1634
ANALYTICAL CHEMISTRY
cludiiig details of modification, if any, would be necessary before valid comparisons of the merits of the procedures could be made. There is evidence that procedures are not commonly written to include all significant detail, as different results can be obtained when using the same method.
dicates the difficulty of the problem. This probably arises from the occurrence of nitrogen in ring compounds, which are more refractory than the compounds commonly analyzed by current procedures for nitrogen determination. ACKNOWLEDGMENT
COMPARISON OF METHODS
Although there has been some dispute as to whethei both Kjeldahl and Dumas methods were suitable for the deter niination of nitrogen in shale oil, the results, aa shoun in Table 11, indicate that either method can be made t o give satisfactory results. The Kjeldahl method on a macro scale gave the greatat percentage of satisfactory results. It also was used to the greatest a t e n t , and experience probably contributes to the amount of sttisfactmy data. The Dumas method, as practiced on a micio ale, waq used less and gave slightly fewer satiifactory data. Insufficient data were submitted to draw any conclusions regardiag Dumas analyses on macro and semimicro scales and Kjeldahl i'alvses on a semimicro scale. The lack of satisfactory results by the Iijeldahl method on a micro scale suggests that difficulties, probably of technique, arise when the scale of the method is reduced. Some difference may be noted in the efficacy of the method8 on the different samplra (see Figure 1 ) . The Kjeldahl maciomethod was consistent 011 all samples. The Dumas micromethod was much better with respect to the crude oil than with the other two samples. The low percentage of satisfactory results by all methods 111-
The following laboratories participated in the exchange program: The Barrett Division, Allied Chemical & Dye Corp.; California Research Corp.; Jackson Laboratory, E. I. du Pont de Nemours & Co., Inc.; Louisiana Division, Esso Standard Oil Co.; Gulf Research & Development Co. ; Huffman Microanalytical Laboiatories; Koppers Co., Inc.; Phillips Petroleum Co.; Shell Development Co.; Socony-Vacuum Laboratories; Process Division, Standard Oil Development Co.; Research Division, Standard Oil Development Co.; The Texas Co.; Union Oil Co. of California; Universal Oil Products Co. ; Oil-Shale Demonstration Plttnt, U. s. Bureau of Mines; and Petroleum and OilShale Experiment Station, U.S.Bureau of Mines. LITERATURE CITED
(1) Racon,
R. F.,and IIamor, W. .I.,"The American Petroleum
Industry," y. 808. Kea- Tork, AIcGraw-Hill Hook Co., Inc., 1916. (2) NcRee,
Ralph H., and co-authors, "Shale Oil." pp. 1 7 6 6 , S e w Tork, Chemical Catalog Co., 1925. (3) Poth, E. J., Armstrong, W. D., Cogburn, C . C'., :tiid Bailey, J. R., I d . Eng. Cheni., 20, 83-5 (1928). RI:CLI\.ED JIarcli 3. lCI31,
(Determination of 2Yitrogen in Shale Oil and Petroleum)
EFFECTS OF DIGESTION TEMPERATURE ON KJELDAHL ANALYSES G. H . L i K E 4 h D PHILIP MCCLTCII4Y, b-nion Oil Co. of Culifornia, Wilmington, calif., HOBI\ \ AY METER AND J. C. YEEL', C . S. Burenit of Wines, Larumie, Wyo. A survey had shown both that present methods for nitrogen determination were unsatisfactor) and that the Kjeldahl method used on a macro scale had promise. Investigation of the variables in the Kjeldahl procedure revealed that the temperature attained in digestion of the sample .was of prime importance. Too low a temperature either requires too long a digestion time or fails to give good results, while too high a temperature may result in loss of
M
ANY attempts have been made to improve t h e Kjeldalil method for nitrogen determination since 1883, when the procedure Mzas first published. Goals sought have included greater accuracy and precision, applicability to more refractor), types of compounds, reduction of digestion time, simplification of the procedure, and others. Bradstreet (2) and, more recently, Kirk ( 4 ) have extensively reviewed the principal published work on the Kjeldahl method. Two significant advances in the method had been made before the turn of the century. Kilfarth (9) has been credited with introducing the use of mercury in the digestion and Gunning ( 3 ) with introducing potassium sulfate. Both of these additions resulted in material improvement in the method by permitting shorter digestion times and by broadening the scope. Much of the later work is contradictory to a point where the confusion now existing in the practice of the Kjeldahl method is quite understandable. Many modifications have been proposed and 1
Piebent address, S.
W.Shattuck Chemical Co.. Denver. Cola.
9ND
nitrogen. Adjustment of the amount of potassium sulfate in the digestion mixture to give the proper temperatures resulted in a procedure which has giien satisfactorj results on a large number of samples. The concept of a narrow temperature range w-ithin which satisfactor) results may be obtained by the Kjeldahl procedure permits modification of the standard procedure on a logical basis to meet requirements of special samples or conditions.
have met with opposition. These include digestion mixtures using selenium oxychloride, selenized pellets, copper sulfate, perchloric acid, and hydrogen peroxide (30:; ), either singly with a form of mercury or in combinations. By a c.ritical study of the operating conditions, a procedure baaed on the Gunning and JVilfarth contributions has been developed which has proved to be satisfactory for analyzing petroleums and shale oils for nitrogen content. Such oils mag contain nitrogen in pyridine ring structures, and in this form the nitrogen is particularly refractory to Kjeldahl digestion. Digestion a t temperatures below 370' C. will not give quantitative recovery of pyridine wit,h 1-hour digestion; a t temperatures abdve 410" C. nitrogen may be lost by decomposition. Since the beginning of this research a somewhat similar concept has been proposed by Ogg and Willitts ( 5 ) rvorking in another field. .4lso, White and Long ( 8 ) , working with sealed digestion tubes on a micro scale, have sucmssfully employed a digestion temperature of -170" C. with still shorter digestion times. Because close attention to procedure is necessary to attain the satisfactory diges-
q O L U M E 2 3 , NO. 11, N O V E M B E R 1 9 5 1 tion temperature, more explicit directions than usual are newssary. Where oxidizd nitrogen may be encountered-a rare instance for shale oil and pet,roleum products-special treatment i a necessary, and this problem is under investigation. APPARATUS
Kjeldahl digestion and distillation rack with fume exhaust or equivalent were used. If distillation procedure B is used, the distillation unit is assembled as shown in Figure 1. Kjeldahl digestion flask, 800 nil. Erlenmeyer flask, 500 ml. Buret, 50 ml., graduated in tenths of a nlilliliter. Buret, 10 ml., graduated in twentieths of a milliliter. Glass beads, 5-mm. diameter. Crucibles, porcelain, small-e.g., Cools 00000 size. Pipet or dropper. neighing. REAGENTS
Sulfuric acid, concentrated, reagent grade. Potassium sulfate, reiigent gr:tdr. Mercury, clean mrt:rilic. Sodium hydmsi&-~odiuni sulfide aqueous solution, 40yo sodiun hydroxide ( C Y . or a grade suitable for Kjeldahl determinations) and 3% Podium sulfide nonahydrate (c.P.). Boric acid solution, &ur:ited. Standard sulfuric acid, 0.1 .Y or, for samples containing less than 0.2% nitrogen, 0.01 .\-. Stund:trdize to four significant figures. Methyl purple indic:itor (not methyl violet), aqueous solution, approsimately 0.1% active constituent (may be purchased from Fleisher Chemicaal (:a,, Benjamin Franklin Station, Washington 4)D. C.). Superior in end point sharpness t o methyl red, broniocresol green, or their iriisture. piirticularl?- with 0.01 &\rtandard
1635 rate of boiling should be such that the condensation of sulfuric acid vapors proceeds approximately one third up the neck of the flask. After a time, the flask contents will have cleared sufficiently for the individual turns of the heating element to be seen through the liquid. \\-hen oil shales or catalysts are digested, the liquid may never become transparent, but’ the disappearance of dark coloration is equivalent to clearing. Time the actual digestion from this point and allow it to proceed for 1 hour f 15 minut.es a t the previously stated rate of boil. Turn off the heater, but the flask should remain in the fume duct or hood until the evolution of sulfur trioxide fumes has subsided. Further cool in air with the flasks removed from the still-warm heaters. Hasten final cooling, if desired, by immersing the bulbs in Jvater. Add approximately 100 ml. of a 100 i 10 ml. quantity of distilled water to the flask and swirl the content8 unt’il the salt cake is substantially dissolved. Warming over a Bunsen burner facilitates dissolving the cake. Then add the balance of the distilled water. Remove the heat of dilution by again cooling the flask with water.
WATER I N L E T + . Y h WATER OUTLET
FRlEORlCHS
COWOENSER
I’HOCEUURE
.I I-gram sample i a recommended to assure satisfactory digestrmperat,ure; however, when high accuracy is required on saniples of low nitrogen content, the size of the sample may be invreased. Conversely, when samples of high nitrogen content are analyzed, it is convenient to reduce the size of the sample so that the titration will not exceed the capacity of the buret. Digestion. Introduce into a Kjeldahl flask 20 i 1 grams of potassium sulfate, 1.3 f 0.2 grams of clean metallic mercury (the appropriate number of drops of mercury approximating this amount delivered from a medicine dropper is a convenient means of measurement and addition), and approximately 15 ml. of a 30fl ml. quantity of concentrated sulfuric acid. Add the acid slowly while the flask is inclined and rotated in order to wet its inner wall. n’eigh the recommended size sample to the nearest 0.0001 gram and introduce it into the flask. Avoid alloffing the sample to come in contact with the neck of the flask. Volatile liquids are best handled in some form of weighing dropper, pipet, or bulb. Solids and liquids of low volatility may be weighed conveniently in small porcelain crucibles-e.g., Coors 00000 size. Introduce the sample and crucible together into the digestion flask, where t,he crucible is allowed to remain during the digestion and subsequent distillation. After introduction of the sample, wash down the neck of the flask with the remaining concentrated sulfuric acid. For 2-gram and 3-gram samples use 35 ml. and 40 ml. of total acid, respectively. Swirl the contents of the flask to facilitate mixing of the sample arid sulfuric acid. Add two or three glass beads to promote smoot>hboiling. Place the charged flask on a digestion rack or in a fume hood, and apply low heat (electrical heating is preferred for uniformity and convenience) until frothing has stopped. During the frothing period rotate the flask frequently to allow uniform charring of the sample. For those samples that do not froth or char, a 45 f 15 minute low heat period is recommended. Apply intermediate heat for 10 f 2 minutes to raise the temperature gradually. Finally, apply full heat, approximately 550 watts, so that the flask contents are brought to a rapid boil. The 1 ioii
800 ml. KJELDAHL DISTILLATION FLASK
Figure 1.
Kjeldahl Distillation Apparatus
Distillation. Distillation method A is presented for those laboratories that use standardized Kjeldahl distillation equipment. Causticization and distillation by this method follow closely usual Kjeldahl practice. Distillation method B, developed by Union Oil Co., offers an additional safety feature by permitting t8headdition of strong caustic solution at a rontrollable rate, with vigorous mixing. DISTILLATIOS .\IETHOLI A. Place a ?iOo-ml. Erlenineyer flask containing 25 ml. of boric acid solution and 5 drops of methvl purple indicator solution under the condenser. so that the tip is well beneath the surface of the liquid. hdd two or three small pieces of mossy zinc to the diluted, cooled contents of the digestion flask. Pour slowly 100 i 5 nil. of sodium hydroxide-sodium sulfide dolution down the inclined neck of the Kjeldahl flask, so that a layer forms on the bottom. This is not difficult to arconiplish, hut failure to maintain discrete layers during the operation may lead to violent reaction, with ejection of part of the contents of the flask, or to loss of ammoilia. Connect the flask to the distillation condenser and swirl the flask contents. Should the boric acid-methyl purple indicator solution be drawn over into the distillation flask (rare), no real harm is done. Add another charge of boric acid and indicator t o the receiver and start the distillation. Apply full heat, approximately 550 watts, to the distillation flash, so that rapid boiling is established and maintained. When the volume of liquid in the receiving flask reaches approximately 130 mi., lower the receiver to expose the condenser tip, and rinse
ANALYTICAL CHEMISTRY
1636 it off with distilled water. After about 1 minute of additional distillation, turn off the heat and allow the condenser tube t o drain. For convenience in titration, the boric acid solution plus distillate plus rinse water should not exceed 150 ml. After adjustment with distilled water t o the 150-ml. mark, titrate the receiver contents with standard sulfuric acid until the green color fades into gray and just beyond to the point that the gray assumes the first faint tinge of purplish pink. DISTILLATIONMETHODB. After addition of two or three small pieces of mossy zinc to the &luted, cooled contents of the digestion flask, connect the flask to the distillation condenser as shown in Figure 1.
more effectivecatalyst than mercury alone, However, in order to reduce complicating factors, mercury was used &s the sole catalyst in these experiments. Later experiments showed that selenium oxychloride would reduce the digestion time from 60 t o 35 minutes, but the advantage was not deemed sufficient, in view of reports that losses of ammonia occur during prolonged digestions in the presence Of Temperahre Necessary for Digestion. From consideration of the compounds likely to be present in shale oil or petroleum, pyridine was selected as probably the most refractory compound to be found. A solution of pyridine in a low-nitrogen I20 hydrocarbon solvent was prepared. Nitrogen determinations were made on this samc: 100 A ple by the method described, z except that the amount of E potassium sulfate was varied ; in order to change the digesw 80 a tion temperatures. These tem> 0 0 W peratures were measured by a thermocouples in glass wells z adjusted so that the junctions 8 60 // a were immersed to maximum t depth in the pool of liquid 0 I HOUR OIGESTIO(I when the flasks were inclined A 2 HOURS DlOESTlON in digestion position. The 40 0 3 HOURS DIGESTION / nitrogen as determined is plotted against digestion temperature in Figure 2. It is 3 30 340 350 360 310 380 390 4 00 20 apparent that satisfactory re3 covery is not attained below DlQESTlON TEMPERATURE, AT 745mm. 360’ C. with 1 hour of digesFigure 2. Nitrogen Recovery V S . Digestion Temperature tion, and that 370” C. gives only a reasonable margin of Place 25 ml. of boric acid solution and 5 drops of methyl purple safety. It is also shown that the temperature specification can indicator solution in a 500-ml. Erlenmeyer flask which is tipped be lowered only slightly by increasing the time of digestion. to give greater depth of acid solution a t the start of the distillaA sample of shale oil tar bases was run in a similar manner: tion and is connected t o the vacuum system. When the appathe results are shown in Table I. The temperature necessary for ratus is connected, apply vacuum to the receiver with the air inlet open to keep the solution well mixed. Add slowly approxirecovery of the nitrogen is in the same range, and the existence of mately 100 ml. of the sodium hydroxide-sodium sulfide solution. an upper temperature limit is indicated. Close off the separatory funnel, disconnect the vacuum line. Another simulated digestion study was made with ammonium apply moderate heat, immediately close the air inlet tube, and oxalate to determine the maximum temperature that could be heat the solution to incipient boiling. At this point turn up the heat. The distillation and titration procedure is exactly the reached while still quantitatively retaining ammonium ions. The same as described in method A. temperature was again controlled by the amount of potassium sulfate added. Sample size was approximately constant for the Blanks. Make blank determinations using 1 gram of sucrose series a t about gram. A 10-minute preboil was given to instead of the usual sample each time a new lot of any of the simulate “heating to clear.” Boiling temperatures a t the end of reagents is employed 8
-
, OC.
DEVELOPMENT OF METHOD
The Union Oil Co. of California and the Bureau of Mines, Laramie, Wyo., participated in a program to study the determination of nitrogen in shale oil through analysis of exchange samples (1). Both laboratories obtained results by the Kjeldahl method that were in good agreement with the selected “best values.” The analytical procedures used by the two laboratories were, however, quite different. Union Oil Co. used a combination of mercury and selenium oxychloride as catalyst, whereas the Bureau of Mines used a combination of mercuric oxide and copper sulfate. Although working independently, both laboratories a t about the same time concluded that the temperature for Kjeldah1 digestions is quite critical. At this point, a cooperative evaluation was made of a number of factors involved in Kjeldahl digestion. A search of the literature on the Kjeldahl method showed that no single element had proved as effective as mercury in elemental or combined form for catalyzing digestions (6). One report ( 7 ) indicated that a combination of mercuric oxide and selenium was
Table I.
Digestion Temperatures Necessary for Determination of Nitrogen in Tar Bases
Digest Temp. after 1 Hour, O C. 358 368 381 405 424
Xtrogen, % 8.28 8.90 8.90 8.90 8.62
Table 11. Retention of Ammonium Ions of Ammonium Oxalate at High Digestion Temperatures Temp. of Digestion, C. After After 60-minute Apparent preboil digestion Xitrogen, yo 19.61 322 322 19.63 361 353 19.62 40 1 399 515 8.05 445 13.11 515 a Digest solidified after 32 minutes a n d so digestion was stopped.
1637
V O L U M E 2 3 , N O . 11, N O V E M B E R 1 9 5 1 60 minutes and the apparcnt nitrogen content of the sample are given in Table 11. Loss of sulfuric acid occurred rapidly a t the higher temperatures, as indicated by the temperature rise during the digestion period. This was accompanied by loss of nitrogen. Based on data in Tables I and 11, an upper limit of 410' C. for the digestion temperature appears to be safe. Control of Temperature. The most practical method of assuring the correct digestion temperature is through control of the digestion mixture roniposition. The elevation of the boiling point of the sulfuric acid by addition of potassium sulfate is shorn in Figure 3. Changes in barometric pressure have the expected etiect. Each digestion proceeded for 1 hour beyond the time of clearing, uniform timing being necessary because the temperature tends to increase with the digestion time, due t,o evaporation of sulfuric acid, The temperatures for no-sample runs indicated in Figure 3 are not so high as exist in typical digestions, because samples consume some sulfuric acid, increasing the relative concentration of potassium sulfate in the digest. Curves indicating temperatures ohtained in the digestion of shale oil tar bases also are shown in Figure 3. Such temperatures, att,ained with 20 grams of potassium sulfate are 20" to 25" C. higher than the temperatures attained by no-sample runs a t the same salt concentration and a t rorreaponding atmospheric pressures. The consumption of acid depends on the type of material, being higher for olefinic or aromatic than for saturated compounds. This is illustrated in Table 111, which gives data on the consumption of acid by a variety of different type compounds. The alkane and cycloalkane appear to volatilize with little or no reaction with the digestion misture. If it is assumed that the osidation products of the sample are carbon dioxide and water and the reduction products of sulfuric acid are sulfur dioside and water, the calculated and observed acid c-onsumptions of sucrose, o-xylene, and dodecene are in reasonably good agreement. Because of the increase in digestion temperature caused by the sample, it is necessary to design the method so that the no-sample temperature will be near the lower end of the satisfactory range and that n-ith samples of the type consuming the most acid the temperat,ure will not esceed the upper limit of 410" C. For low
Table V.
Kjeldahl Procedure Precision Sitrogen, Yo ~~~~~~
Shale-oil products A B C
D E F' Petroleum producte G
.I
Table VI.
~
Table IV.
Effect of Amount of Mercury on Determination of Nitrogen yo Nitrogen" Shale-oil tar base Pyridine
"
I-hour digestion 0 . 5 gram mercury 1.3 grams mercury 2.7 grams mercury %hour digestion 0 , 5 gram mercury 1 . 3 grams mercury 2.7 grams mercury Avcraee of 2 determinations.
8.93 8.91 8.87
14.96 17.33 17.31
8.93 8.91 8.90
17.27 17.26 17.31
0 013 0 017 0 50 1.16 3 30 10 18
0 014 0 020 0 50 1.14 3 29 10 26
0 014 0,019
038 174 396 004
0 0 0 1
0 039 0.176 0.403 1 010
01.5
70
of
Theoretical Actually Found
17.55 4,192 0.815 1.11 1 02 0 94
++
c
1.15 3.30 10.22
010 177 410
xitrogen, 70 Calcd. Found
17.696 Pyridine (99.91 mole %) Pyridine (99.91 mole %) toluene 4.244 Pyridine (99.91 mole %) toluene 0.817 Petroleum naphtha with added nitrogena 1.09 Petroleum naphtha with added nitrogen: 0.99 Petroleum naphtha with added nitrogen 0.93 0
0.50
Kjeldahl Procedure Accuracy
99.2 98.8 99.8 101.8 103.0 101. I
From pyrrole a n d 2,4.6-trimethylpyridine. From 1-methylpyrrole and 2,4,6-trimethylpyridine. From 2,5-dimethylpyrrole a n d 2,4,6-trimethylpyridine.
420
I
~
~
~
BAROMETRIC PRESSURE
~
760 Am. HG
&
~
I
L
3
585 mm HG
w K
t 420 = I
9
BAROMETRIC PRESSURE
f 4601 8 SHALE-OIL
w
a 380'
n
Net Loss of HzSOib Final Digestion Sample Due to Sample, GramsC Temp., C.c Dodecane 0 . .i 367 Methylc yclohexane 0.6 368 Sucrose 6.7 377 Shale oil naphtha 10.0 381 Petroleum (paraffinic) 13.4 389 Shale oil light gas oil 15.7 383 Shale oil naDhtha tar base 15.7 399 o-Xvlene 16.6 388 Crude shale oil 1R 4 394 .. Do d e ce n e i9.9 398 a 1 + 0.05 gram samples. 30 nil. of sulfuric acid, 1.3 g r a m of mercury and 25 grams of potassium sulfate (conducted a t 585 nini. pressure so t h a i more potassium sulfate was needed t o approximate sea level digestiontemperature conditions). b Observed weight loss during digestion minus sample weight minus noSam le digestion weight loss. Cfiverage of 2 or more determinations.
AI..
Sample
dW Consumption of Sulfuric Acid by Different Type Samples"
2
0 0 0 1
H
I
I?
Table I l l .
1
Sample
E
NO SAMPLE
340
Ib
io
io
4b
io
$0
76
POTASSIUM SULFATE, grams per 30rnl. HESO*
Figure 3. Control of Digestion Temperature by Varying the Amount of Potassium Sulfate
altitude laboratories this is accomplished by using the reagent proportions specified under the heading Procedure. However, this causes a problem for laboratories at higher altitudes when predominantly alkane-type samples are analyzed. Care must be exercised in such laboratories to see that the digestion mixture reaches the proper temperature. The temperature is conveniently adjusted up\$-ard by cautious increase in the potassium sulfate dosage. Amount of Mercury Needed. Although the function of the mercury is essentially that of a catalyst, some effect of the amount used is noted. Two samples-a shale oil tar base material and pyridine-were analyzed, using different amounts of mercury. Table IV shows that 0.5 gram of mercury is not enough for conversion of pyridine in 1 hour; however, 1.3 grams were enough in the case of both samples. PRECISION AND ACCURACY
The precision observed for this Kjeldahl procedure is shown by a series of analyses in Table V. These were selected a t random
~
ANALYTICAL CHEMISTRY
1638
from routine analyses of shale oil and petroleum products. Lower precision is to be expected when highly volatile aamplw are anslyaed. Accuracies attained with this procedure are indicated by values given in Table VI. LITERATURE CITED
(1) Ball, J. s..and Van hfeter, Robin, ANAL.C H m f . , 23,1632 (1951). (2) Bradstreet, R. B., Chern. Rez,6.,27,331-50 (1940).
(3) Gunning, J. W., 2. anal. Chem., 28, 188 (1899). (4) Kirk, L., CHEM., 354-8 (1*50). (5) Ogg. C. L.,and Willits, C. O., J . dssoc. Ofi. Agr. Chemisk, 33, 22i
100-3,179-88 (1950). (6) Osborn, R.A., and Krasnitr, A , Ibid., 17,339 (1934). (7) Osborii, R. A., and Wilkie, J. B., Ibid., 18, 604 (1935). ( 8 ) White, L.M., and Long, M. C., ANAL.CHEM.,23,363-5 (1951). (9) Wilfarth, H., Chem. Zentr., 56,17,113 (1885). RFCFJVED March 5, 1951.
(Determination of Nitrogen in Shale Oil and Petroleum)
EVALUATION OF DUMAS PROCEDURES BY MASS SPECTROMETRY ROBIN VAN METER, C. W. BAILEY, AND E. C. BRODIE' CT. S . Bureau of Mines, Laramie, Wyo. Attempts to determine nitrogen in shale oil or its fractions by the Dumas micromethod generally resulted in high and variable results. The mass spectrometer was used to analyze the gases in the nitrometer. The presence of hydrocarbons and nitric oxide indicated that proper techniques were not being used in the combustion. The ability to detect
A
N EXCHANGE program to determine the nitrogen content of certain shale oils was discussed in a preceding paper ( 1 ) . The Dumas analysis results reported for a typical sample range from 1.45 to 2.04%. The scattered distribution of these values indicates that many of the laboratories urgently need a means of evaluating this analytical method. Mass spectrometer analyses of gases produced in the Dumas micromethod (3) have been helpful in diagnosing errors in technique and errors due to the use of insufficiently reactive copper oxide and metallic copper. The analyses have aided in the orderly elimination of the more serious errors by showing the effect of changes in technique and reagents. They have also permitted the salvage of otherwise valueless determinations when corrections for extraneous materials have been applied. .41though the Bureau of Mines laboratories have applied mass spectrometer corrections to Dumas microdeterminations only, they should be equally advantageous for verifying semimicroand macrodeterminations. The mass spectrometer has been used with the Dumas method by Hindin and Groase (g) in a different procedure that employs neon as an internal standard.
extraneous gases and to identify them provides a tool by which the effectiveness of the procedures may be tested. Revision of the procedure until only nitrogen is collected in the nitrometer results in good analyses. In some instances corrections may be applied to nitrometer readings containing extraneous gases, so that acceptable results are obtained.
moisture is present. Remove this by absorption in an anhydrous magnesium perchlorate drying tube during introduction into the inlet system. RESULTS
Table I shows the gases that have been found in the nitrometer and their range of concentrations. The hydrocarbon gas is
-t I
l
95mm
-
1 I
T
50 m m
l
I
PROCEDURE
Standard Dumss micromethod nitrometers are equipped with a small funnel sealed above the stopcock. To facilitate the transfer of gas into the mass spectrometer, cut off this funnel just above the stopcock and seal a glass interjoint connection in its place. Figure 1 shows the result of this simple alteration. As the volume et nitrometer gas is so small (generally approximately 0.2 ml.), the usual method of introducing a gas to the mass spectrometer cannot be used. Figure 2 is a schematic drawing of the portion of the inlet system used in a Consolidated mass spectrometer (Model 21-102). Instead of measuring the preasure of the gas in the metering volume, introduce the gas through volume C into the large sample bottle, A , to a pressure of about 0.07 mm. as measured on the Pirani gage. This pressure gives the most satisfactory peak heights for calculations. As the gas is collected over a saturated solution of potassium hydroxide, considerable 1
Present address, University of Arizona. Tucson, Aria.
UNALTERED DUMAS BELOW SEAL
MICRONITROMETER
Figure 1. Modification of Dumas Micromethod Nitrometer for Introducing Collected Gas into the Mass Spectrometer Scale. One half actual size Material. Borosilicate glass throughout
Compounds Found in Dumas Nitrometer G a s and Their Range of Concentrations Compound Max., % ' Min., % '
Table I.
a b
0.0 81.3'' Hydrocarbons 18.6 100 Nitrogen 0.0 4.0 Nitric orideb 0.0 0.3 Carbon dioxide Methane content in this inetance was 80.8%. The presence of nitric oxide was confirmed by chemical means.
