Flame Chemiluminescence Detection of Nitrogen Compounds K. J. Krost, J. A. Hodgeson, and I?.K. Stevens Division of Chemistry and Physics, Environmental Protection Agency, National Environmental Research Center, Research Triangle Park, N.C. 2771 1
The detection of nitrogen compounds by flame chemiluminescence is described. A hydrogen-rich oxy-hydrogen flame was used as the medium for excitation of characteristic nitrogen bands. The emission observed in the reaction between atomic hydrogen and NO was viewed photometrically above the flame at 690 nm. This paper describes the experimental apparatus and the conditions used in observing the chemiluminescent emissions. A description of detector design parameters and response characteristics is included. The sensitivity of the detector for sulfur compounds was determined by employing a second filter-photomultiplier combination, which viewed the sulfur emission at 394 nm. The detection limits of the apparatus described are 0.150 ppm for nitrogen oxides and 0.004 ppm for sulfur dioxide.
Specific detection of atmospheric nitrogen compounds, including low concentrations of nitrogen oxides, is of considerable interest in determining air quality. Recently, gas phase chemiluminescence approaches have been used in the development of specific and sensitive monitors for the oxides of nitrogen. Fontijn et al. ( I ) described a nitric oxide detector that used the chemiluminescent reaction between ozone ( 0 3 ) and nitric oxide (NO). Wooten and Snyder ( 2 ) developed a chemiluminescent detector for total oxides of nitrogen, NO NOz, which employs the chemiluminescent reaction between atomic oxygen and NO. Both techniques operate at low pressure (1 to 5 Torr) and require the addition of a reactive species, Le., 0 3 or oxygen atoms. Several oxy-hydrogen flame emission detectors have been developed for the detection of compounds or classes of compounds. Crider ( 3 ) discussed detection of organic halides, sulfur compounds, and phosphorous compounds by flame emission photometry. Dagnall et al. ( 4 ) examined the variables affecting the chemiluminescence from sulfur compounds in the hydrogen-rich flame. Brody and Chaney ( 5 ) described a monitor that used the hydrogenrich flame for detection of sulfur and phosphorous compounds. In the hydrogen-rich flame, sulfur compounds burn to yield chemiluminescence that is characteristic of the diatomic sulfur molecule, S z . The emission is observed through a narrow band filter a t 394 nm. Phosphorous compounds bum to yield chemiluminescence from a n excited HPO molecule. The emission is observed through a narrow band filter a t 526 nm. These detectors normally view the chemiluminescent emissions above the center cone of the flame. Braman (6) discussed emission from hydrocarbon and oxy-hydrocarbon compounds in flame plasma. Several bands that might be used to detect
+
(1) A. Fontijn, A. J . Sabadell, and R. J. Ronco. Anal. Chem., 42, 575 (1970). (2) A. 0.Snyder and G. W. Wooten, Final Report, Contract No. CPA 22-69-8, Environmental Protection Agency, Research Triangle Park, N.C.. 1969. (3) W. L. Crider, Anal. Chem., 41, 534 (1969). (4) R. M. Dagnall, K . C. Thompson, and T. S. West, Analysf (London), 92, 506 (1967). (5) S . S . Brody and John E. Chaney, J. Gas Chromafogr.,4, 42 (1966). (6) R. S . Braman, Anal. Chem., 3 8 , 734 (1966). 1800
classes of hydrocarbon compounds were observed. We have used a scanning monochromator t o examine the spectral emission from various atmospheric pollutants in the oxy-hydrogen flame. The infrared emission characteristic of the HNO molecule was observed when NO or NO2 was introduced into the flame. This chemiluminescence probably results from the bimolecular reaction between H atoms and NO, the mechanism of which has been described by Clyne and Thrush (7). H NO-HNO*
+
HNO*---+HNO
+
hv (660 to 770 n m )
(2)
The reaction is first order in both reactants, i.e., the intensity is proportional to the H atom and NO concentrations.
I
= Io[H][NO]
(3) An investigation of the sulfur response was initiated in view of the similar operational flame parameters and the possibility of dual detection.
EXPERIMENTAL A Heath EU-700 scanning monochromator was used to obtain the emission spectra in the flame and to scan the chemiluminescence obtained in the low pressure, gas phase reaction between H atoms and NO. Two photomultipliers were used-an EM1 9558QA for nitrogen compounds and an EM1 9524 for sulfur compounds. Electrometers employed were a Pacific Photometrics Model 110 and a Keithley Model 417. High voltage supplies for the photomultipliers were a Hewlett-Packard 6110A and a Wabco Model LL-1100-1. The atomic hydrogen used in obtaining the H + NO chemiluminescence spectrum was produced with a Raytheon 2450 Mc generator coupled to an Evenson (8)cavity. The chemiluminescence from nitrogen compounds was viewed a t 690 nm through a narrow band interference filter with a 100nm half-width (Infrared Industries Inc., Waltham, Mass.). This filter was blocked on the blue side with transmission less than 0.1% to 200 nm. Additional blocking was obtained with a 650-nm Coming cut-off filter (C. S. 2412). The chemiluminescence from sulfur compounds was viewed a t 394 nm through a narrow band optical filter with a half-width of 14 nm (Baird Atomic). A Model LCT Bell and Gossett air pump was used for gas sampling. The flow controllers used were Fairchild Hiller's Model 59. Standard flow meters from Fischer-Porter and Brooks were employed. Flame and carrier gases used were J. T. Baker extra dry hydrogen, J. T. Baker prepurified nitrogen, J. T . Baker ultra-pure argon, J. T. Baker commercial grade carbon dioxide, and Bureau of Mines Grade A helium. Interference gases used without further purification are listed in Table I. Concentrations of NOa, "3, and monoethylamine in the sub-parts per million range were generated by the permeation tube technique (9). High gas concentrations used in interference studies were generated with an exponential dilution flask (10).
RESULTS AND DISCUSSION Flame Spectra. A scanning monochromator and redsensitive photomultiplier were used to investigate the (7) M. A. A. Clyne and 8. A. Thrush, Discuss. Faraday SOC.,33, 139 (1962). (8) F. C. Fehsenfeld, K. M. Evenson. and H . P. Broida, Rev. Sci. lnsbum., 36,294 (1965). (9) A. E. O'Keeffe and G. C. Ortman, Anal. Chem., 3 8 , 760 (1966). (10)J. E. Lovelock, Anal. Chem., 33, 162 (1961).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
Table I. Interference Gases Used Grade Gas
cis-2-Butene Methane Benzene
99.82% 99.99%
Source Air Products, Inc. Air Products, Inc.
Spectranalyzed
Fisher Scientific Co.
Acetone
ACS ACS analytical
Carbon monoxide
reagent grade C P grade
Ammonia
Ultrapure
Monoethyl amine Nitrogen dioxide Nitric oxide
98.5% 99.5% 99.0%
Mallinckrodt Chemical Works Air Products and Chemicals Air Products and Chemicals J. T. Baker J. T. Baker J. T. Baker WAVELENGTH,
i
Spectra of NO flame chemiluminescence a n d NO -I-H low pressure chemiluminescence Figure 1.
spectra obtained from nitrogen in the hydrogen-rich flame. A complex series of bands was observed between 650 and 760 nm when NO was introduced into the flame. A tentative identification of the emitting species, HNO, was obtained by reference to Pearce and Gaydon ( 1 1 ) . The band spectrum of HNO was originally identified by Clyne and Thrush (7) who generated chemiluminescence from the HNO molecule in the low pressure reaction between hydrogen atoms and NO. In their work, hydrogen atoms were generated with a radio frequency discharge and were pumped downstream, where they were mixed with NO in a flow reactor. The flow reactor was coupled to a photomultiplier and various filters were used to measure the positions of the chemiluminescence band maxima. The band maxima observed by Clyne and Thrush are in substantial agreement with those we observed in the flame chemiluminescence of nitrogen compounds. T o confirm the identity of the emitting flame species, we reexamined the chemiluminescence obtained in the low pressure, bimolecular reaction between hydrogen atoms and nitric oxide. The chemiluminescent emission in the low pressure H NO reaction was scanned with the same monochromator-photomultiplier combination used in obtaining the flame spectrum. Hydrogen atoms, produced by a microwave discharge, were mixed in a flow system with NO in a 1-1. spherical reaction vessel, which was coupled to the entrance slit of the monochromator. The flame chemiluminescent spectrum of NO is compared with the H NO chemiluminescence in Figure 1. The similarity between the two spectra is further evidence that the emitting species in both cases is an excited HNO molecule. The differences observed are probably due to the different conditions of temperature and pressure, ie., 1 atm and elevated temperature in the flame and 5 Torr and ambient temperature in the flow reactor. We have found that the intensity of the flame chemiluminescence is directly proportional to the concentration of NO (cf. Equation 3). The same flame chemiluminescence spectrum was produced with either NO or NOz. Furthermore, the intensity of the chemiluminescence, per unit concentration, was the same for NO or NOp. The emission obtained with NO2 resulted from the rapid conversion of NO2 to NO by hydrogen atoms present in the flame (12). H + N 0 2 d N O + OH
TO AIR PUMP
PHOTO
+
+
h = 2.9
X
10'ol./mol sec
(4)
Pearce and H. G. Gaydon. "The Identification of Molecular Spectra," 3rd ed. Chapman and Hall, London, 1965. (12) L. F. Phillips and H. I . Schiff, J. Chem. Phys., 37,1233 (1962). (11) R. W. B.
IPLIER
- J I b l l L e S A M I Y E
Figure 2.
INLET
Detector schematic
After confirming that a characteristic emission is obtained from the oxides of nitrogen, we constructed a flame detector, the design and response characteristics of which are discussed below. Detector Design and Response Characteristics. Several different detector configurations were evaluated during the course of this work. A shielded flame, similar to that used by Brody and Chaney, provided the maximum signal-to-flame background ratio and is presented here. A schematic of the burner design used in evaluating response characteristics for nitrogen detection is shown in Figure 2. The reactivity of nitrogen dioxide at low levels with metal surfaces dictated the use of an all-glass burner. The burner was mounted inside a stainless steel housing, which provided protection from stray light and coupling to filters and photomultipliers. The burner was held in place by a series of rubber 0 rings. The flame barrel was recessed into a cup design and shielded from the photomultipliers. Tank hydrogen (200 to 300 cm3/min) flowed through a needle valve and flow controller into the side port and diffused around the flame tip. The sample air or the carrier gas plus pollutant was pulled through the barrel with the air pump and mixed with hydrogen in the recessed area. The photomultiplier observed the chemiluminescence approximately 0.5-in. above the center of the
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 11, SEPTEMBER 1973
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PHOTOMULTIPLIER
Table II. Gases Used in Evaluating the Effect of Inert Diluent on Response Factor
D ~ T ~PHOTOMULTIPLIER ~ ~ m TUBE
Third body, M
Argon
K=?l ELECTROMETERS
RECORDERS
Helium
Figure 3. Component diagram for dual detector OXYGEN FLOW, cc mtn
IZ
IS
la
21
24
27
30
Carbon dioxide 3.6W
P
L1
5 3.m
0.000 0.154 0.349
0.500 0.635 0.740
0.000 0.154 0.349 0.500 0.635 0.740
0.000 0.154 0.349
0.0360
P I
Fractional concn, [ M J t [M -+ N21
0.0300 5
0.500
z
e
0.635 0.740
o
60
120
IBO
300
240
HYDROGEN FLOW, cc
360
IZO
480
540
600
660
min
Figure 4. Response factor as a function of H2 and 02 flow
/
.&
‘.20td
02 . I9 cc m m n2 ,650 cc rnin
0.40
NITROGEN FLOW, LC am
Figure 5. Response factor as a function of N z flow
flame. The recessed cup design minimized the flame background signal and confined the hydrogen to the proximity of the barrel. This minimized the flow necessary to attain optimum sensitivity. A schematic of the total system used for the flame detection of nitrogen and sulfur compounds is shown in Figure 3. The dual photomultiplier design enhanced the similarity of operating conditions to provide the optimum flame detection of both classes of compounds. The photomultipliers were mounted on opposite sides of the flame afterglow and observed different spectral regions through interference filters. The power supply, electrometer, and readout systems for both channels can be similar. Low concentrations of NOz, sulfur dioxide (SOz), and ammonia (“3) in air were generated by permeation tube 1802
Background current, nA 46 42 42 45 45 45 40 38 36 44 42 68 45
60 68 77 74 75
Response factor 0.076 0.080 0.077 0.074 0.074 0.074 0.074 0.079 0.080 0.067 0.062 0.044 0.078 0.079 0.080 0.067 0.062 0.044
techniques (9). Concentrations were varied by changing the air flow rate over the tubes. Variable concentrations of other potential interferences were produced with an exponential dilution flask (IO).The response time of the flame detector was sufficiently fast to follow the exponential decay in the concentration of the flask exit. The following components were introduced by means of the exponential flask: cis-2-butene, methane, benzene, acetone, carbon dioxide, and carbon monoxide. The response characteristics were determined by the effects of variable oxygen, hydrogen, and inert gas flow rates on detector output. The parameter used to evaluate response characteristics is the response factor R. This is defined as the ratio of detector signal, in nanoamperes per part per million of NO, (nA/ppm), to the flame background signal. n e t response, nA R = (5) flame background, nA, x[NO,], ppm Figure 4 shows the variation of response factor as a function of hydrogen flow variation and as a function of oxygen flow variation. Hydrogen flow was the least critical parameter in optimizing the response factor. A hydrogen flow rate of 250 to 300 cm3/min was sufficient to attain optimum sensitivity. Below 200 cm3/min, there was a sharp loss in detector performance. The oxygen flow required to maximize the response factor was the minimum flow necessary to sustain combustion. In our experiments, a flow of 19 cm3/min was required. Figure 5 shows the variation of response factor with nitrogen flow. There was a definite improvement in detector performance when a nitrogen-rich atmosphere was used. In atmospheric sampling, the oxygen-to-nitrogen ratio was not controlled since air was pulled through the detector with no nitrogen premixing. There is little loss in response from the optimum Oz/Nz ratio if ambient air is used. Table I1 represents a compilation of gases other than nitrogen used in evaluating the effect of inert diluent on the response factor. Nitrogen gave the lowest background and highest response factor. This “third body” probably influences the flame temperature and the formation and quenching of the excited HNO molecule. Dagnall e t al. ( 5 ) in their work in sulfur compounds also noted a significant background reduction using a nitrogen- or argon-cooled flame.
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 11, SEPTEMBE.R 1973
In the cooled, hydrogen-rich flame, nitrogen fixation to NO should be minimal. Even in the cool flame, production of trace concentrations of nitric oxide is thermodynamically feasible. Trace concentrations of NO could be responsible for some or all of the flame background. Such a mechanism, however, would not explain the high background observed with other inert diluents, such as helium, in which nitrogen could have been present only as a trace impurity. The background signal was apparently due to emission from some flame components because it disappeared as soon as the flame was extinguished. Thus, high frequency black body radiation from hot detector surfaces was not a probable cause for the background signal. Calibration curves for NO and NO2 were determined using the optimum flow conditions defined above. Optimum flow conditions were 250 cm3/min of hydrogen and 100 cm3/min of air. Emission intensity as a function of NO and NO2 concentrations was observed through the 690-nm filter. A logarithmic response curve as a function of NO2 concentration is shown in Figure 6. The upper limit of linear response was 60 ppm, the flame background was 4.6 nA, and the flame noise was 0.125 nA when an electronic time constant of 6 sec was employed. The sensitivity, which is defined as that concentration of NO2 (NO) that gives a response equal to twice the flame noise, is 0.150 ppm. An equivalent response curve was obtained for NO. Interference Studies. Table I11 is a compilation of compounds evaluated as potential interferences in the flame chemiluminescence detection of NO,. The term "interference equivalent" is used as a measure of the degree of interference and is defined as that concentration of interferent necessary to produce a response equivalent to 1.0ppm of NO,. All nitrogen compounds evaluated gave a n interference equivalent of 1.0. Ammonia and organic amines were probably oxidized to NO or NO2 in the flame. Nitrogen dioxide was converted to nitric oxide by rapid reaction with hydrogen atoms. The behavior of organic nitrates and nitrites in the flame has not been evaluated. Nitrous oxide has not been tested as an interference. This oxide is always present in a background concentration in ambient air, and no background signal, above the flame response, has been observed when NO and NOa are absent from the air sample. Thus, nitrous oxide is apparently stable in the low temperature flame. Among the hydrocarbons evaluated, cis-2-butene showed the lowest interference equivalent. Sulfur dioxide gave an interference equivalent of 2.0 a t 0.200 ppm Son. Based on present evidence, measurement of the HNO flame chemiluminescence has provided a signal that is proportional to total gas phase concentrations of nitrogen compounds in the atmosphere (excluding Nz and NZO). With the exception of sulfur dioxide, no significant interferences have been observed from other gas phase species at ambient concentrations. Sulfur Detection. The optimum conditions for the flame chemiluminescence detection of nitrogen compounds are essentially the same as used in the flame photometric detection of sulfur compounds (3-5). Simultaneous detection of nitrogen and sulfur compounds should thus be feasible. A second photomultiplier and filter for sulfur emission were attached opposite the nitrogen combination to view the same flame area. The emission spectrum of SZ shows a series of evenly spaced bands between 350 and 450 nm. The sulfur emission was monitored through a narrow band filter, which transmitted a strong band at 394 nm. A calibration curve for SO2 was obtained using the same detector and condi-
100.0
X
I
/A
10.0
1
I
I I
NITROGEN RESPONSE CURVE I VS. C SULFUR RESPONSE CURVE,dVS. C
I I I
(BACKGROUND 0.83 na NOISE 0.05 naI
I I I
I I
I I
\ I
CONCENTRATION IC], PQm
Figure 6.
Flame response curves for NO2 and SO2
Table Ill. Evaluation of Compounds as Potential Interferences in the Flame Chemiluminescence Detection of NOx Component
cis-2-Butene Methane Benzene Acetone Carbon dioxide Carbon monoxide Ammonia Monornethyl amine S u l f u r dioxide
Concn, PPm
500 500 500 500
Interference equiva (NO detector) 1,600
9,000 18,000
9,000
1250
Infinite
1500 1 .o
10,000 1 .o
3.0
1.0
0.1 26
2.0
Interference equiv (SO2 detector) 14,400 45,000
30,000 45,000 100,000 66,000
NRb NRb 1.0
a Interference equivalent is that concentration of interferent necessary to produce a response equivalent to 1 .O ppm of NO,. NR = no response.
tions described previously for optimum detection of NO,. A permeation tube source was used to generate variable SO2 concentrations. The calibration curve for SO2 is shown in Figure 6. The response was proportional to the square of the sulfur concentration. The sensitivity for sulfur detection was 0.004 ppm. At this concentration, a 2to-1 signal-to-noise ratio was obtained when a 4-sec electronic time constant was used. Because of previous extensive evaluations by Dagnall ( 4 ) and Brody ( 5 ) , no additional work was done on characterization of the flame chemiluminescence detection of sulfur compounds. SUMMARY AND CONCLUSIONS The development of the first flame emission detector for nitrogen compounds has been described. The operating principle of the detector is based on the infrared emission from the excited HNO molecule produced in the hydrogen-rich flame. The detector in its present stage of development has a sensitivity of approximately 0.15 ppm of
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 11, SEPTEMBER 1973
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NO, and an equivalent sensitivity for the other nitrogen compounds that have been tested. By attaching a second appropriate photomultiplier and filter combination, simultaneous detection of nitrogen and sulfur compounds was achieved. The sulfur channel of this dual detector has a sensitivity for so2 of 0.004 ppm. The detector as described is not applicable to ambient air measurements because of inadequate sensitivity and the interference from sulfur dioxide. Further areas of development exist that should yield increased sensitivity and specificity. For example, different regions of the HNO emission spectrum should be examined and compared. The use of correlation techniques to view a wider range of spectral emission and to discriminate against background may be of considerable value.
Even in its present stage of development, this detector should be useful for many applications. The detector has more than adequate sensitivity for the higher concentrations of nitrogen oxides that are present in source emissions. For example, the method should be applicable to the measurement of NO, emissions from light duty gasoline powered vehicles, in which NO, levels are high and SO2 is absent. With additional refinement, it may also be useful as a nitrogen compound detector in gas chromato: graphic applications. Received for review January 15, 1973. Accepted April 4, 1973. Mention of commercial products or brand names does not constitute endorsement by the Environmental Protection Agency.
Accurate Determination of Copper in Mixtures and Ores by Radioisotope-Excited X-Ray Fluorescence Spectrometric Analysis Using Peak Ratios Cesia Shenberg, Aharon Ben Haim, and Saadia Amiel Nuclear Chemistry Department, Soreq Nuclear Research Centre, Yavne, I s r a e l
In a solution containing a single predominant element, the ratio of an X-ray fluorescent line to a target backscattered line varies linearly with the concentration of the element over a wide range. However, the presence of an additional component of lower 2 distorts the proportionality. This work shows that accurate analysis can be carried out in mixtures by suitable treatment of the data. A rough estimate of the lower 2 component was obtained by the usual peak ratio method. The ratio of its fluorescent line to the target backscattered line was largely independent of the concentration of the higher 2 component. This fact and a comparison of mixture solutions of various concentrations with single-element solutions of similar concentrations permitted correction for the distortion introduced by the presence of the lower 2 element. The case studied was the mutual influence of Cu and Fe on the ratios between the fluorescent K X-rays of the individual elements and the backscattered target X-rays in various Cu-Fe mixtures. The X-rays were excited using an 241Am-As source-target assembly and measured with a Si(Li) detector. Using the new method of calculation, satisfactory analytical results were obtained for solutions of 0.6 to 25 w/v YO Cu in the presence of 0.4 to 23 w/v YO Fe. The procedure was extended to solid mixtures and ores containing up to 80% Cu.
Fe-containing Cu ores have been studied by Dziunikowski and Clayton ( I ) using a 3T/Zr source and NaI(T1) scintillation detector. Their method was based on the evaluation of peak ratios obtained from a series of measurements made with different filters. A somewhat differ(1) B. Dziunikowski and C. G. Clayton. AERE-R5914 (1969).
1804
U. S. At. Energy Comm. Rep.,
ent way of using the peak ratio technique has recently been described by Burkhalter (Z),who analyzed silver ores using the Te X-rays from an 1251 source for excitation. In this particular case, the Compton backscattered Te K Xrays were evaluated as internal standards for matrix compensation. A correction procedure for the compensation of interelement effects in X-ray spectrometry was reported by Lucas-Tooth and Pyne ( 3 ) .They describe a method for the accurate determination of major constituents, based on the differences in intensity of the interfering element in the standard and the sample. Lachance and Traill ( 4 ) developed a calculation procedure by using concentrations instead of intensities. A graphical determination method according to Lachance and Traill ( 4 ) was given by Jenkins and Campbell-Whitelaw ( 5 ) . The analytical usefulness of peaks and peak ratios was studied previously (6) and in a single-element solution the ratio of an X-ray fluorescent line to a backscattered line of the target was found to vary linearly with the concentration of the element over a considerable range. The measurement of such ratios could be used to determine elemental concentrations, as illustrated with uranium solutions. However, not all lines are suitable for this determination. For example, individual peaks of uranium La, LP, and Ly lines and their respective ratios to the backscattered K a and KP lines of the iodine target were meaand U LPz/I sured, and only the ratos U La/I KaincOherent KaincOherent varied linearly with uranium concentration over a wide range-about four orders of magnitude (0.11000 mg U/ml). Higher energy uranium L lines could not (2) P. G. Burkhalter, Anal. Chem., 43, 10 (1971). (3) J. Lucas-Tooth and C. Pyne, Advan. X-ray Anal., 7, 523 (1963). (4)G.R. Lachanceand R. J. Traill, Can. Spectrosc., 1 1 , 43 (1966). (5) R. Jenkins and A. Campbell-Whitelaw, Can. Spectrosc., 15 ( 2 ) , 1 (1 970). (6) C. Shenberg and S. Amiel, Anal. Chem., 43,1025 (1971).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
