Determination of recovery rates of adsorbents for sampling very volatile organic compounds (C1–C6) in dry and humid air in the sub-ppb range by use of thermal desorption gas

The reliable measurement of very volatile organic compounds (VVOC) in indoor air by use of thermal desorption gas chromatography (TD-GC) in order to include them into evaluation schemes for building products even nowadays is a great challenge.

Journal of Chromatography A 1626 (2020) 461389 Contents lists available at ScienceDirect Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma Determination of recovery rates of adsorbents for sampling very volatile organic compounds (C1 –C6 ) in dry and humid air in the sub-ppb range by use of thermal desorption gas chromatography-mass spectrometry Matthias Richter∗, Elevtheria Juritsch, Oliver Jann Bundesanstalt für Materialforschung und -prüfung (BAM), Unter den Eichen 87, 12205 Berlin, Germany a r t i c l e i n f o Article history: Received 13 March 2020 Revised July 2020 Accepted July 2020 Available online July 2020 Keywords: VVOC Indoor air: Adsorbent performance Recovery rate Thermal desorption Gas chromatography a b s t r a c t The reliable measurement of very volatile organic compounds (VVOC) in indoor air by use of thermal desorption gas chromatography (TD-GC) in order to include them into evaluation schemes for building products even nowadays is a great challenge For capturing these small molecules with carbon numbers ranging from C1 –C6 , strong adsorbents are needed In the present study, recovery rates of nine suitable adsorbents of the groups of porous polymers, graphitised carbon blacks (GCB) and carbon molecular sieves (CMS) are tested against a complex test gas standard containing 29 VVOC By consideration of the recovery and the relative humidity (50% RH), combinations of the GCB Carbograph 5TD, the two CMS Carboxen 1003 and Carbosieve SII as well as the porous polymer Tenax® GR were identified to be potentially suitable for sampling the majority of the VVOC out of the gas mix The results reveal a better performance of the adsorbents in combination than being used alone, particularly under humid sampling conditions The recovery rates of the chosen compounds on each adsorbent should be in the range of 80–120% © 2021 The Authors Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Introduction In the indoor environment, residents are exposed to a large number of various chemical pollutants originating from both the ingress from the outside and emissions from permanent sources indoors like building materials, furniture, electronic devices or non-permanent sources like household chemicals, etc Most of them are organic compounds, which are classified into very volatile, volatile and semi-volatile organic compounds (VVOC, VOC, SVOC) In the last decades, many studies have shown that these substances are responsible for health complaints often referred to as the Sick Building Syndrome (SBS) [1,2] The study discussed in this paper is focusing on the group of the VVOC, and follows the definition of the European testing standard EN 16516, in which VVOC are defined as “…volatile organic compounds eluting before n-hexane on the gas chromatographic column specified as a 5% phenyl / 95% methyl polysiloxane capillary column, …” (non-polar column) [3] ∗ Corresponding author E-mail address: matthias.richter@bam.de (M Richter) In Europe, the Construction Products Regulation (CPR, 2011/305/EU) sets basic requirements (BR) on how construction works must be designed and built BR “hygiene, health and the environment” states low emissions of toxic gases, VOC, particles, etc from building materials The relevant procedures for the determination of chemical emissions from materials used indoors in emission test chambers are described in the international standard series ISO 160 0 [4–7] and are specified in the harmonized European testing standard EN 16516 [3] This standard focuses on the analysis of pollutants in the VOC range, which it defines as all compounds eluting between C6 and C16 on a slightly polar capillary column with a 5%phenyl-/95%methyl-polysiloxane phase using thermal desorption gas chromatography coupled with a mass selective detector (TD-GC/MS) Measurement and analysis procedures are described in one document, yet it lacks an evaluation of the results To account for this gap, an expert group from EU member states has developed a roadmap towards an EU-wide harmonised framework for the health-based evaluation of indoor emissions from construction products published in the ECA-reports No 24, 27 and 29 [8–10] Relevant target compounds to be identified and traceably quantified in the test chamber air are listed on https://doi.org/10.1016/j.chroma.2020.461389 0021-9673/© 2021 The Authors Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) M Richter, E Juritsch and O Jann / Journal of Chromatography A 1626 (2020) 461389 the EU-LCI list Currently, this list is limited to only a few VVOCs (< C6 ), such as formaldehyde, acetaldehyde, butanal, pentanal and 2-butanone [11], since these analytes are measurable with HPLC using 2,4-dinitrophenylhydrazine (DNPH) as sorbent according to the ISO 160 0-3 procedure The porous polymer type adsorbent Tenax® TA is very well suited for the sampling of compounds in the VOC and SVOC range However, what poses a challenge on the TD-GC/MS method required by the testing standards EN 16516 and ISO 160 0-6 is the decrease of the retention volume of the stipulated adsorbent Tenax® TA with increasing volatility [12,13] Therefore, other adsorbents need to be selected to solve this problem A selection of suitable adsorbents can be found in the literature, e.g in Dettmer and Engewald [13] Woolfenden [14], ISO 16017-1 [15] or in manufacturers’/suppliers’ information, e.g Camsco [16] A standardised method for the analysis of VVOC is currently not available In his review, Salthammer [17] gives a good overview of approaches that have been published to date Only few are dedicated to a systematic validation of adsorbents and combinations of adsorbents to cover a wide VVOC range from carbon number C1 to C6 Schieweck, Gunschera, et al [18] went into this direction by systematically testing six different graphitized carbon blacks (GCB) and carbon molecular sieves (CMS) adsorbents for covering the compounds range of C3 –C6 For testing the suitability of the adsorbents, a recovery rate was determined by referring the arithmetic peak areas of the target compounds for each adsorbent to the arithmetic mean of the areas obtained by measurements on Tenax TA This procedure enables a rating of potentially suitable adsorbents but is neglecting matrix effects affecting measurement uncertainty On the one hand the test standards the adsorbents are spiked with are solutions of methanol, which is beyond sampling practice, and on the other hand the use of an adsorbent serving as reference is improper Pech, Wilke, et al [19] compared the three adsorbents Tenax TA, Carbograph 5TD and Carbopack X as to their suitability to retain a VVOC mix of 20 components in the gas phase However, they used Carbograph 5TD as reference In both studies, the performance of the adsorbents in the presence of water vapour in the sample air was excluded The aim of the present study is to determine the recovery rates of commercially available adsorbents suitable for the sampling of VVOC including compounds with carbon numbers C1 to C6 Nine adsorbents involving porous polymers, GCB and CMS were checked under consideration of relative humidity of the sampled air and loaded with a complex gas standard mixture composed of 29 VVOC and VOC around the C6 limit in the sub-ppb range Finally, based on the values obtained, possible combinations of adsorbents should be tested to get indication if this will lead to improved recovery Methods 2.1 Test gas preparation The gas mixture listed in Table was prepared in a gas collecting tube (GCT) with a volume of 500 mL and equipped with a septum and a valve for additional tightness Benzene, pentanal Table Analytes in gas mixes used for experiments Compound properties, such as retention time (RT), molecular weight (MW) and boiling point (b.p.) are given as well as the absolute mass loaded on adsorbent tubes for injection volumes 60 and 100 μL Substances printed in italic not belong to the group of VVOC according to the definition of ISO 160 0-6 and EN 16516 Carbon No C1 C2 C3 C4 C5 C6 ISTD1 Compound CAS No Formula RT (min) Chlorodifluoromethane Methanol Dichlorodifluoromethane Carbon disulfide Chloroform Vinyl chloride Ethanol Acetonitrile Propene n-Propane Acrolein Propanal Acetone Isopropyl alcohol Methyl acetate 2-Chloro propane 1-Propanol 1,3-Butadiene trans-2-Butene n-Butane cis-2-Butene Furan Diethyl ether Vinyl acetate 2-Butanone Ethyl acetate Isoprene n-Pentane Pentanal 2-Methylpentane Benzene n-Hexane Ethanol-d6 Benzene-d6 75–45–6 67–56–1 75–71–8 75–15–0 67–66–3 75–01–4 64–17–5 75–05–8 115–07–1 74–98–6 107–02–8 123–38–6 67–64–1 67–63–0 79–20–9 75–29–6 71–23–8 106–99–0 624–64–6 106–97–8 590–18–1 110–00–9 60–29–7 108–05–4 78–93–3 141–78–6 78–79–5 109–66–0 110–62–3 107–83–5 71–43–2 110–54–3 1516–08–1 1076–43–3 CHClF2 7.326 7.795 CH4 O CCl2 F2 9.845 18.348 CS2 24.164 CHCl3 11.225 C2 ClH3 C2 H6 O 13.562 14.864 C2 H3 N 7.945 C3 H6 8.635 C3 H8 C3 H4 O 16.788 17.715 C3 H6 O 17.967 C3 H6 O 18.709 C3 H8 O 19.801 C3 H6 O2 20.272 C3 H7 Cl 20.645 C3 H8 O 14.394 C4 H6 15.116 C4 H8 15.396 C4 H10 15.483 C4 H8 C4 H4 O 17.528 21.059 C4 H10 O 23.671 C4 H6 O2 24.296 C4 H8 O 25.333 C4 H8 O2 21.287 C5 H8 C5 H12 21.957 29.962 C5 H10 O 26.800 C6 H14 C6 H6 27.351 27.767 C6 H14 CD3 CD2 OD 13.375 27.044 C6 D6 MW (g mol−1 ) b.p (°C) 86.5 32.0 120.9 76.1 119.4 62.5 46.1 41.1 42.1 44.1 56.1 58.1 58.1 60.1 74.1 78.5 60.1 54.1 56.1 58.1 56.1 68.1 74.1 86.1 72.1 88.1 68.1 72.2 86.1 68.2 78.1 86.2 52.1 48.1 −40.7 64.6 8.9 46.0 61.1 −13.3 78.3 81.7 −47.7 −42.1 52.6 48.0 56.1 82.3 56.8 35.0 97.2 −4.5 0.9 0.5 0.9 3.7 35.0 71.6 79.5 77.1 34.0 36.1 103.1 60.2 80.0 69.0 Loaded mass (ng) Stability1 (%) Note 60 μL 100 μL 22 16 30 61 71 13 17 15 10 17 14 47 38 15 18 33 15 13 14 14 14 44 34 21 38 22 17 18 47 21 42 21 106 22 36 27 50 101 119 26 29 26 17 28 23 78 63 25 31 20 25 22 23 24 23 74 57 36 64 37 28 30 79 36 70 36 177 37 7 12 12 10 11 4 5 5 5 2 47 7 customised gas cylinder customised gas cylinder customised gas cylinder pure compound pure compound customised gas cylinder customised gas cylinder customised gas cylinder customised gas cylinder customised gas cylinder customised gas cylinder pure compound pure compound customised gas cylinder customised gas cylinder customised gas cylinder customised gas cylinder customised gas cylinder customised gas cylinder customised gas cylinder customised gas cylinder pure compound pure compound customised gas cylinder pure compound customised gas cylinder customised gas cylinder customised gas cylinder pure compound customised gas cylinder pure compound customised gas cylinder pure compound pure compound relative standard deviation of samplings out of the gas collecting tubes over a period of 14 days and calculated relative to the ISTD benzene-d6 Direct injection via split/splitless injector2 internal standard M Richter, E Juritsch and O Jann / Journal of Chromatography A 1626 (2020) 461389 Table Adsorbents used for the study Data provided by Woolfenden, manufacturer/supplier and Schieweck, Gunschera et al [14,16,18] Tdes corresponds to the desorption temperature used in this study (Section 2.2) Physical properties Surface area (m² g−1 ) Packing density (g cm−3 ) Tmax Tcond (°C) Tdes Mesh size Volatility range 35 0.28 350 320 300 60/80 C6 –C26 24 0.41 350 320 300 60/80 C7 –C30 Carbograph 5TD 560 n/a >400 350 350 40/60 C3 –C8 Carbopack B 100–200 0.35 >400 350 325 60/80 C5 –C12 Carbopack Z Carbosieve SII 220 1060 0.18 0.61 400 >400 350 350 325 330 60/80 60/80 C3 –C9 C1 –C2 Carboxen 569 485 0.61 >400 350 330 20/45 C2 –C5 Carboxen 1003 1000 0.46 >400 350 330 40/60 C2 –C5 Carboxen 1018 675 0.6 400 350 330 n/a C2 –C3 Adsorbent type Name Porous polymers Tenax TA Tenax GR Graphitized carbon black (GCB) Carbon molecular sieve (CMS) 1 Features Low affinity for water, hydrophobic Lower affinity for water than Tenax TA High thermal stability, low artifacts, hydrophobic High thermal stability, low artifacts, hydrophobic High thermal stability Different data available: some hydrophilicity to significant water retention, low artifacts Different data available: hydrophobic to some hydrophilicity Different data available: hydrophobic to some hydrophilicity, inert Different data available: hydrophobic to some hydrophilicity, inert mixture of Tenax TA and a GCB type adsorbent and n-hexane not belong to the group of the VVOC but were chosen as compounds of the transition region between the VVOC and VOC range The mixture contained 23 compounds taken from a pressurised gas cylinder, custom-made by Linde AG, Germany The remaining 10 compounds were mixed in equal proportions without solvent to two solutions Aliquots were spiked with a gas-tight syringe through the septum of the GCT that was already filled with the gas mix of the pressurized cylinder The temperature was kept at 23 °C For the tests, volumes of 60 or 100 μL of the test gas mix were taken with a gas-tight syringe and injected either directly into the split/splitless injector of the GC or onto the adsorbent to be tested as described in Section 2.4 Resulting amounts are given in Table To compensate measurement-related variations, benzene-d6 and ethanol-d6 were added as internal standards (ISTD) Prior to the experiments, the GCT was thoroughly checked for tightness and the generated test gas mixture for its stability Following a test gas mix injection into the GCT, constant amounts of the mixes were directly injected on a daily basis into the GC’s split/splitless injector over a period of 14 days with the relative standard deviation (RSD) being calculated 2.2 Analysis All test series were carried out on a gas chromatograph equipped with a split/splitless injector (Agilent 7890 N), an automated thermal desorption system (TDS 3/TDS A, Gerstel) using liquefied nitrogen cooling (CIS 4) and a mass selective detector (Agilent MSD 5975 C inert XL) A PLOT column (PoraBond Q, 50 m × 0.32 mm × μm, Agilent) with a polystyrenedivinylbenzene phase suitable for the separation of low boiling compounds was installed flushed with helium (ALPHAGAS, Air Liquide) as carrier gas Additionally, a particle trap was installed between column and MSD The m/z scan range was between 25 and 131 During the analyses, the test gas mix was injected in two ways: a) Directly with a gas-tight syringe via the split/splitless injector (splitless mode) to obtain an unaffected analysis signal (reference value): The oven programme started at 35 °C for min, then heating with °C min−1 to 80 °C for min, further heating with °C min−1 to 230 °C A carrier gas pressure of 0.97 bar was adjusted b) Via thermal desorption of the loaded adsorbent Since sampling of humidified air may have an impact on the analysis, two different thermal desorption modes were applied: b1) the splitless mode when dry air was used and b2) the solvent venting dry purge mode at humid conditions to prevent icing in the cold injection system (CIS) The TDS in both cases was programmed to start at 35 °C for min, then heating with a rate of 60 °C s − to 300–350 °C depending on the used adsorbent (Tdes in Table 2) for The CIS programme started at −150 °C, heating at 12 °C s − to 30 °C for followed by further heating at 12 °C s − to 150 °C held for A quartz wool filled liner was installed For the measurements of the adsorbents the GC oven was programmed to start at 35 °C for min, then heating at °C min−1 to 80 °C for min, further heating at 4.8 °C min−1 to 200 °C immediately followed by further heating at °C min−1 to 230 °C The carrier gas pressure was adjusted to 1.4 bar Fig depicts a chromatogram of the VVOC test gas mixture after injection via the split/splitless injector 2.3 Selection of adsorbents Sampling air always contains water that potentially affects sampling and analysis Helmig, Schwarzer, et al [20] report injected water can cause peak shifting due to restricted flow of carrier gas through the column, changes in carrier gas viscosity, and changes in the stationary phase polarity and split ratios Moreover, water vapour is able to condense in the small pores of molecular sieves [21] Other authors report on competition between analytes and water for active adsorbent sites [14,22], which may impact breakthrough volumes of analytes Vallecillos, Maceira, et al [23] report on significantly decreased breakthrough volumes for 1,3-butadiene on a multi-sorbent bed (Carbotrap B/Carbopack X/Carboxen 569) of 66% at an RH of 56–68% For the present study, mainly hydrophobic or slightly hydrophilic common adsorbents were selected (Table 2) However, M Richter, E Juritsch and O Jann / Journal of Chromatography A 1626 (2020) 461389 Fig Chromatogram of the VVOC test gas mixture analysed after direct injection into the split/splitless injector on a PoraBond Q (50 m × 0.32 mm × μm) the data provided for this parameter diverge in the literature Tenax® TA was used as benchmark Glass tubes (Gerstel, Germany) with an outer diameter of mm and a length of 176 mm were filled with the selected adsorbents Using the manufacturer’s marking, equal volumes of each adsorbent were filled into the tubes This resulted in the exact same bed lengths (60 mm) but in different absolute masses depending on the materials’ densities (Table 3) Tube conditioning was carried out according to the manufacturer’s recommendations (Table 2) Prior to the analysis, blank measurements were carried out injector were connected with the column via a Y-splitter Disactivated pre-columns were used to connect the injector with the Ysplitter This set-up enabled switching between both injectors and allowing a direct comparison of the amount of substance directly injected over the split/splitless injector with the amount that was desorbed from the tested adsorbent The recovery Ri was calculated according to Eq (1) Ri = Ai,T D × AIST D,re f Ai,T D,rel × 100% = × 100%, Ai,re f,rel AIST D,T D × Ai,re f (1) with 2.4 Determination of recovery The recovery is affected by the sorption behaviour, the desorption temperature and the relative humidity at the time of sampling Generally, for a distinct indication of the recovery of compounds from each adsorbent type, a reference value is required that represents 100% of the loaded amount (without losses) The reference value will then be related to the amount of substance desorbed from the adsorbent All effects of above discussed influences can be evaluated with this single value In some studies, clean adsorbent tubes are loaded with a test mixture of known composition and concentration and compared with the performance of other adsorbent types or the same adsorbent type impacted by variations of test parameters [18,24– 28] The adsorbent retaining the highest amounts of the target molecules is then taken as reference These procedures disregard any effects on the reference value obtained that might be resulting from interactions of the test sample molecules with the adsorbent, e.g breakthrough phenomena, insufficient desorption or chemical reactions Similar to the procedure reports by Dettmer, Knobloch, et al [29], the recovery in this study was determined with a test setup depicted in Fig The TD injector as well as the split/splitless Ri Recovery of component i in% Ai,TD Peak area of component i obtained by thermal desorption (TD) of adsorbent tube Ai,TD,rel Ai,TD in relation to the area of ISTD Ai,ref Peak area of component i obtained by direct injection onto GC column via split/splitless injector (reference) Ai,ref,rel Ai,ref in relation to the area of ISTD AISTD,TD Peak area of ISTD obtained by thermal desorption of adsorbent tube AISTD,ref Peak area of internal standard obtained by direct injection onto GC column via split/splitless injector For any experiment as described in this section, the reference value was determined by injection (n = 6) of an aliquot of the test gas mix directly into the split/splitless injector of the GC (route A in Fig 2) by use of a gas-tight syringe The average of the obtained peak areas was taken as Ai,ref and AISTD,ref , respectively The adsorbent tubes from Table were spiked with the same volume of test gas mix by injection into a carrier gas flow (V = L) passing through the adsorbent This spiking took place in the same room as the determination of the reference value to ensure the same ambient conditions The analysis of the adsorbent tubes, also given as peak areas, resulted in the values for Ai,TD and AISTD,TD respec- Table Recovery rates of tested adsorbents under dry (0% RH) and humid (50% RH) sampling conditions in order of their elution from the column The values are related to the internal standard (ISTD) benzene-d6 Recovery rates between 80% and 120% were allowed (bold numbers) Water retention at 50% RH is given as well Compounds in italic not belong to the group of VVOC as to definition in ISO 160 0-6 or EN 16516 Adsorbent (mass per tube) Compound Methanol Propene n-Propane Dichlorodifluoromethane Vinyl chloride Ethanol 1,3-Butadiene Acetonitrile trans-2-Butene n-Butane cis-2-Butene Acrolein Furan Propanal Acetone Carbon disulfide Isopropyl Alcohol Methyl acetate RH (%) 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 Tenax GR (240 mg) Carbograph Carbopack B 5TD (300 mg) (275 mg) Carbopack Z (140 mg) Carbosieve S II (500 mg) n d n d n d (35 ± 17)% (5 ± 4)% (3 ± 4)% (16 ± 3)% n d (2 ± 1)% n d n d n d (3 ± 1)% (73 ± 5)% (12 ± 7)% (2 ± 2)% (17 ± 3)% (71 ± 4)% (72 ± 12)% (1 ± 1)% (15 ± 6)% (1 ± 1)% (14 ± 2)% (1 ± 1)% (18 ± 4)% (83 ± 5)% (85 ± 8)% (12 ± 8)% (67 ± 9)% (114 ± 12)% (94 ± 7)% (101 ± 4)% (89 ± 6)% (7 ± 6)% (74 ± 9)% (120 ± 69)% (53 ± 8)% (89 ± 2)% (75 ± 4)% n d (3 ± 2)% (73 ± 7)% (8 ± 1)% (24 ± 22)% (54 ± 6)% (7 ± 5)% (36 ± 7)% (91 ± 15)% (90 ± 4)% (80 ± 15)% (73 ± 4)% (66 ± 13)% (77 ± 10)% (100 ± 3)% (75 ± 4)% (75 ± 3)% (93 ± 20)% (103 ± 4)% (76 ± 3)% (99 ± 2)% (77 ± 5)% (101 ± 1)% (76 ± 3)% (85 ± 14)% (116 ± 8)% (105 ± 1)% (93 ± 3)% (91 ± 19)% (87 ± 4)% (100 ± 4)% (95 ± 2)% (95 ± 4)% (96 ± 4)% (83 ± 22)% (84 ± 22)% (72 ± 13)% (83 ± 2)% n d (107 ± 5)% (73 ± 4)% (95 ± 17)% (102 ± 18)% (169 ± 43)% (106 ± 11)% (104 ± 6)% (49 ± 5)% (113 ± 6)% (28 ± 2)% (105 ± 6)% (18 ± 2)% (83 ± 4)% (93 ± 15)% (72 ± 20)% (75 ± 5)% (80 ± 12)% (146 ± 20)% (103 ± 7)% (92 ± 6)% (102 ± 5)% (102 ± 6)% (97 ± 4)% (88 ± 5)% (90 ± 7)% (97 ± 6)% (96 ± 8)% (95 ± 3)% (45 ± 30)% (60 ± 5)% (99 ± 3)% (103 ± 5)% (103 ± 4)% (103 ± 5)% (65 ± 36)% (30 ± 9)% (92 ± 2)% (96 ± 4)% (107 ± 2)% (61 ± 16)% (95 ± 17)% (152 ± 39)% (112 ± 4)% (83 ± 30)% (105 ± 3)% (56 ± 17)% (114 ± 0)% (24 ± 6)% (104 ± 4)% (81 ± 14)% (80 ± 6)% (58 ± 5)% (79 ± 19)% (31 ± 8)% (76 ± 6)% (188 ± 40)% (103 ± 2)% (53 ± 8)% (105 ± 2)% (103 ± 20)% (97 ± 6)% (39 ± 6)% (82 ± 10)% (45 ± 7)% (105 ± 3)% (88 ± 5)% (74 ± 21)% (18 ± 6)% (101 ± 3)% (76 ± 8)% (100 ± 5)% (50 ± 9)% (117 ± 66)% (20 ± 12)% (92 ± 1)% (55 ± 8)% (5 ± 5)% (1 ± 2)% (1 ± 1)% n d n d (17 ± 6)% (2 ± 0)% (25 ± 12)% (2 ± 0)% (2 ± 1)% (2 ± 1)% (28 ± 5)% (9 ± 5)% (33 ± 4)% (38 ± 2)% (10 ± 6)% (60 ± 38)% (61 ± 5)% n d n d (49 ± 13)% (17 ± 2)% (8 ± 6)% (12 ± 1)% n d (3 ± 1)% n d n d n d (1 ± 2)% (68 ± 4)% (1 ± 2)% (41 ± 36)% (69 ± 4)% (62 ± 15)% (16 ± 3)% (62 ± 31)% (76 ± 3)% (15 ± 18)% (58 ± 4)% (9 ± 9)% (59 ± 4)% (46 ± 1)% (56 ± 12)% (10 ± 9)% (61 ± 4)% (90 ± 5)% (68 ± 5)% (104 ± 5)% (54 ± 9)% (3 ± 1)% (59 ± 2)% (62 ± 24)% (20 ± 2)% (46 ± 21)% (17 ± 4)% (57 ± 12)% (8 ± 5)% n d n d n d (41 ± 3)% (103 ± 1)% (67 ± 5)% (103 ± 2)% (100 ± 2)% (102 ± 2)% (25 ± 7)% (105 ± 1)% (69 ± 4)% (91 ± 18)% (91 ± 5)% (48 ± 31)% (20 ± 9)% (106 ± 3)% (52 ± 6)% (85 ± 1)% (120 ± 20)% (113 ± 29)% (61 ± 9)% (103 ± 3)% (55 ± 7)% (113 ± 3)% (98 ± 17)% (105 ± 4)% (90 ± 3)% (79 ± 2)% (84 ± 12)% (81 ± 11)% (64 ± 7)% (68 ± 13)% (162 ± 53)% (102 ± 4)% (85 ± 4)% (101 ± 4)% (89 ± 5)% (99 ± 4)% (75 ± 2)% (82 ± 11)% (122 ± 11)% (104 ± 4)% (93 ± 2)% (40 ± 18)% (38 ± 8)% (96 ± 4)% (83 ± 12)% (104 ± 4)% (83 ± 5)% (90 ± 23)% (33 ± 7)% (91 ± 3)% (52 ± 7)% Tx GR/Cx 1003/Cs SII (85/105/115) Carboxen 1018 (570 mg) mg Cg 5TD/Cx 1003/Cs SII (95/95/140) mg (108 ± 3)% (82 ± 3)% 8% (116 ± 4)% 0% (136 ± 5)% 8% (129 ± 3)% 134% (86 ± 4)% 71% (108 ± 3)% 97% (133 ± 0)% 85% (108 ± 2)% 127% (108 ± 5)% 126% (105 ± 0)% 117% (129 ± 5)% 148% (105 ± 3)% 116% (100 ± 1)% 129% (91 ± 1)% 111% (145 ± 6)% 171% (134 ± 2)% 120% (118 ± 1)% 87% (125 ± 5)% 106% (130 ± 1)% 124% (80 ± 9)% (131 ± 28)% (102 ± 4)% (110 ± 4)% (103 ± 5)% (77 ± 7)% (66 ± 17)% (74 ± 1)% (103 ± 3)% (101 ± 3)% (97 ± 4)% (82 ± 4)% (102 ± 4)% (46 ± 13)% (98 ± 3)% (102 ± 5)% (71 ± 28)% (92 ± 2)% (88 ± 1)% 9% (114 ± 5)% 0% (225 ± 4)% 25% (153 ± 6)% 106% (86 ± 1)% 66% (113 ± 3)% 91% (115 ± 10)% 87% (85 ± 5)% 99% (117 ± 2)% 120% (110 ± 2)% 108% (138 ± 4)% 135% (102 ± 4)% 101% (100 ± 3)% 107% (91 ± 6)% 98% (92 ± 15)% 120% (113 ± 12)% 86% (106 ± 1)% 69% (109 ± 9)% 80% (130 ± 5)% 98% Carbotrap 300 (n a.) (93 ± 2)% (11 ± 1)% (87 ± 1)% (100 ± 33)% (189 ± 11)% (190 ± 3)% (134 ± 6)% (148 ± 12)% (97 ± 1)% (83 ± 3)% (92 ± 4)% (107 ± 6)% (77 ± 2)% (73 ± 6)% (35 ± 2)% (80 ± 22)% (99 ± 2)% (127 ± 2)% (85 ± 2)% (117 ± 18)% (114 ± 3)% (158 ± 12)% (83 ± 3)% (109 ± 12)% (37 ± 10)% (93 ± 6)% (79 ± 2)% (93 ± 3)% (86 ± 6)% (128 ± 9)% (124 ± 5)% (119 ± 7)% (80 ± 1)% (70 ± 8)% (94 ± 1)% (82 ± 12)% (110 ± 3)% (95 ± 21)% M Richter, E Juritsch and O Jann / Journal of Chromatography A 1626 (2020) 461389 Chlorodifluoromethane Tenax TA (200 mg) Carboxen Carboxen 569 1003 (440 mg) (365 mg) (continued on next page) Table (continued) Adsorbent (mass per tube) Compound 1-Propanol Diethyl ether Isoprene n-Pentane Vinyl acetate Chloroform 2-Butanone Ethyl acetate 2-Methylpentane Benzene n-Hexane Pentanal Number of retained compounds in the range of 80–120% recovery Water uptake at 50% RH per sampling volume (mg H2 O/g adsorbent) Tenax GR (240 mg) Carbograph Carbopack B 5TD (300 mg) (275 mg) Carbopack Z (140 mg) Carbosieve S II (500 mg) Carboxen 569 Carboxen (440 mg) 1003 (365 mg) Carboxen Tx GR/Cx 1018 (570 mg) 1003/Cs SII (85/105/115) mg Cg 5TD/Cx 1003/Cs SII (95/95/140) mg Carbotrap 300 (n a.) (21 ± 1)% (29 ± 9)% (62 ± 5)% (67 ± 5)% (49 ± 13)% (93 ± 3)% (87 ± 3)% (56 ± 8)% (79 ± 4)% (42 ± 9)% (70 ± 5)% (75 ± 13)% (62 ± 5)% (101 ± 2)% (108 ± 1)% (85 ± 3)% (83 ± 2)% (87 ± 1)% (92 ± 1)% (75 ± 4)% (76 ± 3)% (94 ± 87)% (276 ± 123)% (97 ± 0)% (88 ± 2)% (56 ± 8)% (80 ± 3)% 11 (87 ± 11)% (82 ± 8)% (58 ± 9)% (80 ± 17)% (98 ± 1)% (88 ± 4)% (102 ± 1)% (90 ± 4)% (97 ± 1)% (80 ± 3)% (28 ± 18)% (64 ± 8)% (97 ± 5)% (105 ± 1)% (77 ± 11)% (84 ± 2)% (76 ± 10)% (95 ± 2)% (96 ± 1)% (81 ± 1)% (119 ± 5)% (127 ± 4)% (98 ± 1)% (86 ± 2)% (39 ± 11)% (79 ± 4)% 20 (87 ± 4)% (78 ± 4)% (50 ± 17)% (47 ± 13)% (98 ± 0)% (88 ± 2)% (103 ± 1)% (92 ± 2)% (98 ± 0)% (81 ± 2)% (24 ± 6)% (38 ± 9)% (103 ± 0)% (107 ± 1)% (81 ± 5)% (85 ± 1)% (68 ± 15)% (92 ± 8)% (98 ± 0)% (82 ± 1)% (103 ± 23)% (99 ± 2)% (98 ± 1)% (87 ± 1)% (48 ± 1)% (81 ± 3)% 11 (100 ± 4)% (100 ± 5)% (38 ± 5)% (57 ± 7)% (74 ± 72)% (96 ± 1)% (82 ± 4)% (57 ± 19)% (34 ± 3)% (98 ± 3)% (85 ± 4)% (35 ± 24)% (22 ± 2)% (104 ± 2)% (113 ± 2)% (46 ± 23)% (37 ± 3)% (81 ± 7)% (91 ± 7)% (93 ± 4)% (65 ± 4)% (107 ± 10)% (97 ± 2)% (93 ± 5)% (64 ± 4)% (13 ± 10)% (14 ± 5)% 23 (92 ± 4)% (19 ± 5)% (61 ± 5)% (46 ± 56)% (96 ± 3)% (39 ± 9)% (87 ± 10)% (20 ± 8)% (98 ± 1)% (76 ± 5)% (66 ± 22)% (12 ± 9)% (101 ± 4)% (43 ± 11)% (77 ± 5)% (19 ± 9)% (86 ± 1)% (49 ± 11)% (97 ± 0)% (66 ± 5)% (120 ± 10)% (104 ± 2)% (98 ± 1)% (52 ± 10)% (23 ± 6)% (6 ± 3)% 25 (83 ± 6)% (25 ± 2)% (55 ± 4)% (25 ± 16)% (97 ± 3)% (77 ± 4)% (86 ± 11)% (58 ± 6)% (97 ± 2)% (82 ± 2)% (40 ± 24)% (8 ± 2)% (95 ± 3)% (72 ± 5)% (64 ± 15)% (29 ± 5)% (84 ± 4)% (39 ± 8)% (95 ± 3)% (75 ± 3)% (133 ± 43)% (96 ± 1)% (96 ± 4)% (74 ± 5)% (13 ± 8)% (10 ± 3)% 24 (92 ± 5)% (86 ± 28)% 37% (104 ± 8)% 107% (114 ± 3)% 94% (66 ± 10)% 54% (133 ± 4)% 121% (69 ± 29)% 64% (78 ± 8)% 51% (79 ± 3)% 65% (98 ± 2)% 92% (129 ± 3)% 105% (88 ± 7)% 91% (106 ± 1)% 102% (62 ± 25)% 54% 21 (125 ± 1)% 117% (117 ± 2)% 120% (115 ± 1)% 107% (94 ± 2)% 106% (129 ± 2)% 132% (94 ± 7)% 98% (91 ± 1)% 68% (84 ± 2)% 89% (97 ± 1)% 98% (127 ± 1)% 132% (92 ± 1)% 93% (104 ± 2)% 115% (70 ± 8)% 92% 20 (51 ± 17)% (87 ± 29)% (102 ± 2)% (91 ± 27)% (112 ± 1)% (138 ± 1)% (96 ± 1)% (112 ± 3)% (125 ± 3)% (158 ± 4)% (32 ± 8)% (117 ± 2)% (72 ± 4)% (80 ± 5)% (83 ± 3)% (85 ± 1)% (92 ± 1)% (113 ± 2)% (123 ± 2)% (148 ± 4)% (97 ± 1)% (87 ± 2)% (104 ± 1)% (112 ± 2)% (57 ± 4)% (84 ± 1)% 19 50 1L n d 19 1.5 10 n d 16 20 8.5 11 10.7 19 n a 18 n a 21 n a 3L 5L n d n d 1.7 2.2 n d n d 42 62 12.0 14.4 11.6 12.2 n a n a n a n a n a n a 50 50 50 50 50 50 50 50 50 50 50 50 50 (54 ± 3)% (55 ± 6)% (32 ± 2)% (24 ± 1)% (75 ± 4)% (86 ± 7)% (85 ± 3)% (88 ± 3)% (40 ± 2)% (105 ± 19)% (85 ± 5)% (59 ± 9)% n d.: not detectable measurements under humid conditions carried out without repetition (31 ± 9)% (99 ± 2)% (104 ± 2)% (98 ± 2)% (3 ± 3)% (103 ± 1)% (69 ± 6)% (28 ± 12)% (97 ± 2)% (109 ± 9)% (98 ± 1)% (34 ± 7)% 15 (51 ± 5)% (95 ± 3)% (69 ± 18)% (97 ± 3)% (66 ± 30)% (97 ± 6)% (63 ± 21)% (84 ± 3)% (94 ± 1)% (104 ± 12)% (94 ± 3)% (12 ± 3)% 21 M Richter, E Juritsch and O Jann / Journal of Chromatography A 1626 (2020) 461389 2-Chloro propane Tenax TA (200 mg) M Richter, E Juritsch and O Jann / Journal of Chromatography A 1626 (2020) 461389 Fig Set-up for the determination of the VVOC recovery of the adsorbent tubes tively The calculation of the recovery in% was carried out according to Eq (1) A tolerance of the recovery of ± 20% around 100% was permitted as this variation might be resulting from other effects not necessarily related to the sampling, e g measurementrelated variations The recovery rate was firstly determined under dry conditions (0% RH of carrier gas) with an injection of 60 μL of the test gas mix leading to a first selection of potentially suitable adsorbents These were then investigated with a humidified carrier gas flow adjusted at 50% RH, since this degree of humidity is required in the relevant testing standards mentioned in the introduction section As the analysis is impacted by humidity, the TD method was adjusted by switching to the solvent venting dry purge mode This in turn led to a decrease of the sensitivity of the analysis, which was compensated by an increase of the injected amount of the test gas mix to 100 μL The resulting loading amounts are given in Table Results and discussion 3.1 Stability of gas standards and GCT tightness As shown in Table 1, for the majority of the compounds the stability expressed by the RSD determined by single injections over 14 days was better than 10%, the maximum was obtained for pentanal with 47% Based on the analyses carried out for the experiments, a satisfying explanation for this result cannot be given However, tightness of the GCT and compound stability could be regarded as sufficient for use of the gas standard for at least 14 days 3.2 Determination of recovery under dry and humid sampling conditions In Table 3, the recovery rates determined for dry and humid air sampling on single and combined adsorbents are listed The mean values and standard deviations of four (dry air) and seven (humidified air) repetitions are given, except for the multi-bed tubes Here, the loadings were repeated only three times For the reference values Ai,ref and AISTD,ref , relative standard deviations (RSD) between and 8% throughout both measurement series were obtained From the two ISTD only for benzene-d6 recovery rates near 100% were obtained on all tested adsorbents Benzene-d6 was hence used to compensate variation of measurement performance Chromatograms of the analysis of the adsorbents under both sampling conditions are provided in the supplementary material (S1– S12) 3.2.1 Dry sampling conditions (0% RH) In view of the amount of retained compounds within the average recovery range of 80–120%, the CMS Carbosieve S II, Carboxen 569 and Carboxen 1003 showed the best retention ability for the majority of the VVOCs at 0% RH followed by the GCB Carbograph 5TD The weaker Carbopack B and Tenax GR performed well for the polar compounds 2-butanone [(85 ± 2)% and (81 ± 5)%], propanal [(114 ± 12)% and (90 ± 5)%] and Carbopack B for the less polar compound isoprene (103 ± 1)% compared to the others Finally, these six adsorbents were selected for further tests under humid sampling conditions Although Carboxen 1018 showed as good recovery rates as the other CMS it was not selected, since sulphur dioxide (SO2 ) is produced in the adsorbent (cf Section 3.2.5) 3.2.2 Humid sampling conditions (50% RH) The repetition of the recovery tests under humid sampling conditions revealed a significant impact of water vapour From the two Carboxens, the number of retained compounds with average recoveries between 80 and 120% decreased from 25 to for Carboxen 569 and from 24 to 11 for Carboxen 1003 As reported by Vallecillos, Maceira, et al [23] this may also be linked with the active sites on the adsorbents’ surfaces covered by water molecules and is correlating with the relatively high water uptake compared to the other adsorbents Moreover, breakthrough volumes of the target compounds can also be affected by the presence of humidity during the sampling [30] Carbosieve S II as well showed decreased retention capacity for some VVOC, however, to a much lower extent (from 23 to 16 compounds) and at a significantly higher water uptake as observed for the Carboxens The GCBs Carbopack B, Carbograph 5TD and Tenax GR, which is a mixture of Tenax TA and a graphitised carbon, are only slightly affected by air humidity corresponding to their low water uptake As could be observed, the recovery of some – mainly polar – compounds increased in presence of water vapour in the supply air These are methanol on Carboxen 1003 [increase from (85 ± 1)% to (120 ± 20)%] and ethyl acetate on Carbopack B [(68 ± 15)% to (92 ± 8)%] and Carbograph 5TD [(76 ± 10)% to (95 ± 2)%] For pentanal, which is less polar - and not a VVOC - the recovery increased significantly on Tenax GR [(56 ± 8)% to (80 ± 3)%], Carbopack B [(48 ± 1)% to (81 ± 3)%] and Carbograph 5TD [(39 ± 11)% to (79 ± 4)%] Generally, for all adsorbents, dissatisfying recoveries (< 80%) under humid conditions were observed for chlorodifluoromethane, n-propane, 1,3-butadiene, isopropyl alcohol and vinyl acetate 3.2.3 Testing of multi-bed tubes The high standard deviations of the recovery for a few compounds can either be explained by analytical reasons or by incomplete desorption or breakthrough Therefore, combinations of adsorbents should be taken into consideration Based on the recoveries in Table and under consideration of a relative humidity of 50%, the combinations Carbograph 5TD/Carboxen 1003/Carbosieve SII and Tenax GR/Carboxen 1003/Carbosieve SII were considered for further testing following the procedure described in M Richter, E Juritsch and O Jann / Journal of Chromatography A 1626 (2020) 461389 Section 2.4 and compared to the commercial multi-bed tube Carbotrap 300 (Gerstel, Germany) containing Carbotrap C/Carbotrap B/Carbosieve SIII The results in Table show an improvement of the performance of the multi-bed tubes compared to the single adsorbents However, there is no significant difference between the combinations identified in this study compared to the commercial tube Furthermore, the results are comparable to the recoveries determined for Carbograph 5TD, which is part of one multi-sorbent tube tested It is noticeable that the polar VVOC methanol is not retained apart from Carbotrap 300, although its very good recovery determined on both Carbosieve S II and Carboxen 1003 Since the assumption can be made that the adsorbents in combination will complement each other, optimisation might be obtained by adapting the bed lengths 3.2.4 Water management For an efficient measurement method, water management is highly recommended Some authors propose the use of pre-tubes filled with drying agents, e.g CaCl2 or Nafion® [14,23,24,26] However, since these might serve as adsorbents themselves, losses at non-targeted analysis might be the result Pollmann, Helmig, et al [31] used a Peltier-cooled, regenerable water trap inserted into the sample flow to condensate water prior to analysis Dry-purging of the adsorbents would also be an option [14] During the research for this study, good experiences were made with the solvent venting dry-purge mode of the thermal desorption system, which indeed led to reduced sensitivity of measurement, but which could be compensated with an enhanced sample amount (Table 1) However, to obtain a reliable measurement method, more efforts must be made to solve the humidity issue 3.2.5 Chemical reactions Although Carboxen 1018 showed as good recovery rates as the other CMS it was not selected, since sulphur dioxide (SO2 ) is produced in the adsorbent giving a large peak at the beginning of the chromatogram impacting the analysis The same was also observed in the other Carboxen type sorbents but to a much lower extent The SO2 peak disappeared or reduced at least to a negligible area after the tube was thermally handled prior to use (∼ 20 °C above recommended desorption temperature) Although the test gas mixture was containing CS2 , there was no significant indication for it to trigger any reaction, since SO2 was also occurring in the blank measurements However, Brown and Shirey [32] reported that the formation of SO2 or CO2 is common to most carbon molecular sieves, and does not pose a problem unless the user is trying to sample for these two analytes They not explain why the formation of these molecules takes place but Boehm [33] reports that surface oxides inherent to carbon materials decompose to CO2 and CO on heating to high temperatures and that highly reactive sites remain on the carbon surface After cooling to room temperature, they can react with oxygen (air) or even water vapour, giving new surface oxides It can be assumed that this mechanism is also responsible for the oxidation of sulphur, inherently occurring in carboxen type adsorbents, which are produced from sulfonated polymers [34] Since the group of the VVOC contains highly reactive compounds, a close look into the occurrence of chemical reactions in the employed adsorbents must be taken Some insight into this already is given in the literature, e.g in Schieweck, Gunschera, et al [18] Moreover, for some single adsorbents but particularly for the multi-bed combinations recoveries greater than the tolerated 120% for some compounds were determined These observations can only partially be explained as the blank measurements carried out prior to loading revealed blank values for some components that even did not decrease after repeated desorption Artefacts or residues of propene and n-propane were found on Carbosieve S II as well as on Carbotrap 300 together with n-butane Tenax GR and Carbograph TD showed high benzene blanks, whereupon artefact formation of benzene in Tenax adsorbents is well known Artefact formation might furthermore be promoted by the presence of water However, detailed investigations on this issue are necessary and objective of ongoing work A suitable method for this might be the standard elevation method to get indication on matrix effects Chromatograms of blank measurements of each adsorbent are added to the supplementary material (S13–S24) Conclusions The recovery rates of 29 VVOC and three VOC in nine different adsorbent materials (porous polymers, GCB and CMS) were determined The recovery calculation was obtained by direct and, hence, unaffected measurement of the gas standard mixture This way, any effects that might be resulting from interactions of the test sample molecules with the adsorbent, e.g breakthrough phenomena, insufficient desorption or chemical reactions are considered and evaluated Sampling performance is strongly affected by water vapour in the sample air A comparison between dry (0% RH) and humid (50% RH) sampling conditions revealed that the number of retained VVOC with average recoveries between 80 and 120% dropped significantly for the CMS Carboxen 569 and Carboxen 1003 compared to the recoveries under dry sampling conditions This was furthermore well correlating with the relatively high water uptake compared to the other adsorbents Water management measures are therefore highly recommended In this context, the common practice of calibration with liquid standard solutions followed by flushing with a dry inert gas flow should be rethought Due to the obvious impact of air humidity leading to lower adsorption capacity particularly of the GCB and CMS, underestimations during analysis are likely Chemical reactions in the carbon-based adsorbents themselves or surface reactions with analytes might be a problem In this study, the generation of SO2 in the CMS and particularly in Carboxen 1018 was observed This can be a problem when analytes of interest elute with the same retention time or close to it For the measurement of complex gas samples, combinations of adsorbents should be used With the procedure described here, the combinations Carbograph 5TD/Carboxen 1003/Carbosieve SII and Tenax GR/Carboxen 1003/Carbosieve SII were identified to be potentially suitable The improvement of the performance compared to the single adsorbents particularly under humid sample conditions could be shown The comparison with a commercial tube revealed no significant difference However, one third of the target analytes could not be satisfyingly retained so that potential for optimisation can be seen in the adaptation of the adsorbent bed lengths Future research should focus on investigations on the optimum composition of multi-bed sampling tubes, the recovery under realistic sampling conditions, also including the always present VOC and SVOC, possible chemical reactions, storage effects (compounds migration between sorbent beds) and the loss-free water management These items are objectives of a research project recently started and funded by the German Environment Agency (UBA) Its outcome will be published in a forthcoming paper Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper M Richter, E Juritsch and O Jann / Journal of Chromatography A 1626 (2020) 461389 CRediT authorship contribution statement Matthias Richter: Conceptualization, Methodology, Formal analysis, Supervision, Writing - original draft Elevtheria Juritsch: Methodology, Investigation, Formal analysis, Writing - review & editing Oliver Jann: Conceptualization, Resources, Writing - review & editing Acknowledgements This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors The authors would like to thank Timo Juritsch for proofreading the manuscript Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.chroma.2020.461389 References [1] P Wolkoff, Indoor air 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Set-up for the determination of the VVOC recovery of the adsorbent tubes tively The calculation of the recovery in% was carried out according to Eq (1) A tolerance of the recovery of ± 20% around