Gas Chromatographic Separation of Oxides of Nitrogen J. M. TROWELL Hercules Powder Co., P. 0. Box 98, Magna, Ufah By a unique arrangement of columns, combined with subambient temperature gas chromatography, a method has been developed for the determination of Hz, 02,Nz, NO, CO, NzO, COS, CZH~, CZH4 CZHZ, and NOz(Nz04). The separation is accomplished using three columns in series. The first column temperature is subambient for the separation of NOz(N2O4) and water. The remaining two columns separate the other components in the gas sample. To obtain the complete analysis from one injection it is necessary to fabricate, or modify, a commercial gas chromatograph to accomodate two detectors and two columns. The schematic of a relatively cheap and simple gas chromatograph developed for this analysis is presented.
E
and related industries have had a definite need for methods of analyzing nitrogen oxides. One analytical technique which has been extensively investigated is that of gas chromatography. However, the reported methods have been limited either to the determination of nitric and nitrous oxides with the exclusion of nitrogen dioxide, or to the determination of nitrogen dioxide with the exclusion of nitric and nitrous oxides. Two samples have therefore been required using gas chromatography methods of analysis. The more common methods reported have been at the exclusion of nitrogen dioxide. The work of Frazer and Ernst ( 1 ) is one of the more recent. They separated nitric oxide and nitrous oxide from a mixture of other gases by using a silica gel column in series with a molecular sieve column. Greene and Pust (Z), and more recently Morrison, Rinker, and Corcoran (S), have reported the analysis of nitrogen dioxide in air. Greene and Pust used a wetted molecular sieve column for their analysis; Morrison et al., used a packed column of SF-96 for the separation. A method for analyzing nitric oxide, nitrous oxide, and nitrogen dioxide in a mixture of other gases from a single sample has not been reported. This paper presents a gas chromatographic procedure for determining nitric oxide, nitrogen dioxide, carbon dioxide, carbon monoxide, oxygen, and nitrogen in a sample using series columns with dual detectors to complete the analysis.
RECORDERS COLUMN NO.I*
COOLED AREA
HELIUM GAS
ANALYTICAL CHEMISTRY
CONTROLS
REGU LATl N G
+w--T DETECTOR
I TO REFERENCE
REF. I N
*COLUMN NO. 1-112 CARBOWAX ON G L A S S BEADS COLUMN N O . 2 - 4 0 % DMSO ON GAS CHROME R Z COLUMN NO. 3-13 X MOLECULAR SIEVE
XPLOSIVES
1 152
El
Figure 1.
REF. OUT
REF.OUT
System schematic
EXPERIMENTAL
Apparatus. A gas chromatograph consisting of a series arrangement of three columns and two detectors 'was fabricated. T h e detectofs were GowM a c Model T R l l B thermal conductivity cells with W-2 filaments. T h e detector signal outputs were fed to a Varian two-pen recorder, 0-5 mv. full scale. T h e supply of helium was dried by passing it through a -76' C. cold t r a p packed with 13X molecular sieve. T h e system schematic is shown in Figure 1. Columns. Column No. 1 was a 1-foot length of '/a-inch stainless-steel tubing packed with 0.5% Carbowax 1500 on 60- to 80-mesh silanized glass beads. The column was prepared using standard evaporation procedures. To treat the column, 1 cc. of nitrogen dioxide was passed through the tube. The column was then heated to 100' C . and purged with helium. Column No. 2 was a 20-foot length of 1/4-inch stainless-steel tubing packed with 40% dimethyl sulfoxide (DMSO) on 60- to 80-mesh Gas Chrom RZ. Evaporation procedures using acetone as the solvent were also used to prepare this column; however, Column 2 was packed using the Matronic XL300 pressurized column packer a t a packing pressure of 50 p.s.i.g. As will be shown, this column was superior to a silica gel column for separating nitrous oxide and carbon dioxide. Column No. 3 was an 8-foot length of '/(-inch stainless-steel tubing packed
with 30- to 60-mesh 13X molecular sieve. This column was also packed using the Matronic XL300 pressurized column packer a t a packing pressure of 50 p.s.i.g. T o obtain optimum separation, the column was activated a t 250' C. for 5 minutes with a rapid helium purge of approximately 200 cc. per minute. Overactivation resulted in considerable tailing of the nitric oxide peak. Molecular Sieve 5A was investigated as a possible packing for Column 3 ; however, a prohibitive amount of tailing was found with nitric oxide. Procedure. A helium flow of 75 cc. per minute was established through the columns. Column 1 was controlled at a temperature of -76' C., and Columns 2 and 3 were maintained a t 25' C. A gas sample was introduced into the system a t point A (Figure 1). Injection can be made by a n appropriate means; however, for this study a gas sampling valve was used. The sample volume varied with the type of sample under analysis. When analyzing for trace quantities of the nitrogen oxides, up to 25 cc. were injected with satisfactory results. The injected sample was carried by the helium through Column 1. This column removed the nitrogen dioxide and water vapor from the helium stream. The remainder of the sample was carried to Column 2 where nitrous oxide and carbon dioxide were separated and chromatographed. The exhaust of Detector No. 1 then passed through Column 3 where the order of
" 7
0
20
IO
Figure 2.
30
40
30
€0
80
70
Gas chromatogram; air, NzO,
coz,
W
100
and NOn
o
IO
Figure 3.
20
30
40
50
60
m
80
90
100
Gas chromatogram; air, NzO, COz, and NO2 (NO, peak position chonged)
elution was hydrogen, oxygen, nitrogen, nitric oxide, and carbon monoxide. After elution of carbon dioxide from Column 2 and carbon monoxide from Column 3, the three-way valve No. 1 was turned to channel the helium flow past Column 2, thereby directing the helium flow through a second regulating valve, No. 2. Valve No. 2 reduced the helium flow from 75 cc. per minute t o 30 cc. per minute. After the momentary upset due to the helium flow changes, Column 1 was rapidly heated to 70' C. The nitrogen dioxide and water were vaporiLed and carried directly to Detector 1. (If a measureable peak is desired for water, the column can be heated to 100' C. after elution of the nitrogen dioxide.) Following elution of the water, the three-way valve was returned to the original position. If the sample contained hydrocarbons, such as C i s , CqJs,or acetylene, these components were then eluted from Column 2. The tubing connecting Columns 1 and 2 to the detector was kept at 150' C. by wrapping with a voltage-controlled heating tape. RESULTS AND DISCUSSION
The principle of operation of Column 1 for the separation of nitrogen dioxide
from the other gases was not unique. This technique has been used routinely in the past for the removal of unwanted or interfering materials in carrier gas and samples. However, in this analysis, the trapped components were of interest
other than water, the common tubing between Columns 1 and 2 could be packed and the temperature adjusted from 150' C. to yield the desired separation. A second method which was used for greater separation was to lengthen Column 1. The packing was changed to only glass beads and the column temperature was crudely programmed from -76" C. to 100' C. However, if any one of these modifications is incorporated, time must be allowed for the complete elution of water from the extra packed column. Any water remaining on the column will cause nonreproducible results. A typical gas chromatogram of a mixture of air, nitrous oxide, carbon dioxide, and nitrogen dioxide is shown in Figure 2. There was slight tailing of the nitrogen dioxide peak. Another chromatogram of the same gases is shown in Figure 3 ; however, the position of the nitrogen dioxide peak was changed, from the last peak to the first peak. Figure 4 is similar to Figure 3, except the position of the nitrogen dioxide peak was changed to the second peak. The switching of the nitrogen dioxide peak's position was accomplished by simple manipulation of the bypass valves. A tabulation of peak area us. sample size of nitrogen dioxide is shown in
and were subsequently analyzed. The temperature of -76" C. was chosen because nitrogen dioxide would be adequately removed with very little chance of components freezing, other than water. Mass spectrometer and infrared analysis had shown that the remaining decomposition gases of explosives or double-base propellants would be unaffected a t -76' C. The 0.5% Carbowax 1500 on glass beads was chosen as the column packing material to aid in the separation of the nitrogen dioxide from the water and certain other solvents, such as acetone and ethyl alcohol. I n addition, this column minimized the interaction of nitrogen dioxide and water. A temperature gradient from 25' C. to -76' C. existed in the first two inches of the column. This removed the water from the sample prior to removing the nitrogen dioxide. Thus nitrogen dioxide and water were separated by a combination of temperature and column liquid phase. Therefore, any interaction of water with nitrogen dioxide must occur prior to injection in the instrument. Increased load on the glass beads or a change in the column support of Column 1 to one of the fluorocarbon types resulted in nonreproducible results. If greater separation was desired between nitrogen dioxide and some component,
8
6
z
I
3
8
4
2
INJECTION 0 0
Figure 4.
10
20
30
40
SO
60
70
80
90
0 100
Gas chromatogram; air, N20, COZ, and NO2 (NO2 peak position changed)
0
IO
~
~
I
1
20
30
C&RT
40
1
1
70
80
S P E E D 30 INCHESIHOUR I 50
60
90
100
Figure 5. Chromatogram; gases obtained under optimum conditions VOL. 37, NO. 9, AUGUST 1965
1153
Linearity of NOz Results
Table I.
Sample volume, mm. pressure 20 30 40 50 60 70
8.6
80 -.
100 120 Table It.
Component added N,
N; Nz
Nz
COI CO;
60,
coz
NOz N0z NOz NOz NOz
Peak area, sq. em. 4.9 11.6 15.9 19.8 25 0 21) 9 40.0 53.0
Method Reproducibility
Sample volume, Peak mm. height, mv. pressure 25 0 0.605 0 607 25 0 25 0 0 606 0 607 25 0 25 0 0 792 2.5.0 0.800 25 0 0 799 0 804 25 0 6 00 50 0 5 65 50.0 50 0 5 85 6 15 50 0 5 95 50 0
Peak area, sq. em. 0.443 0 442 0 443 0 442 0 135 0.132 0 136 0 131 1 56 1 51 1 52 1 60 1 55
Table I. The sample size was expressed in millimeters (mm.) of mercury, since the sample injection was accomplished using a gas sample valve connected to a vacuum gauge. A sample size of 100 rnm. would equal approximately 1 cc. of sample. The linearity of the data indicated the freeze-out technique was quantitative. The lower limit of detection for nitrogen dioxide was not determined. However, by increasing sample size or by making several injections it was found that trace quantities (p.p.m.) of nitrogen dioxide could be trapped in Column 1 and subsequently analyzed.
1 154
ANALYTICAL CHEMISTRY
The Columns 2 and 3 and conditions were carefully chosen so that the DMSO column and the 13X column could be operated in series without multiple helium flows. Both of these columns gave optimum separation a t a temperature of 25' C. with a helium flow of 7 5 cc. per minute. A chromatogram of a mixture of gases obtained under optimum conditions is shown in Figure 5 . Chart speed was 30 inches per hour. The separation of the nitrous oxide and carbon dioxide was essentially base line. Further, there was negligible tailing of either component. If ethane and ethylene had been present in the sample, the t a o peaks would have appeared between the composite and the nitrous oxide peak. Acetylene, if present, would appear 20 minutes after appearance of the nitrogen dioxide peak, as explained in the procedure. The separation obtained with the 13X column was equally satisfactory. Although the nitric oxide peak did exhibit tailing, the tailing was minimized by the stringent activation conditions. Increased activation of the 13X increased the difference in the retention times of the nitric oxide and carbon monoxide peaks; however, tailing of the nitric oxide peak was increased. The average useful life of the columns with continuous use was 3 months for the DMSO and 1 month for the 13X. The life of the 13X column can be increased by placing a three-way valve between Detector 1 and Column 3. During the first part of the analysis the valve is turned to direct the exhaust of Detector 1 to Column 3. Following the elution of the carbon monoxide from Column 1, the valve is turned so as to vent the exhaust of Detector 1 to the atmosphere, thus avoiding the contamination of the 13X with nitrogen dioxide, water, and other components retained in Column 1.
A compilation of replicate analyses of carbon dioxide, nitrogen, and nitrogen dioxide is given in Table 11. The data shot$- the 95% confidence limit of the analysis of carbon dioxide and nitrogen to be &0.70% per component at loo%, whereas the calculated 95% confidence limit for the nitrogen dioxide determination was *2.6% a t 100%. This reproducibility range was completely satisfactory for gas chromatographic analysis. The Uacchus Laboratory of Hercules Powder Co. has used the gas chromatographic method for the analysis of nitrogen oxides routinely for over one year. The method has been applied to the analysis of thermal decomposition gases of explosives, double-base propellant firing gases, and effluent gas analysis coupled with differential thermal analysis of propellant. The samples have varied from trace concentrations to extremely high concentrations of various decomposition gases in both inert and air atmospheres; in all instances satisfactory results were obtained. To date, this laboratory has not experienced any interference in the subambient column used for the separation of the nitrogen dioxide. ACKNOWLEDGMENT
The author gratefully acknowledges the suggestion of A. Z. Conner of Hercules Powder Company Research Center, Wilmington, Del., for the use of subambient removal of nitrogen dioxide and subsequent analysis. LITERATURE CITED
(1) Frazer, J. W., Ernst, R., Explosivstoffe
11. 4 i1964). (2) Greene, S. A., Pust, H., ANAL.CHEM. 30, 1039 (1958). (3) Morrison, M. E., Rinker, R. G., Corcoran, W. H., Ibid., 38,2256 (1964). RECEIVEDfor review April 1, 1965. Accepted June 10, 1965.
