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Figure 2.9.18.-8. - Experimental set-up for testing powders for inhalation

Connect a flow meter, calibrated for the volumetric flow leaving the meter, to the induction port. Adjust the flow control valve to achieve steady flow through the system at the required rate, Q (± 5 per cent). Switch off the flow.

Ensure that critical flow occurs in the flow control valve by the following procedure. With the inhaler in place and the test flow rate established, measure the absolute pressure on both sides of the control valve (pressure reading points P2 and P3 in Figure 2.9.18.-8). A ratio P3/P2 < 0.5 indicates critical flow. Switch to a more powerful pump and re-measure the test flow rate if critical flow is not indicated.

Dispense 20 ml of a solvent, capable of dissolving the active ingredient into each of the four upper stages of the apparatus and replace the stoppers. Tilt the apparatus to wet the stoppers, thereby neutralising electrostatic charge. Place a suitable mouthpiece adapter in position at the end of the induction port.

Prepare the dry-powder inhaler for use according to patient instructions. With the pump running and the two-way valve closed, locate the mouthpiece of the inhaler in the mouthpiece adapter. Discharge the powder into the apparatus by opening the valve for the required time, T (± 5 per cent). Repeat the procedure. The number of discharges should be minimised and typically would not be greater than ten. The number of discharges should be sufficient to ensure an accurate and precise determination of fine particle dose. After the final discharge, wait for 5 s and then switch off the pump.

Dismantle the filter stage of the apparatus. Carefully remove the filter and extract the active ingredient into an aliquot of the solvent. Remove the induction port and mouthpiece adapter from the apparatus and extract the active ingredient into an aliquot of the solvent. If necessary, rinse the inside of the inlet jet tube to stage 1 with solvent, allowing the solvent to flow into the stage. Extract the active ingredient from the inner walls and the collection plate of each of the four upper stages of the apparatus into the solution in the respective stage by carefully tilting and rotating the apparatus, observing that no liquid transfer occurs between the stages.

Using a suitable method of analysis, determine the amount of active ingredient contained in each of the six volumes of solvent.

Calculate the fine particle dose (see below).

APPARATUS D - "ANDERSEN" SIZING SAMPLER

The "Andersen" 1 ACFM non-viable ambient sizing sampler consists of 8 aluminium stages together with a final filter. The stages are clamped together and sealed with O-rings. In the configuration used for pressurised inhalers, Figure 2.9.18.-9, the entry cone of the sampler is connected to a right-angle bend metal induction port defined in Figure 2.9.18.-7. A suitable mouthpiece adapter should be used to provide an airtight seal between the inhaler and the induction port. The front face of the inhaler mouthpiece must be flush with the front face of the induction port. In the configuration for powder inhalers, a pre-separator is placed above the top stage to collect large masses of non-respirable powder. It is connected to the induction port as shown in Figure 2.9.18.-10. To accommodate high flow rates through the sampler, the outlet nipple, used to connect the sampler to the vacuum system is enlarged to have an internal diameter > 8 mm.

Procedure for pressurised inhalers. Assemble the "Andersen" Sampler with a suitable filter in place and ensure that the system is airtight. Place a suitable mouthpiece adapter in position at the end of the induction port so that the mouthpiece end of the actuator, when inserted, lines up along the horizontal axis of the induction port and the inhaler unit is positioned in the same orientation as the intended use. Connect a suitable pump to the

Table 2.9.18.-5.

- Calculations for Apparatus C. Use q1

= ^(60/Q), where Q is the flow rate in litres per minute

Cut-off diameter (pm)

Mass of active ingredient deposited per discharge

Cumulative mass of active ingredient deposited per discharge

Cumulative fraction of active ingredient (per cent)

d4 = 1.7q

mass from stage 5, m5*

C4 = m5

f4 = (C4/c)-100

d3 = 3.1-q1

mass from stage 4, m4

C3 = C4 + m4

f3 = (c3/c)-100

d2 = 6.8-q1

mass from stage 3, m3

c2 = c3 + m3

f2 = (c2/c)-100

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