Determination of zwitterionic and cationic surfactants by high-performance liquid chromatography with chemiluminescenscent nitrogen detection

Fabric softeners: nearly instantaneous characterization and quality control of cationic surfactants by easy ambient sonic-spray ionization mass spectrometry

A tiny droplet of typical samples of fabric softeners from different commercial brands placed on a smooth paper surface was subjected to easy ambient sonic-spray ionization mass spectrometry (EASI-MS). With no need for sample-preparation or pre-separation procedures, EASI-MS and EASI-MS/MS identify nearly instantaneously the main surfactants and the homologous series employed in their formulations. Adulterated and low quality samples containing no or less efficient softeners are also easily recognized.

Separation of organic cations using novel background electrolytes by capillary electrophoresis

A background electrolyte for capillary electrophoresis containing tris(-hydroxymethyl) aminomethane (THAM) and ethanesulfonic acid (ESA) gives excellent efficiency for separation of drug cations with actual theoretical plate numbers as high as 300,000. However, the analyte cations often elute too quickly and consequently offer only a narrow window for separation. The best way to correct this is to induce a reverse electroosmotic flow (EOF) that will spread out the peaks by slowing their migration rates, but this has always been difficult to accomplish in a controlled manner. A new method for producing a variable EOF is described in which a low variable concentration of tributylammonium- or triethylammonium ESA is added to the BGE. The additive equilibrates with the capillary wall to give it a positive charge and thereby produce a controlled opposing EOF. Excellent separations of complex drug mixtures were obtained by this method.

A three-factor Doehlert matrix design in optimising the determination of octadecyltrimethylammonium bromide by cation-exchange chromatography with suppressed conductivity detection

A simple and effective chromatographic method with suppressed conductivity detection was developed and validated to determine dissolved samples of octadecyltrimethylammonium bromide (C18H37N+ Me3Br-, ODTAB) for purity testing. A response surface methodology generated with a Doehlert matrix design was applied to optimize the chromatographic and detection conditions in ion-exchange chromatography (IEC) with conductivity detection in the chemical suppression mode. A three-factor Doehlert design was performed to fit a second-order model and jointly optimize the peak intensity and shorten analysis time through a global desirability function. Regenerant flow rate, volume fraction of acetonitrile in the acidic eluent and its flow rate were studied at seven, five and three levels, respectively. The optimized separation and detection conditions were accomplished by using a cation-exchange column eluted at 0.5 mL min(-1) with an isocratic mobile phase composed of CH3CN and 25 mN H2SO4, 82/18 (v/v). Chemical suppression of ionic conductivity was performed by 100 mN tetrabutylammonium hydroxide (TBAOH) as a regenerant at a flow-rate of 4.0 mL min(-1). Remarkably good agreement was found between predicted and experimental values of signal intensity and chromatographic retention. With the developed method, a linear calibration curve of ODTA+ as bromide salt from 5 to 1000 ppm was obtained using hexadecyltrimethylammonium bromide as internal standard. The estimated limit of detection was 0.3 ppm (S/N=3). The effectiveness of electrochemically suppressed conductivity detection of ODTA+ was also demonstrated, thus making easier the whole detection operation and instrumental needs as well.

Structure-dependent response of a chemiluminescence nitrogen detector for organic compounds with adjacent nitrogen atoms connected by a single bond

High-throughput screening (HTS) of chemical libraries is indispensable for drug discovery research. However, the HTS data quality for lead discovery, lead optimization, and quantitative structure activity relationship studies has been severely compromised due to the uncertain compound concentrations in screening plates. In order to address this issue, we compared various high-throughput technologies for quantification of compounds in microtiter plate format without the need for authentic compounds as standards and identified the chemiluminescence nitrogen detector (CLND) as the method of choice at the present time. However, the structure dependence of this detector has not been well studied. A proposed rule suggested that the only exception to equimolar response is for compounds that contain adjacent nitrogen atoms. The response should be zero when the adjacent nitrogen atoms are connected by a double bond and 0.5 when they are connected by a single bond. In this investigation, we studied a broad range of compounds with isolated and adjacent nitrogen atoms. We confirmed that compounds with isolated nitrogen atoms produce an equimolar response with a 15-20% variation depending on structures and compounds with adjacent nitrogen atoms connected by a double bond giving nearly zero response. We discovered that the CLND response for compounds containing adjacent nitrogen atoms that are connected with a single bond is highly structure dependent. Substitutions on the nitrogen atoms or nearby in the molecule can increase the CLND response to approach a value higher than the predicted value 0.5 (maximal value 0.82/nitrogen atom). Without substitution, much lower values than predicted (minimal value 0.0-0.08/nitrogen atom) are obtained. Therefore, the prediction of response of 0.5/nitrogen atom for compounds with adjacent nitrogen atoms connected by a single bond should be abandoned. Compounds with similar structures should be used to generate calibration curves for quantification of this class of compounds.

Separation and quantitative analysis of alkyl sulfobetaine-type detergents by high-performance liquid chromatography and light-scattering detection

An improved high-performance liquid chromatographic (HPLC) method for the separation of zwitterionic detergents is described. It is based on a reversed-phase liquid chromatography with evaporative light-scattering detection (ELSD). The method was shown to be highly specific, allowing the separation of three detergents of the alkyl sulfobetaine family: 3-(N-dodecyl-N,N-dimethyl-ammonio)-propane-1-sulfonate (SB12), 3-(N-tetradecyl-N,N-dimethyl-ammonio)-propane-1-sulfonate (SB14) and 3-(N-hexadecyl-N,N-dimethyl-ammonio)-propane-1-sulfonate (SB16). It was further used to develop a quantitation method for SB14, which was validated for linearity, precision, robustness, limits of detection and quantitation, specificity and accuracy. Linearity was found in the range of 50-500 microg/ml with a correlation coefficient of 0.9938+/-0.0029. The mean value of slope and intercept were 1.567+/-0.06 and 0.1541+/-0.0271, respectively. The limits of detection (LOD) and quantitation (LOQ) were 2 and 10 microg/ml, respectively. The validated method was used to determine the concentration of SB14 in different biological samples, specially in bulks of a recombinant membrane protein, the Klebsiella pneumoniae outer membrane protein A, which is produced at the pilot scale for human clinical studies.

Capillary electrochromatography-mass spectrometry of cationic surfactants

The CEC-MS of alkyltrimethylammonium (ATMA+) ions with chain lengths ranging from C1-C18 is optimized using an internally tapered column packed with mixed mode reversed phase/strong cation exchange stationary phase. A systematic study of the CEC separation parameters is conducted followed by evaluation of the ESI-MS sheath liquid and spray chamber settings. First, the optimization of CEC separation parameters are performed including the ACN concentration, triethylamine (TEA) content, buffer pH and ammonium acetate concentration. Using 90% v/v ACN with 0.04% v/v TEA as mobile phase, the separation of longer chain C6-C18-TMA+ surfactants could be achieved in 15 min. Lowering the ACN concentration to 70% v/v provided resolution of shorter chain C1, C2-TMA+ from C6-TMA+ although the total analysis time increased to 40 min. Furthermore, variation of both the ACN and TEA content as well as ionic strength has found to significantly influence the retention of longer chain surfactants as compared to shorter chains. The optimum CEC conditions are 70% v/v ACN, 0.04% v/v TEA, pH 3.0 and 15 mM ammonium acetate. Next, the optimization of the ESI-MS sheath liquid composition is conducted comparing methanol to isopropanol followed by the use of experimental design for analysis of spray chamber parameters. Overall, the developed CEC-ESI-MS method allows quantitative and sensitive monitoring of ATMA+ from < or =10 microg/mL down to 10 ng/mL. Utilizing the optimized CEC-ESI-MS protocol, the challenging analysis of commercial sample Arquad S-50 ATMA+ containing cis-trans unsaturated and saturated soyabean fatty acid derivatives is demonstrated.

New polar-embedded stationary phase for surfactant analysis

The separation and identification of surfactants is a challenge due to the diversity of surfactants, the complex composition of surfactant raw materials, and the complexity of the sample matrices. High-performance liquid chromatography (HPLC) is the preferred analytical technique, because it allows the characterization of surfactant raw materials according to their composition and the quantitation of individual surfactants in complex mixtures. Although many HPLC columns are available for surfactant analysis, none of these columns provide optimal selectivity for the simultaneous analysis of anionic, non-ionic, and cationic surfactants using the same mobile phase system. In this paper, we describe a new polar-embedded stationary phase for the simultaneous analysis of anionic, non-ionic, and cationic surfactants with a simple and volatile mobile phase system containing ammonium acetate buffer and acetonitrile, utilizing evaporative light-scattering detection (ELSD). Mobile phase ionic strength and pH are important for optimizing chromatographic conditions. The column offers ideal selectivity for different types of surfactants, excellent peak shapes, especially for cationic surfactants, improved resolution for oligomers in ethoxylated surfactants, and compatibility with highly aqueous mobile phases. Thus, it can be used not only for quality assurance of individual surfactants, but also for the analysis of a variety of surfactant-containing formulations.

Capillary Electrochromatography−Mass Spectrometry of Zwitterionic Surfactants

This work describes the on-line hyphenation of a packed capillary electrochromatography (CEC) column with an internally tapered tip coupled to electrospray ionization-mass spectrometry (ESI-MS) and atmospheric pressure chemical ionization-mass spectrometry (APCI-MS) for the analysis of betaine-type amphoteric or zwitterionic surfactants (Zwittergent). A systematic investigation of the CEC separation and MS detection parameters comparing ESI and APCI is shown. First, a detailed and optimized manufacturing procedure for fabrication of the CEC-MS column with a reproducible internally tapered tip (7-9 microm) is presented. Next, the optimization of the separation parameters by varying the C(18) stationary-phase particle size (3 versus 1.5 microm), as well as mobile-phase composition including acetonitrile (ACN) volume fraction, ionic strength, and pH is described. The optimized separation is achieved using 3-microm C(18) packing with 75% ACN (v/v), 5 mM Tris at pH 8.0. Optimization for on-line CEC-ESI-MS detection is then done varying both the sheath liquid and spray chamber parameters while evaluating the use of random versus structured factorial table experimental designs. The more structured approach allows fundamental analysis of individual ESI-MS parameters while minimizing CEC and MS equilibration time between settings. A comparison of CEC-ESI-MS to CEC-APCI-MS using similar sheath and spray chamber conditions presents new insight for coupling of CEC to APCI-MS. The sheath liquid flow rate required to maintain adequate sensitivity is much higher in APCI source (50 microL/min) as compared to the ESI source (3 microL/min). The on-line mass spectra obtained in the full scan mode show that fragmentation in the two sources occurs at different positions on the Zwittergent molecules. For ESI-MS, the protonated molecular ion is always highest in abundance with minor fragmentation occurring due to the loss of the alkyl chain. In contrast, the APCI-MS spectra show that the highest abundant ion resulted by elimination of propane sulfonate from the Zwittergent molecule. A comparison of the sensitivity between the two sources in positive ionization SIM mode shows that CEC-ESI-MS provides an impressive limit of detection (LOD) of 5 ng/mL, which is at least 3 orders of magnitude lower than CEC-APCI-MS (LOD 100 microg/mL). Finally, the optimized CEC-MS methods comparing ESI and APCI are applied for separation and structural characterization of a real industrial zwittergent sample, Rewoteric AM CAS.