Advances in capillary liquid chromatography

Capillary liquid chromatography is an important analytical tool, providing higher sensitivity, lower solvent consumption and more resolution than liquid chromatography (LC) in large bore steel columns. Capillaries permit some separation techniques that are not viable using conventional columns, such as very high pressure liquid chromatography (VHPLC). While standard HPLC is performed at pressures less than 6000 psi, VHFLC is performed at pressures up to 20,000 psi, which can provide better resolution and faster analyses. A VHPLC system is developed and evaluated using protein digests with a variety of detectors, including UV/Vis and mass spectrometry. Peak capacities of more than 200 are shown on separations lasting ∼30 min.; Though there are advantages to using capillary columns for LC, there are also several difficulties encountered. The flow rates in capillary columns are very low, which can cause significant band-broadening in post-column detectors. Because of this limitation, many types of detectors used in standard LC must be adapted to make the transition to use in capillaries. One example is conductivity detection. A contactless conductivity detector for capillaries is improved from previous designs through the addition of a reference conductivity cell to compensate for baseline drift and the use of a lock-in amplifier for added noise rejection.; This improved conductivity detector can be used for several applications. One use presented here is as part of a non-invasive capillary flow meter. The flow meter works by introducing a perturbation to the capillary liquid upstream from a pair of contactless conductivity detectors. The perturbation can be any change to the liquid that provides a corresponding change in conductivity. This perturbation is detected by each contactless conductivity detector, allowing calculation of the liquid linear velocity. Thermal pulses and ionic strength changes are two types of perturbation discussed.; The flow meter is shown to function at the low linear velocities and small inner diameters often used in capillary chromatography and electrophoresis. Results are presented from a 50 μm open capillary and packed capillaries of 75 μm and 10 μm inner diameter. Linear velocity measurements from 1.6 cm/sec to 1.3 mm/sec are presented with a relative standard deviation of less than 1%.