Anal, Chem. 1994,66, 1396-1402
Noncryogenic Purification of Nanomole Quantities of Nitrogen Gas for Isotopic Analysis Stuart R. Boyd,' Agn6s Rejou-Mlchel, and Marc Javoy Laboratoire de Gbochimie des Isotopes Stables, Universiti?de Paris V I I , 4 Place Jussieu, 75251 Paris Cedex 05, France
The noncryogenic purification of nitrogen, prior to isotopic analysis, by a mixture of CaO granules and Cu has been evaluated for nanomole-sized quantities of N. Duplicate experiments were performed on two batches of CaO granules prepared using identical procedures. For both batches, the rate of absorption of C02 by CaO was investigated in the temperature range 400-650 OC. Theinteractions between NO2 and CaO in the temperature range 500-900 OC and between NO2 and CaO, onto which between 47 and 254 m o l of COz had been absorbed, were also investigated. Although the precise behavior of the CaO granules depends upon several factors, such as the length of time that they have been in use and, more importantly, the thermal characteristics of the reaction vessel used to heat the granules, the general characteristics have been observed. A temperature setting (600-725 "C) can be found at which CaO absorbs COz rapidly to leave a temperaturedependent residual pressure of COz of between 1 and 10 mbar. At this temperature setting there is only minimal absorption of NO2 by CaO. When NO2 is exposed to CaO onto which COZhas been absorbed, the residual CO2 apparently catalyzes the reaction NO2 NO + 0.502. At higher temperatures (800-9OO0C), in the absence of C02, the reaction NO + NO Nz 0 2 is catalyzed. After the addition of 1 g of Cu wire to the CaO granules, the rate of reduction of NO2 was investigated at the aforementioned temperature settings (600725 "C). Reduction was complete in under 500 s. On the basis of these results, a new system was designed and built for the extraction and purification of nanomole-sized quantities of nitrogen, and it has been applied successfully to the determination of the nitrogen content of diamonds. Carbon and nitrogen have been separated from a sample with an initial C/N (atomic) of 78 000. Carbon can be recovered quantitatively from the CaO without modification of the initial 13C/ 'ZC. The method offers a cheaper and more efficient means of purifying nitrogen than conventional cryogenic techniques and can probably replace such techniques in high-vacuum lines associated with mass spectrometers for the determination of 14N/15N. This step would aid greatly in the automation of such systems.
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A recent advance in the field of nitrogen isotope analysis has been the introduction of static-vacuum, noble gas type, mass spectrometers.1*2 The great advantage of these systems over conventional dual-inlet mass spectrometers is that they ~~~
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( I ) Frick, U.;Pepin, R. 0. Earrh Planer. Sci. Lex 1981, 56, 64-81. (2) Wright, I. P.; Boyd, S.R.; Franchi, I. A,; Pillinger, C. T. J. Phys. E: Sci. Insfrum. 1988, 21, 865-875. (3) Lewis, R. S.;Anders, E.; Wright, I. P.; Norris, S.J.; Pillinger, C. T. Nature 1983, 305, 767-771.
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can analyze samples which are smaller by -3 orders of magnitude, and such a reduction in sample size requirements has allowed numerous new studies to be undertaken (e.g., refs 1 and 3-11). Although the basic mass spectrometers are simple in principle, their implementation has required much development of procedures for the extraction, purification, and quantification of n i t r ~ g e n . ' J ~ -For ' ~ most samples, extraction and purification are performed in a high-vacuum lineconnected directly to the mass spectrometer. Nitrogen is commonly released from samples by combustion in oxygen,12and the gas has to be separated from other gases such as carbon dioxide and water prior to isotopic analysis, usually with a variabletemperature cryogenic trap.12 However, there are potential problems with this method. Javoy et a1.I6 reported that nitrogen oxides produced during combustion were trapped with carbon dioxide and to transform them to nitrogen gas required repeated passage of all of the condensed gases over copper at 600 OC. Experience has also shown that nitrogen gas may be trapped with carbon dioxide for samples with high C/N such as a diamond containing 50 ppm N. Both of these causes of nitrogen loss will lead to a low estimation of the nitrogen content of a sample and may affect the isotope ratio measurement. Recently, Kendall and Grim" perfected a sealed tube combustion technique for use with conventional dual-inlet mass spectrometers. They found that the addition of calcium oxide and copper to the samples, combined with a slow cooling (1 7 h), resulted in pure nitrogen gas, with no traces of carbon dioxide or water. Their study raised the possibility that a method employing calcium oxide could be used to replace the cryogenic techniques usually used for the purification of (4) Exley, R. A.; Boyd, S. R.; Mattey, D. P.; Pillinger, C. T. Earrh Planer. Sci. Left. 1987, 81, 163-174. ( 5 ) Boyd, S . R.; Mattey, D. P.; Pillinger. C. T.; Milledge, H. J.; Mendelssohn, M. J.; Seal, M. Earth Planer. Sci. Lcrr. 1987, 86, 341-353. (6) Boyd, S. R.; Pillinger, C. T.; Milledge, H. J.; Mendelssohn, M. J.; Seal,M. Nature 1988, 331, 604-607. (7) Wright, I. P.; Grady, M. M.; Pillinger, C. T. Geochim. Cosmochim.Acra 1988 52, 9 17-924. (8) Becker, R. H.; Pepin, R. 0.Geochim. Cosmochim.Acra 1989,53,1135-1146. (9) Boyd, S.R.; Pillinger, C. T.; Milledge, H. J.; Mendclssohn, M. J.; Seal, M. Earrh Planet. Sci. Lerr. 1992, 109, 633-644. (IO) Sugiura, N.; Hashizume, K. Earrh Planer. Sci. Lcrr. 1992, 111, 441454. ( I I ) Boyd, S.R.; Hall, A.; Pillinger, C. T. Geochim. Cosmochim. Acra 1993,57, 1339-1341. (12) Boyd, S.R.; Wright, I. P.; Franchi, I. A.; Pillinger, C. T.J. Phys. E: Sci. Instrum. 1988, 21, 876885. (13) Boyd, S.R.; Pillinger, C. T.Meas. Sci. Technol. 1990, 1. 1176-1183. (14) Boyd, S.R.; Pillingcr, C. T. AMI. Chem. 1991, 63, 1332-1335. (15) Boyd, S. R.; Wright, I. P.; Pillinger C. T. Meas. Sci. Technol. 1993,4, 10001005.
(16) Javoy, M.; Pineau, F.; Demaiffe, D. Earth Planer. Sci. Lcrr. 1984, 68, 399412. (17) Kendall, C.: Grim, E. Anal. Chem. 1990, 62, 526529.
0003-2700/94/03661396$04.50/0
@ 1994 Amerlcan Chemical Sockty
PG CF
@ Young's 'microvalve' Young's
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Flgure 1. Schematic of the vacuum line used during evaluation. Key: SV, bottle for storing NOp;QT, quartz tubing; F, furnace (25-900 "C); TC, temperature controller; T, thermocouple: RV, reaction vessel; CF, cold finger; PG, O-600-mbar piezoresistivepressuregauge: A-C, ports for attaching components to the manifold.
nanomole-sized quantities of nitrogen. This possibility has been evaluated in the present study. EXPERIMENTAL SECTION Reagents. The calcium oxide (Aldrich, 99.95 wt % CaO) was supplied as a fine powder. It was prepared for use in the following way. Deionized water (20 mL) was added to 10 g of calcium oxide, mixed into a paste, and then dried at 250 "C in a muffle furnace. The resulting calcium hydroxide was crushed and sieved, and the 1-2-mm grain-size fraction was retained. This was then placed in a quartz glass tube and heated in a furnace at 1000 OC for 20 min to remove its water. The resulting calcium oxide granules were then sieved again, the 1-2-mm grain-size fraction being retained. NO2 and C02 were prepared by the pyrolysis of Pb(N03)2 and CaC03, respectively (see below). The Cu (Prolabo, 99.7 wt %) was in wire form (0.5-mm diameter, in lengths of 3-10 mm). Apparatus. A schematicof the vacuum line used to evaluate the method is shown in Figure 1. In the following text, numbers or letters refer to Figure 1. The line consisted of a Pyrex manifold (0.d. 12 mm, i.d. 9 mm, length 140 mm) which could be isolated from high vacuum by microvalve 5 (Young's, Acton, U.K.). Three Pyrex ports (A-C) 40 mm in length (0.d. 6 mm, i.d. 4 mm) allowed various components to be attached to the manifold by using thermoretractable tubing.13 Attached to port A was a bottle of 11 cm3 (SV) used to store NO2 prepared in the following way. Between 60 and 90 mg of Pb(N03)2, which had been heated overnight at 375 "C to remove any H20, were placed in the quartz glass tubing (QT, 0.d. 6 mm, i.d. 4 mm, length 150 mm) and attached to valve 2. After the tube and bottle had been pumped out, SV was cooled to liquid nitrogen temperatures, valve 1 closed, valve 2 opened, and the Pb(N03)2 pyrolyzed at 800 "C using the furnace F until no more NO2 evolved. Since oxygen is produced during the pyrolysis of lead nitrate, the sample bottle was pumped out (valve 2 closed) with the liquid nitrogen trap in place. Afterward, valve 1 was closed, the liquid nitrogen trap was removed, and the condensed nitrogen oxides were allowed to expand. Attached to port B was either a sample bottle or a system for generating C02. The latter was identical to the one used
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to produce NO2 (valve 2 + QT) except that CaC03 was placed in the tube. The sample bottle was used to collect gases for subsequent chemical analysis on a Fison's Optima dual-inlet mass spectrometer. For gases condensible at liquid nitrogen temperature, such as NO2, the sample bottle was empty. For noncondensible gases, 200 mg of 5-A molecular sieve was placed in the bottle and degassed at 200 "C prior to use. After condensing of gases and reheating to room temperature, the molecular sieve releases N2, NO, or 0 2 while retaining gases such as C02 or NO2. The reaction vessel was attached to port C. Since rapid mixing of gases in the manifold and reaction vessel was required, a normal Young's valve 3 was used since they have much greater conductance than the microvalves. This valve was connected via a quartz Pyrex graded seal to a 120-mm length of quartz tubing (0.d. 8 mm, i.d. 6 mm). The reaction vessel could be heated from room temperature to 900 "C by a resistance wire furnace wound directly around the reaction vessel. This furnace was controlled automatically by an electronic temperature controller (TC-type LMS; Eroelectronic), the thermocouple (T) being in contact with the bottom of the reaction vessel. The temperature controller maintained the temperature at f 1 OC of the preset value. Adjacent to the reaction vessel was a piezoresistive gauge (PG) with digital output (Keller Type PAA 27) for measuring pressures. The gauge measures pressure directly in millibars in the range0400 mbars (A 0.1 mbar). It served two principal purposes. In conjunction with a liquid nitrogen cold finger (CF), it was used to quantify samples of C02 of up to -300 pmol; during reactions, it was used to monitor the change in pressure with time. The combined volume of the manifold and cold finger was estimated to be 20.2 cm3,that of the reaction vessel being 5.7 cm3,although the latter changed depending upon the quantity of reagents present. Procedure and Results. All of the experiments using the line depicted in Figure 1 followed a similar procedure. The manifold was isolated from high vacuum and aliquots of gas (either C02 or N02) were admitted via port A. With all ports A-C closed, the amount of gas present was quantified using the gauge PG. Valve 3 was then opened, the timer started, and the gas allowed to expand into the reaction vessel which contained 800 mg of CaO granules (later mixed with 1 g of Cu wire) maintained at a preset temperature. The first readings of pressure were taken 10 s after the opening of valve 3, all other timings being given in the figures. Since the pressure in the manifold and the ratios of the volumes were both known, the pressures at t = 0 s could be calculated. For a total volume of -26 mL, the values of these initial pressures are virtually identical to the sample size in micromoles. For example, a sample size of 195 pmol gave an initial C02 pressure of 192 mbar. The manifold was isolated for between 5 and 10 min, and during blank runs the pressure in the line rose by a maximum of 0.1 mbar during a 10-min period. Since the changes in pressure with time would be dependent in part upon the transfer of gas through valve 3, an experiment was performed where the reaction vessel was replaced by a liquid nitrogen trap of similar size. For an initial C02 pressure of 235 mbar in the manifold, total trapping was achieved 7 s after the opening of valve 3. Analytical Chemistry, Vol. 66, No. 9, May 1, 1994
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Time (seconds) Figure 2. Absorption of COPby CaO. COPis absorbed rapidly by CaO to leave a temperaturedependent, residual pressure (glven on the curves in millibar), which was reached after -200 s.
During the course of this work several preparations of CaO were investigated. In the following sections, results are presented for two. Preparation A (Prep A) had been in use for a number of weeks before the experiments were performed whereas preparation B (Prep B) was used immediately after it had been made. The two sets of experiments were performed in different reaction vessels of similar size and design. For Prep A, the reaction vessel had been lined with Pt foil to prevent reaction between the Cu and quartz walls. It will be seen that the results obtained for the two sets of experiments differ slightly. While this may be due in part to differences in the chemical properties of the CaO granules, what appear to be more important are the thermal characteristics of the reaction vessels: one can never know precisely the thermal gradient within a given reaction vessel or where, in relation to the heating coils, the thermocouple is reading the temperature. Each reaction vessel needs to be characterized individually, and as will be shown later, this can be achieved within a short space of time. Carbon Dioxide and Calcium Oxide Experiments. During these experiments the initial C02 pressures were 190 mbar for Prep A and 200 mbar for Prep B, corresponding to approximately 190 and 200 pmol of C02, respectively. The experiments were performed at five temperatures for Prep A and four temperatures for Prep B; the results are given in Figure 2. For Prep A, in the temperature range 450-550 OC, C02 was "pumped" rapidly by CaO to a temperature-dependent equilibrium pressure which was reached 100 s after valve 3 was opened. At 400 "C, the equilibrium pressure was reached after 150 s due to a slower rate of reaction. For the 600 OC experiment, the pressure continued to fall noticeably after 310 s; subsequent experiments showed that, at 600 OC,
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equilibrium was reached after 12 min although the pressure was only 10% less than that after 5 min. For Prep B, the results were similar although shifted to a higher setting on the temperature controller (Figure 2). For example, the 550 OC experiment for Prep A is similar to the 650 OC experiment for Prep B (Figure 2). The difference between the two data sets is probably a reflection of differences in thermal characteristics of the two reaction vessels. Since total removal of C02 was desired, experiments were performed where, after equilibrium was reached, the furnace of the reaction vessel was turned off and the temperature, time, and C02 pressure were monitored. In all cases the pressure dropped to zero (Le.,
