JP2013088367A - Sample cell and spectrophotometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sample cell structure and a spectrophotometer for performing measurement at a higher SN ratio while considering such a problem that measurement at a higher SN ratio requires long cell length, but when the cell length is extended, scatter of light advancing in the long cell should be considered.SOLUTION: The sample cell includes a tube formed like a thin tube and allowing fluid to pass the inside of the tube and a structure for covering the periphery of a capillary tube in contact with a part of an outer wall surface of the tube. The spectrophotometer uses the sample cell.

Description

  The present invention relates to a sample cell and a spectrophotometer used for analyzing the concentration of a substance contained in a liquid sample by a spectrophotometric method. In particular, the present invention relates to a sample cell and a spectrophotometer suitable for use in liquid chromatography and the like to analyze a low concentration substance with a small amount of sample with high sensitivity.

  An ultraviolet / visible spectrophotometer is used as an analyzer for measuring the absorption spectrum of a sample in a predetermined wavelength range or the absorbance of the sample at a specific wavelength.

  The configuration of a conventional typical ultraviolet / visible spectrophotometer is as described in Patent Document 1, for example. These spectrophotometers are equipped with two types of light sources, a deuterium discharge tube for the ultraviolet region and a halogen lamp for the visible region, and the light generated from these light sources is passed through a monochromator to the desired wavelength. After the monochromatic light is taken out, the light beam is split into a sample-side light beam and a reference-side light beam, the former is passed through the sample cell in which the sample is placed, and the latter is passed through the reference cell for reference. A light transmission spectrum is obtained by measuring each light quantity after these two light fluxes pass through each cell and taking a ratio thereof. Furthermore, an absorption spectrum is obtained by logarithmically converting the transmittance, which is the vertical axis of the transmission spectrum, to obtain absorbance.

  In addition, when it is desired to measure an absorption spectrum over the entire desired wavelength range in a short time, instead of a monochromator that takes out monochromatic light as described in Patent Document 2, for example, a spectral spectrum in a wide wavelength range is once used. A spectrophotometer is used that combines a polychromator to be extracted and a one-dimensional or two-dimensional image sensor composed of a plurality of pixels. A spectrophotometer using a polychromator is often used as a detector for a liquid chromatograph.

  When analyzing the concentration of a predetermined component in a liquid sample using these spectrophotometers, for example, a flow cell type sample cell as shown in FIG. The liquid sample is caused to flow through the path, a measurement light beam is made incident from one end of the flow path, and the light amount of the light beam emitted from the other end is measured.

When a liquid sample containing a predetermined component whose concentration is to be analyzed is flowed to the sample cell, the amount of light emitted from the flow path is I, and a blank sample not containing the predetermined component is flowed to the sample cell , The amount of light emitted from the flow path is I ₀ , the concentration of the predetermined component is C, the distance that the measurement light beam passes through the liquid sample, that is, the cell length of the sample cell is L From Lanbert-Beer's law,
[Equation 1]
-Log ₁₀ (I / I ₀ ) ∝ CL
The relationship is established. The amount on the left side of Equation 1 is the absorbance, and Equation 1 indicates that the absorbance measured for the liquid sample is proportional to the product of the concentration C and the cell length L of the predetermined component. .

  From this relationship, it can be seen that when the concentration C to be analyzed is small, the absorbance on the left side can be increased by increasing the cell length L, and the concentration analysis can be made highly sensitive. The structure of a sample cell for increasing the sensitivity of concentration analysis by increasing the cell length L is disclosed in Patent Document 3 and Patent Document 4, for example.

JP-A 61-53527 JP-A-11-108830 JP-A-5-196565 US2009 / 230028

  Here, the structure of a cell for measuring the absorbance with high accuracy will be considered.

  When quantitatively analyzing a desired component substance concentration using the sample cell, the S / N ratio of the measured concentration value is assumed to be a low concentration sample, assuming that the main noise factor is light-derived shot noise. On the other hand, it is determined approximately as shown in Equation 2.

[Equation 2]
S / N ratio ≒ (L / T ₁ ^L ) ^1/2 · (1-T ₁ ^L ) / (I ₀ · V) ^1/2
L: cell length, V: cell volume, T ₁ : transmittance at a cell length of 1 mm,
I ₀ : Illuminance at the sample cell incident side end face (number of photons / mm ² )
FIG. 13 shows the relationship between the cell length of the sample cell and the S / N ratio obtained at that time when the capacity of the sample cell is constant. FIG. 13 shows that if the capacity of the sample cell is constant, it is advantageous to obtain a better S / N ratio by reducing the cross-sectional area of the flow path in the sample cell and increasing the cell length. Suggests.

  In order to measure with a better S / N ratio, it has been found that it is better to increase the cell length. However, if the cell length is increased, the scattering of light traveling in the long cell must be considered. In view of the above problems, the present invention provides a sample cell structure and a spectrophotometer for measuring with a better S / N ratio.

  In order to achieve the above object, the present invention comprises a tube formed in a thin tube shape through which a fluid passes, and a structure that covers the periphery of the capillary tube in contact with a part of the outer wall surface of the tube. And a spectrophotometer using the sample cell.

  In the present invention, while maintaining the flow path volume in the sample cell, the conduction efficiency of the light beam passing through the flow path, and the pressure resistance strength of the entire sample cell, the cell length is lengthened and the concentration analysis is made highly sensitive. A suitable sample cell can be provided.

It is a figure which shows the structure of the sample cell which concerns on the 1st Embodiment of this invention. It is a figure which shows the 1st structure of the structure formed in a cylindrical shape in the sample cell which concerns on the 1st Embodiment of this invention. It is a figure which shows the 2nd structure of the structure formed in a cylindrical shape in the sample cell which concerns on the 1st Embodiment of this invention. It is a figure which shows the 3rd structure of the structure formed in a cylindrical shape in the sample cell which concerns on the 1st Embodiment of this invention. It is a figure which shows the 4th structure of the structure formed in a cylindrical shape in the sample cell which concerns on the 1st Embodiment of this invention. It is a figure which shows the 5th structure of the structure formed in a cylindrical shape in the sample cell which concerns on the 1st Embodiment of this invention. It is a figure which shows the 6th structure of the structure formed in a cylindrical shape in the sample cell which concerns on the 1st Embodiment of this invention. It is a figure which shows the 7th structure of the structure formed in a cylindrical shape in the sample cell which concerns on the 1st Embodiment of this invention. It is a figure which shows the 2nd structure of the structure of the sample cell which concerns on the 1st Embodiment of this invention. It is a figure which shows the structure of the sample cell which concerns on the 2nd Embodiment of this invention. It is a figure which shows the structure of the spectrophotometer which concerns on the 3rd Embodiment of this invention. It is a figure which shows the structure of the sample cell of a prior art. It is a figure explaining the S / N ratio at the time of measuring an absorbance. It is a figure explaining reflection of the light beam in a sample cell. It is a figure explaining a total reflection phenomenon.

  First, the principle of the present invention will be described.

  According to Equation 2, if the capacity of the sample cell is constant, it is advantageous to obtain a better S / N ratio by reducing the cross-sectional area of the flow path in the sample cell and increasing the cell length. I understood. Therefore, the points that must be considered when increasing the cell length are considered.

(I) Prevention of scattering of light traveling through the cell (use of total reflection)
When a measurement beam is introduced into the liquid sample from one end of the sample cell and the beam after passing through the liquid sample is taken out from the other end of the sample cell, the extracted beam is
(1) A light beam that enters from one end of the cell and travels straight to the other end. (2) A light beam that enters from one end of the cell and reflects and travels at least once on the inner surface of the cell. As the length increases, the ratio of the luminous flux (1) decreases, and the ratio of the luminous flux (2) increases. In the case of (2), the number of reflections in the cell increases. As shown in FIG. 14A, the reflection of the light beam in the cell occurs at the interface between the liquid sample and the cell inner surface, and the sample cell is made of a transparent material like a capillary tube. In the case of a tubular structure, it also occurs at the interface between the cell outer surface and the outer region as shown in FIG. 14 (b). If the reflectivity shown in FIGS. 14A and 14B is low, the light quantity of the light beam extracted from the other end of the sample cell is reduced, and a good S / N ratio cannot be obtained.

  Therefore, as a technique for increasing the reflectivity when reflected in a cell, in order to prevent the amount of light flux extracted from the other end of the sample cell from decreasing even if the cell length is increased, Patent Document 3 Alternatively, there is a technique described in 4 that uses the total reflection phenomenon.

In the total reflection phenomenon, when the incident angle of the light flux with respect to the reflective interface exceeds a critical angle determined by the relationship of the refractive index of the substance constituting the interface, the reflectivity at the interface is 100%. Here, as shown in FIG. 15A, a case is considered in which a substance 1 having a refractive index n1 and a substance 2 having a refractive index n2 form an interface, and n1> n2 and a light beam is incident from the substance 1 side. At this time, the critical angle θt is
[Equation 3]
Sinθt = n2 / n1
Thus, total reflection occurs when the incident angle θ of the luminous flux is θ> θt.

(Ii) Cell constituent materials (cell constituent materials for using total reflection)
Consideration must be given to the constituent materials of the cell to cause total internal reflection.

  As described in Patent Document 3, if the sample cell itself is made of a material having a refractive index lower than that of the liquid sample, total reflection occurs in this manner.

  Here, in configuring the sample cell, the substance 2 needs to be a solid.

  Next, as shown in FIG. 15B, the substance 1 having a refractive index n1, the substance 3 having a refractive index n3, and the substance 2 having a refractive index n2 constitute an interface, and n1 <n3 and n2 <n3 and n1>. Consider a case where a light beam is incident from the material 1 side at n2. Although the derivation process is omitted, the critical angle θt is again given by the same number 3 as in the above case. As described in Patent Document 4, if the sample cell has a tubular structure and the outer region of the tubular structure is made of a material having a lower refractive index than that of the liquid sample, such a shape is obtained. Total reflection occurs.

  In this case, in configuring the sample cell, the substance 3 needs to be a solid, but the substance 2 can be selected from gas, liquid, or solid as necessary. In Patent Document 4, a liquid is used as the substance 3. Equation 3 also shows that when the refractive index n1 of the liquid sample is constant, the critical angle θt increases as the refractive index n2 of the other carrier for total reflection decreases. This means that the smaller n2 is, the more light can be introduced into the sample cell and conducted to the other end using the total reflection phenomenon.

  Here, there is a possibility that the smallest value of n2 can be realized is a value of about 1.0 in air or other suitable gas besides vacuum. However, in Patent Document 3, the value that can be realized as the refractive index of the solid material constituting the cell itself is 1.2 or more at most, and it is impossible to realize a low refractive index like the above-mentioned air. . Moreover, in the said patent document 4, the outer side of the tubular cell inner tube | pipe is filled with the predetermined | prescribed liquid, and it is impossible to implement | achieve the value below 1.2 also in this case.

(Iii) Cell structure (further, cell structure contributing to S / N improvement)
Further, when air or other suitable gas is employed as the material capable of minimizing the refractive index n2, the following configuration is further preferable.

  As described above, in the configuration of FIG. 15 (a), the portion of refractive index n2 must be a solid to form a sample cell. Therefore, air or an appropriate gas can be used for the portion of refractive index n2 as shown in FIG. 15 (b). Limited to. By the way, in the structure of FIG. 15 (b), the light beam traveling in the sample cell hardly propagates through the liquid sample and mainly travels only in the substance 3 (cell constituent substance). Can exist. Since such a light flux does not reflect the concentration information of the liquid sample, it causes an error in the concentration analysis of the liquid sample. It is easily expected that the ratio of this component increases as the layer of the substance 3 is thicker. Therefore, it is preferable that the layer of the substance 3 is thin.

  At this time, when the substance 2 is air or other suitable gas, the layer of the substance 3 is thinned, that is, when the thickness of the tubular structure constituting the channel of the sample cell is reduced, When the pressure of the flowing liquid sample is increased, the tubular structure may be deformed or broken.

  Therefore, a structure that increases the pressure resistance of the cell in a state where air or other gas is present around the cell is disposed around the cell.

  Embodiments of the present invention will be described below with reference to the drawings.

<First Embodiment>
FIG. 1 shows a configuration of a sample cell according to a first embodiment of the present invention. In the sample cell of this embodiment, the liquid sample 1 is introduced from the inlet 14 into the flow path 11 in the capillary tube 10 made of a transparent material in a desired wavelength range such as quartz or sapphire, and directed toward the outlet 15. Shed. The measurement light beam 2 is introduced into the flow path from one end face 16 of the flow path 11 and is emitted from the other end face 17. The outer wall surface 13 of the capillary tube 10 is covered with a structure 20 having a plurality of irregularities on the inner surface over the entire length of the capillary tube. As shown in FIG. 2, the structure 20 is formed by cutting a plurality of grooves having a triangular cross section on one surface of a metal plate 21, and each of the plurality of triangular ridge lines is an outer wall surface 13 of the capillary tube 10. Is wound in a cylindrical shape so as to be inscribed substantially without a gap. In the region between the outer wall surface 13 of the capillary tube 10 and the plurality of triangular grooves, gas having a small refractive index, here air, is present. Since the area where the ridge line of the structure 20 is in contact with the outer wall surface 13 of the capillary tube is small, most of the area of the outer wall surface 13 of the capillary tube 10 is in contact with air. In this state, the form of propagation of the light beam introduced into the flow path 11 from one end face 16 of the flow path 11 is
(1) A light beam totally reflected at the interface between the outer wall surface 13 of the capillary tube 10 and the air outside thereof (2) A light beam reflected at the interface between the inner wall surface 12 of the capillary tube 10 and the liquid sample 1 (3) The structure 2 in contact with the outer wall surface 13 of the capillary tube 10 on the outside thereof
There are three types of light flux reflected at the interface with the 0 ridgeline. When the area where the ridgeline of the structure 20 is in contact with the outer wall surface 13 of the capillary tube 10 can be ignored, only two types (1) and (3) need be considered, and (1) and (2 ) Is added, the light beam introduced from one end face 16 of the flow path 11 into the flow path 11 propagates to the other end face 17 without loss.

  Actually, the area where the ridge line of the structure 20 is in contact with the outer wall surface 13 of the capillary tube 10 is not zero but has a finite area, but the structure 20 is made of a highly reflective metal such as aluminum. The loss due to the presence of (3) can be minimized, and most of the light beam introduced from one end face 16 of the flow path 11 into the flow path 11 is the other end face 17. Can be propagated efficiently.

  In addition, as described above, the lower the refractive index of the substance in contact with the outer wall surface 13 of the capillary tube 10, the larger the critical angle, so that more light is taken into the channel 11 and propagates through the channel 11. However, in this embodiment, most of the outer wall surface 13 of the capillary tube 10 is in contact with the air existing outside thereof, and the refractive index of air is approximately 1.0. It is possible to capture a larger amount of light than when it is in contact with a solid.

  Furthermore, in the present embodiment, the outer wall surface 13 of the capillary tube 10 is evenly supported by a plurality of contact points, contact lines, or structures 20 that are in contact with the contact surface, which are substantially uniformly distributed over the entire length thereof. Even when a high-pressure liquid is allowed to flow through the flow path 11, the risk of the capillary tube 10 being deformed or damaged can be suppressed.

  Regarding the structure 20, in the first embodiment, the structure 20 having a plurality of irregularities on the inner surface is formed by winding a plate-shaped metal shown in FIG. 2 into a cylindrical shape. However, the structure 20 is formed in a cylindrical shape. Of course, it is possible to use a metal having the same groove as that shown in FIG.

  In the present embodiment, the plurality of concave and convex shapes on the inner surface of the structure 20 are a plurality of grooves having a triangular cross section as shown in FIG. 2, but instead of the ridges or grooves shown in FIG. The structure has a trapezoidal or rectangular cross section, a plurality of cylinders or prisms shown in FIG. 4, a plurality of triangular pyramids or cones shown in FIG. 5, and a rough surface finished in an irregular shape shown in FIG. However, it is clear that substantially the same effect can be obtained. Further, instead of finishing the inner surface rough as shown in FIG. 6, as shown in FIG. 7, the wavelength of the long wavelength end of the wavelength range in which the structure 20 itself is analyzed using the sample cell of this embodiment is used. The inner surface of the structure 20 may have a plurality of irregularities by forming it with a foam or porous material 22 containing bubbles having a larger diameter. Furthermore, instead of providing the structure 20 with irregularities on the inner surface of one type of material, as shown in FIG. 8, objects that substantially function as protrusions are individually prepared, and these are formed as plate-like structures. You may make it arrange | position discretely on one surface or the inner surface of a cylindrical structure. As such an object that substantially functions as a protrusion, a fine particle object or a fine wire object 24 can be used.

  Further, as shown in FIG. 9, the outer side of the structure 20 may be covered with a protective tube 30, a tube, or a coating, which impairs the above-described effects of the present embodiment. And the strength of the entire sample cell can be further enhanced.

  In the first embodiment, the material of the structure 20 having a plurality of projections and depressions on the inner surface is metal in order to increase the reflectance with the outer wall surface of the capillary tube. It is made of a material having a refractive index lower than the refractive index of the flowing liquid sample, for example, a material such as porous silica or Teflon (registered trademark) AF so that total reflection occurs between the outer wall surface 13 of the capillary tube 10. Anyway. These porous silica and Teflon (registered trademark) AF cause total reflection instead of the air layer.

<Second Embodiment>
A second embodiment of the present invention will be described with reference to FIG. FIG. 10 shows a configuration of a sample cell according to the second embodiment of the present invention. Here, regarding the elements constituting the sample cell of FIG. 10 excluding the capillary tube 10 and the end fixing member 40, their structures, materials, and functions are shown in the first embodiment. Detailed description will be omitted.

  In the present embodiment, the structure 20 having a plurality of irregularities on the inner surface and formed in a substantially cylindrical shape so that the diameter of the maximum inscribed circle of the plurality of irregularities is substantially equal to the outer diameter of the capillary tube 10 is: The capillary tube 10 is covered in the remaining sections excluding the coating exclusion sections A and B at both ends of the capillary tube 10. As shown in the first embodiment, air exists in the region between the irregularities on the inner surface of the structure 20 and the outer wall surface 13 of the capillary tube 10, and this air is totally reflected as described above. It works to increase the critical angle.

  However, at the ends A and B of the structure 20, the region is openly connected to the external environment, and foreign matter and contaminants may flow into the region from the external environment. On the other hand, since the structure 20 is not fixed to the outer wall surface 13 of the capillary tube 10 with an adhesive or the like, the fixing is not strong, and external force is applied to the structure 20 or the capillary tube 10. If it works, the relative positional relationship between the structure 20 and the capillary tube 10 may be shifted. Therefore, in the present embodiment, the structure 20 is fixed to the capillary tube 10 at the ends A and B by the end fixing member 40 so as not to be displaced. Further, the end fixing member 40 also has a function as a sealing material that seals and seals the region between the irregularities on the inner surface of the structure 20 and the outer wall surface 13 of the capillary tube 10 from the external environment. .

  In the present embodiment, the cell length is increased while suitably maintaining the channel volume in the sample cell, the conduction efficiency of the light beam passing through the channel, and the pressure resistance of the entire sample cell obtained in the first embodiment. In addition to the effect that can be achieved, the effect that the structure 20 does not shift with respect to the capillary tube 10 and the external environment in the region between the irregularities on the inner surface of the structure 20 and the outer wall surface of the capillary tube 10 The effect of preventing the inflow of foreign matter and contaminants from is obtained. Moreover, in this embodiment, since the area | region between the unevenness | corrugation of the said structure 20 inner surface and the outer wall surface 13 of the said capillary tube 10 is sealed, the air which exists in the said area is kept higher than atmospheric pressure as needed. Is also possible. Furthermore, when there is a concern that the inner surface of the structure 20 or the outer wall surface 13 of the capillary tube 10 may be altered by the action of oxygen, moisture, or the like in the air, the region is replaced with nitrogen or the like. It is also possible to substitute with an inert gas.

<Third Embodiment>
A third embodiment of the present invention will be described with reference to FIG. FIG. 11 shows a configuration of a spectrophotometer suitable for use as a detector of a liquid chromatograph equipped with the sample cell 100 shown in the first embodiment or the second embodiment of the present invention. A halogen lamp is used as the light source 201 and a deuterium discharge lamp is used as the light source 202. Light emitted from the light sources 201 and 202 is combined by the dichroic mirror 203 and enters the sample cell 100. The sample cell 100 has a quartz capillary tube 10 with a flow path 11 having an inner diameter of 0.2 mm and an outer diameter of 0.3 mm, a cell length of 300 mm, and a flow volume of about 9.4 microliters. Is used. The light that has passed through the sample cell 100 is collected by the imaging lens 204 and then enters the polychromator 210 through the entrance slit 205. The light dispersed by the polychromator 210 forms a spectral image 211 reflecting the spectral transmission characteristics of the liquid sample in the sample cell 100 on the exit-side focal plane of the polychromator 210. A one-dimensional image sensor 212 having 1024 pixels is installed on the focal plane on the emission side of the polychromator 210, and the spectral image 211 is converted into an electric signal by the one-dimensional image sensor 212 and amplified by an amplifier 223. After that, it is digitized by the A / D converter 226 and taken into the computer 230 as the spectral data 213. In the intensity distribution of the spectral image 211, not only the spectral transmission characteristics of the liquid sample but also the spectral emission characteristics of the light sources 201 and 202, the spectral efficiency characteristics of the polychromator 210, and the like are reflected in the form of multiplication. . In order to obtain a transmission spectrum that reflects only the spectral transmission characteristics of the liquid sample, the computer 230 first acquires spectral spectrum data acquired in a state in which a blank sample that does not contain a predetermined component whose concentration is to be analyzed flows in the sample cell 100. 213 is stored in the memory as reference spectrum data. Next, the computer 230 acquires the spectral data 213 in a state where a liquid sample containing a predetermined component whose concentration is to be analyzed is flowed to the sample cell 100, and stores it in the memory as sample spectral data. By dividing the intensity of each wavelength in the sample spectrum data by the intensity at the corresponding wavelength in the reference spectrum data stored in the memory, transmission spectrum data is obtained. Further, absorption spectrum data can be obtained by logarithmically converting the intensity at each wavelength of the transmission spectrum data. Using the absorbance values at the desired wavelength or wavelengths in the absorption spectrum data, the concentration of a predetermined component in the liquid sample is calculated. Since the absorbance is proportional to the product of the concentration of the predetermined component and the cell length of the sample cell 100 based on Equation 1, when compared with a conventional sample cell having a cell length of 10 mm, this embodiment Then, the absorbance can be expanded 30 times with respect to the same concentration. In addition, as described above, the lower the refractive index of the substance in contact with the outer wall surface 13 of the capillary tube 10, the larger the critical angle, so that more light is taken into the channel 11 and propagates through the channel 11. However, in this embodiment, most of the outer wall surface 13 of the capillary tube 10 is in contact with the air existing outside thereof, and the refractive index of air is approximately 1.0. It is possible to capture a larger amount of light than when it is in contact with a solid. Furthermore, in the present embodiment, the outer wall surface 13 of the capillary tube 10 is evenly supported by a plurality of contact points, contact lines, or structures 20 that are in contact with the contact surface, which are substantially uniformly distributed over the entire length thereof. Even when a high-pressure liquid is allowed to flow through the flow path, the risk that the capillary tube 10 is deformed or broken can be suppressed.

  As described above, in this embodiment, the sample cell having a long cell length is used while suitably maintaining the flow channel volume in the sample cell, the conduction efficiency of the light beam passing through the flow channel, and the pressure resistance of the entire sample cell. Therefore, the concentration analysis can be made highly sensitive.

DESCRIPTION OF SYMBOLS 1 Liquid sample 2 Light beam 10 for measurement Capillary tube 11 Flow path 12 Inner wall surface 13 Outer wall surface 14 Inlet 15 Outlet 16, 17 End surface 20 Structure 21 Plate 22 Porous material 23 Fine particle object 24 Fine wire object 30 Protective tube 40 End fixing material 100 Sample cell 200 Spectrophotometer 201, 202 Light source 203 Dichroic mirror 204 Imaging lens 205 Entrance slit 210 Polychromator 211 Spectral spectral image 212 Image sensor 213 Spectral spectral data 223 Amplifier 226 A / D Converter 230 computer

Claims (13)

  A sample cell comprising: a tube formed in a thin tube shape through which a fluid passes; and a structure that covers the periphery of the tube while being in contact with a part of the outer wall surface of the tube.   The sample cell according to claim 1, wherein an uneven portion is provided on the inner wall surface of the structure, and the protruded portion of the uneven portion and the outer wall surface of the tube are in contact with each other. A sample cell comprising a plurality of projections each having a substantially columnar shape, a cone shape, or a ridge shape.   3. The sample cell according to claim 2, wherein the concavo-convex portion of the wall surface in the structure is provided by roughing the inner surface.   The sample cell according to claim 2, wherein the wall surface of the structure is made of a foam or a porous material containing bubbles having a diameter larger than a wavelength belonging to the wavelength range.   3. The sample cell according to claim 2, wherein an object on a fine particle shape or a fine line is arranged on the wall surface of the structure.   3. The sample cell according to claim 2, wherein a portion of the concavo-convex portion of the structure that contacts the outer wall surface of the tube is made of a material having a refractive index lower than that of the liquid that passes through the tube. A sample cell.   7. The sample cell according to claim 6, wherein the substance constituting the portion of the inner wall surface of the structure that contacts the outer wall surface of the tube is a metal.   2. The sample cell according to claim 1, wherein a portion of the outer wall surface of the tube other than a portion in contact with the structure is in contact with a substance having a refractive index smaller than that of the liquid flowing in the tube. cell.   9. The sample cell according to claim 8, wherein a portion other than the portion in contact with the structure is in contact with a gas.   The sample cell according to claim 1, further comprising a fixing member that prevents a relative displacement between the structure and the tube.   The sample cell according to claim 1, wherein the end fixing member seals between the structure end and the tube. In a spectrophotometer that irradiates the sample cell with light from the light source and analyzes the sample based on the amount of light absorbed by the sample passing through the sample cell,
The sample cell includes a tube formed in a thin tube shape through which a fluid passes, and a structure that covers the periphery of the tube in contact with a part of the outer wall surface of the tube.   13. The spectrophotometer according to claim 12, wherein a portion of the outer wall surface of the tube other than a portion in contact with the structure is in contact with a substance having a refractive index smaller than that of the liquid flowing in the tube. Photometer.