## 2 Results and Discussion
### Chromatographic Characterisation of PS-co-DVB PolyHIPEs  
The chromatographic performance of both PS-_co_-DVB polyHIPE columns was evaluated by the separation of selected alkylbenzenes. To date, polyHIPE materials have only been demonstrated for the separation of large proteins under gradient conditions which are applications where the relatively low surface area is not detrimental to chromatographic performance. The use of polyHIPEs in CEC separations of alkylbenzenes resulted in predictably high separation efficiency (200,000 N/m) due in large part to reduced flow inhomogeneity over the cross-section of the column relative to pressure-driven flow typical of HPLC [28; 29]. [PERSON] _et al._ have demonstrated in packed-column CEC that when the pore size of stationary phase particles is large (\(\sim\)400 nm), a significant portion of the total flow is through the pore of the particles (previously thought to be negligible due to electrical double layer overlap) [54].  
Figure 4: Isocratic separation of alkylbenzenes on PS-co-DVB polyHIPEs. (**a**): Column #1, (**b**): Column #2. Chromatographic conditions: Column: 1.0 mm \(\times\) 100 mm 90% porosity PS-co-DVB polyHIPE, Mobile phase: 50% ACN, Flow rate: 40 \(\mu\)L-min\({}^{-1}\), Injection volume: 1 \(\mu\)L, Detection: UV at 214 nm. Peak assignments: (**1**) toluene, (**2**) ethylbenzene, (**3**) propylbenzene, (**4**) butylbenzene, (**5**) pentylbenzene. All analytes are 0.1 mg\(\cdot\)L\({}^{-1}\).  
Under these conditions, flow velocity through and between particles have roughly the same velocity and direction leading to fully perfusive behaviour and a corresponding increase in flow homogeneity across the column. [PERSON] _et al._[PERSON] _et al._ (2015) reported void diameters and window diameters of 8.9 \(\pm\) 2.7 and 2.0 \(\pm\) 0.9 \(\upmu\)m, respectively, which lead to fully perfusive behaviour described by [PERSON] _et al._[PERSON] _et al._ (2015) and much reduced stationary phase mass transfer resistance.Conversely, this report is the first instance of the isocratic separation of small molecules on a polyHIPE fabricated in a column format using pressure-driven flow. A homologous series of alkylbenzenes from toluene to pentylbenzene were well retained using a mobile phase of 50% ACN and a plot of In(\(k\)) _versus_ carbon number (\(n_{c}\)) demonstrated high linearity (\(\geqslant\)0.9995 for both columns), confirming that retention was based upon methylene group selectivity via dispersive interactions. An overlaid chromatogram for both columns is shown in Figure 4 and illustrates remarkable column-to-column reproducibility. For the most part (as shown in Table 1), retention and selectivity factors were very similar for both columns, and retention was only slightly higher for Column #2 (more noticeable for early eluters, with a retention difference of 6.1% for toluene, down to only 2.6% for pentylbenzene). The column-to-column reproducibility is all the more noteworthy given that each column was prepared on different days from separate batches of HIPE emulsion, rather than the same emulsion being filled into two separate column housings. Efficiency values were very low (3087 N/m for toluene) and decreased with increasing retention, as reported recently with commercial polymer monoliths [PERSON] _et al._ (2015); [PERSON] _et al._ (2015); [PERSON] _et al._ (2015).