Choosing sorbents for thermal desorption (TD) sampling tubes and focusing traps takes into consideration several factors such as the strength of the sorbent–sorbate interaction, hydrophobicity, artefacts, mechanical strength, and inertness.
Filled with suitable sorbent(s), a TD sampling tube is capable of attaining quantitative retention and discharges compounds spanning from freons and C2 hydrocarbons to semi-volatiles such as phthalates, PCBs, and PAHs, without going beyond optimized tube dimensions and without needing liquid cryogen coolant.
Material emissions test techniques were generally targeted at very fine analyte ranges, usually from n- hexane to n-C16 or, in certain cases, from n-hexane to n-C22. Such techniques generally use tubes filled with Tenax® TA.
Though this sorbent has several advantageous qualities such as low inherent artefacts, hydrophobicity, good recovery of semi-volatiles and inertness, it is not appropriate for certain highly polar compounds. It is very weak for quantitative retention of species that is highly volatile than n-hexane.
Hence, the demand to measure highly volatile and semi-volatile toxic compounds is growing, increasing awareness about the use of sampling tubes filled with extra sorbents. In these ‘multi-bed’ tubes, up to four sorbents are stacked according to scaling strength from the sampling end, such that the ‘sticky’ parts that are less volatile meet the weakest sorbent, and are easily discharged when the gas flow is reversed during the following thermal desorption procedure.
Due to this fact, multi-bed sorbent tubes are generally applied in ambient air monitoring, and enable a broader instability range of parts to be quantitatively sampled and tested. A current revision of the main international standard technique for indoor air quality monitoring (ISO 16000-6) and material emissions testing includes the choice to apply multi-bed sorbent tubes.
The type of multi-bed sorbent tube referred to in the new regulation was applied in the two researches mentioned here.
The protocols used are illustrated succinctly here. Comprehensive details are available in the cited references. The experiments applied only stainless steel 3½" × ¼" sorbent tubes from Markes International, filled with quartz wool–Tenax TA– arbograph™ 5TD or Tenax TA.
Prior to using Markes’ TC-20™ off- line tube conditioner, all tubes were rigorously conditioned. The conditioned tubes were capped before and after sampling with two-piece brass long-term storage caps equipped with PTFE ferrules.
Markes’ Micro-Chamber/Thermal Extractor™ was applied to sample volatiles transmitted by polyurethane (PU) foam onto the sorbent tubes, which is followed by testing using Markes’ TD-100™ automated thermal desorber with GC–MS.
Emissions from PU foam were cut using a composite door known to discharge a blend of highly volatile, volatile and semi-volatile organic compounds. Material samples were kept in separate micro-chambers equilibrated at 23°C and the emissions were sampled in the two types of sorbent tubes.
Two similar tubes in sequence were used in each case. A front ‘sampling’ tube, and a rear ‘back-up’ tube to gather any analytes which may have broken through.
Sampling conditions were (a) 60min with an air flow of ~80mL/min, to produce the chromatograms displayed in Figure 1, and (b) 15min with an air flow of 50mL/min, to produce the mean values displayed in Figure 2.
Figure 1. Analysis of emissions from PU foam (sampling volume ~4.6 L), sampled using the Micro-Chamber/Thermal Extractor onto (A) two Tenax TA tubes connected in series, and (B) two multi-bed tubes connected in series. Analysis used a TD-100 automated thermal desorber and GC–MS. Internal standard (IS) = toluene-d8. Adapted from ref. 5 with permission from the Royal Society of Chemistry and the authors.
Figure 2. Mean concentrations (n= 7–9) with standard deviations for eight dominant compounds in the analysis of emissions from PU foam (sampling volume 0.75L), sampled using the Micro-Chamber/Thermal Extractor onto two Tenax TA tubes connected in series (red), and two multi-bed tubes connected in series (blue). Analysis used a TD-100 automated thermal desorber and GC–MS. (a) Values divided by 10 for ease of comparison. (b) No pure standard was available, and so quantitation used toluene equivalents. Adapted from ref. 5 with permission from the Royal Society of Chemistry and the authors.
To produce the results displayed in Figure 3, a blend of chemical standards related to material emissions testing was filled into the sorbent tubes in the gas phase either with or without methanol (as per the requirement) to provide a nominal loading of 100ng per part, with a bubbler used to produce humid atmospheres.
Figure 3. Mean percentage recoveries for n-hexane, methyl isobutyl ketone, toluene, butyl acetate, cyclohexanone, 1,2,3-trimethylbenzene and 4-phenylcyclohexene (nominally 100ng each) from Tenax TA and multi-bed sorbent tubes (n = 5), stored for up to 4 weeks at room temperature, after loading using air of moderate and low relative humidity (40% RH and <3% RH). The individual 4-week recoveries across all seven compounds with a 40% RH loading were between 93% and 104%. Image generated from data in ref. 6 with the permission of the authors.
Results and Discussion
Figure 1 evaluates the PU foam sample emissions using the two types of tubes. It is instantly obvious that the multi-bed tubes demonstrate improved recovery of the lightest analytes, methylcyclobutane (b.p. 36°C) and n-pentane (b.p. 36°C), without any detectable breach of either analyte on the relevant back-up tubes.
These results were substantiated by the analysis shown in Figure 2, which used the same setup apart from a lower flow rate and shorter sampling time, which is more characteristic of a basic sampling protocol.
Earlier, during storage there has been apprehension that less volatile analytes might drift from weaker sorbents to stronger sorbents within the multi-bed tubes. The consequence of this phenomenon is that compounds may become permanently bound to the stronger sorbent.
Nevertheless, the four-week stability data illustrated in Figure 3 reveals that the working of the multi-bed sorbent tubes was minimally equal to that of the Tenax TA tubes.
The researchers observed that while more tests on tubes loaded at higher humidities would be preferred, the results of this research showed that the humidity effect is quite small. Precaution has to be taken with storage for prolonged periods (for over two months) on multi-bed sorbents, as compounds can travel to the strong sorbent and become permanently bound.
The study also points out that, regardless of the sorbent combination applied, it is still advisable to analyze tubes as soon as possible subsequent to sampling, and preferably within a month.
The above results show that multi- bed sorbent tubes packed with Tenax TA, quartz wool, and Carbograph 5TD are well-suited to an extended range of analytes when compared to single-bed Tenax TA tubes.
Apart from allowing concurrent active/pumped sampling of volatiles from n-butane to n-C30, the stability of many volatile analytes is found to match with both tube types, therefore establishing the suitability of these multi-bed tubes for several thermal desorption applications.
This information has been sourced, reviewed and adapted from materials provided by Markes International Limited.
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