Size-exclusion and the void volume of HPLC column

Towards an accurate method for column void volume determination using liquid chromatography-mass spectrometry

Prediction of analyte retention times requires prior knowledge of the column void volume, the measurement of which is still highly contested within the literature and therefore experimental based prediction is often used. In this study, we investigated deuterated acetonitrile as an isotopically labelled mobile phase component to observe its elution behaviour in a binary mixture with water at 25 different mobile phase compositions (from 5 to 95 vol.% of acetonitrile), on two stationary phases (C8 and C18), and at two temperatures (30 and 40 °C) using LC-MS. The same experimental design was additionally used for three commonly used neutral void volume markers: uracil, phloroglucinol and N,N-dimethylformamide. Temperature was observed to influence the elution of acetonitrile in an inversely proportional manner with higher temperatures coinciding with lower elution times. By utilizing a three-way ANOVA, the composition of the mobile phase has been shown to have a significant effect on deuterated acetonitrile and other investigated void volume markers, demonstrating the fact that both void volume markers and acetonitrile itself exhibit retention-like behaviour. Excess adsorption isotherms for acetonitrile were calculated using deuterated acetonitrile elution data. The comparison of void volumes, obtained with conventional neutral void volume markers, revealed the former to be 24-36% lower than the void volume obtained using deuterated acetonitrile, as an isotopically labelled mobile phase component. For a water:acetonitrile mobile phase, the minor disturbance method using deuterated acetonitrile to obtain an integral average void volume (2.08 and 2.05 mL for C18 at 30 and 40 °C, respectively and 2.16 and 2.13 mL for C8 at 30 and 40 °C, respectively) was found to be the most appropriate method for determining the elusive column void volume.

Porous Polymers Based on 9,10-Bis(methacryloyloxymethyl)anthracene-Towards Synthesis and Characterization

Porous materials can be found in numerous essential applications. They are of particular interest when, in addition to their porosity, they have other advantageous properties such as thermal stability or chemical diversity. The main aim of this study was to synthesize the porous copolymers of 9,10-bis(methacryloyloxymethyl)anthracene (BMA) with three different co-monomers divinylbenzene (DVB), ethylene glycol dimethacrylate (EGDMA) and trimethylpropane trimethacrylate (TRIM). They were synthesized via suspension polymerization using chlorobenzene and toluene served as porogenic solvents. For the characterization of the synthesized copolymers ATR-FTIR spectroscopy, a low-temperature nitrogen adsorption-desorption method, thermogravimetry, scanning electron microscopy, inverse gas chromatography and size distribution analysis were successfully employed. It was found that depending on the used co-monomer and the type of porogen regular polymeric microspheres with a specific surface area in the range of 134-472 m²/g can be effectively synthesized. The presence of miscellaneous functional groups promotes divergent types of interactions Moreover, all of the copolymers show a good thermal stability up to 307 °C. What is important, thanks to application of anthracene derivatives as the functional monomer, the synthesized materials show fluorescence under UV radiation. The obtained microspheres can be used in various adsorption techniques as well as precursor for thermally resistant fluorescent sensors.

Pseudomorphic synthesis of bimodal porous silica microspheres for size-exclusion chromatography of small molecules

In this work, mesoporous silica microspheres with bimodal porous structures for size exclusion chromatography (SEC) supports were synthesized via a pseudomorphic transformation method by using 3.5 and 5 μm commercial silica particles as sources and cetyltrimethylammonium bromide (CTAB) as a template. The effects of the synthetic conditions on the pore size distribution were examined, including the temperature, reaction time and the molar ratio of SiO₂:NaOH. Bimodal porous silicas (BPSs) with pore sizes of 3.01 and 12.80 nm were obtained with SiO₂:NaOH:CTAB:H₂O=1:0.1:0.1:20 at 80 °C for 24 h. The BPSs were bonded with diol groups to produce a stationary phase for SEC. The column performance was evaluated with three types of samples, namely, dextran (70 KDa-62 Da), polyethene glycol (PEG) (20 KDa-32 Da) and three biomolecules (36 KDa-1.36 KDa). The column that was packed with a 3.5 μm stationary phase showed excellent resolution for molecular weights of less than 1 KDa with high column efficiency. Carbohydrate samples (dextran (MW=1296), dextran (MW=972), sucrose (MW=342), glucose (MW=180) and glycerol (MW=92)) were separated. Heptaethylene glycol, hexaethylene glycol, pentaethylene glycol, tetraethylene glycol, triethylene glycol, and diethylene glycol were resolved in a PEG200 sample. In summary, this work shows the advantages of bimodal mesopores in SEC for small molecules less than 1 kDa. In the pseudomorphic synthesis, the pore size can be regulated by template micelles. Thus, the development SEC supports with high accuracy for a specified molecular weight range is expected since the pore size can be regulated by the surfactant template.

Evaluation of a linear free energy relationship for the determination of the column void volume in hydrophilic interaction chromatography

The application of a linear free energy relationship (LFER) to a variety of hydrophilic interaction chromatography columns with different bonded ligands and pore sizes was studied in order to determine their void volume V_m. The method was based on the determination of the elution volume of a series of alkylbenzene standards from C1 (toluene) to C17 (heptadecylbenzene). Results were compared with those obtained by injection of toluene alone, which has traditionally been used as a simple V_m marker. V_m was smaller when derived from the LFER plot than when measured with toluene with differences between the two methods ranging from 2.7 to 12.7 % for the columns studied. This result could be due to the small but appreciable retention of toluene due to its solubility in the water rich layer, which partially constitutes the stationary phase in HILIC. Larger pore size columns showed less difference in V_m between LFER and toluene procedures. This result may be due to size sieving effects of non-excluded solutes in the pores of the stationary phase, or to differences in phase ratio between columns of different pore size.