cating our modifications, a few of the more obvious ones will lie included in this description. A 6-volt storage battery was ujed to operate the filaments ir. the original amplifier because of t8he€;reat difficulty experienced in eliminating stray 60cycle pickup. It is a nui::ance to maintain and it could probabIy be replaced with a rectified d.c. source. When the photomultiplier is operat'ed a t high voltages the noi5e level becomes quite significant. A simple integrating circuit can be added n-hi~~b reduces the meter fluctnat'ions. d t one t'ime in the development of this instrument this iva3 done by inserting some large capacitors in the d.c. out'put of the Heathkit meter.
At the present time no integration is used. The analyzer', divided circle is good to approximately O.O0lo and this is what determines how small an angle can be measured. I n wavelength regions n ith adequate light, the sensitivity of the null point mag permit measurements to 0.0001". It would be a relatively simple matter to include a d.c. Faraday cell to compensate sample rotations as small as this. The main drawback of u4ng such a compen.ator iq that i t would have to be calibrated a t every wavelength wed. (One of the very ingenious features of Gillham's 1957 polarimeter design was the use of a feedback loop to a d.c. Faraday compensator, making it a direct-reading inqtrument.)
ACKNOWLEDGMENT
The author thanks John A. Schellmau for the use of his laboratory facilities during the early development of the instrument, L. s. Bartell for the loan of a Rudolph polarimeter, and Joel Hail and Brian hIyhr for assistance in constructing and testing the instrument in its final form. LITERATURE CITED
(1) Fopiano, P. J., Tragerer, W. B., Ph.D.
thesis, Massachusetts Institute of Technology, Cambridge, Mass., 1951. ( 2 ) Gates, J. W., Chem. Ind. (London)
1958, 190. ( 3 ) Gillham, E. J., J. Sci. Inst. 34, 435 (1957). WORK supported by Research Grant Kumber A6067 from the National Institutes of Health, Public Health Service.
Determination of Total Nitrogen b y a Combined Dumas-Gas Chromatographic Technique B. A. Stewart, L. K. Porter, and W. E. Beard, Soil & Water Conservation Research Division, Agricultural Research Service, USDA, Ft. Collins, Colo. ETERZIIXATION of nitrogen by the D D u m a s method has in many cases yielded high values, especially when materials containing long aliphatic carbon chains were analyzed. The high values for these materials h a r e been shown (1, 4, 5) to be due, at least in part, to incomplete combustion of the sample. Rather than csarbon dioxide and nitrogen being the only gases produced, methane, and ;ossibly other gases, escape into the nitrometer and are measured along with the nitrogen gas. Carbon monoxide and nitrogen ovides also have been known to escape into the nitrometer, although these gases generally occur cnly when the coiiper layer has been exhausted or is maintained a t too high a temperature. The possibility of using a gas chromatograph for measuring the nitrogen gas produced by the Dumaq combustion was investigated so that any other gases, if produced, could be separated from the nitrogen.
recorder capable of registering 1 millivolt full-scale. The column was 4 feet long and packed with 5 h Alolecular Sieves (32- to 60-mesh). The cabinet of the chromatograph was maintained at 34.5 + 1' C. Helium was used as the carrier gas with the flow maintained a t 90 cc. per minute. The cur-
rent to the detector cell was maintained a t 250 ma. SITROUETER AND GAS-TRANSFER EQUIPMENT.A diagram of the nitrometer and gas-transfer equipment is presented in Figure 1. The nitrometer mas attached to the combustion apparatus and replaced the nitrometer
--T pf 4-ray
s ~ o p c o c ~ =
-
T O Chromatograph
ne
Serum Cap Port
3 - w a y Teflon Stopcock
To A t m o s p h e r e
From
Combustion
50 % Aqueoui KOH
EXPERIMENTAL
Apparatus. ~ o h 1 s u s - ~ I o r id. Colemail automatic nitrogen analyzer (Model 29, Coleman Instruments, Inc., Maywood, Ill.) W E used as t h e combustion apparatus. Details of this instrument can be found in t h e operating manual of t h e manufacturer. T h e instrument waq used as recommended in t h e mxnual with the excc.ptions t h a t t h e n trometer was replaced and the micros,-ringe was not used. The description of this replacement is included below. GAS CHROUATOGRAPII. The chromatographic equipment consisted of a Beckman GC-1, equipped ivith a Brown
10 c c . Syringe
Figure 1. ment
Diagram of nitrometer and gas-transfer equip-
VOL. 3 5 , NO. 9, AUGUST 1963
1331
designed by the manufacturer. The nitrometer was filled with 65 ml. of 50% aqueous KOH solution and 15 ml. of mercury. The volume of the loop on top of the nitrometer is approximately 4 ml. Procedure. At the beginning of the sample combustion cycle, the twoway stopcock shown in Figure 1 is opened and remains open until the combustion has been completed and all of the nitrogen gas is swept into the nitrometer; then this stopcock is closed. During this period, t h e threeway stopcock is closed, and the nitrogen gas is collected at t h e top of the nitrometer, pushing the KOH solution downward. Also during this period, the four-way stopcock is in the position shown in the diagram which allows the helium to flow through the loop. At completion of the combustion cycle, the four-way stopcock is turned 90" to the position shown by the dotted lines. This allows the He to bypass the loop, but continue to flow a t the same rate as before. The three-way stopcock is then turned slowly clockwise, and at the same time the plunger of the syringe is held downward since the He in the loop is a t a positive pressure and will push the KOH solution downward. With this stopcock open, the H e and S 2 become mixed, and then the gas is pushed back into the loop by pressing downward on the syringe plunger until the KOH level has reached the top of the capillary tube portion of the stopcock. The three-way stopcock is then turned back to the position shown, leaving the nitrogen gas located in the loop. The four-may stopcock is then turned back to the position shown, which injects the sample into the chromatograph. At this time the two-way stopcock can be opened, and another sample started through the combustion cycle. With this procedure, a sample can be analyzed every 10 minutes. During the operation of the apparatus, the three-way stopcock is used only as a two-way stopcock. The purpose of the three-way stopcock is to make available a vent which is very useful in emptying and refilling the KOH solution. A Teflon stopcock was used to eliminate the need for a stopcock grease, which would be attacked by the KOH solution. A serum cap port, shown in Figure 1, is used to introduce known amounts of air by means of a syringe. These are injected periodically to check the performance of the column. The weight of sample used has been varied so that the nitrogen gas resulting from the combustion is approximately 0.5 ml. A standard is then analyzed and the unknowns are compared to the standard. The authors have tried to keep the peak heights of the unknowns
1332
ANALYTICAL CHEMISTRY
within about 5% of those of the standard, since it appears safe to assume linearity over this range. A gas sample of about 0.5 ml. produces a peak height of about 65 when the attenuator is turned to sensitivity 5 . By turning the attenuator to sensitivity 1, very small gas samples can be determined with practically the same precision as the larger samples. Standards should be chosen so that the nitrogen content is somewhat similar to the unknowns. This ensures that the operating conditions and weighing precision remain common to the standards and unknowns. Pure compounds, as well as materials whose nitrogen contents are known, can serve equally well as standards providing it is known that they can be satisfactorily combusted. Since the authors are primarily concerned with determination of nitrogen in soils, they used as a standard a soil known to yield the same nitrogen values by both Kjeldahl and Dumas methods. There is a tendency for the peak heights to decrease slightly when several samples are analyzed. This is undoubtedly due to a collection of moisture on the column caused by passing the combustion gases through aqueous KOH. However, if a standard is analyzed after every four or five samples, these changes do not present any difficulties. The contents of the column are periodically (approximately after 200 samples) emptied and dried a t 500" C. for 30 minutes. The collection of moisture by the column could possibly be eliminated by including a moisture trap between the gas transfer equipment and the column. DISCUSSION
The reproducibility of the method has been extremely satisfactory. Results of soil analyses have shown that values can be consistently reproduced within *lo 1i.p.m. when the total nitrogen of the soil is approximately 1500 p.p.m. A chief advantage of this procedure over the volumetric determination of the nitrogen gas is that if any other gases are produced during the combustion, they are separated and not measured with the nitrogen gas. Both methane and carbon monoxide have been found using this procedure on soils and plant materials. Carbon monoxide is present consistently if the post-heater tube of the combustion apparatus is raised above 600" C. Methane was found when soils high in organic matter or plant materials were analyzed. Methane can also be found when analyzing low organic matter materials after
several samples have been analyzed without changing the contents of the post-heater tube. Apparently, the copper oxide in the tube loses its oxidizing capacity and the methane passes without being combusted. Other advantages of this procedure are the elimination of temperature, barometric pressure, and vapor pressure corrections, all of n hich present possibilities for errors. Also, any leak in the combustion apparatus can now easily be detected by the presence of an oxygen peak. Reitsema and Allphin (3) proposed a method for determination of nitrogen and oxygen simultaneously by means of combustion and gas chromatography, and Nightingale and Walker ( 2 ) have recently developed a similar procedure for simultaneous measurement of carbon, hydrogen, and nitrogen. However, their combustion methods deviate considerably from those of the standard Dumas procedures, whereas the method described in this paper differs only in the technique used for measuring the nitrogen gas produced. Although the technique described appears to be a satisfactory method for quantitatively measuring the nitrogen gas produced, another practical use is qualitatively detecting other gases. This provides a very useful means in determining the upper temperature limit a t which the post-heater tube can be maintained without allowing carbon monoxide to escape, as well as what matetials are resisting complete combustion and allowing methane to pass into the nitrometer. Oxidizing agents, other than the copper oxide commonly used, can be investigated by this technique in an attempt to find those that are better suited to the combustion of materials that are likely to produce methane. LITERATURE CITED
(1) Kirsten,
X. J., Mikrochemie Ver. Wikrochim. Acta. 40, 121 (1952). ( 2 ) Xightingale, C. F., Walker, J. A l . , AKAL.CHEW34,1435 (1962). ( 3 ) Reitsema, R . H., hllphin, K.L., Z b d , 3 3 , 3 5 5 (1961). ( 4 ) Stewart, B. A , , Porter, L. K., Clark, F. E., Soil Scz. SOC. Am. Proc. 27, in pres*. ( 5 ) Steyermark, -41, "Quantitative Orgame Microanalysis," 2nd ed., p. 151, Academic Press. Yew York and London, 1961.
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