COMMUNICATION
Determination of Odor Thresholds in Air Using C"-Labeled Compounds to Monitor Concentrations Hemes 0. Smith and Arthur D. Hochstettler The Dow Chemical Co., Analytical Development Laboratory, Midland, Mich. 48640
The minimum identifiable odor levels of compounds in air were determined using a static dilution technique. Since adsorption of the compounds on the surfaces of equipment during the diluting procedures can potentially cause unknown and high errors, it is important t o determine the true concentration in the air at the minimum identifiable odor level concentration. The authors d o this by using C14-labeled compounds and scintillation counting. Results for styrene, monochlorobenzene, and monochloroacetic acid are reported. 4
thresholds, sufficient analytical sensitivity is often lacking and again surface adsorption makes it difficult to obtain and deliver a n accurate aliquot of the system being tested for odor. The authors, in studies o n the odor threshold of various compounds in air, are using a static method and monitoring the low concentrations by using C 'I-labeled compounds and scintillation counting. This overcomes the above-mentioned difficulties and allows the determination of a concentration in the threshold range regardless of how low this may be. Naturally, the quantities of vapor inhaled must be safe from the standpoint of toxicity. Procedure and Apparatus
T
he determination of the odor threshold of a compound in air is subject to potentially high errors resulting from adsorption of the compound on the surfaces of apparatus and from diffusion losses. Of the two, the surface adsorption is probably of more significance, for both static and dynamic methods. The results leave doubt as to whether the concentration being inhaled is truly the concentration calculated from dilution ratios and known high concentrations. Of course, the lower the threshold, the more significant these sources of error can become. Thus, a n acceptable value for a threshold concentration can be obtained only if a reliable analytical method is available. However, for compounds with very low
Figure 1. Saturated vapor flask
Preparation of Saturated Vapor. A saturated vapor in nitrogen is prepared in vessels similar to the one shown in Figure 1. The concentration is calculated and determined by gas chromatography or total carbon determination (Van Hall and Stenger, 1967). This saturated vapor is diluted by syringing portions directly into 12.5-liter inhalation flasks. Panel Testing. The judge inhales this diluted vapor, using the nosepiece shown in Figure 2, which minimizes dilution with room air. Assuming the lungs hold 2 liters, the highest possible error would be approximately 15%, which is well Hithin the range of individual sensitivities. Mixing is accomplished by having a 1 x 3 inch strip of aluminum foil (2-mil) in the flask and shaking the capped flask about 10 seconds. The
Figure 2. Inhalation flask with inhaling tube Volume 3, Number 2, February 1969 169
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Table I. Minimum Identifiable Odor Levels (MIOL) MIOL Panel Averane. Purity of Mg./Liter Compound Scintillation Compound Inhaled, % Theory counting L
c.pac,t>
Styrene Monochlorobenzene Monochloroacetic acid
--
12. 5
Figure 3. Inhalation flask with sampling unit attached motion of the foil strip relative to the flask accelerates the mixing process. The inhalation test is made immediately after this mixing. The end point is the recognizable odor threshold, commonly referred to as the minimum identifiable odor level (MIOL). This end point is used, at the present time, because the authors feel it is more meaningful than the absolute odor threshold for air pollution abatement studies. It represents the lowest concentration at which the odor of a compound can still be recognized as resembling the odor of the same compound at a much higher level, such as a saturated atmosphere at normal room temperatures, Differentiation from other similar compounds or odors is not considered. By this definition, then, other end points are possible, since some compounds d o not retain all their primary odor qualities during dilutions, and the absolute threshold may not coincide with the MIOL. A blank inhalation flask always accompanies the sample, for two reasons: to help monitor the flasks for odorless background, and to give the judge a point of reference. At the MIOL, the olfactory signal is so weak that often one has difficulty in mentally interpreting what the senses are detecting. The sequence of sniffing the blank in respect to the sample is randomized. An experimental design allowing statistical analysis is not considered necessary for the MIOL determination, since the answer is definitely above the absolute threshold. The authors feel that when the MIOL is being approached by subsequent dilutions, a factor of one half each
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Environmental Science & Technology
99.6 99.9
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99.5
1.5
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time is normally adequate. A panel of four men and three women is being used; and the final answer is reported as the panel average, panel range, and individual results. Testing is done in an atmosphere of charcoal-filtered air at 73°F. and 40 to 50% relative humidity. Monitoring MIOL Concentration. To correct for the probable adsorption and diffusion errors mentioned previously, compounds labeled with carbon-14 are diluted and then sampled using the 2-liter evacuated flask shown in Figure 3, and monitored by measurements of their radioactivity. A concentration close to the panel MIOL average is monitored unless the panel range is broad, in which case the extreme values are monitored. The 2-liter aliquot in the sampling flask is transferred quantitatively to an appropriate counting solvent first by sweeping and then by rinsing the flask with the counting solvent. Counting is done on an automatic liquid scintillation spectrophotometer (Nuclear-Chicago Corp.). Results and Discussion
Table I shows results for three compounds. Compounds with sufficiently low activity, safetywise, can be used for the olfactory testing. However, the authors prefer to perform the olfactory testing on an unlabeled compound and run a parallel experiment with a labeled compound for concentration monitoring. The results also indicate the magnitude of errors due to adsorption and diffusion. Acknowledgment
The authors express their appreciation to L. E. Swim, L. E. Bartlett, and V. A. Stenger for their advice and assistance, and to M. E. Mandrel1 and J. Jewel1 for their assistance in obtaining and using labeled compounds and for the scintillation counting. Literufure Cited
Van Hall, C. E., Stenger, V. A., Anal. Clzem. 39, 503-7 (1967). Receiced for reciew March 11, 1968. Accepted Nocember 22, 1968.
