Measurement of Sub-ppb by Volume Levels of Atmospheric Nitrogen Dioxide Using an Alkaline Trapping Solution and Spectrofluorometric Analysis Herman D. Axelrod,’ Richard A. Pickett 11,* and Nancy A. Enge13 National Center for Atmospheric Research, Boulder, Colo. 80303
In these days of energy shortages, there is an effort for the development of new energy sources, usually in geographical areas that are relatively clean and unpolluted. As a result, it may be necessary to assess the quality of air prior to development in order to ensure non-degradation or minimum degradation of the locality. Major emphasis in air quality studies will probably be on the reactive gases such as Son, NOz, and O3 as well as on the particulate matter. The techniques for measuring background levels of these four constituents are at times difficult and, in the case of SO2 and NOz, almost nonexistent. This publication, therefore, is concerned with the measurement of background levels of N02, which has been estimated to be in the range of 0.5-1.0 ppbv (ppbv = parts per billion by volume, 1:109). One of the traditional methods for the measurement of atmospheric levels of nitrogen dioxide is the Jacobs-Hochheiser procedure ( I ) involving the use of an impinger or bubbler filled with dilute sodium hydroxide. A fraction of the nitrogen dioxide passing through this solution is retained as nitrite ion and its concentration is subsequently determined by solution acidification and the formation of an azo dye for colorimetric analysis. The above method has been used, although with serious deficiencies, to measure pollution levels in urban and semiurban areas. Attempts to use this technique to measure background levels in “clean air” have been less than successful because of the method’s lack of adequate sensitivity, which can be traced to the absorptivity of the azo dye. While a variety of azo dyes have been used ( 2 ) ,they all produce about the same overall sensitivity. Unfortunately, instrumentation such as the chemiluminescent nitrogen dioxide monitor also lacks the sensitivity to reach true background levels. I t has been recently reported ( 3 ) that nitrite can be analyzed in the picogram range by using a fluorometric procedure. The coupling of this technique with the sodium hydroxide trapping system would enable one to make background measurements. Indeed, this was accomplished (as described below) such that for a 100-liter air sample one could measure as low as 0.01 ppbv of nitrogen dioxide in air a t 600 Torr. (At this atmospheric pressure 1 ppbv = 1.5 wg/m3.)
EXPERIMENTAL Instrumentation. A Perkin-Elmer Model 203 Spectrofluorometer equipped with a high-pressure xenon lamp source was used for the fluorescence measurements. For the spectrophotometric analyses, a Coleman Model 124 Spectrophotometer was employed. Reagents. The 5-aminofluorescein (F.W. 347) was obtained from Eastman Kodak Co. (Rochester, N.Y.) and was used as received. All other chemicals were reagent grade; the water used was Author to whom correspondence should be addressed. of Colorado, Boulder, Colo. Present address, School of Pharmacy, University of Washington, Seattle, Wash.
* Present address, School of Pharmacy, University
first passed through a deionizing column and then distilled. All glassware was cleaned with Contrad 70 prior to use. Sampling. Air was drawn through a 1-p Nuclepore pre-filter and then through a bubbler (4)a t a flow rate of 2 l./min for 1 hr. The flow rate was controlled by the system of Wartburg e t al. ( 5 ) . The bubbler contained 13 ml of the trapping solution developed by Huygen and Steerman (6) which consisted of 0.1N NaOH, 0.02% 2-naphthol-3,6-disulfonic acid disodium @-naphthol), and 0.124% triethanolamine hydrochloride. Analysis by Fluorometry. The analysis procedure was as follows: after sampling, the bubbler was returned to the laboratory. T h e solution volume remaining in the bubbler after sampling was brought back up to 13 ml with distilled water. A 7-ml aliquot was taken and analyzed by the fluorometric procedure. (The remaining 6 ml was analyzed by a spectrophotometric procedure.) T o the 7 ml was added 1 ml of 1.5 X 10-jM (high range) or 6 X 10-7M (low range) 5-aminofluorescein (5AF). This was followed by 2 ml of 6M HCl. The solution was allowed to stand for 45 min. Afterwards, 5 ml of 5.4M NaOH was added; the solution was mixed and allowed to stand for a t least 10 min (and as long as 2 hr). A standardization curve was prepared in the typical manner using the same reagents and NaN02. The curve is fairly straight and by definition passes through the origin, and the fluorescence intensity increases with increasing NO2- concentration. The fluorescence intensity was measured a t the excitation wavelength of 490 nm and the emission wavelength of 515 nm. For the instrument, the reagent blank was used to set the 0% fluorescence units, whereas the 100% fluorescence units were arbitrarily set by using a standard solution containing the above chemicals plus NaNO2 equal in concentration to the final 5AF concentration. Analysis by Spectrophotometry. The remaining 6 ml of sampling solution was analyzed spectrophotometrically using a modified version of the Saltzman procedure (7). T o this solution was added 1 ml of 1.8% sulfanilamide, 5% H3P04, followed by 0.5 ml of 0.02% N-1-naphthylethylenediamine dihydrochloride. The color was allowed to develop for 30 min and then measured a t 550 nm. The standardization curve was prepared in the usual manner using NaN02.
RESULTS AND DISCUSSION Acidity and Basicity. The pH for the acid and base reactions was not found to be critical, corroborating those data previously presented ( 3 ) . Therefore, the amounts of HCl and NaOH used in the Procedure Section represent mid-range quantities and gave the best analytical results. Reaction Time. Unlike the data reported by Axelrod and Engel ( 3 ) , the fluorescence intensity tended to be lower, the longer the acid reaction was allowed to occur (See Figure 1.).A 45-min reaction time was selected because the reaction had almost stopped a t that point. While shorter reactions can be used, the experimenter must monitor the reaction time for the samples and standards. After the 5 ml of 5.4M NaOH was added and mixed, the solution was allowed to stand for 10 min before being read for fluorescence. Shorter time periods tend to give erratic results; however, the fluorescence intensity is stable from 10 min to 2 hr. Reproducibility and Accuracy. The reproducibility of the method was determined by measuring the amount of NOz- present in 40 aliquots over a range of 2 X 10-8M to 6 X 10-7M NOz- in the sampling solution. The mean percent standard deviation was determined t o be 1.90%.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
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I
0
40
20
60
‘I
80
100
REACTION TIME (min)
Figure 1. Time study of N02--5AF reaction (A) 1 X 10-BMN02-; 1 X 10-eM5AF. (8) 1 X IO-’MNO2-; 5AF. (C) 1 X 10-’MNO2-; 4 X 10-aM5AF
1 X lO-’M
Table I. Materials Investigated for Interferencea Interferent
NH,+
c a2+
cr3+ Mg2+ Mn2+
Hg2’ Ni2’
K+ ~13+ ce3+ Fez’ Fe3‘ Pb2+ Zn2+
Br03Br-
Error, %
0 0 +218 +4 -1 0 -3 6 +4 0
0 +218
-8 -3 -9 -1 0 -1 8 +4
Interferent
Error, %
C 2H302-’ ~0~3-
NO3-
so42-
-8 -6 -1 2
0
CH3COCH3 HCHO CH3CHO d-Glucose Menthol 0-Pinene “,OH O3(1OO ppbv)
so32SO,’-
-8 -1 8 -5 0
0 +34
-80 +400 -2 0 +256 +162 -2 7
a l O - 7 M NOz-, l O - 7 M 5AF; potential interferent a t 100-fold mole excess. 10-fold mole excess. Equal mole Concentration.
The overall accuracy of the sampling procedure and analysis method was not determined. Recent reports (8, 9) on the efficiency of capture of NO2 by alkaline sampling solutions have indicated interferences and efficiency variations due to the presence of NO, COP, and different levels of NO2 in the sampled air. Since the purpose of this report is to present an alternative and more sensitive means for the analysis of samples taken by an alkaline solution, the overall efficiency of this solution was not studied. However, prior to use of this overall method, the experimenter should be aware of the problems associated with using this technique.
Sensitivity a n d Analysis Range. The overall sensitivity of the technique for measuring atmospheric NO2 is about 0.01 ppbv for a 100-1. air sample at 600 Torr, 25 OC. This sensitivity level is more than adequate to measure background concentrations of NO2. For a given dye concentration, the analysis range is limited to about 0.1-2 times the dye concentration employed. I t was found that using the two dye concentrations mentioned in the previous experimental section would allow the experimenter to cover the range of 0.01-7 ppbv for a 100-1. air sample. Interferences. Various anions, cations, and organic materials were checked as possible interferents, the results of which are listed in Table I. As expected, oxidants and reductants could affect the solution NO2- levels. For atmospheric measurements, one of the most serious interferents can be SO2 or S032- in solution. Although dilute NOz- and S032- are compatible in alkaline solution (such as the sampling solution), acidification will cause both of these to react, resulting in a loss of NOz-. The typical procedure of adding a drop of 1%HzOz to oxidize the S032- prior to acidification did not work, because the excess HzOz affected the dye. Other oxidants that were used to remove the S032- did not perform any better than the HzO2. The problem was overcome by adding S032-to both the samples and standards a t a 50-fold concentration excess over the dye concentration. It is surprising that the technique would still be useful with such a large excess of S0s2-. At this point, the reason isn’t very clear except that one would expect the reaction of S0s2- with NOz- in acid solution to be as follows (IO):
2H+
+ NO2- + 2S032- + H 2 0 = NH30H+ + 2SOd2-
Furthermore, the reaction product, NHzOH, as shown in Table I, is a positive interferent and experiments have shown that NHzOH will react with the dye and cause fluorescence enhancement under identical acid and base conditions as the NOz-. At times, the results from sample-taking on the NCAR roof were very erratic, even when a “clean” west wind was present. Samples taken several miles east of the facility did not show this problem. A main difference between the locations was the presence of many conifer trees surrounding NCAR, but not at the eastern site. Because conifer vegetation produces pinenes, it was decided to see if a pinene would be an interferent. Indeed, P-pinene (Table I) can be a serious enough interferent that this technique would probably not be useful for sample-taking in pine forests. No effort was made to try to remove the pinene interference. Analysis Reaction a n d Stoichiometry. A proposed reaction for the N02--5AF analysis procedure has already been reported (3), and the work presented here has nothing
Table 11. NO2 Fleld Sampling D a t a NO2 concentration found, ppbv
Site
NCAR roof
Boulder Reservoir
2022
Date
26 27 30 31 7 8 14 19 13 13
December 1974 December 1974 December 1974 December 1974 January 1975 January 1975 January 1975 January 1975 February 1975 ( A . M . ) February 1975 (P.M . )
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
Fluoromehy
3.9 0.56 3.0 6.6 2.7 6.3 0.35 0.15
i 11%
1% i 29% t 13% i 14% i 5% k 34% i 12% 0.60 i 12% 0.27 34% i
* ”. std dev
Spectrophometry
5.7 0.66 5.2 9.0 3.3 7.7 0.74 0.48 1.3 0.74
i 7% i 38% i 21% i 8%
* 3% i
13%
i 32% f 83%
i 54% f 69%
Fluommetriclspecbophotomebic mean
0.68 0.85 0.58 0.74 0.83 0.82 0.50 0.31 0.46 0.37
to add concerning that reaction. However, there is apparently a different reaction of NHzOH with 5AF. The major difference appears to be the requirements of the presence of (?-naphthol in solution along with the 5AF while the N02- reaction does not require it. Without the presence of the (?-naphthol, the NH20H has no affect on the 5AF. Exactly what reactions are taking place is not known at this time. The stoichiometry for this reaction in water was found to be 1 NO2-:1 5AF (3). For the analysis procedure using the Huygen-Steerman sampling solution as presented here, the reaction required 7 NOz-:l 5AF. By isolating the various reagents used in the sampling solution, it was found that the triethanolamine or an impurity present in it was responsible for increasing the stoichiometric ratio. Spectra. The fluorescence spectra for this procedure was not too different from the data previously reported ( 3 ) . However, the presence of (?-naphtholcaused a high residual absorption and fluorescence in the 250- to 450-nm range which trailed off after 450 nm. No analysis problems were encountered using a double monochromator spectrofluorometer, but it is anticipated that, with the presence of the (?-naphthol fluorescence, the use of a filter-type spectrofluorometer might be difficult. Field Sampling. Comparative field sampling between this new procedure and the modified Jacobs-Hochheiser procedure ( 1 , 6) was conducted a t NCAR (Boulder, Colo.) and a t Boulder Reservoir (Boulder, Colo.), the latter site being 12 miles northeast of NCAR. As indicated earlier, the trapping solution developed by Huygen and Steerman (6) was used in field sampling. This formulation was chosen over the ones reported by Mulik et al. (11) based upon the work of Nash (12), because the 2methoxyphenol present in the sampling solution interfered with the analysis. The arsenic-type trapping solution developed by Christie et al. (23) was not checked for suitability. For each run six samples were taken in parallel at 2 1./ min for 1 hr. They were returned to NCAR and usually analyzed on the following day. (Samples stored for a t least one week showed no loss.) Each sample was split roughly in half and analyzed by the 5AF technique and the azo-dye procedure. The results were compared and are presented in Table 11.
At atmospheric concentrations above 2 ppbv, the azo-dye colorimetric and the fluorometric procedures had about the same reproducibility, while a t lower concentrations the 5AF performed better since the azo-dye procedure was a t its limit of sensitivity. Furthermore, it appeared that the 5AF procedure gave consistently lower results. At NO2 concentrations above 1 ppbv, the 5AFIazo-dye ratio was about 0.7. At lower NO2 levels, the ratio changes to about 0.5. The reason for this response ratio cannot be explained at this time. The percent standard deviation for NO2 field measurements (Table 11) at times is quite high. These data can only be partially attributed to t h e analysis data since variations in mechanical sampling equipment in the parallel apparatus will contribute to the total variation found. It is apparent, however, that the 5AF procedure can be used to measure very low levels of NO2, provided that the sampling solution is satisfactorily removing the NO2 from the air passing through the bubbler.
LITERATURE CITED (1) M. 6.Jacobs and S. Hochheiser, Anal. Chem., 30,426 (1958). (2) E. Sawicki, T. W. Stanley, J. Pfaff. and A. D'Amico, Talanta, 10, 641 (1963). (3) H.D. Axelrod and N. A. Engel, Anal. Chem., 47, 922 (1975). (4) H. D. Axelrod, A. F. Wartburg, R. J. Teck, and J. P. Lodge, Jr., Anal. Chem., 43, 1916 (1971). (5) A. F. Wartburg. H. D. Axelrod. R. J. Teck, M. D. LaHue, and J. P. Lodge, Jr., Anal. Chem., 45, 423 (1973). (6) C. Huygen and P. H. Steerman. Atmos. Environ., 5, 887 (1971). (7) 6.E. Saltzman, Anal. Chem., 26, 1949 (1954). (8) L. J. Purdue, J. E. Dudley, J. 6.Clements, and R. J. Thompson, Environ. Sci. Techno/., 6, 152 (1972). (9) E. L. Merryman, C. W. Spicer, and A. Levy, Environ. Sci. Techno/., 7, 1056 (1973). 110) . . A. C. Rutenbera. J. HalDerin. and H. Taube, J. Amer. Chem. Soc., 73, 4487 (1951). (11) J. Mulik, R. Fuerst, M. Guyer, J. Meeker, and E. Sawicki, Int. J. Environ. Anal. Chem., 3, 333 (1974). (12) T. Nash. Atmos. Environ., 4, 661 (1970). (13) A. A. Christie, R. G. Lidzey, and D. W. F. Radford, Analyst (London),95, 519 (1970).
RECEIVEDfor review March 21, 1975. Accepted June 13, 1975. The National Center for Atmospheric Research is sponsored by the National Science Foundation.
Spectrophotometric Determination of Triphenylphosphine in Dilute Solutions Daniel B. McDonald' and Joel 1. Shulman2 The Procter & Gamble Company, Miami Valley Laboratories, P.O. Box 39 175,Cincinnati, Ohio 45247
The determination of triphenylphosphine (TPP) in dilute solutions is generally difficult, especially where concentration or extraction of samples is undesirable or unfeasible. The most sensitive methods for tervalent phosphorus determination require that samples be oxidized exhaustively to orthophosphate, which can be analyzed colorimetrically as a molybdate complex (1-4). This oxidation procedure is laborious and it is not sufficiently sensitive to allow the detection of TPP at the ppm level. Some methods Present address, Department of Chemistry, University of Chica 0 , Chicago, Ill. 60637. Author to whom correspondence should be addressed. l
which do not involve conversion to orthophosphate are applicable to the determination of TPP; but these methods are either gravimetric (5, 6) or titrimetric (7, 8) and as such are less sensitive than the spectrophotometric analyses mentioned above. Recently Barral et al. ( 9 ) , reported the reduction of dioxobis(diethyldithiocarbamato)molybdenum(VI) by TPP according to the following equations:
-
+ Mo(IV)O(dt)2 + T P P O Mo(IV)O(dtj2+ Mo(VI)On(dt)s s MO2(Vj03(dt)4 Mo(VI)Oz(dt)~ TPP
(1) (2)
where d t = diethyldithiocarbamato and T P P O = triphen-
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