Note: Descriptions are shown in the official language in which they were submitted.
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TIT~E
Gaseous Contaminant Dosimeter
BACKGROUWD OF THE INVENTION
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Field of the Invention
; This invention is related to a personal
dosimeter for registering gaseous contaminants in the
atmosphere. More particularly, it is related to a
dosimeter capable of collecting a quantity of con-
taminant in proportion to its average atmospheric
concentration to aid in determining the integrated
exposure level to the contaminant.
Description ôf The Prior Art
In response to the increasing concern about
the health of workers who are exposed to harmful
pollutants in the air, it has become necessary to
monitor the concentration of the air-borne contaminants.
One developm nt for this purpose involved a rather
large air pump which would force air to be sampled
through a filter, trapping particulate contami~ants.
This obviously is unavailing for the monitoring
of gaseous contaminants and, even for particles,
is not accurate to determine concentration of the
particles in the sampled atmosphere.
Personal sampling devices which are worn
by individual workers and which passively collect
the contaminants have also been used. For example,
a device which utilized the molecular diffusion of
the gas to be monitored to collect the sample has
been described in ~merican Industrial Hygiene
Association Journal, Volume 34, pages 78-81 (1973).
This device and others like it, called impinging tubes,
are often cumbersome to use since their design and
delicate construction necessitate that they always
be oriented properly to accurately sample the
atmosphere and to prevent dislocation of the sampling
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mechanisms within the tu~e.
Therefore, there remains a need for a
personal dosimeter that simply but accurately
collects gaseous contaminants in proportion to their
average atmospheric concentration.
SUMMAR~ OF THE INVE~TION
According to the present invention, there
is provided a personal dosimeter for collecting a
gaseous contaminant in proportion to its average ambient
concentration during the collection time where the dosimeter
consists essentially of
a closed receptacle;
a collecting medium for the gaseous contaminant
within the receptacle;
a diffusion device, forming a part of the
boundary of said receptacle, the device containing
a plurality of through-and-through channels adapted
for the gaseous contaminant to diffuse therethrough
from the atmosphere to the interior of the receptacle,
said channels each having a length-to-diameter ratio
of at least 3 and said channels providing the only
communication between the atmosphere and the
interior of the receptacle; and
a porous, hydrophobic, inert film covering
the interior openings of said channels.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a magnified perspective view
of a diffusion device usable in the present invention.
Figure 2 is a top view of a gaseous
contaminant dosimeter utilizing the diffusion device
of Figure 1.
Figure 3 is a partial perspective view of the
dosimeter of Figure 2.
Figure 4 is an exploded view of another
dosimeter of the present invention.
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Figure 5 ls a cross-sectional view of the
assembled dosimeter of Figure 4.
DETAILED DESCRIPTIOW OF T~E ~NVENTION
The dosimeters of this invention collect
a gaseous contaminant in proportion to its average
concentration in the atmosphere during the collection
period and provide for the expedient determination
of this concentration. This is achieved by
passively sampling the gaseous contaminant in
ambient air in proportion to its cor.centration
therein by allowing the contaminant to diffuse into
an interior portion of the dosimeter where it is
maintained,by a collecting medium situated therein,
until it is analyzed.
The collecting medium holds the gaseous
contaminant or its ions in a form that is more
readily analyzable than is the gaseous form. After
collection, the medium is removed from the dosimeter
and treated with appropriate reagents to product
color, the intensity of wnich is dependent upon the
amount of gaseous contaminant collected. The time-
average ambient concentration can then be determined,
as later explained, with a previously-calibrated
colorimeter or spectrophotometer. Alternatively,
the contaminant can be separated from the collecting
medium and its quantity determined, for example,
by gas chromatography wherein the results of the
gas chromatography analysis have been previously
calibrated against known time-average ambient
concentrations of the contaminant. The preferred
method of determination is colorimetric.
Generally, the collecting medium is a material
that absorbs, adsorbs, reacts or otherwise combines
with the gaseous contaminant being measured. Regard-
less of the manner in which the medium interacts,
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as above, with the contaminant, the quantity orstrength of the collecting medium in the dosimeter
should be sufficient to interact completely wich the
total quantity of gaseous contaminant which is
anticipated to be collected. The collecting medium
will often be specific to the particular gaseous
contaminant being monitored. Examples, meant to be
representative but not limiting, include aqueous
solutions of oxidizing a~ents or triethanol amine
to absorb nitrogen dioxide, solutions of potassium
or sodium tetrachloromercurate to absorb sulfur
dioxide, solutions of sulfuric or other acids to
absorb ammonia, and distilled water or a solution of
sodium bisulfite to absorb formaldehyde. Charcoal
or powdered carbon of high surface area, powders of
metals, or metal salts can be used to adsorb many
other organic contaminants.
Methods for colorimetric analysis, for
example, for sulfur dioxide, nitrogen dioxide,
ammonia, or formaldehyde, in air, are described in
National Institute for Occupational Safety and
Health method numbers 160 (publication 121, 1975),
108 (publication 136, 1974), 205 (publication 121,
1975) and 125 (publication 136, 1974), respectively.
The techniques therein described are readily adaptable
with respect to absorbing solution and color-forming
reagents for use in connection with collection by
the dosimeter of the present invention.
One preferred embodiment of the present
invention is shown in Figures 2 and 3 and is des-
cribed and can be formed as follows. A base sheet
3 of impermeable polymeric material is provided to
form one side of the receptacle portion of the
dosimeter. The sheet is preferably transparent and
thermoplastic and can be made of polymers of olefin,
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halogenated polymers, polyesters, or ionomer resins.
Preferred materials are the ionomer resins shown
in U.S. Patent No. 3,264,272 issued August 2, 1966
to R. W. Rees. They are the ionic copolymers of
alpha-olefins and alpha, beta-ethylenically
unsaturated carboxylic acids of 3-8 carbon a~oms
having 10-90% of the carboxylic acid ~roups neutralized
with metal ions.
The size of sheet 3 is not cri-tical but is
preferably a size easily adaptable for use in a
personal dosimeter which is to be worn or readily
carried. The collecting medium is placed in tne
central portion of sheet 3. When the medium is a
liquid, this can be accomplished by first forming
a depression in the central portion of the sheet by
applying pressure thereto with an appropriate die,
heated or otherwise. The depressed area of sheet 3
is the central portion of the interior, designated 5,
of the dosimeter.
After the collecting medium has been placed
on sheet 3, a second top-sheet 4 corresponding to
sheet 3 in composition and substantially in size
is placed over sheet 3. Heat and pressure are then
applied to the three areas ~ to provide permanent,
fluid-tight bonding at the three corresponding edges
of sheets 3 and 4. Adhesives or other forms of
bonding can also be used provided the bonds are
permanent and fluid-tight and the adhesive is inert
to the collecting medium.
An elongate gas diffusion device l llaving
a plurality of through-and-through channels 2 is
positioned parallel and proximate to the fourth,
unbonded edge of base sheet 3 and parallel and
flush with the fourth, unbonded edge of top sheet 4.
The open channels 2 of device 1 are thus oriented
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horizontally with respect to the plane of sheet 3 and
perpendicularly with respect to the fourth edges of
sheets 3 and 4. On the interior side 7 of device 1,
covering the channel openings of this interior side,
is a porous, hydrophobic film, described in more
detail below. Diffusion device 1, thus placed between
sheets 3 and ~, is bonded to the sheets by the
application of heat and pressure or by use of
adhesives which should be impermeable and chemically
inert to the collecting medium.
The bond between diffusion device l and
each of sheets 3 and 4 should be liquid-tight and
air-tight, thus completely enclosing the interiox 5
of the receptacle formed by sheets 3 and 4. The
relative positions of diffusion device l and sheets
7 and 8 are such that the channels 2 provide the
only communication between the atmosphere and the
interior 5 of the receptacle.
It is also possible to form the dosimeter
of Figures 2 and 3 saving the placement of the
collecting medium, when it is a liquid, for last.
In such a case, the dosimeter is otherwise formed
as described above. The collecting medium can be
placed by piercing top sheet 4 at an appropriate
spot with a hypodermic needle and injecting a
measured amount of the collecting medium into the
interior. The hole made by the hypodermic needle
can then be thermally sealed.
Diffusiondevice 1 allows the gaseous
contaminant to diffuse through each of channels
2 according to Fick's Law, which is expressed in
relevant form as
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M = D-C t A/L
where
M = quantity of gaseous contaminant
transferred (mg)
S D = diffusion coefficient of ~he gaseous
` contaminant through air (cm2/min)
C = concentration of the contaminant
in the atmosphere (mg/cm3)
t = time of exposure (minutes)
A = cross-sectional area of the channel
( cm2 )
L = distance in direction of diffusion,
herein channel length (cm)
Values of D for various gaseous contami-
nants are readily available from the literature.
The purely diffusional nature of the transfer of the
gaseous contaminant through the channels, at a
rate in linear proportion to its atmospheric con-
centration, provides the integrating character of
the dosimeter.
Gas diffusion device 1 is preferably
made from materials that are non-hygroscopic and
both chemically and physically inert to the
gaseous contaminant and to the collecting medium.
Examples are polyethylene, polypropylene, polymers
or copolymers of tetrafluoroethylene and hexa-
fluoropropylene, and stainless steel. The above-
named polymers are preferred since they can be
easily injection-molded.
As can be seen from Fick's Law, the
number and diameter of the channels affect the
quantity of gaseous contaminant collected since they
affect the total cross-sectional area available for
transfer. The quantity of contaminant collected
is also inversely proportional to the length of
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the channels. Although these parameters are not
necessarily critical to the integrating operation
of the diffusion device, it has been found that
when each of the channels has a length-to-diameter
ratio of at least 3, preferably at least 4.5, the
dosimeter attains the desired insensitivi~ to
relative atmospheric motion caused by wind or move-
ment of the wearer. It has further been found that
the use of about 5-500 channels, preferably lO-100
channels,eaah having a diameter of about 50-1000
microns and a length of about 1.0-25.0 mm, preferably
3.0-8.0 mm, provides a device that is su ficiently
sensitive to low ambient contaminant concentrations
but is still of a conveniently small size.
A porous, hydrophobic film of 15-1000
micron thickness is placed over the channel openings
on the interior side 7 of diffusion device 1, the
side communicating with the interior ~ o the
dosimeter. The film can be made, for example,
of polymers or copolymers of tetrafluoroethylene and
hexafluoropropylene. The function of the film is to
prevent the absorbing solution, if that form of
collecting medium is used, from flowing into the
channels of diffusion device 1 and to further reduce
sensitivity to atmospheric motion. Accordingly, the
porosity of the film and the size of its pores
should be selected so that these functions are
performed without interfering with the passage of the
gaseous contaminant from the interior ends of the
channels to the absorbing solution. That is, the
diffusion of gaseous contaminant through this film
should be significantly greater than the diffusion
through the channels so that the overall rate of
diffusion is essentially controlled only by the
channels. It has been found that a film that is
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50-80% porous with a pore size of 0.1-3.0 microns
is sufficient for this purpose when channels as
previously~described are used.
The diffusion device 1 of Figure 1 and
the dosimeter of Figures 2 and 3 are an example of
preferred embodiments of the present invention but
the invention is not limited thereto The diffusion
device, for example, can be in the shape of a plug
sealed into the face of either sheet 3 or sheet 4.
Similarly, the receptacle of the dosimeter need not
be pouch-like as hereinbefore decribed but, for
example, could be in the form of a rigid cuvette.
Such an embodiment of the present invention
is shown in Figures 4 and 5. With respect to these
Figures, there is shown a personal dosimeter composed
of a circular base cup 8 having cylindrical walls
which define an open cavity 9. Base cup 8 is pro-
vided with flange 13, having circular opening 14,
which permits the assembled dosimeter to be fastened
easily to a person's clothing. A friction-fitting
circular cap ll tightly engages base cup 8 in a
manner forming closure of cavity 9. Cap ll has a
plurality of channels 12 of circular cross-section
extending therethrough towards cavity 9. The channel-
containing cap 11 functions as a gas diffusion
device following Fick's Law and therefore the number,
length, and diameter of channels 12 of cap 11 are selected
as previously described for channels 2 of diffusion
device l.
Cap ll and base cup 8 are preferably made
from materials that are non-hygroscopic and both
chemically and physically inert to the gaseous
contaminant and to the collecting medium. The
materials described for construction of diffusion
device l are equally preferable for use in making
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cap 11 and base cup 8.
A porous, hydrophobic,inert film 10 is
placed over the channel openings on the interior
face of cap 11. This film performs the same fun~tions
as earlier described with respect to the dosimeter
of Fig~res 2 and 3. Film 10 preferably is 15~1000 microns in
thickness and has a porosity of 50-80% with a pore size of 0.1-
3.0 microns. It is preferably made of polymers or copolymers of
tetrafluoroethylene or hexafluoropropylene, although any hydrophobic,
inert material of the above physical characteristics is sufficient.
In assembled form, the friction-fit between
base cup 8 and cap 11 is liquid~tight and air-tight,
thus completely enclosing cavity 9 of cup 8.
Channels 12 of cap 11 thus provide the only com-
munication between the atmosphere and cavity 9 and
the collecting medium therein. The collecting medium
preferably completely fills cavity g and is retained
therein, against seepage through channels 12, by
film 10.
In use, a dosimeter of this invention is
exposed to the air containing the gaseous contaminant
for a period of time for which the average contaminant
concentration is sought. When the collecting medium
is an absorbing solution, for example, a measured
amount of the solution is then withdrawn from thedosimeter by, for example~ a hypodermic syringe.
When the analysis is to be made photo-
metrically, the withdrawn absorbing solution is
mixed with appropriate color forming reagents which
change the color of the absorbing solution. The
intensity of color so formed is dependent upon the
amount of gaseous contaminant collected. Although
it is often desirable to have a self-contained
dosimeter, as shown in U.S. Patent 4,208,371, in
which the reagents are contained in the dosimeter
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and no withdrawal of material is necessary, this
is sometimes impractical. An example o this is
where the reagents are highly acidic, as in the
color-production for formaldehyde where the reagents
are chromatropic acid and sulfuric acid. In such
cases, it is difficult to package the reagents in
a s~able and safe form, and the simple dosimeter
of the present invention is well suited for these
applications.
The dosimeter of this invention can be
calibrated to give a direct relationship between
colorimetric or spectrophotometric readings and
average ambient concentration of the gaseous
contaminant. This can be accomplished by follGwing
a calibration procedure similar to that described
in U.S. Patent 4,208,371. In such a procedure,
several dosimeters are exposed over a given period
of time to various known concentrations of a con-
taminant for which calibration is sought. The
dosimeters contain the same kinds and amounts of
collecting medium. Spectrophotometric readings,
for example, are determined or at least two
dosimeters at each of several known concentrations,
and a straight-line is plotted, using a least-squares
analysis, through the data points thus obtained.
