Patent Publication Number: US-7897917-B2

Title: Methods and apparatus for performing chromatography and mass spectroscopy with supercritical fluid samples

Description:
RELATED APPLICATIONS 
     This application is a continuation in part of a prior provisional application entitled “An Improved Interface Between a Supercritical Fluid Chromatograph and a Mass Spectrometer,” Ser. No. 60/941,417, filed Jun. 1, 2007. 
    
    
     STATEMENT REGARDING FEDERAL SPONSORSHIP 
     The present invention was made without Federal funds. 
     FIELD OF THE INVENTION 
     The present invention relates to chemical and biological analysis by chromatography and mass spectroscopy where the sample is dissolved or suspended in a supercritical, critical or near critical fluid. 
     BACKGROUND OF THE INVENTION 
     For the purpose of this application, terms presented below will be used in the manner as defined herein. “Chromatography” is a separation technique which uses differences in affinity exhibited by compounds to different materials to separate such compounds. By way of example, without limitation, compounds dissolved in a solution will exhibit different affinity for an immobile or stationary phase through which such solution is flowing. The solution is often referred to as the mobile phase. A common immobile or stationary phase is a packed bed of particles, fibers or a porous monolith held in a vessel, column, cartridge, tube, or other conical or cylindrical device or even the walls of the device. 
     Gas chromatography (GC) refers to solutions, mobile phases, comprised of gas. Liquid chromatography (LC) refers to solutions, mobile phases comprised of liquid. High performance liquid chromatography (HPLC) refers to methods of chromatography in which the solutions are forced through or around the stationary phase under pressure. 
     A material can exist as a solid, gas and liquid. A gas will become a liquid at a critical temperature and a critical pressure. A compound at such critical temperature and critical pressure is a critical fluid. However, above a critical point, a temperature above which the compound will not exist as a liquid at any pressure, compounds take on unique properties. Compounds which are at a pressure and temperature at or above the critical point are supercritical fluids. Supercritical fluids exhibit the solvation and density properties of liquids and yet have viscosity and diffusivity of gases. These properties can be modified or altered by changes in pressure, temperature or the addition of co-solvents. 
     The term “near critical” will be used to denote a gas compound that approaches the critical pressure and temperature which compound has substantial properties of density, viscosity and diffusivity of a critical or supercritical fluid but is below the critical pressure or temperature. For example, a near critical fluid may have approximately 5-100% of the density of the compound as a liquid but is below the critical temperature. 
     This application will refer to compounds as near critical fluids, critical fluids and supercritical fluids collectively as NSC Fluids. 
     NSC Fluids are used analytically and industrially for solvation properties. It would be useful to have methods and apparatus that couple NSC Fluid chromatography devices and methods with mass spectrometry devices and methods. Mass spectrometry methods and devices often operate at atmospheric pressure at an inlet and at high vacuum within. These large pressure differentials are complicated by the higher pressures used in NSC Fluid chromatography. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention are directed to devices and methods for receiving NSC Fluids having at least one analyte from a chromatograph and directing analyte ions into the vacuum regions of a mass spectrometer. One embodiment of the invention directed to a device has a housing having at least one wall defining a chamber, sample inlet, an ionization media inlet and an outlet. The housing is constructed and arranged to have an affixed position to a mass spectrometer at the mass spectrometer vacuum region orifice. In the affixed position, the chamber is substantially closed with the outlet in fluid communication with the mass spectrometer vacuum region orifice. The sample inlet is constructed and arranged to have a position in communication with a chromatograph receiving a NSC Fluid. At least one analyte is dissolved or suspended or potentially dissolved or suspended in the NSC Fluid. The sample inlet receives the NSC Fluid and directs the NSC Fluid into the chamber to form a sample jet of NSC Fluid. The ionization media inlet is constructed and arranged to be placed in fluid communication with a source of ionization media and directing the ionization media into the chamber and the sample jet to create analyte ions. The analyte ions are received in the mass spectrometer vacuum region orifice. 
     As used herein, the term “analyte” is used to denote a compound which one desires to detect or quantify. And, an analyte ion is such compound bearing at least one charge. 
     Preferably, the sample inlet is a capillary. A preferred capillary has a restriction to form a spray of NSC Fluid. 
     The ionization media inlet preferably injects a flow of ionization media into the flow of the NSC Fluid. Ionization media inlet may take several forms including, without limitation, a second capillary, a concentric opening surrounding the sample inlet or the sample inlet may surround the ionization media inlet. 
     The ionization media is selected from the group of compounds consisting of water, ammonia, carbon dioxide and mixtures thereof. The presence of the ionization media promotes the formation of positive and negative ions of analyte molecules which can thereafter be received through the orifice of the vacuum region of a mass spectrometer and analyzed. 
     Preferably, the housing has a vent for receiving excess gases and removing excess gases from said chamber. NSC Fluids are held under pressure and the chamber is essentially at atmospheric pressure. Thus, the vent allows such excess gases to be removed. 
     A further embodiment of the present invention is directed to a method of introducing ions into a mass spectrometer. The method comprises the steps of providing a device for receiving NSC Fluids having or potentially having at least one analyte from a chromatograph and directing analyte ions into the vacuum regions of a mass spectrometer. The device has a housing having at least one wall defining a chamber, sample inlet, an ionization media inlet and an outlet. The housing is constructed and arranged to have an affixed position to a mass spectrometer at the mass spectrometer vacuum region orifice. In the affixed position, the chamber is substantially closed with the outlet in fluid communication with the mass spectrometer vacuum region orifice. The sample inlet is in communication with a chromatograph receiving a NSC Fluid in which the at least one analyte is dissolved or suspended. The sample inlet receives the NSC Fluid and directs the NSC Fluid into the chamber to form a sample jet of NSC Fluid. The ionization media inlet is constructed and arranged to be placed in fluid communication with a source of ionization media and directing the ionization media into the chamber and the sample jet to create analyte ions. The analyte ions are received in the mass spectrometer vacuum region orifice. The method has the further steps of placing the device in the affixed position and directing a NSC Fluid into the sample inlet as an ionization media is received in the ionization media inlet to form analyte ions which are received in the outlet and the orifice of the mass spectrometer vacuum region for mass analysis. 
     These and other features and advantages will be apparent to those skilled in the art upon reading the detailed description that follows and viewing the drawing briefly described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a device, in cross section, embodying features of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will be described in detail as devices and methods for receiving NSC Fluids having at least one analyte from a chromatograph and directing analyte ions into the vacuum regions of a mass spectrometer. The descriptions will be directed to preferred embodiments with the understanding that such methods and devices are capable of alteration and modification and may have utilities and applications apart from chromatographs and mass spectrometers. 
     One embodiment of the invention directed to a device, generally designated by the numeral  11 , for receiving a NSC Fluid from a chromatograph or similar analytical instrument [not shown]. Chromatographs for separating samples held in a NSC Fluid are known in the art and available from several vendors, including Thar Instruments of Pittsburgh, Pa. and Jasco of Easton, Md. Such instruments are normally comprised of the following major elements, a source of NSC Fluid, pump for moving such NSC Fluid, sample injection apparatus and a chromatographic column. 
     The device  11  is depicted mounted to the vacuum region orifice  13  of a mass spectrometer, generally designated by the numeral  15 . The inlet  13  is in fluid communication with the vacuum regions  17  of a mass spectrometer  15 . As used herein the term “mass spectrometer is used broadly to denote any instrument that produces a signal directed to the ratio of mass to charge. Mass spectrometers include tandem mass spectrometers, time of flight mass spectrometers, ion trap mass spectrometers, Fourier transfer mass spectrometers, tandem mass spectrometers and the like. Mass spectrometers are available from several venders including, by way of example, Waters Corporation of Milford, Mass. under the trademarks SYNAPT and Thermo Electron under the mark ORBITRAP. The vacuum region of the mass spectrometer is understood by those skilled in the art as those sections of the mass spectrometer  15  that operate at a pressure less than atmospheric pressure. 
     The device  11  has a housing  21  having at least one wall, of which four are depicted  23   a ,  23   b ,  23   c , and  23   d , defining a chamber  25 . The housing  21  is made of metal, ceramic or plastic. Preferred metals are steel, stainless steel or aluminum. The housing  21  may have any convenient shape and preferably has a volume of about 0.1 Liter to 1 Liter. This volume accommodates the expansion of the NSC Fluid entering the chamber  25 . As depicted, the chamber has a generally rectangular shape defined by four walls  23   a ,  23   b ,  23   c , and  23   d , and a back wall and a front wall implied in the drawing but not separately numbered for the purpose of clarity. A preferred chamber would comprise a hinged opening [not shown] to permit the user to access and service the chamber  25 . 
     The chamber  25  has sample inlet  31 , an ionization media inlet  33  and an outlet  35 . The housing  21  is constructed and arranged to have an affixed position to a mass spectrometer at the mass spectrometer vacuum region orifice  13 . In the affixed position, the chamber is substantially closed with the outlet  35  in fluid communication with the mass spectrometer vacuum region orifice  13 . Those skilled in the art will immediately recognize that the device  11  may share one or more walls with the mass spectrometer  15  to define a chamber  25  in which event outlet  35  is the open area affixed to the mass spectrometer  15 . 
     The housing  21  is affixed to the mass spectrometer  15  in the manner of an atmospheric pressure electro-spray housing [not shown] by suitable means such as bolts, screws, pins, cam devices [not shown]. Thus, the device  11  may be removed and an atmospheric pressure electro-spray device substituted to allow the user to utilize a mass spectrometer for liquid or NSC Fluid samples. 
     The sample inlet  31  is constructed and arranged to have position in communication with a chromatograph receiving a NSC Fluid. At least one analyte is dissolved or suspended or potentially dissolved or suspended in the NSC Fluid. By way of example without limitation, for applications in which the mass spectrometer  15  is used to identify the presence of the analyte, the analyte may or may not be present. The chromatograph separates the analyte from other compounds in a manner known in the art to allow the analyte to be identified more readily. 
     The sample inlet  31  receives the NSC Fluid and directs the NSC Fluid into the chamber  25  to form a sample jet of NSC Fluid. As depicted, the sample inlet  31  is a capillary  37 . The capillary  37  is secured in an opening  41  in the chamber  25  and sealed with a gasket  43 . Suitable fittings [not shown] known in the art can be used to secure the capillary  37  or to angle the capillary  37 , if desired. A preferred capillary has a restriction shown as the tip  45  of the capillary  37  to form a spray of NSC Fluid. 
     The ionization media inlet  33  is constructed and arranged to be placed in fluid communication with a source of ionization media [not shown] and directing the ionization media into the chamber  25  and the sample jet to create analyte ions. The ionization media is a compound or mixture of compounds that facilitate the formation of positive and/or negative ions in the sample jet. Preferred ionization media comprise one or more of the following compounds, water, ammonia, carbon dioxide and nitrous oxide, often in conjunction with an inert carrier gas such as nitrogen. The source of ionization media is normally a reservoir of such compounds equipped with a pump or pressure device for propelling such media through the ionization media inlet  33  into chamber  25 . 
     The ionization media inlet  33  injects a flow of ionization media into the flow of the NSC Fluid. Ionization media inlet  33  may take several forms including, without limitation, as depicted a second capillary  51 , a concentric opening [not shown] surrounding the sample inlet  31  or the sample inlet may surround the ionization media inlet [not shown]. 
     The analyte ions are received in the mass spectrometer vacuum region orifice. 
     The presence of the ionization media in the chamber  25  promotes the formation of positive and negative ions of analyte molecules which can thereafter be received through the orifice  13  of the vacuum region  17  of a mass spectrometer  15  and analysed. 
     Preferably, the housing  21  has a vent  53  for receiving excess gases and removing excess gases from the chamber  25 . NSC Fluids are held under pressure and the chamber  25  is essentially at atmospheric pressure. Thus, the vent  53  allows such excess gases to be removed to waste. 
     A further embodiment of the present invention is directed to a method of introducing ions into a mass spectrometer and is exemplified by the operation of the device  11 . The method comprises the steps of providing the device  11  for receiving NSC Fluids having or potentially having at least one analyte from a chromatograph [not shown] and directing analyte ions into the vacuum regions  17  of a mass spectrometer  15 . 
     The device  11  has a housing  21  having at least one wall  23   a ,  23   b ,  23   c  and  23   c  defining a chamber  25 . The chamber  25  has a sample inlet  31 , an ionization media inlet  33  and an outlet  35 . The housing  21  is constructed and arranged to have an affixed position to a mass spectrometer  15  at the mass spectrometer vacuum region orifice  13 . In the affixed position, the chamber  25  is substantially closed with the outlet  35  in fluid communication with the mass spectrometer vacuum region orifice  13 . The sample inlet  31  is communication with a chromatograph [not shown] receiving a NSC Fluid in which the at least one analyte is dissolved or suspended. The sample inlet  31  receives the NSC Fluid and directs the NSC Fluid into the chamber  25  to form a sample jet of NSC Fluid. The ionization media inlet  33  is constructed and arranged to be placed in fluid communication with a source of ionization media [not shown] and directing the ionization media into the chamber  25  and the sample jet to create analyte ions. The analyte ions are received in the mass spectrometer vacuum region orifice  13 . The method has the further steps of placing the device  11  in the affixed position and directing a NSC Fluid into the sample inlet  31  as an ionization media is received in the ionization media inlet  33  to form analyte ions which are received in the outlet  35  and the orifice  13  of the mass spectrometer vacuum region  17  for mass analysis. 
     These and other features and advantages will be apparent to those skilled in the art upon reading the detailed description that follows and viewing the drawing briefly described below.