Chemistry: analytical and immunological testing – Including sample preparation – Gaseous sample or with change of physical state
Reexamination Certificate
1999-08-02
2002-05-28
Warden, Jill (Department: 1743)
Chemistry: analytical and immunological testing
Including sample preparation
Gaseous sample or with change of physical state
C422S068100, C422S083000, C422S089000, C436S054000, C436S174000, C073S023200, C073S023350, C073S023410, C073S023420
Reexamination Certificate
active
06395560
ABSTRACT:
TECHNICAL FIELD
This invention relates to devices and methods of chemical analysis of materials. Specifically this invention relates to headspace analysis and apparatus and methods used to increase sensitivity in conducting such analysis.
BACKGROUND ART
Gas chromatography is one of the most powerful methods available for the separation of compounds for the purpose of purification, identification, and quantification. A volatile liquid sample is injected through a rubber septum into a heated glass injector port, which vaporizes the sample. The sample is swept through the column by an inert carrier gas. After passing through the column, the separated solutes flow through a detector the output of which is displayed on a recorder or computer. However, the samples that contain non-volatile constituents (polymers, dissolved salts, soils, biological matrices, etc.) cannot be directly introduced into a gas chromatograph. Such materials often require a tedious sample preparation procedure to remove non-volatile species.
One technique for delivering sample vapor to an analytical instrument is a ‘purge and trap’ apparatus. A purge and trap apparatus is used for capturing and identifying volatile organic compounds in a sample. Referring now to a purge and trap system, generally indicated
10
, shown in
FIG. 1
, liquid or solid samples containing volatile organic compounds are sparged at a controlled temperature with a regulated flow of inert gas for a fixed period of time. Sparging gas enters through the needle adaptor
18
and passes through a sparging needle
12
which is inserted into a test tube containing a sample therein
14
. The sparging needle
12
is adjusted so that the outlet is immersed in the sample. The sparging gas passes through the sample which produces a bubbling effect. Analytes stripped from the sample are accumulated and concentrated on a cool sorbent trap
24
comprised of a material suitable to collect the material to be accumulated in the application. The trap
24
functions as a sample concentrator which thermally traps and selectively later desorbs organic compounds for analysis by a gas chromatograph
30
. Material which passes through the trap is initially directed to a vent
28
through a valve
26
.
All analytes of interest are preferably completely transferred to the trap. Their quantification becomes ambiguous if this does not occur. This is associated with the fact that the vapor pressures in the bubbles depend on the radius of the bubble and the surface tension of the liquid (classical Kelvin Equation). Both vapor pressure and bubble radius are highly variable between samples. This is true even for materials such as environmental water samples which often contain salts, soaps or other materials. For these reasons the quantification of partially removed analytes becomes very difficult, and purge and trap procedures generally require intensive quality control to ensure that all analytes of interest are completely transferred to the trap. This limits the usefulness of the purge and trap approach for many volatile substances that are poorly removable from the matrix of substances included in the sample (alcohols in water for example). Often highly volatile materials are difficult to trap and material may be lost.
After the material of interest is accumulated in the trap
24
, the trap
24
is then rapidly heated. The valve
26
is changed to fluidly connect a column of a gas chromatograph thereto. The analytes are desorbed from the trap as a plug and are moved by a flow of carrier gas which passes through a fitting
15
, through the trap and valve, and into the gas chromatograph. The gas chromatograph provides an output indicative of the substances in the sample.
After passage of the sample to the gas chromatograph, the condition of valve
26
is again opened to vent
28
. The trap is then baked above the desorption temperature so that water and heavier volatile chemicals that it is desirable not to introduce into the analytical instrument, are passed to vent
28
. This clears the trap, reducing interference with subsequent reconcentration, separation, or detection of the analytes from other samples.
Further disadvantages of the conventional purge and trap apparatus are foaming during purge of the sample, contamination from the re-use of the sample holding container and safety hazards. Foaming of the sample during passage of the sparging gas results in net sample loss and less accuracy when calculating sample concentration. Re-use of the sample holding container increases the likelihood of contamination from prior samples. Contamination may result in incorrect sample concentration indications and may falsely indicate that certain chemicals are present in the sample, when actually these chemicals are left over from previous samples due to insufficient cleaning. Safety hazards may arise when the glass vial containing a sample shatters due to a possible pressure overload from sparge gas or other problem. Thus a safety shield is often needed to provide protection in the event of breakage.
Headspace technology is a relatively new technique which allows the sampling of the vapor phase of a sample for analysis in a gas chromatograph. This headspace sampling ensures that only volatile species that can be eluted from the column of the gas chromatograph will be introduced into the instrument. In headspace sampling a volatile non-vapor phase (liquid or solid) sample attains equilibrium with a vapor phase in a sealed vial. Equilibrium is established when the level of liquid in the vial no longer changes so that the total quantity of liquid and vapor remains constant. A syringe may be used to retrieve a small amount of vapor for analysis. Headspace technology is advantageous over conventional direct injection because it allows only vapor to enter the gas chromatograph. This eliminates the chance of contamination, or destruction of the instrument due to introduction of unevaporated sample. Since the sample is in the vapor form, sample volumes may be greater. Increased sample size generally results in increased sensitivity.
Samples of headspace vapor may be extracted from a sample vial using a number of other techniques. Such techniques often involve equiliberating the vapor and non-vapor phase of a substance for analysis within a closed vial. A sample needle is moved to pierce a septum on the vial such that a fluid passage through the needle is in fluid communication with the vapor phase of the sample in the headspace. To extract a headspace sample it is usually necessary to first pressurize the headspace with a suitable gas.
After the headspace has been pressurized, the pressure is released allowing the sample material to pass out of the vial and into an analytical instrument or other device. Examples of techniques for extracting sample vapor from a vial is shown in allowed U.S. patent application Ser. No. 09/131,291) filed Aug. 10, 1998 the disclosure of which is incorporated by reference as if fully rewritten herein.
A drawback associated with conventional techniques for the extraction of a vapor sample from a headspace vial is that variations in pressure must be induced to extract the sample material. Such variations in pressure often affect the equilibrium between the vapor phase and the non-vapor phase of the substance being analyzed. Changes in equilibrium may change the makeup of the headspace vapor. Such changes which result from the sampling process often affect the results in ways that are undesirable.
Thus there exists a need for a headspace apparatus and method which reduces the disadvantages and limitations associated with prior art devices and methods.
OBJECTS OF INVENTION
It is an object of the present invention to provide a sampling apparatus that provides better sampling.
It is a further object of the present invention to provide a sampling apparatus that eliminates foaming associated with the purge and trap technique.
It is a further object of the present invention to provide a sampling apparatus with improved sensitivity which minimizes sample loss.
I
Handy Dwayne K.
Jocke Ralph E.
Walker & Jocke
Warden Jill
Wasil Daniel D.
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