An automatic monitor of formaldehyde in air by a monitoring tape method Nobuo Nakano*a and Kunio Nagashimab aRiken Keiki Co., Ltd., 2-7-6, Azusawa, Itabashi-ku, Tokyo 174-8477, Japan bFaculty of Engineering, Kogakuin University, 2665-1, Nakanocho Hachioji-shi, Tokyo 192-0016, Japan Received 14th January 1999, Accepted 30th March 1999 An automatic monitor has been developed for measuring formaldehyde in air using a sensitive tape for formaldehyde.It is based on the color change of the tape on reaction with formaldehyde. The porous cellulose tape, containing silica gel as an absorbent and impregnated with the processing solution containing hydroxylamine sulfate, Methyl Yellow (pH indicator; pH 2.9–4.0, red–yellow), glycerin and methanol, was found to be a highly sensitive means of detecting formaldehyde and maintains a stable sensitivity.When the tape was exposed to a sample of air containing formaldehyde, the color of the tape changed from yellow to red. The degree of color change was proportional to the concentration of formaldehyde at a constant sampling time and flow rate, and it could be recorded by measuring the intensity of reflected light (555 nm).The tape could be used to detect down to 0.08 ppm (World Health Organization standard) of formaldehyde with a sampling time of 30 min and a flow rate of 100 mL min-1. Reproducibility tests showed that the relative standard deviation of response (n=10) was 3.8% for 0.1 ppm formaldehyde. The monitor is simple, specific, capable of unattended operation and is recommended for both laboratory and field operation.The standard HCHO gas mixture was generated continu- Introduction ously by purging the diVusion tube containing paraformal- Formaldehyde (HCHO) is a carcinogenic chemical that is dehyde (Tokyo Kasei Kogyo, Tokyo, Japan) with a constant emitted from furniture and is also used in hospitals as a flow of purified air. The gas concentration was calculated from preservative. The World Health Organization (WHO) has set the flow rate and the mass loss of the paraformaldehyde.The a standard of 0.08 ppm averaged over 30 min, and the diVusion tube in the gas generating system (Gastec, Ayase, American Conference of Government Industrial Hygienists Japan; PD-1B) was kept at 30±0.1 °C in a thermostatically (ACGIH) has set a ceiling exposure value of 0.3 ppm.1 HCHO controlled water-bath.aVects residents of newly built houses or oYces, and is Humidified standard HCHO mixtures12 were prepared by considered as a major cause of sick building syndrome. The passing dry air through a Gore-Tex (porous Teflon; 4 mm id, establishment by WHO of the standard of 0.08 ppm HCHO 6 mm od and 50 mm in length) tube immersed in water in air2 has emphasized the need for a sensitive, reliable and (25±2 °C), and then purging the air into the diVusion tube specific method for the determination of HCHO in environ- holder.The relative humidity of the standard gas mixture was mental air. Various methods for the detection of HCHO have determined by a humidity sensor (Visala, Helsinki, Finland; been reported,3–7 but to achieve widespread routine use, the HMI32).method should be simple, specific, capable of unattended operation and also inexpensive. However, few methods exhibit Monitoring tape all of these desirable properties. We chose a tape monitor The porous cellulose tape containing silica gel (Whatman, because of its high sensitivity, selectivity, easy maintenance Maidstone, Kent, UK; SG-81 papers, 20 mm wide, 0.27 mm and easy operation in the automatic mode.8–10 A tape containthick and 25 m in length) was immersed in the processing ing silica gel as an absorbent and impregnated with a prosolution for 1 min, oven dried for 1–10 min at 40 °C and cessing solution containing hydroxylamine sulfate, Methyl stored in a desiccator.Yellow (pH indicator), glycerin and methanol was studied11 in order to determine HCHO concentrations of less than Apparatus 0.1 ppm in air with a shorter sampling time than 30 min.This paper describes the development of an automatic monitor for Fig. 1 shows a block diagram of the monitor (Riken Keiki, the determination of HCHO in air. Tokyo, Japan; FP-250FL, 106 mm wide, 78 mm high and 141 mm deep) and the detector is shown schematically in Experimental Reagents and samples All the chemicals used were of reagent grade quality and were employed without further purification.A processing solution was prepared as follows. In 6 mL of water, 1.0 g hydroxylamine sulfate and 0.02 g Methyl Yellow were dissolved; then 15 mL Fig. 1 Block diagram of monitor for formaldehyde. of glycerin and 79 mL of methanol were added to the solution.J. Environ. Monit., 1999, 1, 255–258 255Fig. 2. The sample gas was passed through a filter (pore size, Reflectance spectra 1 mm) to remove dust and sucked through the sampling After the tape had been exposed to 0.1 ppm HCHO for a chamber at a constant flow rate (100 mL min-1) and a consampling time of 30 min at a flow rate of 100 mL min-1, the stant sampling time (30 min).When the tape was exposed to visible reflectance spectrum of the exposed tapes was recorded HCHO, the Methyl Yellow on the tape reacted with sulfuric with a UV-2200 spectrophotometer (Shimadzu, Kyoto, Japan), acid produced by the reaction of hydroxylamine sulfate with using freshly prepared barium sulfate disk as a reference HCHO to produce a change in color.The reaction of this standard (Fig. 4). The full and broken lines show the reflec- method is represented by tance spectra of the tape exposed to 0.1 ppm of HCHO in air 2HCHO+(NH2OH)2·H2SO4�2H2C=NOH and air (without HCHO), respectively. +H2SO4+2H2O (1) Gas flow rate The color of the tape changed from yellow to red with H2SO4 liberated by this reaction. The degree of color change Table 1 shows the eVect of the sample gas flow rate on the was recorded by measuring the reflected light at 555 nm.Then response for zero gas (40% RH N2) and HCHO (40% RH). the pathway of the tape was renewed by moving the tape The response for N2 (background) increased with increasing every 30 min. The response is defined by A=-logV1/V0, sample gas flow rate, while the response for HCHO (net) where V0 and V1 are the outputs of the blank (atmospheric decreased with increasing sample gas flow rate above air) and of the sample, respectively. A period of 30 min was 300 mL min-1.In this experiment, the sample gas flow rate required to measure the responses after the tape was set in the was fixed at 100 mL min-1. The relative standard deviation tape monitor.All the measurements were carried out at was 3.8% for 0.1 ppm HCHO with a sampling time of 30 min 25±2 °C. and a flow rate of 100 mL min-1. Sampling time Results and discussion The response for a fixed concentration of HCHO was plotted We initially studied a cellulose tape without silica gel against various sampling times. Non-linear graphs between (Whatman; 1Chr) using hydroxylamine sulfate and Methyl response and sampling time were obtained in the range Yellow (pH indicator; pH 2.9–4.0, red–yellow) for the determi- 0.2–1.0 ppm and 10–30 min (Fig. 5).nation of HCHO. However, it was unsuccessful in detecting 0.1 ppm of HCHO within a sampling time of 30 min. In this experiment, we examined a porous cellulose tape containing silica gel (Whatman; SG-81).Fig. 3 shows a scanning electron microprobe photograph of the tape surface. White granules of SiO2 are scattered on the surface of the tape. As shown in Fig. 3, the granule size is between 2 and 10 mm. We concluded that the tape containing silica gel impregnated with hydroxylamine sulfate and Methyl Yellow could serve as a sensitive monitoring tape for HCHO.Fig. 2 Schematic diagram of gas detector. Fig. 4 Reflectance spectra of the tape exposed to HCHO (full line) and air (broken line) (reference, BaSO4 disk). Table 1 EVect of sample gas flow rate on response. Concentration of HCHO, 0.2 ppm; sampling time, 30 min Response Gas flow rate/ mL min-1 N2 HCHO/N2 Net 50 0.006 0.056 0.050 100 0.007 0.056 0.049 150 0.008 0.058 0.050 200 0.008 0.059 0.051 250 0.010 0.057 0.047 300 0.011 0.058 0.047 400.014 0.057 0.043 Fig. 3 Scanning electron microprobe photograph of surface 500 0.017 0.058 0.041 appearance. 256 J. Environ. Monit., 1999, 1, 255–258Table 3 EVect of temperature of sample gas on output of monitor. Concentration of HCHO, 0.2 ppm; sampling time, 30 min Temperature/°C Reading (ppm) 5 0.23 10 0.22 15 0.20 25 0.20 35 0.22 Table 4 Output of monitor for various gases Concentration of Reading Gas examined gas (v/v) (ppm) Ethanol 1% 0 Methanol 1% 0 Trichloroethylene 1% 0 Toluene 1% 0 Xylene 1% 0 Benzene 1% 0 Fig. 5 Relationship between sampling time and response at various Dichlorobenzene 1000 ppm 0 concentrations of HCHO: (a) 1.0 ppm; (b) 0.8 ppm; (c) 0.6 ppm; (d) Ethylbenzene 1% 0 0.4 ppm; (e) 0.2 ppm.Gas flow rate, 100 mL min-1. Hexane 1% 0 Carbon monoxide 100 ppm 0 Calibration graph Nitrogen monoxide 50 ppm 0 Nitrogen dioxide 105 ppm 0 Typical calibration graphs for HCHO using the optimum Sulfur dioxide 15.2 ppm 0 experimental conditions are shown in Fig. 6. By using optimum Carbon dioxide 4.9% 0 Hydrogen 100% 0 conditions, the tape could be used to detect 0.08 ppm of Acetic acid 24 ppm 0 HCHO with a sampling time of 30 min (S/N=3). Hydrogen sulfide 32 ppm 0 Reproducibility tests (n=10) showed that the relative standard Hydrogen fluoride 6.0 ppm 0 deviation of the response was 3.8% for 0.1 ppm HCHO.Hydrogen chloride 0.1 ppm 0.16 Acetone 100 ppm 0.13 Humidity of sample gas Acetaldehyde 13 ppm 0.52 Formaldehyde 0.2 ppm 0.2 Table 2 shows the eVect of the humidity of the sample gas on the response of the tape.The tape was hardly aVected by humidity in the region 0–70% RH used in this experiment at 25 °C. It can be concluded that there is no eVect of humidity Fig. 7 Continuous measurement of the sample gas using the monitor. Sampling time, 30 min. Table 5 Concentration of HCHO measured by the HCHO monitor and acetylacetone method HCHO monitor Acetylacetone method Furniture (ppm) (ppm) Fig. 6 Calibration graphs for HCHO at various sampling times: (a) A 0.04 0.06 30 min; (b) 20 min; (c) 10 min. Gas flow rate, 100mL min-1. B 0.05 0.07 C 0.15 0.11 Table 2 EVect of humidity of sample gas on output of monitor. D 1.00 (over) 1.23 Concentration of HCHO, 0.2 ppm; sampling time, 30 min Humidity (%) Reading (ppm) on the response of the tape under normal conditions (30–60% RH). 0 0.22 20 0.19 Temperature of sample gas 40 0.20 60 0.20 Table 3 shows the eVect of temperature variation of the sample 70 0.18 gas within the range 5–35 °C on the response of the tape. The J. Environ. Monit., 1999, 1, 255–258 257tape is hardly aVected by temperature in the region 5–35 °C was very suitable for the determination of HCHO in air.This monitoring tape method is simple, specific, capable of used in this experiment. It can be concluded that there is no eVect of temperature on the response of the tape under normal unattended operation and is recommended for field operation. We hope that this monitor will be used to establish better the conditions (5–35 °C). HCHO concentration in air.Selectivity The response of the tape for various gases is given in Table 4. References The tape is hardly aVected by gases in the concentration ranges used in this experiment, except for hydrogen chloride, acetone 1 ACGIH, Threshold Limit Values for Chemical Substances and Biological Exposure Indices, American Conference of Government and acetaldehyde. Industrial Hygienists, Cincinnati, OH, 1998. 2 M.Maroni, B. Seifert and T. Lindvall, Indoor Air Quality: A Continuous measurement Comprehensive Reference Book, Elsevier Press, Amsterdam, 1995, Fig. 7 shows the continuous measurement of HCHO using the p. 804. 3 F. Sawicki and T. R. Hauser, Anal. Chem., 1960, 32, 1434. monitor. The monitor showed errors of less than ±10% 4 A. C. Rayner and A. C. Jephcott, Anal.Chem., 1961, 33, 627. continuously and no noticeable lowering of the performance 5 K. Fung and D. Grosjean, Anal. Chem., 1981, 53, 168. in the test period. 6 R. H. Still, K. Wilson, and B. W. J. Lynch, Analyst, 1968, 93, 805. 7 M. E. J. Baker and R. Narayanaswamy, Analyst, 1994, 119, 959. Determination of formaldehyde emitted from furniture 8 N. Nakano, A. Yamamoto and K. Nagashima, Analyst, 1996, 12, 1939. The concentration of HCHO emitted from furniture was 9 N. Nakano, A. Yamamoto, Y. Kobayashi and K. Nagashima, measured by the monitor and by the acetylacetone method13 Talanta, 1995, 42, 641. (absorptiometric method). The results are shown in Table 5. 10 N. Nakano, M. Ishikawa, Y. Kobayashi, and K. Nagashima, The concentration of HCHO emitted from furniture obtained Anal. Sci., 1994, 10, 641. 11 N. Nakano, A. Yamamoto, T. Kawabe and K. Nagashima, by the monitor was in good agreement with that of the Nippon Kagaku Kaishi, 1998, 1998, 506. acetylacetone method. 12 T. Otagawa, S. Zaromb, and J. R. Stetter, J. Electrochem. Soc., 1985, 132, 2951. Conclusions 13 T. Nash, Biochem. J. (London), 1953, 55, 416. The automatic monitor using the sensitive tape for HCHO impregnated with hydroxylamine sulfate and Methyl Yellow Paper 9/00410F 258 J. Environ. Monit., 1999, 1, 255–258