The advantage of employing this parameter is that X(c) would have a constant value for each proposed core structure regardless of the degree of alkylation, and thus visual representation and structural interpretations of the spectra become advantageous for characterizing and comparing complex samples. In addition, the proposed parameter complements the AI classification and identification of aromatic and condensed aromatic structures in complex matrices.

Surfactants such as didodecyldimethylammonium bromide (DDAB) adsorb onto fused-silica capillaries to form semipermanent bilayer coatings. However, such coatings must be regenerated between runs to maintain efficiency and reproducibility. In this paper, chemical and physical factors affecting the stability of DDAB coatings are investigated. Chemical factors such as ionic strength and the nature of the buffer anion (e.g., from acetate to phosphate), which decrease the critical micelle concentration of DDAB, improve the coating stability. Increasing buffer pH also increases the coating stability. Finally, reducing the capillary diameter and reducing the volume of buffer flushed through the capillary enhance the coating stability. Using 50 mM acetate, pH 5.0, in a 25-microm-i.d. capillary, cationic proteins were separated with efficiencies of 1.05 million plates/m and a run-to-run migration time reproducibility of 0.6-0.8% RSD for 10 successive runs without regeneration of the DDAB coating between runs.

Semipermanent coatings were generated within fused-silica capillaries by flushing the capillary with a 0.1 mM solution of the double-chained cationic surfactants didodecyldimethylammonium bromide, dimethylditetradecylammonium bromide (2C(14)DAB), dihexadecyldimethylammonium bromide, and dimethyldioctadecylammonium bromide (2C(18)DAB) and the triple-chained surfactant tridodecylmethylammonium iodide. All of these coatings were semipermanent, whereby the coating remained intact after the unadsorbed surfactant was removed from the capillary. The separation efficiencies for four model cationic proteins ranged from 1.2 to 1.4 million plates/m for the 2C(14)DAB coating to 0.3-0.4 million plates/m for the 2C(18)DAB coatings. The stability of the coating increased with increasing hydrophobicity of the surfactant (i.e., increasing chain length and decreasing cmc). Over 60 successive separations were performed on a 2C(18)DAB-coated capillary over 12 days, without any regeneration of the coating. The migration times varied by less than 2.3% over this period with no loss in efficiency.

A novel approach using a combination of capillary electrophoresis/mass spectrometry (CE/MS) and off-line Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) revealed the structural details of acidic constituents of atmospheric organic aerosol. Both techniques utilized electrospray ionization (ESI), a soft ionization method, to facilitate the analysis of complex mixtures of organic compounds. CE/ESI-MS using an UltraTrol LN-precoated capillary and acidic background electrolytes at different pH values (2.5 and 4.7) was used to differentiate between weak (carboxylic) and strong (sulfonic) organic acids. On the basis of the electrophoretic mobility, m/z constraints from CE/ESI(−)-MS, and elemental composition information retrieved from off-line FTICR-MS, a variety of aliphatic and aromatic carboxylic acids (CHO-bearing molecules), nitrogen-containing carboxylic acids (CHON-bearing molecules), organosulfates (CHOS-bearing molecules), and (nitrooxy)organosulfates (CHONS-bearing molecules) were tentatively identified in the Oasis-HLB-extracted urban PM 2.5 (particulate matter with an aerodynamic diameter of <2.5 μm). The chemical known/unknown structures of detected compounds were confirmed by the semiempirical Offord model (effective mobility linearly correlated to Z/M 2/3 ). The majorities of the identified compounds are products of atmospheric reactions and are known contributors to secondary organic aerosols.

A CE‐ESI/quadrupole‐MS method using an ammonium acetate‐based BGE (pH 4.7) was developed for the determination of isomeric benzoic acids in atmospheric aerosols and vehicular emission. UltraTrol™ LN was employed as the pre‐coating polymer to suppress the EOF (0.3×10−9 m2 V−1 s−1) and achieve a baseline separation of the studied acids. Good repeatability for migration time (RSD<1%, N=10) was obtained without coating regeneration. The high pre‐coating stability allowed coupling of CE to MS without ion suppression in the MS. In scanning mode and using field‐amplified sample injection with electrokinetic injection (−5 kV for 60 s), LODs (S/N=3) ranged from 2.5 to 6 μg/L for standard target analytes prepared in DI water. In the presence of 100 mg/L of sulfate (added to simulate a sample matrix), LODs ranged from 8 to 90 μg/L. Several isomeric aromatic acids could be separated in atmospheric and diesel‐engine‐emitted particulate matter extracts based on their different acidities. Additional measurements with a flow infusion ESI Fourier transform ion cyclotron resonance MS were used for further structural information acquisition on the unknown compounds and allowed their formula to be proposed.