Passive dust sampler and method of dust estimation

A personal dust sampler hangs from a ribbon (10) attached to the user's clothing and requires no pump or power source. Dust in the air is captured by a charged PVC sheet (2) exposed at (2a) but framed by a metal foil (25) and surmounted by a metal conductor plate (6) parallel to the charged PVC sheet (2). The plate (6) is supported by conductive gridwork (3) removably attached by a flange (3) to a metal base (21). The dust captured is determined in a method by weighing or by measuring the charge loss of the PVC sheet (2).

This invention relates to a passive dust sampler. 
BACKGROUND OF THE INVENTION 
Dust samplers are known in which the atmosphere containing the dust to be 
sampled is pumped through a collector (such as a filter medium), which 
traps the dust for subsequent analysis or measurement. Such a sampler 
requires a pump, which is bulky, and the pump in turn requires a power 
source, which adds to the mass and bulk. Because these samplers positively 
transport the dust to the collector, they may be called active dust 
samplers. 
For personal use, it would be desirable to have a passive dust sampler, 
i.e. one without a pump or power source, which would sample dust at a rate 
proportional to its atmospheric concentration in the immediate vicinity, 
biassed (ideally) towards smaller particles, these generally being more 
harmful to the human lung. A personal sampler should also be lightweight. 
A passive dust sampler must work by moving the sampled material relative to 
the air. Passive gas samplers make use of the diffusive motion of 
molecules to transport them to an adsorbing surface. Since the masses of 
all molecules of any chemical species are virtually identical, the average 
velocity of transport can be easily and accurately calculated. Dust 
particles also undergo diffusive motion but their average velocity is many 
orders of magnitude smaller than that of molecules, and so this is not an 
effective transport process. Capture must be brought about by external 
forces, and the only forces sufficiently large to be effective are gravity 
and electrostatic attraction. 
J. Pich in Aerosol Science (1966) Ed. C. N. Davies, Academic Press, London 
1966, discloses that a uniformly charged fibre captures charged particles 
at a rate dependent on the number of the charged particles passing per 
unit volume but independent of their velocity. In addition, the rate at 
which the charged body will collect charged particles of the opposite sign 
will be proportional to the electrical mobility of the particles. The 
electrical mobility of a particle is the velocity of the particle caused 
by an electric field, divided by the field strength. 
If capture takes place by gravity, the same velocity-independence is 
observed, but the capture rate is proportional to the square of the 
aerodynamic diameter of the particles, and this would favour the capture 
of large particles and selectively exclude small ones, the opposite of 
what the invention seeks. 
Vincent et al. in the Institute of Occupational Medicine's Report TM83/15 
(1983) describe the variation of electrical mobility with particle size of 
industrial aerosols and find that although there is a tendency for larger 
particles to have a higher electrical mobility the distribution would lead 
to a much lower bias than that of gravitational capture. 
SUMMARY OF THE INVENTION 
The dust sampler according to the present invention comprises an insulator 
(for example an electret) in the form of a collector sheet having a known 
electrical charge when in use, mounted on a non-insulating base (e.g. 
certain glasses are sufficiently non-insulating), the sheet being 
surmounted by a conductor parallel thereto and preferably of at least the 
area of the sheet, the conductor being supported, and preferably 
electrically connected to the base, by (therefore preferably conductive) 
spacer means such as pillars or gridwork around the sheet, but preferably 
enclosing an area at most 110% of, or, in other cases, at least double or 
not more than quadruple, the sheet area. It will be understood that 
"non-insulator" and "conductor" are used interchangeably herein and can 
include not only metals etc. but also any material sufficiently conductive 
to be unable to retain a permanent charge, such as a semiconductor. The 
spacer means is for mechanical protection of the insulator and for support 
of the conductor, and is such that air can flow through the space between 
the insulator and the conductor. The spacer means may take the form of 
gridwork preferably not larger than 11/2 cm normal to the sheet, more 
preferably not larger than 1 cm. Its extent in that direction is however 
preferably at least one-fifth of the square root of the sheet area. The 
gridwork may have the form of a rectangular or square-pattern mesh, in 
which case its extent between sheet and conductor preferably does not 
exceed four wefts and could be 1 weft (=pillars). Pillars could be spaced 
at intervals, perhaps simply at corners of the conductor. 
The conductor, rectangular, nummular or otherwise, is most preferably a 
plate, or it may be apertured, with e.g. 0-60% of the overall area of the 
conductor being made up of apertures. The apertures if present are 
preferably at least 1 mm, but preferably not exceeding 5 mm, across, more 
preferably 2 to 3 mm across. The conductor, if in the form of a mesh, may 
be finer than the gridwork. The conductor may be physically protected by 
relatively stout bars (e.g. 1 mm.sup.2 in cross-section and spaced apart 
just less than a finger's width) fixed to the base somewhat outside the 
gridwork. 
The conductor and gridwork may be fabricated as an integral blank 
subsequently folded into shape, out of a mesh or by photoetching, the 
latter thus allowing the gridwork and the conductor to be of different 
patterns or pattern sizes while yet being mass-produced in one piece. 
The sheet is preferably removable from the base. The conductor and gridwork 
are preferably removable from the base. 
The sheet could be for example of polyvinyl chloride, polypropylene, 
polyester, PVC-acrylonitrile copolymer or polycarbonate. Other possible 
materials are listed in Appendix 1. Its surface electric potential should 
be uniform or, if not, its mean surface electric potential must be known, 
and could suitably be for example 1 kV. It may be either positively or 
negatively charged. In the case of a dust known to have an excess of 
particles of one sign, an electret of the opposite charge would be a good 
choice. 
The conductor may be, or may be attached (permanently or removably) to, a 
carbon fibre cloth capable of adsorbing gas and hence being usable as a 
gas sampler, and in such a situation the conductor is preferably a plate 
without apertures. 
The dust sampler may be for example cuboidal or nummular. 
The purpose of the structure formed by the parallel conductor and 
supportive gridwork is to protect the collector sheet physically, to fix 
the electric field by which dust particle capture is effected and to 
reduce turbulence in any air flow past the collector sheet. The electric 
field is fixed in the sense that the conductor-and-grid has the same 
electric potential as the base, as they are in electric contact. The plane 
of the sheet is preferably upright during wear, to minimise the effects of 
gravity. Preferably the dust sampler is provided with a mounting such that 
it hangs upright in use. 
The sampler may further comprise a second insulator in the form of a 
collector sheet mounted on said conductor facing the first insulator, and 
facing sides of the insulators may carry opposite electric charges. The 
invention also provides a method of dust estimation, comprising exposing a 
collector sheet from a sampler as set forth above, preferably while the 
sheet is upright, and then measuring the loss of charge from the sheet or, 
preferably, from a central portion only of the sheet, or otherwise 
determining the dust captured e.g. by weighing; other possible techniques 
are listed in Appendix 2). Weight gains or charge losses observed in this 
way tend to be small, and care must be taken to minimise gains/losses due 
to other causes. Where the sheet is stuck to the base, for example, an 
unpredictable mass of adhesive may be carried on it when it is removed for 
weighing. That problem can be overcome by using a metal-foil-backed 
insulator as the sheet; the foil, if slightly oversize, may be folded over 
the front of the insulator, "framing" it, and the insulator may be 
retained by a clip grasping the "frame" to the base or by a metal flange 
to the gridwork clamping the "frame" to the base. Alternatively, the 
collector sheet may be stuck to a rigid (e.g. glass or metal) plate, the 
unit (sheet+plate) taking the place of the foil and being easily removable 
from the base in the same way; because of its rigid format, the sheet 
would be easier to measure by charge scanning or microscopy, but its 
larger tare would make weight measurement more difficult. Where a second 
insulator is also present, either or both may be used to take these 
measurements. 
Typically, a calibration factor is determined when putting this method into 
effect, usually from a comparison or measurements as described above with 
more rigorous techniques or determination of the average electrical 
mobility of the specific dust question. "Reference" dusts of known 
properties may be used to "calibrate" the samples. In certain situations, 
the electrical mobility may be sufficiently constant for repeated such 
comparisons to be dispensed with. Previous measurements by Brown et al. 
(Ann. Occ. Hyg. 32. 271-294 (1988)) suggest that small particles are more 
effective than large in reducing charge levels, and so this technique 
should show a bias towards small particles. Smaller particles tending to 
be physiologically more damaging, this bias is an advantage of the 
invention. 
An estimating technique suitable for piezoelectric collector sheets such as 
PVDF is to irradiate the deposit with pulses of light of a wavelength 
absorbed by the deposited dust but not by other parts of the system. The 
energy absorbed will be converted to a quantity of heat proportional to 
the mass of deposited dust and this will cause the collector sheet to 
become heated. The consequent change in volume will cause a change in 
surface potential, which can be measured and related to the mass of the 
dust. The electret may or may not be neutralised by ionizing radiation or 
other means after collection and before measurement. 
A further possible analysis technique might be to combine the electret 
substrate with a piezoelectric crystal, the natural frequency of which 
would be altered by the dust deposited.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Turning to FIG. 1, a sampler has a square base 1 of conductive glass (3 
cm).sup.2, on which is centrally mounted an insulator 2 (acting as a 
collector sheet) having a known electric surface potential, in this case 
1.0 kV. The insulator 2 is (29 mm).sup.2 and is of polypropylene, suitably 
50 microns thick. A (1/2 cm).sup.2 sheet would suffice for in-line mask 
sampling, but miniaturisation is not a virtue here. 
Metallic gridwork 3 (part cut away for clarity) is fixed around the whole 
periphery of the base 1 and upstanding from it, to a "depth" of 1 cm, with 
wires 1 mm thick and (for averagely turbulent conditions) at 21/2 mm 
centres. Grids finer than 21/2 mm gave rise to a more uniform electric 
field but this was outweighed by the diminished efficiency of the sampler 
at collecting dust. Increasing the grid size beyond 21/2 mm showed little 
effect. For a sampler intended for hanging on a wall in a relatively 
stagnant location, however, a coarser mesh could be used, e.g. 41/2 mm 
centres, especially if the sheet does not need physical protection from 
poking fingers or other interference. The "depth" could be increased to 
say 3 cm in such an application, whereby dust depletion would be 
eliminated, as a problem, but a sampler to be worn on clothing would, for 
practical convenience, be limited to the 1 cm depth mentioned. The 
gridwork supports a metal sheet 6 surmounting and parallel to the electret 
2 and spaced 1 cm from it. In an alternative version 6a (shown in the 
bottom far corner of the sheet 6), the sheet is a coarse mesh, e.g. 2 mm 
wires at 5 mm centres. The parts 3 and 6/6a can be integrally made as a 
photoetched blank which is then folded into shape. The parts 3 and 6/6a, 
and the electret 2 are demountable from the base 1. 
The gridwork 3 and sheet/mesh 6/6a form a Faraday cage around the electret 
2. A charcoal cloth (not shown) may be mounted on the outer side of the 
sheet 6, to adsorb gases which can then be desorbed in the laboratory and 
analysed. A sub-base (not shown) behind the base 1 may be larger in area 
and carry relatively stout bars arching in front of the sheet 6 to protect 
the sampler and spaced apart just less than say a finger's width. 
The whole is mounted on a ribbon 10 fixed behind the base 1 and equipped 
with a safety pin 11 for attaching to clothing, so that the sampler, in 
use, is worn in the orientation shown, in particular with electret 
upright. 
In use, the electret 2 is corona-charged, preferably uniformly, and allowed 
to stabilise for a week, (the stabilisation period depending on what 
electret material is used). It is sealed in a dust-free sachet for 
distribution to the user. At the place of use, it is removed from its 
wrapping and its charge is then assessed by a measurement of surface 
electric potential (e.g. 1000 V), and noted in a register. The sampler is 
then issued to and worn by a person exposed to dust. The conditions will 
normally be such that bulk drift of dust-laden air past the electret 
exceeds the critical velocity referred to earlier, and in the unlikely 
event that it is not, i.e. the air is relatively stagnant, this is easily 
detected by a visual or electrostatic check, which would reveal heavier 
dust accumulation at the edge(s) of the electret and relative depletion at 
the centre; such samples are noted, and will usually understate the dust 
concentration. 
Even with the mesh 6a and if the wearer runs forwards, the increased number 
of dust particles passing through the Faraday cage is exactly compensated 
by their reduced residence time. 
As to choosing between a continuous sheet 6 or a grid 6a, 6 usually leads 
to a more uniform dust deposit on the sheet 2, and the 
stagnation/depletion problem noted above is unlikely when the sampler is 
worn by a person undertaking normal activities. 
After a predetermined period of exposure to the dusty atmosphere, the 
sampler is taken to (or sealed in a sachet and sent to) a non-dusty room, 
and the electret 2 is removed (as also the charcoal cloth if present). The 
loss of charge on the sheet 2 is determined by capacitative means and 
hence the amount of dust captured on it is estimated. A typical charge 
will now be 970 V, i.e. loss of 30 V. From this, the dust in the 
atmosphere can be assessed. Alternatively, the sheet 2 could be weighed, 
the weight gain since new being attributable to dust, or the deposit could 
be subjected to chemical analysis or, especially if the substrate is 
transparent, to microscopic analysis. 
A fresh electret collector sheet 2 (or an old one recharged by a corona 
device and allowed to stabilise) is mounted on the substrate. The sampler 
is re-assembled and stored in a dust-proof sachet ready for issue to the 
next user. 
The sampling rate of this device depends on the electrical mobility (and 
therefore the charge) on the dust particles sampled. In order for the 
sampler to operate correctly in all environments, the charge distribution 
on particles of any particular size needs to be approximately constant 
although, because of different mechanisms of dust formation, different 
atmospheric conditions, and different chemical composition and physical 
properties of the dust the average charge carried by particles of 
different dusts is likely to vary. 
In order for an assessment to be quantitative the electrical mobility must 
be known, and so a separate assessment of this must be carried out, unless 
the parameter is sufficiently constant for it to be known in advance. 
The measurement can be made with a device as shown in FIG. 2 consisting of 
a metal duct with an electret 20 (the wall furthest from the reader) 
having in particular a parallel conductor 26 (=wall nearest the reader) 
held a fixed distance above it to form a channel approximately equal in 
width to the electret (i.e. the electret 20 is full-height in the duct). 
Air from the industrial location under investigation is pumped through at 
a known rate, and charged particles are precipitated on to the electret 
forming a deposit. The extent of this deposit will depend on the 
electrical mobility of the particles and can be used to estimate it. The 
extent may be measured by means of charge scanning a reduced charge 
corresponding with aerosol deposit or any of the means described in 
relation to FIG. 1; even simply looking at the electret obliquely will be 
informative for an experienced operator. 
If the electrical mobility of an aerosol is relatively constant at any 
industrial situation, this measurement could be dispensed with. In any 
case, even without it the passive sampler would give a good indication of 
relative exposure between two workers at different positions. The passive 
sampler may also be used in a screening mode, such that measurements made 
with it could be used to decide whether a rigorous conventional 
(expensive) dust sampling exercise should be undertaken. 
FIG. 3 shows a modification of the FIG. 1 sampler, still according to the 
invention. Unchanged parts have the same reference numerals. 
The insulator 2 is backed with a metal foil 25, which is a couple of 
millimetres oversize and the excess part of which is folded over the front 
of the insulator 2, somewhat in the manner of a picture frame. 
The metal grid 3 and conductor sheet 6 span an area larger than the exposed 
front of the insulator 2 but smaller than the whole sheet including its 
"frame" 25a. The grid 3 is integral with a metal flange 30 equipped with 
holes for grub screws 24 for fastening the flange 30 to the base 21 (in 
this case of metal). 
By this means, the flange 30 can clamp the insulator 2 in position without 
adhesive, and consistency of exposed area 2a of insulator 2 is assured by 
the frame 25a. 
If desired, the flange 30 and/or the conductor sheet 6 may be gridwork 
instead of continuous sheet, the gridwork optionally being the same as 3. 
As is clear from the foregoing, the device can be produced in a variety of 
shapes or forms. However, the analysis of collected samples will be much 
easier if the electret 2 is similar in shape and size to a conventional 
sampling filter. It should, therefore, take the form of a 25 mm diameter 
disc, and this consideration gives rise to the sampler according to FIG. 
4, which will now be described. 
The electrets 2 used in the samplers of FIGS. 1 and 3 are square, and so 
the grid 3/6a and base 1 natural form a cuboid, which can be formed by 
cutting and folding a sheet of grid material. If the electret is a disc 
the grid should be nummular, see FIG. 4. This shape has the advantage of 
having no sharp corners, but it has the drawback that it cannot easily be 
made from a single sheet of metal. 
The field at the edge of the electret is at its weakest, especially if the 
grid encompasses an area not much larger than the electret, and this 
reveals itself by a low aerosol deposit in this region. Since uniformity 
of deposit will aid analysis, this observation suggests the use of a large 
grid. From a practical point of view however, it is important that the 
grid should be no larger than necessary, or it may become less convenient 
to wear. 
The electret 2 takes the form of 25 mm diameter 23 .mu.m-thick discs of 
MELINEX (Trade Mark) polyester film attached centrally to a 40 mm diameter 
stainless steel backing plate or base 1. No adhesive is used, 
electrostatic attraction being sufficient to hold the electret in place. 
A cylindrical grid 3 of diameter 40 mm and height 10 mm consists of 
stainless steel rectangular mesh with 22/3 mm-square holes separated by 
1/2 mm-wide metal. Where the loss in strength can be accepted in the 
interests of better air flow, the intermediate circumferential metal can 
be omitted, as shown in part, so that the grid then consists merely of two 
1/2 mm-wide circular strips spaced by 9 mm-high axial pillars at 31/6 mm 
intervals circumferentially. The grid 3 is, in either version, capped at 
one end by a stainless steel disc 6 of diameter 40 mm with drilled lugs 
6a, corresponding to through-holes la in the base 1, for interconnecting 
the disc 6 and base 1 by suitably long axial tamper-proof nuts and bolts 
7. Air can flow easily through the 10 mm-deep space between the electret 2 
and the disc 6. A pin or clip 11 attaches the sampler to clothing. 
Small holes (not shown) may be provided in the base 1 to allow the electret 
to be poked out. 
The through-holes 1a have a further function; they act as locating holes, 
allowing the electret to be scanned before and after dust loading, with 
confidence that it has remained in exactly the same position, which is 
useful if mechanical automated charge-monitors are used for taking the 
measurements. 
FIG. 5 shows in partial section (to a different scale) a modification to 
the FIG. 4 sampler. This modification permits the electret 2 to be stuck 
to a metal disc or to a glass disc such as a 25 mm diameter microscope 
cover slip, for subsequent X-ray or optical examination respectively. The 
metal or glass disc is located by a metal annulus 8 of similar thickness 
and 251/2 mm inside diameter fitting onto the base 1, and the disc is 
retained by a slightly smaller overlapping annulus 9 cemented centrally on 
the first annulus 8. The first annulus 8 has an external diameter 
exceeding the 40 mm of the grid 3 and has lugs located by the nuts and 
bolts 7, whereby the annuli 8 and 9 are retained against sliding out of 
position by the action of the bolts 7 and are retained against coming off 
the base 1 by the presence of the edge of the grid 3, whereby in turn the 
electret 2 is retained in place by an inner ledge 89 formed between the 
annuli. The annuli form an outer step 98 which assists in locating the 
grid 3 when assembling the sampler. 
Alternative samplers in which the grid 3 and disc 6 had diameters of 26 mm 
and of 33 mm were made and are useful, but the 40 mm model here described 
showed consistently in use the most uniform deposit of particles over the 
greatest area of the electret 2. 
In a modification, where the conductor 6 is a continuous sheet, it too may 
bear an insulator e.g. electret, facing the electret 2. The sides of the 
two electrets facing each other are preferably of opposite charge. This 
arrangement assists capture of dust samples regardless of the sign of any 
charge which the particles may carry, and either or both of the electrets 
may be used in the subsequent measurements. 
Appendix 1 
Polymers suitable for electret production include the following: 
______________________________________ 
Polycyclohexyl methacrylate 
(PCHMA) 
Polyethyl methacrylate (PEMA) 
Polymethyl methacrylate (PMMA) 
Polyphenyl methacrylate (PPhMA) 
Polyethylene (PE) 
Polypropylene (PP) 
Polyvinyl chloride (PVC) 
PVC-acrylonitrile copolymer 
(PVC-A) 
Polyvinylidene chloride (PVDC) 
Polyvinyl fluoride (PVF) 
Polyvinylidene fluoride (PVDF) 
Polybisphenol A carbonate 
(PC-n) 
Polyethylene terephthalate 
(PET) 
Polytetrafluoroethylene (PTFE) 
Polyfluoroethylene propylene 
(FEP) 
Tetrafluoroethylene-hexa- 
(Teflon-FEP) 
fluoropropylene copolymer 
Tetrafluoroethylene-hexa- 
(Teflon-PFA) 
fluoromethoxyethylene copolymer 
Polyester (MELINEX) 
Polycarbonate 
Cellulose ester 
______________________________________ 
Appendix 2 
Analysis techniques suitable for determining dust include the following: 
1. Gravimetric 
a) direct weighing 
b) solvent extraction, evaporation, weighing residue. 
2. Chemical Analysis--In situ 
a) X-ray spectrometry (including total reflectance X-ray spectrometry and 
X-ray fluorescence spectroscopy such as proton-induced X-ray emission 
spectroscopy)--elements with atomic number.ltoreq.8 or 10 
b) X-ray powder diffraction--measures compounds rather than elements, 
detection limit poor--10 .mu.g 
c) Reflected light microscopy oblique incidence X-ray or light scattering, 
and for translucent sheets: light extinction coefficient or transmission 
optical or infrared microscopy and spectroscopy 
d) Scanning electron microscopy (with energy) dispersive X-ray spectrometry 
and selected area diffraction)--size, shape, composition of particles. 
e) Auger spectrometry 
f) Reflectrance infra-red spectroscopy 
g) UV spectroscopy 
h) Colorimetry--if the sheet has some chemical reagent that will react with 
the dust to provide measurable colour change 
3. Dissolution of the sheet followed by Chemical Analysis--the full armoury 
of analytical chemistry can then be employed. For example: 
______________________________________ 
a) gas chromatography 
b) high performance liquid chromatography 
c) thin layer chromatography organics 
d) atomic spectroscopy 
e) inductively coupled plasma spectrometry 
f) ion chromatography - inorganic ions 
(sulphates, chlorides, etc.) 
g) Nuclear magnetic resonance 
h) micro-biological techniques 
i) any of the in-situ techniques 
mentioned at 2 above. 
______________________________________ 
in addition, electron microscopy could be used as well as optical 
microscopy, and automatic counting techniques could be implemented as 
well. 
As discussed above, it is possible to attach the electret to the body of 
the sampler by its electrostatic attraction. In this form the sampler is 
at its simplest, and there will be no adhesive to interfere with the 
analysis. However, certain types of analysis may require the electret to 
be attached to a rigid substrate. Some of the foregoing analysis 
techniques are discussed below, with reference to the chemical nature of 
the electret required and in particular to the physical form in which it 
is needed. 
X-ray Fluorescence Spectroscopy 
This technique is suitable for analysis of elements with atomic number 
above 8 and is used in the Occupational Medicine & Hygiene Laboratory of 
the GB Health & Safety Executive mainly for toxic metal analysis. During 
the analysis the electret, without backing, is mounted between two 
molybdenum masks specially designed for holding 25 mm diameter membrane 
filters. Polyester is a commonly used material in such analyses, and it is 
also suitable as an electret. 
It is important that the captured material does not become detached from 
the substrate when it is neutralized by exposure to X-rays. Particle 
detachment is not, however, thought to be likely to occur because although 
it is the electric forces that attract particles and bring them into 
contact with the electret, it is principally van der Waals forces that 
will keep them there. These forces are not affected by neutralization of 
the electret material. 
X-ray Diffraction 
This technique is used for the analysis of both crystalline and amorphous 
silica. It requires the sampling substrate to be placed on an aluminium 
disc, which is then exposed to X-rays. The electret can be detached from 
the sampling base and then placed on the aluminium disc, but it would have 
to be neutralized before detachment. Alternatively the electret could be 
stuck to a thin disc of metal, which could be used in the sampler and then 
transferred to the X-ray equipment for analysis. 
PVC-acrylonitrile copolymer is known to be a suitable material for this 
analysis. Polyester may be tried. 
Optical Microscopy 
Optical microscopy is used for the assessment of natural and man-made 
fibres. The sample is usually collected on a clearable membrane, which may 
be polycarbonate or cellulose ester. An electret with a grid ruled on it, 
like certain types of membrane filters, would be useful in helping the 
microscopist to locate the plane of the sample, especially if the deposit 
is sparse. It is possible that the MELINEX membrane could be used directly 
instead. Alternatively an electret might be stuck to a microscope cover 
slip (25 mm diameter 0.2 mm thick cover slips are readily available) which 
could then be stuck upside down on a microscope slide. The refractive 
index of the polymer may be important in analyses of this sort. The 
optical characteristics of the collector will be less important if 
reflected light microscopy is used. 
Scanning Electron Microscopy 
Preliminary measurements suggest that the electrets we have used are 
beam-stable, and so electron microscopy is a suitable analysis tool. 
Cellulose ester filters would be preferred since their use is well 
established. Polycarbonate would also be acceptable. Analysis of the 
MELINEX electret could be carried out in much the same way as that of a 
membrane filter sample, by examining the deposit after coating it with a 
thin layer of gold. 
Weighing 
Gravimetric analysis is most likely to be successful if samples are 
relatively large. Weighing of the sampler will be easier if the electret 
is stuck to a stable substrate like a stainless steel disc. Weighing a 
detached electret may be a problem because of the electric charge that it 
holds or might develop during handling, and in any case weighing is best 
carried out in a Faraday cage. Polypropylene is likely to be a good 
electret for this purpose because its moisture retention is low, probably 
about one quarter that of MELINEX (polyester). Errors caused by humidity 
could be reduced by sample equilibration and the use of controls, which 
are standard techniques in filter weighing. 
Standard five-figure microbalances would give measurements to within about 
20 .mu.g for an electret weighed in its free state, and about 100 .mu.g 
for an electret attached to a rigid substrate. However, the use of a 
six-figure microbalance--like that employed in the gravimetric estimates 
made on early prototypes, would give a limit of .ltoreq.10 .mu.g. 
The surface of an electret is almost impermeable to deposit and so it 
should, in principle, be possible to remove the deposit by dissolving it, 
and to analyse the solution. A very important advantage of this technique 
is that it allows analysis to be carried out by Inductively Coupled Plasma 
Mass Spectrometry, which is an extremely sensitive technique capable of 
detecting nanogram quantities of deposit. 
It is usually beneficial to dissolve the filter (or electret) along with 
the deposit because this makes it easier to effect quantitative transfer, 
improving the efficiency of the analysis. This means that the electret is 
better in an isolated form since the adhesive used to stick to a backing 
plate may complicate the analysis, and cause contamination. The ideal 
polymers are cellulose esters, because of their high solubility. PVC and 
PVC-acrylonitrile copolymer are also useful. In this technique, the 
suitability of polyester is uncertain, and polypropylene is unsuitable.