Referring to
The negative mass spectrometer 104 includes a flight tube 116 and a negative ion detector 120 that detects negative ions 106 traveling through the flight tube 116. The positive mass spectrometer 108 includes a flight tube 118 and a positive ion detector 122 that detects positive ions 110 traveling through the flight tube 118. The negative and positive mass analyzers 104 and 108 are positioned on opposite sides of the ion generator 102 and can be, e.g., symmetrical with respect to the ion generator 102. Output signals 290 and 292 of the detectors 120 and 122, respectively, are sent to a signal acquisition device 192 (e.g., a digital storage oscilloscope or a computer), to record the mass spectra of the negative and positive ions.
The sample material 146 can be, for example, salts, alloys, semiconductor materials, semiconductor chips, particles, chemicals, biomolecules, physiological fluids, biological tissues, skins, metals, and plasma (which can include a gaseous beam composed of charged particles). The mass spectrometer 100 can analyze the surface properties of the sample material 146 by configuring the laser beam 124 to energize just the surface layers to produce the positive and negative ions. The mass spectrometer 100 can also analyze deeper portions of the sample material beneath the surface layers by configuring the laser beam 124 to successively peel off layers of materials to reveal the inner portions of the sample material, and produce the positive and negative ions from the inner portions.
Using the mass spectrometer 100 to analyze a sample material does not require generating small neutral particles from the sample material prior to ionization, as is the case for aerosol time-of-flight mass spectrometers (ATOF MS). In aerosol TOF MS, neutral particles are derived from the sample material and accelerated along a path and ionized by a laser beam when the flying particles reach an ionization location. Thus, it may be difficult to use the aerosol TOF MS to analyze the surface properties of a bulk sample material without dividing the sample material into very small pieces. By comparison, the sample material used in the mass spectrometer 100 may have dimensions of several millimeters, or even larger, as long as the sample material can be accommodated in the ion source electrode described below. Thus, it is easier to use the mass spectrometer 100 to examine the surface properties of, e.g., a semiconductor chip or a piece of biological tissue.
The ion generator 102 includes an ion source electrode 130 and extraction electrodes 126a, 126b, 128a, and 128b. The source electrode 130 includes a sample surface 150 (see
The electric fields direct the negative and positive ions 106, 110 toward the negative and positive mass spectrometers 104 and 108, respectively, such that particles having similar mass to charge ratios enter the mass spectrometers at substantially the same speed.
In some examples, the extraction electrodes 126a and 126b are positioned on opposite sides of the ion source electrode 130 and are symmetrical with respect to the ion source electrode 130. Similarly, the extraction electrodes 128a and 128b are positioned on opposite sides of the ion source electrode 130 and are symmetrical with respect to the ion source electrode 130.
There are five electric fields generated by the source electrode 130 and extraction electrodes 126a, 126b, 128a, and 128b. A first electric field is located in the open region 300 surrounded on three sides by the sample surface 150 and the inner surfaces of the walls 160 and 162. A second electric field is located between the source electrode 130 and the extraction electrode 126a. A third electric field is located between the source electrode 130 and the extraction electrode 126b. A fourth electric field is located between the extraction electrodes 126a and 128a. A fifth electric field is located between the extraction electrodes 126b and 128b. The second and third electric fields are symmetrical with respect to the ion source electrode 130, except that the polarities of the second and third electric fields with respect to the source electrode 130 are opposite. Similarly, the fourth and fifth electric fields are symmetrical with respect to the ion source electrode 130, except that the polarities of the fourth and fifth electric fields with respect to the source electrode 130 are opposite.
In this description, a Cartesian coordinate system having x-, y-, and z-axes is used to describe the orientations of the components of the mass spectrometer 100. The origin of the axes is at the center of the sample surface 150 (see
In some examples, the extraction electrode 126a has a voltage higher than the ion source electrode 130 to generate an electric field that forms a first acceleration stage 166a to accelerate negative ions 110 toward the −x direction. The extraction electrode 128a has a voltage slightly lower than the extraction electrode 126a to generate an electric field that focuses the negative ions 106 and adjusts the trajectory of the ions 106 so that the ions 106 travel along paths parallel to the axis of the flight tube 116.
The extraction electrode 126b has a voltage lower than the ion source electrode 130 to generate an electric field that forms a first acceleration stage 166b to accelerate positive ions 110 toward the +x direction. The extraction electrode 128b has a voltage slightly higher than the extraction electrode 126b to generate an electric field that focuses the positive ions 110 and adjusts the trajectory of the ions 110 so that the ions 110 travel along paths parallel to the axis of the flight tube 118.
The voltages applied to the extraction electrodes 126a and 128a and the voltages applied to the extraction electrodes 126b and 128b are symmetrical with respective to the voltage of the ion source electrode 130, except that they have opposite polarities with respect to the voltage of the ion source electrode 130. This means that, for example, the voltage of the extraction electrode 126a is higher than the ion source electrode 130 by an amount that is the same as the amount that the voltage of the extraction electrode 126b is lower than the ion source electrode 130.
The negative ion detector 120 can be, e.g., a microchannel plate detector. Similarly, the positive ion detector 122 can be, e.g., a microchannel plate detector. The negative and positive mass analyzers 104 and 108 are positioned on opposite sides of the ion generator 102. The negative and positive mass analyzers 104 and 108 can be, e.g., symmetrical with respect to the ion generator 102. The ion generator 102 is housed in a source chamber (not shown), e.g., a six-way cube chamber, having openings for coupling to the flight tubes 116 and 118.
The output signal 292 of the positive ion detector 122 is measured by a first channel of the data acquisition device 192. The output signal 290 of the negative ion detector 120 is terminated through a circuit 194 and measured by a second channel of the data acquisition device 192. As will be described later, the circuit 194 includes voltage isolation circuitry to prevent the high voltages applied to the negative ion detector 120 from damaging the data acquisition device 192.
Referring to
The ion source electrode 130 and the extraction electrodes 126a and 128a form two acceleration stages 166a and 168a for the negative ions. The ion source electrode 130 and the extraction electrodes 126b and 128b form two acceleration stages 166b and 168b for the positive ions. The ion source electrode 130 and the extraction electrodes 126a, 128a, 126b, and 128b can be, e.g., stainless steel electric plates that are spaced equally apart from one another. The surface of the steel electric plates can be parallel to one another.
A feature of the ion generator 102 is that the desorbed ions are emitted from the sample surface 150 in a generally upwards (+z) direction. The ions are then guided by the electric fields produced by the electrode 130 and the extraction electrodes 126a, 126b, 128a, and 128b. Negative ions are focused and directed towards a direction parallel to the axis of the flight tube 116. Positive ions are focused and directed towards a direction parallel to the axis of the flight tube 118.
Another feature of the ion generator 102 is the use of rectangular slots 154a and 154b near the sample surface 150. The rectangular slots 154a and 154b are defined by surfaces 160 and 162, respectively, of the ion source electrode 130. Using a rectangular opening is better than using a circular opening or a wide-open structure (without the upper portion of the surfaces 160 and 162) because a rectangular opening generates a field-gradient that is less distorted along the y-axis. The electric field generated by the ion source electrode 130 and the extraction electrodes 126a and 126b has a better shape that can guide the positive and negative ions along trajectories toward the flight tubes 118 and 116, respectively.
Having openings that are elongated in the y direction, where the openings are positioned near the sample material 146, can result in an electric field that is substantially constant along the y axis in the vicinity of the sample material 146. This helps in focusing the ions and directing the ions toward the flight tubes 116 and 118.
When the ions are desorbed from the sample 146, a large portion of the ions may initially travel along the +z direction, then gradually turn toward the x axis (negative ions toward −x direction and positive ions toward +x direction). Using positive ions 110 as an example, when the ions 110 are emitted from the sample surface 150, the ions 110 may initially travel in the +z-direction and then be slightly pulled back in the −z direction by the electric field gradient. After the positive ions 110 pass the rectangular slot 154b, the positive ions 110 travel through the first and second acceleration regions 166b and 168b and enter the field-free flight tube 118.
The arrangement of the rectangular slot 154b and the circular openings 156b and 158b provides adequate transmission efficiency, meaning that a large portion of the positive ions 110 can reach the flight tube 118 without hitting the walls of the ion source electrode 130 and the extraction electrodes 126b and 128b. The voltage of the second extraction electrode 128b is higher relative to the flight tube 118 and the first extraction electrode 126b. This configuration produces an ion-focusing effect near the opening 158b and can increase the transmission efficiency of the positive ions 110 by, e.g., about a factor of two.
The arrangements of the extraction electrodes 126a and 128a, and holes 156a and 158a, mirror those of the extraction electrodes 126b and 128b, and holes 156b and 158b, respectively, with respect to the ion source electrode 130.
The ion source electrode 130 and the extraction electrodes 126a and 126b generate an electric field having a particular distribution that adjusts the trajectories of the negative and positive ions after the ions are emitted from the sample surface 150. The electric field forms a trajectory adjustment stage for each of the negative and positive ions 106, 110. For example, the negative and positive ions 106, 110 initially travel along generally +z direction when emitted from the sample surface 150. The electric field distribution adjusts the trajectory of the negative ions 106 and guides the negative ions 106 from the generally +z direction to a generally −x direction toward the rectangular slot 154a. Similarly, the electric field distribution adjusts the trajectory of the positive ions 110 and guides the positive ions 110 from the generally +z direction to a generally +x direction toward the rectangular slot 154b.
When the negative and positive ions 106 and 110 travel from the sample surface 150 to the rectangular slots 154a and 154b, respectively, the acceleration of the negative and positive ions 106, 110 is small compared to the acceleration of the ions in the acceleration stages 166a and 166b. For example, the average kinetic energy of the negative ions 106 in the acceleration stage 166a can be 10, 100, or more than 1000 times greater than the average kinetic energy of the negative ions 106 in the trajectory adjustment stage (i.e., when the negative ions 106 are traveling from the sample surface 150 to the rectangular slot 154a).
The electric field in the region surrounded by the sample surface 150 and the walls 160 and 162 redirects the negative ions 106 from traveling in generally +z directions to generally −x directions. Therefore, negative ions 106 having substantially the same mass to charge ratios will have substantially the same speeds when passing the rectangular slot 154a, have substantially the same acceleration in the first and second acceleration regions 166a and 168a, and have substantially the same speeds when entering the flight tube 116. Similarly, the positive ions 110 having substantially the same mass-to-charge ratios will enter the flight tube 118 with substantially the same speeds.
Referring to
The following describes an example of the dual-polarity time-of-flight mass spectrometer 100 that was used to conduct the experiments. The ion source electrode 130 and extraction electrodes 126a, 126b, 128a, and 128b each has a width×length of 40 mm×100 mm, and are equally spaced apart by 6 mm from each other. The sample electrode 130 has a thickness of 6 mm. The extraction electrodes 126a, 126b, 128a, and 128b each has a thickness of 3 mm. Each of the rectangular slots 154a and 154b has a dimension of 26 mm×3 mm, and is located at 18 mm away from the front side 131 of the sample plate. Each of the circular openings 156a, 156b, 158a, and 158b has a diameter of 5 mm. The centers of the openings 156a and 156b are spaced 1.5 mm away from the x-axis in the +z direction, and the centers of the openings 158a and 158b are spaced 2.5 mm away from the x-axis in the +z direction.
The flight tubes 116 and 118 each has an inner diameter of 32 mm and a length of 1123 mm, and are electrically isolated from the extraction electrodes 128b and 128a, respectively. The pressure in the source chamber was maintained below 3×10−7 mbar during measurement. Both of the flight tubes 116, 118 have center axes that are parallel to the x-axis and aligned 2.5 mm offset from the x-axis in the +z direction, and they are differentially pumped to below 5×10−7 mbar. The microchannel plate detectors 120 and 122 are located about 25 mm away from the flight tubes 116 and 118, respectively, without additional differential pumping stages.
The voltages are applied continuously to the source electrode 130 and the extraction electrodes 126a, 126b, 128a, and 128b. A reference voltage of +5.9 kV is applied to the ion source electrode 130. The voltages applied to the extraction electrodes and the ion detectors are symmetrical with respect to the reference voltage except for having opposite polarities. The voltages applied to the first set of extraction electrodes 126a and 126b are +2.5 kV and +9.3 kV, respectively. The voltage potential of the second set of extraction electrodes 128a and 128b are +3.8 kV and +8 kV, respectively. The voltages applied to the flight tubes 118 and 116 are 0 V and +11.8 kV, respectively.
The circuits of the detectors 120 and 122 are different because the positive ion detector 122 is operated at a lower voltage range, while the negative ion detector 120 is operated at a higher voltage range. The microchannel plate detector 122 has entrance side 140, exit side 142, and anode 144 that are connected to voltages −2200 V, −200 V, and 0 V, respectively. By comparison, the microchannel plate detector 120 has entrance side 134, exit side 136, and anode 138 that are connected to voltages +14 kV, +16 kV, and +16.2 kV, respectively.
Because of the high bias voltages used in the negative ion detector 120, the microchannel plate assembly was isolated and positioned 67 mm away from the vacuum chamber (of the flight tubes) by using an 8-inch acryl flange adaptor. The frame of the detector assembly was biased at +14 kV to reduce the voltage differences around the electrodes, thereby preventing the negative ion detector 120 from high voltage breakdown during operation.
For the data acquisition device 192, a 500 MHz digital storage oscilloscope was used. Because the oscilloscope 192 accepts signals of a few volts, a DC decoupling circuit was used to isolate the high bias voltages of the microchannel plate detector 120 from the oscilloscope 192.
Referring to
The decoupling circuit 180 includes two capacitors 188 and 190 that have high voltage ratings. For example, the capacitors 188 and 190 can be high voltage ceramic capacitors having capacitances 2 nF and 10 nF, respectively, each having a rating of 40 kV. The circuit 180 is enclosed in a glass housing that is electrically isolated from the ambient environment. Most of the conducting wires at the high-voltage side of the capacitors are silicone jacked with voltage ratings of, e.g., 100 kV. The capacitors are not shielded with grounding jackets to prevent short circuiting the circuit 180.
The signal 290 from the microchannel plate detector 120 passes the DC decoupling circuit 180 and is terminated by a resistor 310. The signal 290 is measured by a first channel of the digital storage oscilloscope 192. By comparison, the signal 292 from the microchannel plate detector 122 is directly terminated by another resistor and measured by a second channel of the digital storage oscilloscope 192.
A pulsed frequency-triplet Nd:YAG laser (355 nm) is used as the laser source 114. The power of the laser beam 124 is attenuated to about 2-10 μJ, depending on the sample 146 to be examined. The laser beam 124 passes a fused silica window of the source chamber to irradiate the sample 146. The laser beam 124 is aligned perpendicular to the sample surface 150.
The following describes results from experiments using the example of the mass spectrometer 100 described above. A number of biological samples were used in the experiments, including insulin chain B (M.W.=3495.9 Da), equine skeletal muscle myoglobin (M.W.=16951.5 Da), and a calibration protein mixture that includes angiotensin I (M.W.=1296.7 Da), adrenocorticotropic hormone (ACTH) clip 1-17 (M.W.=2093.1 Da), ACTH clip 18-39 (M.W.=2065.2 Da), ACTH clip 7-38 (M.W.=3657.9 Da), and insulin (M.W.=5730.6 Da). Here, “M.W.” refers to molecular weight.
The experiments measured proteins and protein mixtures of various molecular weights.
The spectrometer 270 includes a negative mass spectrometer 104 for analyzing the negative ions, a positive mass spectrometer 108 for analyzing the positive ions, and a third mass analyzer 272 for analyzing neutral particles. The third mass analyzer 272 includes an ionization region 280 defined by electrodes 274 and 276 that are positioned in front (i.e., in the +z direction) of the ion source electrode 130. When neutral particles emitted from the sample material reach a location (marked by “X” in
Instead of using time-of-flight mass analyzers, each of the mass analyzers 106, 108, and 272 can use, e.g., a quadrupole mass analyzer, an ion trap mass analyzer, a magnet sector mass analyzer, a Fourier-transform ion-cyclotron-resonance mass spectrometer, or a momentum analyzer. The dimensions of the various components of the mass spectrometer 100 is not limited to those described above. The type of laser source 114 can be different from what is described above. Instead of using microchannel plates, each of the detectors 120 and 122 can include, e.g., a scintillation detector, an electron multiplier, or an electric current detector.
In
Also, instead of using a MALDI source as in
The voltages applied to the ion source electrode 130 and the extraction electrodes 126a, 126b, 128a, and 128b can be different from those described above. In
Different configurations of the ion source electrodes 130 may be used for different types of ion sources. For each type of ion source, the geometry and dimensions of the ion source electrode 130, as well as the voltage(s) applied to the ion source electrode 130 are adjusted so as to generate an electric field distribution that directs the positive and negative ions 110 and 106 toward generally +x and −x directions, respectively, before the ions enter the acceleration regions. The positive and negative ions do not necessarily have to travel in a direction parallel to the x-axis when entering the acceleration regions, and can be tilted at a slight angle with respect to the x-axis.
The geometry of the ion source electrode 130 and the extraction electrodes 126a, 126b, 128a, and 128b can be different from those described above. In
It is to be understood that the foregoing description is intended to illustrate and not to limit the scope of the invention, which is defined by the scope of the appended claims. Other embodiments are within the scope of the following claims.