JP2001083134A - Method for determining nitrogen in organic compound by kjeldahl method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To simplify a shortening operation of an analyzing time and to improve safety by decomposing by a microwave decomposing device and determining by an ion chromatography. SOLUTION: First, a sample, a fluoric acid and a decomposition accelerator are introduced into a microwave decomposition container, and decomposed. A hydrogen peroxide of 1/10 to 1/2 of a range by a volume reference and a copper sulfate as a catalyst are used for a sulfuric acid amount to be used. Its decomposing time of about 5 to 30 min is sufficient. After the decomposition is finished, a decomposed liquid is cooled and suitably diluted. A cation column is used as a separation column. A calibration curve solution and a diluted sample solution are analyzed by an ion chromatography, an ammonium ion is determined by the curve to obtain a nitrogen content in the sample. Thus, its analysis is performed by an apparatus having high general purposes. A distillation or capturing of free ammonia or a titration of ammonia by a sulfuric acid is eliminated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for determining nitrogen in organic compounds by the Kjeldahl method, and more particularly to a method for determining nitrogen using ion chromatography for measuring nitrogen concentration.

[0002]

2. Description of the Related Art A commonly used method for measuring the nitrogen content in an organic compound is a Kjeldahl nitrogen determination method. This Kjeldahl nitrogen determination method has good analytical accuracy and is widely used for the determination of various nitrogen-containing organic compounds such as proteins, and the Japanese Pharmacopoeia has adopted the Kjeldahl method as a nitrogen determination method. As is well known, the principle of the Kjeldahl method is that a sample is heated and decomposed with concentrated sulfuric acid in the presence of a catalyst, which is a suitable decomposition accelerator, and nitrogen in the sample is fixed as ammonium sulfate, and a strong alkali such as sodium hydroxide is added thereto. In addition, the released ammonia is collected by distillation and titrated with sulfuric acid to determine the amount of ammonia in the sample. However, thermal decomposition may take a long time, and if distillation and titration are performed, a considerable amount of time is required when analyzing a plurality of samples. In addition, it is pointed out that the operation is complicated and requires the skill of an analyst, and there is a danger that concentrated sulfuric acid is used in thermal decomposition of a sample or titration at the time of quantifying ammonia. Therefore, various improvements have been made in the Kjeldahl nitrogen determination method in order to shorten the analysis time and to avoid the complexity of the analysis operation.

[0003] For example, as a method of shortening the decomposition time,
JP-A-54-30891, JP-A-55-451, and JP-A-61-258166 describe the use of potassium persulfate, antimonate and inorganic peroxosalt as a decomposition catalyst. I have. Also, Japanese Patent Application Laid-Open No. 50-2
No. 5286, Japanese Unexamined Patent Publication No. Sho 63-286763, etc., describe a method in which some or all steps are automated by combining the respective devices, such as an automatic decomposition device, an automatic distillation titration device, sulfuric acid, and an automatic catalyst addition. Has been described.

[0004] Also, JP-A-63-72336, Food Research Institute Research Report, 61, 15 (1997) states that
It describes that an organic compound is decomposed using a microwave device in order to shorten the time required for the decomposition step and to simplify the operation.

[0005]

As described above, as an attempt to improve the analysis time and the operability, which are the drawbacks of the conventional Kjeldahl nitrogen determination method, as an attempt to improve the thermal decomposition of the sample, the distillation, collection, and titration of the released ammonia. However, there are problems that these automated devices are expensive, and that they lack versatility in applying this device to other analyses. Was.
Although the improvement of the decomposition catalyst and the method using a microwave device have the effect of shortening the decomposition time of the sample, the conventional method is still used for the determination of ammonia, and there is room for improvement. I have. An object of the present invention is to provide a method for quantifying nitrogen in an organic compound by the Kjeldahl method using a device with high versatility, which shortens the analysis time, simplifies the analysis operation, and improves safety.

[0006]

Means for Solving the Problems The present inventors have made intensive studies to solve the above problems, and as a result, decomposed a nitrogen-containing organic compound with a microwave decomposition apparatus and quantified a decomposition liquid as ammonium sulfate by ion chromatography. By doing so, analysis time can be shortened, analysis operation can be simplified, safety can be improved, and nitrogen in organic compounds can be quantified using a highly versatile apparatus, thereby completing the present invention. Was.

That is, the present invention provides a method for determining nitrogen in an organic compound using the Kjeldahl method, wherein a microwave decomposition apparatus is used to decompose the organic compound using concentrated hydrogen sulfate, hydrogen peroxide, and copper sulfate. The present invention relates to a method for quantifying nitrogen (claim 1), which comprises a step of quantifying ammonium ions using ion chromatography.

In the step of decomposing an organic compound using a microwave decomposition apparatus, the amount of hydrogen peroxide used is 1/10 to 10% by volume based on the amount of concentrated sulfuric acid used.
The present invention relates to the method for quantifying nitrogen according to claim 1, wherein the amount is set to １ ／.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The microwave decomposition apparatus used in the present invention is not particularly limited as long as it can be rapidly decomposed by irradiating a microwave having an energy amount sufficient to decompose a nitrogen-containing organic compound. Used as a multi-purpose sample pretreatment device in wet chemical analysis, especially for atomic absorption, ICP, ICP-M which requires high precision
A general-purpose device widely used for the sample decomposition of S analysis can be used. Further, both an open system and a closed system can be used.

The microwave decomposition method utilizes the rapid resolution of microwaves. Particularly in a closed system, decomposition can be carried out without receiving external contamination and without loss of the target element due to volatilization. Also, unlike the conventional Kjeldahl method, it is not necessary to pay special attention such as not to adhere to the container wall when adding sulfuric acid, catalyst or the like. As the device, a device capable of programming the decomposition temperature, time, microwave heating power, and the like due to recent advances in microwave decomposition devices and having a sufficient safety function is preferable.

[0011] The ion chromatography used in the present invention is one of ion exchange chromatography.
Separation of ionic components in the sample using a separation column, and removal column connected after the separation column converts the eluent used as the mobile phase to water or a low-conductivity one to reduce the eluent background This is a chromatographic method that makes it possible to detect the target ionic component by an electric conductivity cell by performing a general method commonly used for the analysis of cations, anions, or transition metals in a sample. Can be used. Since the retention time of ammonium ions is about 4 to 5 minutes, quantification can be performed in a short time even when analyzing a large number of samples at once. Also, the use of an automatic injection device reduces the time bound by the analysis.

In the method of the present invention, first, a sample, sulfuric acid, and a decomposition accelerator are placed in a microwave decomposition vessel and decomposed by a microwave decomposition apparatus. The amount of the sample, sulfuric acid, and the amount of the decomposition accelerator can be adjusted according to, for example, the semi-micro Kjeldahl method employed in the Japanese Pharmacopoeia, but depending on the size of the microwave decomposition container, it is necessary. It needs to be adjusted appropriately. The sample can be measured irrespective of its properties, and the directly weighed sample can be directly transferred to a container, and when weighing a small amount, it can be collected after dilution with a solvent. The solvent to be diluted is not particularly limited as long as it does not contain nitrogen, but dilute sulfuric acid is particularly preferable as long as it is soluble.

As the sulfuric acid, it is preferable to use concentrated sulfuric acid having a low water content. The amount used is limited depending on the amount of the sample to be decomposed and the reaction vessel, but usually the amount of nitrogen contained is 2 to 2.
It is used in the range of 2 to 10 ml of sulfuric acid for 3 mg.
As a decomposition accelerator, for example, for 100 mg of a sample,
The use of 1 g of a mixture of 10 g of potassium sulfate and 1 g of copper sulfate and 1 ml of hydrogen peroxide can be exemplified. In addition, various sulfates, heavy metals such as mercury oxide, and group 15 typical metals such as selenium oxide and antimony oxide are generally used. However, in the method of the present invention, excess amount is used because the amount is determined by ion chromatography. Potassium ions may interfere with the determination of ammonium ions,
Further, in consideration of the influence of the waste after analysis on the environment, the optimum decomposition conditions were found by adjusting the selection and amount of the salt that does not affect the determination of ammonium ion and the amount of hydrogen peroxide.

That is, hydrogen peroxide is used in a range of 1/10 to 1/2, preferably 1/5 to 2/5, based on the volume of sulfuric acid used, and copper sulfate is used as a catalyst. Thus, the nitrogen-containing organic compound can be decomposed at a recovery rate of 93% or more without using potassium sulfate, which is most widely used as a decomposition accelerator. When a large number of samples are analyzed at once, the smaller the amount of salts such as potassium sulfate, the more the amount of waste can be minimized, thereby improving the analysis efficiency.

The hydrogen peroxide used is usually 30%, but 50% can also be used. The copper sulfate used is usually used as a hydrate in consideration of solubility. The amount to be used is usually used in the range of 10 times to 1000 times the sample.

When a closed type microwave decomposition apparatus is used, a sample, sulfuric acid, a catalyst, and hydrogen peroxide are put in a microwave decomposition container, and a lid is closed. In a method using a microwave decomposition apparatus, it is usually unnecessary to further add a catalyst, hydrogen peroxide or the like after irradiation with microwaves, but when the reaction is insufficient, irradiation can be stopped and a reagent can be added.

The decomposition vessel can be installed in a decomposition apparatus, the microwave output can be set appropriately according to the type and amount of the sample, and irradiation can be performed to perform decomposition. The microwave output is related to the decomposition time to some extent, but can be set to an appropriate range as long as the safety standard of the device is not exceeded, and can be changed over time.

Although the decomposition time depends on the amount of the sample to be decomposed, it can be sufficiently decomposed in about 5 to 30 minutes. After decomposition, remove the microwave decomposition container from the device, cool it, and appropriately dilute the decomposition solution so that it is within the range of the calibration curve concentration of ion chromatography, and detect the nitrogen content of organic compounds as ammonium ions. can do.

As a procedure for detecting ammonium ions by ion chromatography, a calibration curve is first prepared. The concentration of the calibration curve is determined in consideration of the nitrogen content in the sample, the sensitivity of the detector for ion chromatography, and the like. The calibration curve solution and the diluted sample solution are analyzed by ion chromatography. As the separation column, a cation column is used, and an eluent suitable for the separation column is used. For example, when a cation exchange column having a carboxylic acid or a sulfonic acid as a functional group is used as a polymer-based filler, 10 to 10
An acidic solution such as methanesulfonic acid or hydrochloric acid of 20 mmol / l can be exemplified.

It is also desirable to use a cation analysis suppressor which has the effect of converting the electrolyte used for the eluent to water or a substance having a low conductivity to increase the detection sensitivity of the target ionic component. If the sulfuric acid concentration is high, the sulfate ion affects the chromatogram. Ammonium ions are quantified from the calibration curve to determine the nitrogen content in the sample.

Hereinafter, the present invention will be described in detail with reference to examples, but the scope of the present invention is not limited to the examples.

[0022]

EXAMPLES Example 1 1.50 g of DL-methionine (special reagent grade) was precisely weighed to give 0.1 g.
It was dissolved in 50 ml of 1 mol / l sulfuric acid. 0.3 ml of this sample solution is placed in a 100 ml microwave decomposition vessel, and copper (II) sulfate pentahydrate (special grade reagent) 100 m
g, concentrated sulfuric acid (96-98%), 5 ml, and 30% hydrogen peroxide, 1 ml, were sealed with a lid. The microwave disassembly vessel is set in the disassembly device and disassembly is started. Microwave decomposition conditions are 250W: 1 minute, 0W: 2 minutes, 25
Decomposition was performed by a decomposition program of 0 W: 5 minutes, 400 W: 6 minutes, 650 W: 10 minutes, and cooling 5 minutes. After completion of the decomposition, the decomposition vessel is cooled, the decomposition liquid is transferred to a 100 ml measuring flask, and water is added to make 100 ml. In addition, the solution
Dispense 3 ml into a 50 ml volumetric flask and add water to
0 ml, which was used as a diluted sample solution.

To determine the amount of ammonium ion in the sample by ion chromatography, a solution for a three-point calibration curve was prepared. Commercially available ammonium ion standard solution (1000
μg / ml) and 0, 0.08, 0.2
A solution having a concentration of μg / ml is prepared and used as a calibration curve solution. To this solution, 0.03 ml / 100 ml of sulfuric acid was added to combine the diluted sample solution and the matrix. The calibration curve solution and the diluted sample solution were each analyzed by ion chromatography, the ammonium ion in the sample was quantified from the obtained calibration curve, and the recovery rate was determined by comparing with the nitrogen content calculated from the sample. The results are shown in Table 1.

Examples 2, 3 and 4 4 ml of concentrated sulfuric acid, 2 ml of aqueous hydrogen peroxide (Example 2), 5 ml of concentrated sulfuric acid, 0.5 ml of hydrogen peroxide (Example 3), 5 ml of concentrated sulfuric acid, 2 ml of hydrogen peroxide ( The analysis was carried out in the same manner as in Example 1 except that Example 4) was used, and the recovery rates are shown in Table 1.

Comparative Examples 1 and 2 The reaction was carried out in the same manner as in Example 1 except that 6 ml of concentrated sulfuric acid, 4 ml of aqueous hydrogen peroxide (Comparative Example 1), 4 ml of concentrated sulfuric acid, and 4 ml of hydrogen peroxide (Comparative Example 2) were used. The recovery is shown in Table 1.

[0026]

[Table 1]

Example 5 The operation described in Example 1 was performed twice for four days with two samplings, and the recovery rate was determined and is shown in Table 2.

[0028]

[Table 2]

Analysis of variance is performed using the obtained data.
The results are shown in Table 3.

[0030]

[Table 3]

When the accuracy of the analysis was estimated using the above results, the recovery rate was almost 100% of the theoretical value of the nitrogen content.
, And there was no significant difference between the days, confirming that the analysis accuracy was good.

[0032]

According to the present invention, nitrogen analysis by the Kjeldahl method has become possible with a device more versatile than the present invention. In addition, by quantifying ammonium ions by ion chromatography, the analysis can be performed safely because the distillation and collection of free ammonia and the titration of ammonia with sulfuric acid become unnecessary, and the analysis time was shortened. Therefore, by using a microwave decomposition device to decompose nitrogen-containing organic compounds and using ion chromatography to quantify ammonium ions in the decomposition solution, the analysis time required for decomposition and quantification or the time restricted by analysis is reduced. You. Since the operation of distilling and collecting the decomposed and released ammonia is not required, the analysis time is shortened, and there is no danger in the operation of releasing and distilling ammonia using a strong alkali. Since the determination of ammonia is performed by ion chromatography, the operation can be performed safely without handling sulfuric acid used for titration. When five samples were analyzed from these, it took about 7 hours in the conventional method, but was reduced to about 3 to 4 hours in the method of the present invention. Microwave digesters or ion chromatography are commonly used for other analyses. And so on.

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Claims (2)

[Claims] In a method for determining nitrogen in an organic compound using the Kjeldahl method, a step of decomposing an organic compound in concentrated sulfuric acid using hydrogen peroxide and copper sulfate in a microwave decomposition apparatus, and ion chromatography. A method for quantifying nitrogen, comprising a step of quantifying ammonium ions using the method. 2. In the step of decomposing an organic compound using a microwave decomposition apparatus, the amount of hydrogen peroxide used is 1/10 to 1/1 / volume based on the amount of concentrated sulfuric acid used.
2. The method for quantifying nitrogen according to claim 1, wherein the method is 2.