Author's personal copy Environ Sci Pollut Res DOI 10.1007/s11356-013-2010-5
RESEARCH ARTICLE
A new device for formaldehyde and total aldehydes real-time monitoring Maria Sassine & Bénédicte Picquet-Varrault & Emilie Perraudin & Laura Chiappini & Jean François Doussin & Christian George
Received: 10 April 2013 / Accepted: 12 July 2013 # Springer-Verlag Berlin Heidelberg 2013
Abstract A new sensitive technique for the quantification of formaldehyde (HCHO) and total aldehydes has been developed in order to monitor these compounds, which are known to be involved in air quality issues and to have health impacts. Our approach is based on a colorimetric method where aldehydes are initially stripped from the air into a scrubbing solution by means of a turning coil sampler tube and then derivatised with 3-methylbenzothiazolinone-2-hydrazone in acid media (pH=−0.5). Hence, colourless aldehydes are transformed into blue dyes that are detected by UV–visible spectroscopy at 630 nm. Liquid core waveguide LCW Teflon® AF-2400 tube was used as innovative optical cells providing a HCHO detection limit of 4 pptv for 100 cm optical path with a time resolution of 15 min. This instrument showed good correlation with commonly used techniques for aldehydes analysis such as DNPH derivatisation chromatographic techniques with off-line and on-line samplers, and DOAS techniques
Responsible editor: Gerhard Lammel M. Sassine : C. George (*) Université Lyon 1; CNRS, UMR5256, IRCELYON, Institut de Recherches sur la Catalyse et l’Environnement de Lyon, Villeurbanne, France e-mail:
[email protected] B. Picquet-Varrault : E. Perraudin : J. F. Doussin Laboratoire Interuniversitaire des Systèmes atmosphérique (LISA), UMR-CNRS 7583, Institut Pierre Simon Laplace, Université Paris-Est Créteil (UPEC) et Université Paris Diderot (UPD), 94010 Créteil Cedex, France L. Chiappini Institut National de l’Environnement Industriel et des Risques, Parc Technologique ALATA, 60550 Verneuil-en-Halatte, France Present Address: E. Perraudin CNRS, UMR5805, Equipe LPTC (Laboratoire de Physico-et Toxico-Chimie de l’Environnement), EPOC, University of Bordeaux, 33400 Talence, France
(with deviation below 6 %) for both indoor and outdoor conditions. This instrument is associated with simplicity and low cost, which is a prerequisite for indoor monitoring. Keywords Formaldehyde . Total aldehydes . Air quality . Liquid core waveguide
Introduction It is well known that large urban area megacities such as Mexico City (Wood et al. 2009), Los Angeles (Haagen-Smit 1952), Santiago, Chile (Elshorbany et al. 2009) or even Paris (Vautard et al. 2003) are affected by smog events, which are associated with the photochemical production of ozone. The chemistry taking place during such events involves aldehydes, and especially formaldehyde, among others, as major sources of hydroxyl radicals (OH) which is one of the key players in atmospheric chemistry (Michoud et al. 2012). Aldehydes are either emitted directly into the atmosphere by plants (Lipari et al. 1984) or through incomplete combustion processes (Ciccioli et al. 1993), or are formed in situ during the oxidation of VOCs (Carlier et al. 1986). The tropospheric sinks of aldehydes include photolysis (Calvert and Pitts 1966), daytime reaction with OH radicals, or with Cl and Br radicals in the marine boundary layer, and finally reaction with nitrate radicals during night-time (FinlaysonPitts and Pitts 2000). Besides their key role in atmospheric chemistry, aldehydes are considered highly toxic. Skin complications, eyes and mucous irritations, asthma and respiratory troubles are the essential health impacts of aldehydes (Leikauf 2002; Morgan 1997). In 2004, formaldehyde was classified in group 1 (human carcinogen) by the International Agency for Research on Cancer due to its carcinogenicity. Therefore, exposure to formaldehyde can lead to some acute effects, especially indoors (Rancière et al. 2011).
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Indoor environments are often associated with high levels of aldehydes combined with long periods of human exposure (Guo et al. 2004). Sources of aldehydes indoors include building materials, insulation, combustion appliances, tobacco smoke and various consumer products (Nazaroff and Weschler 2004; Loh et al. 2006; Gustafson et al. 2007; Brown 1999). These sources emit aldehydes in substantial amounts. As a result, indoor aldehyde concentrations almost always exceed outdoor concentrations (Feng et al. 2004; Clarisse et al. 2003). In French dwellings, the average formaldehyde levels vary from 3 to 60 μg m−3 (Observatory on Indoor Air Quality (OQAI) 2006). Indoor air guide values have been published by the High Council for Public Health (HCPH 2009) and formaldehyde surveillance in schools and child care centres has been requested by the French Ministry of the Environment since 2011 on the basis of these guidelines: 10 μg m−3 as a target value to be reached in 2019, 30 μg m−3 as a quality guide value below which no corrective action is required, 50 μg m−3 as a recommendation and information value, 100 μg m−3 as an action value above which immediate measures shall be taken. Various analytical methodologies have been developed in the last two decades for monitoring aldehydes (Gilpin et al. 1997; Cardenas et al. 2000). The more common strategies can roughly be listed as: (1) off-line and on-line derivatisationchromatography methods such as 2,4-dinitrophenylhydrazine (DNPH) derivatisation followed by high pressure liquid chromatography (HPLC) (Grosjean 1991; Grosjean et al. 1996) or gas chromatography (GC) (Sirju and Shepson 1995; Hasegawa et al. 2000); (2) in situ spectroscopic methods such as Fourier transform infrared spectroscopy (FTIR) (Hak et al. 2005; Lawson et al. 1986), differential optical absorption spectroscopy (DOAS) (Heckel et al. 2005; Wagner et al. 2011) and tunable diode laser absorption spectroscopy (TDLAS) (Fried et al. 1997; Wert et al. 2003; Li et al. 2004); (3) on-line chemical ionisation methods such as proton-transfer-reaction mass spectrometry (PTR-MS) (Inomata et al. 2008; Warneke et al. 2011); (4) on-line fluorimetric techniques based on the Hantzsch reaction (Nash 1953; Dasgupta et al. 2005; Dasgupta et al. 1999b; Li et al. 2005) or the formaldehyde dehydrogenase catalysed reduction of NAD+ to NADH (Lazrus et al. 1988a; Mucke et al. 1996; Slemr et al. 1996) and (5) off-line or on-line derivatisation chemical methods followed by spectroscopic detection based on chromotropic acid, J-acid and phenyl-J-acid reactions and the Schiff reaction (Dasgupta et al. 1980; Miksch et al. 1981; Walters 1983) or the 3methyl-2 benzothiazolone hydrazone (MBTH) reaction (Sawicki and Hauser 1960; Sawicki et al. 1961). All of the techniques mentioned above have been tested, and sometimes inter-compared (Cardenas et al. 2000; Gilpin et al. 1997; Hak et al. 2005; Mucke et al. 1996; Ferrari et al. 1999; Hanoune et al. 2006; Kleindienst et al. 1988; Wisthaler et al. 2008), and have successfully measured formaldehyde and many other aldehydes over a wide range of concentrations
and time responses. However, real-time measurements were rarely associated with simplicity and low cost, which is a prerequisite for indoor monitoring. In fact, despite the large attention given to the measurements of formaldehyde, there is still a need (especially for indoor monitoring) for autonomous affordable, simple and small devices that are able to perform the real-time analysis of aldehydes, as this remains a current challenge. Addressing such a challenge was the purpose of this work. We developed a simple and portable instrument for real-time aldehyde analysis. As pioneered by Dasgupta et al. (1999a), this instrument takes advantage of known titration chemistry and inexpensive spectroscopic detection methods that were made sensitive by the use of a liquid core waveguide (LCW), as an innovative optical cell for highly sensitive aldehyde measurements. The use of LCW as environmental useful photometric tools still require further studies and in-depth investigation of its properties and limitations. Especially memory effects have to be understood and addressed. This was investigated here and the titration chemistry was modified by use of new catalyser to prevent those memory effects. This instrument was then inter-compared and tested in two different simulation chambers covering both outdoor and indoor conditions.
Methods Reagents All solutions were prepared with Milli-Q water. All chemicals were purchased from Sigma-Aldrich. Aqueous solutions of 3methyl-2-benzothiazolinonhydrazone hydrochloride hydrate were prepared in the range (1–20 mM) under acidic conditions using hydrochloric acid. Hydrogen peroxide (35 %wt) was used to prepare aqueous solutions in the range from 0.1 to 5 M. Iron (III) chloride hexahydrate (analytical grade, 97 %) was used to prepare diluted solutions (13 μM) at a pH below 1 (by adding hydrochloric acid), hence preventing the precipitation of the ferric ions. Acid blue (C20H13N2NaO5S) was chosen for the LCW optical pathway calibration, since it absorbs in the same wavelength range as the aldehyde derived dyes (600 nm; see below) and its molar absorptivity coefficient is available from the literature (Mazet et al. 1990). A home-made silicone permeation tube, maintained at 298 K was used to generate a constant gaseous flow of formaldehyde from a 3.7 %wt HCHO aqueous solution. Sample collection A schematic diagram of the analytical set-up is given in Fig. 1, which shows that the device comprises three main parts, i.e. a collection unit, a chemical converter and a UV–vis detector.
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FM Air out
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P RC 2 Outlet solution
Inlet solution MBTH H2O2
FeCl3
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Fig. 1 System set up schematic for gaseous formaldehyde and aldehydes continuous measurement. A air pump, FM flow meter, RC1 reaction coil 1, RC2 reaction coil 2, TCU temperature control unit, TC thermo couple, P peristaltic multi-channel pump
The collection unit (Fig. 1) performed the continuous sampling of gaseous aldehyde into an aqueous stripping solution by means of a glass stripping coil of 150 cm length with an inner diameter of 0.6 cm. The acidic stripping solution contained 3methylbenzothiazolinone-2-hydrazone (Lazrus et al. 1988b; Lazrus et al. 1986; Lee et al. 1991; Lee and Zhou 1993; Francois et al. 2005; Sauer et al. 2001). After passing through a debubbler, the solution was pumped with a peristaltic pump (ISMATEC, ECOline) into the chemical converter. The gaseous flow was pumped into this collection unit by means of a membrane pump (KnF Laboport) and the flow adjusted by a valve in the range 0.1 to 6 l min−1.
(RC2; 1.64 cm i.d.×10 cm, residence time 7 min, 323 K) and mixed with hydrogen peroxide (0.9 M) and ferric chloride (13 μM). This second step led to the final product via the reaction between hydrazones and an intermediate product resulting from the oxidation of MBTH by hydrogen peroxide, which is catalysed by the ferric ions. The final product had a large π electron conjugation leading to a strong absorption at 630 nm, with a molar absorptivity of 6.5×104 M−1 cm−1. This dye was detected and quantified by means of UV–vis spectroscopy in the detection unit. The different parameters for the aldehyde collection and conversion were fully optimised, as detailed in further sections (see “Wetderivatisation optimisation” to “Phase transfer optimisation”).
Chemical converter Detection unit The chemical converter is made of two reaction coils (RC1 and RC2) where the aldehydes are converted into a dye that is easily detectable in the visible spectrum. The chemical conversion of formaldehyde into a dye started in the collection unit as soon as it was transferred to the MBTH (9 mM) solution. MBTH reacted with the carbonyl group of the aldehyde(s) to form the corresponding hydrazones. However, this reaction was slow at room temperature, so it only became significant in the heated (323 K) reaction coil 1 (RC1; 1.64 cm i.d.×4.75 cm). The liquid flow conditions typically led to a 10-min residence time in RC1. Then, this solution was pumped, at a rate of 1 ml min−1, into a second reaction coil
The detection unit consists of a long path cell made of Teflon AF2400, which has a refraction index lower than water allowing light conduction and therefore acts as a liquid core waveguide. The original idea was raised in the work of Dasgupta et al. (1999a) where waveguides were exclusively used for physical studies. In this work, the LCW was a 810 μm o.d., 610 μm i.d., Teflon® AF-2400 tube (BioGeneral, San Diego, CA), varying in length from 3 to 100 cm. Visible light was focused into the tubing via an optical fibre and, with the refractive index of the tubing material being lower than that of the liquid, underwent
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multiple total reflections on the inner walls of the tubing dependent on the angle of incidence and hence stayed inside the liquid for absorption. At the opposite end of the LCW, the light was collected again by a glass fibre and detected and analysed by means of a spectrograph/CCD camera combination (iDUS, ANDOR Tech). All data treatment was performed by means macro developed using the ANDOR iDUS acquisition software.
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Concentration of the cationic dye (×10-6M)
Fig. 2 a Influence of the MBTH concentration on the formation of the cationic dye. HCHO=1 μM, (H2O2)=0.5 M, (FeCl3)=13 μM, T=323 K, pH=1, t1 =t2 =10 min and an optical pathway l=30 cm. b Effect of H2O2 concentration on the cationic dye formation. Results are obtained in the following conditions: (HCHO)=1 μM, (MBTH)=9 mM, (FeCl3)=10μM, at 333 K and pH=1, t1 =t2 =10 min and l=30 cm. c Effect of the ferric ions concentration on the conversion of 1 μM of dissolved HCHO into its corresponding dye in the following conditions: (MBTH)=9 mM, (H2O2)=0.9 M, T=333 K pH=1, t1 =t2 =10 min and l=30 cm
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The atmospheric chamber at LISA
The exposure chamber at INERIS The National Institute for Industrial Environment and Risks (INERIS) dynamic exposure chamber is a borosilicate glass loop (200 l volume) described in detail by Gonzalez-Flesca and Frezier (2005). It has been specifically designed to generate single or multi-component gaseous species atmospheres at known and stable concentrations (from hours to weeks). Clean and dry air was generated using a zero air generator (Claind, AZ Air purifier 2010). An equimolar mixture of eight aldehydes (formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, valeraldehyde, the hexaldehyde, acrolein and benzaldehyde) was generated by continuously diluting a 10 ppmv gaseous standard (Air Liquide France). The dilution process was controlled and monitored by mass flow meters.
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The photoreactor at LISA is an evacuable chamber, consisting of a Pyrex reactor of 977L coupled to two multiple optical reflection systems interfaced to an FTIR spectrometer and a UV–visible spectrometer. The main features of this chamber and its spectroscopic devices have previously been described (Doussin et al. 1999; 1997); thus, only the relevant details are given here. Since formaldehyde has an intense and structured spectrum in the UV region (250–350 nm), it was monitored using the DOAS technique during the experiments. The UV–visible spectrometer consists of a light source (high pressure xenon arc lamp, Osram XBO, 450WXe UV), a multipass optical system inside the reactor (path length of 72 m), a monochromator (HR 320 JobinYvon) and a charge-coupled device (CCD) camera (CCD 3000, Jobin-Yvon) as a detector. In this configuration, the CCD covers a spectral range of ca. 60 nm, and the instrument has a maximum resolution of 0.15 nm. UV spectra were treated using DOASIS software (S. Kraus, University of Heidelberg) and using the cross-sections of formaldehyde published by Gratien et al. 2007.
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(c) Relative humidity was generated from a pressurised bulb containing ultrapure water (18.2 MΩ, ElgaPure-Lab Flex,
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Veolia water) connected to a heated liquid mass flow controller system. The temperature in the chamber was maintained at around 293 K by circulation of a temperature conditioned mixture of water/ethylene glycol in the chamber double walls. A wind speed of 1 m s−1 was continuously maintained with a fan. Temperature, pressure, wind speed and relative humidity were continuously monitored. The experiment performed in this chamber is fully detailed in Chiappini et al. (2011). Briefly, formaldehyde concentrations were set to about 10 μg m−3 to simulate low formaldehyde levels, which are usually measured in indoor air, and about 25 μg m−3 to simulate medium French indoor concentration levels ((Observatory on Indoor Air Quality (OQAI) 2006)). The new technique was compared to passive (ISO 16000-4:2001 2004) and active (ISO 16000-3 2001) standard techniques used for formaldehyde measurement and was based on DNPH derivatisation, acetonitrile extraction and HPLC-UV detection.
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(a) 1,0 Concentration of the catioinc dye (×10-6M)
Fig. 3 a Effect of the temperature on the formation of the dye. Results were obtained with a pH media=1 and t1 =t2 =10 min. b Influence of the media pH on the conversion of 1 μM of HCHO. Derivatisation conditions are the following: T=323 K, pH=1, t1 =t2 =5 min. c Influence of the conversion time of each step onto the formation of the cationic dye relative to the HCHO. The results are obtained in these conditions at 323 K and pH=1. The HCHO respective concentrations for the first and the second step are 0.95 and 1 μM. All above experiences have been driven with 1 μM of HCHO except for 4.c 9 mM of MBTH, 0.9 M of H2O2, 13 μM of FeCl3 and with an optical pathway l=30 cm
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The chemical titration used here is well known but was slightly modified in order to prevent memory effects arising from the Teflon AF-2400 porous tubes. Accordingly, we have strongly reduced the levels of the conventional oxidant (FeCl3) by adding hydrogen peroxide (H2O2). FeCl3 absorbs in the visible and its absorption in the porous waveguide I, also reducing the light conducting properties of the LCW. H2O2 is also a strong oxidant and the kinetics of the reaction between MBTH and H2O2 have already been studied (Tomiyasu et al. 1999). It is catalysed by the presence of traces of metal ions such as ferric (Fe3+) and cupric (Cu2+) ions (Tomiyasu et al. 2005, 1999). We had to optimise the operating conditions and especially the aqueous phase reagent in order to maximise the dye production within the 10 min residence time in each reaction coil. As shown in Fig. 2a, the optimum concentration of MBTH was determined to be 9 mM. The optimum H2O2 concentration was found at 0.9 M (see Fig. 2). The FeCl3 level was adjusted to have the maximum conversion and conversion rate while still
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(c) having the minimum levels of iron in the solution. As seen in Fig. 2c, this was achieved with a FeCl3 concentration of 3 μM. As already mentioned, this derivatisation reaction is slow at room temperature. Figure 3a depicts the effect of temperature on
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the titration yield. Increasing the temperature up to 333 K enhances the oxidation kinetics, allowing shorter reaction times. At 333 K and above, a decrease in the signal can be observed due to the degradation of the dye (or of the intermediates leading to it). The chemical converter is operated under acidic conditions favouring the proton transfer to the aldehyde from the MBTH, which is considered a weak nucleophilic compound. Figure 3b indicates that a pH less than 1 is optimal. Under optimum conditions (9 mM MBTH, 0.9 M H2O2, 13 μM FeCl3, pH=1 and temperature=323 K), the reaction times in RC1 and RC2 are 5 and 7 min, respectively, with 99 % HCHO conversion (as shown in Fig. 3c).
UV–visible absorption properties of aldehyde derivatives
Table 1 Summary of the dye derivative conversion yield of the different aldehydes (with an initial concentration of 1 μM) determined in the optimal derivatisation conditions except for t1 =10 min instead of 5 min Aldehydes compounds
Cationic derivative concentration (M)
Conversion yield (%)
Formaldehyde Acetaldehyde n-propanal n-butanal n-pentanal n-hexanal Crotonaldehyde Acrolein Benzaldehyde
(9.84±0.47)×10−7 (9.83±0.49)×10−7 (9.99±0.50)×10−7 (1.00±0.50)×10−7 (9.82±0.49)×10−7 (9.60±0.48)×10−7 (9.45±0.47)×10−7 (9.51±0.46)×10−7 (9.44±0.47)×10−7
(98.4±4.7) (98.3±4.9) (99.9±5.0) (100.0±5.0) (98.2±4.9) (96.0±4.8) (94.5±4.7) (95.1±4.7) (94.4±4.7)
Errors are indicated at the 2σ level
Nine of the most abundant aldehydes have been tested using the above described procedure, i.e. formaldehyde, acetaldehyde, npropanal, n-butanal, n-pentanal, n-hexanal, crotonaldehyde, acrolein and benzaldehyde. Surprisingly, Fig. 4 shows that the observed end spectrum varies slightly for each aldehyde. These differences can be due to the different structure of the carboneous chain leading to small shifts in the wavelength of maximum absorption (Carunchio et al. 1985; Sawicki et al. 1961). Table 1 summarises the conversion yield of each aldehyde. Taking into account previous studies (Sawicki et al. 1961), the observations made here and on the basis of aldehydes spectral features, it becomes possible to classify these nine aldehydes into three main families (see Fig. 4): formaldehyde, saturated (acetaldehyde, n-propanal, n-butanal, n-pentanal and n-hexanal) and unsaturated aldehydes (crotonaldehyde, acrolein and benzaldehyde). This procedure allows a qualitative determination of their amount by using their absorption at three wavelengths (630, 500 and 730 nm) as:
0,7 Formaldehyde Acetaldehyde Propanal Butanal Pentanal Hexanal Crotonaldehyde Acrolein Benzaldehyde
Absorbance (U.A)
0,6 0,5 0,4
A630 ¼ l ðε630;HCHO C 1 þ ε630;RCHO saturated C 2 þ ε630;RCHO unsaturated C 3 Þ ð1Þ where l is the optical path length, C1, C2 and C3 are the concentrations of HCHO, saturated aldehydes and unsaturated aldehydes and ε630,RCHO saturated, (48±3)×103 M −1 cm−1 and ε630,RCHO unsaturated, (20±5)×103 M −1 cm−1 are the molar absorptivity at 630 nm for each aldehyde class measured here. In addition, the absorbance at 500 nm (A500) is due to only two families and can be expressed as: A500 ¼ l ε500;HCHO C 1 þ ε500;RCHO saturated C 2 ð2Þ where ε500,HCHO is the HCHO derivative molar extinction coefficient estimated at 500 nm to be (18±2)×103 M −1 cm−1 and ε500, RCHO saturated is the average of molar extinction coefficients of saturated aldehydes derivatives at 500 nm evaluated as (9±1)×103 M −1 cm−1. Finally, the contribution at 730 nm is identical for all aldehydes leading to: A730 ¼ 1 ε730 ðC 1 þ C 2 þ C 3 Þ
ð3Þ
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Fig. 4 UV spectra for aldehyde derivatives obtained for 1 μM injected aldehyde. Optimal reagents concentrations have been used at 323 K at pH=1; 10 cm is the optical pathway and t1 =t2 =10 min
Resolving Eqs. 1–3 allows then the determination of C1, C2 and C3. Obviously, this is based on tiny shifts on their absorption features and provides only a qualitative basis for the determination of the concentration of the abovementioned three families. Limitations arise from the fact that only nine aldehydes were tested (as mentioned) and that interferences from other aldehydes (e.g. glyoxal) during the sampling and conversions processes cannot be excluded. The interference of ketones was investigated
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by studying the response of acetone. Which did not show any interference as expected from its chemical structure.
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The aldehydes have a relatively low intrinsic solubility in water. However, they react with water to form gem diols. In equilibrium, the concentrations of gas-phase (g) and the liquid phase (aq) species are governed by the equilibrium constants as shown in the following equation:
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Figure 5b represents the variation of the concentration of trapped HCHO in solution as a function of pH in the range −1 to 1. Low pH improves HCHO uptake even at high gaseous flow (i.e. 1.3 l min−1) which is supposed to be insufficient for HCHO trapping at pH 1 (see Fig. 5a). In this case, the increase in the HCHO trapping kinetics influenced by protonation is strongly favoured since the RCH2O+ formation in the aqueous phase is very fast. The limiting step of acid catalysis is the regeneration of the diol form and H+ (Jayne et al. 1992; Jayne et al. 1996; Esteve and Noziere 2005; Tolbert et al. 1993) as given in the relation (6): RCH 2 Oþ þ H2 O→RCH ðOH Þ2 þ H þ
ð6Þ
These results are in agreement with Swartz et al. (1997) where the rate constant of the HCHO protonation increases three orders of magnitude at 293 K (from 103 to 106 s−1) for the same pH range (1 to −1). In addition, we expect that the
Trapped HCHO concentration (ppbv)
where H is the Henry's law coefficient and Khyd is the equilibrium constant of aldehyde hydration into gem diol. How the formation of diol greatly increases effective aldehyde solubility in water has been previously described (Betterton and Hoffmann 1988). The gem-diol formation rate is both acid and base catalysed. Under the conditions described above, increasing the gaseous flow induced a linear response of the detector in the range 0.05–1.2 l min−1, as observed in Fig. 5a. Above 1.2 l min−1, this relationship is no longer linear, indicating that aldehydes may not be quantitatively sampled any more. pH was also studied as a parameter affecting gem-diol formation and therefore total uptake coefficient. As mentioned previously, such uptake is catalysed by acids and bases (Betterton and Hoffmann 1988; Bell and Clunie 1952). We studied the effect of acid media on the capture of HCHO since derivatisation occurs at low pH.
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(b) Fig. 5 a Influence of gaseous flow rate on the quantity being stripped (circles) and the corresponding absorbance (square—left scale) and b pH media on HCHO uptake in solution at 298 K. Injected HCHO concentration=40 ppbv. Fg =1.3 l min−1, Faq =1 ml min−1, (MBTH)=9mM, (H2O2)=0.9 M, (FeCl3)=13 μM, derivatisation temperature=323K, t1 =5 min and t2 =7 min, l=15 cm
uptake rate in this study would be drastically enhanced due to the formation of hydrazones in the first derivatisation step. Characteristics of the analyser As mentioned, under optimum conditions (9 mM MBTH, 0.9 M H2O2, 13 μM FeCl3, pH=1 and 323 K), the time needed to achieve 99 % HCHO conversion is 12 min for both steps. The conversion efficiency depends on chemical nature of the aldehyde and varies from 99 % for HCHO and propanal to 94 % for benzaldehyde, with no interferences from ketones (as tested with acetone). For HCHO, with a 150-cm long stripping coil, combined with a gas in-flow lower than 1.2 l min−1, under acidic conditions, the overall collection and conversion efficiencies were larger than 97 %.
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The concentration of HCHO were then determined by measuring the absorbance of the dye, corrected by the overall efficiency of the device (i.e. 97 %) determined by the measured yields of each step. Calibration of the instrument could then be performed using standard aqueous solutions of HCHO. This calibration procedure led to a nicely linear behaviour over the entire concentration range investigated with a correlation factor of 0.998 and no significant intercept. With 72.3 cm long optical pathway, the quantification limit for aqueous HCHO was 2.5×10−9 M with a lower limit of measurable absorbance of 10−3. The repeatability r of the device was also determined. This parameter defined as: r¼
t 0;
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characterises the threshold differences between two successive determinations under exactly the same conditions and over a short period of time, and be regarded the absolute difference between these two measures at the 97.5 confidence interval. Under these conditions and with seven successive measures, Student’s law defines t as 2.447. The standard deviation S was determined as S=1.11×10−3, leading to r=1.02×10−3 based on the measured absorbance, corresponding to an overall accuracy of 96.8 %. Combining with the accuracy of the gas and liquid flow measurements (2 % of reading), uncertainties in the temperature of the reaction cells (±2 K), we estimate the overall precision of our device to be around 94 % with a high repeatability for HCHO only. Detection limits have been calculated in relation to the CCD limit of quantification (i.e. for a minimal absorbance detected of 10−3) with a signal to noise ratio of 3. As shown in Table 2, the detection limit for formaldehyde is in the order of 3 to 4 pptv with a temporal resolution of 15 min. The detection limit of the other classes worsen to ca. 20 pptv due to their slower time response and for some of them (unsaturated family) to a significantly lower absorption in the investigated wavelength region. Table 2 Limits of detection (LOD) of total aldehydes calculated with regard to the CCD sensitivity and in the following conditions: (MBTH)=9 mM, (H2O2)=0.9 M, (FeCl3)=13 μM, pH=−0.5, T=323 K, t1 =10 min, t2 =7 min, l=100 cm, Faq =1ml min−1, Fg =1 L min−1
Aldehyde
LOD (pptv)
Formaldehyde Acetaldehyde n-propanal n-butanal n- pentanal n- hexanal Glyoxal Acrolein Crotonaldehyde Benzaldehyde
4 5 5 5 5 6 9 11 10 15
Potential interferences not taken into account are however still possible and would lower the overall accuracy of the approach. Such interferences would mainly arise from other aldehydes as discussed above. However, the identify small changes in the spectral features of the dye would allow to overcome partly these interferences allowing the determination of total aldehydes and formaldehyde with an overall accuracy of 20 % (rough estimate) in complex air masses. Measuring the level of total aldehydes or classes of aldehydes is possible but requires longer conversion times (up to 30 min), reflecting decreasing reactivity of most aldehydes (with the exception of glyoxal) as compared to formaldehyde. Taking into account the difference in time and spectral responses, this technique has the potential of measuring both HCHO and total aldehydes in one given air mass, which correspond to an added features compared to the widely used Hantzsch approach. The difference in spectral responses assumes the identical response for formaldehyde, saturated and unsaturated aldehydes at 730 nm which comes with a 5 % uncertainty, which increases to 10 % at 500 nm and peaks at 20 % at 630 nm. The above accuracies were tested against standard concentrations in simulation chambers (see also below for more details) in increasing complex mixtures. The observed differences between the injected concentrations and the measured ones were lightly concentration dependent with accuracy better than 5 % for HCHO, 13 % or saturated aldehydes and 20 % for unsaturated aldehydes, validating the differences in the spectral features for the limited number of aldehydes investigated.
Validation under realistic conditions Inter-comparison with DOAS for HCHO measurement The instrument was inter-compared with the DOAS technique, which is installed on the simulation chamber at LISA. HCHO was chosen for this inter-comparison. However, since the DOAS technique (with a path length of 72 m) is less sensitive than the instrument developed here, this inter-comparison was indirect and was carried out in several steps. First, after having filled the chamber at atmospheric pressure with nitrogen, about 1 ppmv of formaldehyde was introduced into the chamber and its concentration was precisely monitored using the DOAS technique. Then, several successive dilutions were applied to this mixture to generate new mixtures with concentrations of formaldehyde ranging from 0.2 to 68 ppbv, which are representative of remote and polluted areas, respectively. The concentration of formaldehyde in these diluted mixtures was determined by calculating the dilution factor which is the ratio of the pressures measured before and after the pumping. Finally, samplings with the LWC instrument ran for 30 min. Two flow rates were used (0.3 and 1 l min−1), depending on the measured concentration leading to sampled volumes of 9 and 30 L,
Author's personal copy HCHO concentration given by the analyzer (ppbv)
Environ Sci Pollut Res 70 60 50 40 30 20 10 0
0
20 40 60 80 Mean HCHO concentration given by DOAS (ppbv)
Fig. 6 Inter-comparison plot of formaldehyde measurements with FTIR and DOAS methods. Trapping was realised with Faq =1 ml min−1 and Fg =0.3 l min−1 and 1 l min−1 (the latter is only used for the two lowest concentrations) at pH=0. Derivatisation was driven in the optimal conditions with a 65-cm of waveguide length
respectively. Under these conditions, dilution caused by the sampling was neglected considering the large volume of the chamber. Figure 6 illustrates the comparison of HCHO concentrations measured with the LWC instrument and those calculated from the DOAS technique and the dilution factors. Uncertainties on HCHO concentrations obtained by this last method were calculated as the sum of the relative uncertainties on the UV cross-sections of HCHO (5 %, Gratien et al. 2007) and on the dilution factor (between 2 and 10 %). The correlation between the data is strong (r2 =0.99) with a slope very close to unity (0.99). Finally, other successful inter-comparison experiments have been performed in the INERIS exposure chamber and in an indoor environment with the standard passive and active DNPH-HPLC-UV chromatographic techniques. The results of these experiments have been presented by Chiappini et al. (2011). Briefly, a good agreement between the four different measurement techniques could be observed with relative standard deviation ranging from 6 to 18 %. Also, whatever the environment, smog chamber or indoors, the continuous on-line analyser has been able to measure formaldehyde concentration changes even if its response could be altered by memory effect.
0.3 L min−1 for 30 min. The results showed that for 7 ppbv of hexaldehyde, the response was 96.8 %. Even though the Henry's Law constant of the latter is 1,000 times lower than that of formaldehyde, its solubility in solution is improved by the low sampling pH. To confirm this result, a binary mixture of 9.8 ppbv formaldehyde and 13.7 ppbv hexaldehyde was produced in the simulation chamber. In this case, formaldehyde is trapped completely (ca. 99 %) while the hexaldehyde collection coefficient reached 97 %. Therefore, we can assume that the response of other aldehydes will be more than 97 % since hexanal has the lowest aqueous solubility of the aldehydes considered here. In addition to static chamber experiments, several experiments were conducted in dynamic chamber at the National Institute for Industrial Environment and Risks. An equimolar mixture of eight aldehydes (formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, valeraldehyde, the hexaldehyde, acrolein and benzaldehyde) was produced by diluting 10 ppmv of gaseous standards purchased from Air Liquide. The mixture was continuously injected into the INERIS dynamic atmospheric simulation chamber. These experiments were conducted with complex mixtures at two different levels of concentrations. The first is called “poor” and the second “rich” in aldehydes. The chamber temperature was conditioned at (293±0.5)K with a relative humidity set at (50±5)% and a wind speed fixed at 1 m s−1 to simulate real atmospheric conditions. Trapping was carried out at room temperature (298±2) K by pumping a flow of 0.1 l min−1 diluted at the entrance of the trapping system of the analyser in a 1 l min−1 of N2. Table 3 reports the measured mean concentrations in both rich and low conditions for the three aldehyde classes. For Table 3 Mean concentrations measured for aldehydes Compounds
Theoretical concentration (ppbv)
Formaldehyde 8.0 16.0 Saturated 40.0 aldehydes 80.0 Unsaturated 13.0 aldehydes 32.0
Measured Relative Reference concentrationa concentration errorb (ppbv) (ppbv) (%) 6.5±1.3 13.5±2.0 24.9±3.7 62±9.3 – –
6.4±0.7 14.6±1.7 31.0±3.2 63.4±9.5 12.8±1.9 29.0±4.3
1.6 7.3 16.45 2.3
Trapping conditions: Fg =1 L min−1 ; Faq =1 mL min−1 pH=−0.5; derivatisation conditions: [MBTH]=9 mM; [H2O2]=0.9 M; [FeCl3]=13μM; pH=−0.5; t1 =10 min; t2 =7 min; l=3.7 cm
Total aldehydes response
a
The procedure described above was tested, at room temperature, against a mixture of several gaseous aldehydes in the LISA chamber. Binary mixtures of HCHO and either hexaldehyde or benzaldehyde in the ppbv range were generated by successive dilutions of 1 ppmvv of the gaseous aldehydes in N2 as a carrier gas (Gratien et al. 2007), which were sampled at a flow rate of
b The error refers to the difference between the measured and reference concentrations
Standardised methods, i.e. (ISO 16000-3 2001) based on sampling on Sep-Pack cartridges, DNPH derivatisation, acetonitrile extraction and HPLC-UV detection (see Chiappini et al. 2011). An uncertainty of 20 % has been estimated for this measurement technique. The reference concentration may differ from its theoretical value as some compounds are sticky and could be lost within the chamber. Also this method did not measured the unsatrurated compounds
Author's personal copy Environ Sci Pollut Res
both cases, the measurements are in agreement, within the estimated accuracy, with the concentrations measured by a standardised methods, i.e. (ISO 16000-3 2001) based on sampling on Sep-Pack cartridges, DNPH derivatisation, acetonitrile extraction and HPLC-UV detection (see Chiappini et al. 2011). An uncertainty of 20 % has been estimated for this measurement technique. The agreement is especially good for HCHO while the distinction of the other two classes is associated with larger errors (as expected). The relative ratio between these classes is also correctly measured.
Conclusion A revisited method based onto MBTH-derivatisation followed by UV detection technique in a LWC showed good correlation with commonly used techniques for aldehyde analysis, such as DNPH derivatisation chromatographic techniques with offline and on-line samplers, and DOAS techniques. The agreement within the latest experiments was within 6 %. This wet technique is simple and easy to run and is associated with low cost instrumentation. In a context of increasing concern for indoor air quality and systematic surveillance of formaldehyde in schools and child care centres, this kind of technique would be a suitable tool for formaldehyde sources identification and concentration monitoring. Acknowledgements The authors would like to thank the French Ministry of the Environment for supporting this work. We acknowledge the region Rhône-Alpes (Cluster 5) and the French research ministry for the DALD grant through ACI. Finally, all of our collaborators are thanked for their support and their contribution to this work. EP, BPV and JFD acknowledge the European Commission for supporting the intercomparison experiments through the EUROCHAMP project (contract number RII3-CT-2004-505968).
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A new device for formaldehyde and total aldehydes real-time monitoring
Maria Sassine, Bénédicte PicquetVarrault, Emilie Perraudin, Laura Chiappini, Jean François Doussin & Christian George Environmental Science and Pollution Research ISSN 0944-1344 Environ Sci Pollut Res DOI 10.1007/s11356-013-2010-5
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