Analytical correction
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When analyzing samples on a regular basis, we usually employ the area normalization method, and correction is generally not considered. However, when analyzing methanol and toluene, the difference in their response values is quite large, which leads to results that differ significantly from the actual values. If the external standard method is used, manual sampling errors are also substantial. Those who have experience with this process, could you share your insights with me?
Are methanol and toluene not separated and mixed together? I remember that during our actual testing, the extraction solutions from the methanol plant also showed poor performance; the area external standard method was used, but the standard sample had to be entered into the chromatograph again every time it was used. Let’s take a look at others’ insightful opinions
Why not use the internal standard method? The internal standard method is relative and does not require all peaks to appear. The error is also small. The internal standard method (external standard method) involves accurately weighing the sample and adding a certain amount of internal standard. The content of a specific component is determined based on the weights of the component under analysis and the internal standard, as well as their corresponding peak areas. Generally, the internal standard is used as a reference, in which case fs=1. Requirements for the internal standard: a. Its peak should be close to that of the component being analyzed, but still fully separated from it; b. A pure sample of the internal standard is required, and accurate weighing is necessary; c. It must be miscible with the sample, and the amount of the internal standard should be similar to the amount of the component being analyzed. Steps of the internal standard method: a. Select an internal standard; b. Measure fw(i/s) (fi); c. Weigh the unknown sample W, add the internal standard Ws, and then measure Ai and As. Advantages of the internal standard method: It is a relative method that does not require all peaks to be detected. Disadvantages: A pure sample of the internal standard is needed, and two weighings are required, which makes it somewhat cumbersome. Normalization method vs. internal standard method: Both are relative methods that are independent of operating conditions, thus resulting in lower errors. External standard method: It is simple and belongs to the absolute method; its accuracy depends on the operating conditions
Using the internal standard method is indeed more accurate, but finding an appropriate internal standard is troublesome, and high requirements are placed on such standards.
Does the poster use gas-phase or liquid-phase analysis? If it’s in the gas phase, there shouldn’t be any issue with differences in response! Generally, a moderate concentration is sufficient; the proportions of each component and their respective contents should be consistent, right? If it’s in the liquid phase, the difference in response might be due to an inappropriate wavelength selection! It is recommended to choose detection at the wavelength where absorption is maximum for both.
As for the external standard method, there is a quantitation loop in the liquid phase, so the errors associated with manual sampling do not affect the results; however, the concentration preparation must be precise.