Abstract:
An apparatus and method for improved inline and automated chemical analysis is provided, in particular disclosing signal optimization for an electrospray ionization mass spectrometer apparatus. A substantially inert pathway for ion analysis is provided by using substantially inert metals or polymers for pathway parts. Other enhancements and advantages are also disclosed, including an advantageous probe profile and metal foil cover.

Description:
TECHNICAL FIELD 
     The present invention relates generally to chemical analysis and, more particularly, to an apparatus and method for inline and automated chemical analysis with signal optimization. 
     BACKGROUND 
     Systems and apparatus for measuring the concentration of analytes in a sample have been developed using a number of analytical techniques such as chromatography, spectroscopy, and mass spectrometry. In particular, mass spectrometry is often the technique of choice to achieve sensitivity for trace and ultra-trace analysis in which the analyte concentration may be as small as parts per billion (ppb) or sub-ppb such as parts per trillion (ppt). For example, commonly assigned U.S. patent application Ser. Nos. 10/086,025 and 10/094,394 disclose automated analytical apparatuses that measure contaminants or constituents present in trace concentrations, the full disclosures of which are hereby incorporated by reference for all purposes. 
     Trace contaminant metrology (TCM) and chemical composition metrology (CCM) tools, both available from Metara Inc. of Sunnyvale, Calif., rely on an electrospray ionization time-of-flight mass spectrometer (ESI TOF MS) for the measurement and quantitation of analytes. Commercial ESI TOF MS instruments are not suitable for measurement of trace metals in industrial process solutions, for example those used in the semiconductor industry, due to interfering contamination that is introduced from component parts and due to degradation of trace contamination signal strength that can be caused by contamination build up in certain locations along the sample pathway. With continued sampling and analysis, contaminants from the ESI TOF MS system and residue from the process solution may accumulate in the sampling and analysis pathway, increasing inaccuracy, lowering spectrometer resolution, and further reducing sensitivity for trace contamination measurement. In order to acquire high resolution data with high sensitivity, a clean and easily maintained mass spectrometer system is required. 
     Accordingly, apparatus modifications and methods for instrument induced contamination reduction and signal optimization are essential for successful inline and automated chemical analysis of trace contamination. 
     SUMMARY 
     An apparatus and method for improved inline and automated chemical analysis is provided, in particular disclosing signal optimization for an electrospray ionization mass spectrometer apparatus. A substantially inert pathway for ion analysis is provided by using substantially inert metals or polymers for pathway parts. Other enhancements and advantages are also disclosed, including an advantageous probe profile and metal foil cover. 
     In one embodiment of the present invention, a mass spectrometer apparatus for monitoring a fluid system is provided, the apparatus comprising: a sample introduction tube; an electrospray ionization (ESI) probe with a shroud; a union coupling the sample introduction tube and the ESI probe; a capillary assembly with an endcap to which the ESI probe is aligned; a skimmer cone operably coupled to the capillary assembly; an ion guide operably coupled to the skimmer cone; and an ion detection module operably coupled to the ion guide. The ESI probe, the shroud, the union, the endcap, and the skimmer cone are comprised of a substantially inert metal or polymer to provide a substantially inert pathway for ion analysis. 
     In accordance with another embodiment of the present invention, a method of monitoring a fluid system via electrospray ionization time-of-flight mass spectroscopy is provided, the method comprising: providing a substantially inert pathway from a sample introduction tube, through an electrospray ionization probe with a shroud, through a capillary assembly, through a skimmer cone, through an ion guide, and to an ion detection module; flowing a sample through the substantially inert pathway; and analyzing the sample via electrospray ionization time-of-flight mass spectroscopy. 
     Advantageously, the present invention enables trace contamination signal optimization without the introduction of interfering contamination from the ESI components enabling high sensitivity inline and automated chemical analysis of industrial process solutions. (It should be noted that industrial process solutions are not the only application of this high sensitivity capability, for example the same capability may be applied in biological and environmental applications.) 
     The scope of the invention is defined by the claims, which are incorporated into this section by reference. A more complete understanding of embodiments of the present invention will be afforded to those skilled in the art, as well as a realization of additional advantages thereof, by a consideration of the following detailed description of one or more embodiments. Reference will be made to the appended sheets of drawings that will first be described briefly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an electrospray ionization mass spectrometer in accordance with an embodiment of the present invention. 
         FIG. 2  illustrates an electrospray ionization probe assembly in accordance with an embodiment of the present invention. 
         FIG. 3  illustrates the exterior of a spray chamber in accordance with an embodiment of the present invention. 
         FIG. 4  illustrates the interior of a spray chamber in accordance with an embodiment of the present invention. 
         FIG. 5  illustrates an endcap in accordance with an embodiment of the present invention. 
         FIG. 6  illustrates a nose piece after the spray chamber has been removed in accordance with an embodiment of the present invention. 
         FIG. 7  illustrates a capillary assembly in accordance with an embodiment of the present invention. 
         FIGS. 8A through 8C  illustrate different embodiments of an ESI probe profile in accordance with an embodiment of the present invention. 
     
    
    
     Embodiments of the present invention and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures. It should also be appreciated that the figures may not be necessarily drawn to scale. 
     DETAILED DESCRIPTION 
     The present invention provides apparatus and methods for improved inline and automated chemical analysis, in particular disclosing signal optimization for trace contamination measurement by mass spectroscopy. 
     Referring now to  FIGS. 1-8 , an electrospray ionization (ESI) mass spectrometer system  100  and some of its parts are illustrated in accordance with an embodiment of the present invention. System  100  includes sample introduction through a tube  102  (e.g., comprised of perfluoro-alkoxyalkane) and a nebulizing gas line  104 , both operably coupled to an ESI probe assembly  110 , which includes an ESI probe  112  and a probe shroud  114 . A capillary assembly  120 , to which the ESI probe  112  is operably aligned, includes a capillary  126  with an end cap  122  and a nose piece  124  being positioned between the ESI probe  112  and the capillary assembly  120 . A skimmer cone  132  and an ion guide  142  are operably coupled to capillary assembly  120  for guiding ions of interest to an ion detection module  152  (e.g., a TOF MS detector module). In one embodiment, skimmer cone  132  is a cone shaped metal piece with a small hole at the cone tip for allowing ions to pass, the skimmer cone acting as an interface between two different vacuum stages (e.g., from about 1 torr to about 10 −3  torr). The skimmer cone may be positioned with appropriate accessories or parts such as retaining rings and spacers. 
     These components may be housed in a housing  201 , which may be a single housing or separate housings operably combined. In one embodiment, ESI probe  112  and probe shroud  114  are partially housed within a spray chamber  203  of housing  201  and a drain  302  is provided for spray chamber  203 . Capillary assembly  120  may be housed in a capillary housing  205 , skimmer cone  132  and ion guide  142  may be housed in a housing  207 , and the ion detection module  152  may be housed in a housing  209 . 
       FIG. 2  illustrates ESI probe assembly  110  in accordance with an embodiment of the present invention. Probe assembly  110  includes a union  111  which provides an interface/connection between ESI probe  112  and sample introduction tube  102 , an ESI probe nut  103 , and probe shroud  114 . 
       FIG. 3  illustrates the exterior  203   a  of spray chamber  203 , and  FIG. 4  illustrates the interior  203   b  of spray chamber  203  in accordance with an embodiment of the present invention. As noted above, spray chamber  203  houses ESI probe  112  with probe shroud  114  and endcap  122 , shown in close-up in  FIG. 5 .  FIG. 6  illustrates the mass spectrometer system  100  with the spray chamber  203  and endcap  122  removed revealing nose piece  124  and a nut  127  in accordance with an embodiment of the present invention. 
       FIG. 7  illustrates capillary assembly  120  of mass spectrometer system  100  in accordance with an embodiment of the present invention. Capillary assembly  120  includes nose piece  124 , a nut  127 , a washer  125 , an O-ring  123 , glass capillary  126 , and a metal foil  121  serving to cover and protect the capillary entrance and provide electrical conductivity. Portions of capillary assembly  120  may be housed in capillary housing  205  ( FIG. 1 ). 
     In one embodiment of the present invention, contamination is minimized by utilizing inert or high purity polymer material or highly inert metal or metal alloy for parts associated with or that come into direct contact with the sample and/or ion pathways. For example, union  111 , ESI probe  112 , probe shroud  114 , endcap  122 , skimmer cone  132 , and parts of ion guide  142  may be comprised of such inert or high purity material. 
     In one example, union  111  may be comprised of polyetheretherketone (PEEK), ESI probe  112  may be comprised of platinum and/or platinum-iridium alloy, probe shroud  114  may be comprised of polychlorotrifluoroethylene (PCTFE) resin (e.g., Kel-F®, Diaflon®, or Aclon®), and endcap  122  and skimmer cone  132  may include a platinum coating. Previously, most parts were disadvantageously comprised of stainless steel, which is not optimally inert and therefore does not minimize contamination. Of serious concern is the fact that a pathway part is composed of the materials that must be measured to trace contamination levels in process solutions, for example in the semiconductor industry. Even trace amounts of contamination from the ESI-TOF MS components may be at or above the levels that must be measured in process solutions. Thus, highly inert or high purity material is needed. 
     In another embodiment, the overall performance of the mass spectrometer is improved by various other enhancements. Referring to  FIGS. 8A-8C , changing the shape of ESI probe  112  from a tapered shape (e.g., similar to a cone shape) ( FIG. 8A ) or a stepped shape ( FIG. 8B ) to a straight or linear profile ( FIG. 8C ) advantageously reduces erosion and provides an improved signal that is stable for longer periods, as well as for easy manufacture. 
     The use of a metal foil with a hole in the middle  121  ( FIG. 5 ) (e.g., platinum, nickel, aluminum, or other substantially inert metal or metal alloy) provides the necessary electrical conduction to the end cap while being easily changed as contamination builds up on its surface. This contamination occurs during normal spectrometer operation and may be an insulating layer that can charge up and deflect ions away from the desired path. Since the foil can be changed easily and quickly (vs. breaking vacuum and removing and cleaning the capillary end), this avoids significant downtime and more complicated maintenance procedures. 
     Embodiments described above illustrate but do not limit the invention. It should also be understood that numerous modifications and variations are possible in accordance with the principles of the present invention. Accordingly, the scope of the invention is defined by the following claims.