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6 E. Gallego 1* , F. X. Roca 1 , J. F. Perales 1 , X. Guardino 2
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1 Laboratori del Centre de Medi Ambient. Universitat Politècnica de Catalunya
9 (LCMA-UPC). Avda. Diagonal, 647. E 08028 Barcelona. Phone: 34934016683, Fax:
10 34934017150, e-mail: Lcma.info@upc.edu
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2 Centro Nacional de Condiciones de Trabajo. INSHT. C/Dulcet, 2-10. E 08034
13 Barcelona. Phone: 34932800102, Fax: 34932803642, e-mail: cnctinsht@mtin.es
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* Author to whom correspondence should be addressed
16
17 Abstract
18 A comparison between two types of adsorbent tubes, the commonly used Tenax TA and
19 a multi-sorbent bed (Carbotrap, Carbopack X, Carboxen 569) tube developed in our
20 laboratory, has been done to evaluate their usefulness in the analysis of VOCs in
21 ambient air. Duplicate indoor and outdoor samples of Tenax TA and multi-sorbent tubes
22 of 10, 20, 40, 60 and 90 litres were taken in Barcelona city (Spain) on July and October
23 of 2009. Breakthrough values (defined as %VOCs found in the back tube) were
24 determined for all sampling volumes connecting two sampling tubes in series. The

25 analysis was performed by automatic thermal desorption (ATD) coupled with capillary
26 gas chromatography (GC)/mass spectrometry detector (MSD). Significant differences
27 between the concentrations obtained form multi-sorbent bed and Tenax TA tubes are
28
29 observed for the very volatile compounds (56ºC<boiling point<100ºC and
¿47kPa<vapour pressure (20ºC)<4kPa?) (e.g. acetone, isopropanol, n -hexane) and for
30 alcohols and chlorinated compounds (e.g. 1-butanol, carbon disulphide,
31 dichloromethane, chloroform, carbon tetrachloride, trichloroethylene,
32 tetrachloroethylene), being the concentrations higher in multi-sorbent bed than in Tenax
33
34
TA tubes. On the other hand, mainly all compounds with boiling points higher than
100ºC (except α-pinene, chlorinated and polar compounds) do not show significant
35 differences between the obtained multi-sorbent bed and Tenax TA tubes concentrations.
36 For the concentrations obtained, Tenax TA present high breakthrough values (from 0 to
37 77%) for mainly all compounds and sampling volumes studied. On the other hand,
38 multi-sorbent bed tubes do not exhibit important breakthrough values for these
39 compounds, except the VVOCs ethanol (for all sampled volumes), and acetone,
40 dichloromethane and isopropanol (for sampling volumes over 40 litres). The
41 concentration differences observed between Tenax TA and multi-sorbent bed tubes are
42 directly related to the high breakthrough values determined for Tenax TA adsorbent.
43
44 Keywords: Tenax TA, Carbotrap, Carbopack X, Carboxen 569, volatile organic
45 compounds, TD-GC/MS
46
47
1. Introduction
48
Sorbent materials have a wide range of chemical forms and surface and porous
49 structures, as it is observed in the variety of adsorbents that are commercially available

50 both for industrial/occupational and environmental applications. For this reason, in air
51 quality determination and pollution control it is necessary to establish the proper
52 adsorbents to be used to determine the target compounds chosen. Ambient air is a very
53 complex mixture of compounds, and has a very variable composition and concentration
54 of pollutants. Hence, a good choice of sorbent or a good combination of different
55 sorbents may allow the determination of a wide range of target compounds in air
56 samples [1, 2, 3, 4, 5, 6, 7, 8], as well as achieve high breakthrough volumes [9].
57 Nowadays, multi-sorbent beds are used in a high amount of validated methods for
58 determining volatile toxic organic compounds in ambient air (e.g. NIOSH 2549 [10]
59 and EPA TO-17 [11]).
60 Sampling through adsorbent materials serves to enrich the analytes in the sample, as
61 they are generally found in trace and ultra-trace quantities in ambient air. The selective
62 characteristics of the sorbent chosen would determine the removing of the target
63 compounds from the air matrix [12]. On the other hand, a choice of a proper sorbent for
64 the range of the studied target compounds would eliminate problems derived from
65 breakthrough [2]. Other factors different from the sorbent will influence in the choice of
66 a proper sorbent, such as the type and concentration of target pollutants, the sampling
67 equipment and the analytical technique (thermal desorption or solvent extraction), and
68 the environmental conditions of sampling (mainly temperature and humidity) [13].
69 Tenax TA has been determined to be a not suitable adsorbent for very volatile organic
70 compounds (VVOCs, 0<boiling point<50-100ºC [14]), mainly due to the displacement
71 of the adsorbed volatile and polar compounds for non-polar high molecular weight
72 pollutants [15], as it has been described in previous studies [1, 13, 16, 17, 18, 19].
73
However, Tenax TA continues being one of the most widely used adsorbents for the
74 preconcentration of VOCs [19, 20], in spite of its limited specific surface area (20-35

75 m 2 g -1 ) and the possibility of suffering chemical decomposition and degradation of
76 reactive analytes during sampling [1,18]. In addition to that, generally, a single
77 adsorbent cannot be appropriate for the majority of compounds present in ambient air.
78 Hence, a combination of several adsorbents, preferably carbon-based materials [17],
79 may result in better performances. Consequently, if the analysis of the air sample would
80 be done exhaustively, adsorbents that assure us a complete gathering without loss of
81 sample would be needed.
82 In this paper, a comparison between two types of adsorbent tubes, one containing a
83 mixture of three adsorbents (Carbotrap, Carbopack X, Carboxen 569) [7, 21] and
84 another containing Tenax TA were compared to evaluate their usefulness as active
85 adsorbents of ambient air VOCs, including VVOCs.
86
87 2. Materials and methods
88 2.1 Chemicals and materials
89 Standards of VOCs, with purity not less than 98%, were obtained from Aldrich
90 (Milwaukee, WI, USA), Merck (Darmstadt, Germany) and Fluka (Buchs, Switzerland).
91 Perkin Elmer glass tubes (Pyrex, 6 mm external diameter, 90 mm long), unsilanised
92 wool, Carbotrap (20/40 mesh), Carbopack X (40/60 mesh), Carboxen 569 (20/45 mesh)
93 and Tenax TA (60/80 mesh) adsorbents were obtained from Supelco (Bellefonte, PA,
94 USA).
95 2.2 Sampling
96 Duplicate samples of multi-sorbent bed and Tenax TA tubes of 10, 20, 40, 60 and 90
97 litres of indoor and outdoor air were taken in Barcelona city (Spain) during the months
98 of July and October 2009, respectively. Flow sampling rates were set at 70 ml min -1 .
99 VOCs were dynamically sampled connecting custom packed glass multi-sorbent

100 cartridge tubes (Carbotrap 20/40, 70 mg; Carbopack X 40/60, 100 mg and Carboxen
101 569 20/45, 90 mg) [7] and Tenax TA (60/80, 200 mg) tubes to air collector pump
102 samplers specially designed in the LCMA-UPC laboratory [22]. To evaluate
103 breakthrough values, two tubes were connected in series for each sample, letting us
104 determine the amount of the studied compounds that were present in the back tube. On
105
106 the other hand, the flow sampling rate was also evaluated. In October-2009, outdoor air samples of 90 litres were taken at flow rates of 70 ml min
-1
and 90 ml min
-1
, for multi-
107 sorbent bed and Tenax TA tubes,.
The temperature and relative humidity during the
108
109 sampling days ranged between 28-31ºC and 35-48%, and 20-27ºC and 50-63%, for July and October 2009, respectively.
110 Collected ambient air samples were further analysed by thermal desorption and gas
111 chromatography-mass spectrometry (TD-GC/MSD) [7, 21].
112 2.3 Analytical instrumentation
113 The analysis of VOCs was performed by Automatic Thermal Desorption coupled with
114 capillary Gas Chromatography/Mass Spectrometry Detector, using a Perkin Elmer ATD
115 400 (Perkin Elmer, Boston, Massachusetts, USA) and a Thermo Quest Trace 2000 GC
116 (ThermoQuest, San Jose, California, USA) fitted with a Thermo Quest Trace Finnigan
117 MSD.
118
The methodology, validated for 57 compounds, is described elsewhere [7, 21]. Mass
119 spectral data are acquired over a mass range of 20-300 amu. Qualitative identification of
120 target compounds is based on the match of the retention times and the ion ratios of the
121 target quantification ions and the qualifier ions (Xcalibur 1.2 validated software
122 package). Quantification of field samples is conducted by the external standard method
123
[7]. Limits of detection (LOD), determined applying a signal-to-noise ratio of 3, range
124 form 0.001 to 10 ng. The studied compounds show repeatibilities (% relative standard

125 deviation values)

25% [7], accomplishing the EPA performance criteria [11]. Extreme
126 precautions are established for quality assurance, injecting periodically blank samples
127
128 and a known concentration of standards.
All concentration values were normalized by temperature (273

K) and pressure (760
129 mm Hg).
130 3. Results and discussion
131 3.1 Multi-sorbent bed (Carbotrap, Carbopack X, Carboxen 569)-Tenax TA
132 concentrations comparative
133 Differences between multi-sorbent bed and Tenax TA tubes concentrations are shown in
134 Table 1 and Table 2 for indoor and outdoor air, respectively. Average ± standard
135
136 deviation and median values for all sampled volumes are shown. In addition to that, boiling point (ºC) and Vapour pressure at 20ºC (KPa) are also presented for each
137 evaluated compound. Significant differences between the concentrations obtained form
138 multi-sorbent bed and Tenax TA tubes ( t -test, p<0.05 (normal data) and U of Mann-
139
140
Whitney, p<0.05 (not normal data)) are observed for the very volatile compounds
(56ºC<boiling point<100ºC and 47kPa<vapour pressure (20ºC)<4kPa) (e.g. acetone,
141 isopropanol, carbon disulphide, n -hexane) and for alcohols and chlorinated compounds
142 (e.g. 1-butanol, phenol, dichloromethane, chloroform, carbon tetrachloride,
143 trichloroethylene, tetrachloroethylene); being higher the concentrations found in multi-
144
145 sorbent bed than in Tenax TA tubes. On the other hand, mainly all compounds with boiling points higher than 100ºC (except α-pinene, chlorinated and polar compounds)
146 do not show significant differences between the concentrations obtained through multi-
147
148 sorbent bed and Tenax TA tubes, both for indoor and outdoor air concentrations. The boiling point of 100ºC is often advised as a guidance value below which the adsorption
149 of the compounds is not satisfying for Tenax TA [19]. In addition to that, a

150 displacement of the adsorbed volatile and polar compounds for non-polar high
151 molecular weight pollutants in Tenax TA adsorbent has been observed in previous
152 studies [15]. Hence, the behaviour observed for alcohols and chlorinated compounds
153 may be determined by their polarity, being polar pollutants displaced by high-molecular
154 weight compounds.
155 In Figure 1, multi-sorbent bed and Tenax TA concentrations are plotted for some of the
156 studied compounds both for indoor and outdoor air. For the very volatile and polar
157 compounds (e.g. ethanol and isopropanol) differences are observed between the
158 regression line obtained form multi-sorbent bed and Tenax TA correlation and the 1:1
159 line; however, compounds with boiling points over 100ºC, such as ethylbenzene and
160 m + p -xylenes, show good correlations between multi-sorbent bed and Tenax TA
161 obtained concentrations. In Table 3, correlation coefficients between multi-sorbent bed
162 and Tenax TA tubes concentrations, slope and intercept values for all the studied
163
164 compounds are shown. Mainly all compounds that present boiling points above 100ºC and vapour pressures (20ºC) lower than 2-3 kPa, exhibit significant correlations and do
165 not show significant differences in concentrations between multi-sorbent bed and Tenax
166 TA tubes (Table1, Table 2). The correlation coefficients between the compounds that do
167 not present significant differences between the two types of adsorbents and show good
168 correlations range between 0.773-0.954 and 0.950-0.997 for indoor and outdoor air,
169 respectively (Table 3). On the other hand, these compounds present slope values near to
170 1 both for indoor and outdoor concentrations.
171 Ethanol, acetone, isopropanol and dichloromethane present significant correlations
172 between multi-sorbent bed and Tenax TA tubes concentrations in indoor air. However,
173 slope values are really different from 1.
174 3.2 Breakthrough comparative

175 Multi-sorbent bed and Tenax TA breakthrough values for the different volumes sampled
176 are shown in Table 4 and Table 5 for indoor and outdoor air, respectively. Typical
177 VOCs recommended breakthrough value is <5% [11]. For the concentrations obtained,
178 Tenax TA present high breakthrough values for mainly all compounds and sampling
179 volumes studied. However, butyl acetate, ethylbenzene, m + p -xylene, o -xylene, 2-
180 butoxyethanol present acceptable Tenax TA breakthrough values for sampling volumes
181 up to 20 litres. In addition to that, limonene and p -dichlorobenzene present
182 breakthrough acceptable values up to 40 and 90 litres, respectively. For compounds
183 with boiling points below 100ºC and vapour pressures (20ºC) higher than 2 kPa,
184 breakthrough values for Tenax TA tubes are very similar for each sample volume.
185
186
However, breakthrough concentrations increase when increasing the sample volume for compounds that present boiling points above 100ºC and vapour pressures (20ºC) lower
187 than 2-3 kPa (Table 4, Table 5).
188 On the other hand, multi-sorbent bed tubes do not exhibit important breakthrough
189 values for these compounds, except the VVOCs ethanol (for all sampled volumes), and
190 acetone, dichloromethane and isopropanol (for sampling volumes over 40 litres).
191 The significant differences observed between multi-sorbent bed and Tenax TA
192 concentrations, being the concentrations higher in multi-sorbent bed tubes, are directly
193 related to the high breakthrough values determined for Tenax TA adsorbent. High
194 breakthrough values represent a transfer of the target compounds from the front tube to
195
196
197
198 the back tube; hence, lower concentrations are expected in the front tube. Tenax TA has a surface area of 20-35 m
2
g
-1
, whereas Carbotrap, Carbopack X and Carboxen 569 have surface areas of 95-100 m
2
g
-1
, 240-250 m
2
g
-1
and 387-485 m
2
g
-1
, respectively. Total surface areas of the tubes are approximately of 6 and 70 m 2 for Tenax TA and multi-
199 sorbent bed, respectively. Therefore, multi-sorbent bed tubes have approximately 12

200 times more surface area to retain compounds than Tenax TA tubes. Due to its low
201 specific surface area, Tenax TA has low adsorption capacity, and it is only suitable for
202
203 the sampling of medium to high boiling compounds (50-100ºC<boiling point<240-
260ºC and 3-4 KPa<vapour pressure (20ºC)<0.1KPa, [14]), e.g. C
6
-C
26
hydrocarbons,
204 as it has been observed in several previous studies [1, 4, 23].
205
206
3.3 Sampling flow rates comparative
Two sampling rates, of 70 ml min
-1
and 90 ml min
-1
, were evaluated to determine if
207 differences in breakthrough values were observed for the same volumes sampled using
208 different sampling rates. The sample volume was established at 90 litres, being the
209 worst case. In Table 6 and Table 7, average ± standard deviation of concentrations and
210 breakthrough values at the two different sampling rates are shown for multi-sorbent bed
211 and Tenax TA tubes, respectively. Significant differences are observed between Tenax
212
213
TA breakthrough values for compounds with boiling points above 100ºC and vapour pressures (20ºC) lower than 2-3 kPa,being higher the ones corresponding to 90 ml min -1
214
215
216
(Table 7). On the other hand, no differences are observed between breakthrough values of 70 ml min -1 and 90 ml min -1 sampling rates for multi-sorbent bed tubes (Table 6).
Hence, even at 90 litres of sample volume and at a sampling rate of 90 ml min
-1
, multi-
217 sorbent bed tubes present acceptable breakthrough values for the majority of studied
218 compounds (except ethanol, acetone, isopropanol and dichloromethane) as it has been
219 said previously. However, Tenax TA tubes present unacceptable breakthrough values
220 for mainly all studied compounds, and worse values are obtained when increasing the
221 sampling rate.
222
4. Conclusions
223
Multi-sorbent bed tubes show better performance than Tenax TA tubes for very volatile
224 organic compounds, being the first type of sorbent tube more appropriate for the

225
226
227 adsorption of this kind of compounds, especially for those presenting boiling points lower than 100ºC and vapour pressures (20ºC) above 3 kPa. On the other hand, compounds with boiling points higher than 100ºC and vapour pressures lower than 2-3
228 kPa, show similar achievements in multi-sorbent bed and Tenax TA tubes. Tenax TA
229 present unacceptable breakthrough values for mainly all compounds and sampling
230 volumes studied. However, multi-sorbent bed tubes do not exhibit important
231 breakthrough values for these compounds. Hence, if an exhaustive analysis of ambient
232 air would be done, sorbent tubes that assure us a complete gathering of very volatile
233 organic compounds without loss of sample would be more appropriate, such as a
234 multisorbent bed of carbonaceous adsorbents.
235
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