3.4 - Method Evaluation and Validation

In the following, two selected procedures are evaluated with respect to LLD, MDA and uncertainty budget.

²²⁶Ra through ²²²Rn ingrowth (method 2.2.1.4., 24 h storage)

Lower Limit of Detection

The Minimum Detectable Activity MDA is calculated through LLD and the background without considering the uncertainties for analysis by

where f_v represents the volume factor, f_t1 the daughter ingrowth factor and f_t2 the decay factor between Rn extraction and mean measuring time. The following experimental parameters have to be considered for the calculation of MDA for the procedure of ²²⁶Ra determination through Radon ingrowth with 24 h ingrowth storage:

R₀ 0.5 count / hour

t 1 hour (measurement of sample and background)

ε 300 %

f_v 8/7 x 1000/12

f_t1 1 / (1- exp-(24/92.16) _* ln 2)

f_t2 1 / exp-(3/92.16) _* ln 2

V 12 mL

LLD = 2 _* 1.645 _* (2 _* 0.5 / 60² _* 60²)^1/2

LLD = 0.00091 sec^-1 per sample

Minimum Detectable Activity:

MDA [Bq/L] = 0.00091/3 _* 95.2 _* 6.06 _* 1.02

MDA = 0.178 Bq/L

Analysis of uncertainty

Several parameters influence the variability of the analysis. The list of parameters and its relative standard uncertainty are:

The uncertainty parameters are assumed to be independent from each other; therefore the calculation of the analysis uncertainty U or u(y) is done by the law of propagation of errors (sum of errors in square)

Total analysis uncertainty: U = 6.5 %

²²⁶Ra by RAD Disk filter (method 2.2.1.5.)

(a) Gel measurement of the EDTA eluate:

Lower Limit of Detection

LLD = 2 _* 1.645 _* (2 _* 120 / 60² _* 60²)^½ = 0.014 sec^-1 per sample

Minimum Detectable Activity:

MDA [Bq/L] = (LLD / e _* V) _* f_t = (0.014 / (0.9 _* 3) _* 1.02

MDA = 5.3 mBq/L

Analysis uncertainties

The following uncertainty parameters influence the analysis:

Total analysis uncertainty: U = 6.3 %

Remark: No influence by other radionuclides in absence of ²²⁴Ra

(b) Extraction of ²²²Rn from EDTA eluate after equilibration (storage >25 days):

Lower Limit of Detection (LLD)

After equilibration with ²²²Rn (> 25 days, <1 % error), the LLD is calculated according to:

LLD = 0.00091 sec^-1 (see ²²⁶Ra) per sample

Minimum Detectable Activity

MDA [Bq/L] = (LLD / e _* V) _* f_t = (0.00091 / (3 _* 3) _* 1 _* 1.02

MDA = 0.1 mBq/L

The additional uncertainties of this method through Rn losses by emanation (4 %) and durig equilibration are compensated by a lower uncertainty for calibration of organic phases (no quenching and better PSD).