Separation of homologues and isomers of linear alkylbenzenesulfonates by capillary electrophoresis with sodium dodecyl sulfate, carboxylic acids and bile salts

Optimization and validation of a new method for analysis of linear alkylbenzene sulfonates in sewage sludge by liquid chromatography after microwave-assisted extraction

A new approach has been developed for the monitoring of linear alkylbenzene sulfonates (LAS) (C10-C13) in sewage sludge. It is based on microwave-assisted extraction (MAE) prior to high performance liquid chromatography coupled with diode array (HPLC-DAD) and fluorescence (HPLC-FL) detectors. Column Zorbax XDB-C8, 150 mm x 4.6 mm i.d., 5 microm particle size was used as well as acetonitrile-water containing 0.1 M NaClO4 (65:35) and isocratic elution. Compounds were isocratically eluted over 6 min runtime at a flow rate of 1 mL min(-1). Since polar interferences are eluted between 0 and 2 min, they do not interfere in the analysis. The column was thermostated at a temperature of 25 degrees C. For the determination of LAS, DAD-UV (lambda(ex) = 225 nm) and fluorescence (lambda(ex) = 225 nm, lambda(em) = 295 nm) detectors were employed. The main factors affecting the extraction efficiency (extractant volume, microwave power and irradiation time) were optimized using a compost sludge. The detection limit for total LAS in the sludge was < 5 mg kg(-1). The extraction of C10-C13 homologues is carried out by using an extraction time of 10 min and 5 mL of methanol; whereas Soxhlet needs a 12 h extraction. The method did not require clean-up or preconcentration steps. Concentration levels of LAS were between 13,037 mg kg(-1) for digested sludge and 2492 mg kg(-1) for compost sludge.

Recent advances in capillary electrophoresis and capillary electrochromatography of pollutants

Recent advances in the CE and CEC separation, detection, and sample preparation methodologies applied to the determination of a variety of compounds having current or potential environmental relevance have been overviewed. The reviewed literature has illustrated the wide range of CE applications, indicating the continuing interest in CE and CEC in the environmental field.

Analysis of linear alkylbenzene sulfonate homologues in environmental water samples by mixed admicelle-based extraction and liquid chromatography/mass spectrometry

Hemimicelles and admicelles of cetyltrimethylammonium bromide (CTAB) and cetylpyridinium chloride (CPC), adsorbed onto silica, were tested as sorbents for the solid phase extraction (SPE) of linear alkylbenzene sulfonate (LAS) homologues from environmental water samples. LASs were quantitatively retained on both surfactants due to high hydrophobic and ionic interactions, which led to the formation of analyte-extractant mixed aggregates. Parameters affecting the SPE of LASs were optimised. Recoveries of analytes from wastewater influent and effluent and river water samples ranged between 86 and 110%. Combination of SPE with liquid chromatography/mass spectrometry provided detection limits for the different LAS homologues of about 4 ng L(-1). The precision of the method, expressed as relative standard deviation, ranged from 5 to 9%. The method was applied to the analysis of LASs in wastewater and river samples using sample volumes between 10 and 25 mL. The LAS concentrations found ranged from 9 to 503 microg L(-1). No cleaning step was required to get accurate results.

Capillary electrophoresis enhanced by automatic two-way background correction using cubic smoothing splines and multivariate data analysis applied to the characterisation of mixtures of surfactants

Mixtures of the surfactant classes coconut diethanolamide, cocamido propyl betaine and alkylbenzene sulfonate were separated by capillary electrophoresis in several media containing organic solvents and anionic solvophobic agents. Good resolution between both the surfactant classes and the homologues within the classes was achieved in a BGE containing 80 mM borate buffer of pH 8.5, 20% n-propanol and 40 mM sodium deoxycholate. Full resolution, assistance in peak assignment to the classes (including the recognition of solutes not belonging to the classes), and improvement of the signal-to-noise ratio was achieved by multivariate data analysis of the time-wavelength electropherograms. Cubic smoothing splines were used to develop an algorithm capable of automatically modelling the two-way background, which increased the sensitivity and reliability of the multivariate analysis of the corrected signal. The exclusion of significant signals from the background model was guaranteed by the conservativeness of the criteria used and the safeguards adopted all along the point selection process, where the CSS algorithm supported the addition of new points to the initially reduced background sample. Efficient background modelling made the application of multivariate deconvolution within extensive time windows possible. This increased the probability of finding quality spectra for each solute class by orthogonal projection approach. The concentration profiles of the classes were improved by subsequent application of alternating least squares. The two-way electropherograms were automatically processed, with minimal supervision by the user, in less than 2 min. The procedure was successfully applied to the identification and quantification of the surfactants in household cleaners.

Importance of the counterion in optimization of a borate electrolyte system for analyses of anions in samples with complex matrices performed by capillary zone electrophoresis

Borate buffers are common background electrolytes for analyses of anions in capillary zone electrophoresis. Usually, sodium borate at a given pH is used and this specification seems to be sufficient for a successful analysis. In this paper, we show that free migration of OH(-) may deteriorate the analysis of a typical anionic analysis of clinical samples due to uncontrolled migration of OH(-) throughout the systems of analyzed zones and may damage the stacking of anionic analytes of interest. We have proven that the use of ammonium borate may remedy the situation where the presence of ammonium may selectively stop the free migration of OH(-) ions, slow down their effective mobility and bring their safe behavior resulting in reproducible stacking of clinically important anions. Results of real analyses of human serum samples confirmed the proposed method and proved that substitution of sodium for ammonium in borate buffers offers reliable analyses of clinical samples having chloride as the bulk component. The experimental results given in this paper are supported also by computer simulation, which can not only support the positive results but also show the dynamics of the separation that is otherwise hidden to any detection possibilities.

Resolution of overlapped non-absorbing and absorbing solutes using either an absorption null-balance detection window or multivariate deconvolution applied to capillary electrophoresis of anionic surfactants

Non-absorbing alkyl ether sulfates (AES) can be separated using anthraquinone-2-carboxylic acid (AQCA) as a probe; however, absorbing alkyl benzene sulfonates (ABS), if present, interfere indirect detection of most AES oligomers. Overcoming of this interference, as well as the simultaneous characterisation and evaluation of AES, fatty acids and ABS, was accomplished by using a diode-array detector and the procedures here discussed. First, it was shown that ABS can be made undetectable by using a 9 nm wide and 227 nm centred charge-absorptivity null-balance detection window (NBDW), where its contribution to the absorbance cancels the dilution effects that its presence induces on the signal of the background chromophore (BGC). Two other procedures, not requiring any prior knowledge on the nature of the absorbing interference, were also addressed. In the first one, the NBDW procedure was emulated by software, by treating the time-wavelength data matrix stored during the experimental run, and in the second one, both the ABS and BGC spectra, and the concentration profiles of ABS and the non-absorbing solutes, were recovered by orthogonal projection approach (OPA) and alternating least squares (ALS). The OPA-ALS processing provided the deconvolved signals and the wavelengths required to implement the experimental and software-emulated NBDW procedures. A composite ABS spectrum and a mixed concentration profile of the non-absorbing solutes, that involves mutual ABS-BGC dilution effects are enclosed in the OPA-ALS straightforward solutions. The pure spectra and concentration profiles were finally retrieved by crossed orthogonalisation. For the NBDW procedures, the limits of detection (S/N = 3) for AES oligomers overlapped by 1500 microg ml(-1) ABS were of ca. 10 microM AES. Using decyl sulfate as internal standard, the relative standard deviation for AES in an ABS containing industrial sample was 4.5%. The procedures here described are useful to remove the interference produced by any absorbing solute when overlapped with indirectly detected solutes in both capillary electrophoresis (CE) and HPLC.

Differentiation and quantification of linear alkyl benzenesulfonate isomers by liquid chromatography-ion-trap mass spectrometry

Discrimination and quantitation of the 20 positional isomers of C10-C13 linear alkyl benzenesulfonates (LASs), based on the use of reversed-phase liquid chromatography-electrospray ionisation in negative ion mode ion-trap mass spectrometry, was undertaken. Discrimination was achieved by LAS MS-MS analysis into the ion trap, by monitoring specific fragment ions resulting from the benzylic cleavage of the carbon alkyl chain on both side of the LAS phenyl group. Parameters affecting the electrospray ionisation source and the ion-trap operation were optimised. Calibration curves for the different isomers were established and this permitted their quantitation by mass spectrometry for the first time. MS-MS responses were dependent on both the position of the phenyl group on the alkyl chain and the length of this alkyl chain, these responses being higher for the external isomers and the longer alkyl chain homologues. The precision, expressed as relative standard deviation ranged between 9 and 13%. Detection limits for LAS isomers were between 0.03 and 0.07 mg/l and therefore the method is sensitive enough to be applied to environmental samples.