Anal. Chem. 1986, 58, 1533-1537
1533
Sublimation Sources for Nitrous Acid and Other Nitrogen Compounds in Air Robert S. B r a m a n * a n d M a r i a A. d e la C a n t e r a
Department of Chemistry, University of South Florida, Tampa, Florida 33620
Sources of nitrous acid and nitric acld In alr In the partsper-biiilon concentratlon range are provided by means of room-temperature oxallc acld subllmatlon onto solid sodlum nitrlte and potassium nltrate, respectively. Nitrogen dioxide is not present and nitric oxide Is present only at a small percentage in the nltrous acid source at relative humidities in the 30-60 % range. The presence of nltrous acid Is vertfied by spectroscoplc and chemlcal reactlon studles. Nitric oxlde, hydrogen cyanide, and other compounds can be simliariy produced with appropriate target compounds. Nitrogen dioxide from permeation devices must be used in plastic systems to minlmire Impurlties.
Development and testing of specific air analysis methods for several nitrogen-oxygen compounds of interest, "OB, HN02, NOz, and NO, require a low-concentration, reliable, and reasonably pure dynamically generated source of these compounds in air. Permeation devices or diffusion-type sources have been developed and are widely used for all of these but nitrous acid. Because of its instability, nitrous acid is not available as either a diffusion or permeation device source. It is difficult to prepare in high purity at low concentrations in air by either of the two general methods, reported in the literature. One involves the mixing of NO, NOz, and HzO in the vapor form (1)
NO
+ NO2 + HzO
2HN02
Since this equilibrium reaction is not quantitative, the result is a mixture of the nitrogen oxides with nitrous acid. Preparation by solution reaction of nitrites with sulfuric acid has also been used but also results in the formation of much NOz and NO. Using this approach, Cox and Derment (2) report that only approximately 50% of the NO,-type compounds in the headspace above the reaction solution is HNOz with the balance being NOz and NO. This is quite inconvenient in attempting to demonstrate specific reactions of "02. We have found that the sublimation of oxalic acid at a slow rate onto NaN02produces a low, reasonably constant vapor concentration of nitrous acid in air, having only small amounts of NOz,NO, and HNO, copresent, if any. Both spectrophotometric and chemical reaction evidence verified the presence of nitrous acid. Nitric acid is produced in low concentration by a similar sublimation of oxdic acid onto potassium nitrate. Nitric oxide is produced by sublimation of oxalic acid onto sodium nitrite with dry air, at less than 5% relative humidities. A reduced molybdenum oxide hollow tube can also be used with a HNO, sublimation source to convert nitrous acid to nitric oxide. During the investigations of producing a low-concentration source of nitrous acid in air, a study was also done on NOz sources. Unless precautions are taken, large amounts of nitric acid and nitrous acid can be present in a NOz in air dynamic gas generation system. This can lead to confusion as to the 0003-2700/86/0358-1533$0 1.50/0
specificity of specific analysis methods under test and development. Use of NOz in polyethylene plastic containers minimizes nitric and nitrous acid contaminants, but even in the best circumstances some scrubbing of HNO3 and HNOz is needed to obtain only NOz. Finally, tests were also successful in producing HCN and HSCN in air using oxalic acid sublimation onto the appropriate target compounds, NaCN and KSCN, respectively. EXPERIMENTAL SECTION Chemicals. Oxalic acid (HzCZO4.2H20), sodium nitrite, and
potassium nitrate were reagent grade chemicals. Nitrogen dioxide was from a lecture bottle cylinder, also reagent grade. Hollow tubes used were prepared from reagent grade nickel nitrate, tungsten wire, molybdenum wire and cobalt nitrate, copper(1) iodide, and potassium ferrocyanide. Equipment. Figure 1shows the NO, analysis system used. A Bendix commercial model NO, analyzer was used as a nitrogen compound detector with a heated gold-coated hollow tube catalyst bed for oxidation and conversion of NH, and other compounds to nitric oxide, The oxygen make-up line provides the oxidant to the system. A modular single-beam-type spectrophotometer system was assembled for use in observing the absorption spectrum of HNOP Hollow Tube Preparation, Blanking, and Analysis. Tungstic acid and molybdic acid interior coated hollow tubes were prepared by vacuum deposition from W or Mo wire as reported previously (3). Coated lengths were 20-40 cm on 6-mm-o.d., 4-mm-i.d. Vycor-brand glass tubes. Nickel oxide coated hollow tubes were prepared from saturated solutions or molten Ni(N03)z.6H20.The liquid nickel solutions were drawn up the tubes and allowed to run out leaving a liquid coating on the inside walls. This was slowly dried by using an air stream and finally thermally decomposed to NiO using a wire heating coil and an air stream. After the ends of the tubes were cleaned with a pipe cleaner, the tubes were heated in a glassblowing-type torch while passing a slow stream of oxygen through the hollow tube. This removes essentially all traces of nitrogen oxides left from the initial decomposition of the nickel nitrate. KFe (potassium-iron oxide) tubes were prepared by first coating the interior of tubes with 50% potassium ferrocyanide in water, drying, and then oxidizing in air at high temperature to remove cyanide from the coating. Copper(1)iodide coated tubes were prepared from a copper(1) iodide slurry in water. After air-drying to remove water, the tubes were heated to blank in an inert atmosphere. Cobalt(II1) oxide tubes were prepared in a similar manner from a saturated solution of cobalt(I1) nitrate. Sodium carbonate coated hollow tubes were prepared similarly using a 20% NaZCO3solution in water. Only a drying step and tube end cleaning are needed. After preparation each tube was blanked prior to use by simply analyzing each tube by heating to the several temperaturesneeded. Tungstic acid and CUItubes were heated to approximately 320 "C, nickel oxide tubes to 360 "C, and cobalt(II1) oxide tubes to 380 "C. Tubes under analysis were placed in the apparatus shown in Figure 1 and heated with a wire coil and autotransformer. Ammonia, if present, is trapped on the transfer tube. All chemisorbed nitrogen oxide type compounds are desorbed as nitric oxide, which is not absorbed by the transfer tube. Figure 2 shows a typical multitube analysis from one of the HNOz sublimation sources. The detection limit is near 0.03 nmol. 0 1986 American Chemical Society
1534
ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986 02
source 10 rnm.
0.
d.
e
air
analysis
Figure 1.
tube
R
bed
-
Apparatus arrangement for analysis of hollow tubes. 7
I
glass wool
Figure 3. Nltrous acid In air generation systems: (top)double-tube system and (bottom) humidified single-tube system.
Table I. Analysis of HNOz Sublimation Sourcesa
composition, mol % "0, HNOz NO2 NO RH,b%
comments
24
137 ng/min HNOz just prepared (N = 1) 27 ng/min HNOz after 6
8.6
24
34ng/min HNOpafter7
6.5
24
37 ng/min HNOz after 9
0
79.5
0
83
0
0
91.4
0
1.4
89.2
2.9
0
52
5
0
24.1
3.6 72.2
0
89.4
3.4
18.4 18.4 17
31
weeks
182 ng.
weeks
weeks
3.9 ng.
0 ng.
KFe Tube
Figure 2. Analysis of permeation oven HNO, at 1.02 L/min.
43
0
88 ng/min HNOz just
0
81 ng/min HNOz dry air 2
24
111 ng/min HNOz larger diameter source (N = 3)
changed to dry air
Tube
Tube.
source: 4.5-min sample
The development of this sequence of hollow tubes in analyses for "OB, HN02, NOz, and NO and analytical procedures is reported in detail in the companion article to this work ( 4 ) . Tungstic acid, KFe, CUI(or NiO), and Co tubes used in sequence remove "OB, HN02, NOz, and NO from air streams in that order.
RESULTS AND DISCUSSION Nitrous Acid in Air Generation System. Preliminary experiments indicated that oxalic acid dihydrate loses waters of hydration in dry air or just above room temperature (24 "C). It is reported to sublime completely at 157 "C. It was decided to try to make use of an acid-base reaction N2C204+ NaN02 = H N 0 2 + NaHCzO, in generation of HN02 in air by slow sublimation of oxalic acid onto solid NaN02. An oxalic acid packed tube-sodium nitrite interior coated hollow tube system as shown in Figure 3 (top) was assembled. The hollow tube was prepared by coating the interior with a 20% NaN02 in water solution and drying with air. Activated carbon filtered air passing through the double tube combination at 0.5-1.2 L/min was tested for nitrogen compounds by passing it through a sequence of hollow tubes. Experiments with a sequence of W0,-NiOCo20, tubes gave nitrogen compounds upon thermal analysis. Essentially all of the nitrogen compounds' signal was on the NiO tube. Therefore, no nitric acid was present and the presence of H N 0 2was suggested. Identity of the HN02 effluent was later confirmed by use of absorption spectroscopy and other specific chemical reactions on hollow tubes. The oxalic acid-sodium nitrate double-tube system produced a reasonably pure low concentration of "02 at 25-35% relative humidity. A single-tube system as shown in Figure 3 (bottom) was later assembled providing for some increase in the feed air humidity (to 35% relative humidity at 24 'C). The closer proximity of the oxalic acid to the NaNOz than in the double-tube assembly produced a sub-
days later
9.2
OAir carrier gas at 0.8-1.2 L/min, humidity measured at sample outlet, room-temperature source (23-24 "C). *Relative humidity.
stantially higher concentrationof HN02,was more convenient to use, and gave a more constant concentration of HNOz in air. The room-temperature HNOz experimental sources were analyzed for compound composition using the sequential hollow tube system. Results of typical analyses are given in Table I. Reasonably high percentages of HNOzwere generally produced at the approximately 40 ng/min rate at 22-24 "C and a t moderate humidities. Nitric oxide appears to be the main impurity. A larger diameter, two-layer sublimation source (14 mm i.d. vs. 8 mm i.d.) produced a similar source composition but at a higher HNOz evolution rate. The compound evolution rate was proportional to the cross sectional area of the source tube. Presumably this can be extrapolated to much higher compound evolution rates if needed. Humidity has a strong effect on source composition. Dry tank air results in large amounts of NO as well as some NO2 and HN02from the sublimation sources. Similar experiments with dry nitrogen gas produced similar results. In the absence of water it is likely that the oxalic acid dihydrate is slowly converted to the anhydrous acid, which then sublimes much more rapidly into the sodium nitrite. This apparently then increases the redox reaction:
+
H2C204 2NaN02 = 2NaHC03 + 2 N 0 Several experiments were run with water-saturated and 70-71 % relative humidity air. After several hours, high-humidity air results in the dissolution of the sodium nitrite as a result of its hygroscopic character. Humidities should be kept well below 70% to avoid this problem. A nitrous acid in air sublimation system was also adapted to use with a Bendix Model 8850-1 permeation system. The
ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986
composition, mol % temp, RH; HNOs HNOz NO2 NO "C % 0
82.0
3.2 14.8
30
30
0
79.6
5.3 15.1
29
41
0
86.3
2.9 10.8
29
44
1.4
91.9
1.7
5.0
29
44
0 1 3.2
50.1 54.2 20.5
0
49.9 3.3 41.5 8.6 67.6
32 34 36
38 38 38
--
------I
Table 11. Analysis of Permeation Oven HNOz Sublimation Source"
1535
H2C204
?"
U T Teflon Fittings
comments 105ng/min HNOz 1 day after start (N= 2) 80ng/min HNOzafter 4 days 45 ng/min HNOz after 3 weeks ( N = 1) 53 ng/min HNOz after 4 weeks (N = 1) 49ng/minHNOz 54 ng/min HNOz 26 ng/min HNOz
Figure 4.
Permeation oven sublimation source attachment design.
'Air carrier gas at 1.02 L/min for all testa; humidity measured at samnle outlet. bRelativehumiditv. Table 111. Absorption Spectrum Data for HNOz wavelength, nm obsd from ref 6 371.6 364.3 364.8 357.6 354.3 351.0 341.8 338.8 330.9 328.1
cm-I ppm-"
N
2.27 X lo4 3.13 X lo4 2.63 0.03 3.23 f 0.56 4.43 f 0.42 1.43 0.26 3.15 f 0.72
1 1 2 3 3 3 3
6,
371.6 368.3 364.7 357.7 354.3 350.9 341.8 338.8 330.8 327.8
*
I Figure 5.
'Experimental values. two-layer sublimation source was fitted into the permeation oven chamber as shown in Figure 4. The permeation oven provided good control over the temperature of the sublimation source. A humidity of 38-44% was also provided by using a humidifier on the air feed to the permeation system. Table I1 gives results of the analysis of the permeation system source over a period of time and with a short-range temperature study. Good long-term use in expected to continue with the permeation system source. Temperature exhibits a strong influence on the composition of the source effluent mixture. Generally, higher temperatures tend to increase the amount of NO present. An optimum set of conditions appears to be 29 O C and 40-44% relative humidity. Spectroscopic Verification of HNOP A single-beam absorption spectroscopic system was assembled to verify the evolution of HNOz from the sources. A 92.5-cm-path-length cell was used and the absorption spectrum obtained in the
I
330
I
340
I
350
380 nm
Single-beam absorption spectrum of
I
HNOp In
air.
320-400-nm range (tunsten light source). Figure 5 shows the absorption spectrum obtained. Table I11 correlates this absorption spectrum with the reported (2,5) absorption spectrum of HN02 The agreement of this absorption spectrum with that of published data supports the production of "02 by the sublimation source. In order to obtain the absorption spectrum, it was necessary to use anhydrous oxalic acid and to warm the source to increase the vapor concentration of HN02 to detectable levels in the 100 ppm range. Analyses of the effluent gas from the absorption cell were used to calculate the absorptivities given in Table 111. Large amounts of NO were copresent with some NO2 from the air oxidation of NO. Specifio Hollow Tube Reaction Studies of HNOP After the presence of HNOz was verified by absorption spectroscopy, chemical verification was explored at the much lower parts per billion HNOz generation concentration range. Several types of interior-coated hollow tubes were prepared that were known to have specific absorption reactions with NOz, "OB, and NHS. These were NiO tubes, W 0 3 tubes (tungstic acid), CoZO3tubes, Moo3(molybdic acid), and Na2C03tubes. A NO2 in air source was also prepared. As reported in previous work ( 3 , 6 ) ,NO2 is not absorbed by tungstic acid tubes, while nickel oxide chemisorbs NOz in dry air with Gormley-Kennedy efficiency (efficiency calculated
Table IV. Reactions of HNOa and NO2 with Na&O, and Reduced Molybdenum Oxide HNOa
compd, ng/min HNOz NO2
NO
comments
HNOz Source 0
1.3 f 0.03 0 0
43.1 f 3.5 2.4 f 0.3 43.8 f 0.9 4.5 f 1.0
1.9 f 1.9 2.2 f 0.7 1.4 f 0.1 0.54 f 0.12
3.6 f 1.4 3.4 f 0.4 4.8 f 1.5 5.9 f 0.8
NoNazC03or Mo (reduced) ( N = 2), 44% RH" with NaZCOztube ( N = 2), 44% RH no NaZCO3or Mn (reduced) ( N = 3) with Mo (reduced) ( N = 3)
NOz Source 133 f 9.3 21.7 f 2.6 26.5 f 14.8
'Relative humidity.
92 f 1.5 37 f 18 149 f 59
458 f 3 591 f 2 439 f 33
28 f 6 28 f 3 33 f 3
no NazC03 or Mo (reduced) ( N = 2), 27% RH with NaZCO3( N = 2), 27% RH with Mo (reduced) ( N = 2), 27% RH
1536
ANALYTICAL CHEMISTRY, VOL. 58,
NO. 7, JUNE 1986
Table V. Sublimation Source for Nitric Acid in Air Source Air Analysis" compd, mol % HN03 HNOz NOz NO 100 84 100 98.8
0 0 0 0
0 0 0 0
T, "C RH,* %
0 2 4 11 24 0 2 4 1.2 37
77 29 18 18
comments
4 ng/min HN03 8.4 ng/min HN03 50 ng/min HN03 538 ng/min HN03
Flow rate, 1.0 L/min; 10-mm-0.d. sublimation source tube. Relative humidity.
from the Gormley-Kennedy equations). The nonabsorption of NOz by tungstic acid was again demonstrated by analyzing the NOz in air stream using sequential selective hollow tubes with NaZCO3and the NOz source. Hollow tube experiments with the nitrous acid in air generation system gave the results as shown in Table IV. Sodium carbonate tubes absorb most of the NHOz but appear not to absorb NOz. This, then, is chemical indication that the oxalic acid-sodium nitrite source produces HN02. Similarly, as shown in Table IV, reduced molybdic oxide hollow tubes provide a contrast in chemical activity of the nitrous acid and NOz sources. Reduced molybdenum appears to reduce HNOz to NO or perhaps N2,which is not detected. Experiments with NOz indicate that reduced molybdenum oxide reduces some of it to HNOZ. This difference again supports the presence of HNOz in the oxalic acid-sodium nitrite sublimation source. Nitric Acid in Air Sublimation Source. When oxalic acid is sublimed with potassium nitrate as the target compound in a two section source (Figure 3, bottom) a low concentration of nitric acid in air is produced. This indicates that the acid-base reaction predominates rather than some redox reaction. This also occurs despite the fact that the first hydrogen of oxalic acid is considered to be weak, pK,, = 1.27. The driving force of this reaction appears to be the volatilization of the nitric acid. Both temperature and humidity influence the rate of nitric acid evolution via influence on the sublimation rate of oxalic acid. Table V presents some typical results of the analysis of the nitric acid sublimation source including some humidity effects and one elevated temperature condition. The degree of purity of the nitric acid produced is quite good with only occasional traces of NO present. Optimum conditions for nitric acid generation call for camparatively low relative humidities of the air carrier gas. This is opposite of the optimum conditions for nitrous acid generation.
Nitrogen Dioxide in Air Generation System Study. Metal permeation devices using liquid NOz are frequently used to produce low concentrationsin air for testing and calibration of air analysis methods. Prior experience suggested that NO2 in glass manifold gas mixing systems produced substantial amounts of NO,-type compounds other than NOz. Some
preliminary experiments indicated that only approximately 50% of the NO, compounds were actually NOz. Consequently, a polyethylene gas mixing system with a polythylene diffusion bottle was assembled and used for testing specific reactions of NOz ie the developmental work described here and in the development of the specific, sequential hollow tube system ( 4 ) . Dry air is passed through the diffusion bottle and then mixed with humidified air. Table VI gives analysis data on a glass manifold system and the plastic gas mixing system. The use of a plastic system results in a much improved NOz in air gas mixture. The use of a sodium carbonate scrubber should remove the nitric and nitrous acid present to further improve sample purity. Even with the use of dry air and the plastic system, substantial amounts of NO, compounds are still emitted in the NO2 sources. This renders questionable the use of NOz permeation systems as a gravimetric standard for this compound unless the systems under test do not discriminate between NOz and the other compounds of interest. Sublimation Sources for HCN, HSCN, and NO. When sodium cyanide is used as the sublimation target compound, hydrogen cyanide is produced. Several analyses of HCN produced in a two-layer system gave an evolution rate of 50 ng/min with 18% relative humidity air at 24 "C. When potassium thiocyanate is used as a target compound thiocyanic acid is the likely product. An evolution rate of 25 ng of HSCN/min was obtained at 18% relative humidity at 24 "C. Both HCN and HSCN were collected on silver oxide hollow tubes. Nitric oxide is prepared in reasonable purity if dry air is passed through the sublimation oxalic acid-sodium nitrite sublimation source. As indicated in Table I, mostly NO is produced with some HNOZ. Nitrous acid and nitric acid can be removed by use of a sodium carbonate containing tube.
CONCLUSIONS Sublimation sources for nitrogen compounds can provide reasonably constant, low concentrationsin air for experimental testing of analysis techniques and for studying air chemistry. Especially important is the development of a high-purity nitrous acid in air source. The development of good purity nitrous acid in air and nitrogen dioxide in air sources in fact greatly aided the development of the selective, sequential hollow tube system for nitrogen compounds in air (4). Sublimation sources appear to be useful for a variety of volatile compounds as indicated by the few experiments with NaCN (produces HCN) and KSCN (produces HSCN). Obvious extensions could be predicted for producing COz,HF, SOz, and HzS from appropriate target compounds. The major precaution is that while this type of source can be made to produce a reasonably high purity, low concentration of the desired compounds for immediate sampling and analysis, the use of manifolds and chambers is very likely to result in wall reactions that can drastically change the sample composition under study. This is particularly true of NOz.
Table VI. Nitrogen Dioxide Source Analysis" compd, mol % HNOz NO2
HN03 18.5 12.2 f 0.8
0.4 7.4
53.7 6.3 A 0.8 13.6
Relative humidity.
9.8
23.4 77.7 f 1.7 77.7 80.7
NO
RH,O %
4.4 3.7 f 0.3 8.2 2.1
77 71 70 66
comments
188 ng/min 430 ng/min 124 ng/min 849 ng/min
NOz glass system NO, plastic system ( N = 3) NO, plastic system after several weeks NOz plastic system
Anal. Chem. 1986, 58,1537-1541
Registry No. HNOz, 7782-77-6; HN03, 7697-37-2; NOz, 10102-44-0; HCN, 74-90-8; HSCN, 463-56-9; NO, 10102.43-9; NaCN, 143-33-9; KSCN, 333-20-0; H2Cz04,144-62-7; NaN03, 7632-00-0; KNOB,7757-79-1. LITERATURE CITED (1) Johnston, H. .; Greham, R. Can. J . Chem. 1974, 52, 1415-1423. (2) Cox, R. A.; Derment, R. G. J . Photochem. 1076/1077, 6 , 23-24. (3) Braman, R. S.; Shelley, T. J.; McCienny, W. A. Anal. Chem. 1982, 38
358-364. (4) Braman, I?.S.; de la Cantera, M. T.; Han, Q. X. Anal. Chem. 1006, 58, 1637-1541.
1537
(5) King, G. W.; Moule, D. Can. J . Chem. 1062. 4 0 , 2057-2065. (6) Braman, R. S.; Trlndaie, M. Han, Q. X. "A Sequential and Specific Hollow Tube System for Tracing Nitrogen Compounds in Air". Presented at the Third National Symposium on Recent Advances in Pollutant Monitoring of Amblent Air and Stationary Sources, May 3-6, 1983.
RECEIVED for review November 19, 1985. Accepted January in part by the Air Re277 1986* This work was sources Board, State of California.
Sequential, Selective Hollow Tube Preconcentration and Chemiluminescence Analysis System for Nitrogen Oxide Compounds in Air Robert S. Braman* and Maria A. de la Cantera
Department of Chemistry, University of South Florida, Tampa, Florida 33620 Qing Ximg Han
Fushun Petroleum College, Peoples' Republic of China
A serles of hollow tubes havlng, In sequence, tungstlc acld, potasslum-iron oxide, copper( I)Iodide, and cobah( I II) oxide interior coatings preconcentrate, in order, "OB, HNO,, NO2, and NO from amblent air. Thermal desorption and detection of the NO released by a chemiluminescence detector provided an analysts of air for these NO, compounds ai subparts-per-billion concentrations. Ammonla assoclated with these analytes may also be determlned. Initial use of the method lndlcates that "0, Is a major NO, component and that the Saltzman method for NO, measures the sum of NO2 and "0,.
The importance of nitrogen oxide compounds as a component of air pollution is evidenced by extensive monitoring and research on the subject. The main NO2 and NO, monitoring approaches have employed the use of colored dye formation by nitrites, Le., the Saltzman method or its modifications (1-3) and the chemiluminescence monitor (4, 5). Automated chemical analysis methods have also been reported (6). Although some attempt at detection selectivity has been made, the term NO, has appeared, recognizing the difficulty of selectively detecting "OB, HNOZ,NOz, and NO in air by these methods. The colorimetric method (I), which is considered to be selective for NO2, actually analyzes the sum of HNOz and NOB. The chemiluminescence monitors employ catalytic reactors and attempt to be selective for NO2 and NO by selection of the gas route through a molybdenum catalyst bed in the analyzer. The fate of peroxyacetyl nitrate (PAN) and HNOz in such a system is not clear and likely is split between detection as NO and NOz and nondetection if the molybdenum causes reduction to N2, More recently considerable effort has gone into the devblopment of specific methods for nitric acid and particle nitrate in air. The so-called denuder difference methods (7) measure the difference between total nitric acid plus nitrates collected on a nylon filter and the nitrates only collected on a nylon filter after removal of nitric acid on a MgO interior 0003-2700/86/0358-1537$01.50/0
coated hollow tube. Hollow tube methods have also been developed using nylon coatings (8),tungstic acid (9), and sodium carbonate (10, 11). The use of sodium carbonate (11,12)interior coated hollow tubes provides also a possible method for nitrous acid as the tube coating can be analyzed by ion chromatography. The integrity of the collected nitrates and nitrites on long exposure to photochemical oxidants during sampling is unknown and bears examination. Long-path optical absorption methods have been developed and studied (13, 14). While this may be the best approach when the concentrations are above 1 ppb from the point of view of selectivity, the method is complex experimentally and requires substantial equipment. Continuation of research on selective hollow tubes for NO, compounds (15) has led to the development of a series of tube coatings that when used in proper sequence, can separately preconcentrate "OB, HN02, NO2, and NO. This work started with the development of the tungstic acid tube system for nitric acid and ammonia in air (9) and continued with work on NiO tubes and Co203tubes for NOz and NO in air (15). The development of a high-purity source of HNOz in air and NO2in air (16) was key to the study of candidate selective hollow tube coatings. During the development of the hollow tube system reported here a number of different tube coatings were prepared and tested for NO, absorption selectivity. These included carboh, MgO, A1203,Ni, NiO, Cu, CuO, MnO, Fez03,Na2C03,NaOH, W03.Hz0, CozO3, and mixed oxides such as K20.Fe203and Na20Coz03,the latter being prepared from metal complex salts. Most of the above except the tungstic acid, mixed oxide, and molybdic acid tubes were found to absorb to some extent both HNOz and NOz. Nitric acid is absorbed at least to some extent by all of the tube coatings. Nitric oxide was found to be absorbed only by a specially prepared cobalt(II1) oxide. The mixed oxide tubes were specific and selective for HNOz but only after removal of nitric acid by a tungstic acid tube. After establishing the sequential tube order of W03, POtassium-iron oxide (KFe), NiO, and CoZO3(C0)tubes for isolating the NO, analytes, tube capacity and other tests were 0 1986 American Chemical Society
