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Synthetic oligonucleotides have become increasingly popular as a result of the recent discovery of ribonucleic acid interference (RNAi), a natural process for silencing gene expression. As biomedical researchers evaluate the [...]

To optimize separations in capillary electrophoresis, it is important to control the electroosmotic mobility of the running buffer and the factors that affect it. Through the application of a site-dissociation--site-binding model, we demonstrated that the electroosmotic mobility could be controlled qualitatively and quantitatively by the parameters related to the physical and chemical properties of the running buffer: pH, cation valence, ionic strength, viscosity, activity, and dissociation constant. Our study illustrated that the logarithm of the number of apparent silanol sites on a fused-silica surface has a linear relationship with the pH of a buffer solution. The extension of the chemical kinetics approach allowed us to obtain the thickness of the electrical double layer when multivalent inorganic cations are present with monovalent cations in a buffer solution, and we found that the thickness of the electrical double layer does not depend on the charge of anions. The general equation to predict the electroosmotic mobility suggested here also indicates the increase of electroosmotic mobility with temperature. The general equation was experimentally verified by three buffer scenarios: (i) buffers containing only monovalent cations; (ii) buffers containing multivalent inorganic cations; and (iii) buffers containing cations and neutral additives. The general equation can explain the experimental observations of (i) a maximum electroosmotie mobility for the first scenario as the pH was varied at constant ionic strength and (ii) the inversion and maximum value of the electroosmotic mobility for the second scenario when the concentration of divalent cations was varied at constant pH. A good agreement between theory and experiment was obtained for each scenario.

The effect of liquid chromatographic separation on matrix-related signal suppression in electrospray ionization mass spectrometry (LC-ESI-MS) was investigated. A method incorporating on-line two-dimensional liquid chromatography mass spectrometry (LC/LC-MS) was developed to compensate for matrix effects and signal suppression in qualitative and quantitative analysis. The LC/LC-MS(MS) approach was successfully applied for single-component and multicomponent analysis in a variety of complex matrixes. It was demonstrated that matrix-related signal suppression could be induced solely by (i) column overload, (ii) matrix component--nalyte coelution, or a combination of each. Application of on-line orthogonal LC/LC separations can be effective in reducing both causes of matrix-related signal suppression effects i.e., column overload and matrix-analyte coelution for a variety of L[C.sup.n]/M[S.sup.n] applications.

One of the limitations that has restricted the applicability of micellar liquid chromatography (MLC) is the weak eluting power of micellar mobile phases compared to conventional hydro-organic mobile phases used in reversed-phase liquid chromatography. This may be the result of Donnan or steric exclusion of the micelles from the pores of the stationary phase, within which nearly all (>99%) of the stationary phase resides and the analytes spend most of their time. To determine whether wide-pore stationary phases would overcome this limitation in MLC, several C8 and C18 stationary phases ranging from 100 to 4000 Angstroms were investigated using a diverse set of test solutes and micellar solutions of anionic, neutral, and cationic surfactants as mobile phases. With the larger pore size stationary phases, the eluting power of the MLC mobile phases was enhanced with all surfactant types, the greatest effect being with the neutral surfactant. Differences in retention behavior were observed between various solute types and between the C8 and C18 stationary phases. These differences appear to be related to the relative hydrophobicity of the solutes and to differences in the surfactant-modified stationary phases. Partitioning behavior of representative solutes on the large-pore C8 and C18 columns was shown to follow the three-phase partitioning model for MLC. Methylene group selectivity data showed only minor differences in the stationary-phase characteristics between the small- and large-pore size C18 columns. The true eluting power of micellar mobile phases was revealed with wide-pore stationary phases and was demonstrated by the separation and elution of an extended series of alkylphenones on C18 columns.

Two-dimensional liquid chromatography (2DLC) is an increasingly popular technique which has the potential to provide a far more detailed separation and characterization of alcohol ethoxylates (AE) than has been shown by previously utilized separation techniques. The AE are unique in that these molecules have distributions in both alkyl and ethylene oxide chain lengths. In this paper, we compare the single-column techniques of open-tubular SFC, normal- and reversed-phase HPLC, and the multiple column technique of 2DLC in terms of the efficacy of separation and characterization of the alkyl and ethoxylate distributions in a select group of AE. The combination of normal- and reversed-phase HPLC in a 2DLC system accomplishes the simultaneous alkyl and ethylene oxide distribution analysis. The advantage of using 2DLC over one-dimensional chromatographic techniques is clearly demonstrated in the increased selectivity resulting in the ability to produce the ethylene oxide distributions of each alkyl component in an AE. In addition, 2DLC chromatograms are easier to interpret due to ordering of the chromatograms.

A review of published literature on liquid chromatography for the period Oct 1995 to Oct 1997 is presented. The papers covered topics on peak purity, deconvolution, solute retention, mobile-phase studies, capillary electrochromatography, support activation and affinity ligand immobilization. Papers on stationary-phase studies, ion chromatography, ion pairing, optical isomers and postcolumn derivatization were also included in the review.

In 'comprehensive' two-dimensional liquid chromatography, the column effluent from the first separation system (the first dimension) is sequentially sampled by the second dimension separation system. The total analysis time is largely determined by the speed of the second dimension separation system; the most retained component must elute before the least retained component of the next second dimension separation. Optimization of multidimensional separation systems requires that one understand the relationship between system resolution and the number of second dimension samples across a first dimension peak. In this paper, we study the theoretical and experimental aspects of this sampling process. To obtain high two-dimensional resolution, each peak in the first dimension should be sampled at least three times into the second dimension when the sampling is in-phase. ff the sampling is maximally out of phase, there should be at least four samples per peak for high-fidelity separation. The sensitivity of the resolution with respect to the sampling phase is discussed in detail and shown to be insignificant when four or greater samples are taken across the first dimension peak width. These results suggest optimal criteria for method development with multidimensional chromatography.

The electrokinetic chromatography (EKC) of a novel mixed surfactant system consisting of oppositely charged surfactants, sodium dodecyl sulfate (SDS) and n-dodecyltrimethylammonium bromide (DTAB), was investigated. The chromatographic characteristics of large liposome-like spontaneous vesicles and rodlike mixed micelles formed from the mixture were explored and compared with those of SDS micelles. Separations of a series of n-alkylphenones showed that the spontaneous vesicles provided about a 2 times wider elution window than SDS micelles. Both vesicle and mixed micelle systems were found to provide larger methylene selectivity than SDS. The different elution order of a group of nitrotoluene geometric isomers with DTAB/SDS spontaneous vesicles and SDS micelles pseudostationary phases suggested the possibility of different separation mechanisms with these two systems. Comparisons of polar group selectivity, retention, and efficiency were made between vesicles, mixed micelles, and SDS micelles. The correlation between the logarithms of the retention factors (log k[prime]) and octanol-water partition coefficients (log [P.sub.ow]) for a group of 20 neutral compounds was also studied with DTAB/SDS vesicles. Spontaneous vesicles have great potential as a pseudostationary phase in electrokinetic chromatography.

A fundamental review of liquid chromatography literature addresses major changes for the time period of Oct. 1993 to Oct. 1995. Most publications are gathered from on-line CAS database with additions from personal data gathering by the reviewers. The subject matter is generally treated in Scott's Liquid Chromatography for the Analyst, Riley's Pharmaceutical and Biomedical Applications of Liquid Chromatography, and Subramanian's Process Scale Liquid Chromatography.

In micellar electrokinetic chromatography (MEKC), the pseudostationary phase used most often consists of sodium dodecyl sulfate (SDS) micelles. We investigated the effects of using different counterions, [Li.sup.+] and [K.sup.+], with the dodecyl sulfate micelles in order to ascertain the influence of the counterion on efficiency, selectivity, retention, elution range, and resolution in MEKC. A typically used concentration of 50 mM surfactant was employed for all of the surfactants studied. The effect of acetonitrile was also investigated. Due to the high Krafft point of the potassium dodecyl surfate (KDS), a minimum of 15% acetonitrile had to be used in order to solubilize the KDS surfactant monomers. Comparisons between the [Na.sup.+], [Li.sup.+], and [K.sup.+] counterions are made with 15 and 20% acetonitrile added, while comparisons between [Na.sup.+] and [Li.sup.+] are also made with 0 and 10% acetonitrile added. Operating currents were lowest with the LiDS system. The LiDS system provided efficiencies almost 2.5 times that generated by SDS, while the KDS system generated efficiencies much lower than both the SDS and LiDS micellar phases. The more hydrophobic test analytes exhibited the greatest enhancement in efficiency with the use of the LiDS micelles. Retention factors for the various test analytes dropped appreciably with the use LiDS and acetonitrile in comparison to the acetonitrile-modified SDS system in which retention factors appeared to stabilize with 15 and 20% acetonitrile. The KDS micelies provided the largest elution range followed by SDS and then LiDS.

The feasibility of increasing the elution range in micellar electrokinetic capillary chromatography (MEKC) was investigated using a nonionic/anionic mixed micellar system. The micellar 'pseudostationary' phase was comprised of a mixture of nonionic (poly(oxyethylene)(23) dodecanol) and anionic (sodium dodecyl sulfate) surfactants. Variation in poly(oxyethylene)(23) dodecanol (Brij 35, Aldrich) concentration affected the electroosmotic and micellar electrophoretic mobilities differently. Equal but opposite electroosmotic and micellar electrophoretic velocities were obtained at a specific Brij 35 concentration, thereby producing a 'stationary' micellar phase. Retention characteristics of a series of n-alkylphenone homologues suggested a nearly infinite elution range. Infinite elution ranges are possible at neutral pH (7.0) using deactivated fused-silica capillaries and at a pH of 6.2 using untreated fused-silica capillaries.

A review of selected literature on the chemistry of the separation process in liquid chromatography (LC), published during Dec. 1991-Dec. 1993, is presented. The review covers research and application papers involving normal-phase LC, reversed-phase LC, biopolymer separations, affinity chromatography, ion chromatography, secondary chemical equilibria, microcolumn and opentubular LC, trace analysis and preparative LC.

Many scientific papers written between Dec 1989, and Dec 1991, indicated that the application of various theoretical and procedural concepts aimed to improve the efficiency and selectivity of liquid chromatography were proven to be successful. New materials and methodologies were tested on the different types of liquid chromatography. The improvements in the technique's performance made it useful and valuable in a number of compounds and analytical experiments.