Technical Note pubs.acs.org/ac
Sorbent Coated Glass Wool Fabric as a Thin Film Microextraction Device Farhad Riazi Kermani and Janusz Pawliszyn* Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, N2L 3G1, Canada S Supporting Information *
ABSTRACT: A new approach for thin film microextraction (TFME) with mixed-phase sorptive coating is presented. Carboxen/polydimethylsiloxane (CAR/PDMS) and polydimethylsiloxane/divinylbenzene (PDMS/DVB) TFME samplers were prepared using spin coating and glass wool fabric mesh as substrate. The samplers were easily tailored in size and shape by cutting tools. Good durability and flat-shape stability were observed during extraction, stirring in water, and thermal desorption. The latter characteristic obviates the need for an extra framed holder for rapid TFME and makes the samplers more robust and easier to deploy. The samplers combine the advantages of adsorptive solid-phase microextraction (SPME) and TFME, including one-step solvent-free extraction and preconcentration, direct thermal desorption, and enhanced sensitivity without sacrificing analysis time due to thin film geometry. The analytical performance of these new devices was demonstrated using water samples spiked with N-nitrosamines (NAs) as model compounds. Over an order of magnitude enhancement of extraction efficiencies was obtained for the model compounds compared with the SPME fibers of similar coatings and PDMS thin film membrane. The results of this study indicate that these novel thin film devices are promising for rapid and efficient microextraction of polar analytes in water.
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transferred, it is normally sufficient to produce a significant analytical signal with modern detectors. In some applications, however, the SPME fiber technique may not provide the desired sensitivity due to low distribution coefficient of some analytes between the coating and sample matrix. To address this limitation, polydimethylsiloxane (PDMS)-based SBSE and TFME with larger volumes of the extraction phase (typically 25−125 times more than fiber geometry) were developed which proved to have considerably higher efficiencies than the fiber approach. Further, TFME was shown to achieve higher extraction rates and shorter equilibration times than SBSE, owing to larger surface area-to-volume ratio and small thickness of the extraction phase.8,11 Although SBSE with more polar absorptive phases (e.g., polyacrylate and ethylene glycol− silicone) have recently been introduced,12 the applications of this technique and PDMS thin film membrane microextraction are still limited in terms of sensitivity for polar analytes in water samples. This study reports, for the first time, development of mixedphase sorptive TFME devices as novel analytical tools for sampling and sample preparation of trace amounts of polar analytes in water. The fabrication method using PDMS, adsorptive particles, glass wool fabric mesh, and spin coating technique is demonstrated. An extraction setup is presented
ample preparation has been recognized as the main bottleneck of analytical processes in many applications, including environmental and water analysis.1 It is often the slowest, most complicated step of the process that has a direct impact on precision, accuracy, sensitivity, and overall performance of analytical methods. The conventional sample preparation methods for analysis of micropollutants in water are liquid−liquid extraction (LLE), purge-and-trap (P&T), and solid-phase extraction (SPE). While these methods have been frequently used for water analysis, they have some drawbacks such as the need for organic solvent, multistep time-consuming labor-intensive procedures, and the cost associated with the techniques. During the past two decades, efforts have increasingly been devoted to the development of simpler, faster, and more sustainable methods, with focus on miniaturization, reduction, or elimination of organic solvent and automation to enable higher throughput.2 These efforts have resulted in a number of novel microextraction techniques such as solid-phase microextraction (SPME),3,4 liquid-phase microextraction (LPME), 5,6 stir bar sorptive extraction (SBSE),7 and thin film microextraction (TFME).8 SPME fiber technique in particular has evolved very quickly appearing in almost every field of analytical chemistry, especially in environmental and water analysis.9 This stems from advantages offered by the technique including integration of extraction, isolation, preconcentration, and sample introduction into an easy single solvent-free step.10 Despite the fact that the sample capacity of the fiber approach is low and in most cases a small portion of analyte is extracted and © 2012 American Chemical Society
Received: July 4, 2012 Accepted: October 7, 2012 Published: October 7, 2012 8990
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using the mass spectra library of the National Institute of Standard and Technology (NIST, USA) and their retention times. The quantifications were done using selected ions for each compound. The ions used were 74 for NDMA, 88 for NMEA, 102 for NDEA, 100 for NPYR, 70 for NDPA, 114 for NPIP, and 84 for NDBA. The TFME and comparative analyses were carried out using an Agilent 6890 GC coupled with a 5973 MSD system. The Agilent GC was equipped with a MPS-2 multipurpose autosampler and a TDS-2 thermal desorption system (Gerstel GmbH, Mullheim, Germany) which was mounted on the GC via a CIS-4 cooled injection system inlet. The thermal desorption unit (TDU) served as a large volume injection (LVI) unit, and the CIS-4 served as a programmed temperature vaporizer (PTV) with cryogenic trapping capability for refocusing of the thermally desorbed analytes. The thermal desorption was performed under TDU-splitless/CIS-split mode with high gas flow rate (80 mL/min). The TDU initial temperature, delay time, and desorption time were set as 30 °C, 2 min, and 7 min, respectively. The transfer line situated between the TDU and the CIS was set at 300 °C. The CIS-4 cryogenic cooling temperature for all thin film analyses was set at −150 °C. The chromatographic separation was performed on a 30 m × 0.25 mm × 0.25 μm SLB-5MS capillary column from Supelco (Bellefonte, USA) with a Helium constant carrier gas flow rate at 1.6 mL/min. The column was initially set at 40 °C for 2 min, ramped at 5 °C/min to 75 °C, held for 5 min, then ramped at 25 °C/min to 200 °C, and held for 6 min, giving a total GC run of 25 min. The mass spectrometry was performed using selected ion monitoring (SIM) mode with electron impact ionization (EI). The ion used for quantification and qualification were 74 and 42 for NDMA, 88 and 42 for NMEA, 102 and 42 for NDEA, 100 and 68 for NPYR, 70 and 130 for NDPA, 114 and 55 for NPIP, and 84 and 57 for NDBA, respectively. Data acquisition was started after 3 min. Spin Coating. The novel samplers were prepared by the spin coating technique owing to the fastness, low volume fluid requirement, and high level of uniformity by which thin films can be prepared.13 The technique consists of four basic stages: (1) deposition of the coating fluid onto the center of a silicon wafer disk already mounted on the rotating platform, (2) acceleration to a high rotation speed or spin up stage, (3) continuation of the high speed rotation to gradually thin the fluid by the radial flow, referred to as spin off stage, and (4) evaporation, as illustrated in Figure 1A. The coating thickness can be precisely controlled by the speed and time of rotation. Generally, faster spin speeds and longer rotation times result in thinner coating layers. In this study, a Cee Model 200 precision spin coater (Brewer Science Inc., Rolla MO, USA) was used to prepare thin film coatings. Scanning Electron Microscopy (SEM). Surface characterization of the TFME devices was performed on a LEO 1530 field emission SEM (Carl Zeiss NTS GmbH, Germany) using 10 nm of gold deposition on the surface before the microscopy. Thin Film Fabrication Technique. To prepare mixedphase sorptive TFME samplers, PDMS gum (0.25 g) was initially diluted in methylene chloride (5−10 times) in a 10 mL screw cap vial. Fine particles of the adsorptive material were then suspended in the diluted PDMS at ratios similar to those of commercial fiber coatings (property of Supelco, Bellefonte, USA) by vigorous shaking for a few hours. A thin layer of glass wool fabric mesh was silanized with dimethyldichlorosilane (DMDCS, 5% in toluene) and then covered over a 4-in. wafer
which can be used for both direct and headspace TFME. The analytical performance of these new microextraction devices combined with direct thermal desorption, cryogenic trapping, and gas chromatography−mass spectrometry (GC/MS) is discussed. Also, their extraction performance is compared with those of commercial SPME fibers and PDMS thin film membrane using water samples spiked with N-nitrosamines (NAs) as model compounds.
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EXPERIMENTAL SECTION Chemicals and Supplies. A standard mix solution of Nnitrosamines (EPA 521) [N-nitrosodimethylamine (NDMA), N-nitrosomethylethylamine (NMEA), N-nitrosodiethylamine (NDEA), N-nitrosopyrrolidine (NPYR), N-nitrosodipropylamine (NDPA), N-nitrosopiperidine (NPIP), and N-nitrosodibutylamine (NDBA)] containing 2000 μg/mL of each compound in methylene chloride from Supelco (Bellefonte, USA) was used to prepare secondary stock solutions in methanol. Standard aqueous solutions were prepared by spiking appropriate volumes of the stock solutions into ultrapure water. Ultrapure water was obtained from a Barnstead/Thermodyne water system (Dubuque IA, USA). High density polydimethylsiloxane gum (PDMS), Carboxen 1006 (CAR), divinylbenzene (DVB) particles, dimethyldichlorosilane (DMDCS 5% in toluene), sodium chloride, vials (10 and 20 mL), and the SPME fibers used in this study, including carboxen/polydimethylsiloxane (CAR/PDMS 75 μm), and polydimethylsiloxane/divinylbenzene (PDMS/DVB 65 μm), were all supplied by Supelco (Bellefonte, USA). Low density PDMS was provided by Dow Corning Co. (Midland, USA). PDMS thin film membrane (75 μm) was purchased from Specialty Silicone Products Inc. (Ballston NY, USA). Analytical grade methanol and glass bottles (250 mL) were purchased from Fisher Scientific (Ottawa, Canada). Ultrapure Helium for gas chromatography and liquid nitrogen for cryogenic trapping were supplied by Praxair (Kitchener, Canada). Glass wool fabric mesh (58 μm) was provided by BGF Industries, Inc. (NC, USA). Silicon wafer disks (4 in.) were purchased from Montco Silicon (PA, USA). Stainless steel cotter pins were supplied by Spaenaur (Kitchener, Canada). Teflon holder and custom-made thin film cutters were created by University of Waterloo Science Shop (ON, Canada). The thin film-to-fiber volume ratio of similar coatings was estimated indirectly by weighing using a Radwag MXA 21 microbalance (North Miami Beach, FL). GC/MS Analysis. The SPME optimizations13 were carried out using a Varian 3800 gas chromatograph coupled with a 4000 ion trap MS detector (Varian, Mississauga, Canada). The gas chromatograph was equipped with a CTC Analytics CombiPAL autosampler having a temperature-controlled SPME agitator (Zwingen, Switzerland) and operated by Cycle Composer software (Version 1.4.0). The injection port was equipped with a SPME insert and was kept splitless during injection. The chromatographic separation was performed on a 30 m × 0.25 mm × 0.25 μm RTX-5 amine capillary column from Restek (Bellefonte, PA, USA). The column was initially set at 45 °C for 1 min, ramped at 5 °C/min to 100 °C, held for 1 min, and then ramped at 1 °C/min to 105 °C. Temperature was held at 105 °C for 1 min, then ramped at 30 °C/min to 250 °C, and held for 3.17 min, giving a total GC run of 28 min. The Helium carrier gas flow rate was constant at 1 mL/min. The mass spectrometry was performed using full scan mode (40 to 250 m/z) with electron impact ionization (EI). Data acquisition was started after 2 min. The NAs were identified 8991
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movable along the rod. The disk had a diameter equal to inner size of the bottle screw cap and was settled at the top of the bottle serving to hold the assembly during extraction and to seal the bottle. The grooved rod at the lower part was used for fitting the cotter pin. The separate tube served to tighten or loose the cotter pin end in the groove by moving along the rod. A schematic representation and typical images of the novel samplers, the Teflon holder, and the setup for TFME are presented in Figure 2. The sealed bottle setup designed in this study can be used for both direct and headspace TFME.
Figure 1. Schematic representations of (A) spin coating technique and (B) preparation of mixed-phase sorptive TFME sampler with glass wool fabric mesh substrate.
using adhesive tape. After complete dispersion of the particles in PDMS, the curing agent was added and the mixture was vortexed for a few minutes. The solvent was then allowed to evaporate so that the viscosity of the mixture could be adjusted to the desired level. Approximately 1.5 mL of the coating mixture was deposited on the fabric mesh in the middle of the wafer, already over the spin chuck, using a disposable pipet (Figure 1B). The spin coater lid was then closed, and the wafer was accelerated to the desired speed, typically 1000−1250 rpm, and kept rotating for a set period of time (30−40 s). Following the spin cycle, the lid was opened and the wafer was transferred into a vacuum oven. The thin film coating was thermally cured (80 °C) under vacuum for approximately 2 h. Afterward, the coated glass wool fabric mesh was removed carefully from the wafer, transferred over a thin sheet of Teflon, and cut into a house shape at the desired size (2 cm × 2 cm square with a 1 cm height triangle on top). SPME Procedure. The SPME fibers were conditioned before use, according to the manufacturer’s instructions. Extractions were performed by direct immersion (DI) in 10 mL of spiked water samples at room temperature. The agitation speed was 250 rpm, and the incubation and desorption times were 5 and 2 min, respectively. The desorption temperatures were 300 and 265 °C for CAR/PDMS and PDMS/DVB fibers, respectively. TFME Procedure and Setup. All TFME samplers were conditioned before use. The conditioning was done under a stream of helium in a tube inside an oven and later in a large volume GC injection port for a minimum of 1 h at temperatures typical of similar commercial SPME fiber coatings. The TDU liners containing conditioned samplers were stored in sealed 20 mL vials to minimize contamination from air. Prior to extraction, the samplers were reconditioned for 15 min, at temperatures appropriate of each coating. Extractions were performed manually by direct immersion in 250 mL glass bottles containing 240 mL of spiked water samples. A magnetic stirrer and Teflon-coated stir bar (4 cm length) were used for agitation. To facilitate the handling, each sampler was attached to a stainless steel cotter pin, initially cleaned by sonication, water, and methanol. To position and hold the samplers inside water in a reproducible way during extraction, a specially designed Teflon-made holder was used. The Teflon holder was composed of a thin disk at the upper part, a grooved rod at the lower part, and a separate tube
Figure 2. (A) Schematic representations and (B) images of mixedphase sorptive TFME samplers, Teflon holder, and the extraction setup.
Immediately after exposing the devices to the sample, the bottle was tightly capped and extraction began for a set period of time. After extraction, the TFME assembly was removed from the bottle and the sampler with the cotter pin was detached from the Teflon holder. The sampler was then dried gently and thoroughly by a soft lint-free tissue and rolled into a TDU glass liner (60 mm length, 6 mm o.d. and 4.8 mm i.d.) for TDU-CIS-GC/MS analysis. The liner was transferred from the MPS-2 tray into the TDU by the autosampler, where it was heated to the desired temperature (265 °C for PDMS/DVB or PDMS and 300 °C for CAR/PDMS coating) to desorb and transfer the analytes to the precooled (−150 °C) CIS-4 liner. For efficient desorption of analytes, a TDU-splitless/CIS-split mode with high gas flow rate (80 mL/min) was used. This combination mode with the two split points and a narrow liner cold trap (0.75 mm i.d. packed with silanized glass wools) placed between enabled high flow rate desorption of analytes without significant loss in the CIS-4 split. After desorption, the TDU liner containing the sampler was cooled and transferred back to the TDU tray by the autosampler. The CIS split was then closed, and the trap temperature was rapidly increased automatically to transfer the analytes to the GC column. 8992
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Technical Note
RESULTS AND DISCUSSION
Development of Mixed-Phase Sorptive TFME Devices. In the course of preparation of the new samplers, several different particle-to-PDMS ratios were tested. For slightly particle-loaded (up to ∼0.20 w/w ratio) samples and low density PDMS, the strength and integrity of thin films were less affected by mixing with the particles. On the other hand, the coatings prepared with higher loads of particles or high density PDMS exhibited poor cohesion and strength, to the extent that they were not practical without a support. Therefore, a flexible thermally resistant substrate had to be devised in order to make such devices. In search for a suitable support, thin samples of stainless-steel mesh, Teflon sheet, and glass wool fabric mesh were examined. Among these, the latter was found to be superior with ease of handling during extraction and analysis. The fabric mesh supported sample was easy to roll into the TDU liner and preserved its original flat shape upon removing from the liner. However, the other two substrates did not exhibit as much flexibility and flat-shape reversibility to deploy. Since one of the objectives of this work was to compare the extraction efficiencies of the new devices with those of commercial SPME fibers, samplers were prepared with particle-to-PDMS ratios equal to those of the commercial fiber coatings using glass wool fabric mesh support. Figure 3 presents SEM images of typical mixed-phase sorptive TFME samplers fabricated by spin coating and the mesh substrate. As shown, the coatings were quite uniform and the particles were tightly dispersed in PDMS. The particles and their porous surface are clearly visible in Figure 3 B. Both Carboxen and DVB particles were found to be spherical with a diameter size of less than 5 μm. Device Stability. The CAR/PDMS and PDMS/DVB TFME samplers fabricated in this study exhibited good mechanical stability during extraction and analysis. No detachment of the particles was observed due to stirring and extraction in water. As well, the samplers showed no deterioration with good repeatability after 50 times extraction and thermal desorption (Supporting Information, Figure S-1 and Figure S-2). Additionally, remarkable flat-shape stability was observed during stirring in water, which can be attributed to the reinforcement made by incorporation of the mesh substrate in the structure of these devices. Previous works in this group concerning TFME with PDMS thin film membranes had shown that these samplers were of poor rigidity and therefore could not maintain a flat-shape at rotational speeds commonly used for extraction.11 This causes a reduction in effective extraction surface area and consequently a decrease in the extraction rate. As a result, application of an extra framed holder had been proposed in order to preserve the thin film flat-shape and better kinetics during extraction.11 In an assessment of the stability during rotation in water, it was observed that the PDMS thin film membrane (75 μm) began to twist around itself even at a low stirring rate of 50 rpm. However, a PDMS sampler of similar thickness fabricated with the mesh substrate preserved its flat-shape up to 800 rpm stirring and only slightly twisted at rotation speeds up to 2000 rpm. Similar stabilities were observed of the particle-loaded samplers during rotation in water. This characteristic is clearly advantageous for fast active TFME, particularly for on-site applications, by obviating the need for a framed holder during extraction and making the samplers more robust and userfriendly.
Figure 3. SEM images of typical mixed-phase sorptive TFME samplers; (A) CAR/PDMS coating using 51× magnification and (B)DVB/PDMS coating using 18870× magnification; particle-toPDMS ratios were equal to that of the similar commercial SPME fiber coating.
Comparative Performance. During SPME optimization, it was found that the most appropriate coating for extraction of the smaller molecular size NAs is CAR/PDMS mixed phase.13 Due to higher polarity and solubility, these NAs represent the main challenge in extraction from water. Therefore, further discussion will be more focused on these compounds and CAR/PDMS coating. As illustrated in Figures S-3 and S-4 (Supporting Information), long extraction time profiles were observed using both the SPME fiber and TFME approach. However, as is seen from the slopes of the profiles, extraction rates were much higher at short sampling time with the thin film sampler compared to the SPME fiber. Table 1 compares the extracted amount ratios of the same analytes at short (30 min) and long (960 min) sampling times by the two approaches. As shown, a marked difference in the extraction efficiencies was observed for both sampling times with more than 1 order of magnitude enhancement by the CAR/PDMS TFME sampler. Also, the amount ratios for the short sampling time were close to the surface area ratio (∼110) whereas the ratios for the long sampling time were closer to the volume ratio (∼40), as predicted by the theory of TFME.8 Some observed deviations are likely associated with how close the extractions were to equilibration times. Similar enhancement of 8993
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Table 1. Amount Extracted by CAR/PDMS TFME (nt) Relative to CAR/PDMS SPME Fiber (nf)a with Corresponding Detection Limitsb LDL (ng/L) (RSD %)c
amount ratio (nt/nf) compound
acronym
N-nitrosodimethylamine N-nitrosomethylethylamine N-nitrosodiethylamine N-nitrosopiperidine
NDMA NMEA NDEA NPIP
log
Kow14
30 min
960 min
117 107 77 69
73 89 51 46
−0.57 −0.24 0.48 0.63
TFME 10 10 2 7
(3) (5) (6) (8)
SPME fiber 90 80 70 110
(5) (5) (6) (5)
a Direct extraction from 500 ng/L spiked water sample with 250 rpm stirring at room temperature; extraction-phase volume ratio (Vt/Vf) ∼ 40; surface area ratio (St/Sf) ∼ 110. b30 min sampling from 240 mL spiked water samples at room temperature. cRelative standard deviations (n = 7) for 2 μg/L spiked water are given in parentheses.
low as 40 mL can be extracted by a device of similar size used in this work. Samples of lower volumes can also be extracted in smaller vials provided that the size of the device would be tailored and reduced, which is convenient by the technique, yet at the cost of relatively lower sensitivity. Fabrication by spin coating and glass wool fabric mesh substrate could include a number of different directions. Singlephase sorptive TFME devices with enhanced flat-shape stability can be prepared with PDMS or other absorptive (liquid-like) materials such as polyacrylate (PA) and polyethylene glycol (PEG) at desired thicknesses. Other adsorptive particles such as Carbopack Z (a porous graphitized carbon material), hydrophilic−lipophilic balance (HLB) particles, e.g., divinylbenzeneN-vinylpyrrolidone (DVB-NVP) copolymer, C18,15 and a wide variety of other particles can be used for fabrication of mixedphase sorptive TFME devices. Samplers with dual-type coating such as DVB/CAR/PDMS of desired thickness can be prepared by sequential spin coating procedures. These can be fabricated as either layer over layer or as individual layers coated on each side of the sampler, which is an advantage in that it provides free access of analytes to each coating. The new TFME approach can be applied to other challenging micropollutants in water for more sensitive extraction and analysis. This technique can also be extended to fabric materials other than the glass wool. Single synthetic fibers have already been used successfully as extraction phases for a range of analytes,16 so the fabrics made of these materials might have similar properties with the advantage of higher surface areas, higher volume, and therefore better sensitivity. In that case, the fabric role is not only to support but also to extract. The extraction properties of the fabric can be enhanced as it has been showed in this work by impregnating the fabric with appropriate sorbents as discussed above.
extraction efficiencies was observed for more hydrophobic analytes (NDEA, NPIP, NDPA, NDBA) using the PDMS/ DVB TFME sampler compared with commercial PDMS/DVB fiber (Figure S-5, Supporting Information). In another assessment, the efficiencies of the PDMS/DVB TFME sampler and a PDMS thin film membrane were compared for the extraction of NDPA and NDBA, the NAs having the highest hydrophobic characters in the group, under similar experimental conditions. As shown in Figure S-6 (Supporting Information), the PDMS/DVB TFME sampler was several times to more than of 1 order of magnitude more efficient for the extraction of the analytes from water compared to the PDMS thin film membrane. Analytical Parameters. The precision of the technique represented by relative standard deviation (RSD) % of seven replicates at 2 μg/L spike level was 8% or better for CAR/ PDMS and 5% or better for PDMS/DVB TFME samplers. The calibration curves for the compounds were established in concentration ranges varying between 0.01 and 5 μg/L based on the analyte and the type of coating. Good linearities were obtained with both CAR/PDMS and PDMS/DVB TFME samplers with correlation coefficients (R2) exceeding 0.9923 and 0.9962, respectively. The lowest detectable levels (LDLs) for NDMA, NMEA, NDEA, and NPIP were in the range of 2− 10 ng/L using the CAR/PDMS sampler. The LDLs for NDPA and NDBA were 3 ng/L and for NPYR was 40 ng/L using the PDMS/DVB sampler. These LDLs were a result of detector signal responses at low concentrations that were 3 times the noise level (signal-to-noise ratios of 3). These analytical data along with the comparative results described earlier indicate that quantification of the NAs in water with mixed-phase sorptive TFME is reliable and more sensitive than SPME fiber.
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CONCLUSIONS The mixed-phase sorptive TFME samplers were conveniently prepared by spin coating and could be tailored in thickness by the spin procedure and in size and shape by cutting tools. The samplers were durable and exhibited enhanced flat-shape stability induced by incorporation of glass wool fabric mesh substrate in their thin film structure. This new sampling and sample preparation technique is promising for rapid and efficient microextraction of polar analytes in water because of ease of preparation, good analytical performance, higher sensitivity, and wider range of application offered by mixedphase coatings. The technique, however, is not fully automated compared with SPME fiber approach and (like SBSE and PDMS membrane TFME) requires a TDU-CIS interface for efficient thermal desorption and reconcentration of analytes. The 240 mL sample volume employed in this work is not a requirement for TFME from water. Samples with volume as
ASSOCIATED CONTENT
S Supporting Information *
Additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: 1-519-888-4567, ext. 84641. Fax: 1-519-746-0453. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the support from Canadian Water Network (CWN) and University of Waterloo Science 8994
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Shop (Harmen Vander Heide) as well as the supply of glass wool fabric mesh by BGF Industries, Inc. (Mike Bryant).
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REFERENCES
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