Detection method of 6-hydroxy-8-chloro ethyl caprylate chiral isomer

Abstract

The invention discloses a method for detecting a chiral isomer of 6-hydroxy-8-chloro ethyl caprylate. The method comprises the following steps: carrying out gas chromatography detection on a test solution containing R-6-hydroxy-8-chloro ethyl caprylate and S-6-hydroxy-8-chloro ethyl caprylate, wherein the gas chromatography detection is carried out on the test solution containing R-6-hydroxy-8-chloro ethyl caprylate and S-6-hydroxy-8-chloro ethyl caprylate; the conditions of the gas chromatography detection are as follows: a chromatographic column is selected from a chiral chromatographic column containing beta-cyclodextrin; the chromatographic column adopts programmed heating in the detection process, the initial column temperature is 120-180 DEG C, the final column temperature is 190-220 DEG C, and the heating rate is 0.5-2.0 DEG C / min. The method is based on a gas chromatographic method, adopts a beta-cyclodextrin-containing chiral chromatographic column, and is combined with programmed heating at a specific initial column temperature heating rate, so that the two chiral isomers of 6-hydroxy-8-chloro ethyl caprylate can be separated and detected, the sensitivity is high, the specificity is good, and the separation degree meets the standard; the method has important significance on preparation of single-configuration lipoic acid.

Description

Technical Field

[0001] The invention belongs to the technical field of detection and analysis, and in particular relates to a method for detecting chiral isomers of 6-hydroxy-8-chlorooctanoic acid ethyl ester. Background Art

[0002] Lipoic acid (lipoic acid) is a natural product with biological activity. As a safe and effective strong antioxidant, it is widely used in medicine, health products, food additives and cosmetics, etc. Its two chiral isomers show different biological activities, and its R-type enantiomer is much more effective than S-type. Therefore, the R-type lipoic acid of synthetic high optical purity is of great significance. And 6-hydroxy-8-chlorocaprylic acid ethyl ester is an important intermediate in the lipoic acid synthetic route, and it also has two chiral isomers (R-6-hydroxy-8-chlorocaprylic acid ethyl ester, S-6-hydroxy-8-chlorocaprylic acid ethyl ester), and chiral purity detection is carried out to control its quality, which is of great significance for preparing single configuration lipoic acid.

[0003] R / S-6-hydroxy-8-chlorooctanoic acid ethyl ester belongs to chain compound, and ultraviolet absorption is weak, and polarity is small, so the establishment of analytical method is relatively difficult. For this chain compound, current analytical method mainly includes pre-column derivatization method, introduces ultraviolet absorption group, or adopts special universal detectors such as evaporative light scattering detector (ELSD), differential refractive index detector (RID), electrospray ionization detector (CAD), but there are problems such as complicated pre-treatment steps and long reaction time in derivatization method, and special detector has corresponding restrictions on use conditions. At present, there is no report on the chiral purity detection method of R / S-6-hydroxy-8-chlorooctanoic acid ethyl ester, and only its final product thioctic acid has been reported to detect chiral content using differential refractive index detector, but there is poor peak shape, relatively long equilibrium time, and the problem that the instrument is greatly affected by external conditions. The United States Pharmacopoeia adopts thin layer chromatography to control chiral isomers to the final product thioctic acid, and the sensitivity is relatively poor and the accuracy is not high. Therefore, the development of a simpler, faster and more sensitive analytical method is of great significance for the quality control of R / S-6-hydroxy-8-chlorooctanoic acid ethyl ester, and is also of great significance for the preparation of single-configuration lipoic acid. Summary of the invention

[0004] In order to overcome at least one of the problems existing in the above-mentioned prior art, one of the objects of the present invention is to provide a method for detecting chiral isomers of 6-hydroxy-8-chlorooctanoic acid ethyl ester, which has high sensitivity and specificity, simple operation and separation degree that meets the standards.

[0005] The second object of the present invention is to provide an application of the above detection method in the preparation of single-configuration lipoic acid.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] The first aspect of the present invention provides a method for detecting chiral isomers of 6-hydroxy-8-chlorooctanoic acid ethyl ester, comprising the following steps: subjecting a test solution containing R-6-hydroxy-8-chlorooctanoic acid ethyl ester and S-6-hydroxy-8-chlorooctanoic acid ethyl ester to gas chromatography detection;

[0008] The gas chromatography detection conditions include: the chromatographic column is selected from a chiral chromatographic column containing β-cyclodextrin; the chromatographic column adopts programmed temperature rise during the detection process, the initial column temperature is 120-180°C, the final column temperature is 190-220°C, and the heating rate is 0.5-2.0°C / min.

[0009] Preferably, the initial column temperature is 130-177°C; more preferably, the initial column temperature is 140-175°C; more preferably, the initial column temperature is 150-170°C.

[0010] Preferably, the final column temperature is 192-215°C; more preferably, the final column temperature is 195-210°C; more preferably, the final column temperature is 195-205°C.

[0011] Preferably, the heating rate is 0.6 to 1.8°C / min; more preferably, the heating rate is 0.7 to 1.5°C / min; more preferably, the heating rate is 0.8 to 1.2°C / min.

[0012] Preferably, the column temperature of the chromatographic column during the detection process is 130-210°C; further preferably, the column temperature of the chromatographic column during the detection process is 140-200°C; further preferably, the column temperature of the chromatographic column during the detection process is 160-200°C.

[0013] Preferably, the chromatographic column for gas chromatography detection is selected from a CP-Cyclodextrin-β-2,3,6-M-19 chromatographic column.

[0014] In some specific embodiments of the present invention, the chromatographic column for gas chromatography detection is selected from Agilent J&W CP-Cyclodextrin-β-2,3,6-M-19 chromatographic column.

[0015] In some specific embodiments of the present invention, the column length of the CP-Cyclodextrin-β-2,3,6-M-19 chromatographic column is 20 to 40 m.

[0016] In some specific embodiments of the present invention, the inner diameter of the CP-Cyclodextrin-β-2,3,6-M-19 chromatographic column is 0.2-0.3 mm.

[0017] In some specific embodiments of the present invention, the liquid film thickness of the CP-Cyclodextrin-β-2,3,6-M-19 chromatographic column is 0.2-0.3 μm.

[0018] Preferably, the solvent of the test solution includes at least one of dimethyl sulfoxide, dimethylformamide, methanol or acetonitrile; further preferably, the solvent of the test solution is selected from methanol.

[0019] Preferably, the concentration of the test solution is 1-10 mg / mL; further preferably, the concentration of the test solution is 2-8 mg / mL; further preferably, the concentration of the test solution is 3-6 mg / mL.

[0020] Preferably, the carrier gas flow rate of the gas chromatography detection is 0.2-2 mL / min; further preferably, the carrier gas flow rate of the gas chromatography detection is 0.5-1.5 mL / min; further preferably, the carrier gas flow rate of the gas chromatography detection is 0.8-1.2 mL / min.

[0021] Preferably, the carrier gas for gas chromatography detection includes at least one of nitrogen, helium, argon or hydrogen; further preferably, the carrier gas for gas chromatography detection is selected from nitrogen.

[0022] Preferably, the injection volume of the gas chromatography detection is 0.1-2 μL; further preferably, the injection volume of the gas chromatography detection is 0.5-1.5 μL; further preferably, the injection volume of the gas chromatography detection is 0.8-1.2 μL.

[0023] Preferably, the split ratio of the gas chromatography detection is (0.01-100):1; further preferably, the split ratio of the gas chromatography detection is (0.1-50):1; further preferably, the split ratio of the gas chromatography detection is (1-20):1.

[0024] Preferably, the injection port temperature of the gas chromatography detection is 180-250°C; further preferably, the injection port temperature of the gas chromatography detection is 185-230°C; further preferably, the injection port temperature of the gas chromatography detection is 190-210°C.

[0025] Preferably, the detector for gas chromatography detection is selected from a hydrogen flame detector.

[0026] Preferably, the detector temperature of the gas chromatography detection is 180-250°C; further preferably, the detector temperature of the gas chromatography detection is 185-230°C; further preferably, the detector temperature of the gas chromatography detection is 190-210°C.

[0027] In some specific embodiments of the present invention, the detection method is used to detect the enantiomeric excess (ee%) of the chiral isomer of 6-hydroxy-8-chlorooctanoic acid ethyl ester, and the calculation formula of the enantiomeric excess (ee%) is as follows:

[0028] ee%=(RS) / (R+S)×100%;

[0029] Wherein, R refers to the peak area of ​​R-6-hydroxy-8-chlorooctanoic acid ethyl ester detected by the gas chromatography, and S refers to the peak area of ​​S-6-hydroxy-8-chlorooctanoic acid ethyl ester detected by the gas chromatography.

[0030] The second aspect of the present invention provides an application of the detection method described in the first aspect of the present invention in the preparation of single-configuration lipoic acid.

[0031] The beneficial effects of the present invention are as follows: the present invention is based on gas chromatography, adopts a chiral chromatographic column containing beta-cyclodextrin, and combines a specific initial column temperature and a programmed temperature rise rate to achieve separation and detection of two chiral isomers of 6-hydroxy-8-chlorooctanoic acid ethyl ester, has high sensitivity and specificity, is simple to operate, and has a separation degree that meets standards, can be used for quality control of 6-hydroxy-8-chlorooctanoic acid ethyl ester, and has very important practical significance for preparing single-configuration lipoic acid. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is the gas chromatogram of the test solution in Example 1.

[0033] Figure 2 It is the gas chromatogram of the test solution in Example 2.

[0034] Figure 3 It is the gas chromatogram of the test solution in Comparative Example 1.

[0035] Figure 4 It is the gas chromatogram of the test solution in Comparative Example 2. DETAILED DESCRIPTION

[0036] The content of the present invention is further described in detail below through specific examples. It should also be understood that the following examples are only used to further illustrate the present invention and cannot be construed as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the principles set forth in the present invention all belong to the scope of protection of the present invention. The specific process parameters and the like in the following examples are also only examples within a suitable range, that is, those skilled in the art can make a selection within a suitable range through the description herein, and are not limited to the specific data exemplified below. The raw materials, reagents or devices used in the following examples and comparative examples, unless otherwise specified, can all be obtained from conventional commercial sources, or can be obtained by existing known methods.

[0037] It should be noted that in the gas chromatograms obtained in the following examples, the peak positions of R-6-hydroxy-8-chlorooctanoic acid ethyl ester and S-6-hydroxy-8-chlorooctanoic acid ethyl ester were confirmed by testing the corresponding reference substances under the same testing conditions.

[0038] Preparation Example 1

[0039] Preparation of test solution:

[0040] The test sample was dissolved in dimethyl sulfoxide (DMSO), dimethylformamide (DMF), methanol and acetonitrile respectively to obtain test sample solutions using different solvents. Among them, methanol had a good dissolution effect when used as a solvent and had no interference with the detection.

[0041] Preparation Example 2

[0042] Preparation of test solution:

[0043] The test sample was dissolved in methanol and prepared into test sample solutions with concentrations of 1 mg / mL, 5 mg / mL and 10 mg / mL. When the concentration of the test sample solution was 1 mg / mL, the response was too weak and the sensitivity was insufficient; when the concentration was 10 mg / mL, the concentration was too high and was not conducive to sample separation; the concentration of 5 mg / mL could obtain the appropriate response intensity and baseline separation effect.

[0044] Example 1

[0045] A method for detecting the chiral purity of 6-hydroxy-8-chlorooctanoic acid ethyl ester, the specific steps are as follows:

[0046] The test samples containing two configurations of R-6-hydroxy-8-chlorooctanoic acid ethyl ester and S-6-hydroxy-8-chlorooctanoic acid ethyl ester were dissolved in methanol to obtain a test solution with a concentration of 5 mg / mL, and analyzed by gas chromatography. The chromatographic conditions are as follows:

[0047] Chromatographic column: Agilent J&W CP-Cyclodextrin-β-2,3,6-M-19, capillary column 30.0m×0.25mm×0.25μm;

[0048] Inlet temperature: 200°C;

[0049] Carrier gas: nitrogen;

[0050] Gas flow rate: 1.0 mL / min;

[0051] Detector: Hydrogen flame detector, detector temperature is 200℃;

[0052] Split ratio: 10:1;

[0053] Column temperature: 170-200 °C, using programmed temperature method, the temperature program is as follows: the initial column temperature is 170 °C, and the temperature is increased to 200 °C at a heating rate of 1.0 °C per minute;

[0054] Injection volume: 1.0 μL;

[0055] Gas chromatograph: Agilent 8860 gas chromatograph.

[0056] Accurately measure 1.0 μL of the test solution, inject it into the gas chromatograph, and record the gas chromatogram. The results are as follows: Figure 1 shown.

[0057] Depend on Figure 1 It can be seen that the separation degree of the two configuration samples is 1.65, which is greater than 1.5. It can be seen that the detection method in Example 1 can achieve baseline separation.

[0058] The peak areas of the two isomers in the gas chromatogram were integrated and calculated respectively, and the enantiomeric excess (ee%) of the isomers was calculated by the following formula:

[0059] ee%=(RS) / (R+S)×100%

[0060] Here, R refers to the peak area of ​​R-6-hydroxy-8-chlorooctanoic acid ethyl ester detected by gas chromatography in this example, and S refers to the peak area of ​​S-6-hydroxy-8-chlorooctanoic acid ethyl ester detected by gas chromatography in this example.

[0061] In this example, after integral calculation, the peak area (S) of S-6-hydroxy-8-chlorooctanoic acid ethyl ester is 4443.48, and the peak area (R) of R-6-hydroxy-8-chlorooctanoic acid ethyl ester is 4385.45, so the ee% of S-6-hydroxy-8-chlorooctanoic acid ethyl ester is 0.65%.

[0062] Example 2

[0063] A method for detecting the chiral purity of 6-hydroxy-8-chlorooctanoic acid ethyl ester, the specific steps are as follows:

[0064] The test samples containing two configurations of R-6-hydroxy-8-chlorooctanoic acid ethyl ester and S-6-hydroxy-8-chlorooctanoic acid ethyl ester were dissolved in methanol to obtain a test solution with a concentration of 5 mg / mL, and analyzed by gas chromatography. The chromatographic conditions are as follows:

[0065] Chromatographic column: Agilent J&W CP-Cyclodextrin-β-2,3,6-M-19, capillary column 30.0m×0.25mm×0.25μm;

[0066] Inlet temperature: 200°C;

[0067] Carrier gas: nitrogen;

[0068] Gas flow rate: 1.0 mL / min;

[0069] Detector: Hydrogen flame detector, detector temperature is 200℃;

[0070] Split ratio: 10:1;

[0071] Column temperature: 160-200 °C, using programmed temperature method, the temperature program is as follows: the initial column temperature is 160 °C, and the temperature is increased to 200 °C at a heating rate of 1.0 °C per minute;

[0072] Injection volume: 1.0 μL;

[0073] Gas chromatograph: Agilent 8860 gas chromatograph.

[0074] Accurately measure 1.0 μL of the test solution, inject it into the gas chromatograph, and record the gas chromatogram. The results are as follows: Figure 2 shown.

[0075] Depend on Figure 2 It can be seen that the separation degree of the two configuration samples is 1.79, which is greater than 1.5, and baseline separation can be achieved.

[0076] Comparative Example 1

[0077] A method for detecting the chiral purity of 6-hydroxy-8-chlorooctanoic acid ethyl ester, the specific steps are as follows:

[0078] The test samples containing two configurations of R-6-hydroxy-8-chlorooctanoic acid ethyl ester and S-6-hydroxy-8-chlorooctanoic acid ethyl ester were dissolved in methanol to obtain a test solution with a concentration of 5 mg / mL, and analyzed by gas chromatography. The chromatographic conditions are as follows:

[0079] Chromatographic column: BGB Analytic BGB-175, capillary column 30.0m×0.25mm×0.25μm;

[0080] Inlet temperature: 200°C;

[0081] Carrier gas: nitrogen;

[0082] Gas flow rate: 1.0 mL / min;

[0083] Detector: Hydrogen flame detector, detector temperature is 200℃;

[0084] Split ratio: 10:1;

[0085] Column temperature: 170-200 °C, using programmed temperature method, the temperature program is as follows: the initial column temperature is 170 °C, and the temperature is increased to 200 °C at a heating rate of 1.0 °C per minute;

[0086] Injection volume: 1.0 μL;

[0087] Gas chromatograph: Agilent 8860 gas chromatograph.

[0088] Accurately measure 1.0 μL of the test solution, inject it into the gas chromatograph, and record the gas chromatogram. The results are as follows: Figure 3 shown.

[0089] Depend on Figure 3 It can be seen that using BGB Analytic BGB-175 as the chromatographic column, the two configurations of 6-hydroxy-8-chlorooctanoic acid ethyl ester samples cannot achieve baseline separation.

[0090] On the basis of Comparative Example 1, the gas chromatography conditions, for example, the temperature, column temperature and split ratio, were adjusted. It was found that no matter how the gas chromatography conditions were adjusted, the two configuration samples of 6-hydroxy-8-chlorooctanoic acid ethyl ester could not achieve baseline separation.

[0091] Comparative Example 2

[0092] A method for detecting the chiral purity of 6-hydroxy-8-chlorooctanoic acid ethyl ester, the specific steps are as follows:

[0093] The test samples containing two configurations of R-6-hydroxy-8-chlorooctanoic acid ethyl ester and S-6-hydroxy-8-chlorooctanoic acid ethyl ester were dissolved in methanol to obtain a test solution with a concentration of 5 mg / mL, and analyzed by gas chromatography. The chromatographic conditions are as follows:

[0094] Chromatographic column: Agilent J&W CP-Cyclodextrin-β-2,3,6-M-19, capillary column 30.0m×0.25mm×0.25μm;

[0095] Inlet temperature: 200°C;

[0096] Carrier gas: nitrogen;

[0097] Flow rate: 1.0 mL / min;

[0098] Detector: Hydrogen flame detector, detector temperature is 200℃;

[0099] Split ratio: 10:1;

[0100] Column temperature: 200°C, isothermal method was used, and 200°C was maintained for 30 min.

[0101] Injection volume: 1.0 μL;

[0102] Gas chromatograph: Agilent 8860 gas chromatograph.

[0103] Accurately measure 1.0 μL of the test solution, inject it into the gas chromatograph, and record the gas chromatogram. The results are as follows: Figure 4 shown.

[0104] Depend on Figure 4 It can be seen that using the isothermal method, the two configuration samples of 6-hydroxy-8-chlorooctanoic acid ethyl ester cannot achieve baseline separation.

[0105] On the basis of Comparative Example 2, the isothermal method was used to adjust other gas chromatography conditions, such as column temperature, carrier gas flow rate, split ratio and other chromatographic conditions. The results showed that no matter how the gas chromatography conditions were adjusted, the two configuration samples of 6-hydroxy-8-chlorooctanoic acid ethyl ester could not achieve baseline separation.

[0106] Example 3

[0107] This example verifies the effectiveness of the method provided in Example 1.

[0108] 1. Specificity test:

[0109] Preparation of specific solution: Dissolve the test samples containing two configurations of R-6-hydroxy-8-chlorooctanoic acid ethyl ester and S-6-hydroxy-8-chlorooctanoic acid ethyl ester in methanol to obtain a specific solution with a concentration of 5 mg / mL, and inject 1.0 μL.

[0110] Acceptable criteria: Separation of two configuration samples ≥ 1.5.

[0111] The sample was injected according to the gas chromatography conditions in Example 2. The specificity result was: the separation degree was 1.65.

[0112] 2. Precision test (expressed as repeatability):

[0113] Preparation of reproducible solution: Dissolve the test samples containing two configurations of R-6-hydroxy-8-chlorooctanoic acid ethyl ester and S-6-hydroxy-8-chlorooctanoic acid ethyl ester in methanol to obtain a reproducible solution with a concentration of 5 mg / mL, and inject 1.0 μL.

[0114] Acceptable criteria: The RSD% of the peak area of ​​the six-injection solution should be less than 10.0%.

[0115] The sample was injected according to the gas chromatography conditions in Example 2. The specific results are shown in Table 1.

[0116] Table 1 Repeatability experimental data

[0117] 3. Linearity and range test:

[0118] Preparation of linear solution: Take about 250 mg of R-6-hydroxy-8-chlorooctanoic acid ethyl ester and S-6-hydroxy-8-chlorooctanoic acid ethyl ester respectively, weigh accurately, dissolve in a 25 mL volumetric flask with methanol to 25 mL. As the 200% mother solution, i.e., stock solution A, dilute according to Table 2 below and inject 1 μL, with methanol as the diluent.

[0119] Table 2 Linear solution preparation process

[0120]

[0121]

[0122] Acceptable criteria: Linear regression of concentration versus response, correlation coefficient should be ≥ 0.999.

[0123] The sample was injected according to the gas chromatography conditions in Example 2. The specific results are shown in Tables 3 and 4.

[0124] Table 3 Linearity and range test results of R-6-hydroxy-8-chlorooctanoic acid ethyl ester

[0125]

[0126] Table 4 S-6-hydroxy-8-chlorooctanoic acid ethyl ester linearity and range test results

[0127]

[0128] As can be seen from Comparative Example 1, the application uses a specific chromatographic column for detection, in order to achieve the baseline separation of two chiral isomers of 6-hydroxy-8-chlorooctanoic acid ethyl ester, so as to measure its chiral purity; and as can be seen from Comparative Example 2, in the process of programmed temperature rise, the setting of the initial column temperature, the selection of the heating rate and other factors will interfere with the separation of the two chiral isomers of 6-hydroxy-8-chlorooctanoic acid ethyl ester, the initial column temperature is too high, the heating rate is too fast, which is not conducive to the separation between the two chiral isomers of 6-hydroxy-8-chlorooctanoic acid ethyl ester, resulting in a separation degree of less than 1.5, and it is impossible to achieve complete separation. And as can be seen from Examples 1 to 3, the method provided by the present invention can be used to achieve the effective separation of two chiral isomers of 6-hydroxy-8-chlorooctanoic acid ethyl ester, the baseline separation degree is all above 1.5, and then the chiral purity of the test sample can be obtained by measuring the peak area, and the specificity of the measurement is good, the repeatability is high, and the linearity is high, which is of great significance for the quality control of 6-hydroxy-8-chlorooctanoic acid ethyl ester and the preparation of a single configuration lipoic acid.

[0129] In summary, the present invention is based on gas chromatography, adopts a chiral chromatographic column containing beta-cyclodextrin, combines a specific initial column temperature and a programmed temperature rise rate, can achieve separation and detection of two chiral isomers of 6-hydroxy-8-chlorooctanoic acid ethyl ester, has high sensitivity and specificity, is simple to operate, and has a separation degree that meets the standard (that is, the separation degree is greater than 1.5), can be used for quality control of 6-hydroxy-8-chlorooctanoic acid ethyl ester, and has very important practical significance for preparing single-configuration lipoic acid.

Claims

1. A method for detecting chiral isomers of ethyl 6-hydroxy-8-chlorooctanoate, characterized in that: The following steps are involved: The test solution containing R-6-hydroxy-8-chlorooctanoic acid ethyl ester and S-6-hydroxy-8-chlorooctanoic acid ethyl ester was subjected to gas chromatography detection; The conditions for gas chromatography detection include: the chromatographic column is selected from a chiral chromatographic column containing β-cyclodextrin; The chromatographic column adopts programmed temperature heating during the detection process, with the initial column temperature of 120-180°C, the final column temperature of 190-220°C, and the heating rate of 0.5-2.0°C / min.

2. The detection method according to claim 1, characterized in that: The chromatographic column for gas chromatography detection is selected from CP-Cyclodextrin-β-2,3,6-M-19 chromatographic column.

3. The detection method according to claim 1, characterized in that: The solvent of the test solution includes at least one of dimethyl sulfoxide, dimethylformamide, methanol or acetonitrile.

4. The detection method according to claim 1, characterized in that: The concentration of the test solution is 1-10 mg / mL.

5. The detection method according to claim 1, characterized in that: The carrier gas flow rate of the gas chromatography detection is 0.2 to 2 mL / min; And / or, the carrier gas for gas chromatography detection includes at least one of nitrogen, helium, argon or hydrogen.

6. The detection method according to claim 1, characterized in that: The injection volume of the gas chromatography detection is 0.1-2 μL.

7. The detection method according to claim 1, characterized in that: The split ratio of the gas chromatography detection is (0.01-100):

1.

8. The detection method according to claim 1, characterized in that: The injection port temperature of the gas chromatography detection is 180-250°C.

9. The detection method according to claim 1, characterized in that: The detector for gas chromatography detection is selected from a hydrogen flame detector; And / or, the detector temperature of the gas chromatography detection is 180-250°C.

10. Use of the detection method according to claims 1 to 9 in the preparation of single-configuration lipoic acid.

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