Inverse least-squares modeling of vapor descriptors using polymer-coated surface acoustic wave sensor array responses.

In previous work, it was shown that, in principle, vapor descriptors could be derived from the responses of an array of polymer-coated acoustic wave devices. This new chemometric classification approach was based on polymer/vapor interactions following the well-established linear solvation energy relationships (LSERs) and the surface acoustic wave (SAW) transducers being mass sensitive. Mathematical derivations were included and were supported by simulations. In this work, an experimental data set of polymer-coated SAW vapor sensors is investigated. The data set includes 20 diverse polymers tested against 18 diverse organic vapors. It is shown that interfacial adsorption can influence the response behavior of sensors with nonpolar polymers in response to hydrogen-bonding vapors; however, in general, most sensor responses are related to vapor interactions with the polymers. It is also shown that polymer-coated SAW sensor responses can be empirically modeled with LSERs, deriving an LSER for each individual sensor based on its responses to the 18 vapors. Inverse least-squares methods are used to develop models that correlate and predict vapor descriptors from sensor array responses. Successful correlations can be developed by multiple linear regression (MLR), principal components regression (PCR), and partial least-squares (PLS) regression. MLR yields the best fits to the training data, however cross-validation shows that prediction of vapor descriptors for vapors not in the training set is significantly more successful using PCR or PLS. In addition, the optimal dimension of the PCR and PLS models supports the dimensionality of the LSER formulation and SAW response models.

Hydrogen bond acidic polymers for surface acoustic wave vapor sensors and arrays.

Four hydrogen bond acidic polymers are examined as sorbent layers on acoustic wave devices for the detection of basic vapors. A polysiloxane polymer with pendant hexafluoro-2-propanol groups and polymers with hexafluorobisphenol groups linked by oligosiloxane spacers yield sensors that respond more rapidly and with greater sensitivity than fluoropolyol, a material used in previous SAW sensor studies. Sensors coated with the new materials all reach 90% of full response within 6 s of the first indication of a response. Unsupervised learning techniques applied to pattern-normalized sensor array data were used to examine the spread of vapor data in feature space when the array does or does not contain hydrogen bond acidic polymers. The radial distance in degrees between pattern-normalized data points was utilized to obtain quantifiable distances that could be compared as the number and chemical diversity of the polymers in the array were varied. The hydrogen bond acidic polymers significantly increase the distances between basic vapors and nonpolar vapors when included in the array.

Selective vapor sorption by polymers and cavitands on acoustic wave sensors: is this molecular recognition?

Selectivity patterns for the sorption of organic vapors from the gas phase into cavitand monolayers on acoustic wave sensors are very similar to those seen for sorption of the same vapors by amorphous polymers, demonstrating that the vapor/cavitand selectivity patterns are determined primarily by solubility interactions. The amorphous polymers serve as controls demonstrating that the three-dimensional structure of a cavitand layer is not primarily responsible for the selectivity observed. Binding and selectivity in the examples cited are governed primarily by general dispersion interactions and not by specific oriented interactions that could lead to molecular recognition.

Optimal coating selection for the analysis of organic vapor mixtures with polymer-coated surface acoustic wave sensor arrays.

A method for determining the optimal set of polymer sensor coatings to include in a surface acoustic wave (SAW) sensor array for the analysis of organic vapors is described. The method combines an extended disjoint principal components regression (EDPCR) pattern recognition analysis with Monte Carlo simulations of sensor responses to rank the various possible coating selections and to estimate the ability of the sensor array to identify any set of vapor analytes. A data base consisting of the calibrated responses of 10 polymer-coated SAW sensors to each of six organic solvent vapors from three chemical classes was generated to demonstrate the method. Responses to the individual vapors were linear over the concentration ranges examined, and coatings were stable over several months of operation. Responses to binary mixtures were additive functions of the individual component responses, even for vapors capable of strong hydrogen bonding. The EDPCR-Monte Carlo method was used to select the four-sensor array that provided the least error in identifying the six vapors, whether present individually or in binary mixtures. The predicted rate of vapor identification (87%) was experimentally verified, and the vapor concentrations were estimated within 10% of experimental values in most cases. The majority of errors in identification occurred when an individual vapor could not be differentiated from a mixture of the same vapor with a much lower concentration of a second component. The selection of optimal coating sets for several ternary vapor mixtures is also examined. Results demonstrate the capabilities of polymer-coated SAW sensor arrays for analyzing of solvent vapor mixtures and the advantages of the EDPCR-Monte Carlo method for predicting and optimizing performance.

Characterization of polymeric surface acoustic wave sensor coatings and semiempirical models of sensor responses to organic vapors.

Responses from an array of four polymer-coated surface acoustic wave sensors exposed to a series of 39 organic vapors were used to investigate sensor response models based on vapor boiling point, solubility parameters, and solvation parameters in conjunction with linear solvation energy relationships. As part of this effort, sensor response data were used to estimate the solubility parameters and solvation parameters of the sensor coatings by adaptation of methods originally developed for use with gas-liquid chromatographic retention data. Values of these parameters were found to be consistent with the structures of the coatings though in some cases different from those determined by other methods. Discrepancies were attributed to differences in the conditions used for the determinations. Sensor responses were linear over the concentration ranges examined and could be summarized using the empirically determined partition coefficient, Ke, for each vapor-coating pair. Linear correlations were found between log Ke and vapor boiling point, and the slopes of the regressions lines were similar to those expected for ideal behavior. The strength of the correlations decreased with increasing coating polarity, and it was necessary to divide the vapors into two or three broad chemical classes in order to obtain satisfactory results. Improved correlations were found by use of Hildebrand solubility parameters in a model based on regular solution theory which attempts to account for nonideal vapor-coating interactions. The use of solvation parameters in linear solvation energy relationships, however, provided the strongest correlations, with modeled K values falling within a factor of 2 of experimental values in all cases and within +/- 25% of experimental values in 83% of the cases. Application of these models to the prediction of sensor array response patterns appears promising.

Glove permeation by semiconductor processing mixtures containing glycol-ether derivatives.

Results of permeation tests of several glove materials challenged with semiconductor processing formulations containing glycolether derivatives are described. Commercial glove samples of nitrile rubber (Edmont), natural rubber (Edmont and Baxter), butyl rubber (North), PVC Baxter), a natural rubber/neoprene/nitrile blend (Pioneer), and a natural rubber/neoprene blend (Playtex) were tested according to the ASTM F739-85 permeation test method (open-loop configuration). The liquid formulations examined included a positive photoresist thinner containing 2-ethoxyethyl acetate (2-EEA), n-butyl acetate, and xylene; a positive photoresist containing 2-EEA, n-butyl acetate, xylene, polymer resins, and photoactive compounds; a negative photoresist containing 2-methoxyethanol (2-ME), xylene, and cyclized poly(isoprene); and pure 2-methoxyethyl acetate (2-MEA), which is the solvent used in a commercial electron-beam resist. With the exception of the negative photoresist, butyl rubber provided the highest level of protection against the solvent mixtures tested, with no breakthrough observed after 4 hr of continuous exposure at 25 degrees C. Nitrile rubber provided the highest level of protection against the negative photoresist and reasonably good protection against initial exposure to the other solvent mixtures. Gloves consisting of natural rubber or natural rubber blends provided less protection against the mixtures than either nitrile or butyl rubber. For most of the glove samples, permeation of the glycol-ether derivatives contained in the mixtures was faster than that predicted from the permeation of the pure solvents. Increasing the exposure temperature from 25 to 37 degrees C did not significantly affect the performance of the butyl rubber glove. For the other gloves, however, exposures at 37 degrees C resulted in decreases in breakthrough times of 25-75% and increases in steady-state permeation rates of 80-457% relative to values obtained at 25 degrees C. Repeated exposure of nitrile rubber samples resulted in shorter breakthrough times for all mixture components. In fact, exposure for as little as one-half of the nominal breakthrough time followed by air drying overnight resulted in measurable quantities of one or more of the component solvents at the inner surface of the gloves at the beginning of the next exposure. This effect was not observed with the butyl rubber samples. With the exception of the negative photoresist, heating previously exposed nitrile rubber samples at 70 degrees C for 20 hr prior to retesting reduced or eliminated the effects of residual solvents, permitting reuse of the gloves. The use of thin PVC or natural rubber gloves adjacent to the nitrile gloves provided moderate increases in permeation resistance.(ABSTRACT TRUNCATED AT 400 WORDS)