Mass spectrometer for entrained particles, and method for measuring masses of the particles

A particle mass spectrometer includes a supersonic flow expansion nozzle, and a source of a gas having particles entrained therein. The source is in gas-flow communication with an inlet of the expansion nozzle. The particle mass spectrometer further includes a vacuum chamber in gas-flow communication with an outlet of the expansion nozzle, wherein the vacuum chamber has a sufficient vacuum that a gas flow through the expansion nozzle is supersonic, and a microphone having an active element and an output signal responsive to a movement of the active element. The active element is disposed within the vacuum chamber and is positioned so that particles that flow from the outlet of the expansion nozzle impact upon the active element. The output signal of the active element of the microphone is a measure of the masses of the entrained particles.

This invention relates to the measurement of the masses of small particles and, more particularly, to a mass spectrometer that entrains the particles in a supersonic gas flow and impacts the particles on an active element of a microphone.

BACKGROUND OF THE INVENTION

It is important to identify precisely the nature of certain types of small particles. For example, various biologically active species such as different types of viruses have widely varying effects on living organisms. Some may have little effect, and others may be deadly. Persons who are potentially exposed to viruses need an accurate approach to rapidly and accurately identify the nature of such viruses, so that preventative measures or countermeasures may be employed as necessary. In other cases no action need be taken with viruses that are not potentially injurious.

A number of chemical and physical techniques are useful for identifying small particles such as viruses. Chemically active small particles may be analyzed by observing their chemical reactivity. In one technique, the particles are captured in a filter and then chemically tested by determining their reactivity with other species or with particular chemicals. Such chemical testing may be quite slow in providing an identification, however. Small particles may also be analyzed according to physical properties, such as by X-ray diffraction to determine their internal structure. This approach also requires a considerable time and also cannot be performed readily on small numbers of very small particles. Small particles may also be captured and visually analyzed to estimate their masses, using a powerful microscope such as a scanning electron microscope. This technique does not yield information in real time, and it requires the availability of a scanning electron microscope.

In another approach which is used to analyze larger particles and has the potential for real-time analysis, a light beam is passed through an entrained flow of the particles. The scattered light beam is used to obtain a distribution of sizes of the particles, which may be approximately related to a distribution of particle masses. However, light scattering is not useful to detect particles which are much smaller than the wavelength of light, as is the case for viral particles.

There is therefore a need for an approach to rapidly and accurately analyzing for the presence and nature of small particles in the atmosphere and elsewhere. The present invention fulfills this need, and further provides related advantages.

SUMMARY OF THE INVENTION

The present invention provides an apparatus and method for measuring the masses of small particles that are entrained in a gas flow. The approach allows the rapid measurement of the masses of very small particles, such as viruses. There is substantially no delay between the time when the particles are encountered and the time when the mass information is available. For viruses, the mass information correlates well with the nature of the virus, so that the nature of the virus may be determined essentially instantaneously.

A knowledge of the mass of a small particle may be useful in identifying the nature of the particle. Viruses typically have characteristic masses. That is, the mass of each particle of a particular type of virus is substantially the same, as the virus does not grow or fragment during its life, unlike a bacterium where the mass changes over time and there may be a range of masses for each type of bacterium, and unlike other types of particles which do not have discrete masses. The characteristic virus masses are smaller than the masses of almost all other types of particles that are routinely encountered.

In accordance with the invention, a particle mass spectrometer comprises a supersonic flow expansion nozzle having an inlet and an outlet, and a source of a gas having particles entrained therein. The expansion nozzle is preferably a converging-diverging expansion nozzle. The particles may be viruses with a mass of from about 106–1010Daltons. The source is in gas-flow communication with the inlet of the expansion nozzle.

The particle mass spectrometer further includes a vacuum chamber in gas-flow communication with the outlet of the expansion nozzle. The vacuum chamber has a sufficiently low pressure, typically in the range of from about 10−3Torr to about 10−1Torr, that a gas flow through the expansion nozzle is supersonic. Pressures below 10−3Torr are acceptable, but it is preferred that the pressure not be greater than 10−1Torr so that the mean free path of the gas molecules is long.

A microphone has an active element and an output signal responsive to a movement of the active element. The active element typically moves in response to the impact by the particles. The active element of the microphone is preferably either a piece of a piezoelectric material or a flexible diaphragm. The active element is disposed within the vacuum chamber and is positioned so that particles that flow from the outlet of the expansion nozzle impact upon the active element. The output signal of the microphone is an indicator of the masses of the individual particles impacting upon the active element.

In some cases, the flow of particles to the active element of the microphone may be so great that the output signals associated with the individual particles overlap and cannot be readily analyzed. The particles may be angularly spread out to aid in the analysis, as by electrostatically deflecting the particles. A set of electrostatic deflection plates is disposed so that the particles that flow from the outlet of the expansion nozzle toward the microphone must pass between the deflection plates and are deflected. The electrostatically deflected particles impact upon an array of microphones. Each microphone of the array has an active element and an output signal responsive to a movement of the active element. Each active element is disposed within the vacuum chamber and positioned so that particles that flow from the outlet of the expansion nozzle impact upon the active element. The electrostatically deflected particles are laterally spread out so as to impact different ones of the active elements of the microphones, thereby reducing the particle impacts for each individual microphone and reducing the overlap in signals to make analysis easier.

A method for determining the masses of particles comprises the steps of entraining the particles in a flow of gas, passing the flow of gas with the particles entrained therein through a supersonic flow expansion nozzle into a vacuum, and impacting the flow of gas with the particles entrained therein on an active element of a microphone that is maintained within the vacuum, after the flow of gas with the particles entrained therein leaves the expansion nozzle. The output signal of the microphone is associated with the masses of the particles. Other compatible features discussed herein, such as the electrostatic deflection plates and the array of microphones, may be used in relation to the method.

The present approach essentially instantaneously determines the masses of particles entrained in a gas flow and supplied to the apparatus. In some cases, such as for some viruses, the masses are directly associated with the nature of the virus particles, which in turn are associated with the chemical and/or biological properties and effects of the virus particles. The present approach thus provides a technique for instantaneously determining the type, and the chemical and/or biological nature of such particles.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2illustrate two embodiments of a particle mass spectrometer20. The two embodiments are the same, except as will be discussed. In each case the particle mass spectrometer20includes a supersonic flow expansion nozzle22having an inlet24and an outlet26. The expansion nozzle22is preferably a converging-diverging expansion nozzle, having a narrow throat28, a converging region30of decreasing cross-sectional area from the inlet24to the throat28, and a diverging region32of increasing cross-sectional area from the throat28to the outlet26.

There is a source34of a gas36having particles38entrained therein that is in gas-flow communication with the inlet24of the expansion nozzle22. The pictured source34includes a sample inlet40and an optional dilutent inlet42. The particles38in a sample gas flow41are drawn into the sample inlet40to serve as a sample. The sample gas flow is optionally mixed with a dilutent gas flow43that is first filtered through a filter44and drawn into the dilutent inlet42, to create the source34of the gas36having the particles38therein. The gas36is typically air, but it may be any operable gas. The gas flow from the sample inlet40is preferably diluted with the dilutent gas flow43from the dilutent inlet42to reduce the number density of the particles38in the gas source34and thence reduce the possible overlaps in the measured output signal to be discussed subsequently. Otherwise, the high mass flow of the particles and the resulting signal overlap may obscure important information. The relative flow rates of the sample and the dilutent are used to calculate back to the density of particles in the sample.

The particles38may be of any type operable with the present approach, and may have a wide range of particle masses. Typically, there is a wide range of particle masses in any sampling. A preferred application of the present approach is to analyze samples that may have virus particles38therein. The mass of individual virus particles is typically from about 106to about 1010Daltons, depending upon the type of virus, which is quite small compared to the mass of a bacterium, a dust particle, or a soot particle, but large compared to the mass of an air molecule or many types of aromatic organic molecules. The mass of a virus particle is a good indicator of the type of virus, since the particles of any one type of virus have essentially the same mass because the virus particles do not grow or fragment over time.

A vacuum chamber45is in gas-flow communication with the outlet26of the expansion nozzle22. The vacuum chamber45is pumped by a vacuum pump46that exhausts through a filter48. The vacuum chamber45and the pump46are sized so that, considering the provided gas flow rate from the source34(i.e., the sum of the sample gas flow41and the dilutent gas flow43), the vacuum chamber has a sufficient vacuum that a gas flow through the expansion nozzle22is supersonic. In a typical case, the vacuum chamber45has a pressure of from about 10−3Torr to about 10−1Torr, most preferably about 102Torr. A substantially higher pressure in the vacuum chamber45will not produce a supersonic gas flow through the expansion nozzle22. A vacuum in communication with the outlet26of the expansion nozzle22must be used to provide the driving force for the gas flow through the expansion nozzle22. A positive pressure applied to the inlet24, without a sufficiently high vacuum communicating with the outlet26, may not be used. The vacuum on the outlet side is necessary to reduce the mean free path for molecular collisions so that a shock wave does not build up that prevents the supersonic flow.

A microphone50, also shown inFIG. 3in more detail, has an active element52and an output signal54responsive to a movement of the active element52. When particles38impact the active element52, the pressure associated with each individual particle impact produces the electrical (or optical) output signal54from the active element52, as the active element deflects. The output signal54is amplified by an amplifier56(FIG. 3) in an analysis electronics58to produce an output VOUT. The active element52may be of any operable type, such as a flexible diaphragm in a capacitance microphone as illustrated inFIG. 3, or a piezoelectric material. The active element52of the microphone50is disposed within the vacuum chamber45and is positioned so that particles38that flow from the outlet26of the expansion nozzle22impact upon the active element52.

An important feature of the present approach is the achieving of a supersonic velocity of the gas36and particles38that flow through the supersonic flow expansion nozzle22. When the gas36moves at a supersonic velocity, the particles38move at a supersonic velocity. The particles38have comparable velocities (ideally exactly the same velocities), regardless of their masses. The momentum and pressure of the impact of each particle38on the active element52is therefore largely a function only of the mass of the particle38, and is generally proportional to the mass of the particle. Thus, the pulsed output signal54for each particle impact is largely a function of the mass of the impacting particle38.

The embodiments ofFIGS. 1 and 2differ in that the particle mass spectrometer20ofFIG. 1has a single microphone50, and the particle mass spectrometer20ofFIG. 2has an array51of a plurality of microphones50. The particle mass spectrometer ofFIG. 2also includes a set of electrostatic deflectors, here electrostatic deflection plates57, disposed such that the particles38that flow from the outlet26of the expansion nozzle22toward the microphone50must pass between the deflection plates57. A voltage VD, typically on the order of 500–1000 volts, is applied between the electrostatic deflection plates57. The particles38are charged as they pass through the expansion nozzle22and are deflected slightly by the electrostatic deflection plates57according to their respective masses and charges. This causes particles38, even particles of identical masses, to be deflected by slightly different amounts to impact different ones of the microphones50of the array. The angular and spatial separation is not the spectrometer effect (and therefore the deflection plates57are not necessary for practicing the present approach), but instead simply reduces the flux of particles38to any one of the microphones50to reduce the signal rate in the output signal54. The dilution of the sample by the dilutent gas flow43has largely the same effect. This reduction in signal rates ensures that the effect of each individual particle38is clearly distinguishable in the output signal54, and avoids superposition of the signals of the individual particles38. Deflection plates57may also be used with the embodiment ofFIG. 1.

FIG. 4illustrates the output signal VOUTof two different particles striking the active element52. A smaller particle has an output signal59with a smaller integrated area than a larger particle with an output signal60.

In operation, a large number of particles may be measured and the results combined in a frequency-mass distribution as shown inFIG. 5. For the particles of larger masses (smaller reciprocal masses), there is typically a continuous distribution62of frequencies of occurrence above a background level64, because larger particles normally do not correspond to discrete masses. That is, a larger particle such as a dust particle or a soot particle typically does not have a specific mass—there are usually dust and soot particles of a variety of masses. For the particles of smaller masses, such as virus particles, there are typically discrete peaks66in the distribution because the only particles in that mass range are the specific types of particles such as viruses in which the mass of each virus particle is the same as the mass of each other virus particle of the same type. These discrete peaks66may therefore often be associated with different specific types of particles38, such as specific types of virus particles.

To assist in identifying the nature of the small particles, the analysis electronics58may further include an optional data processor70that receives the amplified output signal (VOUT)72as shown, or the unamplified output signal54, and analyzes the output signal72or54. The analysis performed in the data processor70may be of any relevant type, but typically associates the output signal with the known masses of particular particle types. In the case of interest where the small particles are viruses, the data processor70may contain a lookup table of virus mass as a function of the virus type, and the peaks66in the output signal may be associated with these virus types.

The data processor70may also serve to compensate for any velocity nonuniformities in the particles38that pass through the expansion nozzle22. That is, if one type of small particle always moves at a greater velocity than another type of small particle, so that the momentum impact on the active element52is not proportional solely to the mass of the particles, the lookup table may take this effect into account by being based on a parameter that is not purely mass, but instead takes into account the velocity dependence as well. Such an approach is implemented by performing calibration studies wherein measurements are serially performed on the source gas34that controllably includes only a single type of a known small particle at a time.

FIG. 6depicts a method for determining the masses of particles. The method involves entraining the particles in a flow of gas, step80, and passing the flow of gas36with the particles38entrained therein through the supersonic-flow expansion nozzle22into a vacuum, step82. The flow of gas36with the particles38entrained therein is impacted on the active element52of the microphone50(either a single microphone50or an array51of microphones50) that is maintained within the vacuum, after the flow of gas36with the particles38entrained therein leaves the expansion nozzle, step84. The output signal54of the microphone50is associated with the masses of the particles38, step86. The association step86may include forming relations such as shown inFIGS. 4 and 5, including but not limited to performing calibration studies to associate particular masses of the particles with specific types of viruses. Other operable features of the present approach, as discussed herein, may be utilized in relation to the method.