A method to estimate the octanol-air partition coefficient of semivolatile organic compounds

A multicolumn method for the estimation of the octanol-air partition coefficient (K(OA)) of semivolatile compounds is described. The method is based on the retention time of a compound on gas chromatographic columns of different selectivity and polarity. Log K(OA) values of polychlorinated biphenyls (PCB) were estimated with the novel multicolumn method. The results were in excellent agreement with previously published data using a generator column method. Therefore, we used our multicolumn method to estimate K(OA) for more than 100 PCB congeners.     

Prediction of gas chromatographic relative retention times of stimulants and narcotics.

The ADAPT software system was used to create models for the prediction of gas chromatographic relative retention times (RRTs) of stimulants and narcotics that are analyzed in doping control of athletes. The two main methods that were followed for building the models were the quantitative structure-retention relationship (QSRR) and multiple linear regression analysis. The main proposed model for the entire data set had a multiple correlation coefficient R = 0.991 and standard error s = 0.046 or approximately 4.5%. Because of the relatively high standard error of the main model, a second model was built on a subset of compounds with R = 0.982 and s = 0.027 or approximately 2.5%.     

Collection of nonpolar organic compounds from ambient air using polyurethane foam-granular adsorbent sandwich cartridges.

Glass cartridges containing 6-10 g of Tenax-GC or 15 g of XAD-2 resin packed between two slices of polyurethane foam (PUF) were used in a General Metal Works PS-1 high-volume sampler to collect chlorobenzenes (CBs) containing three to six chlorine atoms, hexachlorocyclohexanes (HCHs), and two-ring aromatic hydrocarbons from 35-385 m3 of air. Laboratory experiments were run by vaporizing known quantities of analytes into a clean airstream for sampling by the PS-1 system at 20 degrees C. Collection efficiencies, determined from mass balance of the quantities introduced and recovered, ranged from 70 to 120% for individual compounds and averaged 93% overall. Penetration of analytes to backup adsorbent traps showed an inverse correlation to vapor pressure. The method was used to collect the above compounds from urban and rural air.     

Prediction of gas chromatographic retention indexes for polychlorinated dibenzofurans

A model has been developed by using molecular connectivity to describe the relationship between the molecular structure of polychlorinated dibenzofurans (PCDFs) and their gas chromatographic linear temperature-programmed retention characteristics on a 30-m fused silica column coated with a DB-5 stationary phase. Model variables account for the number of chlorine atoms present, the position and the relationship on each aromatic ring, the chlorine atom interaction between the two rings, and the skeletal structure of each PCDF isomer.     

In-membrane preconcentration/membrane inlet mass spectrometry of volatile and semivolatile organic compounds

The on-line determination of volatile and semivolatile organic compounds (SVOCs) is reported using membrane inlet mass spectrometry with in-membrane preconcentration (IMP-MIMS). Semivolatile organic compounds in aqueous samples are preconcentrated in a flow-through silicone hollow-fiber membrane inlet held in a GC oven. The sample stream is replaced with air, and the SVOCs are thermally desorbed into the mass spectrometer by rapid heating of the membrane. The method is evaluated for the on-line determination of 4-fluorobenzoic acid, 3,5-difluorobenzoic acid, 2-chlorophenol, p-tert-butylphenol, and dimethyl sulfoxide (DMSO) in water. The selectivity of the IMP-MIMS technique for SVOCs in the presence of VOCs is demonstrated. Cryotrapping and a rapid gas chromatographic separation step were added between the membrane and the mass spectrometer ion source for the determination of SVOCs in complex mixtures. The procedure is demonstrated for the determination of dimethyl sulfoxide (DMSO) in equine urine, using internal standardization with DMSO-d6. Full-scan electron ionization (EI) mass spectrometric detection showed good linearity (R = 0.998) and RSDs, relative to the internal standard, of 2.2% for desorption only and 4.6% for desorption and cryotrapping.     

Direct analysis of semivolatile organic compounds in air by atmospheric pressure chemical ionization mass spectrometry.

Atmospheric pressure chemical ionization is employed for direct air analysis, without ion source modification, by using the sheath gas as the sample transport agent. A simple modification of the sheath gas inlet line allows introduction of gaseous samples into a commercial atmospheric pressure chemical ionization source. Optimization and testing of this novel air sampling method are described and detection of semivolatile compounds is shown. The analytical performance of the technique is established with methyl salicylate, including a limit of quantification of 100 pptr, a limit of detection of 50 pptr, a linear response from 100 pptr to 20 ppb, and rise and fall times of 12 and 20 s, respectively. Using reagent ion monitoring, it is shown that the protonated methanol dimer is the principal CI reagent ion leading to protonated dimethyl methylphosphonate, while the monomer is mainly responsible for protonating methyl salicylate. Since the formation of the CI reagent (methanol clusters) can be controlled by simple variation of experimental parameters, the selectivity of the method can be easily adjusted to suit the targeted analyte. Performance is found to be independent of the choice of air or nitrogen as the sheath gas (and thus as the sample matrix) and this, together with the sensitivity and speed of the technique, make it promising for field studies.     

Predictions of micelle-water partition coefficients and retention in micellar electrokinetic chromatography from solute structure. 2. Fragmental constant approach

The fragmental constant approach (FCA) was used to calculate water-sodium dodecyl sulfate (SDS) micelle partition coefficients, K(mw), for uncharged solutes from their structure. Subsequently, the availability of K(mw) values allows prediction of retention factor, k, in micellar electrokinetic chromatography (MEKC) using the simple relationship k = K(mw)phi, where phi is the phase ratio. The FCA model describes a micelle-water partition coefficient as the sum of the partition coefficients of the constituent atomic/molecular fragments, measured by fragmental constant values, f (i), as well as correction factors to account for various "intramolecular effects" that cause deviations from the predicted partition coefficients as, log K(mw) = sum(n)(i=1)aif i+sum(m)(i=1)kiCm. The fragmental constants for a set of 41 fragments were determined using a training set of 229 aromatic solutes and 198 aliphatic compounds. The K(mw) of the aromatic compounds in the training set were determined by MEKC, while the K(mw) of the aliphatic solutes were estimated using the linear solvation energy relationship (LSER) for the SDS micelles. The fragments consisted of both aromatic fragments (i.e., directly attached to an aromatic ring) and aliphatic fragments. The FCA predictions agree nicely with the observed and LSER partition coefficient values, even for complex molecular structures such as beta-blocker drugs. The results show the great potential of the FCA for a priori prediction of retention behavior in MEKC from solute structure.     

Design and evaluation of a new thermospray liquid/liquid extractor for the extraction of semivolatile and nonvolatile organic compounds from water

The recovery of several semivolatile organic compounds (SVOCs) using a new thermospray liquid/liquid extractor (TSLLE) was investigated. The base system includes a 300 mL multiport extraction vessel, jacketed in a 500 mL cooling flask, a dual-stage condenser for progressive cooling, several thermospray probes, and solvent/sample delivery systems. Aqueous mixtures of SVOCs were used to evaluate the TSLLE. For most compounds, recovery values of 80-100% were obtained during a single cycle in <1 h. The design, evaluation, and extraction capability of the TSLLE are discussed.     

Comparisons of polymer/gas partition coefficients calculated from responses of thickness shear mode and surface acoustic wave vapor sensors

Apparent partition coefficients, K, for the sorption of toluene by four different polymer thin films on thickness shear mode (TSM) and surface acoustic wave (SAW) devices are compared. The polymers examined were poly(isobutylene) (PIB), poly(epichlorohydrin) (PECH), poly(butadiene) (PBD), and poly(dimethylsiloxane) (PDMS). Independent data on partition coefficients for toluene in these polymers were compiled for comparison, and TSM sensor measurements were made using both oscillator and impedance analysis methods. K values from SAW sensor measurements were about twice those calculated from TSM sensor measurements when the polymers were PIB and PECH, and they were also at least twice the values of the independent partition coefficient data, which is interpreted as indicating that the SAW sensor responds to polymer modulus changes as well as to mass changes. K values from SAW and TSM measurements were in agreement with each other and with independent data when the polymer was PBD. Similarly, K values from the PDMS-coated SAW sensor were not much larger than values from independent measurements. These results indicate that modulus effects were not contributing to the SAW sensor responses in the cases of PBD and PDMS. However, K values from the PDMS-coated TSM device were larger than the values from the SAW device or independent measurements, and the impedance analyzer results indicated that this sensor using our sample of PDMS at the applied thickness did not behave as a simple mass sensor. Differences in behavior among the test polymers on SAW devices are interpreted in terms of their differing viscoelastic properties.     

Helical sorbent microtrap for continuous sampling by a membrane and trap interface for on-line gas chromatographic monitoring of volatile organic compounds

A helical sorbent microtrap consisting of a helical sorbent fixed inside a silicosteel capillary tube is presented. The main parameters that affect the safe sampling time of the helical sorbent microtrap in continuous sampling by a membrane and trap interface for on-line gas chromatographic monitoring of organic volatiles in gaseous samples are examined, taking into account the helical configuration of the sorbent, the presence of the membrane in system, and the properties of the analytes. Thermal desorption of analytes from the helical sorbent trap was also examined having regard to the influence of the turbulent flow generated by the helical sorbent in the heat transfer and the effect of thermal backward flow on the peak shape. The practical application of the helical sorbent microtrap in a membrane and trap interface was demonstrated by on-line gas chromatographic monitoring of four volatile organic compounds in the fume hood air and of volatile organic compounds from a diesel engine exhaust. The limit of detection was in the picogram per milliliter range, depending on the time of trapping and the parameters that affect the permeation through the membrane.     

Simultaneous ultrasound-assisted emulsification-extraction of polar and nonpolar compounds from solid plant samples

A new approach to solid sample preparation for the simultaneous isolation of polar and nonpolar compounds using a microemulsion as leaching medium is proposed. Methanol/water (dispersed-phase)-hexane (continuous-phase) emulsions formed in the presence of ultrasound and a solid sample allow polar and nonpolar compounds to be transferred to the dispersed and continuous phase, respectively. The efficiency of this dual sample preparation approach was assessed in the characterization of natural products of variable hardness including acorns, grape seeds, and alperujo (a residue of olive oil production). The time needed for quantitative extraction of the target fractions (phenol compounds and fatty acids) is 9 min for acorns and alperujo and 20 min for grape seeds; the longer time needed for grape seeds can be attributed to higher matrix hardness. Such good performance can be ascribed to the ultrasound-enhanced formation of methanol/water microdroplets 1-15 microm in size, which act as solid-liquid microextractors spanning a large surface area. The presence of the sample was found to greatly improve emulsion stability, which can be ascribed to the amphiphilic nature of the fatty acids in the samples. Following leaching and separation of the two phases by centrifugation, the polar and nonpolar fractions were analyzed by HPLC-diode array detection and GC/MS, respectively. The proposed approach provides extraction efficiency similar to the Folch method (reference method for fat extraction, 4.5 h) in a shorter time and extraction efficiency equal to or higher than the stirring-based method (reference method for phenol compounds extraction, 24 h).     

Novel hydrophobicity ruler approach for determining the octanol/water partition coefficients of very hydrophobic compounds via their polymer/solvent solution distribution coefficients

A novel hydrophobicity ruler approach for determining the octanol/water partition coefficients of very hydrophobic compounds is proposed, which is an indirect method that measures the polymer/solvent solution distribution coefficients (log Kp/s) of reference and unknown compounds. The log Kp/s values of the unknown compounds can be calibrated to their log Ko/w values via the correlation of the log Kp/s values of the reference compounds with their log Ko/w values. An organic solvent was used to increase the solubility of the very hydrophobic compounds in the aqueous solution, so that their concentrations and absorption amounts were high enough to be measured precisely. The solvent also reduced the hydrophobicity scale of the very hydrophobic compounds and controlled the amounts absorbed into the polymer phase, so that compounds spanning a very wide range of log Ko/w values could be measured in a single measurement and the coexisting compounds would not interfere each other. Poly(dimethylsiloxane) (PDMS), aqueous methanol solutions, and a series of 21 PCB (polychlorinated biphenyl) compounds were used to demonstrate the principle of the hydrophobicity ruler approach. The PCB compounds with known experimental log Ko/w values served as reference compounds, whereas the PCB compounds without known log Ko/w values were determined. The log Ko/w values determined for PCB126, PCB187, PCB197, PCB180, PCB170, and PCB195 were 6.94, 7.84, 8.33, 8.17, 7.92, and 8.49, respectively. The correlation of the log Kp/s values of the reference PCB compounds with their log Ko/w values was linear (log Ko/w=2.56 log Kp/s+1.08, R2=0.95). The hydrophobicity ruler approach is also a valuable tool for validating the experimental and theoretical log Ko/w values and identifying outliers in log Ko/w databases.