Fig. 1. Schematic representation of the DDMAU coating formed upon the reversed-phase surface showing (a) possible bi-layer formation, (b) the formation of hemi-micelles upon the surface.

Fig. 2. Overlaid chromatograms of 288 consecutive injections of a 50 mg/L standard mixture of nitrite, chloride, bromide and nitrate on a 5 cm DDMAU coated column, using a 1 mM phthalic acid eluent (pH 3.0) at a flow-rate of 2.0 mL/min and direct conductivity detection.

Fig. 3. Overlaid chromatograms of a 1.0 mM mixture of iodate, bromate, nitrite, bromide, nitrate, iodide and thiocyanate, obtained using 10 mM sodium phosphate buffer mobile phase at pH 3.0 (lower trace) and pH 6.0 (upper trace). Other conditions: flow rate, 2.0 mL/min; wavelength of detection, 214 nm.

Fig. 4. Graph showing the relationship between retention and mobile phase pH using a 10 cm DDMAU coated monolithic column. Eluent: 10 mM sodium phosphate.

Fig. 5. Chromatogram of a 1.0 mM mixture of iodate, bromate, nitrite, bromide, nitrate, iodide and thiocyanate obtained obtained using a DDMAU coated 2.5 cm monolithic column and a mobile phase comprised of 10 mM sodium phosphate buffer at pH 4.0, a flow-rate of 2.0 mL/min and a wavelength of detection of 214 nm.

Fig. 6. (a) Overlaid chromatograms of a 1.0 mM mixture of iodate, bromate, nitrite, bromide, nitrate, iodide and thiocyanate obtained using initial flow-rates of 2.0 mL/min and a starting mobile phase pH of 4.0, with a dual gradient from 1.5 to 4.5 min. Gradient details: (lower trace)—100% pH 4.0 to 100% pH 8.0, 2.0–6.0 mL/min; (upper trace)—100% pH 4.0 to 100% pH 6.0, 2.0–6.0 mL/min. (b) As above using a starting mobile phase pH of 4.0, with a pH gradient (100% pH 4.0 to 100% pH 6.0) and a flow gradient (2.0–6.0 mL/min) from (lower trace) 1.5 to 4.5 min, and (upper trace) 1.5 to 2.0 min.

Fig. 7. Column outlet pH determined during application of pH gradients (pH 4.0–6.0 over 1.5–4.5 min) using the DDMAU coated 2.5 cm monolithic column, overlaid with pH profiles obtained without column. Flow-rate = 4 mL/min.

Retention factors (k) of ionic and zwitterionic surfactants on C₁₈ modified columns

Comparison of efficiency values for all seven analyte anions using reversed-phase monolithic columns of different lengths (10 and 2.5 cm) coated with DDMAU

Mean, standard deviation and % RSD values of retention times of all seven analytes (n = 6), using a dual gradient from 1.5 to 4.5 min

Limits of detection for anions employing a dual gradient^a applied between 1.5 and 4.5 min of the chromatographic run time

Linearity data obtained using dual gradient run for all seven analyte anions

All data shown averaged from three repeat runs.