In the third Plackett–Burman design, the volumes of the sample
and of TCA were set at 6 and 15 ml, respectively, whereas agitation
and centrifugation times varied as indicated in Table 1. Treatment
2, which consisted of 6 ml sample, 15 ml TCA, 4 min agitation and
no centrifugation, provided the best recoveries of the five amines.
The elimination of the centrifugation step is advantageous as it
decreases analysis time as well as its costs.