Hach AutoCAT 9000 - MDL, Accuracy, and Precision Determination

AutoCAT 9000 Validation Study
AutoCAT 9000 Validation Study
Page 162
50081_AutoCAT 9000 Validation Study.fm
A comparison graph at a chlorine concentration of 16 µL Cl
2
is shown in Figure 2.
Figure 2 Titration Curve—16 µg/L Chlorine
Spreadsheet analysis of the data pairs, utilizing linear regressions of the line segments
prior to and after the endpoint, and results calculation, resulted in equivalent results to
those displayed on the instrument, again with excellent correlation and signal to noise.
MDL, Accuracy, and Precision Determination
After these trials were concluded, demonstrating the instrument capability to measure
low µg/L chlorine concentrations, per Standard Method 4500 Cl D, experiments were
conducted to determine the Method Detection Limit (MDL), according to 40 CFR Part
136, Appendix B.
This MDL calculation involves replicate analyses, where the sample concentration must
be within the range 1 to 5 times the calculated MDL. The MDL is calculated as the
Student's t value (at the 99% confidence interval), times the standard deviation. For
seven replicates, the Student's t value (at the 99% confidence interval) is 3.143.
The experimental conditions, and apparatus for this series of experiments was the same
as for the screening studies described above. Great care was observed during the required
sample manipulations to minimize experimental error.
Seven replicate measurements were initially run, at a sample chlorine concentration of
6.5 µg/L. An analysis of the standard deviation resulting from these trials determined that
the sample concentration was too high, outside the range 1 to 5 times the calculated
MDL. A second set of seven replicate measurements was then conducted on a chlorine
sample concentration of 4.1 µg/L. An analysis of the calculated MDL from this series of
trials showed that the sample concentration was within the stipulated range.
The resulting MDL, in accordance with 40 CFR, Part 136, Appendix B, was thus
determined to be 1.2 µg/L using this instrument. A summary of these trials can be seen in
Table 1.
Table 1 Method Detection Limit
Method Standard Standard Deviation Student t Calculated MDL
4500 Cl D 6.5 µg/L ±0.40 µg/L 3.143 1.3 µg/L
4500 Cl D 4.1 µg/L ±0.37 µg/L 3.143 1.2 µg/L