Use of the blank test for detecting threshold effects in the indirect spectrophotometric determination of sulfate ions in tap water

Four different types of blank measurements, two of which were obtained directly (placebo and method blanks) and the other two (total Youden and system blanks) by extrapolation, are examined. The latter are usually more suitable for analytical work in dynamic ranges far above the detection limit, whereas the former two are only pertinent in dealing with detection limits. However extrapolated blanks can indicate when there is a risk of threshold effects emerging as the analyte concentration approaches zero, because neither the total Youden nor the system blank are affected by such effects. In addition, the presence of threshold effects can impair both the measurement of detection limits and the application of the method, obscuring the one-to-one correspondence between analytical signal and analyte amount. One case of a significant threshold effect is demonstrated in the indirect spectrophotometric determination of the sulfate ion in tap water. Also, a general approach is put forward for establishing the detection limit as the intercept of the lower confidance band with the analyte-amount axis (x-axis) from a calibration plot which includes the detection limit in its dynamic range, once the absence of threshold effects has been confirmed. Finally, the absence of direct interference from the matrix in the results of the spectrophotometric method is demonstrated by comparison with the results of the gravimetric determination of barium sulfate.

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