Quantitative methods for chromatographic analysis
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This post was last edited by zbx on 2010-6-13 at 17:48. One of our intermediate products needs to be quantitatively analyzed, but its activity is very high; it is not possible to find an internal standard, so quantitative analysis using the internal standard method cannot be applied. We have standard samples with a purity of 99.5%, but the purity of the intermediate products must be greater than 99.95%; using the external standard method results in large errors. How can such problems be solved?
Chromatography won’t work; let’s think of another method.
The purity is so high; it’s better to normalize it by area, hehe!
What did you use to do this before? In other words, how did you conduct this analysis in the past? If you haven’t done it before, you can first figure out the analysis methods on your own, after all, you have standard samples
Chromatographic analysis of pure substances is not aimed at analyzing the main components, but rather at analyzing impurities. The normalization method mentioned above simply won’t work, as the chromatographic peak of the main component may heavily mask or interfere with the peaks of impurities, resulting in large errors for the trace components. First, based on the characteristics of other analysis methods or synthesis processes, it is determined what impurities might be present, while simultaneously conducting an accurate analysis of any possible moisture content. Then, depending on the composition of the impurities, appropriate chromatographic methods are used to accurately analyze these impurities. Finally, the purity is calculated by performing a subtraction operation on the data: Purity, % = 100.00 - Total chromatographic impurities - Moisture. The qualitative identification of impurities should be carried out using the external standard method; additionally, chromatographic conditions should be optimized for the peaks near the main component, or sufficient amounts of the main component should be added to the standard sample. Many high-purity substances in industry are analyzed in this way, offering accuracy and reliability. This analysis method is used for substances such as polymer-grade ethylene and propylene, the purity of monomeric aromatic hydrocarbons in triphenyl compounds, as well as the purity of certain high-purity gases.
3rd floor: As mentioned on the 5th floor, using the area normalization method results in large errors. 4th floor: The method is still under exploration; the problem is that there are no suitable reference materials. 5th floor: The samples are highly reactive and react vigorously upon contact with water. The impurities are also unstable and present in very low amounts, ranging from 100PPM to 20PPM.
It is recommended to use the method from Floor 5, focusing solely on the qualitative and quantitative analysis of impurity components. Also, a control accuracy of 99.95% is questionable.
As mentioned above, if there is no better method, using a good chromatograph such as the Agilent 7890 with a high split ratio, along with area normalization, can also yield good results. Generally, the normalization method is quite accurate with the 7890 when a high split ratio is used (especially for the main component); the levels of impurities can generally be reduced to a few tenths of ppm； Of course, you can also look for substances with similar properties to those of the sample, but that have better stability or higher purity, to use as references; this will yield more accurate results.
7th floor: It cannot be diluted as it reacts with water, and no compatible solvent can be found. 8th floor: Since electronic-grade products are manufactured, the intermediate products have a purity level of over 5N. Floor 9: Thank you for the method you provided. We are using an Agilent 7890; how can we use a larger split ratio? Could you describe it further?
I usually set the split ratio to 1:90 (one microliter of sample enters; beyond that, it becomes more difficult to manage). If the properties of the substances are similar or their peak times are close to each other, the error for the main peak is generally not too large. For the peaks of impurities, the error is also acceptable in the absence of correction factors. If the results are still unsatisfactory, the sample volume can be reduced, provided that all the impurities required can be separated into distinct peaks. Find the conditions yourself and try a few more times. After all, if this method succeeds, your workload will **decrease** (the moisture content is usually calculated separately).