Recommended General Procedure. Transfer a solution containing 0.3 to 1.8 mg of molybdenum(V1) to a 125-1111 separatory funnel containing 10 ml of the acetate buffer solution. Add 5.0 ml of a 2 APDTC solution by means of a transfer pipet and mix well. Allow the mixture to stand until the yellow precipitate which first forms disappears and the solution becomes a red-purple color. Add 25 ml of reagent grade chloroform and shake vigorously for at least 30 seconds. Let the funnel set until the chloroform layer has cleared of dispersed water droplets and transfer the chloroform to the separatory funnel and repeat the extraction. When the organic layer has become clear, transfer the chloroform layer to a volumetric flask. Dilute to the mark with chloroform. Prepare a reference blank solution by following the same procedure but substituting an equivalent volume of distilled water for the molybdenum(V1) solution. Measure the absorbance of the chloroform extract from the sample solution at 388 mk against the reagent blank solution in the reference cell. RESULTS AND DISCUSSION
Effect of Variables. MOLYBDENUM CONCENTRATION. The absorbance maximum for the molybdenum-PDTC complex was at 388 rnb when a reagent blank solution was used in the reference cell (Figure 4). (The chloroform extract used for the reference solution exhibits moderate absorbance in the 350-400 mk.) Although there are also absorbance maxima in the visible region, they are not as reproducible and their possible analytical significance was not investigated. Conformity to Beer’s law is found for 0-30 ppm of molyb-
denum and the optimum concentration range is 3-15 ppm. The molar absorptivity is 8.50 X l o 3 liter/mole-cm. REAGENTCONCENTRATION. In studying the amount of a 1.O% APDTC solution required for maximum development of maximum absorbance in solution at pH 4.5, it was found that 5 ml of the 1 % APDTC solution was sufficient. pH. It was found that the molybdenum-PDTC complex did not form above p H 6. At p H below 4, the reagent and the complex tend to decompose more rapidly. The recommended p H is 4.5 + 0.5. DIVERSE IONS. Five hundred ppm of the following ions caused no interference in determining 18 ppm of molybdenum: ammonium, calcium, cerium(III), magnesium, potassium, sodium, mercury(II), chloride, bromide, perchlorate, nitrate, sulfate, arsenate, acetate, citrate, and tartrate. The interfering ions are listed in Table IV. PRECISION.An estimate of the reproducibility obtainable with this proposed method can be obtained from the results of seven determinations of solutions containing 18 ppm molybdenum which gave a relative standard deviation of 0.8%. The exact nature of the complexation reaction and the composition of the complex has not been determined as yet. Preliminary evidence is that molybdenum(V1) is reduced by the reagent to molybdenum(II1) and that this reduction is faster in more acidic solutions. Further study of this sequence of reactions is contemplated. RECEIVED for review January 26, 1968. Accepted March 13, 1968. Presented a t the 15th Anachem Conference, Detroit, Mich., October 1967.
Thermogravimetric-Mass Spectrometric Analysis Fred Zitomer Celanese Research Company, Box 1000, Summit, N . J . 07901 Thermogravimetric mass spectrometric (TGA-MS) analysis can be applied to a variety of problems involving thermal degradations, structure elucidations, and the determination of volatiles. An interfacing similar to ones used with VPC-mass spectrometer combinations can be utilized with most mass spectrometers having rapid scan capabilities. The method has the advantage of allowing degradation studies and volatile determinations to be run in controlled atmospheres, while providing order of liberation data with minimum time lapse between evolution and mass spectrometer detection. The ability of TGA-MS analysis to provide data concerning the identification of degradation volatiles, their order of evolution, the detection of volatile impurities in high boiling substances, and the elucidation of structure i s illustrated with the thermal degradations of polymethylene sulfide and a maleic hydrazide methyl vinyl ether copolymer.
THEMASS SPECTROMETER has been shown to be a useful tool for monitoring effluent gases and the volatiles liberated from a variety of systems (1-6). Most often, a time-of-flight mass spectrometer, because of its rapid scan facility and ion source accessibility, has been the instrument of choice for this type of study. Improvements in instrumentation have, however, established the feasibility of using almost any type of mass spectrometer geometry. Consequently, many laboratories now have the capability of monitoring effluent gas mixtures
with mass spectrometers which can scan beyond m/e 500 in a matter of seconds. In this laboratory, two mass spectrometers, a Bendix Timeof-Flight Model 12, modified to include an extended flight tube and Studier continuous ion source (7), and a Consolidated Electrodynamics Model 21-104, have been coupled to a DuPont Model 950 thermogravimetric analyzer (8). This combination, in effect, adds a spectrometric dimension to TGA analysis, and it is being used for both thermal degradation studies and as a routine analytical tool.
(1) R. S. Gohlke and H. G. Langer, ANAL.CHEM., 37, 25A (1965). (2) H. G. Langer and R. S. Gohlke, ibid., 35,1301 (1963). (3) D. R. Peterson, H. W. Rinn, and S. T. Sutton, J. Phys. Chem., 68, 3057 (1964). (4) L. C. Scala, W. M. Hickam, and A. Langer, Reu. Sci. Znstr., 29, 988 (1958). ( 5 ) W. W. Wendlandt, “Thermal Methods of Analysis,” WileyInterscience, New York, 1964. (6) P. E. Slade, Jr., and L. T. Jenkins, “Techniques and Methods of Polymer Evaluation,” Vol. I, “Thermal Analysis,” Marcel Dekker, New York, 1966. (7) M. H. Studier, Paper, 11th Annual Conference on Mass Spectrometry and Allied Topics of ASTM E-14, San Francisco, Calif., May 19-24, 1963. (8) F. Zitomer, Paper, DuPont Thermal Methods Conference, Wilmington, Del., May 23-24, 1967. VOL. 40, NO. 7, JUNE 1968
1091
a RECORDER
I
MELTING POINT
3-WAY METERING VALVE
80
70
3
I MINUTE
40
M
SPECTROMETER PURGE
I
OSCILLOSCOPEI 00
Figure 1. Block diagram of TGA-MS instrumentation Perhaps the most useful applications of the technique are in areas where data concerning the order of product evolution are required. The common practice of collecting or trapping fractions for subsequent analysis by spectroscopic or other methods poses several problems : secondary reactions can lead to the formation of products other than those initially collected; cross contamination can result if several products are liberated at short intervals; the most opportune time for collection is not always obvious. TGA-MS, on the other hand, provides a n alternative to these practices by affording a continuous monitor for dynamic systems. An important advantage that TGA-MS offers is the ability to control degradation atmospheres. It has been pointed out (9) that the restriction on atmospheric conditions imposed by the vacuum requirements of the mass spectrometer can place a limitation on mass spectrometric thermal analysis. With TGA-MS, however, the atmosphere can be varied according to the needs of the experiment through the selection of appropriate purge gases. Another important feature of the TGA-MS dynamic system is its relatively short dead time--i.e., the time between product evolution and introduction into the mass spectrometer ion source. With degradations or pyrolyses conducted in a static system, especially when a solid is involved, the time for a volatile product to reach the ion source can be in the order of minutes or longer. Under proper flow conditions, and with suitably heated connecting lines, TGA-MS can reduce this interval to seconds or less. It should also be noted that the method provides a means for immediately sweeping volatiles away, thus decreasing the probability of reaction with undegraded material. For all practical purposes, the sample environment remains constant for the duration of the experiment; this minimizes the effect of varying atmospheric conditions, caused by fuel consumption, which can be a factor in batch-type thermal degradation studies. The most obvious applications of TGA-MS, particularly in the polymer field, are in the areas of thermal degradation, trace solvent or impurity analysis, additive analysis, kinetics and polymer classifications. Undoubtedly, future workers will find more areas t o investigate. The applications presented in this paper are designed to illustrate the versatility of TGA-MS. For this reason, some examples were selected in which samples of questionable purity and drastic TGA conditions were employed. (9) P. D. Garn and G. D. Anthony, ANAL.CHEM., 39, 1445 (1967):
1092
ANALYTICAL CHEMISTRY
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Figure 2. Thermogram of polymethylene sulfide containing solvent impurities EXPERIMENTAL
Instrumentation. Figure 1 is a block diagram of the apparatus used for most of the work presented here. It is a schematic representation of the TGA unit coupled to a Bendix Time-of-Flight mass spectrometer. The purge gas serves the dual function of controlling the atmosphere in the furnace cell and of sweeping the volatiles into the mass spectrometer. Flow rates of up to 2.5 liters per minute can be tolerated without creating turbulence sufficient to cause base line drift on the TGA recorder; however, rates below 100 ml per minute are normally used. Optimum flow rate conditions will vary with the system being studied, and they should be established accordingly. Selection of the purge gas will be dictated largely by the nature of the experiment and possible interference with the mass spectra of the products being generated. Helium, nitrogen, oxygen, and air are most commonly chosen; however, other rare gases and most fixed gases could also be used. Subambient pressures can be attained in the sample cell area by connecting a vacuum pump t o the exhaust line and regulating the purge gas flow with a needle valve. The furnace area itself can be evacuated t o about 5 p for in-vacuo studies if the exhaust line is blanked off. The lines between the TGA unit, metering valve, and exhaust are l/r-inch 0.d. stainless steel. The connection between the quartz TGA furnace tube and the coupling line is made with a l/&-ich Swagelok union and either ferrules made of Teflon (Du Pont) or a silicone rubber “0” ring in conjunction with a back stainless steel ferrule. The quartz T G A tube, as supplied by the manufacturer, terminates with a ball joint which must be removed and replaced with l/r-inch quartz tubing before it can be connected to the union. The metering valve is a Nupro, cross pattern series S model with vernier handle. Equipped with a Viton “A” “0” ring it can be operated up to 450 O F . A new bellows metering valve, also available from Nupro, which has a temperature limit of 900 O F . is currently being evaluated here for the same purpose. The diaphragm valve serves as main shut-off to the mass spectrometer source. The oscilloscope and oscillograph permit continuous display and permanent recording of spectra from the gaseous products as they enter the mass spectrometer. As is the case with coupling a gas chromatograph to a mass spectrometer, the most critical dimension is the length of tubing between the effluent gas port and the metering valve. For best resolution and least hold-up this distance should be as short as possible. With the materials described above, it is possible t o complete the coupling line in less than three inches.
Wt
z
remaining 100 99.8 99.7 99 99 98.5 98 96.5 94 90 86 80.5 75
69 62.5 58.4 42 36 29 24 17 0
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T, "C.
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45
48
Table I. Compilation of TGA-MS Data m/eo 60 62 64 73 76 92 94 104
108
124
126
138
140
W W W W W M M I I M M
W W W
M M M I I I I I I
W W M M M W W W M M M M
I M I-Intense
M M M M M M
M M M M M M W
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M
The distance of the line between the metering valve and mass spectrometer source is not as critical. Ideally, it should be short and of wide diameter. Negligible dead times can be realized, however, with a line as long as several feet for most volatile components. In the apparatus described here, satisfactory results have been obtained with 3/s-inch 0.d. stainless steel tubing. Temperature is regulated on the entire interfacing unit with variable control heating lines. Procedure. Purge gas flow, which is continuous through the metering valve and exhaust, is adjusted to the desired level after sample weighing and furnace tube connections have been completed. An appropriate amount of purge gas is admitted into the mass spectrometer through the metering valve. Some latitude, depending on sensitivity requirements, exists in the selection of purge gas sampling rates; however, it has been found that analyzer or flight tube torr can be tolerated before pressures of up to about 5 X appreciable loss of mass spectrometer resolution becomes a limiting factor. Once a steady sampling rate has been achieved, the TGA unit is actuated. When a weight loss is detected on the X-Y recorder, attention is shifted to the oscilloscope for indications of the presence of species other than purge gas. A mass spectrum is then recorded with the oscillographic recorder. As operated here, a mass spectrum of m/e 12 to 300 can be recorded in about five seconds with the time-of-flight instrument. When the Consolidated Electrodynamics 21-104 mass spectrometer was used, similar plumbing and operating conditions were employed except that the monitoring functions of the oscilloscope were transferred to a total ionization monitor. Spectra can be recorded with this instrument at a maximum rate of 0.3 octave per second.
RESULTS AND DISCUSSION Figure 2 shows a TGA curve obtained from an impure sample of polymethylene sulfide which was heated in a helium atmosphere at a rate of 15 "Cper minute. Under these conditions the polymer degrades rapidly near the melting point t o yield a multitude of primary and secondary products. Mass spectra of the volatiles evolved were recorded at each point shown on the curve. Tabulated in Table I are the molecular ions, with the exception of m/e 45 which is a fragment ion from thioformaldehyde,
146
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W W M M I I I I I
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W M W M W W W
M
w
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M M M M M M M
W
w
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Table 11. Mass Assignments for Polymethylene Sulfide Degradation Products Ion mie 146
01;
140
CH~--S-CHZ-S-CH-SH
126 124
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108 104 94 92
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78
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of the components detected by mass spectrometry as a function of temperature and weight loss. The ion intensity factors have no real quantitative significance other than to indicate roughly the presence of major or minor components and their order of appearance and disappearance. Mass assignments for the ions in Table I are given in Table 11. All of the assignments shown in Table I were made from mass spectra as VOL. 40, NO. 7, JUNE 1968
1093
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Figure 3. Thermogram of polymethylene sulfide containing solvent impurities and residual monomer
70-
-
60
So-
they were generated directly on line with the T G A unit. Structural configurations for some of the components, those with asterisks in Table 11, were confirmed by infrared, NMR, and mass spectrometry following chromatographic separation. From the chromatographic data, it was possible to account for at least 23 components in some of the degradation mixtures. It is apparent from these data that a n initial weight loss of about 2 0 x is caused primarily by evolution of two solvents, o-dichlorobenzene and dimethylformamide. The first degradation product, thioformaldehyde, appears near the melting point of the polymer at about 256 "C. Continued heating leads to the formation of numerous other sulfur compounds, chief of which are carbon disulfide and thioformaldehyde. Toward the end of the degradation, a small amount of odichlorobenzene evolution suggests that some of the solvent is more tightly bound to the polymer than the initial weight loss would indicate. Figure 3 is the thermogram of a sample of polymethylene sulfide which was contaminated with residual monomer. Mass spectra for this series were scanned magnetically at the numbered points with a Consolidated Electrodynamics 21-104 mass spectrometer. Presented in Figure 4 are the mass spectra, in bar graph form, which were recorded a t points 0, 1, 7, and 11 of Figure 3. The mass spectrum at point 0 shows only low levels of air and water which are the result of instrument background and residual air in the furnace area. This spectrum was recorded after sweeping the sample with helium for about one hour. It is obvious from the contributions t o the air peaks in spectrum 7, which are fairly close to the actual mass spectrometer background, that the amount of air remaining in the furnace area, although quite low, could be significant in some studies. For this reason, a standard procedure has been adopted in this laboratory of evacuating the furnace area before bleeding in the purge gas when atmospheres other than air are used. At point 1, the molecular ions of the first two volatiles evolved, trithiane and o-dichlorobenzene, can be seen at m/e 138 and 146. The source of trithiane is unpolymerized starting material and o-dichlorobenzene was used as a solvent. At point 7, just before degradation, the mass spectra of both compounds are strongly evident. The last spectrum in Figure 4, recorded at point 11 after the start of decomposition, shows the appearance of the two major degradation products, thioformaldehyde and carbon disulfide along with lesser amounts of some of the same secondary products reported above. The degradation of polymethylene sulfide has been studied extensively in this laboratory with the techniques described above. Degradations were carried out thermally and oxida1094
ANALYTICAL CHEMISTRY
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-
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-
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Figure 4. Mass spectra of volatiles generated from corresponding points shown in Figure 3 tively under isothermal and programmed temperature conditions on samples of high purity. From these studies, it has been possible t o propose thermal and oxidative degradation mechanisms, identify the primary and secondary degradation products, determine the effect of various stabilizers, establish conditions for efficient solvent removal, and, in conjunction with gas chromatography, obtain quantitative estimates of the products formed. These studies, have made it possible to propose that the thermal degradation of polymethylene sulfide proceeds through a n initial homolytic cleavage of the carbon-sulfur bond followed by evolution of carbon disulfide and thioformaldehyde. A detailed report on the degradation of polymethylene sulfide will be forthcoming in a future paper. In the present context, these data serve mainly to illustrate applications of the TGA-MS technique.
-
8070
-
E
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e
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-
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100
150
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300
350
400
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TEMPERATURE, O C
Figure 5. Thermogram of maleic hydrazide methyl vinyl ether copolymer Figure 5 shows a TGA curve which was obtained from the thermal degradation of a substance presumed to be a maleic hydrazide methyl vinyl ether copolymer. Of prime interest in the study of this polymer was the acquisition of data which would indicate the presence of a methoxy group. From the degradation products listed in Table 111, it is apparent that the evolution of methanol offers some confirmatory support for the proposed structure. Aside from illustrating the applicability of TGA-MS to polymer characterizations, this study also demonstrates the ability of the instrumentation to follow a complicated degradation scheme. Mass spectra were recorded at all the numbered points shown in Figure 3, and it is obvious from the list of products in Table I11 that the components responsible for the weight losses and inflection points have been described adequately. As noted above, TGA-MS is finding wide application in this laboratory in a variety of special analyses. It is being used routinely to determine the volatiles in polymeric and high boiling substances. Additives, such as benzophenones and phenolic stabilizers, have been identified in a number of competitive type analyses, and the thermal conditions required for the evolution of specific products have been established for some reactive systems. An example of the latter
Table 111. TGA-MS Data for Maleic Hydrazide Methyl Vinyl Ether Copolymer TGA curve point Mass spectrometer analysis 1-4 Toluene (solvent) only product detected Methanol first detected 4 4-8 Methanol increases to major component 8 Carbon dioxide evolution commences 9 Methanol, major product, and carbon dioxide only volatiles detected 10 Ammonia first detected 11 Acetonitrile first detected 12 Cot becomes major volatile product Hydrogen cyanide first detected 13 Methanol no longer detected 14 15-17 Carbon dioxide major product along with lesser amounts of acetonitrile, hydrogen cyanide, ammonia, and possibly carbon monoxide
application was the determination of the temperatures required for the selective generation of water and ammonia from polyacrylamide. Sample requirements for TGA-MS vary according to the amount of volatiles liberated and the sensitivity of the mass spectrometer for a given component. This parameter generally has to be established for each system being studied. For the polymethylene sulfide system described above, adequate spectra of all the components could be obtained from 1 mg of sample. It is quite often necessary, however, when determining low level additives, to weigh out as much as 100 mg of material. ACKNOWLEDGMENT
Grateful acknowledgment is given to A. H. DiEdwardo for infrared and NMR interpretation of some of the polymethylene sulfide degradation products, and to C . S. H. Chen for synthesis of the polymethylene sulfide samples. RECEIVED for review December 18, 1967. Accepted March 7, 1968.
VOL 40,
NO. 7, JUNE 1968
1095
