Good day. Could you please help me with a question about systematic error in spectrophotometry? I have registered the spectra of phenol red in a range of five orders – from 10^-10 M to 10^-5 M. At concentrations below 10^-8 M, I observe a significant number of errors. I have characterized most of them, but my professor wants me to describe the error in determining optical density against a water background. He suggests that if the absorption of phenol red is at the same level as the absorption of water, a systematic error will occur. However, I cannot find anything similar in the literature. Could you assist me?
My professor said that it's easier to determine 0.1 against a background of 10 or 100 against a background of 10?
Systematic errors occur when sample absorption approaches water's background level due to low signal-to-noise ratio and stray light interference. To minimize, calibrate against water baseline, verify stray light limits, and use longer path lengths for low-concentration samples.
The systematic error occurs when the signal from phenol red approaches the background signal level of water, resulting in a low signal-to-background ratio. This overlap affects the limit of quantification (LoQ), as accurate determination requires the analyte signal to be significantly above the background noise. To calculate the systematic error, compare the measured absorbance of phenol red to the absorbance of water, and determine at what concentration the analyte signal exceeds the background signal by a defined factor (e.g., 10 times the standard deviation of the background signal) to reliably reach the LoQ.
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