Pharmaceutical Technology - November 2018

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Pharmaceutical Technology LABORATORY BEST PRACTICES 2018 29 ALL FIGURES COURTESY OF THE AUTHORS. Vortex mixers have a relatively simple design with a rotating motor attached to a base plate, which often has some type of vial holder attached. Vials are placed in the vial holder, and depending on the style of vortex mixer, the start conditions may vary. The two most common types of vortex- ers are: • A Type I vortex mixer is pressure sensitive; once a vial comes in contact with the vial holder, the vortex mixer initiates when the re- quired pressure has been applied. • A Type II vortex mixer has an on/off switch and initiates when the switch has been turned on. For the purpose of this study, only the Type I vortex mixer was used. Once a vial has liquid in it and is placed on the vortex mixer, the rotation causes a vortex to form within the vial. Figure 1 shows the expected flow of fluid within a vial once the vortex has been created. The shear is generated by the parallel force applied by particle-particle and particle-f luid interaction. The shear is calculated using Equation 1, which calcu- lates the shear, τ, as a function of the fluid viscosity, η, and the frequency of revolution, γ , more com- monly referred to as the flow velocity or shear rate. τ = ηγ [Eq. 1] This study had two objectives: • Measure the rotational velocities as a function of instrument setting for a number of differ- ent vortexers. Convert the rotational velocities to measurements of shear. Then compare the vortexers to determine if they are equivalent. • Provide an example of how the particle size of certain morphologies are prone to breakage as a function of shear. Vortexer shear measurement For this study, three common vortexer mixers were utilized. The viscosity used in Equation 1 was that for water at 20 °C, which is 1.0 cP (1X10 -3 Pa s). The user manuals for the vortex mixers list a ro- tational velocity in the range of 300 rpm to 3200 rpm; however, these values correspond to the dial settings for the individual vortex mixers. The rota- tional velocity for each vortexer at various settings was measured using a strobe light tachometer and a ring stand with clamps, as the setup shows in Figure 2. The values were measured two times on sepa- rate days using different tachometers and averaged. The averaged values were then used to calculate the shear by first converting to frequency of revolution by multiplying the measured rpm by a factor of 1.67X10 -5 Pa (see Equation 2). τ = ηγ = (1x10 -3 Pa s) ( ) ( ) = 1.67X10 -5 Pa 1 min 1 min 60 s [Eq. 2] The results have been included in Table I; Figure 3 shows a graphical comparison of the vortex mixers. One of the vortexers used did not have a dial for vari- Figure 1: Flow of fluid in a vial. Continued on page 32

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