Extent of Sample Loss on the Sampling Device and the Resulting Experimental Biases When Collecting Volatile Fatty Acids (VFAs) in Air Using Sorbent Tubes

Not all volatile organic compounds (VOCs) are suitable for sampling from air onto sorbent tubes (ST) with subsequent analysis by thermal desorption (TD) with gas chromatography (GC). Some compounds (such as C2 hydrocarbons) are too volatile for quantitative retention by sorbents at ambient temperature, while others are too reactive – either for storage stability on the tubes (post-sampling) or for thermal desorption/GC analysis. Volatile fatty acids (VFAs) are one of the compound groups that present a challenge to sorbent tube sampling. In this study, we evaluated sample losses on the inner wall surface of the sorbent tube sampler. The sorptive losses of five VFA (acetic, propionic, <i>n</i>-butyric, <i>i</i>-valeric, and <i>n</i>-valeric acid) were tested using two types of tubes (stainless steel and quartz), each packed with three sorbent beds arranged in order of sorbent strength from the sampling end of the tube (Tenax TA, Carbopack B, and Carbopack X). It showed significantly higher losses of VFAs in both liquid phase and vapor phase when using stainless steel tube samplers. These losses were also seen if vapor-phase fatty acids were passed through empty stainless steel tubing and increased dramatically with increasing molecular weight, e.g., losses of 33.6% (acetic acid) to 97.5% (<i>n</i>-valeric acid). Similar losses of VFAs were also observed from headspace sampling of cheese products. Considering that stainless steel sampling tubes are still used extensively by many researchers, their replacement with quartz tubes is recommended to reduce systematic biases in collecting VFA samples or in their calibration

Test on the Reliability of Gastight Syringes as Transfer/Storage Media for Gaseous VOC Analysis: The Extent of VOC Sorption between the Inner Needle and a Glass Wall Surface

A gastight syringe (GTS) is commonly used as a medium for transfer or storage of gaseous standards (or samples) in the analysis of volatile organic compounds (VOCs). In this study, the sorptive loss patterns of 21 VOCs were examined, using GTS as the transfer medium. The results of the test were evaluated with respect to a number of key variables including concentration, sampling volume, and physicochemical properties (molecular weight and boiling point). The VOCs with relatively high volatility (Group 1: aldehyde, ketone, ester, alcohol, and aromatic hydrocarbons (<i>n</i> = 12)) showed low sorptive losses with a mean (±SD) of 2.56 ± 2.87%, regardless of differences in the aforementioned key variables (<i>p</i>-value by <i>t</i>-test before and after using GTS = mean 0.15 ± 0.13). Conversely, the sorptive losses of seven semi-VOCs (Group 2: carboxyl and cresol (<i>n</i> = 9)) were significantly high, ranging from 18.0 ± 4.10% (propionic acid) to 65.4 ± 10.9% (<i>n</i>-heptanonic acid). In addition, we also measured the sorptive losses on the syringe needle (mean sorptive loss of Group 2 = 5.94 ± 5.63%). A linear regression analysis showed that the sorptive losses for Group 2 increased as molecular weight (or boiling point) increased, exhibiting a highly significant correlation (<i>R</i>² value (0.804 ± 0.084) and mean <i>p</i>-value (0.002 ± 0.003)

Generation of Sub-Part-per-Billion Gaseous Volatile Organic Compounds at Ambient Temperature by Headspace Diffusion of Aqueous Standards through Decoupling between Ideal and Nonideal Henry’s Law Behavior

In the analysis of volatile organic compounds in air, the preparation of their gaseous standards at low (sub-ppb) concentration levels with high reliability is quite difficult. In this study, a simple dynamic headspace-based approach was evaluated as a means of generating vapor-phase volatile organic compounds from a liquid standard in an impinger at ambient temperature (25 °C). For a given sampling time, volatile organic compound vapor formed in the headspace was swept by bypassing the sweep gas through the impinger and collected four times in quick succession in separate sorbent tubes. In each experiment, a fresh liquid sample was used for each of the four sampling times (5, 10, 20, and 30 min) at a steady flow rate of 50 mL min^–1. The air–water partitioning at the most dynamic (earliest) sweeping stage was established initially in accord with ideal Henry’s law, which was then followed by considerably reduced partitioning in a steady-state equilibrium (non-ideal Henry’s law). The concentrations of gaseous volatile organic compounds, collected after the steady-state equilibrium, reached fairly constant values: for instance, the mole fraction of toluene measured at a sweeping interval of 10 and 30 min averaged 1.10 and 0.99 nmol mol^–1, respectively (after the initial 10 min sampling). In the second stage of our experiment, the effect of increasing the concentrations of liquid spiking standard was also examined by collecting sweep gas samples from two consecutive 10 min runs. The volatile organic compounds, collected in the first and second 10 min sweep gas samples, exhibited ideal and nonideal Henry’s law behavior, respectively. From this observation, we established numerical relationships to predict the mole fraction (or mixing ratio) of each volatile organic compound in steady-state equilibrium in relation to the concentration of standard spiked into the system. This experimental approach can thus be used to produce sub-ppb levels of gaseous volatile organic compounds in a constant and predictable manner

Ultimate Detectability of Volatile Organic Compounds: How Much Further Can We Reduce Their Ambient Air Sample Volumes for Analysis?

To understand the ultimately lowest detection range of volatile organic compounds (VOCs) in air, application of a high sensitivity analytical system was investigated by coupling thermal desorption (TD) technique with gas chromatography (GC) and time-of-flight (TOF) mass spectrometry (MS). The performance of the TD-GC/TOF MS system was evaluated using liquid standards of 19 target VOCs prepared in the range of 35 pg to 2.79 ng per μL. Studies were carried out using both total ion chromatogram (TIC) and extracted ion chromatogram (EIC) mode. EIC mode was used for calibration to reduce background and to improve signal-to-noise. The detectability of 19 target VOCs, if assessed in terms of method detection limit (MDL, per US EPA definition) and limit of detection (LOD), averaged 5.90 pg and 0.122 pg, respectively, with the mean coefficient of correlation (<i>R</i>²) of 0.9975. The minimum quantifiable mass of target analytes, when determined using real air samples by the TD-GC/TOF MS, is highly comparable to the detection limits determined experimentally by standard. In fact, volumes for the actual detection of the major aromatic VOCs like benzene, toluene, and xylene (BTX) in ambient air samples were as low as 1.0 mL in the 0.11–2.25 ppb range. It was thus possible to demonstrate that most target compounds including those in low abundance could be reliably quantified at concentrations down to 0.1 ppb at sample volumes of less than 10 mL. The unique sensitivity of this advanced analytical system can ultimately lead to a shift in field sampling strategy with smaller air sample volumes facilitating faster, simpler air sampling (e.g., use of gas syringes rather than the relative complexity of pumps or bags/canisters), with greatly reduced risk of analyte breakthrough and minimal interference, e.g., from atmospheric humidity. The improved detection limits offered by this system can also enhance accuracy and measurement precision

Understanding Size-Dependent Morphology Transition of Triangular MoS₂ Nanoclusters: The Role of Metal Substrate and Sulfur Chemical Potential

Molybdenum disulfide (MoS₂) nanoclusters have recently attracted enormous interest, due to their promising applications as catalysts in hydrodesulfurization of fossil fuels. It has been demonstrated that the catalytic activity of MoS₂ nanoclusters closely relates to their equilibrium morphology, which is in turn quite sensitive to various factors, such as the synthesis environments, the cluster size, and the substrates. Here, we carry out the density functional theory (DFT) calculations to study the size-dependent morphology change of triangular MoS₂ nanoclusters with all these factors systematically considered. Our results indicate that the stability of triangular MoS₂ nanoclusters is mainly determined by their edge and corner energies, and the variation of the ratio of the edge to corner energies with respect to the cluster size, chemical potential of sulfur, and substrates could induce a structural transition for their equilibrium morphology. By setting the chemical potential to fit experimental conditions, our calculations reveal a size-dependent morphology transition of triangular MoS₂ nanoclusters on Au(111) substrate, which is quantitatively consistent with experiments. In addition, the electronic structures of triangular MoS₂ nanoclusters are carefully studied. The results indicate that the metallic edge states, which is important for the hydrodesulfurization catalysis, are very sensitive to the substrates and only the clusters with Mo edge on Au(111) is found to have the one-dimensional metallic edge states. This result implies that in addition to the Mo edge, the metallic substrates may also play an important role in understanding the experimentally observed catalytic activity of MoS₂ nanoclusters, which has never been considered before

Development of the Detection Threshold Concept from a Close Look at Sorption Occurrence Inside a Glass Vial Based on the In-Vial Vaporization of Semivolatile Fatty Acids

Headspace (HS) analysis has been recommended as one of the most optimal methods for extracting and analyzing volatile organic compounds from samples in diverse media such as soil and water. Short-chain volatile fatty acids (VFA, C₃–C₇) with strong adsorptivity were selected as the target compounds to assess the basic characteristics of the HS analysis through simulation of HS conditions by in-vial vaporization of liquid-phase standards (VL) in 25 mL glass vials. The reliability of the VL approach was assessed by apportioning the in-vial VFA mass into three classes: (1) vaporized fraction, (2) dynamic adsorption on the vial walls (intermediate stage between vaporization and irreversible absorption), and (3) irreversible absorptive loss (on the vial wall). The dynamic adsorption partitioning inside the vial increased with n-VFA carbon number, e.g., 43% (C₂: acetic acid, extrapolated value), 65% (C₃: propanoic acid), and 98% (C₇: heptanoic acid). The maximum irreversible losses for the studied n-VFAs exhibited a quadratic relationship with carbon number. If the detection threshold limit (DTL: the onset of mass detection after attaining the maximum irreversible loss) is estimated, the DTL values for target VFAs were in the range of 101 ng for i-valeric acid to 616 ng for propionic acid, which are larger than the method detection limit by about 3 orders of magnitude. Consequently, quantitation of VFAs using the VL approach should be critically assessed by simultaneously considering the DTL criterion and the initial VFA masses loaded into the vial

Steric-Hindrance-Driven Shape Transition in PbS Quantum Dots: Understanding Size-Dependent Stability

Ambient stability of colloidal nanocrystal quantum dots (QDs) is imperative for low-cost, high-efficiency QD photovoltaics. We synthesized air-stable, ultrasmall PbS QDs with diameter (<i>D</i>) down to 1.5 nm, and found an abrupt transition at <i>D</i> ≈ 4 nm in the air stability as the QD size was varied from 1.5 to 7.5 nm. X-ray photoemission spectroscopy measurements and density functional theory calculations reveal that the stability transition is closely associated with the shape transition of oleate-capped QDs from octahedron to cuboctahedron, driven by steric hindrance and thus size-dependent surface energy of oleate-passivated Pb-rich QD facets. This microscopic understanding of the surface chemistry on ultrasmall QDs, up to a few nanometers, should be very useful for precisely and accurately controlling physicochemical properties of colloidal QDs such as doping polarity, carrier mobility, air stability, and hot-carrier dynamics for solar cell applications

Steric-Hindrance-Driven Shape Transition in PbS Quantum Dots: Understanding Size-Dependent Stability

Ambient stability of colloidal nanocrystal quantum dots (QDs) is imperative for low-cost, high-efficiency QD photovoltaics. We synthesized air-stable, ultrasmall PbS QDs with diameter (<i>D</i>) down to 1.5 nm, and found an abrupt transition at <i>D</i> ≈ 4 nm in the air stability as the QD size was varied from 1.5 to 7.5 nm. X-ray photoemission spectroscopy measurements and density functional theory calculations reveal that the stability transition is closely associated with the shape transition of oleate-capped QDs from octahedron to cuboctahedron, driven by steric hindrance and thus size-dependent surface energy of oleate-passivated Pb-rich QD facets. This microscopic understanding of the surface chemistry on ultrasmall QDs, up to a few nanometers, should be very useful for precisely and accurately controlling physicochemical properties of colloidal QDs such as doping polarity, carrier mobility, air stability, and hot-carrier dynamics for solar cell applications

Steric-Hindrance-Driven Shape Transition in PbS Quantum Dots: Understanding Size-Dependent Stability

Ambient stability of colloidal nanocrystal quantum dots (QDs) is imperative for low-cost, high-efficiency QD photovoltaics. We synthesized air-stable, ultrasmall PbS QDs with diameter (<i>D</i>) down to 1.5 nm, and found an abrupt transition at <i>D</i> ≈ 4 nm in the air stability as the QD size was varied from 1.5 to 7.5 nm. X-ray photoemission spectroscopy measurements and density functional theory calculations reveal that the stability transition is closely associated with the shape transition of oleate-capped QDs from octahedron to cuboctahedron, driven by steric hindrance and thus size-dependent surface energy of oleate-passivated Pb-rich QD facets. This microscopic understanding of the surface chemistry on ultrasmall QDs, up to a few nanometers, should be very useful for precisely and accurately controlling physicochemical properties of colloidal QDs such as doping polarity, carrier mobility, air stability, and hot-carrier dynamics for solar cell applications

First-Principles Study of Electronic Structure and Hydrogen Adsorption of 3d Transition Metal Exposed Paddle Wheel Frameworks

Open-site paddle wheels, comprised of two transition metals bridged with four carboxylate ions, have been widely used for constructing metal–organic frameworks with large surface area and high binding energy sites. Using first-principles density functional theory calculations, we have investigated atomic and electronic structures of various 3d transition metal paddle wheels before and after metal exposure and their hydrogen adsorption properties at open metal sites. Notably, the hydrogen adsorption is impeded by covalent metal–metal bonds in early transition metal paddle wheels from Sc to Cr and by the strong ferromagnetic coupling of diatomic Mn and Fe in the paddle wheel configurations. A significantly enhanced H₂ adsorption is predicted in the nonmagnetic Co₂ and Zn₂ paddle wheel with the binding energy of ∼0.2 eV per H₂. We also propose the use of two-dimensional Co₂ and Zn₂ paddle wheel frameworks that could have strongly adsorbed dihydrogen up to 1.35 wt % for noncryogenic hydrogen storage applications