Portable system for near-real time measurement of gaseous formaldehyde by means of parallel scrubber stopped-flow absorptiometry

Analytica Chimica Acta 531 (2005) 41–49

Portable system for near-real time measurement of gaseous formaldehyde by means of parallel scrubber stopped-flow absorptiometry Kei Todaa,∗ , Ken-Ichi Yoshiokaa , Kotaro Moria , Shizuko Hiratab a b

Department of Environmental Science, Faculty of Science, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860 8555, Japan National Institute of Advanced Industrial Science and Technology (AIST) Chugoku, 2-2-2, Hiro-Suehiro, Kure 737 0197, Japan Received 19 July 2004; received in revised form 30 August 2004; accepted 30 August 2004 Available online 7 December 2004

Abstract Formaldehyde, HCHO, is one of the important causal agents of sick-building syndrome. It is also an important product of ambient air photochemistry. We report here a portable instrument capable of a 0.08 ppbv limit of detection (LOD) and a time resolution of 5 min that is useful for both indoor and ambient air applications. The detection is based on efficient gas collection and chromogenic reaction with 3methyl-2-benzothiazolone hydrazone (MBTH) through a pair of alternately sampling small-bore porous-membrane tube diffusion scrubbers (DS). The chemistry is well established, requires no special reagent preparation or elevated reaction temperatures and permits the use of inexpensive light emitting diode (LED)-based detectors without need for long path cells. Stopped flow alternate sampling allows an HCHO collection performance, an order of magnitude better than any previous system with high throughput and high sensitivity. Results for indoor and ambient air analyses are presented. © 2004 Elsevier B.V. All rights reserved. Keywords: Flow analysis; Formaldehyde; Diffusion scrubber; Spectrophotometry; Stopped-flow; Portable instrument

1. Introduction Over the last decade, formaldehyde (HCHO) has emerged as one of the principal agents responsible for sick-building or sick-house syndrome [1–4]. Present building practices result in airtight, thermally and acoustically insulated buildings that are energy efficient, this is especially important in countries with high population density, such as Japan. Fresh air turnover rate is deliberately low to achieve energy conservation; volatile organic compounds such as HCHO that are emitted from various building materials, including adhesives, paints, floors, walls and curtains, etc. tend to build up to high concentrations. In addition, HCHO is contained in pesticides, antiseptics, cosmetics and perfumes; it is also a combustion product of tobacco and present in exhaust from stoves [5]. Complaints of occupants of brand new houses ∗

Corresponding author. Fax: +81 96 342 3389. E-mail address: [email protected] (K. Toda).

0003-2670/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2004.08.070

of headaches, ocular and thoracic pain, asthma, and atopic dermatitis [6] are common. The World Health Organization (WHO) has recommended a maximum permissible level of indoor HCHO to be <80 ppbv (30 min average) [7]. In Japan, the building environment health control standard has required measurements of HCHO in special public buildings such as department stores, schools, and hotels since 2003. Measurements are required when they are initially built or repaired on a large scale [8]. Recently, even for individual family residences in Japan, some manufacturers provided indoor HCHO data to prospective customers as an assurance towards safety and quality concerns. Also in the ambient air, HCHO plays important role in photochemical reactions. The atmospheric HCHO level is mostly less than 10 ppbv and fluctuates with time by the effect of sunlight and source situations. A convenient method for the measurement of gaseous HCHO, capable of continuous on-site measurement or in a short time is needed. In current practice, a gas detector

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tube is used for indoor HCHO measurement; 30 min sampling is needed to attain an LOD of 20 ppbv. Recently, an HCHO monitor was reported; this uses a disposable reagentimpregnated tablet with a LED photodiode-based reflectance detector [9]. However, indoor HCHO levels tend to change with temperature or humidity, because of its high volatility and water solubility. The source conditions also have a significant effect on the measured concentration. All of these suggest that one should use continuous measurement for some period rather than a spot check for building monitoring purposes. Electrochemical sensors [10] and FTIR instruments can provide continuous measurements for HCHO but exhibit poor sensitivity and/or humidity dependence. Laboratory methods for low ppbv levels of HCHO, in addition to the traditional hourly or daily measurements [11,12], include collection into a 2,4-dinitrophenylhydrazine (DNPH) cartridge or DNPH solution and subsequent analysis by HPLC with absorbance detection [13–15], solid phase microextraction (SPME) with GC–FID analysis [16], tunable diode laser absorption spectrometer [17], new spectroscopic technique based on difference frequency generation [18–20], glass coil collector combined with fluorometric measurement [21–23]. SPME/GC–FID analysis gives 1 ppbv LOD with 10 min sampling. These systems are large and expensive, and inappropriate for on-site applications. On the other hand, gas measurement based on gas collection and subsequent chemical analysis is relatively simple, and some novel methods have been reported for HCHO analysis. For example, Zhang et al. collected HCHO into a membrane cell (sometimes called a chromato-membrane cell) and measured it by adsorptive polarographic determination after batch process reaction at 60 ◦ C for 15 min [24]. Sakai et al. developed a high sensitivity fluorometric FIA system using 5,5dimethylcyclohexane-1,3-dione (dimedone) chemistry and measured gaseous HCHO [25]. Air samples were collected in advance using conventional impingers. Li et al. coupled a Nafion membrane collection system with a fluorometric flow system to attain automatic collection/measurement in which 1,3-cyclohexanedione (CHD) was used as the reagent [26]. The sensitivity of CHD system is very high and LOD is 30 pptv with 10 min measurement cycle. Very recently, Motyka and Mikuˇska developed a new HCHO instrument using a wet effluent denuder and fluorescence 2,4-pentandione (PD) method [27]. The PD system can measure the HCHO level continuously with 2.5 min delay and LOD 1.2 ppbv. These fluorometric methods for HCHO determination require high reaction temperatures, so that high backpressure, a postcooling device or a debubble diffusion cell is used to prevent bubble generation and consequent noise. The fluorometric methods also need an expensive photomultiplier tube detector. The purpose of this work was to develop a simple and inexpensive portable instrument for highly sensitive on-site formaldehyde measurements capable of easily monitoring ppbv levels of HCHO. We preferred an absorbance base method that would permit the use of inexpensive photodetec-

tors and a chemistry that would not require involved reagent preparation, purification or maintenance and highly elevated reaction temperatures. In general, absorptiometry has less sensitivity compared to fluorometry. It has not generally been possible to perform trace gas measurement at the ppbv level on a (near)-continuous basis even though there are many colorimetric methods for gas analysis. Long path absorbance cells, often based on liquid core waveguides (LCW), have been applied to gas measurement in order to increase the sensitivity [28,29]; this has its own limitations. In this work, we use a commercially available reagent without purification, an inexpensive LED-photodiode based absorbance detector of modest path length and a stopped flow collection system with a modest reaction temperature. This results in a simple and affordable portable power-efficient instrument that is capable of high sensitivity and throughput.

2. Experimental 2.1. Reagents The absorbing or carrier solution was 3 mM 3-methyl2-benzothiazolone hydrazone hydrochloride (MBTH hydrochloride) obtained from Tokyo Kasei (Tokyo) with 1 or 10 mM HCl, and the reagent solution was a mixture of 10 mM FeCl3 , 0.1 M HCl and 50% (v/v) 2-propanol. A gaseous standard containing 1 ppmv HCHO was prepared by bubbling purified air through a 5.53 mM HCHO solution (50 ml) at 0.2 l min−1 , and was diluted to generate test gas concentrations in the range of 10–100 ppbv [30,31]. The HCHO source solution was put in a water bath maintained at 20 ◦ C. Gaseous CH3 CHO (5 ppmv) was also prepared in the same way as for HCHO, using 1 l of 75 ␮M CH3 CHO solution. The CH3 CHO gas concentration could not be kept constant for a long period of time because the CH3 CHO concentration in the source solution is very low and perceptibly decreases. Decreasing in the aqueous source concentration was 0.01 and 3.6% h−1 for HCHO and CH3 CHO, respectively. The HCHO and CH3 CHO solutions were prepared everyday and every 2 h, respectively. 2.2. Gas diffusion scrubbers The diffusion scrubber (DS) unit was composed of a hydrophobic membrane tube. Two types of membrane tubes were tested: (a) a porous polytetrafluoroethylene (pPTFE) tube (Poreflon, Sumitomo Electric, Osaka, Japan), 1 mm i.d. × 2 mm o.d., filled with 0.52 mm monofilament [32,33] and (b) a porous polypropylene (pPP) tube (Accurel, Membrana, Wuppertal, Germany), 0.5 mm i.d. × 0.9 mm o.d. Either membrane tube was jacketed in a 3 mm i.d. PTFE tube and an air sample flowed between the membrane tube and the jacket tube. The active lengths were 18 and 50 cm, respectively, to obtain the same solution volumes (∼100 ␮l) in the two DS units. The absorbing solution flowed through

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Fig. 1. Absorbance detector. LED: orange LED; PD: photodiode; BP: black plastic plate; TP: transparent plate; CP: connector plate; N: nut for liquid tube connection; LI and LO: liquid inlet and outlet.

or was contained in the lumen of the DS and collected the HCHO. 2.3. Absorbance detector The absorbance detector comprised of a photodiode (OPT301, Texas Instruments) and an orange LED (HLMPEG08-X1000, Hewlett Packard) with an emission maximum of 626 nm (FWHM 17 nm). Some initial work was done with a previously developed detector design [34], but the final design adopted is shown in Fig. 1. A transparent plate (0.5 mm thick) was sandwiched between two opaque plates. The transparent plate acted as both a gasket and an optical cell. It had a 10 mm wide channel for the solution to flow through. The LED and the photodiode were fixed facing each other in the black plates. This configuration was simplified version of the hybrid absorbance detector designed by Dasgupta et al. [35]. The LED current was adjusted to 17 mA, and photocurrent of the photodiode was amplified with a 1 M feedback resistor. 2.4. Flow system A schematic diagram of the final flow system is given in Fig. 2. The carrier and reagent solution were both aspirated at 0.2 ml min−1 with a miniature peristaltic pump (P) (MRP-P2, Minato Concept Co., 12 V dc). The carrier bag (CB) fed two parallel DS units simultaneously but solenoid valve (V1) (EXAK-3, Takasago Electric, Nagoya, Japan), which switched every 5 min, allowed the carrier solution to flow through only one DS at a time. This way, while one DS collected the sample, the collected sample was swept from the other DS. Simultaneously as V1 switches, air valve (V2) ((MTV-3R-NN4, Takasago Electric) also switches such

Fig. 2. System setup: (a) flow diagram and (b) outside appearance of the instrument. CB: carrier solution bag; RB: reagent solution bag; WS: waste bottle; DS: diffusion scrubber; V1, V2: three-way solenoid valves for liquid and air controls, respectively; P: two-channel peristaltic pump; I: sample injector; RC1, RC2: reaction coils; H: heater; TC: thermo couple; CC: cooling coil; DP: debubble port; D: absorbance detector made with LED and photodiode; BPC: back pressure coil; AP: air pump; FM: flow meter; DT: digital timer; TCU: temperature control unit. Thick solid lines and dashed lines show the liquid and air flows, respectively.

that the miniature air aspiration pump (AP) (T2-03 HP, TeeSquared MFG) always draws the air sample through the DS in which there is no liquid flow. The gas flow rate was maintained at 1.0 l min−1 by adjusting the air pump voltage. After the collected HCHO from the DS is pumped via injector (I) (6-port V-451, Upchurch Scientific), used for liquid phase calibration) to reaction coil (RC1) (0.38 mm i.d. × 2 m, residence time 1 min, 50 ◦ C), it reacts there with the MBTH

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already present. Next, it was mixed with the reagent solution from RB and reacted with FeCl3 in RC2 (0.56 mm i.d. × 0.9 m, residence time 0.5 min, 50 ◦ C). Both reaction tubes were coiled around a single heater maintained at 50 ◦ C by temperature control unit (TCU). Finally, the mixed solution went through a cooling coil CC (0.38 mm i.d. × 0.3 m, residence time 0.1 min) the absorbance detector (D) and backpressure coil (BPC) to waste. A debubble port (DP) was provided for cleaning the detector as needed. A digital timer controlled valve timing and data were acquired using a data logger (model 8420, Hioki E. E.) or a notebook computer with an AD converter card (CBI-3133A, Interface Co.). All components were powered by 12 V dc, so it could be operated off a storage battery in the field or generated from a 100 V ac powered 12 V dc supply. The whole system was contained in an aluminium case, and the solution bags were hung from a demountable pole installed on the box side. The weight of the complete system (including enough reagents for 20 h use) was 8.5 kg, of which half the weight was from the case itself.

Fig. 3. Absorbance spectra of product from HCHO after reactions with AHMT (dashed lines), MBTH (solid lines) and Fluoral-P (dotted line). Final concentrations of HCHO were 10 ␮M for AHMT and MBTH and 1 mM for Fluoral-P. These spectra are blank corrected. Blank absorbance of MBTH was 0.04, and those of AHMT and Fluoral-P were both less than 0.01.

3.2. Optimum conditions for the HCHO–MBTH reaction 3. Results and discussion 3.1. Choice of chemistry The most popular reagent for colorimetric HCHO measurement is 4-amino-3-hydrazino-5-mercapto-1,2,4-triazole (AHMT) [11,36]. The product of AHMT method has larger molar coefficient than that of chromotropic acid method [37] and not sticky as pararosaniline product [38]. 4-Amino-3pentene-2-one (Fluoral-P), which was originally used as a fluorescent reagent, can also be used for HCHO absorptiometric determination [39,40]. MBTH is less commonly used because it reacts with other aldehydes and the sample solution should be measured immediately after the collection due to the instability of the MBTH–HCHO intermediate [41,42]. However, the instability is not an issue if the measurement immediately follows collection [41]. We investigated all three above chemistries prior to a final choice. Fluoral-P reacts with HCHO in one step, whereas AHMT and MBTH need twostep reactions. However, not only molar absorption coefficient of Fluoral-P was small, but also this needed acetonitrile as a solvent, this leaks through a hydrophobic membrane tube. The molar absorption coefficient of MBTH was the highest, twice that of AHMT, as shown in Fig. 3. The AHMT chemistry also needs a very strong base (5 M NaOH) as the reaction medium, this is not desirable especially as carbonate formation will occur during air sampling and membrane pores can be filled by carbonate deposits. Accordingly, MBTH was the final choice. HCHO adds to the amino group of MBTH and then couples to another MBTH molecule in the presence of an oxidant such as FeCl3 . The final product has a large ␲ electron conjugation area, and has its maximum absorption at a long wavelength (λmax 629 nm) with a molar absorptivity of 6.5 × 104 M−1 cm−1 .

The effect of the reaction coil temperature was firstly examined as preliminary studies showed that the first and second reactions need ∼10 and 5 min at room temperature, respectively, to reach completion. The reaction tubes were coiled around a small U-shaped heater (10 cm long × 6 cm wide) consuming 3 W when on. A thermistor was imbedded in the coil and the whole assembly covered with Al-tape. As the first reaction took longer, the first and second reaction coils were set to lengths that would provide a residence time twice as large in RC1 compared to RC2. The effect of the temperature on the second reaction step was examined first by moving the injector (I) between RC1 and RC2 and injecting HCHO pre-reacted with MBTH just upstream of RC2. The results are shown in Fig. 4(a). The second reaction was relatively efficient, and good conversion was obtained even at low temperatures. The dependence of the overall reaction on temperature was now examined. As shown in Fig. 4(b), this temperature dependence was much greater. Relative to injection of solution phase HCHO, in the case of the gas measurement, the first reaction also takes place during the collection and transport steps and optimum reaction temperature is attained at a relatively lower temperature. The chosen reaction temperature of 50 ◦ C was significantly lower than recommended reaction temperatures for fluorometric methods in the literatures [25–27]. With the reactor temperature and the each flow rates were fixed to 50 ◦ C and 0.2 ml min−1 , the reagent composition was optimized using injected samples of aqueous 10 ␮M and 20 ␮M HCHO (Fig. 5). A lower HCl concentration in the reagent solution resulted in a slightly higher sensitivity, however, problems with precipitation of Fe(OH)3 were encountered. We chose a 0.1 M HCl concentration to avoid these problems. The optimum FeCl3 and MBTH concentrations were determined to be 10 and 3 mM, respectively (Fig. 5(b)

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sample gas Cg . CF =

Cs Cg

(1)

Higher signals result from a higher CF value. However, CF itself does not include the collection time over which the sample is collected and analyzed. The concentration factor per unit time (CFT) is given by dividing CF by the time t needed for one measurement cycle. CF (2) t The CFT value represents total performance of the collection method. The CF and CFT values are shown in Table 1 in nM ppbv−1 and nM ppbv−1 min−1 , respectively. Previously, we investigated gas measurement with tubular [32,43] and planar [44–46] gas diffusion scrubbers. In the 1 mm i.d. tubular membranes, porous polytetrafluoroethylene (pPTFE) tube was better than expanded polytetrafluoroethylene (ePTFE) tube in its collection efficiency, collection stability against temperature and durability [33,47]. Therefore, we started this work using pPTFE tube with a simple continuous-flow method. Zero gas and sample gas were introduced for 5 min each meanwhile the absorbing solution was flowed continuously. The signal intensity was not enough for the trace HCHO analysis. Hence, the absorbing solution was stopped for 5 or 10 min in the DS to improve the sensitivity. As shown in Fig. 6(a), the peak height for 10 min collection was twice that for 5 min. The increased peak height came at the sacrifice of the sampling rate. Even with 5 min collection, the signal and CF were five times larger than that of the continuous method as shown in Table 1. During this initial study, the absorbing solution was allowed to flow through a bypass to obtain a stable baseline during the collection step. It occurred to us that if another DS is placed in the bypass as in Fig. 2, there is no wasted time. This arrangement, henceforth as parallel scrubber stopped-flow, and the data obtained in this mode is shown in Fig. 6(b). In this way, a good throughput, 12 samples/h, was obtained. After finishing most of the investigation with pPTFE, we obtained a small pPP membrane tube and tested this tube. Fig. 6(c) shows the CFT =

Fig. 4. Temperature effects on (a) the second reaction step and (b) the overall reaction in the proposed scheme. Solutions of 3.3 mM MBTH and 7.2 mM FeCl3 flowed at 0.2 ml min−1 . Aqueous HCHO solutions (10 and 20 ␮M) were injected and signal peaks were plotted as the symbols of 䊉 and
, respectively. In the case of the investigation into the second reaction, HCHO and MBTH were mixed beforehand and injected just before RC2 after the first reaction was complete. The temperature effect on the total reaction was also examined using 10 ppbv HCHO gas measured by parallel scrubber stopped-flow mode, and their data are shown as .

and (c)). 2-Propanol (IPA) was added to the FeCl3 solution to 50% (v/v) to prevent adsorption of the product onto the tube wall and the absorbance cell. With these optimum conditions, the slope of the calibration curve for 0–25 ␮M aqueous HCHO was 0.0072-absorbance units ␮M−1 . 3.3. Parallel scrubber stopped-flow and choice of membrane tube The efficiency of the gas collection step is described by the apparent concentration factor CF: the ratio of the analyte concentration in the absorbing solution (Cs ) to that of the

Fig. 5. Optimization of reagent conditions. Effects of (a) HCl and (b) FeCl3 concentrations in the reagent solution, and (c) MBTH concentration in the absorbing solution were tested using 䊉 10 ␮M and
20 ␮M aqueous HCHO samples. Dashed vertical lines indicate the values determined as the optima.

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Table 1 Performance of the gas collections Method

Time sequence

Cycle time (min)

Collector

CF (nM ppbv−1 )

Continuous

5 min zero + 5 min sample

10

pPTFE DS pPP DS

60.0 216

6.00 21.6

Stopped-flow

5 min stop + 5 min flow

10

pPTFE DS pPP DS

293 979

29.3 97.9

Parallel scrubber SF

5 min stop + 5 min flow

5

pPTFE DS pPP DS

293 979

58.6 196

responses to the same concentrations as Fig. 6(b) with additional injections of the aqueous standard. Solution volumes of both scrubbers were almost identical, and the same signal peak widths were obtained. On the other hand, it is obvious that higher peaks were obtained by using the pPP scrubber. The main reason was higher mass transfer rate and longer effective area of pPP DS. Surface area of pPP (11.8 cm2 ) and pPTFE (8.48 cm2 ) DSs were not different much, but the mass transfer rates were 310 and 102 fmol s−1 ppbv−1 for pPP and pPTFE DSs at those conditions. The collection efficiencies of these scrubbers were estimated from the peak areas and they were ca. 13 and 40% for pPTFE and pPP, respectively. The amount of HCHO collected was not affected by the gas flow

CFT (nM ppbv−1 min−1 )

rate, because permeation through the membrane was the ratedetermining step at a sample gas flow rate of around 1 l min−1 . This is true for both ePTFE and pPTFE over the same ranges of flow rates [32,47]. Fig. 5(d) shows the response to a very low level of HCHO where response intensity is enlarged 10 times. It can be seen that this instrument can measure subppb levels of HCHO. Standard deviation of the zero response was equivalent to 27 pptv and from this an S/N = 3 LOD of 0.08 ppbv HCHO is estimated. The CF could be enlarged five times by the stopped-flow compared to the continuous flow, and three times by the pPP scrubber compared to the pPTFE scrubber. Also CFT in the parallel scrubber stopped-flow was twice that in the normal stopped-flow, and the performance has been enlarged totally 30 times better. In this way, the gas collection has been dramatically improved. 3.4. Investigation of interferences Other aldehydes coexist with HCHO in air, especially acetaldehyde, which is the major carbonyl compound after HCHO. Therefore, interference by CH3 CHO was examined, and the results are shown in Fig. 7. The response to CH3 CHO is very small, and was 10% of that of HCHO for aqueous CH3 CHO and 1% of that of HCHO for gaseous CH3 CHO. There are some reasons for this low sensitivity, e.g. smaller absorptivity, lower diffusion coefficient, smaller Henry’s law solubility [48] and slower reaction with MBTH (Table 2).

Fig. 6. Response curves obtained by (a) normal stopped-flow and (b)–(d) parallel stopped-flow. (a) and (b) were obtained with pPTFE in the different modes, and (c) and (d) were obtained by the parallel stopped-flow using pPP scrubbers. Responses to aqueous HCHO in (c) were obtained by manual injections of the sample solutions via the injector, and responses to gases were obtained by automatic operation. The response intensities in (d) are enlarged 10 times.

Fig. 7. Response to CH3 CHO. Gaseous samples of 250 ppbv and 500 ppbv CH3 CHO were tested. Responses to 20 ␮M HCHO and 100 ␮M CH3 CHO obtained by the liquid injections are also presented.

K. Toda et al. / Analytica Chimica Acta 531 (2005) 41–49 Table 2 Characteristics of HCHO and CH3 CHO (M−1

cm−1 )

ε of MBTH product Diffusion coefficient (cm2 sec−1 ) [15,42] Henry’s law constant (M atm−1 ) at 25 ◦ C [48] Rate of reaction with MBTH

47

can measure HCHO without significant interference in the normal conditions. HCHO

CH3 CHO

6.5 × 0.171

4.8 × 104 0.141

104

2.97 × 103

1.14 × 101

Relatively fast

Slow

It was confirmed in a batch test that only half of 10 ␮M CH3 CHO reacted with MBTH in 10 min of the first reaction time at room temperature, while HCHO reacted completely in the same conditions. The second reaction rate of CH3 CHO was about one third of that of HCHO. Here, the reaction time was only 1 min, and selective HCHO measurement could be performed. Like CH3 CHO, larger gas molecules are not expected to be major interferents because of their smaller diffusion coefficient and the consequently lower effective mass transfer rates [33]. This is one of the merits of DS-based collection. Small molecule gases might interfere if they react with MBTH or HCHO rapidly. In liquid phase, it was confirmed that 20 ␮M of nitrite, nitrate, sulfate and ammonium did not give detectable change in 10 ␮M HCHO response. Also, HCOOH, CH3 COOH and acetone solutions (20 ␮M) were injected by themselves, and they showed no detectable interference. Addition of 20 ␮M H2 O2 into 10 ␮M HCHO made the signal slightly smaller (∼10%). However, the HCHO signal decreased seriously with addition of sulfite. This interference is due to the reaction of HCHO with sulfite, and SO2 was expected to interfere in the gas measurement. Actually, when the sample was collected into water, negative interference from gaseous SO2 was observed though the Henry’s law constant of SO2 is more than 1000 times smaller than that of HCHO. However, the effect of SO2 could be eliminated by lowering pH of absorbing solution as mentioned in the experimental. SO2 does not dissolve well into the acidic solution and reacts with HCHO slowly at low pH. In this way, this system

3.5. Measurement of HCHO in indoor air and the ambient atmosphere The instrument was evaluated for indoor air monitoring. As a comparison measurement using the impinger-based AHMT method was carried out concurrently. Sample air was bubbled through 10 ml of water at 1 l min−1 for 30 min, and then 2 ml each of 5 M NaOH and 34 mM AHMT solutions were added immediately. After 20 min of reaction, 1 ml of 65 mM KIO3 was added, and then the absorbance at 550 nm was measured using a conventional spectrophotometer. The experiment was conducted over half a day, as shown in Fig. 8. The circles and horizontal lines show the data obtained automatically by our instrument every 5 min, and by the AHMT batch procedure every 30 min, respectively. The experimental laboratory was relatively large (7.2 m × 8.5 m × 3.3 m), and a small gas stove was turned on at 12:02. With the stove combustion, the HCHO level gradually increased and reached three times the initial value, even though the stove was small and was fueled by natural gas. After turning off the stove and opening the windows, the HCHO level decreased rapidly back to the initial level. This fast transition could be monitored with our instrument, but the bubbler method did not have the time resolution required to recognize this change. The results obtained by the methods were in good agreement, and the time resolution of our instrument was obviously much better than that of the conventional method. Next, the instrument was applied to ambient atmospheric analysis. Though ambient HCHO levels are lower than those inside a building, this instrument is capable of measuring atmospheric levels. Outdoors, HCHO is generated not only directly from automobile exhaust [14], but also as a secondary product of photochemical reactions of volatile organic compounds. In the latter case, atmospheric HCHO is formed by ozonolysis of terminal olefin and photochemical reaction of

Fig. 8. Indoor monitoring of HCHO. HCHO was measured in our laboratory (w 7.2 m × d 8.5 m × h 3.3 m) on April 15, 2003. A small gas stove was turned on at 12:02, and at 16:36 it was turned off and the windows were opened. The  symbols indicate the HCHO values automatically taken every 5 min by our instrument. The horizontal lines were obtained every 30 min by a batch process AHMT method.

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Fig. 9. Measurement of atmospheric HCHO. Circles represent HCHO values, and dashed lines show hourly sunshine time in minutes at Kumamoto city (obtained from Japan Meteorological Agency). The measurements span November 22–28, 2003.

hydrocarbons in the presence of NOx . HCHO plays as an essential originators of the free radicals and is a significant intermediate that triggers chain reactions [49,50]. The measurements were conducted for 1 week each and repeated at intervals of 1 or 2 months. The MBTH solution was prepared every two days. An example of a week-long study is shown in Fig. 9 along with hourly sunshine data. Diurnal cycles can be readily seen. For example, the HCHO level rose after sunrise to several ppbv, and decreased to ca. 1 ppbv in the night. It can also be surmised that the HCHO that we see is a photochemical product; the level was lower when it was cloudy, e.g. on November 27. This instrument can continuously measure atmospheric levels of HCHO for a week, without difficulty. The instrument was also used outside in the fully portable mode, powered by a 12 V storage battery. Atmospheric HCHO and soil gas HCHO were measured in the tidal flats of the Ariake Sea in the west of Japan (33.10◦ N, 130.15◦ E). In this area, HCHO levels were 3–4 ppbv during the daytime. When mud/soil samples were evacuated and the offgas was measured, the HCHO content was found to be 40–60 pmol kg−1 .

4. Conclusion A simple and reliable instrument for measurement of HCHO is proposed. Even though this is based on the absorbance measurements, both high sensitivity and high throughput are achieved by the parallel scrubber stoppedflow. The instrumental configuration proposed here has the potential to be applicable to other absorbance-based trace gas measurement applications. It is clearly a useful device for indoor and ambient monitoring of HCHO.

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