Patent Publication Number: US-8970840-B2

Title: Method and apparatus for aerosol analysis using optical spectroscopy

Description:
GOVERNMENT INTEREST 
     The invention described herein may be manufactured, used, and licensed by or for the United States Government. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to methods and apparatus for the collection and analysis of particles of a flow of aerosol. 
     BACKGROUND OF THE INVENTION 
     An aerosol is a suspension of fine solid particles or liquid droplets in a gas. There is a need to collect and analyze the particles of an aerosol especially where the particles of aerosol are unidentified or include pollutants that must be monitored or limited. There have been numerous attempts to provide instruments far the collection and analysis of particles of an aerosol. However, each of these approaches has had certain drawbacks. As such, there is a need for improved real-time or semi-continuous methods and apparatus for the collection and analysis of particles of an aerosol. 
     SUMMARY OF THE INVENTION 
     The present invention provides methods and apparatus for the collection and analysis of particles of an aerosol. In a first embodiment of such a method, a housing is provided having an inlet area and an outlet area. The housing further has a collection and analysis area interconnecting the inlet area with the outlet area. A flow path for an aerosol is defined from the inlet area, through the outlet and collection area, and out the outlet area. A collection electrode is provided having a tip disposed in the flow path in the collection and analysis area. A flow of aerosol is introduced along the flow path and particles of the aerosol are collected on the tip of the collection electrode. A second electrode is provided spaced from the collection electrode tip so as to define a spark gap in the collection and analysis area. A high voltage pulse generator is provided connected to the collection electrode and second electrode for generation of a spark across the spark gap. A high voltage spark is applied across the spark gap so as to ablate the particles collected on the tip of the collection electrode thereby creating atomic emissions. An optical window is provided in the housing, with the window being aligned with the spark gap. Atomic emissions are collected for analysis. 
     In some versions of this method, the particles of the aerosol passing along the flow path are charged and during the collection step the collection electrode is held at a bias voltage relative to the charged particles such that the charged particles collect on the tip of the collection electrode. During this collection step, a corona may be produced by the second electrode such that the second electrode creates a cloud of ions and the particles of the aerosol are charged by the ions from the second electrode. Alternatively, during the collection step, the second electrode may be held at a voltage sufficient to create an electrical field so as to urge the charged aerosol particles towards the tip of the collection electrode. 
     In some versions, a collection electrode assembly is provided, with the collection electrode being a central electrode of the collection electrode assembly. The assembly further includes at least one concentric electrode surrounding the central electrode and a dielectric layer separating the central electrode from the concentric electrode. The concentric electrode is held at a potential greater than the central electrode when the central collection electrode is held at the bias voltage relative to the charged particles. 
     In some embodiments, the collection electrode and second electrode are each elongated and are coaxial with one another. The housing may also be elongated, with the electrodes extending coaxially along the housing and the window being disposed in a side wall of the housing. 
     In some embodiments, the inlet area of the housing has a first surface and a second surface spaced therefrom. The first and second surfaces are each formed of an electrically conductive material. An aerosol inlet is defined adjacent to the first surface and a sheath flow inlet is defined between the aerosol inlet and the second surface of the inlet area. The flow path for the aerosol is defined from the aerosol inlet of the inlet area, through the collection and analysis area, and out the outlet area. A sheath flow (free of any particles) path is defined from the sheath inlet of the inlet area, through the collection and analysis area, and out the outlet area. During the collection step, the first surface of the inlet area is grounded relative to the charged particles, a sheath flow is introduced along the sheath flow path, and the second surface is held at a classification voltage chosen such that a portion of the charged particles are deflected toward the second surface and towards the collection electrode tip. In some versions, the housing is an elongated tube defining at least the inlet area and the collection and analysis area, with the first surface of the inlet area being defined by the inner surface of the tube. A central member is provided which extends axially along the inlet area of the housing to a downstream end, with the central member having an outer surface defining the second surface of the inlet area. A coaxial flow guide may be provided in the inlet area between the central member and the elongated tube, with the coaxial flow guide having a downstream end disposed upstream of the downstream end of the central member. The aerosol flow path extends between the flow guide and the elongated tube and the sheath flow path extends between the flow guide and the central member. The second electrode may have a tip that is adjacent the downstream end of the central member. 
     In a further embodiment, the housing has a first surface and an opposed second surface, with the first surface formed of an electrically conductive material and the second surface formed of a dielectric material. An aerosol inlet is defined adjacent the first surface in the inlet area, with the aerosol flow path defined from the aerosol inlet of the inlet area, through the collection and analysis area, and out the outlet area. A sheath flow inlet is defined between the aerosol inlet and the second surface, with a sheath flow path being defined from the sheath flow inlet, through the collection and analysis area, and out through the outlet area. The collection electrode comprises a plurality of collection electrodes having tips disposed in or extending from the second surface, with these tips being spaced apart along the second surface of the collection and analysis area. The second electrode comprises a plurality of second electrodes disposed in or extending from the first surface, with these second electrodes being spaced apart along the first surface and registered with the collection electrodes so as to define a plurality of spark gaps. During the collection step, the plurality of collection electrodes are each held at a bias voltage relative to the charged particles, a sheath flow is introduced along the sheath flow path, and the first surface is held at a classification voltage such that a portion of the charged particles are deflected toward the second surface for collection on the tips of the plurality of spaced apart collection electrodes, wherein each electrode tip collects particles of a different charge. During the spark applying step, a high voltage spark is applied across each of the plurality of spark gaps. In some versions, the high voltage spark is applied across each spark gap at a different time. A flow guide may be disposed in the inlet area between the aerosol flow path and the sheath flow path. 
     In yet a further embodiment, a charger is provided for charging particles of the flow of aerosol and a size classification unit is provided to sort particles of the flow of aerosol into a plurality of sorted aerosol flows. Each of the sorted flows has particles within a predetermined size range. In this embodiment, the collection electrode and the second electrode are part of a plurality of collection electrodes and second electrodes each separated so as to define a spark gap, with each collection electrode tip being disposed in a flow path for one of the sorted flows. The size classification unit may comprise a plurality of cyclone separators. 
     In some embodiments, the method further comprises providing a broadband optical spectrometer in optical communication with the optical window and analyzing the atomic emissions in the spectrometer. A fiber optic cable may be provided with one end of the fiber optic cable forming or being in optical communication with the optical window and the opposite end of the fiber optical cable being in optical communication with the spectrometer. 
     In another embodiment, the method further comprises providing a laser light source and using the laser light source to ablate particles on the collection electrode tip prior to applying the high voltage spark. 
     A spark emission spectroscopy device in accordance with an embodiment of the present invention includes a housing having an inlet area and an outlet area. The housing further has a collection and an analysis area interconnecting the inlet area with the outlet area. A flow path for an aerosol is defined from the inlet area, through the collection and analysis area, and out the outlet area. A collection electrode has a tip disposed in the collection and analysis area and in the flow path for collection of particles of an aerosol thereon. A second electrode is spaced from the collection electrode tip so as to define a spark gap in the collection and analysis area. An optical window is defined in the housing, with the window being aligned with the spark gap. A high voltage pulse generator is connected to the collection electrode and second electrode for generation of a spark across the spark gap. The collection electrode is held at a bias voltage during the collection step, whereby charged particles of a flow of aerosol passing along the flow path are collected on the collection electrode tip. During a spark ablation step, a high voltage pulse is applied between the second electrode and collection electrode tip so as to create a spark across the spark gap, thereby ablating aerosol particles collected on the tip of the collection electrode. 
     According to a further embodiment of the present invention, a method is provided for collecting and analyzing particles of a flow of aerosol. A housing is provided having an inlet area and an outlet area. The housing further has a collection and analysis area interconnecting the inlet area with the outlet area. The inlet area of the housing has a first surface and a second surface spaced therefrom. An aerosol inlet is defined adjacent the first surface and a sheath flow inlet is defined between the aerosol inlet and the second surface of the inlet area. A flow path for an aerosol is defined from the aerosol inlet of the inlet area, through the collection and analysis area, and out the outlet area. A sheath flow path is defined from the sheath inlet of the inlet area, through the collection and analysis area, and out the outlet area. A collection electrode is provided having a tip disposed in the aerosol flow path in the collection and analysis area. During a collection step, the flow of aerosol is introduced along the aerosol flow path, with the particles of the aerosol being charged. A sheath flow is introduced along the sheath flow path. The collection electrode is held at a bias voltage relative to the charged particles. The first or second surface of the inlet area is held at a classification voltage chosen such that a portion of the charged particles are deflected toward the tip of the collection electrode. As such, some of the charged particles deflected toward the tip of the collection electrode are collected on the tip. During an analysis step, particles collected on the tip of the collection electrode are ablated, thereby creating atomic emission, and these atomic emission signals are collected during at least part of the spark step, such as after a short delay from the start of the spark, for analysis of the ablated particles. 
     In some versions, a second electrode is spaced from the collection electrode tip so as to define a spark gap in the collection and analysis area, a high voltage pulse generator is provided, and a high voltage spark is applied across the spark gap so as to perform the ablating step. 
     In certain versions, the first and second surfaces are each formed of an electrically conductive material. During the collection step, the first surface of the inlet area is grounded relative to the charged particles and the second surface is the surface that is held at the classification voltage such that a portion of the charged particles are deflected towards the second surface and towards the tip of the collection electrode. 
     In an alternative embodiment, the first surface of the inlet area is formed of an electrically conductive material and the second surface of the inlet area is formed of a dielectric material. The collection electrode comprises a plurality of collection electrodes each having tips disposed in or extending from the second surface. The tips of the plurality of collection electrodes are spaced apart along the second surface in the collection and analysis area. During a collection step, the plurality of electrodes are each held at a bias voltage relative to the charged particles and the first surface is the surface held at the classification voltage such that a portion of the charged particles are deflected towards the second surface for collection on the tips of the plurality of spaced apart collection electrodes. Each collection electrode tip collects particles of a different charge. During the ablating step, the particles are ablated on the tips of each of the collection electrodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a testing setup for an embodiment of the present invention; 
         FIG. 2  is a cross sectional perspective view of a portion of an embodiment of the present invention; 
         FIG. 3  is a cross sectional view showing part of an embodiment of the present invention; 
         FIG. 4  is a cross sectional view showing part of an alternative embodiment of the present invention; 
         FIG. 5  is a cross sectional view showing part of a further embodiment of the present invention; 
         FIG. 6  is a cross sectional view showing a portion of yet another embodiment of the present invention; 
         FIG. 7  is a cross sectional view of an embodiment of the present invention utilizing a sheath flow; 
         FIG. 8  is a cross sectional view showing a portion of an alternative embodiment of the present invention utilizing a sheath flow; 
         FIG. 9  is a cross sectional view of an embodiment of the present invention utilizing a sheath flow and a plurality of collection electrodes; 
         FIG. 10  is a top view of a portion of the embodiment of  FIG. 9 ; 
         FIG. 11  is a cross sectional perspective view of a portion of yet a further embodiment of the present invention wherein the aerosol flow is sorted by cyclone separators; 
         FIG. 12  is a perspective view of an embodiment of the present invention utilizing a plurality of cyclone separators; and 
         FIG. 13  is an exploded view of the embodiment of  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention provides both methods and apparatus for the collection and analysis of particles of an aerosol. In preferred embodiments, particles of the aerosol are collected on the tip of a collection electrode and these particles are then ablated so as to create atomic emissions that are collected for analysis of the particles. In preferred embodiments, the particles of the aerosol are charged either as part of the present invention or prior to introduction to the apparatus or method, and the collection electrode is held at a bias voltage, which may be ground, relative to the charged particles such that these charged aerosol particles collect on the tip of the collection electrode. In some preferred embodiments, a second electrode is provided spaced from the collection electrode tip and after particles are collected on the tip a high voltage spark is created between the collection electrode and second electrode so that the spark ablates the particles. In certain embodiments, charged particles of the aerosol are deflected by a surface held at a classification voltage such that particles collected on the tip of the collection electrode are sorted. Multiple collection electrodes may be provided such that each collection electrode collects particles of a different charge and/or size. 
     Referring to  FIG. 1 , a test setup for testing an embodiment of the present invention is illustrated schematically. An embodiment of the present invention is shown generally at  10 . This embodiment consists of an apparatus for collection and analysis of particles of an aerosol. An aerosol is introduced to the apparatus at inlet  12  and exits through an outlet shown at  14 . In this testing configuration, an aerosol is provided to the apparatus  10  using a pneumatic atomizer  16  to create an aerosol and a diffusion dryer  18  to remove excess moisture. The resulting aerosol is passed through a differential mobility analyzer  20  and then introduced to the inlet  12  of the apparatus  10 . As will be described in more detail hereinbelow, particles of the aerosol are collected inside the apparatus  10  and then ablated to create atomic emissions  22 . These atomic emissions are then collected for analysis, such as by a spectrometer  24 . In this test setup, the remaining flow exits outlet  14  and is introduced into a condensation particle counter  26 . By using the differential mobility analyzer  20  to analyze the flow prior to introduction to the apparatus  10 , and the condensation particle counter  26  to analyze the flow remaining after the apparatus  10 , the apparatus may be tested and/or calibrated. 
       FIG. 2  illustrates a cross-sectional view of an apparatus in accordance with an embodiment of the present invention. This apparatus corresponds to the apparatus  10  in FIG.  1 . The apparatus  10  includes a housing  30  having an inlet area  32 , an outlet area  34 , and a collection and analysis area  36  interconnecting the inlet area with the outlet area. An aerosol flow path is generally illustrated at A. This aerosol flow path extends from the inlet area, through the collection and analysis area, and out the outlet area. A collection electrode  40  has a tip  42  that is disposed in the flow path in the collection and analysis area  36 . In this embodiment, a second electrode  44  is spaced from the collection electrode tip  42  so as to define a spark gap  46  in the collection and analysis area  36 . In this embodiment, the overall housing  30  is generally tubular and elongated and the collection electrode  40  and second electrode  44  are each elongated and coaxial with each other and with the housing. At least one optical window  50  is provided in the housing in alignment with the spark gap  46 . By saying that the optical window is in alignment with the spark gap  46  it is meant that the window has an optical view of the spark gap, which may be either direct or indirect, such that atomic emissions from the spark gap may be collected through the optical window  50 . In this embodiment, a second optical window  52  is provided on an opposite side of the spark gap. A high voltage pulse generator  54  is connected to the collection electrode  44  and second electrode  40  for generation of a spark across the spark gap  46 . 
     During operation, a flow of aerosol is introduced along path A. In some versions, the particles of the aerosol are charged prior to entering the inlet area  32 , such as by passing the flow of aerosol through a unipolar or bipolar charger. During a collection step, the collection electrode  40  is held at a bias voltage relative to the charged particles such that particles collect on the tip of the electrode. The bias voltage may be approximately zero or ground or may be a different voltage for attracting the charged particles. As used herein, “ground” means that an electrode or other component is at ground relative to the high voltage electrode but may not be at an absolute ground. In another version, the flow of aerosol introduced along path A is charged by the second electrode  44 . In this version, during a collection step, the second electrode is provided with a corona current so as to hold the second electrode at a corona voltage sufficient to create a cloud of ions around the second electrode. These ions then mix with the particles of the aerosol causing the particles to be charged and thereby attracted to the collection electrode  40 . In an alternative version, the second electrode is held at a voltage that creates an electrical field that generally urges the particles towards the collection electrode. 
     During a subsequent step, particles collected on the tip  42  of the collection electrode  40  are ablated so as to create atomic emissions for analysis of the particles. In some versions, the high voltage pulse generator  54  creates a high voltage spark across the spark gap, thereby ablating the particles. A spectrometer  24  may be in optical communication with the window  50  so as to collect atomic emissions from the ablated particles. Optical communication between the spectrometer  24  and the spark gap may be achieved by a fiber optic cable with one end in optical communication with the window  50 , or actually forming the window, and the other end connected to the spectrometer. 
     In an alternative embodiment, a laser  56  is provided so as to provide a laser light source for ablating particles on the tip  42  of the collection electrode  40 . The laser  56  and pulse generator  54  may be used in combination, such as by using the laser to first ablate the particles from the tip  42  and then using the pulse generator  54  to form a plasma to create the atomic emissions. 
     Referring now to  FIG. 3 , a further embodiment of the present invention will be described. An apparatus for collection and analysis of particles in aerosol is shown at  58  and includes a housing  60  with an inlet area  62 , an outlet area  64 , and a collection and analysis area  66 . In this embodiment, the housing  60  is generally tubular. In some exemplary embodiments, the tube forming the housing  60  has a diameter which can range from a few millimeters to centimeters. A collection electrode  70  and a second electrode  76  are coaxial with the housing  60  and with one another. They each are insulated with a dielectric layer,  72  and  78  respectively, covering their side surfaces. In one exemplary embodiment, the dielectric sheathing has a thickness in the range of 0.5 to 1 millimeter. The collection electrode  70  has an exposed tip  74 . In this embodiment, this tip  74  is flat with the tip surface being generally perpendicular to the axis of the collection electrode  70  and housing  60 . The diameter of this tip may vary in size, with some exemplary embodiments having a diameter d in the range of 1 to 1000 microns. In this embodiment, the second electrode  76  has a sharp tip  80  with the tip having a tip radius in the range of 5 to 100 microns. The tips  74  and  80  are spaced apart by a spark gap. The distance of the spark gap in an exemplary embodiment is in the range of 1 to 10 millimeters. An optical window is shown at  82  in a sidewall of the housing  60 . A portion of the aerosol flow path is shown at A. As will be clear to those of skill in the art, the aerosol flows along the housing from the inlet area  62 , through the collection and analysis area  66 , and out through the outlet area  64 . As with the prior discussed embodiments, during a collection step, particles of an aerosol flowing along the path A are collected on the tip  74  of the collection electrode  70 . In some embodiments, the second electrode  76  is held at a high corona voltage so as to create a cloud of ions for charging of the aerosol flow. During a subsequent step, a spark is generated across the spark gap so as to ablate particles collected on the tip  74 . As will be clear to those of skill in the art, such an approach allows a near time analysis, since collection and ablation of particles from the tip may be done sequentially in rapid succession, and repeated as necessary. 
       FIG. 4  shows an alternative embodiment in which the collection electrode  84  and second electrode  86  extend generally perpendicular to the axis of the housing  88 . An optical window is shown at  90  and the spark gap is shown as B. 
       FIG. 5  shows an alternative embodiment of an apparatus  100  for collection and analysis of particles of an aerosol. An aerosol is introduced at  102  through a unipolar aerosol charger  104 , which is operable to charge the particles of the aerosol. The unipolar charger may impart either negative or positive charge to the particles. The housing  106  has an inlet area  108 , an outlet area  110 , and a collection and analysis area  112 . A coaxial collection electrode  114  has a tip  116  in the collection and analysis area  112 . In this embodiment, the collection electrode  114  is held at a bias voltage having a polarity opposite to the charge imparted by the unipolar aerosol charger  104 . This attracts particles of the aerosol to the tip  116 . In this embodiment, at least a portion of the walls of the housing  106  is electrically conductive. This electrically conductive portion forms the complementary second electrode for use during a subsequent step in which a spark is used to ablate particles on the tip  116 . During the ablation step, the conductive part of the housing may be held at ground while the collection electrode  114  is energized at a high voltage so as to cause a spark between the tip  116  and the housing  106 , thereby ablating particles from the tip  116 . The electrically conductive portion of the housing may also be held at ground during the collection step. 
     Referring now to  FIG. 6 , a further alternative embodiment of an apparatus  120  will be described. The apparatus  120  differs from the embodiment of  FIG. 3  in that the collection electrode is surrounded by concentric assembly of electrodes  122 . A central electrode  124  of the electrode assembly  122  serves as a collection electrode. It has a tip  126 . A concentric electrode  128  surrounds the central electrode  124  and is separated therefrom by a dielectric layer  130 . Additional concentric electrodes, separated by adequate dielectric layers, may be provided, if needed. During the collection step, the central electrode  124  is held at a bias voltage. This may be ground or may be a bias voltage opposite to the potential of the other electrode. In the example in which the central electrode is held at a bias voltage, the concentric electrodes may be held at the same voltage or at lower levels with the same polarity so as to act as an electrostatic lens that focuses particle capture on the central electrode tip  126 . Further concentric electrodes may be provided. This arrangement is used to create an electrostatic lens or funnel to improve particle deposition characteristics on the tip  126 . 
     Referring now to  FIG. 7 , yet another alternative embodiment of an apparatus in accordance with the present invention will be described. The apparatus has a housing  140  with an inlet area  142 , an outlet area  144 , and a collection and analysis area  146  interconnecting the inlet area with the outlet area. The inlet area  142  may be said to have concentric inner  148  and outer  150  surfaces with the surfaces spaced apart from one another. In the illustrated embodiment, the housing  140  is generally tubular with the surface  150  being an inner surface of the tubular housing and the surface  148  being an outer surface of a central member  152  that extends coaxially along the housing from the inlet area to a downstream end  154  at the collection and analysis area  146 . The surface  148  is defined by the outer surface of the central member  152 . In this embodiment, an aerosol inlet  156  is defined adjacent the surface  150  and a sheath inlet  158  is defined between the aerosol inlet and the surface  148 . In this embodiment, a coaxial flow guide  160  separates the aerosol inlet  156  from the sheath inlet  158 . An aerosol flow path is defined from the aerosol inlet  156  through the collection and analysis area  146  and out through the outlet area  144 . A sheath flow path is defined from the sheath inlet  158  through the collection and analysis area  146  and out through the outlet area  144 . In preferred embodiments, the surfaces  148  and  150  are each electrically conductive. During the collection step, an aerosol flow is introduced at aerosol inlet  156  and a sheath flow is introduced at sheath inlet  158 . The surface  150  may be held at ground relative to surface  148 , which is held at a classification voltage. The classification voltage, V CL  is opposite in polarity to the charge of the particles in the aerosol flow, and is chosen such that these particles are deflected towards the surface  148  such that they cross the sheath flow. This approach allows separation of particles based on their electrical mobility—and hence size, thereby allowing size-resolved particle collection on the collection electrode tip  162 . This tip may be held at ground or at a bias voltage for collection of the particles. By choosing the dimensions L l , r in , r out , and gap Z, along with the classification voltage, bias voltage, and flow rates, particles within a predetermined size range may be collected on the collection electrode tip  162 . As will be clear to those of skill in the art, a charged aerosol may have particles in which the charge on the individual particles corresponds to the size of the particle and therefore particles may be sorted by size in this embodiment. Following the collection step, particles collected on the tip  162  may be ablated as discussed in accordance with prior embodiments. 
     Referring now to  FIG. 8 , a further embodiment of a collection and analysis apparatus that allows size-resolved particle collection is shown at  170 . In this embodiment, aerosol enters at an aerosol inlet  172  as sheath flow enters at sheath inlet  174 . The aerosol flow path is diverted concentrically outwardly and then reversed so as to join the sheath flow. This design may be considered parallel disk geometry with axial symmetry. Some of the sheath flow exits through outlet  176  while some of the aerosol flow exits through outlet  178 . As with prior embodiments, particles are collected on tip  180  and then ablated for analysis. 
     Referring now to  FIGS. 9 and 10 , a further alternative embodiment of an apparatus for collection and analysis of particles of an aerosol will be described. The apparatus is generally shown at  200  in  FIG. 9  and has a pair of opposed surfaces  202  and  204 .  FIG. 10  is a view looking down on surface  204 . An aerosol inlet is provided at  206  adjacent the surface  202  as a point source. A sheath inlet is provided at  208  adjacent the surface  204 , as a line source. A flow separator is provided at  210  to separate the two inlets. In this embodiment, the collection electrode takes the form of a plurality of collection electrodes  212 - 220 . These collection electrodes have tips that are disposed in or extend from the surface  204  and are insulated therefrom by dielectric layers. The collection electrodes are spaced apart along the surface  204  in the direction of flow. The second electrode takes the form of a plurality of second electrodes  222 - 230 . In this embodiment, the surface  202  is formed of a dielectric material while the surface  204  is formed of metal or another electrically conductive material. The second electrodes  222 - 230  are spaced apart along the surface  202  in the direction of flow and are positioned in register with the collection electrodes  212 - 220 . This defines a plurality of spark gaps between aligned pairs of electrodes, such as between electrodes  212  and  222 . The spark gap has a dimension b. During a collection step, an aerosol flow is introduced through aerosol inlet  206  and a sheath flow is introduced at sheath inlet  208 . The aerosol flow is preferably a narrow stream while the sheath flow is a flat and wider flow. The surface  202  is nonconductive, while the surface  204  is held at a classification voltage chosen to cause particles of the aerosol flow to be deflected towards the surface  204 . Because particles of different sizes have different charges and masses, these particles will be deflected more or less strongly towards the surface  204 . This allows the particles to be sorted by size for collection on the various collection electrode tips  212 - 220 . During a subsequent ablation step, a spark is created between aligned pairs of electrodes so as to ablate the particles on each of the collection electrodes  212 - 220 . Preferably, this is done sequentially such that a spark is first created between electrodes  212  and  222 , then a spark is created between electrodes  214  and  224 , etc.  FIG. 10  shows the tips of the collection electrodes  212 - 220  and optical windows  232 - 240  aligned with the various spark gaps. 
     Referring now to  FIGS. 11-13 , a further embodiment of the present invention will be discussed. It is desirable to separate the analysis of various types of particles within an aerosol flow. The apparatus  300  shown in  FIG. 12  has an aerosol introduced at A through inlet  302  into a charging unit  304 . This charging unit may be a unipolar or bipolar charger. The aerosol then flows to a manifold  306 , best shown in the exploded view of  FIG. 13 . The flow is then divided into multiple flows, each of which is introduced to a separator and analysis unit  308 . Such a unit  308  is shown in cross section in  FIG. 11 . The flow enters through inlet  310  and into a cyclone section  312 . The cyclone unit  312  is designed such that the flow circulates in a chamber and only particles within a particular size range exit the separator unit  312  through outlet  314 . From there, the aerosol flow may be considered to be a sorted aerosol flow and will contain only particles within a certain size or mass range. They are then introduced to a collection and analysis unit as described previously. The plurality of cyclone units  312  may be considered to serve as a size classification unit for separating the flow of aerosol into a plurality of sorted aerosol flows. This approach allows multiplexing of the aerosol collection and analysis method. This allows simultaneous collection of different size fractions of the aerosols, which can be subsequently analyzed instantaneously. This considerably reduces measurement time, and improves the time resolution of size-resolved measurements. The multiplexing can also be done with respect to other aerosol properties, such as humidity or temperature to name a few. 
     In some embodiments, analysis of particles may be conducted at different temperatures, such as by heating the collection tip to different temperatures to study how spectroscopic signal changes as a function of temperature. As will be clear to those of skill in the art, this temperature change may have many applications. 
     As will be clear to those of skill in the art, the herein described embodiments of the present invention may be altered in various ways without departing from the scope or teaching of the present invention. It is the following claims, including all equivalents, which define the scope of the present invention.