High Efficiency Narrow-Bore Columns Packed with 1.6 and 2.7 μm Solid Core Particles

It has previously been shown that it is possible to obtain remarkably high efficiencies for standard-bore (4.6 mm diameter) columns packed with 2.6 – 2.7 μm solid-core particles (SCP). Minimum reduced plate heights of 1.3 – 1.6 have been reported for these columns. However, similarly low reduced plate heights have been much more difficult to achieve for 2.6 – 2.7 μm SCP in narrow-bore (2.1 mm diameter) columns. Packing < 2 μm SCP into narrow-bore columns has proven to be even more challenging. In this article, we demonstrate that it is possible to achieve reduced plate heights of 1.5 – 1.6 for 2.7 and 1.6 μm SCP in narrow-bore columns. Examples are shown to demonstrate that these high efficiencies may provide faster or higher resolution separations.

One of the most discussed recent advances in columns for HPLC and UPLC is the use of solid-core particles (SCP) instead of fully porous particles (FPP) [1 - 9]. Remarkably high optimum efficiencies have been reported for standard-bore (4.6 mm diameter) columns packed with 2.6 or 2.7 μm SCP, in the range of 230,000 – 300,000 plates per meter [2, 5]. Expressed as reduced plate heights (h), a measure of efficiency that is normalised for differences in particle size, these efficiencies translate to h values in the range of 1.3 – 1.6. The reduced plate height is inversely proportional to efficiency, so low h values correspond to high efficiencies for a given particle size. For comparison, standard-bore columns packed with similar size FPP typically have reduced plate heights in the range of 1.8 – 2.2 [1]. The lower h values of columns packed with SCP have been shown to arise primarily from reductions in longitudinal diffusion and eddy dispersion [2].

Achieving similarly high efficiencies for narrow-bore (2.1 mm diameter) columns has proven to be more difficult. Narrow-bore columns are preferred for use with mass spectrometry (MS) detection because their lower optimum flow rates are a better match for electrospray ionisation [10]. Narrow-bore columns are also attractive for use with optical detectors because they give higher sensitivity when the same mass of analyte is injected as on a standard-bore column [11]. Narrow-bore columns also use much less mobile phase. The consumption of mobile phase is reduced by a factor of 4.8 for 2.1 mm vs 4.6 mm diameter columns operated at the same linear velocity.

Several investigations of the performance of 2.6 or 2.7 μm SCP in narrow-bore columns have reported optimum efficiencies in the range of 160,000 – 250,000 plates per meter, corresponding to reduced plate heights of 1.5 – 2.5 [2, 4, 5, 12]. This places them in the range of narrow-bore columns packed with 2.5 μm FPP [13]. It has been shown that narrow-bore columns packed with 2.6 or 2.7 μm SCP have greater eddy dispersion than their standard-bore counterparts, which is the major cause of their lower efficiencies [12]. More specifically, the increased trans-column contribution to the eddy dispersion is the issue for the narrow-bore columns. The trans-column eddy dispersion is a measure of the radial homogeneity of the packed bed structure. The radial homogeneity is affected by the column packing process, and may be improved by optimisation of the packing conditions.
The difficulty of column packing optimisation increases not only with decreasing column diameter, but also with decreasing particle size. Minimum reduced plate heights of 1.9 - 2.9 have been reported for 1.7 μm SCP in narrow-bore columns [3, 5, 12, 14]. We have worked to optimise the packing of narrow bore columns with < 2 μm particles over the last ten years, since the introduction of UPLC® [15]. With the knowledge gained from packing < 2 μm FPP, we have been able to achieve reduced plate heights for SCP narrow bore columns that are similar to the remarkable values previously reported for 2.6 or 2.7 μm SCP standard-bore columns. In this article, we report examples of the efficiencies and reduced plate heights that we have achieved for narrow bore columns packed with 1.6 and 2.7 μm SCP. Examples are also shown to demonstrate the benefits of these higher efficiencies for increasing separation speed and resolution.

Unlock and download the full article for more...