United States Patent (19) 11 Patent Number: 4,667,100 Lagna 45 Date of Patent: May 19, 1987 METHODS AND APPARATUS FOR MASS 4,209,696 6/1980 Fite ..................................... 250/281 (54) 4,300,044 11/1981 Iribarne ............................... 250/282 SPECTROMETRICANALYSES OF FLUIDS 4,531,056 7/1985 Labowsky et al. ................. 250/281 76 Inventor: William M. Lagna, 11209 Sandyvale Primary Examiner-Bruce C. Anderson Rd., Bradshaw, Md. 21021 Attorney, Agent, or Firm-Schwartz, Jeffery, Schwaab, 21) Appl. No.: 724,166 Mack, Blumenthal & Evans 22 Filed: Apr. 17, 1985 57 ABSTRACT 51 Int, Cl." .............................................. B01D 59/44 In accordance with the invention, an electrode is held at 52 U.S. C. .................................... 250/282; 250/281; high voltage potential within a chamber constructed of 250/288 high dielectric material. A sample is sprayed past the 58) Field of Search ........... 250/281, 282,288, 423 R, electrode and at least a portion of the sample is ionized. 250/424 Some of the ions are directed through a suitable inlet into the high vacuum portion of the mass to charge 56) References Cited analyzer. U.S. PATENT DOCUMENTS 4,144,451 3/1979 Kambara ............................. 250/281 42 Claims, 5 Drawing Figures SYYYYYYYYYYYYY s - U.S. Patent May 19, 1987 Sheet 1 of 5 4,667,100 2// assan asara Ny U.S. Patent May 19, 1987 Sheet 2 of 5 4,667,100 N Z-Z-Z-Z-ZZzz,N N, U.S. Patent May 19, 1987 Sheet 3 of 5 4,667,100 U.S. Patent May 19, 1987 Sheet 4 of 5 4,667,100 SZN NZf SN Z> -->2 U.S. Patent May 19, 1987 Sheet 5 of 5 4,667,100 OO F.G. 5 4,667,100 1. 2 tile, evaporation will occur until the critical charge to METHODS AND APPARATUS FOR MASS radius ratio is exceeded, and Rayleigh emission will SPECTROMETRICANALYSS OF FLUIDS occur again. This happens because evaporation pro ceeds with virtually no loss of electrolyte. Aqueous BACKGROUND OF THE INVENTION 5 electrolytic solvation energies are typically of the range The invention relates to a method and apparatus for 3-6 eV, and the probability of an ion escaping from the mass spectrometric analysis of gases and liquids and surface is calculated to be in the order of 1050. If the constituents thereof such as may be received from a gas droplet is in the micron size range, a competing process or liquid chromatograph. of ion evaporation can take place. In ion evaporation, Major limitations exist in current mass spectrometric O ion clusters can be emitted from a charged droplet expe ionization techniques in the methods used to volatilize riencing a large electric field applied at the surface. For the analytes. Electron impact, photoionization, ion these small droplets, the net charge on the droplet com molecule charge transfer, and thermal ionization are a bined with its small radius is sufficient to produce an number of methods by which the ionization can be electric field at the surface of enough energy to allow accomplished. However, heat is used almost universally 15 ions to evaporate. to effect volatilization. Many large molecules and bio By passing gases through a very high potential elec logically important compounds can not be determined tric field, non-thermal ionization can be accomplished using mass spectrometry due to their thermally sensitive by conduction and induction. If the field potential is nature. Heat induced decomposition and fragmentation greater than 105 volts per meter, ionization of the gases of these unstable compounds occurs prior to detection 20 and constituents of the gases occurs primarily by induc by the mass analyzer. tion and independent of ion-molecule charge transfer Electron impact, chemical ionization, thermospray, reactions. and direct liquid introduction require the input of ther Ion emission by Rayleigh instability occurs in ther mal energy to accomplish or maintain the volatility of mospray, atmospheric pressure ionization, ion evapora the analyte. Atmospheric pressure ionization also uses 25 tion and electrospray liquid chromatographic/mass heat to assist in the volatilization of liquid samples. The spectrometric interfaces. However, all of these methods plasma desorption techniques, laser desorption, fast rely on the existence of preformed ions, or require addi atom bombardment, and californium-252 desorption are tional electrolytes or buffers within the solvent from at least partially dependent on thermal energy to ac which a proton can be transferred to effectionization of complish volatilization and ionization of the analyte. 30 the analytes. Low dielectric, non-aqueous and aprotic Both fast atom bombardment and californium-252 are solvents do not supportion formation, and as a result, further restricted because no method currently exists to few compounds exist in ionized form in these solvents. interface them directly with any chromatographic sepa Thermospray, atmospheric pressure ionization, ion ration techniques. Ion evaporation, a non-thermal ion evaporation and electrospray are therefore primarily separation method, and thermospray require either that 35 limited to aqueous solvent systems. Also, because of the analytes exist in ionized form in aqueous solvent or dependence on ion-molecule reactions to accomplish that the analytes can be protonated through ion charge transfer, these methods, particularly atmo molecule proton transfer reactions from an aqueous spheric pressure ionization, ion evaporation and elec buffered solvent for detection by the mass analyzer. trospray are limited to operational pressures near ambi Gases and compounds which are insoluble or un 40 charged in aqueous solvents are not analyzed by these ent. At reduced pressures, fewer ion-molecule collisions methods. This restriction limits the application of these result in fewer charge transfer reactions. At increased techniques, particularly in the analysis of many ther pressures, evaporation of droplets is reduced. Droplet mally unstable compounds. evaporation is necessary in these methods to accomplish However, a thermally independent volatilization pro 45 ion emission by decreasing the droplet volume until the cess, known as Rayleigh ion emission, provides a means critical Rayleigh charge to radius limit is exceeded. to effect non-thermal volatilization of charged species The use of an induction electrode in atmospheric from liquids. When the electric field at the surface of a pressure ionization and ion evaporation serves to in droplet is of sufficient energy that the surface potential crease the net charge on a droplet. In ion evaporation, can be overcome, emission of charged species occurs 50 the induction electrode is positioned adjacent to the from the droplet. This ion emission reduces the electro liquid spray orifice. This type of system is disclosed in static repulsion experienced by an ion at the surface of U.S. Pat. No. 4,300,044 to Iribarne et al. The charge on the droplet. For ion emission to occur, the field at the the electrode is opposite to the droplet charge at a po surface of the droplet must exceed the Rayleigh instabil tential of 1.5 to 3 kilovolts. This serves to increase the . 55 relative field strength experienced by ions at the surface ity number. of the droplet, to assist the ion emission process. The The conditions for Rayleigh instability are described field generated is of insufficient strength to ionize either in the following equation: the solute or the solvent. Therefore, only polar solvents containing preformed ions can be used with this 60 method. Instability occurs when a =4 where q is the charge on The induction electrode in atmospheric pressure ioni a drop, V is the volume, t is the surface tension and e is zation liquid chromatography/mass spectrometry is the dielectric constant. The critical radii for ion emis positioned within the path of the sprayed droplets. This sion from water or other solvents or mixtures by Ra type of system is disclosed in U.S. Pat. No. 4,144,451 to leigh instability can, therefore, be calculated directly. 65 Kambara. The electrode is of the same polarity as the A droplet undergoing Rayleigh ion emission will lose ions to be analyzed, at an electric potential of typically a considerable fraction of the charge with only a small 1.5 to 3.0 kilovolts. The electrode serves to increase the change in the radius. If the solvent is sufficiently vola net charge on a droplet, primarily by conduction. How 4,667,100 3 4. ever, the electromagnetic field generated by the induc potentials of 15 to above 120 kilovolts. Typical total ion tion electrode is of insufficient strength to ionize non currents are in the range of hundreds of microamps for polar, organic and aprotic solvents or compounds. Wa ionization of the liquid effluent. Most gas effluents re ter, or another polar or ionic compound is usually added quire total ion currents in the microamp range. to non-polar solvents to increase the relative amount of 5 charge transfer in order to accomplish ionization. As BRIEF DESCRIPTION OF THE DRAWINGS such, non-polar solvents are observed as protonated FIG. 1 is a schematic of a differentially pumped quad molecular ions or ion clusters in positive ion mode. rupole mass analyzer suitable for interface with the In electrospray and related processes, electric poten described ion generation source. tial is applied to the capillary which carries the liquid 10 FIG. 2 is a cross-sectional side view of a preferred effluent. This type of system is disclosed in U.S. Pat. embodiment of the ion source volume portion of the No. 4,209,696 to Fite. Charge transfer occurs by con mass analyzer. duction through the solvent. High dielectric and non FIG. 3 is a schematic of a differentially pumped quad polar solvents are not conductive by nature, and as a rupole mass analyzer suitable for interface with the result, little charge is transfered to these solvent types.