Automated Vacuum Fusion Apparatus J. W. Frazer, J. W. Fischer, C. J. Morris, R . W. Crawford, and E. R. Fisher Lawrence Licermore Laboratory, Uniaersity of California, Liuermofe, Calif., 94550 A completely automated vacuum fusion system has been constructed for the determination of oxygen, hydrogen, and nitrogen in metals. Automation was accomplished by interfacing the component parts of the vacuum fusion apparatus to a computer. The system analyzes up to 16 successive samples and reports the data in terms of parts per million by weight.
A NEW, COMPLETELY AUTOMATED VACUUM fusion system has been designed and constructed. It was built t o provide increased accuracy and precision as well as to relieve the operator of the routine tasks involved in precision vacuum fusion analysis (1-5). This new vacuum fusion system is capable of the simultaneous determination of H2, 02,and N2 in metals in the range from many thousand parts per million t o less than one part per million. Automation was accomplished by interfacing the sample-dropping mechanism, valves, pressure transducer, residual gas analyzer (mass spectrometer), and the interactive interface of the vacuum fusion apparatus t o a Digital Equipment Corp. PDP-7 computer (6, 7). The computer simultaneously handles all of the control, data acquisition, data reduction, and output functions. It also handles these functions for several other instruments simultaneously. The software, which includes immediate data reduction, was written with the versatility necessary t o analyze samples of varying composition. The program follows the kinetics of the gas evolution and makes logical decisions based on the data obtained. Included in these decisions is the determination of the point at which the analysis is complete. EXPERIMENTAL
Apparatus. A schematic diagram of the vacuum system is shown in Figure 1 . For simplicity, the system can be viewed as four main units, one for each of the basic control operations: dropping of the sample into the fusion crucible; operation of the valves for gas manipulation; measurement of the total quantity of gas evolved; and determination of the composition of the evolved gas. Sample-Dropper Assembly. The sample dropper (Figure 2) consists of a vacuum tight, stainless steel container attached t o the main system by a high-vacuum Cajon fitting. A rotatable disk containing 16-sample holes is enclosed in the dropper. On command from the computer, an electric motor rotates the disk 22.5” via a n O-ring sealed vacuum feed-through connector. The computer initiates the rotation, but the extent of rotation is controlled by a microswitch and notched cam attached to the electric motor. With each 22.5” of rotation, a sample is moved over the opening in the bottom of the dropper. The sample falls through the open gate valve on the dropper assembly, through the pneumatic (1) J. A . James, Met. Rec., 9,93 (1964). (2) L. L. Kunin, J. Anal. Cliem. USSR,20,774 (1965).
G. S. Tankins, in “High Temperature Refractory Metals, 1965,” W. A. Krivsky, Ed., Gordon and Breach Science Publishers, New York, N.Y., 1968, Vol. 34, Part 1, p 325. (4) H. A . Sloman, J. Inst. Metals, 80, 391 (1952). (5) R. Lesser. in “Transactions of the Eighth Vacuum Symposium and Second International Congress,” 1961, Pergamon Press, New York, N.Y., 1962, p 782. (6) J. W. Frazer, ANAL.CHEM., 40, (S), 26A (1968). (7) J. W. Frazer, Chem. I/zstrum.,Z, 271 (1970). (3)
gate valve on the main system, and into the sample insertion tube, which guides it into the fusion crucible. The dropper assembly can be removed from the main system. Under normal operating procedures, this unit is loaded in an inert-gas dry-box with properly prepared samples. The unit is then sealed, removed from the dry-box, and attached to the main system. The inert gas is pumped out of the sample-dropper assembly through an auxilliary vacuum system (not shown in Figure 1). This operation is an important feature of our routine determinations since most samples are cleaned by cathodic etching (8) in an argon drybox and cannot then be exposed to air if accurate results are t o be obtained. Gas Evolution System. A 15-kW induction motor-generator is used to heat the graphite fusion crucible (A) up to temperatures of 2300 “C. Graphite felt is used t o insulate the fusion crucible from the quartz tube in which it is held. A graphite lid covers the crucible t o reduce the distillation of volatile samples. Removal of the lid, suspended on a 10-mil tantalum wire, is accomplished by a pneumatic linear actuator and a magnetic vacuum feed-through connector. The lid is automatically raised and lowered whenever a sample is dropped. A 60-liter/sec glass mercury-diffusion pump (B) removes the evolved gases from the furnace region. Between the furnace and the pump, 4-in. 0.d. glass tubing is used to increase pumping speed and thus decrease gettering. The calculated pumping speed is -20 liters/sec at the crucible. A three-stage glass mercury-transfer pump (C), capable of operation against 10 Torr backpressure, pumps the gas into the holding reservoir. Gas Quantity Measurement System. Another glass mercury-diffusion pump (D) is used t o transfer the gas from the holding reservoir into a calibrated volume. This pump, which will operate against 1-Torr backpressure, also functions as a one-way valve to confine the gas in the calibrated volume. In addition to the primary volume E, the calibrated volume includes two expansion chambers (F and G) that are used when large quantities of gases are evolved. Three volumes are available for gas quantity measurement: E = 440 ml, E F = 840 ml, and E F G = 4150 ml. A pressure transducer (MKS Baratron Model 77H-3) with a range of 0 to 3000 mTorr is used to measure the pressure of the evolved gases in the calibrated volume. (The volume is held at constant temperature of 21 f 0.5 “C.) The transducer is connected to a MKS-100A auto digital power supply via a 10-channel multiplex unit. The computer selects the appropriate channel and initiates the pressure determination cycle of the 100-A power supply. For the 3-Torr transducer on the vacuum fusion system, the pressure is determined to the nearest 0.1 mTorr. Gas Composition Analysis System. Gas composition is determined by a fully automated AEI MS-10 residual gas analyzer. The mass scans are accomplished by varying the value of ion accelerating voltage, which is determined by a potentiometer in the high-voltage circuit of the mass spectrometer. A separate potentiometer exists for the mass 2 and mass 12-45 ranges. A stepping motor, which is interfaced to the computer, is connected by a gear train to these potentiometers. The evolved gas is admitted to the spectrometer for analysis through the valve that isolates the calibrated volume from
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(8) J. H. Hill, Analyst (London),95,215 (1970). ANALYTICAL CHEMISTRY, VOL. 44, NO, 6, MAY 1972
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LVacuum manifold Vacuum gage
Figure 1. Vacuum fusion system
X A i r operated valve 3 Hand operated valve Induction heated Diffusion crucible pump
A
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Figure 2. Sample-dropper assembly
the leak valve. The mass spectrum is then taken, with the output of the electrometer measured by means of a 1 MHz voltage-to-frequency converter and a digital counter. The mass spectrometer is pumped by a 4-in. oil-diffusion pump with a n 8 liter/sec ion pump for standby operation. General Specifications. A gas inlet for the introduction of standards is also included in the system. Gas standards are manually bled into the calibrated volume through a bellows-sealed needle-valve inlet and analyzed on the mass spectrometer to establish cracking patterns and relative sensitivities. All of the vacuum lines in the system, not previously described, are either 3i4-in. 0.d. stainless steel or to 3/4-in. 0.d. glass. The valves are all bellows-sealed, stainless steel high-vacuum valves with either pneumatic or manual actua1028
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
B
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Transfer Diffusion pump pump ( v a l v e )
C
D
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tors, depending o n the required control function. Computer control of the pneumatic valves is restricted to either fully open or fully closed positions. All mercury-diffusion pumps in the system are cooled by water chilled to 5 "C to promote optimum pumping. Procedure. The computer is run in a time-shared mode, simultaneously controlling several instruments and types of apparatus, including the vacuum fusion system. A realtime monitor handles the required time-response characteristic, data rate, and inputioutput. The program for the vacuum fusion system is written entirely in assembly language. The approximately 1330 instructions include all functions for control and data reduction. A condensed flow diagram of the program is shown in Figure 3. For a typical analysis, the program is initiated and the mass spectrometer calibration data are typed in on the teletypewriter (TTY) located at the apparatus. These data consist of the cracking patterns and relative sensitivities of the calibration gases. The program then asks, via the TTY, for the maximum acceptable blank pressure. This pressure, in tenths of millitorr, is entered, followed by the weights of all samples. The computer then takes over full control of the system. As the first control operation, gas from furnace outgassing is collected in the holding reservoir for a predetermined time to establish the blank. At the end of this time, the gas is pumped into the calibrated volume and the pressure determined. If the pressure is above the maximum acceptable blank value, as entered o n the TTY, the gas is immediately removed by the main vacuum system and another blank is accumulated. If this pressure is below that of the maximum acceptable blank value, the gas is admitted to the mass spectrometer, and a spectrum is taken. The gases analyzed for are: H2, CO, N?, CHd, CO, as determined by the 2, 12, 14, 15, and 44 nzle peaks, respectively. (The 28 m/e peak is determined but is not used for data reduction.) The 2 mje peak is scanned first, followed by a scan of mass range 12 to 45. The computer controls the mass spectrometer through the following automated functions : 1. Range selection. Either mass 2 or mass range 12 to 45 can be chosen. 2. Electrometer amplifier sensitivity selection. The choices range from 10-10 to 10-13 A of ion current for full-scale amplifier output. I t is presently set at A for mass 2 and lo-" A for mass range 12 to 45. The
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Continue sample gas
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Transfer gas to calibration volume and measure pressure
Reduce pressure to
