MICROPIPETS continued
Figure 2 – Imprecision (CV) of a 20-μL pipet set to 20 μL.
Figure 5 – Inaccuracy of a 100-μL pipet set to 20 μL. Table 1 – Experimental conditions
• All tests reported in Figures 2–5 were conducted in a controlled calibration laboratory, at 20.0 ± 1.0 °C and 45–65% relative humidity.
• Volume measurements were performed with an Artel PCS® Pipette Calibration System (Westbrook, ME), using the photometric method according to ISO 8655-7.
Figure 3 – Inaccuracy of a 20-μL pipet set to 20 μL.
• Only one parameter of the pipetting technique was varied in each experiment and compared to the control method described below; compounding of technique errors was not investigated in this study.
• Two pipets were evaluated in this study: – 1) 20-μL pipet set at 20 μL – 2) 100-μL pipet set at 20 μL
• The manufacturer’s recommended tips were used. • Experiments were carried out by trained operators. • The control method used the following pipetting technique: – Each pipet tip was prewet 3 times
Figure 4 – Imprecision (CV) of a 100-μL pipet set to 20 μL.
pipet tip, temperature, sample type (vapor pressure), speed of aspiration, and sample volume.
Tip wiping The practice of wiping the pipet tip after aspiration with a laboratory
cloth is widespread. Due to the high propensity of introducing large errors through this technique, one should be very carefully evaluate whether this step is really necessary. If it is determined that a particular sample is prone to forming droplets on the outside of the pipet tip that must be wiped off, extreme care should be taken not to touch the tip orifice, since it is very easy to wick out some of the sample solution. In our study, tip wip- ing introduced over 2.3% of CV and led to underdelivery of up to –1.3% RI.
– The tip was immersed 2 mm below the meniscus in the sample solution
– Pipet was held in a vertical position during aspiration, and at a 45° angle during the dispense against the glass wall of the measurement cuvette
– Forward mode of pipetting was used, with blow-out during dispensing
– Each experiment was conducted with 30 replicates – A new pipet tip was used for each data point.
• Accuracy is reported as relative inaccuracy (RI) as percent difference to the set volume of the pipet.
• Precision is reported as the coefficient of variance (CV). AMERICAN LABORATORY • 16 • JUNE/JULY 2014
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