Table I . Comparison of Methods for Nitrate Ppm NO,-found (S,P Sample
Synthetic Synthetic Synthetic River
Ppm NO,added
5.0 5.0 5.0
-
Polarographic, UO,CI,
5.0 (2.8%) 4.9 4.6
2.0 (2.2%)
Potentiometric
4.9 10.9 5.0 4.7
(2.4%) (4.5%) (3.2%) (7.8%)
Colorimetric, brucine sulfate
4.9 5.0 5.2 2.0
Other components
(2.4%) (2.8%) (2.3%) (2.0%)
30 ppm SO,250 ppm CI5 ppm F-
a Relative standard deviation.
5. A 0.1-ml sample of a 0.7M stock solution of ZrOCl2 (previously heated) was added, and after a short additional purge the output was read. The output normally requires about 15-20 sec to stabilize. Repetitive readings made by allowing the output voltage to stabilize after shorting s-1to ground normally agree to within 1%relative. Analysis was done by means of calibration curves. A typical calibration curve is shown in Figure 3.
Results Comparison with Other Methods. Table I shows analytical results of the U0z2+ polarographic method compared to other methods for real and synthetic samples. The high results in the potentiometric measurement are not unexpected in view of the known chloride interference in this case. A survey was carried out of water treatment effluent samples that had been previously analyzed for nitrate using the cadmium column-diazotization method. In all cases, the polarographic analysis was carried out after the colorimetric analysis a t times varying from a few hours to a few days. Since no preservative was added and there was apparent loss of nitrate, a valid comparison cannot be made. For a total of 16 samples analyzed by both methods, the median result for Zr02+ polarographic measurements was 23% low. Excellent agreement between the two polarographic methods (Zr02+ and U0z2+) were obtained in all except one of the samples. A more valid comparison between the polarographic (ZrOZ+)and the cadmium reduction method is shown in Table 11. In this case the polarographic data were obtained within 4 hr after the spectrophotometric determinations.
Table I I . Comparison of. Polarographic (ZrOCI,) and Spectrophotometric (Cd-Column Diazotization) Methods for Effluent Samples Ppm NO,-found Sample
G-704 G-682 G-692 A-459
Polarographica
Spectrophotometrica
3.1
2.9 2.6b 2.8
4.0 2.4 2.5 2.9
apolarographic determination done 4 h r after spectrophotom e t r i c . b Relative s t a n d a r d d e v i a t i o n 6.1%, s i x d e t e r m i n a t i o n s .
Acknowledgment The assistance of Carol Jeffcoat in providing analyzed water samples is gratefully acknowledged. Literature Cited (1) Coleman, D. M., Van Atta, R. E., Klatt, L. N., Enuiron. Sci. Technol., 6,452 (1972). (2) Tokuoka, M., Collect. Czech. Chem. Commun., 4,444 (1932). (3) Tokuoka, M., Ruzicka, J., ibid., 6,339 (1932). (4) Rand, M. C., Heukelekian H., Anal. Chem., 25,878 (1953). ( 5 ) Kolthoff, I. M., Harris, W. E., Matsuyama G . , J. Am. Chem. SOC.,66,1784 (1944). (6) American Public Health Association, New York, N.Y., “Standard Methods for the Examination of Water and Wastewater.” 12th ed., pp 202 and 398. (7) Wharton, H. W., J.Electroanal. Chem., 9, 134 (1965). (8) Brezina, M., Zuman, P., “Polarography in Medicine, Biochemistry and Pharmacy,” p 114 Interscience, New York, N.Y., 1958. Received for review November 6,1974. Accepted July 25,1975.
Identification of Soil Denitrification Peak as N20 P. Zimmerman’ and R. Rasmussen Air Pollution Research Section, College of Engineering, Washington State University, Pullman, Wash. 99 163
Researchers working with soil samples in closed systems should be aware of the possible production of high levels of N20 by some soils. The measurement of this compound is possible during FID gas chromatographic analysis, although high COz levels may interfere with quantitation.
Many researchers in the soil science and air pollution fields have become increasingly aware of the important role soils play in emitting and scavenging hydrocarbons in the
atmosphere. Among these hydrocarbons, ethylene is unique. It is the major component of the light hydrocarbons in auto exhaust. It is also the most actively removed light hydrocarbon from auto exhaust by soils. It has been recognized to be an important plant hormone (1). Recent evidence indicates that it may also serve as a microbial population regulator, by maintaining aerobic conditions in soil (2). Therefore, the observation of a peak just prior to and/or interfering with the elution of ethylene attracted considerable attention in our laboratory during studies quantifying the rates of emission and scavenging of light hydrocarbons in the soil airspace. Volume 9, Number 12, November 1975
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Figure 1. Detector: FID. Column: 6 ft X y8in. 0.d. Carrier gas: nitrogen at 30 mllmin. Attenuation: 1X. Sample: hydrocarbon caii-
ics, high concentrations of inorganic species, such as water, have been reported to cause an FID response (3). The FID response for oxygen and N20 has also been extensively studied. Proposed mechanisms to explain the FID signal for inorganics include: thermionic emission, chemiionization, charge transfer ionization, flame temperature response, hyperoxygenation, vacancy response, and dilution response (4). Mapy of these theories have been discounted as causative factors for the FID response to nitrogen compounds. The most likely mechanism for the FID response to nitrogen compounds such as N2O and NH3 is chemi-ionization ( 5 ) . Further information concerning the identity of the unknown component was provided through the use of a Hewlett-Packard Model 5930 quadrapole mass spectrometer and a Carle Model 800 gas chromatograph equipped with a thermal conductivity detector. A 6-ft Porapak S column operated with a helium flow of 20 ml/min at 22OC gave good separation of COS from N2O in the Carle GC. N2, NO,
Flgure 2. Sample: hydrocarbon calibration standard (1 ml) spiked
with N20 SECONDS
The flame-ionization-detector-(F1D)-equippedgas chromatograph, typically used in the analysis of hydrocarbons in auto exhaust, is an ideal instrument for the measurement of low levels of hydrocarbons in soil airspaces. The combination provides directness of operation with the sensitivity, accuracy, and precision needed for soil systems analysis at the parts-per-billion level. The specificity of the FID is often taken for granted and many researchers believe that the FID responds only to hydrocarbons. However, it will be shown that although the FID has a greater sensitivity for hydrocarbons, it may also respond to high levels of inorganic species in a quantitative manner. Results and Methods An experiment was designed to measure the variation in ethylene production for different soil systems. Five-gram soil samples were placed in separate 14-ml glass bottles, saturated with water to their field capacities, and sealed with rubber septa. Periodically, 1-ml gas samples of the head space above the soil were extracted and analyzed using a Hewlett-Packard Model 5700A gas chromatograph (GC) equipped with a flame ionization detector and a 6-ft X %-in. 0.d. Porapak N column. This column was operated at 55OC with a nitrogen carrier gas flow of 30 ml/min. Typical chromatograms (Figure 1) show a “pressure disturbance” and the sequential elution of methane, ethylene, ethane, and acetylene (if present). However, several soil samples consistently produced another compound, which eluted just before ethylene. This peak had been noted previously, correlated with anaerobic conditions, and labeled a “denitrification peak.” No hydrocarbons are known that elute between methane and ethylene under these conditions, and the fiame ionization detector is expected to respond only to C-H bonds; thus, the identity of the denitrification peak was quite puzzling. The most logical explanation seemed to be that the flame ionization detector (FID) was responding to some inorganic species. Although the FID is expected to respond to organ1078
Environmental Science & Technology
I BO
:i
Figure 3. Linear relationship of the FID response to the N20 concentra-
tion (1-ml samples)
CONCENTRATION (%)
and NO2 were eluted very early. The elution times for COz and N20 were determined using Scott Research Laboratory, Inc., calibration standards. A 0.5-ml soil head space sample collected in Pressure-Lok syringes was then injected into the chromatograph, and discrete samples of the effluent were collected at the detector exit port a t the elution times corresponding to those of COBand N20. The collected samples were sealed and immediately analyzed on the mass spectrometer. The mass spectra of the sample corresponding to the elution time of N20 indicated peaks at mass 44 and mass 30 in the ratio of 10 to 1. An N20 calibration standard containing 98% pure NzO revealed similar mass spectra and 10 to 1 ratio. The mass spectrum of the sample collected at the elution time of COz also yielded an ion of mass 44. The COz fragmentation ion of mass 28 was masked by the ubiquitous presence of N2; however, no peak occurred at mass 30. The mass spectral analysis thus verified the presence of NzO and COz in the soil headspace sample. Literature reports also document the emission of high concentrations of NzO by some soil systems. It therefore was suspected that the denitrification peak measured by the FID was the result of high levels of N20 in the soil head space. T o test this hypothesis, standard mixtures containing 10-20% NzO and CO2 were injected into the FID-equipped GC. The C 0 2 injections produced a negative response a t 100 sec after injection, while N2O produced a positive response at 110 sec (Figure 2). The NzO peak exactly matched the elution time of the denitrification peak. To determine if the peak could be used as a quantitative indicator of N20, a calibration curve was constructed by plotting peak height responses for concentrations between 0.025% and 1.96% NzO in ambient air (Figure 3). The data in this figure indicate a linear response with a minimum detectability for 1-ml sample8 close to 0.025%(250 ppm). Although the FID response to N20 appears linear and therefore quantitative, some interference may occur with samples containing high C 0 2 levels (1%).The negative response to C02 overlaps slightly with N20 elution, affecting the peak area and probably peak height (Figure 4). For this
Flgure 4. Soil head space sample (1 ml) containing high levels of Con and showing the N20 “dentrification peak”
L
i 8
Flgure 5. Carrier: helium at 10 ml/ min. Column: 6 ft X ’&in. 0.d. Attenuation: IOOX. Sample: soil headspace (0.5 ml)
6 W v)
2
0
85 W a 4
2
L
* SECONDS
reason, all samples were cross-calibrated on the Carle Model 800 gas chromatograph. Under these conditions, separation of COz and NzO could be achieved, and good quantitation was realized (Figure 5). Some soil samples showed NzO head space levels approaching 1% (10,000 PPm).
Summary A denitrification peak may be noted when soil head space samples are analyzed with a flame-ionization-detector-equipped gas chromatograph. The flame ionization detector is thought to respond primarily to hydrocarbons, yet the denitrification peak cannot be accounted for by any organic species. Mass spectroscopy coupled with gas chromatographic techniques identified the denitrification peak as NzO. A flame-ionization-detector-equipped gas chromatograph using a 6-ft Porapak N column at 5OoC responds to levels of NzO in the concentration range of 250 ppm or greater (1-ml sample). The response is linear to 2%,although COP levels greater than 1%may interfere with quantitation. Literature Cited
0
a N n * W W
>
(1) Abeles, F. B., “Ethylene in Plant Biology,” Academic Press, 1973. (2) Smith, A. M. and Cook, R. J., “Implications of Ethylene Production by Bacteria for Biological Baiance o f Soil.” Nature, Dec. 20/27 (252,703,1975). (3) . . Walker. J. 8..Jackson M. T. Y.. Mavnard. J. B.. “Chromatographic Systems,” Academic Press; 197i, pp. 159-162. (4) Schaefer, B. A., “Response of the Flame Ionization Detector t o Oxvgen and Nitrous Oxide.” J. C h r o m a t o n Sci.. 10. 110-20. Fe6ruary 1972. (5) Blades, A. T., “bsponse of the Flame Ionization Detector to Nitrogenous Compounds,” ibid , 693-5, November 1972.
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v
::
SECONDS
Received for review March 10, 1975. Accepted August 8, 1975. This project has been financed in part by the Environmental Protection Agency Grant No. AP802565. The contents do not neeessarily reflect the views and policies of the Environmental Protection Agency, nor does mention of trade names or commercial products constitute endorsement or recommendation for use. Volume 9, Number 12. November 1975
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