CN110987899A - Method for measuring content of total acid total esters in white spirit and identifying quality grade of white spirit - Google Patents
Method for measuring content of total acid total esters in white spirit and identifying quality grade of white spirit Download PDFInfo
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Abstract
一种测定白酒中总酸总酯含量及鉴别白酒质量等级的方法，步骤如下：采集待测样品及与待测样品的酒精度对应同等体积分数的乙醇‑水溶液的拉曼光谱；用待测样品的拉曼光谱减去乙醇‑水溶液的拉曼光谱得到待测样品的差谱；分别对待测样品的差谱的C＝O伸缩振动峰及待测样品的拉曼光谱的C‑C‑O对称伸缩振动峰进行积分；计算待测样品差谱的C＝O伸缩振动峰积分值与待测样品拉曼光谱的C‑C‑O对称伸缩振动峰的积分值的比值；根据检测公式计算待测样品的总酸总酯浓度。将计算结果和待测样品的质量等级对应的总酸总酯最低含量比较，即可判断出待测样品的质量是否符合质量等级要求。本发明不需样品预处理，操作简单，检测成本低，可实现无损检测。
A method for measuring total acid total ester content in liquor and identifying liquor quality grade, the steps are as follows: collecting a Raman spectrum of a sample to be tested and an ethanol-water solution of the same volume fraction corresponding to the alcohol content of the sample to be tested; The difference spectrum of the sample to be tested is obtained by subtracting the Raman spectrum of the ethanol-aqueous solution from the Raman spectrum of Integrate the stretching vibration peak; calculate the ratio of the C=O stretching vibration peak integral value of the differential spectrum of the sample to be tested and the integral value of the C-C-O symmetrical stretching vibration peak of the Raman spectrum of the sample to be tested; The total acid and total ester concentration of the sample. By comparing the calculation result with the minimum content of total acid total ester corresponding to the quality grade of the sample to be tested, it can be judged whether the quality of the sample to be tested meets the quality grade requirements. The invention does not need sample pretreatment, has simple operation, low detection cost, and can realize non-destructive detection.
Description
Technical Field
The invention relates to identification of liquor quality grades, in particular to a method for determining total acid total ester content in liquor based on Raman difference spectrum and an identification method of liquor grades.
Background
The quality of white spirit has always been a great concern for producers and consumers. The content of total acid total ester is one of important indexes in the physicochemical indexes, quality grades and flavor types of the white spirit. At present, the content of total acid and total ester of white spirit is mainly detected by the following methods: indicator method, potentiometric titration, chromatography, near infrared spectroscopy.
Indicator methods and potentiometric methods are the methods used in the national detection standards. The principle of the two methods for detecting the total acid is to use phenolphthalein as an indicator, perform neutralization titration by using a sodium hydroxide solution, and calculate the total acid content through the volume change of the phenolphthalein. The principle of detecting total ester is that after the titration of total acid, a certain amount of alkali liquor is added, then the ester is saponified by heating and refluxing, the titration is carried out by using sulfuric acid solution, and the content of the total ester is calculated by the volume change of the solution. Indicator method and potentiometric titration accuracy are higher, but need use more kinds of medicine when detecting, and the standard solution that titrates and use requires highly, and heating reflux unit operation is complicated, and whole testing process is long consuming time, and the operation degree of difficulty is great, is not suitable for short-term test.
Gas chromatography is a common method for detecting the content of total acid and total ester in white spirit by chromatography. The components to be measured in the white spirit have different distribution coefficients in gas-liquid two phases, so that the components are separated by forming a difference of migration velocity in a chromatographic column, and then the content of the total acid total ester is detected by a detector. When the method is used together with a mass spectrometer, the gas chromatography can more accurately measure a plurality of groups of components in the white spirit, so that the content of the total acid total ester is measured to be closer to a true value. However, the method needs to set experimental parameters and pretreat the sample, and has the defects of long detection time and unsuitability for rapid detection.
The near infrared spectrum technology is a detection technology which is rapidly developed in the 80 th of the 20 th century, and the near infrared spectrum detection method can analyze multiple components without preprocessing a sample and loss. However, when the near-infrared spectroscopy is used for processing, a proper model and an algorithm need to be selected and verified, the operation process is complex, professional operation is required, and the requirements of market detection on simplicity, convenience and rapidness in operation are not met.
Disclosure of Invention
The invention aims to provide a method for rapidly and accurately measuring the content of total acid total esters in white spirit.
The invention also aims to provide a method which has high accuracy and can realize the rapid identification of the quality grade of the white spirit by measuring the content of total acid total esters in the white spirit.
In order to achieve the first object, the invention adopts the following technical solutions:
a method for measuring the content of total acid total esters in white spirit comprises the following steps:
collecting the Raman spectrum S of a sample to be detected and the Raman spectrum S of an ethanol-water solution with the same volume fraction corresponding to the alcoholic strength of the sample to be detected1；
Subtracting the Raman spectrum S of the ethanol-water solution from the Raman spectrum S of the sample to be detected1And obtaining a difference spectrum delta S of the sample to be detected: Δ S-M S1；
Integrating the C ═ O stretching vibration peak of the difference spectrum delta S of the sample to be detected to obtain the C ═ O stretching vibration peak integral value I of the difference spectrum of the sample to be detectedcoIntegrating the C-C-O symmetric stretching vibration peak of the Raman spectrum S of the sample to be detected to obtain an integral value I of the C-C-O symmetric stretching vibration peak of the Raman spectrum S of the sample to be detectedcco；
Calculating the C ═ O stretching vibration peak integral value I of the difference spectrum of the sample to be measuredcoIntegral value I of C-C-O symmetric stretching vibration peak of Raman spectrum of sample to be measuredccoRatio of (A) R: r is ═ Ico/Icco；
According to the detection formula R ═ KnCcoCalculating the total acid total ester concentration of the sample to be measured, wherein C in the formulacoIs the total acid total ester concentration, K, of the sample to be testednR is C ═ O stretching vibration peak integral value I of the difference spectrum of the sample to be measured as a fitting coefficientcoIntegral value I of C-C-O symmetric stretching vibration peak of Raman spectrum of sample to be measuredccoThe ratio of (a) to (b).
Further, when the C ═ O stretching vibration peak of the difference spectrum of the sample to be detected is integrated, the selected integration waveband range is 110 wave numbers with the C ═ O peak as the center, and/or when the C-C-O symmetrical stretching vibration peak of the Raman spectrum of the sample to be detected is integrated, the selected integration waveband range is 100 wave numbers with the C-C-O peak as the center.
Furthermore, when the C ═ O stretching vibration peak of the difference spectrum delta S of the sample to be detected is integrated, the wave number range is 1660-1770 cm-1And/or the wave number range is 840-940 cm when the C-C-O symmetrical stretching vibration peak of the Raman spectrum of the sample to be detected is integrated-1。
Further, the detection formula R ═ KnCcoFitting according to the following steps:
preparing a standard sample solution, wherein the standard sample solution comprises a plurality of groups of ethyl acetate-n% ethanol aqueous solutions with different ethyl acetate concentrations, and n% is the volume fraction of ethanol in the standard sample solution;
collecting Raman spectra S of each standard sample solutioncCollecting the Raman spectrum S of the ethanol-water solution with the volume fraction of n percent under the same condition0The Raman spectrum S of each standard sample solution was analyzedcSeparately subtract the Raman spectra S of n% ethanol-water0Correspondingly obtaining a difference spectrum Delta S of each standard sample solutionc：ΔSc＝Sc-S0；
Integrating the C ═ O stretching vibration peak of the difference spectrum of each standard sample solution respectively to obtain the integral value I of the C ═ O stretching vibration peak of the difference spectrum of each standard sample solutionc(co)Respectively integrating the C-C-O symmetric stretching vibration peaks of the Raman spectrum of each standard sample solution to correspondingly obtain the integral value I of the C-C-O symmetric stretching vibration peaks of the Raman spectrum of each standard sample solutionc(cco)；
Calculating the integral value I of the C ═ O stretching vibration peak of the difference spectrum of each standard sample solution of the corresponding concentrationc(co)Integral value I of C-C-O symmetric stretching vibration peak of Raman spectrum of standard sample solutionc(cco)Ratio R ofc：Rc＝Ic(co)/Ic(cco)Establishing a standardAnd (3) carrying out linear fitting on the relation curve of the ethyl acetate concentration of the sample solution and the integral value ratio to obtain a detection formula: r is KnCco。
Further, when a Raman spectrum of each standard sample solution is collected, the collected Raman spectrum has a Raman shift of 800cm-1～2000cm-1A spectrum within the range.
Furthermore, the volume fraction of the ethanol in the standard sample solution corresponds to the alcoholic strength of the sample to be detected.
Further, when the volume fraction of ethanol in the standard sample solution does not correspond to the alcoholic strength of the sample to be detected, the fitting coefficient Kn’The value of (a) is calculated according to the following coefficient conversion formula:
In order to achieve the second object, the invention adopts the following technical solutions:
after the total acid total ester concentration of a sample to be detected is obtained by adopting the method for determining the total acid total ester content in the white spirit, the total acid total ester concentration of the sample to be detected is compared with the minimum total acid total ester content corresponding to the quality grade marked on a label of the sample product to be detected, and whether the quality grade of the sample to be detected meets the quality grade marked on the package of the sample to be detected is judged.
Further, when the concentration unit used by the content of the substance marked on the product label is g/L, the concentration unit is converted into mol/L, and then the calculated total acid total ester concentration of the sample to be detected is compared with the minimum content of the total acid total ester corresponding to the quality grade marked on the product label of the sample to be detected;
the conversion formula is as follows:
m in the formula1M is the minimum content of total acids corresponding to the quality grade marked on the label of the sample product to be tested2Is the minimum content of total esters corresponding to the quality grade marked on the label of the sample product to be tested, M1Relative molecular mass of acetic acid, M2Is the relative molecular mass of ethyl acetate.
From the above, the invention takes the C ═ O bond of acid and ester in the Raman spectrum of the white spirit as the detection index, the C-C-O stretching vibration characteristic peak of ethanol and the C ═ O vibration characteristic peak of total acid total ester in the spectrum are taken as the observation objects by collecting the Raman spectrum of the sample to be detected, and the difference spectrum analysis method is utilized, the measurement of the content of the total acid total ester and the identification of the quality grade are realized by observing the spectrum change of the ethyl acetate-ethanol water solution with different concentrations, the ratio of the integral values of the two spectrum peaks is adopted in the measurement process, the difference between different concentrations is increased, the change value is convenient to measure, the accuracy is high, the detection formula is obtained by fitting and calculating in advance, the spectrum is only needed to be collected during the detection, the sample is not needed to be pretreated, the operation steps and the experimental data processing process are simple, the invention has the advantage of rapid detection, and can realize the nondestructive detection, the method reduces the detection cost, and solves the problems of complicated experimental operation, long detection time, pretreatment of samples, high equipment cost, inapplicability to marketization and the like in the conventional method for determining the content of the total acid total ester.
Drawings
FIG. 1 is a schematic diagram of a Raman spectrum acquisition system according to an embodiment of the present invention;
FIG. 2 is a flow chart of the method of the present invention;
FIG. 3 is a Raman spectrum of an ethyl acetate-ethanol aqueous solution having a concentration of 0.05 to 0.7 mol/L;
FIG. 4 is a Raman spectrum of a C ═ O stretching vibration peak of a difference spectrum obtained by subtracting a Raman spectrum of an ethyl acetate-ethanol aqueous solution with a concentration of 0.05 to 0.7mol/L and a Raman spectrum of a 45% ethanol-aqueous solution;
FIG. 5 is a Raman spectrum of a C-C-O symmetric stretching vibration peak of an ethyl acetate-ethanol aqueous solution with a concentration of 0.05-0.7 mol/L;
FIG. 6 is a graph showing the relationship between the ratio of the integrated values and the ethyl acetate concentration;
FIG. 7 is a difference spectrum of a set of samples to be measured after subtracting an ethanol-water solution from a Raman spectrum.
The following describes embodiments of the present invention in further detail with reference to the accompanying drawings.
Detailed Description
In order to make the aforementioned and other objects, features and advantages of the present invention more apparent, embodiments of the present invention will be described in detail below with reference to the accompanying drawings.
Fig. 1 is a schematic diagram of a raman spectroscopy system used in the method of the present invention, as shown in fig. 1, the raman spectroscopy system comprising: the device comprises a laser 1, a sample area 2, a light splitting component 3, a CCD camera 4 and a host 5, wherein the host 5 can be a computer, or a portable intelligent device with a data processing function, such as a tablet computer and a smart phone. The laser wavelength of the laser 1 of this embodiment is 532 nm. Other spectroscopic measurement systems known in the art may also be used to collect the raman spectra of the sample.
Laser emitted by the laser 1 sequentially passes through a reflector 6, a Glan prism 7, a half-wave plate 8, a triple prism 9 and a first convex lens 10 (focal length 5cm) which are arranged between the laser 1 and the sample area 2, and then is incident to the sample area 2. The first convex lens 10 is simultaneously positioned between the sample area 2 and the light splitting assembly 3, and a second convex lens 11 (with a focal length of 20cm) is also arranged between the first convex lens 10 and the light splitting assembly 3. After the scattered light formed by the incident light irradiating the sample to be measured is changed into parallel light beams through the first convex lens 10, the parallel light beams are converged on the light inlet slit of the light splitting assembly 3 through the second convex lens 11 and then enter the light splitting assembly 3. The light splitting component 3 of the embodiment is a three-level spectrometer, which is purchased from Tokyo Toolhaman optical instruments Co., Ltd., model number Omin lambda-180D & Omin lambda-5008 i. The scattered light passes through the light splitting component 3, and then is collected by the CCD camera 4 to obtain a spectrum signal, and is transmitted to the host 5, and the CCD camera of the embodiment is an electric refrigeration CCD.
The authentication method of the present invention will be described with reference to fig. 2. As shown in fig. 2, the method for identifying the quality grade of the white spirit comprises the following steps:
collecting the Raman spectrum S of the sample to be measured and the Raman spectrum S of the ethanol-water solution with the same volume fraction corresponding to the alcoholic strength of the sample to be measured by a Raman spectrum measurement system1Taking C-C-O symmetric stretching vibration peak from ethanol in Raman spectrum S of sample to be detected and Raman spectrum S of ethanol-water solution1The C-C-O symmetric stretching vibration peak in the medium is taken as an observation object; the corresponding equal volume fraction means that the volume fraction (concentration) of the ethanol in the ethanol-water solution corresponds to the alcoholic strength of the sample to be detected, for example, if the sample to be detected is 45-degree white spirit, the volume fraction of the ethanol in the ethanol-water solution corresponds to 45%;
subtracting the Raman spectrum S of the ethanol-water solution from the Raman spectrum S of the sample to be detected1And obtaining a difference spectrum delta S of the sample to be detected: Δ S-M S1M in the formula is an adjustment coefficient, the spectrum of ethanol and water is subtracted from the difference spectrum, and the difference spectrum has a peak (fig. 7) of C ═ O stretching vibration from acid or ester at about 1700 wave numbers and is used as an observation object; ideally, when the raman spectrum of the white spirit (sample to be measured) is subtracted from the raman spectrum of the ethanol-water solution corresponding to the same volume fraction, the signals of ethanol and water should be completely subtracted, but in actual operation, the ethanol-water solution completely corresponding to the alcoholic strength of the white spirit is difficult to prepare, and in the ethanol-water solutions with different volume fractions, the microstructure of ethanol in water changes, so that spectral peaks can generate displacement, and positive and negative peaks can occur during subtraction, so that errors are generated; the adjustment coefficient M is used for reducing errors, the value of the adjustment coefficient M is an empirical value, the adjustment coefficient M is taken near 1, the M values corresponding to different samples are different, and the adjustment coefficient M can be determined according to specific spectral analysis and according to the fact that signals of positive and negative peaks which are equal in height and minimum in area based on a background base line appear in a C-C-O symmetric stretching vibration peak in a difference spectrum, for example, the M value is arbitrarily set to be a few points and zero, if the negative peak in the difference spectrum after subtraction is large and the M value is too large, the M value is reduced, if the positive peak is too small and the M value is increased, and the positive and negative peaks are basically the same in height, the M value can be determined;
integrating the C ═ O stretching vibration peak of the difference spectrum delta S of the sample to be detected to obtain the C ═ O stretching vibration peak integral value I of the difference spectrum of the sample to be detectedcoIntegrating the C-C-O symmetric stretching vibration peak of the Raman spectrum S of the sample to be detected to obtain an integral value I of the C-C-O symmetric stretching vibration peak of the Raman spectrum S of the sample to be detectedcco(ii) a Further, when the C ═ O stretching vibration peak of the difference spectrum of the sample to be measured is integrated, the selected integration waveband range may be about 110 wave numbers with the C ═ O peak as the center, and when the C-O symmetric stretching vibration peak of the raman spectrum of the sample to be measured is integrated, the selected integration waveband range may be about 100 wave numbers with the C-O peak as the center; for example, the wave number range of the C ═ O stretching vibration peak of the difference spectrum delta S of the sample to be detected can be 1660-1770 cm-1The wave number range of 840-940 cm when the C-C-O symmetrical stretching vibration peak of the Raman spectrum of the sample to be detected is integrated-1；
Calculating the C ═ O stretching vibration peak integral value I of the difference spectrum of the sample to be measuredcoIntegral value I of C-C-O symmetric stretching vibration peak of Raman spectrum of sample to be measuredccoRatio of (A) R: r is ═ Ico/Icco；
Using the calculated R value, according to the detection formula R ═ KnCcoCalculating the total acid total ester concentration of the sample to be measured, wherein C in the formulacoIs the total acid total ester concentration, K, of the sample to be testednIt is a constant for the fitting coefficient.
After the total acid total ester concentration of the sample to be detected is obtained, the quality grade of the white spirit can be identified according to the total acid total ester concentration of the sample to be detected, and the method comprises the following specific steps: the total acid total ester concentration C of the sample to be detected is obtained through calculationcoAnd comparing the minimum content of the total acid total esters corresponding to the quality grade marked on the label of the sample product to be detected, so as to judge whether the quality grade of the sample to be detected meets the quality grade marked on the package of the sample to be detected. The national standard used by the product is marked on the product label of the white spirit product, and the minimum content of the total acid total ester of the product specified by the standard can be determined according to the used national standard, so that comparison can be carried out.
If the substance marked on the product labelWhen the concentration unit of the substance on the label is not consistent with the calculated total acid total ester concentration unit, for example, the concentration unit of the substance on the label is g/L, and when the calculated total acid total ester concentration unit is mol/L, the concentration unit of the substance on the label is converted correspondingly and then compared. The conversion formula is as follows:
m in the formula1M is the minimum content of total acids corresponding to the quality grade marked on the label of the sample product to be tested2Is the minimum content of total esters corresponding to the quality grade marked on the label of the sample product to be tested, M1Relative molecular mass of acetic acid, M2Is the relative molecular mass of ethyl acetate.
Detection formula R ═ KnCcoAccording to C-R of standard sample solutioncAnd fitting the relation curve. The standard sample solution comprises a plurality of groups of ethyl acetate-n% ethanol water solutions with different ethyl acetate concentrations and is used for simulating white spirits with different total acid total ester concentrations, n% is the volume fraction of ethanol in the standard sample solution, and the volume fraction of ethanol in the standard sample solution can correspond to the alcoholic strength of a sample to be detected. For example, when a 45 ° spirit is to be identified, a series of ethyl acetate-45% ethanol aqueous solutions with different ethyl acetate concentrations can be prepared as standard sample solutions to simulate 45 ° spirits with different total acid total ester concentrations. The amount of the standard sample solution can be determined according to the measurement requirement, and when the amount of the ethyl acetate-ethanol aqueous solution with different ethyl acetate concentrations is more in the standard sample solution, the concentration difference is smaller, and the measurement result is more accurate.
Collecting Raman spectrum S of each standard sample solution by adopting Raman spectrum measurement systemcThe subscript c is used to indicate the ethyl acetate concentration of the standard sample solution and the raman spectrum S of an n% ethanol-water solution was taken under the same conditions0The Raman spectrum S of each standard sample solution was analyzedcSeparately subtract the Raman spectra S of n% ethanol-water0Correspondingly obtaining a difference spectrum Delta S of each standard sample solutionc：ΔSc＝Sc-S0(ii) a Further, it is characterized byThe Raman spectrum collected is the Raman shift at 800cm-1～2000cm-1A spectrum within the range;
integrating the C ═ O (ethyl acetate) stretching vibration peaks of the difference spectrum of each standard sample solution, and obtaining the integral value I of the C ═ O stretching vibration peaks of the difference spectrum of each standard sample solutionc(co)Simultaneously integrating the C-C-O (ethanol) symmetric stretching vibration peaks of the Raman spectrum of each standard sample solution respectively to obtain the integral value I of the C-C-O symmetric stretching vibration peaks of the Raman spectrum of each standard sample solution correspondinglyc(cco)；
Respectively calculating the integral value I of the C ═ O stretching vibration peak of the difference spectrum of each standard sample solution with corresponding concentrationc(co)Integral value I of C-C-O symmetric stretching vibration peak of Raman spectrum of standard sample solutionc(cco)Ratio R ofc＝Ic(co)/Ic(cco)Establishing C (ethyl acetate concentration) and R of the standard sample solutionc(integral value ratio) and fitting the relation curve to obtain a detection formula: r is KnCco。
In fitting the detection formula, the volume fraction of ethanol in the prepared standard sample solution is fixed, for example, the volume fraction of ethanol in all the standard sample solutions is n%, n-degree white spirit is simulated, and the fitting coefficient K in the detection formula isnThe method is a coefficient corresponding to n% volume fraction (liquor degree), and can be used for identifying the liquor with n degrees, and when the liquor to be detected has other degrees (n' degrees), if a standard sample solution containing ethanol with the volume fraction corresponding to the liquor degree is prepared in addition and then is fitted again, the operation steps are excessively complicated, the workload is high, and the practicability is greatly reduced. The applicant provides a coefficient conversion formula, and the fitting coefficient K in the detection formula can be obtained on the basis of the detection formula obtained on the basis of the standard sample solution of the ethanol volume fraction corresponding to the degree of a certain white spiritnPerforming conversion, and calculating the fitting coefficient value Kn’Replacing the fitting coefficient K in the original detection formulanThe white spirit with n' degree can be identified.
Kn’Value according to KnAnd the ethanol concentration corresponding to the alcoholic strength is obtained by conversion calculation according to a coefficient conversion formula, and the derivation process of the coefficient conversion formula is as follows:
carrying out formula fitting on a standard sample solution with the volume fraction of the ethanol being n%, and obtaining a detection formula as follows:
in the process of fitting the detection formula, R is equal to the ratio of the integral value of the difference spectrum of a certain standard sample solution to the integral value of the C-C-O symmetric stretching vibration peak of the corresponding standard sample solution Raman spectrum, namely
In the formula sigmaCORaman scattering cross section of C ═ O stretching vibration, σ, representing raman spectrum of standard sample solutionCCORaman scattering cross section representing C-C-O symmetric stretching vibration of Raman spectrum of standard sample solution, NccoDenotes the amount of C-O bond (ethanol), CcoIs the ethyl acetate concentration of the standard sample solution, CccoIs the concentration of ethanol in the standard sample solution;
σCOand σCCOThe ratio of (A) to (B) is constant for standard sample solutions of different volume fractions (ethanol) in the same optical path, and K is the value of K when a detection formula corresponding to a certain ethanol volume fraction (n%) is obtainednKnowing, then σCOAnd σCCOThe ratio can be determined, the alcohol degree (n' degree) of the sample to be detected is known, namely the alcohol concentration C of the sample to be detected corresponding to the alcohol degreecco(n’)When known, then:
ethanol of sample to be detected corresponding to alcoholic strengthConcentration of
Wherein m is the mass fraction of ethanol corresponding to the alcoholic strength of the sample to be detected, and ρ is the ethanol density corresponding to the alcoholic strength of the sample to be detected.
In order to improve the accuracy of the measurement, after the Raman spectrum of the relevant solution is obtained, cosmic rays can be removed from the obtained Raman spectrum; and/or removing background drift of the two peaks before integrating the C-C-O symmetrical stretching vibration peak and the C ═ O stretching vibration peak. The method for removing cosmic rays and processing background drift can refer to the method disclosed in the chinese patent application with publication number CN 109001182A. The spectrum may be acquired a plurality of times and then the average value may be taken as the processing object.
The validity of the method of the invention is verified in a specific example below. Firstly, preparing a standard sample solution, wherein the standard sample solution comprises 17 parts of ethyl acetate-n% ethanol aqueous solution with different ethyl acetate concentrations, the concentration range of ethyl acetate in the standard sample solution is 0.05-0.7 mol/L, the concentration difference of ethyl acetate in the standard sample solution with adjacent concentrations is about 0.03-0.04 mol/L, namely the concentration of ethyl acetate in the standard sample solution of the 1 st part is 0.05mol/L, the concentration of ethyl acetate in the standard sample solution of the 2 nd part is 0.08mol/L or 0.09mol/L, and the like. In this example, n% ═ 45%.
The Raman shift of 17 standard sample solutions collected respectively is 800cm-1～2000cm-1Raman spectrum S in the regioncWith a C-C-O symmetric stretching vibration peak in the spectrum as an observation object, each spectrum is collected 5 times, and Raman spectra of 17 collected standard sample solutions are shown in FIG. 3. When the spectrum is collected, all the solution is moved into a cuvette for collection, the Raman spectrum of the cuvette containing the solution is collected under the same condition, the collected spectrum is preprocessed, cosmic rays collected in the spectrum collection process are removed, and a background signal of the cuvette is subtracted; simultaneously collecting Raman spectrum S of 45% ethanol-water solution under the same condition0。
The Raman spectra S of standard sample solutions with different ethyl acetate concentrationscThe Raman spectra S of the 45% ethanol-water solutions were subtracted0Correspondingly obtaining the difference spectrum Delta S of the standard sample solution with different ethyl acetate concentrationsc(as shown in FIG. 4), the difference spectrum Δ S is obtainedcRespectively integrating C-O stretching vibration peaks, wherein the wave number range of the integration is 1660-1770 cm-1Obtaining 17 difference spectra C ═ O integral value I of expansion vibration peakc(co). Integrating C-C-O symmetric stretching vibration peaks of Raman spectra of standard sample solutions with different concentrations, wherein the integrated wave number range is 840-940 cm-1Obtaining the integral value I of C-C-O symmetric stretching vibration peak of Raman spectrum of 17 standard sample solutionsc(cco)(as shown in fig. 5).
Calculating the C ═ O integral value I of the stretching vibration peak of each difference spectrumc(co)Integral value I of C-C-O symmetric stretching vibration peak of Raman spectrum of standard sample solution with corresponding concentrationccoRatio R ofc＝Ico/IccoTable 1 shows the ratio R of ethyl acetate concentration C to integral value of 17 standard sample solutionscAccording to the experimental data, C and R are plottedcThen the relation curve is subjected to straight line fitting to obtain the detection formula R of the white spirit with the alcoholic strength of 45 degrees, wherein the detection formula R is 0.1995Cco. The detection formula of 42 degrees and 52 degrees is calculated by a coefficient conversion formula as R-0.2137Cco，R＝0.1726Cco。
TABLE 1
6 bottles of liquor of 3 different brands with first-grade and high-grade quality grades are purchased from the market, wherein the quality grades of the samples 1, 3 and 5 are first-grade, and the quality grades of the samples 2, 4 and 6 are high-grade. 6 bottles of wine contain 42 °, 45 ° and 52 °; samples 1, 2, 3, and 4 are the same national standard, and samples 5 and 6 are the same national standard. Respectively sampling 6 bottles of white spirit, putting the samples into a cuvette, collecting the Raman spectra of the 6 bottles of white spirit and the Raman spectra of a 42% ethanol-water solution, a 45% ethanol-water solution and a 52% ethanol-water solution, and collecting the Raman spectra of the cuvette containing the ethanol-water solution under the same condition. And (3) preprocessing the spectra of 6 groups of white spirits, removing cosmic rays collected in the spectrum collection process, and subtracting a background signal of a cuvette.
The raman spectra of the ethanol-water solutions of the same concentrations were subtracted from the raman spectra of 6 types of white spirits to obtain 6 sets of difference spectra, one set of which is shown in fig. 7, where M is 1.04 in this example and at 800cm-1The positive and negative peak heights of (a) are substantially the same.
After 6 groups of difference spectrums are subjected to background drift removal processing, C-O stretching vibration peaks in the difference spectrums are integrated, and after 6 groups of white spirits are subjected to background drift removal processing, C-C-O symmetrical stretching vibration peaks in the Raman spectrums of the 6 groups of white spirits are integrated.
Calculating the ratio of the integral value of the C-O stretching vibration peak of each group of sample difference spectrum to the integral value of the C-C-O symmetrical stretching vibration peak of the corresponding sample Raman spectrum, and respectively substituting the integral values into a detection formula R-0.1995Cco，R＝0.2137Cco，R＝0.1726CcoThe total acid total ester content was calculated and the measurement results are shown in table 2.
TABLE 2
In Table 2, the quality grade identification value is the quality grade identification value on the package of the white spirit, and the measured value is the total acid total ester content obtained by using the method of the invention, and the unit is mol/L. As can be seen from Table 2, the total acid total ester content determined by the method of the present invention is very close to the national standard value, and meets the quality grade marked on the package, and the result is reliable.
Although the present invention has been described with reference to a preferred embodiment, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.
Claims (9)
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