Fourier Transform Infrared Spectrometer Measurements Research Articles

Atmospheric propane (C3H8) column retrievals from ground-based FTIR observations in Xianghe, China

Abstract. Propane (C3H8) is an important trace gas in the atmosphere, as it is a proxy for oil and gas production and has a significant impact on atmospheric chemical reactions related to the hydroxyl radical and tropospheric ozone formation. In this study, solar direct absorption spectra near 2967 cm−1 recorded by a ground-based Fourier transform infrared spectrometer (FTIR) were applied to retrieve C3H8 total columns between June 2018 and July 2022 in Xianghe in north China. The systematic and random uncertainties of the C3H8 column retrieval are estimated to be 18.4 % and 18.1 %, respectively. The mean and standard deviation of the C3H8 columns derived from the FTIR spectra in Xianghe are 1.80 ± 0.81 (1σ) × 1015 molec. cm−2. Good correlations are found between C3H8 and other non-methane hydrocarbons, such as C2H6 (R=0.84) and C2H2 (R=0.79), as well as between C3H8 and CO (R=0.72). However, the correlation between C3H8 and CH4 is relatively weak (R=0.45). Moreover, the FTIR C3H8 measurements in Xianghe are also compared against MkIV measurements at several sites around the world. The new FTIR measurements in Xianghe provide us with insight into C3H8 column variations and the underlying processes in north China.

Corrosion resistance of polymer-modified hardened cement paste and phosphoric acid-activated metakaolin geopolymer in carbonic acid solution

Corrosion resistance of hardened Portland cement paste (HCP), polymer-modified hardened cement paste (PHCP) and phosphoric acid-activated metakaolin (PMK) geopolymers in carbonic acid solution was comparatively evaluated by multiple techniques of thermogravimetry (TG), energy dispersive spectrometer (EDS), fourier transform infrared spectrometer (FTIR), inductively coupled plasma optical spectrometer (ICP), microindentation and compressive tests. The incorporated polymer kinetically slows down the corrosion process of Portland cement paste as the carbonation depth of PHCP is lower than those of HCP characterized by all techniques. Meanwhile, the addition of polymer slightly hinders leaching of substances from HCP by blocking the pores. Despite the increase of elastic modulus, decreases of compressive strength of both HCP and PHCP samples caused by carbonation are observed probably due to the generation of microcracks and uneven brittleness across the section. PHCP presents less mechanical loss after carbonation treatment than HCP. PMK geopolymer samples soaked in deionized water and carbonic acid solution have comparatively identical results of TG, EDS, FTIR and mechanical measurements, indicating that the PMK geopolymer is highly resistant to carbonic acid, thanks to its inherent acidic nature. In addition, the leaching amount of PMK geopolymer in carbonic acid solution is also much less than the HCP and PHCP.

A comparison of carbon monoxide retrievals between the MOPITT satellite and Canadian high-Arctic ground-based NDACC and TCCON FTIR measurements

Abstract. Measurements of Pollution In The Troposphere (MOPITT) is an instrument on NASA's Terra satellite that has measured tropospheric carbon monoxide (CO) from early 2000 to the present day. Validation of data from satellite instruments like MOPITT is often conducted using ground-based measurements to ensure the continued accuracy of the space-based instrument's measurements and its scientific results. Previous MOPITT validation studies generally found a larger bias in the MOPITT data poleward of 60∘ N. In this study, we use data from 2006 to 2019 from the Bruker IFS 125HR Fourier Transform Infrared spectrometer (FTIR) located at the Polar Environment Atmospheric Research Laboratory (PEARL) in Eureka, Nunavut, Canada, to validate the MOPITT version 8 (V8) retrievals. These comparisons utilize mid- and near-infrared FTIR measurements made as part of the Network for the Detection for Atmospheric Composition Change (NDACC) and the Total Carbon Column Observing Network (TCCON), respectively. All MOPITT version 8 retrievals within a radius of 110 km (1∘) from the PEARL Ridge Laboratory and within a 24 h time interval are used in this validation study. MOPITT retrieval products include those from the near-infrared (NIR) channel, the thermal infrared (TIR) channel, and a joint product from the thermal and near-infrared (TIR–NIR) channels. Each channel's detector has 4 pixels. We calculated the MOPITT pixel-to-pixel biases for each pixel, which were found to vary based on the season and surface type (land or water). The systematic bias for pixel 1 over land is larger than that for other pixels, which can reach up to 20 ppb. We use a small-region approximation method to find filtering criteria. We then apply the filters to the MOPITT dataset to minimize the MOPITT pixel bias and the number of outliers in the dataset. The sensitivity of each MOPITT pixel and each product is examined over the Canadian high Arctic. We then follow the methodologies recommended by NDACC and TCCON for the comparison between the FTIR and satellite total column retrievals. MOPITT averaging kernels are used to weight the NDACC and TCCON retrievals and take into account the different vertical sensitivities between the satellite and PEARL FTIR measurements. We use a modified Taylor diagram to present the comparison results from each pixel for each product over land and water with NDACC and TCCON measurements. Our results show overall consistency between MOPITT and the NDACC and TCCON measurements. When compared to the FTIR, the NIR MOPITT retrievals have a positive bias of 3 %–10 % depending on the pixel. The bias values are negative for the TIR product, with values between −5 % and 0 %. The joint TIR–NIR products show differences of −4 % to 7 %. The drift in MOPITT biases (in units of % yr−1) relative to NDACC and TCCON varies by MOPITT data product. In the NIR, drifts vs. TCCON are smaller than those vs. NDACC; however, this scenario is reversed for the MOPITT TIR and joint TIR–NIR products. Overall, this study aims to provide detailed validation for MOPITT version 8 measurements in the Canadian high Arctic.

The efficient recovery of molybdenite fines using a novel collector: Flotation performances, adsorption mechanism and DFT calculation

In this work, a novel collector N-(N-butyl) thiophosphoric triamide (NBPT) was explored to improve the flotation of molybdenite fines. The flotation performance was evaluated through micro-flotation tests, and the interaction mechanism was investigated through Zeta potential, Fourier Transform Infrared Spectrometer (FTIR), adsorption capacity, Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy (XPS), Atomic Force Microscopy (AFM) and density functional theory (DFT) calculations. Flotation results manifested NBPT could greatly improve the flotation of molybdenite fines. Zeta potential, FTIR, adsorption capacity and AFM measurements proved NBPT was more inclined to adsorb on molybdenite surface. XPS and ToF-SIMS analysis showed NBPT chemically adsorbed on molybdenite surface via the bonding between PS groups with Mo atoms. DFT calculations reconfirmed NBPT had much stronger affinity towards edges (Eads = -657.69 kJ/mol) than faces (Eads = -39.96 kJ/mol). Moreover, NBPT strongly interacted with molybdenite edges through the hybridization of the S 3p orbital of NBPT with the Mo 4d orbital of molybdenite to form strong covalence bond. As a result, NBPT could be utilized as a potential collector to enhance the flotation of molybdenite fines.

Experimental Evaluation of Volatile Organic Compound Quantification Methods for Reciprocating Natural Gas Engines

Abstract Compressor stations utilizing large-bore natural gas engines transport natural gas through pipelines worldwide. One emission class regulated by the Environmental Protection Agency (EPA) is volatile organic compounds (VOCs), which are nonmethane, nonethane, nonaldehyde hydrocarbons. The combination of a gas chromatograph (GC) and a flame ionization detector (FID) can measure VOCs, following EPA Method 18/25A. The Fourier transform infrared spectrometer (FTIR) also measures VOCs, following EPA Method 320. Multiple VOC calculation techniques are utilized, some combining measurements from separate analyzers. Two basic methods of extracting exhaust gas are direct extraction and Tedlar bag sampling. In this study, various VOC quantification methods are evaluated. Exhaust gas was sampled from a Cooper-Bessemer GMV lean-burn engine and a Caterpillar G3304 rich-burn engine. The GMV was tested in three configurations: open chamber spark ignition, precombustion chamber (PCC) ignition, and PCC ignition with high-pressure fuel injection. Ignition timing sweeps were performed on both engines, and a fuel variability test was performed on the GMV. Results showed that the Gasmet and MKS FTIRs’ (Method 320) VOC measurements deviate significantly from the HP GC when measuring low molar concentrations, albeit below regulatory limits. A common VOC quantification approach is subtracting the sum of methane and ethane FTIR measurements from a total hydrocarbon measurement utilizing a FID. This method produces uncertainties of 190% and overestimates VOC concentration by an average of 100%. The Tedlar bag sampling method produced VOC measurements within −2% of the direct extraction method.

Super-chiral vibrational spectroscopy with metasurfaces for high-sensitive identification of alanine enantiomers

Chiral nature of an enantiomer can be characterized by circular dichroism (CD) spectroscopy, but such a technique usually suffers from weak signal even with a sophisticated optical instrument. Recent demonstrations of plasmonic metasurfaces showed that chiroptical interaction of molecules can be engineered, thereby greatly simplifying a measurement system with high sensing capability. Here, by exploiting super-chiral field in a metasurface, we experimentally demonstrate high-sensitive vibrational CD spectroscopy of alanine enantiomers, the smallest chiral amino acid. Under linearly polarized excitation, the metasurface consisting of an array of staggered Au nano-rods selectively produces the left- and right-handed super-chiral fields at 1600 cm−1, which spectrally overlaps with the functional group vibrations of alanine. In the Fourier-transform infrared spectrometer measurements, the mirror symmetric CD spectra of D- and L-alanine are clearly observed depending on the handedness of the metasurface, realizing the reliable identification of small chiral molecules. The corresponding numerical simulations reveal the underlying resonant chiroptical interaction of plasmonic modes of the metasurface and vibrational modes of alanine. Our approach demonstrates a high-sensitive vibrational CD spectroscopic technique, opening up a reliable chiral sensing platform for advanced infrared inspection technologies.

Deriving Temporal and Vertical Distributions of Methane in Xianghe Using Ground-based Fourier Transform Infrared and Gas-analyzer Measurements

Methane (CH4) is one of the most important greenhouse gases in the atmosphere, making it worthwhile to study its temporal and vertical distributions in source areas, e.g., North China. For this purpose, a ground-based high-resolution Fourier transform infrared spectrometer (FTIR), the Bruker IFS 125HR, along with an in-situ instrument, the Picarro G2301, were deployed in Xianghe County (39.8°N,117.0°E), Hebei Province, China. Data have been recorded since June 2018. For the FTIR measurements, we used two observation modes to retrieve the mole fraction of CH4: the Total Carbon Column Observing Network (TCCON) method (retrieval algorithm: GGG2014), and the Network for the Detection of Atmospheric Composition Change (NDACC) method (retrieval algorithm: SFIT4). Combining FTIR with in-situ measurements, we found the temporal and vertical distributions of atmospheric CH4 within three vertical layers (near the ground, in the troposphere, and in the stratosphere), and throughout the whole atmosphere. Regarding the diurnal variation of CH4 near the ground, the concentration at night was higher than during the daytime. Regarding the seasonal variation, CH4 was low in spring and high in summer, for all three vertical layers. In addition, there was a peak of CH4 in winter near the ground, both in the troposphere and the whole atmosphere. We found that variation of CH4 in the tropospheric column was close to that of the in-situ measurements near the ground. Furthermore, the variations of CH4 in the stratospheric column could be influenced by vertical motions, since it was higher in summer and lower in winter.

Design, fabrication and optical characterizations of large-area lithography-free ultrathin multilayer selective solar coatings with excellent thermal stability in air

A sub-micron-thick selective multilayer solar thermal absorber made of tungsten, SiO2 and Si3N4 multilayer thin films was theoretically designed and experimentally demonstrated in this work. The optical performance was optimized by the particle swarm optimization algorithm for this multilayer absorber, whose spectral selectivity is associated with the Fabry-Perot resonance and anti-reflection effects. The designed multilayer absorber was deposited by sputtering and chemical vapor deposition techniques on the wafer scale. Its spectral absorptance characterized by a Fourier Transform Infrared spectrometer (FTIR) was demonstrated to be greater than 0.95 in the solar spectrum and less than 0.1 emittance in the mid-infrared with angular insensitivity. Temperature dependent FTIR measurements with an optical fiber setup revealed stable optical performance up to 600°C in ambient, while thermal cycle testing showed its long-term thermal stability at 400°C. Theoretical analysis of solar to power efficiency for a Carnot heat engine driven by the Solar heat was performed, which clearly shows that the proposed ultrathin selective multilayer absorber with spectral selectivity, angular insensitivity as well as high temperature stability could significantly boost the conversion efficiency of solar thermal systems at mid to high temperatures.

Background CO<sub>2</sub> levels and error analysis from ground-based solar absorption IR measurements in central Mexico

Abstract. In this investigation we analyze two common optical configurations to retrieve CO2 total column amounts from solar absorption infrared spectra. The noise errors using either a KBr or a CaF2 beam splitter, a main component of a Fourier transform infrared spectrometer (FTIR), are quantified in order to assess the relative precisions of the measurements. The configuration using a CaF2 beam splitter, as deployed by the instruments which contribute to the Total Carbon Column Observing Network (TCCON), shows a slightly better precision. However, we show that the precisions in XCO2 ( = 0.2095 ⋅ Total Column CO2Total Column O2) retrieved from > 96 % of the spectra measured with a KBr beam splitter fall well below 0.2 %. A bias in XCO2 (KBr − CaF2) of +0.56 ± 0.25 ppm was found when using an independent data set as reference. This value, which corresponds to +0.14 ± 0.064 %, is slightly larger than the mean precisions obtained. A 3-year XCO2 time series from FTIR measurements at the high-altitude site of Altzomoni in central Mexico presents clear annual and diurnal cycles, and a trend of +2.2 ppm yr−1 could be determined.

Facile fabrication of PVA composite fibers with high fraction of multiwalled carbon nanotubes by gel spinning

Poly(vinyl alcohol) (PVA) composite fibers with high fraction of multiwalled carbon nanotubes (MWCNTs) were prepared by gel spinning process. Here, a modified process was introduced to prepare concentrated PVA/MWCNTs/DMSO spinning dope, and to attain good dispersion of MWCNTs in the fibers. The final composite fibers were studied by thermogravimetric analyzer (TGA), Fourier transform infrared spectrometer (FTIR), Raman spectroscopy, differential scanning calorimetry (DSC), and WAXD analysis. The total content of MWCNTs in PVA composite fibers, from 5 to 30 wt%, was confirmed by TGA analysis. FTIR and Raman measurements demonstrated the existence of strong hydrogen interaction between MWCNTs and PVA matrix. SEM images of composite fibers showed smooth surface, regular cross‐section shape and good dispersion of MWCNTs in the fibers. DSC analysis showed that the crystallinity first increased and then decreased with the increase of MWCNTs contents. It can be concluded that low concentration of MWNCTs can act as nucleation sites for crystallization of PVA component, and large amount of MWCNTs may impede the crystallization of PVA component. The WAXD analysis results indicated that the crystal orientation of the PVA component in PVA composite fibers is almost identical at the same drawn ratio. Polarized Raman analysis indicated a small increase in MWCNTs orientation for the composite fibers. The mechanical properties tests showed that the composite fibers exhibit significant improvement in tensile strength and modulus as compared to the neat PVA fibers. The composite fibers also showed sustained growth in electrical conductivity. POLYM. ENG. SCI., 58:37–45, 2018. © 2017 Society of Plastics Engineers

Information content of space-borne hyperspectral infrared observations with respect to mineral dust properties

In principle, observations from hyperspectral infrared (IR) sounders such as IASI (infrared atmospheric sounding interferometer) can be used to simultaneously retrieve dust aerosol optical depth (AOD) and properties such as dust particle size, composition, emission temperature and height. Starting from a compilation of “typical” mineral dust particle size distributions and mineralogical compositions, the information content of dust spectra from Mie simulations and from FTIR (Fourier-transform-infrared spectrometer) measurements (provided by the University of Iowa) is analysed. While the Mie spectra provide a higher number of degrees of freedom for signal (up to 6.7) than the FTIR spectra (up to 5.7), the Shannon information content is slightly lower (3.4) from Mie than from FTIR (3.5). The analysis shows that the spectra provide information on particle size and composition, but information about both cannot be extracted independently owing to the correlations between the different spectra. A dust retrieval approach for IASI probing the spectral shape of extinction has been updated using the Mie and FTIR spectra. Dust properties provided by the retrieval algorithm are: AOD (at 0.55μm and 10μm), effective radius, mass-weighted mean diameter, weight-fractions of mineralogical components, IR single scattering albedo and dust layer effective emission temperature. The retrieval uncertainty in each of these parameters is calculated for each IASI pixel. From the retrieved dust layer temperature the dust layer altitude is also inferred using temperature profiles from the WRF numerical weather prediction model.To evaluate the impact of using the Mie and FTIR spectra within the algorithm, AODs determined using each are compared to AERONET and SEVIRI dust observations. This evaluation suggests that the overall performance of the retrieval in terms of AOD is better for the FTIR version. Evaluation (of the FTIR version) with AERONET coarse mode AOD shows a correlation of 0.73 with RMSD of 0.18 and bias of −0.07. 85% of IASI AOD retrievals are found to be within ±0.2 of AERONET coarse AOD. Evaluating the quality of the other retrieved parameters is more difficult, but we find that the values obtained do show a strong dependence on whether the Mie or FTIR spectra are used. For example, using FTIR spectra results in higher spatial variability in the clay fraction of the retrieved dust compared to Mie. Similar sensitivity is seen in the retrieved particle sizes and single scattering albedo. Indeed, assumptions made concerning the absorption properties of the Mie spectra result in the retrieval of unrealistically low dust layer altitudes, while reasonable values are obtained when using FTIR spectra. Thus it is important to acknowledge that good AOD agreement with independent validation sources does not automatically imply a similar level of quality in the remaining variables.

Preparation and thermal properties of polyphenylene sulfide/organic montmorillonite composites

Cetyl trimethyl ammonium bromide (CTAB) was used for the intercalation modification of nano-montmorillonite (MMT). The spectra of Fourier transform infrared spectrometer (FTIR) and X-ray diffraction (XRD) suggested that CTAB had intercalated into the layers of MMT. And the interlayer spacing of MMT increased from 1.198 to 3.616 nm. The thermogravimetric analysis (TGA) curves of organic montmorillonite (OMMT) showed three-step degradation. The scanning electron microscope (SEM) photographs showed that surface morphology of MMT remained unchanged. Then polyphenylene sulfide (PPS)/organic montmorillonite (OMMT) composites were prepared by melt blending. And composites were treated at 180 °C for 24 h. The results of SEM showed that OMMT had good dispersion behavior and adhesion with PPS. The results of differential scanning calorimetry (DSC) showed the addition of OMMT resulted in heterogeneous nucleation crystallization of PPS. The results of TGA showed there was a good improvement of the thermal stability of PPS. The FTIR and color measurement analysis showed the addition of OMMT could improve the thermal oxidation stability.

Fabrication and characterization of chitosan‐gelatin blend nanofibers for skin tissue engineering

Tissue engineering scaffolds produced by electrospinning feature a structural similarity to the natural extracellular matrix. Polymer blending is one of the effective methods to provide new and desirable biocomposites for tissue engineering applications. In this study chitosan was blended with gelatin and the effect of processing parameters of electrospinning and the solution properties of the polymer on the morphology of the fibers obtained were investigated. The morphology of the electrospun chitosan, gelatin and the chitosan-gelatin blend were characterized using a scanning electron microscope (SEM). The miscibility of the blend was determined using a SEM, and differential scanning calorimetry (DSC) Fourier transform Infrared spectrometer (FTIR). Further the tensile properties of the blend nanofibers were studied and compared with chitosan and gelatin fibers. In this study we have been able to electrospin defect-free chitosan, gelatin and chitosan-gelatin blend nanofibers with smooth morphology and diameter ranging from 120 to 200 nm, 100 to 150 nm, and 120-220 nm, respectively by optimizing the process and solution parameters. Chitosan and gelatin formed completely miscible blends as evidenced from DSC and FTIR measurements. The tensile strength of the chitosan-gelatin blend nanofibers (37.91 +/- 4.42 MPa) was significantly higher than the gelatin nanofibers (7.23 +/- 1.15 MPa) (p < 0.05) and comparable with that of normal human skin. Thus the novel chitosan-gelatin blend nanofiber system has potential application in skin regeneration.

Evaluation of photothermal effects in cartilage using FT-IR spectroscopy

Photothermal effects after laser irradiation of cartilage are investigated using an infrared focal plane array (IR-FPA) camera and a Fourier transform infrared (FT-IR) spectrometer. The IR-FPA camera records radiometric temperature profile, while the local laser heating is applied to the sample; whereas the FT-IR spectrometer analyzes absorption peaks of cartilage constituents. As the major effect of photothermal heating in cartilage is water evaporation, spectral changes because of dehydration between control and laser-irradiated cartilage are recorded by FT-IR spectrometer measurements. Additionally, another interest was the observation of the spectral changes from macromolecules such as collagen and proteoglycans because of phase transformation and/or conformational changes after laser irradiation. The methodology may be useful for quantitative investigation of the relationship between the clinically important phenomenon of accelerated stress relaxation and the kinetics of macromolecular denaturation.

Validation of ILAS‐II version 1.4 O3, HNO3, and temperature data through comparison with ozonesonde, ground‐based FTS, and lidar measurements in Alaska

Version 1.4 data from the Improved Limb Atmospheric Spectrometer‐II (ILAS‐II) were validated through comparison with profile data measured by several instruments at Poker Flat (65.1°N, 147.5°W), Alaska. The height profiles of O3 and HNO3 provided by ILAS‐II were compared with those retrieved from spectra measured by a ground‐based Fourier transform infrared spectrometer (FTS). The O3 and HNO3 abundances measured by ILAS‐II and FTS in the 17–35 km altitude range agreed within the precision of the two measurements. The O3 volume mixing ratios measured by ILAS‐II above (below) 20 km were 10% lower (higher) than those from the FTS measurements. The HNO3 values from ILAS‐II and FTS agreed to within 10% above 17 km. The O3 profiles obtained from electrochemical concentration cell (ECC) ozonesondes launched from Fairbanks (64.8°N, 147.9°W) were also compared to ILAS‐II data. The ILAS‐II ozone abundance agreed with the ozonesonde values. Comparison of mesospheric and upper stratospheric temperature data obtained by ILAS‐II and Rayleigh lidar indicated that the temperature derived from ILAS‐II was significantly higher (lower) than that from lidar at about 60 (40) km.

Evolution of ozone and ozone‐related species over Kiruna during the SOLVE/THESEO 2000 campaign retrieved from ground‐based millimeter‐wave and infrared observations

Vertical profiles of stratospheric ozone and ozone‐related trace gases were measured in winter 1999/2000 using the ground‐based millimeter‐wave radiometer MIRA 2 and a Fourier‐transform infrared spectrometer (FTIR) located at the Swedish Institute of Space Physics at Kiruna. The MIRA 2 measurements covered all three SOLVE/THESEO 2000 flight phases. An almost complete time series of O3 profiles and complementary profiles of ClO, HNO3, and N2O were achieved. Profiles of O3, HCl, HNO3, and N2O, as well as stratospheric column amounts of NO2, ClONO2, and ClO, were obtained from the continuous ground‐based FTIR measurements between January and March 2000. From the measurements of N2O and HF, a diabatic subsidence inside the polar vortex of about 1.2 km between January and March for altitudes of about 20 km was deduced. On 26 and 28 January, an uptake of about 25% of stratospheric HNO3 by polar stratospheric clouds (PSCs) could be measured and between January and March, an significant denitrification of the lower stratosphere was derived from the measurements. Strong chlorine activation was detected by both instruments in January and March, resulting in an ozone loss of more than 1 ppmv in a layer below 23 km. In comparison to recent cold winters, this layer was found to be quite thin. Therefore the total loss measured in column amount of (1.2–1.4) × 1022 molec/m2 was smaller than the losses in previous cold winters.

Automated Detection of Acetone, Methyl Ethyl Ketone, and Sulfur Hexafluoride by Direct Analysis of Fourier Transform Infrared Interferograms

The lack of a valid background reference spectrum for many Fourier transform infrared (FT-IR) spectrometric open-air monitoring applications limits the ability to perform quantitative measurements. Direct interferogram analysis suppresses the broad-band detector envelope and maintains the spectral signature of interest to circumvent this limitation. The background suppression approach uses a combination of interferogram segment selection, digital filtering, and pattern discrimination techniques. The spectral band location, width, and contour of the vapor determine the parameters necessary for background suppression. Interferogram segment selection relies principally on the spectral bandwidth. Digital filter design employs inputs of both spectral band location and width. Pattern discrimination methods consider the variation in the spectral band contour with band intensity. FT-IR spectrometer measurements from both laboratory and open-air trials demonstrate the utility of the background suppression approach with analytes of acetone, methyl ethyl ketone (MEK), and sulfur hexafluoride (SF6).