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Characterization of an Improved Thermal Ionization Cavity Source for Mass Spectrometry

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Characterization of an Improved Thermal Ionization Cavity Source for Mass Spectrometry View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Characterization of an Improved Thermal Ionization Cavity Source for Mass Spectrometry Yixiang Duan,* Ray E. Danen,† Xiaomei Yan,‡ Robert Steiner,§ Juan Cuadrado,* David Wayne,§ Vahid Majidi,* and Jose´ A. Olivares* Los Alamos National Laboratory, Los Alamos, New Mexico, USA A new thermal ionization source for use with a quadrupole mass spectrometer has been designed and characterized. The new source provides significant advantages over the previously reported prototype source and traditional filament-type thermal ionization sources. The operational interface between the source and the quadrupole mass spectrometer has been redesigned. A vacuum interlock, a translational stage, and an adjustable insertion probe are added to improve the source performance. With these modifications, the source is easier to operate while maximizing sample throughput. In this work, the performance of the newly developed source is examined. The ionization efficiencies are measured with a quadrupole mass spectrometer. The efficiency values obtained with this system are comparable to those obtained from a large scale isotope separator. The relationships among the ionization potential, vapor pressure, and measured ionization efficiency results are discussed. The crucible lifetime has been quantitatively estimated by measuring the crucible sputtering rate. Diagnostic studies of the new source show that the crucible position is a crucial parameter for sensitivity and performance. Stability tests demonstrate that the source can be run several weeks at a fixed emission current without significant degradation. (J Am Soc Mass Spectrom 1999, 10, 1008–1015) © 1999 American Society for Mass Spectrometry hermal ionization mass spectrometry (TIMS) is circumvents this problem by placing the sample on one one of the most powerful techniques for isotopic or two filaments and leaving the third filament free for Tanalysis and isotope dilution measurements. It ionization. In this way, it is possible to set the sample has been extensively employed in geochemistry, envi- evaporation rate and ionization temperature indepen- ronmental chemistry, material science, and nuclear dently. Alternative filament shapes or geometry have studies. Traditionally, TIMS involves the use of a resis- been widely explored for TIMS [4–7]. White et al. [4] tively heated filament for sample evaporation and ion- employ a V-shape filament, in which the sample is ization [1]. Single, double, and triple flat ribbon-type placed in a shallow groove. With this arrangement, filaments have been broadly employed for thermal neutral atoms evaporated in the hot tungsten groove ionization [1, 2]. The pioneering work of Inghram [3], have more opportunities to contact the walls of the which incorporates three filaments into a thermal ion- filament before leaving the immediate region of the ization source, demonstrates that multiple filaments filament surface, and are ionized more efficiently. As a have obvious benefits over the single ones. This is result, both the V-shaped filament and the multiple especially true in the analysis of elements with high filament sources have improved the absolute efficiency ionization potentials. In such cases, the vapor pressure of ion production to some extent, relative to single of the compound, containing the elements of interest, filament sources, especially for elements with high reaches high values before suitable ionization temper- ionization potentials. Rokop et al. [8] use a rhenium atures can be achieved. The multiple filament source filament with platinum electroplating on the surface to increase the work function of the filament. To achieve greater interaction between the analyte atoms and the Address reprint requests to Jose´ A. Olivares, Inorganic Trace Analysis filament surface, the sample is sandwiched between Group, CST-9, Chemical Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545. E-mail: [email protected] platinum layers. Although these methods improve the *Inorganic Trace Analysis Group, CST-9, Chemical Science and Technology performance of the filament-type ion sources some- Division. what, there is still little marked improvement in the † Currently at Intel Corp., Albuquerque, NM. absolute ionization efficiency achieved by these meth- ‡Currently at the Cytometry Group, LS-5, Life Sciences Division. §Analytical Chemistry Group, NMT-1, Nuclear Materials Technology Divi- ods. Typically, the reported values of ionization effi- sion. ciency (i.e., ions detected to atoms loaded or the degree © 1999 American Society for Mass Spectrometry. Published by Elsevier Science Inc. Received February 4, 1999 1044-0305/99/$20.00 Revised April 29, 1999 PII S1044-0505(99)00065-3 Accepted April 29, 1999 J Am Soc Mass Spectrom 1999, 10, 1008–1015 IMPROVED THERMAL IONIZATION CAVITY SOURCE 1009 tantalum filament surrounding the crucible. Adjust- ment of the electron emission current makes it possible to control the energy and the power applied to the crucible, and therefore, the ionization temperature. We named this ion source a thermal ionization cavity (TIC). The ionization efficiencies of some elements were mea- sured using a high transmission isotope separator. One to two orders of magnitude improvement in sample utilization efficiency have been obtained with the TIC source, compared to traditional filament-type sources. Although the advantages of the prototype source were Figure 1. Schematic of the thermal ionization cavity (TIC) system. obvious when compared with filament-type sources and traditional cavity-type sources, sample loading and positioning were awkward and problematic, and no of ionization of the analyte atoms) for filament-type electron emission regulation was available, which lim- sources wander around 10% for the alkali metals, and ited the use of the prototype source. less than 1% for uranium, and even much lower for More recent developments in our laboratories have refractory elements [1–8]. Other techniques used to led to a series of modifications from the original TIC improve the sample utilization efficiency in filament- source design. In this paper, we describe a modified type sources include improvements in ion transport [7], and improved TIC source design that overcomes the and the uses of chemical ionization enhancement [9, 10]. major limitations of the previous prototype. A special A significant improvement in sample utilization ef- interface for the new TIC source and the quadrupole ficiency can be obtained through the use of cavity-type mass spectrometer has been designed, built, and char- sources, which can provide orders of magnitude en- acterized. We have also added electron emission regu- hancement in ionization efficiency [11–15]. The first lation to the power supply system. The performance of cavity-type source was developed by Beyer et al. [11, the newly developed source is examined in this work. 12] in the early 1970’s in the USSR, and by Johnson et al. [13] at Lawrence Livermore National Laboratory. As Experimental shown in Figure 1, in the cavity-type sources a refrac- tory metal tube (or crucible) is used instead of a Instrumentation filament to evaporate and ionize the samples. High The experimental setup used in this work is shown in ionization temperatures are achieved by using high- Figure 2. This system consists of three sections, an ion energy electron bombardment, generated by a heated source and its power supply, a quadrupole mass spec- filament, to heat the crucible. As the sample evaporates, trometer, and a vacuum pumping system. A quadrupole inside the crucible, the gaseous atoms produced interact mass spectrometer system (QMG 420, Pfeiffer Vacuun, with each other and with the inner wall of the cavity Asslar, Germany) is used in this work. The system has chamber to produce ions. Compared with the filament- both faraday and ion counting detection capabilities. type source, the interactions between the atoms and the Data collection and processing programs are based on walls are greatly increased due to the high temperature, LabView (National Instruments, Austin, TX) software. the limited volume inside the crucible, and relatively The typical experimental operating conditions are sum- abundant surface area. Based on Kirchner’s calculations marized in Table 1, unless otherwise stated. [16, 17] on a 0.2 cm3 tungsten cylinder with an orifice of 0.7 mm i.d. operated at a temperature of about 2850 K, Power Supply and Electron Emission Regulation atoms have about 700 wall collisions before escaping through the exit hole. The high temperature achieved in In an attempt to control the ion beam current in the TIC the cavity-type source also accounts for the efficiency source, an electron beam power supply (Model 2684, enhancement. Such high temperatures are difficult to Bertan HV, Hicksville, NY) has been designed specifi- reach in a filament-type source. However, traditional cally for electron emission regulation of this ion source. cavity-type sources are relatively complex in design The electron beam power supply consists of two por- and fabrication, and are costly. These aspects inevitably tions,a0toϪ3 kV dc electron beam power supply and limit the applications of the cavity-type sources, and as a filament power supply that is electrically referenced such, these traditional cavity-type sources have only to the potential given by the beam power supply been used with large
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