Differences between internal standard method and external standard method in gas chromatographic analysis
Internal standard method:
An indirect or relative calibration method. In the analysis of a certain component content in the samples, will be a certain weight of pure substance as internal standard substance added to a certain amount of analysis of samples of mixture, then the samples containing internal standard substance of chromatographic analysis, determination of internal standard substance and peak area of the component under test (or peak height) and relative correction factor, according to the formula to calculate percentage of components in the sample being tested. An internal standard substance is added to calibrate and eliminate the influence of fluctuations in operating conditions on the analysis results to improve the accuracy of the analysis results.
Differences between internal standard method and external standard method in gas chromatographic analysis
Internal standard method is an important technique in quantitative analysis of gas chromatography. It is a more accurate quantitative method in chromatographic analysis, especially when there is no reference material, this method shows its superiority.
When using internal standard method, a certain amount of standard substance is added to the sample, which can be separated by the chromatographic column, and is not interfered by the peaks of other components in the sample. As long as the peak area and the relative response value of the internal standard substance and the components to be measured, the percentage content of the components to be measured in the sample can be calculated.
External standard method:
The quantitative method using the pure substance of the component to be measured as the reference material and comparing the response signal of the component to be measured in the sample is called external standard method. For example, in chromatography, you want to know the concentration of the sample being measured. The external standard method can be used to draw the working curve with the standard sample of the components to be measured first, measure the peak height of each peak or the sample concentration corresponding to the peak area, and draw the standard curve. In practice, the sample concentration can be obtained by measuring the peak height or peak area corresponding to the standard curve.
Differences between internal standard method and external standard method in gas chromatographic analysis
This method can be divided into working curve method and external standard one point method.Working curve method is to prepare a series of concentration of reference substance solution to determine the working curve, find the slope, intercept. Under exactly the same conditions, accurately injection sample solution with the same volume of reference solution, according to the signal of the components to be measured, the concentration can be found out from the standard curve, or calculated by regression equation, the working curve method can also be replaced by the external standard two-point method. In general, the intercept should be zero. If it is not zero, a systematic error exists. When the intercept of the working curve is zero, the external standard one point method (direct comparison method) can be used for quantification.
External standard method is simple, does not need correction factor, regardless of the other components in the sample peak, can be used to measure the quantitative. However, the accuracy of this method is affected by the sampling repeatability and the stability of experimental conditions. In addition, in order to reduce the experimental error of the external standard one-point method, the concentration of the reference solution should be as close as possible to the concentration of the components in the sample.
External standard method is a quantitative method in chromatographic analysis. It is not to add the standard substance to the tested sample, but to determine separately under the same chromatographic conditions with the tested sample. The chromatographic peak area obtained is compared with the chromatographic peak area of the tested component to get the content of the tested component. The concentration of the external standard substance should be close to the concentration of the tested substance in order to facilitate the accuracy of quantitative analysis.
To put it simply: the internal standard method is used to quantitatively add the substance to be analyzed in the sample, and quantify the sample content to be analyzed by measuring the ratio of the actual sample amount to the added sample amount.
The main advantages of internal standard method are simplicity and speed.The disadvantage is that there is no standard curve method quantitative accuracy.
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