Photoionization Detector Research Articles

Characterization of nitrogen-containing radical products from the photodissociation of trimethylamine using photoionization detection

The outcome of neutral and photoionized N(CH3)2 primary products of trimethylamine photodissociation at 193 nm is determined by combining photoionization detection with supporting G3 theoretical calculations. N(CH3)2 primary products with very little internal energy show an experimentally observed ionization onset of 9.1±0.2 eV, but do not appear at the parent ion (m/e=44). Instead, the parent ion is unstable and easily fragments to m/e=42, where the signal is observed. N(CH3)2 radicals with higher internal energies undergo H-atom loss from the neutral to give CH2NCH3, which has an observed ionization onset at parent (m/e=43) of <9.3 eV. At slightly higher ionization energies, these secondary products also appear at m/e=42 (where their appearance energy is roughly 9.8–9.9 eV, uncorrected for internal energy). Finally, N(CH3)2 radicals with the highest internal energy in this study appear to undergo H2 loss as neutrals, giving rise to a species whose parent ion has m/e=42. The ionization onset of this species at m/e=42 is found to be in the range of 9.5–9.6 eV.

Atmospheric pressure photoionization: an ionization method for liquid chromatography-mass spectrometry

Atmospheric pressure photoionization (APPI) has been successfully demonstrated to provide high sensitivity to LC-MS analysis. A vacuum-ultraviolet lamp designed for photoionization detection in gas chromatography is used as a source of 10-eV photons. The mixture of samples and solvent eluting from an HPLC is fully evaporated prior to introduction into the photoionization region. In the new method, large quantities of an ionizable dopant are added to the vapor generated from the LC eluant, allowing for a great abundance of dopant photoions to be produced. Because the ion source is at atmospheric pressure, and the collision rate is high, the dopant photoions react to completion with solvent and analyte molecules present in the ion source. Using APPI, at an LC flow rate of 200 microL/min, it is possible to obtain analyte signal intensities 8 times as high as those obtainable with a commercially available corona discharge-atmospheric pressure chemical ionization source.

Determination of furan fatty acids in extra virgin olive oil.

The presence of 4 different furan fatty acids (F-acids) was detected in 18 samples of transmethylated monovarietal extra virgin olive oil: methyl 10,13-epoxy-11,12-dimethyloctadeca-10,12-dienoate [diMeF(9,5)], methyl 12,15-epoxy-13,14-dimethyleicosa-12,14-dienoate [diMeF(11,5)] and both olefinic derivatives of diMeF(11,5) with one unsaturation on the side chains conjugated with the furan ring. Transmethylated oils were analyzed by normal phase high-performance liquid chromatography coupled on-line with capillary gas chromatography. After the gas chromatographic separation step, a more selective detection of F-acids was achieved by using a photoionization detector mounted in series with a flame ionization detector. The concentration of F-acids ranged between 50 ppb (detection limit of the method) and 2.1 ppm in the oil. The olefinic derivatives of diMeF(11,5) acids detected were not artifacts created during the sample preparation or during the chromatographic analysis.

An Evaluation of Employee Exposure to Volatile Organic Compounds in Three Photocopy Centers

Personal and area samples from three copy centres were collected in thermal desorption tubes and analyzed using gas chromatography–mass spectrometry. Real-time personal total volatile organic compounds (TVOC) were measured using a data-logging photoionization detector. Fifty-four different VOCs were detected in the area samples. The maximum concentration measured was 1132.0 ppb (toluene, copy center 3, day 1). Thirty-eight VOCs were detected in the personal samples and concentrations ranged from 0.1 ppb (1,1-biphenyl, p-dichlorobenzene, propylbenzene, styrene, and tetrachloroethylene) to 689.6 ppb (toluene). Real-time TVOC measurements indicated daily fluctuations in exposure, ranging from <71 to 21,300 ppb. The time-weighted average exposures for the photocopier operators on days 1 and 2 were 235 and 266 ppb and 6155 and 3683 ppb, in copy centers 2 and 3, respectively. Personal exposure measurements of individual VOCs were below accepted occupational standards and guidelines. For example, the maximum concentration was 0.3% of the permissible exposure limits (toluene, copy center 3). Exposures were highest in copy center 3; this is likely due to the presence of offset printing presses. It is concluded that photocopiers contribute a wide variety of VOCs to the indoor air of photocopy centers; however, exposures are at least 100 times below established standards.

Pressure-tunable column selectivity for high-speed vacuum-outlet GC.

A pressure-tunable ensemble of two series-coupled capillary columns operated at subambient outlet pressure is described. The ensemble consists of a 4.5-m length of nonpolar dimethyl polysiloxane column followed by a 7.5-m length of polar trifluoropropylmethyl polysiloxane column. Air at an inlet pressure of 1.0 atm is used as carrier gas, and a vacuum pump is used to pull the carrier gas and injected samples through the column ensemble. Detection is provided by a photoionization detector operated at a pressure of 0.3 psia. Ensemble selectivity is controlled by means of an electronic pressure controller located at the junction point between the columns. The minimum pressure step size is 0.1 psi, and 50 different set-point pressures can be used, each one producing a different pattern of peaks eluting from the column ensemble. Measured ensemble retention factors for a set of target compounds produce straight lines when plotted versus the ratio of the calculated holdup time of the first column in the ensemble to the total ensemble holdup time. A component band trajectory model is used to describe the effects of ensemble junction-point pressure on the elution patterns generated by the ensemble. Ensemble retention times predicted by the model are in good agreement with values obtained from chromatograms. The use of on-the-fly set-point pressure changes during a separation (selectivity programming) is demonstrated and used to improve the quality of the separation of a 19-component test mixture.

A pulsed Laval nozzle apparatus with laser ionization mass spectroscopy for direct measurements of rate coefficients at low temperatures with condensable gases

A pulsed Laval nozzle, low Mach number supersonic expansion kinetics apparatus has been constructed to study neutral–neutral kinetics by a rather general laser photolysis initiation and laser photoionization detection of the product species. This new apparatus permits laboratory studies of low temperature rate coefficients (e.g., 70–170 K) on condensable gases that have insufficient vapor pressures at low temperatures for conventional methods of kinetic measurements. The design considerations, the uniformity of the reaction zone over 10–20 cm, and the skimmer sampling of the pulsed Laval expansion are examined. The direct measurement of a rate coefficient at 90 K is also demonstrated using this new apparatus.

Field evaluation of a portable photoionization detector for assessing exposure to solvent mixtures.

To evaluate a portable photoionization detector for assessing personal exposure to solvent mixtures, a set of 26 side-by-side, time-weighted average (TWA) personal breathing zone samples were collected during various construction painting tasks by two different sampling methods: (1) standard charcoal sorbent tubes analyzed by gas chromatography (CST/GC), and (2) a direct-reading photoionization detector coupled with an extended data-logger (PID). The TWA concentrations of the hydrocarbons detected by CST/GC analysis were summed for comparison with the TWA concentration obtained from the direct-reading PID. Based on linear regression between the log TWA concentrations of the two sampling methods, the data were highly correlated (r2 = 0.95). Since the solvents had effects that may be considered additive, threshold limit values (TLVs) for mixtures were developed using American Conference of Governmental Industrial Hygienists formulas to evaluate solvent exposure. The logs of the TLV mixture data from the sampling methods were highly correlated (r2 = 0.94). Based on the linear regression analyses, the response of the portable PID was highly correlated to the CST/GC results for hydrocarbon mixtures encountered during various painting tasks. Due to the short duration of tasks, highly fluctuating exposures, and complexity of the mixtures, the PID may provide the most cost-effective, detailed exposure assessment for solvent mixtures.

Pressure-Tunable Dual-Column Ensembles for High-Speed GC and GC/MS

A series-coupled ensemble of two capillary GC columns of different selectivity with an adjustable pressure at the column junction point is used to obtain tunable selectivity for high-speed GC and GC/ TOFMS. An electronic pressure controller with a 0.1-psi step size is used to obtain numerous computer-selected unique selectivities. System configurations for conventional, atmospheric-pressure outlet operation with flame ionization detection and for vacuum-outlet operation with photoionization detection are described for GC-only experiments. Polydimethylsiloxane is used as the non-polar column and polyethylene glycol (atmospheric outlet) or triflouropropylpolysiloxane (vacuum outlet) is used as the polar column. For GC/ TOFMS experiments, 5% phenyl polydimethylsiloxane was used as the non-polar column, and polyethylene glycol was used as the polar column. The time-of-flight mass spectrometer can acquire up to 500 complete mass spectra per second. Since spectral continuity is achieved across the entire chromatographic peak profile, severely overlapping peaks can be spectrally deconvoluted for high-speed characterization of completely unknown mixtures. For mixture components with significantly different fragmentation patterns, spectral deconvolution can be achieved for chromatographic peak separations of as little as 6.0 ms. This can result is very large peak capacity for time compressed (not completely resolved) chromatograms. The use of columns with tunable selectivity allows for precise peak-position control, which can result in more efficient utilization of available peak capacity and thus further time compression of chromatograms. The limits of tunability and deconvolution are tested for near co-elutions of different classes of hydrocarbon compounds as well as for more multi-functional mixtures.

Field Evaluation of a Portable Photoionization Detector for Assessing Exposure to Solvent Mixtures

Field Evaluation of a Portable Photoionization Detector for Assessing Exposure to Solvent Mixtures

Pulsed discharge emission detector: an element-selective detector for gas chromatography

An element-specific pulsed discharge emission detector (PDED) has been coupled directly with a vacuum UV monochromator so that vacuum UV atomic emissions from Cl, Br, I and S can be observed. The observed sensitivities for the elements are in the range of mid to high pg/s, but can be lowered by direct absorption of the radiation using a vacuum UV radiation photomultiplier tube. A helium pulsed discharge photoionization detector (He-PDPID) was run simultaneously in parallel with the PDED. The chromatograms recorded with the two detectors had similar peak shapes, suggesting that there is no peak tailing in the PDED. The ratio of the detector responses PDED/He-PDPID can be used for qualitative identification of the Cl-, Br-, I- or S-containing compounds.

Characterization of chlorinated compounds using a dual chlorine-selective pulsed discharge emission detector–helium-pulsed discharge photoionization detector system

The Cl-selective pulsed discharge emission detector (Cl-PDED) response is dependent only upon the Cl content, irrespective of the molecular structures of the compounds. This provides a simple, fast quantitative method of analysis for chlorinated compounds. The response of the helium-pulsed discharge photoionization detector (He-PDPID) is a function of the molecular structure and the number of photoionizable electrons using the He 2 band at 13.5–17.5 eV. The ratio of the responses of the two detectors is independent of concentration and can be used to characterize the Cl-containing compounds along with the retention time, or the ratio can be used as evidence for coelution. The dual Cl-PDED–He-PDPID detector system is a useful tool for peak identification. The effect of coeluting hydrocarbons on the Cl-PDED response was evaluated by spiking a gasoline sample with US Environmental Protection Agency mixture 502. All Cl-PDED responses were greater than 90% of the response in the absence of the hydrocarbons.

Dynamics of H2 and C2H4 Elimination in the Y + C2H6 Reaction

The reaction between ground-state atomic yttrium (a2DJ) and ethane (C2H6) has been studied in crossed molecular beams with 157 nm photoionization detection of products. Studies have been carried out at several collision energies, ranging from 〈Ecoll〉 = 16.1 kcal/mol to 〈Ecoll〉 = 29.8 kcal/mol. At 〈Ecoll〉 ≥ 18.1 kcal/mol, reactive scattering is observed, with both YH2 + C2H4 and YC2H4 + H2 products being formed. Detected products of the YH2 + C2H4 channel are found to exhibit very little translational energy, suggesting that there is little or no potential energy barrier above the final product energy to ethylene elimination from an (H)2Y(C2H4) intermediate. Translational energy distributions for the YC2H4 + H2 channel, resulting from elimination of H2 from a common intermediate, demonstrate that a slightly larger fraction of energy available to these products is channeled into translation. The YH2 + C2H4 channel is found to dominate the H2 elimination channel by greater than an order of magnitude, which suggests that a small potential energy barrier to the less endoergic H2 elimination channel exists. In lower collision energy studies, only nonreactive scattering of Y atoms from ethane is found to occur. This observed collision energy threshold behavior for the onset of YH2 and YC2H4 product formation allows determination of the barrier to C−H bond insertion of Y atoms into ethane to be 19.9 ± 3.0 kcal/mol.

Quantitative analysis of benzene, toluene, and m‐xylene with the use of a UV–ion mobility spectrometer

An ion mobility spectrometer (IMS) equipped with a 10.6 eV low-pressure gas-discharge lamp usually used in photoionization detectors for gas chromatographic applications was developed for the continuous detection of benzene, toluene, and m-xylene. To improve the resolution of the IMS for single substances a customized IMS with a doubled drift tube length was built. The responses of both IMS (drift tube lengths of 6 and 12 cm) to the compounds selected were determined and compared. The advantages of the combination of multi-capillary columns with IMS are discussed with the aim of achieving further enhancements to the resolution of the instrument. This coupling leads to a significant increase in the scope of application of ion mobility spectrometry for environmental applications. The influence of intermolecular charge-transfer reactions on the peak can be reduced and more complex matrices considered. The presented three-dimensional correlation supports the interpretation of the spectra acquired from the mixtures. © 2000 John Wiley & Sons, Inc. Field Analyt Chem Technol 4: 157–169, 2000

Competing reaction pathways from Y+C2H2 collisions

The crossed molecular beams method with 193 and 157 nm photoionization detection was used to study the competing reaction pathways resulting from collisions of ground state Y atoms with acetylene (C2H2). Three channels, corresponding to nonreactive decay of collision complexes, H2 elimination, and H atom elimination, were studied as a function of collision energy (〈Ecoll〉=6–25 kcal/mol). Production of YC2+H2 and decay of long-lived complexes back to reactants were observed at all collision energies studied. Product translational energy distributions for the H2 elimination channel demonstrate that a substantial fraction of excess energy available to the YC2+H2 products is channeled into relative translational energy. Analogous H2 elimination channels were studied in reactions of Zr and Nb with C2H2 at 〈Ecoll〉=6.0 kcal/mol. For these reactions, the H2 elimination product translational energy distributions were found to peak near zero kinetic energy, in contrast to the behavior observed for the YC2+H2 products. This suggests that a significant potential energy barrier exists in the exit channel of the YC2+H2 elimination step, whereas no exit channel barrier exists in forming ZrC2+H2 and NbC2+H2. The reformation of Y + C2H2 reactants following decay of long-lived collision complexes was found to transfer 40%–50% of the initial relative translational energy into C2H2 internal excitation. The YC2H+H product channel was only observed to occur above a collision energy threshold of 21.5±2.0 kcal/mol. Since YC2H+H production is fully spin-allowed and involves simple Y–H bond fission in the intermediate HYC2H complex, it is unlikely that any significant potential energy barrier is present in excess of the reaction endoergicity. Additional studies of Y+C2D2 reactions confirm that the observed collision energy threshold for the H or D atom loss channel corresponds to the energetic threshold for reaction, allowing determination of D0(Y–CCH)=110.2±2.0 kcal/mol.

Rotatable source crossed molecular beams apparatus with pulsed ultraviolet/vacuum ultraviolet photoionization detection

A newly constructed universal crossed molecular beams apparatus for studies of bimolecular chemical reaction dynamics is described. The apparatus employs two rotatable molecular beam sources and a fixed ultrahigh vacuum (UHV) quadrupole mass spectrometer with electron impact or pulsed photoionization of reaction products. Electronically cold neutral supersonic transition metal atomic beams are produced in one of the rotatable sources using laser vaporization. The beams are characterized by laser induced fluorescence spectroscopy, photodepletion spectroscopy, and time-of-flight analysis. Photoionization of the ZrC2H2 products from the crossed beam reaction Zr+C2H4→ZrC2H2+H2 is carried out using the pulsed 157 nm radiation from a F2 excimer laser in the UHV region of the mass spectrometer. Compared to conventional electron impact ionization, 157 nm photoionization improves signal-to-noise ratios by more than two orders of magnitude for experiments using pulsed beam sources where cross correlation methods cannot be used. We also demonstrate the use of 1+1 resonance enhanced multiphoton ionization for state selective detection of nonreactively scattered transition metal atoms from decay of long-lived collision complexes. Due to the small reaction cross sections for Y+C2H6→YC2H4+H2 and Y+CD3CDO→DYCD3+CO, these reactions could not be studied using electron impact ionization. However, photoionization detection permitted direct studies of the reactions with excellent signal-to-noise ratios. The greatly improved sensitivity of the photoionization technique facilitates studies of transition metal systems not previously amenable to the crossed beams method.

Source-identification investigations of petroleum contaminated groundwater in the Missouri Ozarks

The growing population in the Missouri Ozarks rely on the abundant supply of potable groundwater found in the fractured bedrock underlying the region. Protection of this valuable resource from surface, or near-surface contamination is essential to the health and safety of the citizens of Missouri. Petroleum products stored in underground storage tanks pose a threat to groundwater if a release occurs. “Pools” of free product of petroleum can become trapped in voids commonly found on the karst bedrock of the Ozarks. These pools become underground sources of contamination by slowly dissolving into the groundwater. Although the free product rarely travels far from the surface source, dissolved-phase contamination can migrate thousands of meters through underground conduits. Dissolved phase petroleum degasses and partitions out of the groundwater in these conduits. Consequently, contaminated groundwater is not commonly found more than a kilometer or two (few thousand feet) from the source. The Missouri Department of Natural Resources, Division of Geology and Land Survey's Leaking Underground Storage Tank Unit (DGLS/LUST) investigates sites where petroleum-contaminated groundwater has been found in springs or private wells but a source of the contamination is unknown. These investigations start at the regional scale by determining the recharge area of the contaminated groundwater and by identifying the potentially responsible parties (PRPs). A review of DGLS documents and well logs supplemented by field measurements of water levels in private wells throughout the area is used to construct a potentiometric map and determine regional groundwater flow direction. Water tracing is used to confirm or deny the validity/accuracy of the potentiometric map and to establish hydrogeologic connections between the potential surface source of the contamination and the affected groundwater. Fluorescent dyes are injected into the subsurface, recovered in packets containing activated carbon and analyzed by a synchronously scanning spectrofluorometer. These results are compared to background fluorescence conditions established at the site prior to injection. The background conditions and dye recovery results must be submitted to the Missouri Water Trace Committee for approval as an independent quality assurance/quality control measure. Each of the PRPs found to be within the recharge area of the contaminated groundwater are investigated to determine the source, or sources, of the contamination. Initial field screening techniques are followed up by confirmatory sampling of soil and groundwater. Geophysical methods such as resistivity and electromagnetic conductivity are used to locate the bedrock surface, fracture patterns and areas of extensive petroleum contamination. On-site drilling activities are performed at each of the PRPs with on-site contamination screening methods such as analyzing soil gas within the borehole with a photoionization detector (PID) and colorimetric tubes. Soil samples are collected and can be analyzed on-site with immunoassay kits with confirmatory samples sent to the lab. Temporary monitoring wells are installed in boreholes to collect on-site groundwater data. Depth-to-water levels are measured, and the relative elevations of the temporary wells are surveyed so that a more detailed potentiometric map can be constructed. Water samples are collected from the temporary wells, and the boreholes are abandoned per Missouri Law. Petroleum fingerprinting by fluorescence is another tool used to investigate PRPs and determine the source of contamination. Petroleum is extracted from soil or water samples collected during on-site drilling and compared to the petroleum which is extracted from a sample collected at the contaminated spring or well.

Quantification of re-evaporated mass from loaded fibre-mist eliminators.

Airborne lubricant emissions are a serious health hazard for employees in the metal working industry. The basic components of lubricants are oils and additives for adapting the properties to achieve the process demands. The oils used in lubricants are either mineral, synthetic or of biological origin. The lubricants are used as water-emulsions and also as straight oils. Extreme process conditions cause considerable amounts of aerosol and vapour emissions of lubricants into the working environment. Fibre filters are used in industrial demisters for pollution control and also for sampling purposes. Re-evaporation of separated lubricants from loaded fibre filters causes increased vapour emissions. Quantification of lubricant vapour emission was the subject of this research. An apparatus and an appropriate procedure for measuring the dynamic behaviour of evaporative losses from fibre filters were developed. The test piece of a loaded fibre filter was fixed in a sampling probe according to VDI 2066. The vapour concentration of organic compounds in downstream air was measured in real-time by using a photo-ionisation detector (PID). The PID was checked by sampling the vapour on an oleophilic adsorbent resin, solvent extraction and quantitative IR analysis and also gravimetrically. The two basic processes of filtering volatile aerosols are, on the one hand, collecting droplets on the fibres and, on the other hand, the evaporation of collected liquid. These two processes had to be separated in order to measure the increase of the vapour concentration caused by the tested fibre filter. The experiments were carried out using pure dodecane and hexadecane in order to avoid difficulties due to the unknown chemical composition of lubricant vapour. The variation of the air flow and the initial liquid mass on the filter covered the relevant range for industrial fibre demisters and for sampling methods based on collecting aerosols on fibre filters. It was found that the downstream air was saturated with lubricant vapour for a wide range of filter loads and filtration velocities. From the results obtained it can be concluded that loaded industrial filter systems emit air with saturation vapour concentration throughout their operation. Hence, vapour emissions can be estimated easily if the saturation vapour pressure and the mean relative molecular mass are known. Moreover, results obtained from measurements of filters with low loads confirm that the phase transition from liquid to vapour is influenced by the identified process parameters. This is relevant for measurement methods using fibre filters for sampling volatile aerosols. A semi-empirical model to estimate the evaporative losses for low loadings is proposed. However, further research is necessary to determine the full range of parameters that are relevant for evaporative losses from filters loaded with low masses of aerosols.

Characterisation and On-Site Monitoring of Odorous Organic Compounds in the Environment of a Landfill Site

This study presents an analytical procedure developed in order to monitor specific volatile organic compounds (VOC) causing olfactive nuisances in the environment of a landfill site. l-Methyl-4-iso-propenyl-l-cyclohexene (limonene) and l-methyl-4-iso-propyl-benzene (p-cymene) detected by GC-MS in the landfill gas emissions were considered as tracers of the malodour. An on-line analytical system including a gas chromatograph coupled with a photoionisation detector was installed “on-site” to detect these markers at μ/m3 level in the atmosphere after pumping up the ambient air at close intervals. Quantitative results of the semi-continuous monitoring were strongly correlated with olfactive nuisances reported in the vicinity of the source. The chemical characterisation of odorous organic tracers and their “on-site” monitoring was shown to be a useful technique to detect malfunctions in the bio-gas collecting facilities and to prevent air pollution and olfactive nuisances in the environment.

PID-based estimation of the concentration of aromatic and chlorinated VOCs in soil

A rapid method for estimating the total concentration of aromatic and chlorinated volatile organic compounds (VOCs) in soil with a hand-held photoionization detector (PID) was developed to assist with recently adopted protocols for sample collection and handling. The results from sample screening can be used to rank the total concentration of VOCs or to answer a simple yes or no question with regard to a single value. Coupling a rapid method for estimating the total VOC concentration with sample collection and preparation procedures that limit substrate dissaggregation and exposure complements efforts to achieve site representative estimates for vadose zone contamination and efficient laboratory analyses. © 1999 John Wiley & Sons, Inc. Field Analyt Chem Technol 3: 193–200, 1999

The Validation of a Permeation Cell for Testing Chemical Protective Clothing

The performance of a simple rugged permeation cell for testing chemical protective clothing was compared with the reference cell suggested by the American Society for Testing and Materials and using their validation protocol. The new cell overcomes some of the limitations of the reference cell including ruggedness, low dead space, ease and speed of use, and small sample size. The testing of the new cell was performed under standard conditions using acetone against reference neoprene with an automated test system incorporating a photoionization detector. The performance of the new cell was found to be within the acceptance criteria for normalized breakthrough time and steady state permeation rate. The normalized breakthrough time index was a major impediment to the automated testing of more than one cell at a time, as it required a very low degree of cross contamination between cells, if a shared detector was used. It is suggested that lag times rather than normalized breakthrough times form part of the ba...