- Email: [email protected]
Rapid Detection of Toluene-2,4-diisocyanate in Various Sports Fields Using Extractive Electrospray Ionization Mass Spectrometry
CHINESE JOURNAL OF ANALYTICAL CHEMISTRY Volume 36, Issue 9, September 2008 Online English edition of the Chinese language journal
Cite this article as: Chin J Anal Chem, 2008, 36(9), 1300–1304.
RESEARCH PAPER
Rapid Detection of Toluene-2,4-diisocyanate in Various Sports Fields Using Extractive Electrospray Ionization Mass Spectrometry LI Jian-Qiang1, ZHOU Yu-Fen1, DING Jian-Hua1,2, YANG Shui-Ping1, CHEN Huan-Wen1,2,* 1 2
Applied Chemistry Department, East China Institute of Technology Institution, Fuzhou 344000, China College of Chemistry, Jilin University, Changchun 130023, China
Abstract: Plastic materials that release free toluene-2,4-diisocyanate (TDI) can be commonly found in sports venues. As the presence of TDI in the atmosphere will result in severe air pollution, the WHO has enforced strict regulations to ensure that the proportion of the free TDI monomer in the isocyanate-polymer is lower than 0.5%. In this study, ambient air samples containing trace amounts of TDI-water aerosols were collected with glass syringes on the spot and then directly injected into a home-made extractive electrospray ionization (EESI) source. The EESI source was installed on a LTQ-MS instrument for the direct analysis of samples with complex matrices. The raw samples were directly analyzed by tandem EESI-MS without any sample pretreatment under the following working conditions: spray voltage, 3 kV; capillary voltage, 21 V; and capillary temperature, 175 °C. A single sample analysis, along with tandem MS experiments for enhanced specificity, was completed within 1 s. The limit of detection was found to be less than 0.04% (n = 5, S/N = 100), considerably lower than that allowed in the air. The relative standard deviation of the analysis results was no higher than 1.4%. Key Words:
1
Extractive electrospray ionization; Tandem mass spectrometry; Toluene-2,4-diisocyanate; Rapid analysis
Introduction
Ion source is one of the most important components of a mass spectrometer. Practical applications of mass spectrometry rely heavily on source performance[1]. Well-established ionization techniques such as electrospray ionization (ESI)[2], matrix-assisted laser desorption ionization (MALDI)[3], atmospheric pressure chemical ionization (APCI)[4,5], and secondary ion mass spectrometry (SIMS)[6] have allowed the applications of mass spectrometry to be extended over multiple disciplines. Traditionally, complex samples require tedious sample pre-treatment before they are analyzed using mass spectrometer, and result in a bottleneck for high throughput analysis. In recent years, several novel techniques such as desorption electrospray ionization (DESI)[7–10] and surface desorption atmospheric pressure chemical ionization
(DAPCI)[11–13] have been successfully applied to direct desorption/ionization of non-volatile compounds on solid surfaces without sample pre-treatment. Extractive electrospray ionization (EESI)[14–18] is the choice for samples that are not in the solid state. EESI is a newly developed technique for direct analysis of complex samples without any sample pre-treatment. The first example of EESI was demonstrated with the continuous real time online monitoring of undiluted milk, urine, and waste water. An advanced feature of EESI is its capability for the in-vivo analysis of virtually any type of samples, including biological surfaces, biofluids, and aerosols. By implementing ion/molecule reactions in EESI, along with multiple-stage tandem mass spectrometry, both polar and non-polar compounds can be selectively detected with enhanced specificity. Thus, EESI ion trap mass spectrometry is particularly suitable for the rapid analysis of trace analytes in
Received 19 January 2008; accepted 28 March 2008 * Corresponding author. Email: [email protected] This work was supported by the grants from the National Science Foundation of China (No. 20505003) and The Ministry of Science and Technology of China (No. 2006SJ156100). Copyright © 2008, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
LI Jian-Qiang et al. / Chinese Journal of Analytical Chemistry, 2008, 36(9): 1300–1304
complex matrices. Toluene-2,4-diisocyanate (TDI) polyurethane plastic can be commonly found in domestic sports venues. At high temperature or in light, a notable amount of free monomer TDI is released from the plastic material. TDI is a toxic compound as its active group (–N=C=O) can react with human proteins, resulting in the formation of malfunctioned proteins. Furthermore, TDI may cause serious allergic reactions in man, resulting in breathing difficulties and even death. There are existing methods to detect free TDI in air. However, a colorimetry-based method is not suitable for the detection of TDI in the atmosphere[19] because of its low sensitivity, while gas chromatography (GC), high-performance liquid chromatography[20], and GC-MS[21] require time-consuming sample pre-treatment processes and involve toxic reagents. Extractive electrospray ionization mass spectrometry (EESI-MS) tolerates extremely complex matrices, and provides high sensitivity and high specificity for the rapid analysis of trace analytes. However, no determination of TDI using EESI-MS has been reported so far. This study demonstrates that the rapid detection of free TDI in air can be conveniently done with high sensitivity and high specificity without any sample pre-treatment using EESI tandem mass spectrometry.
2 2.1
Experimental Instruments and reagents
LTQ XL ion trap mass spectrometer (Finnigan, San Jose, CA); EESI source (a home-made device); LTQ Xcalibur 1.2 software system (Finnigan, San Jose, CA); micro-vortex-mixed (WH 2, Huxi Shanghai Analytical Instrument Co., Ltd., Shanghai); sample airbags (1 L, Atlantic Scientific Co., Inc. New Jersey, USA); toluene-2,4-diisocyanate (WM174, Tianjin Fu-cheng Reagent Limited, Tianjin); syringe (1 μl, Hamilton Co., Reno. Nevada, USA); glass syringe (100 ml, Hamilton Co., Reno. Nevada, USA); acetone (HPLC Grade, Dima Technology Inc, USA). The standard gas sample of toluene-2, 4-diisocyanate (0.04%) was prepared using the method as reported in literature[22]. The water used was twice-distilled water provided by the chemistry facilities of the institute. The purity of all reagents used was of analytical grade or higher.
samples were directly injected into the EESI source without any sample pre-treatment, and a good signal of TDI was immediately observed. 2.3 Set-up of EESI-LTQ-MS Previous studies have demonstrated that extraction electrospray ionization mass spectrometry is particularly suitable for real time on-line analysis[14–18] of trace substances in complex matrices. The experiments described here were carried out using a home-made EESI source, which was coupled to a commercially available LTQ-MS instrument. The experimental setup is shown schematically in Fig.1. The core components of the EESI source included a sample introduction channel, a 3-way stainless steel union tee, and an electrospray assembly consisting of a high-voltage connector and other necessary accessories to generate reagent ions. A quartz capillary was used in the sample introduction channel to guide the neutral samples into the EESI source, in which pure solvents were electrosprayed by the electrospray assembly to generate primary ions. The union tee assembly is able to sustain a gas pressure higher than 20 MPa, which facilitates the nebulization of neutral liquids. Nitrogen gas is generally used as the sheath gas for nebulization, and the reagent ions for EESI are produced by continuously electrospraying solvents such as water or dilute acetic acid solution with non-complex matrix. The EESI source was configured to allow in-line liquid-liquid micro-extraction and charge transfer reaction occurring between the tiny droplets produced by the two independent channels. This resulted in efficient ionization of the analytes in complex sample. The analyte ions formed were then introduced into the LTQ-MS for mass analysis. The angle between the two channels of the EESI source was optimized at 60º for better sensitivity. In this study, the other experimental parameters of the EESI source were as follows: mass range, m/z 50–200; EESI high voltage, 3 kV; voltage for the MS inlet capillary, 21 V; temperature of heating capillary, 175 °C; and sheath gas (N2) flow, 40 arbitrary units.
2.2 Sample collection On the selected sampling spots, 100 ml of air was directly collected using a 100 ml glass syringe. The sample was tightly sealed inside the syringe up to the point of analysis. The sample was collected directly from an environment with a high relative humidity (70%); it contained a mixture of air, water droplets, and solutes inside the water droplet. After collection, the
Fig.1
Schematic diagram of an extractive electrospray ionization (EESI) source
LI Jian-Qiang et al. / Chinese Journal of Analytical Chemistry, 2008, 36(9): 1300–1304
The instrument was operated in the positive ion detection mode, and the mass spectra were recorded using either full scan mass spectrometry or two-stage tandem mass spectrometry (MS2). During the collision-induced dissociation (CID) experiments, the parent ions were isolated using a mass/charge window of 1.6 units, and the collision energy was optimized to be 20%. For the remaining experimental parameters, the default values were directly used and no further optimization was done.
3 Results and Discussion 3.1 EESI-MS and MS2 Spectra of TDI Unlike the conventional methods such as ESI, EESI tolerates highly complex matrices and provides high sensitivity for mass spectrometric analysis of trace amounts of analytes in complex matrices. In this study, the sample was the moist air containing trace amounts of TDI. Although the matrix was relatively simple, it is still necessary to perform sample pre-treatment (e.g., pre-concentration and pre-separation) for TDI detection using a conventional mass spectrometry-based method. However, the mixed aerosol was directly introduced and used for EESI-MS analysis. It took no more than 1 s to record a mass spectral fingerprint, as shown in Fig.2, in which TDI mainly formed protonated molecules with a corresponding peak at m/z 175. Although the sensitivity of EESI mass spectrometry is high, the result obtained may be falsely positive if only the first-stage mass spectrum was considered. Therefore, two-stage tandem mass spectrometry analysis of TDI was applied to the ions of m/z 175. The ions of m/z 175 provided two major fragments of m/z 147 and 133 by the loss of CO and NCO, respectively (as shown in the inset of Fig.2). Under the same CID conditions, the identical fragmentation pattern was observed using the authentic TDI compound. These data confirmed that the peak at m/z 175 detected by MS was the TDI signal, demonstrating the successful detection of TDI in the samples. The experimental findings showed that a sample analysis could be completed within 1 s using this method.
This is the most rapid method reported for the detection of TDI in air. Besides its fascinating speed, EESI tandem mass spectrometry can provide the characteristic fragments of virtually any analyte of interest, allowing the confident and sensitive detection of trace analytes in complex matrices. 3.2 Limit of detection and precision of measurement The commercial SNR calculation software of the LTQ-MS instrument work station was used to study the limit of detection for TDI. At SNR 100, the TDI signal (m/z 175) corresponded to a concentration of 0.04% in the aerosol sample. Under these conditions, the CID mass spectra of the precursor ions (m/z 175) were still in good accordance with that of Fig.2, with the exception of a lower signal level. The WHO has rigorously regulated the content of the free TDI in the isocyanate-polymer to be lower than 0.5%[19], which is a level considerably higher than the limit of detection of this method. Therefore, samples of lower concentrations were not further investigated. The sample was analyzed eight times using the same procedure, and signal levels of 274, 271, 272, 274, 273, 275, 274 and 272 were obtained, respectively, resulting in an average peak intensity of 273. The relative standard deviation of the 8 measurements was calculated to be 1.4%, demonstrating a good repeatability and a high precision. 3.3 Vertical TDI distribution in air The concentration of free TDI varies widely with different heights in air, because TDI is a semi-volatile chemical denser than air. In this study, the air samples were collected from different heights at different venues and the TDI contents were analyzed by EESI-MS. The relative contents of free TDI in the air at different heights are summarized in Table 1. The result obtained shows that the amount of TDI present decreases with increasing vertical height from the sports ground. However, the concentrations of TDI are relatively high in the region below 0.5 m, with no notable differences between different sampling spots. Similar trends were found for air samples collected both indoors and outdoors. For a given outdoor venue, the TDI content in the air above 1 m is Table 1 Relative abundance of TDI at different altitudes Vertical height (m)
Venue Ground (0.0)
Fig.2 EESI-MS spectra of toluene-2,4-diisocyanate (TDI) sampled directly from ambient air
Synthetic-rubber sports track outdoors Synthetic-rubber sports track indoors Basketball court outdoors Volleyball hall indoors Student Health Testing Center
0.5
1.0
1.6
5.87
5.65
4.32
4.01
4.12
4.03
3.98
3.96
8.23
8.12
5.76
4.32
3.96
3.89
2.81
2.76
3.43
3.41
2.16
2.12
LI Jian-Qiang et al. / Chinese Journal of Analytical Chemistry, 2008, 36(9): 1300–1304
Table 2 Relative abundance of TDI in different locations Locations Relative Abundance (E2)
Lakeshore
Outdoor synthetic-rubber sports track
0.046
4.27
Indoor synthetic-rubber sports track
significantly lower than that below 0.5 m. However, for the indoor venues, the concentration of TDI only varies slightly with height, probably because the air circulation is not adequate. It is thus clear that particularly in indoor venues with poor ventilation; TDI can seriously affect athletes below 1.6 m in height (such as teenagers). 3.4 Distribution of TDI in different locations Using the method reported above, unique air samples were collected at the same temperature from the same height in different places, such as from a public park (300 m from a coliseum), an indoor volleyball hall, a Student Health Testing Center, an outdoor synthetic-rubber sports track, and an outdoor basketball court. The TDI concentrations of these samples were evaluated by EESI-MS under the same experimental conditions. The results are shown in Table 2. In Table 2, it is clear that there is no significant difference of free TDI content in the air sampled from the indoor volleyball hall and the Student Health Testing Center, because the two venues were constructed using the same plastic materials. The EESI mass spectra indicate a noticeable difference between the indoor and outdoor synthetic-rubber sports venues. The lakeshore air also contained a low concentration of TDI. This can be accounted for by the fact that the lake was close to the Coliseum, resulting in the air quality around the lake being affected by the air from the Coliseum.
Outdoor basketball court
3.59
Indoor volleyball hall
6.89
Student health testing center
2.56
2.23
3.5 Effects of climatic conditions on the sampling The release of free TDI from sports venues constructed from materials containing polyurethane plastic increases at high temperatures, resulting in an increase in the TDI content in the air. The TDI concentration of air is affected by the air flow rate even in the same venue under the same climatic conditions. The samples were collected from different locations under different weather conditions for fast TDI detection by EESI-LTQ-MS. The major findings are shown in Fig.3. Figure 3 shows that the total amount of free TDI released into the air increases significantly when the temperature rises or when the plastic is directly exposed to sunlight. In hot sunny weather, the highest concentration of TDI is detected outdoors owing to sunlight exposure and the high plastic temperature. The exposure to sunlight plays a more significant role than a high temperature in the release of TDI from plastics. For example, the content of TDI in the air during a sunny day at 26 °C is considerably higher than that of a cloudy day with a temperature of 28 °C. In addition, the free TDI content of air obtained indoors does not change significantly with temperature, unlike the cases investigated outdoors. This may also be the result of the exposure to sunlight in the latter.
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Fig.3 Relative abundance of TDI under different climate conditions a, outdoor synthetic-rubber sports track; b, indoor synthetic-rubber sports track; c, outdoor basketball court; d, indoor volleyball hall; e, student health testing center
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Cite this article as: Chin J Anal Chem, 2008, 36(9), 1300–1304.
RESEARCH PAPER
Rapid Detection of Toluene-2,4-diisocyanate in Various Sports Fields Using Extractive Electrospray Ionization Mass Spectrometry LI Jian-Qiang1, ZHOU Yu-Fen1, DING Jian-Hua1,2, YANG Shui-Ping1, CHEN Huan-Wen1,2,* 1 2
Applied Chemistry Department, East China Institute of Technology Institution, Fuzhou 344000, China College of Chemistry, Jilin University, Changchun 130023, China
Abstract: Plastic materials that release free toluene-2,4-diisocyanate (TDI) can be commonly found in sports venues. As the presence of TDI in the atmosphere will result in severe air pollution, the WHO has enforced strict regulations to ensure that the proportion of the free TDI monomer in the isocyanate-polymer is lower than 0.5%. In this study, ambient air samples containing trace amounts of TDI-water aerosols were collected with glass syringes on the spot and then directly injected into a home-made extractive electrospray ionization (EESI) source. The EESI source was installed on a LTQ-MS instrument for the direct analysis of samples with complex matrices. The raw samples were directly analyzed by tandem EESI-MS without any sample pretreatment under the following working conditions: spray voltage, 3 kV; capillary voltage, 21 V; and capillary temperature, 175 °C. A single sample analysis, along with tandem MS experiments for enhanced specificity, was completed within 1 s. The limit of detection was found to be less than 0.04% (n = 5, S/N = 100), considerably lower than that allowed in the air. The relative standard deviation of the analysis results was no higher than 1.4%. Key Words:
1
Extractive electrospray ionization; Tandem mass spectrometry; Toluene-2,4-diisocyanate; Rapid analysis
Introduction
Ion source is one of the most important components of a mass spectrometer. Practical applications of mass spectrometry rely heavily on source performance[1]. Well-established ionization techniques such as electrospray ionization (ESI)[2], matrix-assisted laser desorption ionization (MALDI)[3], atmospheric pressure chemical ionization (APCI)[4,5], and secondary ion mass spectrometry (SIMS)[6] have allowed the applications of mass spectrometry to be extended over multiple disciplines. Traditionally, complex samples require tedious sample pre-treatment before they are analyzed using mass spectrometer, and result in a bottleneck for high throughput analysis. In recent years, several novel techniques such as desorption electrospray ionization (DESI)[7–10] and surface desorption atmospheric pressure chemical ionization
(DAPCI)[11–13] have been successfully applied to direct desorption/ionization of non-volatile compounds on solid surfaces without sample pre-treatment. Extractive electrospray ionization (EESI)[14–18] is the choice for samples that are not in the solid state. EESI is a newly developed technique for direct analysis of complex samples without any sample pre-treatment. The first example of EESI was demonstrated with the continuous real time online monitoring of undiluted milk, urine, and waste water. An advanced feature of EESI is its capability for the in-vivo analysis of virtually any type of samples, including biological surfaces, biofluids, and aerosols. By implementing ion/molecule reactions in EESI, along with multiple-stage tandem mass spectrometry, both polar and non-polar compounds can be selectively detected with enhanced specificity. Thus, EESI ion trap mass spectrometry is particularly suitable for the rapid analysis of trace analytes in
Received 19 January 2008; accepted 28 March 2008 * Corresponding author. Email: [email protected] This work was supported by the grants from the National Science Foundation of China (No. 20505003) and The Ministry of Science and Technology of China (No. 2006SJ156100). Copyright © 2008, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
LI Jian-Qiang et al. / Chinese Journal of Analytical Chemistry, 2008, 36(9): 1300–1304
complex matrices. Toluene-2,4-diisocyanate (TDI) polyurethane plastic can be commonly found in domestic sports venues. At high temperature or in light, a notable amount of free monomer TDI is released from the plastic material. TDI is a toxic compound as its active group (–N=C=O) can react with human proteins, resulting in the formation of malfunctioned proteins. Furthermore, TDI may cause serious allergic reactions in man, resulting in breathing difficulties and even death. There are existing methods to detect free TDI in air. However, a colorimetry-based method is not suitable for the detection of TDI in the atmosphere[19] because of its low sensitivity, while gas chromatography (GC), high-performance liquid chromatography[20], and GC-MS[21] require time-consuming sample pre-treatment processes and involve toxic reagents. Extractive electrospray ionization mass spectrometry (EESI-MS) tolerates extremely complex matrices, and provides high sensitivity and high specificity for the rapid analysis of trace analytes. However, no determination of TDI using EESI-MS has been reported so far. This study demonstrates that the rapid detection of free TDI in air can be conveniently done with high sensitivity and high specificity without any sample pre-treatment using EESI tandem mass spectrometry.
2 2.1
Experimental Instruments and reagents
LTQ XL ion trap mass spectrometer (Finnigan, San Jose, CA); EESI source (a home-made device); LTQ Xcalibur 1.2 software system (Finnigan, San Jose, CA); micro-vortex-mixed (WH 2, Huxi Shanghai Analytical Instrument Co., Ltd., Shanghai); sample airbags (1 L, Atlantic Scientific Co., Inc. New Jersey, USA); toluene-2,4-diisocyanate (WM174, Tianjin Fu-cheng Reagent Limited, Tianjin); syringe (1 μl, Hamilton Co., Reno. Nevada, USA); glass syringe (100 ml, Hamilton Co., Reno. Nevada, USA); acetone (HPLC Grade, Dima Technology Inc, USA). The standard gas sample of toluene-2, 4-diisocyanate (0.04%) was prepared using the method as reported in literature[22]. The water used was twice-distilled water provided by the chemistry facilities of the institute. The purity of all reagents used was of analytical grade or higher.
samples were directly injected into the EESI source without any sample pre-treatment, and a good signal of TDI was immediately observed. 2.3 Set-up of EESI-LTQ-MS Previous studies have demonstrated that extraction electrospray ionization mass spectrometry is particularly suitable for real time on-line analysis[14–18] of trace substances in complex matrices. The experiments described here were carried out using a home-made EESI source, which was coupled to a commercially available LTQ-MS instrument. The experimental setup is shown schematically in Fig.1. The core components of the EESI source included a sample introduction channel, a 3-way stainless steel union tee, and an electrospray assembly consisting of a high-voltage connector and other necessary accessories to generate reagent ions. A quartz capillary was used in the sample introduction channel to guide the neutral samples into the EESI source, in which pure solvents were electrosprayed by the electrospray assembly to generate primary ions. The union tee assembly is able to sustain a gas pressure higher than 20 MPa, which facilitates the nebulization of neutral liquids. Nitrogen gas is generally used as the sheath gas for nebulization, and the reagent ions for EESI are produced by continuously electrospraying solvents such as water or dilute acetic acid solution with non-complex matrix. The EESI source was configured to allow in-line liquid-liquid micro-extraction and charge transfer reaction occurring between the tiny droplets produced by the two independent channels. This resulted in efficient ionization of the analytes in complex sample. The analyte ions formed were then introduced into the LTQ-MS for mass analysis. The angle between the two channels of the EESI source was optimized at 60º for better sensitivity. In this study, the other experimental parameters of the EESI source were as follows: mass range, m/z 50–200; EESI high voltage, 3 kV; voltage for the MS inlet capillary, 21 V; temperature of heating capillary, 175 °C; and sheath gas (N2) flow, 40 arbitrary units.
2.2 Sample collection On the selected sampling spots, 100 ml of air was directly collected using a 100 ml glass syringe. The sample was tightly sealed inside the syringe up to the point of analysis. The sample was collected directly from an environment with a high relative humidity (70%); it contained a mixture of air, water droplets, and solutes inside the water droplet. After collection, the
Fig.1
Schematic diagram of an extractive electrospray ionization (EESI) source
LI Jian-Qiang et al. / Chinese Journal of Analytical Chemistry, 2008, 36(9): 1300–1304
The instrument was operated in the positive ion detection mode, and the mass spectra were recorded using either full scan mass spectrometry or two-stage tandem mass spectrometry (MS2). During the collision-induced dissociation (CID) experiments, the parent ions were isolated using a mass/charge window of 1.6 units, and the collision energy was optimized to be 20%. For the remaining experimental parameters, the default values were directly used and no further optimization was done.
3 Results and Discussion 3.1 EESI-MS and MS2 Spectra of TDI Unlike the conventional methods such as ESI, EESI tolerates highly complex matrices and provides high sensitivity for mass spectrometric analysis of trace amounts of analytes in complex matrices. In this study, the sample was the moist air containing trace amounts of TDI. Although the matrix was relatively simple, it is still necessary to perform sample pre-treatment (e.g., pre-concentration and pre-separation) for TDI detection using a conventional mass spectrometry-based method. However, the mixed aerosol was directly introduced and used for EESI-MS analysis. It took no more than 1 s to record a mass spectral fingerprint, as shown in Fig.2, in which TDI mainly formed protonated molecules with a corresponding peak at m/z 175. Although the sensitivity of EESI mass spectrometry is high, the result obtained may be falsely positive if only the first-stage mass spectrum was considered. Therefore, two-stage tandem mass spectrometry analysis of TDI was applied to the ions of m/z 175. The ions of m/z 175 provided two major fragments of m/z 147 and 133 by the loss of CO and NCO, respectively (as shown in the inset of Fig.2). Under the same CID conditions, the identical fragmentation pattern was observed using the authentic TDI compound. These data confirmed that the peak at m/z 175 detected by MS was the TDI signal, demonstrating the successful detection of TDI in the samples. The experimental findings showed that a sample analysis could be completed within 1 s using this method.
This is the most rapid method reported for the detection of TDI in air. Besides its fascinating speed, EESI tandem mass spectrometry can provide the characteristic fragments of virtually any analyte of interest, allowing the confident and sensitive detection of trace analytes in complex matrices. 3.2 Limit of detection and precision of measurement The commercial SNR calculation software of the LTQ-MS instrument work station was used to study the limit of detection for TDI. At SNR 100, the TDI signal (m/z 175) corresponded to a concentration of 0.04% in the aerosol sample. Under these conditions, the CID mass spectra of the precursor ions (m/z 175) were still in good accordance with that of Fig.2, with the exception of a lower signal level. The WHO has rigorously regulated the content of the free TDI in the isocyanate-polymer to be lower than 0.5%[19], which is a level considerably higher than the limit of detection of this method. Therefore, samples of lower concentrations were not further investigated. The sample was analyzed eight times using the same procedure, and signal levels of 274, 271, 272, 274, 273, 275, 274 and 272 were obtained, respectively, resulting in an average peak intensity of 273. The relative standard deviation of the 8 measurements was calculated to be 1.4%, demonstrating a good repeatability and a high precision. 3.3 Vertical TDI distribution in air The concentration of free TDI varies widely with different heights in air, because TDI is a semi-volatile chemical denser than air. In this study, the air samples were collected from different heights at different venues and the TDI contents were analyzed by EESI-MS. The relative contents of free TDI in the air at different heights are summarized in Table 1. The result obtained shows that the amount of TDI present decreases with increasing vertical height from the sports ground. However, the concentrations of TDI are relatively high in the region below 0.5 m, with no notable differences between different sampling spots. Similar trends were found for air samples collected both indoors and outdoors. For a given outdoor venue, the TDI content in the air above 1 m is Table 1 Relative abundance of TDI at different altitudes Vertical height (m)
Venue Ground (0.0)
Fig.2 EESI-MS spectra of toluene-2,4-diisocyanate (TDI) sampled directly from ambient air
Synthetic-rubber sports track outdoors Synthetic-rubber sports track indoors Basketball court outdoors Volleyball hall indoors Student Health Testing Center
0.5
1.0
1.6
5.87
5.65
4.32
4.01
4.12
4.03
3.98
3.96
8.23
8.12
5.76
4.32
3.96
3.89
2.81
2.76
3.43
3.41
2.16
2.12
LI Jian-Qiang et al. / Chinese Journal of Analytical Chemistry, 2008, 36(9): 1300–1304
Table 2 Relative abundance of TDI in different locations Locations Relative Abundance (E2)
Lakeshore
Outdoor synthetic-rubber sports track
0.046
4.27
Indoor synthetic-rubber sports track
significantly lower than that below 0.5 m. However, for the indoor venues, the concentration of TDI only varies slightly with height, probably because the air circulation is not adequate. It is thus clear that particularly in indoor venues with poor ventilation; TDI can seriously affect athletes below 1.6 m in height (such as teenagers). 3.4 Distribution of TDI in different locations Using the method reported above, unique air samples were collected at the same temperature from the same height in different places, such as from a public park (300 m from a coliseum), an indoor volleyball hall, a Student Health Testing Center, an outdoor synthetic-rubber sports track, and an outdoor basketball court. The TDI concentrations of these samples were evaluated by EESI-MS under the same experimental conditions. The results are shown in Table 2. In Table 2, it is clear that there is no significant difference of free TDI content in the air sampled from the indoor volleyball hall and the Student Health Testing Center, because the two venues were constructed using the same plastic materials. The EESI mass spectra indicate a noticeable difference between the indoor and outdoor synthetic-rubber sports venues. The lakeshore air also contained a low concentration of TDI. This can be accounted for by the fact that the lake was close to the Coliseum, resulting in the air quality around the lake being affected by the air from the Coliseum.
Outdoor basketball court
3.59
Indoor volleyball hall
6.89
Student health testing center
2.56
2.23
3.5 Effects of climatic conditions on the sampling The release of free TDI from sports venues constructed from materials containing polyurethane plastic increases at high temperatures, resulting in an increase in the TDI content in the air. The TDI concentration of air is affected by the air flow rate even in the same venue under the same climatic conditions. The samples were collected from different locations under different weather conditions for fast TDI detection by EESI-LTQ-MS. The major findings are shown in Fig.3. Figure 3 shows that the total amount of free TDI released into the air increases significantly when the temperature rises or when the plastic is directly exposed to sunlight. In hot sunny weather, the highest concentration of TDI is detected outdoors owing to sunlight exposure and the high plastic temperature. The exposure to sunlight plays a more significant role than a high temperature in the release of TDI from plastics. For example, the content of TDI in the air during a sunny day at 26 °C is considerably higher than that of a cloudy day with a temperature of 28 °C. In addition, the free TDI content of air obtained indoors does not change significantly with temperature, unlike the cases investigated outdoors. This may also be the result of the exposure to sunlight in the latter.
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