Column Selection Considerations in Compact Capillary Liquid Chromatography

Reduction in solvent usage is a primary goal of green analytical chemistry [1], [2], [3], [4], and for liquid chromatography (LC), this can be most easily achieved through a reduction in mobile phase consumption [5,6]. With analytical-scale high pressure LC (i.e., HPLC), this has been achieved by reducing column internal diameter (i.d.) from 4.6 mm down to 3.0 mm [7]. In ultra-high pressure HPLC (UHPLC), where 2.1 mm i.d. columns have been typically used because of performance losses that can occur due to viscous friction at pressures exceeding 400 bar, 1.5 mm i.d. columns have recently demonstrated comparable separation performance while reducing mobile phase flow rates by approximately 50% [8,9]. However, to truly achieve the greatest reduction in mobile phase consumption, capillary- and nano-scale LC columns are most effective because of their dramatically reduced flow rates [6].

With the recent advent of integrated, compact capillary LC instruments [10], [11], [12], [13], which simplify operation and minimize extra-column volume, there is now greater potential to broaden the adoption of capillary-scale LC techniques. These systems have recently been used for the characterization of biocides [14], cannabinoids [13,15], polycyclic aromatic hydrocarbons (PAH) [16], perfluoroalkyl substances (PFAS) [17], pharmaceutical compounds [13,18], and illicit drug compounds [13,19], as well as for on-line reaction monitoring [11,18]. To enhance UV-absorbance detector sensitivity when using capillary LC, flow cells with long pathlengths have been developed [20,21]. However, longer pathlength, which leads to greater detector flow cell volume, can reduce chromatographic performance due to increased extra-column band broadening. Placing an on-capillary light-emitting diode (LED) UV-absorption detector directly adjacent to the column outlet frit adds minimal extra-column volume, but limits the pathlength to the capillary column i.d. (e.g., 150 µm for a 150 µm i.d. column) [22].

In this report, a re-designed version of an on-capillary LED UV-absorption detector that uses similar optics but integrates a z-cell design that increases the pathlength to 1.2 mm is described. To accommodate the increase in detector volume, column internal diameters of 200 µm and 300 µm were investigated. Specifically, the effects of column length, column i.d., and particle size/morphology on separation performance were measured within the limitations of each column maximum operating pressure limit, the LC instrument maximum pressure limit, and the balance between mobile phase flow rate and analysis time. For such comparisons, the kinetic plot approach with emphasis on the overall kinetic performance limit (KPL) for a given set of operating conditions was adopted [23]. These results provide guidance on column selection within the given parameters afforded by modern compact capillary LC instrumentation.