Research Article Optimization of cyclical electrical field flow fractionation Cyclical electrical field flow fractionation (CyElFFF) is a variation on electrical field flow fractionation (ElFFF) where cyclical electrical fields are used instead of steady DC fields to increase the effective field experienced by particles in the flow channel. Even though the effective field increases more than 20-fold compared to normal ElFFF, the retention and resolution in CyElFFF has not been shown to be better than in ElFFF. In this paper we report how one can optimize operational parameters in CyElFFF to obtain good retention and resolution in CyElFFF. The effects of offset voltage, frequency, flowrate, concentration of particles and sample size on retention, resolution and retained peak/ void peak ratio have been observed. The results obtained from these experiments were analyzed and suggestions have been made to improve both retention and resolution. A 4-fold improvement in retention without a significant increase in band broadening is reported. Keywords: Cyclical electrical field-flow fractionation / Field-flow fractionation / Optimiza- tion / Separation technique and nanoparticles DOI 10.1002/elps.201000024 1 Introduction Field flow fractionation (FFF) is a well-established separa- tion technique developed by Giddings at the University of Utah [1]. In FFF, the separation field is applied perpendi- cular to the direction of separation, which allows high- resolution separations to be obtained with relatively small applied fields. FFF techniques are subdivided based on the type of applied field with the major subtypes being: electric [2], magnetic [3], gravitational [4], thermal [5] and flow [6]. Unlike standard FFF, an alternating field is applied in cyclical FFF. Cyclical FFF was first conceptualized [7] by Giddings and later demonstrated using gravitational FFF [8]. Applying cyclical fields in flow, thermal and most of the other subtypes of FFF is difficult and complicated due to the required instrument design complexities; so little research has been done in this area for a significant period of time. A decade after the initial research, cyclical fields were again used but this time on an electrical field flow fractionation (ElFFF) channel [9]. This technique, known as cyclical electrical field flow fractionation (CyElFFF), produces higher effective fields and separates particles based solely on elec- trophoretic mobility [7]. Applying cyclical fields using an ElFFF system is easier than for any of the other subtypes as no modifications to a conventional ElFFF instrument are required. In the published initial CyElFFF work, Gale and Merugu explored the influence of various operational para- meters on retention and demonstrated the first particle separations [9]. Later, Kantak et al. demonstrated that similar results can be obtained in a microsystem [10]. In their subsequent work, Kantak et al. demonstrated the impor- tance of carrier conductivity on particle retention [11] and developed a better mathematical model of physics in CyElFFF [12]. The model was based on a simple electric circuit representing the system and they justified their model by comparing the model with experimental results. At nearly the same time, Lao et al. developed a slightly modified version of a microfabricated CyElFFF system [13]. They applied different duty cycle pulses rather than the symmetrical waveforms used by Merugu and Kantak. Later, Lao proved that electrophoresis dominates the particle motion in CyElFFF when devices with planar electrodes are used [14]. Subsequent work has primarily focused on developing a mathematical model for CyElFFF. Among these efforts, Biernacki et al. modified the electrical circuit model for the CyElFFF system by replacing the constant valued double layer capacitance and interface resistance with the time varying functions [15]. In the mean time, Anuj Chauhan developed a mathematical model based on the electrochemical response obtained from a microfabricated system [16]. Even though the effective field was observed to be significantly better in CyElFFF than in ElFFF, CyElFFF Merugu Srinivas Himanshu J. Sant Bruce K. Gale Department of Mechanical Engineering, University of Utah, Salt Lake City, UT, USA Received January 16, 2010 Revised May 26, 2010 Accepted July 19, 2010 Abbreviations: CyElFFF, cyclical electrical field flow fractionation; ElFFF, electrical field flow fractionation; FFF, field flow fractionation Correspondence: Dr. Bruce K. Gale, University of Utah, Depart- ment of Mechanical Engineering, 50 S. Central Campus Drive Room 2110, Salt Lake City, UT 84112-9202, USA E-mail: bruce.gale@utah.edu Fax: 11-801-585-9826 & 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.electrophoresis-journal.com Electrophoresis 2010, 31, 3372–3379 3372