Quantitative analysis by microchip capillary electrophoresis: current limitations and problem-solving strategies.

Obstacles and possible solutions for the application of microchip capillary electrophoresis in quantitative analysis are described and critically discussed. Differences between the phenomena occurring during conventional capillary electrophoresis and microchip-based capillary electrophoresis are pointed out, with particular focus on electrolysis, bubble formation, clogging, surface interactions, injection and aspects related to the power supply. Current drawbacks are specified and improvements for successful quantitative microchip capillary electrophoresis are suggested.

Quantitative analysis of thiols in consumer products on a microfluidic CE chip with fluorescence detection.

A microchip CE-based method for the quantification of the thiols mercaptoethanoic acid (MAA) and 2-mercaptopropionic acid (2-MPA) in depilatory cream and cold wave lotions was developed. The thiols were first derivatized with the fluorogenic reagent ammonium-7-fluorobenzo-2-oxa-1,3-diazole-4-sulfonate (SBD-F). The derivatives were separated within only 20 s by microchip CE and detected by their fluorescence. Conventional CE with diode array detection and LC with fluorescence detection were used for validation. The internal standard 3-mercaptopropionic acid (3-MPA) provided RSDs of multiple injections of only 4% or less for the MCE approach. LOD is 2 microM, LOQ 6 microM, and the linear range comprises nearly three decades of concentration starting at the LOQ.

Quantitative on-chip determination of taurine in energy and sports drinks.

A new method for the quantitative determination of taurine in beverages by microchip electrophoresis was developed. A rapid and simple sample preparation procedure, only including two dilution steps and the addition of the fluorogenic labeling reagent NBD-Cl (4-chloro-7-nitrobenzofurazan), is applied. Using a home-built wavelength-resolved fluorescence detector, the separation and determination of the taurine derivative could be achieved in only 12 s, while the additional spectral information was utilized to ensure peak purity. Spanning from 0.1 to 50 mmol L(-1), the linear dynamic range of the applied method was adapted to the apparent contents in common taurine containing beverages. The smallest detectable amount of the taurine derivative actually injected into the separation channel was as low as 60 amol. The method was successfully validated by an independent liquid chromatographic method.

Bilayer vesicles of amphiphilic cyclodextrins: host membranes that recognize guest molecules.

A family of amphiphilic cyclodextrins (6, 7) has been prepared through 6-S-alkylation (alkyl=n-dodecyl and n-hexadecyl) of the primary side and 2-O-PEGylation of the secondary side of alpha-, beta-, and gamma-cyclodextrins (PEG=poly(ethylene glycol)). These cyclodextrins form nonionic bilayer vesicles in aqueous solution. The bilayer vesicles were characterized by transmission electron microscopy, dynamic light scattering, dye encapsulation, and capillary electrophoresis. The molecular packing of the amphiphilic cyclodextrins was investigated by using small-angle X-ray diffraction of bilayers deposited on glass and pressure-area isotherms obtained from Langmuir monolayers on the air-water interface. The bilayer thickness is dependent on the chain length, whereas the average molecular surface area scales with the cyclodextrin ring size. The alkyl chains of the cyclodextrins in the bilayer are deeply interdigitated. Molecular recognition of a hydrophobic anion (adamantane carboxylate) by the cyclodextrin vesicles was investigated by using capillary electrophoresis, thereby exploiting the increase in electrophoretic mobility that occurs when the hydrophobic anions bind to the nonionic cyclodextrin vesicles. It was found that in spite of the presence of oligo(ethylene glycol) substituents, the beta-cyclodextrin vesicles retain their characteristic affinity for adamantane carboxylate (association constant K(a)=7.1 x 10(3) M(-1)), whereas gamma-cyclodextrin vesicles have less affinity (K(a)=3.2 x 10(3) M(-1)), and alpha-cyclodextrin or non-cyclodextrin, nonionic vesicles have very little affinity (K(a) approximately 100 M(-1)). Specific binding of the adamantane carboxylate to beta-cyclodextrin vesicles was also evident in competition experiments with beta-cyclodextrin in solution. Hence, the cyclodextrin vesicles can function as host bilayer membranes that recognize small guest molecules by specific noncovalent interaction.

Characterization of a pressurized C5-C16 hydrocarbon gas calibration standard for air analysis.

A compressed gas standard containing parts-per-billion (ppb) amounts of the volatile hydrocarbons methylpentadiene (isoprene, 540 ppb) and isooctane (259 ppb) and a series of less volatile C12-C16 n-alkanes (n-dodecane, 349 ppb; n-tridecane, 340 ppb; n-tetradecane, 202 ppb; n-pentadecane, 271 ppb; and n-hexadecane, 308 ppb) was prepared by a one-step (no further gas dilution) microgravimetric method. The gravimetric mixing ratios were confirmed by referencing to a capillary diffusion method. The cylinder was heated to 75 degrees C to minimize condensation losses of analytes to the cylinder walls. Mixing ratios were monitored over a 2.5-year period. Some initial analyte losses (approximately 3 to 20%) were observed for the heavier C14-C16 n-alkanes. Subsequently, analyte loss rates were found to be in the range of <1.0% per year for n-dodecane to n-hexadecane. The developed guidelines for preparation, storage, and retrieval of these semivolatile analytes enable the use of compressed gas standards for calibration and method development purposes in the environmental gas-phase analysis of these and related compounds.

Calibration system and analytical considerations for quantitative sesquiterpene measurements in air.

Sesquiterpenes (C15H24, SQT) are semi-volatile organic compounds emitted from vegetation and are of interest for air quality considerations because of their suspected contribution to the formation of secondary aerosol. This article investigates the application of a capillary diffusion method for the generation of standard atmospheres of 16 SQT and four other related semi-volatile compounds. This instrument subsequently has been used in the testing of analytical materials, protocols and calibration of air sampling methods. SQT DB-1 retention indices, vapor pressures at 25 and 75 degrees C, and diffusion coefficients were determined. A quantitative, on-line GC method yielded improved results (median relative standard deviation of 5.0-6.1%) for the diffusion rate determination in comparison to a gravimetric approach (median relative standard deviation 18%). The GC method also allowed identifying errors in the gravimetric method stemming from residual solvent evaporation, impurities, and chemical analyte losses. Stainless steel, glass, nickel and PTFE tubing that were tested for transfer lines and a sampling loop had to be kept at temperatures in excess of approximately 110 degrees C in order to prevent significant analytical errors from the stickiness of SQT to these materials. In addition to SQT analysis, results from this research provide general guidelines for gas-phase analysis of related compounds in the C14-C16 volatility range.