Abstract:
A liquid chromatography column distributor spreads liquid over the internal cross sectional area of the column to facilitate separation of the liquid in the column into its constituent components for analysis, particularly where the volume of liquid is small and the rate of flow through the column is high. The distributor is useful for small volumes, less than 100 micro liters. The distribution is preferably done in two or more stages by two or more plates arranged in series. This arrangement keeps the flow paths short and volumes of channels in the plate carrying the liquid small. The distributor may be used in a high performance liquid chromatography (HPLC) in which liquid flows at high rates through the column. The distributor is constructed to protect the liquid sample from damage.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates generally to handling of liquid samples, and more particularly to a distributor for distributing a liquid sample over an area for liquid chromatography.  
           [0002]    Liquid chromatography is well known in the art for characterizing properties of liquid samples. In general, liquid chromatography involves the separation of one or more components of a liquid sample by selective adsorption, partitioning, ion exchange, size exclusion or other techniques. Typically, the components of a sample are separated by passing a liquid mobile phase containing the sample through or across a solid stationary phase, e.g. a “packing” in a chromatographic column. For example, in size exclusion chromatography, separation is achieved by the differential exclusion or inclusion within the pores of porous packing particles, of the sample molecules as the mobile phase moves past the stationary phase. The separation causes the components of the sample to move through the column at different rates; thus, the components exit the column at different times, which allows the separated components to be characterized, for example, with a flow-through detector.  
           [0003]    Approaches for liquid chromatography can vary with respect to the basis of separation and the basis of detection. For example, liquid chromatography can generally be characterized as either preparative chromatography or analytical chromatography. As used herein, the terms preparative chromatography and analytical chromatography are meant to have their normal meanings within the art of liquid chromatography. As such, preparative chromatography involves the high capacity purification or isolation of impurities in a sample, typically a biological sample, prior to using the sample for further analysis or another technique. On the other hand, analytical chromatography involves separating the components of a pure sample for the identification and determination of the percentage composition of the constituents of the sample (i.e., quantitative analysis) or other characterizing properties such as molecular weight, mass, particle size or conversion.  
           [0004]    The present invention has particular, although not exclusive application for use in analytical chromatography, and in particular, high speed analytical chromatography such as that used in combinatorial chemistry methods. Combinatorial chemistry refers generally to the methods for synthesizing a collection of chemically diverse material and rapidly testing or screening the collection for desirable performance characteristics and properties. A detailed discussion of a particular application of combinatorial chemistry (including liquid chromatographic analysis) to polymer science may be found in co-assigned U.S. Pat. No. 6,260,407, the disclosure of which is incorporated herein by reference.  
           [0005]    Combinatorial chemistry requires in its best application that the samples be characterized as quickly as possible. The impact on the liquid chromatography side of such a process is that samples need to be moved as rapidly as possible through one or more chromatographic columns and detectors. However, the column must still be able to perform its separation function, and the detector must have a minimum quantity of liquid in order to detect the characteristic or property of the liquid. To reduce the amount of time it takes the liquid sample to pass through the column, the column is made short. However in order to have sufficient volume, the column has a relatively large diameter with respect to the volume of the liquid sample.  
           [0006]    A typical chromatographic column generally comprises a tube having an internal separation material or “packing” as the stationary phase, which acts to separate the components of the sample in the mobile phase. To ensure adequate separation, it is important that the mobile phase be equally distributed across the stationary phase. Thus, the liquid sample must be spread out over substantially the full internal cross sectional area of the column. Absent adequate distribution of the liquid sample volume at the top or entry point of the column, the mobile phase may tend to channel inside the column. As a result, components of the sample are not sufficiently separated, resulting in the detection of broad adsorption bands which make the characterization of individual components or properties of the sample difficult or impossible.  
           [0007]    Accordingly, it is known to provide distributors which receive the liquid sample from a relatively small diameter capillary entering the column and spread it out over the internal cross sectional area of the column before entering the packing. The distributor may take the form of a plate having channels extending to numerous outlet ports extending from the channels through the plate to the other side. In order to achieve the necessary distribution the outlet ports are arranged over an area substantially equal to the internal cross sectional area of the column. The channels must be of a certain minimum length and have a certain minimum volume to reach those outlet ports. If the volume of the liquid sample plug within the mobile phase is less than the total volume of the channels in the distributor, the distribution of sample will tend to be non-uniform, i.e., more of the sample will flow to some outlet ports than to others.  
         SUMMARY OF THE INVENTION  
         [0008]    Among the several objects and features of the present invention may be noted the provision of a distributor for use in a chromatographic column which uniformly distributes liquid samples of small volumes over the internal cross sectional area of the column; the provision of such a distributor which distributes the liquid over an area which is large in proportion to the volume of the sample; the provision of such a distributor which can operate at high flow rates without damaging the liquid in the sample; the provision of such a distributor which distributes the liquid sample in multiple stages; and the provision of a liquid chromatographic column and a liquid chromatography system employing such a distributor.  
           [0009]    In one aspect of the present invention, a chromatographic column comprises a tube having first and second open ends and packing disposed in the tube and substantially filling the tube between the ends. First and second closures located at the first and second ends of the tube, respectively, have passages for receiving a sample volume of liquid therethrough. A distributor is located in the tube generally adjacent to the first closure and between the passage in the first closure and the packing at the first end. The distributor includes a first member disposed to receive liquid from the passage of the first closure and having channels and outlet ports therein. A second member disposed between the first member and the packing for receiving liquid from the outlets of the first member has channels and outlet ports therein. The number of outlet ports in the second member is greater than the number of outlet ports in the first member so that liquid of a liquid sample is distributed over a wider area into the packing. The channels in the second member being arranged to direct liquid from the outlet ports of the first member to the outlet ports of the second member.  
           [0010]    In another aspect of the present invention, a liquid distributor for use in distributing a small sample volume of liquid over an area in a chromatographic column comprises a first thin, flat plate having multiple channels and outlet ports therein. The channels extend across the first plate to the outlet ports for delivering the sample volume to the outlet ports. A second thin, flat plate has channels and outlet ports therein. The number of outlet ports in the second plate being greater than the number of outlet ports in the first plate so that liquid of the liquid sample is distributed over a wider area into the packing. The channels in the second plate are arranged to direct liquid from the outlet ports of the first plate to the outlet ports of the second plate.  
           [0011]    In still another aspect of the invention, a liquid chromatography system for identifying at least one property of a sample liquid volume comprises a liquid delivery device for delivering a volume of a liquid sample at a selected rate. A column having packing therein for receiving the liquid sample has a distributor in the column to distribute the sample liquid volume substantially over the cross sectional area of the column. The distributor has a thin flat plate including channels and outlet ports therein. The total volume of the channels in the plate is less than about {fraction (1/20)}th of the volume of the liquid sample. A detector detects at least one property of liquid from the column. The liquid delivery device is adapted to move the sample liquid volume through the column at the rate of at least about 2 cm/min.  
           [0012]    In a further aspect of the invention, a method of characterizing a liquid in a flow analysis system comprises the steps of delivering a sample liquid in a volume to a column. The sample liquid volume is conducted in channels on a surface of a thin, flat distributor plate to outlets on the plate. The channels have a total volume less than about {fraction (1/20)}th the sample liquid volume. The sample liquid volume is dispersed over an area within the column by the distributor plate. The liquid sample volume passes from the distributor plate into a separation medium in the column, and the effluent of the liquid sample from the column is analyzed to determine a property of the liquid sample.  
           [0013]    Other objects and features of the present invention will be in part apparent and in part pointed out hereinafter.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a block diagram of a liquid chromatography system including a liquid chromatography column having distributor of the present invention;  
         [0015]    [0015]FIG. 2A is an enlarged, fragmentary, schematic section of the column illustrating a sample plug of liquid approaching the column;  
         [0016]    [0016]FIG. 2B is an enlarged, fragmentary, schematic section of the column illustrating the sample plug of liquid being distributed in the distributor of the column;  
         [0017]    [0017]FIG. 2C is an enlarged, fragmentary, schematic section of the column illustrating the sample plug flowing through the column spread over substantially the full internal cross sectional diameter of the column;  
         [0018]    [0018]FIG. 3 is an enlarged exploded view of the distributor;  
         [0019]    [0019]FIG. 4 is a top plan view of a first member of the distributor;  
         [0020]    [0020]FIG. 5 is a top plan view of a second member of the distributor. 
     
    
       [0021]    Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]    Referring now to the drawings, and in particular to FIG. 1, a liquid chromatography system is shown to include a sample source  1  from which samples of liquid to be analyzed are drawn. The sample source  1  may be sample vials, a reactor in which the samples are formed by chemical reaction, a chemical process line or may be a sample manipulator or other source for liquid samples. An injector  3  injects the samples into the flow of a mobile phase liquid supplied from a mobile phase liquid source  5  by a mover such as a pump  7  past the injector, which injects samples into the flow, and thence to a chromatographic column, indicated generally at  9 . The present invention has particular, although not exclusive, application to liquid chromatography on samples of small volumetric size. More particularly, samples which are less than about 100 microliters, more preferably samples which are less than about 50 microliters and most preferably less than about 20 microliters. Sample sizes of this range are more typical of analytical liquid chromatography as distinguished from preparative chromatography. However, the present invention is not narrowly limited in application to analytical chromatography.  
         [0023]    Preferably, the system is configured to perform high performance liquid chromatography (HPLC), in which samples are contained in a mobile phase and are moved rapidly through the system for analysis. The liquid sample is preferably moved through the column  9  at a rate of at least about 2 cm/min and more preferably at least at about 5 cm/min. In a preferred embodiment, the volume flow rate is about 1 to 50 ml/min and more preferably about 3-15 ml/min. Generally speaking the linear flow rate, which is independent of the cross sectional area of the column, is calculated by dividing the volumetric flow rate by the volume of the column and multiplying by the length. These flow rates are those encountered in HPLC for which the present invention is particularly useful. The construction, configuration and operation of the liquid chromatography system, except as further described herein, are conventional. A discussion of different exemplary techniques useful in HPLC may be found in co-assigned U.S. Pat. No. 6,260,407.  
         [0024]    The chromatographic column separates the sample into various components in a suitable manner that is well understood by those of ordinary skill in the art. In a preferred embodiment, gel permeation chromatography (GPC), a form of size exclusion chromatography, is used. In gel permeation chromatography, the sample is separated into components according to the hydrodynamic volume occupied by each component in solution. Typically, the GPC separation medium comprises a “packing” of porous beads which receive components with molecules of lower hydrodynamic volume, thereby impeding their passage through the column  9 . Components having molecules of larger diameter are not received in the pores of the packing beads and pass more rapidly through the column  9 . Although not necessary or critical to the invention, the remainder of the discussion will be phrased in the context of GPC as an example of a particular chromatographic technique. However, it is to be understood that different separation media and different separation techniques may be employed without departing from the scope of the present invention. For example, reference is made to the aforementioned U.S. Pat. No. 6,260,407 for an additional discussion of the liquid chromatography system and alternative separation strategies. The system further includes a suitable flow through detection device  11  through which liquid sample exiting the column  9  passes.  
         [0025]    Referring now to FIG. 2A, the chromatographic column  9  is shown to comprise a cylindrical tube  13  which has open ends closed by first and second closures (designated  15  and  17 , respectively) threaded onto the tube. Although the tube  13  is shown as cylindrical, tubes of other cross sectional shapes may be used. A first end of the tube  13  is further closed by a distributor constructed according to the principles of the present invention and indicated generally at  18 . The tube  13  is preferably relatively wide in relation to its length as compared with traditional columns. For example, the ratio of column diameter to column height is preferably about 0.1 to 1.0. In one preferred embodiment, the diameter of the tube  13  is about 1 cm and its height is about 4.6 cm, but the exact dimensions of the tube are not narrowly critical to the present invention. The aspect ratio, defined as the ratio of width to height (e.g., D/L for right-cylindrical columns), can preferably range from about 0.3 to about 1.0, from 0.4 to about 1.0 or from about 0.5 to about 1. In some embodiments, columns having a broader range of aspect ratios can be employed in connection with distributor of the present invention. For example, the aspect ratio (e.g., D/L) can more generally range from about 0.01 to about 3.0, from about 0.05 to about 2.0, and from about 0.07 to about 1.5. The first closure  15  has a central passage  19  which threadably receives an end  21  of a fitting  23  connected to a capillary  25  extending from the injector  3  (as shown in FIG. 1) where the liquid sample is injected into the mobile phase liquid. Liquid from the capillary  25  passes from the fitting  23  to the distributor  18  located immediately adjacent to the closure and preferably engaging the end  21  of the fitting. A separation medium or packing, indicated generally at  27 , in the form of porous beads fills the remaining internal volume of the tube  13 . The second closure  17  is substantially identical in construction to the first closure in the illustrated embodiment. The second closure receives a fitting  29  attaching an exit flow capillary  31  to the second closure  17 . A collector  33  can be located in the tube  13  between the packing  27  and the second closure  17  has the same construction as the distributor  18  (more fully described hereinafter). The collector  33  essentially works in reverse of the distributor  18  to collect the sample exiting the packing  27  into a single stream passing into the exit capillary  31 . However, it is to be understood that other forms of collectors may be used. Moreover, the volume of channels (not shown) in components of the collector  33  for conducting the liquid can be larger without adversely affecting the operation of the chromatography system. It is to be understood that the collector  33  is optional and the distributor  18  can be used with or without the collector. Flow of a liquid sample S through the column  9  will be described in more detail hereinafter with reference to FIGS.  2 A- 2 C.  
         [0026]    Although the invention is described herein in connection with columns or tubes having a circular cross-sectional geometry, and preferably being right-cylindrical in shape, it is contemplated that the distributor of the invention may be advantageously applied in connection with other column or tube geometries as well. For example, the column or tube can have a cross-sectional geometry other than circular—such as polygonal (dodecagonal, octagonal, hexagonal, pentagonal, square, triangular, etc.). Moreover, regardless of the cross-sectional geometry (e.g., whether circular or otherwise), the cross-sectional area of the column or tube can be the same over the entirety of its length (e.g., as is the case for a right-cylindrical column), or alternatively, can vary over its length (e.g., funnel-shaped columns having a first end of relatively large circular cross-sectional area relative to its second end, or columns having a necked-down entrance portion, followed by a portion of substantially the same cross-sectional area for a lower portion of the column).  
         [0027]    Referring now to FIG. 3, it may be seen that the distributor  18  of the illustrated embodiment comprises multiple parts which are sandwiched together in layers. That is, the distributor comprises a plurality of laminae, in integral relation to form a common body, with each of the laminae being configured and arranged relative to each other to provide multiple distribution flow paths, originating from one or more common inlets in a top surface of the first laminate, and terminating with multiple, distributed outlets in a bottom surface of a subsequent laminate (i.e.,, a second, third, fourth, fifth, etc. laminate corresponding to the number of laminates in the plurality of laminae that form the distributor). The actual number of laminates used to form the distributor is not critical, and can vary depending on the desired degree of distribution, total desired volume of the distributor flow-paths (taken cumulatively), column geometry, etc. The laminates of the distributor can be held together by pressure (e.g., compression from the closures  15  to the column  13 ), preferably with sealed gaskets therebetween, or alternatively, can be physically or chemically joined to each other (by appropriate fasteners or by bonding such as by anodic bonding), using techniques well know in the art of microfabrication. More specifically, as shown the distributor  18  includes a first thin, flat plate (or “member”)  39 , a second thin, flat plate (or “member”)  41 , an upper gasket  43  and a lower gasket  45 . The first plate  39  is formed of a suitable material such as stainless steel and has channels  47  and outlet ports  49  formed in a first, upwardly directed face  51  of the plate. Because the channels  47  are small in size, quartz is another suitable material because of its ability to be micromachined. The channels  47  are generally V-shaped grooves formed in and extending along the face  51 , and the outlet ports  49  extend entirely through the first plate  39 , opening on the other side of the plate. The particular channel geometry is not critical to the invention, and can be selected from among many possible geometries that are known in the art of microfabrication An inlet port  53  generally in the center of the first plate  39  and can extend through the plate for manufacturing convenience. The inlet port  53  constitutes part of the channels  47 . In the case where the inlet port  53  extends through the first plate  39 , liquid flow through first plate  39  at the inlet port  53  is blocked by engagement with the second plate  41  so that all of the liquid sample S is passed into the channels  47 . As best seen in FIG. 4, there are three channels  47  extending from the inlet port  53 , each including two branches  47 A at its outer end terminating at respective outlet ports  49 . Thus, it may be seen that from a single entry location (inlet port  53 ), liquid is distributed to multiple outlet ports  49 , as indicated by arrows in FIG. 3. The outlet ports  49  are arranged over a first area, which is preferably markedly less than the entire surface area of the first plate  39  and confined to an annular ring. Flow paths for liquid in each channel  47  extend from the inlet port  53  to a respective outlet port  49 . The length of all of the flow paths is the same. Preferably, the total resistance to flow for each of the flow paths is substantially the same, such that a sample provided at the common inlet port  53  will be distributed approximately equally, by volume or mass, to each of the respective outlet ports  49 . The total flow resistance includes resistance to flow based on length, geometric factors (number of turns and angles of turns), and surface finish associated with each channel  47 , among other factors. Moreover, the total volume of the channels  47  is preferably substantially less than the volume of the liquid sample S being analyzed.  
         [0028]    The upper gasket  43  is optional, but if employed fits over and sealingly engages the first face  51  of the first plate  39  to close the channels  47  to prevent liquid in the channels from leaving the channels except through the outlet ports  49  of the first plate. In one embodiment, the upper gasket  43  is made of polytetrafluoroethylene, but may be made of any material which is sufficiently non-reactive and can form a liquid seal. The upper gasket  43  has a central opening  55  (FIG. 3) in registration with the inlet port  53  of the first plate  39  and with the end  21  of the fitting  23 . The liquid sample S may flow from the fitting  23  in the central passage  19  of the first closure  15  through the upper gasket  43  and into the channels  47  of the first plate  39 .  
         [0029]    The second plate  41  is formed of a suitable material such as stainless steel or quartz, and has channels  57  and outlet ports  59  formed in a first, upwardly directed face  61  of the plate. The channels  57  are generally V-shaped grooves formed in and extending along the face  61 , and the outlet ports  59  extend entirely through the plate, opening on the other side of the plate. In the illustrated embodiment, there are six inlet ports  63  aligned with respective outlet ports  49  of the first plate  39  for receiving liquid from the first plate into the channels  57  of the second plate  41 . The flow of liquid from the outlet ports  49  of the first plate  39  into the inlet ports  63  of the second plate  41  are indicated by the vertically downwardly directed arrows in FIG. 3. The inlet ports  63  constitute part of the channels  57 , and can extend entirely through the second plate  41 , in which case liquid flow through the second plate  41  at the inlet ports  63  is blocked by engagement with the lower gasket  45 . As best seen in FIG. 5, there are three channels  57  extending from each inlet port  63 , each channel including two branches  57 A at its outer end terminating at respective outlet ports  59 . Flow of liquid in the channels  57  is indicated by arrows for one of the inlet ports  63  and connected group of outlets  59  in FIG. 3. The lower gasket  45  is optional, but if employed has one opening  65  for each of the outlet ports  59  in the second plate  41  to allow liquid to pass through the lower gasket to the packing in the column  9  (FIG. 3). The lower gasket  45  may be made of the same material as the upper gasket  43 .  
         [0030]    Thus, it may be seen that from the six inlet ports  63  in the second plate  41 , liquid is distributed to  36  outlet ports  59  arranged over a second area. The second area is preferably much larger than the first area and substantially equal to the internal cross sectional area of the tube  13 . Flow paths for liquid in each channel  57  extend from the inlet port  63  to a respective outlet port  59 . The length of all of the flow paths is the same. Preferably, the total resistance to flow for each of the flow paths is substantially the same, such that a sample provided at the common inlet port  63  will be distributed approximately equally, by volume or mass, to each of the respective outlet ports  59 . The total flow resistance includes resistance to flow based on length, geometric factors. Dividing the distribution into two or more stages, for example on the first and second plates  39 ,  41 , respectively, allows the length and therefore volume of the channels  47 ,  57  to be kept low. The total volume of the channels  47 ,  57  of each plate  39 ,  41  is substantially less than the volume of the liquid sample S to be analyzed. Preferably the volume of the channels in each plate (considered independently) is less than about {fraction (1/20)} of the sample volume, more preferably less than about {fraction (1/40)} of the sample volume, still more preferably less than about {fraction (1/80)} of the sample volume and most preferably less than about {fraction (1/100)} of the sample volume. For example, the total volume of the channels  47 ,  57  of each plate  39 ,  41  is less than about 10 microliters, more preferably the volume of the channels may be less than about 1 microliter. However, still smaller volumes of 0.5 microliters or 0.1 microliters are envisioned. The precise volume of the channels may be other than described without departing from the scope of the present invention. Also, the volume of the channels  47 ,  57  in each plate  39 ,  41  is about the same, but the invention is not limited to this equality of volume.  
         [0031]    However the cross sectional area of the individual channels  47 ,  57  may be kept sufficiently large enough by use of multiple plates to inhibit shearing macromolecules flowing through the channels at relative high flow rates. Macromolecules typically are present in the liquid sample when polymers are being analyzed. “Macromolecules” are molecules composed of an aggregation of hundreds or thousands of atoms. See,  Hawley&#39;s Condensed Chemical Dictionary  684 (14th ed., Richard J. Lewis, Sr. ed., 2001). It is to be understood that although two plates  39 ,  41  are shown, a greater number of plates may be used to further multiply the number of outlet ports discharging liquid into the packing. Moreover, the use of a single plate (not shown) is contemplated. Hence, although described herein in detail as a two-laminate configuration, the distributor can comprise larger numbers of laminae, including three or more, four or more, five or more or six or more laminate, held together to form an integral, multi-laminate body as described above. Moreover, the particular configuration of the flow paths is not critical, and is not limiting on the invention. In general, the number of common inlet ports on the first surface of the first laminate can be one or more, and is preferably one common inlet port, adapted for fluid communication with the capillary (or other tubing) of the chromatographic system. The number of outlet ports associated with each particular laminate is generally greater than the number of inlet ports associated therewith. The number of inlet ports of the second laminate (and each succeeding laminate) preferably corresponds to the number (and spatial arrangement) of outlet ports from the immediately-preceding laminate. Using the system and geometric configuration shown herein, each additional plate would increase the number of outlets six times. However, some base number of openings other than six may be used. More generally, the number of outlet ports in the lowest plate is equal to the number of outlet ports in the uppermost plate raised to a power equal to the total number of plates. However, it is to be understood that other arrangements and numbers of outlet ports are envisioned without departing from the scope of the present invention.  
         [0032]    Referring again to FIGS.  2 A- 2 C the handling of a single liquid sample plug S is shown. In FIG. 2A, the sample plug S is passing through the capillary  25  toward the column  9 . Different sample plugs S are separated on either side by the mobile phase liquid, as is known in the art. A given liquid sample S passes through the first closure  15  to the distributor  18 . The flow of the sample S through the distributor  18  is schematically illustrated in FIG. 2B. The sample passes in a single stream through the opening  55  in the upper gasket  43  to the inlet port  53  of the first plate  39 . The flow is divided into the three channels  47  emanating from the inlet port and further divided into two branches  47 A at the end of the channels to the outlet ports  49  of the first plate  39 . At the six outlet ports  49 , the liquid sample passes through the first plate  39  to the inlet ports  63  of the second plate  41 . The liquid sample is directed there to three channels  57  from each inlet port  63 , or  18  total channels. The ends of the channels  57  have two branches  57 A leading to respective ones of the  36  outlet ports  59 . A fewer number of outlet ports  49 ,  59  in both the first and second plates  39 ,  41  have been illustrated in FIG. 2B for clarity. The representation of FIG. 2B is schematic and not intended to comport with the actual construction of the distributor  18 . The liquid sample S exits the distributor  18  by passing through the openings  65  in the lower gasket  45 . As shown in FIG. 2C, the sample S has been converted by the distributor  18  from a cylindrical stream to a thin, flat disk within the packing  27 , having a diameter which is substantially the same as the internal cross sectional diameter of the tube  13 . Thus, the packing  27  is given the best opportunity to separate the liquid sample S into its components for analysis by the detector  11 .  
         [0033]    When introducing elements of the present invention or the preferred embodiment(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than those listed.  
         [0034]    As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.