CA1222500A - Diphenylamine coated sorbent sampling cartridges for the determination of no.sub.2 in air - Google Patents

Abstract

DIPHENYLAMINE COATED SORBENT
SAMPLING CARTRIDGES FOR THE

Abstract of the Disclosure A simple, low cost determination of nitrogen dioxide in air can be made by contacting a known volume of air with diphenylamine-coated sorbent particles to quantitatively remove the nitrogen dioxide. The nitrogen dioxide-diphenylamine reaction products are then analyzed to measure the amount of nitrogen dioxide present in the air volume.

Description

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DIPHENYLAMINE COATED SORBENT
SAMPLING CARTRIDGES FOR THE
DETERMINATION OE` NO 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 tNOX ~ 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 NOX emitted by combustion sources is NO which is subsequently oxi-dized 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 peroxyacety nitrate (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 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 method for NO2 analysis.

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Summary of the Invention In accordance with a preferred practice of my invention, the above and other objects and advantages can be accomplished as follows. A coating of diphenyl-amine (hereinafter sometimes 'IDPA'') is applied from asolution 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 poly-ethylene 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 85F. The diphenylamine--- coated particles ~uantitatively and selectively absorb NO2 from the air stream. The NO2 reacts in the presence of an acid catalyst to form various deriva-tives of diphenylamine, such as 4-nitrodiphenylamine,

2-1~itrodiphenylamine and N-nitrosodiphenylamine, with N-nitroso DPA accounting for ~4% 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 NO2in 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 sub-stances 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 my invention will become more apparent ~rom a detailed description thereof which follows. Reference will be made to the drawings, in which:
Figure 1 is a sectional view of a suitable cartridge employing my sorbent material;
Figure 2 is an illustration of the removal of absorbed NO2-diphenylamine compounds from the sorbent bed in the practice of my method;
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 o~ 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-diphenylamine or N-methyl-aniline. 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. It is an extremely white, hard, granular material. It suitably has a grain size smaller 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 (Wa]tham, MA) under the trade name "Thermosorb"~. The cartridges are depicted schematically in Figure l at l0~ They have a cylindri-cal body portion 12 with an air inlet 14 and an air outlet 16. The cartridges may be molded of poly-ethylene 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 gla~s 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 conEiguration of standard luer~lok fittings to facil-itate 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 (CH2C12). 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 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 corre-sponds to about 24 ~moles or 4 mg of DPA in each cartridge. Ordinarily this is more than enough DPA for most NO2 analysis applications. Higher loadings o~ 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 L/min for a 16.5 hour sampling period. For field studies, air sampling was done ~ith 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 guantitatively collected. The DPA reacts with ~2 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 prod-ucts are removed from the bed. This may be accom-plished 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 cartridgeO An injection of about 4 mL of methanol fills the bed and produces about 2 mL of eluate through the air inlet fitting 14, attached hypodermic needle 24 and into sample collectGr vial 26. A small amount of mineral acid catalyst is added to the collected sample solution to catalyze the derivatization reaction ~2;~

between N02 andl DPA. Hydrochloric acid is the pre-ferred catalyst and suitably about 50 L of lN HCl catalyst is addled to the collection vial for each mL of eluate collected. The sample is then centrifuged briefly to settle any sorbent from the collection vial.
The sample is then ready for analysis of the N02-DP~
products.
Generally, 1-2 mL of methanol is required to quantitatively elute all the N02-DPA products from the Thermosorb cartridge. A smaller amount of methanol may be used i the expected amount of N02 collected is low.
If th~e intended analysis of the eluate requires that the reaction between N02 and diphenyl-amine be complete, ;t is necessary that an acid catalyst be empLoyed. Any mineral acid, other than nitric acid, is suitable. However, hydrochloric acid is preferred. 2~bout 50 ~L of O.lN HCl per mL of collected eluant is usually sufficient to promote complete recovery of the expected N02-DPA products. In general use 9 50 ~L of l.ON HCl is used per mL of collected eluant. In general, I have treated the methanol eluate 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 N02-DPA prodlucts in the eluate to correlate the N02 content therein with the volume of air sampled. It has been found that reverse-phase high performance liquid chromatography (EIPLC~ is particularly suitable for this purpose.
The HPI,C system used consisted of a Varian Model-5060 liquid chromatograph ~ith a Vista 401~ data ,. .

~ ~2250[) station ~Varian Associates, Palo Alto, CA), a Perkin-Elmer Model ~C-85~ (Perkin-Elmer Corporation, Norwalk, CT) variable wavelength UV-v~sable absorbance detector with a 2.4 ~L flow cell, and a Valco~ (Valco Instru-ments, 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 ~m prefilter.
A sample of the eluate was injected into a 55%/45% (by volume) acetonitrile/water mobile phase flowing at 2.0 mL/min 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 result~ng in the least interference from the excess diphenS~lamine reagent. N-nitroso-diphenylamine is the maljor 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 effectivemess of the absorption cartridge and HPLC analysis method were demonstrated and verified by generating standard ]~2 atomspheres and analyzing them~ A Metronics (VICI Metronics, Santa Clara, CA) certiied NO2 permeation wafer device along with a Metronics Mod~el 350 Dynacalibrator permeation system ~2;~25C~

was used to generate known atmospheres f NO2 in air.
The permeation rate of the wafer devices was checked by bubbling the NO2 gas product from the permeation system through an a~ueous solution of 'triethanolamine and analyzing the subsequent nitrit~e produced. The permeation rate ~as also determined by monitoring the weight loss of the device with a recording electro-balance over a one week period. The permeation rates determined by these two methods were 82 and 85 ng/min, respectively, which aigreed well with the Metronics certified value of 88 + 5 ng/min at 30C. The average of these numbers gives a permeation rate oE 85 ng/min at 30C which was equivalent to 44 ppb NO2/L-min under the experimental conl~itions.
Figure 3 shows a chromatogram of a sample obtained by flGwing 2,000 L of an 11 ppb NO2 air stream at 4.0 L/min throughl one of the cartridges coated with diphenylamine, as described above. The air-NO2 mixture was prepared using l:he NO2 permeation system described above. Sensitivity is 0.0~ 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 pea~s 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 samiples. The 4-nitro and 2-nitro-diphenylamine prodlucts are usually minor reaction products that occur at about the same molar ratio in many of the samples.

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]:t was also found that other nitrodiphenyl-amine reac'cion 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 l in Figure 4) and 7.~ minutes (peak 4 in Figure 4).
Figure 4 is a chromatogram obtained for a l,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 i~; N-nitroso DPA, peak 5 is DPA and peak 6 is 2-nitro OPA. 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 q~lantities 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 a chromatogram obtained in a like 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 ~PLC will consisl:ently 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.

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This corresponds to about 96 ppb NO2 for a l,000 L air sample. Accordinglyr at ambient NO2 concentrations of 5 to 10 ppb a single cartridge should last about six days at 1.0 L/min and about thr~ee days at 2.0 L/min.
For extended sampling periods greater than one day, a backup cartridge should be used to insure that no breakthrough due to channeling has occurred.
The detection limit for NO2 using the subject 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 L/min 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 subject 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, sulfur 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 ~uite low, the interferences should be minimal. Its interference may be minimized with alkaline pretraps ups~ream of the cartridges.
My apparatus is intended to be used at substantially normal room temperatures. The DPA-coated sorbent beds failed to trap NO2 at temperatures in excess of 90F.

,, , Cartridges which have trapped NO2-DPA
products can be stored for an extended period before analysis, for example, up to one monkh without refriyeration and up to two months with refrigeration.
In summary, suitable 21CidiC sorbent beds of particles coated wikh dlphenylamine and/or N-methyl-aniline may be used to analyz~ ambient air ~amples of known volume Eor NO2. Any acid~c ~orbent, such as siltca gel or ma~nesium silicat~, as described, may be used so long as it does not interfere with the NO2-DPA
type react.ion. 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 product~. are then analyzed to correlate the quantity of NO2 removed Erom the air sample with the volume of the sample.
While my invention has been described in terms of a few speciic embodiments thereof, it will be appreciated that other forms could readily be adapted by those skilled in the art. The scope of my invention is to be considered limited only by the following claims.

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Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A cartridge for the analysis of NO2 in ambient air comprising a bed of acidic sorbent particles in the size range of 30 to 100 mesh and coated with a secondary amine selected from the group consisting of diphenylamine and N-methylaniline, 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.

2. A method of measuring the NO2 content of a known volume of air comprising flowing the air through a bed of acidic sorbent particles coated with a secondary amine taken from the group consisting of diphenylamine and N-methylaniline, the size of the bed and the quantity of the secondary amine being sufficient to remove all of the NO2 from the air, eluting the NO2-secondary amine reaction products from the bed, and analyzing for the amount of said products in relation to said volume of air.

3. A method of measuring the NO2 content of a known volume of air comprising contacting a stream of air with a bed of acidic sorbent particles coated with diphenylamine, the quantity of DPA being sufficient to remove all of the NO2 from the air, eluting the NO2 DPA reaction products from the bed, and measuring the amount of said products for relation to the said volume of air.