Industrial Hygiene Sampling and Analytical ... - ACS Publications

1 Industrial Hygiene Sampling and Analytical Methods for Formaldehyde Past and Present

Downloaded by 190.109.130.119 on January 31, 2016 | http://pubs.acs.org Publication Date: August 21, 1985 | doi: 10.1021/ba-1985-0210.ch001

E U G E N E R. K E N N E D Y , A L E X A N D E R W. TEASS, and Y V O N N E T. G A G N O N National Institute for Occupational Safety and Health, Cincinnati, O H 45226

National Institute for Occupational Safety and Health (NIOSH) method Ρ&CAM 125 (chromotropic acid), one of the first industrial hygiene sampling and analytical methods for formaldehyde, was adapted from Intersociety Method 116 and is still widely used to day, although it is susceptible to many interferences. Several other adaptations of this method have been used in the development of passive monitors. Other methods for formaldehyde, including those using pararosaniline and 2,4-dinitrophenylhydrazine, have been developed and adapted to industrial hygiene sampling and do offer advantages over the chromotropic acid method. During field and laboratory evaluations of NIOSH method Ρ&CAM 318 (oxidative charcoal), this method was found to have sample instability prob lems. Samples collected with NIOSH method Ρ&CAM 354 [2-(benzylamino)ethanol] were found to be stable, but sensitivity was lim ited. Recent work on this method has allowed a sampling rate of 0.08 L/min for 8 h. On the basis of this work, measurement of for maldehyde at low levels (0.1 μg/L) should be possible with the 2-(benzylamino)ethanol method.

- F O R M A L D E H Y D E IS O N E O F T H E M O R E W I D E L Y P R O D U C E D and used chemical intermediates in the U.S. chemical industry. Production in 1983 was ap proximately 5.45 billion lb and has averaged around 5 billion lb over the past 3 years (i). Because of this large-volume production and usage of formaldehyde and its possible exposure-related health effects (2), there has been much concern over the sensitivity, accuracy, and applicability of sam pling and analytical methodology for this compound. In this chapter a brief summary of some of the more widely used industrial hygiene sam pling and analytical methods for formaldehyde is presented. 0065-2393/85/0210/0003$06.00/0 © 1985 American Chemical Society

In Formaldehyde; Turoski, Victor; Advances in Chemistry; American Chemical Society: Washington, DC, 1985.

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FORMALDEHYDE: ANALYTICAL CHEMISTRY A N D TOXICOLOGY


Chromotropic Acid Methods One of the first industrial hygiene sampling and analytical methods for formaldehyde was adapted directly from air pollution monitoring. The impinger-chromotropic acid method [National Institute for Occupational Safety and Health (NIOSH) method P & C A M 125 (3)] for formaldehyde was taken from Intersociety method 116 (4). The method in its original form recommended the use of distilled water in an impinger for collection of formaldehyde. To an aliquot of this sample, chromotropic acid and concentrated sulfuric acid were added sequentially. The color of the reaction product of the chromotropic acid and the formaldehyde was purple. The absorbance of this purple chromophore was measured and related to the concentration of formaldehyde by the relationship of Beer's law. Further refinements of the method, such as use of a 1 % sodium bisulfite solution as a collecting medium to increase collection efficiency from 80 % to > 95 % and heating during the color development to ensure complete reaction, have been incorporated in the latest version of the method as published by NIOSH (5). With the original version of the method, a problem with longterm sample stability was encountered (6). Further work in our laboratory (7) indicated that when samples were stored in Nalgene cross-linked polyethylene (CPE) bottles with screw caps instead of glass scintillation vials, samples were stable for at least 2 weeks. The type of vial used for storage of samples was critical. If the vial cap did not seal well, then leakage of formaldehyde from the headspace was possible. With metal-lined caps, reaction of the bisulfite with metal cap liners and invalidation of the sample due to precipitate formation was possible (8). The measurement limit of the method was 1.5 /ig/sample. A sample volume of 60 L collected at 1 or 0.25 L/min, in accordance with the sampling time desired, was recommended to keep samples in the linear range of the calibration curve without the need for dilution when measuring air concentrations around the Occupational Safety and Health Administration (OSHA) permissible exposure limit [3.7 mg/m (9)]. This sample volume permitted a limit of quantitation of 0.025 mg/m . Although susceptible to many interferences, such as phenol, ethanol, higher molecular weight alcohols, and olefins, the method is still widely used today. These interferences create a negative bias in the method because they inhibit the color formation reaction. A different sampling technique was combined with the chemistry of the chromotropic acid procedure in NIOSH method P & C A M 235 (10), which used an alumina sorbent tube instead of an impinger for collection of formaldehyde. The formaldehyde was desorbed from the tube with 1 % aqueous methanol solution and determined by the chromotropic acid procedure described earlier. The reported measurement limit for this method was 1.0 jig/sample. The capacity of the sampler was limited to 6 L , collected at 0.2 L/min, to prevent sample breakthrough. On the basis of this volume, the limit of quantitation was 0.17 mg/m . The samples collected 3

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Industrial Hygiene Sampling

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on alumina required immediate desorption at the field site to prevent loss of formaldehyde. Several adaptations of the chromotropic acid procedure have been used in the development of diffusive monitors. The major advantage of these devices over more conventional sampling and analysis methods was that they did not require sampling pumps and impingers or sorbent tubes. The formaldehyde diffused into the monitor and was collected by adsorption onto a reactive sorbent or by absorption into a liquid medium separated from the air by a gas-permeable membrane. The sample was either desorbed with distilled water, or an aliquot of the absorbing liquid was taken and the formaldehyde was determined by modifications of the chromotropic acid procedure. Various modifications in the sample workup and measurement procedure, such as use of a more concentrated chromotropic acid solution and changes in aliquot volume, were incorporated to reduce the effect of interferences in these sampling devices (11-12). Also, because the diffusion coefficient for formaldehyde was larger than those of potential interferences, the ratios of sampling rates of formaldehyde to interferences were > 1, and the sampling devices exhibited a sampling selectivity for formaldehyde. The reported measurement limits for these types of devices were 0.25-0.8 /ig/sample. The sampling rates for these devices were 0.0017-0.0569 L/min to give limits of quantitation of 0.3-0.03 mg/m for 8-h sampling periods. Tests in our laboratory showed that the collection efficiency of one of these commercial badges was sensitive to relative humidity (13). Our findings indicated that the collection efficiency tended to decrease with decreasing relative humidity. 3

Pararosaniline Methods The procedure for formaldehyde using pararosaniline was an adaptation of a method for sulfur dioxide (14). In this adapted method (15) formaldehyde is collected in sodium sulfite solution in an impinger and then reacted with sodium tetrachloromercurate and pararosaniline to yield a purple chromophore that is determined spectrophotometrically at 560 nm. Acetaldehyde and propionaldehyde were positive interferences at the 5-ppm levels, but not at low levels. A major drawback of this method was the use of the toxic tetrachloromercurate salt. In a subsequent modification of this method, the use of the salt was not required, but the modified method was sensitive to temperature and subject to interferences from hydrogen cyanide, sulfite ion, hydroxylamine, and sulfur dioxide (16). The reported measurement limits for the original method (15) and the modification (16) were 2.0 and 1.8 jug/sample, respectively. With sample volumes of 28 and 60 L , collected at 1 L/min, the limits of quantitation for the methods were 0.07 and 0.03 mg/m , respectively. This analytical procedure was used to analyze samples collected on molecular sieve sorbent tubes (17). The major drawback to this collection procedure was that the capacity of the sorbent 3


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varied inversely with the amount of water collected by the tube. In highhumidity environments, sampling time before breakthrough of formaldehyde was quite limited, with a maximum volume of 30 L collected over a 15-min period at 2 L/min recommended. This sample volume allowed a limit of quantitation of 0.03 mg/m . The pararosaniline method also was adapted for a commercial continuous air-monitoring device (18). Subsequently, this monitor was modified to improve sensitivity (19) and remove the need for the tetrachloromercurate salt (20). 3


2,4-Dinitrophenylhydrazine

Methods

With the 2,4-dinitrophenylhydrazine (2,4-DNPH) procedure, formaldehyde reacted with 2,4-DNPH in either aqueous (21) or acetonitrile solution (22) to form the 2,4-dinitrophenylhydrazone, which was then determined by high-pressure liquid chromatography with UV detection. Other analytical techniques, such as spectrophotometry at 440 nm (23) and gas chromatography (24) have been used, but the liquid chromatographic analysis provided the sensitivity and selectivity required for formaldehyde. This method also was used for the simultaneous determination of certain other aldehydes and ketones. We experienced problems with low capture efficiency with an aqueous solution of 2,4-DNPH for low formaldehyde concentrations. This problem of low capture efficiency was overcome with the use of an acetonitrile solution of 2,4-DNPH (22); however, with the use of the acetonitrile solution, the inherent problems of solvent evaporation during sample collection and safe transport of samples back to the laboratory for analysis were encountered. The reported measurement limits for these methods with the aqueous and acetonitrile 2,4-DNPH solutions were 0.05 and 0.25 jug/sample, respectively. With a sample volume of 30 L collected at 0.5 and 1.5 L/min, the limits of quantitation were 0.002 and 0.008 mg/m , respectively. 3

The 2,4-DNPH sampling approach was also used in several sorbent tube methods for formaldehyde in which the 2,4-DNPH was coated on X A D - 2 or silica gel (25-26). However, the stability of the 2,4-dinitrophenylhydrazone derivative on the 2,4-DNPH coated sorbents was questionable. In our laboratory, sample loss from the silica gel tubes has been observed after 7 days. Also, the tubes had a shelf life limited to 30 days. Experiments in our laboratory with the 2,4-DNPH coated silica gel tube for analysis of ketones indicated that the sampling tube did not trap methyl ethyl ketone. Although other ketones were not investigated, we concluded that other ketones would behave similarly. Reported measurement limits for formaldehyde with the X A D - 2 and silica gel sorbents were 0.64 and 2.5 jug/sample, respectively. With sample volumes of 5 and 20 L collected at 0.2 L/min, the limit of quantitation was 0.13 mg/m for each method. 3


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3-Methyl-2-benzothiazolone

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Hydrazone Method

Another method for the measurement of formaldehyde adapted from air pollution monitoring was the 3-methyl-2-benzothiazolone hydrazone (MBTH) method (27). An air sample was collected in an impinger containing 0.05 % aqueous M B T H solution. During analytical workup of the solution, addition of ferric chloride and acid caused the formaldehyde-MBTH reaction product to form a blue cationic dye. Formaldehyde concentration was then determined by the absorbance of this blue dye at either 635 or 670 nm by spectrophotometry. This blue color was not stable for periods longer than 4 h. Because other aldehydes reacted in a similar manner and, therefore, presented a positive interference in the method, this method was used for the determination of formaldehyde only when formaldehyde was the sole aldehyde present in the environment to be sampled. The method was used routinely for the measurement of total aldehydes. The reported measurement limit for formaldehyde was 5 /xg/sample. With a sample volume of up to 250 L collected at 0.5 L/min, the limit of quantitation was 0.02 mg/m . A modification of this method in which sulfamic acid was added in the oxidation step to reduce sample turbidity has reduced the measurement limit to 2.0 ptg/sample (28). With the same sampling conditions just described, the limit of quantitation for this modification was 0.008 mg/m . 3

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Girard T Method Under the joint NIOSH-OSHA Standards Completion Program a method for formaldehyde was developed and included in the NIOSH Manual of Analytical Methods as method S327 (29). Formaldehyde was collected with a bubbler filled with Girard T reagent (trimethylammoniohydrazide chloride) and determined by polarography of the resulting hydrazone. The major advantage of the method was that the sample required no preliminary workup and could be transferred directly to the polarographic cell for analysis. Because they form hydrazones, other volatile aldehydes, such as acrolein, crotonaldehyde, and benzaldehyde, were likely interferences in the analysis. The reported measurement limit for this method was 6 figlsample. With a sampling rate of 0.1-0.2 L/min and a recommended sample volume of 18 L , the limit of quantitation was 0.3 mg/m . 3

Hydrazine Method A method for formaldehyde developed recently by OSHA specifies sampling air with a bubbler containing aqueous methanol (30). The collected formaldehyde is reacted with hydrazine and determined by polarography. Because of the differences in half-wave potentials for the formaldehyde derivative and other low molecular weight aldehydes (e.g., acetaldehyde, propionaldehyde, and acrolein), the method is selective for formaldehyde.


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The method has sufficient sensitivity for ceiling (15 min) measurements. The reported measurement limit is 1.8 pig/sample. When the maximum recommended sample volume of 160 L is collected at 0.7-1.15 L/min, the limit of quantitation is 0.01 mg/m . The major drawback of the method is that the collection media pose the problem of transportation of an organic solution back to the laboratory for analysis. 3


Oxidative Charcoal Tube Method The NIOSH oxidative charcoal tube method [NIOSH P&CAM 318 (31)] used an oxidant-coated sorbent to react with the collected formaldehyde and partially oxidize it. The formaldehyde was recovered from the sorbent as formate by using aqueous hydrogen peroxide and determined by ion chromatography. Early commercial sample tubes had blank levels that were high and caused the analytical measurement limit to be high also. For later commercial lots of the tubes, the blank was greatly reduced and the reported measurement limit was 3 /xg/sample. With a sampling rate of 0.2 L/min and a recommended sample size of 100 L , the limit of quantitation was 0.03 mg/m . Recent NIOSH field and laboratory evaluations of this method with commercial tubes revealed that the samples were not stable for more than 5 days (32). In the original work reported by NIOSH, samples were good for up to 30 days (33). Scientists believe that the original batch of sorbent with which the method was developed was never duplicated for the preparation of the commercial tubes. This method is no longer used by NIOSH. 3

2-(Benzylamino)ethanol-Coated Sorbent Tube Method With the development of the 2-(benzylamino)ethanol (BAE)-coated sorbent tube method [NIOSH P & C A M 354 (34)], a different derivatization approach was taken for formaldehyde sampling. Formaldehyde was known to react with secondary aminoethanols to form cyclic derivatives called oxazolidines. This reaction was fast enough to be developed into a sampling method for formaldehyde. The major disadvantage of the method is the lack of sensitivity of the method, a result of the low sampling rate and volume and high blank values found in the unexposed tubes. The method is selective for formaldehyde because of the resolution of the capillary gas chromatographic analysis of the sample. Samples were stable for at least 4 weeks when stored at 25 ± 5 ° C . Suspected interferences in the method are acid gases or mists, which will react with the BAE to convert it to the ammonium salts and thus prevent its reaction with formaldehyde. With tubes prepared in our laboratory, the measurement limit was approximately 5 jitg/sample. With a sampling rate of 0.05 L/min and a recommended sample volume of 12 L , the limit of quantitation was 0.42 mg/m . Work by commercial vendors on the sampling tube has reduced the blank 3


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level to below 2 jug/sample with good precision. This reduction, in effect, has lowered the measurement limit to 2 jug/sample. To attempt to improve the sensitivity of the method, capacity and sampling studies were conducted with the commercially available tubes. When an atmosphere of 9.4 mg/m of formaldehyde was sampled at 0.08 L/min for up to 15 h, no breakthrough was observed. On the basis of this finding, the sampling volume can be increased from 12 to 38 L (0.08 L/min X 480 min) with the commercial tubes. To study low sample loadings, an atmosphere of 0.36 mg/m of formaldehyde, as determined by the chromotropic acid method (3), was generated. This atmosphere was sampled with three sets of six BAE-coated sorbent tubes for time intervals of 70, 286, and 482 min at flow rates around 0.08 L/min. The concentrations determined by these sets of samples with their 95% confidence limits were 0.33 ± 0.03, 0.31 ± 0.008, and 0.33 ± 0.012 mg/m , respectively. Levels of formaldehyde collected by the tubes were 1.6-10.8 /ig/sample. The results of the 70-min set suggest that the limit of quantitation for a 38-L air sample is 0.05 mg/m . 3


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Summary In Table I several of the more commonly used sampling and analytical methods for formaldehyde are summarized. As can be seen in this table, the sampling devices most frequently used are the bubbler and impinger. These devices are selected because many of the industrial hygiene sampling and analysis methods have been adapted from methods of air pollution monitoring that were in use before sorbent tube sampling was common. Also, the impinger, instead of the bubbler, is specified in several of these methods. In the literature no rationale was ever given for this preference. The use of less than optimal flow rates and mismatched impinger stembody assemblies may affect collection efficiency of the sampler adversely. For the collection of area samples, the impinger is adequate when the cautions stated earlier are observed, but when personal samples are required, this collection technique is much less appealing because of its cumbersome nature and potential for spillage. This device can also present major shipping problems when samples must be transported to the laboratory for analysis. If long-term samples are taken, then the volume of the absorbing solution should be measured both before and after sampling to correct for solution evaporation. If the volume loss is significant (e.g., > 10%), then sampling efficiency may be reduced, and pollution of the environment with the solvent being used may occur. The method of choice for personal sampling is a solid sorbent-based device or a sealed liquid-containing device. This approach reduces the bulk of the sampling device and usually solves sample-handling and shipping problems. Close study of Table I reveals that the most sensitive methods use an


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F O R M A L D E H Y D E : A N A L Y T I C A L CHEMISTRY A N D TOXICOLOGY

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