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Anal. Chem. 1986, 58, 1581-1583
Design of a Pulsed Source for Supercritical Fluid Injection into Supersonic Beam Mass Spectrometry Ho Ming Pang, Chung Hang Sin, and David M. Lubman* Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 48109
Jens Zorn Department of Physics, The University of Michigan, Ann Arbor, Michigan 48109 Supercritical fluid injection has become an important means of dissolving nonvolatile molecules into a compressed gas for introduction into mass spectrometry (1-5). The coupling of a dense gas chromatograph to a mass spectrometer using a supersonic molecular beam expansion was first constructed by Wahrhaftig et al. (1, 2). The instrument designed by Wahrhaftig (1)successfully demonstrated the use of a mass spectrometer for detection of the effluent from a dense gas chromatograph (DGC/MS), which could serve as an alternative to HPLC/MS or GC/MS for analysis of nonvolatile or thermally labile molecules. Although supercritical fluids under high pressure attain liquidlike densities and thus dissolving properties similar to those of liquids, upon expansion into vacuum they become gases again and thus can be converted into supersonic beam expansions with the presence of single, gas-phase solute molecules. The original work of Randall and Wahrhaftig (1) sought to utilize these unique features of supercritical fluids with the great sensitivity and selectivity in detection afforded by mass spectrometry. In their instrumental design of a DGC/MS, injection at pressures of a t least 300 atm at temperatures of 10-60 "C was achieved. The authors a t the time outlined the requirements pertinent to the design of such an instrument based upon the properties of supersonic expansions and the pumping requirements necessary. The main problem with their design for routine analytical use was the complexity of the system and the rather large pumping capacity needed to handle the continuous flow of gas. In addition, a small orifice diameter ( 10 pm) was used because the throughput of gas increases as D2,the square of the orifice diameter. The small orifice limits the on-axis density and thus the sensitivity of the method. At the time further development of the DGC/MS was recommended by the authors. More recently, Smith and co-workers have successfully developed a supercritical fluid chromatography interface to a mass spectrometer that utilizes capillary chromatography columns (4). In their work nozzles on the order of 10 pm or smaller can be routinely used for injection and analysis of nonvolatile compounds. In this article, we describe the design of a pulsed valve capable of withstanding supercritical fluids of C02or N20of up to at least 380 atm and 100 "C for pulsed injection of these high-pressure samples into a mass spectrometer system. The use of this pulsed injection system together with extensive liquid N2cryopumping provides a sufficiently low background pressure for use with our mass spectrometer using a simple, differentially pumped vacuum system with moderate sized oil diffusion pumps. This pulsed valve allows the use of a 150-pm orifice thus increasing the on-axis density and ultimately the sensitivity by over 2 orders of magnitude compared to a continuous expansion through a 10-pm orifice. The use of a pulsed source for supersonic beam introduction of supercritical fluids is particularly appropriate when using a pulsed laser ionization source (6-8). The advantage of this method for chemical analysis as shown in our previous work (9) is that selective identification of molecules can be achieved by the unique sharp wavelength ionization spectrum of N
0003-2700/88/0358-1581$01.50/0
molecules cooled in supercritical expansions of supersonic beams.
EXPERIMENTAL DESIGN The ultra-high-pressure supersonic beam setup has been described previously (9) and consists of a differentially pumped vacuum system with a time-of-flight mass spectrometer (TOFMS) sitting vertically on top of the chamber. The novel feature here is the pulsed valve which is a modified Bosch fuel injector, based on the original design of Otis and Johnson (10) that has been modified for special high-pressure operation. The valve (Figure 1)has been designed with a stainless-steel body 0.65 cm thick, and the face plate is fastened to the main body with 12 8-32 stainless-steelscrews. This body has been successfully tested up to 400 atm pressure COz with no leakage problem. A metal-tometal seal is used (Figure lA), and when the metal seal is lapped properly the leak is negligible compared to the gas emitted per pulse up to 380 atm. There is minimum surface area available for the COz pressure against the plunger in the direction of the orifice, so the valve operates up to at least 380 atm with no major degradation of performance. Typically a -400-V square wave pulse that is 20 ps in duration is applied to the solenoid resulting in a measured gas pulse of -700 ps monitored by following the laser ionization signal as a function of delay at a distance of -22 cm from the orifice. Under these conditions the pressure in the torr using extensive liquid Nz ionization chamber is 3 X cryobaffling, although generally a voltage of 250-300 V is used to limit the opening of the valve somewhat so that the pressure is kept at -1.5 X torr. In our previous work (9),the original electrical driver circuit of Otis and Johnson (10) was used to pulse the solenoid. With this driver circuit the nozzle was not able to open properly above 270 atm, and an accompanying drop in throughput resulted. The nozzle was barely able to open at above 330 atm even at 400-V input. The gas pulse width was found to be 1450 pus, although the throughput at 300 atm backpressure was found to be at least a factor of almost 10 lower than in the present configuration shown in Figure 2. The circuit was modified to deliver more current in a shorter period of time, and the capacitor is now able to charge up to the full voltage input, whereas before it was unable to do so. Thus, the new driver provides sufficient voltage delivery in a shorter time period to drive the solenoid so that it opens to allow sufficient flow for use at backing pressures up to 380 atm. The electrical feedthrough used to provide the current is constructed of machinable ceramic insulation (Macor, Corning Glass Works) that is threaded so that it screws into the stainless-steelbody; a threaded stainless-steelrod expoxied into the ceramic conducts the current to the solenoid. The valve is heated with a thermocoax heater, which is a resistively heated wire insulated with a ceramic coating and surrounded by a conducting outer sheath. Generally the valve is operated at -45 "C, which is a temperature slightly higher than the sample reservoir, in order to prevent clogging of the orifice. However, it can operate at least up to 100 "C where the temperature is monitored by use of a chromel-alumel thermocouple. The orifice used in our experiments is a laser-drilled hole -150 pm in diameter in a 0.05-mm steel stock. The size of the orifice was chosen as a compromise between several conflicting parameters including throughput, sensitivity, pumping speed, and the attainable Mach number. An orifice of 150 pm was chosen, since it allowed a large improvement in throughput over a much smaller torr could be maintained orifice, yet a pressure of -1.5 X in the acceleration region of the TOFMS. If an orifice 500 pm
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0 1986 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986
A.
TIME (p)
Flgure 3. Time-of-flight mass spectra of (a) tyramine (T) and benzimidazole (B) and carbazole (C)expanded from a jet at 350 atm back pressure of N20at 60 "C and (b) tryptophan (R) expanded from a jet of 380 atm back pressure of COP at 40 "C with the background acenaphthene (A) as the calibrant. The flight time of the compounds will vary from a to b due to the different acceleration voltages used.
Flgure 1. (A) Ultrahigh-pressure pulsed nozzle assembly. (B) Close-up of nozzle aperture.
1000 mfd 25V
High Voltage In IN4007
pansion gas is COz. In our experiment X = 22 cm, D = 150 pm, so that M = 32. A significantlylarger orifice would require a much longer expansion distance to achieve the same Mach number, and, since the density drops on axis as 1/X2,no real gain would be made in sensitivity. A smaller orifice would allow us to reach a higher M in our ionization region at the expense of sensitivity. Thus the compromise decided upon here is between a M needed to obtain sufficient cooling for spectroscopy vs. the sensitivity needed to obtain a reasonable S I N ratio in our spectrum. This compromise depended basically on the configuration of our particular differentially pumped chamber. In order to reach the terminal Mach number, MT,before the beam reaches the ionization region an orifice of 22 pm diameter at 400 atm back pressure would be needed; this would probably lead to frequent clogging of the orifice. The ultraviolet laser source used to characterize the compounds dissolved in the supercriticalfluid supersonic beam is a frequency quadrupled (266 nm) Nd:YAG laser (Quanta-Ray DCR-1A) at an energy of
