Radiant energy – Ion generation – Field ionization type
Reexamination Certificate
2002-02-27
2004-11-30
Wells, Nikita (Department: 2881)
Radiant energy
Ion generation
Field ionization type
C250S424000, C250S42300F, C250S281000
Reexamination Certificate
active
06825477
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to desorption and ionization methods and apparatuses to produce gas phase ions for subsequent analysis. More particularly, it relates to ionization of gaseous analytes subsequent to adsorption of the gaseous analyte to an ionization surface.
BACKGROUND OF THE INVENTION
This invention generally relates to methods and apparatuses for the adsorption, desorption, and ionization of an analyte for analysis of the ionized analyte by such analytical methods as, for example, mass spectrometry.
How analytes are ionized depends on the volatility of the analyte. That is, volatile analytes are typically ionized into the gas phase, by methods such as electron ionization (EI), chemical ionization (CI), or photoionization by lasers. Involatile analytes are either desorbed from surfaces by energy input or desorbed in liquid sprays and detected as ions. Desorption from surfaces occurs in methods such as laser desorption (LD), secondary ion mass spectrometry (SIMS), and matrix-assisted laser desorption (MALDI). Desorption from liquid sprays occur in methods, such as electrospray (ES) and thermospray (TSP).
These methods of analyte ionization produce a variety of both positive and negative analyte ions. Positive ions include molecular ions (M
+
), protonated molecules (MH
+
), cationized molecules (A
+
(M)), and various fragment ions (F
1
+
). Negative ions include molecular ions (M
−
), deprotonated molecules ((M−H)
−
), anionized molecules (X
−
(M)), and fragment ions (F
−
).
The positively and negatively charged analyte ions can then be extracted from the ion source by an electric field, and separated according to their m/z ratios, using magnetic sector, quadrupole, time-of-flight, Fourier-transform, ion trap, or other types of mass spectrometers. The molecular identity of the analyte can usually be determined from the measured mass-to-charge ratios of the structurally significant ions.
Some ionization methods involve deposition of the analyte on a surface. In LD, analytes are deposited on a surface, usually metal, which is then irradiated with a laser to produce structurally significant ions for lower molecular weight materials (generally less than about 1,000 Da). In MALDI, which is primarily used for larger analytes, such as proteins, analytes are deposited on a surface together with a large excess of matrix. Isolating the analyte in the matrix is considered necessary in order to observe analyte ions. In SIMS, analytes are deposited on a surface, which is then bombarded with keV-kinetic energy primary ions, which cause secondary ions to be emitted from the surface. Another ionization method that involves surfaces is surface-assisted laser desorption ionization (SALDI). In SALDI laser irradiation is used to desorb ions from suspensions of solid carbon powders in a liquid matrix and from beds of carbon powders immobilized on a substrate. Here it is believed that the individual particles protrude above the surface of the liquid matrix.
Yet another surface ionization method is “hot wire” surface ionization. In this method, gaseous molecules are ionized in the vicinity of a hot (ca. 600° C.) filament. This method gives primarily fragment ions and the surface topography is not a factor.
Another surface ionization method set forth in U.S. Pat. No. 6,288,390 is desorption ionization from porous silicon (DIOS) wherein analyte ions are obtained by irradiating a porous silicon substrate loaded with analyte. The silicon substrate is described as having a porosity, measured gravimetrically, from approximately 4% to substantially 100%, with 60-70% porosity most preferred. The porous silicon substrate is formed by chemical etching with an ethanol/hydrofluoric acid mixture. Analyte is adsorbed or loaded onto the porous substrate prior to inserting the loaded substrate into the analyzing instrument where the substrate is irradiated, which then causes desorption of ionized analyte. The porosity of the substrate surface is critical to the ability to load the substrate with analyte molecules.
When gas phase analytes are analyzed, the analyte typically is maintained in the gas phase for both the ionization process and subsequent analysis. However, where the gaseous analyte has associated with a substrate surface prior to ionization such as in, for example, field ionization (FI) and field desorption (FD) methods, the ionization of the interacted gaseous analyte is accomplished by application of electric voltage to the substrate itself. Critical to this method is the presence of very sharp edges and tips on the substrate. The electric potential applied to the substrate creates extremely high electric fields at such tips and edges. The ionization of the analyte is a direct result of these high electric fields. Application of external sources of energy, such as laser light, onto the surface has limited or no effect on the mass spectra obtained.
Thus, except for FI, FD, and surface ionization, where ionization occurs by application of either an electric field or temperature in the immediate vicinity of the surface, mass spectrometry procedures used to analyze gases and gaseous mixtures rely on the ionization in the gas phase of the analyte.
All of the above described procedures for producing ionized particles are limited by low ionization efficiency for structurally significant ions. In the gas phase, ionization efficiency is determined by the cross section for the elementary ionization process, the flux of ionizing particles or photons, and the time that the gaseous analyte molecules spend in the ionization region. For the most commonly used ionization methods (electron ionization and chemical ionization) the fraction of analyte molecules ionized is approximately 10
−4
. In the electron capture method, typically, used for high electron-affinity compounds, the ionization efficiency is higher, about 10
−2
. However, few if any fragment ions are usually formed by this method. While an ionization efficiency close to 1 can be achieved with photoionization, it requires very high power lasers and mainly forms atomic ions. Those ions contain very little structural information, which makes identifying more complex molecules virtually impossible.
Mass spectrometers, such as magnetic sector or quadrupole instruments, are almost always used with continuous ionization where analyte ions are continually being formed and analyzed by the appropriate mass spectrometers. Commonly these instruments have a disadvantageous low ion detection efficiency particularly when analyzing ions with a wide range of m/z values. The low ion detection efficiency results because only ions within a limited range of m/z values can be detected at any one time.
In contrast, other types of mass spectrometers, such as time-of-flight instruments, have high ion detection efficiencies, that is, detection of essentially all ions formed. Efforts have been made to use such high ion detection efficient mass spectrometers for the analysis of gaseous compounds by using either a pulsed ionization scheme, or ion storage devices, or a combination thereof. Presently, among the approaches that are not limited to selected compounds, the highest sensitivity method is probably gas phase ionization with subsequent analysis in an ion trap.
Despite the very high sensitivity of current mass spectrometric methods for gas phase analysis, there remains a need to detect even smaller amounts of analytes. Such detection would provide for improved monitoring of gas purity, detection and quantification of trace compounds in the atmosphere, and ultra-sensitive gas chromatographic detection.
There further remains a need for an ionization method that utilizes a microscopically rough surface of a solid substrate to promote in situ adsorption of analyte, ionization, and desorption of the ionized analyte to achieve both increased ionization efficiency of gaseous analytes and increased detection efficiency of the ions formed. Such a method would be an advancement over the known method
Alimpiev Sergey
Nikiforovl Sergey
Sunner Jan
Kalivoda Christopher M.
Morgan & Lewis & Bockius, LLP
Wells Nikita
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