Mass Spectrometric Analysis of low Concentrations of Vapors in Air G. P. HAPP, D. W; STEWART, A N D H. F. BROCKMYRE Kodak Research kboratories, Rochester, N . Y . opened to the main expansion volume and to the leak into the spectrometer. With this procedure, there was no indication of fractionation in the condensation-evaporation process. Despite the use of stopcocks and lubricated ground joints, there was negligible carry-over from one sam le to the next, if care was taken to u8e the sample bulbs only a t pow concentrations, and if back-diffusion from the oil of the mechanical pump was prevented by the second liquid nitrogen trap. Pump-out times between samples varied from 5 to 15 minutes, depending upon the compounds present and their concentrations. In the most recent work, 200-ml. sample bulbs, with break-seals instead of stopcocks, have been used with good results.
LTHOUGH it is well known that the mass spectrometer can determine low concentrations of one gas in the presence of large amounts of another, as in t,he analysis of atmospheres in annealing furnaces for traces of oxygen, or of soil gases for hydrocarbons, no description has been found of techniques suitable for concentrating and determining contaminants in air in the range below 100p.p.m. The following procedure has given useful results when substances that can be trapped by liquid nitrogen are to be determined. This would appear to include most of the ordinary atmospheric contaminants with the exception of carbon monoxide. Typical results are given for several mixtures of solvent vapors in air, sampled in a small test room. The inherent advantages of the mass spectrometric method of gaa analysis are rapidity and freedom from interference by chemically related substances. In addition, the spectrum provides a complete qualitative record, which should be particularly important when the source of atmospheric contamination is not cospletely known. Two recent discussions of mass spectrometry outline the principles and techniques of this instrumental method of analysis (4,6).
A
I""
I 0 Methylene chloride El Acetone x Ethylene dichloride ,Propylene dichloride Q Cyclohexone
APPARATUS AND PROCEDURE
The mass spectrometer was a 60 sector-type instrument constructed in this laboratCoryand used successfully for general analytical work. The an-glass inlet manifold, including several stopcocks and a 1-liter expansion volume, operated without auxiliary heating, as did the ion source. A glass capillary leak was located a t the end of the inlet tube inside the ion source. For calibration, pure liquids in volumes up to 0.0005 mi. were admitted, through a sintered-glass valve, from a graduated micropipet. Ion currents were amplified and then recorded on a b e d s & Northrup Speedomax recorder, with manual selection of shunts. All scanning was done by varying the magnetic field. The maximum over-all sensitivity of the apparatus as used in this work WBS approximately 30 chart divisions for the principal peak of acetone, mass 43, with a n expanded sample pressure of 1 micron, This corresponds to anacetone content of 30 p.p.m. in the 50-ml. sample bulb. The observed peak heights may be measured to 0.2 chart division, or 0.2 p.p.m. for acetone. Air samples were taken from a static atmosphere in a small, tightly'closed laboratory room with painted walls and a volume of 290 cubic feet. Mixed solvents were evaporated on a hot plate, and the air was circulated with an electric fan. Sampling started 20 minutes after completion of the evaporation. Instantaneous samples for the mass spectrometer were taken in previously evacuated 50-ml. bulbs with stopcocks. For comparative chemical analysis, continuous samples were drawn from the same point in the room into suitable absorbers. The volume of air was measured with rotameters. Activated silica gel was used to absorb acetone, chloride-free activated carbon to absorb chlorinated hydrocarbons, and a sulfuric acid-potasaium nitrate nitrating mixture for aromatics. The chemical determination of acetone was by the iodometric method ( S ) , of chlorinated solvents by decomposition with liquid ammonia and sodium ( 5 ) , followed by precipitation with silver nitrate, and of aromatics by extraction and weighing of the nitrated products ( 2 ) . The bulb samples for the mass spectrometer were attached to the inlet, manifold through an &mm. glass U-trap, 4 inches (10 ( m.) deep, which vias cooled in liquid nitrogen. The contents of the bulb were exhausted through the U-trap by means of a mechanical pump preceded by a second liquid nitrogen trap. Careful manipulation of the stopcock on the bulb prevented any large initial surge. Evacuation was continued for a total of 5 minutes. The U-trap and manifold were then isolated (eliminating the volume of the sample bulb), the liquid nitrogen was removed, and the collected solvents were vaporized into the fixed volume of the fore-part of the sample system. After 2 minutes, this volume was 1224
t
01
0
I
5
I
I
15
IO Minutes
Figure 1. Variation of Solvent Concentrations in Air with Time Five-component mixture
Typical curves of concentration L'S. time of sampling for a five-component misture, shown i n Figure 1, indicate a dieaway effect due to absorption by the walls and contents of the room. ilverage concentrations obtained graphically from these and other similar results are shown in Tubk I, together with the
Table 1. Analysis of Test-Room Atmospheres Volume Found, P.P.M. Sample No. 1
Component Acetone (see Figure 1) Cyclohexane Methylene chloride Ethylene dichloride Propylene dichloride Total chlorinated solvents
Mass
spectrometer 44.4 30.6 53 6 43.4 36.4 133 4
Chemical 48a 2
145 5
-2
Acetone Methylene chloride
19.8 36.1
19.3 39.1
3
Acetone Methylene chloride
41.8 11.6
46.1 13.3
4
Toluene Benzene Total aromatics
229.1 161.8
Not determined separately.
300.4
360
V O L U M E 22;NO.
1225
9, S E P T E M B E R 1 9 5 0
comparable chemical analyses. Direct comparison with the amount evaporated was not possible because of the absorption during the necessary mixing period in the closed room. The results deviate, on the average, by 8.3% from the amount found chemically, with the spectrometer low in five out of the seven c:tses. Although this accuracy is sufficient for many studies in industrial hygiene, particularly when conventional methods are not suitable because of the complexity of the mixture or the need to take instantaneous samples, further work is expected to result in considerable improvement. A further description of this technique and its application will be published
LITERATURE CITED
(1) Happ, G. P., Stewart, D. W., and Brockmyre, H. F., Am. I d . Hug. Assoc., Quart., in press. (2) Manning, A. B., J. Chem. SOC.,1929, 1014. (3) Messinger, J., BeT., 21, 3366 (1888). (4) Stewart, D. W., “Mass Spectrometry,” Chap. XXXI in “Physical Methods of Organic Chemistry,” 2nd ed., Part 11. A. Weiss-
berger, ed., New York, Interscience Publishers, 1949. (5) Vaughn, T. H., and Nieuwland, J. A., IND.ENQ.CREM.,ANAL. ED.,3, 274 (1931); 4, 22 (1932). (6) Washburn, H. W., “Mass Spectrometry,” in “Physical Methods in Chemical Analysis,” W. G. Berl, ed., Vol. I, New York,
Academic Press, 1950. RECEIVED April 14, 1950.
(1).
Cyclotetramethylene Tetranitramine (HMX)
36.
Contributed by WALTER C. MCCRONE, Armour Research Foundation, Illinois Institute of Technology, Chicago 16, Ill. Interfacial Angleso(Polar). 101 A 101 = 8X” Beta Angle. 103 . Density. 1.96.
OPTICAL PROPERTIES
Refractive Indexes (5893 A.; 25’ C.). 01 = 1.589 * 0.002 ; * 0.002; y = 1.73 0.01. ODtic Axial Andes (5893 A.; 25’ C.). 2V = 20”; 2 E = 33”. Dispersion. Scght’horizontal, T > v. ODtic Axial Plane. 1 0 1 0 : rhc = 30” in acute 8. Si’gn of Double RefraFtion.’ Positive. Acute Bisectrix. y.
@ = 1.594
\
CH2-T
/ I
SO? Structural Formula for 1IUX
f
c
IIMX is a high melting by-product in the manufacture of RDX. The cystallography of H M S is therefore important iu order to recognize this component in IlDS products. I t is also particularly interesting because it exists in four polymorphic forms, each of which can be obtained at will from a variety of solvents by varying the rate of cooling during crystallization. HMX I
The room-temperature-stable form of HMX can be prcpwrd by very slow cooling of solutions of HMX in acetic acid, acetone, nitric acid, or nitromethane. .%gitation favors formation of I.
Figure 2. Orthographic Projection of HMX I1 Molecular Refraction (R) (5893 A,; 25“ C.). R (calcd.) = 58.0. R (obsd.) = 56.1. FUSIONDATA
w
Figure 1. Orthographic Projection of HMX I
CRYSTAL MORPHOLOQY Crystal System. Monoclinic. Form and Habit, Massive crystals showing the forms: prism, { 110 ; clinopinacoid, (010) ; clinodome, [ 011 ) ; agd both the positive and negative hemiorthodomes, [ 101) and 11011.
v(YaY = 1.64.
The three low temperature modifications of H M X transform through the solid phase to H M X IV a t about 156’ C.; hence, the sublimate and the crystals formed on cooling are of the latter modification. Some decomposition occurs during melting a t 279” C. and the preparation should be quickly cooled. The sublimate shows slightly rounded hexagonal pimcoid and bipyramid combinations. All possible orientations are usually shown with a few giving a centered uniaxial negative interference figure. The crystals from the melt show the same random orientations, but again some six-sided outlines can be recognized showing low birefringence and a centered figure. HMX I1
HMX I1 can be prepared from the same solyents as I but with more rapid cooling. It is the stable form from about 115’ to 156’ C. CRYSTAL MORPHOLOGY Crystal System. Orthorhombic
