Patent Application
20070164209
This invention relates generally to combining ionization modes produced by, for example, electrospray (ESI), atmospheric pressure chemical ionization (APCI), and thermospray for analysis of molecules. In particular, this invention relates to the creation of a new source apparatus combining APCI and ESI which will interface with existing mass spectrometers, as well as the creation of new mass spectrometers where the present invention would be the ionization source. Examples of applications which will benefit from this invention include creation of fist and accurate sample characterization of pharmaceuticals, organic intermediates, as well as the creation of sample libraries produced from combinational chemistry and high throughput biological screening.
Mass spectrometry is an analytical methodology used for qualitative and quantitative chemical analysis of material and mixtures of materials. An analyte, usually an organic, inorganic, biomolecular or biological sample, is broken into electrically charged particles of its constituent parts in an ion source. Next, the analyte particles are separated by the spectrometer based on their respective mass-to-charge ratios. The separated particles are then detected and a mass spectrum of the material is produced. The mass spectrum is analogous to a fingerprint of the sample material being analyzed by providing information about the masses and quantities of various analyte ions that make up the sample. Mass spectrometry can be used, for example, to determine the molecular weights of molecules and molecular fragments within an analyte. In addition, mass spectrometry can be used to identify molecular structures, sub-structures, and components of the analyte based on the fragmentation pattern, which occurs, when the analyte is broken into particles. Mass spectrometry is an effective analytic tool in chemistry, biology, material science, and a number of related fields.
Many challenges remain in building a mass spectrometer having high sensitivity, high resolution, high mass accuracy, and efficient sample use. One challenge is to efficiently maximize the ionization of a sample as well as allow a dynamic range of analyte samples to be used.
Problems have occurred with various ionization methods creating identifiable differences in mass spectra. For example, the introduction of various solution chemistries during the use of Liquid Chromatography/Mass Spectrometry (LC/MS) can cause notable differences in the mass spectra because one or more ions can exist simultaneously in the mass spectrometer source. During electrospray, the liquid is introduced through a metal capillary which carries an extremely high voltage. This environment creates an electrochemistry cell since the resulting spray or plume or jet is a result of the liquid exceeding its rayleigh limits as it is drawn towards a counterelectrode. Also, the redox reaction occurring during electrospray produces identifiable differences in the mass spectra such as the adduction of metal ions, M+Na. There are several different methods of ionization which have been developed.
Ion sources include methods such as APCI, ESI, and thermospray. Generally, APCI derives ions by heating the liquid flow and creating an aerosol. It is worth noting that APCI does not exhibit such adduction as described above, but will promote background ionization since it ‘uses’ the solvent as a vehicle to transfer charge to the analyte of interest. For example, hydronium ions are created in a plasma through which the analyte travels to become ionized and often tell-tale products such as M+NH4 are created if the liquid contains ammonium acetate. ESI creates the aerosol or plume as a product of the excessive charge. Also related to APCI is thermospray. In general, thermospray is APCI without high voltage (HV) and no APCI needle. (See MDS Parma ASMS poster, 2000). In this method, ions escape the aerosol droplets as they are desolvated.
Of these sources, electrospray sources are amongst the most successful. Although the basic technique of electrospray was known much earlier, the first practical source designs suitable for organic mass spectrometry appeared in 1984 (see e.g., EP 0123552A). Various improvements to this basic electrospray ion source have been proposed. Bruins et al., (34th Ann. Confr. on Mass Spectrometry and Allied Topics, Cincinnati, 1986, pp 585-6) and (U.S. Pat. No. 4,861,988) describes a pneumatically assisted electrospray source wherein a coaxial nebulizer fed with an inert gas is used in place of the capillary tube of the basic source to assist in the formation of the aerosol. In practice however, sources of this type are often operated with the capillary tube inclined at an angle to the optical axis of the mass analyzer, usually at about 30°, but still directed towards the orifice. U.S. Pat. No. 5,015,845 discloses an additional heated desolvation stage which operates at a pressure of 0.1-10 torr and is located downstream of the first nozzle. While U.S. Pat. Nos. 5,103,093, 4,977,320 and Lee, Henion, Rapid Commun. in Mass Spectrum. 1992, vol. 6 pp. 727-733, and others, teach the use of a heated inlet capillary tube. Furthermore, U.S. Pat. No. 5,171,990 teaches an off-axis alignment of the transfer capillary tube and the nozzle-skimmer system to reduce the number of fast ions and neutrals entering the mass analyzer, and U.S. Pat. No. 5,352,892 discloses a liquid shield arrangement which minimizes the entry of liquid droplets entering the mass analyzer vacuum system.
It has been realized that a major factor in the success of electrospray ionization sources for high-molecular weight samples is that, in contrast with most other ion sources, ionization takes place at atmospheric pressure. Furthermore, ionic and polar compounds ionize by ESI while neutral and weakly-polar compounds typically do not. For this reason, there has been a revival of interest in APCI sources which are also capable of generating stable ions characteristic of high molecular weight, typically <1000 Da, thermally labile species. Such sources are generally similar to electrospray sources except for the ionization mode.
APCI provides a unique method of ionization by a corona discharge (see Yamashit & Fenn, J Phys Chem., 1984). APCI maintains a corona pin at high potential, allowing the APCI to provide a source of electrons, for example, a beta-emitter, typically a Ni foil or a corona discharge (see McKeown, Siegel, American Lab. November 1975 pp. 82-99, and Horning, Carroll et al, Adv. in Mass Spectrom. Biochem. Medicine, 1976 vol. 1 pp. 1-16; Carroll, Dzidic et al, Anal. Chem. 1975 vol. 47 (14) pp. 2369). In early sources, the high-pressure ionization region was separated from the high vacuum region containing the mass analyzer by a diaphragm containing a very small orifice disposed on the optical axis of the analyzer. Later APCI sources developed into incorporating a nozzle-skimmer separator system in place of the diaphragm (see e.g., Kambara et al., Mass Spectroscopy (Japan) 1976 vol. 24 (3) pp. 229-236 and GB patent application 2183902 A).
Atmospheric pressure ionization sources, in particular electrospray and atmospheric pressure chemical ionization, interfaced with mass spectrometers have become widely used for the analysis of compounds. Ion sources which ionize a sample at atmospheric pressure rather than at high vacuum are particularly successful in producing intact thermally labile high-molecular weight ions.
Previous attempts have been described that create a dual ESI/APCI ionization source. In particular, the dual source ionization relies on a switching box. This modification allows a user to use a control box and two input BNC (bayonet Neill Concelman) connectors of the instrument to either manually or automatically select the voltage for the ESI and APCI modes. Operation of the dual ESI/APCI requires the adjustment of source voltage. Both the ESI and the APCI modes function simultaneously. The most significant parameter controlling the behavior of the source is the temperature and flow rate of the gas (see Seigel et al, J. AM. Soc. Mass Spectrom. 1998, 1196-1203).
The present invention is based, at least in part, on the discovery that a solid state switch can be used for directing the electrical potential from a power supply to either an electrospray probe or the corona discharge needle(s) creating a multi-mode ionization source. The multi-mode ionization source provides significant advantages over prior ionization sources and techniques. The multi-mode ionization source enables automatic, rapid switching from a first ionization mode to a second ionization mode without compromising results and without requiring modification of the equipment. High-speed switching is provided by the use of a solid-state switching device. Furthermore, due to source design, there is no need to elevate the temperature of the nebulizing gas to effect ionization; the source is capable of rapid switching between techniques without waiting for heating to occur. The multi-mode ionization source allows for optimal techniques and conditions to be applied to a sample during a single run. Thus, the multi-mode ionization source realizes significant savings in cost and time while increasing efficiency.
In one embodiment of the invention, an ionization source for a mass spectrometer contains an ion chamber defining an ion path, an electrospray probe for ionizing a sample, and a corona discharge needle for ionizing a sample using atmospheric pressure chemical ionization. The present invention uses a power supply for applying an electrical potential to the electrospray probe or the corona discharge needle that is run by a solid state switch for directing the electrical potential from the power supply.
Further disclosed by the present invention is a method of ionizing a sample for analysis by a mass spectrometer. This method may include introducing a sample to a probe; ionizing the sample using a first ionization mode; and then switching to a second ionization mode. In one embodiment the ionization of the sample has a duration of less than one tenth (0.1) of a second. Furthermore, switching or interscan delay can be faster or slower depending on desired speed or fidelity.
Also taught by the present invention is a system for ionizing a sample using a multi-mode ionization source. This method may include computer implemented steps such as obtaining information related to the multi-mode ionization source, and ionizing a sample based on the information related to the multi-mode ionization source. A further embodiment of this invention is a system for ionizing a sample using a multi-mode ionization source using a computer. In yet another embodiment, a multi-mode ionization source uses a plurality of ionization modes, and may have an interface for displaying information related to the multi-mode ionization source.
Also taught by the present invention is a computer readable medium for allowing, for example, a user to ionize a sample for analysis by a mass spectrometer using a plurality of different ionization modes utilizing instructions for running a multi-mode ionization source in response to information entered into a graphical user interface.
Examples of practical applications which will benefit from this invention include creation of fast and accurate sample characterization of pharmaceuticals, organic intermediates, as well as sample libraries produced from combinational chemistry and high throughput biological screening.
The present invention provides a multi-mode ionization source for ionizing samples for analysis via mass spectrometry.
The multi-mode ionization source 100 allows different ionization techniques to be applied to a sample within a single analysis. The multi-mode ionization source 100 combines the ability to generate ions in different modes of ionization into a single source and is capable of switching quickly between two or more ionization modes without modifying the equipment and without requiring external heating of the nebulizing gas used to assist formation of charged droplets. In one particular embodiment, the multi-mode ionization temperature ranged from 60-70° C. The multi-mode ionization source 100 provides a transition time between modes on the order of milliseconds, while providing accurate results. This provides the advantage of providing quality results under a broad range of speed and fidelity interscope delay conditions.
a,
2
b and 2c show a multi-mode ionization source according to an illustrative embodiment of the invention. The illustrative source 100 is a combined APCI-ESI source to enable the source to alternate between APCI and ESI scans (in both positive and negative modes). One skilled in the art will recognize that alternate ionization modes, e.g. photoionization, may be implemented in addition to or in place of the APCI mode or the ESI mode. The multi-mode ionization source interfaces to the mass analyzer to produce ions from continuously flowing liquid samples. The multi-mode ionization source 100 includes a source chamber 101 defining a region of atmospheric pressure, enclosing an electrospray probe 110 to provide electrospray ionization of molecules, a corona discharge needle 120, forming a sharply pointed discharge electrode, to provide atmospheric pressure chemical ionization of molecules and an ion inlet port 19 to a chamber 160. The chamber 160 defines an ion path for conveying ions to the mass analyzer. The source 100 is connected to a power supply 130 (shown in
A liquid inlet line 181 is provided, which connects the sample source to the ESI probe 110 to deliver the sample to be analyzed to the ESI probe 110. The ion source further includes a plurality source block heaters 182 for heating the ionization region, as well as a probe heater 186. A source exhaust port 185 is also formed in the source chamber 101. The source further includes a diffusion baffle 115 formed around the outlet end of the electrospray probe 110 for directing the flow of vaporized sample from the probe to the ion chamber inlet 19.
As shown in
The chamber 160 may be configured similar to the ionization path of the source described in U.S. Pat. No. 5,756,994, the contents of which are herein incorporated by reference, though the invention is not limited to the illustrated chamber. One skilled in the art will recognize that the chamber for conveying ions to the mass analyzer may have any suitable size and configuration according to the teachings of the present invention allowing for post-aerosol desolvation effects as taught by the presently claimed invention.
In ESI mode, the switch 150 connects the power supply 130 to the ESI probe, so that the power supply applies a high voltage to the ESI probe 110 to effect ionization of molecules, to be described in detailt below. In APCI mode, the switch 150 connects the power supply 130 to the corona discharge needle, such that the power supply applies a high voltage to the corona discharge needle 120 to effect ionization of molecules, to be described below. A data system, such as the MassLynx™ system, enables automatic switching between the different modes and polarities. Control signals from the data system further select and control the techniques and parameters of operation.
Electrospray ionization generates ions directly from solution by creating a fine spray of highly charged droplets in the presence of a strong electric field. The electrospray probe assembly 110, shown in detail in
A supply of nebulizing gas, such as nitrogen, is fed via a nebulizing channel 171 from the nebulization source (170 in
The probe assembly is clamped adjacent to the entrance port 19 of the chamber 160, such that the resulting ions pass through the entrance port 19, through the chamber. 160 and into the mass analyzer.
In APCI mode, ionization occurs through a corona discharge or plasma, creating reagent ions from the sample vapor. In APCI mode, the switch 150 activates the corona discharge needle 120 and as a consequence of the gas and heat dynamics of the source chamber/enclosure and ESI probe, the droplets are further desolvated thereby producing gaseous phase molecules at ambient temperature. The power supply establishes a corona discharge between the corona discharge needle 120 and the chamber 160 to effect ionization. Vaporized sample molecules from the probe 110 are carried through the corona discharge, creating reagent ions from the solvent vapor, which are conveyed through the chamber 160 to the mass analyzer.
In yet a further embodiment, the process of ionizing a sample using the multi-mode source of the present invention is automatically controlled by the MassLynx™ system or other suitable software system.
In one preferred embodiment, the source enclosure measures 53 inches by volume and the present shape and contour contribute to the dynamics. (See
The multi-mode ionization source provides significant advantages over prior ionization sources and techniques. The multi-mode ionization source enables automatic, rapid switching from a first ionization mode to a second ionization mode without compromising results and without requiring modification of the equipment. High-speed switching is provided by the use of a solid-state switching device. Moreover, multi-mode ionization allows the unique opportunity to acquire valuable data during short time constant events such as chromatographic peak transitions. Furthermore, because there is no need to elevate the temperature of the nebulizing gas to effect ionization, the source is capable of rapid switching between techniques without waiting for heating to occur. The multi-mode ionization source allows for optimal techniques and conditions to be applied to a sample during a single run. Thus, the multi-mode ionization source realizes significant savings in cost and time while increasing efficiency.
While there are many compounds that are ionized by both ESI and APCI, they may not ionize with equal success. Furthermore, some compounds may not ionize by ESI at all. The present invention provides a solution for ionization of compounds of this nature.
For example, the performance of the ZQ™ Mass Spectrometer with an ESCi™ ionization source has yielded successful results of polycyclic aromatic hydrocarbons (PAHs). PAHs such as naphthalene do not ionize by ESI because there is no opportunity for a proton to attach to form M+H.
Further demonstrating the capacity and diversity of the present invention was the results of sampling 50 ng daidzein isolavomoid on-column. This example showed the accuracy and fidelity of the results of all four modes. While the practice of sample preheating is common during electropsray, this example illustrates that ESCi proceeds exceptionally well with inordinate amounts of heat introduced. In fact, this example illustrates that the heat settings were identical to normal ESI operation. The ESI desolvation temperatures were near 120° C., as opposed to the 400-600° C. range needed by standard MS configurations.
This example demonstrated that the ESCi new technology may be adapted easily to current operating systems such as the GSK (RTP) Open Access. Here, output was a valid MassLynx™ data file which allowed the ESCi technology to be added transparently to open access and high throughput environments. Previously, these environments had to be operated in one mode or another using different devices. This allowed the collection of data and results as well as an invaluable ability to compare both modes. (See
One of the most important applications of the present invention is the ability to use the results to create accurate sample libraries. This example set out to characterize 500,000 compounds in one year ensuring a purity level of >70%. The results are used to label a correct molecular weight as determined from the result of positive and/or negative mass spectra.
The experimental detail was run on a short LC gradient. There was a generic 2 minute gradient (0.05% formic Acid/MeCN), with 3 minute run time. The flow rate was 0.7 ml/min with injected volume of 1 ul. The compounds were detected at a UV of 225-320 nm and the mass spectra was run at 150-800 amu. The scans were taken at 00.2 sec (3250 amu/sec) with a 0.2 sec. ISD (inter scan delay).
This example further illustrated that with a slower flow rate, the acquisitions times were actually increased due to the lack of high heat necessary for the ESCi to perform. Thus, the very high acquisitions rate capability of the embedded PC on the ZQ allowed more functions to be carried out during the brief passage of the chromatographic peak or band by scanning at speeds far above what was normal prior to the instant invention.
The present example proceeded by taking a 96-well test plate containing a variety of compounds covering molecular weight from 150-500 amu. These compounds were analyzed in three phases; (a) traditional ESI source alone, (2) traditional APCI source alone and by (3) reanalyzed using ESCi™ technology.
The results showed the advantages and improvement of results for the sample libraries via the ESCi method versus other traditional modes of analysis. In
Another advantage of the present invention is that a single injection captures multiple data points. As illustrated in
There also has been experimentation with this method and extending the ionization mode capability beyond ESI and APCI to include other forms of ionization such as photoionization detector (APPI). APPI will promote ionization of weekly polar or neutral analytes, monomers, hydrocarbons or organo-heteroatom species and other compounds which to not “spray” readily. This device used ultraviolet light as a means of ionizing an analyte exiting from a gas chromatography (GC) column. Electrodes collected the ions produced by this process. The current generated was therefore a measure of the analyte concentration.
Further advantages of the ESCi™ multimode-ionization are illustrated by the comparison of polymer additives. As illustrated in
In sum, the advantages of the invention are that the ESCi apparatus used existing mass spectrometers. The addition of the apparatus discharge mechanism and power supply has proven successful in experimental runs. The ESCi Source ran at 100 ms inter scan delay for polarity and ionization switches. There is no apparent loss of performance for both ESI and APCI under these experimental conditions. The present invention reduced annalist times and was incorporated into open access instruments.
Equivalents
Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed by the following claims.
The entire contents of all references, patents, and patent applications cited herein are expressly incorporated by reference.
This application claims priority to U.S. Provisional Application No. 60/385,419 entitled “A High Speed Combination Multi-Mode Chemical Ionization Source for Mass Spectrometers” filed on May 31, 2002, the entire contents of which are incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US03/16892 | 5/30/2003 | WO | 11/11/2004 |
| Number | Date | Country | |
|---|---|---|---|
| 60385419 | May 2002 | US |
