Palladium may lead to thumbnail-scale GC Already established techniques could put chromatograph on chip Maxwell's sorting demon—the one who works the gate to separate hot molecules from cold ones—hasn't been caught yet. But a research team at California Institute of Technology's Jet Propulsion Laboratoiy, Pasadena, has found another kind of sorting de mon, one that can segregate mole cules of different species. It has been put to work in gas chromatography. The demon is already being used to make a miniaturized gas chromatograph-mass spectrometer-computer system, 7 by 9 by 11 inches, sched uled to travel to Mars in 1975 (C&EN, July 13, page 5 4 ) . Soon it may make possible a complete gas chromato graph laid down on an integrated cir cuit chip no bigger than a thumbnail. The demon is palladium. It's long been known that palladium and its alloys are selectively permeable to hy drogen. Dr. D. P. Lucero and Dr. F. C. Haley suggested in 1968 that this characteristic of palladium could be used to advantage to couple a gas chromatograph to a mass spectrom eter. At JPL, Dr. J. E. Lovelock, Dr. P. G. Simmonds, Dr. G. R. Shoemake, Dr. K. W. Charlton, and Samuel Rich seized upon the suggestion as one that might help them in their project of designing a miniature GC-MS system for the Viking Lander Mars Probe. Heated. In an early experiment, the JPL team connected a hydrogen supply to one end of a heated palla dium-silver tube and a vacuum sys tem to the other. The hydrogen was turned on until the pressure at the tube inlet was 30 torr. At that stage, the pressure in the vacuum system was 5 Χ 10 _β torr. The inlet pres sure was gradually raised to 760 torr, but the pressure remained constant in the vacuum system—the hydrogen was passing through the tube wall, reacting with the oxygen outside to form water vapor. Finally, at a pres sure of 780 torr, the hydrogen "broke through," and the pressure in the vac uum system rose from 5 X 1 0 ° torr to 0.1 torr in less than 2 seconds. The result of the experiment—the demonstration that it is possible to maintain an active yet open barrier between gas at ambient pressure and a hard vacuum—"was far beyond our expectations," a team member says. The team calls their first palladium device the transmodulator. With the
Palladium's promise: GC on a chip -Column: Cross section 0.0025 cm. Length J 8 cm.
-Sample inlet port
Detector volume: 6 nanoliters -Detector section (second anode)
-Cathode and hydrogen generator
0.0075-cm. strips of palladium-silver alloy 'Transmodulator section (anode) ~MgO chip
transmodulator, hydrogen is used as a first carrier gas. As the effluent stream emerges from a chromato graphic column, a second carrier gas is injected into the stream. The mix ture is passed through a palladium al loy tube, heated in air, which removes the hydrogen quantitatively and thus "transmodulates" the components of interest to the second carrier gas. By choosing a lower flow rate for the second carrier, it is possible, with concentration-sensing detectors, to in crease detectivity 40- or 50-fold. Also, by replacing hydrogen with a constant-flow secondary carrier, inac curacies caused by flow-rate changes typical of programed-flow and pro gramed-temperature GC procedures can be avoided. Next step was to construct a GC MS interface, using a short length of palladium-silver tube heated in air. The separator has potentially 100CA efficiency for hydrogen removal, and can deliver sample components to the mass spectrometer without loss. Single cell. Air is not essential to the operation of the palladium de vices. An oxidizing environment can be furnished by an electrolytic cell. This makes it possible to generate and remove hydrogen in a single cell—in effect "closing the loop." An unexpected bonus was the dis covery that the generator-separator
cell also acts as a GC detector. With the cell polarized at a fixed potential from a low-impedance source, current flow in the cell drops when a com ponent emerges. The palladium "demon" may also make it possible to build an integrated circuit gas chromatograph in which all components, including the column, are laid down on a small chip perhaps 1 x 2 cm. The palladium cathode is the source of hydrogen and the means of controlling flow rate and pressure. The anode is the recycling pump and sample concentrator. A second anode serves as detector. The sample chamber could be simply a known fixed volume between the cath ode and the column, with a valve to the outside. To load the sample chamber, the hydrogen in it would be removed by reversing the polarity of the power supply to the cell, in a brief, large pulse. Then the valve would be opened and the sample drawn into the chamber. The valve would be closed and the chromato graph started by applying the normal potential to the cell. The IC GC is only a conjecture so far, but, the JPL workers assert, the principle is sound and the requisite microelectronic techniques are already well established. "There seems to be no reason why such chromatographs could not be made." AUG.
24, 1970 C&EN
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