Note: Descriptions are shown in the official language in which they were submitted.
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Method for solid-phase microextraction and analysis, and a collector for this
method
Field of the Invention
The invention relates to a method for solid-phase
microextraction and analysis of substances originating for example from a
carrier fluid and to a collector for use within this method. The invention
also
relates to a collector, which serves as a passive collector in a gaseous, for
example loaded environment containing substances to be analyzed and a
method for solid-phase microextration and analysis of this substances.
Description of the Related Art
A method of this type is known from Boyd-Boland et al.,
Environ. Sci. Technol. Vol. 28, No. 13, 1994, 569A-574A and from
EP 0,523,092 B1, in which a special syringe is used, which has a fiber
which can be telescoped through the syringe needle. The fiber, which is
expediently coated, is brought into contact with the carrier fluid which
contains the substances to be examined and at the same time is being
stirred, after which the fiber is retracted and the syringe needle is
introduced into a feeding device of an analyzer, followed by desorption of
adhering substances using a carrier gas. The fiber has only a very limited
absorption capacity for substances which are to be examined and,
moreover, is only dipped into the stirred carrier fluid, so that consequently
the sensitivity of the analysis itself leaves something to be desired if the
coated fiber is vibrated. In addition, it is known from DE 196 19 790 C2 to
have the microfiber functioning as a collector rotate about its own axis by
means of an electric motor with a rotational speed of one's choice.
Summary of the Invention
An object of the invention is to provide a method for solid-
phase microextraction and analysis of substances to be analyzed and
originating for example from a carrier fluid, which provides a significantly
improved sensitivity.
Another object of the invention is to provide a collector for
solid-phase microextraction and analysis of substances to be analyzed and
originating for example from a carrier fluid.
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A further object of invention is to provide a collector which may be used
within the method for solid-phase microextraction and analysis of substances.
A still other object of the invention is to provide a collector as a passive
collector for solid-phase microextraction and analysis of substances for
example
pollutants to be analyzed and originating, for example from an environment.
A still other object of the invention is to provide a method for solid-phase
microextraction and analysis of substances to be analyzed and originate from
an
environment, for example pollutants.
According to an aspect of the present invention is a method for solid-
phase microextraction and analysis of substances in a carrier fluid, in which
a
collector is brought into contact with a stirred fluid containing the
substances for a
sufficient time to adhere at least one of the substances and is then subjected
to a
solid-phase extraction directed at at least one substance adhering to the
collector, and desorbed substances are transported for analysis by means of a
carrier gas, wherein the carrier fluid containing the substances is stirred in
a
receptacle of a magnetic stirrer by means of a coated stirring element as
collector, and/or the carrier fluid is made to move relative to the collector
by
means of ultrasound, and afterwards the stirring element is arranged in a
solid-
phase extraction device.
According to another aspect of the invention is a collector for a solid-
phase microextraction and analysis of substances to be examined contained in a
carrier fluid, comprising a stand alone rod-like carrier, sheathed in a
flexible tube
of material selected from the group consisting of a sorbent, an adsorbent and
a
combined sorbent and adsorbent, being capable of being picked up out of the
carrier fluid.
A still other subject of the invention is a coated magnetic collector, which
serves as a passive collector in a gaseous, for example loaded environment
containing substances to be analyzed.
A still other subject of the invention is a method for solid-phase
microextraction and analysis of substances in an environment, in which a
collector is exposed to the environment for a sufficient time to adhere at
least one
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of the substances as a passive collector and then the collector is positioned
in a
solid-phase extraction device, desorbed substances being transported for
analysis by means of a carrier gas.
The use of a stirring element which is actuated by a magnetic stirrer
and/or an ultrasonic agitator increases the accuracy of analysis very
considerably, and moreover it is possible to use large-volume receptacles,
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for example liter receptacles, for the fluid containing the substances to be
examined.
Further objects, advantages and embodiments of the
invention are evident from the following description.
The invention is explained in more detail below with reference
to exemplary embodiments of the invention, which are illustrated in the
attached drawings, of a device for carrying out the method and of passive
collectors.
Brief Description of the Drawings
Fig. 1 is a diagrammatic view of a device for carrying out the
method for solid-phase microextraction and analysis of substances which
are in a carrier fluid.
Figs. 2 to 4 show various embodiments of passive collectors,
in section.
Fig. 5 shows a comparison of two equilibrium curves relating
to the prior art and the invention.
Fig. 6 is a diagrammatic view of a further device for carrying
out the method for solid-phase microextraction and analysis of substances
which are in a carrier fluid.
Fig. 7 is a diagrammatic view of a headspace receptacle.
Description of Preferred Embodiments
In accordance with Fig. 1, a magnetic stirrer 1 is provided, which
comprises a receptacle 3 which is positioned on a base 2, is in the form of
a beaker and may expediently be closed at its top side by a septum 4.
Before it is closed by the septum 4, the receptacle 3 receives a carrier fluid
containing substances which are to be analyzed. The receptacle 3 may
have been filled and closed in advance at a sampling point. The carrier fluid
may be water and/or an organic solvent or a mixture thereof or liquefied
gas.
The base 2 comprises an electric motor 5, the shaft of which
bears an magnet 6 eccentrically. In the receptacle 3, there is a stirring ball
7 made from ferromagnetic material, such as iron or steel, which is
advantageously glass- or plastic-sheathed and has a diameter in the range
of a few millimeters. The plastic sheating 7a used may, for example, be
polytetrafluoroethylene or another fluorinated hydrocarbon polymer. The
stirring ball 7 is preferably covered with an active phase 7b for
sorption/adsorption of substances contained in the carrier fluid. This may
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be a coating selected from the group comprising polyethylene glycol,
silicone, polyimide, octadecyltrichlorosilane, polymethylvinylchlorosilane,
liquid-crystal polyacrylates, grafted self-organized monomolecular layers
and inorganic coating materials.
The stirring ball 7 can be stirred for a sufficient time during
which it comes into intensive contact with the carrier fluid and therefore
with
the substances contained therein and sorbs and/or adsorbs the latter, so
that it serves as a collector. After the end of stirring, the stirring ball 7
is
picked up and introduced in a solid-phase extraction device, preferably a
desorption device 8. The latter advantageously comprises a desorption
tube 9 with a diameter section whose diameter is slightly greater than that
of the stirring ball 7, followed, via a frustoconical section, by a diameter
section whose diameter is smaller than that of the stirring ball 7. The
desorption device 8 is part of an analyzer 10, for example of a gas
chromatograph, connected to a carrier gas port 11, so that carrier gas flows
through the desorption tube 9 passing the stirring ball 7, desorbing
substances adhering thereto, and can supply these substances for
analysis. The desorption device 8 preferably comprises a heating device
12, in order to carry out a thermal desorption.
The stirring ball 7 can be removed from the receptacle 3
automatically by means of a discharging device 13 which penetrates the
septum 4 and may be designed in the form of a gripping device, suction
device or also as a magnet, and can be positioned in the desorption tube 9,
which can then be placed automatically in the desorption device 8, so that
the entire solid-phase microextraction and analysis can be performed
automatically. For this purpose, appropriate receptacle 3 for a plurality of
samples can be arranged on a turntable which can rotate in steps and
beneath which, in one position, the base 2 of the magnetic stirrer 1 is
arranged.
In order to achieve reproducible measurements, stirring times
of approx. 45 to 60 min are generally required.
As an alternative to a stirring element in the form of a stirring
ball, it is also possible to use an element in the form of a stirring rod 14.
This may have a coated rod-like carrier 15 made from ferromagnetic or
paramagnetic material; in the latter case, it should have a minimum length
of approximately 2 cm, while shorter lengths are also possible if
ferromagnetic material is used. For example, it may be a rod-like carrier 15
which is rounded at the ends and is entirely coated with the active phase
15b (Fig. 3) or also a carrier comprising a section of wire which is
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surrounded by a cylindrical sheath 16 made from material of the active
phase which is in the form of a flexible tube (Fig. 4). By way of example,
the rod-like carrier 15 may have a diameter of approx. 3 to 6 mm.
It is also possible to achieve improved analysis if the carrier
fluid which contains the substances and is situated in a receptacle
containing a collector, which is preferably a stirring ball 7, is
alternatively or
additionally set in motion by means of ultrasound. Fig. 6 shows one
embodiment of such an ultrasonic stirring device as a resonator unit 17 in
which one or more ultrasonic generators (vibrators) 19, which are shielded
by means of insulating plate 18, are incorporated at the bottom and/or on
the sides of a housing 20. A metal wall 21 which is mounted in front of the
ultrasonic generator 19 and has a wall thickness of d = n*k/2 (k:acoustic
wavelength) transmits the vibrations to a coupling liquid 22 in the resonator
unit 17, preferably water, which is set in motion. The receptacle containing
the substances to be analyzed is introduced into the resonator unit17.
In this case, the receptacle 3 is also advantageously one of a
magnetic stirrer, in which case stirring is carried out by means of the coated
magnetic stirring ball 7 as collector, so that the ultrasound is applied in
addition to the magnetic stirring.
In general, thermal, liquid or desorption by means of
supercritical gases is possible.
As an alternative to being introduced into a thermal desorption
device 8, the stirring element can be arranged in a headspace receptacle
23 (Fig. 7), the diameter of which is only slightly greater than the diameter
of the stirring ball 7, by means of the discharging device 13. The
headspace receptacle 23 is then closed by means of a septum 24 and a
sealing ring 25, using a closure instrument, and is introduced into a
headspace head 26. In the latter, the headspace receptacle 23 is
preheated by means of a heating device 27 and there is a build-up of
pressure, during which an equilibrium for the volatile substances to be
examined with the gas phase 28 above the stirring ball 7 is established.
These substances can be removed by means of a syringe which
penetrates the septum 24 and can be fed to the separation column of, for
example, a gas chromatograph.
As an alternative to being desorbed in a thermal desorption
device 8, the stirring element may also be introduced into an extraction
device containing an organic liquid, the organic liquid used exhibiting a high
level of interaction with the substances to be examined and absorbing the
latter - if necessary during a stirring movement of the stirring element with
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respect to this liquid, after which the liquid which is enriched with the
substances to be examined is taken up by means of a syringe and taken to
a feeding device of, for example, a gas chromatograph, in order to be fed
for analysis, for example using a gas-chromatography separation column,
by means of a carrier gas.
Due to the use of a stirring element in a magnetic stirrer or,
alternatively or additionally, in an ultrasonic agitator and its intensive
contact with the carrier fluid containing the substances to be examined, it is
possible to achieve a sensitivity of analysis which is orders of magnitude,
for example about 1000 times, better than the use of the known fiber. Fig. 5
shows a diagram comparing the yield (plotted on the ordinate) of absorbed
substances for a known fiber coated with active phase (curve A) and a
stirring rod which is sheathed by active phase and has been stirred by
means of the magnetic stirrer according to the invention (curve B) at mass
equilibrium, the concentration quotient (K(o/w)) of a substance in octanol
and water being plotted on the abscissa. This coefficient (for normal
temperature) can be found in the literature for a wide range of substances.
For example, if this concentration quotient is 100, it can be seen from the
diagram shown in Fig. 5 that in this case the coated fiber provides a yield of
approximately 1 % and the invention provides a yield of approximately 50%.
At a concentration quotient of below 100, the coated fiber cannot generally
carry out any reliable measurement, while the sheathed stirring element
generally still allows absolute reliable measurements. Generally, the
covered stirring element improves the measurement accuracy
considerably, i.e. by powers of 10, and widens the measurement range
considerably, in that the sensitivity of analysis is improved approximately by
a factor of 1000. With the sheathed stirring element, there is generally no
need for the sensitivity of analysis to be improved by heating the liquid
containing the substances to be examined, as is necessary in many cases
for a coated fiber, and cause moreover measurement errors.
Moreover, a stirring element of this type may be arranged as
a passive collector in a gaseous, for example loaded environment
containing substances to be examined or may be carried by a person
working in the environment, in which case the passive collector is exposed
to the environment for a sufficient time and afterwards the substances
which it has sorbed and/or adsorbed are subjected to extraction, after
which desorbed substances are transported for analysis by means of a
carrier gas via a feeding device, for example in order to monitor personal
exposure to pollutants.
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Although the foregoing has been a description of preferred
embodiments of the invention, it will be apparent to those skilled in the art
that numerous variations and modifications may be made in the invention
without departing from the scope as described herein.
