A Rapid Combustion Method for Nitrogen Determination Utilizing Gas Chromatography M. L. PARSONS, S. N. PENNINGTON, and J. M. WALKER Department of Chemisfry, Kansas Sfate College o f Piffsburg, Piffsburg, Kan.
b This method utilizes the rapid combustion of a sample by means of a high frequency induction furnace and the simplicity of gas chromatography to detect combustion gases. The procedure i s based upon sample combustion in a helium atmosphere, and subsequent conversion of the nitrogen oxides to elemental nitrogen. Other combustion products are absorbed prior to reaching the chromatographic column. The nitrogen i s passed through a 5-A. Molecular Sieve column, detected by a thermal conductivity detector, and recorded on a strip chart recorder. The incorporation of a double O-ring seal allowed the construction of an all metal system and resulted in a greater applicability of the method to various chemical compounds. The method i s general for all compounds thus far analyzed: acetanilide derivatives; aniline derivatives; heterocyclic compounds of pyridine, caffeine, and triazole; nitro compounds; amines; ammonium nitrate; sodium nitroferricyanide, and associated materials such as blood. Other elements present include carbon, hydrogen, oxygen, sulfur, sodium, and iron, and they did not interfere. Sample sizes ranged from 1 to 10 mg. for the compounds and 10 to 20 mg. for the blood. The method has an over-all precision to 0.1% absolute. One person can make about 30 analyses in an 8-hour working day.
Meloan (1) determined carbon and sulfur with an analogous method also utilizing a liquid nitrogen trap. Nightingale and Walker (3) introduced the use of a high frequency induction furnace for the rapid combustion of samples and eliminated the need for a liquid nitrogen trap, thus allowing the simultaneous analysis of carbon, hydrogen, and nitrogen. I t was the purpose of this investigation to develop a rapid, accurate method for the determination of nitrogen which would be general for a wide variety of compounds. The system developed by Kightingale and Walker has been modified to accomplish this intention. The method described here incorporates a double O-ring seal on the quartz combustion tube permitting the construction of an all-metal system which could be pressurized up to 20 p.s.i. The water and carbon dioxide from the combustion gases are absorbed by magnesium perchlorate and Ascarite, eliminating the need for purging the column between runs. -4 method has been de-
veloped to purge the combustion tube of atmospheric gases entrapped upon changing samples. These modifications allowed the nitrogen to be determined isothermally a t 50" C., thus reducing the time for a single analysis to less than 15 minutes. At least 30 samples may be run by one person in a normal 8-hour working day. EXPERIMENTAL
Apparatus and Materials. The system for this method is shown diagrammatically in Figure 1. It consists of a helium supply, Bureau of Mines helium furnished by the Linde Co. The helium is purified by scrubbing with Arthur H. Thomas Co. Ascarite (8-20 mesh), Mallinckrodt wire form, analytical reagent grade cupric oxide held a t 800' C. plus by a Burrell, Model T-2-9, electric furnace, anhydrous Fisher reagent grade magnesium perchlorate, F & M Scientific 5-A. Molecular Sieve moisture trap, and Leco specially prepared manganese dioxide. A Leco Model 521, high frequency induction furnace with a
S
methods which apply gas chromatography t o the microdetermination of certain elements in chemical compounds have appeared in the literature. Walish (5) describes a method for carbon, hydrogen, and nitrogen determination utilizing two thermal conductivity detectors. Duswalt and Brandt ( 9 ) burned samples in an oxygen atmosphere and concentrated the combustion products in a liquid nitrogen trap with subsequent determination of carbon and hydrogen by gas chromatography. Sundburg and Maresh (4) eliminated the use of an oxygen atmosphere by the utilization of an internal oxidant in a helium atmosphere, and then analyzed carbon and hydrogen in much the same manner as Duswalt and Brandt. Beuerman and EVERAL
842
ANALYTICAL CHEMISTRY
h Figure 1.
Schematic diagram of system
a. Helium supply b. Ascarite tubes C.
Burrell furnace with cupric oxide tube
d. Magnesium perchlorate tubes e.
5-A. Molecular Sieve moisture trap
f. Differential flow regulalor
g. Leco specially prepared manganese dioxide
lube
h. Leco induction furnace
Quartz combustion tube with double 0 ring seal i. Hevi-Duty electric furnace containing brass oxidation-reduction tube k. Colcium carbide tube 1. 5-A. Molecular Sieve column rn. Thermistor block n. Reference side 0. Sample side V,, V 2 , V3, V?, Vb, VS,V7. Valves
i.
7 - - 8 cm. A-
B
- -+
0.64 c m .
-1 I
t
I_ 1-45-72 Figure 3.
I\ crn.~-j
'
Brass oxidation-reduction tube
Arrows point to silver soldered joints,, A. Volume which contains cupric oxide B . Volume which contains elemental copper
Figure 2.
Quart;: combustion tube with O-ring seal
A. Quarter-inch brass tube connection B. Bolt C. Stainless steel O-ring holder D. Three Kont8.s Glass Co. No. 9, Viton O-rings
E. F.
Quartz-enclosed carbon igniter Quartz conibustion tube G. Regular Leco O-ring seal
quartz combust,ion tuh:, a double O-ring seal modification as sl-.on.n in Figure 2 , and a n F 8r. M Scientific hloael 202 gas chromatograph equipped rrith a -l-foot, 5-A. Ilolecu1:tr Sieve column arid a circulating air w e n were incorporated into the system. The reJ brass, type A, oxidat'ionreduction tube was 18 inches in length and contained about 5% Mallinckrodt analytical reagent p t d e cupric oxide follomed by 9570 elemental copper, purified by reduction of Mallinckrodt reagent cupric oxide with hydrogen (See Figure 3). The oxidation-reduction tube was heated by a Hevi-Duty Electric Co. Multiple Cnit tube heater with controls set on four coarse and six fine (approximately 800" C.). Before liein2 placed into the system, this combustion tube was conditioned by oxidation and reduction using air and hydrogen. rcrpectivelj-, wh de being heated hy the Hevi-Duty furnace with a setting of three coarse and three fine. To prevent the con3:nsat'ion of water bctneen the combustion and absorption tubes, the copper tubing in this section em \\-as henlied with Electrotliermal S o . HTX2 heat,inq tape, and two Sj-lvania Electric I50-watt projetator spotlights PAR 38 N e d . Skt. w r e focnsetl on the wmbustion tube during each run. The absorption tubes (Icigure 1) contained anhydrous Fisher reagent, gratlc niagnesium wchlorate and
Arthur H. Thomas Co. Ascarite (8-20 mesh). Another tube filled with Fisher laboratory chemical calcium carbide (Electrolite 20-30 mesh) was used to convert a n y water, not previously absorbed, to acetylene which does not interfere with the column. Samples were combusted in Leco quartz-enclosed carbon crucibles. They were cleaned between each run with
5
Retentlo"
0
1 1 m e
(Mtn
I
Figure 4. Trace of typical nitrogen chromatog ra m
aqua regia, rinsed with deionized, distilled water and dried on a n electric hot plate. Silver pei,manganate, used as the oxidizing agent, was prepared by the reaction of Mallinckrodt analytical reagent grade silver nitrate and Baker analyzed reagent grade potassium permanganate in hot aqueous solution. The silver permanganate was washed several times with hot deionized, distilled water for further purification. The entire system consisted of copper, brass, and stainless steel with the exception of the quartz Combustion tube (Figure 2 ) . Procedure. T h e system was connected as shown in Figure 1 with valves VI and V 3closed and all others opened. T h e chromatograph was operated under the following conditions: attenuation setting, 8 ; column temperature 50' C.; thermistor block temperature, 205" C. thermistor bridge setting, 9 ma.; injection port', off; helium pressure, 10 p s i ; helium flow rate-reference 13 ml./minute, sample, 36.5 ml./minute; chart speed, 1 inch/ minute. Initially, the system was conditioned by programming the column several t'imes to 400' C. and then holding i t at 400" C. overnight (at the end of each day the column was held at 400" C. until the next day), and all furnaces were allowed 1 day in which to reach equilibrium. The samples n-ere weighed either on a Cahn Electrobalance, Model M-10, or the lLlettler Gram-Atic-Balance (sample sizes ranged from 1 t o 20 mg. depending upon the relative amount of nitrogen present) and placed in a quartz-enclosed carbon crucible, mixed wit,h 0.8 gram of silver permanganate; and a n-ad of cupric oxide, made by oxidizing Baker and Adamson copper metal light turnings, Code 1619, over a flame, \vas tightly inserted over the silver permanganate-sample mixture. I-alve P2 n-as shut and valve Tig opened; the sample was placed in the high frequency induction furnace; valve VI was opened to purge out the entrapped atmospheric gases. After about 30 seconds, valve l i p was opened and valves VI and V S were shut. The sample was then combusted for 35 hecorids and the resulting nitrogen gas n-as detected by the thermal conductivity detector and its peak recorded o n a strip chart recorder. Xi1 actnal t r a w of a typical chromatogram is slion-n in Figure 4. The areas under the peak.s were integrated with a Keuffel and Esser Co. VOL. 35, NO. 7 , JUNE 1963
843
Results of Nitrogen Determination for Several Compounds
Table 1.
Sitrogen, Theory Found
Compound p-Bromoacetanilide p-Nitroaniline Siacin (nicotinic acid) Xicinamide (nicotinamide) a-Bromo-4-nitrotoluene Thiourea Caffeine Amnionium nitrate Sodium nitroferricyaiiide lI2,3-Benzotriazole
Table
Sample no.
II.
11.38 23 94 6 48 36.80
2s.85 3 5 . 00 28 21 36 28
Results of Ten Blood Determinations
&lean error
4.19 2.07 4.90 3.49 2.02 4.99
-0.04 $0.02
4.T6
+0.15 -0.12 -0.09 tO.08
I I1 I11 IV
4.16 2.05 4.80 3.54 2.08 4.96 4.61 12.03 6.72 3.06
v
VI 1-11 VI11
IS
s
6.55 20.26 11.31 23. 2iJ 6.48 X6, 80 28.60 34. SS 28, 19 35.09
6 54
20.2s
Xitrogen, 70 Theory Found
+0.10 -0.05 -0.06
t0.03
11.91 6.63 3.14
Mean error
N o . 4212 planimeter. 11 mean value of area per milligram !vas used to calculate the per cent nitrogen in the samples. RESULTS
h series of runs was made to determine the amount of silver permanganate needed for complete combustion of the various samples. The results of these determinations for several typical compounds are given in Figure 5. These results show the need for a measured excess of silver permanganate, especially for compounds in which the nitrogen is incorporated into a ring system. It was decided to use 0.8 gram of osidant, and this has proved to be quite adequate.
Other elements C, H, 0, Br
+0.01
c, H, 0 c, H, 0
-0.02 -0.07
C, H, 0 C, H, 0. Ur C, 14, s C, H, 0 H, 0 C, H, 0 , Xa, Ice C, H
+0.35 - 0.05 0.00 -0.16 --0.1"
-0.02 -0.19
Several blanks were run, using only silver I)ermanganate (0.8 gramj and cupric oxide wads, and it was found that a smail, reproducible amount of nitrogen remained in silver permanganate as an iin,iurit>y. The average area caused by this imilurity was subsequently subtracted from the areas of all analyzed samiks. Ten organic and inorganic nitrogencontaining compounds representing a variety of nitrogen structures were analyzed. The results were calculated either according to the minimum assay purity of each particular compound as stated by the supplier or purified in this laboratory and considered as 100yo pure. In ail cases the compounds were dried over phosphorous pentoxide. Sample sizes ranged from 1 to 10 milligrams, and were weighed on the Cahn Electrobalance in order to have four 1)lace weighing accuracy on the higher ~~ercentage nitrogen compounds. The results are given in Table I. Each value listed in Tables I and I1 is a mean of three experimental runs. Ten powderrd commercial blood samI)lcs (Thorn Smith Co., Royal Oak, JIich.! ranging from 2 to 12% nitrogen, were analyzed on a wet or dry basis according to the supplier's instruct'ions. Sample sizes were kept above 10 milligrams so that the four-place llettler balance could be used, thus keeping the
method simple and more applicable to routine laboratory analysis. The results are given in Table 11. The theoretical nitrogen percentage, given by the supplier, was obtained by the Kjeldahl method. The precision of these analybes is of the same order as the mean error shown in Table 11. DISCUSSION
'lie conilwinds analyzed iricludcd ncetaiiilide derivatives; aniline derivatives; heterocyclic structures including pyridine, purine, and triazole; nitro compounds; amines; ammonium nitrate; sodium nitrokrricyanide; and associated materials such as powdered blood. One can see from the above results that this method gives relatively uniform accuracy for the compounds thus far analyzed and seems to be general for a variety of nitrogen-containing compounds. The speed and accuracy appears to be comparable to that of the modified micro-Dumas methods in use today. The simplicity and ease in operation should also be noted. Furt,her, the use of the double O-ring seal has allowed the analysis to include explosive-type compounds such as ammonium nitrate and thiourea. The incorporation of a threeway solenoid-type valve to purge atmospheric gases from the combustion tube could well reduce the time required for each run. Since other combustion products are absorbed prior to reaching the chromatographic column, i t is the opinion of the authors that the column length could be reduced considerably, or, under an ideal set of parameters, eliminated from the system. ,4much shorter column might well bring total analysis time to less than 3 minutes. The ultimate sensitivity of this method (attenuat'ion set a t one) is ai)proximately 6T square inches per milligram of nit'rogen (1 p g . of nitrogen is equivalent to 0.067 square inch). It is felt that further investigation will find that this method can be extended to include samples of much smaller size and much less nitrogen content without appreciable loss of speed or accuracy. ACKNOWLEDGMENT
4 Figure 5. Graph of recorder response vs. grams of silver permanganate used for several typical compounds A. Ammonium nitrate B. Thiourea C. S o d i u m n i t r o f e r r i cyanide D. Caffeine 0.1
0.2
0.3
Grams
844
0.4
AgMnO,
ANALYTICAL CHEMISTRY
0
5
0.6
0.7
OS
(1) Beuerman, D. R . , RIeloan, C. E , ANAL.CHEM . 34, 319 (1962). (2) Duswalt, A A , Brandt, W. W., Ibid., 32, 27: i (1960) (3) Xightingab C. F., Walker, J. M., Ibid., 34 , 14ik (1962). (.4 ,) SundbIurg, 0 C., Maresh, C., Ibid , 32, 274 (1'360). ( 5 ) Walisl1, W., Chem. Ber. 94, 2314 0
(1961).
RECEIVED for review December 31, 1962. Accepted April 1, 1903. Presented in part before Division of -4nalytical Chemistry, 144th Meeting, ACS, Los Angeles, Calif., April 1963. Research sponsored, in part, by the National Science Foundation's Undergraduate Science Education Program.
