JP6331140B2 - Moisture content measurement method - Google Patents

Description

  The present invention relates to a moisture content measuring method, and more particularly to a stratum corneum moisture content measuring method.

  Until now, various methods such as a method using electrical characteristics, nuclear magnetic resonance imaging, near-infrared spectroscopy, and confocal Raman spectroscopy have been proposed for measuring skin moisture. However, with these methods, problems such as difficulty in separating water from substances contained in cosmetics and skin external preparations, or absorption spectra of substances in the skin, and being susceptible to substances other than stratum corneum moisture, etc. In view of the effects of cosmetics on the moisture content of the skin, particularly the stratum corneum, the conventional methods have not been sufficient.

  By the way, the terahertz wave having a frequency of 0.1 to 10 THz is an electromagnetic wave having a characteristic of being strongly absorbed by water. In recent years, in view of this characteristic, attempts have been made to measure moisture in biological samples using terahertz waves. For example, in Patent Document 1, the absorption coefficient of a sample is obtained from the transmittance of a thin-section sample such as a biological sample and the thickness of the thin-section sample, and the volume fraction of moisture contained in the thin-section sample is calculated from the absorption coefficient of moisture. Attempts have been made to seek. Further, Non-Patent Document 1 attempts to determine the moisture content of the skin by irradiating terahertz waves from the skin surface and measuring the intensity difference of echo pulses generated at the interface of each layer constituting the skin. Yes.

  However, since the former method is a method using a thin section of a biological sample, human or animal skin is injured and it cannot be said that it is a preferable method. The latter method can be said to be a preferable method because it can be measured without damaging the skin, but because this method cannot temporally separate the two echo pulses of the air-corneal interface and the stratum corneum-skin interface. , Stratum corneum moisture can not be measured. In the latter method, the change in the group refractive index is determined from the echo pulse intensity at the air-corneal layer interface, and the change in the moisture content on the skin (horny layer) surface is determined. It can be said that it is difficult to measure (absolute amount).

  In addition, a terahertz wave is incident on the sample under total reflection conditions, and the attenuation of the terahertz wave due to the interaction between the surface wave (evanescent wave) of the terahertz band generated at that time and the sample is measured. Attempts have been made to obtain a complex dielectric constant (Patent Document 2, Non-Patent Document 2). This method is a so-called Attenuated Total Reflection Spectroscopy (ATR method). In Patent Document 2, a complex refractive index of a sample is obtained by measuring two spectra having different incident angle conditions without requiring a liquid layer having an extremely thin thickness as in the transmission method. In Non-Patent Document 2, in order to eliminate the influence of water stacked on an extremely thin thin film sample such as a single cell layer, the so-called two-interface model considering reflection at the interface between the thin film sample and water is used. The complex permittivity is obtained.

  However, both documents only show that the complex refractive index or the complex dielectric constant was measured. In addition, the stratum corneum of the skin is not a single cell layer, but is a complex stack of dead keratinocytes, and it was not known whether it could be used in a method for measuring the water content of the stratum corneum.

JP 2009-122007 A JP 2008-304444 A

Yasui et al., Biomedical Engineering, Vol. 42, no. 4, 190-194, 2004 K. Shiraga et al., Applied Physics Letters, 102, 053702 (2013)

  By the way, the thickness of the stratum corneum varies among humans and animal species, and there are individual differences. In addition, the thickness of the human stratum corneum varies greatly between parts and individuals. The attenuation of the terahertz wave is also related to the depth of the evanescent wave, and even if the moisture content is obtained from the attenuation of the terahertz wave, only the absorption in the very shallow part of the stratum corneum may be measured. In addition, the skin is roughly divided from the outside and has a layered structure of the stratum corneum, epidermis layer, dermis layer, and subcutaneous tissue layer. Where the stratum corneum is thin, it may be affected by moisture in the epidermis layer under the stratum corneum. There is also.

  The present invention has been made in view of such background art, and the present invention aims to provide a method for measuring the moisture content of a thin film sample such as a stratum corneum as accurately as possible using terahertz waves. And

  A measuring method according to the present invention is a method for measuring the amount of water in a thin film sample using terahertz wave total reflection attenuation spectroscopy, wherein a prism is used to apply a terahertz wave to a measurement object in a state where the laminated sample is laminated on the thin film sample. Irradiating from the thin film sample side to observe the attenuation of the terahertz wave, correcting the attenuation of the terahertz wave using a reflection coefficient at the interface between the thin film sample and the laminated sample, and the corrected terahertz wave Obtaining an absorption coefficient of the thin film sample from a wave.

  According to the present invention, a method for measuring the amount of water in the stratum corneum using terahertz waves is provided.

FIG. 1 is a schematic diagram showing a measurement concept of a two-interface model. FIG. 2 is a diagram showing an absorption spectrum of water. FIG. 3 is a diagram showing an absorption spectrum of an aqueous sucrose solution. FIG. 4 is a graph showing the relationship between the mass water content of the aqueous sucrose solution and the absorption coefficient, where (a) shows the measurement result at 0.5 THz and (b) shows the measurement result at 1.0 THz. ● indicates experimental values and ○ indicates literature values. FIG. 5 is a diagram showing an absorption spectrum of the stratum corneum model. FIG. 6 is a diagram showing the relationship between the mass moisture content and the absorption coefficient using the stratum corneum model, where (a) shows the measurement result at 0.25 THz, (b) shows the measurement result at 0.5 THz, (C) is a figure which shows the measurement result in 1.0 THz. FIG. 7 is a diagram showing analysis results obtained by dropping distilled water on a dielectric thin film, where (a) shows a 10 μm dielectric thin film, (b) shows a 20 μm dielectric thin film, and (c) shows It is an analysis result at the time of using a 30 micrometers dielectric thin film. FIG. 8 is a diagram showing an analysis result measured by dropping distilled water on a dielectric thin film, where (d) shows a 40 μm dielectric thin film, and (e) shows a case where a 50 μm dielectric thin film is used. It is a result. FIG. 9 is a graph showing an absorption spectrum of pig skin (dried for 48 hours). FIG. 10 is a graph showing the relationship between the mass moisture content of pig skin and the number of stripping times. (A) shows the measurement result at 0.25 THz, (b) shows the measurement result at 0.5 THz, c) is a diagram showing a measurement result at 1.0 THz. FIG. 11 shows the absorption spectrum of pig skin (dried for 48 hours) after application of the two-interface model. FIG. 12 is a graph showing the relationship between the absorption coefficient of pig skin after the application of the two-interface model and the number of stripping, where (a) shows the measurement result at 0.25 THz and (b) the measurement at 0.5 THz. (C) is a figure which shows the measurement result in 1.0 THz. FIG. 13 is a diagram showing the complex refractive index of the stratum corneum obtained using a two-interface model, where (a) shows its real part and (b) shows its imaginary part. FIG. 14 is a diagram showing a change in absorption coefficient before and after application of the two-interface model, and shows a change rate with respect to before application. (A) shows the measurement result at 0.25 THz, (b) shows the measurement result at 0.5 THz, and (c) shows the measurement result at 1.0 THz. FIG. 15 is a diagram showing an absorption spectrum of an albumin aqueous solution. FIG. 16 is a diagram showing the relationship between the mass water content of the aqueous albumin solution and the absorption coefficient. (A) shows the measurement result at 0.25 THz, (b) shows the measurement result at 0.5 THz, and (c) shows the measurement result at 1.0 THz. FIG. 17 is a diagram showing the relationship (0.5 THz) between the mass water content of pig skin and the number of stripping when an aqueous albumin solution is used as a stratum corneum model.

  In the total reflection attenuation spectroscopy, the method according to the present invention is preferably a measurement of a laminated body including a thin film sample and a laminated sample so that an evanescent wave having a greater bleed depth than that of the thin film sample whose moisture content is to be obtained is generated. This is a method of performing analysis in consideration of reflection at the interface between the thin film sample and the laminated sample after irradiating the object with terahertz waves. A method of analyzing in consideration of reflection at the interface between the thin film sample and the laminated sample is already known and is referred to as a so-called two-interface model (see Non-Patent Document 2).

FIG. 1 is an explanatory diagram of the two-interface model. In total reflection attenuation spectroscopy, the terahertz wave is incident on the prism under the condition of total reflection at the interface between the surface of the ATR prism (hereinafter referred to as “prism”) and the thin film sample, and is reflected at the interface between the prism surface and the thin film sample. The terahertz wave is observed by a detection device (not shown). In this case, the electric field strength E ~ _out of the reflected terahertz wave is represented as multiplied by coefficient a to the electric field strength E _in the incident terahertz wave as shown in Equation 1. If not using a two-interface model, the coefficients a prism and the Fresnel reflection coefficient at the interface of the thin film sample r ~ ₁₂ is used, the analysis relation holds and to the _{E ~ out = r ~ 12 ×} E in is performed. On the other hand, when the two-interface model is used, the interface between the thin film sample and the laminated sample is treated as having an attenuation proportional to the Fresnel reflection coefficient r- ₂₃ at the interface, and the attenuation of the thin film sample is eliminated by removing the influence of this attenuation. Is taken out and analyzed. For this reason, the coefficient a has a Fresnel reflection coefficient r ~ ₁₂ at the interface between the prism and the thin film sample, and an overall reflection coefficient r ~ ₁₂₃ considering the Fresnel reflection coefficient r ~ ₂₃ at the interface between the thin film sample and the laminated sample. used, analysis is performed by the relation _{E ~ out = r ~ 123 ×} E in holds. In other words, in the analysis using the two-interface model, the Fresnel reflection coefficient r ~ ₁₂ at the interface between the prism and the thin film sample is corrected by the Fresnel reflection coefficient r ~ ₂₃ at the interface between the thin film sample and the laminated sample. ₁₂₃ is used. In other words, it can be said that the two-interface model is a model obtained by correcting the attenuation of the observed terahertz wave using the Fresnel reflection coefficient at the interface between the thin film sample and the laminated sample. The absorption coefficient α of the thin film sample necessary for calculating the water content of the thin film sample is calculated by calculating the overall reflection coefficient r to ₁₂₃ from the reflection spectrum and the phase difference spectrum observed by the detector as described below. Further, the complex refractive index n ~ ₂ of the thin film sample is calculated from the total reflection coefficient r ~ ₁₂₃ .

(Equation 1)
E ~ _out = a × E _in (1 ≧ a ≧ 0)

Here, when the refractive index of the prism is n ₁ , the complex refractive index of the thin film sample is n to ₂ , the complex refractive index of the laminated sample is n to ₃ and the incident angle of the terahertz wave is θ, at the interface between the thin film sample and the laminated sample. The Fresnel reflection coefficient r ~ ₂₃ is expressed by the following equation 2.

Further, since the Fresnel reflection coefficient r ~ _{12 at} the interface between the thin film sample and the prism is expressed by Equation 3, when the thickness of the thin film sample is d, the overall reflection coefficient r ~ ₁₂₃ when there are two interfaces is It is expressed by the following formula 4.

Here, when the sample is a substance having absorption of terahertz waves, if the thickness of the sample on the prism surface is t and the absorption coefficient of the sample is α, the absorption intensity A (t of the ATR spectrum observed from the reflected terahertz wave ) Is represented by the following equation (5). The evanescent wave oozing depth d _p is a depth at which the electric field intensity of the incident wave is attenuated to 1 / e, the wavelength λ of the incident terahertz wave, the refractive index n _{1 of the} prism, and the refractive index n˜ of the sample. _{2. It} is uniquely determined by the incident angle θ with respect to the prism surface.

  On the other hand, the propagation of electromagnetic waves in a substance having absorption can be expressed using a complex refractive index n˜. The complex refractive index n˜ is expressed by Equation 6 using the extinction coefficient κ of the substance. It is known that the extinction coefficient κ and the absorption coefficient α of the substance have a proportional relationship shown in Expression 7. In Equation 6, n is the refractive index of the substance, and i is an imaginary unit.

  That is, the absorption intensity A (t) of the ATR spectrum is proportional to the absorption coefficient α (Equation 5), and the absorption coefficient α is proportional to the extinction coefficient κ (Equation 7). Therefore, the absorption coefficient α can be obtained by directly measuring the absorption intensity A (t) of the ATR spectrum or by obtaining the extinction coefficient κ of the substance.

The ATR spectrum after reflection can be observed by terahertz time domain spectroscopy. In this method, a reflection spectrum and a phase difference spectrum are obtained by performing Fourier transform on the acquired time waveform of the terahertz wave. When the penetration depth d _{p of the} evanescent wave is larger than the thickness t of the sample, the reflection spectrum R (ω) and the phase difference spectrum Φ (ω) are expressed by the following equations 8 and 9, respectively. Further, Fresnel reflection coefficient r ~ ₂₃ at the interface between the thin film sample and the laminated sample is expressed by Expression 2, Fresnel reflection coefficient r ~ _{12 at} the prism / sample interface is expressed by Expression 3, and the overall reflection coefficient r ~ ₁₂₃ is expressed by Expression 4. It is. Therefore, from the reflection spectrum R (ω) and the phase difference spectrum Φ (ω) obtained by the terahertz time domain spectroscopy, the solutions of the continuous cubic numbers of Equation 2, Equation 3, Equation 4, Equation 8, and Equation 9 are obtained. Thus, the complex refractive index n ~ ₂ of the thin film sample is obtained. In Equations 8 and 9, r _ref is the Fresnel reflection coefficient when the measurement object is not placed. Then the complex refractive index n ~ ₂ of the imaginary part (extinction coefficient kappa) and Equation 7 obtained, the absorption coefficient of the thin film sample α is obtained. That is, the measurement method according to the present invention is a reflection spectrum obtained by irradiating a terahertz wave while bringing a sample (a sample in which a laminated sample is laminated on a thin film sample) into contact with a prism surface that is a terahertz wave emitting surface. And measuring the phase difference spectrum and calculating the absorption coefficient α of the sample from the obtained reflection spectrum and phase difference spectrum, and calculating the absorption coefficient α, the Fresnel at the interface between the prism and the thin film sample. It is also a method in which the overall reflection coefficient r to ₁₂₃ obtained from the reflection coefficient r to ₁₂ and the Fresnel reflection coefficient r to ₂₃ at the interface between the thin film sample and the laminated sample is used. In other words, the step of measuring the time domain spectrum by irradiating the thin film sample with the terahertz wave from the thin film sample side through the prism from the thin film sample side with respect to the measurement object in a state where the laminated sample is laminated; Using the complex refractive index of the laminated sample, the refractive index of the prism, and the thickness of the thin film sample, a step of calculating a reflection coefficient of the measurement object in the laminated state, the measured time domain spectrum, and the calculated measurement It can also be said that the method has a step of obtaining the absorption coefficient of the thin film sample from the reflection coefficient of the object. It should be noted that the thickness d of the thin film sample and the complex refractive index n- ₃ of the laminated sample necessary for calculating the reflection coefficient r- ₁₂₃ and the complex refractive index n- ₂ of the thin film sample are actually measured values and estimated values. It can be a literature value or the like.

  The absorption coefficient α obtained above represents the absorption coefficient of the thin film sample. However, since the terahertz wave is extremely absorbed by water and hardly shows absorption by substances other than water, It can be said that the obtained absorption coefficient α represents absorption due to moisture contained in the thin film sample. In particular, when a laminated sample containing moisture is laminated on the thin film sample, the influence of moisture contained in the laminated sample is eliminated by applying the two-interface model.

  The measurement object in the present invention is an object including a thin film sample. The measurement object is a laminated body including a thin film sample and a laminated sample laminated on the thin film sample, and may be an object on which the laminated sample is placed on the thin film sample during measurement. The laminate is not limited to a two-layer laminate including a thin film sample and a laminate sample, and may be a multilayer laminate including two or more laminate samples. The laminated sample may also be a liquid containing water, such as an aqueous solution. Examples of the measurement object that is a laminate include human and animal skins. As described above, the skin has a structure in which the horny layer, the skin layer, and the like are laminated from the outermost layer, and the horny layer can be handled as a thin film sample and the other layers as a laminated sample. When applied to the skin, the stratum corneum moisture is not affected by the absorption of proteins, lipids, blood components, etc. that make up the skin tissue and hair, and the influence of moisture on the epidermis is eliminated as much as possible. The quantity is measured. In the present invention, it can be said that an object having a clear interface between the thin film sample and the laminated sample is suitable, but an object having no clear interface may be used. In the present invention, the thickness of the thin film sample is not particularly limited, and the thickness is, for example, about 1 mm or less, 0.5 mm or less, and 0.1 mm or less.

The frequency of the terahertz wave used in the present invention is 0.1 to 10 THz which is a frequency band of the terahertz band. In the present invention, in order to measure the moisture content in the entire thickness direction of the thin film sample to be measured, it is preferable that the depth of the evanescent wave generated is deeper than the thickness of the thin film sample. Therefore, the frequency band of the terahertz wave appropriate for the thickness of the thin film sample is selected so that the depth of evanescent wave bleeding is greater than the thickness of the thin film sample. The depth d _p of the evanescent wave oozing is proportional to the wavelength λ of the terahertz wave and inversely proportional to the frequency as shown in Equation 10. Since the bleeding depth d _p increases on the low frequency side, a terahertz wave on the low frequency side is preferable. As described below, the higher the frequency of the terahertz wave, the higher the sensitivity because the water absorption coefficient α increases, but the higher the frequency, the more the bleeding depth d _p becomes smaller and the horny layer moisture content is sufficient. Tend not to be reflected. On the other hand, the evanescent wave oozing depth d _p becomes too large on the low frequency side, and correction is not sufficient even with the two-interface model, and there is a possibility that the absorption is mainly caused by the laminated sample laminated on the thin film sample. For example, since the thickness of the skin stratum corneum is about 10 to 70 μm, the upper limit of the frequency in the measurement of the moisture content of the skin stratum corneum is 5.0 THz, 3.0 THz, and 2.0 THz, 1.5 THz or less, and may be 1.0 THz.

  The refractive index (material) of the prism and the incident angle of the terahertz wave can be adjusted as appropriate. Specifically, silicon (n = 3.4) and MgO (n = 3.1) are preferable. Further, the condition for total reflection is that at least the refractive index of the medium on the exit side is larger than the refractive index of the medium on the incident side. That is, the refractive index of the prism needs to be larger than the refractive index of the thin layer sample. Therefore, when considering irradiation on the skin, it varies depending on the frequency of the terahertz wave used for the measurement, but is generally at least 2.0 or more, preferably 2.5 or more, desirably 3.0 or more at a frequency lower than 1 THz. A prism having a refractive index of 2.0 or higher is used in a high-frequency band of about 1 THz. In other words, if the silicon or MgO prism has a refractive index of 3.0 or more, a terahertz wave of 0.1 THz or more can be used. As described below, according to the actual measurement by the present inventors, when the terahertz wave of 0.1 THz or more is irradiated, the real part of the complex refractive index of the stratum corneum of the pig skin is about 2.5 or less. (See FIG. 13). More specifically, it is determined from the complex refractive index of the stratum corneum calculated by applying a two-interface model using the complex refractive index obtained by measuring the skin layer obtained by stripping the skin and the thickness of the stratum corneum. Can be done. As described above, the thickness of the stratum corneum necessary for calculating the complex refractive index can be measured, but the thickness may be obtained from the number of stripping, or the literature value may be used. . For application to humans, calculation may be performed using model animal skin such as pigs in addition to human skin. In particular, porcine skin is very similar to the composition and structure of human skin, and it can be said that the complex refractive index of the stratum corneum of human skin is almost the same as that of porcine skin. The incident angle varies depending on the refractive index of the prism and the frequency of the terahertz wave, and is set to an angle that satisfies the total reflection condition.

  The obtained absorption coefficient α is a value reflecting the moisture content of the thin film sample, and the relative moisture content of the thin film sample can be known from the calculated absorption coefficient α. Since the absorption coefficient α is proportional to the molar concentration of the substance, the molar concentration of the substance can be obtained from the absorption coefficient α. That is, using the relationship between the known mass water content and the absorption coefficient α, the mass water content (water content) of the thin film sample is obtained from the obtained absorption coefficient α. For example, when measuring the stratum corneum of the skin, it can be obtained by comparison with the absorption coefficient α measured using a stratum corneum model whose mass water content is known. An arbitrary model is used as the stratum corneum model. Since the stratum corneum is a layer of dead keratinocytes, for example, a sheet model obtained by compressing protein component powder collected from the skin and containing moisture, the stratum corneum is composed of protein and water. The model which consists of an albumin protein aqueous solution assumed to be comprised is illustrated.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on the following Example, this invention is not limited by the following Example.

[Measurement of water absorption coefficient]
In measuring the moisture in the stratum corneum, first, the water absorption coefficient α was measured. The ATR module of the terahertz total reflection attenuation spectrometer used for the measurement has a silicon (n = 3.4) prism, and the incident angle is 57 ° (the same applies hereinafter). Measurement was performed by dropping water on the prism surface. The reference was air. The results are shown in FIG.

Next, sucrose aqueous solutions having different moisture contents were measured, and the relationship between the absorption coefficient α and the mass moisture content (sucrose concentration) of the sucrose aqueous solution was examined. Measurement was performed by dropping 1 mL of a sucrose aqueous solution onto the prism surface. The results are shown in FIGS. From this result, it was confirmed that the mass water content of the sucrose aqueous solution and the absorption coefficient α were proportional (FIG. 4). The coefficient of determination ^{R 2} is for example 0.5THz at this time, and at each 0.99 or more at 1.0 THz, the absorption coefficient at 0.5THz literature value (Keiichiro Shiraga et al., Food Chemistry, 140,315-320, 2013).

[Water absorption coefficient in the stratum corneum model]
The water absorption coefficient contained in the stratum corneum model was measured using the stratum corneum model. A sheet obtained by pressing interstitial powder (hide powder non-chromated, manufactured by SIGMA) collected from the skin dermis was used as a stratum corneum model (sheet model). Applying a pressure of 294 Pa or 589 Pa to 0.12 g of powder for 2 seconds was repeated 3 times to produce a sheet. 200 to 500 μL of distilled water was dropped onto the sheet, and left in a container with a humidity of 100% for 48 hours, so that moisture was uniformly distributed in the sheet. The water content was calculated from the wet mass immediately after measurement and the dry mass after drying at 70 ° C. for 24 hours.

  The obtained sheet was placed on a prism, and a weight was placed on the sheet for measurement. The results are shown in FIGS. As shown in FIG. 6, it was shown that the mass moisture content and the absorption coefficient are proportional, and the moisture content in the stratum corneum model can be calculated from the absorption coefficient.

[Analysis using a two-interface model]
Next, an analysis using a two-interface model was attempted. A dielectric thin film (manufactured by Nitto Denko Corporation) having a thickness of 10 μm is pasted on the prism so as to have a thickness of 10 μm, 20 μm, 30 μm, 40 μm, and 50 μm, and 1 mL of distilled water is dropped on the dielectric film. As shown in FIG. 2, there are two interfaces (the interface between the prism and the dielectric thin film and the interface between the dielectric thin film and the water stack). In addition, the dielectric thin film was measured by being attached to the prism until the thickness became sufficiently deeper than the depth of evanescent wave bleeding. From this measurement, the absorption coefficient of the dielectric thin film was determined. The results are shown in FIGS. On the low frequency side, the calculated value obtained using the two-interface model and the absorption coefficient of the actually used dielectric thin film were almost the same, but when the thickness of the dielectric thin film was 40 μm or 50 μm, the dielectric thin film alone The value was smaller on the high frequency side than the measured value. From this result, it is considered that the moisture of the thin film can be accurately measured by the two-interface model if a terahertz wave in a frequency band in which the oozing depth d _p is larger than the thickness of the thin film is used. It can also be said that the influence of moisture on the dielectric film is also eliminated.

[Measurement of water content of pig skin]
Prior to applying the two-interface model, the water content of the stratum corneum of pigs was determined without using the two-interface model. Then, the change in the amount of water due to stripping was measured. The frozen porcine skin (2 cm × 2 cm) was thawed at room temperature (about 20 ° C.) and then left to dry at room temperature (about 20 ° C.) for 48 hours with the side of the skin covered with an adhesive tape. Next, a paper waste (trade name “Kimwipe”, manufactured by Nippon Paper Claire Co., Ltd.) containing 2 mL of distilled water was placed on the dried skin for 150 seconds and left standing. Thereafter, the surface of the skin was lightly wiped with a paper waste cloth to obtain water-containing skin. In addition, stripping using a cellophane tape was performed after the measurement, and the measurement was repeated until 25 stripping was performed. In this stripping, it is considered that one stratum corneum (thickness of about 1 μm) is peeled off by one stripping, and the stratum corneum is almost removed by stripping 25 times.

  The pig skin was placed on a prism, and the measurement was performed with a weight. The moisture content was calculated from the measured absorption coefficient using a graph showing the relationship between the absorption coefficient obtained in Example 2 and the mass moisture content. The results are shown in FIG. 9 and FIG. As in the case of water or an aqueous sucrose solution, the obtained absorption coefficient tended to increase on the high frequency side. As the stripping was repeated, the water content of the pig skin decreased temporarily, but then tended to increase. As a result of water absorption in the dried skin in a very short time, water absorption occurred in the outermost layer, but in the subsequent intermediate layer, water was not sufficiently absorbed and the moisture content was low, and the deep part of the skin In this case, it was considered that the moisture content remained high as a result of insufficient drying.

[Measurement of water content of pig skin (horny layer) using two-interface model]
From the measurement data obtained in Example 4, the absorption coefficient and the mass water content of the stratum corneum of pig skin were calculated using a two-interface model. The results are shown in FIG. 11 and FIG. An example of a diagram showing the relationship between the refractive index n (real part) and extinction coefficient κ (imaginary part) of the complex refractive index n ~ ₂ of the stratum corneum derived to calculate the absorption coefficient of the stratum corneum and the frequency. Is shown in FIG. In the two-interface model, the stratum corneum thickness d of pig skin necessary for calculating the reflection coefficient r to ₁₂₃ was assumed to be 30 μm. As the complex refractive index n to ₃ of the skin layer, the complex refractive index obtained by stripping 25 times in Example 4 was used.

As shown in FIG. 13, it was measured that the real part of the complex refractive index n- ₂ of the stratum corneum was 2.5 or less. The relationship between the absorption coefficient of the skin and the number of stripping times and the absorption coefficient showed the same tendency as in Example 4. On the other hand, when examining whether or not the two-interface model is applied, as shown in FIG. 14, when the two-interface model is applied, the skin absorption coefficient tends to be smaller as a whole, and compared with the high frequency side on the low frequency side. Became smaller. It can be said that the evanescent wave oozes deeper on the low frequency side, and the effect of the skin layer is eliminated. Further, the change in the absorption coefficient depending on the presence or absence of application was smaller on the high frequency side than on the low frequency side.

[Chorn layer model using water-soluble albumin solution]
There was a concern that the stratum corneum model used in Example 2 could not measure a moisture content with a mass moisture content of 30% or less due to the presence of voids. Therefore, the same measurement was performed using an aqueous solution of water-soluble albumin having a known mass concentration (w / w%) as a model without voids. FIG. 15 shows the relationship between the absorption coefficient and the frequency. FIG. 16 shows the relationship between the absorption coefficient and the mass moisture content. Moreover, the relationship between the mass moisture content calculated | required using the 2 interface model from the measurement data obtained in Example 4 and stripping frequency was shown in FIG. The relationship between the mass moisture content and the absorption coefficient in FIG. In Equation 11, M _W is the mass of water, the M _D mass of albumin, V is the volume of the aqueous solution, is W _M is the mass concentration of water. In the terahertz band, the absorption coefficient is considered to have a linear relationship with the molar concentration of water molecules, but in an albumin aqueous solution, not only the mass of water molecules but also the mass of albumin should be considered in calculating the mass water content. The mass moisture content is not proportional to the molar concentration. On the other hand, the decrease in the absorption coefficient of water when albumin concentration increases (mass moisture content decreases) is considered to be proportional to the increase in albumin (absorption is negligibly small), so albumin concentration and water absorption It is considered that the coefficient also has a linear relationship. If it does so, the density | concentration of water and albumin density | concentration can be represented as Numerical formula 12 and Numerical formula 13, respectively, using A, B, C, and D as constants. Accordingly, A, B, C, and D are obtained by approximating the measured absorption coefficient and the mass concentration of water, and the measured absorption coefficient and the albumin mass concentration by linear functions, respectively, and the relationship shown in FIG. )

  By using the albumin aqueous solution as a stratum corneum model, the effect of voids was reduced, and the water content of the stratum corneum could be measured even when the mass moisture content was 30% or less. Further, when the relationship between the mass water content and the number of stripping was determined from this calibration curve, the same tendency as in Example 5 was observed.

  The present invention provides a new measurement method for measuring the amount of water in a thin film sample.

Claims (6)

A method for measuring a moisture content of a thin film sample using terahertz wave total reflection attenuation spectroscopy,
Irradiating a terahertz wave from a side of the thin film sample to the measurement object in a state where the laminated sample is laminated on the thin film sample, and observing the attenuation of the terahertz wave;
Correcting the attenuation of the terahertz wave using a reflection coefficient at the interface between the thin film sample and the laminated sample;
Obtaining an absorption coefficient of the thin film sample from the modified terahertz wave;
Method for measuring water content in thin film sample having A method for measuring a moisture content of a thin film sample using terahertz wave total reflection attenuation spectroscopy,
Irradiating terahertz waves from the thin film sample side through the prism from the thin film sample side to the measurement object in a state where the laminated sample is laminated on the thin film sample, and measuring a time domain spectrum;
Using the complex refractive index of the laminated sample, the refractive index of the prism, and the thickness of the thin film sample, calculating the reflection coefficient of the measurement object in the laminated state;
Obtaining an absorption coefficient of the thin film sample from the measured time domain spectrum and the calculated reflection coefficient of the measurement object;
Method for measuring moisture content of thin film having   The moisture measuring method according to claim 1 or 2, wherein a terahertz wave in a frequency band that generates an evanescent wave having a oozing depth larger than a thickness of the thin film sample is irradiated.   The moisture content measuring method according to claim 1, wherein the measurement object is skin, the thin film sample is a stratum corneum of skin, and the laminated sample is a skin epidermis layer.   The moisture content measuring method according to claim 4, wherein a prism having a refractive index of 2.0 or more is used.   The moisture content measuring method according to any one of claims 1 to 5, further comprising a step of obtaining a moisture content of the thin film sample from a correlation between a known moisture content and an absorption coefficient.