Patent Publication Number: US-2019170629-A1

Title: Cascade impactor plate coating

Description:
FIELD 
     The present invention relates to a cascade impactor and methods of using the same. 
     BACKGROUND 
     Cascade impactors have been used for a number of years to determine the aerodynamic particle size distribution of aerosol particles. The aerodynamic particle size of inhaled products is important since the particle size directly influences the regional deposition of the aerosol in the lungs and respiratory tract. 
     Therefore, the measurement of the particle size distribution of aerosol particles is of importance during the development and manufacture of inhaler products which generate aerosols for delivery of a substance to the lungs or respiratory tract of the user. A number of suitable techniques for measuring the particle size of such aerosol particles have been developed. These include, for example, particle time-of-flight spectrometry, laser diffractometry, Phase-Doppler particle size analysis, and inertial techniques such as cascade impaction. 
     As described in the article “ Understanding cascade impaction and its importance for inhaler testing ”, Mark Copley, www.copleyscientific.com, July 2007, the fraction of a dose that will deposit in a user&#39;s lung is defined as the fine particle fraction (FPF). The FPF is dependent on the particle size of the aerosol. The upper limit used to define the FPF varies, but tends to lie between four and six microns, the optimal size for central airway deposition. Peak deposition in the peripheral airways of the lung occurs at between two and four microns. In general, there is no lower limit defined for FPF, although there is an argument for one since very small particles below one micron may be exhaled. The measurement range of interest for inhalation product development is therefore around ten microns and below. 
     The primary advantage offered by inertial techniques compared with the alternative methods described above is that they allow determination of an assay for the FPF and other size fractions. The other methods provide no differentiation between active pharmaceutical ingredients (APIs) and any other components in the formulation; they simply measure the overall particle size distribution. 
     In addition, inertial techniques are also able to directly measure aerodynamic particle diameter, which is the parameter most closely correlated with particle behaviour during inhalation. Time-of-flight analysers may also measure this variable, but with other techniques it must be calculated from the volume equivalent diameter. 
     In particular, cascade impactors have been adopted as the standard technique recommended by regulatory bodies for the validation of new inhalation products since they uniquely provide the required degree of resolution in the particle size range of greatest interest for inhalation products: 0.5 to 5 microns. 
     In some embodiments, the present invention seeks to provide an improved cascade impactor for measuring the particle size distribution of an aerosol comprising nicotine, and a method for using the same. 
     SUMMARY 
     In accordance with some embodiments described herein, a cascade impactor for measuring particle size distribution of an aerosol comprising nicotine is provided, the cascade impactor comprising:
         an induction port for receiving an inhaler; and   at least one sample collection stage comprising a nozzle assembly and a collection means;
 
wherein the collection means of at least one sample collection stage is coated with isopropyl alcohol.
       

     In accordance with some embodiments described herein, a method is provided for measuring the particle size distribution of an aerosol comprising nicotine, the method comprising the steps of: 
     (a) attaching an inhaler for generating the aerosol comprising nicotine to a cascade impactor, wherein the cascade impactor comprises:
         an induction port for receiving an inhaler; and   at least one sample collection stage each comprising a nozzle assembly and a collection means;
 
wherein the collection means of at least one sample collection stage is coated with isopropyl alcohol; and
       

     (b) dispensing an aerosol comprising nicotine from the inhaler into the cascade impactor. 
     In accordance with some embodiments described herein, there is provided the use of isopropyl alcohol for reducing the extent of evaporation of nicotine during use of a cascade impactor for measuring the particle size distribution of an aerosol comprising nicotine. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Embodiments of the present invention are described, by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  shows the principle of operation of a multi-stage cascade impactor. 
         FIG. 2  shows a collection curve efficiency for a sample collection stage in a cascade impactor. 
         FIG. 3  shows an exemplary cascade impactor in accordance with the present invention. 
         FIG. 4A  shows a schematic diagram of the orientation of sample collection stages in an NGI (Copley Scientific). 
         FIG. 4B  shows a schematic diagram of an NGI (Copley Scientific). 
         FIG. 5  shows a schematic diagram of an ACI (Copley Scientific). 
         FIG. 6  shows a schematic diagram of an MMI (Copley Scientific). 
         FIG. 7  shows a schematic diagram of a Twin Impinger (Copley Scientific). 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments are shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the drawings and detailed description of the specific embodiments are not intended to limit the invention to the particular forms disclosed. On the contrary, the invention covers all modifications, equivalents and alternatives falling within the scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     The present invention relates to a cascade impactor. In particular, the present invention relates to a cascade impactor for measuring the particle size distribution of an aerosol comprising nicotine. 
     Generally, a cascade impactor comprises at least one sample collection stage, each stage having a nozzle assembly and collection means. The collection means typically includes an impaction surface for the collection of the particles. 
     The principle of operation of cascade impactors is described in the 2015 edition of Copley Scientific, Quality Solutions for Inhaler Testing Brochure. Cascade impactors separate particles based on the principle of inertial impaction. In a cascade impactor, particles in an aerosol are removed from an air stream based on their inertia when the air stream changes direction. Particles above a certain size possess so much momentum that they cannot follow the air stream and thus strike a collection surface which is available for later analysis of mass and composition. The inertia of a particle is a function of particle size and velocity. 
     In a cascade impactor, sample-laden air is accelerated through an orifice or nozzle towards a collections means at some distance below the nozzle; this causes the air stream to abruptly change direction. Particles which are below a certain size, and therefore have a low enough inertia, remain suspended in the air stream, while larger particles with a higher inertia impact on the collection means. In general, particles larger than the cut size diameter of the impactor impinge or impact on the collection means, where the cut size diameter is the size of particles which are collected with 50% collection efficiency. Collection efficiency increases for particles larger than the cut size diameter, and decreases for smaller particles. The cut size diameter for an impactor is dependent on the diameter of the orifice or nozzle and the flow rate. 
     Some cascade impactors have multiple collection stages (e.g. two or more) arranged in series, each stage having a nozzle assembly where the nozzle orifice decreases in size relative to that of the previous stage. 
     During use of a cascade impactor having multiple stages, the sample-laden air is introduced into the impactor and is drawn through the impactor. The air stream flows sequentially through the sample collection stages. At each sample collection stage, the smaller the nozzle orifice, the higher the velocity of air or gas particles moving through the orifice. The higher the velocity, the smaller the particles that are collected on the impaction plate. Therefore, particles of decreasing size may be collected on each sequential sample collection stage. 
     Particles larger than the cut size diameter of the impactor impinge or impact upon the impaction surface of the collection means. The smaller particles pass with the air stream out of the first impaction region and proceed to the next stage. This procedure continues through the cascade impactor with each stage having higher velocity and collecting smaller size particles. 
     This principle of operation of a typical multi-stage cascade impactor is shown in  FIG. 1  (Copley Scientific, Quality Solutions for Inhaler Testing). The sample-laden air stream  3  passes through a nozzle  2  of the cascade impactor  1 . The particles in the air stream  3  either subsequently impact on the collection means  4  or are carried along in the air stream which is directed at an angle to the flow-stream through the nozzle. The impaction depends on the inertia of the particles. 
     As described in the article “ Understanding cascade impaction and its importance for inhaler testing ”, Mark Copley, www.copleyscientific.com, July 2007, ideally collection efficiency would be a step function—all of the particles above a certain size would be captured and those below it would pass through to the next collection stage. However, in practice, there is generally a curve from which the D50 particle size, the cut size diameter of the sample collection stage, is determined.  FIG. 2  shows a collection efficiency curve for a typical cascade impactor stage as shown in the Copley reference article. 
     In some instances, particles may “bounce” as opposed to impact when they contact the collection means of a sample collection stage. As used herein, “bounce” refers to the phenomenon where the aerosol particles impact the collection means of a sample collection stage, but subsequently become re-entrained in the air stream rather than remaining on the collection means; i.e. they bounce off the surface of the collection means. The “bounce” of particles leads to them being re-entrained in the sample-laden air stream such that they are carried to a sample collection stage further downstream, where they ultimately impact a sample collection stage having a cut size diameter configured to collect smaller particles. 
     Previously, this problem of re-entrainment has been addressed by coating the collection means of the sample collection stages with a suitable surface coating, such as glycerin, Tween 80 or silicone oil. 
     As used herein the term “cascade impactor” includes devices which act as an impactor or as an impinger. As used herein the term “impactor” describes an instrument where the particles impact on a dry impaction collection means. As used herein the term “impinger” describes an instrument where the particles impact on a liquid collection means. 
     The cascade impactor in accordance with the present invention comprises an induction port for receiving an inhaler and at least one sample collection stage, wherein each sample collection stage comprises a nozzle assembly and a collection means. 
     “Inhaler” as used herein includes an aerosol-generating means which generates an aerosol comprising nicotine. 
     A cascade impactor in accordance with the present invention is shown in  FIG. 3 . During use of the cascade impactor  30 , an inhaler  31  is received into the induction port  32  of the cascade impactor  30 . Upon actuation of the inhaler  31 , an aerosol comprising nicotine is produced resulting in a nicotine-laden air stream entering the cascade impactor  30 . The nicotine-laden air stream subsequently passes through the at least one collection stage  33 . 
     In some embodiments, a vacuum pump or air pump is attached at the outlet end  34  of the cascade impactor. The outlet end  34  of a cascade impactor is further downstream than the final sample collection stage in the cascade impactor. The vacuum pump or air pump may be used to draw the air stream released from the inhaler through the cascade impactor. 
     In some embodiments, the cascade impactor comprises at least two sample collection stages. In some embodiments, the cascade impactor comprises at least three sample collection stages. In some embodiments, the cascade impactor comprises at least four sample collection stages. In some embodiments, the cascade impactor comprises at least five sample collection stages. In some embodiments, the cascade impactor comprises at least six sample collection stages. In some embodiments, the cascade impactor comprises at least seven sample collection stages. 
     In some embodiments, the cascade impactor comprises two sample collection stages. In some embodiments, the cascade impactor comprises three sample collection stages. In some embodiments, the cascade impactor comprises four sample collection stages. In some embodiments, the cascade impactor comprises five sample collection stages. In some embodiments, the cascade impactor comprises six sample collection stages. In some embodiments, the cascade impactor comprises seven sample collection stages. In some embodiments, the cascade impactor comprises eight sample collection stages. 
     In some embodiments, the cascade impactor further comprises an internal filter or an external filter. Such filters may be used to collect ultra-fine particles that are not impacted on any of the sample collection stages in the cascade impactor. This may be useful, for example, where the particle size of the aerosol particles is less than the cut size diameter of the sample collection stage having the smallest cut size diameter (i.e. the sample collection stage furthest downstream in the cascade impactor). 
     In some embodiments the cascade impactor is selected from a Next Generation Impactor (NGI), an Andersen Cascade Impactor (ACI), a Multi-stage Liquid Impinger (MSLI), a Marple-Miller Cascade Impactor (MMI) and a Twin Impinger. 
     In some embodiments, the cascade impactor is an NGI. A “Next Generation Impactor” as used herein includes a cascade impactor that comprises seven sample collection stages. In some embodiments, the sample collection stages are arranged in a planar configuration. The NGI may include a horizontal tray for receiving each of the sample collection stages. The sample collection stages may each comprise a nozzle assembly, and a collection means in the form of a cup. 
     In some embodiments, the NGI operates at a flow rate of between 30 and 100 L/min. The particle size range measurable by an NGI may be between 0.24 to 11.7 μm depending on the flow rate of the NGI. 
     In some embodiments, the NGI comprises seven sample collection stages and a micro-orifice collector (MOC). The MOC may comprise a nozzle assembly such that any particles remaining in the air stream after passing through the seventh sample collection stage may be impacted on the MOC. The MOC may thus be able to collect all remaining particles having a particle size below the cut size diameter of the seventh collection stage. This eliminates the need for a final filter paper to be present downstream of the seventh collection stage. 
     In some embodiments, the NGI comprises a cup tray containing the seven sample collection stages in the form of cups, and the MOC. The cups may be used to collect the samples prior to analysis. In some embodiments, the NGI comprises a frame to support the cup tray. In some embodiments, the NGI comprises a lid containing the inter-stage pathways for directing the air stream between the sample collection stages. In some embodiments, the lid holds the nozzle assemblies of each sample collection stage in place. 
     A schematic of an exemplary NGI as marketed by Copley Scientific is shown in  FIGS. 4A and 4B . As shown in  FIG. 4A , the NGI comprises seven sample collection stages and an MOC. The stages are arranged such that the air stream passing through the impactor travels in a saw tooth pattern. The NGI  40  shown in  FIG. 4B  comprises a lid  45  with a seal body attached for holding the nozzle assemblies  41  of each sample collection stage in place. The lid  45  is hingedly attached to a frame  42  comprising a removable cup tray  43  which houses eight cups  44  as collection means for each sample collection stage and the MOC. The inter-stage passageway  46  of the air stream is shown as a dotted line in  FIG. 4B . 
     In some embodiments, the cascade impactor is an NGI as produced by Copley Scientific (see  FIGS. 4A and 4B ). In this embodiment, the cut size diameters of each of the sample collection stages in the NGI are as follows: 
     
       
         
           
               
               
               
            
               
                   
                   
               
               
                   
                 Cut Size Diameter at flow rate of: 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 15 L/min 
                 30 L/min 
                 60 L/min 
                 100 L/min 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Stage 1 
                 14.10 μm  
                 11.76 μm  
                 8.06 μm 
                 6.12 μm 
               
               
                   
                 Stage 2 
                 8.61 μm 
                 6.40 μm 
                 4.46 μm 
                 3.42 μm 
               
               
                   
                 Stage 3 
                 5.39 μm 
                 3.99 μm 
                 2.82 μm 
                 2.18 μm 
               
               
                   
                 Stage 4 
                 3.30 μm 
                 2.30 μm 
                 1.66 μm 
                 1.31 μm 
               
               
                   
                 Stage 5 
                 2.08 μm 
                 1.36 μm 
                 0.94 μm 
                 0.72 μm 
               
               
                   
                 Stage 6 
                 1.36 μm 
                 0.83 μm 
                 0.55 μm 
                 0.40 μm 
               
               
                   
                 Stage 7 
                 0.98 μm 
                 0.54 μm 
                 0.34 μm 
                 0.24 μm 
               
               
                   
                 MOC 
                 0.70 μm 
                 0.36 μm 
                 0.14 μm 
                 0.07 μm 
               
               
                   
                   
               
            
           
         
       
     
     In other words, there are seven sample collection stages in the Copley Scientific NGI, five with cut size diameters between 0.54 and 6.12 μm at flow rates from 30 to 100 L/min. 
     As shown in  FIG. 4B , the sample aerosol air stream passes through the impactor in a saw tooth pattern. Particle separation and size measurement may be achieved by successively increasing the velocity of the air stream as it passes through each sample collection stage by forcing it through a series of nozzles containing progressively reducing jet diameters. 
     In some embodiments, the cascade impactor is an ACI. An “Andersen Cascade Impactor” as used herein includes a cascade impactor that comprises eight sample collection stages that are typically arranged in a stacked design. The stacked design of an ACI makes for easy handling, and damaged stages can simply be removed and replaced when necessary. 
     In some embodiments, the ACI operates at a flow rate between 28.3 and 100 L/min. In some embodiments, the ACI operates at a flow rate of 60 L/min. In some embodiments, the ACI operates at a flow rate of 90 L/min. 
     A schematic of an exemplary ACI attached to a vacuum pump as marketed by Copley Scientific is shown in  FIG. 5 . The ACI  50  comprises eight sample collection stages  51  arranged in a stack, one on top of the other. An induction port  52  is positioned on top of the first sample collection stage such that when an inhaler  53  is attached to the induction port, and an aerosol released from the inhaler, the aerosol air stream flows through the sample collection stages sequentially from the first stage to the eighth stage. The vacuum pump  54  is used to draw the aerosol air stream through the ACI. 
     In some embodiments, the ACI has eight sample collection stages: stage  0 , stage  1 , stage  2 , stage  3 , stage  4 , stage  5 , stage  6 , and stage  7 . In some embodiments, stage  0  has a cut size diameter such that it collects particles with particle sizes of at least 9 μm. In some embodiments, stage  1  has a cut size diameter such that it collects particles with particle sizes of from 5.8 to 9 μm. In some embodiments, stage  2  has a cut size diameter such that it collects particles with particle sizes of from 4.7 to 5.8 μm. In some embodiments, stage  3  has a cut size diameter such that it collects particles with particle sizes of from 3.3 to 4.7 μm. In some embodiments, stage  4  has a cut size diameter such that it collects particles with particle sizes of from 2.1 to 3.3 μm. In some embodiments, stage  5  has a cut size diameter such that it collects particles with particle sizes of from 1.1 to 2.1 μm. In some embodiments, stage  6  has a cut size diameter such that it collects particles with particle sizes of from 0.7 to 1.1 μm. In some embodiments, stage  7  has a cut size diameter such that it collects particles with particle sizes of from 0.4 to 0.7 μm. 
     In some embodiments, the cascade impactor is an MSLI. A “Multi-stage Liquid Impinger” as used herein includes a cascade impactor (or liquid impinger) comprising five sample collection stages which can be used for determining the particle size (aerodynamic size distribution) of inhalers. An MSLI may typically have the sample collection stages arranged in a stacked design, similar to the ACI. The sample collection stages of the MSLI are typically kept moist in order to reduce the problem of re-entrainment. 
     In some embodiments, the MSLI operates at a flow rate between 30 and 100 L/min. In some embodiments, the cascade impactor is an MSLI marketed by Copley Scientific. The Copley Scientific MSLI comprises four sample collection stages having cut size diameters decreasing in increments from 13 to 1.7 microns; the fifth stage comprises an integral paper filter to capture the remaining fraction of particles having a D50 particle size of less than 1.7 microns. 
     In some embodiments, the cascade impactor is an MMI. A “Marple-Miller Cascade Impactor” as used herein is a cascade impactor including five sample collection stages and an induction port. In some embodiments, the collection means of the sample collection stages are in the form of cups for collecting the aerosol particles. 
     In some embodiments, the MMI operates at a flow rate of between 4.9 and 90 L/min. In some embodiments, the MMI operates at a flow rate between 60 and 90 L/min (Model 160). In some embodiments, the MMI operates at a flow rate between 30 and 60 L/min (Model 150). In some embodiments, the MMI operates at a flow rate between 4.9 and 12 L/min (Model 150P). 
     In some embodiments, the MMI comprises a paper filter downstream of the fifth sample collection stage in order to ensure that all aerosol particles of are collected in the impactor. 
     The cut size diameter of each sample collection stage in an MMI may be calculated using the following equation: 
     
       
         
           
             
               D 
                
               
                   
               
                
               50 
                
               
                 ( 
                 Q 
                 ) 
               
             
             = 
             
               D 
                
               
                   
               
                
               50 
                
               
                 ( 
                 
                   Q 
                   n 
                 
                 ) 
               
                
               
                 
                   ( 
                   
                     
                       Q 
                       n 
                     
                     Q 
                   
                   ) 
                 
                 
                   1 
                   / 
                   2 
                 
               
             
           
         
       
     
     where D50(Q) is the cut size diameter of the sample collection stage at the flow rate Q, Q n  is a flow rate of 60 L/min, and D50(Q n ) is the cut size diameter of the sample collection stage at a flow rate of 60 L/min. 
     A schematic diagram of an exemplary MMI marketed by Copley Scientific is shown in  FIG. 6 . The MMI  60  comprises five sample collection stages  61  in the form of cups, and an induction port  62  for receiving an inhaler. A filter  63  is also included downstream of the fifth sample collection stage. A port at the outlet end  64  enables connection of the cascade impactor to a vacuum pump or air pump for drawing the aerosol from the inhaler through the MMI. 
     In some embodiments, the cascade impactor is an MMI marketed by Copley Scientific. The cut size diameters of each of the five sample collection stages at a flow rate of 60 L/min (Model 160) are as follows: 
     Stage  1 —10 μm 
     Stage  2 —5 μm 
     Stage  3 —2.5 μm 
     Stage  4 —1.25 μm 
     Stage  5 —0.63 μm. 
     In some embodiments, the cascade impactor is a Twin Impinger. A “Twin Impinger” as used herein includes a cascade impactor comprising two sample collection stages. A schematic diagram of an exemplary Twin Impinger is shown in  FIG. 7 . The Twin Impinger  70  comprises a first stage  71  which is configured to collect aerosol particles that have a particle size such that they would impact on the oropharynx of the user inhaling the aerosol. The Twin Impinger  70  also comprises a second stage  72  which is configured to collect aerosol particles that would penetrate the lungs of a user inhaling the aerosol. 
     In some embodiments, the cut size diameter of the first stage  71  is 6.4 μm. Particles smaller than 6.4 μm may pass through to the second stage  72 . 
     In some embodiments, the outlet end  73  of the Twin Impinger is configured to be attached to a vacuum pump such that the aerosol is drawn through the Twin Impinger. 
     In accordance with the present invention, at least one sample collection stage of the cascade impactor is coated with isopropyl alcohol. 
     “Isopropyl alcohol” as used herein is a compound with the chemical formula C 3 H 8 O and the following chemical structure: 
     
       
         
         
             
             
         
       
     
     Isopropyl alcohol may also be referred to as isopropanol or 2-propanol. Isopropyl alcohol is a colourless, flammable chemical compound with a strong odour, and has a wide variety of industrial and household uses. 
     In accordance with the present invention, at least one sample collection stage in the cascade impactor is coated with isopropyl alcohol. In some embodiments where there is more than one sample collection stage in the cascade impactor, at least two sample collection stages are coated with isopropyl alcohol, such as at least three sample collection stages are coated with isopropyl alcohol, such as at least four sample collection stages are coated with isopropyl alcohol, such as at least five sample collection stages are coated with isopropyl alcohol, such as at least six sample collection stages are coated with isopropyl alcohol, such as at least seven sample collection stages are coated with isopropyl alcohol. 
     In some embodiments, each of the sample collection stages in the cascade impactor is coated with isopropyl alcohol. 
     In some embodiments, the cascade impactor is an NGI. In some embodiments, the NGI comprises seven sample collection stages. In some embodiments, the NGI comprises seven sample collection stages and an MOC. In some embodiments, at least one of the sample collection stages in the NGI is coated with isopropyl alcohol. In some embodiments, at least two of the sample collection stages in the NGI are coated with isopropyl alcohol. In some embodiments, at least three of the sample collection stages in the NGI are coated with isopropyl alcohol. In some embodiments, at least four of the sample collection stages in the NGI are coated with isopropyl alcohol. In some embodiments, at least five of the sample collection stages in the NGI are coated with isopropyl alcohol. In some embodiments, at least six of the sample collection stages in the NGI are coated with isopropyl alcohol. In some embodiments, all seven of the sample collection stages in the NGI are coated with isopropyl alcohol. In some embodiments, the MOC is not coated with isopropyl alcohol. In some embodiments, the MOC is coated with isopropyl alcohol. 
     In some embodiments, the cascade impactor is an ACI. In some embodiments, the ACI comprises eight sample collection stages. In some embodiments, at least one of the sample collection stages in the ACI is coated with isopropyl alcohol. In some embodiments, at least two of the sample collection stages in the ACI are coated with isopropyl alcohol. In some embodiments, at least three of the sample collection stages in the ACI are coated with isopropyl alcohol. In some embodiments, at least four of the sample collection stages in the ACI are coated with isopropyl alcohol. In some embodiments, at least five of the sample collection stages in the ACI are coated with isopropyl alcohol. In some embodiments, at least six of the sample collection stages in the ACI are coated with isopropyl alcohol. In some embodiments, at least seven of the sample collection stages in the ACI are coated with isopropyl alcohol. In some embodiments, all eight of the sample collection stages in the ACI are coated with isopropyl alcohol. 
     In some embodiments, the isopropyl alcohol is present in an amount of no greater than 5 mL in each of the sample collection stages that are coated with isopropyl alcohol, such as in an amount of no greater than 4 mL in each of the sample collection stages that are coated with isopropyl alcohol, such as in an amount of no greater than 3 mL in each of the sample collection stages that are coated with isopropyl alcohol. 
     In some embodiments, the isopropyl alcohol is present in liquid form. In some embodiments, the isopropyl alcohol is present in solution. In some embodiments, the isopropyl alcohol is present in solution with a liquid organic compound. In some embodiments, the isopropyl alcohol is present in solution with a liquid hydrocarbon alkane compound. In some embodiments, the isopropyl alcohol is present in solution with a liquid C 5 -C 17  alkyl compound. In some embodiments, the isopropyl alcohol is present in solution with heptadecane. 
     In some embodiments, each of the sample collection stages that is coated with isopropyl alcohol is coated with a solution of a liquid organic compound in isopropyl alcohol. In some embodiments, each of the sample collection stages that is coated with isopropyl alcohol is coated with a solution of a liquid hydrocarbon alkane compound in isopropyl alcohol. In some embodiments, each of the sample collection stages that is coated with isopropyl alcohol is coated with a solution of a liquid organic compound in isopropyl alcohol. In some embodiments, each of the sample collection stages that is coated with isopropyl alcohol is coated with a solution of a liquid C 5 -C 17  alkyl compound in isopropyl alcohol. In some embodiments, each of the sample collection stages that is coated with isopropyl alcohol is coated with a solution of heptadecane in isopropyl alcohol. 
     In some embodiments, each of the sample collection stages that are coated with isopropyl alcohol is coated with a solution of heptadecane in isopropyl alcohol. In some embodiments, the concentration of heptadecane in isopropyl alcohol is from 10 μg/mL to 50 μg/mL, such as from 10 μg/mL to 40 μg/mL, such as from 15 μg/mL to 40 μg/mL, such as from 15 μg/mL to 30 μg/mL, such as from 15 μg/mL to 25 μg/mL, such as about 20 μg/mL. 
     The use of small amounts of isopropyl alcohol in the sample collection stages may be advantageous since such small amounts do not significantly reduce the distance between the nozzle assembly and the collection means in the sample collection stage. Therefore, the presence of the isopropyl alcohol coating may not significantly affect the impaction properties of the aerosol particles on the collection means. 
     An advantage of the present invention is that the extent of evaporation of the nicotine collected on the sample collection stage(s) may be reduced such that the collection efficiency and measurement accuracy of the impactor may be increased. Nicotine is a readily volatile chemical compound, which has a vapour pressure of 5.5 Pa at 25° C. The inventors thus found that, when using a cascade impactor to measure the particle size distribution of an aerosol comprising nicotine, the degree of accuracy and the collection efficiency was lower than for other aerosol substances. Without wishing to be bound, this may be attributed to the nicotine impacting on the sample collection stage(s) being evaporated and subsequently being lost into the vacuum pump at the end of the impactor, or being deposited at a collection stage further down the impactor (i.e. a sample collection stage with a smaller cut size diameter than the D50 particle size of the nicotine particle). 
     The use of the isopropyl alcohol in the present invention advantageously decreases the extent of evaporation of the nicotine from the sample collection stages, and thus increases the proportion of nicotine particles which remain impacted on the sample collection stage having the appropriate cut size for the size of the nicotine particles. This results in a higher degree of accuracy of the cascade impactor when measuring the particle size distribution of an aerosol comprising nicotine. 
     Therefore, in one embodiment, there is provided use of isopropyl alcohol for reducing the extent of evaporation of nicotine during use of a cascade impactor for measuring the particle size distribution of an aerosol comprising nicotine. 
     In some embodiments, at least one sample collection stage of the cascade impactor is coated with glycerin, silicone oil, Tween 80 or a mixture thereof. 
     The coating of the sample collection stages with any one or more of these components may advantageously reduce the bounce of the particles off the collection means of the sample collection stage, thus increasing the collection efficiency of the cascade impactor. 
     In some embodiments where there is more than one sample collection stage in the cascade impactor, at least two sample collection stages are coated with glycerin, silicone oil, Tween 80 or a mixture thereof, such as at least three sample collection stages are coated with glycerin, silicone oil, Tween 80 or a mixture thereof, such as at least four sample collection stages are coated with glycerin, silicone oil, Tween 80 or a mixture thereof, such as at least five sample collection stages are coated with glycerin, silicone oil, Tween 80 or a mixture thereof, such as at least six sample collection stages are coated with glycerin, silicone oil, Tween 80 or a mixture thereof, such as at least seven sample collection stages are coated with glycerin, silicone oil, Tween 80 or a mixture thereof. 
     In some embodiments, each of the sample collection stages present in the cascade impactor are coated with glycerin, silicone oil, Tween 80 or a mixture thereof. 
     In some embodiments, the cascade impactor is an NGI. In some embodiments, the NGI comprises seven sample collection stages. In some embodiments, the NGI comprises seven sample collection stages and an MOC. In some embodiments, at least one of the sample collection stages in the NGI is coated with glycerin, silicone oil, Tween 80 or a mixture thereof. In some embodiments, at least two of the sample collection stages in the NGI are coated with glycerin, silicone oil, Tween 80 or a mixture thereof. In some embodiments, at least three of the sample collection stages in the NGI are coated with glycerin, silicone oil, Tween 80 or a mixture thereof. In some embodiments, at least four of the sample collection stages in the NGI are coated with glycerin, silicone oil, Tween 80 or a mixture thereof. In some embodiments, at least five of the sample collection stages in the NGI are coated with glycerin, silicone oil, Tween 80 or a mixture thereof. In some embodiments, at least six of the sample collection stages in the NGI are coated with glycerin, silicone oil, Tween 80 or a mixture thereof. In some embodiments, all seven of the sample collection stages in the NGI are coated with glycerin, silicone oil, Tween 80 or a mixture thereof. In some embodiments, the MOC is not coated with glycerin, silicone oil, Tween 80 or a mixture thereof. In some embodiments, the MOC is coated with glycerin, silicone oil, Tween 80 or a mixture thereof. 
     The present invention also provides a method for measuring the particle size distribution of an aerosol comprising nicotine, the method comprising: 
     (a) attaching an inhaler for generating the aerosol comprising nicotine to a cascade impactor, wherein the cascade impactor comprises:
         an induction port for receiving an inhaler; and   at least one sample collection stage each comprising a nozzle assembly and a collection means;
 
wherein the collection means of at least one sample collection stage is coated with isopropyl alcohol; and
       

     (b) dispensing an aerosol comprising nicotine from the inhaler into the cascade impactor. 
     In some embodiments the method further comprises the steps of: 
     (c) removing the sample collection stages from the cascade impactor; and 
     (d) analysing the sample collected on each sample collection stage to determine the particle size distribution of said sample. 
     In some embodiments, the analysing step (d) comprises a chromatographic method. In some embodiments, the analysing step (d) comprises high pressure liquid chromatography of the sample collected on each sample collection stage. 
     In some embodiments, the inhaler is attached to cascade impactor by inserting the inhaler into the induction port of the cascade impactor. The inhaler may subsequently be actuated to dispense the aerosol comprising nicotine into the cascade impactor, where the particle size distribution of the aerosol comprising nicotine may then be measured. 
     EXAMPLES 
     The invention will now be described with reference to the following non-limiting example. 
     A cascade impactor in accordance with the present invention was set up in order to determine the particle size distribution of an aerosol comprising nicotine. 
     The apparatus was set up as detailed below. The experiment was conducted using a Copley Scientific NGI, as described in detail above, and as shown in  FIGS. 4A and 4B . An isopropyl alcohol solution was added to the sample collection stages of the NGI, and an aerosol comprising nicotine introduced into the device. Subsequently, the samples collected on each stage in the NGI were collected and analysed using gas chromatography. 
     Diluent Preparation 
     A 1 mg/mL stock solution of heptadecane in IPA was prepared. 100 mg heptadecane was weighed into a 100 mL volumetric flask. The volumetric flask was made up to volume using IPA. 
     A 1 L solution of heptadecane in IPA (20 μg/mL) was prepared. 20 mL heptadecane was diluted in IPA (1 mg/mL) in a volumetric flask, and then made up to volume using IPA. This solution was stable for 1 month. 
     Preparation of Nicotine Stock Solutions 
     100 mg (±1 mg) nicotine was weighed and transferred to a 200 mL volumetric flask, which was made up to volume using IPA. This stock solution was prepared in duplicate. 
     Flow Adapter Preparation 
     A Dose Unit Sampling Apparatus (DUSA) was assembled as per the following steps, and the airflow through the DUSA adjusted to 30 L/min:
         i) A vacuum pump was connected to a Copley TPK (a critical flow controller valve that has a timed solenoid valve). The other side of the TPK was connected to the filter holder of a DUSA. With the filter holder clamped in a retort stand in the vertical position, a Type NE Glass Fibre filter (Pall Corp) was placed on the filter support, and a DUSA tube attached to hold the filter in place. The DUA assembly was rotated so that the DUSA tube was in the horizontal orientation.   ii) An unused inhaler device was inserted into the mouthpiece of a Flow Adapter, and the needle valve was fully closed.   iii) The Flow Adapter was attached to the DUSA using the associated sleeve of the Flow Adapter, and the air flow was started using the TPK.   iv) A flow meter (Copley Scientific model DFM3) was attached to the orifice of the Flow Adapter, and the needle valve slowly opened until the flow reached the desired value of 23 L/min (where 23 L/mine equates to the desired flow rate into the NGI of 30 L/min, minus the desired flow rate through the inhaler device of 7 L/min.       

     NGI Preparation 
     
         
         
           
             i) The NGI was assembled and an induction port inserted into the inlet of the NGI. A flow meter was attached to the induction port using a suitable adapter (e.g. Oxette adapter). 
             ii) The air pump was turned on for 5 minutes to allow the apparatus to warm up prior to testing. 
             iii) The air flow was started using the TPK, and the flow rate adjusted to 30 L/min (±0.2 L/min). 
             iv) The air flow was stopped, and the flow meter and induction port removed. 
             v) The solution of heptadecane in IPA (as formulated under “Diluent Preparation”) was added to the cups in quantities as provided in Table 1. 
             vi) The prepared Flow Adapter was attached to the induction port, ensuring that the throat and Flow Adapter surfaces were flush. 
           
         
       
    
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Stage 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 MOC 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Volume 
                 3.0 
                 1.5 
                 1.5 
                 1.5 
                 1.5 
                 1.5 
                 1.0 
                 0.0 
               
               
                 (mL) 
               
               
                   
               
            
           
         
       
     
     Device Actuation
         i) The inhaler device was filled with aerosol formulation.   ii) The inhaler device was attached to the Flow Adapter.   iii) The air flow was started using the TPK.   iv) The device was removed from the Flow Adapter and weighed.   v) The above steps were repeated so that 2 doses were fired into the NGI.       

     Sample Recovery 
     Each section of the NGI was disassembled and washed with a defined volume of diluent (shown in Table 2) to recover the nicotine. A disposable syringe was used to draw sample from the wash volume. The solution was put into vials and analysed using Gas Chromatography with Flame Ionisation Detector (GC-FID). 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Stage 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Oxette 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 adapter 
                 Throat 
                 Stage 1 
                 Stage 2 
                 Stage 3 
                 Stage 4 
                 Stage 5 
                 Stage 6 
                 Stage 7 
                 MOC 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Wash 
                 10 
                 10 
                 7 
                 9 
                 9 
                 9 
                 9 
                 9 
                 9 
                 10 
               
               
                 volume 
               
               
                 (mL) 
               
               
                   
               
            
           
         
       
     
     The various embodiments described herein are presented only to assist in understanding and teaching the claimed features. These embodiments are provided as a representative sample of embodiments only, and are not exhaustive and/or exclusive. It is to be understood that advantages, embodiments, examples, functions, features, structures and/or other aspects described herein are not to be considered limitations on the scope of the invention as defined by the claims or limitations on equivalents to the claims, and that other embodiments may be utilised and modifications may be made without departing from the scope of the claimed invention. Various embodiments of the invention may suitably comprise, consist of, or consist essentially of appropriate combinations of the disclosed elements, components, features, parts, steps, means, etc., other than those specifically described herein. In addition, this disclosure may include other inventions not presently claimed, but which may be claimed in the future.