Patent Publication Number: US-2023160801-A1

Title: Measurement cell and centrifugal sedimentation-type particle-size distribution measuring device using said measurement cell

Description:
TECHNICAL FIELD 
     The present invention relates to a centrifugal sedimentation type particle size distribution measuring device, and particularly, relates to a measurement cell used in a line start mode of a centrifugal sedimentation type particle size distribution measuring device. 
     BACKGROUND ART 
     As a conventional centrifugal sedimentation type particle size distribution measuring device, there has been a device that uses a line start mode in which a sample suspension is supplied into a cell that contains a density gradient liquid, and particles in the sample suspension are centrifugally precipitated in the density gradient liquid, as disclosed in Non Patent Document 1. 
     Specifically, this particle size distribution measuring device uses a disk shaped, hollow cell. The cell storing therein a density gradient liquid is then rotated at a constant rotation speed, and a sample suspension is injected from the center of the rotation. 
     However, because the sample suspension is injected into the rotating cell from the outside, it is more likely for a variation to be introduced to the timing at which the sample suspension is injected. Furthermore, there are extensive variations in the time required for the sample to reach the density gradient liquid contained in the cell. Therefore, the resultant measurements suffer from some inaccuracy. 
     CITATION LIST 
     Non Patent Literature 
     
         
         Non Patent Literature 1: Stephen T. Fitzpatrick, “Measurement of particle size distribution by frequency dependent centrifugal sedimentation: advantages and disadvantages, current situation, and future outlook” [searched on May 30, 2018], Internet (URL: https://www.nihon-rufuto.com/myadmin/rufuto_catalog/wp-content/uploads/2017/06/CPS-Polymer-News.pdf) 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     The present invention has been made to solve the problem described above, and a main object of the present invention is to improve measurement accuracy of a particle size distribution measuring device that measures a particle size distribution in a line start mode. 
     Solution to Problem 
     In other words, a measurement cell according to the present invention is a measurement cell used in a line start mode of a centrifugal sedimentation type particle size distribution measuring device, the measurement cell being characterized by including: a cell main body that has an opening on one end and that stores therein a density gradient liquid that is a solution having a density gradient; a cell cap that closes the opening of the cell main body and that is provided with an internal passage for holding a sample liquid, wherein the sample liquid is introduced into the density gradient liquid through the internal passage by receiving an application of a centrifugal force. 
     With such a configuration, because the internal passage provided inside the cell cap holds the sample liquid, and the sample liquid is introduced through the internal passage into the density gradient liquid by receiving an application of a centrifugal force, it is possible to establish the timing of the application of the centrifugal force to the measurement cell or the timing of rotating the measurement cell at a predetermined rotation speed, as the timing of the start of the measurement in the line start mode. Hence, it is possible to improve the measurement accuracy. In addition, because the sample liquid is held internal of the cell cap, it is possible to reduce the distance between the sample liquid and the density gradient liquid. Therefore, it is possible to reduce variations in the time required for the sample liquid to reach the density gradient liquid. With this, too, the measurement accuracy can be improved. 
     In a configuration in which the measurement cell is configured removably from the device main body, a thought is given to reduce the size of the measurement cell so as to make it easier to attach and to remove the measurement cell. 
     In such a case, the configuration in which the sample liquid is introduced from the outside could be a cause of a measurement error because the time required for the sample liquid to reach the measurement cell becomes extended. In addition, an introducing mechanism for introducing the sample liquid into the rotating measurement cell from the outside becomes complicated. 
     By contrast, according to the present invention, because the sample liquid is held internal of the cell cap of the measurement cell, it is possible to solve such a problem caused by making the measurement cell removable from the device main body, advantageously. 
     To allow the internal passage internal of the cell cap to hold the sample liquid with the cell cap attached to the cell main body, it is preferable to provide a sample inlet on one end of the internal passage and to provide a sample outlet to the other end, and for the sample inlet to be positioned external of the cell main body and for the sample outlet to be positioned internal of the cell main body when the cell cap is attached to the cell main body. 
     As a specific embodiment of the internal passage, it is preferable for the internal passage to include a liquid reservoir that holds the sample liquid introduced from the sample inlet, and a main passage that communicates with the liquid reservoir and that communicates with the sample outlet. When an application of a centrifugal force is received, the sample liquid stored in the liquid reservoir preferably flows out into the main passage, passes through the main passage, and is introduced into the density gradient liquid. The content volume of the liquid reservoir is determined based on the volume of the sample liquid. 
     By providing the liquid reservoir in the internal passage in the manner described above, the position where a predetermined amount of the sample liquid is held inside the internal passage becomes constant, so that it is possible to improve the reproducibility of particle size distribution measurements. 
     A configuration in which the main passage has a constant diameter has a risk of causing a problem that the sample liquid is introduced into the density gradient liquid as a lump, and the measurement of the particle size distribution fails to be carried out accurately due to a streaming phenomenon. 
     In order to solve this problem advantageously, it is preferable for the main passage to include a small diameter passage portion connected to the liquid reservoir and a large diameter passage portion provided downstream of the small diameter passage portion and communicating with the sample outlet. 
     With this configuration, the sample liquid from the small diameter passage portion spreads inside the large diameter passage portion, and is introduced into the density gradient liquid through the sample outlet. As a result, it is possible to prevent the streaming phenomenon, and to improve the measurement accuracy. 
     As a specific embodiment of the internal passage, it is preferable for the internal passage to also include an inlet passage portion communicating with the sample inlet and the liquid reservoir, for the inlet passage portion to be provided along a direction of a rotating shaft of the measurement cell, and for the main passage to be provided along the direction of the centrifugal force applied to the measurement cell. If the main passage is provided along the direction of the centrifugal force, it is possible to allow the sample liquid to flow through the main passage quickly. 
     In order to keep the sample liquid inside the liquid reservoir reliably before the measurement cell is rotated, and to allow the sample liquid to flow out of the liquid reservoir smoothly into the main passage when the measurement cell is rotated, it is preferable for the main passage to be connected to an upper part of the liquid reservoir, and for the liquid reservoir to have an inclined surface inclined upwards toward the main passage on the side connected with the main passage. 
     In order to facilitate machining of the liquid reservoir in the cell cap, it is preferable for the liquid reservoir to have a conical shape or a partially conical shape tapered toward the bottom. 
     With this configuration, the liquid reservoir may be formed in the cell cap by machining a hole into the cell cap using a drill. 
     When the sample outlet of the internal passage is in contact with the density gradient liquid, a surface tension acting near the sample outlet may prevent spreading (dispersion) of the sample liquid into the density gradient liquid. 
     In order to solve this problem advantageously, it is preferable for a space to be ensured between the cell cap and the density gradient liquid when the cell cap is attached to the cell main body. 
     With this configuration, the sample liquid introduced into the density gradient liquid is allowed to spread reliably, so that it becomes possible to improve the measurement accuracy. 
     Preferably, the cell main body and the cell cap are provided with a restricting mechanism for restricting the orientation in which the cell cap is attached to the cell main body. 
     With this configuration, it is possible to prevent the cell cap from being attached to the cell main body in an incorrect orientation. 
     Furthermore, a centrifugal sedimentation type particle size distribution measuring device according to the present invention is characterized by including the measurement cell described above, and a cell rotating mechanism that rotates the measurement cell in such a manner that a centrifugal force in a direction from a low density side toward a high density side of the density gradient is exerted on the measurement cell. 
     A possible configuration of the cell rotating mechanism includes a cell holder on which the cell is mounted. In this configuration, it is preferable for the measurement cell to be configured to be removable from the cell holder. 
     With such a configuration, only the component removed from the device main body is the cell, so that the removing operation can be simplified, and the subsequent cleaning operation can also be simplified. In addition, it is also possible to configure the cell to be removable from the device main body by using a configuration having a cell holder that is removable from the device main body. 
     The measurement cell and the cell holder are preferably provided with a restricting mechanism for restricting the orientation in which the measurement cell is attached to the cell holder. 
     With this configuration, it is possible to prevent the measurement cell from becoming attached to the cell holder in an incorrect orientation. 
     Furthermore, a measurement cell according to the present invention is a measurement cell used in a centrifugal sedimentation type particle size distribution measuring device, the measurement cell being characterized by including: a storage space where a dispersion medium is stored, and an internal passage communicating with the storage space and holding a sample liquid, and the sample liquid is introduced through the internal passage into the dispersion medium by receiving an application of a centrifugal force. 
     With such a measurement cell, the internal passage communicating with the storage space holds the sample liquid, and the sample liquid is introduced through the internal passage into the dispersion medium by receiving an application of a centrifugal force. Therefore, it is possible to reduce the distance between the sample liquid and the dispersion medium, so that it becomes possible to suppress variations in the time required for the sample liquid to reach the dispersion medium. With this, too, the measurement accuracy can be improved. 
     Advantageous Effects of Invention 
     According to the present invention described above, it is possible to improve the measurement accuracy of a particle size distribution measuring device that performs a particle size distribution measurement in a line start mode. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram schematically illustrating a configuration of a centrifugal sedimentation type particle size distribution measuring device according to an embodiment of the present invention. 
         FIG.  2    is a plan view of a cell holder having a cell according to the embodiment attached. 
         FIG.  3    is a cross sectional view schematically illustrating a configuration in which a cell cap is removed from a cell main body of a measurement cell according to the embodiment. 
         FIG.  4    is a cross sectional view schematically illustrating a configuration in which the cell cap is attached to the cell main body of the measurement cell according to the embodiment. 
         FIG.  5    is a plan view, a front view, and a cross sectional view taken along the line A-A, illustrating a configuration of the cell cap according to the embodiment. 
         FIG.  6    is a cross sectional view illustrating (1) a stage of sample preparation and (2) a configuration before rotation is started, in the embodiment. 
         FIG.  7    is a cross sectional view illustrating a process in which a sample liquid reaches a density gradient liquid after the rotation is started, in the embodiment. 
         FIG.  8    is a plan view and a cross sectional view taken along the line A-A, illustrating a configuration of a cell cap according to another embodiment. 
     
    
    
     REFERENCE SIGNS LIST 
     
         
         
           
               100  centrifugal sedimentation type particle size distribution measuring device 
               2  measurement cell 
               3  cell rotating mechanism 
               31  cell holder 
             W SS  sample liquid 
             W DGS  density gradient liquid 
               21 H opening 
               21  cell main body 
             R internal passage 
               22  cell cap 
               2 A sample inlet 
               2 B sample outlet 
             R 1  liquid reservoir 
             R 2  main passage 
             R 2   a  small diameter passage portion 
             R 2   b  large diameter passage portion 
             R 3  inlet passage portion 
             R 1   x  inclined surface 
             S 1  space 
           
         
       
    
     DESCRIPTION OF EMBODIMENTS 
     A centrifugal sedimentation type particle size distribution measuring device according to an embodiment of the present invention will now be explained with reference to some drawings. 
     &lt;1. Device Configuration of Particle Size Distribution Measuring Device&gt; 
     A centrifugal sedimentation type particle size distribution measuring device  100  according to the present embodiment performs a particle size distribution measurement in a line start mode, and includes, as illustrated in  FIG.  1   , a measurement cell  2  where a density gradient liquid W DGS  and a sample liquid W SS  are stored, a cell rotating mechanism  3  that rotates the measurement cell  2 , and a light emitting unit  4  and a light detecting unit  5  that are provided facing each other across a section where the measurement cell  2  being rotated by the cell rotating mechanism  3  passes. The sample liquid W SS  according to the present embodiment is a sample suspension in which particles are dispersed. 
     The measurement cell  2  is a hollow rectangular cell having a substantially cuboid shape, for example. A density gradient liquid W DGS  that is a solution with a density gradient is stored in the measurement cell  2 . The density gradient liquid W DGS  is formed using, for example, a plurality of sucrose solutions having different concentrations, and is stored as a plurality of layers in such a manner that the density gradually increases toward the bottom of the measurement cell  2 . In the present embodiment, a reference cell  6  is also provided, and water is stored in the reference cell  6 . 
     The cell rotating mechanism  3  is configured to rotate the measurement cell  2  in such a manner that a centrifugal force in a direction from a low density side toward a high density side of the density gradient is exerted on the measurement cell  2 . 
     Specifically, the cell rotating mechanism  3  includes a cell holder  31  to which the measurement cell  2  and the reference cell  6  are removably attached, and a rotating unit  32  that rotates the cell holder  31 . 
     As illustrated in  FIG.  2   , the cell holder  31  has a disk like shape, for example, and the measurement cell  2  and the reference cell  6  are attached facing each other with a rotational center of the cell holder  31  therebetween. At this time, the measurement cell  2  is attached in such a manner that the direction of the density gradient extends along the radial direction of the cell holder  31 . In addition, the cell holder  31  has attaching recesses  31   a  each having a shape corresponding to the shape of a cell, and the cells  2  and  6  are attached by being fitted into the attaching recesses  31   a , respectively. Furthermore, guiding mechanisms (not illustrated) are provided between the measurement cell  2  and the reference cell  6 , and the respective attaching recesses  31   a . Each of the guiding mechanisms includes a guide rail provided to one of the attaching recess  31   a  and the cells  2  and  6 , and a guide groove provided to the other. The guide rail or the guide groove provided to the cell  2 ,  6  may be provided integrally with the cell  2 ,  6 , or may be provided to a member attached to the cell  2 ,  6 . The guiding mechanism also functions as a restricting mechanism that restricts the orientation in which the measurement cell  2  or the reference cell  6  is attached to the cell holder  31 . 
     The measurement cell  2  and the reference cell  6  are configured to be removable from the cell holder  31 . By configuring the cells  2  and  6  to be removable from the cell holder  31 , the cells  2  and  6  are configured to be removable from the device main body. An opening and closing lid  13  is opened when the cells  2  and  6  are to be removed from the device main body. 
     A cover body  33  for preventing the measurement cell  2  and the reference cell  6  from being unexpectedly detached during the rotation is provided on the top surface of the cell holder  31  (see  FIG.  1   ). 
     As an orientation which the measurement cell  2  is attached to the cell holder  31 , the measurement cell  2  is attached in such a manner that the high density side is on the radially outer side. As a result, when the cell holder  31  is rotated, a centrifugal force in a direction from a low density side toward a high density side of the density gradient is exerted on the measurement cell  2 . 
     As illustrated in  FIG.  1   , the rotating unit  32  includes a rotating shaft  321  that is connected to the center of the bottom surface of the cell holder  31 , and a motor  322  that rotates the rotating shaft  321 . The rotation speed of the motor  322  is controlled by the control unit  10 . Note that the rotating shaft  321  may be provided integrally with or separately from the cell holder  31 . Furthermore, the rotating shaft  321  may be made from one member or may be made by connecting a plurality of members. 
     The cell holder  31  is housed in a housing space  100 S provided inside the particle size distribution measuring device  100 . The rotating shaft  321  of the rotating unit  32  is passed through a bottom wall  11  by which the housing space  100 S is formed. A top wall  12  by which the housing space  100 S is formed is provided as an opening and closing lid  13  that is opened and closed when the measurement cell  2  is to be attached and removed. 
     As illustrated in  FIG.  1   , the light emitting unit  4  is provided below a section across which the rotating cells  2  and  6  pass (below the cell holder  31 ). The light emitting unit  4  according to the present embodiment is provided under the bottom wall  11  of the housing space  100 S, and emits light into the cells  2  and  6  through a light transmission window  11 W provided to the bottom wall  11 . Specifically, the light emitting unit  4  includes a light source  41  such as an LED, and a condenser lens  42  that condenses the light emitted from the light source  41 . The light emitted from the light emitting unit  4  passes through a light passage hole  31   h  provided to the cell holder  31 , and the measurement cell  2  or the reference cell  6  is irradiated therewith. 
     As illustrated in  FIG.  1   , the light detecting unit  5  is provided above the section across which rotating cells  2  and  6  pass (above the cell holder  31 ). The light detecting unit  5  according to the present embodiment is provided above the top wall  12  of the housing space  100 S, and detects light transmitted through the cells  2  and  6 , via a light transmission window  12 W provided on the top wall  12 . Specifically, the light detecting unit  5  includes a light detector  51  and a condenser lens  52  that condenses the light to be detected by the light detector  51 . The light to be detected by the light detecting unit  5  passes through the cells  2  and  6 , passes through a light passage hole  33   h  provided to the cover body  33 , and is condensed by the condenser lens  52 . 
     A light intensity signal obtained by the light detector  51  is acquired by the particle size distribution computing unit  15 , and the particle size distribution computing unit  15  calculates particle size distribution data. The particle size distribution computing unit calculates particle size distribution data using the light intensity signals and a rotation start signal acquired from the control unit  10 . The particle size distribution data is displayed on a display by a display unit, not illustrated. 
     &lt;2. Specific Configuration of Measurement Cell  2 &gt; 
     As described above, the measurement cell  2  according to the present embodiment is used in the line start mode of the centrifugal sedimentation type particle size distribution measuring device  100 . 
     Specifically, as illustrated in  FIGS.  3  and  4   , the measurement cell  2  includes a cell main body  21  that has an opening  21 H provided on one end and that stores therein the density gradient liquid W DGS , and a cell cap  22  that closes the opening  21 H of the cell main body  21  and internal of which is provided with an internal passage R for holding the sample liquid W SS . The measurement cell  2  is configured in such a manner that, when an application of a centrifugal force is received, the sample liquid W SS  is introduced into the density gradient liquid W DGS  through the internal passage R. 
     The cell main body  21  is made of a metal such as aluminum, for example, and is provided with the light transmission windows W 1  and W 2  for transmitting light, on opposing walls  21   a  and  21   b  facing each other, respectively. The opposing walls  21   a  and  21   b  face each other in a direction orthogonal to the direction of the centrifugal force. By using a metal as the material of the cell main body  21 , it is possible to ensure that the cell main body has a strength to withstand the centrifugal force, and an excellent chemical resistance. 
     The light transmission windows W 1  and W 2  are configured by installing glass window members  211  in through holes that are provided in the opposing walls  21   a  and  21   b  of the cell main body  21 , respectively. Because the window members  211  are made of glass, the window members  211  have excellent chemical resistance. The window members  211  are liquid tightly fixed to through holes, respectively, that are provided in the opposing walls  21   a  and  21   b , respectively, each with a seal member  212  and a fixing member  213  therebetween. At this time, among the light transmission windows W 1  and W 2 , the size of the light transmission window W 2  that is the outgoing side of the light is larger than the light transmission window W 1  on the incident side of the light. In this manner, it is ensured that, even when the cell rotating mechanism  3  experiences rotational deviations (deviations of the rotating shaft), the light comes out of the light transmission window W 2  on the outgoing side of the light. 
     The cell cap  22  is made of a resin, for example, and is inserted into the opening  21 H of the cell main body  21  and closes the opening  21 H, as illustrated in  FIGS.  3 ,  4 , and  5   . At this time, the cell cap  22  has an inserted portion  22   a  to be inserted into the opening  21 H of the cell main body  21 , and a rear end  22   b  that is positioned outside the cell main body  21 . 
     A sample inlet  2 A is provide on one end of the internal passage R provided to the cell cap  22 , and a sample outlet  2 B is provided on the other end. With the cell cap  22  attached to the cell main body  21 , the sample inlet  2 A is positioned external of the cell main body  21 , and the sample outlet  2 B is positioned internal of the cell main body  21 . Specifically, the sample inlet  2 A is provided on the top surface of the rear end  22   b , and the sample outlet  2 B is provided on a tip surface  22   x  of the inserted portion  22   a . With this configuration, when the cell cap  22  is attached to the cell main body  21 , the sample liquid W SS  can be introduced into the internal passage R from the sample inlet  2 A. 
     The internal passage R includes a liquid reservoir R 1  for holding the sample liquid W SS  introduced from the sample inlet  2 A, and a main passage R 2  communicating with the liquid reservoir R 1  and communicating with the sample outlet  2 B. When the centrifugal force is applied, the sample liquid W SS  held inside the liquid reservoir R 1  flows into the main passage R 2 , passes through the main passage R 2 , and is introduced into the density gradient liquid W DGS . 
     At this time, the internal passage R also includes an inlet passage portion R 3  communicating with the sample inlet  2 A and the liquid reservoir R 1 . The inlet passage portion R 3  is provided along the direction of the rotating shaft (in  FIGS.  3  and  4   , the up and down direction on the paper surface) of the measurement cell  2 , and the main passage R 2  is provided along the direction of the centrifugal force (the radial direction of the rotation, in  FIGS.  3  and  4   , the left to right direction on the paper surface) applied to the measurement cell  2 . In other words, in the present embodiment, the inlet passage portion R 3  and the main passage R 2  are orthogonal to each other. The inlet passage portion R 3  may be provided in a manner inclined with respect to the main passage R 2 . 
     The main passage R 2  includes a small diameter passage portion R 2   a  that is an upstream passage communicating with the liquid reservoir R 2 , and a large diameter passage portion R 2   b  that is provided downstream of the small diameter passage portion R 2   a  and that communicates with the sample outlet  2 B. The small diameter passage portion R 2   a  is a linear passage having a constant diameter, and the large diameter passage portion R 2   b  is a linear passage having a constant diameter that is larger than the diameter of the small diameter passage portion R 2   a . The small diameter passage portion R 2   a  and the large diameter passage portion R 2   b  are coaxially provided. 
     The liquid reservoir R 1  is provided to a pointed end (bottom end) of the inlet passage portion R 3 , and can temporarily store therein a predetermined amount (for example, 10 μL) of the sample liquid W SS . At this time, by being connected to the inlet passage portion R 3 , the liquid reservoir R 1  is configured as a space having an opening on the top. The liquid reservoir R 1  stores therein substantially the entire sample liquid W SS  to be introduced into the inlet passage portion R 3  via the sample inlet  2 A. 
     The positional relationship between the liquid reservoir R 1  and the main passage R 2  is configured in such a manner that the main passage R 2  is connected to an upper part of the liquid reservoir R 1 . In other words, the main passage R 2  communicates with the liquid reservoir R 1  by being connected to the inlet passage portion R 3 . 
     At this time, the liquid reservoir R 1  has an inclined surface R 1   x  inclined upwards toward the main passage R 2 , on the side where the liquid reservoir R 1  is connected to the main passage R 2  (radially outer side). This configuration allows the sample liquid W SS  stored in the liquid reservoir R 1  to flow into the main passage R 2  smoothly when an application of a centrifugal force is received. The liquid reservoir R 1  according to the present embodiment has a conical shape or a partially conical shape that is tapered toward the bottom, thereby forming the inclined surface R 1   x.    
     Furthermore, in the present embodiment, when the cell cap  22  is attached to the cell main body  21 , a space S 1  is ensured between the cell cap  22  and the density gradient liquid W DGS . The space S 1  is ensured between the tip surface  22   x  of the cell cap  22  and the liquid surface of the density gradient liquid W DGS  stored in the cell main body  21 . At this time, the surface tension of the density gradient liquid W DGS  stored inside the cell main body  21  keeps the shape of the surface, and even when the measurement cell  2  is tilted sideways, the density gradient liquid W DGS  does not flow toward the side near the cell cap  22 , and the space S 1  between the tip surface  22   x  of the cell cap  22  and the liquid surface of the density gradient liquid W DGS  is maintained. 
     In addition, hollows  221  are provided to the cell cap  22 , as illustrated in  FIG.  5   , to reduce the weight. At this time, the hollows  221  are provided on both the left and the right sides of the inserted portion  22   a  of the cell cap  22 , with the internal passage R provided at the center interposed therebetween. The hollows  221  according to the present embodiment are through holes. Each of the hollows  221  is a long hole provided across a predetermined range in the direction in which the cell cap  22  is attached. 
     Furthermore, the cell cap  22  is provided with air vents  222  for letting the air out when the cell cap  22  is attached to the cell main body  21 . The air vents  222  are communicating passages each having an opening on the tip surface  22   x  of the cell cap  22  and communicating with the through hole that is the corresponding hollow  221 . When the cell cap  22  is attached to the cell main body  21 , the air inside the cell main body  21  passes through the air vents  222  and discharged from the through holes that are the hollows  221 . 
     Furthermore, on both of the left and the right side surfaces of the cell cap  22 , protrusions  223  are provided to ensure adhesion with the left and right inner side surfaces of the cell main body  21 , respectively, when the cell cap  22  is attached to the cell main body  21 . At this time, because the hollows  221  are provided on the inner sides of the side surfaces where the protrusions  223  are provided, respectively, the side surfaces provided with the respective protrusions  223  become elastically deformed inwards, and the protrusions  223  are elastically brought into close contact with the left and right inner side surfaces of the cell main body  21 , respectively. 
     In addition, the cell main body  21  and the cell cap  22  are provided with a restricting mechanism that restricts the orientation in which the cell cap  22  is attached to the cell main body  21 . Specifically, the restricting mechanism is achieved by the shape of the opening of the cell main body  21  and the shape of the inserted portion  22   a  inserted into the opening  21 H of the cell cap  22 . For example, at least one of the four rounded portions of the opening  21 H of the cell main body  21  has a different shape, and four round portions  22 R 1  and  22 R 2  of the inserted portion  22   a  of the cell cap portion  22  are provided with the shapes corresponding thereto, respectively (see the front view of  FIG.  5   ). As a result, it is possible to make sure that orientation in which the cell cap  22  is inserted into the cell main body  21  is restricted to only one. 
     A line start mode using the measurement cell  2  will now be explained briefly. 
     To introduce the sample liquid W SS  into the measurement cell  2 , as illustrated in  FIG.  6    ( 1 ), the sample liquid W SS  is introduced from the sample inlet  2 A, with the cell cap  22  attached to the cell main body  21 . After the sample liquid is introduced, the sample liquid W SS  passes through the inlet passage portion R 3 , and is held in the liquid reservoir R 1  (see  FIG.  6    ( 2 )). 
     After the sample liquid is introduced, the sample inlet  2 A may be closed with a lid body  23 . By closing the sample inlet  2 A with the lid body  23 , it is possible to prevent evaporation of the sample liquid W SS  and prevent leakage of the density gradient liquid W DGS  into the internal passage R due to an air pocket formed inside the internal passage R. 
     When the measurement cell  2  is then rotated, the sample liquid W SS  receives the centrifugal force and moves from the liquid reservoir R 1  toward the small diameter passage portion R 2   a  of the main passage R 2 , as illustrated in  FIG.  7    ( 1 ). The particle size distribution measurement is then started at the same time as when the measurement cell  2  starts being rotated. In the present embodiment, the timing at which rotation of the measurement cell  2  is started is used as the timing at which the particle size distribution measurement is started. At this time, because the sample liquid W SS  passes through the inlet passage portion R 3  and is held in the liquid reservoir R 1 , the sample liquid W SS  is positioned away from the rotational center of the measurement cell  2 , and the sample liquid is more likely to be subjected to the centrifugal force generated by the rotation. Therefore, the sample liquid flows into the main passage R 2  more smoothly. In a device configuration in which the sample liquid is introduced from the rotational center, the centrifugal force is hardly received immediately after the sample liquid is introduced, and variations prior to the sample liquid being introduced into the density gradient liquid become increased. 
     The sample liquid W SS  then flows through the small diameter passage portion R 2   a  to the large diameter passage portion R 2   b , spreads inside the large diameter passage portion R 2   b , as illustrated in  FIG.  7    ( 2 ), and is introduced into the density gradient liquid W DGS  via the sample outlet  2 B, as illustrated in  FIG.  7    ( 3 ). As a result, it is possible to prevent a streaming phenomenon. Furthermore, because the space S 1  is ensured between the sample outlet  2 B and the density gradient liquid W DGS , it is possible to ensure that the sample liquid W SS  having been introduced spreads across the density gradient liquid W DGS  reliably. Hence, it is possible to improve the measurement accuracy. 
     Advantageous Effects Achieved by Present Embodiment 
     With the centrifugal sedimentation type particle size distribution measuring device  100  according to the present embodiment, the sample liquid W SS  is held inside the internal passage R provided inside the cell cap  22 , and when a centrifugal force is applied thereto, the sample liquid W SS  is introduced through the internal passage R into the density gradient liquid W DGS . Therefore, it is possible to establish the timing of the application of the centrifugal force to the measurement cell  2  or the timing of rotating the measurement cell  2  at a predetermined rotation speed as the timing of the start of the measurement in the line start mode. Hence, the measurement accuracy can be improved. Furthermore, because the sample liquid W SS  is held inside of the cell cap  22 , it is possible to reduce the distance between the sample liquid W SS  and the density gradient liquid W DGS , so that it is possible to reduce variations in the time required for the sample liquid W SS  to reach the density gradient liquid W DGS . With this, too, the measurement accuracy can be improved. Furthermore, because the sample liquid W SS  is held in the measurement cell  2  that is configured to be removable from the device main body, an introducing mechanism for introducing the sample liquid W SS  into the rotating measurement cell  2  from the external can be rendered unnecessary. 
     In the present embodiment, because the liquid reservoir R 1  is provided to the internal passage R, a predetermined amount of the sample liquid W SS  can be held at a constant position in the internal passage R, so that the reproducibility of the particle size distribution measurement can be improved. 
     Furthermore, because the main passage R 2  is connected to the upper part of the liquid reservoir R 1 , and the liquid reservoir R 1  has the inclined surface R 1   x  on the side connected with the main passage R 2 , the inclined surface being inclined upwards toward the main passage R 2 , it is possible to hold the sample liquid W SS  reliably inside the liquid reservoir R 1  before the measurement cell  2  is rotated, and to allow the sample liquid W SS  to flow out of the liquid reservoir R 1  smoothly into the main passage R 2  while the measurement cell  2  is being rotated. 
     At this time, because the liquid reservoir R 1  has a conical shape or a partial conical shape tapered toward the bottom, the liquid reservoir R 1  can be easily machined into the cell cap  22 . 
     Furthermore, because, when the cell cap  22  is attached to the cell main body  21 , the space S 1  is ensured between the cell cap  22  and the density gradient liquid W DGS , it is possible to alleviate a streaming phenomenon caused by the contact of the density gradient liquid W DGS  with the cell cap  22 , to spread the sample liquid W SS  into the density gradient liquid W DGS  reliably, and to improve the measurement accuracy. 
     Other Embodiments 
     Note that the present invention is not limited to the embodiment described above. 
     For example, in the above embodiment, the particle size distribution is measured in the line start mode, but according to the present invention, it is possible to configure so as to enable the particle size distribution measurement not only in the line start mode but also in a uniform sedimentation mode. In this mode, sample dispersion stored in the measurement cell  2  contains particles dispersed across a medium. In the uniform sedimentation mode, too, the timing of rotating the measurement cell containing the sample dispersion is used as the timing of the start of the measurement. 
     In such an example, as a configuration of the measurement cell, the measurement cell includes a storage space storing therein the dispersion medium, and an internal passage communicating with the storage space and holding the sample liquid, and the sample liquid is introduced through the internal passage into the dispersion medium by receiving an application of a centrifugal force. Specifically, it is possible to configure the measurement cell to include a cell main body that has an opening on one end and that stores therein a dispersion medium, and a cell cap by which the opening of the cell main body is closed and that is provided with an internal passage for holding a sample liquid, and the sample liquid is introduced through the internal passage into the dispersion medium by receiving an application of a centrifugal force. The cell main body and the cell cap may have the same configuration as those according to the embodiment described above. With such a measurement cell, the internal passage communicating with the storage space holds the sample liquid, and the sample liquid is introduced through the internal passage into the dispersion medium by receiving an application of a centrifugal force. Therefore, it is possible to reduce the distance between the sample liquid and the dispersion medium, so that it becomes possible to suppress variations in the time required for the sample liquid to reach the dispersion medium. With this, too, the measurement accuracy can be improved. 
     Furthermore, the liquid reservoir R 1  according to above embodiment described above has a conical shape or a partial conical shape tapered toward the bottom, but may have various other shapes. For example, the liquid reservoir R 1  may have a partial spherical shape, or a polygonal columnar shape, for example. Even with such configurations, it is desirable to have the inclined surface R 1   x  having an upward gradient toward the main passage R 2 . 
     Furthermore, the measurement cell according to the embodiment described above is a rectangular cell having a rectangular cuboidal outer shape, but may also have any other shape. 
     In the measurement cell  2  according another embodiment, as illustrated in  FIG.  8   , a recess (specifically, a groove) G may be provided across the side surfaces of the inserted portion  22   a  of the cell cap  22  so as to prevent leakage of the density gradient liquid W DGS  oozing out of the gap between the cell main body  21  and the cell cap  22 , due to the capillary phenomenon. The recess G preferably has an annular shape in a manner extending around the side surfaces of the inserted portion  22   a , about its axis. By providing such a recess G across the side surface of the inserted portion  22   a  and increasing the gap with respect to the main body  21 , it is possible to stop oozing of the density gradient liquid W DGS  due to the capillary phenomenon. 
     In addition, various modifications and combinations of the embodiments may be made within the scope not deviating from the gist of the present invention. 
     INDUSTRIAL APPLICABILITY 
     According to the present invention, it is possible to improve the measurement accuracy of a particle size distribution measuring device that performs particle size distribution measurement in a line start mode.