2.4. Measurements of repeatability and reproducibility
The repeatability and reproducibility studies, either between-
days (for 1 month) or during the day, were investigated by
consecutively collecting the sensor results of the same sample
(250 ppm of ethanol diluted in paraffin oil). The signal produced
by the standard was comparable to the signals produced by the
samples. The stability in time from this standard was checked by
the R/R0 ratio of the measurements. The repeatability study during
the day yielded a mean %RSD (relative standard deviation) of 5.7%
with a maximum value of 11.6%. For the between-days repeatabil-
ity study, the maximum deviation was 18.1% and the mean 10.2%.
Similar values were found for the perfume samples where only the
counterfeit samples showed higher values (7.9% mean RDS dur-
ing the day and 13.2% mean RDS between-days) due to their lower
sensor response.