Page:NIOSH Manual of Analytical Methods - 6700.pdf/3
NITROGEN DIOXIDE: METHOD 6700, Issue 2, dated 15 January 1998 - Page 3 of 5
7. 8.
NOTE: If 2.1 mL is not sufficient to completely cover the exit slit of the spectrophotometer, a larger volume can be used provided the same volume is used for both standards and unknowns. Recap the samplers and mix manually or with a mixer. Allow 10 min for the color to develop. Transfer the solution to a cuvette and read the absorbance at 540 nm within 30 min from time reagent was added. NOTE: If sample reads beyond calibration graph, dilute sample with combined reagent or extend calibration range.
CALCULATIONS: 9.
From calibration graph, read nanomoles nitrite ion (NO 2 ) collected by the sampler. Divide by 2.3 nanomoles/ppm-h (the diffusive collection rate [1]) and the sample exposure time, t (h), to obtain time-weighted average concentration, C (ppm NO 2), of NO2:
—
C
NOTE 1: NOTE 2:
nanomoles NO2 2.3 t
If sampler dimensions are different from those specified in the APPENDIX, use 2.3 (actual At/L [cm]÷ 0.1 cm) nanomoles/ppm-h as the diffusive collection rate. The assumption is made that NO2 is completely converted to NO2 , because of the small quantity collected [1].
EVALUATION OF METHOD: This method is based on a method developed by E. D. Palmeset al at New York University [1]. Analytical precision and useful range were estimated from a laboratory evaluation conducted by NIOSH (1982) [2]. Overall precision ( rT = 0.06) was estimated from side-by-side replicate samples collected in an underground salt mine [3]. In a laboratory study, this method gave results averaging 94 ± 4% (mean ± sr) of a reference method over the range 1.3 to 79 ppm-h [2]. A field study found results for this method of 109 ± 9% (mean ± sr) vs. a reference method in the range 12 to 19 ppm-h [3]. Sampling errors may exist in this method when the concentration is not constant in time and the sampling period is short [6, 7]. For example, the value of s r associated with estimating the TWA of an isolated random 10-sec concentration pulse within a 15-min sampling period may be calculated [6] to equal 0.5. Secondly, reference [6] reports a specific set of real-time concentration data measured in an industrial environment. For these data, the error sr in making 15-min TWA estimates is calculated to equal 0.12. Although these values are large, similar sampling errors due to time variations are expected to be better controlled for longer sampling periods as the variance of the sampling error varies inversely with the sampling period.
REFERENCES: [1] [2] [3] [4] [5]
Palmes ED, Gunnison AF, DeMattio J,Tomczyk C [1976]. Personal sampler for nitrogen dioxide. Am Ind Hyg Assoc J37:570-577. Woebkenberg ML [1982]. A comparison of three personal passive sampling methods for NO 2. Am Ind Hyg Assoc J 43:553-561. Jones W, Palmes ED, Tomczyk C, Millson M [1979]. Field comparison of two methods for the determination of NO2 concentration in air. Am Ind Hyg Assoc J40:437-438. Lindenboom R, Palmes ED [1983]. Effect of reduced atmospheric pressure on a diffusional sampler. Am Ind Hyg Assoc J44:105-108. NIOSH [1994]. Nitric oxide and nitrogen dioxide: Method 6014. In: Eller PM, Cassinelli ME, Eds. NIOSH Manual of Analytical Methods (NMAM), 4th ed. Cincinnati, OH: National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 94-113.
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition