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Calculating a method detection limit.

  • 04-01-2011 10:03pm
    #1
    210
    Registered Users, Registered Users 2 Posts: 186 ✭✭


    Calculation Method detection Limits.
    I have had a go at calculating a MDL for the colour test. What I have done is carried out 21 repeats of the test and recorded the results below. The results are in Hazens and the standard I have used was a 25 hazen standard. The results were as follows;

    Rep 1 23
    Rep 2 25
    Rep 3 23
    Rep 4 24
    Rep 5 25
    Rep 6 23
    Rep 7 24
    Rep 8 24
    Rep 9 25
    Rep 10 24
    Rep 11 25
    Rep 12 24
    Rep 13 26
    Rep 14 25
    Rep 15 26
    Rep 16 26
    Rep 17 26
    Rep 18 26
    Rep 19 27
    Rep 20 25
    Rep 21 27

    The Standard Deviation works out to be 1.220850601. I think the multiplication factor used for 21 reps to calculate the MDL is 2.528 so my MDL works out to be 3.08631032 [st dev * 2.528 (21 replicates)]. So for this test am I correct in saying that I should quote any result less than 3 as being < 3 Hazen ?


Comments

  • #2
    El Siglo
    Registered Users, Registered Users 2 Posts: 3,803 ✭✭✭El Siglo


    210 wrote: »
    Calculation Method detection Limits.
    I have had a go at calculating a MDL for the colour test. What I have done is carried out 21 repeats of the test and recorded the results below. The results are in Hazens and the standard I have used was a 25 hazen standard. The results were as follows;

    Rep 1 23
    Rep 2 25
    Rep 3 23
    Rep 4 24
    Rep 5 25
    Rep 6 23
    Rep 7 24
    Rep 8 24
    Rep 9 25
    Rep 10 24
    Rep 11 25
    Rep 12 24
    Rep 13 26
    Rep 14 25
    Rep 15 26
    Rep 16 26
    Rep 17 26
    Rep 18 26
    Rep 19 27
    Rep 20 25
    Rep 21 27

    The Standard Deviation works out to be 1.220850601. I think the multiplication factor used for 21 reps to calculate the MDL is 2.528 so my MDL works out to be 3.08631032 [st dev * 2.528 (21 replicates)]. So for this test am I correct in saying that I should quote any result less than 3 as being < 3 Hazen ?

    Have you analysed blank samples before doing these ones?
    According to a wikipedia article I've just looked at (I'm too lazy to get my analytical chemistry book from the shelf, apologies for that. However, this is still fairly accurate):
    The practical method for determining the MDL is to analyze 7 samples of concentration near the expected limit of detection. The standard deviation is then determined. Then a one-sided t distribution* is determined and multiplied versus the determined standard deviation. For seven samples (with six degrees of freedom) the t value for a 99% confidence interval is 3.14. Rather than performing the complete analysis of seven identical samples, if the Instrument Detection Limit is known, the MDL may be estimated by multiplying the Instrument Detection Limit or Lower Level of Detection by the dilution prior to analyzing the sample solution on the instrument. This estimation, however, ignores any uncertainty that arises from performing the sample preparation and will therefore probably underestimate the true MDL.

    *Students' T-test.

    Source.

    Hope that's what you're looking for.


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