Anal. Chem. 1980,52, 2290-2293
2290
Field Desorption Emitter Temperature Regulator for Constant Temperature Control David
F. Fraley, W.
Stephen Woodward, and Maurlce M. Bursey"
William Rand Kenan, Jr. -Laboratories
of Chemistry. The University of North Carolina, Chapel Hill, Norih Carolina 275 14
An electronic power supply called an emitter temperature regulator (ETR) has been developed to provide a constant reproducible temperature for field desorption emitters. The device is configured to regulate the emitter heating current in either the new constant temperature mode or the tradnionai constant current mode. The power supply employs a feedback system that utilizes the temperature coefficient of resistivity for tungsten and tungsten carbide. By maintaining a constant ratio of high temperature to ambient temperature resistances, a constant temperature is likewise maintained. Since dtfferent manufacturers produce FD instruments utiPring dtfferent emttter geometries (typically either 4 or 5 mm between posts), different lengths of tungsten wire will produce different temperatures for a given constant current. For emmers of dtfferenl geometry, this device can produce the same reproducible temperatures by varying the heating current from emitter to emitter.
Field desorption mass spectrometry (FDMS) is frequently the method of choice for the analysis of many types of nonvolatile, thermally labile organic compounds ( I ) . In this technique, a sample is coated on an emitter anode (normally a 10-Km tungsten wire covered with microneedles) and placed in a high electric field in the source of a mass spectrometer. Ion production occurs on the sharp needle tips because of the distortion of the potential well and consequently the energy levels of the sample molecule by the large electric field gradient. The resulting loss of an electron to the emitter by a quantum mechanical tunneling process and the immediate desorption of the positive ion from the positive emitter simultaneously ionize and vaporize the sample molecules. For aid in mobility of the sample to the needle tips and desorption of the sample from the emitter, the FD process frequently is assisted by passing a heating current of 5-50 mA through the FD emitter. As the current passes through a typical 10-Km tungsten wire, Joulean (PR)heating occurs due to the finite resistance of the wire. FD mass spectra depend very sensitively on this heating current to control the emitter temperature. The best anode temperature (BAT) has been defined (2) as the temperature a t which predominantly molecular ions are generated and thermally or field-induced fragmentation is reduced. This emitter temperature, however, is often characterized in terms of the emitter heating current in milliamps, from which a rough estimate of the emitter temperature can be derived (3). For precise information on this temperature, primarily the length of the tungsten wire and the size of the microneedles must be considered in detail. The effect of the length of the wire and a new emitter temperature regulator will be considered in this paper. Since the accuracy and reproducibility of control of the emitter temperature are important, several authors have designed different analog or digital devices to control either the FD ion current or the emitter temperature. The electrical heating current may be produced by a simple battery or by 0003-2700/80/0352-2290$0 1.OO/O
a more elaborate constant current or constant voltage power supply. The actual adjustment of the current frequently is by manual control through a simple potentiometer. Alternatively, automatic adjustment of the current may be achieved by one of two methods, (1) automatic control through a feedback loop (4,5) that regulates the heating current to maintain a set level of ion emission from the emitter or (2) computer control of the heating device (6, 7) or an emitter current programmer (8) that precisely controls the magnitude and the rate of change of current in the emitter wire but without utilizing emission controlled feedback. Problems with Constant-CurrentPower Supplies. The typical FD emitter consists of a short piece of 10-km tungsten wire spot welded to the ends of two parallel support posts. The distance between the center of the two posts varies from manufacturer to manufacturer: DuPont and VG Micromass emitter posts are 4 mm apart, Varian posts are 5 mm, Kratos posts are 5.1 mm, and our homemade posts are 6 mm. Wire length for a single instrument can vary by h0.4 mm if the posts are bent or if the spotwelding position is not very carefully controlled. Since the diameter of the tungsten wire is constant, the total resistance of the tungsten is a function of the length of the wire. Information on the critical heating current and best anode temperature (BAT) is not transferable from one instrument to another which utilizes a different emitter geometry. This dilemma is known as the constant current power supply problem: emitters with different distances between the posts produce different temperatures for the same heating current. The same problem will exist for constant-power or constant-voltage power supplies also.
Calculated Temperature Distribution along the Emitter. A computer program has been developed to solve the nonlinear second-order differential equation that describes the temperature distribution along a thin wire electrically heated in vacuo (9) and will be described elsewhere. The results for a bare tungsten wire (no dendrites) heated with 30 mA dc indicated that the maximum center temperature for a wire 5 mm long is 788 K; for a wire only 4 mm long it is 471 K. This temperature differential is greater in the 30-40 mA range and less at lower milliamperes and a t higher milliamperes. It may be observed from Figure 1that a large temperature gradient exists along the emitter wire. Due to the large thermal cooling mass of the support posts, the two ends of the wire are very near room temperature. The BAT actually represents an average value of dendrite tip temperatures. Indirect heating of the emitter either by infrared sources (IO) or by a laser (11) offers promise for a more uniform temperature distribution but only if the entire wire and support posts are heated. If only the center of the wire is heated, an analogous temperature distribution problem occurs. EXPERIMENTAL SECTION The entire electronic device is electrically isolated in a Lucite container for operator protection since the power supply is floating at 10 kV or at the desired FD emitter voltage. Isolation and simplification are obtained by using a 12-V battery power supply. Control of switch 1 (Figures 2 or 3), the balance potentiometer, 0 1980 American Chemical Society
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Figure 1. Temperature vs. distance across a bare tungsten wire electrically heated in vacuo. Arrows show points of average temperature.
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