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.
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 xe2x80x98purge and trapxe2x80x99 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.
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.
It is a further object of the present invention to provide a sampling apparatus which minimizes the risk of false readings due to contamination from prior samples.
It is a further object of the present invention to provide a sampling apparatus which includes two cooperative needles.
It is a further object of the present invention to provide a sampling apparatus which accomplishes a two-in-one function of purging and sweeping the headspace for retrieval of sample.
It is a further object of the present invention to provide a sampling apparatus which occupies less overall space than conventional purge and trap.
It is a further object of the present invention to provide a sampling apparatus which includes a dual needle for extracting sample vapor from a headspace in a vial.
It is a further object of the present invention to provide a sampling apparatus which reduces safety hazards and the potential shattering of sample containers.
It is a further object of the present invention to provide a sampling apparatus which utilizes a regular, inexpensive headspace vial as a disposable purge vessel.
It is a further object of the present invention to provide an improved method of sampling.
It is a further object of the present invention to provide a method of sampling that reduces the effects of pressure changes on the headspace sample.
It is a further object of the present invention to provide a method of sampling that achieves greater sample size.
It is a farther object of the present invention to provide a method of sampling which achieves increased sensitivity.
It is a further object of the present invention to provide an apparatus and method of sampling under conditions close to true thermodynamic equilibrium.
The foregoing objects are accomplished by a system and method of the present invention in which sample vapor is extracted from the headspace of a vial in which material to be analyzed is contained. Sample vapor is extracted from the headspace using a dual needle apparatus that in one exemplary embodiment includes two concentric overlaid sampling needle structures with two separate fluid passages: a first fluid passage and a second fluid passage. In other embodiments separate, parallel needles, each of which includes a fluid passage may be used.
Material to be analyzed is held in the vials generally in liquid or solid form. A headspace is maintained above the non-vapor phase of the material in each vial. The headspace contains a vapor phase of the substance to be analyzed. The non-vapor phase of the substance in the vial and the vapor phase of the substance in the headspace preferably achieves the equilibration between the condensed non-vapor phase and the vapor phase prior to sampling. The sample headspace vapor may be retrieved using the dual needle apparatus. Using one approach, a purge gas passes through one fluid passage in the needle into the headspace. The entering purge gas displaces the vapor from the substance to be analyzed in the headspace, and moves such material out of the headspace through the second passage in the dual needle. The sample headspace vapor is passed from the headspace through a six-port valve. The function of the six-port valve is to direct the sample headspace vapor extracted from the vial to various devices fluidly connected to the valve.
In one configuration of the apparatus the sample vapor extracted from the vial may be conducted directly to an analytical instrument. An example of such an instrument is a gas chromatograph. The pressure of the headspace is preferably maintained at the column pressure of the instrument to which the headspace vapor is conducted. This pressure remains generally balanced and may provide a greater sample size to achieve increased sensitivity.
In other configurations sample headspace vapors are passed through and collected on a cool sorbent trap or other trap device. Some vapor and purge gases pass through the trap without collection. Such gas and vapor pass through the six-port valve and are either discharged into the atmosphere through a vent or are otherwise processed or collected in a helium trap bake. The trap functions as a concentrator for material in the sample headspace vapor. After collection, the material from the sample headspace vapor is thermally desorbed off the trap by the activation of a trap heater or other release mechanism. The sample vapor is carried into an analytical instrument with the aid of a carrier gas. The instrument analyzes the material in the sample.
The system and method of the invention enables sweeping of the material in the headspace of the vial to maximize the collected volume of sample material from the headspace. The effects of pressure changes within the vial due to sampling are also reduced. The vapor sample is preferably obtained and delivered under conditions which maintain conditions close to thermodynamic equilibrium. The sensitivity of the system and method may also be selectively optimized for particular substances by controlling the ratios of headspace and sample volumes.