Nylon filters interferents

An informal intercomparison of these two CIMS methods with a filter pack method shows generally excellent agreement between the mass spectrometric approaches and often, but not uniformly, good agreement with the filter pack method (Fehsenfeld et al., 1998). The latter was often high, which was attributed to interference from decomposition of ammonium nitrate to HNO-, + NH3 on the Teflon particle prefilter, followed by absorption of the HNO-, by the nylon filter. 

Potential interferents collected by nylon filters include organic nitrates (e.g., PAN), N02, and HONO. Each can yield N02, which can be oxidized to N03" by 03 or other oxidants. Although not related directly to nitrate sampling, S02 retention in varying degrees was also of concern. 

Other Interferent Studies with Nylon Filters. Retention of HONO on Gelman Nylasorb (nylon 6) filters was tested 0.3 to 1 xg/m3 HONO was sampled in air (—50% RH) at 20 L/min with 47-mm filters for 16-23 h. At saturation, the filters contained 1.8 xg/cm2 N02 this result indicates less than 50% efficiency for HONO retention (19). Sampling 440 xg/m3 HONO at 20 L/min with 47-mm Gelman Nylasorb filters (nylon type unstated), Sickles and Hodson reported about 25% retention efficiency, with a similar saturation value, and noted that such retention was reversible. Substantial oxidation of HONO to N03 on the filter surface by 03 was demonstrated in laboratory and atmospheric trials (21). Perrino et al. (20) observed with 47-mm Gelman Nylasorb (nylon type unstated) filters an efficiency for HONO that increased from about 25 to about 90% as the flow rate decreased from 12 to 2 L/min, with saturation at —50 xg/filter. Nitrite to nitrate conversion on the nylon filter surface increased from 13% at 45 ppb 03 to 93% at 200 ppb 03. 

PAN is a potential interferent with daytime concentration maxima that can coincide with the HN03 maxima. However, early work by Joseph and Spicer (23) and later work by Fahey et al. (24) indicated that PAN is not a significant interferent in HN03 measurements relying on nylon-filter collection. The change in nylon-filter composition introduces a small degree of uncertainty to this conclusion. 

Because of the labor-intensive nature of annular denuder sampling, alternative techniques deserve further scrutiny. The DDM, relying on Teflon-nylon filter packs (or nylon filters alone) and an anodized aluminum annular denuder ahead of one of a pair of modified dichotomous samplers (46), merits evaluation for the influence of potential interferents (e.g., HONO). The DDM with NaCl-coated denuders would probably be nearly free of interference from HONO, but this conclusion requires evaluation. Because coarse particulate N03 must be excluded to achieve improved accuracy for HN03 and fine particulate N03, supplementary samplers are needed to obtain improved estimates of total as well as coarse particulate... 

Eluent entering the pump should not contain any dust or other particulate matter. Particulates can interfere with pumping action and damage the seal or valves. Material can also collect on the inlet frits or on the inlet of the column, causing pressure buildup. Eluents or the water and salt solutions used to prepare the eluents are normally filtered with a 0.2 or 0.45 pm nylon filter. 

This reaction produces p-nitrophenoxide which strongly absorbs 404 nm radiation. The sensor tip is constructed with alkaline phosphatase covalently immobilized on a nylon membrane. This membrane is positioned at the common end of a bifurcated fiber-optic bundle. One arm of this bundle is connected to the source optics and the other is connected to the detector optics. Incident radiation is transported from a 100 watt tungsten-halogen lamp source to the sensor tip. A fraction of this incident radiation is back scattered off the nylon mesh and a fraction of this back scattered radiation is collected by the fiber-optic bundle and directed to a 404.7 nm interference filter and then to a photomultiplier tube detector. 

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