Sub-femtomole determination of phenylthiohydantoin-amino acids: capillary electrophoresis and thermooptical detection

Determination of sequence and absolute configuration of peptide amino acids by HPLC-MS/CD-based detection of liberated N-terminus phenylthiohydantoin amino acids

We report a method for the simultaneous determination of the sequence and absolute configuration of peptide amino acids using a combination of Edman degradation and HPLC–MS/CD. Phenylthiohydantoin (PTH) derivatives of 20 pairs of standard d- and l-amino acids were synthesized by the Edman reaction. The CD spectra of the derivatives revealed that each pair of the PTH derivatives exhibited the absorption with opposite signs at around 270 nm. These standard PTH derivatives showed well-resolved resolution without interference from byproducts in the ion chromatogram and clear positive/negative CD absorptions when subjected on a reversed phase HPLC–MS system coupled with a CD-2095 HPLC detector. This method was applied for the detection of a synthetic pentapeptide and a natural depsipeptide (halicylindramide C). The sequence and configuration of the pentapeptide and up to eight residues of halicylindramide C were successfully analyzed by this method. The amino acid configuration of the pentapeptide was also determined successfully by subjecting its acid hydrolysates to the Edman reaction followed by HPLC–MS/CD.

Direct Photothermal Measurement of Optical Absorption in a Flow System

We describe here a simple photothermal detection scheme in a flow stream based on the temperature difference upstream and downstream of the point of illumination. We use a single, two-junction 25 μm diameter thermocouple to measure the temperature change. The baseline standard deviation in the dark is ∼0.001 °C that increases up to 0.0016 °C depending on the illumination source. We demonstrate the detection of several chromatographically separated dyes both with a 1.5 mm and a 0.1 mm i.d. detection cell, respectively, with a white LED and a solid-state laser source. With an inexpensive 660 nm, 19 mW laser as the light source, the estimated detection limit for methylene blue (MB) was 30 nM, corresponding to 120 amol in the illuminated volume. The dimerization constant of MB and the quantum efficiency of the monomer was determined.

Chemometric analysis of MALDI mass spectrometric images of three-dimensional cell culture systems

As imaging mass spectrometry (IMS) has grown in popularity in recent years, the applications of this technique have become increasingly diverse. Currently there is a need for sophisticated data processing strategies that maximize the information gained from large IMS data sets. Traditional two-dimensional heat maps of single ions generated in IMS experiments lack analytical detail, yet manual analysis of multiple peaks across hundreds of pixels within an entire image is time-consuming, tedious and subjective. Here, various chemometric methods were used to analyze data sets obtained by matrix-assisted laser desorption/ionization (MALDI) IMS of multicellular spheroids. HT-29 colon carcinoma multicellular spheroids are an excellent in vitro model system that mimic the three dimensional morphology of tumors in vivo. These data are especially challenging to process because, while different microenvironments exist, the cells are clonal which can result in strong similarities in the mass spectral profiles within the image. In this proof-of-concept study, a combination of principal component analysis (PCA), clustering methods, and linear discriminant analysis was used to identify unique spectral features present in spatially heterogeneous locations within the image. Overall, the application of these exploratory data analysis tools allowed for the isolation and detection of proteomic changes within IMS data sets in an easy, rapid, and unsupervised manner. Furthermore, a simplified, non-mathematical theoretical introduction to the techniques is provided in addition to full command routines within the MATLAB programming environment, allowing others to easily utilize and adapt this approach.

Detection of Separated Analytes in Subnanoliter Volumes Using Coaxial Thermal Lensing

A collinear-beam thermal lens detector has been constructed and its properties were characterized. Its application to the high-performance liquid chromatography (HPLC) separation of a mixture of five anthraquinone dyes dissolved in water shows a linear response over 3.5 orders of magnitude and a detection limit that is subnanomolar in the dye concentrations. These results are compared with those obtained previously using cavity ring-down spectroscopy (CRDS) in a Brewster's angle flow cell (Bechtel, K. L.; Zare, R. N.; Kachanov, A. A.; Sanders, S. S.; Paldus, B. A. Anal. Chem. 2005, 77, 1177-1182). The peak-to-peak baseline noise of the thermal lensing detection is 3.5 x 10(-8) absorbance units (AU) with a path length of 200 microm, whereas the peak-to-peak baseline noise of CRDS detection is approximately 2 x 10(-7) AU with a path length of 300 microm. Both of these figures of merit should be compared to the peak-to-peak baseline noise of one of the best commercial UV-vis HPLC detection systems, which is approximately 5 x 10(-6) AU with a path length of 10 mm (1-s integration time). Therefore, the thermal lensing technique has a demonstrated sensitivity of subnanomolar detection that is approximately 140 times better than that of the best commercial UV-vis detector and approximately 5 times better than that of CRDS.

Monitoring the Electroosmotic Flow in Capillary Electrophoresis Using Contactless Conductivity Detection and Thermal Marks

The fundamental aspects and the capillary electrophoresis usage of thermal marks are presented. The so-called thermal mark is a perturbation of the electrolyte concentration generated by a punctual heating of the capillary while the separation electric field is maintained. The heating pulse is obtained by powering tungsten filaments or surface mount device resistors with 5 V during a few tens to hundreds of milliseconds. In the proposed model, the variation of the transport numbers with the rising temperature leads to the formation of low- and high-concentration regions during the heating. After cooling down, the initial mobilities of the species are restored and these regions (the thermal mark) migrate chiefly due to the electroosmotic flow (EOF). The mark may be recorded with a conductivity detector as part of a usual electropherogram and be used to index the analyte peaks and thus compensate for variations of the EOF. In a favorable case, 10 mmol/L KCl solution, the theory suggests that the error in the measurement of EOF mobility by this mean is only -6.5 x 10(-7) cm2 V-1 s-1. The method was applied to the analysis of alkaline ions in egg white, and the relative standard deviations of the corrected mobilities of these ions were smaller than 1%. This is a challenging matrix, because albumin reduces the EOF to 20% of its initial value after 11 runs. The combination of thermal mark, electrolysis separated, and contactless conductivity detection allowed the measurement of the EOF of a silica capillary with unbuffered KCl solution with constant ionic strength. The overall approach is advantageous, because one can easily control the chemical composition of the solution in contact with the inner surface of the capillary.

Sensitivity improvement in capillary electrophoresis using organo-aqueous separation buffers and thermal lens detection

It is shown that organo-aqueous separation buffers show much promise when used in capillary electrophoresis separations with photothermal (thermal lens) detection systems. Acetonitrile-water and methanol-water mixtures were selected, as conventionally used in capillary electrophoresis. It is shown that, despite more sophisticated experimental conditions (significant heat outflow from the capillary body) and peak detection, the theoretical ratio of the thermal lens signal for a binary mixture to the thermal lens signal for an aqueous solution (or the corresponding ratio obtained experimentally under bulk batch conditions) can be used to predict the sensitivity of thermal lens detection in capillary electrophoresis. The limits of detection for 2-, 3-, and 4-nitrophenols selected as model compounds in 70% v/v acetonitrile separation buffers are 1 x 10(-6) M, 1 x 10(-6) M and 3 x 10(-7) M, respectively, and are therefore decreased by a factor of six compared to thermal lens detection in aqueous separation buffers. The overall increase in the thermal lens detection sensitivity in a 100% ACN buffer is a factor of 13.

Development of a Conductivity-Based Photothermal Absorbance Detector for Capillary Separations

A contactless conductivity-based absorbance detector has been developed for use with capillary separations. Detection is based on a photothermal process. As analytes pass through the detector they absorb light, producing a thermal perturbation. This thermal event results in a change in the solution conductivity. The measured change in conductivity is directly related to the absorption of light. The major advantage to this type of detector is that the measured absorbance is, to a first approximation, independent of optical path length, allowing small-diameter capillaries to be used. This approach combines the optical simplicity of traditional transmission-based instruments with the path length independence of similar refraction-based photothermal detectors. In addition to the initial development and characterization of the photothermal absorbance detector, multiphysical modeling of the heat transfer within the conductivity cell was performed.

Limitations arising from two-photon absorption of solvent in pulsed-laser thermal lens detection: determination of the two-photon absorption coefficient of ethanol at 266 nm

Two-photon absorption of the solvent under pulsed-laser excitation at 266 nm produces a high background thermal lens signal interfering with the analyte signal. Discrimination of both solvent and analyte signals along with calibration of the photothermal response has allowed the determination of the two-photon absorption coefficient of ethanol. The obtained value, 3.0x10(-10) cm W-1, is close to the literature values obtained from transmittance measurements using picosecond or femtosecond laser pulses.

Optimization of instrumental parameters of a near-field thermal-lens detector for capillary electrophoresis

The optical scheme of a near-field dual-beam mode-mismatched thermal-lens detector for capillary electrophoresis with a crossed-beam configuration employing a multimode HeCd laser (325 nm) as an excitation source was optimized. It is shown that a multimode laser can be successfully used as an excitation source in thermal lensing with minimal deviations in thermal responses from Gaussian excitation sources. An equation for diffraction thermal-lens theory for near-field measurements is deduced, and the experimental results agree with the deduced equation. The temperature rise in the capillary was estimated, and the exponential decrease of the signal with time for static conditions and low flow velocities was explained. The optimum configuration of the detector from the viewpoint of the maximum sensitivity and beam sizes was found. The detector provides a significant improvement in the detection limits for model compounds absorbing at 325 nm (nitrophenols) compared to the results obtained with a commercial absorbance detector operating at the same wavelength.

Amino acid analysis by micellar electrokinetic chromatography with laser-induced fluorescence detection: application to nanolitre-volume biological samples from Arabidopsis thaliana and Myzus persicae

Amino acids were derivatised with 4-fluoro-7-nitrobenzo-2,1,3-oxadiazol (NBD-F), separated by micellar electrokinetic chromatography (MEKC), and detected by argon-ion (488 nm) laser-induced fluorescence. The optimised MEKC background electrolyte conditions were: 40 mM sodium cholate, 5 mM beta-cyclodextrin in 20 mM aqueous borate buffer, pH 9.1, with 7% v/v acetonitrile. Using these conditions, 19 amino acids were separated within 17 min. The limits of detection were in the range of 7.6-42.2 pmol/mL and limits of quantitation from 0.05-0.14 nmol/mL. The method was systematically validated for injection volume error, migration time variation, calibration linearity, accuracy, precision, and recovery. Nanolitre volume samples of phloem sap of individual sieve element cells from the plant Arabidopsis thaliana and honeydew from the aphid Myzus persicae were directly analysed with this method. Quantitative amino acid concentrations in these two biological matrices were profiled for the first time. This method is particularly important because it allows the complete profile of the amino acids obtained from individual phloem elements, allowing cell to cell and plant to plant variation to be quantified, which to date has not been possible with Arabidopsis thaliana.

Photothermal spectrometry for detection in miniaturized systems: relevant features, strategies and recent applications

There is a growing interest in using miniaturized analytical devices because they allow to execute the different steps of an analytical process within very short times and with drastic reduction in the amounts of solvents, reagents and samples. As for capillary electrophoresis, these systems require detectors which are sensitive, versatile and adaptable to very small detection volumes. In this respect, photothermal spectrometry which is complementary to fluorescence seems to be a promising detection method. This review describes the basic principle of photothermal spectrometry along with the related methods based on colinear-beam or crossed-beam configuration of the pump and probe lasers. Two experimental set ups especially designed for microfluidic systems as well as for capillary electrophoresis are described. Their characteristics and key features are discussed and the main applications are outlined.

Quantification of Etoposide and Etoposide Phosphate in Human Plasma by Micellar Electrokinetic Chromatography and Near-Field Thermal Lens Detection

A method employing micellar electrokinetic chromatography in combination with near-field thermal lens detection (NF-TLD) was developed for the rapid simultaneous determination of etoposide phosphate and etoposide in human blood plasma, taking teniposide as internal standard. The method developed allows the baseline separation of the solutes of interest from each other and from potential interfering matrix constituents within 4 min. The NF-TLD device employed permits detection of solutes absorbing electromagnetic radiation at lambda = 257 nm in fused-silica capillaries with 75-microm i.d. via the near-field thermal lens effect with LODs of 100 microg L(-1) for etoposide phosphate and 170 microg L(-1) for etoposide. Comparison of the performance of this detector to the performance of a commercial absorption spectrometric detector working at lambda = 257 nm showed a substantial improvement in detection limits (up to 60-fold improvement) for the near-field thermal lens detector.

Laser-based non-fluorescence detection techniques for liquid separation systems

Over the last two decades, the possibility to use lasers for detection purposes in column liquid chromatography (LC) and capillary electrophoresis (CE) received much attention in the analytical chemistry literature. Most attention has been devoted to laser-induced fluorescence. The present review covers developments on non-fluorescence techniques for LC and CE. The techniques considered are thermal lens spectrometry, photoacoustic detection, refractive index detection including refractive index backscattering, Raman spectroscopy and degenerate four-wave mixing (a special mode of transientholographic spectroscopy). The paper starts with an outline of the characteristics of lasers; it ends with an overall evaluation and a discussion of the perspectives of the techniques dealt with.

Near-field thermal lens detection at 257 nm as an alternative to absorption spectrometric detection in combination with electromigrative separation techniques

A device is presented that permits detection of analytes absorbing electromagnetic radiation at lambda = 257 nm (in fused-silica capillaries with 75-microm i.d.) via the near-field thermal lens effect. The detector was realized by using a frequency-doubled argon ion laser as pump laser and a laser diode (emission wavelength, 633 nm) coupled into a monomode optical fiber as probe laser. Comparing the performance of this detector to the performance of a commercial absorption spectrometric detector working at lambda = 257 nm equipped with a unit for on-column detection in fused-silica capillaries showed a substantial improvement in detection limits (up to 30-fold improvement) for the near-field thermal lens detector (NF-TLD). The feasibility of the NF-TLD for sensitive detection of nonfluorescent analytes in real samples after separation by micellar electrokinetic chromatography was shown taking the determination of nitroaromatic compounds in contaminated water from a former ammunition plant as an example. Dependence of the thermal lens signal on pump laser power, velocity of the mobile phase, and chopper frequency was investigated. A linear calibration range over 2 orders of magnitude was obtained.

Amino acid sequence and D/L-configuration determination methods for D-amino acid-containing peptides in living organisms

D-amino acid-containing peptides with biological activities have been isolated from invertebrates and amphibians, and partial racemization of amino acid residues in mammalian peptides associated with aging and diseases have been discussed. Here, we review the amino acid configuration determination methods in these peptides and recent progress of simultaneous determination method for sequence and configuration of amino acid residues. The applicability of C-terminus sequence analysis and mass spectrometry to configuration determination of amino acids is also discussed.

Highly sensitive and direct detection of DNA fragments using a laser-induced capillary vibration effect

A pulsed laser-induced stationary wave capillary vibration detection method was applied to the sensitive detection of capillary gel electrophoresis, and the direct detection of non-labeled nucleic acids, such as DNA sequencing products, was demonstrated. An excimer laser operating at 248 nm was used as a CVL excitation source, and polynucleotides were sensitively detected without derivatization. From an investigation on the endurance of several matrixes to pulsed laser irradiation, a polyacrylamide without a cross-linker (0%C) was found to have adequate endurance, and it exhibited no serious damage during an analysis. A cytosine-terminated sequence reaction product was detected with a sensitivity close to that of laser-induced fluorometry (LIF). These results suggest the feasibility of the highly sensitive detection of ultramicro amounts of biological materials without a pre- or post-column derivatization, which has usually been required in sensitive detection procedures, such as LIF. Furthermore, the feasibility of a novel DNA sequencing method is also suggested.

Non-aqueous capillary electrophoresis of tamoxifen and its acid hydrolysis products

Tamoxifen and its acid hydrolysis products were separated and tentatively identified by non-aqueous capillary electrophoresis with thermooptical absorbance and electrospray ionization mass spectrometry. Acid hydrolysis is a convenient method of generating tamoxifen degradation products. The parent compound and seven hydrolysis products were separated in 9 min.

Electromigration methods for amino acids, biogenic amines and aromatic amines

Methods of electromigration in laboratory apparatus of small-bore size have recently undergone development at a remarkably rapid pace, leading to a variety of new analytical techniques. One such technique is called "capillary electrophoresis" (CE), which is further classified on the basis of electromigration mode, viz., "capillary zone electrophoresis" (CZE), which, in turn, has several variations. This review aims to give a short overview of the various electromigration methods for amino compounds by using CE. Firstly, this review briefly summarizes the detection methods employed for detection of monoamines and polyamines by CE for both native and derivative forms. Next, current CE methods are described, and their applications to detection of amino acids, biogenic amines, aromatic amines, including heteroaromatic amines and their enantiomers, are introduced from representative papers. Finally, new methods for single-cell analysis and microchip CE techniques are focused on.

The chemistry of protein sequence analysis

N-terminal sequence analysis by Edman chemistry continues to play an important role in the structural analysis of proteins and peptides. Improvements in the sensitivity of the method have been achieved mainly at the level of increasing the sensitivity of the on-line analysis of PTH amino acids by RP-HPLC (reverse phase high performance chromatography). Using microbore columns (0.8-1.0 mm), it is possible to run standards at the 0.5-1.0 pmol level and to sequence samples in the 1-5 pmol range. Due to constraints in current chromatographic methods, it is unlikely that further improvements in sensitivity will be achieved by this approach alone. Although alternative Edman reagents, including fluorescent chemistries, have promised to increase the sensitivity of sequencing into the low femtomole range, none of the methods have progressed into routine usage. These reagents and chemistries are critically evaluated in this review, and the problems which have prevented their further development discussed. Instrumental constraints are also considered. It is concluded that the development of more sensitive methods requires further research into both the chemistry and the instrumentation, and that alternative separation and detection methods may also play a role.

Optimization of a capillary electrophoresis-electrospray mass spectrometry method for the quantitation of the 20 natural amino acids in childrens blood

This paper describes useful information on the capillary electrophoresis-electrospray ionization mass spectroscopy (CE-ESIMS) interfacing for the analysis of amino acids (AAs) in standard mixtures and in child plasma blood serum. The developed procedure allows quantitation of the 20 natural AAs, in a single run, without any derivatization. Limits of detection as low as 3-20 micromol/l (5-30 pg injected) per analyte were obtained with an efficiency of about 100,000 plates and a peak area relative standard deviation below 4%.

Detection in capillary electrophoresis

A review of the four major, on-line, capillary electrophoresis (CE) detection modalities is presented. It is shown that each detection method, fluorescence, absorbance (conventional and nonconventional), electrochemical and refractive index, have distinct advantages and limitations when applied to analysis in a CE format. Various aspects of CE detection are considered and a perspective regarding the applicability of the technique is provided. It is shown that because of widely varying detection limits (ranging from single molecule to 10(-5) M) and detection scheme complexity, the particular application should dictate the selection of detection methodology in CE.

Migration time correction for the analysis of derivatized amino acids and oligosaccharides by micellar capillary electrochromatography

Migration-time reproducibility is essential in the use of capillary electrophoresis to identify components in mixtures. Two methods based on the migration time of either one or two reference markers are proposed for improving migration time reproducibility. These methods were evaluated to determine the migration time reproducibility for phenylthiohydantoin-amino acids, fluorescein thiohydantoin-amino acids, and tetramethylrhodamine labeled oligosaccharides. In the best case, the relative standard deviation of the migration time was reduced from >3% without correction to <0.04% with the two-marker correction.

Capillary electrochromatography with thermo-optical absorbance detection for the analysis of phenylthiohydantoin-amino acids

Capillary columns were packed with 3 microns C18 stationary phase, interfaced with an ultraviolet-laser based thermo-optical absorbance detector, and evaluated for separation of a mixture of phenylthiohydantoin-amino acids. These columns demonstrated consistent performance with a relative standard deviation (RSD) for migration time of less than 1.5% and a separation efficiency of 216,000 plates/m for the electroosmotic flow marker, thiourea. The thermo-optical absorbance detector was based on a 248 nm krypton-fluoride excimer laser. Detection limits (3 sigma) ranged from 1.6 to 4.8 x 10(-7) M phenylthiohydantoin (PTH)-amino acid injected onto the column, which is a factor of three superior to those obtained in micellar electrokinetic chromatographic analysis of these compounds. A mixture of 17 PTH amino acids was injected onto the capillary; 13 components were nearly baseline resolved in 14 min.

Nonaqueous capillary electrophoretic separation and thermo-optical absorbance detection of five tricyclic antidepressants and metabolism of amitriptyline by Cunninghamella elegans

We developed a technique based on nonaqueous capillary electrophoresis and laser-based thermo-optical absorbance detection to assay five antidepressants with similar structures and mass-to-charge ratios. A mixture of methanol and acetonitrile with ammonium acetate was essential to achieve baseline resolution of these compounds. We investigated the effects of ammonium acetate concentration, temperature, applied voltage, and capillary length on separation efficiency. The nonaqueous capillary electrophoresis and laser-based thermo-optical absorbance detection technique was used to study the metabolism of amitriptyline by Cunninghamella elegans. Sample preparation procedures were simplified for fast screening of the parent drug and its metabolites. Reproducible electropherograms were obtained from replicate cultures of C. elegans growing in the presence of amitriptyline.

Development of an on-line preconcentration method for the analysis of pathogenic lipopolysaccharides using capillary electrophoresis-electrospray mass spectrometry. Application to small colony isolates

The present investigation describes the use of on-line chromatographic preconcentration coupled to capillary zone electrophoresis-electrospray mass spectrometry (cPC-CZE-ES-MS) for trace level analysis of negatively charged lipopolysaccharides (LPS) obtained from pathogenic strains of Haemophilus influenzae. The analytical performance of two different types of adsorption media [i.e., C18 irregular particles and poly(styrene-divinylbenzene) membrane] for anionic analytes was first evaluated using a mixture of peptide standards to determine the overall sensitivity of this approach. These chromatographic preconcentrators provided an enhancement of sample loadings of up to 5 microliters with good linear response and low nM concentration detection limits for most peptides investigated. The application of cPC-CZE-ES-MS is further demonstrated for extracts of O-deacylated LPS obtained from H. influenzae strain Eagan. In combination with novel enzymatic releasing methods using proteinase K, this technique provides unparalleled sensitivity and enabled the identification of LPS surface antigens from as little as five bacterial colonies.

Improving the recovery of lysine in automated protein sequencing

Automated protein sequencing is an important tool in protein characterization. Most instruments use tetrahydrofuran (THF) as the HPLC eluent for separation of the derivatized amino acids residues. THF is highly perishable when exposed to air, generating peroxides which can degrade amino acids, mainly lysine, leading to uncertainty in chromatogram interpretations. Modifications of the existing HPLC equipment were introduced to create a permanent inert atmosphere inside the bottle of THF solution. This was carried out by changing the argon plumbing and some electrical connections and by reprogramming the software of the protein sequencer. The positive results of this procedure were demonstrated by comparing the decreasing lysine peak area during 28 days before and after the modifications. In the modified instrument, lysine recovery was much better as a function of the age of the THF eluent. Since these modifications improved the instrument performance, they have been adopted for routine use in our laboratory.

Miniaturized thermal lens device for capillary electrophoresis

In this paper we will report on the combination of a miniaturized fiber optic modified photothermal sensor (size 2 x 2 x 2 cm) to capillary electrophoresis (CE) as detection device. The properties and the performances of the thermal lens detector, based on a double beam absorption scheme, were studied in a capillary electrophoresis system with different types of toxic pollutants, e.g. pesticides. A tunable Argon ion laser, which emits between 364 and 514 nm, was adapted to a commercial injection and high voltage CE system. The miniaturization of the thermal lens detector head with optical waveguides allows the combination with typical CE systems, therefore pesticides in microliter volumes can be treated. The detector performance is demonstrated with the monitoring of pesticides in capillaries with various diameter. The limit of detection is one of two orders of magnitude better than conventional absorption detector.

Developments in amino acid analysis using capillary electrophoresis

The current status in the analysis of amino acids using capillary electrophoresis is addressed. This area of biological analysis has received increased attention with more than 200 articles being published in the last five years. This review discusses pre-, post-, and on-column derivatization techniques used to tag amino acids providing a detectable moiety. Several separation methodologies which provided resolution for large sets of amino acids are presented. An overview of advances in the enantiomeric resolution methodologies for amino acids is given. Both direct and indirect enantiomeric separation schemes are summarized. Recent advances in detection strategies for both derivatized and underivatized amino acids are presented. Applications utilizing amino acid analysis by capillary electrophoresis are described. This review covers articles published between 1991 and 1996.

Micellar electrokinetic chromatographic study of the interaction between enkephalin peptide analogs and charged micelles

The relative hydrophobicity/hydrophilicity of the pentapeptides leucine enkephalin (LE), methionine enkephalin (ME) and five analogs, differing in their uncharged side chain and/or chirality, was investigated by micellar electrokinetic capillary chromatography (MEKC) employing anionic and cationic surfactants. The effect of sodium dodecyl sulfate (SDS) concentration on peptide mobility was studied at pH 8.8, a value that is well above the peptide isoelectric point, to minimize electrostatic interaction with the anionic micelles. Similarly, the effect of cetyltrimethylammonium bromide (CTAB) cationic micelles on peptide migration was studied at pH 4.1. The migration order from MEKC experiments was compared to the peptide hydrophobicity calculated from reversed-phase HPLC-derived hydrophobicity coefficients. Although relative peptide hydrophobicity was, in general, positively correlated with effective electrophoretic mobility, a tryptophan-containing analog showed only weak interaction with micelles compared to the less hydrophobic peptides. The enkephalins studied were zwitterionic in character from pH 3 to 8, and their migration as a function of pH under MEKC conditions demonstrated that electrostatic forces were at least as important as hydrophobic interactions in pentapeptide-micelle complexation.

Sensitive absorbance detection method for capillary electrophoresis based on laser wave-mixing

Forward-scattering four-wave mixing is demonstrated as a sensitive absorbance detection method for capillary electrophoresis, using an argon ion laser operating at 457.9 nm. Since this four-wave mixing laser technique utilizes only two input laser beams, it offers important advantages, including ease of optical alignment, high wave-mixing efficiency and low excitation power requirements. In addition, since the analytical signal is a laser-like coherent beam, highly efficient optical signal detection can be performed with minimum optical background noise. Excellent detection sensitivity and short absorption path lengths, and hence, small detector probe volumes, are some of the useful features this absorbance detection method offers for on-column detection of both fluorescing and non-fluorescing analytes in capillary electrophoresis and liquid chromatography. Preliminary "detected" concentration detection limit of 8.5.10(-8) M, mass detection limit of 13 amol and an absorbance-unit detection limit of 1.35.10(-5) AU are determined for dabsyl-glycine using this absorbance detection method.

Investigation of a pulsed-laser thermo-optical absorbance detector for the determination of food preservatives separated by capillary electrophoresis

Thermo-optical absorbance (TOA) detection using a KrF excimer waveguide laser for detection of benzoic acid, dehydroacetic acid and sorbic acid separated by capillary electrophoresis (CE) was studied. Detection limits were, on average, ten times better than those for on-column UV absorbance methods with CE, and two or more times better than those for UV absorbance with HPLC. The influence of increased laser power on TOA detection sensitivity was found to be strong for benzoic and dehydroacetic acids but quite weak for sorbic acid. It was discovered that photoisomerization of sorbic acid (2,4-hexadienoic acid) occurred readily in the detection volume at moderate laser powers (P(ave) = 3 mW) and increased with slow electroosmotic flows (< 6 cm/min). The TOA method described here shows improved detection sensitivity for CE analyses of compounds having only weak absorptivities (< 5% of maximum) at lambda = 248 nm, and thus demonstrates its utility for determination of a variety of analytes in a single separation.