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ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
AIDS FOR ANALYTICAL CHEMISTS Simultaneous Determination of Trace Amounts of Hydrogen, Oxygen, Nitrogen, Carbon Monoxide, Carbon Dioxide, Methane, Ethane, Ethylene, and Acetylene by Two Gas Chromatographic Columns in Parallel and One Detector Fikry F. Andrawes" Lockheed Electronics Go., Inc., 1830 Nasa Road I , Houston, Texas 77058
Everett K. Gibson, Jr. SN7, Geochemistry Branch, NASA Johnson Space Center, Houston, Texas 77058
The determination of trace amounts of Hz, 02,Nz, CO, COS, CH4, and Cz hydrocarbons has wide application and is of interest to many analysts. Unfortunately, this suite of gases cannot be separated isothermally using a single chromatographic column. Analysis and separation of this group of gases can be carried out using temperature programming with carbon molecular sieve ( I ) or various Porapak columns ( 2 ) . However, temperature programming suffers from some limitations and is not suitable for all applications. Isothermal methods of analysis using a combination of multiple columns with one or more detectors have been previously described ( 3 , 4 ) .For example, two columns with two detectors connected in parallel (commonly called a dual system) are frequently used to separate the gases mentioned above. A molecular sieve column is used to separate H2, Oz, Nz, CH,, and CO while a Porapak or carbon molecular sieve column is used to separate COz and the Cz hydrocarbons. T h e use of a dual system requires the use of a dual channel recorder and two integrators if quantitative analysis is required. The use of two columns connected in series with two nondestructive detectors has been reported ( 3 ) ,but this arrangement is not always suitable for trace analysis and it also requires two detectors. The use of more than one chromatographic column with one detector has previously been reported ( 4 ) . In this arrangement a switching, valving system is used to allow insertion of independent columns in the carrier gas circuit. A distinct disadvantage of this system is the complicated switching required to carry out an analysis. This method of analysis cannot be used with the highly sensitive helium ionization detector because of different gas compositions a t the outlet of different chromatographic columns. This difference changes the background current in the detector and prevents proper operation. Our laboratory has been carrying out trace gas analysis in terrestrial and extraterrestrial materials (e.g., volcanic rocks, meteorites, and lunar samples). We have recently developed a gas chromatographic analysis system in which the trapped gases are released upon crushing of the sample and analyzed using a dual column-dual helium ionization detector system ( 5 ) . With the present analytical system, it is difficult to perform simultaneous analysis of any sulfur gases that may be released from the samples. Because of this difficulty, we have attempted the separation of Hz, O z , N2, CO, C 0 2 , CH4, and C2 hydrocarbons using separate molecular sieve and Porapak columns operating in parallel with a single detector; the second detector is then available for the analysis of the sulfur gases on a separate column. The molecular sieve and Porapak columns have been tailor-made so that the gases eluted from the two columns would not overlap in the detector. EXPERIMENTAL A Varian 1700 gas chromatograph equipped with helium ionization detector was used in this study. The detector was
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Figure 1. Analysis using 3 m Porapak Q with flow of 40 mL/min, and 6 m molecular sieve with flow of 55 mL/min
operated at an applied potential of 480 V. A Valco 8-port sample injection valve was adapted to the gas chromatograph with a 100-pL sample loop. In order to obtain positive responses for all of the gases of interest, 1-5 ppm hydrogen was added to the purified helium carrier gas using the permeation tube technique described previously (6). Chromatographic columns were made of stainless steel and of the following specifications: two columns of 6 m and 4.5 m long of 2.1-mm i.d. by 3.2-mm 0.d. packed with molecular sieve 13X 60/80 mesh; one column of 3 m of 1.8-mm i.d. and 3.2-mm 0.d. packed with Porapak Q 80/100 mesh; and one 1.5-m column of 2.1-mm i.d. and 3.2-mm 0.d. packed with Porapak N sO/lOO mesh. The columns were conditioned at 200 "C with helium flowing at 40 mL/min. The columns were operated at room temperature during this study. The helium carrier gas line from the gas sampling valve was split into two fractions, (1) the Porapak column and (2) the molecular sieve column, via a two-way sample splitter. Between the sample splitter and chromatographic columns, a fine metering valve was placed in the line. The Swagelok connections at the outlets of the two chromatographic columns were connected using a Swagelok union modified by drilling into the center of the union (normal to the gas flow) and silver soldering a 1-cm stainless steel tubing of 3.2-mm 0.d. to the union. The modified union was attached to the helium ionization detector using a Swagelok connection. The standard reference gas contained the following concentrations of gases: 11 ppm H2, 20 ppm 0 2 ,29 ppm N2, 23 pprn CO, 5 ppm CO,, 14 ppm CHI, 5 ppm C2H2,4 ppm C2H4,and 5 PPm GHs. RESULTS AND DISCUSSIONS Typical analyses of Hz, 02,N2, CH,, and CO using a molecular sieve column are achieved with columns of l- to 2.5-m
0003-2700/79/035 1-0462$01.00/0 0 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
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Figure 2. Analysis using 1 5 m Porapak N with flow of 43 mL/min, and 4.5 m molecular sieve with flow of 46 mL/min
length. However, in our work, we used molecular sieve columns of 4.5- to 6-m length in order to obtain a wide separation of the elution peaks to permit the gases eluted from t h e Porapak column to "fit" between the gases eluted from t h e molecular sieve column. In order to determine the operating parameters required for the operation of the dual columnssingle detector system, both columns (molecular sieve and Porapak) were connected to the helium ionization detector. The needle valve on one of the columns is closed. The sample is analyzed on the open column. Chromatograms for the analysis are obtained using gas flow rates of 30 to 60 cm3/min. After the analysis is completed on the first column, the needle valve on the first column is closed. The needle valve of the second column is opened and the analysis is carried out on the second column a t different flow rates, and a second set of chromatograms is obtained. With the proper matching between the two sets of chromatograms, the ap-
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propriate flow rates can be determined. When the two columns are operated together (at the experimentally determined flow rates), small adjustments in the flow rate of either column may be required to enhance separation. T h e analysis of a mixture of gases is shown in Figure 1 with the flow through the 3-m Porapak Q column at 40 cm3/min and the 6-m molecular sieve 13X column at 55 cm3/min. The flow rate measured a t the detector was 95 cm3/min. The Porapak Q does not separate C2H2from C2H,. If this separation is required, a Porapak N column can be used as shown in Figure 2 . The flow rate through the 1.5-m Porapak N column was 43 cm3/min and 46 cm3/min through the 4.5-m molecular sieve 13X column. The flow rate measured a t the detector was 89 cm3/minute. The use of two parallel columns with one detector has been previously described by Lysyj and Newton ( 7 ) for the separation of oxygen, nitrogen, and carbon dioxide. Brenner and Cieplinski ( 8 ) also describe the use of two parallel columns to separate oxygen, nitrogen, methane, propane, n-butane, and isobutane. The system we describe here utilizes molecular sieve and Porapak columns to analyze nine gases in about 10 min. The system is simple and can be used with either the sensitive helium ionization detector or the thermal conductivity detector. The use of two columns in parallel connected to one detector could be employed whenever two columns are needed for the analysis.
LITERATURE CITED ( 1 ) A. Zlatkis, H. R. Kaufman, and D. E. Durbin, J . Chromatogr. Sci., 8 , 416
(1970). (2) 0. L . Hollis. Anal. Chem.. 38. 309 (1966). (3) E. W Cieplinski, W. Averill, and L. S. Ettre, J . Chromatogr., 8, SSO (1962). (4) A . Di Lorenzo, J . Chromatogr. Sci., 8 , 224, (1970). (5) F. F. Andrawes and E. K . Gibson, A m . Mineral., in press ( 6 ) F. F. Andrawes and E. K . Gibson, Anal. Chem., 50, 1146 (1978). (7) I . Lysyj and P. R . Newton, J . Chromatogr., 11, 173 (1963). (8) N. Brenner and E. Cieplinski, Ann. N . Y . Acad. Sci., 72, 705 (1959).
RECEIVED for review October 2 , 1978. Accepted November 10, 1978. This work was performed in part under the auspices of the National Aeronautics and Space Administration, Contract NAS 9-15200 to Lockheed Electronics Co., Inc.
Preparing Nitrogen Gas for Nitrogen-I5 Analysis Richard J. Volk" and William A. Jackson DepartfnefIt of soil Science, North Carolina State University, Raleigh, North Carolina 27607
Ammonium nitrogen can be converted to N2 for mass spectrometric 'jN analysis by oxidation with NaOBr ( I ) or LiOBr ( 2 ) . Both reagents contain dissolved N2 which must be removed just prior to use. This is commonly achieved by evacuating the hypobromite reagent either in a Rittenberg Y tube ( I ) or in a glass dispensing apparatus ( 2 , 3 ) . R e have recently developed a freeze-layering procedure which eliminates the need for specialized glass apparatus. It consists of separating a dry NHICl sample from &-free NaOBr by a layer of N2-free ice. Nitrogen is subsequently generated from the sample by evacuating the container (a disposable test tube) and thawing its contents.
Table I. Atom % ISN Analysis of NZ Samples Prepared by the Rittenberg Procedure ( I ) and by the Proposed Freeze-Layering Method atom % ''N i'reezpN/sam pl p , kitten. layer berg sample tissue mg A B C D E F
shoot shoot shoot root root root
1.10 1.05
0.80 0.55 0.50 0.46
0.639 0.681 0.512 1.898 2.990 1.878
0.641 0.678 0.514 1.891 2.992 1.881
EXPERIMENTAL Two reagents are required: ammonia-free redistilled uater and NaOBr solution ( I ) . The latter contains KI to minimize O2 liberation ( 4 , s )and is diluted with 1.5 volumes of water to hasten subsequent freezing. Dissolved gases are displaced from both reagents by bubbling with argon for 10 min prior t o and during sample preparation. The samples, consisting of 0.2 t o 1.0 mg N 0003-2700/79/035 1-0463$0 1 O O / O
as NH4C1,are evaporated to dryness at 95 " C i n disposable. 1 2 mm 0.d. x 75 mm borosilicate glass test tubes. During freezelayering, the sample tubes are cooled in ethanol--dry ice slush. and are continuously flushed with argon (Figure 1). Each NH,C'I sample is covered with 0.3 mL of S,-free water, which freezes rapidly. followed hy 0.5 mL of N2-freeNaOBr solution. Two 6-mm c 1979 American Chemical Society
