JPS5942823B2 - Graphite cuvette for atomic absorption spectrometer and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a graphite cuvette for an atomic absorption spectrophotometer and a method for producing the same.

As means for atomizing a sample in atomic absorption spectrometry, there are a flame method in which a sample solution is sprayed into an acetylene flame and a method in which a graphite atomizer is used. The graphite atomizer has the following features compared to the flame method.

(1) High detection sensitivity. (2) A small amount of sample comes out. (
3) High safety. (4) It has good operability and can be used with an autosampler. However, its biggest drawback is that it is inferior to the flame method in terms of reproducibility, which is the most important analytical method. Graphite atomizers hold a small graphite cuvette with graphite electrodes,
A sample is injected with a micropipette through the sample injection hole in the center of the cuvette, and when a current is passed through the electrodes, heat is generated due to the Joule effect and the sample is atomized. Graphite conventionally used in graphite atomizers is produced by chemical vapor deposition (CVD-Chemical V) on artificial graphite or artificial graphite.
A material coated with pyrographite (hereinafter referred to as a pyrographite cuvette) by the apour deposition method is used. Ordinary artificial graphite cuvettes have the following drawbacks, which are inevitably caused by the method of manufacturing artificial graphite. (1) Porous. (2) There are many variations in density, and therefore there are large variations in electrical resistance. In addition, pyrographic cubes have drawbacks such as (1) the pyrographic parts tend to peel off during use, and (2) electrical resistance varies widely because it depends on the artificial graphite used as the base material. . These drawbacks of conventionally used cuvettes are closely related to the fatal drawback of graphite atomizers, which is poor reproducibility and short life of a single cuvette. Being porous allows the sample solution to penetrate not only on the surface of the cube but also into the inside of the graphite material.

The state of atomization changes with each measurement due to the so-called memory effect, and cuvette graphite is easily consumed by oxidation.
Associated with short lifespan. Further, the large variation in electrical resistance causes the temperature of the cube to vary from cube to cube, causing changes in the atomization state, resulting in poor reproducibility. Artificial graphite is generally produced through the following steps.

Carbonaceous raw materials such as petroleum coke and coal coke are calcined, pulverized and sieved, and powders and particles are mixed in a desired ratio, a binder such as coal tar or pitch is added, and a temperature exceeding the melting point of the binder is added. Temperature, generally 150-250
Soak at ℃. Next, a raw product is produced by either extruding the nectar as it is by passing it through an extruder under heating, or by cooling and pulverizing the anagen, and then putting it into a mold and pressurizing it. Next, the raw product thus produced is primarily fired in a non-oxidizing atmosphere and then further graphitized at 2,000 to 3,000°C. Due to this manufacturing process, volatile components in the binder in general artificial graphite are volatilized during the primary firing and graphitization processes, resulting in the generation of pores. Even if the conditions of the summer heating, primary firing, and graphitization processes are carefully controlled, the general characteristics of graphitized products vary widely due to complex decomposition reactions and delicate atmospheric influences. The range of variation in the general properties of artificial graphite used in cubes is usually as follows. Bulk specific gravity approximately 1.65-1.85,
Electrical specific resistance: about 700-1400 μΩ-Cm, coefficient of variation: about 10%, porosity: about 10-20% by volume. The measurement reproducibility accuracy with the graphite atomizer is approximately ±7%, which is less than half of the approximately ±3% of the flame method.

In addition, the lifespan of the cuvette is approximately 3
0 to 70 times, about 100 times with pyrographic eye cubes
~150 times.

Measurement reproducibility with club art atomizer is ±3%
The lifespan of the cuvette is required to be 200 times or more.

SUMMARY OF THE INVENTION The object of the present invention is to improve the above-mentioned drawbacks and to provide a graphite cube that satisfies the requirements.

As mentioned above, the artificial graphite and birographite used in current graphite atomizers have a high porosity, a two-layer structure consisting of a surface coating and an internal layer, and large variations in electrical resistivity. It has properties such as things.

These properties are thought to be the cause of the fact that the measurement reproducibility of the graphite atomizer method is inferior to that of the flame method, and that the life span is short due to oxidative consumption of the cuvette and peeling of the coating layer. Therefore, the present inventors have discovered that by improving the properties of the above-mentioned artificial graphite using the method described below, it is possible to improve the measurement reproducibility and cube life of the graphite atomizer method to a level that exceeds the level required by the market. I found out.

The pores of the artificial graphite used in the cube are made finer and the porosity is less than 6% by volume, which is smaller than that of conventional products.This prevents the sample solution from penetrating into the interior, prevents oxidative consumption, and improves the electrical resistivity. By reducing the variation width to a coefficient of variation of 3% or less, the heat generation due to the Joule effect is kept constant,
The reproducibility of the atomization state is improved.

The present invention has a porosity of 6% by volume or less and a coefficient of variation of electrical resistivity of 3.
A graphite cuvette for an atomic absorption spectrophotometer consisting of less than 10% graphite and various pits are thermally polymerized and crystallized to produce a carbon precursor material, which is pulverized to an average particle size of 10 μm or less. Atomic absorption is produced by molding, firing and graphitizing without adding any other binder to produce artificial graphite with a porosity of 6% by volume or less and a coefficient of variation of electrical resistivity of 3% or less, and then machined into a predetermined shape. Method for producing spectrophotometer graphite; After producing heavy pitches made by making various pitches heavy in a non-oxidizing atmosphere, they are crushed, and then an acid aqueous solution and an oxidizing gas are added separately at a temperature lower than the softening point. or introducing two or more types to produce a carbon precursor substance having a functional group,
This is pulverized to an average particle size of 10 ttm or less, and then molded, fired, and graphitized without adding any other binder to create an artificial product with a porosity of 6% by volume or less and a coefficient of variation of electrical resistivity of 3% or less. The present invention relates to a method of manufacturing a graphite cuvette for an atomic absorption spectrometer, in which graphite is manufactured and then machined into a predetermined shape.

In the present invention, the reason why the porosity of the cuvette is 6% by volume or less and the coefficient of variation of the electrical resistivity is 3% or less is because if the porosity exceeds 6% by volume, (a) a memory effect occurs in which the sample solution permeates into the cuvette. Therefore, the reproducibility of each measurement becomes poor,
(1:]) This is because the oxidation consumption of the cuvette is rapid and the life cannot be maintained at more than 200 cycles, and if the coefficient of variation of electrical resistivity exceeds 3%, (c) when heating the sample solution to atomize it. This is because the heating temperature differs from cuvette to cuvette, resulting in poor reproducibility for each cuvette, making it impossible to maintain measurement reproducibility within ±3% due to (a) and (c).

The details of the present invention will be explained below.

Coal tar, coal pitch, high-temperature pyrolysis tar of petroleum, ethylene bottom oil, by-product tar pitch during ethylene production, polyvinyl chloride pyrolysis pitch, pitch obtained by thermal polymerization of asphalt, etc. 35
A carbon precursor substance is obtained by thermal polymerization crystallization at a temperature of 0 to 550°C, or a heavy pitch is produced by heavyening the above various pitches in a non-oxidizing atmosphere such as nitrogen or hydrogen. After that, it is crushed, and then at a temperature lower than the softening point, an acid aqueous solution such as nitric acid, sulfuric acid, or hypochlorous acid, or an oxidizing gas such as chlorine gas, air, or sulfur vapor is introduced singly or in combination with oxygen,
A carbon precursor material having functional groups such as sulfur, halogen, and nitrogen is obtained.

Next, the carbon precursor material obtained in this way was adjusted to an average particle size of 10 μm to prevent defects such as cracks from occurring during carbonization.
The artificial graphite is pulverized as follows, and then formed, fired, and graphitized using conventional methods to produce artificial graphite with a porosity of 6% by volume or less and a coefficient of variation of electrical resistivity of 3% or less. Graphite cuvettes can be obtained by machining into shapes. In the present invention, the carbon precursor material is pulverized to an average particle size of 10 μm or less, and then molded, fired, and graphitized in a conventional manner without adding any other binder to achieve a porosity of 6 volume % or less and an electrically Artificial graphite with a specific resistance variation coefficient of 3% or less can be obtained.

The present invention will be explained below with reference to Examples.

Example 1 Pitch beads with an average diameter of 1 m were made by a melting method from high-temperature pyrolysis pitch of crude oil with a softening point of 270°C, and low boiling point components were extracted and removed with benzene.

Thereafter, the sample was treated at 220°C for 2 hours in an air fluidized bed, and then at 500°C for 10 minutes in a nitrogen stream. This treated pitch was pulverized to an average particle size of 5 μm or less. Thereafter, it was molded in a mold in the same manner as in the usual artificial graphite manufacturing method, and the resulting molded product was primarily fired to 1,000°C at a heating rate of 5°C/hour in a non-oxidizing atmosphere. 2 more baked products
, 800'C in a non-oxidizing atmosphere. The porosity of the artificial graphite obtained in this way is 1.5% by volume, and the electrical resistivity is = 1,550μΩ-o, coefficient of variation 2
, 0% (n = 20. Example 2 Ethylene bottom oil was made heavy in a nitrogen stream at 410°C to obtain a hydrogen/carbon (H/C) atomic ratio = 0.63 and a softening point of 30.
A pitcher at 0°C was produced.

This pitch was pulverized to 200 mesh or less, and while being careful not to melt the pitch, it was heated to 300° C. at a heating rate of 5°C/hour to introduce oxygen into the pitch.

This pitch was pulverized to an average particle size of 10 μm or less. Artificial graphite was produced in the same manner as in Example 1.
The artificial graphite thus obtained had a porosity of 4.0% by volume, an electrical resistivity of x=3,500 μΩ20, and a coefficient of variation of 1.2% (n−20). Comparative Example 1 Petroleum pitch coke powder with an average particle size of 40 μm has a softening point of 75
The volatile content of the molded powder after kneading at 210°C with a double-arm kneading machine is 15% by weight.
After kneading for a period of time, the mixture was pulverized to 150 meshes or less to obtain a molding powder, and then processed and molded to obtain brick-shaped blocks.

This block was fired to 950°C in a non-oxidizing atmosphere, and then heated to 300°C in a non-oxidizing atmosphere.
It was heated to 0°C to graphitize it. Comparative Example 2 The firing block obtained by the method of Comparative Example 1 was placed in an impregnating tank, and the inside of the tank was 5 [!] of mercury. I! After degassing under reduced pressure until the temperature reached 1, tar pitch having a softening point of 75° C. was heated, liquefied and injected, and the firing block was impregnated with tar pitch under pressure of 1 kg/CTIi.

Next, firing and graphitization were performed under the same conditions as in Comparative Example 1. Comparative Example 3 Petroleum pitch coke powder finely pulverized to an average particle size of 10 μm and tar pitch with a softening point of 150°C were kneaded at a temperature of 250°C using a double-arm kneader. After kneading the mixture for a time to reach 10% by weight, the powder was finely pulverized to less than 400 mesh to form a molded powder, and then pressure molded to obtain a brick-shaped block.

This process was fired and graphitized under the same conditions as the comparative example. For the artificial graphite obtained in the above comparative example, the porosity, electrical resistivity, and coefficient of variation of electrical resistivity (n=20) were determined in the same manner as in the examples.

These values are shown in Table 1 together with the values of Examples. In proportional example 2, the pores of the firing block were filled with tar pitch carbide, but the decrease in the coefficient of variation of the porosity and electrical resistivity was slight.

Comparative Example 3 uses fine raw material powder to reduce the number of pores, but as shown in Table 1, both the porosity and the coefficient of variation of electrical resistivity are lower than those of the examples. Next, manufacture it in this way. A tube-shaped graphite cuvette 1 having the shape shown in FIG. 1 was manufactured by machining the artificial graphite obtained.

In FIG. 1, 2 is a sample injection hole, and 3 is a graphite electrode. In addition, a Zeeman atomic absorption spectrophotometer (
K. K.

The above-mentioned cuvette was installed in a model (trade name: Model 170-70, manufactured by Nippon Seisakusho), and continuous analysis was performed using an aqueous solution containing 1 ng of lead. The measurement conditions at that time were a wavelength of 283.3 nm and a magnetic field of 1 kilogauss. The results are shown in FIG.
In Example 1, the reproducibility of each measurement was ±2%, as shown by the solid line 4 in FIG. 2, as measured using the produced artificial graphite cuvette.

Furthermore, the cube was used more than 300 times.
The results measured using the artificial graphite cube produced in Example 2 also showed good reproducibility as in Example 1. The results of measurement using a cuvette made of conventionally available artificial graphite in Comparative Example 1 are as shown by the dotted line 5 in FIG. 2, and the reproducibility of the measured values up to 50 times was ±6%. Furthermore, the maximum number of times a cuvette could be used was about 60 times. Similarly, for Comparative Examples 2 and 3, the reproducibility and service life (number of uses) for each measurement in 50 measurements were determined as shown in (a) of Table 2. Even when densification was achieved using

Furthermore, one cube from each of the 10 artificial graphite blocks produced in Example 1 was taken out, and a total of 100 measurements were performed.

The measurement reproducibility of each cuvette was within ±3%, and the variation width of the average value of 100 measurements between each cuvette was 2%. The comparative cuvette was also measured in the same manner as above.

The results are shown in Table 2. Table 2 (mouth)
The reproducibility shown is the maximum value for those that can be used up to 50 times.

Even in Comparative Example 3, which showed relatively good values from the analysis results in Table 2, the reproducibility of measurement was worse than the target ±3%, and the lifespan was 2.
It does not reach 00 times. As shown in the examples, by using the graphite cube of the present invention with a porosity of 6% by volume or less and a coefficient of variation of electrical resistivity of 3% or less, the reproducibility of the intended measurement is within ±3% and the life span is improved. It is possible to achieve 200 times or more. According to the present invention, it is possible to obtain a graphite cube for an atomic absorption spectrophotometer, in which a sample solution to be analyzed is difficult to enter the cube, and therefore has excellent reproducibility of measurement and has a long life.

[Brief explanation of the drawing]

Fig. 1 is a plan view showing the shape of a graphite cuvette for explaining the present invention, and Fig. 2 is a comparative test of reproducibility between a cuvette made of artificial graphite manufactured in Example 1 of the present invention and a conventional cuvette. It is a graph showing the results.

Claims (1)

[Scope of Claims] 1. A graphite cube for an atomic absorption spectrophotometer made of graphite having a porosity of 6% by volume or less and a coefficient of variation of electrical resistivity of 3% or less. 2. Various pitches are thermally polymerized and crystallized to produce a carbon precursor material, which is pulverized to an average particle size of 10 μm or less, and then molded, fired, and graphitized without adding any other binder. A method for producing a graphite cuvette for an atomic absorption spectrophotometer, which comprises producing artificial graphite with a porosity of 6% by volume or less and a coefficient of variation of electrical resistivity of 3% or less, and then machining it into a predetermined shape. . 3. Heavy pitch is produced by heavyizing various pitches in a non-oxidizing atmosphere, and then pulverized, and then at a temperature lower than the softening point, an acid aqueous solution or an oxidizing gas is introduced singly or two or more to create functional groups. A carbon precursor material having a porosity of 6% by volume or less is produced, pulverized to an average particle size of 10 μm or less, and then molded, calcined and graphitized without adding any other binder to give a carbon precursor material with a porosity of 6% by volume or less, A method for producing a graphite cube for an atomic absorption spectrophotometer, which comprises producing artificial graphite having a specific resistance variation coefficient of 3% or less, and then machining it into a predetermined shape.