Patent Application
20140061459
The present invention relates to devices and methods for ion mobility systems. More specifically, the invention relates to a method and apparatus for detecting chemicals using a Field Asymmetric Ion Mobility Spectrometry (FAIMS) system.
The ability to ionize gases using ion mobility systems is useful for a wide range of applications including many chemical detection applications. Ionization techniques, in which a gas sample is ionized and then separated into constituent parts that can be detected individually, are widely used for gas composition sensing. Two well-known examples are Ion Mobility Spectrometry (IMS) and Field Asymmetric Ion Mobility Spectrometry (FAIMS), also known as Differential Mobility Spectrometry (DMS). Ion mobility detection techniques tend to be very well suited to measuring trace constituents of gas mixtures that often consist of a carrier gas with additional gases mixed in at low concentrations (for example part-per-million or part-per-billion levels). Ion mobility techniques can also be used effectively over a range of gas pressures, including pressures close to one atmosphere. This makes them useful for, amongst other things, measuring low-level impurities in air. Because they work by measuring properties of ionized molecules and because gas samples for analysis generally consist mainly of neutral molecules, ion-mobility-based detectors generally incorporate an ionizer. The sample gas is passed through the ionizer to produce a population of ionized molecules that are then manipulated in some way involving separation or selection of ionized molecules according to their behavior in an electric field, before being detected. Ionizers commonly in use include radioactive sources, light-based devices such as ultra-violet lamps, and electrostatic devices such as corona discharge ionizers.
Practical chemical detectors such as Field Asymmetric Ion Mobility Spectrometry (FAIMS) systems compensate for varying environmental conditions as these can affect the output of the spectrometer. There are also a number of implementation challenges that the designer must overcome in order to produce a repeatable and reliable detection system. As one skilled in the art would recognize, a wide variety of non-idealities are encountered in practice. For instance, some of those most relevant to FAIMS systems fall into the following categories:
In this regard, it is to be understood and appreciated changes in ambient conditions such as temperature, pressure, and ambient humidity often lead to changes in FAIMS spectra. It is noted theoretical and/or empirical corrections exist for these effects. While theoretical approaches work well for pressure and, to a lesser extent, temperature, not all environmental conditions can easily be corrected in this manner. Empirical corrections can be determined for all ambient variables, but this approach necessitates prior characterization of analytes across a multi-dimensional matrix of conditions and the storage of this data for each analyte, which can be a time consuming and laborious process. It is thus desirable to provide a system or approach that enables conditions at the sensor head to be controlled in such a system such that each analyte only has to be characterized under a small set of environmental conditions.
In regards to a FAIMS system, since it is a highly sensitive system, FAIMS spectrometers often detect down to parts-per-billion levels or more. Direct sampling from the ambient environment can thus be problematic for such highly sensitive systems, as they can be saturated by higher concentrations of analytes or interferents, thus hiding the analytes of actual interest. High concentrations of some analytes can persist within the system as contamination due to absorption onto internal surfaces, adversely affecting performance.
It is to be further understood that in Ion Mobility Spectrometry (IMS) systems (which work at a constant electric field) the peak location is used as an identifier and the parameter of Reduced Mobility (K0) “factors” response variations due to temperature and pressure. Previous DMS/FAIMS systems (which work under variable electric field) have used the EC:ED spectrum for identification but operate over a much limited field range (35 kV·cm−1 c.f. 75 kV·cm−1).
The purpose and advantages of the invention will be set forth in and apparent from the description that follows. Additional advantages of the invention will be realized and attained by the devices, systems and methods particularly pointed out in the written description and claims hereof, as well as from the appended drawings.
One aspect of the invention relates to an apparatus, system and method for detecting, identifying, classifying and/or quantifying chemical species in a gas flow using a micro-fabricated ion filter coupled to a system adapted to apply drive signals to the ion filter. Coupled to the ion filter is a system adapted to measure the output of the ion filter, which in turn is coupled to a system adapted to extract numerical parameters from the measured output of the ion filter to facilitate chemical detection, identification, classification and/or quantification of the gas flow.
Other aspects of the invention is that inclusion of a FAIMS system in which the output of the FAIMS system is a two dimensional plot indicating the signal as a function of the RF voltage (Dispersion Field—ED) and the compensation voltage (Compensation Field—EC). This plot is used to discriminate between different chemicals. With reference to
It is to be appreciated that another aspect of the present invention relates to finding information from plots characteristic of individual chemicals. An apparatus and method is provided to process variations induced by ambient conditions (specifically pressure, temperature and humidity). It is to be understood that when more than one species is present, chemical interactions and competitive ionization causes changes to both the spectra and signal strength. A noted advantage over the prior art is that the electric field strength translating to the Dispersion Field (ED) can be much higher, causing fragmentation of molecules. Fragmentation patterns provide characteristic, molecular specific classification features.
The accompanying appendices and/or drawings illustrate various non-limiting, example, inventive aspects in accordance with the present disclosure:
The present invention is now described more fully with reference to the accompanying drawings, in which an illustrated embodiment of the present invention is shown wherein like reference numerals depict like elements. Before the present invention and specific exemplary embodiments of the invention are described, it is to be understood that this invention is not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims.
Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either both of those included limits are also included in the invention.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present invention, exemplary methods and materials are now described. All publications mentioned herein are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited.
It must be noted that as used herein and in the appended claims, the singular forms “a”, “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a stimulus” includes a plurality of such stimuli and reference to “the signal” includes reference to one or more signals and equivalents thereof known to those skilled in the art, and so forth.
With reference now to
Device 10 further includes a drive signal system 30 for applying drive signals to the ion filter 20 and a measurement system 40 for measuring the output of the ion filter 20. An extraction system 50 is coupled to measurement system 40 adapted and configured to extract numerical parameters from the measured output 40 of the ion filter 20. A detection system 60 is preferably coupled to the extraction system 50 being adapted and configured to facilitate chemical detection, identification, classification and/or quantification of the gas flow of ion filter 20.
As described further below, it is to be understood and appreciated that device 10 is adapted to use measurements of ion current as a function of one or more of compensation field and dispersion field to facilitate chemical detection, identification, classification and/or quantification which may preferably include the use of one or more ion peaks in FAIMS spectrum. It may further be adapted to classify ion peak trajectories into one or more of a number of categories and to use characteristic features of the trajectory of one or more ion peaks in the FAIMS spectrum to facilitate chemical detection, identification, classification and/or quantification wherein the characteristic features include ion peak position at one or more dispersion field values. Additionally, the characteristic features may be attributable to one or more of: ion fragmentation, dimer breakdown, monomer resurgence, monomer breakdown, field heating or changes in ion mobility. The characteristic features may include a point at which the gradient of compensation field versus dispersion field changes sign and/or a change in one or more peak characteristics at or in the vicinity of a particular dispersion field wherein the one or more peak characteristics include peak width and/or peak height. The one or more peak characteristics may further include peak location.
In additional embodiments of the invention, the rate of gas flow through the ion filter 20 is adjustable wherein the rate of gas flow is preferably selected to achieve a desired sensitivity and/or selectivity. Additionally, device 10 uses the relative peak amplitudes at a plurality of sample flow dilutions to infer one or more relative affinities wherein the one or more inferred relative affinities is preferably used to calculate one or more quantification correction factors. Still further, the one or more inferred relative affinities is used in combination with one or more identified or partially identified chemical responses to calculate the affinity of unknown analytes to facilitate detection, identification, classification and/or quantification.
Additional embodiments of the invention are to be understood to include using measurements of ion current as a function of one or more of compensation field and dispersion field. Device 10 may monitor a vicinity of a reactant ion peak to detect and/or device 10 uses the presence of a dimer peak to facilitate chemical detection, identification, classification and/or quantification. Further, device 10 may use the dispersion field value at which a monomer peak undergoes one or more of; a characteristic shift in compensation field position, a split into two or more separate peaks to identify a point of monomer fragmentation. Device 10 may also be adapted and configured to use measurements of peak width along the dispersion field axis wherein the device 10 uses the dispersion field and compensation field co-ordinates of a point at which ion current drops below a threshold value to facilitate chemical detection, identification, classification and/or quantification.
Other embodiments of the invention may include using a character table and/or characteristic table to associate features of the spectrum with one or more classes of analytes to facilitate chemical detection, identification, classification and/or quantification wherein the features include one or more of dimer breakdown, monomer resurgence, a turning point in peak trajectory, monomer zero compensation field crossing point, and other FAIMS spectral features. Device 10 may be adapted and configured to compare analyte peak position to a position of the reactant ion peak to compensate for environmental effects wherein an analyte peak property is used in combination with a reactant ion peak property to ascertain identity, class and/or concentration of the analyte. Device 10 may further use an analyte peak property in conjunction with a reactant ion peak property to ascertain identity, class and/or concentration of the analyte.
It is also to be understood and appreciated the device 10 is operational such that the width of the ions species peak under zero dispersion field conditions is estimated by extrapolating from the measurements of the width under two or more non-zero field conditions wherein signal levels are normalized to the reactant ion peak intensity or total ion current and wherein one or more applied fields are adjusted to account for changes in number of molecules per unit volume of gas in proximity to the FAIMS ion filter 20. Further, a number of molecules per unit volume are calculated by applying Van der Waal's equations for one or more gaseous components.
Device 10 may be further operational such that the relationship between peak parameters at a plurality of dispersion field frequencies is used to facilitate chemical detection, identification, classification and/or quantification wherein the region of the spectrum at which peak width starts to increase is also used to facilitate chemical identification, classification and/or quantification.
Device 10 may be adapted to adjust the frequency of the dispersion field waveform to modify the effects of humidity on resulting spectrum. The relationship between two peak intensities may be used to facilitate chemical detection, identification, classification and/or quantification. Device 10 may use the dispersion field value at which a dimer peak falls below a threshold value to identify a point of dimer breakdown and/or use an increase in a monomer peak intensity above a certain threshold dispersion field to confirm dimer breakdown to facilitate chemical detection, identification, classification and/or quantification wherein the relationship between the two peak intensities is that of one peak increasing while the other decreases and wherein the dispersion field is adjusted to account for changes in one or more of pressure and the effective temperature of ions.
Device 10 may further detect and use changes in peak properties as a function of one or more of compensation field and dispersion field to infer a value for the mobility of an ion species, and/or device 10 may further detect and use the inferred mobility value associated with ion trajectory to create a temperature dependent correction factor that can be applied to the trajectory to facilitate chemical detection, identification, classification and/or quantification.
With the basic components of device 10 in accordance with an illustrated embodiment being described above, particular features of the present invention is further provided below.
In accordance with an embodiment of the invention, it is to be understood that by extracting the peaks (i.e. their Gaussian parameters) on each Compensation Field (EC) line the path of a particular chemical is tracked. It is noted there are various different types of information that can be extracted from a chemical track, which are discussed below and can be used in singly or in combination depending on the degree of similarity of the chemicals under test.
With regards to the peak trajectories shown in
It is to be appreciated the Point of Inflection, Zero Crossing Point and End Point can be used to distinguish chemicals. The Point of Inflection is the point at which a curve turns back on itself. In the above example of
Additionally, chemicals can be identified by matching the unknown track against previously collected tracks of known chemicals, which is termed Peak Location. It is to be understood the peak height (or area) plotted along the track of the peak contains distinctive features. Analytical systems in accordance with the present invention preferably operate at much higher fields (ED) and the ED dependant ion transmission contains much richer information than prior art systems (the kinetic and thermodynamic properties of ions are revealed and since these are molecular specific useful classification features are derived).
In accordance with the algorithm, the ED/N scale may be converted to effective ion temperature and Teff scales to (ED/N)2:
T
eff
=T+ζ·M·K
(E/N)
2
·N
0
2(ED/N)2/(3kb)
With reference to
Additionally, physical constants for the ion generated from the chemical may be determined and include:
A
I
=A
I(in)
·Q·exp(−tres·π2·DII/geff2)
Additional features of the present invention include:
It is to be appreciated that environmental conditions affect the track and the ion current (which is important for quantification). A noted tool for this purpose is the track called the RIP (Reactant Ion Products) which are the ions of nitrogen, oxygen and water that are ionised along with the sample of interest.
As the quantity of ions is set by the ionisation method and the other ions that are presently working with ion currents normalised against the RIP will enable more accurate quantification. In practice the chemical ion current is scaled against the height of the peak at low DF.
By measuring peak locations relative to the RIP it is possible to reduce the effect of environmental changes. This is likely to be most useful for humidity as temperature and pressure can be compensated for via physical models.
If there are chemicals with high electron/proton affinities present, the RIP will change characteristics enabling identification.
Effective Temperature and/or Pressure
It is noted that one method of finding chemical tracks that are invariant to ambient conditions is to use a y-axis scale that corresponds to the effective temperature and/or pressure. This includes a contribution from heating by the high field RF. Townsend units (i.e. field over number density—E/N), the effective ion temperature (Teff) and the ion neutral collisional efficiency factors (ζ) may be used in the correction of temperature and/or pressure.
If compensation for humidity cannot be achieved then it is preferable to operate the device at a small number of fixed humidities which rules specific to each fixed humidity.
It is to be appreciated that flow rate changes allow improved sensitivity (important when working near the detection limit) and can also be used to confirm if a high affinity chemical is present.
Townsend correction divides the electric field by the number density of the gas. Normally the calculation of the number density is performed by assuming that all the gases are ideal. In accordance with an embodiment of the invention, improved results are obtained by using van der Waal's equations for each component.
It is noted that the impact of humidity on chemicals is thought to be due to the formation of clusters of water molecules around the chemical of interest in the low field and the breaking up of them in the high field. In accordance with an embodiment of the invention, by using higher frequencies the chemicals preferably remain clustered thus reducing the variability.
It is to be appreciated that in the same way that Infra-Red is able to map specific frequencies to particular bonds, in accordance with an embodiment of the invention, particular tracks are associated with classes of molecules. This has been mentioned above regarding identifying chemicals as Type A, B or C. For instance, a non-exhaustive example of a UH-FAIMS character Table is shown below as:
It is to be appreciated Character Tables are used in many forms of molecular spectroscopy to qualitatively define where different types of functional molecules give discrete responses in different parts of the available spectral window. They start off basic and become more detailed as more data is available, as illustrated in the positive and negative ion mode spectral character tables depicted in
It is also to be appreciated that there may be chemicals which have very similar characteristics, in which case the conditions can be changed (humidity, flow rate, temperature) to provide information to distinguish them.
The above presents a description of a best mode contemplated for carrying out the present invention, and of the manner and process of making and using them, in such full, clear, concise, and exact terms as to enable any person skilled in the art to which it pertains to make and use these devices and methods. The present invention is, however, susceptible to modifications and alternative method steps from those discussed above that are fully equivalent.
Consequently, the present invention is not limited to the particular embodiments disclosed. On the contrary, the present invention encompasses all modifications and alternative constructions and methods coming within the spirit and scope of the present invention. Optional embodiments of the present invention may also be said to broadly consist in the parts, elements and features referred to or indicated herein, individually or collectively, in any or all combinations of two or more of the parts, elements or features, and wherein specific integers are mentioned herein which have known equivalents in the art to which the invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.
This application claims priority to U.S. Patent Application Ser. No. 61/617,395 filed Mar. 29, 2012 which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61617395 | Mar 2012 | US |
