ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
I 0 TIME,
I
I
5
IO
minutes
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- t o 6-m length in order t o obtain a wide separation of the elution peaks to permit the gases eluted from t h e Porapak column t o "fit" between t h e gases eluted from t h e molecular sieve column. In order t o determine the operating parameters required for the operation of the dual columnssingle detector system, both columns (molecular sieve a n d Porapak) were connected t o the helium ionization detector. The needle valve on one of the columns is closed. The sample is analyzed on the open column. Chromatograms for t h e analysis are obtained using gas flow rates of 30 to 60 cm3/min. After the analysis is completed on the first column, t h e needle valve on t h e first column is closed. T h e needle valve of t h e 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 t h e two sets of chromatograms, the ap-
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propriate flow rates can be determined. When t h e 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 t h e flow through the 3-m Porapak Q column a t 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 . T h e 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. T h e 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. T h e system we describe here utilizes molecular sieve and Porapak columns to analyze nine gases in about 10 min. T h e system is simple and can be used with either t h e 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 t h e 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 t h e National Aeronautics and Space Administration, Contract NAS 9-15200 to Lockheed Electronics Co., Inc.
Preparing Nitrogen Gas for Nitrogen-I 5 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 t o 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. I t 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) a n d 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
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ANALYTICAL CHEMISTRY, VOL. 51, NO.
3,MARCH 1979
the residual level, it was assumed that the sample contained air, and the nitrogen peaks were corrected accordingly. Correction factors were obtained from the mass spectrum of ambient air (5, 8). These corrections, although commonly used, are only approximate because the N2/02ratio in ambient air may differ from that in the contaminant air, especially if part of the latter came from the solutions.
TO M A S S SPECTROMETER
O P T ! O N A L STOPCOCK 'RUBBER TEST T U B I2 X 75mm
SLEEVE
E 1
FROZEN
NaOBr
6mm G L A S S BEADS
RESULTS AND DISCUSSION
MANIFOLD A r i
4
I
I
NoOBr
DRY ICE
?
- ETHANOL
Figure 1. Preparation of freeze-layered samples using an argon manifold to minimize contamination with atmospheric nitrogen
,
diameter borosilicate glass beads are then placed on the frozen NaOBr to prevent its upward movement during subsequent Nz generation. When sufficient samples have been prepared, they are stoppered and stored on dry ice until analyzed. Just prior t o analysis, a sample tube is attached to the mass spectrometer inlet system with a short length of 3-mm wall rubber tubing and an appropriate standard taper joint, which may include a stopcock for convenience (Figure 1). Applying a light coat of high vacuum grease improves the seal between the rubber and glass parts and minimizes leakage of air into the sample tube. The tube is evacuated, either with the mass spectrometer pump or with an auxiliary pumping system, thawed to permit NaOBr oxidation of the NH,Cl, and refrozen prior to admitting N2 into the instrument. To test the procedure, six corn tissue samples, known to be enriched in 15N, were digested using a semi-micro Kjeldahl procedure ( 6 ) . The ammonia was recovered by distillation into 0.1 N HCl, using a silver condenser to avoid I5N cross contamination between samples (7). Half of the distillate was analyzed by a Rittenberg procedure ( I ) and half by the proposed freeze-layering procedure, using an electron ionization mass spectrometer. When the oxygen peak in any sample exceeded
Comparable atom % 15Nvalues were obtained on samples run by the Rittenberg ( I ) and freeze-layering procedure (Table I). In no case was the A atom 70 15Ngreater than 0.007, nor was there a tendency for one method to exhibit higher values than the other. The freeze-layering procedure as described here has proven to be both rapid and precise during six months of routine use. In addition, the oxygen values obtained with the proposed method are usually so low (e.g. m l e 32 < 0.1% of the m / e 28) t h a t correction of the m / e 28 peak for air leakage is unnecessary. This is not the case with the Rittenberg tube procedure, where the oxygen values are considerably higher (e.g., m l e 32 > 0.3% of m / e 28). When using the latter procedure, we routinely correct for air leakage, based on the m l e 32 value obtained.
ACKNOWLEDGMENT The authors gratefully acknowledge the technical assistance of Penelope V. Windsor.
LITERATURE CITED (1) D. 6.Sprinson and D. Rittenberg, J. Biol. Chem., 180, 707 (1949). (2) P. J. Ross and A. E. Martin, Analyst, (London),95, 817 (1970). (3) L. K. Porter and W. A. O'Deen, Anal. Chem., 49, 514 (1977). (4) M. P. Fleury, Compt. Rend., 171, 957 (1920). (5) A. P. Sims and E. C. Cocking, Nature(London), 181, 474 (1958). (6) H. A. McKenzie and S. Wallace, Aust. J . Chem., 7 , 5 5 (1954). (7) A. C. D.Newman, Chem. Ind. (London), 115 (1966). ( 8 ) D. Rittenberg in "Preparation and Measurement of Isotopic Tracers", D. W. Wilson, A. 0. C. Nier, and S. P. Reimann, Ed., J. W. Edwards, Ann
Arbor, Mich.. 1948, pp 31-42.
RECEIVED for review October 31, 1978. Accepted December 8,1978. Paper number 5806 of the Journal Series of the North Carolina Agricultural Experiment Station, Raleigh, N.C. This research was supported in part by National Science Foundation grant P C M 77-03152.
