PTFE matrix in a sample inlet liner and method of use

Chemistry: analytical and immunological testing – Including sample preparation

Reexamination Certificate

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C073S023390, C073S023410, C095S089000, C096S105000, C422S089000, C422S091000, C436S161000, C436S180000, C436S181000

Reexamination Certificate

active

06498042

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to chromatographic analysis systems and, more particularly, to a sample introduction system and related apparatus for analyzing a sample comprised of one or more components to determine the identity and concentration of each component in the sample. More specifically, this invention is related to a sample inlet liner and related apparatus fabricated from inert material(s) to reduce sample adsorption or decomposition.
BACKGROUND OF THE INVENTION
Chromatography is a known method of analyzing a sample comprised of one or more components to qualitatively determine the identity of the sample components as well as quantitatively determine the concentration of the components.
A typical chromatographic apparatus includes: a sample introduction system having an injection port, into which the sample is injected and mixed with an inert fluid at high temperature; a column through which the various dissolved components of the sample will travel at a rate related to the characteristics of the specific components; and a detector for measuring the presence of each component.
Analytical chromatography involves a series of steps: sample collection, sample preparation, sample introduction into the chromatographic system, chromatographic separation into individual components, detection of those components, and data acquisition and reduction. For each step, the analyst must make appropriate choices among accepted procedures and available instrumentation. Improper selection or use of the sample introduction system can dramatically limit the performance of the system and, therefore, the ultimate performance of the analytical method. Because of the great variety of columns and the diversity of samples that can be analyzed with modern chromatography, several sample introduction systems and related injection modes are known; no single system can best satisfy all analytical requirements.
However, a common function of the sample introduction system is to provide accurate, reproducible, and predictable introduction of sample into the column. Usually the sample introduction system includes a device known as a sample inlet, whereby a quantity of sample in a liquid form is injected using a syringe. There are other sample introduction devices that introduce samples into the chromatographic column when syringe injection is inappropriate (for example, with solid samples): a gas or liquid sampling valve, head space autosampler, thermal desorber, purge and trap sampler, or pyrolyzer.
Sample inlets are usually divided into two major categories: packed column inlets and capillary column inlets. The types of capillary column inlets are known to include: capillary direct, split/splitless, programmed temperature vaporizing (PTV), and cool on-column direct. Nearly all capillary inlets are vaporizing, including on-column direct injection, except for cool on-column injection, which deposits condensed sample directly into the column.
A useful technique for introducing a liquid sample into a chromatographic system is by injection into a hot inlet wherein it is quickly vaporized (flash vaporization). The benefits of flash vaporization includes the transfer of the liquid sample to a gas and a quick transfer of a sample into the column. However, typical problems associated with vaporizing inlets include band broadening, needle discrimination, inlet discrimination, and sample decomposition. Sample decomposition is indicated by lost or misshapen peaks, or by the generation of unwanted peaks. Decomposition is exacerbated by high inlet temperatures, long residence time of the sample in the inlet, and chemical activity of the sample with the inlet. These problems are exacerbated by the use of high inlet temperatures and low boiling point solvents.
Accordingly, an inlet liner may be positioned in the inlet to reduce chemical activity between the sample and the inlet. The selection and construction of the inlet liner has a direct effect on the success of an analysis, and each type of inlet is designed to function best with a certain type of liner. For example, splitless inlets may require straight liners with no packing, whereas liners constructed for other techniques, such as programmed temperature vaporization, require baffled or packed liners to retain a liquid sample during cold sample introduction.
The inlet liner and any packing material therein is expected to be non-reactive with respect to the compounds and solvents that may be present in an injected sample. One conventional approach is to chemically deactivate the liner and its contents. Deactivation reagents such as hexamethyldisilazane (HMDS), dimethyldichlorosilane (DMDS), and polymethylhydrosiloxane (PMHS) are typical examples. In another approach, a stationary phase coating may be applied to a particulate (such as Dexsil-300 on Chromosorb 750) to achieve a deactivated surface. Such a procedure may only be effective for a limited series of analytical runs, after which the liner must be cleaned and again be deactivated. Thus, conventional approaches can be unsatisfactory, as will now be described.
Note that any sample decomposition will undesirably degrade the minimum detectable level (MDL) of the chromatograph. Low detection levels are important in environmental, pharmaceutical, food analysis, and other gas chromatography applications. Improvements in sample handling, sample injection techniques, and detectors have all contributed to the ability to measure compounds at decreasing levels.
Sample decomposition is especially undesirable in large volume injection (LVI), which is an important technique for lowering the minimum detection level. In LVI, a large volume of sample is injected. The bulk of the solvent is evaporated before the transfer of the sample to the analytical column is initiated. Large volume injection is especially useful in trace analysis to improve analyte detectability, for analysis of, e.g., pesticides and pollutants. Very often it can replace an off-line evaporation step carried out to concentrate a diluted sample extract.
New inlets and injection techniques supporting large volume injection (LVI) have been developed in recent years. Accordingly, there are two primary techniques used to eliminate solvent: 1) via a programmable temperature vaporizer (PTV) inlet; and 2) via a cool on-column injection with solvent vapor exit (COC-SVE). Programmed temperature vaporizing injectors (PTV) have been shown to be well-suited for large volume sample introduction in capillary gas chromatography (cf., e.g., Wilson et al., “Large Volume Injection for Gas Chromatography Using a PTV Inlet”, Application Note 228-374, Hewlett-Packard Company, March 1997). LVI with PTV is ideal for trace analysis of later-eluting solutes (i.e., solvents having boiling points approximately 100° C. higher than the solvent) and for dirty samples.
Note that the typical injection volume for capillary column analysis is 0.5 to 2 &mgr;l. The Hewlett-Packard HP 5890 and HP 6890 series gas chromatographs allow approximately two times the normal injection volume (up to 5 &mgr;l, depending on the solvent) using “pulsed” splitless injection, injecting still larger volumes with standard techniques can lead to contamination of the system, irreproducible results, and loss of sample. In contrast, the typical injection volumes employed in a large volume, solvent elimination PTV inlet are 25 to 100 &mgr;l; even up to 1 ml have been demonstrated. Multiple injections can be used with the PTV inlet when even larger volumes are required.
Hence, for large volume injections, the PTV inlet is often used in a “solvent vent” or “solvent elimination” mode. Sample is introduced into the inlet with the inlet temperature near the boiling point of the solvent and with a relatively high split ratio. The solvent (and low-boiling solutes) is vented while the higher boiling solutes (more than about 100° C. above the solvent boiling point) remain and are concentrated in the inlet. After a predetermined period, the split vent is closed and the inlet temperature is increa

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