Abstract:
A source of ions for an analyzer includes a reservoir for containing a liquid and a channel having a first end opening into the reservoir. A nozzle element adjacent a second end of the channel includes a plurality of tips for producing individual droplets from the liquid. The plurality of tips reduces the likelihood that individual droplets will coalesce, increases the overall flow of material or analyte to the mass spectrometer and provides a level of redundancy in the delivery of liquid for producing droplets. The tips also may produce a higher current output and greater signal from the system, as well. With micro-miniaturization, the individual droplets are relatively small, thereby increasing the likelihood that ions would be ejected from the droplet surfaces under the influence of an electric field. Multiple nozzle elements can be used to more selectively deliver fluid droplets to the analyzer, or to increase the overall flow rate of droplets from the reservoir. The tips may have a volcano or truncated cone shape for the desired fluid delivery, electrostatic effects and manufacture ability.

Description:
BACKGROUND OF THE INVENTIONS  
         [0001]    1. Field of the Inventions  
           [0002]    The present inventions relate to methods and apparatus for producing ions, and have particular application to structures and methods including micro-electronic micro-structures used for producing ions from liquids, for example to produce ions for mass spectrometers and the like.  
           [0003]    2. Related Art  
           [0004]    Mass spectrometers and other analyzers have been used to determine the properties or characteristics and quantities of unknown materials, many of which are present in only minute quantities. Many such analyzers function by determining the quantity of material present in an unknown solution as a function of the relationship between the mass and the charge on ions provided to the analyzer by a source of ions. The ability of the analyzer to produce reliable results depends in part on the ability of the source of ions to produce a maximum number of individual ions for a given amount of input material.  
           [0005]    Electro-spray ion sources are one type of source of ions for analyzers. Typical ion generation from electro-spray ion sources peaks at a certain ion generation level for a given system due to coalescing or nucleation of charged and un-charged droplets as the droplet density increases in the high electrostatic field. Most of the coalesced, larger-than-original droplets fail to eject ions from their surfaces due to new conditions and subsequently larger droplets. Larger droplets mean that their kinetic inability to reach a critical minimal volume reduces the likelihood that ions will be ejected, regardless of the liquid flow rate available for electro-spray. For example, typical liquid ion source devices have a single liquid conduit producing droplets in a range of sizes from sub-micron diameters to hundreds of microns in diameter. Ions are ejected from smaller aerosol droplets when and if the droplet reaches a critical smaller dimension and if the repulsive internal charge becomes greater than the surface tension holding the droplet in its spherical shape. Absent a critical dimension and a suitable repulsive internal charge, few or no ions are ejected. A high percentage of the droplets do not reach critical volume, resulting in a low yield.  
         SUMMARY OF THE INVENTIONS  
         [0006]    Methods and apparatus are described for improving the production of ions from bulk liquids and other materials, for example for use in mass spectrometers and other analyzers, and providing for greater control and redundancy in ion delivery systems. One or more aspects of these methods and apparatus also provide for ion production which may approach linearity in proportion to flow rate. Moreover, these methods and apparatus may be particularly suited to micro-miniaturization.  
           [0007]    In accordance with one aspect of the present inventions, a source of ions for an analyzer includes a liquid source such as a reservoir for containing a liquid and a channel having a first end opening into the reservoir. The source of ions also preferably includes a droplet emission element or assembly such as a nozzle element adjacent a second end of the channel that preferably includes a plurality of tips for producing individual droplets from the liquid. The plurality of tips reduces the likelihood that individual droplets will coalesce, increases the production of ions from bulk liquids and other materials in an approximately linear relationship, and increases the overall flow of material or analyte to the mass spectrometer, which gives a higher current output and a greater signal for the analyzer. They also provide a level of redundancy in the delivery of liquid for producing droplets. With micro-miniaturization, the individual droplets are relatively small, thereby increasing the likelihood that ions would be ejected from the droplet surfaces under the influence of an electric field.  
           [0008]    In one preferred form of one aspect of the present inventions, the channel may feed into a manifold which can be used to more efficiently provide fluid to the nozzle element. Additionally, multiple nozzle elements can be used to more selectively deliver fluid droplets to the analyzer, or to increase the overall flow rate of droplets from the reservoir.  
           [0009]    In another form of one aspect of the present inventions, the plurality of tips are arranged linearly with respect to each other for ease of use and for ease of manufacture. Additionally, or alternatively, tips may be arranged so that all of the tips are spaced apart from each other in all directions from a center point. Such an arrangement may define a circle filled with spaced apart tips extending outwardly from a surface. In one form, the tips have a volcano or truncated cone shape for the desired fluid delivery, electrostatic effects and manufacture ability. Additionally, parallel arrangements of tips may produce parallel beams or streams of ions with a lower probability of coalescing in the path between the tips and a counter electrode and the analyzer.  
           [0010]    In another form of one aspect of the present inventions, a source of ions for an analyzer includes a liquid supply for supplying analyte to a nozzle or nozzles pointing in a first direction and a counter electrode spaced from the nozzle in the first direction. Means are provided for creating an electric field in the vicinity of the nozzle for producing ions from droplets ejected from the nozzle. Preferably, each nozzle includes a plurality of tips extending in the first direction for producing droplets from each of the tips. Supplying the analyte as a liquid and producing multiple droplets improves the efficiency and the ion production of the system, and also allows operation of the system at ambient pressures. Consequently, the ion delivery system is easier to manufacture, use and maintain.  
           [0011]    In a further form of one aspect of the present inventions, ions are produced from a liquid by passing a liquid along a first channel and into a plurality of second channels terminating in respective openings facing at least partly toward a counter electrode. An electric field is produced so that there is a potential difference between the fluid at the respective openings and the counter electrode. As before, supplying the analyte as a liquid and producing multiple droplets improves the efficiency and the ion production of the system. Additionally, the method of producing ions can be carried out at ambient pressures. Preferably, the counter electrode is spaced sufficiently from the tips to allow sufficient time for the ions to be ejected from the droplets and/or for the droplets to evaporate. The counter electrode can be facing the tips or can be oriented at an angle relative to the tips. For example, the counter electrode can be approximately perpendicular to the plane defined by the ends of the tips.  
           [0012]    These and other aspects of the present inventions will be further understood after consideration of the drawings, a brief description of which follows, and the detailed description of the preferred embodiments. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a schematic and block diagram of an analyzer and an ion generation system in accordance with one aspect of the present inventions.  
         [0014]    [0014]FIG. 2 is a schematic diagram of an ion generation element showing reservoirs and nozzles in accordance with one aspect of the present inventions.  
         [0015]    [0015]FIG. 3 is a schematic depiction of a nozzle such as that shown in FIG. 2 in accordance with a further aspect of the present inventions.  
         [0016]    [0016]FIG. 4 is a partial cutaway isometric view of several tips or openings on the nozzle of FIG. 3 in accordance with a further aspect of the present inventions.  
         [0017]    [0017]FIG. 5 is a plan view of a nozzle having a plurality of tips in accordance with a further aspect of the present inventions.  
         [0018]    [0018]FIG. 6 is an isometric, partial cutaway view and partial schematic of a further embodiment of an ion generation assembly in accordance with another aspect of the present inventions.  
         [0019]    [0019]FIG. 7 is a partial vertical section and schematic of a further alternative embodiment of an ion generation assembly in accordance with another aspect of the present inventions. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    The following specification taken in conjunction with the drawings sets forth the preferred embodiments of the present inventions in such a manner that any person skilled in the art can make and use the inventions. The embodiments of the inventions disclosed herein are the best modes contemplated by the inventor for carrying out the inventions in a commercial environment, although it should be understood that various modifications can be accomplished within the parameters of the present inventions.  
         [0021]    The apparatus and methods of the present inventions improve the production of ions and give improved control and redundancy in ion delivery systems. One or more aspects of these methods and apparatus may also provide for ion production that can be linear in proportion to flow rate. Additionally, micro-miniaturization and micro-fabrication techniques can be used to advantage with these methods and apparatus.  
         [0022]    The following discussion will focus primarily on electro-spray ion delivery systems for use with mass spectrometers, with particular emphasis on those that can be made using micro-electronic fabrication techniques. It is believed that one or more aspects of the present inventions can be easily implemented in any number of different analyzers while still achieving the results obtained with the configurations of the ion delivery systems described herein. However, it should be understood that this specification focuses on preferred applications of the inventions as they may be implemented as an electro-spray ion delivery system for mass spectrometers.  
         [0023]    In accordance with one aspect of the inventions, an ion delivery system  30  (FIG. 1) is provided which improves production of ions from bulk liquids and other materials and which provides more flexibility in the control and ongoing supply of liquid for producing ions. The ion delivery systems described herein can be used with any number of devices, but will be described herein in conjunction with an analyzer  32 , which may be a mass spectrometer such as an ion trap, quadrupole mass filter, time-of-flight, magnetic sector and mobility mass spectrometers, or the like. The analyzer may include a trap, filter or other discrimination element  34  for separating the ions of interest from the remaining particles. The ions of interest are then collected, detected or otherwise analyzed in a detector  36 , which sends signals to and is controlled by a controller and power supply assembly  38 , which also may have any number of configurations. The controller and power supply assembly  38  provides through an interface  39  whatever power and control signals are necessary for operating the analyzer  32 , as well as the ion delivery system  30 . The assembly  38  also may receive signals representing the ongoing status of the ion delivery system and the analyzer, and can be configured to respond accordingly. The analyzer is maintained within an enclosure  40  preferably at sub atmosphere pressure by a suitable pump or other vacuum source  42 . Typical pressures in the analyzer may be in the range of 10 −3  to 10 −9  Torr (one Torr equals 1/760 Atmosphere).  
         [0024]    The ion delivery system  30  is also preferably housed within its own enclosure  44 , preferably above the pressure of the analyzer  32 , and preferably at ambient pressure, as indicated at  43 . In other configurations, the ion delivery system  30  can be maintained at about 0.1 atmospheres to about 1.5 atmospheres, while operation could occur outside this range depending on design. Typical operation would be at about one atmosphere. The enclosure  44  can be maintained above the pressure of the analyzer  32  because the ion delivery system is preferably holding and operating on liquids instead of gases. Consequently, the ion delivery system is easier and less expensive to manufacture and easier to use with the analyzer  32 . The interface between the ion delivery system  30  and the analyzer  32  can take any number of forms, depending on the type of analyzer being used.  
         [0025]    The ion delivery system  30  preferably includes an electro-spray droplet source  46  and a counter electrode or counter electrode assembly  48  maintained at an electric potential Delta V relative to the droplet source  46 . The droplet source  46  can be maintained at ground, but it should be understood that the potential difference between the droplet source and the counter electrode  48  can be maintained in any number of ways. For example, the counter electrode can be grounded, or both the droplet source and counter electrode can be at different potentials other than ground.  
         [0026]    The voltage difference Delta V can be any number of values from a few volts to thousands of volts. In one embodiment, the voltage can be between 700 to 800 volts and possibly as high as 1400 volts, but preferably still avoiding any electric break down between the tips of the ion source and the counter electrode assembly  48 . As will be apparent from some of the dimensions provided herein, the electric field experienced by a droplet produced by the droplet source  46  relative to the counter electrode can be relatively high given the surface areas of the nozzle tips. Consequently, significant latitude in selecting the voltage differences is possible.  
         [0027]    The droplet source  46  is preferably oriented so as to eject droplets in a direction  50  approximately perpendicular to a central axis  52  of the analyzer  32 . The preferred angle can range from about 70 and 115 degrees, for example, while other angles can be used as well. The benefits of a perpendicular orientation are described in U.S. Pat. No. 5,495,108, the description and drawings of which are incorporated herein by reference.  
         [0028]    In one preferred embodiment, the droplet source  46  includes a liquid source and a droplet emission system in the form of a reservoir and nozzle array  54  (FIG. 2) for containing liquid and passing the liquid to outlets such as tips for ejecting droplets from the liquid. The array  54  can have one or more reservoirs, such as reservoirs  56 ,  58 ,  60 ,  62 , and  64  for containing or holding liquid analyte to be analyzed by the analyzer  32 . The reservoirs can be any shape, size or configuration but typically may be circular in plan view and have a depth as may be determined by the particular application or the analyte or analyte samples under consideration. Additionally, in the case of more than one reservoir, the relative positions of the reservoirs can vary according to their size, shapes and according to the size of the array, and also according to their functions or use. However, it is preferred that the positions and configurations of the reservoirs are such as to optimize the delivery of liquid to the outlets or tips while still maintaining adequate control over the flow of liquid and still allowing access to the reservoirs.  
         [0029]    The array also preferably includes one or more nozzle elements or assemblies  66  for receiving liquid from one or more of the reservoirs and ejecting the liquid as droplets into an electric field created between the nozzle elements and the counter electrode. Each nozzle can receive liquid from one or more of the reservoirs through any number of flow channel configurations, conduits or the like, as may be determined by the layout of the array, the material from which the array is formed or constructed and the dimensions of the flow channels. As with the size and orientations of the reservoirs, the layout, configurations and dimensions of the flow channels may be determined in part by the desire to optimize the control and the ease of flow of liquid from the reservoir to the nozzle or nozzles. In the embodiment shown in (FIG. 2), the flow channels include a first flow channel  68  having a first end  70  coupled to the first reservoir  56  and a second end  72  opening into a manifold  74  for passing liquid from the first reservoir  56  to the nozzles  66 . Preferably, the channel is a straight line between the reservoir  56  and the manifold  74 . The second end  72  opens out into the manifold  74  preferably at a location which optimizes the flow of liquid from the reservoir  56  to the desired nozzle or nozzles without being affected by and without affecting other channels.  
         [0030]    In a preferred embodiment, the manifold  74  is sufficiently small to minimize excess volume or dead volume while still permitting sufficient flow of liquid to the nozzles. The manifold may include a first wall  76  at which the second end  72  of the channel  68  opens out, along with any other channels coming from respective reservoirs. The wall  76  may be flush or co-linear with a forward wall  78  of the array or may be slightly arcuate or partly circular. Also in the preferred embodiment, the nozzles  66  are formed on, mounted to or extend from a manifold forward wall  80 . The depth of the manifold may be defined by the spacing between the wall  76  and the manifold forward wall  80 . The length of the manifold is defined by a first manifold side wall  82  and a second manifold side wall  84 , and the width is defined by a top wall and a bottom wall.  
         [0031]    A second channel  86  includes a first end  88  opening into the reservoir  58  and a second end  90  opening into the manifold for allowing liquid to flow from the reservoir  58  to the manifold. Likewise, a third channel  92  may include a first end  94  opening into the reservoir  60  and a second end  96  opening into the manifold. A fourth channel  98  includes a first end  100  and a second end  102  for allowing liquid to flow from the reservoir  62  to the manifold. A fifth channel  104  includes a first end  106  and a second end  108  for allowing liquid to flow from the reservoir  64  to the manifold.  
         [0032]    One or more contacts, conductors or conductive regions  110  are associated with respective reservoirs so that an electric potential Delta V x  can be generated between the respective reservoir and the counter electrode so that fluid flows from the reservoir to and out of one or more of the nozzles  66 . Each reservoir can then be controlled by appropriate respective voltages Va, Vb, Vc, Vd and Ve to induce liquid flow from the selected reservoir through electrophoresis, where the variable “x” in V x  represents “a”, “b”, “c”, “d” or “e”, respectively. Liquids from the appropriate reservoirs can then be selectively caused to flow down the respective channel, into the manifold  74  to be ejected as droplets from the nozzles  66  and into the region between the nozzles  66  and the counter electrode  48 .  
         [0033]    The array  46  can be constructed or formed in any number of ways. In one approach, the array can be formed from one or more plates of glass or quartz appropriately bonded together. Other non-conductive materials can be used as well. For example, the array can be formed by a first plate substantially square or rectangular along with a projection to form the manifold and nozzles. A second plate having the same outline is formed, cut or etched to include holes to form the reservoirs and a bottom surface is also formed, cut or etched to form respective channels in the bottom surface of the plate. Channels or reservoirs can also be formed in other ways as well, to provide the desired configurations. The first plate then becomes the bottom for the reservoirs and a bottom portion of the channels. The second plate is also formed, cut or etched in the bottom surface thereof to form the manifold and to form channels or openings to form the nozzles. Alternatively, the array can be formed through microelectronic machining or fabrication such as lithography on non-conductive surfaces.  
         [0034]    The nozzle  66  (FIG. 3) preferably includes a wall  112  defining a channel  114  extending from the manifold  74  to a nozzle manifold  116  for passing liquid from the manifold  74  to one or more outlets, ports or tips  118  at the far or distal end  120  of the nozzle. The channel  114  can be a single channel or multiple channels extending from the manifold  74  to the manifold  116  for supplying liquid to the tips  118 .  
         [0035]    The tips  118  can be arranged linearly with respect to each other, as depicted in the sectional view of FIG. 3, they may be arranged spaced apart from each other in all directions from a center  122  (FIG. 5), or they may be arranged to have any number of other configurations. Preferably, each tip  118  is spaced apart from each adjacent tip an equal amount so as to minimize the effects produced on a given tip by adjacent tips. Other configurations are possible as well for distributing or positioning the tips over the surface of the nozzle, including symmetrical and/or asymmetrical.  
         [0036]    The dimensions and configurations of the tips are preferably such as to minimize the restriction to flow of liquid to the tip, minimize the size of the droplets ejected from the tips and to minimize the depositing of residue on the surface on the nozzle. The tips can take any number of forms, and may be substantially straight with a constant wall thickness or they may have a varying wall thickness, but they preferably have a volcano shape (FIG. 4) or a converging tip end. Each tip preferably includes an outer surface  124  sloping inwardly toward a central axis  126  and outwardly away from the manifold  116  (FIG. 3) generally in the direction of the counter electrode. The outer surface  124  converges to a substantially cylindrical wall  128 , which is substantially circular in cross-section. The cylindrical wall  128  preferably terminates at a flat or squared-off end face  130  and has a thickness “t” (FIG. 4) preferably as small as possible to minimize the surface area defined by the end face  130  and to minimize obstructions to uniform flow. The interior wall of the tip  132  preferably has a diameter D of an appropriate size to minimize the size of the droplets ejected from the tip. The diameter D may be constant throughout much of the length of the channel to the tip or may be converging to a similar extent as the outside of the tip, in other words the thickness “t” is relatively constant near the face  130 . The diameter of the channel  114  (FIG. 3) may be about 20 micrometers, or other dimensions producing an approximately similar cross sectional area.  
         [0037]    The height “h” of each tip is preferably sufficient to properly form and eject droplets while minimizing spread or flow of liquid across the surface of the nozzle or depositing of liquid on the nozzle. The height may be approximately similar to or greater than the inside diameter of the tip, and is preferably about or greater than one and one-half times the diameter D. The spacing S between each tip is preferably sufficient to allow formation and ejection of droplets from each tip without interference from the formation and ejection of droplets from adjacent tips, and so that each tip has its own electric field point. The spacing S may be about or greater than one and one-half times the diameter D, to take into account the relationship between the dynamics of the formation of the spherical droplet as it leaves the tip, which droplet diameter depends on the diameter D, and the spacings for adjacent droplets if droplets formed simultaneously.  
         [0038]    In one aspect of the preferred embodiments, the tips are spaced from the counter electrode a distance sufficient to allow ions to be ejected from the droplets or for the droplets to evaporate. The counter electrode is preferably positioned closer to the analyzer than to the tips and is preferably spaced in a direction from the tips that is at least partly in the same direction as the line of flight of the droplets, and preferably at least partly in a direction coaxial with the tips. The spacing between the tips and the counter electrode is preferably about one to five mm, and may be more depending on the mode of operation, the temperature and similar parameters.  
         [0039]    In operation, liquid analyte is placed in one or more of the reservoirs  56 - 64  and the array  46  placed in the ion generator  30 . Voltages are applied to the counter electrode and the array, and to one of the reservoirs, such as reservoir  56 , to cause liquid to flow from the reservoir along the channel  68  to the manifold  74  and to the nozzles  66 . Liquid flows through the channel  114  in the appropriate nozzle out to the manifold  116  and to the tips  118 . Droplets are formed through each tip and ejected under the influence of the voltage difference V x  created between the end face  130  and into the droplets and the relative voltage on the counter electrode. Ionized portions of the analyte are then ejected from the droplet and taken into the analyzer. The remainder of the droplet passes the counter electrode and is either deposited or leaves the assembly  30 .  
         [0040]    Exemplary dimensions can be given for the preferred embodiments, but other dimensions can be used for the same or different configurations while still achieving one or more of the benefits of the present inventions. In one example, the inside diameter of the tip is between about 0.1 and one micro-meter and the outside diameter is about 2 micro-meters. However, the outside diameter is preferably as close to the inside diameter as possible. The center to center distance between tips can be as small as two micro-meters or less, but is preferably more. For example, the center to center spacing can be twice or three times or more that of the outside diameter of a tip. The channels to each of the manifolds can be about 20 micro-meters in diameter.  
         [0041]    In a further form of one aspect of the present inventions, a source of ions  134  (FIG. 6) includes a liquid source  136  such as a reservoir and pump for containing a liquid and transporting the liquid to a manifold  138 . The source of ions also includes a droplet emission assembly  140  having a plurality of tips  142 ,  144 , and  146  for producing droplets  148  and ejecting the droplets into an electric field between the tips and a collector  150 , which generically may be considered the analyzer, well known to those skilled in the art, but where the analyzer is used simply to measure the flow of ions from the tips, it may take the form of an ammeter  151 . The collector may include a power supply, source or generator  152  for producing the electric field between the collector  150  and the tips  142 ,  144  and  146 . In the example shown in FIG. 6, the tips are placed at a potential different from the collector  150  through a copper wire  154  or other conductor to complete the circuit. The wire  154  preferably encircles and electrically contacts tips  142 ,  144  and  146 , such as by way of respective tubes  156 .  
         [0042]    In this aspect of the inventions, the tips  142 ,  144  and  146  can be formed by a well-known drawing process such as is known to those skilled in the art of manufacturing small tubes. The drawing process is carried out on a plurality of quartz tubes in a bundle to produce a plurality of tubes  156  that are cut at one end  158  and convergent or necked down to the tips  142 ,  144  and  146  at the other. The tubes are then made somewhat conductive by application of a conductive coating on the outer surfaces of the tubes, such as through a conductive paint or electro-deposition of a suitable conductive material. The wider-diameter ends  158  are press fit into an elastomeric disk  160 , such as a Teflon disk, to form a suitable seal between the disk and the tubes. The Teflon disk  160  is then fit into a tube  162  made of plastic or other material to serve as a channel and manifold for liquid before entering the quartz tubes  156 . In this embodiment, the outer diameter of the each of the tips were about two micro-meters and the inside diameter of the tip was about one micro-meter. The inlet diameter of the tube was about 200 micro-meters. The tips were separated from each other by a distance of about 1230 micro-meters, and the distance ratio between tips was between 600 and 1200; however, a ratio of separation of about 100 may be preferred between the tips. The particles produced ranged in size from sub-micro-meters in diameter to about two micro-meters. The separation ratio provided a large distance between aerosol particles to reduce their ability to coalesce prior to the ions being collected at the collector.  
         [0043]    The tube array was separated from the collector by a distance of between three and 9 mm, with a suitable distance being about 8 mm. In this configuration, the tubes and the collector were oriented with respect to each other to be coaxial. A voltage was applied to the tube array of between 1000 and 1400 volts. With this arrangement, ion detection as measured by observed current was found to have a direct correlation to the number of tubes.  
         [0044]    In a further form of the present inventions, a source of ions may include tubes  163  having tips  164  similar to the tips  142 ,  144  and  146 , having opposite ends  166  in fluid communication with a manifold  168  for supplying liquid to the tips  164 . The tubes  163  pass through respective openings in a lower housing  170  and are sealed and held in place by respective O-rings  172 . The ends  166  of the tubes are pressed or otherwise fit into respective openings in a seal plate  174 , which is then preferably pressed or otherwise placed against the O-rings  172  to help seal the tubes and hold them in place. An upper housing  176  seals with and covers the lower housing  170  to form the manifold  168 . A fitting  178  couples with a tube or other liquid supply for supplying liquid analyte to the manifold.  
         [0045]    The O-rings may also take the form of gaskets, and they are preferably formed from conductive polymers, such as graphite or silver impregnated polymer, such as polyimide. The conductive O-rings or gaskets are preferably about 1.2 mm inside diameter.  
         [0046]    Having thus described several exemplary implementations of the invention, it will be apparent that various alterations and modifications can be made without departing from the inventions or the concepts discussed herein. Such operations and modifications, though not expressly described above, are nonetheless intended and implied to be within the spirit and scope of the inventions. Accordingly, the foregoing description is intended to be illustrative only.