A method for rapid detection of lactic acid in liquor Daqu and fermented grains using ion mobility spectrometer

Abstract

The present invention relates to the field of separation, analysis and detection technology, and specifically to a method for quickly detecting lactic acid in liquor Daqu and fermented grains using an ion mobility spectrometer. The ion mobility spectrometer comprises an ion mobility tube, which comprises an ionization zone and a drift zone. The ionization zone comprises a photoionization source, a vacuum pump interface, a chemical dopant interface, and an injection interface. A drift gas inlet is provided at the tail of the ion mobility tube. The vacuum pump interface, the chemical dopant interface, and the injection interface are sequentially arranged in a vertical direction of the central axis of the ion mobility tube, and the injection interface is connected to an injection device. The ion mobility spectrometer is used for detection in negative ion mode, and the sample is extracted with isopropanol as an extractant to achieve high-sensitivity detection of lactic acid. The method of the present invention has simple sample pretreatment, saves time, does not require filtering operation, and will not contaminate the detector or block the separation column. The sample is extracted with isopropanol, which can not only regulate the air reagent ion peak, but also solve the corrosion effect of lactic acid on the entire system material.

Description

Technical Field

[0001] The invention relates to the technical field of separation, analysis and detection, and in particular to a device for detecting lactic acid in liquor daqu and fermented grains. Background Art

[0002] The production of Daqu liquor focuses on quality and requires a high yield. The yield depends on the quality of fermentation. The main factors affecting the fermentation quality are the temperature, acidity, water, and starch of the fermented grains before entering the cellar. Water and starch are easy to control, while temperature and acidity are difficult to control and are the key. If the temperature entering the cellar is high, the temperature rises quickly and the acid rises quickly; if the acidity entering the cellar is high, fermentation is hindered and the yield is low; but if the acidity entering the cellar is low, although it is beneficial to the yield, it lacks the base material for producing esterification substances, the aroma components are insufficient, and the product quality is poor.

[0003] The acid substances in wine are mainly composed of organic acids, which mainly come from acetic acid, propionic acid, butyric acid, lactic acid, caproic acid and higher acids in the fermentation process of the mash. Most of them exist in a free state, and a small part exists in the form of salts. When calculating the total acid, organic acids are mainly used and converted into acetic acid content. Organic acids are all weak acids. Only a small part of weak acids is ionized in aqueous solution, and most of them still exist in the form of unionized molecules. For example, the acid content of a strong acid in an aqueous solution should show a good linear relationship with the pH value. However, the acids in wine are mostly organic acids. Affected by many factors such as fermentation and distillation, the type and content of acids are not fixed. Even for the same winemaking group, different cellars will produce wines with difficult to have fixed values. Lactic acid, an organic acid, has a molecular formula of C 3 H 6 O 3 . Gas chromatography is commonly used to detect lactic acid, a flavor component in liquor. However, due to the thermal instability of lactic acid, it cannot be accurately quantified by gas chromatography. Since lactic acid is easily decomposed by heat in the vaporization chamber, mass spectrometry only detects its decomposition product dioxane. Therefore, it cannot be directly sampled and analyzed, and can only be solved by other means. Lactic acid is an important acid, and the detection method mostly uses the colorimetric method used in conventional chemical analysis for quantitative analysis, but it does not belong to the concept of acidity in food analysis. According to literature reports, the detection methods of lactic acid in liquor include liquid chromatography, benzyl esterification, ion chromatography, near-infrared spectroscopy, etc. The detection cost of the existing technology is high, the sample amount required is large, the pretreatment is complex, the operation is cumbersome, it cannot be accurately quantified, and a large number of samples cannot be detected at the same time. The use is limited by instruments, consumables, resources, etc.; the market needs a detection method that can quickly, accurately, and a large number of lactic acid content detection with a small amount of sample, and the present invention solves such problems. Finding a method that is simple, fast and accurate to detect lactic acid in liquor has very important guiding significance for the production of liquor base liquor and the improvement of liquor quality. Summary of the invention

[0004] In order to solve the problems existing in the prior art, the purpose of the present invention is to provide a device and method for analyzing lactic acid detection in liquor Daqu and fermented grains, including a method for extracting lactic acid from liquor Daqu and fermented grains, a sampling and injection process, and an analysis and detection device and method. Liquor Daqu and fermented grains samples are subjected to a simple solvent extraction pretreatment process to obtain a lactic acid solution, and a negative pressure sampling ion migration tube injection technology is used to achieve high-sensitivity detection of lactic acid.

[0005] In order to achieve the above-mentioned invention object, the present invention adopts the following technical solutions:

[0006] The invention provides an ion migration spectrometer for rapid detection of lactic acid, comprising an ion migration tube, wherein the ion migration tube comprises an ionization zone and a drift zone, wherein the ionization zone comprises a photoionization source and through hole one, through hole two and through hole three, wherein a drift gas inlet is arranged at the tail of the ion migration tube, wherein the through hole one, through hole two and through hole three are sequentially arranged in a direction perpendicular to the central axis of the ion migration tube; the through hole one is an air pump interface, the through hole two is a chemical dopant interface, and the through hole three is an injection interface; and the injection interface is connected to an injection device.

[0007] In the above technical solution, further, the sampling device includes a sampling tube and a sampling bottle; the sampling tube is 2-10 cm long, the sampling tube is an inner and outer outer tube, and the inner tube is a cross-sectional diameter The outer tube is a PTFE tube with a cross-sectional diameter of PTFE tube.

[0008] Another aspect of the present invention provides a method for rapid detection of lactic acid in liquor Daqu and fermented grains, using an ion mobility spectrometer in negative ion mode for detection, the method comprising the following steps:

[0009] (1) Analysis of lactic acid standard: Take a lactic acid standard and detect it using an ion mobility spectrometer in negative ion mode to obtain a peak migration time spectrum of lactic acid;

[0010] (2) Sample pretreatment: Dissolve the sample using isopropanol, which is selective for the ionization of air reagent ions in the negative ion mode ion mobility spectrometer, as the extraction solvent;

[0011] (3) Sample analysis: The sample obtained in step (2) is detected in the negative ion mode of an ion mobility spectrometer. By comparing the obtained sample peak migration time with the spectrum lactic acid signal peak migration time in step (1), the quantitative analysis target peak tracking signal intensity data is selected for quantitative analysis.

[0012] In the above technical solution, further, in the step (1), the lactic acid standard is diluted stepwise with isopropanol to prepare 10, 25, 50, 100, 200, and 400 μg / ml solutions for detection.

[0013] In the above technical scheme, further, in the step (2), the concentration of the sample dissolved in isopropanol is: 0.2-0.3 g / ml for Daqu and 2-3 mg / ml for mash; the sample extraction solution is vortexed for 2-10 min, centrifuged for 2-10 min, and the supernatant is used for sample injection analysis.

[0014] In the above technical scheme, further, in the step (3), 2-5 μL of sample is taken into a sampling bottle, and the sampling tube is directly inserted into the bottom of the sampling bottle during sampling, and the sample is brought into the ionization zone of the ion migration tube by the negative pressure sampling of the vacuum pump gas; the detection and analysis are carried out in the negative ion mode of the ion migration spectrometer; the gas is air, the vacuum pump flow rate is 670-750 ml / min, the chemical dopant (dopant) gas flow rate is 50 ml / min, the drift gas flow rate is 600 ml / min, and the sampling flow rate is 20-100 ml / min.

[0015] In the above technical solution, further, the chemical dopant dopant is acetone or butanone, and isopropanol is added and mixed with the acetone or butanone, and the volume ratio of isopropanol to the chemical dopant is (1-2):1.

[0016] In the above technical solution, further, after the sample injection is completed, the sampling tube directly takes the isopropanol headspace vapor for 1 minute to eliminate the residual lactic acid in the ion transfer tube detection system; a single continuous experiment is replaced The PTFE sampling tube is used to eliminate the memory effect of isopropyl alcohol.

[0017] In the above technical solution, further, in the step (3), the sampling time is ≤1 min, and the quantitative analysis is calculated according to the cumulative peak intensity of the tracking signal within 1 min.

[0018] Beneficial effects of the present invention:

[0019] 1. As an analytical method for lactic acid in Daqu and fermented grains, ion mobility spectrometry is simple and time-saving for sample pretreatment. There is no filtration operation and it will not pollute the detector or clog the separation column. The sample is extracted with isopropanol as the extractant, which can regulate the air reagent ion peak (eliminating CO 3 - , increasing CO 4 - ion concentration), and can solve the corrosion effect of lactic acid on the entire system materials.

[0020] 2. The detection mode of ion mobility spectrometer for lactic acid analysis is injection for 1 min, cleaning for 1 min, and system equilibrium for 0.5-1 min; the process cycle and single analysis are within only 3 minutes, which can ensure high-throughput application.

[0021] 3. Lactic acid is an important acid and is very unstable when heated. The injection method of the present invention avoids the influence of high temperature heating on the stability of the sample.

[0022] 4. Based on the high lactic acid content in the mash, the target peak of the lactic acid quantitative analysis method is to ignore the monomer peak and select the dimer peak signal range for linear fitting, which is more suitable for the detection of high concentration lactic acid in the mash.

[0023] 5. The position design of the vacuum pump interface, dopant interface, and injection interface in the present invention can realize the chemical dopant entering the ion migration tube separately, ensuring the stability of the ion migration spectrum reagent ion peak, and at the same time realizing unidirectional airflow injection, thereby improving the injection efficiency and ionization efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of the structure and principle of the ion mobility spectrometer device for detecting lactic acid;

[0025] 1 is the interface for connecting the vacuum pump; 2 is the interface for connecting the chemical dopant (dopant tube); 3 is the interface for connecting the sampling device; 4 is the drift gas inlet; 5 is the photoionization source; 6 is the ion transfer tube detector; 7 is the ionization zone; 8 is the drift zone; 9 is the sampling tube; and 10 is the sampling bottle.

[0026] Figure 2 Detecting the ion migration spectrum of air signal peak by ion migration spectrometer;

[0027] Figure 3 The ion mobility spectrometer is used to detect the ion mobility spectrum of the lactic acid standard;

[0028] Figure 4 The standard curve fitting diagram for the quantitative analysis of lactic acid detected by ion mobility spectrometer;

[0029] Figure 5 This is the ion migration spectrum of lactic acid extracted from different amounts of fermented grains samples. DETAILED DESCRIPTION

[0030] The ion mobility spectrometry operations involved in the following examples are all performed using Figure 1 The ion migration spectrometer device of the structure shown in the figure is used for detection; the experimental conditions of the ion migration spectrometry in the following embodiments are: the ion migration spectrometry with the ionization source as the photoionization source, and the headspace negative pressure sampling and injection method of the sample volatilization in the sample bottle. During the experiment, the migration tube temperature was kept at 100°C, and the sample carrier gas (air), float gas (air), and dopant gas (acetone) gas flows were 100, 600, and 50 mL / min, respectively. The injection environment temperature was 22-25°C. The ion gate opening time was 50 μs. The negative high voltage was loaded with 8000V, and the electric field was uniform.

[0031] In order to achieve accurate, rapid and sensitive detection of lactic acid in Daqu and fermented grains, the present invention uses an ion mobility spectrometer as an analytical means, dissolves a lactic acid standard in an organic solvent, isopropanol (to form a solution of 10-400 μg / ml), uses the lactic acid standard as a standard reagent, and uses the ion mobility spectrometer in negative ion mode as a detection instrument to obtain detection signals, respectively, and finally calculates the lactic acid content of the Daqu and fermented grains samples.

[0032] Example 1

[0033] Air background (RIP) detection:

[0034] The air sample is carried by the carrier gas in the sampling tube into the ionization zone and is ionized into positive and negative ions. It passes through the periodically opened ion gate and enters the drift zone composed of a uniform electric field. It is separated and detected in the drift zone to obtain the instrument air background detection signal. Figure 2 The ion migration spectrum of the air background detected by the negative ion mode ion migration spectrometer of the device of the present invention is given. It can be seen from the figure that the negative ion mode air reagent ion peak (RIP) has two peaks, which are at 2.9ms and 3.1ms respectively.

[0035] Example 2

[0036] Qualitative and quantitative analysis of lactic acid standards:

[0037] Accurately measure 3 μl of 200 μg / ml lactic acid standard solution at the bottom of the sample bottle. The solution is carried into the ionization zone by the carrier gas in the sampling tube and ionized into positive and negative ions. The ions pass through the periodically opened ion gate and enter the drift zone composed of a uniform electric field. The ions are separated and detected in the drift zone to obtain instrument detection signals. Figure 3 The ion migration spectra of lactic acid detected by the negative ion mode ion migration spectrometer of the device of the present invention are given. It can be seen from the figure that the negative ion mode lactic acid detection peaks appear at 3.5ms, 3.7ms, and 4.5ms. A series of concentration experiments found that the lactic acid dimer peak (4.5ms) conforms to the linear equation relationship in the concentration range of 20-200μg / ml (ppm), such as Figure 4 The equation is: Y = 101952.7 + 4079.8 * X, and the correlation coefficient is 0.9916. Each single point concentration was repeated 3 times, and the calculated relative standard deviation RSD was within 20%.

[0038] Example 3

[0039] Detection of lactic acid in fermented grains:

[0040] Accurately measure 24.0 mg of a certain fermented grain sample in a 5 ml centrifuge tube, add 4 ml of isopropanol reagent, vortex for 10 minutes, centrifuge at 12000 rpm for 10 minutes, dilute the supernatant 10 times, and 3 μl is carried by the carrier gas in the sampling tube into the ionization zone to be ionized into positive and negative ions. Through the periodically opened ion gate, it enters the drift zone composed of a uniform electric field, is separated and detected in the drift zone, and obtains the instrument detection signal. Figure 5 The ion migration spectrogram of lactic acid detected by the negative ion mode ion migration spectrometry of the device of the present invention is given, and the 4.5ms signal peak is continuously collected for 1 minute. The cumulative sum signal of the recorded lactic acid dimer peak (4.5ms) is brought into the standard curve equation: Y=101952.7+4079.8*X, and the lactic acid concentration is calculated. Then the lactic acid content in the fermented grains sample is calculated to be 5.4g / 100g.

[0041] Detection of lactic acid in Daqu:

[0042] Accurately measure 0.5099g of a ground Daqu sample into a 5mL centrifuge tube, add 2ml of isopropanol reagent, vortex for 10min, centrifuge at 12000rmp for 10min, and 3μl of the supernatant is carried by the carrier gas in the sampling tube into the ionization zone for ion mobility spectrometry analysis. The cumulative sum signal of the recorded lactic acid dimer peak (4.5ms) is brought into the standard curve equation: Y=101952.7+4079.8*X, and the lactic acid concentration is calculated, and then the lactic acid content in the Daqu sample is calculated to be 0.01g / 100g.

[0043] In the lactic acid detection method of the present invention, a single sample needs 20 minutes of sample pretreatment for a single analysis, and 2 minutes of analysis and detection; the sample pretreatment can be batch processed without secondary centrifugation and filter membrane operation.

[0044] It can be seen from this that the device and method of the present invention are feasible methods for completing lactic acid content detection quickly, accurately and with high throughput.

[0045] For any technician familiar with the art, without departing from the scope of the technical solution of the present invention, the technical content disclosed above can be used to make many possible changes and modifications to the technical solution of the present invention, or modified into equivalent embodiments with equivalent changes. Therefore, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention should still fall within the scope of protection of the technical solution of the present invention.

Claims

1. An ion mobility spectrometer for rapid detection of lactic acid, comprising an ion mobility tube (6), wherein the ion mobility tube comprises an ionization region (7) and a drift region (8), wherein the ionization region (7) comprises a photoionization source (5) and through hole 1, through hole 2, and through hole 3, wherein a drift gas inlet (4) is provided at the rear of the ion mobility tube (6), characterized in that: The through hole 1, through hole 2 and through hole 3 are sequentially arranged in a direction perpendicular to the central axis of the ion migration tube; the through hole 1 is a vacuum pump interface (1), the through hole 2 is a chemical dopant interface (2), and the through hole 3 is a sample injection interface (3); the sample injection interface (3) is connected to a sample injection device; The detection steps of the ion mobility spectrometer for rapid detection of lactic acid are as follows: (1) Analysis of lactic acid standard: Take lactic acid standard and detect it using an ion mobility spectrometer in negative ion mode to obtain the peak migration time spectrum of lactic acid; (2) Sample pretreatment: Dissolve the sample in isopropanol, which is selective for the ionization of air reagent ions in the negative ion mode ion mobility spectrometer, as the extraction solvent; (3) Sample analysis: The sample obtained in step (2) is detected in the negative ion mode of the ion mobility spectrometer, and the obtained sample peak migration time is compared with the peak migration time of the lactic acid signal in the spectrum in step (1), and the quantitative analysis target peak tracking signal intensity data is selected for quantitative analysis; In step (1), the lactic acid standard is diluted stepwise with isopropanol to prepare 10, 25, 50, 100, 200, and 400 μg / ml solutions for detection; In the step (2), the concentration of the sample dissolved in isopropanol is: 0.2-0.3 g / ml of Daqu and 2-3 mg / ml of mash; the sample extraction solution is vortexed for 2-10 min, centrifuged for 2-10 min, and the supernatant is used for injection analysis; In the step (3), 2-5 μL of the sample is taken into a sampling bottle, and the sampling tube is directly inserted into the bottom of the sampling bottle during sampling, and the sample is brought into the ionization zone of the ion migration tube by the negative pressure sampling of the vacuum pump gas; the detection and analysis are performed in the negative ion mode of the ion migration spectrometer; the gas is air, the vacuum pump flow rate is 670-750 ml / min, the chemical dopant gas flow rate is 50 ml / min, the drift gas flow rate is 600 ml / min, and the sampling flow rate is 20-100 ml / min; The chemical dopant is acetone or butanone, and isopropanol is added and mixed with the acetone or butanone, and the volume ratio of isopropanol to the chemical dopant is (1-2):1; After the sample injection is completed, the sampling tube directly takes the isopropanol headspace vapor for 1 minute to eliminate the residual lactic acid in the ion migration tube detection system; a φ3 polytetrafluoroethylene sampling tube is replaced for a single continuous experiment to eliminate the memory effect of isopropanol.

2. An ion mobility spectrometer for rapid detection of lactic acid according to claim 1, characterized in that: The sampling device comprises a sampling tube (9) and a sampling bottle (10); the sampling tube has a length of 2-10 cm, and is an inner and outer tube; the inner tube is a polytetrafluoroethylene tube with a cross-sectional diameter of φ3, and the outer tube is a polytetrafluoroethylene tube with a cross-sectional diameter of φ4.

3. An ion mobility spectrometer for rapid detection of lactic acid according to claim 1, characterized in that: In the step (3), the sampling time is ≤1 min, and the quantitative analysis is calculated according to the cumulative peak intensity of the tracking signal within 1 min.

4. A rapid detection method for lactic acid in liquor Daqu and fermented grains, characterized in that: The method is carried out using the negative ion mode ion mobility spectrometer according to claim 1 or 2 for detection.

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