Abstract:
An apparatus for analyzing a measuring substance which is dissolved in a solvent, comprising a conduit ( 4 ) for transporting the dissolved measuring substance from a feed means ( 3 ) to a measuring location ( 12 ), wherein the feed means ( 3 ) is designed to optionally feed a solvent or dissolved measuring substance into the conduit ( 4 ), is characterized in that the conduit ( 4 ) is designed, at least partially, as a polycapillary area ( 9 ) which has N parallel connected capillaries ( 21   a,    21   b,    21   c;    62   a,    62   b ), such that the individual capillaries ( 21   a,    21   b,    21   c;    62   a,    62   b ) have identical flow times from the feed means ( 3 ) to the measuring location ( 12 ) and wherein N≧2. The apparatus improves the signal-to-noise ratio of the analysis.

Description:
[0001]    This application claims Paris Convention priority of DE 10 2006 023 223.2 filed May 18, 2006 the complete disclosure of which is hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The invention concerns an apparatus for analyzing a measuring substance which is dissolved in a solvent, comprising a conduit for transporting the dissolved measuring substance from a feed means to a measuring location, wherein the feed means is designed to optionally feed a solvent or a dissolved measuring substance into the conduit. 
         [0003]    An apparatus of this type is disclosed in DE 10 2004 029 632. 
         [0004]    LC-NMR (hyphenated liquid chromatography nuclear magnetic resonance) or FI-NMR (flow injection nuclear magnetic resonance) can be used to analyse soluble substances. A liquid sample (generally a mixture of a solvent and the substance to be measured) is thereby guided from a feed means through a conduit, in particular, a glass or plastic capillary, to a measuring location in an NMR spectrometer. 
         [0005]    The liquid sample is thereby typically pushed by subsequently fed transport solvent and the liquid sample is, in turn, pushed by the transport solvent. A so-called “plug flow” is thereby desired, i.e. the liquid sample moves through the conduit like a plug with flat interfaces relative to the transport solvent which extend perpendicularly to the transport direction. 
         [0006]    When the liquid sample advances through the conduit, the sample is spread. The speed of laminar flow is maximum in the center of a capillary and drops off towards the edges, approximately like a parabola. The sample front then assumes an approximately parabolic cross-sectional shape. 
         [0007]    When the sample advances, the liquid sample is consequently distributed over an extending section of the capillary, i.e. the concentration of substance to be measured in the conduit decreases. This decreased concentration of substance to be measured deteriorates the signal-to-noise ratio (SNR) of the NMR measurement at the measuring location. 
         [0008]    Similar problems also occur in other measuring methods using liquid samples, such as UV (ultra violet) absorption or UV fluorescence. 
         [0009]    The “Nanotight Y Connector” by Upchurch Scientific Inc., Catalog of Chromatography &amp; Fluid Transfer Components 2006, page 39, discloses a distributor element for splitting a sample flow from a large capillary into two individual capillaries. This distributor element is used to distribute a liquid sample to two measuring means to which the individual capillaries are guided. 
         [0010]    It is the underlying purpose of the present invention to provide an apparatus for analyzing a measuring substance which is dissolved in a solvent, wherein the apparatus improves the signal-to-noise ratio of the analysis. 
       SUMMARY OF THE INVENTION 
       [0011]    This object is achieved by an apparatus of the above-mentioned type which is characterized in that the conduit is at least partially designed as a polycapillary area which has N parallel connected capillaries, such that the individual capillaries have identical flow times from the feed means to the measuring location, wherein N≧2. 
         [0012]    The flow profile in a capillary depends on the inner diameter (ID) of the capillary. At identical average flow speeds (transported liquid volume per cross-sectional area and unit time), a sample in a capillary with a smaller diameter is spread less than in a capillary with a larger diameter. Thus, by using a capillary with small diameter, the sample smears less. However, a smaller diameter also reduces the overall liquid sample flow. The conduit resistance of the capillary is also increased which must be compensated for by increasing the pressure. 
         [0013]    In accordance with the invention, the desired overall liquid sample flow is therefore distributed over several capillaries which extend parallel to each other. Each individual capillary has a relatively small inner diameter in the polycapillary area compared to the inner diameter of an equivalent capillary whose cross-sectional area is equal to the sum of the cross-sectional areas of the individual capillaries of the polycapillary area. The liquid sample is smeared less during transport through the conduit in each individual capillary and therefore also over the entire conduit. 
         [0014]    The individual capillaries are designed such that when a sample is fed into the polycapillary area, the sample fronts in each individual capillary require the same time to pass through the polycapillary area. The front tips of the sample fronts may thereby be e.g. compared. In the simplest case, the individual capillaries therefore have identical design, in particular the same length, the same inner diameter, and the same inner surface. 
         [0015]    The liquid sample is distributed to the individual capillaries at the entry of the polycapillary area, and the liquid sample is reunited at the end of the polycapillary area with little smearing and typically directly before the measuring location. A conduit in accordance with the invention may comprise one polycapillary area or several polycapillary areas which are disposed consecutively in series. 
         [0016]    Cleaning of the inventive apparatus is also accelerated due to reduced smearing, requiring less cleaning and rinsing liquid. 
         [0017]    When, in an existing apparatus, one single large capillary of one conduit is to be replaced by a polycapillary area, the sum of the cross-sectional areas of the individual capillaries of the polycapillary area should correspond to the cross-sectional area of the large capillary. Retrofitting of existing apparatus can thereby be easily realized. Four individual capillaries with an inner diameter of 250 μm correspond e.g. to a large capillary of a diameter of 500 μm. 
         [0018]    In one particularly preferred embodiment of the inventive apparatus, the individual capillaries have identical inner diameters and identical lengths. Moreover, the individual capillaries preferably also have identical inner surfaces, e.g. an identical inner coating or the same capillary material. The inner diameter and the inner surface determine the flow profile or the flow speed in the capillary. Identical flow speeds and identical lengths produce identical flow times of the liquid sample. This embodiment easily realizes identical flow times for a liquid sample in the individual capillaries. Quartz glass capillaries maintain particularly narrow inner diameter (and outer diameter) tolerances. 
         [0019]    In an alternative embodiment, the length of one single capillary is adjusted to the flow speed in this capillary. Due to production tolerances, e.g. of the inner diameter, the flow speeds in the individual capillaries of a polycapillary area may slightly differ. This can be easily determined through previous measurement of the individual capillaries. In accordance with the invention, the individual capillary with the highest flow speed is e.g. left unchanged and the individual capillaries with a smaller flow speed are shortened in correspondence with their deviation. This also ensures identical flow times through all individual capillaries of the polycapillary area. The flow speed can be measured e.g. using the advance of the tip of a sample front. 
         [0020]    In another preferred embodiment, the overall conduit is designed as a polycapillary area. Spreading of the sample is reduced to a maximum extent. Further components such as a chromatography column may be contained in the conduit, which are each connected to polycapillary areas. 
         [0021]    In a preferred embodiment, the conduit contains a chromatography column. The chromatography column separates different components of a sample. 
         [0022]    In another preferred embodiment, the conduit terminates in a measuring cell at the measuring location. The measuring cell is one single chamber, wherein the actual analysis measurement of liquid sample takes place in the chamber. The measuring cell, in particular its dimensions, is adjusted to analysis measurement. 
         [0023]    In an alternative embodiment, the measuring location may be designed as a polycapillary area. The capillaries have very thin walls compared to typical measuring cells (capillaries typically 70 to 170 μm, measuring cells 150 to 500 μm). Absorption losses can thereby be reduced, in particular, in NMR measurements. 
         [0024]    In one particularly preferred embodiment, the capillaries have a hydrophobic inner coating. The hydrophobic (non-polar) inner coating, e.g. of FEP, improves the flow profile, in particular, when polar (transport) solvents such as water are used. A liquid sample will then spread less than without a hydrophobic inner coating, i.e. the parabolic flow profile is flatter. A coating also means surface treatment (passivation) of the inner surface. This has proven to be advantageous for quartz glass capillaries. An inventive non-polar capillary inner surface may also be obtained by a non-polar base material of the capillary itself. 
         [0025]    In another preferred embodiment, the number N of capillaries in the polycapillary range is between 2 and 7. This number is easy to handle, in particular, simultaneous supply of the individual capillaries with liquid sample is still easily possible. 
         [0026]    One embodiment has proven to be useful in practice, with which the inner diameter of the capillaries is between 50 μm and 500 μm. This realizes good flow performance without excessively increasing the transport pressure or excessively spreading the sample. 
         [0027]    One advantageous embodiment comprises a plastic coating in which the capillaries extend. The plastic coating protects the capillaries of the polycapillary area from damage. 
         [0028]    In another preferred embodiment, the capillaries are individually guided in some areas through bores of guiding elements. The guiding elements may e.g. be inserted at the start and end of a polycapillary area, i.e. as a part of a distributor element. The guiding elements can preferably be elastically deformed. Slight compression seals the bores from the capillaries and the individual capillaries can be held. The capillaries may be slightly shifted within the bores for adjustment, if required. 
         [0029]    In another preferred embodiment, the inlet of the polycapillary area has a distributor element for the N capillaries. Typically, a large capillary leads to the distributor element, and opens in a widening distributor chamber. The individual capillaries of a polycapillary area exit opposite to the opening of the large capillary. The distributor chamber is preferably sufficiently small (e.g. ≦20 nL volume) such that the flow paths within the distributor chamber are small compared to flow paths within the individual capillaries of the polycapillary area. 
         [0030]    In a further development of this embodiment, the N capillaries are disposed symmetrically about a central axis in the distributor element and at identical separations from the central axis. A large capillary preferably terminates on the central axis opposite to the individual capillaries. The symmetry ensures simultaneous feeding of the individual capillaries of the polycapillary area. 
         [0031]    In one particularly preferred embodiment, the apparatus is designed as an NMR spectrometer or UV detection apparatus. These sensitive methods profit especially from a higher measuring substance concentration in the analysis. 
         [0032]    Further advantages of the invention can be extracted from the description and the drawing. The features mentioned above and below may be used in accordance with the invention either individually or collectively in arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but have exemplary character for describing the invention. 
         [0033]    The invention is shown in the drawing and explained in more detail with reference to embodiments. 
     
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0034]      FIG. 1  shows an inventive apparatus with a conduit having a polycapillary area; 
           [0035]      FIG. 2  shows a schematic perspective exploded view of a polycapillary area of a conduit as used in accordance with the invention; 
           [0036]      FIG. 3   a  shows a schematic side view of the distributor element for the invention; 
           [0037]      FIG. 3   b  shows the distributor element of  FIG. 3   a  with clamped guiding element and capillaries; 
           [0038]      FIGS. 4   a - 4   e  show schematic cross-sectional views through guiding elements for the invention; 
           [0039]      FIG. 5   a  shows a schematic view of a feed means for the invention in a valve position for filling a sample buffer; 
           [0040]      FIG. 5   b  shows the feed means of  FIG. 5   a  in a valve position for feeding sample from the sample buffer into the conduit to the detector; 
           [0041]      FIG. 6   a  shows a schematic view of a flow profile of a large capillary; 
           [0042]      FIG. 6   b  shows a schematic view of the flow profiles of two capillaries with the same total overall cross-sectional area as the large capillary of  FIG. 6   a;    
           [0043]      FIG. 6   c  shows a schematic view of the flow profile of a capillary with the same diameter as the capillary of  FIG. 6   a  but with hydrophobic inner coating. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0044]      FIG. 1  schematically shows the structure of an inventive apparatus for investigating a measuring substance dissolved in a solvent, using instrumental analysis, in particular using NMR spectrometry. 
         [0045]    Transport solvent is supplied from a supply container  1  for a transport solvent or mobile liquid, using a pump  2 , to a feed means (or injector)  3 . The transport solvent is either directly supplied through the feed means  3  into a line  4  or via a sample storage in the feed means  3 . In the latter case, the content of the sample storage, which is pushed by the transport solvent, is supplied to the conduit  4 . The sample storage can be filled with a liquid sample via an injection connection  5 . The liquid sample comprises a solvent which contains the dissolved substance to be measured (measuring substance). Excessive amounts of sample can be discharged into a waste container  6 . 
         [0046]    The conduit  4  is designed as a large capillary in a first short transition section  7   a  directly behind the feed means  3 . The first transition section  7   a  terminates in a chromatography column  11 . The supplied liquid is further supplied to a distributor element  8  via a second transition section  7   b.  The conduit  4  extends from the distributor element  8  in the form of a polycapillary area  9  with several (in the present case three) individual capillaries, in which the supplied liquid is propagated in parallel. The flows of the individual capillaries are united at a unification element  10  which has a similar structure as the distributor element  8 , and are transferred to a third transition section  7   c.    
         [0047]    The supplied liquid is supplied to a measuring location  12  via the third transition section  7   c.  The measuring location  12  is disposed in a detector  13 , in the present case an NMR spectrometer. The actual analytical measurement takes place at the measuring location  12 . The supplied liquid is finally disposed into a waste container  14 . 
         [0048]    The conduit  4  connects the feed means  3  to the measuring location  12  of the apparatus. The major part of the conduit  4  thereby extends as a polycapillary area  9  to reduce spreading of the liquid sample during transport. 
         [0049]      FIG. 2  shows an exploded schematic view of a polycapillary area  9 . The polycapillary area  9  comprises three individual capillaries  21   a,    21   b,    21   c,  which are produced from fused silica. The capillaries  21   a - 21   c  have an outer plastic coating, e.g. of PEEK (polyether ether ketone) (not shown) in order to guarantee the flexibility of the capillaries  21   a - 21   c.  The inner surfaces of the capillaries  21   a - 21   c  are coated with a hydrophobic substance, e.g. FEP (tetrafluoroethylene perfluoropropylene) (not shown). All capillaries  21   a - 21   c  have identical lengths, identical inner diameters and identical outer diameters. The production tolerance of the capillaries relative to the inner diameters is ±2% or less over the entire length, the maximum length difference between the capillaries should be at most ±1%. 
         [0050]    The three capillaries  21   a  to  21   c  are disposed in a flexible plastic coating  22 . This reduces the mechanical load on a comparably sensitive individual capillary  21   a,    21   b,    21   c  in handling the polycapillary area  9  (e.g. when the conduit is laid). Illustration of the plastic coating  22  is interrupted for simplification. The plastic coating  22  is slightly shorter than the capillaries  21   a - 21   c.    
         [0051]    Circular cylindrical, elastically deformable guiding elements  23  are provided at the upper and lower ends of the polycapillary region  9 . Three bores  24   a,    24   b,    24   c  are provided in the guiding elements  23 , whose bore diameters are slightly larger than the outer diameters of the capillaries  21   a - 21   c  (e.g. with a bore diameter 370 μm and outer diameter of the capillaries 363 μm). The capillaries  21   a - 21   c  can therefore be easily inserted into the bores  24   a - 24   c.    
         [0052]    For connection to the apparatus, the guiding elements  23  are clamped when the capillaries  21   a - 21   c  are inserted, wherein the guiding elements  23  are compressed in a radial direction along an annular ring (or a cylindrical jacket area). This may be effected e.g. in a distributor element. The inner walls of the bores  24   a - 24   c  thereby abut the outer walls of the capillaries  21   a - 21   c  in a liquid-tight fashion. The capillaries  21   a - 21   c  themselves are sufficiently stiff to prevent them from being compressed. 
         [0053]    The illustrated polycapillary area  9  may also be called multi-lumen capillary. The multi-lumen capillary can extend over large distances, in particular, more than 1 m, wherein liquid sample in the multi-lumen capillary is only minimally mixed (and diluted) with advancing and trailing transport solvent. 
         [0054]      FIGS. 3   a  and  3   b  show schematic lateral cross-sections through a distributor element  8 , on its own ( FIG. 3   a ), and with clamped capillaries  21   a,    21   b,    34 , guiding element  23 , and holding element  33  ( FIG. 3   b ). 
         [0055]    A first recess  31  is provided for first fitting  39  of the holding element  33  to one individual, large capillary  34 . A second recess  32  is provided for second fitting  39  of the guiding element  23  to the capillaries  21   a,    21   b  of the polycapillary area. The holding element  33  and guiding element  23  are fixed through clamps thereby utilizing the elastic properties of the holding element  33  and the guiding element  23 . Towards this end, a connecting technique, which has proven to be useful in chromatography, i.e. fitting  39 , is used which is screwed into the distributor element  8  using a thread  37 . The conical extension of the fitting  39  at its front area and the conical inner profile of the distributor element  8  produce a clamping effect (i.e. narrowing of the fitting  39  and also narrowing of the guiding element  23  or the holding element  33 ) which seals the capillaries  21   a,    21   b  and  34  and keeps them in position. 
         [0056]    A connecting bore  35  and a funnel-shaped distributor chamber  36  are provided in the distributor element  8 , through which liquid can be guided from the large capillary  34  to the capillaries  21   a,    21   b  of the polycapillary area (or vice versa). In the clamped state, the large capillary  34  and the connecting bore  35  are disposed centrally on a central axis  38  of the distributor element  8 . The capillaries  21   a,    21   b  of the polycapillary area are grouped symmetrically about the central axis  38  such that the flow paths from the opening of the connecting bore  35  to the opening of each capillary  21   a,    21   b  in the distributor chamber  36  have the same length. In the illustrated embodiment, there is, in particular, no capillary on the central axis  38 . 
         [0057]    When the dimensions of the distributor chamber are sufficiently small compared to the lengths of the capillaries  21   a,    21   b,  a symmetrical arrangement of the capillary openings in the distributor chamber  36  can be omitted without producing noticeable running time differences between the capillaries of the polycapillary area. 
         [0058]    PEEK has proven to be useful as distributor element  8  material. 
         [0059]      FIGS. 4   a  to  4   e  show some guiding elements which can be used in a recess in a distributor element (see  FIG. 3   a,    FIG. 3   b ) in accordance with the invention, in a cross-section along the plane A of  FIG. 3   b.  They show examples with three ( FIG. 4   a ) to seven ( FIG. 4   e ) bores  24   a,    24   d  for capillaries. 
         [0060]    All bores  24   a,    24   d  are disposed symmetrically relative to a central axis  38  in order to obtain uniformly good sealing of all capillaries for radial clamping. The embodiments of  FIGS. 4   a,    4   b  only have bores  24   a  having identical separations from the central axis  38 , which guarantees a very uniform flow distribution to the capillaries. In contrast thereto, the embodiments of  FIGS. 4   c - 4   e  each also have a bore  24   d  on the central axis  38  (with a separation “zero” from the central axis). This increases the overall flow. When the polycapillary areas are sufficiently long (e.g. 1 m or more), a running path difference in the distributor element (of e.g. 500 μm) compared to other running path differences among the capillaries of a polycapillary area (e.g. due to production tolerances in the inner diameter) can be neglected. 
         [0061]      FIGS. 5   a  and  5   b  show the function of a feed means  3  which can be used in connection with the invention. The feed means  3  has a total of four connections to the outside:
       a feed line from the pump  2  for introducing transport solvent;   a discharge line via the conduit to the detector  13 ;   the injection connection  5  via which liquid sample can be filled into the sample storage  51 ; and   a discharge line to a waste container  6 .       
 
         [0066]    The sample storage  51  moreover has two access openings  52   a,    52   b.    
         [0067]    The feed means  3  has a disc-shaped rotary valve  53  with three connecting channels  54   a,    54   b,    54   c  which can connect neighboring connections  2 ,  13 ,  52   b,    6 ,  5 ,  52   a  to each other. 
         [0068]    In the position of the rotary valve  53  of  FIG. 5   a,  liquid sample can be filled from the injection connection  5  via the connecting channel  54   a  and the opening  52   a  into the sample storage  51 . The sample storage  51  is designed as a spiral capillary. Excessive liquid sample flows through the opening  52   b  and the connecting channel  54   c  into the waste container  6 . At the same time, transport solvent is guided from the pump  2  via the connecting channel  54   b  to the detector  13 . 
         [0069]    In the position of the rotary valve  53  of  FIG. 5   b,  the sample is fed into the system. The rotary valve  53  was turned to the right through 60° (clockwise direction) as compared to  FIG. 5   a.  Transport solvent is guided from the pump  2  via the connecting channel  54   b  into the sample storage  51 . The liquid sample previously introduced into the sample storage  51  is forced out via the connecting channel  54   a  and to the detector  13 . 
         [0070]      FIGS. 6   a  through  6   c  show smearing of liquid samples during movement in the capillaries under different conditions.  FIG. 6   a  represents a capillary  61  of a typical size,  FIG. 6   b  shows two small capillaries  62   a,    62   b  whose overall cross-sectional area corresponds to the cross-sectional area of the large capillary  61  of  FIG. 6   a,  and  FIG. 6   c  shows a large capillary  63  like in  FIG. 6   a  but with a hydrophobic inner surface (e.g. due to coating). 
         [0071]    A thin, disc-shaped sample (e.g. produced by NMR excitation in only one disc-shaped area in the capillary) is pushed through each capillary by following transport solvent, in  FIGS. 6   a  through  6   c  from the top to the bottom. In  FIGS. 6   a,    6   b,    6   c,  the same amounts of solvent volume were added over the same time period and uniformly distributed to the two parallel capillaries  62   a,    62   b  ( FIG. 6   b ). 
         [0072]    In most cases smearing of the sample can observed. The liquid sample attains an approximately parabolic profile. In the center of the capillary, the sample advances more quickly and the sample advances more slowly at the capillary edge. 
         [0073]    Maximum smearing takes place in the uncoated large capillary  61  of  FIG. 6   a.  The smearing can be reduced through using a hydrophobic inner coating (see  FIG. 6   c ) and also through distribution of the liquid transport to several (in the present case two) capillaries  62   a,    62   b  in accordance with the invention. 
         [0074]    The following table shows quantitative values for the smearing degree in an individual capillary and a double capillary (dual lumen capillary) which were determined through experiments. The position of the tip of the sample front is thereby compared to the position of a flat sample line with a hypothetic absolutely non-smeared sample (“plug flow”) with a certain amount of added solvent. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Inner diameter 
                 Volume in 
                 Smearing 
               
               
                   
                 in mm 
                 μl/mm 
                 in % 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Individual 
                 0.512 
                 0.206 
                 154.5 
               
               
                   
                 capillary (FEP) 
               
               
                   
                 Double capillary 
                 0.363 
                 2 × 0.104 
                 111.4 
               
               
                   
                 (FEP) 
               
               
                   
                   
               
             
          
         
       
     
         [0075]    By using two smaller capillaries of each 0.363 μm instead of one single large capillary of a diameter of 512 μm, smearing can be considerably reduced (by approximately 43%) with the same overall cross-sectional area which is available for the flow. 
         [0076]    In summary, the invention describes a measuring apparatus for investigating liquid samples, wherein the liquid samples are transported via a conduit from a feed means to a measuring location. The conduit is thereby largely designed as a polycapillary area in which several capillaries take over parallel transport of liquid sample and transport solvent. The individual capillaries are designed such that liquid sample flows through them within the same time. 
         [0077]    In the simplest case, the capillaries are of identical design, so that all capillaries have the same flow velocity and also identical lengths. The several individual capillaries can have a smaller inner diameter than one single capillary of the same overall cross-sectional area. This reduces spreading of liquid sample. The higher sample concentration after transfer to the measuring location improves the SNR of the analysis.