Role of the carbohydrate moieties in chiral recognition on teicoplanin-based LC stationary phases

For this study, we used the macrocyclic antibiotic teicoplanin, a molecule consisting of an aglycone peptide "basket" with three attached carbohydrate (sugar) moieties. The sugar units were removed and the aglycone was purified. Two chiral stationary phases (CSPs) were prepared in a similar way, one with the native teicoplanin molecule and the other with the aglycone. Twenty-six compounds were evaluated on the two CSPs with seven RPLC mobile phases and two polar organic mobile phases. The compounds were 13 amino acids or structurally related compounds (including DOPA, folinic acid, etc.) and 13 other compounds (such as carnitine, bromacil, etc.). The chromatographic results are given as the retention, selectivity, and resolution factors along with the peak efficiency and the enantioselective free energy difference corresponding to the separation of the two enantiomers. The polarities of the two CSPs are similar. It is clearly established that the aglycone is responsible for the enantioseparation of amino acids. The difference in enantioselective free energy between the aglycone CSP and the teicoplanin CSP was between 0.3 and 1 kcal/mol for amino acid enantioseparations. This produced resolution factors 2-5 times higher with the aglycone CSP. Four non amino acid compounds were separated only on the teicoplanin CSP. Six and five compounds were better separated on the teicoplanin and aglycone CSPs, respectively. Although the sugar units decrease the resolution of alpha-amino acid enantiomers, they can contribute significantly to the resolution of a number of non amino acid enantiomeric pairs.     

Chiral stationary phase based on a biostable L-RNA aptamer

An immobilized anti-L-arginine d-RNA aptamer, used as a target-specific chiral stationary phase (CSP), was found to be very quickly degraded by RNases under usual chromatographic utilization and storage. To overcome this severe limitation for a practical use, a CSP based on the L-RNA aptamer, that is, the mirror image of the D-RNA aptamer, was created. It was shown that this mirror-image approach was a very simple and powerful strategy to develop a highly stable stationary phase due to the intrinsic insensitivity of l-RNA to the RNase degradation. In addition, such an approach allowed one to reverse the enantiomer elution order relative to that obtained with the corresponding d-RNA CSP.     

Direct high-performance liquid chromatographic separation of peptide enantiomers: study on chiral recognition by systematic evaluation of the influence of structural features of the chiral selectors on enantioselectivity

All-R/all-S enantiomers of oligoalanines (Ala(n), n = 1-10) with N-terminal protection group have been separated by HPLC on chiral stationary phases based on various cinchona alkaloid selectors. Structure-enantioselectivity relationships derived by extensive selector structure optimization provided insights into binding mechanisms and chiral recognition. Their interpretation was supported by X-ray crystal structures of amino acid and dipeptide, respectively, in complex with chiral selector. Optimized selectors have bulky elements representing steric barriers and deep binding pockets that afforded very high enantioselectivities; e.g., for the all-R and all-S enantiomers of N-(3,5-dinitrobenzoyl)alanylalanine, an alpha-value of 20.0 (corresponding to deltadeltaG of -7.43 kJ/mol) was obtained with a chiral stationary phase based on 6'-(neopentoxy)-9-O-tert-butylcarbamoylcinchonidine. Further, a chiral stationary phase based on 1,4-bis(9-O-quinidinyl)phthalazine was able to distinguish between the all-R and all-S enantiomers of hepta- to decaalanine peptides with enantioselectivity values between 1.8 and 1.9, corresponding to deltadeltaG of -1.46 and -1.59 kJ/mol, respectively.     

Use of a modified cyclodextrin host for the enantioselective detection of a halogenated diether as chiral guest via optical and electrical transducers

In an alkaline rebreathing circuit, the inhalation anesthetic sevoflurane degrades into at least two products, one of them being the chiral halodiether 1,1,1,3,3-pentafluoro-2-(fluoromethoxy)-3-methoxypropane (halodiether B). Using octakis(3-O-butanoyl-2,6-di-O-n-pentyl)-y-cyclodextrin (Lipodex E) as chiral host diluted in the polysiloxane PS255, an exceptional large chiral separation factor alpha of 9.7 at 30 degrees C was found for halodiether B by capillary gas chromatography (cGC). Hence, the interaction of the single enantiomers and the racemic mixture of the halodiether B with Lipodex E was selected as a model system to study the enantioselective recognition by thickness shear mode resonators (TSMR), surface acoustic wave sensors, surface plasmon resonance (SPR), and reflectometric interference spectroscopy. Further investigations of the recognition process by using chemical sensors confirmed the preferential enrichment of the S-enantiomer resulting in 9-fold higher signals. Based on the distinction between enantioselective and nonenantioselective sorption, thermodynamic complexation constants of the single enantiomers with Lipodex E could be determined. The difference in Gibbs free energy -deltaE2,E1(deltaG) of the complexation of the enantiomers of halodiether B with pure Lipodex E was determined at 30 degrees C by TSMR and SPR to be 5.7 or 5.9 kJ/mol, respectively, agreeing well with that determined by cGC, i.e., 5.7 kJ/mol at 30 degrees C.     

Polar-liquid, derivatized cyclodextrin stationary phases for the capillary gas chromatography separation of enantiomers

A new class of hydrophilic, relatively polar liquid, cyclodextrin (CD) derivatives have been used as highly selective chiral stationary phases (CSPs) for capillary gas chromatography (GC). Several possible requirements for liquidity in CD derivatives are discussed. O-(S)-2-Hydroxypropyl derivatives of alpha-, beta-, and gamma-cyclodextrins were synthesized, exclusively characterized, permethylated, and evaluated for enantioselectivity. Seventy pairs of enantiomers were resolved. They represent a wide variety of structural types and classes of compounds including chiral alkyl amines, amino alcohols, epoxides, pyrans, furans, sugars, diols, esters, ketones, bicyclic compounds, alcohols, and so on. Many of these compounds were not aromatic and cannot be resolved on any known liquid chromatographic CSP. Often, these enantiomers had far less functionality than required for LC separation. General properties of these CSPs as well as possible insights into the separation mechanism are discussed.     

Enantiomeric separation using an l-RNA aptamer as chiral additive in partial-filling capillary electrophoresis

In this paper, we report the chiral resolution of arginine using an anti-arginine l-RNA aptamer chiral selector in partial-filling CE. The effects of the capillary temperature, sample load, and aptamer plug length on the enantiomeric separation were assessed. Very high chiral resolving capability was observed at low or moderate capillary temperatures (the target peak being not detected in the separation window), whereas the practical chiral resolution was achieved only at high enough temperatures (50-60 degrees C). Over this high-temperature range, the electrophoretic behavior of the target enantiomer appeared to result from a combination of binding site heterogeneity, slow desorption kinetics, and concentration overload of aptamer binding sites. From additional thermal UV melting experiments, three RNA conformations were identified for the 50-60 degrees C temperatures. It was suggested that the presence of these different RNA conformations was a plausible source of the binding site heterogeneity.     

Empirical procedure that uses molecular structure to predict enantioselectivity of chiral stationary phases.

A total of 121 racemic compounds were separated in the normal-phase mode on a (S)-(1-naphthylethyl)carbamoylated beta-cyclodextrin (S-NEC-beta-CD) bonded phase and 74 on the R equivalent (R-NEC) chiral stationary phase (CSP). All compounds are of the type that have four substituents on a stereogenic center, rather than an "axis of chirality". It is shown that the binary solvent pair used as the mobile phase has a significant influence on chiral recognition. However, the proportions of the components of a specific pair have little effect. From the results, the individual contributions to chiral recognition by these CSPs were estimated for 81 different substituents of the stereogenic center. Varying the arrangement of these 81 substituents could produce over 1.6 million compounds. Hydrogen was chosen as the reference substituent and was assigned a 0 cal/mol free energy. The chiral recognition increased when sp2-hybridized carbons were connected to the stereogenic center. Conversely, sp3-hybridized carbons decreased the enantioselectivity. Amido groups increased the chiral recognition, especially when associated with pi-acid (3,5-dinitrobenzoyl) or pi-basic (naphthyl) groups. This approach does not allow one to know which enantiomer elutes first. However, the "substituent energy" list for chiral compounds can be used to obtain an estimated value for the enantioselectivity of a compound by adding the energy contributions of the four substituents connected to the stereogenic center. In this way one can predict a priori whether or not a compound will separate on a CSP and estimate its separation factor (alpha). Theoretically, this approach can be used for most CSPs, provided a sufficient data base is generated on them.     

Microcalorimetry of chiral surfactant-cyclodextrin interactions

The interactions of the chiral surfactants taurodeoxycholate (TDOCA) and deoxycholate (DOCA) with a range of cyclodextrins in aqueous solution have been investigated by isothermal titration microcalorimetry. In the presence of β-cyclodextrin, the apparent critical micelle concentration (cmc) of taurodeoxycholate is increased, and the enthalpy of demicellization decreased, in a manner consistent with 1:1 complexation of TDOCA with β-CD at low concentrations. There is no evidence for direct interaction of cyclodextrins with surfactant micelles. This is confirmed by more direct binding titrations. Below the cmc, TDOCA forms 1:1 host-guest complexes with β-cyclodextrin (ΔH°(bind) = -32 kJ mol(-)(1), K(diss) = 0.38 mM; 25 °C, pH 7), methyl-β-cyclodextrin (ΔH(bind) = -13 kJ mol(-)(1), K(diss) = 0.36 mM), hydroxypropyl-β-cyclodextrin (ΔH°(bind) = -12 kJ mol(-)(1), K(diss) = 0.51 mM), and γ-cyclodextrin (ΔH°(bind) = -7.3 kJ mol(-)(1), K(diss) = 0.08 mM), but not with the smaller α-cyclodextrin. At higher cyclodextrin concentrations, the calorimetric binding data are more ambiguous, suggesting 2:1 cyclodextrin/TDOCA complexation. Similar results are found with DOCA, though experiments here are limited by the tendency of DOCA to form gels in aqueous buffers. Enhanced chromatographic or electrophoretic chiral resolution observed in mixed chiral surfactant/cyclodextrin phases could be the result of increased solubility and/or the multiplicity of chiral complexes in such systems.     

Evaluation of a vancomycin chiral stationary phase in capillary electrochromatography using polar organic and reversed-phase modes

A vancomycin chiral stationary phase (CSP) was fully evaluated in capillary electrochromatography (CEC) in reversed-phase and polar organic modes for a number of racemic pharmaceutical compounds. High efficiency and resolution values were obtained for a number of compound classes including thalidomide in both the polar organic mode (190000 plates meter(-1) and Rs = 13.8) and reversed-phase mode (125000 plates meter(-1) and Rs = 13.0). Experimental parameters, including organic modifier, organic solvent ratio, ionic strength, pH, temperature, and voltage, were examined in both the aqueous and nonaqueous modes to deduce their effect on the resultant EOF, retention times, resolution, and efficiency of chiral separations. All results were consistent with and found to be a combination of what is known from existing literature on CEC theory and experience obtained with macrocyclic antibiotic CSPs in LC. Column stability was excellent, and each column packed was found to offer repeatable separations even when switching from the aqueous to the nonaqueous mode.     

Combining the enantioselectivities of L-valine diamide and permethylated beta-cyclodextrin in one gas chromatographic chiral stationary phase

An L-valine diamide chiral selector was attached to a polysiloxane through a long hydrocarbon spacer giving rise to a chiral stationary phase (CSP), Chirasil-Val-C11. The enantioselective properties of this readily accessible diamide CSP under gas chromatographic conditions were found to be similar to that of the commercially available Chirasil-Val CSP prepared by a polymer-analogous route. A new binary CSP, Chirasil-DexVal-C11, was synthesized by means of simultaneous attachment of both the L-valine diamide and permethylated beta-cyclodextrin selectors to a polysiloxane using platinum-catalyzed hydrosilylation, thereby overcoming the immiscibility problem known for Chirasil-Val and Chirasil-Dex. This binary CSP retained both the enantioselectivity of Chirasil-Val-C11 toward alpha-amino acid derivatives and the unsurpassed enantioselectivity of Chirasil-Dex toward underivatized chiral alcohols, ketones, and hydrocarbons. Furthermore, it was shown that the presence of the cyclodextrin selector in Chirasil-Val-C11 significantly improved the enantioseparation of proline, which represented a problematic amino acid on diamide CSPs.     

Distinction of amino Acid enantiomers based on the basicity of their dimers

Mixtures of several amino acid pairs, in all four chiral combinations, were studied. The protonated trimers (A(2)BH(+)) fragment, forming ABH(+) and A(2)H(+) dimers. Abundance ratios of these fragments were measured in the mass-analyzed ion kinetic energy spectra of the trimers. These were found to depend on the stereochemistry (homo- or heterochiral form) of the ABH(+) dimer. The results were evaluated using the kinetic method, and the chiral discrimination was related to a difference in gas-phase basicity (GB) between the homo- and the heterochiral dimers. Four amino acid pairs (proline-tryptophan, phenylalanine-alanine, phenylalanine-proline, and phenylalanine-valine) were studied. Chiral discriminations were observed in all cases, relating to 0.4-4 kJ/mol differences in GB. The technique described here can generally be used to study enantiomers by mass spectrometry and is capable of reliably distinguishing energy differences as small as 0.2 kJ/mol in cluster ions.     

On the behavior of indole-containing species sequestered within reverse micelles at sub-zero temperatures

We report on the effects of temperature (+30 to -100 degrees C) on the fluorescence from N-acetyl tryptophanamide (NATA) and human serum albumin (HSA) sequestered within Aerosol-OT (AOT) reversed micelles. NATA reports simultaneously from the polar and non-polar side of the reverse micelle interface. As the sample temperature decreases, the relative fraction of NATA molecules associated with the polar side increases. This redistribution process is characterized by DeltaH = -14.8 +/- 0.6 kJ/mol and DeltaS = -54 +/- 2 J/(K mol). The activation energy for thermal quenching (E(a,TQ)) associated with the polar side NATA molecules is 6.7 kJ/mol before the micelles have shed water and 1.0 kJ/mol after water shedding (below approximately -20 degrees C). The time-resolved fluorescence intensity decay for tryptophan-214 in HSA is triple exponential. We suggest that these lifetimes arise from three indole residue conformations in equilibrium. Cooling the sample causes a freezing-in of the least quenched conformer; the other conformers are frozen out. The E(a,TQ) value for the shortest lifetime component is 6 kJ/mol. The E(a,TQ) for the long and intermediate lifetime components are equivalent (approximately 1.5 kJ/mol).     

Assay of trace D-amino acids in neural tissue samples by capillary liquid chromatography/tandem mass spectrometry

A sensitive chiral capillary HPLC-MS/MS method well suited for the determination of amino acid enantiomers in biological samples was developed. The method involved precolumn derivatization of the sample with 7-fluoro-4-nitrobenzoxadiazole (NBD-F). After derivatization, NBD-amino acids were stacked on a C18 reversed-phase extraction microcolumn, thus enriching and cleaning up the analytes. Various chiral stationary phases (CSPs) including cyclodextrin-bonded silica, Pirkle-type, vancomycin, and teicoplanin-bonded silica particles were evaluated for resolving NBD-F tagged amino acid enantiomers with mobile phases compatible with MS detection. It was found that only teicoplanin aglycon CSP provided sufficient resolution of NBD-Asp and NBD-Ser enantiomers to quantify trace levels of D-Asp and D-Ser in tissue samples. MS/MS detection of NBD-amino acid derivatives was very sensitive and selective. The high selectivity allowed the use of a stable isotope-labeled analyte analogue (i.e., L-aspartic acid-2,3,3-d3) as internal standard for the quantitation to improve assay reproducibility and reliability. Neural tissue samples dissected from rat brain and the central nervous system (CNS) of Aplysia californica, a widely used neuronal model, were analyzed to determine the chirality of glutamic acid (Glu), aspartic acid (Asp), and serine (Ser). The former two are major excitatory amino acids in the brain, and the last one has been recently identified as a neuromodulator. Both D-Ser and D-Asp were detected in rat brain. While the D-Asp level decreased rapidly through the developmental stages of the rat, the D-Ser level increased steadily from 82.3 microg/g of wet tissue in 3-day prenatal rats to 241.3 microg/g of wet tissue in 90-day-old rats. Interestingly, no D-Ser was detected in the CNS of Aplysia, a "primitive" invertebrate. However, the D-Asp level in this animal was found to be high. In a particular connective nerve sample, D-Asp was at 323.2 microg/g of wet tissue and constituted 60.2% of total Asp. D-Glu was not detected either in rat brain or in Aplysia's CNS.     

On-bead combinatorial approach to the design of chiral stationary phases for HPLC

A library of 36 L-amino acid anilides, which are potential selectors for chiral HPLC, was synthesized in solution and attached to functionalized macroporous polymer beads. The best selector from the library was identified by a deconvolution process using the HPLC separation of several racemic N-(3,5-dinitrobenzoyl)-alpha-amino acid alkylamides as a probe. In each deconvolution step, a series of chiral stationary phases (CSPs) containing a subset of the amino acid anilide selector library was screened for enantioselectivity. After the best CSP was chosen, the library was further deconvoluted until the single best selector was found. The highest selectivity was obtained with a L-proline-1-indananilide that exhibited alpha values up to 23 under normal-phase HPLC conditions. In addition, six CSPs were prepared using individual selectors from the library, and screening results indicate that the deconvolution process indeed led to the most selective receptor.     

Surface-immobilized peptide aptamers as probe molecules for protein detection

We demonstrate the use of surface-immobilized, oriented peptide aptamers for the detection of specific target proteins from complex biological solutions. These peptide aptamers are target-specific peptides expressed within a protein scaffold engineered from the human protease inhibitor stefin A. The scaffold provides stability to the inserted peptides and increases their binding affinity owing to the resulting three-dimensional constraints. A unique cysteine residue was introduced into the protein scaffold to allow orientation-specific surface immobilization of the peptide aptamer and to ensure exposure of the binding site to the target solution. Using dual-polarization interferometry, we demonstrate a strong relationship between binding affinity and aptamer orientation and determine the affinity constant KD for the interaction between an oriented peptide aptamer ST(cys+)_(pep9) and the target protein CDK2. Further, we demonstrate the high selectivity of the peptide aptamer STM_(pep9) by exposing surface-immobilized ST(cys+)_(pep9) to a complex biological solution containing small concentrations of the target protein CDK2.     

Sorption profile of Cd(II) ions onto beach sand from aqueous solutions

Sorption of traces of Cd(II) ions onto beach sand is investigated as a function of nature and concentration of electrolyte (10(-4) to 10(-2)M nitric, hydrochloric and perchloric acids, pH 2-10 buffers and deionized water), shaking time 5-40min, shaking speed 50-200strokes/min, dosage of sand (50-1000mg/15cm(3)), concentration of sorbate (1.04x10(-6) to 1.9x10(-4)M) and temperature (293-323K). Maximum sorption of Cd(II) ions (approximately 66%) is achieved from deionized water using 300mg/15cm(3) sand in 20min. The data are successfully tested by Langmuir, Freundlich and Dubinin-Redushkevich (D-R) sorption isotherms. The values for characteristic Langmuir constants Q=13.31+/-0.20micromol/g and of b=(6.56+/-0.53)x10(3)dm(3)/mol, Freundlich constants A=2.23+/-1.16mmol/g and 1/n=0.70+/-0.05 of (D-R) constants beta=-0.005068+/-0.000328kJ(2)/mol(2), X(m)=46.91+/-11.91micromol/g and energy E=9.92+/-0.32kJ/mol have been estimated. Kinetics of sorption has been studied by applying Morris-Weber, Richenberg and Lagergren equations. The sorption follows first order rate equation resulting 0.182+/-0.004min(-1) The thermodynamic parameters DeltaH=32.09+/-2.92kJ/mol, DeltaS=111.0+/-9.5J/molK and DeltaG=-1.68+/-0.02kJ/mol are evaluated. The influence of common ions on the sorption of Cd(II) ions is also examined. Some common ions reduce the sorption while most of the ions have very little effect. It can be concluded that beach sand may be used as an alternative for the expensive synthetic sorbents.     

Characterization of a thermally induced irreversible conformational transition of amylose tris(3,5-dimethylphenylcarbamate) chiral stationary phase in enantioseparation of dihydropyrimidinone acid by quasi-equilibrated liquid chromatography and solid-state NMR

A thermally induced irreversible conformational transition of amylose tris(3,5-dimethylphenylcarbamate) (i.e., Chiralpak AD) chiral stationary phase (CSP) in the enantioseparation of dihydropyrimidinone (DHP) acid racemate was studied for the first time by quasi-equilibrated liquid chromatography with cyclic van't Hoff and step temperature programs and solid-state ((13)C CPMAS and (19)F MAS) NMR using ethanol and trifluoroacetic acid (TFA)-modified n-hexane as the mobile phase. The conformational transition was controlled by a single kinetically driven process, as evidenced by the chromatographic studies. Solid-state NMR was used to study the effect of the temperature on the conformational change of the solvated phase (with or without the DHP acid enantiomers and TFA) and provided some viable structural information about the CSP and the enantiomers.     

Signaling aptamer/protein binding by a molecular light switch complex

A novel method of signaling aptamer/protein binding for aptamer-based protein detection has been developed using a molecular light switch complex, [Ru(phen)2(dppz)]2+. The method takes advantage of the sensitive luminescence signal change of [Ru(phen)2(dppz)]2+ intercalating to the aptamer upon protein/aptamer binding. A 37-nt DNA aptamer against immunoglobulin E (IgE) was first tested as a model system. The luminescence of the [Ru(phen)2(dppz)]2+/IgE aptamer decreased with the increase of IgE. By monitoring the luminescence change, we were able to detect the binding events between the aptamer and IgE for IgE quantitation in homogeneous solutions as well as in serum. The assay was highly selective and sensitive with a detection limit of 100 pM for IgE. This new method is very simple and without the need for the covalent coupling of fluorophores to aptamers. The generalizability of the method was demonstrated by the direct detection of two other proteins, oncoprotein platelet derived growth factor-BB (PDGF-BB) using its DNA aptamer and alpha-thrombin using its RNA aptamer. This new approach is expected to promote the exploitation of aptamer-based biosensors for protein assays in biochemical and biomedical studies.     

Absolute configuration modulation attenuated total reflection ir spectroscopy: an in situ method for probing chiral recognition in liquid chromatography

A method to selectively probe the different adsorption of enantiomers at chiral solid-liquid interfaces is applied, which combines attenuated total reflection infrared spectroscopy and modulation spectroscopy. The spectral changes on the surface are followed while the absolute configuration of the adsorbate is changed periodically. Demodulated spectra are calculated by performing a subsequent digital phase-sensitive data analysis. The method is sensitive solely to the difference of the interaction of the two enantiomers with the chiral surface, and the small spectral changes are amplified by the phase-sensitive data analysis. Its potential is demonstrated by investigating an already well-studied system in liquid chromatography, namely, the enantiomer separation of N-3,5-dinitrobenzoyl-(R,S)-leucine (DNB-(R,S)-Leu) using tert-butylcarbamoyl quinine (tBuCQN) as the chiral selector immobilized on the surface of porous silica particles. The performed experiments and density functional theory calculations confirm an interaction model that was proposed earlier based on solution NMR and XRD in the solid state. It emerges that the ionic interaction is the strongest one, but the main reason for the potential for enantioseparation of the chiral stationary phase (CSP) is the distinct formation of a hydrogen bond of the (S)-enantiomer with the chiral selector. This H-bond is established between the amide N-H of DNB-(S)-Leu with the carbamate C=O of the CSP. The (R)-enantiomer instead shows no specific hydrogen bonds. Only the unspecific ionic bonding between the protonated quinine part of the tBuCQN and the carboxylate of the DNB-(R)-Leu (holds also for DNB-(S)-Leu) is observed.     

Kinetic Resolution of d,l-Amino Acids Based on Gas-Phase Dissociation of Copper(II) Complexes

Chiral recognition of d- and l-amino acids is achieved in the gas phase on the basis of the kinetics of competitive fragmentations of trimeric Cu(II)-bound complexes. The singly charged copper(II)-amino acid trimeric cluster ions [A(2)BCu(II) - H](+) dissociate to form [A(2)Cu(II) - H](+) and [ABCu(II) - H](+) upon collision-induced dissociation (CID) in a quadrupole ion trap. The abundance ratios of these fragments depend strongly on the stereochemistry of the ligands in the [A(2)BCu(II) - H](+) complex ion. The kinetic method was used to calculate relative Cu ion affinities (ΔCu(II)') for homo- and heterochiral copper(II)-bound dimeric cluster ions as the indicator of chiral discrimination. Six amino acids of four different types showed chiral distinctions which ranged from 0 to 6.5 kJ/mol in terms of values of ΔCu(II)' with abundance ratios, referenced to the other enantiomer, ranging from 1 to 9.2. Amino acids with aromatic substituents displayed the largest chiral distinction, which correlates well with reported chromatographic results. The methodology presented here provides a sensitive means to study enantiomers by mass spectrometry, and initial results show that it is applicable to measurement of enantiomeric excess.