Instrumentation for hand-portable liquid chromatography

Liquid chromatography (LC) has lagged behind gas chromatography (GC) in developments related to hand-portable instrumentation. In this work, a new battery-operated (24 V DC) nano-flow pumping system with a stop-flow injector was developed and integrated with an on-column UV-absorption detector (254 nm) that was reduced in size to an acceptable weight and power usage for field operation. The pumping system, which includes nano-flow pump, stepper motor and high-pressure valve weighs only 1.372 kg (3 lbs) and can generate up to 110.32 MPa (16,000 psi) pressure. A major advantage of this pump is that it does not employ a splitter, since it was specifically designed for capillary column use. The volume capacity of the pump is 24 μL, and a sample volume as low as 10 nL can be injected. Flow rate calibration (300 nL to 6.12 μL per min) was performed, and an accuracy >99.94% was obtained. The percent injection carry-over was found to be low (RSD 0.31%), which makes it practical for quantitative analysis. The detector linear range and limit of detection (LOD) were determined using sodium anthraquinone-2-sulfonate. A linear regression coefficient (R) of 0.9996 was obtained for a plot of log peak area versus log concentration over the range of 3.2 μM to 6.5 mM, and the LOD (S/N = 3) was found to be 7.8 fmol (0.13 μM). The short term noise of the detector is comparable to commercially available detectors (∼10−5 AU). In this work, the system was tested in the laboratory using regular line power (120 V AC) with an AC to DC adapter. Reversed-phase isocratic separations were performed using a 15.5 cm × 75 μm i.d. fused silica capillary column containing a monolithic stationary phase synthesized from 1,6-hexanediol dimethacrylate. Good retention time repeatability (RSD 0.09–0.74%) was obtained for a mixture containing an unretained marker (i.e., uracil) and a homologous series of alkyl benzenes.