Detection limits of optical gas imagers as a function of temperature differential and distance

Optical gas imaging (OGI) is an effective tool for detecting gas leaks from process equipment. Despite the fact that OGI has been used for leak detection for over a decade, its detection limit is an elusive performance metric and has not been systematically characterized and quantified like other detection instruments. A substantial body of research has been performed that has shed some light on the OGI detection limits and the factors that dictate the detection limits. The OGI detection limit expressed as ppm-m and ppm now can be quantified as a function of ΔT (differential temperature between the gas plume and the background), the OGI camera type, and the specific gas in question. Furthermore, the OGI detection limit expressed as grams per hour can be calculated based on the ΔT and the distance from the OGI camera to the leak location under common leak survey conditions. For the same OGI camera, the detection limit can vary by several orders of magnitude due to ΔT and distance. The present work has demonstrated how different OGI detection limits can be. More importantly, this work has, for the first time, formulated equations that can be used to determine OGI detection limits with a given set of leak detection conditions. Being able to quantify OGI detection limit and understand the variables that dictate the detection limit is a significant advancement. It will help OGI to become accepted as a mature field instrument. The variables characterized in this work should have an impact on the development of OGI leak survey protocols, such as Appendix K to Code of Federal Regulations 40 CFR Part 60 in the United States. Established detection limits will also help emission inventory for fugitive emissions when OGI is used as the sole leak detection method.

Implications: Optical gas imaging (OGI) has been used for leak detection and control of fugitive volatile organic compound (VOC) emissions and methane emissions due to equipment leaks. However, detection limits of OGI have not been characterized and quantified like other detection instruments. The lack of well-understood detection limits has hindered broader applications of OGI. The work presented in this paper represents important steps that will enable OGI users and policymakers to establish (1) OGI detection limits under various conditions, (2) OGI leak survey criteria for a desired minimum detectable leak size, and (3) maximum potential emissions from the nondetect sources in emission inventory studies.     

Humic acid complexation of basic and neutral polycyclic aromatic compounds

Complexation by humic acid (HA) of basic (quinoline) and neutral (naphthalene) polycyclic aromatic compounds (PACs) was compared using fluorescence spectroscopy and equilibrium dialysis (ED). These compounds sorb to HA via cation exchange and hydrophobic interactions, respectively. Ionization of quinoline strongly affects its sorption to HA; maximum sorption is observed at pH close to logKb (4.92), and competition with H+ and electrolyte cation (Li+) is evident. Spectroscopic experiments indicate that quinolinium (QH+) cation fluorescence is quenched via a static mechanism (i.e., a dark complex is formed) when the protonated form is adsorbed via ion exchange to HA. The extent of sorption, calculated from fluorescence data using the Stern-Volmer equation, was compared to independent ED measurements. Although both methods indicated the same trends with solution chemistry, fluorescence quenching data suggested more extensive complexation than that measured using ED. In contrast to ionizable PACs, studied here and previously, interaction of naphthalene with HA is unaffected by changes in solution conditions (pH, ionic strength).     

Analysis of anaerobic BTX biodegradation in a subarctic aquifer using isotopes and benzylsuccinates

In situ biodegradation of benzene, toluene, and xylenes in a petroleum hydrocarbon contaminated aquifer near Fairbanks, Alaska was assessed using carbon and hydrogen compound specific isotope analysis (CSIA) of benzene and toluene and analysis of signature metabolites for toluene (benzylsuccinate) and xylenes (methylbenzylsuccinates). Carbon and hydrogen isotope ratios of benzene were between -25.9 per thousand and -26.8 per thousand for delta13C and -119 per thousand and -136 per thousand for delta2H, suggesting that biodegradation of benzene is unlikely at this site. However, biodegradation of both xylenes and toluene were documented in this subarctic aquifer. Biodegradation of xylenes was indicated by the presence of methylbenzylsuccinates with concentrations of 17-50 microg/L in three wells. Anaerobic toluene biodegradation was also indicated by benzylsuccinate concentrations of 10-49 microg/L in the three wells with the highest toluene concentrations (1500-5000 microg/L toluene). Since benzylsuccinate typically accounts for a very small fraction of the toluene present in groundwater (generally <1 mol%), the signature metabolite approach works best at higher toluene concentrations when it is not constrained by detection limits. In wells with lower toluene concentrations (410-640 microg/L), carbon and hydrogen isotopic values were enriched by up to approximately 2 per thousand for delta13C and approximately 70 per thousand for delta2H. This evidence of isotopic fractionation verifies the effects of biodegradation in these low concentration wells where metabolites may already be below detection limits. The combined use of signature metabolite and CSIA data is particularly valuable given the challenge of verifying biodegradation in subarctic environments where degradation rates are typically much slower than in temperate environments.     

Protandry in western cicada killer wasps, (Sphecius grandis,Hymenoptera: Sphecidae): an empirical study of emergence time and mating opportunity

Summary Sphecius grandis is a univoltine, colonial wasp. Females mate once and are sexually receptive when they emerge in July and early August. Males generally emerge earlier in the summer than females. The opportunity for each male to acquire mates is a function of the number of females emerging during his lifetime and the number of competitiors that are active when the females emerge. I determined a mating opportunity index (MOI) for each male in an aggregation of wasps for three separate summers, and correlated the MOI of individual males with their emergence date. The MOI of a male estimates the potential contribution that timing of emergence makes to his reproductive success. In 1984 males emerging near the mean of the male emergence distribution had the highest MOI. These males emerged between one and two weeks prior to the mean female emergence date. However, in 1981 late emerging males had the highest MOI. In 1983 there was no significant difference in MOI among males. As a result of between-year variation in female emergence schedules and in the duration of male lifetime, the selection pressures influencing male emergence time vary between years.     

Flexibility,learning, and irreversibility in environmental decisions: A bayesian approach

The Bayes decision is characterized and studied for a two-period decision problem involving a proposed resource development project. It is assumed that development is irreversible and that expectations about benefits and costs during the second period vary with experiences resulting from the first-period action. The amount and types of learning achieved depend upon the particular action taken during the first period. Flexible sequential decision procedures are compared with fixed once-and-for-all decisions. It is shown that under this specification, optimal development decisions are not necessarily more conservative when a sequential procedure is used. The possibility of reneging on a fixed decision is formally specified and studied.     

Does male harassment of females contribute to reproductive synchrony in the grey seal by affecting maternal performance?

We investigated the possibility that male harassment of lactating females differed in relation to time of birth in the grey seal, Halichoerus grypus, on Sable Island, Nova Scotia. This was done by comparing the frequency of male disturbances, maternal performance and pup growth for females that either gave birth during the peak of the pupping season or after the peak. Of the females, 58% gave birth in a 7-day period near the beginning of the pupping period, when the operational sex ratio was 2–4 females per male. Late in the pupping period the operational sex ratio reversed to about 1 female for every 2 males. The relative frequency of disturbances by males was significantly greater for late-pupping mothers than for peak-pupping ones (1.9 vs. 1.4 encounters/h). Females that gave birth late also were disturbed by males 3 times more often than females that gave birth during the peak (3.4 vs. 1.1 % of observation time). Late-pupping mothers spent 22% less time suckling (4.0 vs. 5.1 % of observation time), had 30% slower growing pups (1.7 vs. 2.4 kg/d), and weaned pups that were 16% lighter (45.6 vs. 54.0 kg). The effect of birth time on pup mass gain and weaning mass was not attributable to factors such as maternal mass, pup birth mass or pup sex. We conclude that the reduced maternal performance is likely the result of the increased male harassment. As reduced weaning mass can lead to reduced juvenile survival, male harassment may have contributed to the enhanced reproductive synchrony in this species.     

Flexible seed selection by individual harvester ants, Pogonomyrmex occidentalis

Summary Individual seed harvester ants (Pogonomyrmex) have been shown to specialize on specific seed types. We examined possible mechanisms for seed specialization and tested whether fidelity to food type limits the foraging decisions of individual western harvester ants, Pogonomyrmex occidentalis. The seed selection regimes of individually marked ants foraging at piles of two seed types were described and related to differences in seed quality and colonial dietary history. Individual foraging choices were affected by multiple factors, including seed caloric rewards, the previous seed selected, and the dietary history of the colony. Individual seed choices generally converged on the most energetically profitable species, suggesting that foragers exhibit labile preference. However, for a portion of the foragers, seed specialization was also partially due to constancy, defined as a tendency to select seed species that were previously collected. When colonies were presented with one seed type for 1 h and then were offered a mix of that seed and a novel seed type, individuals showed a strong preference for the novel seeds. Such rapid changes in seed preference argue strongly that individual P. occidentalis ants are highly flexible in seed choice and that resource assessment by these ants is more complex than simple maximization of net energetic return.Offprint requests to: J.H. Fewell at the current address     

2,4,6-Trinitrotoluene detection using recombinant antibodies

The ability to detect low levels of 2,4,6-trinitrotoluene (TNT) in aqueous samples is important due to the toxicity of both TNT and its breakdown products. We have been characterizing recombinant anti-TNT antibodies isolated from the Griffin library of phage displayed scFvs by selection for binders to the TNT-surrogate 2,4,6-trinitrobenzene (TNB) coupled to the protein bovine serum albumin. Two candidate antibody fragments, TNB1 and TNB2, were isolated and evaluated by ELISA for their ability to bind to TNB coupled to the protein ovalbumin. Competition ELISA was then used to demonstrate antibody fragment binding to TNT in solution and to examine cross-reactivity towards several TNT-related compounds and other explosives. Both recombinant antibody fragments were incorporated into a continuous flow assay for the detection of TNT. TNB2, the best single chain antibody, showed a limit of detection of 1 ng ml(-1), comparable to a commercially available anti-TNT antibody in the same assay format.     

Toward an Understanding of the Development of Ownership Feelings

Psychological ownership is increasingly recognized as a core feeling in the experience of work. Within jobs and the work context, there is a wide range of opportunities to experience psychological ownership. Yet empirical work on how feelings of ownership develop is lacking, and thus ways to develop psychological ownership in the workplace are not well understood. We explore the routes traveled to feelings of ownership by using job complexity as one example of work environment structure that affects the formation of psychological ownership. In two studies, we develop measures of the routes and confirm that perceived differences in one's work meaningfully predict psychological ownership. Collectively, the two studies provide insight into and offer suggestions for how ownership develops and ways in which managers might foster employee feelings of ownership toward their work. Copyright © 2013 John Wiley & Sons, Ltd.