Methods, apparatus, and system for mass spectrometry

A miniature, low cost mass spectrometer capable of unit resolution over a mass range of 10 to 50 AMU. The mass spectrometer incorporates several features that enhance the performance of the design over comparable instruments. An efficient ion source enables relatively low power consumption without sacrificing measurement resolution. Variable geometry mechanical filters allow for variable resolution. An onboard ion pump removes the need for an external pumping source. A magnet and magnetic yoke produce magnetic field regions with different flux densities to run the ion pump and a magnetic sector mass analyzer. An onboard digital controller and power conversion circuit inside the vacuum chamber allows a large degree of flexibility over the operation of the mass spectrometer while eliminating the need for high-voltage electrical feedthroughs. The miniature mass spectrometer senses fractions of a percentage of inlet gas and returns mass spectra data to a computer.

BACKGROUND

Mass spectrometry is one of the leading chemical analysis tools. A mass spectrometer, often used as a detector in conjunction with another instrument (e.g., a gas chromotograph), may be capable of determining the relative abundances of the chemical species present in a gaseous sample by separating the species by atomic mass.

Mass spectrometry is widely used across many disciplines. Mass spectrometers have been sent aboard unmanned spacecraft; both of the Viking landers carried gas chromotograph/mass spectrometer (GCMS) packages, and the Cassini-Huygens probe dropped into Titan's atmosphere carried a GCMS as well. Mass spectrometers are heavily used in the biological sciences; they are one of the commonly used methods of determining protein structure and sequence.

In the medical field of pharmacokinetics, mass spectrometry has been used to track extremely small quantities of drugs through the human body.

Mass spectrometers have been designed for chemical and biological defense; the Block II chemical biological mass spectrometer (CBMS) was designed to be a portable, vehicle mounted instrument capable of detecting chemical and biological threats (e.g., nerve agents, bacteria) in the field. More recently, mass spectrometers have been carried aboard unmanned submersibles to aid in the tracking of hydrocarbons released by the Macondo oil well failure in the Gulf of Mexico on Apr. 20, 2010.

Many other fields have employed mass spectrometry as well. As early as 1976, a mass spectrometer was used to continuously analyze the respired gases of patients on ventilators in intensive care for potentially dangerous complications.

SUMMARY

The Applicants have recognized that the conventional mass spectrometer is an extremely versatile instrument, but it is not without some drawbacks. A conventional mass spectrometer is generally a large, complex, and expensive instrument that may consume a substantial amount of electrical power.

In view of the foregoing, inventive embodiments disclosed herein relate in part to improved mass spectrometers, which, in various aspects, may be small enough to be handheld, capable of running in remote usage on minimal power for a useful length of time, and inexpensive enough to build and assemble such that it can be widely deployed. An illustrative instrument may be deployed in large numbers to blanket wide areas for air or water quality monitoring, installed in industrial exhaust stacks for combustion process feedback control, or attached to hospital ventilators or used as first response tools in emergency rooms.

Embodiments of the present invention include mass spectrometers and corresponding methods of mass spectrometry. One illustrative mass spectrometer includes a vacuum housing defining a vacuum cavity to support a vacuum of about 10−5mm Hg or less along with an electrode and a conversion circuit disposed within the vacuum cavity. A feedthrough with a dielectric strength of about 36 V or below provides an electrical connection between the conversion circuit and a power source outside the vacuum cavity. In some examples, the feedthrough may provide the only electrical connection between the inside of the vacuum cavity and the outside of the vacuum cavity. The conversion circuit receives an input voltage (e.g., at a first value of about 1 V to about 36 V) from the power source via the feedthrough and converts the input voltage to an electrode potential (e.g., at a second value of about 100 V to about 5 kV) and charges the electrode to the electrode potential. Once charged to the electrode potential, the electrode controls acceleration of a charged particle propagating through the vacuum cavity.

In one example, the charged particle is an electron. In such an example, the mass spectrometer may further include an electron source, disposed within the vacuum cavity, to provide the electron; a cathode to repel the electron; and an anode, disposed on a side of the control electrode opposite the electron source, to accelerate the electron toward a particle to be analyzed. The conversion circuit may be configured to provide: an anode potential of about 100 V to about 5 kV for the anode; a cathode potential of about 70 V below the anode potential for the cathode; and the electrode potential of about 0 V and about 140 V below the anode potential.

Such a mass spectrometer may also include electronics (e.g., a microprocessor, an analog-to-digital converter, or a digital-to-analog converter), disposed within the vacuum cavity, to control or vary the electrode potential (e.g., to control acceleration of the electron). The electronics may also be coupled to a detector that determines a mass of the charged particle based on the acceleration of the charged particle.

Another illustrative mass spectrometer and corresponding method of mass spectrometry includes a magnet in a magnetic yoke to generate a magnetic field having a first strength (e.g., about 0.1 T) in a first region and a second strength (e.g., about 0.7 T) in a second region. It also includes a vacuum housing defining a vacuum cavity, an ion pump disposed in the first region to maintain the vacuum pressure of the vacuum cavity, and a mass analyzer (e.g., a magnetic sector analyzer) disposed in the second region to determine the mass of a particle propagating through the vacuum cavity. A control electrode disposed within the vacuum cavity controls acceleration of an electron that ionizes the particle, and a conversion circuit disposed within the vacuum cavity provides one or more voltages to the ion pump, the electrode, and/or the mass analyzer.

A further example of the illustrative mass spectrometer may include control electronics, disposed within the vacuum cavity and in electrical communication with the control electrode, to vary a potential of the control electrode. It may also include signal processing electronics, disposed within the vacuum cavity and powered by the conversion circuit, to process signals provided by the mass analyzer.

Such a mass spectrometer may also include an electron source, disposed within the vacuum cavity, to provide the electron; a cathode to shield the electron source from the vacuum cavity; and an anode, disposed on a side of the control electrode opposite the electron source, to accelerate the electron toward a particle to be analyzed. The conversion circuit may be configured to provide an anode potential of about 100 V to about 5 kV for the anode, a cathode potential about 70 V below the anode potential for the cathode, and the electrode potential, which may be about 0 V and about 140 V below the anode potential. In addition, the conversion circuit may be configured to step up the input voltage, with a first value of about 1 V to about 36 V, to the electrode potential at a second value of about 100 V to about 5 kV.

DETAILED DESCRIPTION

Following below are more detailed descriptions of various concepts related to, and embodiments of, inventive systems, methods and apparatus for mass spectrometry. It should be appreciated that various concepts introduced above and discussed in greater detail below may be implemented in any of numerous ways, as the disclosed concepts are not limited to any particular manner of implementation. Examples of specific implementations and applications are provided primarily for illustrative purposes.

1.0 OVERVIEW OF MASS SPECTROMETRY

Many different implementations of mass spectrometers exist, and the configuration often depends on the intended application. Generally, however, they include the same basic functional blocks: an inlet, an ion source, a mass analyzer, a detector, and a vacuum system. Samples entering the inlet are ionized, usually by bombardment with an electron beam, then separated by mass using one or more electric and/or magnetic fields, then analyzed for relative abundance.

Ultimately, all of the implementations of the mass spectrometer produce a graph relating the atomic mass-to-charge (m/z) ratios of the components of the ionized sample to the relative abundance of each component. For example, a mass spectrometer measuring a sample of atmosphere would find components at masses 28, 32, 40, and 44, and possibly others depending on the sensitivity of the instrument. These masses correspond to nitrogen, oxygen, argon, and carbon dioxide. The mass spectrometer output will show the highest signal strength for mass 28, nitrogen, which comprises 70% of atmospheric gas, followed by about ⅓ the signal strength of the nitrogen peak for oxygen, at 32 (22% of the atmosphere), and lower signal strengths still for argon and carbondioxide.

Mass spectrometers are generally designed for specific mass ranges and resolutions, depending on the application. Mass ranges might be 10 to 50 AMU for an instrument designed for environmental gas monitoring, or many tens of thousands of AMU for in instrument used in protein analysis. The mass spectrometer often scans through this mass range by varying one of the electric or magnetic field parameters, producing a spectrum in both mass-to-charge (m/z) ratio and, undesirably, time. The scan will produce peaks in signal intensity where masses are present. The resolution of the mass spectrometer is determined by how narrow these peaks are; some mass spectrometers may only resolve unit masses while some may resolve extremely small fractions of mass (e.g., for distinguishing different species that appear at the same nominal unit mass, such as carbon monoxide at 28.010 and nitrogen at 28.0134). Peaks are often characterized by full-width half-maximum (FWHM) measurements; the width of the peak at half of its amplitude can help in determining which masses will be visible. In general, mass spectrometers that produce narrower peaks have better resolving power than those with wide peaks.

FIG. 1Ashows an isometric view of a computer-aided design (CAD) model of an exemplary mass spectrometer100, shown without the vacuum housing for purposes of illustration. The components shown inFIG. 1Aare within a vacuum cavity defined by the vacuum housing and a vacuum flange170unless noted otherwise. A vacuum housing seal172extending along a surface of the vacuum flange prevents leaks, allowing the vacuum pressure to reach 1e-5 torr or less. An inlet180extending through the vacuum flange170to permit introduction of samples for analysis.

The mass spectrometer100includes a shared magnetic circuit110formed of one or more magnets112within a magnetic yoke114. The yoke114couples magnetic flux from the magnets112into two or more magnetic field regions111aand111b. An ion pump (shown inFIG. 1Aas integrated ion pump electrodes120) in the first region111amaintains the vacuum pressure inside the vacuum cavity, and a magnetic sector mass analyzer130in the second region111bseparates ionized sample particles according to mass as understood in the art. An ion source104generates the ions, which are collimated with ion optics300, by ionizing particles admitted through the inlet180with electrons from an electron source (not shown). An ion detector140at one end of the magnetic mass analyzer130generates a current that varies with the number of ions collected by the detector140.

The mass analyzer130and ion detector140are mounted on a planar substrate190, which can be made from printed circuit board (PCB) material as described below, that also supports a conversion circuit (high-voltage power supplies)150. The substrate190is mounted to the vacuum flange170via the magnetic yoke114. Those of ordinary skill in the art will readily appreciate that other mounting configurations are possible as well.

The conversion circuit150converts, or steps up, an input voltage of about 1-36 V (e.g., 12 V) from an external power supply to a voltage high enough to charge the electrodes inside the vacuum cavity (e.g., 100 V to 5 kV), including any electrodes in the electron source, ion source104, ion optics300, and ion detector140. The conversion circuit150may be coupled to the external power supply via a single feedthrough (not shown) that has a relatively low dielectric strength (e.g., a dielectric strength of equal to or less than about 36 V or less, equal to or less than about 24 V, equal to or less than about 12 V, or equal to or less than about 9 V). In at least one embodiment, this low-dielectric-strength feedthrough is the only electrical connection between the interior and exterior of the vacuum cavity defined by the vacuum flange170and the vacuum housing (not shown).

FIG. 1Bshows a low-dielectric-strength feedthrough174suitable for use with the conversion circuit150, vacuum housing, and vacuum flange170ofFIG. 1A. Such a low-dielectric-strength feedthrough174can be made quickly and inexpensively with epoxy and may have a dielectric strength of equal to or less than about 36 V or less. To make the feedthrough174, a small hole is drilled through the vacuum housing (e.g., through the vacuum flange170), tapered towards the vacuum side to a diameter just large enough to accept a feedthrough wire178, which may be bare or coated with conformal insulation (e.g., magnet wire). The wire178is positioned and the hole backfilled with low outgassing epoxy to form an epoxy seal or plug176. In this configuration, the epoxy plug176sees little force; the vacuum flange170or housing still carries the load because the hole is mostly filled by wire178and the epoxy176holds the wire178in place. Using bare or conformally coated wire reduces the change of vacuum leaks between the wire178and its insulation layer, as might happen with wire that is insulated with a separate jacket.

Placing the conversion circuit150inside the vacuum cavity is counterintuitive because even the most efficient conversion circuit150dissipates energy in the form of heat. This heat raises the temperature of other components in the cavity, including the substrate190. As the other components heat up, they may release absorbed or adsorbed gases, which causes the pressure inside the cavity to rise, increasing the load on the ion pump120.

But placing the conversion circuit150inside the vacuum cavity makes it possible to eliminate high-voltage electrical feedthroughs, which are typically expensive and difficult to manufacture. Unlike low-dielectric-strength feedthroughs, high-voltage electrical feedthroughs typically need to provide a vacuum-tight electrical connection that can withstand hundreds or thousands of volts with respect to the vacuum housing, and can be baked at hundreds of degrees Celsius. They are often fashioned out of Kovar and brazed to a ceramic dielectric, which is in turn brazed to a stainless steel housing or fitting.

2.0 TYPES OF MASS SPECTROMETERS

Many different types of mass spectrometer exist, generally classified by the method used to separate the different masses. This section briefly covers some of the simpler types of mass spectrometer, and although nowhere near comprehensive, describes those that have potential to be manufactured inexpensively.

2.1 Types of Mass Analyzers

A magnetic-sector mass spectrometer (e.g., mass analyzer130shown inFIG. 1A) produces a spatial separation in mass. In this design, ionized samples are accelerated in an electric field and injected into a region with a perpendicular magnetic field. The radius of curvature of the ion's trajectory in the magnetic field is proportional to its mass and inversely proportional to its charge state. By scanning either the electric field, and therefore varying the ion's kinetic energy, or scanning the magnetic field and varying the ion's trajectory, the various masses can be separated and detected independently. Many variants of this design exist, including some with separate or combined electric and magnetic sectors, producing improved resolution.

A time-of-flight mass spectrometer is another design that produces a temporal separation in mass. Ions are injected into a drift region by a fixed electric field; the separation in ultimate ion velocity and therefore arrival time at the far end of the drift region is proportional to ion mass.

A quadrupole mass spectrometer uses two pairs of electrodes parallel to an ion flight path; by applying a variable-frequency RF field using one electrode pair and a DC bias on another, and tuning the RF field for a specific mass, only one mass at any given time has a stable trajectory through the fields.

A similar type of mass spectrometer, the ion trap mass spectrometer, uses principles similar to the quadrupole mass spectrometer to trap clouds of ions in a volume and selectively make the orbits of specific masses unstable. The unstable masses are then ejected from the ion volume and measured.

2.2 Ion Sources

Mass analyzers typically rely on ionized samples injected into the mass spectrometer to function properly. Once the sample is ionized, the ionized sample molecules (the ions) may be manipulated and separated by electromagnetic fields.

Common ion sources use electron ionization. In this type of source, an electron beam, usually generated thermionically, is aimed into a gaseous sample. Electrons interacting with sample molecules remove electrons from the sample, producing positively charged sample ions, although negative ion mass spectrometry is practical for some electronegative chemical species.

Once a sample has been ionized and the resulting ions separated by mass, the ions can be detected with a detector (e.g., detector140inFIG. 1A). The simplest detector is a Faraday cup followed by a high gain transconductance amplifier. Ions striking the Faraday cup produce a tiny but measurable current that is then amplified and recorded. However, since these detectors provide no intrinsic gain, the noise floor is that of the amplifier.

3.0 MASS SPECTROMETER DESIGN OVERVIEW

An illustrative embodiment of the miniature mass spectrometers disclosed herein may have a simple, robust design that can be made without complicated or labor-intensive manufacturing techniques. Each design choice may involve a tradeoff among multiple factors, among them performance, size, weight, power consumption, complexity, ease of manufacture, and cost. Such a design may be manufacturable using automated machine tools. Manufacturing can be simplified further by creating a planar design that relies on two-dimensional (2D) machining; any features in the third dimension can be built or approximated by stacking multiple layers of 2D-machined components. Eliminating secondary machining operations can help to eliminate extra fixturing, time, and waste. Thus, in at least one case, the design incorporates many co-fabricated features.

In one example, an inventive mass spectrometer comprises a single unit that may be operated in a simple, cylindrical vacuum chamber with a port for gas inlet, several low-voltage cables, and a port for a roughing vacuum pump. These ports may be implemented with thin tubing or cabling fed through the vacuum chamber wall and embedded in epoxy.

An exemplary mass spectrometer can be designed with a number of potential applications in mind, but for the most part, with common performance requirements. For instance, it could be designed and built for unit resolution (i.e., it can discriminate between ions one or more integer mass units distant) with enough sensitivity to detect species comprising of 0.5% or more of the analyte gas at an operating pressure of 1e-4 Pa (1e-6 torr). It can also carry its own high vacuum pump onboard; while slightly less versatile than a design incorporating both the high vacuum and roughing pumps, the substantial savings in cost, weight, and complexity may be invaluable. Such an exemplary mass spectrometer may be able to run on its own for long periods of time with low power consumption as well as low maintenance.

An instrument providing this level of performance is of limited utility if the production cost is comparable to that of an existing commercial instrument (e.g., tens of thousands of dollars). The mass spectrometer can be quite inexpensive (e.g., on the order of $1000), making it suitable for large-scale deployment in novel applications. Figuring into the cost of the mass spectrometer is ease of manufacture and complexity; difficult or skilled manufacturing techniques and/or large numbers of parts may make the design more expensive to build.

Minimizing power consumption is also important for certain applications. For instance, a mass spectrometer meeting the above specifications may be well-suited for a variety of remote or portable applications, in which the mass spectrometer can run for long periods of time off batteries, solar power, wind power, or another energy source.

In one embodiment, the miniature mass spectrometer is a single-focusing, 180-degree magnetic sector mass spectrometer. A magnetic sector mass spectrometer can be constructed using layers of planar components, greatly reducing the cost of the instrument, as most simple manufacturing techniques are two-dimensional. The geometries involved are simple and no high power RF oscillators or high speed timing abilities are needed, as may be the case with a quadrupole or time-of-flight mass spectrometer, respectively. Other mass spectrometer types, such as ion trap or Fourier-transform types, can be demanding in terms of geometry, power, or complexity.

A set of permanent magnets and yoke creates the magnetic field for the mass analyzer. With the ready availability of NdFeB magnets this is an obvious choice; an electromagnet requires too much power for a small instrument. Additionally, a second benefit is available with a permanent magnet. By carefully choosing the sizes of the pole pieces for the yoke, the design can incorporate an ion pump into the same magnetic circuit that encloses the analyzer, thus saving on complexity, size, and parts count. The length of the magnetic sector analyzer may be 180 degrees, simplifying the layout and minimizing the size of the design by placing the ion source and detector on the same side of the instrument. The design of each subsystem of the mass spectrometer is detailed in the following sections.

In another embodiment, the upper and lower mass analyzer include an electric sector, changing the overall mass spectrometer topology to that of a Nier-Johnson double-focusing mass spectrometer, possibly more than doubling the mass resolution.

3.1 Vacuum System Design

During operation, the entire length of the ion flight path is kept at high vacuum, i.e., at pressures below 1e-4 Pa (1e-6 torr). At higher pressures (lower vacuum), the mean free path for an ion becomes too short for enough of them to transit the entire length of the flight path. This criterion alone necessitates the use of a vacuum system with very tight tolerances to reduce the leak rate, as well as a vacuum pump capable of producing the high vacuum.

At the same time, the mass spectrometer's vacuum system may have to contend with a constant influx of gas; the gas entering the system from the inlet should be continuously pumped back out or captured lest the vacuum chamber pressure rise to an unacceptable level. Thus, the vacuum system may also incorporate a one or more vacuum pumps capable of pumping faster than the inlet leak rate.

In most mass spectrometers, the vacuum system is a very expensive part of the design. Compared to the cost of a typical instrument, the vacuum system may not be a large percentage of the overall cost, but for a miniature inexpensive design, the vacuum components alone may easily dominate the budget. High vacuum components, even standard fittings, are extremely expensive. Nearly every component is constructed of machined or formed stainless steel, typically with welded junctions. Mass spectrometers often use custom vacuum components just due to the geometry of the instrument. For example, a magnetic sector mass spectrometer often has a formed, thin-walled, welded section of stainless steel tubing welded to high vacuum flanges for the mass analyzer. This is typically because the mass analyzer's flight path should fit between the poles of the magnet, and the gap is rarely a standard size.

Moreover, electrical signals are fed into and out of a typical mass spectrometer vacuum system, with one feedthrough for every voltage in the system. In a conventional mass spectrometer, there may be anywhere from five to ten or more separate potentials at different points within the vacuum system. Feedthroughs for high voltages can be especially expensive because they are made by brazing Kovar conductors with ceramic insulators and stainless steel flanges. Because of the cost and complexity using multiple feedthroughs (including high-voltage feedthroughs), illustrative mass spectrometers may be designed and built to operate with a small number (e.g., one or two) of signals penetrating the vacuum chamber.

One way to reduce vacuum system cost and complexity is to reduce the number of components involved. For instance, a miniature mass spectrometer can be designed to fit, in its entirety (including magnets, power and control electronics, high vacuum pump, and ion optics, etc.), within a vacuum chamber that has a 100 mm diameter and a 150 mm length. An exemplary mass spectrometer can be mounted on a single vacuum flange through which all of the electrical signals and the inlet pass, and the vacuum chamber can therefore include a 100 mm diameter cylindrical pipe for simplicity. Indeed, a simple but smaller vacuum chamber could be constructed that follows the contours of the instrument to reduce size and weight.

To reduce the number of electrical feedthroughs, the data can be handled digitally and control signals can be generated inside the vacuum housing by an onboard control system. In this manner, the system uses one, two, or three low-voltage electrical signals (e.g., power and one or two data lines) fed through the vacuum chamber walls. These electrical lines may be simple lengths of cable embedded in low-outgassing epoxy, since high isolation is not necessary. Ground reference can be the chamber itself.

Alternatively, or in addition, the system may be capable of transmitting data wirelessly (e.g., via infrared or RF channels) through the vacuum chamber walls, making only a single electrical feedthrough for power necessary. In addition, the system could be powered inductively (e.g., via coil loop antennas), eliminating any need for a feedthrough to connect the inside and outside of the vacuum chamber.

In another example, the miniature mass spectrometer incorporates a co-fabricated ion pump, designed to use the same permanent magnet and yoke assembly that the mass analyzer uses, to maintain a high vacuum within the vacuum chamber. An ion pump by itself may not be sufficient to pump down a mass spectrometer from atmospheric pressure, so a valved port can be provided for rough-pumping the chamber to a point at which the ion pump can start. This port can be mounted on the same flange as the electrical feedthrough and inlet.

3.2 Mass Analyzer Design

The resolution of the mass spectrometer may depend heavily on the design of the mass analyzer. Generally speaking, the stronger the magnetic field, the smaller the radius of curvature. In one example, the mass analyzer in the mass spectrometer is a 180° magnetic sector, with an ion flight centerline radius of 23 mm. This is in part a practical consideration; 50 mm×25 mm NdFeB magnets are available without requiring custom fabrication, and some clearance between the ion flight radius and the edge of the magnet accommodates any imperfections in the ion's nominally circular flight due to nonlinearities of the magnet's field.

Choosing the sector length to be 180° makes it possible to increase the spatial separation between ion beams of adjacent mass, as more of each ion's flight is within the sector. Second, with a 180° sector, both the ion source and the detector are located on the same side of the mass analyzer, leading to a more compact design and fewer complications (if any) with locating the magnet yoke. Larger instruments typically don't enjoy this benefit because they have separate vacuum compartments for the ion source and detector and because the sector length in these instruments is typically limited by the size of the magnet.

There is a tradeoff between field strength and weight and cost. The maximum magnetic field strength using permanent magnets is in the range of 0.5 to 1 T, using high grade (N52) neodymium-iron-boron magnets. Higher fields require more coercive force: more magnet thickness in the direction parallel to the gap, and more iron in the magnet's return path. This can lead to a heavier and larger design. But a stronger magnetic field, e.g., created with a vanadium permandur yoke or a Hallbach array of neodymium-iron-boron magnets, increases the resolution at low masses, while the achievable higher voltages preserve the upper, light mass resolution.

Likewise there is a tradeoff between resolution and signal strength and cost. Narrowing the filter slits leads to higher resolution, but fewer ions complete the flight, causing detector gain and sensitivity to become more important. Furthermore, as the slit becomes narrower, alignment of the slit with the axis of the ion beam becomes more critical, leading to tighter tolerances and larger cost.

One illustrative design eliminates the need for filter fixturing and alignment by co-fabricating slits160with the chassis of the analyzer. Furthermore, the slits160are themselves mounted on flexures integral to the analyzer chassis such that the geometry may be varied at assembly; the slit width can be modified to change the operating point on the signal/resolution curve. In some cases, an actuator, such as a lead screw, piezo, or shape memory alloy component, changes the slit width actively, e.g., in response to feedback during calibration, operation, or both.

FIG. 1Cshows a computer-aided design (CAD) model of the magnet yoke114ofFIG. 1A. It can be made of 1008 mild steel, and holds a pair of 50×50×10 mm N52 neodymium-iron-boron magnets112in the magnetic-sector mass analyzer130. In one embodiment, the yoke114increases in cross section from the leading edge of each magnet112to 25×50 mm at the trailing edge of each magnet112. The yoke mass, including the magnets112, is approximately 1.4 kg. The yoke114also incorporates features for mounting; a pair of holes in the return path allows the magnet, itself the heaviest part of the mass spectrometer, to be bolted to the vacuum flange.

As shown inFIG. 1C, the cross section of the yoke114may be approximately constant beyond the magnet. A 10 mm gap is left between the trailing face of the magnet112and the yoke114to avoid shorting the magnet112. The gap between pole faces is 10 mm, approximately the same air gap as magnet thickness. This configuration produces a field ranging from approximately 0.6 T at the edges of the pole face to about 0.8 T in the center. Non-uniformity of this field may leads to trajectory errors in the ion beam and lower resolution.

FIG. 1Dshows an alternative yoke214suitable for holding one or more magnets212in position around the mass analyzer130. The yoke214channels magnetic flux generated by the magnets212into two field regions211aand211bof different field strengths. The ion pump120is disposed within the first field region211a, which may have a strength of about 0.1 T, and the mass analyzer130sits in the second field region211b, which may have a strength of about 0.7 T.

Given the field strength and ion flight radius, it is a simple matter to calculate the range of ion energies, and therefore the ion acceleration potentials, required to run the mass spectrum sweeps. First is a force balance: in the mass analyzer, the force required to keep an ion on a circular trajectory is equal to the ion's mass multiplied by the centripetal acceleration, and is provided by the Lorentz force due to the ion's charge and the applied magnetic field,
qvBsin θ=mv2/r

where B is the magnetic field strength in Tesla; v is the ion velocity in m/s; θ is the angle between ion beam plane and magnetic field in radians; m is the ion mass in kg; q is the elementary charge in C; and r is the ion curvature radius in m.

The velocities give the range of voltages required to accelerate the ions. Final ion velocity, that is, the velocity of the ion as it exits the ion source into the analyzer, is proportional to the voltage E across the electrodes in the ion source,
qE=½mv2

These equations can be combined to give the relationship between ion mass and the potential required to accelerate the ion in order for it to reach the detector,

So there is an inverse relationship between the required electric field and the mass of the ion, as expected. Heavier ions require more kinetic energy to traverse the analyzer with the proper radius, given constant charge. Assuming each molecule is singly ionized (i.e., q=1.6e-19 C) and within the intended mass range, 10 to 44 AMU (m=1.66e-26 to 8.3e-26 kg), an analyzer radius r of 23 mm, and a perpendicular B field (θ=0) the equation can be simplified to,

For an operating point of B=0.6 T and mass range of 10-44 AMU, the voltage E to accelerate the ions should sweep from about 208 V to 915 V. These potentials are attainable, given the dielectric strength of high vacuum. Moreover, there are many methods capable of generating these voltages efficiently. Voltage generation will be discussed in a later section.

FIG. 2is a plot of ion source potential versus ion mass for different magnetic field strengths. Note that since this is an inverse power function, resolution will decrease as ion source potential decreases because the same change in ion source potential will span a much larger mass range. This is a feature intrinsic to magnetic sector mass spectrometers, and this design is no different. This issue is discussed in more detail below.

3.3 Ion Source Design

The ion source affects both the efficiency and the performance of the mass spectrometer. Ions are typically formed by electron ionization; an electron gun generates an electron beam that interacts with the sample gas to form positive ions. This type of ion source has historically been called electron impact ionization; however, due to the wavelike nature of electrons, the exact mechanism of ionization is not related to particle impact.

The ion source may be located far enough from the magnet yoke structure such that the fringing fields from the magnet do not affect the trajectory of the electrons. In some cases, the distance between the ion source and the yoke is approximately 30 mm. Furthermore, the ion source is designed with an electron beam oriented vertically, essentially in parallel with the fringing fields of the magnet. This reduces the chance that the electron beam will be sent off course by stray fields.

3.4 Electron Source Design

The electron beam is typically generated thermionically by heating a hot wire, usually tungsten or an alloy, to incandescence, so as to add enough thermal energy to some of the electrons in the wire such that they can overcome the work function of the bulk metal and escape into the surrounding vacuum. The escaped electrons are removed from the area surrounding the wire using electrostatic fields. This process of generating electrons is typically inefficient; furthermore, the probability of an interaction between an electron in the beam and a molecule in the sample gas resulting in the formation of an ion is also low, on the order of 0.1%.

Ideally, these ions emerge from the ion source in a collimated beam of appropriate geometry for subsequent flight through the analyzer. In practice, however, ionized molecules have a random distribution within the ionization region, and only a small fraction of the ions produced emerge from the ionization region in the appropriate direction to be analyzed.

To compensate, many conventional mass spectrometers employ an electrostatic field produced by an electrode, typically called the repeller, in the ionization region to sweep ions towards the analyzer; however, the field produced by this electrode is relatively low. The result is that the ion yield of a mass spectrometer using a thermionic electron gun is extremely low. Thus, a high current electron beam is desirable to increase the total production of ions, but this may require a large investment in electrical power.

There are at least three techniques by which the efficiency of the ion source may be improved. The yield of electrons for a given filament power may be increased, through the use of improved emissive materials. The yield of ions may be improved by increasing the probability of interaction between the electron beam and the sample gas, by changing the trajectories of the electron beam (e.g., a helical instead of straight trajectory). Finally, it might be possible to capture more of the ions that would form but otherwise not be swept into the analyzer. Both high efficiency emissive materials and methods of increasing ion yield were examined.

In one or more embodiments of the inventive mass spectrometers, the ion source is designed to improve ion yield. An illustrative ion source operates by ionizing a large volume of ions using a large diameter electron beam, producing an ion beam with a wide dispersion, and then using a series of electrostatic lenses to collect and collimate these ions into a uniform ion beam. A large, cylindrical electron beam is produced by a simple, low power tungsten filament and a circular aperture in an anode. This structure is called a Pierce diode and well understood; it was studied extensively in the days of vacuum tubes and appears in reference literature. The diameter of the electron beam is quite large, at 3 mm, and is used to ionize a large volume of sample gas. However, instead of directing these resultant ions through an adjacent, narrow mechanical filter, the entire volume is gathered and focused with electrostatic lenses.

In the Pierce diode, the current density of the current emitted from the anode hole is,

where Imaxis the current density in A/m^2; V is the voltage between anode and cathode in volts; r is the radius of anode hole in m; and d is the distance between anode and cathode in m. For a distance of d=5 mm between the filament and the entrance of the ion source and a potential of V=70 V, the emission current is 120 μA. The emission angle of the Pierce diode is θ=r/3d, where θ is the beam angle in degrees; r is the radius of anode hole in m; and d is the distance between anode and cathode in m. In one example, the Pierce diode may have a beam angle is 0.1°. The emissive material generating the electrons may be capable of producing 120 μA of electron current within a 3 mm diameter circle, which is the diameter of the hole in the anode.

The space-charge limited emission from an incandescent tungsten filament, as a function of temperature, is

where imaxis emission current density in A/m2of emissive surface and T is the surface temperature in K. At 2500 K, the current density from a tungsten emitter is 3170 A/m2. In one example, the ion source includes an emissive surface with an area of 4e-6 m2, which is disposed in a anode hole (window) of 7.1e-6 m2, that can produce a 120 to electron current. In one case, the emissive surface is formed of a tungsten filament 3 mm in length and 0.4 mm in diameter.

Alternatively, the emissive surface area can be produced using a thinner, coiled tungsten wire. The thinner wire of a coiled filament is less thermally conductive, leading to a more efficient system because less of the heat is carried out of the filament power leads, and for the same power input can be run at a higher voltage and lower current. Fifteen turns of 12 μm diameter tungsten wire, with a turn diameter of 1 mm and pitch of 0.2 mm has a surface area of 4 mm^2 and a length of 3 mm. Such a coiled filament may be supported by a support structure made of glass or ceramic insulators and copper conductors.

A filament with essentially this configuration is already mass produced as a flashlight bulb, typically designated PR-2. The PR-2 draws 0.5 A at 2.4 V, and has a coiled filament approximately 1 mm in diameter and a length of approximately 3 mm. In one example, the mass spectrometer's ion source includes a PR-2 flashlight bulb with the glass bulb carefully removed. Application of vise jaws allows the bulb to be shattered without damaging the delicate filament structure in the middle.

The electric field across the Pierce diode can be set to 70 V. As a result, the electrons emitted from the Pierce diode anode hole are at approximately 70 eV. This value of kinetic energy is a commonly accepted value for maximizing the number of ions produced by electron ionization for a given electron current. This is due to the fact that the de Broglie wavelength of an electron at 70 eV is 14 nm, which is approximately the length of the bonds between atoms in many molecules. At 70 eV, the de Broglie wavelength of the electron is given by λ=h/mv, where λ is the de Broglie wavelength in m, h is Planck's constant, m is the particle mass in kg, and v is the particle velocity in m/s.

FIG. 3is a diagram of an ion source lens system300that focuses ions generated by the electron beam. The ion source lens system300includes an inlet302that admits ions into a ionization region308and a trap electrode306opposite the inlet302. A repeller electrode304charged to a potential whose polarity is opposite the polarity of the ions repels the ions. The repeller electrode's weak electrostatic field sweeps the ions from the ionization region towards a three-element symmetric electrostatic lens310, also known as an Einzel lens, that focuses the ion stream on a large slit (filter)312. These ions diverge again beyond the filter312, but a second two-element lens320defocuses the ion beam slightly, changing the focal point to a point infinitely distant from the filter312. In other words, the first lens310and filter312spatially filter the ion beam, and the second lens320collimates the ion beam to make it better-suited for analysis.

Inventive ion sources include a control electrode (also called a grid) that screens the anode of the Pierce diode from the cathode. An electrical potential, or control potential, on this control electrode can enhance or prevent the emission of electrons from the cathode. The control potential, applied to an electrostatic element, may be rapidly modulated with electronics disposed inside or outside the vacuum chamber, and can operate in much the same way as a control grid in a vacuum tube. The signal used to modulate the thermionic emitter can be used with advanced signal processing techniques such as synchronous detection or stochastic system identification to improve the signal to noise ratio of the mass spectrometer.

3.7 Sample Jet

One of the unknowns is how well the electron beam interacts with the incoming sample gas. To increase the interaction between the sample gas and electron beam, a hole is provided in the center of the trap electrode. The sample is then directed downward through the trap, while electrons are beamed in the opposite direction.

3.8 Detector Design

An exemplary mass spectrometer includes a detector to sense ions in the mass analyzer. The ion beam that reaches the detector may be equivalent to a current on the order of tens to hundreds of femtoamperes (fAs). The detector at the outlet of the mass analyzer can detect these minute currents and produce a signal above its intrinsic noise floor.

In one embodiment, the detector is a Faraday cup followed by a transconductance amplifier with a gain of 50e9. The Faraday cup captures the incident ion beam as well as recapturing any electrons produced by secondary emission. Since the incident ion beam can have quite large energies, on the order of hundreds of eV, secondary emission is a concern. The Faraday cup electrode shape is designed to capture secondary emission by providing a deep cavity into which the incident ion beam travels that recaptures all electrons that are emitted in any direction but perpendicularly back out. However, since the Faraday cup is still within the fringing field produced by the permanent magnet, secondary emission electrons may be captured by the cup.

The transconductance amplifier can be built around a National Semiconductor LMP7721 low input bias operational amplifier (op-amp) or any other suitable op-amp. Operating with supplies of ±2.5 V, the LMP7721's input bias currents are on the order of 3 fA. A 50 GΩ resistor in parallel with a 5 pF silver-mica capacitor for stability provide the amplifier's feedback path. The output of this transconductance amplifier drives the front end of an analog-to-digital converter (e.g., a Texas Instruments ADS1278 24-bit analog-to-digital converter). By placing these components in close proximity and under appropriate shielding, the intrinsic noise may be reduced.

Alternatively, the mass spectrometer may include an electron multiplier-type detector400, shown inFIG. 4, that operates in a fashion similar to that of a photomultiplier tube without the photocathode. Ions striking a first dynode402adislodge electrons, which fall down a series of increasingly higher voltage dynodes402bthrough402n, each iteration producing twice or more the number of electrons. This electron cloud is then captured and measured by a transconductance amplifier404, but the signal can be many orders of magnitude larger than a simple Faraday cup detector, without a significantly higher noise floor, thus allowing for much more sensitive detection. For instance, a four- or five-stage discrete-dynode electron multiplier, appropriately placed, may give a signal-to-noise ratio boost of just over 16-32, while the low dynode count reduces the dark current.

3.9 High Vacuum Pump Design

The miniature mass spectrometer uses a pump, such as an ion pump or turbo-molecular pump, to maintain the high vacuum of the vacuum envelope. Ion pumps are silent, clean, and employ no moving components. In an ion pump, two pumping mechanisms, both capture and sorption, are in operation. While pumping, gases are ionized by high field ionization in cylindrical anodes and accelerated into titanium or sometimes tantalum cathodes. Upon impact, the ions are either buried or cause titanium to sputter back to the anode. This constantly renewed layer of titanium is chemically reactive and captures gases by sorption.

The electrodes for the ion pump are located within a magnetic field, which generally adds mass to the system and complexity to the vacuum chamber. However, the miniature mass spectrometer is already designed with a magnetic circuit located within the vacuum chamber. In at least one embodiment, the size of the pole faces of the magnet are large enough to encompass the footprint of the mass analyzer and the ion pump to add pumping capability without a significant increase in complexity.

In one case, the ion pump is a diode pump, which includes a set of stainless steel hollow cylinders, open on each end, suspended between a pair of titanium plates. The pump is designed to produce the maximum pumping speed in the area available. Specific geometries and tradeoffs are discussed in below.

The ion pump keeps the system pressure low enough such that the mean free path of the ions is greater than the entire flight length of the mass spectrometer. For this miniature mass spectrometer, the length of the flight path is approximately 200 mm. The mean free path of an ion is given by, 1=3.71e-7/p, where 1 is the mean free path length in m and p is the pressure in Pa.

In general, the vacuum should be high enough (i.e., the pressure should be low enough) to keep each ion's mean free path about an order of magnitude larger than the flight length of the mass spectrometer. For a mean free path of 2 m, the minimum system pressure is 3.3e-3 Pa (2.48e-5 torr).

As shown inFIG. 1A, the mass spectrometer100includes an inlet180to admit the sample to be analyzed. The inlet180may be of any suitable type. For instance, it may include an inlet chamber500formed of a semi-permeable hydrophobic plastic membrane502supported by a perforated stainless steel plate504as shown inFIG. 5. The membrane502allows sample particles P to diffuse into the vacuum chamber (not shown) at a rate proportional to its exposed surface area while preventing the influx of water vapor and liquids. The inlet rate can be chosen such that the mass spectrometer's pumping system can handle the inlet gas load at an appropriate vacuum chamber pressure.

An exemplary miniature mass spectrometer ion optics design was extensively simulated using SIMION 8.0, a commercial ion optics modeling software package. These simulations can be used to modeling the ion flight and to make or change device parameters, including instrument geometry, magnet strength, ion radius, etc.

Simulation can be used to iterate through design choices (e.g., by simulating choices that affect the electrode voltages to properly focus the ion beam). In one example of simulation, the overall height of the mass spectrometer's analyzer was set first. The vertical dimension is somewhat arbitrary. The permanent magnets used are both 10 mm in height, and the gap was chosen to match this figure. Leaving some 1.5 mm for the thickness of each of the top and bottom covers of the mass analyzer, the vertical dimension was then set to 7 mm.

FIG. 6Ais a diagram of the ion source104(FIG. 1A) and ions source optics300(FIG. 3) captured from the SIMION simulation of the mass spectrometer. The radius of the mass analyzer was set to 23 mm (as above). Using this as a controlling dimension, the remainder of the mass spectrometer ion optics300and flight path was designed to be no more than 50 mm in length. The electron beam was placed as far from the magnetic sector mass analyzer130(FIG. 1A) as possible, to reduce the influence of the stray magnetic field on the operation of the electron beam.

The next decisions involved the size of the first lens310. The first lens310collimates the volume of ions created by the electron beam and focuses them on a mechanical filter. This lens310is a three-element symmetric lens, otherwise known as an Einzel lens, and described as symmetric because the first and third lens elements are at the same potential. This type of lens was chosen because it is a variable focus lens that does not change the energy of the ion that emerges from the other side. Typically, electrostatic lenses are built with approximately the same width as element length, with an element spacing equal to a tenth of the length. Such lenses typically have focal lengths that are of equal distances on both sides of the lens; hence, the filter312following the first lens310is the same distance from the lens310as the ionization region.

The second lens320, used to defocus the beam slightly (e.g., placing its focal point at infinity), is a two-element lens that roughly equally subdivides the region between the first mechanical filter and the second mechanical filter. The longer electrode faces provide a slightly more uniform field; the exact placement of the electrodes is slightly less crucial.

A second mechanical filter322after the second lens320further limits the beam dispersion to reduce stray ions reaching the detector. This filter322was placed 10 mm from the nominal entrance340to the magnetic sector mass analyzer130(FIG. 1A; not shown), since the fringing fields from the magnet are quite strong, and may nudge the ion beam off course before it reaches the filter322.

Note that all of the electrodes, rather than being simple flat faces along the ion flight path, extend perpendicularly well away from the flight path. Although a flat plate would behave identically in this simulation, in practice it would be nearly impossible to fabricate. The depth of the electrodes allows them to be mounted to a common plane; the simulation is done this way as a reminder that the electrodes need to be mounted somehow. The shapes of the back sides of the electrodes are not critical.

4.2 Ion Flight Simulations

The entire mass spectrometer design was simulated and found to conform to the initial design work. Simulations were done for ions of mass 10 AMU to 44 AMU. The voltages required on the various electrodes roughly conform to the predictions.

FIG. 6Bis a simulation showing the flight of carbon dioxide molecules from the ion source104through the ion source optics300and the mass analyzer130. SIMION does not simulate either space charge, ion collisions, or secondary electron emission; the simulations are done on single, isolated ions in the geometry provided. The effects of fringing electric fields are simulated.

It is important to note that the simulation is done under ideal conditions, and one can easily be led off track by improper choice of initial conditions. For example, a simulation done on a stationary ion beginning dead center in the ion beam is likely to behave much more favorably than an ion near the edge of the ionization region with an initial velocity perpendicular to the intended path. An improper choice of initial conditions may lead to a belief that a design will work with much higher ion efficiency and resolution than the design can realistically produce. Thus, the initial conditions for ions in the flight path should be chosen carefully.

Ion initial energies were chosen to have a Gaussian spread centered around the thermal energy of a gas molecule at room temperature. The average translational energy of a gas molecule of an ideal gas is E=3kT/2, where E is the kinetic energy in J, k is Boltzmann's constant (8.617e-5 eV/K), and T is the temperature in K. At room temperature, E is approximately equal to 0.015 eV. Therefore, the later trajectory simulations were done using a Gaussian distribution of initial kinetic energy with a mean of 0.015 eV and a standard deviation of 0.005 eV.

Ion initial direction was set using a uniform distribution across 360 degrees radially. Ion initial position was set using a uniform distribution across a cylinder above the projection of the hole through which the electron beam enters the ionization region.

FIG. 6Cis a detailed view of the ion source104and first lens310of the mass spectrometer100(FIG. 1A). Ions originate in the center of the ionization region308, generated by a vertical, cylindrical electron beam, directed vertically out of the page. The initial trajectories of the ions are generated with random direction and random kinetic energy. The repeller electrode304directs the ions towards the first lens310, which focuses the ions on a slit312(FIGS. 3,6A, and6B; not shown). The black traces in the simulation diagrams are computed trajectories of ions given a realistic set of initial conditions. It is reasonable to neglect space charge, due to the low magnitude of the ion current.

FIG. 7is an isometric view of the mass spectrometer100, with the physical layout represented in two dimensions and potential energy represented in the third, vertical dimension. The potential energy is highest in the ion source104, then decreases at the first filter312, increases again in the second lens320, before decreasing in the mass analyzer130. Here, the benefits of a longer, lower-voltage second lens320becomes more apparent; any slight misalignment in a higher voltage lens could cause a much larger trajectory error in the ion beam, as the potential energy ‘obstacle’ the over which the beam climbs becomes much steeper.

4.3 Electron Source Simulations

FIGS. 8 and 9show simulations of an electron source assembly800, or Pierce diode, that includes an electron source102, which may be a filament or any other suitable type of electron source. The electron source102is disposed with a region bounded by a cathode810on three sides and an anode830on the fourth side and is simulated here as a cylindrical source of electrons 1 mm in diameter a 3 mm in length. A control electrode820sits between the electron source102and the anode830. Slits or apertures in the control electrode820and the anode830allow electrons to propagate to the ionization region in the ion source104(FIGS. 1A,3, and6A).

In operation, the cathode810is held at a potential about 70 V below the potential of the anode830, which can be at a potential of about 100 V to about 5 kV. Control electronics (not shown), which may be disposed inside the vacuum chamber, vary the control electrode's potential from about 140 V below the anode potential to about 0 V below the anode potential. When the control electrode is off (i.e., at a potential equal to the anode potential), the cathode810and anode830act to propel electrons out of the assembly, as shown inFIG. 8.FIG. 8shows the focusing effect of the anode; the emitted electron beam is collimated with a narrow beam angle. The electron beam narrows slightly as the source potential climbs from 150 to 900 V. Applying a voltage to the control electrode820reduces the intensity of the electron beam. For instance, holding the control electrode820at a potential 100 V below the anode potential, as shown inFIG. 9.

The mass spectrometer uses a number of electrostatic elements that are held in alignment while remaining electrically isolated. To reduce parts count, a single, inexpensive substrate was chosen to maintain both alignment and isolation of all of the electrodes.

FR-4 printed circuit board material was chosen as the substrate onto which the mass spectrometer is built. The reasons for this choice are numerous. FR-4 fiberglass printed circuit boards (PCBs) are inexpensive in large quantity, due to the large number of facilities dedicated to producing custom boards and the highly automated processes involved. PCBs can be made with very small feature sizes and extremely high accuracy; typical PCB houses such as Sunstone (www.sunstone.com) can produce feature sizes down to about 0.15 mm in prototype quantities and smaller features in large production quantities, with positioning accuracy to a tenth of that. PCBs, nominally designed for electrical components, have a very high dielectric strength, on the order of 1e7 V/m to 2e7 V/m, which is sufficient for the voltages involved in this mass spectrometer design. Finally, PCBs are mechanically very strong, being primarily composed of woven fiberglass mat and epoxy resin, and are a good choice for keeping electrodes separated.

Since PCBs are designed for the implementation of electrical circuitry, both the electrodes of the mass spectrometer and the circuitry that drives it may be incorporated onto the same substrate. An additional benefit of using PCB material for a substrate is that multiple variants of printed circuit board composition exist, including ceramic printed circuit boards, and the underlying material could be changed relatively easily should the potential drawbacks of FR-4 prevent the design from working properly.

PCBs do have a couple of potential drawbacks, however. FR-4 printed circuit boards are made of copper over glass-reinforced epoxy sheets. As such, the substrate material has the potential to absorb and adsorb water and gases (diffusion into the bulk material and adhesion to the surface, respectively). These absorbed and adsorbed molecules could then be released slowly into the mass spectrometer's vacuum system, preventing the system pressure from falling low enough such that this background concentration of gas remains visible on top of the inlet gas spectrum. These potential problems are not without solutions. Two primary countermeasures to these problems exist; driving the absorbed and adsorbed gases off the material, or encapsulating the material in a low-outgassing conformal coating.

It is well known that raising the temperature of a material tends to aid in the removal of both absorbed and adsorbed gases in vacuum. Standard procedure when constructing vacuum tubes is to degas the tube by heating the elements while the tube is still on the exhaust vacuum manifold. Degassing is usually done either by operating the tube's filament, which heats the tube's electrodes by radiation, or by drawing electron current, which heats the tube's anodes and other electron collecting electrodes, or by bombing the tube. Bombing involves heating electrodes by Joule heating using eddy currents induced in the electrodes by an RF coil held external to the tube envelope.

Encapsulating outgassing materials has precedent as well. Outgassing of materials is often a problem on spacecraft, especially satellites, where gases may be emitted by one surface and re-adsorbed by other critical surfaces, such as sensors. As such, conformal coatings are often tested for outgassing properties. A standard test method for determining outgassing properties exists, ASTM E595-07. One well known low-outgassing conformal coating is parylene, and parylene coating is a service offered by many job shops.

An embodiment of the inventive mass spectrometer may include a distributed network of resistive heaters added to the bottom of the PCB substrate. These heaters enable heat to be added at points all across the PCB simultaneously. In another embodiment, these resistive heaters are replaced or augmented by a simple network of thin traces, similar to the resistive array on the rear windows of most automobiles.

5.2 PCB Design and Construction

FIG. 10shows a CAD layout of the printed circuit board, with all of the pieces concatenated (to be cut apart after build to reduce cost). To reduce the overall size of the mass spectrometer, several layers of PCB were used. A bottom layer of printed circuit board carries the electronics package, described in detail in the next chapter, while the two upper PCBs form the bottom and top covers of the mass analyzer.

FIG. 11shows a CAD model of an exemplary mass analyzer assembly1100. The substrate190is sandwiched between a top cover1102and a bottom cover1104, with an analyzer electrode1110in between. The substrate190is connected to an electronics board1120by standoffs (e.g., 20 mm long M3 hex standoffs). The screws go through the mounting holes in the analyzer ring, the mass analyzer's lower PCB, and the hex standoff. Cutouts in the mass analyzer's upper PCB allow the screw heads to seat without interference. This allows the top cover of the mass analyzer to be removed for electrode alignment without necessitating the removal of the mounting hardware.

Electrical feedthroughs connect the mass analyzer boards to the electronics boards. The low voltage digital and analog supply pins are carried on two rows of 20 mm tall, 2.54 mm spacing pin header. The high voltages used for electrostatic lenses were more difficult; electrical mezzanine connectors rated for 2 kV do not exist. Instead, a properly spaced row of holes in the mass analyzer board and the electronics board are fitted with 25 mm M2 hardware after the two boards are mechanically mounted together. The copper rings around each hole serve as electrical contacts.

Using PCB as a substrate, electrodes can be fabricated and fitted to the PCB. The geometries for these electrodes and their relative spacing can be taken directly from the simulations described above. The electrodes have a symmetry through the vertical axis (the axis out of the plane of the ion flight path). Most of the simple manufacturing techniques are greatly simplified when carried out in two dimensions; the fixturing or complicated machine required to mount a component to carry out operations on more than two axes adds to the cost of the finished part.

The electrodes are cut from Type 303 stainless steel. This stainless steel has multiple beneficial properties; the bulk metal and its surface oxide are electrically conductive, unreactive, and have a low affinity for gas adsorption. It is a common material used for high vacuum work; most high vacuum components are constructed of 303 stainless steel or similar materials.

Type 303 stainless steel is one of the easiest stainless steels to machine. However, some of the features required to produce these electrodes are quite small, on the order of hundreds of micrometers, and these sorts of features are not conducive to fabrication by cutting tools. Generally, the cutting tool imparts too much force for making thin walled features. Thus, the manufacturing technique chosen for fabrication of mass spectrometer electrodes is wire electrical discharge machining (wire EDM). Alternatively, symmetric components of the mass spectrometer may be built, possibly with a change of materials, as an extrusion. The extrusion could then simply be chopped into segments, leading to a very economical method of construction.

The electrodes at different potentials are separate components, but an effort was made to simplify the manufacturing for the mass spectrometer by allowing all of the electrodes that are at the same potential to be cut as one piece from the same stock. Additionally, all of the features necessary for mounting the components were designed into the tool paths so that each electrode could be cut in a single pass.

FIG. 12is a CAD model of the mass analyzer electrode. Since the mass analyzer is at ground potential, its structural loop encircles all of the other in-plane electrodes in the system for both structural rigidity of itself and of the mass spectrometer, and for electrical shielding. Fields produced by the electrodes within the mass analyzer should be shielded from the outside, thus theoretically preventing some stray fields that might otherwise interfere with the electronics.

The mass analyzer also has a pair of delicate features at the entrance and exit of the magnetic sector. These features are the mechanical filters that limit the width of the detected ion beam, maximizing the likelihood that a detected ion is of the intended mass. The filters are slits that are tens to hundreds of μm wide, and as seen from the simulations, have a direct bearing on the sensitivity and resolution of the mass spectrometer. Generally, the slits are manufactured and installed separately in most mass spectrometers; here, they are co-fabricated with the mass analyzer, both ensuring that they are collinear with the ion optics and minimizing costs by minimizing parts count and eliminating any need for slit alignment.

FIG. 13illustrates a thin-walled adjustable flexure1300formed using wire EDM from the same piece of material (e.g., PCB material) as the substrate190(FIG. 1A). The flexure1300includes an L-shaped member1304connected to the substrate190via a hinged portion (hinge)1302. Pushing the upright portion of the L-shaped member1304with an actuator, such as a lead screw1310, causes the L-shaped member1304to rotate about the axis of the hinge1302, which in turn reduces the width of a slit1308in the ion (or electron) path. A stop1306prevents the L-shaped member1304from closing down slit1308too much. Unscrewing the lead screw1310causes the hinge1302to return to a relaxed position with the L-shaped member1304no longer closing the slit1308. This flexure can be positioned before or during operation to give tremendous control over the resolution and sensitivity of the instrument.

In another embodiment, the flexures are actuated, e.g., by motorized lead screws or by piezo actuators. This allows the mass spectrometer to automatically optimize its sensitivity to resolution on the fly, expanding the slits to increase ion current for weak signals and narrowing them for better resolution when analyzing ions of adjacent mass.

The smaller electrodes used in the ion source, mass analyzer, and detector can also cut from the same stock as the mass analyzer using wire EDM. In addition to the active faces, at least two mounting features can be cut into each electrode, corresponding to features in the mass analyzer PCB, thus minimizing the chance of angular misalignment.

5.7 Electron Beam Electrodes

The electron beam in the mass spectrometer's ion source requires electrodes for proper function as well, and these electrodes are out of the plane of the ion source electrodes. Since the electron beam runs perpendicularly to the ion beam, from bottom to top, the ion source electrodes may be fabricated using a different fabrication technique. For instance, the electron beam electrodes, the trap and the electron focusing ring, can be printed on small PCBs and mounted to the main PCBs with M2 hardware.

The electron focusing ring doubles as the physical mounting for the PR-2 flashlight bulb that provides the tungsten filament; the focusing ring allows the filament and its supports to penetrate the electronics PCB while keeping the bulb's mounting flange captive. M2 screws 25 mm in length run through the focusing ring PCB, past the flashlight bulb base and through the electronics PCB. The M2 screws are kept under tension, which fixes the flashlight bulb in place while allowing for alignment; the bulb base can be moved slightly before the mounting screws are tightened.

The trap electrode is mounted above the upper mass analyzer PCB, spaced 200 um distant by M2 washers, and through-bolted to the mass analyzer. A long M2 screw, constructed of a 30 mm length of M2 threaded rod and jam nuts, electrically connects the trap electrode to the electronics board where the trap potential is generated.

The electrodes of the miniature mass spectrometer fit to the printed circuit board substrate like standard electrical components. For example, they can be mounted by cutting notches in each electrode and brazing small stainless steel pins to the electrode body using a hydrogen flame torch and silver solder. This approach allows the electrodes to be mounted with no protrusions above the top of the electrode, so that there was no issue of aligning the mounting features of each electrode with the mass analyzer's top PCB cover. Alternatively, the upper PCB cover may include cutouts to provide clearance for mounting screw heads. The finished version of the mass spectrometer uses a combination of M2 and M1.6 hardware to affix each electrode to the PCB.

FIG. 14shows two steps from the process of assembling the mass spectrometer electrodes. Upon removal from the wire EDM (at left inFIG. 14), the cut surfaces of each electrode are covered with a thick oxide layer. Electrodes were bathed in a 30% nitric acid solution for 30 minutes, followed by two changes of anhydrous ethanol for 30 minutes at 50 degrees Celsius in an ultrasonic cleaning bath (at right inFIG. 14). This procedure removes the oxide layer, leaving bright metal beneath.

In one example, the mass analyzer includes a pair of NdFeB magnets held in alignment by a soft iron yoke as described above. A mounting face is provided on one edge of the magnet yoke, drilled and tapped for M3 hardware. This mounting face can be attached to the electronics PCB.

The co-fabricated ion pump can fit in a volume that is small enough to encompass just the unused half of the magnet face. Since ion pumps operate at high voltage, the printed circuit board is used to insulate the magnet pole faces from the ion pump electrodes. As such, the entire ion pump can fit within a 50×25×7 mm volume.

FIG. 15is a CAD model of the ion pump anode120. Typically, ion pumps are designed with bunches of stainless steel tubes bonded together to form the anode. Such a process is costly and labor-intensive; the anode for the miniature ion pump on this mass spectrometer includes a series of cells cut from stainless steel plate in one pass by wire EDM.

Pumping speed is proportional both to diameter and number of cells; increasing these values, to a point, improves the speed of the ion pump. Given the limited space available, as well as the higher than standard B field strength, more cells were added instead of increasing the diameter of the cells. Another guideline indicates that the length of each cell should be on the order of 1.5 times larger than the diameter of the cell; with a 3.5 mm plate, this is difficult to do without designing extremely small cells.

The ion pump's cathode includes a pair of titanium plate cathodes, 0.5 mm thick, with mounting tabs located such that they interleave with the four mounting tabs of the anode. The mounting holes in the ion pump electrodes mate with holes in the PCB substrate.

FIG. 16is a photograph of the complete mass spectrometer with the top cover and magnet yoke removed. As designed, the mass spectrometer can be assembled without any complicated tools or techniques. All mounting hardware can be attached with a single 1.5 mm slotted screwdriver and long nosed pliers. Alignment features on the printed circuit board in the form of outlines of each electrode ease the assembly, and a jig can be inserted into the ion flight path upon which the electrodes can be pressed before the screws are fully tightened, ensuring that the electrode faces remain parallel. Other electrodes can be spaced with 0.5 mm shim stock, as all the electrodes were designed with a 0.5 mm gap between adjacent features.

FIG. 17shows a photograph of the filament (left) illuminated from the side with a flashlight and a photograph of the entrance slit (right) to the mass analyzer. Filament alignment can be done optically; a bright flashlight can be shined towards the filament from the side of the partially-assembled mass spectrometer, and the electron focusing ring electrode moved in plane until the center of the filament is clearly visible from above. Due to the large volume ion source and large diameter electron beam, this is a relatively simple procedure as visibility through the electron beam path is good. The slits on flexures that form the mechanical filters can be adjusted by tightening or loosening the lead screws. A macro photograph of the analyzer entrance slit, illuminated from above by a Mag-Lite flashlight, is shown in the right photograph inFIG. 17.

Once the electrodes are assembled, the mass analyzer's top cover can be fitted and through-bolted with a single M2 screw. The trap electrode is then fitted above the analyzer cover and through-bolted as well. The PCB assembly is then bolted to the magnet yoke; alignment diagrams indicating the relative positions of the magnet poles are etched in the printed circuit board copper layers on the outer sides of the analyzer PCB assembly. Slightly oversized mounting holes allow the magnet to be adjusted slightly to match the diagrams on the outside, thereby ensuring alignment with the now-covered mass analyzer.

FIG. 18Ais a photograph of the assembled mass spectrometer attached to a 6″ ConFlat flange. The final assembly of the mass spectrometer involves the vacuum chamber, which may be as simple as a steel or glass cylinder. The mass spectrometer's magnet yoke was through-bolted to tapped holes in the ConFlat flange. A piece of 1.29 mm outer diameter stainless steel hypodermic tubing for an inlet and a few low voltage wires were fed through holes in the flange and epoxied in place. The inset photograph is the side of the vacuum flange opposite the mass spectrometer, showing the electrical and gas connections to the instrument. (A port for a roughing pump could be used on this flange; in this case, however, the roughing port was provided on another end of the vacuum chamber.)

FIG. 18Bis a photograph of the mass spectrometer mounted on the flange and inserted into the end of a 6″ ConFlat tee. The far face of the tee was fitted with an ion gauge (Duniway Stockroom, www.duniway.com) connected to an ion gauge controller (Varian model 843, www.varianinc.com/vacuum). The third face of the tee was used for the roughing system.

As the initial gas load from this mass spectrometer was expected to be rather high, a powerful roughing system was used. A 0.2 m^3/s turbo-molecular pump (Varian V-200) was connected to the ConFlat tee, and the turbopump's exhaust connected to a mechanical roughing pump (Welch Vacuum1402) and cooling provided by a temperature controlled recirculator (VWR Scientific Products) with distilled water as the working fluid.

The electronics that control the miniature mass spectrometer, aside from the detector, sit on a printed circuit board beneath the mass analyzer board. As with the mass analyzer, the electronics board is fabricated without a solder mask to facilitate outgassing. Physically, the electronics board is laid out such that 20 mm M3 standoffs can be used to mate it to holes in the analyzer board, and electrical feedthroughs connect the electronics board to the electrostatic elements and detector on the mass analyzer board. The electronics board includes two major sections: a power supply section (conversion circuit) and a digital controller. Multiple independent, isolated power supplies operate all of the subsections of the electronics board.

The mass spectrometer may operate at a single input supply of +12 VDC, at up to 1.1 A, although typical supply current while operating under normal conditions is 0.5 A. Multiple different supplies are generated internally via one or more dc/dc converters (conversion circuits150inFIG. 1A). The +12 V supply also serves as the main supply for the lens drivers, as detailed in a section below. The ground of this supply serves as the system ground and is also tied to the vacuum envelope.

In one example, the conversion circuit generates voltages for the various spectrometer electrodes and components, including but not limited to: the microprocessor, the digital-to-analog converters (DACs), and the analog-to-digital converters (ADCs) that are used to control the mass spectrometer; the analog stages of the detector140(FIG. 1A); the electron source and electron source electrodes (e.g., the filament102, cathode810, control electrode820, and anode830inFIGS. 8 and 9); the ion pump120(FIG. 1A); ion optics300(FIG. 3); and ion source104electrodes, such as the repeller304(FIGS. 3 and 6A). Suitable voltages include digital logic voltages (e.g., +3.3 V, +5 V) and potentials of about 100 V to about 5 kV for the ion pump120, electron source, ion optics300, and ion source104. The spectrometer may also include filters and regulators to compensate or correct ripple in the input voltage from the external power supply.

The spectrometer may also include one conversion circuit150for each component and electrode or conversion circuits150that generate voltages for groups of components and electrodes. For instance, it may include an isolated +3.3 V/1 W dc/dc converter supplies the digital logic. The digital logic includes the microprocessor and the analog input/output (I/O) modules, such as the DACs and the ADCs that are used to control the mass spectrometer. The digital side of the detector's ADC is also run from the digital logic supply. The ground side of the logic supply is tied to the system ground at a single point.

The spectrometer may also include an isolated ±5 V/1 W dc/dc converter followed by a pair of linear regulators provides a ±2.5 VDC supply for the analog stages of the detector. This supply is heavily filtered and lightly loaded, providing supply current for a pair of op-amps and the analog half of the detector ADC. The ground of this supply is tied to the system ground right at the detector electrode to reduce noise.

The spectrometer may also include an isolated +3.3 VDC/3 W dc/dc converter provides supply voltage for the filament, which draws nominally a 2.4 V/500 mA. The ground of this supply is tied to the filament bias supply, which is in turn 70 V below the ion source supply.

The spectrometer may also include an isolated +3.3 VDC supply, with its ground biased to the trap potential, provides supply voltage for the ADC that measures the mass spectrometer's trap current. The spectrometer may also include an isolated +5.0 VDC supply, with its ground biased to the ion source potential, that provides the supply voltage for the op-amp that drives the repeller electrode304(FIG. 3). It may also include an isolated 3 kV/3 W dc/dc converter provides the anode voltage for the onboard ion pump120.

6.2 Ion Optics Drivers

Five high voltage proportional dc/dc converters (conversion circuits) provide the electrostatic element potentials. A proportional dc/dc converter generates an output voltage that is linearly proportional to the converter's input voltage, and is useful when a range of output voltages is desired. The input voltage of these dc/dc converters is supplied by operational amplifiers configured such that a fraction of the output voltage of each dc/dc converter is fed back to each op-amp, stabilizing the output. The reference for each op-amp is provided by a DAC from the digital controller or from a potentiometer for potentials that can be calibrated once and may remain unchanged during operation.

These dc/dc converters (conversion circuits) supply potentials for the ion source, the ion source's electrostatic lenses, the trap, and the bias for the filament. All of these converters' outputs are referenced to system ground. Although it would have been easier to tie the outputs together appropriately (e.g., reference the trap supply to the ion source supply instead of ground), the output isolation rating of each of these dc/dc converters was not sufficient to do so.

The electrometer connected to the Faraday cup electrode is a sensitive transconductance amplifier, as a National Semiconductor LMP7721 operational amplifier in transconductance configuration with a gain of 5e10, connected to an analog-to-digital converter. In parallel with the feedback path is a 5 pF silver mica capacitor; the capacitor decreases the amplifier's gain at high frequency, thereby cutting down on the high frequency noise present at the amplifier's output.

Due to the electrometer's high gain, leakage currents can cause drift in the electrometer output. To help reduce this, a guard ring surrounds the junction connecting electrometer's input pin, one end of the feedback resistor and capacitor, and the Faraday cup electrode. This guard ring is driven by a second operational amplifier, such as a National Semiconductor LMP7715, in unity gain voltage mode whose input is derived from the non-inverting and nominally grounded (and slightly offset due to bias currents) input of the electrometer. The output of the transconductance amplifier is digitized directly by an ADC (e.g., a Texas Instruments ADS1281 24-bit ADC).

The entire electrometer circuit is mounted on the analyzer PCB inside a pocket cut into the mass analyzer electrode. The electrode, in conjunction with copper on the two PCBs, serve to encase the electrometer inside a Faraday cage. The close proximity of the electrometer to the Faraday cup detector electrode reduces the opportunity for noise to disrupt the signal.

The printed circuit boards in the vacuum chamber were expected to carry a fairly large gas load. As such, a network of distributed resistors was added to the printed circuit boards to ensure that the board temperatures could be raised far enough to help remove the gases absorbed and adsorbed by the PCB. Multiple 1 W resistors, operated by the main +12 VDC supply, are placed in strategic locations and gated by a P-channel FET as an on/off or PWM heating control.

6.5 Digital Controller

FIG. 19is a block diagram of the mass spectrometer's digital controller1900, which is built around a processor1902(e.g., a 32-bit ARM Cortex-M3 microprocessor manufactured by STMicroelectronics (STM32F103CBT6)). The processor1902is powered by power supplies (conversion circuits)150and coupled to a radio-frequency (RF) communication module1920, which acts a wireless communications interface for relaying data and instructions between the inside and outside of the vacuum chamber. The controller1900also includes DACs1904a-1904c(collectively, DACs1904), ADCs1906a-1906c(collectively, ADCs1906), and field-effect transistors (FETs)1908a-1908c(collectively, FETs1908) coupled to the processor1902via a common serial peripheral interface (SPI) bus1910on the microcontroller1900. The entire controller1900may be contained within the vacuum cavity defined by the mass spectrometer's vacuum cavity. For instance, the controller1900may be mounted or coupled to the electronics board1120shown inFIG. 11.

In one exemplary controller1900, there are three DACs1904a-1904c(e.g., AD5662 DACs) used to set the potentials on the ion source supply and the two electrostatic lenses. There are two ADCs1906aand1906b(e.g., AD7680 ADCs) used to measure the filament drive current and the trap current. The two ADCs1906aand1906bare both operating on supplies biased at high voltage; the SPI bus for these devices is isolated from the logic-level bus by opto-isolators (e.g., Avago Technologies ACSL-6410 bidirectional (3/1 channel) opto-isolators). Another ADC1906cis coupled to an electrometer.

The DACs1904and ADCs1906are connected to the microprocessor's SPI bus1910. Each DAC1904and ADC1906has its own dedicated microprocessor GPIO pin for addressing. Additionally, several GPIO lines run to the electrometer ADC for other functions (e.g., data ready, reset). A port expander/LED driver1912(e.g., a Maxim Integrated Products MAX6696 port expander/LED driver) is also connected to the SPI bus1910and three RGB LEDs1914, used for user feedback.

A pin connected to a hardware timer on the microprocessor1902is used as the gate drive for a P-channel FET1908aconnected to the filament. The filament is driven in a pulse-width modulated manner for maximum efficiency. Switching frequency is 100 kHz, but can be changed during operation if interference is detected.

Other pins on the processor are used to control other peripherals. Several of the power supplies, including the filament and most of the high voltage supplies, and the degas heater, are gated by large P-channel FETs (e.g., FETs1908band1908c). The FETs1908are driven by microprocessor pins, such that the filament and high-voltage supplies can be shut down to save power when the mass spectrometer is not being used.

A pair of pins is used to control and monitor the ion pump. One pin enables the ion pump so that the controller can be run at atmospheric pressure without the ion pump arcing. The other pin is used, as an analog input connected to the microprocessor's onboard 12-bit ADC, to monitor the terminal voltage of the ion pump supply.

Two pins connected to the hardware USART transceiver in the microprocessor are the mass spectrometer's means of communication with the outside world. These pins pass through the wall of the vacuum chamber (although the data could be passed optically if the vacuum housing were made of glass).

In this example, the three serial wire programming (SWP) pins specific to the Cortex-M3 were also passed through the vacuum housing, so that the microprocessor's code could be reconfigured without requiring venting the vacuum chamber.

6.6 Control Software

In one example, the control software for the mass spectrometer is written in the computer language C and compiled for the Cortex-M3 core using the IAR Systems Embedded Workbench IDE and compiler. The main execution loop is a finite-state machine that controls the basic operations required to produce mass spectra. During each loop cycle, the mass spectrometer reads all of the available data indicating the states of the external variables and then executes code that depends on the state of the instrument. One of the LEDs is tasked with blinking a color depending on the state of the machine. The blinking speed is controlled by the main execution loop, providing visual feedback that the code has not locked up. The following sections describe the states in more detail.

At boot time, the mass spectrometer checks the state of the state of all of the peripherals attached to the buses. Most of the peripherals, the ADCs and the various power supplies, can be checked by interpreting the data they provide. Failure of any of the self checks causes the mass spectrometer to go into fault mode.

In standby mode, the microprocessor shuts down all of the peripherals except, optionally, the ion pump and degas heaters. In this minimal power consumption mode, the system may draw less than 1 W.

In idle mode, the microprocessor brings the high voltage supplies and the filament supply online. The filament is operated at reduced voltage to increase its lifespan. In this mode, the microprocessor can ensure that the high-voltage supplies are functioning properly and that the filament has not burned out. During transitions to idle mode, the filament is brought to temperature slowly to reduce thermal shock. The filament warm-up time may be about 0.5 s.

In sweep mode, the microprocessor is actively driving the electrodes and measuring the ion currents. The ion source supply is brought to the minimum voltage achievable by the hardware, approximately 150 V, and swept through to about 800 V at about 20 V/s. The electrostatic lens voltages are also constantly changed to properly focus the ion beam at each ion source potential.

Electrometer current is sent out the serial port to a laptop or other computing device connected to the mass spectrometer. Data may be collected with a simple terminal program; when running mass scans, the data are outputted as columns of text which may be captured on the laptop and opened as a data file (e.g., a comma-separated-variable (.CSV) file) in a data analysis program.

The mass spectrometer is controlled by a serial terminal interface that is accessed via a computer. The terminal program on the mass spectrometer allows commands to be sent and interpreted, mostly for debugging purposes, but also for controlling the state of the machine. The command “mode” with an argument specifying a new state, allows the user to switch between the modes of operation as detailed above. The commands “filament,” “repeller,” “ionbox,” “lens1,” and “lens2,” with an argument such as a floating point number or on/off (e.g., “filament off” or “ionbox 500.0”), allow the user to directly control the various electrodes in the vacuum chamber. Other commands, “degas,” “ionpump” allow the user to turn these peripherals on and off remotely, as the microprocessor can't know when these features should be enabled or not.

The mass spectrometer was subjected to extensive testing of both subassemblies as well as the complete system.

7.1 Power and Control Systems

All of the power supplies were powered on and tested for nominal voltage. Particular attention was paid to the ±2.5V analog electrometer supply, as the noise figure of this supply directly impacts the electrometer noise floor by the CMRR of the electrometer op-amp.

The control software was tested by verifying that the mass spectrometer could run in all modes for several days without crashing. Then the various modes of operation were examined for power consumption. TABLE 1 (below) shows power consumption of each operating mode at 12 VDC. Note that in every mode of operation the instrument draws less power than any other existing miniature mass spectrometer. The ion pump draws 3 W, although this amount of power was not quite enough to sustain the pump.

Operation of the electron beam is the first diagnostic of a mass spectrometer. Operation is generally characterized by the trap current. The trap current is the fraction of the electron current that is emitted from the filament, passes entirely through the ionization region, and collected at the trap electrode. The trap current should be directly proportional to filament brightness, which is itself a strongly nonlinear function of filament power. Above a certain power level, trap current begins to rise rapidly while filament life decreases.

Filament intensity as a function of filament voltage V is proportional to V^(3.4) while filament lifetime is proportional V^−16, giving a strong incentive not to overdrive the filament. The filament used in this mass spectrometer is that of a standard PR-2 tungsten flashlight bulb. This type of bulb is rated for a 15 hour lifespan at 2.4 V and 0.5 A. Operating at reduced voltage will increase its lifespan. For example, at 2.3 V the filament will retain 86% of its brightness while doubling its lifespan to 30 hours.

The trap current was measured at two different filament voltages, summarized in Table 2.

TABLE 2Trap Current as a Function of Filament VoltageFilament Voltage (V)Trap Current (μA)2.0102.2192.425

The trap current was somewhat rather variable during different experiments, dropping to 25 μA to during some tests even at an operating voltage of 2.4 V, possibly due to the fact that the mass spectrometer was frequently disassembled and reassembled, changing the exact orientation of the filament with respect to the ionization region.

FIG. 20is a diagram of a degas heater2000formed of a network of resistive heaters2002connected to a mass spectrometer substrate board2004, such as substrate190(FIG. 1A) or the electronics board1120(FIG. 11). The heater2000can be used to remove at least some the absorbed and adsorbed gases on these boards by raising the boards' temperature. Turning the heater2000involves running a current through the resistive heaters2002, which in turn causes the resistive heaters2002and the board2004to heat up. As the board2004heats up, it release absorbed and adsorb gas, which is pumped out of the vacuum cavity by the ion pump120(FIG. 1A), a separate turbopump attached to the vacuum chamber, or both. When the heater is working properly, it should be possible to turn the heater on under vacuum, see a rise in chamber pressure as gas is driven off, then see the pressure fall to a level below the initial level when the heater is turned off again.

FIG. 21is a plot of pressure versus time for an experiment run to test the degas heater. The mass spectrometer was installed in a vacuum housing and pumped down. When the chamber pressure had stabilized, the heater was switched on, then off again approximately three hours later. Note the relatively slow initial decrease in chamber pressure followed by the rise in chamber pressure when the heater was switched on. The gas is driven off and the chamber pressure begins to fall, at which point the heater is then switched off. At this point the power electronics are activated, which produce their own heat and drive gases off the electronics board. In the future these two cycles can be run concurrently, however, they currently produce too much heat to operate simultaneously without damage.

FIG. 22shows infrared images of a the analyzer board at different time intervals after the heaters are turned on. The mass analyzer board was placed beneath a thermal imaging camera (e.g., a FLIR ThermoVision A40 camera), and the thermal transient behavior observed over ten minutes (600 s). While the temperature rise is modest in absolute value in this series of frames, this experiment was run in air. In vacuum there is no convection to cool the surfaces and the temperature rise should be substantially faster, though the heat will flow roughly in the pattern observed here.

FIG. 23shows the relative calibrations of each lens driver. Despite attempts to ensure that the feedback control loop wrapped around each of the lens drivers was accurate, there was some variation between lens commands and lens voltages. A calibration was thus run on the ion source potential and the two electrostatic lenses. This calibration curve was linearized and programmed into the mass spectrometer controller's code to ensure that the correct voltages are being output to the lenses. While the lens drivers were similar, as they should have been given that they were constructed using identical hardware, they varied by a few volts. This may not seem very important, but the potential energy surface described above indicates how carefully some of these voltages should be aligned; a lens tuned incorrectly can severely limit or block the ion beam, eliminating the signal.

The miniature co-fabricated ion pump was tested on its own after the system had been pumped down to 2.6e-6 Pa [2.0e-8 torr]. The ion pump was started at 2.6e-4 Pa [2.0e-6 torr] and operated in conjunction with the vacuum chamber's turbopump until the pressure reached 2.6e-6 Pa, at which point a valve inserted between the turbopump and the chamber was closed.

FIGS. 24 and 25are plots of the vacuum chamber pressure, pump voltage, and ion current during the commissioning process. At first, the miniature ion pump is heated to drive off the adsorbed gases and is run in conjunction with a second high vacuum pump until the ion pump is ready carry the gas load. This commissioning process takes approximately 15 hours without using the mass spectrometer's onboard heater.

FIG. 26includes photographs of the ion pump disassembled afterward commissioning test. The titanium cathode plates were pitted in the center of each pump cell, and the anode was plated with sputtered titanium.

For an inventive mass spectrometer, spectra may appear as ion beam current as a function of ion source potential. While the microprocessor may be programmed to output ion current versus mass to charge ratio, for this example the mapping between ion source potential and m/z is done in post-processing of the data. Alternatively, an inventive mass spectrometer may measure high-voltage biased parameters (e.g., filament current, trap current).

Large numbers of mass sweep tests were run on the miniature mass spectrometer. Between tests, many optimizations were made based on the resultant data. Optimizations were generally minor and included adjusting the variable-geometry mass analyzer slits, electrometer hardware (e.g., feedback resistor, capacitor), and modifying the software to optimize filament power, electrostatic lens potentials and ion source voltage sweep rate and range.

FIG. 27shows a mass spectrum collected from an exemplary miniature mass spectrometer. The large centered peak is likely nitrogen while the peak on the right side of the graph is water. Oxygen likely appears, as a peak protruding from the left shoulder of the nitrogen peak; this exemplary mass spectrometer did not have sufficient resolution to separate masses that were distant by 4 AMU. This spectrum shows that the ion beam has been chopped using the digital controller to modulate one of the electrodes.

FIG. 28is a mass spectrum captured by the another version of the mass spectrometer, with prominent peaks highlighted. The data have been corrected for the inverse relationship between acceleration potential and mass/charge ratio. Note the peak at m/z of 29, this is likely an isotope of nitrogen, 15N14N, which is present in air with a 0.36% abundance relative to 14N14N.

One interesting feature observed is that the mass spectrometer functions, albeit with a lower signal to noise ratio, even if the electrostatic lenses are disabled (e.g., the lens is programmed not to alter the beam). This result was used to characterize the effect of the electrostatic lenses.

FIG. 29is a pair of spectra, one run with the lenses off, and another run with the lenses on. The lenses give nearly a factor of ten increase in signal strength without increasing the noise floor. This is extremely valuable in mass spectrometry, and shows how attention to capturing and analyzing a larger fraction of the ions generated can produce a stronger signal. The lenses were tuned initially by hand; the ion source was set to a potential with a known ion species, and the lenses were then tuned for maximum signal. Several ions were tuned and the resultant curve fitted with a linear interpolation.

FIG. 30is a mass spectrum of air indicating the effectiveness of the variable geometry slits. Although several other factors have changed, including the overall gain of the system, the salient features of this comparison are visible at the bases of the peaks. The peak for m/z 27 and 26 are both visible in the red curve, with narrower slits, while they are completely invisible in the blue curve, made with wider slits.

FIG. 31is a plot that shows that the illustrative mass spectrometer can detect a new species entered into the inlet.FIG. 31is a test of the mass spectrometer's detection capabilities. A sample of nitrous oxide (N2O) was injected into the inlet and the mass spectrum sweep run. The control run, in blue, shows the standard spectrum; water, nitrogen, oxygen. The run containing nitrous oxide shows several new peaks. N2O shows up quite clearly at m/z 44, and another species, the fragmentary ion NO shows up between oxygen and nitrogen, at m/z 30.

FIG. 32is a series of spectra generated using the grid as a modulation source. The grid (control electrode) of the ion source was used to remove the background drift, or 1/f noise, of the electrometer. The blue curve is the baseline curve, generated when the grid is biased such that the ion beam is cut off. The red curve is the signal curve, generated with the ion beam enabled. The green curve is a subtraction of the two, the signal with the baseline offset and drift removed.

These plots show inventive mass spectrometers work with resolution that is sufficient for many tasks, including, but not limited to use as a medical, environmental, or industrial tool. In at least one case, the experimental results indicate that the mass spectrometer is sensitive enough to detect species comprising less than 0.5% of the inlet sample gas, and with a mass resolution of 1 AMU. The noise floor is extremely low, below 10 fA, as indicated on the graph inFIG. 28. Deconvolution with an appropriate function may yield even narrower spectra.

The above-described embodiments can be implemented in any of numerous ways. For example, the embodiments may be implemented using hardware, software or a combination thereof. When implemented in software, the software code can be executed on any suitable processor or collection of processors, whether provided in a single computer or distributed among multiple computers.