Extension of Golay's plate height equation from laminar to turbulent flow I – Theory

Emerging Separation Techniques in Supercritical Fluid Chromatography

Supercritical fluid chromatography (SFC) has unique separative characteristics distinguished from those of HPLC and gas chromatography. At present, SFC is widely used and there are many applications in various biological, medical, and pharmaceutical fields. In this review, we focus on recently developed novel techniques related to SFC separation including: new column stationary phases, microfluidics, two-dimensional separation, and gas-liquid separation. In addition, we discuss the application of SFC using a water-containing modifier to biological molecules such as amino acids, peptides, and small proteins that had been challenging analytes.

Recent advancements in open-tubular liquid chromatography and capillary electrochromatography during 2014-2018

This review critically discusses the developments on open-tubular liquid chromatography (OT-LC) and open-tubular capillary electrochromatography (OT-CEC) during 2014-2018. An appropriate Scopus search revealed 5 reviews, 4 theoretical papers on open-tubular format chromatography, 29 OT-LC articles, 68 OT-CEC articles and 4 OT-LC/OT-CEC articles, indicating a sustained interest in these areas. The open-tubular format typically uses a capillary column with inner walls that are coated with an ample layer or coating of solid stationary phase material. The ratio between the capillary internal diameter and coating thickness (CID/CT) is ideally ≤ 100 for appropriate chromatographic retention. We, therefore, approximated the CID/CT ratios and found that 22 OT-LC papers have CID/CT ratios ≤100. The other 7 OT-LC papers have CID/CT ratio >100 but have clearly demonstrated chromatographic retention. These 29 papers utilised reversed phase or ion exchange mechanisms using known or innovative solid stationary phase materials (e.g. metal organic frameworks), stationary pseudophases from ionic surfactants or porous supports. On the other hand, we found that 68 OT-CEC papers, 7 OT-LC papers and 4 OT-LC & OT-CEC papers have CID/CT ratios >100. Notably, 44 papers (42 OT-CEC and 2 OT-LC & OT-CEC) did not report the retention factor and/or effective electrophoretic mobility of analytes. Considering all covered papers, the most popular activity was on the development of new chromatographic materials as coatings. However, we encourage OT-CEC researchers to not only characterise changes in the electroosmotic flow but also verify the interaction of the analytes with the coating. In addition, the articles reported were largely driven by stationary phase or support development and not by practical applications.

High-resolution turbulent flow chromatography

The resolution power of turbulent flow chromatography using carbon dioxide as the mobile phase and coated (crosslinked methyl phenyl polysiloxane) open tube columns (OTCs) as the stationary phase was investigated under retentive conditions (0 <k< 1). The improvement in column efficiency from a laminar to a turbulent flow regime was accurately measured for small molecules (coronene and benzo[a]anthracene). This relative increase in column performance decreased from 9 to 5, 3, and to 3 with increasing the retention factor from 0 to 0.2, 0.5, and to 1.0, respectively. Despite a four to five orders of magnitude larger sample dispersion coefficient in turbulent than in laminar flow, the mass transfer in turbulent flow chromatography is still controlled and limited by the slow sample transport across the viscous layer at the column wall. The benefit of turbulent flow chromatography is then restricted to small retention factor (k< 0.2). From a practical viewpoint, turbulent flow chromatography using carbon dioxide as the mobile phase and 20 m long × 180 μm i.d. × 0.2 μm film thickness OTCs provides ultra-fast (analysis time < 10 s) and high-resolution (plate counts of 33,000) separations of weakly retained compounds (k∼0.1) at Reynolds number around 5000 (3.75 mL/min, 3000 psi back pressure).