The analysis results for Coulomb chlorine in the same sample are getting lower and lower
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First, let’s complain about how fussy the coulometer is – it’s always getting upset! Analysis item: Coal tar Coulomb chlorine. Sample: Low-temperature coal tar, with a density of around 1.03 and a moisture content of about 2%. Solvent: Xylene, analytical grade. Instrument settings: temperature 820/620, nitrogen 200, oxygen 100; electrolyte: glacial acetic acid : water 7:3 (water is distilled water); bias voltage 260, gain 2000, resistance 1k; sample diluted 5-fold. Abnormal issue: The control samples give a conversion rate of 100–110; when moving on to the actual samples, the results obtained starting from the first injection become progressively lower, until no results can be obtained at all by the end. I remember that for the first dose it was 28 ppm, for the second dose it was 20, then it dropped to 15, 9, 4, and eventually it became impossible to determine the value. Previously, carbon buildup often accumulated at the injection port; it was cleaned by heating it in a muffle furnace, washed with distilled water, and then dried. I urge experts in analysis to offer some guidance; I would be extremely grateful. :$
I feel that the effect of the carrier gas flow rate on the conversion rate isn’t very significant :( Has anyone encountered the problem I mentioned? I would appreciate your guidance.
The conversion rate is low. What could be the reason? The column has been burned and washed, and both the standard solution and the electrolyte have been replaced
What is the concentration of the standard sample? Does this situation occur once or frequently? I wonder if your syringe was first contaminated with a substance of high concentration, which is why the concentration gradually decreased over time
The standard sample is a chlorine standard at 50 ppm; using this standard sample to determine the conversion rate results in peaks that are quite parallel, and the reverse calibration yields satisfactory results. Many injection needles have been replaced. Next time I do it, I’ll prepare to reverse-label it several times to see if there are any issues with the reversed labeling results. Also, I’m not sure if the solvent is inappropriate; I don’t know what solvent to try using. I’m going to use the normal samples from before to conduct another test to see if there will be any problems. I’ve been busy lately and don’t have time to experiment.
There are many possible factors; check them one by one: 1. High-concentration sample analysis was conducted previously, and the injection system was contaminated (including the syringe) ； 2. The sampling container was not properly inerted, resulting in severe adsorption of chlorine elements in the sample ； 3. In previous analyses, in addition to chlorine, the samples also contained other high-concentration pollutants such as sulfur ； 4. Each time, two samples are taken using the same container; after the results for the first sample begin to decrease with each successive injection, the second sample is injected for observation……
Thank you for the teacher’s explanation. I would also like to ask about the principle and procedures of container inerting Thank you again.
1. Handled by professional agencies ； 2. Use inerted containers directly.