Effective use of molecular recognition in gas sensing: results from acoustic wave and in situ FT-IR measurements.

To probe directly the analyte/film interactions that characterize molecular recognition in gas sensors, we recorded changes to the in situ surface vibrational spectra of specifically functionalized surface acoustic wave (SAW) devices concurrently with analyte exposure and SAW measurement of the extent of sorption. Fourier transform infrared external-reflectance spectra (FT-IR-ERS) were collected from operating 97-MHz SAW delay lines during exposure to a range of analytes as they interacted with thin-film coatings previously shown to be selective: cyclodextrins for chiral recognition, nickel camphorates for Lewis bases such as pyridine or organophosphonates, and phthalocyanines for aromatic compounds. In most cases where specific chemical interactions--metal coordination, "cage" compound inclusion, or pi-stacking--were expected, analyte dosing caused distinctive changes in the IR spectra, together with anomalously large SAW sensor responses. In contrast, control experiments involving the physisorption of the same analytes by conventional organic polymers did not cause similar changes in the IR spectra, and the SAW responses were smaller. For a given conventional polymer, the partition coefficients (or SAW sensor signals) roughly followed the analyte fraction of saturation vapor pressure. These SAW/FT-IR results support earlier conclusions derived from thickness-shear mode resonator data.     

ATR-FTIR spectroscopic analysis of sorption of aqueous analytes into polymer coatings used with guided SH-SAW sensors

Attenuated total internal reflectance Fourier transform infrared (ATR-FTIR) spectroscopy was used for the investigation of sorption of aqueous solutions of analytes into polymer coatings. A series of simple model polymers, such as poly(dimethylsiloxane), poly(epichlorhydrin), and poly(isobutylene), and films and analytes, such as aqueous solutions of ethylbenzene, xylenes, toluene, and nitrobenzene, were used to evaluate the use of ATR-FTIR spectroscopy as a screening tool for sensor development. The ratios of integrated infrared absorption bands provided a simple and efficient method for predicting trends in partition coefficients. Responses of polymer-coated guided shear horizontal surface acoustic wave (SH-SAW) sensor platforms to the series of analytes, using polymer coatings with similar viscoelastic properties, were consistent with ATR-FTIR predictions. Guided SH-SAW sensor responses were linear in all cases with respect to analyte concentration in the tested range. Comparison of ATR-FTIR data with guided SH-SAW sensor data identifies cases where mass loading is not the dominant contribution to the response of the acoustic wave sensor. ATR-FTIR spectra of nitrobenzene, coupled with computational chemistry, provided additional insight into analyte/polymer interactions.     

Real-time detection of organic compounds in liquid environments using polymer-coated thickness shear mode quartz resonators

The selection of sensitive coatings is a critical task in the design and implementation of chemical sensors using coated thickness shear mode quartz crystal resonators (QCRs) for detection in liquid environments. This design or selection is performed through a study of the sorption process in terms of the partition coefficients of the analytes in the coatings. The partition coefficient, which is controlled by the chemical and physical properties of the coating materials, determines the inherent selectivity and sensitivity toward analyte molecules. The selection of the coatings is logically determined by the interactions between coating and target analyte molecules, but can also be made through a systematic variation of the coating's properties. The determination of the partition coefficients is only accurate if all contributions to the total measured frequency shifts, deltafs, of the coated QCR can be established. While mass loading is often assumed to be the dominant factor used in determining partition coefficients, viscoelastic effects may also contribute to deltafs. Both the effect of viscoelastic properties and the effect of mass loading on the sensor responses are investigated by using a network analyzer and oscillator circuit and by characterizing the total mechanical impedance of the loaded sensor. Different types of coatings including rubbery and glassy polymers are investigated, and the targeted analytes include classes of polar compounds (methanol), nonpolar compounds (toluene, xylenes), and chlorinated hydrocarbons (trichloroethylene, tetrachloroethylene, etc). It is seen that changes in viscoelastic properties due to analyte sorption may be significant enough to place the sensor in the nongravimetric regime. However, for most applications involving the detection of relatively low concentrations of organic compounds and the use of acoustically thin films, changes in the complex shear modulus of the coatings contribute less than 5% of the total shift in the series resonant frequency, depending on the coating. In that case, the measured deltafs and, hence, the calculated approximate classification and selection of the coatings for operation in a complex solution of water/analyte molecules.     

Polypyrrole-coated capillary coupled to HPLC for in-tube solid-phase microextraction and analysis of aromatic compounds in aqueous samples

In-tube solid-phase microextraction (SPME) based on a polypyrrole (PPY)-coated capillary was investigated for the extraction of aromatic compounds from aqueous solutions. The PPY-coated capillary was coupled on-line to HPLC that was programmed with an autosampler to achieve automated in-tube SPME and HPLC analysis. Three groups of aromatics, including both polar and nonpolar compounds, were examined. The results demonstrated that the PPY coating had a higher extraction efficiency than the currently used commercial capillary coatings, especially for polycyclic aromatic compounds and polar aromatics due to the increasing pi-pi interactions, interactions by polar functional groups, and hydrophobic interactions between the polymer and the analytes. In addition to the functional groups in the PPY coating, which contributed to the higher extraction efficiency and selectivity toward analytes, the coating's porous surface structure,which was revealed by electron microscopy experiments, provided a high surface area that allowed for high extraction efficiency. It was found that the extraction efficiency and selectivity could be tuned by changing the coating thickness. The preliminary study of the extraction mechanism indicated that analytes were extracted onto the PPY coating mainly by an adsorption mechanism. The method was used for the extraction and analysis of both polar and nonpolar aromatics in aqueous samples.     

Theory and use of the pseudophase model in gas-liquid chromatographic enantiomeric separations

The theory and use of the "three-phase" model in enantioselective gas-liquid chromatography utilizing a methylated cyclodextrin/polysiloxane stationary phase is presented for the first time. Equations are derived that account for all three partition equilibria in the system, including partitioning between the gas mobile phase and both stationary-phase components and the analyte equilibrium between the polysiloxane and cyclodextrin pseudophase. The separation of the retention contributions from the achiral and chiral parts of the stationary phase can be easily accomplished. Also, it allows the direct examination of the two contributions to enantioselctivity, i.e., that which occurs completely in the liquid stationary phase versus the direct transfer of the chiral analyte in the gas phase to the dissolved chiral selector. Six compounds were studied to verify the model: 1-phenylethanol, alpha-ionone, 3-methyl-1-indanone, o-(chloromethyl)phenyl sulfoxide, o-(bromomethyl)phenyl sulfoxide, and ethyl p-tolylsulfonate. Generally, the cyclodextrin component of the stationary phase contributes to retention more than the bulk liquid polysiloxane. This may be an important requirement for effective GC chiral stationary phases. In addition, the roles of enthalpy and entropy toward enantiorecognition by this stationary phase were examined. While enantiomeric differences in both enthalpy and entropy provide chiral discrimination, the contribution of entropy appears to be more significant in this regard. The three-phase model may be applied to any gas-liquid chromatography stationary phase involving a pseudophase.     

Determination of air-to-water partition coefficients using automated multiple headspace extractions

Air-to-water partition coefficients are experimentally determined using a multiple headspace extraction procedure and an automated headspace cell. The approach is first validated with 2-butanone and then applied to a homologous series of methyl ketones. As adsorptions of the most hydrophobic compounds occurred in the sampling cell, technical improvements have been tested. This study represents the first attempt to overcome analyte adsorptions by studying and minimizing the effect of the cell's adsorption of hydrophobic analytes on the determination of their partition coefficients. The present method allows the measurement of several analytes at the same time, in the ppm range, without calibration, and with a limited manpower.     

Molecular dynamics study of chiral recognition for the whelk-O1 chiral stationary phase

In this article, we examine the docking of 10 analytes on the Whelk-O1 stationary phase. A proper representation of analyte flexibility is essential in the docking analysis, and analyte force fields have been developed from a series of B3LYP calculations. Molecular dynamics simulations of a representative Whelk-O1 interface, in the presence of racemic analyte and solvent, form the basis of the analysis of chiral selectivity. The most probable docking arrangements are identified, the energy changes upon docking are evaluated, and separation factors are predicted. From comparisons between the analytes, the mechanism of chiral selectivity is divided into contributions from hydrogen bonding, ring-ring interactions, steric hindrance, and molecular flexibility. We find that both hydrogen bonding and ring-ring interactions are necessary to localize the analyte within the Whelk-O1 cleft region. We also identify one docking mechanism that is often dominant and analyze the conditions that lead to alternate docking modes.     

Absorption of hydrophobic compounds into the poly(dimethylsiloxane) coating of solid-phase microextraction fibers: high partition coefficients and fluorescence microscopy images

The use of solid-phase microextraction with poly(dimethylsiloxane) (PDMS)-coated glass fibers for the extraction and analysis of hydrophobic organic analytes is increasing. The literature on this topic is characterized by large discrepancies in partition coefficients and an uncertainty of whether highly hydrophobic analytes are retained by absorption into the fiber coating or by adsorption to the fiber surface. We applied a new method, which minimizes the impact of experimental artifacts, to determine PDMS water partition coefficients of 17 hydrophobic analytes including chlorinated benzenes, PCBs, PAHs, and p,p'-DDE. These partition coefficients are several orders of magnitude higher than some reported values. Two observations strongly suggest that the retention of hydrophobic organic substances is governed by partitioning into the PDMS coating. (1) The partition coefficients are proportional with octanol/water partition coefficients. (2) The fluorescence of fluoranthene was observed to be homogeneously distributed within the polymer coating when studied by means of fluorescence microscopy. Implications of these findings for the application of solid-phase microextraction with respect to potential detection limits, with respect to biomimetic extraction, and with respect to measurements in multicompartment systems are discussed.     

Detection and discrimination capabilities of a multitransducer single-chip gas sensor system

The performance of a single-chip, three-transducer, complementary metal oxide semiconductor gas sensor microsystem has been thoroughly evaluated. The monolithic gas sensor system includes three polymer-coated transducers, a mass-sensitive cantilever, a thermoelectric calorimetric sensor, and an interdigitated capacitive sensor that are integrated along with all electronic circuits needed to operate these sensors. The system additionally includes a temperature sensor and a serial interface unit so that it can be directly connected to, for example, a microcontroller. Several multitransducer chips have been coated with various partially selective polymers and then have been exposed to different volatile organic compounds. The sensitivities of the three different polymer-coated transducers to defined sets of gaseous analytes have been determined. The obtained sensitivity values have then been normalized with regard to the partition coefficients of the respective analyte/polymer combination to reveal the transducer-specific effects. The results of this investigation show that the three different transducers respond to fundamentally different molecular properties, such as the analyte molecular mass (mass-sensitive), its dielectric coefficient (capacitive), and its sorption heat (calorimetric) so that correlations between the determined sensitivity values and the different molecular properties of the absorbed analytes could be established. The information as provided by the system, hence, represents a body of orthogonal data that can serve as input to appropriate signal processing and pattern recognition techniques to address issues such as the quantification of analytes in mixtures.     

Evanescent fiber-optic chemical sensor for monitoring volatile organic compounds in water

The transport of trichloroethylene, 1,1,1-trichloroethane, and toluene in aqueous solutions through a polydimethylsiloxane film was modeled using a Fickian diffusion model to fit data obtained from an evanescent fiber-optic chemical sensor (EFOCS). The resultant diffusion coefficients for these analytes were respectively 3 × 10(-)(7), 5 × 10(-)(7), and 1 × 10(-)(7) cm(2)/s. Inclusion of an interfacial conductance term, defined as the ratio of the mass transport coefficient across the polymer surface and the analyte diffusion coefficient in the polymer, was required to accurately model the data. It was determined that the interfacial conductance terms were generally of the same order of magnitude for the analytes examined, suggesting a constant transport mechanism for the analytes. Linear chemometric algorithms were used to model the EFOCS response to aqueous mixtures of the three analytes with individual analyte concentrations between 20 and 300 ppm. Both partial least-squares and principal component regression algorithms performed comparably on the calibration sets, with cross-validated root-mean-squared errors of prediction for trichloroethylene, 1,1,1-trichloroethane, and toluene of approximately 26, 29, and 22 ppm, respectively. The resultant prediction model was then used to determine analyte concentrations in an independent data set with comparable precision.     

Compound screening for the presence of the primary N-oxide functionality via ion-molecule reactions in a mass spectrometer

A mass spectrometry method is presented for the identification of compounds that contain the primary N-oxide functional group. This method utilizes a gas-phase ion-molecule reaction with dimethyl disulfide that rapidly and selectively derivatizes the protonated primary N-oxide functional group in a mass spectrometer to yield an ionic reaction product (with 31 Da higher mass than that of the protonated molecule) that is diagnostic for the presence of a primary N-oxide functionality. A variety of protonated analytes containing different functional groups were tested in Fourier transform ion-cyclotron resonance and triple quadrupole mass spectrometers to probe the selectivity of the reaction. Only molecules containing the protonated primary N-oxide functional group yielded the diagnostic reaction product; all other protonated molecules gave protonated dimethyl disulfide or no reaction products. The feasibility of this method for compound screening was tested by examining six analytes with the same molecular formula but different atom connectivity. The one analyte that contained the primary N-oxide functional group was readily differentiated from the other analytes.     

Transient signal analysis using complementary metal oxide semiconductor capacitive chemical microsensors

This work explores the possibility to discriminate analytes based on their nonequilibrium signals in polymer-coated capacitive chemical microsensors. The analyte uptake in the chemically sensitive polymer layers of 3-7-microm thickness has been analyzed using a diffusion model and the dynamic sensor response data. The shapes of the response profiles have been calculated analytically. Despite the simplifications in the model, the observed transient signal profiles could be described accurately. Comparison of the measured diffusion coefficients (on the order of 10(-12) m2/s) with literature values measured at similar concentration levels showed good agreement. Concentration-independent diffusion coefficients for several analyte/polymer combinations (poly(etherurethane)/all analytes; poly(epichlorohydrin)/alcohols) as well as slightly concentration-dependent diffusion coefficients (poly(epichlorohydrin)/toluene or ethyl cellulose/toluene) have been found in the investigated concentration range of tens to hundreds of pascals gas-phase partial pressure. The diffusion times of water and the first aliphatic monohydric alcohols in the polymers are strongly correlated to their molecular size. The discrimination of these substances based on dynamic sensor data of a single sensor could be demonstrated. In particular, the analysis of mixtures of analytes with similar chemical behavior (water/ethanol or methanol/ethanol) by means of analyzing the response profile of single-exposure steps or by applying a series of decreasingly long alternating target gas exposure and carrier gas exposure steps has been performed.     

Model of vapor-induced resistivity changes in gold-thiolate monolayer-protected nanoparticle sensor films

An investigation of the modulation of charge transport through thin films of n-octanethiolate monolayer-protected gold nanoparticles (MPN) induced by the sorption of organic vapors is presented. A model is derived that allows predictions of MPN-coated chemiresistor (CR) responses from vapor-film partition coefficients, and analyte densities and dielectric constants. Calibrations with vapors of 28 compounds collected from an array of CRs and a parallel thickness-shear-mode resonator are used to verify assumptions inherent in the model and to assess its performance. Results afford insights into the nature of the vapor-MPN interactions, including systematic variations in apparent film swelling efficiencies, and show that the model can predict CR responses typically to within 24%. Using CRs of different dimensions, vapor sensitivities are found to be virtually independent of the MPN film volume over a range of 104 (device-area x MPN layer thickness). Sensitivities vary inversely with analyte vapor pressure similarly for the two sensor types, but the CR sensor affords significantly greater signal-to-noise ratios, yielding calculated detection limits in the low-part-per-billion concentration range for several of the analytes tested. The implications of these results for implementing MPN-coated CR arrays as detectors in microanalytical systems are considered.     

Electrokinetic chromatography utilizing two pseudostationary phases providing ion-exchange and hydrophobic interactions

The electrokinetic chromatographic separation of a series of inorganic and organic anions was achieved by utilizing an electrolyte system comprising a cationic soluble polymer (poly(diallydimethylammonium chloride, PDDAC) and a neutral beta-cyclodextrin (beta-CD) as pseudostationary phases. The separation mechanism was a combination of electrophoresis, ion-exchange (IE) interactions with PDDAC, and hydrophobic interactions with beta-CD. The extent of each chromatographic interaction was independently variable, allowing for control of the separation selectivity of the system. IE interactions could be varied by changing either the PDDAC concentration or the concentration of a competing ion (e.g., chloride) in the BGE, while the hydrophobic interactions could be varied by changing the concentration of beta-CD. The separation system was very robust, with the reproducibility of the migration times being <0.7% RSD. A mathematical model that predicted the mobilities of analytes under varying experimental conditions was derived and was shown to give good correlation (r2 = 0.9804) between predicted and experimental migration times. Parameters derived from the model were in good agreement with the ion-exchange and hydrophobic characteristics of the analytes. The model was also applied successfully to the optimization of conditions for the separation of a mixture of analytes or for conditions under which particular analytes migrated in a desired order. That is, the opportunity to tune the separation selectivity has been demonstrated.     

Mechanistic aspects of chiral discrimination on modified cellulose

Cellulose and cellulose derivatives are biopolymers which are often used as stationary phases for the separation of enantiomers. Describing the mechanism of such separations is a difficult task due to the complexity of these phases. In the present study, we attempt to elucidate the types of interactions occurring between a diol intermediate for a LTD(4) antagonist and a tris(4-methylbenzoate)-derivatized cellulose stationary phase. Thermodynamic studies indicate that, at low temperatures, the enantioselectivity is entropy driven. At higher temperatures, the separation is enthalpy driven. DSC and IR experiments reveal that the transitions between the enthalpic and the entropic regions of the van't Hoff plots are a result of a change in conformation of the stationary phase. Investigation of chromatographic kinetic parameters reveals that, at low temperature, the second eluted enantiomer undergoes sluggish inclusion interactions. Subtle changes in the structure of the analyte indicates that π-π interactions do not contribute to enantioselectivity. Finally, molecular modeling of (R)- and (S)-diol and the stationary phase suggests that hydrogen bonding is a primary factor in the separation, and the calculated energy values obtained from the molecular modeling correlate well with the chromatographic elution order.     

Modelling enthalpy and entropy of pure and mixed refrigerants with an innovative corresponding states method

In this work an original improvement of the Corresponding States technique is developed and a new model, based on a three parameters CS format, is proposed to predict the enthalpy and the entropy of the new generation halogenated alkanes fluids together with some alkanes. Limiting the analysis of the selected fluids to a specific thermodynamic property behaviour, an appropriate conformality approach can be deduced, which allows to set up a predictive model of high accuracy level on a wide range of the enthalpy and entropy surfaces. The fundamentals of the model are innovative scaling parameters deduced from the enthalpy of vaporization and from two dedicated equations, belonging to the selected family of fluids. This allows to set up innovative models following a CS format. Through the introduction of advanced mixing rules, the models can be simply extended to calculate the corresponding properties for mixtures. The proposed models allow also the calculation of VLE for systems of rather regular behaviour. The required inputs for a pure target fluid are an ideal gas isobaric heat capacity correlation, a single value of saturated liquid density and of vaporization enthalpy; if the last one is lacking, a single value of vapor pressure can be alternatively supplied. For non azeotropic mixtures the enthalpy and entropy models are predictive, whereas in case of azeotropy VLE calculations are possibly only applying regressed interaction coefficients. Due to the lack of accurate experimental enthalpy data and to the particular nature of the entropy function, the validation of the models is proposed against fundamental dedicated EoS available, both for pure and mixtures, for a significant number of the studied family of fluids. The predictive character of the proposed approach as well as the high performances reached, make these models particularly suitable for the new families of fluids regarding advanced technological applications.     

Effect of affinity for droplet surfaces on the fraction of analyte molecules charged during electrospray droplet fission

The effect of uneven fissioning of mass and charge from electrospray droplets on the amount of analyte charged during the electrospray process was explored. A surface selectivity factor (S) was developed to describe the affinity of an analyte for the droplet surface, and both theoretical and experimental response curves were compared for analytes with various S values. The theoretical response curves were generated by calculating the overlap between the charge and analyte spawned from parent droplets to determine the amount of analyte charged. This overlap was then graphed as a function of analyte concentration. Differences in the amount of analyte charged during droplet fission were predicted for analytes of varying surface affinities. The issue of analyte partitioning between the surface and interior phases of the ESI droplet was also included in the discussion. This was accomplished by applying the equilibrium partitioning model to a set of offspring droplets to determine the amount of analyte on their surfaces and then calculating the overlap between fissioning analyte and excess charge. Experimental response curves resembled theoretical ones, and S values predicted from theory were in excellent agreement with those predicted on the basis of the structural characteristics of the analytes.     

A general method to perform a noncompetitive immunoassay for small molecules.

A new general method to perform a noncompetitive immunoassay for low-molecular-mass analytes (less than 6000 Da) is described and checked using cortisol as a model system. The method is based on the use of a "polydentate ligand" (cortisol-poly(L-lysine) conjugate) able to block the antibody sites unoccupied by the analyte, followed by the replacement of an antibody-bound analyte by an enzyme-labeled analyte (cortisol-horseradish peroxidase), and permits the direct measurement of the analyte bound sites. The observed signal shows a near-linear correlation with the analyte concentration. The characteristics of interactions between the analyte and polydentate ligand with the specific antibody were studied to perform a preliminary evaluation of the noncompetitive immunoassay for cortisol. The noncompetitive assay was compared with a competitive immunoassay obtained under the same conditions and using the same reagents. The results of the experiments showed a lower detection limit for the noncompetitive model (0.15 ng mL-1 rather than 0.72 ng mL-1), emphasizing that the model is successful. Moreover, as the polydentate ligand is prepared from the same hapten used for the immunogen synthesis, this type of noncompetitive immunoassay appears generally applicable to all small molecules for which antibodies have been obtained.