Curve problems
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Dear seniors, when using an ultraviolet spectrophotometer to determine the iron content in circulating water, a curve is created based on the relationship between absorbance and mg/L, with a correlation coefficient of 0.9998. However, it was found that varying the sampling volume results in different values; yet the iron content in the same sample of water within the water system remains constant. I would appreciate your advice on this matter, 🙏
I didn’t understand the question clearly: the curves are used for measurement purposes, so why is it fine when different samples give identical results?
Since the volumes are different, wouldn’t the correlation coefficient also change accordingly? It should be the same.
I suggest that you replace the horizontal and vertical axes used to plot the curve with those representing absorbance and mg, and then try again to see if there is any difference in the results. From what you’ve described, it seems to me that in the final calculations you have ignored the difference between the volume of the standard sample used to create the curve and the volume of the sample used for measurement
There might be interferences in the sample; try the standard addition method.
You’re right. Previously, I created curves using absorbance and mg as the measurement parameters, with the calculation formula KA+B/V; regardless of the amount of sample taken, the results were more or less the same. But this time, by using absorbance and mg/L as the measurement parameters and varying the sample volume, the results differed. I don’t understand why this is the case – could you explain it in detail?
You’re right. Previously, I created curves using absorbance and mg as the measurement parameters, with the calculation formula KA+B/V; regardless of the amount of sample taken, the results were more or less the same. But this time, by using absorbance and mg/L as the measurement parameters, different results were obtained depending on the volume of sample taken. I don’t understand why this is the case. Could you explain it in detail?
The key issue may lie in the conversion between the sampling volume unit and mg/L
When preparing the standard curve in accordance with the operating procedures, different sampling volumes still have an impact. Once the analysis data is obtained, it is necessary to determine how large the deviation is and whether it falls within the allowable error range
Let me give an example: Suppose you are creating a curve that relates absorbance to mg/L, and the sampling volume for the standard solution used is 50 ml; then when working with normal samples, the sampling volume must also be 50 ml, so that your results will be accurate； If your sampling volume is 25 ml, then the calculation of the results will be based on a volume of 50 ml; in other words, the sample is effectively diluted by a factor of two (this is based on the 50 ml volume used for preparing the standard curve, and the volumes must be consistent). As a result, the values obtained will definitely be lower. I’m not sure if you understand my explanation; this is also why the standards specify drawing a curve based on the relationship between absorbance and mg. This is because variations in the amount of sample taken can lead to errors when using the relationship between absorbance and mg/L. It’s better to first calculate the mass and then determine the concentration – although the calculation process is a bit more complicated, it avoids errors. Those who are aware of this situation are excluded, but not everyone is familiar with it in daily work, so a comprehensive consideration is needed.
The poster must have made some mistake; otherwise, this situation would never have occurred. Iron standard curves are easy to prepare.