Measuring and testing – Gas analysis – Breath analysis
Reexamination Certificate
1998-09-11
2002-08-20
Noland, Thomas P. (Department: 2856)
Measuring and testing
Gas analysis
Breath analysis
C073S023200, C073S028010, C073S863030, C073S865500
Reexamination Certificate
active
06435004
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an apparatus and process for the measurement of the size distribution of particles in aerosols. More particularly, the present invention relates to an apparatus and process that overcome the measurement limitations of instruments used to measure the size distribution of particles in aerosols, due to the instrument requirements for constant flow rate.
BACKGROUND OF THE INVENTION
Pharmaceutical aerosol delivery devices are often used to generate aerosols for the delivery of drugs to the respiratory tract. Generally, pharmaceutical aerosol delivery devices are of two types; self-propelled, and breath-propelled. Self-propelled aerosol delivery devices, exemplified by most pressurized Metered Dose Inhalers, generate an aerosol cloud by using energy supplied by the delivery device itself, i.e., liquefied gas propellants, battery operated fans, or compressed air. In contrast, breath-propelled aerosol delivery devices depend on externally-supplied negative pressure to generate the aerosol, such as the negative pressure created by the act of inhaling. For both of these types of delivery devices, the size distribution of the particles in the generated aerosol in the airway and lungs depends on the characteristics of the air stream into which the aerosol is dispensed. In addition, the efficiency of the delivery of these types of devices depends on the flow rate of gas which is drawn through the device.
In general, methods for measuring the size distribution of aerosol particles require that the aerosol particles are within a well-defined gas flow field. For example, inertial classifiers, one type of such instruments, operate by suspending aerosol particles in a moving gas stream. The stream is then caused to change directions, which in turn, causes the aerosol particles to cross the flow streamlines. In this manner, the aerosol particles are removed from the gas stream. In order for these types of instruments to operate properly, the flow rate of the gas stream must be constant.
The cascade impactor, which is a type of inertial classifier, is especially well-suited to the measurement of the size of aerosol particles. For example, U.S. Pat. No. 5,343,767 describes one example of a particular kind of a cascade impactor. Generally, cascade impactors comprise a plurality of collection stages arranged in series, with each stage designed to collect particles of successively smaller sizes. Within each stage, the gas stream is caused to flow through one or more nozzles to give the stream a relatively high velocity. The nozzles are directed perpendicular to a flat collection surface that is relatively close (less than about ten times the nozzle diameter) to the nozzle outlet. Depending primarily upon the speed of the gas stream through the nozzle, particles greater than a certain size strike the collection surface where they are retained for subsequent quantitation in a separate step. Successive stages have either fewer nozzles or smaller nozzles, so that the gas stream undergoes higher speeds and hence deposits smaller particles on the collection surface. The total gas flow rate through the cascade impactor must remain at a known and constant value during the course of a measurement because the size of particles deposited on a particular stage depends largely on the speed of gas flow.
However, both types of aerosol delivery devices are intended ultimately to produce aerosols into the varying flow rates which characterize the human breathing cycle. The size measurement of the particles in an aerosol in an air flow stream which is representative of the end-use situation thus presents a challenge. That is, although most measurement devices require constant gas flow, both types of delivery devices (i.e., self-propelled and breath propelled), are ultimately used in a situation with a variable flow rate. Thus, the requirement of constant flow through a measurement instrument cannot be satisfied when measuring the particle size distribution generated by a device operated in a variable flow rate. Thus, prior art methods for measuring the particle size distribution of aerosols are inadequate, in that the flow rate through the delivery device is maintained at a constant flow rate which is not representative of the end-use situation.
Generally, prior art apparatus' for the measurement of the size distribution of particles in an aerosol consist of a measuring instrument, such as an inertial classifier, coupled to an aerosol generating device, such as the inhaler devices described hereinabove. A vacuum source is coupled to the measuring instrument and is utilized to generate a relatively constant gas flow through the measuring instrument. Because the measuring instrument is coupled to the aerosol generating device this relatively constant gas flow must also pass through the aerosol generating device. Inasmuch as the measuring instrument is intended to measure the size distribution of the particles in an aerosol as distributed during a breathing pattern, this is an undesirable result. That is, it would be preferable if the flow from the aerosol generating device were comparable to the flow and pattern of a breathing cycle. More preferably, the flow through the aerosol generating device would be adjustable so as to correspond to different breathing flow rates, i.e., as for a child as opposed to an adult.
Thus, there is a need for an improved system for the measurement of the particle size distribution in an aerosol that includes an instrument utilizing a constant gas flow rate, yet capable of measuring an aerosol of varying flow rate.
SUMMARY OF THE INVENTION
The aforementioned shortcomings and disadvantages of the prior art are overcome by the apparatus and process provided by the present invention. Specifically, the present invention relates to an apparatus and process for measuring the size distribution of particles in an aerosol stream utilizing a measuring instrument that utilizes a generally constant gas flow rate.
Specifically, the apparatus of the present invention is a measuring system comprising a measuring instrument capable of measuring the size distribution of particles in an aerosol. Preferably, the measuring instrument will be one generally utilizing a constant gas flow rate. The measuring system further comprises a vacuum source, sealably coupled to the measuring instrument, that is utilized to force a constant flow rate of gas through the measuring instrument. Also provided is an inlet tube sealably coupled to the measuring instrument at a first end and an aerosol generating device connected to the inlet tube at a second end.
It is preferred that the connection between the inlet tube and the aerosol generating device is gas tight. That is, it is preferred that the connection between the inlet tube and the aerosol generating device is sufficiently leak-free that essentially all of the gas which enters the inlet tube is drawn in through the aerosol generating device. In this manner, the inlet tube functions to receive a first flow of a gas, as well as an amount of an aerosol generated by the aerosol generating device.
It is further preferred that the measuring system of the present invention comprises a mixing chamber that functions to couple the inlet tube to the measuring instrument. Preferably the mixing chamber comprises an inlet, hereinafter referred to as the mixing chamber inlet, capable of receiving a second flow of gas. In operation of the measuring system of the present invention, the first flow and/or the second flow of gas are preferably adjustable so that the combination of the first flow and the second flow are equal to the constant flow rate generated by the vacuum source.
Also provided by the present invention is an improved process for the measurement of the size distribution of particles in an aerosol stream. The process comprises employing a vacuum source to establish a constant flow through a measuring instrument. The vacuum generated by the vacuum source is used to draw an aerosol stream through an inlet incorporating a
Faegre & Benson LLP
Noland Thomas P.
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