Note: Descriptions are shown in the official language in which they were submitted.
20846~
FLOW CBL~ APPAR~TU8 FOR U8B IN HBAD~PACE ANA~Y~IS
The present invention relates to a flow cell device for use
in headspace analysis and more particularly, to a constant
headspace volume cell permitting sampling in a dynamic condition.
Sampling of soils and industrial effluents for hazardous
materials has always been of importance, however, with the advent
of enhanced environmental concerns revolving around pollution,
compliance with more stringent regulations has become important
and required sophisticated analytical equipment.
The primary difficulty presently experienced by industry is
the measurement of mixtures of volatile organic solvents, e.g.
benzene, toluene, xylene and perchloroethylene, CFC-113, etc.
Generally speaking, these substances, when dissolved in
water in a flowing system, are difficult to sample and measure
due to losses incurred by volatilization. In addition, most
chlorinated solvents cannot be detected with flow through
detectors, e.g. ultra-violet, fluorescence or conductivity HPLC
detectors since these compounds do not respond to such
instrumentation.
Instruments which are capable of handling some organic
compounds include, for example, gas chromatographs; a drawback
still exists, however, since most organic solvents have a higher
volatility than water making direct injection into gas
chromatographs impractical.
Presently used methods for solvent collection include
discrete sampling followed by analysis. Clearly, this procedure
is labour intensive and is highly susceptible to partial losses
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of the volatiles.
Schnable et al., in LC-GC Vol. 9, number 12, describes a
purge-and-trap sampling valve for use in the monitoring of small
sample streams. This system does not provide a flow cell for
sampling on-line and further lacks a compensatory fluid line for
equalizing the pressure in the cell, such as that which is
provided in the present invention.
Headspace analysis equipment which is known in the field is
manufactured by Siemens. The Siemens apparatus is useful for on-
line analysis, but does not provide a compensatory air-line or
permit headspace analysis of low flow rate fluid streams.
U.S. Patent No. 5,050,425, discloses an apparatus for use
in the analysis of headspace~ The valve employed in the
apparatus is devoid of a compensatory air-line, and an over flow
vent. In addition, there is no provision for a constant
headspace volume in the valve. Further still, this sytem is not
dynamic and only is useful for soil samples and not water
samples.
Engelhardt et al., disclose in U.S. Patent No. 5,058,416,
an apparatus for the determination of the partial pressure of
gases dissolved in a fluid. There is no teaching of a constant
volume flow cell for headspace analysis in this reference.
A continuous air monitoring apparatus is taught in U.S.
Patent No. 5,014,541. This document teaches the use of a solid
sorbent material to concentrate airborne materials. The
patentees do not discuss headspace analysis in an in-line system.
Other generally relevant references include U.S. Patent Nos.
208~616
4,617,828, 4,791,820 and 5,094,099.
In view of the instruments which have been set forth in the
art to date, there exists a need for an apparatus capable of
collecting headspace material from a continuously flowing fluid
sample without interrupting the flow of the fluid. The present
invention addresses this need and provides a cell for sampling
a stream containing volatile materials to be sampled comprising:
a sample receiving inlet; a sample discharging outlet; a
collection zone intermediate said inlet and said outlet for
collecting volatile material evolved from said fluid; outlet
means for removing collected volatile material; and means for
supplying make-up fluid proximate said collection zone.
The cell according to the present invention has been found
to be particularly useful in on-line analysis procedures, since
the cell is provided with a constant volume headspace collection
zone, thus simplifying the analysis procedure. -
In dynamic headspace analysis, the critical factor is to
sample the headspace without disruptin~ the flow of material
being sampled or draw it up the sampling tube. The present
invention avoids this difficulty by including the compensatory
or make-up air line to provide a stream of air above the air-
water interface in the cell. In addition, the air-line flushes
the volume above the water once the headspace sample has been
taken. Accordingly, contamination of samples subsequently taken
is avoided.
A further advantage attendant with the apparatus of the
present invention is the provision of a vent to permit excess air
to exit the cell and air to enter during a sampling procedure.
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A further aspect of the present invention is to provide a
sampling cell for use in on-line analysis of continuous flow
fluid streams, said cell comprising: a sample receiving inlet;
a sample discharging outlet; a collection zone intermediate said
inlet and said outlet for collecting volatile material evolved
from said fluid; a conduit in communication with said collection
zone, said conduit for supplying a make-up gas to said collection
zone; and vent means for venting excess make-up gas from said
cell.
Yet another aspect of the present invention is to provide
a flow cell for use in dynamic headspace analysis which
facilitates direct and automatic analysis of the headspace
constituents.
The flow cell, as a further feature, permits two phases,
i.e. liquid and gas to be in contact and to equilibrate prior to
sampling. Generally, some volatile compounds at equilibrium will
be more highly concentrated in the headspace above the liquid
than in the liquid itself. Accordingly, such a feature permits
smaller volumes of liquid to be sampled effectively.
Having thus generally described the invention, reference
will now be made to the accompanying drawings illustrating
preferred embodiments and, in which:
Figure l is a cross-sectional view of the flow cell of the
present invention; and
Figure 2 is a schematic representation of the overall
analysis system within which the flow cell is employed.
Referring now to the drawings, Figure 1 shows a cross-
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2084616
sectional view of the flow cell of the present invention
generally represented by numeral 10.
The flow cell 10 includes a T-shaped body having
communicating passageways therein. Contaminated fluid, for
example water containing volatile organic compounds, enters the
cell at inlet 12 and exits the cell at outlet 14. The cell is
designed so that the inlet and outlet are in fluid communication
to the extent that continuous flow is possible therethrough.
Fluid exiting at outlet 14 may be discarded, recirculated or
recycled depending upon the constituents and their value. -
As extension 16 is provided on the body of the cell 10 and
is in fluid communication with the inlet and outlet and more
importantly, the f~uid passing therethrough.
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Within the extension 16, a sufficiently sized collection
area 18 is provided to capture volatile material evolving from
the fluid stream. The collection area 18 is of constant volume
and constitutes the headspace of the cell.
A juncture member 20 is connected to the T-shaped portion
of the cell by a conduit 22 extending therebetween: conduit 22
may be clear to provide the possibility of visual inspection.
A plurality of additional communicating openings 24,26 and
32 are included. ;~
opening 24 comprises an outlet for the volatile materials
collected in collection area 18. Various fittings, e.g. tubes,
etc. may be connected at 24 to remove the volatiles for analysis
by, for example, a gas chromatograph.
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Fluid conduit 30 extends within the cell via opening 26 in
juncture 20 to the collection area 18.
The conduit 30 is releasably and sealingly engaged within
juncture 20 with a septum 31. The septum may comprise silicone
rubber, Teflon or other suitable materials.
Conduit 30 is positioned such that when fluid to be sampled
is in the cell, the conduit 30 uses just above the fluid level.
When the headspace is sampled, there is a slight vacuum within
the cell caused by negative pressure at outlet 24 and
additionally, there is negative pressure included in collection
zone 18 due to the flow of fluid from inlet 12 to outlet 14.
Conduit 30 alters this pressure difference by providing gas,
typically air, nitrogen, helium or other suitable gases, to
partially compensate for the difference. This prevents the fluid
to be sampled from being drawn into the cell. In addition to this
function, the conduit 30 is used to "sweep" or "flush" the
collection zone of volatiles thus ensuring that carryover from
a sample is removed before subsequent samples are taken.
The third opening 32 permits any pressure build up to be
discharged from the cell. To this end, a vent 34 is positioned
over the opening. The opening 32 further aids in supplementing
gas entrance into the cell during sampling to prevent the
sampling fluid from being drawn into the cell.
In terms of materials for constructing the cell, rugged
inert materials are preferred, e.g. glass, with stainless steel
being most desirable.
Figure 2 illustrates a schematic representation of the
analysis system in which fell 10 would be employed.
20~4~B
A fluid sample 40 may be passed in conduit 42 to generator
column 46 via pump 44.
The fluid sample enters cell 10 at 12, the headspace is
sampled with the fluid sample passing into waste collection at
48. Volatiles evolved from the sampled headspace are passed on
via sampling line 50 to, for example, a Photovac~M portable gas
chromatograph. Data generated therefrom may be transferred to
computer, etc.
Gas enters conduit 30 to purge the collection zone 18 (not
shown) for subsequent sample collection.
Although embodiments of the invention have been described
above, it is not limited thereto and it will be apparent to those
skilled in the art that numerous modifications form part of the
present invention insofar as they do not depart from the spirit,
nature and scope of the claimed and described invention.
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