Characterization of a microscale cyclical electrical field flow fractionation system Ameya Kantak, Merugu Srinivas and Bruce Gale* Received 28th November 2005, Accepted 2nd March 2006 First published as an Advance Article on the web 14th March 2006 DOI: 10.1039/b516827a A microscale cyclical electrical field flow fractionation (CyElFFF) channel is characterized with regard to the effect of various operating parameters and comparison made to recent theoretical developments. Challenges associated with various operating conditions are reported along with some of the optimized operating parameters. The effect of retention wall choice, an offset voltage, relaxation steps, and flow rates, along with the basic operating parameters of voltage, frequency, and electrophoretic mobility are reported. Retention of polystyrene nanoparticle standards is accomplished and the first separations using this technique in a microscale system are also demonstrated. Relaxation steps and offset voltages are found to be effective in eliminating early peaks and in improving plate heights. Plate heights were also found to decrease with increasing flow rates, which is the opposite of the behavior seen in most existing chromatographic systems. The experimental results are compared to the analytical and empirical models of CyElFFF and found to be compatible. Suggestions are made for improving the separation and analysis methods used with CyElFFF. Introduction Cyclical electrical field flow fractionation (CyElFFF) is a recently developed 1,2 member of the field flow fractionation (FFF) family of particle analysis, separation and sample preparation tools first described by Giddings in 1966. 3 FFF methods rely on using an external field perpendicular to a parabolic velocity field within very thin channels. There are several different FFF subtypes based on the choice of an externally applied field, such as: electrical, flow, gravitational, magnetic, sedimentation, steric, thermal and so forth. Significant interest has been shown in electrical FFF systems (ElFFF) 4,5 during the past few years, including the introduc- tion of microscale ElFFF systems. 6 ElFFF systems have been shown to suffer from electrode polarization and an associated effective field decrease to only a fraction of the applied field— typically less than 1%. 4–6 Merugu et al. 1 first showed that an alternating electrical field can be applied to reduce the effects of polarization using a macroscale ElFFF system, 2,7 Lao et al. 8 demonstrated pulsed ElFFF in a miniaturized system using asymmetric cyclical fields as guided by the work of Mridha et al. 9 No CyElFFF microsystems have been adequately characterized with respect to the theory for cyclical field flow fractionation (CyFFF) originally proposed by Giddings in 1986 10 and extended and demonstrated in sedimentation FFF in 1988. 11 A simulation of cyclical electrical field flow fractionation (CyElFFF) was published by Stevens in 1990, 12 but not confirmed experimentally. In a recent communica- tion 13 a modified theory of CyFFF for use with electrical systems that was validated using a CyElFFF microsystem was reported and work has recently been published related to instrumentation and carrier effects, 14,15 but little information was provided on the operation and characterization of the system. In this work, a detailed performance evaluation of micro- scale CyElFFF will be presented with respect to the opera- tional parameters of the system. A comparison with published analytical models and experimental data will be provided. Basic particle retentions will be shown along with separations and suggestions will be made for improvements in the design and operation of CyElFFF systems. Theory The theory of CyElFFF from the literature 10,13 will be used in this work for comparison with experimental data and the reader is referred there for details. Only specifically relevant details are provided here. A CyElFFF system is essentially the same as a traditional ElFFF system with two planar electrodes separated by a thin spacer defining a microchannel. The flow in the channel is laminar with Reynolds numbers often less than unity. An oscillating electrical field is provided across the flow channel, such that particles susceptible to the electrical field respond in a manner similar to that shown in Fig. 1. Depending upon the extent of oscillation of the particles in the channel, modes of operation for CyElFFF are defined. A dimensionless parameter l o , defined for a square wave- form by l o ~ mE eff 2fw (1) is used to describe the motion of a particle in a CyElFFF channel, where, m is the particle electrophoretic mobility, f is University of Utah, Department of Mechanical Engineering, 50 S. Central Campus Drive Room 2110, Salt Lake City, UT 84112-9202, USA. E-mail: gale@eng.utah.edu; Fax: (801) 585-9826; Tel: (801) 585-5944 PAPER www.rsc.org/loc | Lab on a Chip This journal is ß The Royal Society of Chemistry 2006 Lab Chip, 2006, 6, 645–654 | 645