GB2158816A - Method and apparatus for the determination of NO2 in air - Google Patents

Abstract

A simple determination of nitrogen dioxide in air can be made by contacting a known volume of air with diphenylamine- or N-methyl-aniline-coated sorbent particles to quantitatively remove the nitrogen dioxide. The nitrogen dioxide-diphenylamine reaction products are desorbed and are then analyzed e.g. by chromatography to measure the amount of nitrogen dioxide present in the air volume. The sorbent particles 22 are preferably silica or magnesium silicate. <IMAGE>

Description

SPECIFICATION Method and apparatus for the determination of NO2 in air This invention relates to a low cost, simple method and apparatus for ambient nitrogen dioxide (NO2) analysis that can be used in both rural and urban environments.

Nitrogen oxides (NO, = NO + NO2) are an important class of nitrogeneous air pollutants which play an important role in photochemical smog production and acid deposition chemistry. Most of the NO, emitted by combustion sources is nitric oxide (NO) which is subsequently oxidized in the atmosphere to NO2. Nitrogen dioxide affects smog chemistry by influencing the photochemical production of ozone and hydroxyl (OH) radicals and by reacting with organic radicals to form peroxyacetylnitrate (PAN) which is a strong eye irritant. NO2 can also be oxidized to HNO3 which is a component of acid precipitation. Presently, the National Ambient Air Quality Standard for NO2 is 53 parts per billion (ppb) (annual average) and, as such, NO2 is routinely measured in air quality studies.

It is an object of this invention to provide a low cost, simple, selective, sensitive and integrative method for ambient NO2 analysis that can be used in both rural and urban environments.

It is another object of this invention to provide an adsorbent material and container useful in the practice of such a method for NO2 analysis.

Summary of the invention In accordance with a preferred practice of the invention, the above and other objects and advantages can be accomplished as follows. A coating of diphenylamine (hereinafter referred to sometimes as "DPA") is applied from a solution to the surface of high purity, small (suitably 60 to 80 mesh) particles of an acidic sorbent material, such as magnesium silicate. A small bed of the sorbent particles is prepared which may be contained in a polyethylene cartridge having an air inlet and air outlet. A known volume of air is caused to flow through the bed at substantially ambient conditions. Preferably, the air is no warmer than about 29.4'C (85"F). The diphenylamine-coated particles quantitatively and selectively absorb NO2 from the air stream.The NO2 reacts in the presence of an acid catalyst to form various derivatives of diphenylamine, such as 4-nitrodiphenylamine 2-nitrodiphenylamine and Nnitrosodiphenylamine, with N-nitroso DPA accounting for 94% to 98% of the total product mixture.

The NO2 products are readily removed from the sorbent bed, suitably analyzed, such as, for example, by reverse- phase high-performance, liquid chromatography with UV detection, and related to the amount of NO2 in the sampled air. The collection efficiency of such a sorbent bed of suitable capacity for the sample size is about 100%. The detection limit of the method is 0.1 ppb NO2 for a 2,000 L air sample which corresponds to an eight hour sampling period at 4.0 L/min. As will be described in more detail below, there are few substances in air that interfere with the subject method. The method has been successfully used to monitor daily ambient NO2 levels ranging from 2.4 to 33.5 ppb NO2.It has also been used to monitor indoor NO2 levels at concentrations ranging from 12.5 to 144.8 ppb NO2.

Other objects and advantages of the invention will become more apparent from a detailed description thereof which follows. Reference will be made to the accompanying drawings, in which: Figure 1 is a sectional view of a suitable cartridge employing sorbent material according to the present invention; Figure 2 is an illustration of the removal of absorbed NO2-diphenylamine compounds from the sorbent bed in the practice of the method of the invention; Figure 3 is a chromatogram obtained for a 2,000 L air sample containing 11 ppb NO2; and Figure 4 is a chromatogram obtained for a 1,430 L sample of a 44 ppb NO2 air stream.

Detailed description of the invention In the practice of this invention NO2 is selectively and quantitatively absorbed from a stream of air using particles of a suitable sorbent coated with a secondary amine-dipheny- lamine or N-methylaniline. The sorbent is a granular acidic material, such as silica or, preferably, a magnesia-silica gel (magnesium silicate). Such a gel is commercially available under the trademark "Florisil", from Floridian Company, U.S.A. It is an extremely white, hard, granular material. It suitably has a grain size smallar than about 30 mesh and coarser than about 100 mesh. A 60;80 mesh material is preferred. Air sampling cartridges packed with high purity 60/80 mesh Florisil are suitable for use and may be purchased from Thermoelectron Corporation (Waltham, MA) under the trade name "Thermosorb".

The cartridges are depicted schemati- cally in Figure 1 at 10. They have a cylindrical body portion 12 with an air inlet 14 and an air outlet 16. The cartridges may be moulded of polyethylene and are about 1.5 cm in diameter and about 2 cm long.

They have a 100 mesh stainless steel screen 18 at the inlet and a glass wool plug 20 at the outlet to retain the granular sorbent bed 22. The air inlet 14 and outlet 16 are located to provide for a flow of air uniformly through the bed 22. They are in the configuration of standard tapered fittings to facilitate sorbent coating and sample elution using a syringe. The Thermosorb cartridges contain about 1.2 grams of dry absorbent.

The sorbent particles are coated with diphenylamine by filling the cartridges with about 2 mL of a 4 mg/mL diphenylamine solution in dichloromethane (CHCl2). A syringe may be used for this purpose. The cartridges are capped and allowed to stand at room temperature for a couple of hours.

The outlet cap is then removed and the cartridges are placed in a vacuum oven with no heat at a pressure of 50.8 - 67.7 kpa (15-20 inches of mercury) for about one hour to volatilize the solvent.

The cartridges are then capped, stored and are ready for use.

The sorbent retains about one-half of the diphenylamine placed in the cartridge. This corresponds to about 24gel moles or 4 mg of DPA in each cartridge. Ordinarily this is more than enough DPA for most NO2 analysis applications. Higher loadings of DPA can be coated on the sorbent in the cartridge by increasing the concentration of the DPA coating solution.

In use, ambient air is drawn through the cartridge using a suitable constant flow sampling pump, not shown in the drawings.

Air sampling was performed with Gilian Model HFS 113 UT (Gilian Instrument Corporation, Wayne, NJ) portable constant flow sampling pumps. These pumps are capable of maintaining a constant flow of 1.0 to 4.0 Limit for a 16.5 hour sampling period. For field studies, air sampling was done with VWR Model 4K Dynapumps (VWR Scientific, Detroit, MI) and the total gas volumes were measured with a Precision Scientific Model 63125 (VWR Scientific) wet-test meter.

When air containing NO2 is drawn through an absorbent bed, such as that depicted at 22 in Figure 1, the NO2 is quantitatively collected. The DPA reacts with NO2 in the air to form in situ N-nitroso DPA, 4-nitro DPA and 2-nitro DPA. In each instance one mole of DPA reacts with one mole of NO2.

After an air sample of desired known volume has been drawn through the cartridge 10 and its sorbent bed 22, the air flow is stopped and the NO2 DPA products are removed from the bed. This may be accomplished by backflushing the cartridge 10 with methanol, as illustrated in Figure 2. A syringe 28 containing methanol 30 is inserted into the air outlet fitting 16 and a small volume of methanol injected into the cartridge. An injection of about 4 mL of methanol fills the bed and produces about 2 mL of eluate which passes through the air inlet fitting 14 and attached hypodermic needle 24 and into sample collector vial 26. A small amount of mineral acid catalyst is added to the collected sample solution to catalyze the derivatization reaction between NO2 and DPA.Hydrochloric acid is the preferred catalyst and suitably about 50 1L of 1 N HCI catalyst is added to the collection vial for each mL of eluate collected. The sample is then cenrtifuged briefly to settle any sorbent from the collection vial. The sample is then ready for analysis for the NO2- DPA products.

Generally, 1-2 mL of methanol is required to quantitatively elute all the NO2-DPA products from the Thermosorb cartridge. A smaller amount of methanol may be used if the expected amount of NO2 collected is low.

If the intended analysis of the eluate requires that the reaction between NO2 and dip henylamine be complete, it is necessary that an acid catalyst be employed. Any mineral acid, other than nitric acid, is suitable. However, hydrochloric acid is preferred.

About 50 > L of 0.1 N HCI per mL of collected eluant is usually sufficient to promote complete recovery of the expected NO2-DPA products. In general use, 501r. of 1.0N HCI is used per mL of collected eluant.

In general, the methanol eluate has been treated with the acid. However, it should be apparent that acid treatment in the sorbent bed would also be suitable.

Any suitable means may be employed to analyze the NO2- DPA products in the eluate to correlate the NO2 content therein with the volume of air sampled. It has been found that reverse-phase high performance liquid chromatography (HPLC) is particularly suitable for this purpose.

The HPLC system used consisted of a Varian Model-5060 liquid chromatograph with a Vista 401 data station (Varian Asso- ciates, Palo Alto, CA), a Perkin-Elmer Model LC-85 (Perkin- Elmer Corporation, Norwalk, CT) variable wavelength UV- visable absorbance detector with a 2.4 zL flow cell, and a Valco (Valco Instruments, Houston,TX) air actuated injection valve with a 25 > L sample loop. The analytical column used was a 4.6 mm x 25 cm Zorbax ODS (DuPont Instruments, Wilmington, DE) reverse-phase column with a Rainin (Rainin Instrument Company, Woburn, MA) 0.5 ,um prefilter.

A sample of the eluate was injected into a 55%1 45% (by volume) acetonitrile/water mobile phase flowing at 2.0 mllmin through the Zorbax-ODS column.The effluent stream was analyzed by UV detection at 254 nm. This wavelength was employed because it was found to produce good UV responses for the nitro and nitroso compounds while resulting in the least interference from the excess diphenylamine reagent. N- nitroso-diphenylamine is the major product formed and the 254 nm wavelength is a good compromise in terms of maximum sensitivity for this species. A chromatogram of a standard mixture of NO2-DPA derivatives was prepared using diphenylamine, N-nitrosodiphenylamine, 4-nitrosodiphenylamine, 4-nitrodiphenylamine and 2-nitrodiphenylamine, purchased from Aldrich Chemical Company.

The effectiveness of the absorption cartridge and HPLC analysis method were demonstrated and verified by generating standard NO2 atmospheres and analyzing them. A Metronics (VICI Metronics, Santa Clara, CA) certified NO2 permeation wafer device along with a Metronics Model 350 Dynacalibrator permeation system was used to generate known atmospheres of NO2 in air. The permeation rate of the wafer devices was checked by bubbling the NO2 gas product from the permeation system through an aqueous solution of triethanolamine and analyzing the subsequent nitrite produced. The permeation rate was also determined by monitoring the weight loss of the device with a recording electrobalance over a one week period. The permeation rates determined by these two methods were 82 and 85 nglmin, respectively, which agreed well with the Metronics certified value of 88 + 5 ng/min at 30cm. The average of these numbers gives a permeation rate of 85 ng/min at 30"C which was equivalent to 44 ppb NO2/L-min under the experimental conditions.

Figure 3 shows a chromatogram of a sample obtained by flowing 2,000 L of an 11 ppb NO2 air stream at 4.0 Cumin through one of the cartridges coated with diphenylamine, as described above.

The air-NO2 mixture was prepared using the NO2 permeation system described above. Sensitivity is 0.04 absorbance units full scale. The three principal products formed are 4-nitro DPA (peak 1 in Figure 3), N-nitroso DPA (peak 2) and 2-nitro DPA (peak 4). Peak 3 is DPA. The 4-nitroso DPA product was not found under ordinary sampling conditions. The peaks at 1.7 and 3.7 minutes are not NO2 related and are due to other unidentified compounds in the air which are trapped and detected at 254 nm.

N- nitrosodiphenylamine was the major product formed, and accounted for 94-98% of the total NO2 product yield in most of the samples. The 4-nitro and 2-nitrodiphenylamine products are usually minor reaction products that occur at about the same molar ratio in many of the samples.

It was also found that other nitrodiphenylamine reaction products may be formed on the cartridges. They are formed in appreciable amounts (about 1% to 2% of total yield) during long term sampling of relatively high NO2 concentrations greater than 40 ppb. for example, Figure 4 shows two dinitro DPA derivatives in the chromatogram corresponding to retention times at 5.7 minutes (peak 1 in Figure 4) and 7.4 minutes (peak 4 in Figure 4).

Figure 4 is a chromatogram obtained for a 1,430 L sample of a 44 ppb NO2 air stream from the permeation system. Sensitivity is 0.04 absorbance units full scale. In Figure 4, peak 2 is 4-nitro DPA, peak 3 is N-nitroso DPA, peak 5 is DPA and peak 6 is 2-nitro DPA. Subsequent analysis of the dinitro DPA peaks by gas chromatography-mass spectroscopy revealed that the 5.7 minute peak was 4,4' dinitro DPA and the 7.4 minute peak was 2,4' dinitro DPA.

In accordance with conventional practice, the molar quantities of the NO2-DPA reaction products trapped in the cartridge may be measured by comparison of the relative absorbance of their HPLC chromatogram peaks with chromatogram obtained in a link manner of a known mixture. Validation experiments of the NO2 collection efficiency of the method and of the HPLC analysis has shown that the trapping of NO2 is substantially quantitative and that HPLC will consistently analyze the trapped products.

Sample capacity studies performed on the Thermosorb cartridges, coated with DPA as described above, revealed that about four micromoles of NO2 can be quantitatively collected in a single cartridge.

This corresponds to about 96 ppb NO2 for a 1,000 L air sample. Accordingly, at ambient NO2 concentrations of 5 to 10 ppb a single cartridge should last about six days at 1.0 Limin and about three days at 2.0 Cumin. For extended sampling periods greater than one day, a backup cartridge should be used to ensure that no breakthrough due to channelling has occurred.

The detection limit for NO2 using the described cartridge and analytical method is determined to be about 0.1 ppb NO2 in a 2,000 L air sample and a 2 mL elution volume. this corresponds to an eight hour sampling period at 4 Limit or to a sixteen hour sampling period at 2 L/min.

While the invention has been described using diphenylamine coatings on a suitable acidic sorbent, another secondary amine, N-methylaniline, may also be used.

The described apparatus and method is quite selective and free from interferences. Nitric oxide is not absorbed on the coated sorbent bed and does not interfere in the method. Similarly, ozone, nitric acid, sulphur dioxide and humidity have no effect.

PAN produces a positive interference in the method. In urban areas where PAN levels may be high, this could present a problem and should be considered. In rural areas where PAN levels are quite low, the interferences should be minimal. Its interference may be minimized with alkaline pretraps upstream of the cartridges.

The apparatus of the present invention is intended to be used at substantially normal room temperatures. The DPA- coated sorbent beds failed to trap NO2 at temperatures in excess of 32.2"C (90"F).

Cartridges which have trapped NO2-DPA products can be stored for an extended period before analysis, for example, up to one month without refrigeration and up to two months with refrigeration.

In summary, suitable acidic sorbent beds of particles coated with diphenylamine andior N-methyaniline may be used to analyze ambient air samples of known volume for NO2. Any acidic sorbent, such as silica gel or magnesium silicate, as described, may be used so long as it does not interfere with the NO2-DPA type reaction. The air stream is passed through the bed at a suitable rate until a desired volume has been processed. The NO2-secondary aromatic amine reaction products are then analyzed to correlate the quantity of NO2 removed From the air sample with the volume of the sample.

Claims (4)

1. A sorbent bed for selectively absorbing nitrogen dioxide (NO2) from ambient air comprising acidic sorbent particles coated with a secondary amine selected from the group consisting of diphenylamine and N-methyl-aniline, the sorbent particles being inert to the reaction between nitrogen dioxide and the secondary amine.

2. A cartridge for the analysis of nitrogen dioxide in ambient air comprising a bed of acidic sorbent particles according to Claim 1, in the size range of 30 to 100 mesh, the bed being contained in a housing with an air inlet and an air outlet disposed to promote air flow through the bed, the acidic sorbent being of a composition taken from the group consisting of silica and magnesium silicate.

3. A method of measuring the nitrogen dioxide (NO2) content of a known volume of air comprising flowing the air through a bed of acidic sorbent particles according to Claim 1, the size of the bed and the quantity of the secondary amine being sufficient to remove substantially all of the nitrogen dioxide from the air, eluting the NO2-secondary amine reaction products from the bed, and analyzing the eiute to determine the amount of said products present in relation to said volume of air.

4. A method of measuring the nitrogen dioxide (NO2) content of a know volume of air comprising contacting a stream of said volume of air with a bed of acidic sorbent particles coated with diphenylamine, the quantity of diphenylamine being sufficient to remove substantially all of the nitrogen dioxide from the air, eluting the nitrogen dioxidediphenylamine reaction products from the bed, and measuring the amount of said products present in relation to said volume of air.