Mass Spectrometric Differential Thermal Analysis SIR: I n the previous article dealing with mass spectrometric thermal analysis (MTA) and differential thermal analysis (DTA) ( 2 ) , it was mentioned that progress was being made toward the incorporation of MTA and D T A into a single instrumental method. The purpose of this conimunication is to describe one of the possible methods, the basic mechanical and physical aspects of the system, and some of the preliminary data which are compared to those previously obtained by MTA experiments. The basis of the system is the remote cell adapter that can be purchased as an accessory to the Du Pont Differential Thermal Analyzer ( I ) . The bell jar, base plate, and furnace assembly can be readi'y transferred between the main instrument and the remote cell adapter. A gasket supplied by the manufacturer separates the base plate-bell jar-furnace assembly from the base plate of the adapter assembly and allows either vacuum or high pressure modes of operation. Mechanical connections are made to the base plate of the adapter (Figure 1) in the following manner: Opening 1 is fitted with 1/4-inch 0.d. copper tubing which is connected to a twoway valve opening to the atmosphere and is the purge gas exit. Opening 2 is plugged. Opening 3 is fitted with l/c-inch 0.d. copper tubing which is connected to a vacuum pump through a valve. Opening 4 is fitted with '/leinch 0.d. stainless steel tubing connected to a three-way valve (S), which connects to a vacuum pump on the second arm and opens to the mass spectrometer on the third arm. Opening 5 is fitted with '/(-inch 0.d. copper tubing connected to a two-way valve and is used to conduct a purge gas into the system as desired. The system is thus equipped to provide a sample to the mass spectrometer from any pressure between roughly loW3torr to 2 atmospheres with any desired purging gas. Openings 1, 3, and 5 open through the gasket directly into the bell-jar and furnace assembly. Opening 4 is connected to the furnace by */*-inch 0.d. tubing as described in Figure 2. This permits direct sampling of any gas evolved from the sample during the heating process into the mass spectrometer. The heater block was drilled as shown to accommodate the tubing. The vacuum pump is connected by one line directly at the b-alve controlling pas flow to the mass spectrometer in an effort to provide as good a control over the gas flow into the mass spectrometer and as small a "dead volume" as was possible. The latter consideration is perhaps the most important if
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NPT.
5.F.
TOP
Figure 1 .
VIEW
Base plate adapter
the changes occurring in the vapors evolved from the sample are to be studied with any degree of fidelity.
endotherm is due to water release from the sample. The slight temperature difference (actually a time lag) is due to the time necessary for the gas to flow from the sample in the DTA well to the ionizing region, a distance of about 70 cm. This time lag, usually of the order of 30 sec., is caused by an excessive gas volume on the high pressure side of the valve. Changes are contemplated to
The first compound studied utilizing this system was triphenyltin hydroxide. An ?(/ITA scan of this compound (4) a t a mass spectrometer (and hence sample) pressure of < l o F 6 torr indkates that water is released in a diffusion controlled process over a wide temperature range. Maximum water release is noted a t 65' C. When the system described above is used, at a pressure of torr, two endotherms are noted on the D T A trace, one occurring a t 100" C., and another occurring a t 124' C. The second of these is the characteristic melting point of bis(tripheny1tin)oxide as previously established by D T A a t atmospheric pressure ( 4 ) . 4 comparison with the trace from the scanner on the mass spectrometer monitoring water at m/e 18 shows a maximum occurring a t about 103' C., indicating that the first
18
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7 SAMPLE
R E F E R E N C E WELLS-
Figure 2.
Furnace assembly
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monitor
WELL
---
endotherm
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20
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l
40
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I
60
Figure 3.
1
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IO0 TEMP., *C.
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Phz SnOH VOL. 37, NO. 3, MARCH 1965
e
433
this work are probably caused by the pressure difference between the two operations; MTA work being done at a factor of lo3 to lo4 lower pressure. This is particularly evident for a fast release of water a t about 310' C. and 10" torr (sharp peak in the MTA scan in Figure 4) as compared with a slow diffusion process a t lop3 torr [gradual increase in base line for both MDTA (mass spectrometric differential thermal analysis) scans]. The technique of M D T A will, then, be particularly useful in the study of pressure dependent thermal reactions.
monitor
DTA endolher m
MDTA
L
LITERATURE CITED I ~ I ' I ' I ' I ' I ~ I ' I ' I ' I ' I ' I ' I ' J ' I J' II
20 40
60
80
1 1 1
100 120 140 160 180 200 220 240 260 280 3 0 0 320 3 4 0 360:
TEMP..
Figure 4.
MgC12.6
OC.
HzO
( 1 ) E. I. du Pont de Nemours & Co., "900 Differential Thermal Analyzer," (2) Langer, H. G., Gohlke, R. S., ANAL. CHEM.35, 1301 (1963). (3) Nuclear Products modified 1-SA or unmodified 1-SX.
p.,
(4) Smith, D. H., Langer, H. G., The
reduce this volume and with this the time lag. Figure 3 compares the various runs. As a second example, MgC12.6HzO was studied, also a t a pressure of torr. Again, HzO a t m/e 18 was mbnitored. A comparison of the data from the M T A scan monitoring water
and from the combined instrumentation can be made by examining Figure 4. The apparatus functions quite well and will prove to be very useful in monitoring thermal decomposition products and phase changes in one operation. Discrepancies initially noted between the M T A and D T A scans in
Dow Chemical Co., Framingham, Mass., unpublished data, 1964.
HORSTG. LANGER S. GOHLKE ROLAND DENNISH. SMITH The Dow Chemical Co. Eastern Research Laboratory Framingham, Mass.
Determination of Americium-243 by Separation and Counting of Neptunium-239 Daughter SIR: A rapid, precise method was developed for determining low concentrations of AmZ43 in actinide-fission product mixtures. The determination is based on the separation and counting of ,56-hour Np239, the decay daughter of Am243. Although Am243is a n active a and y emitter, it is frequently encountered in systems in which its contribution to the total activity is so low as to make direct measurement by a or y spectrometry impossible. Methods involving the separation of Am from Cm and fission products are time consuming and require a n exacting technique. The mass spectrometric isotope dilution method reported previously by Banick, Carothers, and Donaldson (1) is the most precise of those that have been used in this work. However, the method not only requires the use of an expensive mass spectrometer, but requires chemical purification to obtain a sample t h a t is very low in solids and 0-7 radioactivity. A convenient method for determining the concentration of Am243consists of separating and counting Np239, the product of the Am243a decay. The N p 434
ANALYTICAL CHEMISTRY
count at secular equilibrium (Np eq.) is compared with that of samples of known Am243concentration. EXPERIMENTAL
The &io Np eq./Am243 was established by analyses of eight samples standardized by the mass spectrometric method referred to above. Samples
Table
I.
Ratio of Np239Activity to Weight of Am243in Solutions of Irradiated Pu239
Am24a (mg./ml.) 0.0173 0.0173 0.0173 0.0173 0.0173 0,1555 0,1555 0.1555
Average
were undisturbed for a t least twenty days to allow the Am243-Npz39system to reach equilibrium. Aliquots of the samples containing between 0.1 and 5.0 pg of .4mZ43were taken, and Np*39 was then extracted into 2-thenoyltrifluoroacetone (TTA) by the method of Murray ( 2 ) . Of the gamma emitters present (Zrg5-Nb95, Ce144-Pr144 C~134~137, Ru1O3P1O8),only Zrg5-Kbg5was kxtracted with the Np239. The extract was
Np23geq. (c./s./ml.) 2 . 0 1 x 104 1.98 1.93 1.98 1.96 7.0 6.8 6.8
Np939 eq. (c./s./mg.) AmZ43 1 . 1 6 X lo6 1.14 1.12 1.12 1.13 1.09 1.08 1.08 ~
~~
Apparent" counting efficiency,
70
16 23 1.5 94
15.60 15.66 15 80 15 24 15.10 15.10
= 15.59 f 0 . 3 9
Np2sg c./s./mg. AmZ43 Apparent counting efficiency = 7.15 X lo6 d./s./mg. with TL/2Am243= 7950 years.
