Determination of residual acrylonitrile monomer content in styrene-acrylonitrile copolymer by gas chromatography (2)

7 Operation steps

7.

Use a pipette to accurately measure 1.

7.

Weigh about 1g polymer sample (accurate to 1mg), put it into a 25mL volumetric flask, add about 20mL dimethylformamide (4.

7.

7.

In a 100mL volumetric flask containing dimethylformamide (4.

7.

Transfer an appropriate amount of the internal nitrile solution prepared in 7.

7.

According to the sensitivity of the gas chromatograph used, take an appropriate volume of the sample solution prepared in accordance with 7.

Record until the acrylonitrile and internal standard are completely eluted

7.

The retention time of acrylonitrile and propionitrile must be known, and at least the relative retention time of each other must be known.

The peak areas of acrylonitrile and propionitrile are tested or calculated according to the following methods:

a) Electronic integration method;

b) Estimate the area according to the following formula

Area (A)-peak height×half width (see ISO2561);

c) Geometric quadrature method

It is recommended that the method b) be used when the chromatographic peak shape is similar to an isosceles triangle with a horizontal baseline to reduce measurement errors

8 Presentation of results

8.

If several calibration solutions of different concentrations of acrylonitrile are available, the peak area ratio (A'a/A's) can be used to make a calibration chart for each concentration (mg/mL)
.

The relative ratio Aa/As determined from the sample solution can be read from the calibration chart of the acrylonitrile concentration c _a in the sample solution, and the acrylonitrile content w _a in the polymer sample can be calculated from the following relationship :

Where:

A'a—the peak area of ​​acrylonitrile in the calibration solution;

A's—the peak area of ​​the internal standard substance (propionitrile) in the calibration solution;

A _a —The peak area of ​​acrylonitrile in the sample solution;

As—the peak area of ​​the internal standard substance (propionitrile) of the sample solution towel;

c _a ——The concentration of acrylonitrile in the sample solution, in milligrams per milliliter (mg/mL);

m _p —the mass of the polymer sample, in grams (g);

w _a - the content of acrylonitrile in the polymer sample , in milligrams per kilogram (mg/kg)
.

In conventional determination, if the same result can be obtained, the peak height can be used instead of the peak area A'a, A's, Aa and As for calculation
.

8.
2 According to the calculation result of single-point calibration

If there is a linear relationship between the peak area and each concentration of acrylonitrile , w _a can be calculated by the following formula:

A ' _A , A' _S , A _A , A _S , m _P and W _A as defined in 8.
1;

m'a—The mass of acrylonitrile in 25mL calibration solution (7.
3), in milligrams (mg)
.

8.
3 Precision and detection limit

The dispersion of the repeated measurement results of the acrylonitrile concentration in the calibration sample should not exceed ±20% of the average value or the absolute value of 5mg/kg, and the detection limit of this method can reach 3mg/kg
.

9 Test report

The test report should include the following items:

a) Indicate the use of this national standard;

b) Complete identification of the test polymer;

c) Indicate any differences from the gas chromatography equipment and operating methods used in this standard;

d) Methods of testing and calculation;

e) The content of acrylonitrile monomer, in mg/kg, rounded to integer:

f) The dispersion or error range does not exceed the requirements of 8.
3
.