Radiant energy – Ionic separation or analysis – Ion beam pulsing means with detector synchronizing means
Reexamination Certificate
2001-03-05
2004-11-09
Lee, John R. (Department: 2881)
Radiant energy
Ionic separation or analysis
Ion beam pulsing means with detector synchronizing means
C250S281000, C250S282000, C250S287000, C250S288000
Reexamination Certificate
active
06815668
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to spectrometry, and more particularly, to methodology and apparatus for the analysis of compounds by chromatography-high field asymmetric waveform ion mobility spectrometry.
There is a developing interest in making in situ measurements of chemicals present in complex mixtures at industrial or environmental venues. A fully functional chemical sensor system may incorporate a front end, e.g., a gas chromatography (GC) analyzer as a compound separator, and then a detector, i.e., a spectrometer.
Gas chromatography is a chemical compound separation method in which a discrete gas sample (composed of a mixture of chemical components) is introduced via a shutter arrangement into a GC column. Components of the introduced gas sample are partitioned between two phases: one phase is a stationary bed with a large surface area, and the other is a gas which percolates through the stationary bed. The sample is vaporized and carried by the mobile gas phase (the carrier gas) through the column. Samples partition (equilibrate) into the stationary (liquid) phase, based on their solubilities into the column coating at the given temperature. The components of the sample separate from one another based on their relative vapor pressures and affinities for the stationary bed, this process is called elution.
The heart of the chromatograph is the column; the first ones were metal tubes packed with inert supports on which stationary liquids were coated. Presently, the most popular columns are made of fused silica and are open tubes with capillary dimensions. The stationary liquid phase is coated on the inside surface of the capillary wall.
Compounds are discriminated by the time that they are retained in the GC column (the time from sample injection to the time the peak maximum appears). Chemical species are identified from a sample based on their retention time. The height of any one of these peaks indicates the intensity or concentration of the specific detected compound.
A carrier gas (e.g., helium, filtered air, nitrogen) flows continuously through the injection port, and the column. The flow rate of the carrier gas must be carefully controlled to ensure reproducible retention times and to minimize detector drift and noise. The sample is usually injected (often with a microsyringe) into a heated injection port where it is vaporized and carried into the column, often capillary columns 15 to 30 meters long are used but for fast GC they can be significantly shorter (less than 1 meter), coated on the inside with a thin (e.g., 0.2 micron) film of high boiling liquid (the stationary phase). The sample partitions between the mobile and stationary phases, and is separated into individual components based on relative solubility in the liquid phase and relative vapor pressures. After the column, the carrier gas and sample pass through a detector that typically measures the quantity of the sample, and produces an electrical signal representative thereof.
Certain components of high speed or portable GC analyzers have reached advanced stages of refinement. These include improved columns and injectors, and heaters that achieve precise temperature control of the column. Even so, detectors for portable gas chromatographs still suffer from relatively poor detection limits and sensitivity. In addition, GC analyzers combined with any of the conventional detectors—flame ionization detectors (FID), thermal conductivity detectors, or photo-ionization detectors—simply produce a signal indicating the presence of a compound eluted from the GC column. However, presence indication alone is often inadequate, and it is often desirable to obtain additional specific information that can enable unambiguous compound identification.
One approach to unambiguous compound identification employs a combination of instruments capable of providing an orthogonal set of information for each chromatographic peak. (The term orthogonal will be appreciated by those skilled in the art to mean data which enables multiple levels of reliable and accurate identification of a particular species, and uses a different property of the compound for identification.) One such combination of instruments is a GC attached to a mass spectrometer (MS). The mass spectrometer is generally considered one of the most definitive detectors for compound identification, as it generates a fingerprint pattern of fragment ions for each compound eluting from the GC. Use of the mass spectrometer as the detector dramatically increases the value of analytical separation provided by the GC. The combined GC-MS information, in most cases, is sufficient for unambiguous identification of the compound.
Unfortunately, the GC-MS is not well suited for small, low cost, fieldable instruments. Therefore there is still a strong need to be met with a fieldable chemical sensor that can generate reliable orthogonal information. A successful field instrument should include both a small injector/column and a small detector/spectrometer and yet be able to rapidly produce unambiguous orthogonal data for identification of a detected compound.
While GC's are continuously being miniaturized and reduced in cost, mass spectrometers are still very expensive, easily exceeding $100K. Their size remains relatively large, making them difficult to deploy in the field. Mass spectrometers also suffer from the need to operate at low pressures, and their spectra can be difficult to interpret often requiring a highly trained operator. The search therefore has continued for fieldable spectrometer.
Time-of-flight Ion Mobility Spectrometers (TOF-IMS) have been described as detectors for gas chromatographs from early in the development of ion mobility spectrometry and the first successful use of TOF-IMS detectors with capillary chromatography occurred in 1982. High-speed response and low memory effects were attained and the gas phase ion chemistry inside the TOF-IMS can be highly reproducible providing the foundation to glean chemical class information from mobility spectra. Thus, TOF-IMS, as ionization detectors for GC, do exhibit functional parallels to mass spectrometers, except all processes in IMS occur at ambient pressure making vacuum systems unnecessary. The IMS spectra is also simpler to interpret since it contains fewer peaks, due to less ion fragmentation. The usefulness of a gas chromatograph with TOF-IMS detector has been recognized for air quality monitoring, chemical agent monitoring, explosives detection, and for some environmental uses.
Fieldability still remains a problem for TOF-IMS. Despite advances over the past decade, TOF-IMS drift tubes are still comparatively large and expensive and suffer from losses in detection limits when made small. The search therefore still continues for a successful field instrument that includes both a small ion injector/column and a small detector/spectrometer and yet is able to rapidly produce unambiguous orthogonal data for identification of a detected compound.
The high field asymmetric waveform ion mobility spectrometer (FAIMS) is an alternative to the TOF-IMS. In a FAIMS device, a gas sample that contains a chemical compound is subjected to an ionization source. Ions from the ionized gas sample are drawn into an ion filter and subjected to a high field asymmetric waveform ion mobility filtering technique. Select ion species allowed through the filter are then passed to an ion detector, enabling indication of a selected species.
The FAIMS filtering technique involves passing ions in a carrier gas through strong electric fields between the filter electrodes. The fields are created by application of an asymmetric period voltage (typically along with a further control bias) to the filter electrodes.
The process achieves a filtering effect by accentuating differences in ion mobility. The asymmetric field alternates between a high and low field strength condition that causes the ions to move in response to the field according to their mobility. Typically the mobility in the high field differs from that
Eiceman Gary A.
Krylov Evgeny
Miller Raanan A.
Nazarov Erkinjon G.
Tadjikov Boris
Gurzo Paul M.
Lee John R.
Ropes & Gray LLP
The Charles Stark Draper Laboratory Inc.
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