|Publication number||USRE39099 E1|
|Application number||US 10/236,514|
|Publication date||May 23, 2006|
|Filing date||Sep 6, 2002|
|Priority date||Jan 23, 1998|
|Publication number||10236514, 236514, US RE39099 E1, US RE39099E1, US-E1-RE39099, USRE39099 E1, USRE39099E1|
|Inventors||Andrew N. Krutchinsky, Alexandre V. Loboda, Victor L. Spicer, Werner Ens, Kenneth G. Standing|
|Original Assignee||University Of Manitoba|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (15), Non-Patent Citations (15), Referenced by (4), Classifications (13), Legal Events (3)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This invention relates to mass spectrometers and ion sources therefor. More particularly, this invention is concerned with pulsed ion sources and the provision of a transmission device which gives a pulse ion source many of the characteristics of a continuous source, such that it extends and improves the application of Time of Flight Mass Spectrometry (TOFMS) and that it additionally can be used with a wide variety of order spectrometers, in addition to an orthogonal injection time-of-flight mass spectrometer.
Ion sources for mass spectrometry may be either continuous, such as ESI (electrospray ionization) sources or SIMS (secondary ion mass spectrometry) sources, or pulsed, such as MALDI (matrix-assisted laser desorption/ionization sources). Continuous sources have normally been used to inject ions into most types of mass spectrometer, such as sector instruments, quadrupoles, ion traps and ion cyclotron resonance spectrometers. Recently it has also become possible to inject ions from continuous sources into time-of-flight (TOF) mass spectrometers through the use of “orthogonal injection”, whereby the continuous beam is injected orthogonally to the main TOF axis and is converted to the pulsed beam required in the TOF technique. This is most efficiently carried out with the addition of a collisional damping interface between the source and the spectrometer, and this is described in the following paper, having four authors in common with the present invention (Krutchinsky A. N., Chernushevich I. V., Spicer V. L., Ens W., Standing K. G., Journal of the American Society for Mass Spectrometry, 1998, 9, 569-579).
On the other hand, pulsed sources, MALDI sources for example, have usually been coupled directly to TOF mass spectrometers, to take advantage of the discrete or pulse nature of the source. TGIF mass spectrometers have several advantages over conventional quadrupole or ion trap mass spectrometers. One advantage is that TOF mass spectrometers can analyze a wider mass-to-charge range than do quadrupole and ion trap mass spectrometers. Another advantage is that TOF mass spectrometers can record all ions simultaneously without scanning, with higher sensitivity than quadrupole and ion trap mass spectrometers. In a quadrupole or other scanning mass spectrometer, only one mass can be transmitted at a time, leading to a duty cycle which may typically be 0.1%, which is low (leading to low sensitivity). A TOF mass spectrometer therefore has a large inherent advantage in sensitivity.
However, TOF mass spectrometers encounter problems with many widely used sources which produce ions with a range of energies and directions. The problems are particularly acute when ions produced by the popular MALDI (matrix-assisted laser desorption/ionization) technique are used. In this method, photon pulses from a laser strike a target and desorb ions whose masses are measured in the mass spectrometer. The target material is composed of a low concentration of analyte molecules, which usually exhibit only moderate photon absorption per molecule, embedded in a solid of liquid matrix consisting of small, highly-absorbing species. The sudden influx of energy is absorbed by the matrix molecules, causing them to vaporize and to produce a small supersonic jet of matrix molecules and ions in which the analyte molecules are entrained. During this ejection process, some of the energy absorbed by the matrix is transferred to the analyte molecules. The analyte molecules are thereby ionized, but without excessive fragmentation, at least in the ideal case.
Because a pulsed laser is normally used, the ions also appear as pulses, facilitating their convenient measurement in a tune-of-flight spectrometer. However, the ions acquire a considerable amount of energy in the supersonic jet, with velocities of the order of 700 m/s, and they also may lose energy through collisions with the matrix molecules during acceleration, particularly in high accelerating fields. These and similar effects lead to considerable peak broadening, and consequent loss of resolution in a simple linear time-of-flight instrument, where the ions are extracted from the target nearly parallel to the spectrometer axis. A partial solution to the problem is provided by a reflecting spectrometer, which partially corrects for the velocity dispersion, but a more effective technique is the use of delayed extraction, either by itself or in combination with a reflector. In delayed extraction, the ions are allowed to drift for a short period before the accelerating voltage is applied. This technique partially decouples the ion production process from the measurement, making the measurement less sensitive to the detailed pattern of ion desorption and acceleration in any particular case. Even so, successful operation requires careful control of the laser fluence (i.e. the amount of power supplied per unit area) and usually some hunting on the target for a favorable spot. Moreover, the extraction conditions required for optimum performance have some mass dependence; this complicates the calibration procedure and means that the complete range of masses cannot be observed with optimum resolution at any given setting. Also, the technique has had limited success in improving the resolution for ions of masses greater than about 20,000 Da. Moreover, it is difficult to obtain high performance MS—MS data in conventional MALDI instruments because ion selection and fragmentation tend to broaden the fragment peak width. The present inventors have realized that these problems can be overcome by abandoning the attempt to maintain the original pulse width, producing instead a quasi-continuous beam with superior characteristics, and then pulsing the injection voltage of the TOF device at an independent repetition rate.
Although coupling to a TOF instrument is used as an example above, problems also arise in coupling MALDI and other pulsed sources to other types of mass spectrometer, such as quadrupole (or other multipole), ion trap, magnetic sector and FTICRMS (Fourier Transform Ion Cyclotron Resonance Mass Spectrometer). Further, it is also desirable to be able to couple MALDI or other pulsed sources to tandem mass spectrometers, e.g. a triple quadrupole or a quadrupole TOF hybrid instrument, which allows MS/MS of MALDI ions to be obtained. Standard MALDI instruments cannot be configured to carry out high performance MS/MS. The dispersion in, energy and angle of ions produced by a MALDI source, or similar source, accentuates the difficulty of ion injection. Also, because the residence times of ions in most other types of mass spectrometer are considerably longer than in TOF instruments, the large space charge in the pulse can introduce additional problems. These instruments are all designed to operate with continuous sources, so conversion of the pulsed source to a quasi-continuous one solves most of the problems.
Accordingly, it is desirable to provide an apparatus and method enabling a pulse source, such as a MALDI source, to be coupled to a variety of spectrometer instruments, in a manner which more completely decouples the spectrometer from the source and provides a more continuous ion beam with smaller angular and velocity spreads.
More particularly, it is desirable to provide an improved TOF mass spectrometer with a pulsed ion source, in which the energy spread in the ion beam is reduced, in which the source is more completely decoupled from the spectrometer than in existing instruments, in which problems resulting from ion fragmentation are reduced, enabling new types of measurement, and in which the results obtained from the mass spectrometer and its ease of operation are consequently improved.
It is also desirable to provide a TOF mass spectrometer with both continuous and pulsed sources, for example both ESI and MALDI sources, so either source can be selected.
In accordance with the present invention, there is provided a mass spectrometer system comprising:
The invention has particular applicability to time of flight mass spectrometers. As these require a pulsed beam, conventional teaching is that a pulsed source should be coupled maintaining the pulsed characteristics. However, the present inventors have now realised that there are advantages to, in effect converting a pulsed beam into a continuous, or at least quasi-continuous, beam, and than back into a pulsed beam. The advantages are: improvement in beam quality through collision damping; decoupling of the ion production from the mass measurement; ability to measure the beam current by single-ion counting; because it is converted from a few large pulses to many small pulses, for example from about 1 Hz to about 4 kHz., or a factor of 4,000; compatibility with a continuous source , such as ESI, offering the possibility of running both sources on one instrument.
The invention also bas applicability to mass spectrometers that work with or require a continuous beam. Then, the advantage is that a pulsed source can indeed be used with such spectrometers.
Preferably, the ion source provides the analyte for ionization by radiation, and there is provided a source of electromagnetic radiation, more preferably a pulsed laser, directed at the ion source, for generating radiation pulses to cause desorption and ionization of analyte molecules.
Advantageously, the ion source comprises a target material composed of a matrix and analyte molecules in the matrix, the matrix comprising a species adapted to absorb radiation from the radiation source, to promote desorption and ionization of the analyte molecules.
Preferably, the transmission device comprises a multipole rod set There can be two or more multipole rod sets and means for supplying different RF and DC voltages to the rod sets.
Collisional damping can also be accomplished in a chamber where no RF field is present providing there is enough buffer gas pressure. In this case ions with reduced velocities can be moved to the exit of the chamber by gas flow drag or a DC electrostatic field Combinations of electrostatic fields, RF fields and gas flow can also be implemented in a collisional damping chamber.
Another advantage of the invention is that the collisional cooling of the ions helps to reduce the amount of fragmentation of the molecular ions. It is usually desirable to produce a simple mass spectrum containing only ions representative of molecular species. In typical MALDI ion sources, therefore, the laser power must be carefully optimized so that it is close to the threshold of ionization in order to reduce fragmentation. The inventors have observed, however, that the presence of a gas around the sample surface greatly assists in reducing fragmentation, even at relatively high laser power. Presumably this is due to the effect of collisions with gas molecules which remove internal energy from the desorbed species before they can fragment. This means that the laser power can be increased in order to improve the ion signal strength, without causing excessive decomposition. The inventors have observed that the amount of fragmentation is decreased as the pressure is increased up to at least approximately 1 torr. Higher pressures may be even more advantageous, but electric fields may be required to avoid clustering reactions at higher pressure.
The mass spectrometer system can include a continuous ion source, and means for selecting one of the pulsed ion source and the continuous ion source, and this then provides the characteristics of two separate instruments in one instrument. The two ion sources can comprise a MALDI source and an ESI source.
Another aspect of the present invention provides a method of generating ions and delivering ions to a mass spectrometer, the method comprising the steps of:
The gas pressure of the damping gas can be in the range from about 10−4 Torr up to at least 760 Torr. Preferably, step (3) comprises providing an RF rod set within the transmission device. Further, a DC field can be provided between the ion source and the spectrometer to promote movement of ions towards the spectrometer.
The method can include providing two or more rod sets in the ion transmission device, and operating at least one rod set with a DC offset to enable selection of ions with a desired mass-to-charge ratio. A potential difference can be provided between two adjacent rod sets sufficient to accelerate ions into the downstream rod set, to cause collisionally induced dissociation in the downstream rod set.
When a pulsed laser is used, for each laser pulse, a plurality of pulses of ions are delivered into the time-of-flight mass spectrometer.
The ions can first pass through one or more differentially pumped regions that provide a transition from the pressure at the ion source to pressure in the spectrometer. The ion source may be at atmospheric pressure or at least at a pressure substantially higher than that in downstream quadrupole stages and in the mass spectrometer. At least one of these regions can be without any rod set and ion motion towards the mass spectrometer is then driven by gas flow and/or an electrostatic potential.
For a better understanding of the present invention and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, which show preferred embodiments of the present invention and in which:
The first embodiment shown in
A simple example of further manipulation in stage 4 is dissociation of the ions by collisions in a gas cell, so that the resulting daughter ions can be examined in the mass analyzer. This may be adequate to determine the molecular structure of a pure analyte. If the analyte is a complex mixture, stage 4 needs to be more complicated. In a triple quadrupole or a QqTOF instrument (as disclosed in A. Shevchenko et al, Rapid Commun. Mass Spectrom. 11, 1015, (1997)), stage 4 would include a quadrupole mass filter for selection of a parent ion of interest and a quadrupole collision cell for decomposition of that ion by collision-induced dissociation (CID). Both parent and daughter ions are then analyzed in section 5, which is a quadrupole mass filter in the triple quadrupole, or a TOF spectrometer with orthogonal injection in the QqTOF instrument. In both cases stages 1 and 2 would consist of a pulsed source and a collisional damping ion guide.
It will be appreciated that the collisional focusing chamber 2 is shown with a multipole rod set 3, which could be any suitable rod set, e.g. a quadrupole, hexapole or octopole. The particular rod set selected will depend upon the function to be provided.
Alternatively, a radio frequency ring guide could be used for the collisional focusing device, and ion creation could be performed within the volume defined by the radio frequency field in order to contain the ions.
In the embodiment tested, the quadrupole rod sets 31 and 32 were made of rods 4.45 cm in length and 11 mm in diameter, and were separated by 3 mm, i.e. the spacing between rods on adjacent corners of the rod set. The quadrupoles 31 and 32 are driven by a power supply which provides operating sine wave frequencies from 50 kHz to 2 MHz, and output voltages from 0 to 1000 volts peak-to-peak. Typical frequencies an 200 kHz to 1 MHz, and typical voltage amplitudes are 100 to 1000 V peak-to-peak. Both quadrupoles are driven by the same power supply through a transformer with two secondary coils. Different amplitudes may be applied to the quadrupoles by using a different number of turns in the two secondary coils. D.C. Bias or offset potentials are applied to the rods of quadrupoles 31 and 32 and to the various other components by a multiple-output power supply. The RF quadrupoles 31 and 32, with the damping gas between their rods can be run in an RF-only mode, in which case they serve to reduce the axial energy, the radial energy, and the energy spreads, of the ions which pass through it, as will be described. This process substantially spreads the plume of ions out along the ion path, changing the initial beam, pulsed at about 13 Hz, into a quasi-continuous beam as described in more detail below. The first quadrupole 31, can also be run in a mass-filtering mode by the application of a suitable DC voltage. The second quadrupole 32 can then be used as a collision cell (and an RF-guide) in collision-induced dissociation experiments (see below).
From chamber 30, the ions pass along an ion path 27 and through a focusing electrode 19 and then pass through orifice 38, into a vacuum chamber 40 pumped by a pump (not shown) connected to a port 42. There, the ions are focused by grids 44 through a slot 46 into an ion storage region 48 of a TOF spectrometer generally indicated at 50.
In known manner, ions are extracted from the storage region 48 and are accelerated through a conventional accelerating column 51 which accelerates the ions to an energy of approximately 4000 electron volts per charge (4 keV). The ions travel in a direction generally orthogonal to the ion path 27 between the ion storage region, through a pair of deflection plates 52. The deflection plates 52 can serve to adjust the ion trajectories, so that the ions are then directed toward a conventional electrostatic ion mirror 54, which reflects the ions to a detector 56 at which the ions are detected. The ions are detected using single-ion counting and recorded with a time-to-digital converter (TDC). The accelerating column 51, plates 52, mirror 54 and detector 56 are contained in a main TOF chamber 58 pumped to about 2×10−7 Torr by a pump (not shown) connected to a port 60.
The use of orthogonal-injection of MALDI ions from source 13 into the TOF spectrometer 50 has some potential advantages over the usual axial injection geometry. It serves to decouple the ion production process from the mass measurement to a greater extent than is possible in the usual delayed-extraction MALDI. This means that there is greater freedom to vary the target conditions without affecting the mass spectrum, and the plume of ions has more time to expand and cool before the electric field is applied to inject them into the spectrometer. Some improvement in performance might also be expected because the largest spread in velocities is along the ejection axis, i.e. the ion path 27, normal to the target, which in this case is orthogonal to the TOF axis. However, orthogonal injection of MALDI ions into the TOF 50 without collisional cooling has several problems which appear to make the geometry impractical, namely:
In delayed extraction MALDI in the usual axial geometry, i.e. not the orthogonal configuration shown, acceptance is nearly complete, and while the largest velocity spread is along the TOF axis, the well-defined target-plane perpendicular to the TOF axis allows a combination of time-lag focusing (delayed extraction with optimized values of delay and applied voltage) and electrostatic focusing (optimized value of the reflector voltage) in an ion mirror to produce resolution well above: 10,000 in some cases.
Experiments carried out by the present inventors suggest that competitive resolution could not be obtained with an acceptable signal using orthogonal injection, unless collisional cooling according to the present invention is employed. Moreover, some disadvantages of delayed-extraction MALDI—the dependence of optimum extraction conditions on mass, and the more complex calibration required—are still present in orthogonal injection MALDI without cooling although to a lesser extent than with axial injection.
The introduction of an RF quadrupole or other multipole with collisional cooling of the ions between the MALDI target and the orthogonal injection geometry avoids the problems described above while offering additional advantages. These are detailed below with reference to the remaining figures.
By reducing the radial energies of the ions, an approximately parallel beam can be produced, greatly reducing the losses that result from collimation before the ions enter the storage region. This allows the use of a larger entrance aperture to the TOF spectrometer 50, further reducing losses. By reducing the axial energies of the ions, and then reaccelerating them to a uniform energy, the mass discrimination mentioned above is not present.
The uniform energy distributions of the ions after cooling remove any mass dependence on the optimum extraction conditions and allow the simple quadratic relation between TOF and mass to be used for calibration with two calibrant peaks.
The decreasing relative intensity of the molecular ions with mass is to some extent a reflection of the decreasing detection efficiency with increasing mass. Detection efficiency depends strongly on velocity, which decreases with mass for singly-charged ions at a given energy. In this embodiment the energy of singly-charged ions is only about 5 keV (compared to 30 keV in typical MALDI experiments), so the detection efficiency limits the practical range of application to less than about 6000 Da. The relative intensities of the molecular ion peaks in
As mentioned above, the collisional cooling spreads the ions out along the ion beam axis changing the initial beam pulsed at 13 Hz into a quasi-continuous beam. This is illustrated in
Assuming 104 ions of a single molecular ion species are produced with each laser shot, the transmission efficiency of the RF-quadrupole is in the range of 10%. Taking account of the duty cycle, about 2% of the ions produced at the target are detected in the mass spectrometer. This represents significant losses compared to the conventional axial MALDI experiment in which transmission is probably 50% or more. However, from the point of view of data rate, the losses can be compensated to a large extent by the higher repetition rate and higher fluence of the laser. In these experiments, the repetition rate was 13 Hz, but can easily be increased to 20 Hz with the current laser, or in principle up to at least 100 Hz before the, counting system becomes saturated. In contrast, the usual MALDI experiment is run at about 1 or 2 Hz. The laser fluence in a conventional MALDI experiment must be kept close to threshold to achieve the best performance, the threshold being the minimum energy necessary to cause vaporization of the sample to produce a useful signal using a conventional transient recording with analog to digital conversion. In the present invention, the laser fluence can be increased to the fluence at which the ion production process saturates. As the quadrupole serves to smooth out the ion burst produced by the laser, a short intense burst of ions ran be accepted. From the point of view of absolute sensitivity, it seems that the independence of the spectrum on laser conditions (see below) allows more efficient usage of the sample, deposited on the target. Using fluence several times higher than threshold produces ions until the matrix is completely removed from the target probe.
These results indicate that the performance of the invention for peptides is comparable to conventional MALDI experiments but with the advantage of a mass-independent calibration, and a simple calibration procedure. However, the most important advantages result from the nearly complete decoupling of the ion production from the mass measurement. In a conventional MALDI experiment, the location of the laser spot on the target and the laser fluence and location must be carefully selected for optimum performance, and these conditions are typically different for different matrices and even for different target preparation methods. The situation was improved with the introduction of delayed extraction but even so, many commercial instruments have implemented software to adjust laser fluence, detector gain, and laser position, and to reject shots in which saturation occurs. None of these techniques are necessary with the present invention. The performance obtained shows no dependence on target or laser conditions. The laser is simply set to maximum fluence (several times the usual threshold) and left while the target is moved to a fresh position occasionally. This means that alternative targets can easily be tried (including insulating targets), and alternative lasers with different wavelengths or pulse widths can be used.
The decoupling of the ion production from the mass measurement also provides an opportunity to perform various manipulations of the ions after ejection but before mass measurement. One: example is parent ion selection and subsequent fragmentation (MS/MS). This is most suitably done with an additional quadrupole mass filter as described below, but even in the present embodiment of
In the past it has not been possible to use both continuous sources, such as electrospray ionization (EST), and pulsed sources, such as MALDI, in the same instrument, which would have significant advantages. To the inventors' knowledge, the only successful ESI-TOF instruments to date have been the orthogonal injection spectrometers (by the present inventors, Dodonov, and now the commercial machines by PerSeptive and others), so it appears that orthogonal injection is necessary for ESI-TOF, with or without collisional damping, although the former improves the situation, as detailed in Krutchinsky A. N., Chernushevich I. V., Spicer V. L., Ens W., Standing K. G., Journal of the American Society for Mass Spectrometry, 1998, 9, 569-579. Up to now, attempts to put MALDI on an orthogonal injection instrument have been without collisional damping (for example by the present inventors and by Guilhaus' and both gave unpromising results). The present invention enables two such sources to be available in one instrument. Here, the MALDI probe 11 in
Presently, MALDI and EST techniques are often considered to be complementary methods for biochemical analysis, so many biochemical or pharmaceutical laboratories have two instruments in use. Obviously there are significant benefits of combining both ion sources in one instrument, as in the embodiments above. In particular, the cost of a combined instrument is expected to be little more than half the cost of two separate instruments. In addition, similar procedures for ion manipulation, detection and mass calibration could be used, since the ion production is largely decoupled from the ion measurement. This would simplify the analysis and processing of the separate spectra and their comparison.
The ability the use both MALDI and EST sources on a single instrument is not restricted to the spectrometer shown in
While specific embodiments of the invention have been described, it will be appreciated that a number of variations are possible within the scope of the present invention. Thus, the apparatus could include a single multipole rod set as shown in
Reference will now be made to FIG. 11. For simplicity and brevity, components common with the apparatus or spectrometer of
Here, the MALDI target is provided at 100 and generates an ion beam indicated at 102. The MALDI target 100 is located in a differentially pumped chamber 104 connected to a pump as indicated at 106 in known manner. A fast rod set Q0 is located in the chamber 104. An aperture and an interquad aperture plate 108 provides communication through to a main chamber 110. Again, in known manner a pump connection is provided at 112.
Within the main chamber 110, there is a short rod set 111, sometimes referred to as “stubbies”, provided for the purpose collimating the beam. A first quadrupole rod set in the chamber 110 is indicated at Q1 and a second rod set at Q2.
The rod set Q2 is located in it collision cell 114 provided with connection, indicated at 116, for a collision gas.
On leaving the collision cell 114, ions pass through a grid and then an aperture into the storage region 48 of the TOF instrument, again indicated at 50. Here, a TOF instrument 50 is provided with a liner 118 around the flight region.
Here, the differentially pumped chamber 104 is maintained at pressure of around 10−2 torr. The main chamber 110 is maintained at a pressure of around 10−5 torr, while the collision cell 114 is maintained at a pressure of around 10−2 torr. In known manner, the pressure in the collision cell 114 can be controlled by controlling the supply of nitrogen to it through connection 116.
Here, collisional damping of ions generating from the MALDI target 100 is accomplished by the relatively high pressure in the differentially pumped chamber 104. Ions then pass through into the quadrupole rod set Q1, which can be operated to mass select a desired ion.
The mass selected ion then enters the collision cell 114 and the rod set Q2; potentials are such that ions enter the rod set Q2 with sufficient energy to effect collision induced dissociation. The fragment ions generated by this CID are then passed into the TOF instrument 50 for analysis.
Typical spectra obtained in a MALDI-QqTOF instrument are presented in FIG. 12. The spectrum shown in
Referring now to
As before, a short seat of rods or stubbies 140 together with a rod set Q1, are provided in a chamber here indicated at 142.
The alternative collisional damping setup of
Thus, ions generated at the target 150 travel, as indicated at 164, to the collector electrode 162. An approximately homogeneous electric field is established in the region between the target 150 and the collector electrode 162. The field strength is proportional to the applied potential difference U. The distance between the target and collector was about 3 mm. The laser was operated at 20 Hz and the total ion current was measured using an amplifier 166.
These results indicate that the MALDI technique can be used at any desirable pressure, even out of the range in which RF collisional multipoles can be implemented. Collisional damping of the ions can be accomplished at least partially in the region with no RF field adjacent to the sample target. The inventors believe that similar dependence of pressure and electric field can be observed in some other pulsed ion sources and these ionization techniques can be also used with collisional damping at higher pressures.
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|U.S. Classification||250/281, 250/288, 250/282, 250/287|
|International Classification||H01J49/10, H01J49/04, H01J49/40|
|Cooperative Classification||H01J49/063, H01J49/10, H01J49/40|
|European Classification||H01J49/40, H01J49/10, H01J49/06G1|
|May 20, 2009||FPAY||Fee payment|
Year of fee payment: 8
|Mar 14, 2013||FPAY||Fee payment|
Year of fee payment: 12
|Apr 9, 2013||AS||Assignment|
Owner name: APPLIED BIOSYSTEMS, INC., CALIFORNIA
Free format text: LIEN RELEASE;ASSIGNOR:BANK OF AMERICA, N.A.;REEL/FRAME:030182/0677
Effective date: 20100528