A filter assembly

Abstract

The invention provides a filter assembly for a continuous dust monitor, the filter assembly comprising: a filter body comprising an air outlet, the air outlet configured to removably couple with a tubular element of a tapered element oscillating microbalance (TEOM) in the continuous dust monitor; and a filter element mounted on the filter body, the filter element comprising a porous silicon-free filter membrane for capturing dust present in air flowing into the filter assembly, wherein the filter element is detachable from the filter body to allow direct-on-filter analysis of dust captured on the porous silicon-free filter membrane.

Description

[0001] A filter assembly

[0002] Technical Field

[0003]

[0001] The invention relates to a filter assembly for a continuous dust monitor. The filter element comprises a filter element comprising a porous silicon-free filter membrane, wherein the filter element is detachable from the filter body to allow direct- on-filter analysis of dust captured on the porous silicon-free filter membrane. The invention further relates to a kit, and to a method of dust analysis. The invention is particularly compatible with certain existing continuous personal dust monitor designs, and it will be convenient to disclose aspects of the invention in relation to these exemplary applications. However, it should be appreciated that the invention is not so limited and may be more generally applied with a range of continuous dust monitors comprising a tapered element oscillating microbalance.

[0004] Background of Invention

[0005] [2] Exposure to respirable crystalline silica (RCS) is well established as a risk factor for silicosis and lung cancer. In industrial workplaces where RCS is a hazard, such as coal mines, it is therefore critical to conduct respirable dust sampling to monitor the dose and composition of respirable dust in air breathed by workers. Historically, this has been performed via gravimetric sampling techniques.

[0006] [3] Conventional gravimetric personal samplers direct a known volume of air from within a worker’s breathing zone through a cyclone, which excludes larger, non- respirable particulates, and then through a filter containing a removable filter membrane to retain the respirable dust fraction. After a desired period of use, the filter membrane is removed from the sampler and the measured mass of retained dust is used to calculate the mass of respirable dust per unit volume of air. The dust retained on the filter membrane is then subjected to laboratory analysis to determine its crystalline silica (quartz) content.

[0007] [4] The polymeric filter membrane may first be ashed to reduce interference from organic species in the dust sample and the ash is then redeposited on a further filter membrane suitable as a substrate for Fourier transform infrared spectroscopy (FTIR) and / or X-ray diffraction (XRD) analytical methods. Industry-standard methods for gravimetric sampling and analysis have been established by regulatory bodies such as the National Institute for Occupational Safety and Health (NIOSH, a US government agency) which specifies Methods 7603 and 7500 for FTIR and XRD analysis of respirable crystalline silica in coal dust in its Manual of Analytical Methods.

[0008] [5] A significant shortcoming of conventional gravimetric sampling is that it cannot provide immediate feedback on dust exposure risk. For this reason, substantial resources have been invested to develop continuous dust monitoring devices which provide real-time monitoring of dust exposure. The most successful approaches use resonant mass monitoring techniques, such as a tapered element oscillating microbalance (TEOM), wherein the dust filter of the monitoring device is oscillated at its natural resonance frequency in the sampling device. As the mass of the filter increases due to dust capture, the resonance frequency of the oscillating filter decreases. This correlation is used to calculate and track the amount of dust deposited on the filter in real-time.

[0009] [6] An example of a continuous dust monitoring device in commercial use is the PDM3700 personal dust monitor (PDM), available from Thermo Scientific. The TEOM unit of such a device, schematically depicted in Figure 1 , includes inlet 100 through which air 102 comprising respirable dust particulates is drawn. Dust filter 104, located in inlet chamber 106, is mounted on tapered tubular element 108 which is fixed at its opposite end to base 110. The tapered tubular element can thus oscillate at a natural resonance frequency in the direction indicated by arrows 112a, 112b. The PDM electronics control and measure this oscillation, for example via interaction of electromagnetic drivers 114a, 114b with magnets 116a, 116b.

[0010] [7] Dust filter 104 includes a hollow polymeric filter body 118 configured to receive tapered tubular element 108, glass fibre filter membrane 120 and polymer retaining ring 122 which seals filter membrane 120 to the rim of filter body 118. The three components are welded together to form a unitary structure which is mountable on and detachable from tapered tubular element 108.

[0011] [8] In use, air is drawn at a known flowrate from a worker’s breathing zone into the PDM, size-selected in a cyclone to remove coarse particulates, and passed as air 102 into chamber 106, through dust filter 104, and out of the TEOM unit via tapered tubular element 108 and outlet 122. The respirable dust in air 102 is retained on filter membrane 120, thus increasing the mass of dust filter 104. This mass is monitored in real time by measuring the resonance frequency of tapered tubular element 108.

[0012] [9] Personal dust monitors such as the PDM3700 are now in routine operation in many workplaces, including coal mines. Fixed-location or portable dust monitors using a TEOM are also used. While the real-time monitoring capability of these devices provides an advantage over traditional gravimetric samplers, they suffer from the significant disadvantage that the crystalline silica content of the dust retained on the filter cannot be analysed by traditional FTIR and XRD analysis methods, e.g. NIOSH Methods 7603 and 7500. Since the amount and composition of respirable dust particulates are both important factors, the conventional use of continuous PDM devices and other TEOM-based continuous dust monitors does not provide a complete picture of dust exposure risk.

[0013]

[0010] This issue arises from the construction and composition of the dust filter assembly used in continuous PDM devices. Commercially available PDM dust filters, as described above, are not suitable for ashing because of the durable materials of construction, including glass fibres and fluoropolymers present in the filter membrane, and the significant mass of the multicomponent filter assembly by comparison to removable polymeric filter sheets used in traditional gravimetric sampling methods. Silicon-based components in the dust filter, including the glass fibres, would also contaminate the ash and confound any analysis of crystalline silica in the dust.

[0014]

[0011] A previous attempt to address this issue is described in US patent 7,947,503. The approach described therein sought to modify the composition of the dust filter assembly (item 104 in Figure 1 ), thereby rendering all components suitable for ashing. Thus, the filter membrane (item 120 in Figure 1 ) was fabricated entirely from nylon or other organic polymers. Inorganic components such as TiO2 pigments were also excluded from the polypropylene filter body and retaining ring (items 118 and 122 in Figure 1 ). It was claimed that a respirable dust sample retained on such a modified dust filter could be analysed by ashing the entire filter assembly and then subjecting the ash to conventional FTIR or XRD analysis.

[0015]

[0012] However, this approach still has significant shortcomings, including that the dust filter includes structural polymeric components which require extended time periods to ash. The approach described in US patent 7,947,503 has not been commercially successful and alternative PDM filters are not available to replace the glass fibre filters currently in use.

[0016]

[0013] Furthermore, it is desirable in some scenarios to avoid ashing the dust sample altogether. The ashing procedure used in standard analysis methods such as NIOSH Methods 7603 and 7500, or with the approach of US patent 7,947,503, is timeconsuming and must frequently be conducted in an off -site analytical laboratory. Results generated using the above standard laboratory analysis are generally not available for several days to several weeks after sampling, which delays timely and effective intervention for engineering control of dust exposure. Earlier results might be obtained, without unacceptable impacts on accuracy, by conducting a direct-on-filter analysis of the sampled dust, for example with a field-based FTIR method. However, the dust filters of continuous dust monitoring devices (such as the PDM3700) are inherently unsuited for direct-on-filter analysis because the bulky filter assembly blocks IR beam transmission and the glass fibre filter contains silica, which would confound the RCS analysis. Moreover, the three-dimensional PDM filter assembly, whether sold commercially or modified as described in US patent 7,947,503, is not physically compatible with analytical instruments configured to receive planar substrates or for spectroscopic analysis in transmission mode.

[0017]

[0014] The international patent application published as W02024 / 011288A1 proposes a different approach for analysing the crystalline silica content of dust retained in the filter of a continuous dust monitor. The retained dust is initially detached from the filter with an organic solvent such as an alcohol and then redeposited onto a silicon- free polymeric membrane suitable for direct-on-filter dust analysis. This method has the significant advantage of compatibility with existing continuous personal dust monitor designs, but requires that dust detachment and re-deposition steps be carefully performed in a laboratory environment to prepare the dust sample for analysis.

[0018]

[0015] There is therefore an ongoing need for new apparatus and methods for analysing dust collected in a continuous dust monitor, particularly a continuous personal dust monitor, which at least partially address one or more of the above- mentioned short-comings, or provide a useful alternative.

[0019]

[0016] A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that the document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

[0020] Summary of Invention

[0021]

[0017] Disclosed herein is a filter assembly compatible with a continuous dust monitor comprising a tapered element oscillating microbalance, but which is designed to allow analysis of the captured dust via a direct-on-filter analysis method. The filter assembly comprises a detachable filter element comprising a silicon-free filter membrane, thus allowing the filter element comprising captured dust to be transferred directly from the continuous dust monitor into an analytical instrument. There is therefore no requirement for complex or time-consuming laboratory-based steps such as ashing, solvent treatments or dust redeposition. The composition of dust to which a one or more persons, such as a user of a continuous personal dust monitor, has been exposed, for example the respirable crystalline silica content, may thus be rapidly determined in the field. This complements the real-time data obtained from the continuous dust monitor relating to the total amount of dust exposure, allowing dust exposure risks to be managed effectively with the use of a single dust collection apparatus.

[0022]

[0018] In accordance with a first aspect the invention provides a filter assembly for a continuous dust monitor, the filter assembly comprising: a filter body comprising an air outlet, the air outlet configured to removably couple with a tubular element of a tapered element oscillating microbalance (TEOM) in the continuous dust monitor; and a filter element mounted on the filter body, the filter element comprising a porous silicon-free filter membrane for capturing dust present in air flowing into the filter assembly, wherein the filter element is detachable from the filter body to allow direct- on-filter analysis of dust captured on the porous silicon-free filter membrane.

[0023]

[0019] In some embodiments, the continuous dust monitor is a continuous personal dust monitor.

[0024]

[0020] In some embodiments, the filter element comprises a peripheral sealing element surrounding the porous silicon-free filter membrane, wherein the peripheral sealing element forms an air-tight seal against the filter body such that air flowing into the filter assembly passes through the porous silicon-free filter membrane.

[0025]

[0021] In some embodiments, the peripheral sealing element is a polymeric ring.

[0022] In some embodiments, the peripheral sealing element is resilient when flexed.

[0026]

[0023] In some embodiments, the peripheral sealing element is 3D-printed on the porous silicon-free filter membrane.

[0027]

[0024] In some embodiments, the filter body comprises a rim, preferably a discshaped rim. The peripheral sealing element may seal against inner walls of the rim.

[0028]

[0025] In some embodiments, the filter body comprises at least one seat located inside the rim. The filter element is mounted over the at least one seat thereby spacing the porous silicon-free filter membrane apart from a floor of the filter body.

[0029]

[0026] In some embodiments, the filter body comprises one or more spacer elements, for example a plurality of spacer elements intruding radially from the rim, to space the porous silicon-free filter membrane apart from a floor of the filter body.

[0030]

[0027] In some embodiments, a portion of the peripheral sealing element protrudes past the rim, thereby allowing the filter element to be detached from the filter body by gripping and withdrawing the peripheral sealing element.

[0031]

[0028] In some embodiments, the filter assembly further comprises a porous support membrane mounted on the filter body beneath the filter element. The filter element may be separable from the porous support membrane when the filter element is detached from the filter body.

[0032]

[0029] In some embodiments, the porous support membrane has an average pore size of between 4 pm and 30 pm, such as between 5 pm and 15 pm, for example about 10 pm.

[0033]

[0030] In some embodiments, the porous silicon-free filter membrane comprises, or consists of, one or more organic polymers. The one or more organic polymers may be selected from the group consisting of polyvinyl chloride, polyvinyl chloride-acrylic copolymer, polyolefin, nylon, polyester and cellulose.

[0034]

[0031] In some embodiments, the porous silicon-free filter membrane is a polyvinyl chloride membrane.

[0035]

[0032] In some embodiments, the porous silicon-free filter membrane has an average pore size of between 2 pm and 10 pm, such as between 3 pm and 7 pm, for example about 5 pm.

[0033] In accordance with a second aspect the invention provides a kit comprising: at least one filter body, each filter body comprising an air outlet configured to removably couple with a tubular element of a tapered element oscillating microbalance (TEOM) in a continuous dust monitor; and a plurality of filter elements configured to be mounted on the at least one filter body to form a filter assembly for the continuous dust monitor, each filter element comprising a porous silicon-free filter membrane for capturing dust present in air flowing into the filter assembly, wherein each filter element once mounted is detachable from the filter body to allow direct-on-filter analysis of dust captured on the porous silicon-free filter membrane.

[0036]

[0034] In some embodiments, the continuous dust monitor is a continuous personal dust monitor.

[0037]

[0035] In some embodiments, the kit further comprises at least one, and optionally a plurality of, porous support membranes configured to be mounted on the filter body beneath one of the filter elements.

[0038]

[0036] Each filter body and each filter elements may independently have features according to any embodiment of the first aspect already disclosed herein.

[0039]

[0037] In accordance with a third aspect the invention provides a method of dust analysis, the method comprising: coupling the filter assembly according to any embodiment of the first aspect with a tubular element of a tapered element oscillating microbalance (TEOM) in a continuous dust monitor; flowing air through the filter assembly; capturing dust present in the air on the porous silicon-free filter membrane; and monitoring the mass of the captured dust with the TEOM.

[0040]

[0038] In some embodiments, the continuous dust monitor is a continuous personal dust monitor and the method comprises flowing air from the breathing zone of a user of the continuous personal dust monitor through the filter assembly.

[0041]

[0039] In some embodiments, the method further comprises detaching the filter element from the filter body; and analysing the captured dust while still present on the porous silicon-free filter membrane.

[0042]

[0040] In some embodiments, the captured dust is analysed by FTIR spectroscopy and / or XRD analysis.

[0041] In some embodiments, the filter element is retained in a membrane holder configured for mounting in an analytical instrument, such as an FTIR spectrometer.

[0043]

[0042] In some embodiments, analysing the captured dust comprises quantifying an amount of crystalline silica in the captured dust. The method may further comprise applying a correction to correct to correct the amount of crystalline silica for non-uniform distribution of dust captured on the porous silicon-free filter membrane. The correction may be based on a correlation between the measured mass fraction of crystalline silica and the actual mass fraction of crystalline silica determined in calibration experiments.

[0044]

[0043] In some embodiments, the dust is coal dust.

[0045]

[0044] In accordance with a fourth aspect the invention provides a method of producing a filter element comprising a peripheral sealing element surrounding a porous silicon-free filter membrane, for a filter assembly according to some embodiments of the first aspect or a kit according to some embodiments of the third aspect, the method comprising (i) applying a curable polymeric resin to a porous silicon- free filter membrane; (ii) curing the curable resin thereon to produce the peripheral sealing element; and (iii) cutting away the porous silicon-free filter membrane extending beyond the peripheral sealing element.

[0046]

[0045] In some embodiments, the ring of the curable polymeric resin is applied in a 3D printing process. In some embodiments, the curable polymeric resin is an acrylic photopolymer. In some embodiments, the peripheral sealing element is a polymeric ring.

[0047]

[0046] Where the terms “comprise”, “comprises” and “comprising” are used in the specification (including the claims) they are to be interpreted as specifying the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof.

[0048]

[0047] Further aspects of the invention appear below in the detailed description of the invention.

[0049] Brief Description of Drawings

[0050]

[0048] Embodiments of the invention will herein be illustrated by way of example only with reference to the accompanying drawings in which:

[0049] Figure 1 schematically depicts the tapered element oscillating microbalance (TEOM) unit of a commercially available personal dust monitor, including its removable dust filter.

[0051]

[0050] Figure 2 schematically depicts in exploded view the components of a filter assembly according to some embodiments of the present disclosure.

[0052]

[0051] Figure 3 schematically depicts in perspective view a filter body of a filter assembly according to some embodiments of the present disclosure.

[0053]

[0052] Figure 4 depicts in top view a filter body (4A: photograph; 4B: schematic diagram) of a filter assembly according to some embodiments of the present disclosure.

[0054]

[0053] Figure 5 depicts side cross-sectional views (5A: not-to-scale schematic; 5B: scale diagram) taken through section A-A of the filter body of Figure 4.

[0055]

[0054] Figure 6 depicts side cross-sectional views (6A: not-to-scale schematic; 6B: scale diagram) taken through section B-B of the filter body of Figure 4.

[0056]

[0055] Figure 7 depicts in top view a filter element of a filter assembly according to some embodiments of the present disclosure.

[0057]

[0056] Figure 8 schematically depicts a side cross-sectional view taken through section C-C of the filter element of Figure 7.

[0058]

[0057] Figure 9 depicts in top view a filter assembly according to some embodiments of the present disclosure.

[0059]

[0058] Figure 10 schematically depicts side cross-sectional views taken through section D-D of the filter assembly of Figure 9.

[0060]

[0059] Figure 11 schematically depicts relevant dimensions (in mm) of a disassembled filter body from a commercial PDM3700 dust filter, as produced in Example 1 .

[0061]

[0060] Figure 12 schematically depicts relevant dimensions (in mm) of a filter element as produced in Example 4.

[0062]

[0061] Figure 13 is a graph showing 15- and 30-min rolling time sampling concentrations and total mass of collected dust as determined over time by Filter 9 and a commercial PDM filter, as determined in Example 3.

[0062] Figure 14 is a graph showing the TEOM frequency and differential pressure over time for Filter 9 and a commercial PDM filter, as determined in Example 3.

[0063]

[0063] Figure 15 is a graph showing 15- and 30-min rolling time sampling concentrations and total mass of collected dust as determined over time on a filter assembly with detachable filter element and on a commercial PDM filter, as determined in Example 6.

[0064]

[0064] Figure 16 is a graph showing the TEOM frequency and differential pressure over time for a filter assembly with detachable filter element and for a commercial PDM filter, as determined in Example 6.

[0065]

[0065] Figure 17 depicts a membrane holder configured to accommodate a filter element (of a filter assembly according to some embodiments of the present disclosure) for direct-on-fi Iter analysis.

[0066]

[0066] Figure 18 shows the membrane holder of Figure 17, holding a filter element with captured dust.

[0067]

[0067] Figure 19 is a graph showing uncorrected comparative silica analyses (% silica of collected dust) determined for (i) dust captured on a commercial PDM, determined by solvent detachment / redeposition, (ii) dust captured on a filter assembly according to an embodiment of the present disclosure, determined by solvent detachment / redeposition, and (iii) dust captured on a filter assembly according to an embodiment of the present disclosure, determined by direct -on-filter analysis.

[0068] Detailed Description

[0069] Filter assembly

[0070]

[0068] The present invention relates to a filter assembly for a continuous dust monitor, such as a continuous personal dust monitor. The filter assembly comprises a filter body comprising an air outlet which is configured to removably couple with a tubular element of a tapered element oscillating microbalance (TEOM) in the continuous dust monitor. The filter assembly further comprises a filter element mounted on the filter body, the filter element comprising a porous silicon-free filter membrane for capturing dust present in air flowing into the filter assembly. The filter element is detachable from the filter body to allow direct-on-fi Iter analysis of dust captured on the porous silicon-free filter membrane.

[0069] As used herein, a continuous dust monitor refers to a dust monitor which is configured to sample air and provide real-time feedback on the mass of dust collected, and thus the amount of dust in the sampled air. As used herein, a continuous personal dust monitor refers to a dust monitor which is configured to be worn by a human user and sample representative air from the breathing zone of that user, and which provides real-time feedback on the mass of dust collected. A continuous personal dust monitor is thus distinguished from conventional gravimetric sampling devices which do not have real-time monitoring capabilities. The filter assembly disclosed herein is particularly useful for continuous personal dust monitoring applications, but may also be applied with continuous dust monitors not configured for personal use, for example wall- mounted or portable units, which utilise a TEOM to provide real-time monitoring.

[0071]

[0070] As used herein, a porous silicon-free filter membrane is a porous membrane, suitable for trapping and retaining dust present in air passing through the membrane, which is either entirely free of silicon or contains a negligibly low amount of silicon that does not interfere with subsequent direct-on-filter analysis of the dust. Typically, the membrane is configured as a thin, substantially planar sheet of membrane material. In some embodiments, the porous silicon-free filter membrane consists of one or more organic polymers. Non-limiting examples of suitable organic polymers may include polyvinyl chloride (PVC), polyvinyl chloride-acrylic copolymer, polyolefin, nylon, polyester, cellulose etc. An example of a particularly suitable material is PVC. Typically, the porous silicon-free filter membrane does not contain metals, and in some embodiments therefore may be a porous silicon-free and metal-free filter membrane.

[0072]

[0071] The porous silicon-free filter membrane has a suitable porosity to capture dust particles while providing sufficient permeability for air to be drawn into the filter element. In some embodiments, the porous silicon-free filter membrane has an average pore size of between 2 pm and 10 pm, such as between 3 pm and 7 pm, for example about 5 pm. Average pore size is generally specified by membrane manufacturers, or it be determined by routine methods such as capillary flow porometry or scanning electron microscopy. An example of a suitable porous silicon-free filter membrane is a polyvinyl chloride (PVC) membrane filter, 5 pm pore diameter, available from Casella. These membranes have previously been used for dust capture applications, including in NIOSH methods 7603 and 7500.

[0072] As used herein, a filter element which is detachable from the filter body refers to a filter element which can be separated from the filter body during a routine dust analysis procedure without unacceptably disturbing dust captured on the filter element. The filter element is thus typically not permanently bonded to or integrally formed with the filter body. Instead, the filter element and filter body may be discrete components which are designed and fabricated for complementarity so that the filter element can be reversibly mounted onto the filter body. In particular, the filter element may be configured to provide an air-tight seal between the periphery of the filter element and the filter body, such that air flowing into the filter assembly is forced to pass through the porosity of the porous silicon-free filter membrane. The filter element may be friction- fitted to the filter body, thus providing an air-tight seal between the periphery of the filter element and the filter body.

[0073]

[0073] In some embodiments, the filter element thus comprises a peripheral sealing element surrounding the porous silicon-free filter membrane. The peripheral sealing element may be configured to form an air-tight seal against a complementary surface of the filter body, and typically also to support the filter element in a substantially planar configuration while it is manipulated in use.

[0074]

[0074] In some embodiments, the peripheral sealing element is a polymeric ring. The polymeric ring may be resilient when flexed, i.e. it yields elastically when subjected a small degree of flexing or inward compression, thus assisting the polymeric ring to fit snugly and with a suitably air-tight seal against complementary surfaces of the filter element. The complementary surfaces may comprise the internal walls of a discshaped rim of the filter body and / or a seat located inside the rim.

[0075]

[0075] In some embodiments, the filter assembly further comprises a porous support membrane, mounted on the filter body beneath the filter element. The filter element is typically separable from the porous support membrane when the filter element is detached from the filter body. The porous support membrane and the filter body are also typically discrete components, allowing the porous support membrane to be freely mounted on and detached from the filter body, although it is not excluded that the porous support membrane is permanently bonded to the filter body.

[0076]

[0076] The porous support membrane may provide mechanical support to the porous silicon-free filter membrane, preventing it from sagging or collapsing in use. In addition, the porous support membrane may modulate the overall permeability of the filter assembly to air flow. In particular, its permeability may be sufficient to provide a differential pressure across both membranes which is compatible with the operating parameters of the continuous dust monitor, and to allow effective dust capture on the overlying porous silicon-free filter membrane.

[0077]

[0077] The porous support membrane may be configured as a substantially planar sheet of membrane material, and is typically slightly thicker and / or more rigid than the porous silicon-free filter membrane. In some embodiments, the porous support membrane consists of one or more organic polymers. Non-limiting examples of suitable organic polymers may include polyester, nylon and cellulose esters.

[0078]

[0078] The porous support membrane may have a suitable porosity to modulate the permeability of air into the filter element. In some embodiments, porous support membrane has an average pore size of between 4 pm and 30 pm, such as between 5 pm and 15 pm, for example about 10 pm. Pores sizes below or above these ranges may cause excessive or insufficient pressure differentials, respectively. An example of a suitable porous support membrane is a polyester membrane, 10 pm pore diameter, available from Sterlitech.

[0079]

[0079] Figure 2 schematically depicts in exploded view the components of a filter assembly 200 according to some embodiments of the present disclosure. In some embodiments, filter assembly 200 is configured for use in a continuous personal dust monitor as described herein with reference to Figure 1 .

[0080]

[0080] Filter assembly 200 comprises filter body 210 comprising air outlet 212 which is configured to removably couple with the oscillating tubular element of a tapered element oscillating microbalance (TEOM) in a continuous personal dust monitor, for example a PDM3700.

[0081]

[0081] Filter assembly 200 further comprises filter element 250 comprising a porous silicon-free filter membrane 252, for example a porous PVC membrane with an average pore size of 5 pm. Membrane 252 is configured to capture dust present in air flowing into the filter assembly through the porosity of membrane 252.

[0082]

[0082] Filter element 250 is mounted on filter body 210 when filter assembly 200 is fully assembled. The filter assembly is configured to provide an air-tight seal between the periphery of filter element 250 and the filter body, such that air flowing into the filter assembly is forced to pass through the porosity of porous silicon-free filter membrane 252. However, filter element 250 and filter body 210 may be discrete components and filter element 250 is thus detachable from filter body 210. This facilitates direct-on-f ilter analysis of dust captured on porous silicon-free filter membrane 252, since the detached filter element 250 of generally planar configuration can be positioned in a suitable analytical instrument and the silicon-free composition of the membrane may avoid or acceptably limit interference with the analysis.

[0083]

[0083] Optionally, filter assembly 200 further comprises porous support membrane 270 which is mounted on filter body 210 beneath filter element 250 when filter assembly 200 is fully assembled. Support membrane 270 provides mechanical support to membrane 252, thus preventing collapse or sagging in use, but is sufficiently permeable to airflow to avoid an excessive pressure drop when air is drawn into the filter assembly. In embodiments where the filter assembly is designed as a direct replacement for an existing unitary filter design, e.g. for a PDM3700 unit, support membrane 270 may be selected to tune the differential pressure of the entire filter assembly. Porous support membrane may thus have porosity selected to provide a differential pressure of air flowing from the breathing zone of a user through the filter assembly that adequately matches the differential pressure provided by the existing unitary filter design. Either insufficient or excessive permeability of the combined membranes, relative to the exiting filter design, may lead to diagnostic failure of the instrument.

[0084]

[0084] Porous support membrane 270 and filter body 210 may be discrete components, allowing porous support membrane 270 to be freely mounted on and detached from filter body 210. However, filter element 250 is typically separated from porous support membrane 270 when dust captured on silicon-free filter membrane 252 is to be analysed, and it is therefore not essential that porous support membrane 270 is itself detachable from filter body 210. If desired, porous support membrane 270 may therefore be permanently bonded to filter body 210 as part of a unitary structure. It is also envisaged that porous support membrane may be omitted, for example by using a sufficiently mechanically robust filter element 250.

[0085]

[0085] Filter body 210 is depicted in a perspective view in Figure 3 and in top view in Figure 4 (Figure 4A is a photograph; Figure 4B is a schematic diagram). Figures 5 and 6 show cross-sectional views of filter body 210 taken through sections A-A and B- B shown in Figure 4, respectively (Figure 5A, 6A are not-to-scale schematics useful to explain key operating principles; Figures 5B, 6B are scaled engineering diagrams).

[0086]

[0086] Filter body 210 comprises a generally disc-shaped main body portion 214 and air outlet 212 which is configured as tubular stem 216 extending axially from the centre of the main body portion. The air outlet comprises cylindrical bore 218 which opens into the interior of the filter assembly through opening 219 in planar filter body floor 222, and is configured to receive the oscillating tubular element of a tapered element oscillating microbalance (TEOM) in the continuous personal dust monitor.

[0087]

[0087] At its periphery, filter body 210 comprises disc-shaped rim 224 having inner walls 226, outer walls 228 and upper rim surface 230. Filter body 210 further comprises seat 232 which is located inside and directly against rim 224 and beneath upper rim surface 230. Seat 232 preferably extends continuously around inner walls 226 of the rim, thereby having a circular configuration as depicted.

[0088]

[0088] Filter body 210 may further comprise one or more spacer elements 234 formed on floor 222, for example elongated spacer elements 234a, 234c which intrude radially from rim 224 partway towards the centre of the filter body and one elongated spacer element 234b which extends over opening 219 in filter body floor 222. As will be further described hereafter, the role of the seat and spacer elements is to provide a mounting surface for the filter element and support membrane, thereby supporting and spacing the filter membrane 252 apart from filter body floor 222 while still allowing free movement of air through the filter assembly.

[0089]

[0089] Filter body 210, including all the elements described above, may be fabricated as a unitary structure from a suitable material, preferably a polymer such as polypropylene.

[0090]

[0090] Filter element 250 is depicted in top view in Figure 7. Figure 8 depicts a cross-sectional view of filter element 250 taken through section C-C shown in Figure 7. Filter element 250 comprises a porous silicon-free filter membrane 252, for example a porous PVC membrane, and peripheral sealing element 254 surrounding the porous silicon-free filter membrane. Peripheral sealing element 254 is configured to form an air-tight seal against filter body 210, for example against inner walls 226 of rim 224, when filter element 250 is mounted on filter body 210, thus preventing air drawn into the filter assembly from by-passing the filter membrane. Its shape and dimensions are thus dependent on the configuration of filter body 210. In some embodiments, peripheral sealing element 254 is a ring with a substantially circular shape. In crosssection, as seen in Figure 8, sealing element 254 may protrude from at least one side of filter membrane 252, for example with an approximately semi-circular cross-sectional profile.

[0091]

[0091] Peripheral sealing element 254 is typically resilient when flexed, thus allowing it to yield when mounted within rim 224 and form an air-tight seal against the filter body, e.g. against inner walls 226, via a friction fit. However, it is typically also sufficiently robust so that filter element 250 is self-supporting. By this, it is meant that the filter element can maintain an approximately planar configuration (i.e. with only a small degree of bending) when peripheral sealing element 254 is gripped at a single location on one side of the filter element. This allows the filter element to be detached from the filter body, after dust collection, with reduced risk of displacing dust due to flexing or rolling of the filter membrane. Finally, peripheral sealing element 254 is suitably lightweight so that filter assembly 200 remains compatible with the TEOM of the continuous personal dust monitor. In some implementations, for example with a PDM3700 dust monitor, filter element 250 weighs less than 20 mg.

[0092]

[0092] Peripheral sealing element 254 may suitably be formed from a polymeric material. The sealing element may be produced by applying a flowable form of the polymeric material onto one side of the filter membrane in the required configuration and thereafter hardening the polymeric material. In some embodiments, a curable resin is applied to the filter membrane and subsequently cured, for example in a 3D printing process.

[0093]

[0093] The inventors have achieved good results by PolyJet 3D printing with a Stratasys J850 Prime 3D printer, using Verollltra™ White, an acrylic photopolymer 3D- printing resin available from Stratasys, to produce a peripheral sealing element 254 in the form of a polymeric ring of required dimensions on a porous PVC filter membrane. After curing the resin, the PVC membrane extending beyond the ring was cut away to produce a filter element with the configuration depicted in Figure 7.

[0094]

[0094] Fully assembled filter assembly 200 is depicted in top view in Figure 9. Figure 10 depicts a cross-sectional view of filter assembly 200 taken through section D-D shown in Figure 9. Filter assembly 200 comprises filter body 200 as described herein with reference to Figures 3-6, filter element 250 as described herein with reference to Figures 7-8, and support membrane 270.

[0095]

[0095] Filter element 250 and underlying support membrane 270 are both mounted on filter body 200 via seat 232 and spacer elements 234 which space filter membrane 252 apart from filter body floor 222 so as to define internal filter space 280. Peripheral sealing element 254 forms an air-tight seal against inner walls 226 of rim 224 such that air flowing into filter assembly 200 passes through porous silicon-free filter membrane 252. Seat 232, spacer elements 234 and support membrane 270 together support and maintain filter element 250 in a generally planar configuration, even when subjected to inward-directed pressure caused by air flow into the filter assembly.

[0096]

[0096] Filter element 250 is detachable from filter body 210 and separable from support membrane 270. After collecting dust in a continuous personal dust monitor, the filter element can thus be separated from the other components of the assembly for direct-on-filter analysis of the dust captured on filter membrane 252. Preferably peripheral sealing element 254 protrudes past rim 254, i.e. it extends beyond upper rim surface 230, when filter element 250 is mounted on filter body 210. This may advantageously allow filter element 250 to be detached from filter body 210 by gripping and withdrawing peripheral sealing element 254, for example with a pair of tweezers.

[0097] Kit

[0098]

[0097] The invention further relates to a kit comprising at least one filter body and a plurality of filter elements configured to be mounted on the at least one filter body, thereby forming a filter assembly for a continuous dust monitor, such as a continuous personal dust monitor. Each filter body comprises an air outlet configured to removably couple with a tubular element of a tapered element oscillating microbalance (TEOM) in the continuous dust monitor. Each filter element comprises a porous silicon-free filter membrane for capturing dust present in air flowing into the filter assembly. Once mounted on a filter body of the kit, the filter element is detachable from the filter body to allow direct-on-filter analysis of dust captured on the porous silicon-free filter membrane.

[0099]

[0098] The kit may further comprise at least one and typically a plurality of porous support membranes, configured to be mounted on the filter body beneath the filter element. The porous support membranes and the filter elements are preferably discrete components, such that the filter element is readily separable from the porous support membrane when the filter element is detached from the filter body on which it is mounted. The support membranes and filter bodies of the kit are preferably also discrete components, allowing the porous support membranes of the kit to be freely mounted on and detached from a filter body of a kit. Alternatively, each porous support membrane may be integrally permanently bonded to a filter body as part of a unitary structure.

[0100]

[0099] The filter body, filter elements and support membranes of the kit are generally as described herein in the context of the filter assembly, and may thus be according to any embodiments of filter body 210, filter element 250 and support membrane 270 described herein.

[0101]

[0100] The filter elements and support membranes are typically single-use components of the filter assembly in dust monitoring applications, but the filter body may be amenable to multiple uses. For example, after capturing dust on the filter element in a continuous dust monitor, the filter assembly may be removed and disassembled for analysis of the captured dust. The separated filter body may then be cleaned to remove any residual dust, and another filter element from the kit mounted on the cleaned filter body for another dust monitoring use. The kit may thus comprise more filter elements and support membranes than filter bodies, and in principle the kit may comprise only a single filter body for repeated use with fresh membranes in a series of analyses.

[0102] Method of dust analysis

[0103]

[0101] The invention further relates to a method of dust analysis. The method comprises coupling a filter assembly as described herein with a tubular element of a tapered element oscillating microbalance (TEOM) in a continuous dust monitor, such as a continuous personal dust monitor. Air is then flowed through the filter assembly, e.g. from the breathing zone of a user of a continuous personal dust monitor, so that dust present in the air is captured on the porous silicon-free filter membrane. The mass of the captured dust is monitored with the TEOM. The captured dust mass is typically monitored in real-time allowing an immediate response to an unsafe level of dust exposure.

[0102] The dust present in the air may be any dust which is collectable in a continuous dust monitoring device, and in particular any air-born dust comprising respirable particulates of potential health concern if inhaled. In some embodiments, the dust comprises, or possibly comprises, crystalline silica such as quartz.

[0104]

[0103] In some embodiments, the dust is coal dust, for example in air found in a coal mine. In other embodiments, the dust is produced in facilities for mineral or stone mining or manufacturing.

[0105]

[0104] The filter assembly is generally as previously described herein, and may thus be a filter assembly 200 comprising filter body 210, filter element 250 and support membrane 270 according to any of the embodiments disclosed herein.

[0106]

[0105] The continuous dust monitor may be a commercially available device, such as a PDM3700 unit available from Thermo Scientific. In such scenarios, the filter assembly may be designed and constructed for compatibility with the physical configuration and operating constraints of the commercially available device. In particular, it may have a physical configuration, mass and permeability to air through the filter membrane(s) which adequately resemble existing commercially available filters for the device and which allow the installed filter assembly to pass one or more pre-capture diagnostic tests of the device.

[0107]

[0106] Alternatively, any other continuous dust monitor comprising a TEOM for realtime analysis of captured dust mass may be used, provided that the monitor and filter assembly are configured for mutual compatibility.

[0108]

[0107] The continuous dust monitor may include a cyclone to size-select dust particulates, so that dust particulates approximating the respirable dust fraction is collected on the filter assembly. Other well-known components of continuous dust monitoring devices, including pumps, sensors, electronics, etc are also expected to be present.

[0109]

[0108] The steps of capturing and monitoring the dust present in air flowed into the filter assembly, e.g. from the breathing zone of a user, is generally performed by conventional methods associated with the use of a continuous dust monitor. When using an existing commercially available device, such as a PDM3700 unit, these steps may be performed according to the manufacturer’s instructions for use, and implemented by the operating software of the device.

[0109] The methods disclosed herein may comprise performing a series of dust analyses using one or more continuous dust monitors, for example by multiple workers in a work environment and / or by one worker across multiple shifts. In all of these dust analyses, the mass of the dust captured on the filter may be monitored by the TEOM, providing real-time detection of the amount of dust exposure. The dust assemblies disclosed herein, comprising detachable filter element, may be used in all or only some of these real-time dust mass analyses.

[0110]

[0110] In at least some of the dust analyses conducted using the filter assemblies disclosed herein, the filter assembly is removed from the continuous dust monitor after the required period of dust mass monitoring, e.g. at the end of a worker’s shift, the filter element is detached from the filter body and the captured dust is analysed while still present on the porous silicon-free filter membrane (i.e. a “direct-on-filter” analysis). Such direct-on-filter analyses may suitably be performed after each dust mass analysis of the series, for only a pre-determined fraction of such analyses, or as a response to a pre-determined value of dust mass measured by the continuous dust monitor (e.g. when it is determined that a worker has been exposed to an unsafe amount of dust).

[0111]

[0111] In some embodiments, the filter element is detached from the filter body by gripping the peripheral sealing element of the filter element (e.g. peripheral sealing element 254 of filter element 250), for example with a pair of tweezers, and withdrawing the peripheral sealing element. The peripheral sealing element may assist to support the filter element during the detachment process, preventing the filter membrane from flexing or rolling and thus avoiding the loss of dust.

[0112]

[0112] The detached filter element may be transferred to a suitable holder configured for mounting in an analytical instrument. Figure 17 depicts membrane holder 300 configured to accommodate filter element 250 as disclosed herein with reference to Figures 7-8. Membrane holder 300 comprises a generally disc-shaped holder body 310 which is configured for mounting in an analytical instrument, for example a compact FTIR spectrometer suitable for location in the field. Membrane holder 300 includes slotted opening 312 extending inwardly from one side of holder body 310, configured to receive filter element 250. Filter element 250, containing captured dust for analysis, thus slides into slots 314 and along slotted opening 312 until positioned at the centre of holder body 310 as depicted in Figure 18. Holder 300 thus securely holds filter element 250 while exposing at least a substantial portion of the dust capture area thereof (including the centre-point) to analysis, e.g. by FTIR spectroscopic analysis in transmission mode.

[0113]

[0113] In some embodiments, the captured dust is analysed by FTIR spectroscopy and / or XRD analysis. In some embodiments, analysing the captured dust comprises quantifying an amount of crystalline silica (quartz) in the captured dust. Such methods of analysis are well-known in the field of dust analysis, for example as set out in NIOSH Methods 7603 (FTIR analysis) and 7500 (XRD analysis). While these methods are intended for RCS analysis of dust redeposited by solvent-based methods after ashing the filter membrane on which the dust was captured, they can be adapted for direct-on- fi Iter analyses as disclosed herein.

[0114]

[0114] Dust captured on the filter membrane may have a non-uniform distribution, for example in the radial direction extending outwardly from the centre of the membrane, caused by non-uniform flow of air through the filter element in the continuous dust monitor. When the captured dust is then analysed in a direct-on-filter analysis method, the portion of the membrane that is analysed, typically at the centre of the membrane (e.g. within a beam spot where an infrared beam passes through the membrane in an FTIR analysis), may be non-representative. This could lead to an over- or underestimation of the target analyte, e.g. crystalline silica, if it is then assumed that the dust is uniformly distributed across the dust capture area of the filter element.

[0115]

[0115] Quantifying the amount of a target analyte such as crystalline silica (quartz) in the captured dust may therefore comprise applying a correction to correct for non- uniform distribution of dust captured on the porous silicon-free filter membrane. The correction may be based on a correlation between the measured mass fraction of the analyte and the actual mass fraction of the analyte determined in calibration experiments.

[0116]

[0116] The dust collection area of the filter elements disclosed herein may be smaller than in some other reported direct-on-filter dust analyses. Thus, the error introduced by non-uniform dust distribution may advantageously be smaller and / or more systematic, and thus more correctable by correlations as disclosed herein. EXAMPLES

[0117]

[0117] The present invention is described with reference to the following examples. It is to be understood that the examples are illustrative of and not limiting to the invention described herein.

[0118] Example 1.

[0119]

[0118] Commercially available filters (part number 57-009727-0020) for a PDM3700 continuous personal dust monitor (PDM), obtained from Lear Siegler Australasia, were disassembled by breaking off the fibrous filter element and using a tweezer to remove remaining glass fibres, thereby recovering the filter body. Analysis of the separated filter element by scanning electron microscopy indicated two adjacent porous layers: (i) a dust-collecting layer of random borosilicate fiberglass with a polytetrafluoroethylene (PTFE) binder, and (ii) a structural backing layer of woven glass fabric. The disassembled filter bodies, having the configuration of filter body 210 as described herein with reference to Figures 3-6, were used in the subsequent examples. Relevant dimensions, in mm, are shown in Figure 11 .

[0120] Example 2.

[0121]

[0119] Membranes potentially suitable for use as porous silicon-free filter membrane 252 and support membrane 270 in a filter assembly 200 were investigated in initial screening experiments by cutting out suitably sized circles of different membrane materials and gluing them onto seat 232 of 3D-printed filter bodies 210. Each filter assembly thus prepared included a circular support membrane directly mounted on seat 232 and a porous non-silicon filter membrane mounted thereon.

[0122]

[0120] The resulting filter assemblies were then installed in a PDM3700 unit for evaluation. The instrument diagnostics function of the WinPDM software is able to monitor a variety of operating parameters of PDM units after installing the filter. Two key parameters were considered directly attributable to the properties of the filter: the differential pressure across the filter and the TEOM frequency. The frequency is related to the weight of the overall filter assembly while the differential pressure is dependent on the permeability of the membrane filter. Both parameters are required to meet a given range of specified values to pass the diagnostic test, or the PDM will not proceed to the sampling stage. Table 1 shows results for the investigated membrane materials. Table 1. acut from 25 mm diameter polyvinyl chloride (PVC) membrane filters, 5 pm pore diameter, supplied by Casella (part number. P102078).

[0123] Example 3.

[0124]

[0121] The filters with successful filter combinations as identified in Example 2 (see Table 1 ) were then evaluated in dust sampling tests carried out using PDM3700 units in a laboratory dust testing chamber. Parallel dust sampling tests were conducted with PDM3700 units fitted with (i) a commercial glass fibre PDM filter and (ii) the newly fabricated silicon-free glued filters. A mixture of standard respirable quartz (NIST 1878b) with the coal powder sample (<125 pm) was fed into the dust chamber. The content of standard respirable quartz in the feeding mixture was varied at three different levels of 0.25, 0.35 and 0.50 wt.% to vary the respirable crystalline silica (RCS) concentrations of the formed airborne dust inside the chamber. In addition, the concentration of dust exposure inside the chamber was varied by adjusting dust feeding frequency.

[0125]

[0122] As an example, Figure 13 compares the 15- and 30-min rolling time sampling concentrations and total mass of collected dust as determined over time by Filter 9 (see Table 1 ) and the commercial PDM filter. Figure 14 compares the TEOM frequency and differential pressure in the same experiment.

[0126]

[0123] The comparisons of parallel dust sampling results of the commercial PDM filter and new silicon-free filter assemblies are summarised in Table 2. The deviations of average 30 min dust concentration and total dust mass are calculated from equations (1 ) and (2) below:

[0127] Deviation

[0128] Deviation (%) =mcf~mnf(2) mcf where Cntand mnf represent the average 30 min concentration and total dust mass using the new silicon-free filter assembly, cCf and mCf the average 30 min concentration and total dust mass using a commercial PDM filter.

[0129]

[0124] As shown in Table 2, significant differences were observed in the dust sampling results obtained with filters 2 and 6 and their parallel commercial PDM filters. However, filters 7, 9 and 10 provided average 30 min concentrations and total dust mass values close to those of commercial PDM filters, with variations less than 10%. Without limitation by any theory, the choice of support membrane, and particularly its pore size, affects the dust collecting performance by regulating the overall permeability of the filter assembly.

[0130]

[0125] The PDM3700 reported a status code of “high filter load” for filters 7, 9 and 10 during dust samplings, indicative of approaching a maximum dust loading capacity, and the dust sampling data beyond this point was not evaluated. Among these three new filters, filter 9 had the greatest dust loading capacity of 3.44 mg before maximum dust loading was reached.

[0131] Table 2.

[0132]

[0126] Based on the relatively high dust loading capacity compared to other investigated membranes, and comparable dust sampling performance to a commercial PDM filter (as seen in Figures 13 and 14), the membrane combination of filter 9 was selected for further development.

[0133] Example 4.

[0134]

[0127] Filter elements having the configuration of filter element 250 as described herein with reference to Figures 7-8, were prepared by PolyJet 3D printing with a Stratasys J850 Prime 3D printer, using Verollltra™ White resin available from Stratasys. This material is an acrylic photopolymer with a modulus of elasticity of 2-3 GPa when cured. Relevant dimensions, in mm, are shown in Figure 12.

[0135]

[0128] A positioning block configured to hold the PVC membrane filter (5 pm pore diameter, obtained from Casella) was initially prepared by 3D-printing. The top surface of the positioning block included a circular protruding ring configured to securely retain a 25mm diameter circular filter membrane, as purchased. After cleaning the positioning block, a filter membrane was pressed into place in the protruding ring. The z-offset of the J850 3D printer was then set and the polymeric ring (sealing element 254) was printed on the membrane surface. Upon completion, the edge of the filter membrane was pried off the positioning block with tweezers and the membrane removed. The membrane material extending beyond the polymeric ring was then cut away by punch cutting, thereby producing filter element 250 with peripheral sealing element 254 at the edge.

[0136] Example 5.

[0137]

[0129] Filter assemblies having the configuration of filter assembly 200, as described herein with reference to Figures 9-10, were then prepared. A circular support membrane (STERLITECH polyester filter, 10 pm pore size, part number PET1009030, see Table 2) with an outer diameter of 14.5 mm was initially mounted on the filter body (deconstructed commercial filter as produced in Example 1 ). A filter element (as prepared in Example 4) was then pressed into place on the filter body so that it was mounted over the polyester support membrane and an airtight seal was formed between the polymeric ring surrounding the filter membrane and the inner walls of the filter body’s rim. The polymeric ring protruded beyond the rim, allowing the polymeric ring to be gripped with tweezers, thereby facilitating detachment of the filter element from the filter body when required.

[0138] Example 6.

[0139]

[0130] Parallel dust sampling tests were conducted in a laboratory dust testing chamber with PDM3700 units fitted with (i) a commercial glass fibre PDM filter and (ii) a filter assembly with detachable silicon-free filter element produced in Example 5 (“silicon-free filter”), using the methods described in Example 3, as well as with a non- continuous gravimetric personal dust sampler (CASELLA AEPX2ISPRO) commercially available from Casella.

[0140]

[0131] Nine sets of parallel sampling data were obtained from experiments with varied mixed ratios of silica in the dust feed (0.25-0.50 wt.%) and various amounts of dust loading (0.34-7.29 mg using the PDM3700 unit and 0.45-8.56 mg using the gravimetric sampler). Table 3.

[0141]

[0132] The PDM3700 reported a status code of “high filter load” for test 9 when 7.07 mg of dust was collected on the silicon-free filter, indicative of approaching a maximum dust loading capacity. At the end of the experiment, 7.29 mg of dust had been collected: significantly higher than with glued filter 9 (Example 3) due to an increased membrane area.

[0142]

[0133] The results indicate that the silicon-free filter provides average 30 min concentrations and total dust mass values close to those of commercial PDM filters, with variations less than 15% and mostly less than 5%.

[0143]

[0134] The two key instrument parameters for PDM3700 i.e. differential pressure and TEOM frequency, were also monitored. The silicon-free filter exhibited similar variation trends of differential pressure and frequency as the commercial PDM filter in each parallel sampling experiment. Taking parallel test 2 as a typical example, Figure 15 compares the 15- and 30-min rolling time sampling concentrations and total mass of collected dust as determined over time by the silicon-free filter and the commercial PDM filter. Figure 16 compares the TEOM frequency and differential pressure in the same experiment. Initially the silicon-free filter had a different rate of decrease in differential pressure compared to the commercial filter but thereafter (from about 1 hour to the end of 6-hour dust sampling) the variation in differential pressure for both filters was comparable.

[0144] Example 7.

[0145]

[0135] A solvent backflush and redeposition method, as disclosed in W02024 / 011288A1 , was used to measure the RCS content of dust sampled by commercial PDM filters or silicon-free filters as produced in Example 5. In this method, the filter was backflushed with solvent to detach the dust particles, and the dust was then captured from the slurry onto a PVC membrane filter (25 mm diameter, 5 pm pore size) using a vacuum filtration apparatus. The PVC filter with redeposited dust sample was then mounted in a standard FTIR sample holder and placed in a compact FTIR spectrometer (Thermo Scientific Nicolet Summit PRO) for silica analysis. The FTIR analysis of RCS followed NIOSH method 7603 using the IR absorbance peak of 800 cm’1. Spectra were collected in absorbance mode at 1 cm’1resolution from wavenumber 650-1000 cm’1by 32 scans. The standard respirable quartz (NIST 1878b) was used to establish the calibration curve of silica quantification (silica mass content vs. peak height at 800 cm’1). A kaolinite correction was established and applied in the FTIR analysis.

[0146]

[0136] A d irect-on-f i Iter method was developed to measure the RCS content of dust sampled by silicon-free filters as produced in Example 5. In this method, the filter element with loaded dust was detached from the filter body after dust sampling and slotted into a 3D-printed membrane holder configured for mounting in a compact FTIR spectrometer (Thermo Scientific Nicolet Summit PRO) suitable for field applications.

[0147]

[0137] Figure 17 depicts membrane holder 300 configured to accommodate filter element 250 as disclosed herein with reference to Figures 7-8. Membrane holder 300 comprises a generally disc-shaped holder body 310 which is configured for mounting in a compact FTIR spectrometer. Membrane holder 300 includes slotted opening 312 extending inwardly from one side of holder body 310, configured to receive filter element 250. Filter element 250, containing captured dust for analysis, thus slides into slots 314 and along slotted opening 312 until positioned at the centre of holder body 310 as depicted in Figure 18.

[0138] Silica analysis was then conducted on filter elements 250, held within membrane holder 300, by the same methods used for membranes comprising redeposited dust (described above). After the direct-on-filter analysis, the same dust sample was analysed by a solvent detachment and redeposition method (dust detachment in a beaker with ultrasound stimulation to facilitate dust detachment into the solvent) to allow for a direct comparison between the two methods.

[0148]

[0139] Figure 19 shows the comparative silica analyses (% silica of collected dust) as determined by (i) solvent detachment and redeposition method with the PD3700 commercial filter (“Commercial filter”), (ii) solvent detachment and redeposition method with the silicon-free filter (“Silica content-RDP”) and (iii) direct-on-filter method with the detached silicon-free filter element from the new filter assembly (“Silica content-DoF”). The results indicate that the Silica content-RDP and Commercial filter analysis values are closely aligned, but the raw Silica content-DoF analysis values are significantly higher than the Silica content-RDP analysis valued obtained for the same dust sample.

[0149]

[0140] This discrepancy is attributed at least in part to radial variation in the distribution of dust captured on the filter membrane, with a higher dust concentration obtained in the centre where the infrared beam (which may have a spot size of only 6- 8 mm in diameter) passes through the membrane. Similar issues have been encountered in other direct-on-fi Iter analyses of captured dust and may be resolved by mathematical models which correct for uneven dust distribution. Thus, the measured Silica content-DoF values were found to be linearly related to the Silica content-RDP measured values, with R2of 0.998, allowing a correction of the Silica content-DoF using the linear correlation.

[0150]

[0141] Deviations of corrected Silica content-DoF values from the Silica content- Commercial filter values are calculated from equation (3) below.

[0151] Deviation where Snf represents the corrected Silica content-DoF of the dust collected on the silicon-free filter and Set is the measured silica content of redeposited dust from the commercial PDM3700 filter.

[0152]

[0142] The results are shown in Table 4. The deviations between the corrected silica analysis by direct-on-filter method and the silica analysis by solvent backflushing and redeposition with commercial PDM3700 filters is small (<15%) for all parallel tests except for test 9 where the maximum dust loading capacity of the commercial PDM filter was reached, leading to errors.

[0153] Table 4.

[0154]

[0143] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.

Claims

Claims1 . A filter assembly for a continuous dust monitor, the filter assembly comprising: a filter body comprising an air outlet, the air outlet configured to removably couple with a tubular element of a tapered element oscillating microbalance (TEOM) in the continuous dust monitor; and a filter element mounted on the filter body, the filter element comprising a porous silicon-free filter membrane for capturing dust present in air flowing into the filter assembly, wherein the filter element is detachable from the filter body to allow direct-on- filter analysis of dust captured on the porous silicon-free filter membrane.

2. The filter assembly according to claim 1 , wherein the continuous dust monitor is a continuous personal dust monitor.

3. The filter assembly according to claim 1 or claim 2, wherein the filter element comprises a peripheral sealing element surrounding the porous silicon-free filter membrane, wherein the peripheral sealing element forms an air-tight seal against the filter body such that air flowing into the filter assembly passes through the porous silicon-free filter membrane.

4. The filter assembly according to claim 3, wherein the peripheral sealing element is a polymeric ring.

5. The filter assembly according to claim 3 or claim 4, wherein the peripheral sealing element is resilient when flexed.

6. The filter assembly according to any one of claims 3 to 5, wherein the peripheral sealing element is 3D-printed on the porous silicon-free filter membrane.

7. The filter assembly according to any one of claims 3 to 6, wherein the filter body comprises a rim, and wherein the peripheral sealing element seals against inner walls of the rim.

8. The filter assembly according to claim 7, wherein the filter body comprises at least one seat located inside the rim, wherein the filter element is mounted over the at least one seat thereby spacing the porous silicon-free filter membrane apart from a floor of the filter body.

9. The filter assembly according to claim 7 or claim 8, wherein the filter body comprises one or more spacer elements to space the porous silicon-free filter membrane apart from a floor of the filter body.

10. The filter assembly according to any one of claims 7 to 9, wherein a portion of the peripheral sealing element protrudes past the rim, thereby allowing the filter element to be detached from the filter body by gripping and withdrawing the peripheral sealing element.11 . The filter assembly according to any one of claims 1 to 10, further comprising a porous support membrane mounted on the filter body beneath the filter element.

12. The filter assembly according to claim 11 , wherein the porous support membrane has a pore size of between 4 pm and 30 pm.

13. The filter assembly according to any one of claims 1 to 12, wherein the porous silicon-free filter membrane comprises one or more organic polymers.

14. The filter assembly according to any one of claims 1 to 13, wherein the porous silicon-free filter membrane is a polyvinyl chloride membrane.

15. The filter assembly according to any one of claims 1 to 14, wherein the porous silicon-free filter membrane has a pore size of between 2 pm and 10 pm.

16. A kit comprising: at least one filter body, each filter body comprising an air outlet configured to removably couple with a tubular element of a tapered element oscillating microbalance (TEOM) in a continuous dust monitor; anda plurality of filter elements configured to be mounted on the at least one filter body to form a filter assembly for the continuous dust monitor, each filter element comprising a porous silicon-free filter membrane for capturing dust present in air flowing into the filter assembly, wherein each filter element once mounted is detachable from the filter body to allow direct-on-filter analysis of dust captured on the porous silicon-free filter membrane.

17. A kit according to claim 16, further comprising at least one porous support membrane configured to be mounted on the filter body beneath one of the filter elements.

18. A method of dust analysis, the method comprising: coupling the filter assembly according to any one of claims 1 to 15 with a tubular element of a tapered element oscillating microbalance (TEOM) in a continuous dust monitor; flowing air through the filter assembly; capturing dust present in the air on the porous silicon-free filter membrane; and monitoring the mass of the captured dust with the TEOM.

19. The method according to claim 18, wherein the continuous dust monitor is a continuous personal dust monitor and the method comprises flowing air from the breathing zone of a user of the continuous personal dust monitor through the filter assembly.

20. The method according to claim 18 or claim 19, further comprising: detaching the filter element from the filter body; and analysing the captured dust while still present on the porous silicon-free filter membrane.21 .The method according to claim 20, wherein the captured dust is analysed by FTIR spectroscopy and / or XRD analysis.

22. The method according to claim 20 or claim 21 , wherein the filter element is retained in a membrane holder configured for mounting in an analytical instrument.

23. The method according to any one of claims 20 to 22, wherein analysing the captured dust comprises quantifying an amount of crystalline silica in the captured dust.

24. The method according to claim 23, comprising applying a correction to correct the amount of crystalline silica for non-uniform distribution of dust captured on the porous silicon-free filter membrane.

25. The method according to any one of claims 18 to 24, wherein the dust is coal dust.

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