points of the operational switches remain at a virtual ground potential, each trigger point may be adjusted independently. The gain and bias of A I 4are adjusted so that the output ranges from - 10 to +10 volts, as shown in Figure 6. This chopped ramp is then recorded on magnetic tape along with the mass spectral data and the digital control gate. f i e digital cornputer reconstructs the ramp and uses the instantaneous ramp voltage as a time base to assign mass numbers in the mass spectrum. The fall times of the expanded ramp are less than 200 microseconds, which is faster than the 500-microsecond sampling of the analog to digital converter, thus ensuring that the
reconstructed ramp will not have discontinuities at the ramp resets. ACKNOWLEDGMENT
The authors are grateful to Eldon Beran, McDonnell Automation cos, for his assistance in the analog and digital programming. RECEIVED for review June 30, 1969. Accepted January 14, 1970. Research conducted under the McDonnell Douglas Independent Research and Development Program,
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Inert Carrier-Gas Fusion Determination of Total Nitrogen in Rocks and Meteorites Everett K. Gibson, Jr.,l and Carleton B. M o o r e Center for Meteorite Studies and Department of Chemistry, Arizona State University, Tempe, Ariz. 85281 An inert carrier-gas extraction-gas chromatography technique has been adapted for the determination of total nitrogen in rocks and meteorites. Samples are fused in a graphite crucible at 2400 O C and the nitrogen and other evolved gases are separated and analyzed using a gas chromatograph. The excess carbon monoxide produced from the silicates is converted to carbon dioxide and removed from the system. The limit of detection was found to be 2 pg of N and the error of the technique was less than 5% of the normal working range between 10 and 200 pg of N. The method is rapid, accurate, and simpler than other methods now in use for the determination of trace quantities of nitrogen in a wide variety of materials.
COMBUSTION-THERMAL CONDUCTIVITY ANALYZERS for determining total nitrogen in steels and refractory metals have been described by Bryan and Bonfiglio ( I ) , Dallmann and Fassel (2), and others. The sample is heated to 2400 "C in helium in a graphite crucible and the molecular nitrogen and other gases that are formed are swept into a molecular sieve trap and collected. After a fixed collection period, the gases are swept into the molecular sieve column of a gas chromatograph and, upon elution from the column, the molecular nitrogen and the other evolved gases are detected with a thermal conductivity detector. In the determination of nitrogen in silicates and various metal oxides with this technique, problems arise because of the excessively large amounts of carbon monoxide formed upon combustion of the sample in the graphite crucible. The excess carbon monoxide prevents resolution of the nitrogen peak on the chromatogram obtained. In this investigation the technique was modified to remove the carbon monoxide and permit nitrogen determinations in silicate materials. After combustion of the silicate samples, the evolved gases are swept through a gas train in which they pass over a copper oxide-rare earth oxide catalyst held at 400 "C,which converts Present address, TN7, Geochemistry Branch, NASA Manned Spacecraft Center, Houston, Texas 77058 (1) F. R. Bryan and S. Bonfiglio, J. Gas Chromatogr., 2,97 (1964). (2) W. E. Dallrnann and V. A. Fassel, ANAL.CHEM.,39, 133R
(1967).
all of the carbon monoxide to carbon dioxide. After the conversion, the gases are passed through an Ascarite-Anhydrone trap to remove the carbon dioxide and water. The remaining gases are transferred into the molecular sieve column of the chromatograph, where the nitrogen peak is easily resolved. By removing the carbon dioxide and water, the life of the molecular sieve column is increased. Advantages of this method are that high sensitivity is obtained by concentrating the nitrogen in the cold trap from the helium carrier gas, and the chromatographic separation allows the separation of nitrogen from the rare gases which interfere in the v a c u h fusion determination of nitrogen. The samples are fused directly in the graphite crucible and no dissolution of the sample is required. The loss of nitrogen, which often occurs during Kjeldahl dissolution procedures, is eliminated. Although the total nitrogen content of the sample is determined, the technique does not allow determining the form in which the nitrogen occurs. Our previously reported work (3, 4 ) gives the results of the determination of total nitrogen content of chondritic meteorites. The work reported here discusses a technique for the determination of the total nitrogen content of silicate samples of the U S . Geological Survey standard rocks, synthetic silicate standards, National Bureau of Standards standard steels, and meteorites. EXPERIMENTAL
The equipment consists of a Leco Nitroxd Analyzer (Model 534-800). Its operation for nitrogen and oxygen determinations has been described (1). The schematic of the apparatus with the modifications made for this work is given in Figure 1. Combustion Apparatus. A Leco induction furnace (Type 537-100) was used with Leco combustion tube No. 589-625. Samples were melted in graphite crucibles (Leco No. 534-352). Theoperatingtemperature for the silicate samples was 2400 OC. A Leeds & Northrup optical pyrometer was used to calibrate the temperatures of the induction furnace. (3) C. B. Moore and E. K. Gibson, Scierzce, 163, 174 (1969). (4) C. B. Moore, E. K. Gibson, and K. Keil, Earth Planet, Sci. Letters, 6, 457 (1969). ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
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-
Analyze
Figure 1. Schematic of furnace-chromatograph sections with modifications for Leco Nitrox-6 After Bryan and Bonfiglio ( 1 ) a. Cylinder helium 6. Desiccator c. Valve d. Rotameter e. Zirconium sponge (200 "C) f. Induction furnace (2400 "C) g. Dust trap h. Rare earth-copper oxide catalyst (400 "C) 2. Ascarite i. Anhydrone k . Molecular sieve collection trap (- 195 "C) 1. Chromatograph column molecular sieve 5A (45 "C) m , Thermal conductivity detector Analyzer, A Leco nitrogen analyzer (Model 589-100) was used to determine the nitrogen liberated upon combustion of the samples in the furnace. Its chromatograph section consisted of a thermal conductivity detector using a set of matched thermistors. Integrators, used to obtain the areas of the chromatographic peaks, provided the peak areas as numerical counts. The detector and molecular sieve column were housed in a 45 "C constant temperature oven. The chromatograph column was a 2-foot-long column, with l/4inch outside diameter, and filled with 40/50-mesh Molecular Sieve 5A. Operation. The graphite crucibles were packed in carbon black in a thimble (Leco No. 537-236), then degassed by heating at 2500 "C for 3 hours. The bake-out procedure removed atmospheric gases adsorbed on the carbon black powder and the crucible. Calibration curves were obtained from the analysis of either metallic or chemical compound standards. For this work National Bureau of Standards steels with a certified nitrogen content were used to obtain the calibration curves. The linear calibration curve was obtained by plotting the analyzer response (integrator counts) against micrograms of nitrogen in the standard steel. Powdered samples (50 to 300 mg) were prepared for analysis by weighing the sample directly into nitrogen-free tin capsules (Leco No. 501-59). The samples in the capsules were then kept in a vacuum for 48 hours, and sealed in a glove box under a helium atmosphere. For analysis, a sample was placed in the sample loader above the combustion tube and flushed with purified helium for 30 minutes. The helium flowing through the furnace portion of the system had been purified by passing it over zirconium sponge held at 200 "C to remove trace quantities of nitrogen and oxygen. After obtaining the blank, the sample was dropped into the graphite crucible held at 2400 "C. The combustion products were passed over a catalyst, consisting of copper oxide-rare earth oxide (Leco No. 501-170), at 400 "C to convert the carbon monoxide formed to carbon dioxide. The nitrogencontaining compounds and any dissolved nitrogen had been converted to molecular nitrogen. A trap containing Ascarite and Anhydrone in the flow system removed the carbon 462
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1
3
2
1
I
1
0
3
1
2
1
Minuler
hl,n"!er
Blank
Sample
0
Figure 2. Typical chromatogram obtained from analysis of blank and meteorite sample dioxide and water. The remaining combustion products passed on through. This procedure was necessary for resolution of the nitrogen peak on the chromatogram; otherwise the excess carbon monoxide produced from the oxygen in the silicate materials would have prevented resolution. The remaining combustion products were collected in a molecular sieve trap at liquid-nitrogen temperature. After the 5-minute collection period, the trap was warmed, and the condensed gases were instantaneously flushed into the gas chromatography molecular sieve column held at 45 "C. As the gases eluted from the column, they were detected in sequence as they passed over a thermal conductivity cell using matched thermistors in the chromatograph. Typical chromatograms are shown in Figure 2. After the analysis of the sample the system was blanked a second time. The average of the two blank analyses was taken to be the background nitrogen value for the analysis. NBS standard steels, synthetic silicate standards, Leco potassium thiocyanate standards, and iron and stony meteorites and terrestrial materials indicated that the technique was accurate for determination of trace quantities of nitrogen. The precision obtained was within sample inhomogeneity. The standard steels and the iron meteorite samples can be considered as samples which are homogeneous throughout with respect to their nitrogen contents. The nitrogen which they contain is atomic nitrogen dissolved in solid solution in the metal phases. Thus the replicate analyses of the iron meteorites and standard steels is an indication of the precision of the technique. For the silicates and the stony meteorites the nitrogen is inhomogeneously distributed in various mineral phases. The nitrogen may occur as a separate phase-e.g., NH4A1SiaOs,TiN, Si2N20,etc.-as dissolved atomic nitrogen in solid solution of certain mineral phases, or perhaps as molecular nitrogen trapped in small cavities. It is therefore difficult to obtain a sample of a natural silicate material which has nitrogen distributed homogeneously throughout. The limit of detection for the silicate samples was between 2 and 4 pg of N. The instrument background (blank) was around 4 to 6 p g of N, depending upon the condition of the graphite crucible. After 5 to 8 grams of silicate samples (12 to 25 samples, depending upon their size) had been added to the crucible, the background began to increase. When this occurred, a new crucible was packed and placed in the instru-
Table I. Analysis of NBS Steels Reported Recovered Sample %, pg N recovery NBS No. wt, gram l.(g N 3.0 100.0 3.0 0.1011 16 d 4.6 102.3 4.5 0.1496 16 d 4.7 97.6 4.8 16 d 0.1603 7.6 95.2 7.9 0.2622 16 d 6.3 100.0 6.3 0.1045 12 h 28.1 100.0 28.1 0.3119 5k 34.2 101.1 34.3 0.3807 5k 34.5 100.7 34.4 0.2454 129 b 36.5 102.8 35.5 0.2538 129 b 60.8 102.1 59.3 0.4232 129 b 98.2 101.4 96.5 129 b 0.6890 105.6 101.2 104.3 0.2675 101 e 162.5 98.7 164.6 0.4331 101 e 160.0 100.5 159.0 0.2149 343 Av recovery 100.2 cmean 1.9% Table 11. Analysis of Synthetic Silicate Standards (Ottawa sand matrix) Sample No. Calcd pg N Exptl pg N % recovery Ammonium Nitrate Additive 366 98.4 1 372 2 391 395 101.0 3 433 450 103.0 4 250 255 102.0 5 337 325 96.4 c m e a n = 2.6% Av recovery 100.3 1 2 3
4 5
6 7 8 9 cmean
=
2.3%
Titanium Nitride Additive 290 95.1 305 304 98.7 308 425 98.6 43 1 234 233 99.6 305 95.7 319 455 100.4 453 302 99.3 304 368 102.2 360 560 101.8 550 Av recovery 99.1
ment. The life of the graphite crucible could be increased by adding 8 to 10 small chips (3 to 5 mm in size) of a broken new graphite crucible to the packed crucible in the furnace. These chips allowed 2 to 3 additional grams of material (8 to 12 additional samples) to be added to the reaction crucible. RESULTS AND DISCUSSION
Studies on artificially prepared standards containingnitrogen as nitrides, ammoniacal ions, and nitrates, as well as standard NBS steels, with nitrides and dissolved nitrogen, indicated that complete extraction and detection were achieved for each sample. Analyses of six standard steels ranging in nitrogen content from 3 to 165 ppm are given in Table I. Because no silicate standards of certified nitrogen concentration are available, synthetic silicate standards which contained known amounts of nitrogen in the form of nitride, ammoniacal nitrogen, and nitrate were prepared. Ottawa silica sand was selected for the matrix material. It was first analyzed for nitrogen content, which was found to be 40 ppm of N. Known amounts of titanium nitride and ammonium nitrate were added to the matrix and physically mixed using a mortar and pestle. The analysis of the synthetic silicate standards is given in Table 11. The silicate standards prepared
Table 111. Replicate Analysis of an Iron and a Stony Meteorite (erg N per gram)
Mean (10) Range Spread Std dev from mean Confidence level (99 %)
Iron, Arispe Stony, Richardton coarsest octahedrite, chondrite, sample sample size 400 mg size 200 mg 24.2 50.5 22.4 to 26.7 40 to 64 4.3 24 1.6 6.9 24.2 & 1 . 3
50.5 =k 5.6
Table IV. Analysis of USGS Rock Standards Experimental Mean results, Sample USGS value, weight, g standard rock Pg N/g fig N/g 0.1567 51 W-1 diabase 0.1730 53 52 0.1682 51 0.1441 57 G-1 granite 0.1858 59 59 61 0.1635 0.0888 53 G-2 granite 0.1313 65 56 0.1159 50 0.1537 GSP-1 granodiorite 45 0.1506 46 48 0.1407 54 0.1456 49 AGV-1 andesite 0.1216 42 44 0.1284 40 0.2117 27 DTS-1 dunite 0.1504 31 27 0.1975 24 0.1557 PCC-1 peridotite 37 0.1760 49 43 0.1270 43 0.1268 BCR-1 basalt 31 0,1473 28 30 0.1945 30
using titanium nitride also gave excellent per cent recovery values for nine analyses. Titanium nitride is one of the most difficult refractory-like materials in which to determine total nitrogen, because of the difficulties in decomposing the nitride. The precision obtainable by using the technique of inert carrier-gas fusion extraction is indicated by the replicate analyses of NBS steels and stony and iron meteorites. Replicate analyses of 200-mg samples of NBS steel 33d gave an average recovery of 100.6Z for the ten analyses. The standard deviation from the mean was 2.8 %. Table I11 shows the results of the replicate analyses of a stony and an iron meteorite. The large standard deviation for the stony meteorite may be in part due to sample inhomogeneities, the small sample size, and the manner in which nitrogen is found in the stony meteorites. The method developed was used to analyze the suite of rock standards prepared by the U S . Geological Survey. The two standards prepared several years ago (G-1, granite; W-1, diabase) by the survey were analyzed in addition to six new standard rocks (G-2, granite; GSP-1, granodiorite; AGV-I, andesite; DTS-1, dunite; PCC-I, peridotite; BCR-1, basalt). Each of the USGS standard rocks was analyzed in triplicate (Table IV). The values ranged from a mean of 59 pg of N ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
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per gram for a granite (G-1) to 27 for the dunite (DTS-1). There are no previous analyses reported for total nitrogen on any of the USGS standard rocks (5,6). The nitrogen found in terrestrial rocks and silicate minerals is known to exist, in part, as ammoniacal nitrogen (NH4+), which is held within the lattices of the silicate structures (7-9). The ammoniacal nitrogen may substitute for the potassium ion (K+) in the lattice sites because of similarities in size and ionic charge. Nitrogen may also be dissolved as atomic nitrogen in solid solution within various mineral phases of the rocks. Molecular nitrogen may be found in minute crystalline cavities, but is not detected by methods such as the Kjeldahl technique. Vacuum fusion methods can determine molecular nitrogen, but the inherent problems associated with vacuum fusion techniques rule out the method. The methods conventionally used for the determination of nitrogen in rocks and silicate materials are modifications of the Kjeldahl procedure and often lead to erroneous results. The dissolution procedures and temperatures necessary for sample digestion often lead to bumping, contamination with ammonia from the atmosphere of the laboratory, and deposition of the mineral material on the sides of the digestion flasks. Stevenson (10) noted that the difficulties of obtaining satisfactory nitrogen determinations in igneous rocks and silicate materials are further increased by the fact that the small amount of nitrogen ordinarily present is so intimately combined with the mineral phases that prolonged periods of digestion are required for its liberation. Kallmann et al. (11) noted that among the most exasperating obstacles for determining trace quantities of nitrogen in refractory-like materials have been ( 5 ) F. J. Flanagan, Geochim. Cosmochim. Acta, 33, 81 (1969). (6) M. Fleischer, ibid., 33, 65 (1969). (7) D. S. Barker, Amer. Mineral., 49, 851 (1964). (8) F. J. Stevenson, Science, 130, 221 (1959). (9) F. J. Stevenson, Geochim. Cosmochim. Acta, 26, 797 (1962). (10) F. J. Stevenson, ANAL.CHEM., 32, 1704 (1960). (11) S. Kallmann, E. W. Hobart, H. K. Oberthin, and W. C. Brienza, ibid., 40, 332 (1968).
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the inability to dissolve the samples and the relatively high operational blanks. The technique of inert carrier-gas fusion extraction overcomes both problems. The operational blank is a constant 4 to 6 pg of N and the complete sample is fused at the 2400 "C operating temperature employed in the graphite crucible. The problem of reagent contamination is not present with this technique. Inert carrier-gas extraction overcomes most of the problems which have plagued previous methods for trace nitrogen determination in a wide variety of materials. The procedure of passing the evolved gases over a hot copper oxide-rare earth oxide catalyst to convert the excess carbon monoxide to carbon dioxide, followed by absorption on an Ascarite-Anhydrone trap, is easily carried out and permits determination of trace quantities of total nitrogen in both silicate and metallic materials. The perfected method is both rapid and accurate. A sample can be completely analyzed in 30 minutes after calibration and sample preparation. The combination of the technique of inert carrier-gas fusion extraction with a mass spectrometer affords the opportunity for further progress in the rapid growing field of gas chromatography-mass spectrometer systems. With this combination the chromatographic column separates nitrogen from carbon monoxide (both mass 28) and allows resolution of the two species. ACKNOWLEDGMENT
The authors thank C. F. Lewis for assistance in preparation of the meteorite samples. One of the authors (E.K.G.) is an N.R.C.-NASA Resident Research Associate at the Manned Spacecraft Center. RECEIVED for review November 6, 1969. Accepted January 12,1970. Work supported in part by National Science Foundation Grant GA-909.
