase Selectivities and Morphologies for Intact Protein Separations

The central dogma of biology proposed that one gene encodes for one protein. We now know that this does not reflect reality. The human body has approximately 20,000 protein-encoding genes; each of these genes can encode more than one protein. Proteins expressed from a single gene can vary in terms of their post-translational modifications, which often regulate their function within the body. Understanding the proteins within our bodies is a key step in understanding the cause, and perhaps the solution, to disease. This is one of the application areas of proteomics, which is defined as the study of all proteins expressed within an organism at a given point in time. The human proteome is incredibly complex. The complexity of biological samples requires a combination of technologies to achieve high resolution and high sensitivity analysis. Despite the significant advances in mass spectrometry, separation techniques are still essential in this field. Liquid chromatography is an indispensable tool by which low-abundant proteins in complex samples can be enriched and separated. However, advances in chromatography are not as readily adapted in proteomics compared to advances in mass spectrometry. Biologists in this field still favour reversed-phase chromatography with fully porous particles. The purpose of this review is to highlight alternative selectivities and stationary phase morphologies that show potential for application in top-down proteomics; the study of intact proteins.

The rationale for the optimum efficiency of columns packed with new 1.9μm fully porous Titan-C18 particles-a detailed investigation of the intra-particle diffusivity

In a previous report, it was reported that columns packed with fully porous 1.9μm Titan-C18 particles provided a minimum reduced plate height as small as 1.7 for the most retained compound (n-octanophenone) under RPLC conditions. These particles are characterized by a relatively narrow size distribution with a relative standard deviation (RSD) of only 10%. A column packed with classical 5μm Symmetry-C18 particles, used as a reference RPLC column, generated a minimum reduced plate height of 2.1 for the same retained compound. This work demonstrates that this was due to an unusually low intra-particle diffusivity across these particles, which leads to a small longitudinal diffusion coefficient along the column. The demonstration is based on the combination of accurate measurements of the height equivalent to a theoretical plate (HETP), inverse size exclusion chromatography (ISEC), peak parking (PP), and minor disturbance method (MDM) experiments. The experimental results show that the reduced eddy dispersion HETP term (A=0.8 for a reduced velocity of 5), the internal particle porosity (ϵp=0.35), and the enrichment of acetonitrile in the pore volume (75% acetonitrile in the bulk, 85% inside the mesoporous volume) are identical on both the Titan-C18 and Symmetry-C18 columns. The difference between the internal structures of these two brands of RPLC-C18 fully porous particles lies in the values of the internal obstruction factor γp, which is 0.42 for the Symmetry-C18 but only 0.26 for the Titan-C18 particles. This is in part related to the diffusion hindrance due to the small average pore size of the Titan-C18 particles, around 59Å versus 77Å for Symmetry-C18 particles. A simple model of constriction along diffusion paths having the shape of a truncated cone suggests that the width of the pore size distribution (RSD of 30% and 20% for Titan-C18 and Symmetry-C18 particles) is mostly responsible for the difference in their obstruction factors.

The effect of pressure and mobile phase velocity on the retention properties of small analytes and large biomolecules in ultra-high pressure liquid chromatography

A possible complication of ultra-high pressure liquid chromatography (UHPLC) is related to the effect of pressure and mobile phase velocity on the retention properties of the analytes. In the present work, numerous model compounds have been selected including small molecules, peptides, and proteins (such as monoclonal antibodies). Two instrumental setups were considered to attain elevated pressure drops, firstly the use of a post-column restrictor capillary at low mobile phase flow rate (pure effect of pressure) and secondly the increase of mobile phase flow rate without restrictor (i.e. a combined effect of pressure and frictional heating). In both conditions, the goal was to assess differences in retention behaviour, depending on the type or character of the analyte. An important conclusion is that the effect of pressure and mobile phase velocity on retention varied in proportion with the size of the molecule and in some cases showed very different behaviour. In isocratic mode, the pure effect of pressure (experiments with a post-column restrictor capillary) induces an increase in retention by 25-100% on small molecules (MW<300 g/mol), 150% for peptides (~1.3 kDa), 800% for insulin (~6 kDa) and up to >3000% for myoglobin (~17 kDa) for an increase in pressure from 100 bar up to 1100 bar. The important effect observed for the isocratic elution of proteins is probably related to conformational changes of the protein in addition to the effect of molecular size. Working in gradient elution mode, the pressure related effects on retention were found to be less pronounced but still present (an increase of apparent retention factor between 0.2 and 2.5 was observed).

Potential advantage of constant pressure versus constant flow gradient chromatography for the analysis of small molecules

A recent model designed to predict the variation of the flow rate with time in constant pressure (cP) gradient chromatography was validated from an experimental viewpoint for non-retained gradients (methanol-water), incompressible eluent (P<250 bar), and in absence of pressure effects on the analyte retention pattern (small molecules). Experimental data confirmed that cP and constant flow (cF) gradients are strictly equivalent if the analysis time is kept constant. The same model was also used to predict the gradient kinetic performance of cP versus cF gradients when the constraint was the maximum inlet pressure at which the column and/or the HPLC system can safely be run. For linear volume gradients of methanol in water (5-95% in volume) and a maximum pressure of 250 bar, the same peak capacity as that in cF mode is predicted in cP mode. Also, a reduction of the analysis time by 17.3% was expected. These theoretical results were confirmed by separating a real mixture of about twenty small molecules on either one or two 4.6 mm × 150 mm columns packed with 3.5 μm Bridge Ethylene Hybrid (BEH) C(18) particles and run at flow rates smaller than 0.8 mL/min and at a maximum inlet pressure of 250 bar. The experimental gain in analysis time was 17.6% (1 column) and 20.1% (2 columns in series) for a virtually insignificant loss of peak capacity (-4%).

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.

Measurement of hold-up volumes in reverse-phase liquid chromatography

The hold-up volumes, V(M) of two series of RPLC adsorbents were measured using three different approaches. The first method is based on the difference between the volumes of the empty column tube (150x4.6mm) and of the material packed inside the column. It is considered as giving the correct value of V(M). This method combines the results of the BET characterization of the adsorbent before packing (giving the specific pore volume), of carbon element analysis (giving the mass fraction of silica and alkyl bonded chains), of Helium pycnometry (providing silica density), and of inverse size exclusion chromatography (ISEC) performed on the packed column (yielding the interparticle volume). The second method is static pycnometry, which consists in weighing the masses of the chromatographic column filled with two distinct solvents of different densities. The last method is based on the thermodynamic definition of the hold-up volume and uses the dynamic minor disturbance method (MDM) with binary eluents. The experimental results of these three non-destructive methods are compared. They exhibit significant, systematic differences. Pycnometry underestimates V(M) by a few percent for adsorbents having a high carbon content. The results of the MDM method depend strongly on the choice of the binary solution used and may underestimate or overestimate V(M). The hold-up volume V(M) of the RPLC adsorbents tested is best measured by the MDM method using a mixture of ethanol and water.

The bandwidth in gradient elution chromatography with a retained organic modifier

The variance of a chromatographic band is derived in the case of RPLC gradient elution when the organic modifier is significantly retained onto the stationary phase. This derivation is based on the extension of a model due to Poppe et al. [H. Poppe, J. Paanakker, M. Bronckhorst, J. Chromatogr., 204 (1981) 77] which assumes that the gradient front remains unchanged and propagates along the column at the same speed as the mobile phase, following piston flow. Theoretical and experimental results are compared in the case of caffeine on a C(1)-silica stationary phase eluted with an acetonitrile gradient. The actual retention behaviors of caffeine and acetonitrile were implemented in the theoretical calculations. The model predicts compression factors between 0.71 and 0.34 for relatively smooth gradient steepness, betat(0), between 0.009 and 0.054 while the corresponding experimental band compression factors vary between 1.01 and 0.43 for the very same gradient steepness. The model underestimation of these factors arises likely from the strong deviation of the actual retention behavior from the prediction of the Linear Solvent Strength Model (LSSM).