Determination of the content of active ingredients in Chinese medicine (4)

5.

The determination of the content of volatile oil can be divided into the determination of total volatile oil and single volatile oil

The volatile oils contained in traditional Chinese medicines are all mixtures, often composed of a dozen or even hundreds of compounds.

Unless otherwise specified in the gas chromatography method, the suitability test of the chromatographic system is the same as the provisions under the four-part high-performance liquid chromatography (General Principle 0512) of the 2015 Chinese Pharmacopoeia, that is, use the specified reference solution or system suitability test solution Perform tests on the specified chromatographic system.

(1) Determination method

1.

The internal standard method is an important technique in the quantitative analysis of gas chromatography

2.

If the standard curve crosses the origin, the content of the measured components does not change much, and the one-point method of external standard can be used

3.

pt%=A ₁ f ₂ /(A ₁ f ₂ +A ₂ f ₂ +…A _n f _n )×100%.

In the formula, F _.

pt%=h ₁ f ₁ /(h ₁ f ₁ + h ₂ f ₂ +.

In the formula, f ₁ , f ₂ , and f _n are the specific peak height relative correction factors of each component under the operating conditions; h ₁ , h ₂ , h _n are the peak heights of each component

The regulations of the above methods are the same as the corresponding regulations under the four-part high-performance liquid chromatography (General Principle 0512) of the Chinese Pharmacopoeia of the 2015 edition

4.

Precisely weigh (quantity) take an appropriate amount of reference substance for an impurity or component to be tested, prepare it into a reference substance solution of appropriate concentration, take a certain amount, and accurately add it to the solution of the test substance, and determine the impurities or impurities according to the external standard method or internal standard method.

The standard curve method is suitable for situations where the matrix of the standard curve and the matrix of the sample are roughly the same.
The advantage is that it is fast, but the disadvantage is that it is incorrect when the matrix of the sample is complicated
.
The standard addition method can effectively overcome the shortcomings mentioned above, because this method mixes the sample and the standard together for simultaneous determination (the "standard addition method" is called from here), but the disadvantage is that the speed is very slow
.

(2) Calculation formula

The correction factor has the same influence before and after adding the reference solution, namely:

Then the concentration cx of the component to be tested can be calculated by the following formula:

In the formula, cx is the concentration of component X in the test product; Ax is the chromatographic peak area of ​​component X in the test product; △cx is the concentration of the added reference substance of the component to be tested; Ais is the concentration after adding the reference substance peak area of the X component
.

(3) Matters needing attention

1.
Since the injection volume of gas chromatography is generally only a few microliters, in order to reduce injection errors, especially when manual injection is used, since the retention time and room temperature also affect the injection volume, the internal standard method is used for quantification as Appropriate
.

2.
When automatic injection is used, due to the increase in the repeatability of the injection volume, the external standard method can also be used for quantification while preserving the analysis error:

3.
When using headspace injection, because the test product and reference product are in an incomplete matrix, the standard solution addition method can be used to eliminate the influence of matrix effect
.

4.
When the results of the standard solution addition method are inconsistent with other quantitative methods, the results of the standard addition method shall prevail
.

5.
Due to the complex composition of traditional Chinese medicine preparations, a pre-column should be added before the analytical column during analysis
.
After the analysis is completed, generally use water or low-concentration alcoholic water to wash away water-soluble impurities such as sugar, and then rinse the chromatographic column with organic solvents such as methanol
.