An example of establishing the dilution factor
| Sample | Dilution factor % Sample | Neat 100 |
1/2 50 |
1/4 25 |
1/10 10 |
1/20 5 |
|---|---|---|---|---|---|---|
| A | Observed | 391.1 | 200.9 | 91.1 | 36.2 | 22.1 |
| Target | 391.1 | 195.6 | 97.8 | 39.1 | 19.6 | |
| %Recovery | 100.0 | 102.7 | 93.2 | 92.6 | 113.0 | |
| B | Observed | 306.0 | 151.9 | 79.4 | 33.1 | 16.2 |
| Target | 306.0 | 153.0 | 76.5 | 30.6 | 15.3 | |
| %Recovery | 100.0 | 99.3 | 103.8 | 108.2 | 105.9 | |
| C | Observed | 357.4 | 185.2 | 85.8 | 39.9 | 20.1 |
| Target | 357.4 | 178.7 | 89.4 | 35.7 | 17.9 | |
| %Recovery | 100.0 | 103.6 | 96.0 | 111.6 | 112.5 | |
| D | Observed | 361.2 | 171.1 | 83.4 | 39.1 | 16.5 |
| Target | 361.2 | 180.6 | 90.3 | 36.1 | 18.1 | |
| %Recovery | 100.0 | 94.7 | 92.4 | 108.3 | 91.4 | |
| E | Observed | 388.6 | 190.1 | 95.9 | 35.8 | 21.2 |
| Target | 388.6 | 194.3 | 97.2 | 38.9 | 19.4 | |
| %Recovery | 100.0 | 97.8 | 98.7 | 92.1 | 109.1 | |
| Mean %Recovery | 100.0 | 99.6 | 96.8 | 102.5 | 106.4 | |
| SD of %Recovery | 0.00 | 3.64 | 4.64 | 9.42 | 8.87 | |
| N | 5 | 5 | 5 | 5 | 5 | |
| t-test | 2.776 | 2.776 | 2.776 | 2.776 | 2.776 | |
| SE | 0.00 | 1.63 | 2.07 | 4.21 | 3.97 | |
| Upper 97.5% CL | 100.0 | 104.2 | 102.6 | 114.2 | 117.4 | |
| Lower 2.5% CL | 100.0 | 95.1 | 91.1 | 90.9 | 95.4 |
Target value=(Value of neat sample)∙(Dilution factor); %Recovery=100∙(Observed value)/(Target value).
Mean and SD %Recovery are the values calculated using the %recoveries of 5 samples in each dilution factor.
From Appendix B of EP34-ED1 [19], the t-test value for N=5 is 2.776.
SE=SD/(√n); Upper 97.5% CL=mean %recovery+SE∙(t-test value); Lower 2.5% CL=mean %recovery–SE∙(t-test value).
If the goal of %recovery is 10%, the mean %recovery for all dilution factors satisfies the goal. However, in this example, the 97.5 upper CL or the 2.5 lower CL in 1/10 and 1/20 dilutions do not satisfy the goal. Therefore, 1/10 and 1/20 dilution factors cannot be used, and the maximum dilution factor established is 1/4.
Abbreviations: SD, standard deviation; SE, standard error; CL, confidence limit.