EP1320740A1

EP1320740A1 - Impactor

Info

Publication number: EP1320740A1
Application number: EP01967366A
Authority: EP
Inventors: Ari Ukkonen; Risto Luoma
Original assignee: Dekati Oy
Current assignee: Dekati Oy
Priority date: 2000-09-08
Filing date: 2001-09-07
Publication date: 2003-06-25
Also published as: WO2002021104A1; AU2001287754A1; FI113407B; FI20001992A; FI20001992A0

Abstract

The invention relates to an impactor (20) comprising one or more successive stages (4; 3; 2; 1), each stage (4; 3; 2; 1) comprising at least a nozzle part (24a; 23a; 22a; 21a), a collection substrate (24b; 23b; 22b; 21b) arranged behind said nozzle part in the direction of a gas flow (21), and a collecting film (24c; 23c; 22c; 21c) arranged on top of said collection substrate in the flow direction, onto which collecting film (24c; 23c; 22c; 21c) are deposited, in a size selective manner, aerosol particles contained in the gas flow (21) passing through the impactor (20). According to the invention, the surface of said nozzle part (24a; 23a; 22a; 21a) facing the next collection substrate (24b; 23b; 22b; 21b) in the flow direction comprises one or more protruding squeeze means (24d; 23d; 22d; 21d; 24e; 23e; 22e; 21e) to keep the collecting film (24c; 23c; 22c; 21c) in its place on the collection substrate (24b; 23b; 22b; 21b).

Description

IMPACTOR

The invention relates to a device for the size-fractionated measurement of aerosol particles as presented in the preamble of the appended claim 1.

As environmental regulations are becoming stricter, there is an increasing need to measure various particle emissions. The need for measuring is present, for example, in the research and development of various combustion processes, such as combustion engines, as well as in the research and development of methods for decontamination of flue gases and exhaust gases. In addition, particle emissions are measured to control emissions from existing plants etc. Furthermore, particle emissions are measured in connection with, for example, the type approval of vehicles.

In this context, aerosol particles refer to particles whose properties, substantially the small size and weight of the particles, make it possible that the particles are carried with an air or gas flow. The typical size of aerosol particles varies from 10 nm to 10 μm, but depending on the conditions and the process, also larger or smaller particles behaving like aerosol particles may occur. So-called fine particle emissions normally refer to emissions which particularly contain aerosol particles.

It is known that the harmful effect of, for example, dust and exhaust gas emissions caused by traffic on health depends on the size distribution of the aerosol particles contained in said emissions. When inhaled, the smallest particles find their way more easily deep into the lungs, adhere to the tissues and are thus more likely to remain permanently in the body than larger particles. During the process of formation of the aerosol particles, various substances may end up in different particle size fractions, wherein particles of different sizes may have different effects on health, due to their varying composition and thereby varying toxicity.

It is also known that, for example, in the development and research of combustion processes, the measurement of the properties of aerosol particles contained in the flue gases in said process at different parts of the process yields important information about the operation and status of the process. In this respect, it is interesting to be able to measure not only the total quantity or mass of the particles but also the particle size distribution, as well as to be able to determine e.g. the composition of particles of different sizes. This information is helpful in the optima- tion of the combustion process and its operation parameters to achieve a better efficiency, as well as to reduce the emissions from the process.

Measurements involving the size distribution of aerosol particles in connection with processes of filtering and/or decontamination of flue gases yield important information about e.g. the filtering or decontamination capacity of the process. Particle measurements can be used to find out e.g. failure situations in which the filtering or decontamination capacity of the process is decreased. By means of particle size information obtained by the measurements, it is possible to select the filtering and decontamination method which is most suitable for the particles in a given target and process.

The need to measure aerosol particles is also present in other processes than those mentioned above, primarily in processes producing harmful particle emissions. For example, methods for measuring aerosol particles are required by the pharmaceutical industry in its produc- tion and quality control, as well as by the material technical industry which produces pulverulent or powdered materials.

As obvious from the few examples presented above, there is a distinct need to be able to measure aerosol particles in such a way that it is possible to obtain information not only about the total quantity or mass of the particles but also about the size distribution of the aerosol particles, and further to be able to perform more accurate analyses of particles belonging to different size fractions, such as to determine the chemical compositions of the particles.

Consequently, to implement this, a device is needed which, firstly, separates aerosol particles from an air or gas flow according to their size and, secondly, collects particles belonging to different size fractions for analyses to be made on the particles later.

From the prior art, a measuring device which is called an impactor is generally known for this purpose. The impactor collects particles from a gas flow, sorting the particles into different size fractions on the basis of their so-called aerodynamic size. An impactor according to prior art is presented, for example, in US patent 4,327,594. The following is a brief description on the operating principle of the impactor with reference to Fig. 1.

A gas carrying particles from a front chamber 12 flows through orifices contained in a nozzle part 4a to the next chamber 4, which chamber4 is provided with a collection substrate 4b behind the nozzle part 4a in the direction of the gas flow. The flow direction of the gas flow 11 through the orifices of the nozzle part 4a is abruptly changed when it impacts upon the collection substrate 4b. The particles entrained by the flow 11 and having a sufficiently large aerodynamic particle size cannot follow the abrupt change in the direction of the flow 11 , but they impact upon the collection substrate 4b, being deposited on the same. Those aerosol particles whose aerodynamic particle size is such that they can be re-entrained by the flow 11 will bypass the collection substrate 4b and pass through the orifices of the next nozzle part 3a into the next chamber 3.

The impactor shown in Fig. 1 is a so-called cascade impactor which contains several successive chambers, so-called stages. Each stage 4; 3; 2; 1 has a nozzle part 4a; 3a; 2a; 1 a as well as a collection substrate 4b; 3b; 2b; 1 b placed behind said nozzle part. This structure of a cas- cade impactor based on successive stages 4; 3; 2; 1 makes it possible to collect aerosol particles in a size-fractionated way. By selecting, in a known manner, the number and size of orifices in each nozzle part 4a; 3a; 2a; 1a, the distance between the surface of the collection substrate 4b; 3b; 2b; 2b facing the flow direction and the nozzle part 4a; 3a; 2a; 1a of the same degree, as well as the volume flow of the gas passing through the impactor 10, each stage 4; 3; 2; 1 of the impactor can be dimensioned so that only aerosol particles larger than a given particle size are deposited on the collection substrate 4b; 3b; 2b; 1b at each stage. In other words, when the flow 11 meets the collection substrate 4b; 3b; 2b; 1 b, the local flow rate of the gas is raised when proceeding from one stage 4; 3; 2; 1 to another in the flow direction, and/or in the flow 11 , a more abrupt change in the flow direction is caused at each stage 4; 3; 2; 1 than at the preceding stage. Thus, only the aerosol particles with the smallest aerodynamic size will reach the last stage 1. Before the gas flow 11 exits the impactor 10, after the last stage 1 , the gas flow can still be led through a filter 5, if necessary, wherein the rest of the particles contained in the gas flow are deposited in the filter 5. By dimensioning the successive stages 4; 3; 2; 1 of the impactor in a suitable, known manner in relation to each other, it is possible to deposit only particles belonging to desired size fractions at each stage.

The cut diameter of the impactor stage is defined as a value of the particle diameter, the stage collecting 50 % of the particles with said diameter. Particles which are larger than this are collected at said stage with a probability which increases very highly with the particle size and, in a corresponding manner, smaller particles are re-entrained by the flow to the next stage.

The operation of the impactor, the dimensioning of the successive stages in the cascade impactor, as well as the closer determination of the aerodynamic size of aerosol particles are known as such for any- one skilled in the art and do not directly relate to the invention; therefore, they will not be further discussed in this context.

The present invention relates to the structure of the impactor, particularly the solutions used in the fixing of the collection substrate 4b; 3b; 2b; 1b, which, in prior art impactors, make the use of the impactors unnecessarily complex and thereby increase the number of misuse failures and also reduce the accuracy and reliability of the measurement results.

Typically, the collection substrate 4b; 3b; 2b; 1 b used in impactors is a separate thin collecting film placed on a back plate, or the like, used as a mechanical support. Depending on the application, the collecting film can be, for example, an aluminium foil or polycarbonate film. Due to the small size of the aerosol particles, and particularly when measuring relatively pure gases, the total quantity and mass of the aerosol particles deposited on the collecting film are very small. Typically, the change in the mass of the collecting film due to the aerosol particles deposited on the film during the measurement ranges from one milligram to a few milligrams. It is thus important that the inherent mass of the film used as the collection substrate is as small as possible, so that the mass of the particles deposited on the film can be determined by gravimetric analysis as accurately as possible. The accuracy of scales used in the gravimetric analysis typically makes it possible to detect weight differences, at the smallest, in the order of about 10 micro- grams.

The use of a discrete collecting film as the collection substrate 4b; 3b; 2b; 1 b makes it possible to transfer the samples deposited at the different stages 4; 3; 2; 1 of the impactor to further analyses by removing said films from the impactor. Typically, this is performed carefully by using tweezers or the like, wherein the collecting film is not contami- nated or damaged during the handling and the quantity of particles deposited on the film is thus not changed during the removal operation. For transfer and storage, the films intended to be analysed later are properly protected. By replacing the used collecting films with new, clean films in the impactor, the impactor can be quickly made ready for use again, if necessary.

To obtain a reliable measurement result, the collecting film used as the collection substrate 4b; 3b; 2b; 1b, for example an aluminium foil with a thickness of 0.4 mm and a diameter of 50 to 100 mm, must be fixed sufficiently firmly in its position so that the distance between said film and the nozzle part 4a; 3a; 2a; 1b preceding it in the direction of the gas flow remains precisely constant. The attachment must also be sufficiently rigid so that oscillation caused by the gas flow striking the film and/or by the movement of the impactor or the movement of the film will not cause detachment of the aerosol particles deposited on the film from the collecting film. In prior art impactors, the attachment of the collecting films onto the back plates or the like, used as a mechanical support for the films, is typically performed in the following ways.

In a commonly used method, the collecting film is pressed at its edges to the back plate supporting the same, by means of a separate spacer ring. Said spacer ring can itself be supported in the flow direction to the preceding nozzle part, wherein the height of the spacer ring is also used to determine the distance between the collecting film and said nozzle part. The main problem in this attachment method is that a separate component is required for the attachment: the spacer ring. Impactors are typically used for measuring under field and/or industrial conditions. Thus, the impactor is opened for the change of collecting films under conditions which are not the best possible with respect to the work. When said spacer ring is lifted off from the top of the collect- ing film, it has been found in practice that the user often accidentally drops the spacer ring onto the collecting film and/or touches the collecting film with the spacer ring or with the tool used for lifting it, wherein the particle sample deposited on the collecting film is damaged. In many commercial cascade impactors equipped with spacer rings, the impactor can also be assembled in a wrong way so that the spacer rings of the different stages 4; 3; 2; 1 are exchanged, wherein the distances between the collecting films and the respective nozzle parts are incorrect. In this case, the impactor will not operate in the intended way, and the mistake can only be detected when the impactor is opened the next time.

Another method is also known for attaching the collecting film, wherein the back plate used as the mechanical support for the collecting film is equipped with a recess, to which the collecting film, made of for exam- pie aluminium foil, is attached by pressing the collecting film, whose diameter is slightly larger than the diameter of said recess, firmly in its place. Thus, the collecting film remains in its place in the recess, thanks to the tight adjustment of said diameters. This attachment method has the problem that a separate tool is required for pressing the collecting film in its place. Because of the tight fitting, the detachment of the collecting film from the recess with tweezers or the like is difficult and will easily result in damage to the thin collecting film and/or the sample. A collecting film which is attached too loosely may also be detached by itself from the recess during the measurement.

Furthermore, the collecting film can be attached to the recess in the back plate or the like by means of a locking ring, or the like, pressing the edges of the collecting film. Separate fixing means are also known to be attached to the pack plate by means of threads or the like, whereby the collecting film is pressed at its edges against the back plate. Also in these attachment methods, separate components and/or tools are required for the attachment, wherein the collecting film and/or the sample deposited on the collecting film may be damaged during the placement of the components in their place or during their removal.

Typically, adhesive, tacky and/or oily substances cannot be used to attach collecting films to a back plate or another corresponding substrate, because these substances disturb the gravimetric analysis of the collecting film. In other words, when the collecting film is detached from its substrate, the quantity of the adhesive or other substance used in the attachment and remaining on the collecting film will vary inci- dentally, wherein it is not possible to determine the mass of the particles deposited on the film by comparison between the mass of the film used for the measurement and the mass of the same clean film, with sufficient precision.

It is the primary aim of the present invention to provide an impactor in with a structure and, in particular, a solution used for attaching the collecting film used as a collection substrate for aerosol particles, to avoid the above-presented problems of impactors according to prior art. The aim of the invention is thus to provide an impactor having a simple structure and being easy to use, whose good properties at use also make it possible to improve the reliability and precision of the measurement results, especially in the case of gravimetric measurements and analysis.

To attain this purpose, the impactor according to the invention is primarily characterized in what will be presented in the characterizing part of the independent claim 1. The other dependent claims present some preferred embodiments of the invention.

The invention is based on the idea that the separate collecting film is kept in its place on top of the collection substrate supporting the same by means of one or more protruding squeeze means designed in the nozzle part preceding the collecting film and the collection substrate in the direction of the gas flow. In other words, when said stage of the impactor is assembled, the collecting film does not need to be separately attached to its collection substrate, but when the corresponding nozzle part is lifted into its position on top of the collecting film and its collection substrate, the squeeze means at the bottom of the nozzle part press the collecting film tightly against the collection substrate, thereby keeping it tightly in its position during the measurement. Thus, the use of separate attachment components as well as the work required by their handling are totally avoided in the attachment of the collecting film. In a corresponding manner, when said stage of the impactor is opened by lifting the nozzle part from its place, the collect- ing film is ready to be removed from its base without other operations. This will significantly reduce the risk of damaging the collecting film and the sample deposited on the film, when compared with the solutions of prior art.

In an advantageous embodiment of the invention, the nozzle part is provided with designed squeeze means which are arranged at regular intervals on a circumference, whose diameter substantially corresponds to the diameter of the circular collecting film. In this arrangement, the squeeze means interfere as little as possible with the flow coming from the nozzle part towards the surface of the collecting film.

In another advantageous embodiment of the invention, the nozzle part is provided with a designed squeeze means which is arranged to press the collecting film in its centre. In this way, it is possible to efficiently prevent e.g. the buckling of the collecting film and/or the oscillation of the collecting film caused by the flow impacting on the film. This is important particularly in situations in which a collecting film with a wide surface area is used, and/or in which the material of the collecting film causes a tendency of said phenomena to occur. Said squeeze means placed in the centre can be used together with said squeeze means placed on the outer circumference, or also as the only squeeze means. The squeeze means placed in the centre is also advantageous because it does not shade any area of the collecting film from the flow coming from the nozzle part.

In an advantageous embodiment of the invention, the impactor being a cascade impactor comprising several successive stages, the parts contained in the different stages are further designed in such a way that the impactor cannot be wrongly assembled. In other words, the nozzle part at each stage of the impactor is, with respect to its mechanical contact surfaces, designed to match with only one correct preceding and next impactor component in the order.

Consequently, as a whole, the impactor according to the invention contains fewer parts, and particularly small-size parts which are difficult to handle, than a corresponding impactor of prior art, and it is therefore less expensive to manufacture. Upon using, i.e. assembling and disassembling, the impactor, the smaller number of parts will naturally make the impactor easier and faster to use. The smaller number of parts will also facilitate the cleaning of the impactor and reduce the non-desired risk of contamination involved in small parts. These above- mentioned advantages of the impactor according to the invention are significant particularly in such applications and targets in which measurements are taken continually. Because the impactor of the invention cannot be assembled in a wrong way, measurement failures due to said human error are totally avoided.

In the impactor of the invention, the attachment/detachment of the collecting film is performed with very simple measures, and the movable parts have further such sizes and shapes that it is easy to grip them firmly without using separate tools or other auxiliary means. Thus, par- ticularly when working under field conditions, the probability of damage or contamination of the collecting film itself or the aerosol particle sample contained in it is clearly reduced when compared with impac- tors of prior art. This is a very important advantage when the aerosol particle samples are to be analysed using gravimetric methods.

The following, more detailed description of the invention will more clearly illustrate, for anyone skilled in the art, advantageous embodiments of the invention as well as advantages to be achieved with the invention in relation to prior art.

In the following, the invention will be described in more detail with reference to the appended drawings, in which

Fig. 1 illustrates the known operating principle of an impactor in a cross-sectional view,

Fig. 2 illustrates an advantageous embodiment of the impactor according to the invention in a cross-sectional view,

Fig. 3a illustrates the structure of the nozzle part in the impactor of Fig. 2 in a cross-sectional view,

Fig. 3b shows the nozzle part of Fig. 3a in an end view,

Fig. 4a illustrates an alternative implementation of the nozzle part in the impactor of Fig. 2 in a cross-sectional view,

Fig. 4b shows the nozzle part of Fig. 4a in an end view,

Fig. 5a illustrates the structure of a collection substrate in the impactor of Fig. 2 in a cross-sectional view, and

Fig. 5b shows the collection substrate of Fig. 5a in an end view.

Figure 2 illustrates an advantageous embodiment of an impactor 20 according to the invention, comprising four successive stages. In Fig. 2, a gas flow 21 is shown to enter the impactor 20 from above and to exit the impactor 20 from below. The impactor 20 sorts aerosol particles in size fractions on the basis of their aerodynamic size by the same known basic physical principle as has already been described above in connection with Fig. 1. Consequently, there is no need to present said basic principle again herein below, but the advantages of the structure of the impactor 20 according to the invention will be presented by primarily describing the operations related to the assembly and disassembly of the impactor 20. The reference numbers of the stages of the impactor 20 correspond to the number of the stages of the impactor 10 shown in Fig. 1 ; i.e., the stages are numbered 4, 3, 2 and 1 in the direction of the flow.

In the housing of the impactor 20, the lower part 25 and the upper part 26 are attached to each other during the use by means of a band or a corresponding fixing means which presses the lower part 25 and the upper part 26 to each other in a gas tight manner. When said fixing means is detached, the upper part 26 of the housing can be removed from its position on top of the lower part 25. With the upper part 26, the nozzle part 24a of the fourth stage is removed, which is fixed to the upper part 26 by means of screws or the like.

The removal of the upper part 26 will expose the collection substrate 24b of the fourth stage as well as the collecting film 24c on top of the same. When the upper part 26 is in its place, the collecting film 24c is kept in place, according to the invention, by means of protruding squeeze means 24d and/or a squeeze means 24e designed on the lower surface of the nozzle part 24a. Thus, to remove the collecting film 24c, it is only necessary to lift up the upper part 26 and the nozzle part 24a from their place, after which the collecting film 24c can be simply removed by tweezers or the like in a suitable way. It is easy to lift up the upper part 26 manually, because it is easy to grip said upper part firmly, thanks to its size and design.

To expose the collection substrate 23c of the next stage, the nozzle part 23a is removed from its place, wherein the collection substrate 24b of the preceding stage is also lifted up. Thanks to the size of the nozzle part 23a, it can be firmly gripped manually, wherein it is not likely to be dropped onto the other components of the impactor. The removal of the nozzle part 23a will expose the collecting film 23c.

The disassembly of the impactor 20 is continued in a corresponding manner by removing the nozzle part 22a with the collection substrate 23b, and further the nozzle part 21 a with the collection substrate 22b. The last collection substrate 21b of the first stage is removed with a holder 23 which keeps a filter 24 in its place.

The assembly of the impactor is performed in the reverse order. To prevent the wrong assembly of the impactor, it is, at first, only possible to fit the holder 23 in the empty lower part 25 of the housing, when said holder is in the correct position. Thus, the design of the mechanical contact surface of the holder 23 matches with the design of the corre- sponding contact surface of the lower part 24.

Further, only the nozzle part 21 a, installed in the correct position, fits on top of the holder 23; only the nozzle part 22a, installed in the correct position, fits on top of the nozzle part 21 ; and so on, until all the stages of the impactor 20 are assembled. The collection substrates 24b; 23b; 22b; 21 b are preferably all equal, wherein they can be installed at any stage of the impactor. If a recess or the like is used for the collecting film 24c; 23c; 22c; 21c in the collection substrates 24b; 23b; 22b; 21 , they must be installed in the correct position in the respective nozzle parts 24a; 23a; 22a; 21a; in other words, the surface with the recess or the like being against the flow direction, or else the mounting fault will be detected at the latest during an attempt to close the housing of the impactor 20, wherein the lower part 25 and the upper part 26 of the impactor housing cannot be pressed tightly against each other, because the joint height of the parts remaining inside is too great.

Figures 3a and 3b illustrate the structure of the nozzle part 21 a of the first stage of the impactor according to Fig. 2. The cross-sectional view of Fig. 3a corresponds to the section A-A of Fig. 3b.

The nozzle part 21a comprises a sealing means 33, such as an O-ring sealing groove and a corresponding sealing to seal the nozzle part 21 with the preceding nozzle part 22a in the flow direction. The nozzle part 21a is sealed in a corresponding manner with the next component in the flow direction, in this case with the holder 23. Those mechanical contact surfaces of the nozzle part 21 a, by which said nozzle part is connected to the preceding and the next components of the impactor 20 in the flow direction, are designed so that the nozzle part 21a and the preceding and next components of the impactor 20 can be placed tightly on top of each other in the correct order only. In the example case of Fig. 3a, the nozzle part 21a being a round plate-like piece, said mechanical contact surfaces are designed on the outer circumference of the nozzle part 21a, on the upper and lower surfaces of the plate-like nozzle part.

The protruding squeeze means 21 d designed on the lower surface of the nozzle part 21a press the collecting film 21c on top of the collection substrate 21 b tightly against the collection substrate 21b. The collection substrate 21 b is arranged to rest on top of the holder 23, supported in a point-like manner by flexible support means 31 , such as rubber pads or the like. For said support means, the holder 23 is provided with corresponding mounting cavities 32 which are, in Figs. 3a and 3b, shown in the structure of the nozzle part 21. The support means 31 , to be placed in the mounting cavities 32, are then, in a corresponding manner, intended to support the collection substrate 22b of the preceding stage. The purpose of these flexible support means 31 is to allow the thickness of the collecting films 24c; 23c; 22c; 21c to vary within given limits without disturbing the operation of the impactor 20. In other words, the support means 31 are used to secure that the collecting film 21c is always in contact with the squeeze means 21 d and thereby that the distance between the nozzle parts 21a and the col- lecting film 21c, determined by the dimensioning of the squeeze means 21 d, is as intended.

Figure 3b shows the placement of the three squeeze means 21 d designed on the lower surface of the nozzle part 21a at intervals of 120 degrees on a circumference whose diameter substantially corresponds to the diameter of the collecting film 21c. When the nozzle part 21 a is pressed onto the collection substrate 21b, the squeeze means 21 d will squeeze the collecting film 21c tightly against the collection substrate 21b. In connection with said attachment, the collecting film 21c is not subjected to any rotating or lateral movements but only the pressing force directly from above; the collecting film 21c is thus not subject to a risk of being creased or damaged in another way during the attachment.

Figures 4a and 4b illustrate an alternative implementation of the nozzle part 21a of the first stage of the impactor according to Fig. 2. The cross-sectional view of Fig. 4a corresponds to the section A-A of Fig. 4b. The nozzle part 21 of Figs. 4a and 4b is provided with, in addition to the squeeze means 21 d, a designed squeeze means 21 e which is arranged to squeeze the collecting film 21c in its centre. In this way, it is possible to efficiently prevent e.g. the buckling of the collecting film 21c and/or the oscillation of the collecting film caused by the flow striking the film. This is important particularly in situations in which a collecting film with a wide surface area is used, and/or in which the material of the collecting film causes a tendency of said phenomena to occur. The squeeze means 21 e placed in the centre is also advanta- geous because it does not shade any area of the collecting film 21c from the flow coming from the nozzle part 21a.

In the embodiments of the nozzle part 21a shown in Figs. 4a and 4b, the collecting film 21c is also squeezed by four squeeze means 21 d which are placed at intervals of 90 degrees on a circumference whose diameter substantially corresponds to the diameter of the collecting film 21c.

Furthermore, Figs. 5a and 5b illustrate the structure of the collection substrate 24b; 23b; 22b; 21 b of the impactor according to Fig. 2. The upper surface of the collection substrate 24b; 23b; 22b; 21 b against the flow direction is provided with a recess 52 for the collecting film 24c; 23c; 22c; 21c, such as an aluminium foil. The diameter of said recess 52 is selected so that the protruding squeeze means 24d; 23d; 22d; 21 d designed on the lower surface of the preceding nozzle part are placed within the area of said recess, close to its edge. In this way, the squeeze means 24d; 23d; 22d; 21 d squeeze the collecting film 24c; 23c; 22c; 21c closely against the bottom of said recess 52. The collection substrate 24b; 23b; 22b; 21b is provided with openings 51 for leading the flow to the next stage of the impactor. Preferably, upon assembling the impactor, the squeeze means 24d; 23d; 22d; 21 d of the collecting film 24c; 23c; 22c; 21c close to the edge are placed by necks 53 left between the openings 51 , wherein the squeeze means 24d; 23d; 22d; 21 d interfere with the gas flow as little as possible. With a variable number of squeeze means 24d; 23d; 22d; 21 d, the number of the openings 51 can also vary respectively, to make said placing possible.

The impactor presented in the examples and in the figures can be, for example, a cascade impactor comprising four stages whose 50 % cut diameters are 2.5 μm, 1.0 μm, 0.5 μm and 0.2 μm. It is naturally obvi- ous that the use of the invention is not limited solely to this type of cascade impactors nor to 4-stage impactors in general, but the embodiments of the invention may vary within the scope of the inventive characteristics of the claims to be presented hereinbelow.

The cross-section of the impactor perpendicular to the flow direction may have any suitable shape for the purpose, including a shape different from the circular shape shown in the example. In each application, the impactor may have the required number of successive stages, or also only one stage. The collecting film used as the collection substrate can be of any material suitable for the purpose. For gravimetric analysis it is important that the inherent mass of the collecting film is as small as possible, so that the mass of the particle sample deposited on the film can be determined as accurately as possible. The collecting film can be circular or for example annular, which annular shape is obtained when a circular/oval area is removed from the centre of a circular/oval collecting film. Further, the shape of the collecting film can also be any other shape suitable for the purpose, including angular shapes.

At different stages of the same impactor, it is possible to use collecting films made of different materials, if necessary. In the collection substrates, it is possible to use a recess intended for the collecting film according to the example, but it is also possible to use collection substrates without said recess. The flexible support of the collection substrate can be arranged, according to the examples, in a point-like manner with support means 31 , or also, for example, in such a way that the support means 31 is a uniform, flexible O-ring or the like, which is arranged to support and circle the collection substrate at its edges.

Furthermore, it will be obvious for anyone skilled in the art that the number of protruding squeeze means, designed on the lower surface of the nozzle part in the flow direction and holding the collecting film, can be different from that given in the example, which number may also vary in the nozzle parts of the different stages. If the collecting film is sufficiently rigid, it is also possible to use only one squeeze means 24e; 23e; 22e; 21 e placed in the centre of the collecting film to squeeze said collecting film against its collection substrate.

For anyone skilled in the art, it will also be obvious that the invention can be used both in vacuum-operated impactors, in which the flow is led into the impactor by means of a suction pump or the like after the impactor in the flow direction, and in impactors in which the flow led into the impactor is generated by the discharge of overpressurized gas through the impactor into a lower pressure.

Claims

Claims:

1. An impactor (20) comprising one or more successive stages (4; 3; 2; 1), each stage (4; 3; 2; 1) comprising at least a nozzle part (24a; 23a; 22a; 21a), a collection substrate (24b; 23b; 22b; 21b) arranged behind said nozzle part in the direction of a gas flow (21), and a collecting film (24c; 23c; 22c; 21c) arranged on top of said collection substrate in the flow direction, onto which collecting film (24c; 23c; 22c; 21c) are deposited, in a size-fractionated manner, aerosol particles contained in the gas flow (21) passing through the impactor (20), characterized in that the back surface of said nozzle part (24a; 23a; 22a; 21 a), i.e. the surface facing the next collection substrate (24b; 23b; 22b; 21 b) in the flow direction, comprises one or more protruding squeeze means (24d; 23d; 22d; 21 d; 24e; 23e; 22e; 21 e) to keep the collecting film (24c; 23c; 22c; 21 c) in its place on the collection substrate (24b; 23b; 22b; 21 b).

2. The impactor (20) according to claim 1 , characterized in that the collecting film (24c; 23c; 22c; 21c) is of lightweight design suitable for gravimetric analysis.

3. The impactor (20) according to claim 1 or 2, characterized in that the squeeze means (24d; 23d; 22d; 21 d) is arranged, on the surface of the nozzle part (24a; 23a; 22a; 21a), on a circumference whose diameter substantially corresponds to the diameter of the collecting film (24c; 23c; 22c; 21c).

4. The impactor (20) according to claim 3, characterized in that there are three squeeze means (24d; 23d; 22d; 21 d) and they are arranged on said circumference at intervals of 120 degrees with respect to each other.

5. The impactor (20) according to any of the preceding claims, characterized in that the nozzle part (24a; 23a; 22a; 21a) comprises only, or in addition to other squeeze means (24d; 23d; 22d; 21 d), a squeeze means (24e; 23e; 22e; 21 e) arranged in the centre of said nozzle part.

6. The impactor (20) according to any of the preceding claims, characterized in that the nozzle part/parts (24a; 23a; 22a; 21a) included in the impactor (20) are arranged to match, with respect to the design of their mechanical contact surfaces, only with the corresponding com- ponent of the directly preceding and/or next stage (4; 3; 2; 1) in such a way that the nozzle part/parts (24a; 23a; 22a; 21a) can be installed in only one position in their place between said preceding and/or next components.

7. The impactor (20) according to any of the preceding claims, characterized in that the impactor (20) comprises one/several flexible support means (31) to support the collection substrate (24b; 23b; 22b; 21b) in its place against the nozzle part (24a; 23a; 22a; 21a) preceding said collection substrate (24b; 23b; 22b; 21b) in the flow direction.

8. The impactor (20) according to any of the preceding claims, characterized in that said impactor (20) is a cascade impactor comprising four stages, whose 50 % cut diameters are 2.5 μm, 1.0 μm, 0.5 μm and 0.2 μm.