Patent Application
20080078944
The present invention relates to systems and methods for optimizing the control of an ion source utilizing electro-pneumatic superposition connected to a mass spectrometer by feedback from operational data.
Laser Desorption Ionization (LDI) and Electrospray (ES) ion sources have become an essential and component in the modern mass spectrometry of biological macromolecules (e.g. proteins, peptides, and sugars). Both methods revolutionized the application of mass spectrometers in life science, in particular in proteomics but also in functional genomics and metabolomics and drug discovery.
In Matrix Assisted Laser Desorption and Ionization (MALDI) ion sources, typically a UV laser (sometimes IR) are fired at the crystals in the MALDI spot with typical pulse duration on the order of tLP≈10−9 to 10−8 s. The matrix molecules in the spot absorb the electromagnetic laser energy. It is generally thought that the matrix is ionized by this event, and then transfers part of its charge to the analyte molecules (e.g. a protein), thus ionizing them while still protecting them from the disruptive energy of the laser.
One variant of MALDI is sometimes referred to as Surface Enhanced Laser Desorption Ionization (SELDI) in which the matrix is already pre-deposited on the target surface (“MALDI” will refer to both MALDI and SELDI). Originally, LDI/MALDI ion sources were operated under vacuum conditions at pressures where sample ion-background gas collisions are negligible. Later, ion sources operating at elevated pressure and Atmospheric Pressure MALDI (AP MALDI) were introduced for their convenience in terms of sample handling, as well as collisional cooling.
Experiments carried out in the early 90's indicated improved ion transmission within gas-filled multiple ion guides due to “collisional cooling”: Repeated collisions of ions with gas molecules reduce the temperature of the ions and also cause the ion beam to collapse axially inside RF multiple ion guides (D. J. Douglas and J. B. French: “Collisional focusing effects in radiofrequency quadrupoles,” J. Am. Soc. Mass Spectrometry 1992, no. 3, p. 398-408). This collisional cooling effect was subsequently utilized in MALDI ion sources themselves. Simple versions of so called elevated pressure and Atmospheric Pressure MALDI (AP MALDI) ion sources have been described, beginning in the late 1990's. However, their ion-optical design is not completely satisfactory, and a pneumatic design is effectively non-existent due to the lack of appropriate computational design tools capable of modeling the flow field as well as the electro-pneumatic interactions (ion-neutral collisions).
In advanced implementations of the second major ionization method for biological macromolecules, electrospray, electric-pneumatic superposition is equally important. In electrospray ion sources, a liquid in which the sample molecules are dissolved is pressed through a capillary. The sample molecules are already in an ionized state inside the liquid. Upon leaving the capillary, through a nebulization process, the liquid forms a mist (or aerosol) of very small droplets containing ionized sample molecules. Due to coulombic forces, individual ionized sample molecules are eventually released and transported into a mass spectrometer, although the exact mechanism of the ion formation is under debate. However, since the entire process typically occurs in the presence of a gas or, more generally, a gas flow field as well as an electric field, advanced configurations with electro-pneumatic superposition provide distinct advantages. One reason for using gas flow fields and electro-pneumatic superposition, however, is to support nebulization and ion guidance. In addition, controlled moderate collisional heating may be affected.
Due to high laser power densities LDI creates ions with substantial translational and internal temperatures that frequently results in fragmentation and decay that generates molecular fragments, thereby limiting the available ion life time for analysis. It is, therefore, advantageous to employ collisional sample ion cooling by means of intentionally introduced gases. In advanced types of such collisionally-cooled LDI ion sources the actual ion dynamics are the result of so-called electro-pneumatic superposition, which picks utilizes electro-pneumatic elements to create specifically designed electric fields as well as gas flow fields.
Analogously, ES ion sources, which create ions from a liquid by means of high electric fields, typically operate at atmospheric gas pressures or at least at elevated pressures. Again, the use of electro-pneumatic superposition by means of electro-pneumatic elements to create specifically designed electric fields as well as gas flow fields provides significant advantages with respect to ion guidance and introduction in a mass spectrometer with minimal losses.
In both applications the optimal superposition and the resulting ion dynamics will depend on various operational parameters, such as sample composition, sample spot surface chemistry (in case of MALDI), physical and chemical properties of the liquid (in case of ES). Such varied conditions may cause the optimal point of operation of the ion source to shift (within its parameter space); more particularly, changes in gas supply and electric field strength may be required.
Due to the complexity of the ion source behavior, the number of parameters that can be adjusted, and the limited available time, a typical user of such ion sources (connected mass spectrometers) cannot be expected to perform such correcting adjustments in an optimal and rapid fashion. Accordingly, Improvements in systems that offer active control and feedback of operating parameters would be desirable.
Embodiments of this invention relate to systems and methods that optimize the control of ion sources employing electro-pneumatic superposition, the ion sources being operably connected to a mass spectrometer. Methods and systems of control include collecting and analyzing data from the mass spectrometer during its operation, generating signals from the data analysis, and providing the signals as feedback to control various aspects of the operation of the ion source. Data from which informative feedback signals are generated may include the mass spectrum data from a sample being analyzed, and may also include data from sensors reporting conditions from the locale of the ion source, as well as data from other sources.
The ion source of mass spectrometers controlled by embodiments of these systems and methods may include ion sources of the laser desorption ionization type as well as the electrospray type. Some embodiments of the ion source may make use of charge injection (CI-LDI/CI-MALDI), and may further make use of two-dimensional sample chips.
Objectives of the optimization of the control of the ion source include optimally guiding ions, cooling ions collisionally, and optimally guiding droplets containing sample ions. Optimizing control may be effected by various approaches, for example, by changing the multiplicity of the gas reservoir pressures used to supply gas to the ion source region in which the electro-pneumatic superposition occurs, or by controlling changing the total gas flow to the ion source region in which the electro-pneumatic superposition occurs. Optimizing control may further be effected by changing the electric potentials on electro-pneumatic elements.
Optimizing control may further be effected by changing the mechanical arrangement of electro-pneumatic elements such as angles or gap-width by means of active drives such as stepper motors. Optimizing control may still further be effected by changing the timing behavior of the electric or pneumatic parameters. Optimizing control may even still further be effected by changing the operation of a pump connected to the ion source or the gas flow to said pump by means of a throttling valve.
Embodiments of the presently described active control system may assume various configurations, for example, they may be integrated into the control system of the ion source, they may be integrated in the control system of the mass spectrometer, or they may be stand-alone devices.
Embodiments of the active control system may make use of information obtained throughout the entire mass spectrometric data acquisition process to provide feedback information to optimize the performance of the ion source, or they may make use only of information obtained during an initial phase of the mass spectrometric data acquisition process. In these embodiments, the active control system is providing feedback in real time. In other embodiments of the active controls or it may make use of stored information, such stored information may also be encoded in the sample itself or on a bio-chip.
Embodiments of the active control system make use of an algorithm that derives variously from any of the control signals provided to the ion source from the total ion count, from the signal to noise ratio in the mass spectrum, and/or from the amount of fragment or cluster ions in the mass spectrum.
The subsequently described embodiments of an active feedback and control system for ion sources and applies to both LDI and ES technology in conjunction with electro-pneumatic superposition. The following patent applications of Hieke are related to the presently described embodiments of the invention, and are included by this reference: “Ion source with controlled superposition of electrostatic and gas flow fields” (WO05081944A2 and US2005194542A1, both filed on Feb. 22, 2005); and “Methods and apparatus for controlling ion current in an ion transmission device” (US2005194543A1 and WO05081916A2, both filed on Feb. 22, 2005). Provisional U.S. applications that are related and included by this reference are “Laser desorption ionization ion source with charge injection” (U.S. App. No. 60/798,377, filed on May 5, 2006) and “Laser desorption ionization ion source with self-adjusting holder and insertion system for one and two-dimensional sample chips” (U.S. App. No. 60/802,941, filed on May 23, 2006.
As illustrated in
To understand the operation of such ion sources, per aspects of this invention, the visualization of the electric fields and pneumatic flow fields created by the electric-pneumatic elements and the computation of ion trajectories are helpful.
In practical applications, electro-pneumatic ion sources are contained in housings and connected to mass spectrometers. In some particular embodiments of the invention, as shown in the
Mass spectra obtained with such configurations can exhibit many artifacts such as ion fragmentation, ion clustering, or insufficient ion transmission due to superposition breakdown if the ion source is not operating at optimal conditions. An example is shown in
At ρmax=100 Pa the maximum ion count for the true peak has been reached, however, ion fragments are still observed. The highest signal to noise ratio is reached at ρmax=200 Pa although the total ion count is now reduced for this particular electro-pneumatic design. At ρmax=300 Pa the signal to noise ratio decreases again due to the appearance of cluster ions 403. In addition to varying the reservoir pressure the electric potential on the electric-pneumatic elements thereby the electric field inside the ion source has to be modified in order to maintain sufficient ion transmission.
The actual optimal values for gas reservoir pressure and various electric potentials typically depend on the design of the electro-pneumatic elements, sample composition, surface chemistry of the chip as well as laser operation parameters in case of LDI. Further, it is apparent that the optimization can have different goals, such as improving the maximum ion count or the maximum signal to noise ratio.
The aforementioned difficulties may be eliminated by implementing an active control and feedback system, per aspects of some embodiments, as shown in
Another embodiment is shown in
While particular embodiments of the invention and variations thereof have been described in detail, other modifications and methods of using the disclosed workflow management system will be apparent to those of skill in the art. Accordingly, it should be understood that various applications, modifications, and substitutions may be made of equivalents without departing from the spirit of the invention or the scope of the claims. Various terms have been used in the description to convey an understanding of the invention; it will be understood that the meaning of these various terms extends to common linguistic or grammatical variations or forms thereof. It will also be understood that when terminology referring, for example to physical equipment, hardware, or software has used trade names or common names, that these names are provided as contemporary examples, and the invention is not limited by such literal scope. Terminology that is introduced at a later date that may be reasonably understood as a derivative of a contemporary term or designating of a subset of objects embraced by a contemporary term will be understood as having been described by the now contemporary terminology. Further, it should be understood that the invention is not limited to the embodiments that have been set forth for purposes of exemplification, but is to be defined only by a fair reading of claims that will be appended, including the full range of equivalency to which each element thereof is entitled.
The following applications are related to this application: “Ion source with controlled superposition of electrostatic and gas flow fields” (U.S. App. 11/063,485, filed on Feb. 22, 2005), “Methods and apparatus for controlling ion current in an ion transmission device” (U.S. application Ser. No. 11/063,801 filed on Feb. 22, 2005), “Laser desorption ionization ion source with charge injection” (U.S. App. 60/798,377, filed on May 5, 2006), and “Laser desorption ionization ion source with self-adjusting holder and insertion system for one and two-dimensional sample chips” (U.S. App 60/802,941, filed on May 23, 2006). This application is further related to two U.S. Provisional Patent Applications being filed concurrently with this one, the applications entitled “Advanced ion source for macromolecules” and “Ion source with controlled liquid injection,” respectively.
| Number | Date | Country | |
|---|---|---|---|
| 60843205 | Sep 2006 | US |
