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ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
Thin Film Deposition by the Electrospray Method for Californium-252 Plasma Desorption Studies of Involatile Molecules C. J. McNeal” and R. D. Macfarlane” Department of Chemistry, Texas A& M University, College Station, Texas 77843
E. L. Thurston Electron Microscopy Center, Texas A& M University, College Station, Texas 77843
An electrospray system and procedure has been developed for the routine preparation of thin films of involatile molecules for analytical measurements. An anode-cathode design has been developed and condltlons for reproducible performance have been established. Solvent systems for pdar and nonpolar molecules have been investigated and electron microscopy measurements have been made on the microstructure of the deposlts and fraction of surface coverage. The method does not appear to degrade thermally-lablle molecules.
T h e preparation of thin solid films of involatile molecules is a problem frequently encountered in analytical measurements that are sensitive to the nature of the sample surface. T h e problem is particularly important in “solid state” mass spectroscopy where ions are produced a t the surface of a solid film and intensities depend on the fraction of the surface covered. This includes field desorption ( I ) , 262Cf-plasma desorption mass spectrometry (PDMS) (2),secondary ion mass spectrometry (3), and laser desorption ( 4 ) . Various techniques have been employed in the past for producing thin films on a solid backing. These include high temperature vacuum evaporation, electrodeposition from nonaqueous media, and solution evaporation. A summary of the applicability of these techniques has been given by Yaffe (5). A nebulization method has been introduced by Jolly and White (6) as a n alternate approach t o the problem. In our first studies carried out with PDMS (7), sample deposits on metal backings were prepared by solution evaporation of up t o 200 pg of material. Krueger (8)showed that PDMS mass spectra could be obtained using much smaller quantities of material employing the electrospray method of Zeleny (9). This method has been used for the preparation of uniform emulsions by Nawab and Mason (10) and by Brunnix and Rudstam for thin radioactive sources (11). Dole e t al. used the phenomenon as a n ion source in a mass spectrometer designed for high molecular weight species (12). During the course of our PDMS studies, it became apparent that the electrospray method was indeed the method of choice because of its general applicability to a wide range of inorganic and organic solute species as well as to a n extensive combination of solvent systems. We have developed a n electrospray system and procedure that gives considerable control in film preparation, is reproducible, and has a high transfer efficiency. The details of this system and its performance are given in this paper. EXPERIMENTAL Apparatus. The electrospray cell consisted of a Vita Needle hypodermic delivery electrode with a Vita hub and a deposition electrode which was the surface to be covered. A photograph of the system is shown in Figure 1. The hypodermic needle was 0003-2700/79/0351-2036$01 .OO/O
stainless steel, 26 or 27 gauge, and 1-1.3 cm long. A stainless steel wire, 0.134.15 mm o.d., was inserted into the capillary providing a variable constriction for the flow of liquid. This is shown at the top of Figure 1. The deposition electrode was a thin Ni or aluminized polypropylene foil (0.3-1.2 cm2 area) mounted on a support plate and grounded to the ring stand shown in the photograph. The hub connector on the end of the hypodermic needle served as a liquid reservoir with a capacity of 200 gL. The support holder was attached to a X-Y positioner (Stoelting Model SA-853) for centering the electrospray. A Z-positioner operated remotely through a power jack (Precision Scientific Model TS-69306-1) controlled the distance between the anode and cathode. The electrospray cell was mounted in a dust-free, gas-tight Lucite box to minimize turbulence between the anode and cathode and to provide a controlled atmosphere when needed. A Fluke Model 410B (0-10 kV, 3 mA) high voltage supply was connected to the hypodermic electrode through a 50-MQcurrent limiter resistor that eliminated high current, high voltage breakdown. The electrical current carried by the electrospray was monitored by measuring the potential drop across a 1-MQ resistor connecting the cathode to ground. Reagents. Solvents were distilled in glass grade obtained from Burdick and Jackson Laboratories, Inc. Procedure. The configuration of the electrospray cell and surrounding support structures was carefully arranged to minimize interferences with the delicate flow of the electrospray. A low intensity lamp illuminated the space between the anode and cathode to facilitate visualization of the electrospray and to assist in the evaporation of the solvent in the spray. Figure 2 shows a close-up view of the electrospray emerging from the capillary and the surface being coated. The solution to he electrosprayed was loaded into the reservoir of the hypodermic needle and the flow rate adjusted by positioning of the insertion wire. The hypodermic needle was placed into the positioner-holder and approximately centered with respect to the collector foil. The anode-cathode distance was selected according to the area to be sprayed and varied between 2 to 4 cm. To establish the electrospray condition, a movable grounded plate was inserted between the cathode and anode and the voltage applied to the hypodermic electrode. The voltage was increased continuously until a current was detected. Prior to establishing a stable spray, large droplets often emerged from the tip and the spray current was erratic. When a stable spray was achieved, the movable plate was withdrawn, and the spray directed to the collector. The movable plate is shown in the withdrawn position in Figure 1. Final adjustments of the capillary position and cathode-anode distance were made by a visual observation of the location of the electrospray cone and its size relative to the desired electrospray area. The electrospray was continued until the proper amount of material was deposited or until the reservoir was depleted. If more material was to he added, the reservoir was refilled periodically while the electrospraying continued. The electrospray rate was typically 1 gL/min. This was measured by radioisotope dilution using 241Amand recording the a activity of the deposit as a function of time. Typical voltages and currents were f5-7 kV and 0.14.3 PA. Either polarity was used depending on which produced the better electrospray for the solvent-solute system being used. By electrospraying in a controlled environment 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
Figure 1. Electrospray assembly. The scale of the assembly can be estimated from the length of the 6-cm long horizontal rod on the right
Figure 2. Detail of the electrospray area during deposition, showing the area being covered. The diameter of the foil contained in the ring is 1 cm
such as dry nitrogen, the conditions for electrospray were not dependent on the fluctuation in atmospheric conditions such as relative humidity. RESULTS AND DISCUSSION G e n e r a l Comments. Detailed studies have been carried out by Brunnix and Rudstam (11) on the parameters affecting the performance of the electrospray. This included design of delivery electrode, influence of solvent, voltage, and compound to be sprayed. The application was for the preparation of thin uniform films of inorganic compounds. Biological molecules present different problems because of difficulties with solubility and possible thermal and chemical instabilities. Hypodermic Delivery Electrode. The requirement established for the selection of the delivery electrode was that it be applicable for all solvent systems used, commercially available without modification, and inexpensive. The hypodermic needle-wire combination described in the Experimental section met these criteria. T o prevent cross-contamination between samples, the hypodermic needle was discarded after use. One of the critical properties of the delivery electrode operation was the flow rate through the metal capillary. This is dependent on the viscosity and surface tension of the solvent system. Since this was a variable quantity, the positioning of the wire in the capillary acted as
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a controllable throttle for the fluid flow. In the work of Nawab and Mason (IO)on the electrospray method, the effect of the shape of the tip of the capillary on the electrospray was studied. We found that the machined bevel of the Vita hypodermic tip produced a uniform conical electrospray with a cone angle of - 4 5 O . Collector Electrode. In our work with 252Cf-PDMS,the film backing had to be very thin (
