Patent Application
20210319992
This application is directed to the field of membrane mass spectrometry, and more particularly, to an improved membrane inlet for a gas analyzer, such as a chromatograph mass spectrometer.
MI/MS (Membrane Interface/Mass Spectrometry) was first described by Hoch and Kok in 1963 and has become a well-known sample introduction technique in the field of mass spectrometry and most importantly in the field of portable mass spectrometry. In portable mass spectrometry, low detection levels are made challenging by the performance limitations inherent to geometrically compact mass spectrometers with limited power and vacuum pumping. MI/MS has been often used to counteract these performance effects, as noted in Voss et al. U.S. Pat. No. 5,426,300.
For instance, a semi-permeable polymer membrane separates the gaseous sample matrix—which is often the effluent of a gas chromatograph (GC)—from the high vacuum region of the mass spectrometer. Analyte molecules pass through the semi-permeable polymer membrane preferentially to the carrier gas because of higher analyte permeability in the membrane relative to the carrier gas permeability in the membrane. Permeability is the product of a molecule's membrane-molecule diffusion coefficient and the molecule's membrane-carrier gas partition coefficient. Semi-permeable polymer membranes are typically silicone-based, however, other polymeric materials have been examined, yielding different advantages and disadvantages. (See Amy J. Maden and, Mark J. Hayward. Sheet Materials for Use as Membranes in Membrane Introduction Mass Spectrometry. Analytical Chemistry 1996, 68 (10), 1805-1811. DOI: 10.1021/ac9509216.). A number of geometries have been explored ranging from planar sheets to column-shaped tubes where the gas flows through or around the tubular membrane, i.e., Cooks et al. U.S. Pat. No. 4,791,292, to systems having multiple membranes, i.e., Wright et al. U.S. Pat. No. 10,755,909. Few of these semi-permeable membranes or columns were suitable for use in a GC/MI/MS.
A limitation to MI/MS in defense and hazardous material (HAZMAT) applications relates to the fact that many low vapor pressure chemicals of concern are larger in size than the environmental targets where MI/MS has historically been most successful. For example, chemical warfare agents like VX, and drugs of abuse like Fentanyl® (is a registered trademark of Janssen Pharmacuticals), these molecules are typically much larger in size than, for example, benzene or tetrachloroethylene. Molecules of larger size typically exhibit very low membrane-analyte diffusion coefficients and as a result these molecules take longer to move through the membrane than smaller molecules. A low membrane-analyte diffusion coefficient degrades the performance or analysis of a GC/MI/MS system inasmuch as the analyte GC peaks are spread or broadened as the molecules diffuse through the semi-permeable membrane. This reduces the resolving power of the GC/MI/MS system as time-resolved peaks exiting the GC overlap, and are smeared together as they enter the MS. The result is reduced detection performance in applications with complex or multi-chemical compounds. It will also be appreciated that such broadened peaks also lower the signal-to-noise ratios compared to peaks which are sharp, i.e., molecules which pass through the membrane quickly so as to rise and fall sharply in the GC/MI/MS analysis. According to Fick's Second law of Diffusion, the broadening of a peak is proportional to the square of the membrane's thickness, so that a thin membrane better preserves chromatographic performance. On the other hand, it can be challenging to make a membrane extremely thin inasmuch as the membrane must support a pressure differential of at least atmospheric pressure on the GC side while maintaining a low vacuum pressure on the MS side. That is, the pressure differential can rupture a very thin membrane, particularly if the membrane is heated to speed diffusion, or reduce other sticking-time constants of a molecule as it passes through the interface. A solution to this problem is taught in Wolcott, et al. U.S. Pat. No. 8,956,696, wherein a method is described for fabricating an ultrathin MI/MS membrane. Therein, large molecule analysis becomes more accessible in a GC/MI/MS analysis. However, a limitation to the method described in the Wolcott et al. '696 patent relates to the increased amount of carrier gas flowing into the GC/MI/MS system. Small pumps and consumable getter pumps can only tolerate limited gas loads. To counteract the vulnerabilities of thin or ultrathin membranes, the area of the membrane must be correspondingly small. This is due to the relationship between the gas loads and the membrane thickness, i.e., the steady-state gas load is inversely proportional to the membrane thickness and directly proportional to the membrane area as described by Fick's First law of Diffusion
For low vapor pressure chemicals, a small membrane area is generally ideal because high partition coefficient analytes are rapidly depleted from the vapor stream as they exit the GC column. A larger area of membrane provides no advantage for low volatility chemicals because the low vapor pressure analyte is depleted from the column effluent stream within a short distance where the vapor stream first makes contact with the membrane. A larger membrane area allows more carrier gas to flow into the mass spectrometer without the additional benefit of more analyte. Excess carrier gas is also undesirable because it must be pumped away to maintain acceptable vacuum levels, i.e., requiring larger pumps or more rapid consumption of getter pumps. Excess carrier gas can also degrade the performance of the mass spectrometer because of an increased space charge and increased ion-neutral collisions.
A limitation with respect to the current state-of-the-art GC/MI/MS systems relates to the same ultrathin interface which is not well-suited for high vapor pressure analyte applications (e.g., HCN or Sarin). This is due to the fact that: (i) high vapor pressure chemicals have a propensity for lower membrane-air partition coefficients and (ii) these chemicals require more membrane contact before fully depleting the vapor stream. However, since these chemicals are typically smaller, they tend to have higher polymer-analyte diffusion coefficients. This means that a thicker membrane is tolerable inasmuch as the broadening of GC peaks will not be significant i.e., the GC peak-shape will not degrade. Accordingly, in those cases where optimization is required for volatile chemicals, a thicker membrane is advantageous because it reduces the amount of carrier gas flowing into the mass spectrometer over the required membrane area to fully extract the sample analyte from the carrier.
U.S. Pat. No. 8,648,293 teaches the use of a MI/MS for calibration of an MS system wherein a substantially planar membrane is constructed from silicon (Si), and/or silicon nitride (SiN) with a thick region and a thin region in the center. A system of this type does not benefit from enhancement of analyte transport afforded by differential permeation rates of analyte and carrier through a semipermeable membrane. Additionally, no use can be made of the thicker region to also permit higher vapor pressure gases into the MS because this region is gas-tight.
U.S. Pat. No. 10,755,909 teaches of a MI/MS system using of multiple membranes having different thicknesses to enhance the sensitivity and time response to distinguish between different chemicals. A disadvantage of this system relates to the fact that intervening components/volumes degrade the GC peak shape or otherwise interact unfavorably with chemicals prone to degradation. Another disadvantage is that for optimization of many chemicals having differing permeation behavior, many separate membranes are needed but are not compatible with chromatographic separation. For example, with 2 membranes, only one low vapor pressure and one high vapor pressure chemical can be simultaneously optimized. A method to optimize for a wide range of chemicals are still necessary to deliver sharp GC peaks.
A need, therefore, exists for a semipermeable membrane useful for analyzing chemicals having a wide spectrum of vapor pressure characteristics but which is suitably thin and/or small so as to be compatible with the low carrier gas flow rates used in modern gas chromatography.
In one embodiment, a gas analyzer is provided for analyzing a sample having one or more analytes flowing in a carrier gas. The gas analyzer includes a membrane configured to receive an input flow of carrier gas. The membrane defines a variable thickness region between first and second positions along an input face of the membrane and functions to produce a concentrated or enriched flow of analyte molecules. That is, the membrane rejects most of the carrier gas and allows a highly concentrated flow of analytes molecules through the membrane. The membrane is supported by a housing defining an aperture for directing an output flow of analyte molecules into a mass spectrometer which is disposed downstream of the membrane. The mass spectrometer performs and analysis of the analyte molecules to identify the chemical elements of the analyte sample.
In another embodiment, the gas analyzer includes a housing for receiving the membrane, which housing includes an output nozzle directing the analyte sample over the input face of the membrane. The output nozzle generally complements the geometric shape of the input face to evenly distribute the analyte sample thought the membrane. That is, the output nozzle of the housing in combination with the input face of the membrane channels carrier gas across the variable thickness region from the first to the second positions.
In another embodiment, a method is provided for membrane mass spectroscopy analysis of an analyte sample comprising the steps of: directing the sample analyte to an input face of a membrane defining first and second positions, which membrane defines a variable thickness region between the first and second positions along an input face of the membrane. The variable thickness region of the membrane is functionally operative to concentrate an output flow of analytes for analysis by a mass spectrometer. The mass spectrometer produces a response profile for identifying the analytes of the analyte sample.
In another embodiment, a membrane is provided for a gas analyzer which membrane is disposed between an output nozzle and a support surface of a housing. The membrane comprises a variable thickness region between a first position and a second position, wherein the variable thickness region defines a first thickness dimension at the first position and a second thickness dimension at the second position. The first thickness dimension is less than the second thickness dimension so as to produce a concentrated output flow of analytes in the analyte sample.
The above embodiments are exemplary only. Other embodiments as contemplated and fall within the scope of the disclosed subject matter and appended claims.
So that the manner in which the features of the disclosure can be understood, a detailed description may be had by reference to certain embodiments, some of which are illustrated in the accompanying drawings. It is to be noted, however, that the drawings illustrate only certain embodiments and are therefore not to be considered limiting of its scope, for the scope of the disclosed subject matter encompasses other embodiments as well. The drawings are not necessarily to scale, emphasis generally being placed upon illustrating the features of certain embodiments. In the drawings, like numerals are used to indicate like parts throughout the various views, in which:
Corresponding reference characters indicate corresponding parts throughout several views. The examples set out herein illustrate several embodiments, but should not be construed as limiting in scope in any manner.
The present disclosure relates to gas analyzers and membranes used in gas analyzers. Specifically, different membrane geometries are presented that represent a marked improvement over conventional membranes when used in membrane interface mass spectroscopy (MI/MS).
Advantageously, various embodiments demonstrate how to geometrically optimize a GC/MI/MS interface for the detection of chemicals spanning a range of vapor pressures while minimizing the carrier gas load into the instrument. The gas analyzer is characterized by a compact design envelope which is compatible with fast chromatography.
Advantageously, the present disclosure provides a portable GC/MI/MS system that permits large, medium, and small analyte molecules into the mass spectrometer while minimizing the flow of carrier gas and preserving the shape of the signal amplitude vs. time domain output plot. By providing a single membrane with a variable thickness, the membranes described herein can be geometrically optimized for a wide range of vapor pressures while minimizing carrier flow into the mass spectrometer. By contrast, and as mentioned in the Background section of this disclosure, conventional systems include membranes with a single thickness, such as thin membranes or thick membranes, configured as tubes or flat membranes.
As explained in further detail below, the present disclosure provides a GC/MI/MS interface with a stepped or graded membrane, achieved either through variable thickness, temperature gradients, materials selection, or a combination thereof. In one embodiment, the GC/MI/MS interface is configured such that column effluent is directed at the thinnest portion of a variable thickness membrane with the gas then flowing substantially in the direction of a gradient of increasing membrane thickness, for example laterally across an input face of the membrane. The membrane may be axially symmetric or horizontally asymmetric, i.e., in a plane defined by the membrane. In one example, the membrane is supported by a single support. In yet another example, the membrane is in a single housing. In still another embodiment, the GC/MI/MS interface is configured such that the effluent is directed from the column onto the thinnest portion of a semi-permeable membrane having two (2) or more layers so that the gas passes substantially over a thin region prior to passing over a thicker region. In a further embodiment, the GC/MI/MS interface is configured such that the effluent is directed from the column onto a region of a semi-permeable membrane so that the gas passes substantially over a portion with the highest analyte permeability prior to passing over a region containing polymers of lower analyte permeability. In any of the above examples, rather than GC, a gas analyzer may use air as a carrier gas and the air is directed over the membrane interface with a pump, e.g., applicable to other sources of the sample analyte.
By way of general overview,
Next, at block 16, the analytes are directed to a membrane, which may be a semi-permeable membrane as noted above. The membrane separates the analytes from the carrier gas. In one example, the membrane has a first thickness at a first position and a second thickness at a second position. In such a case, a first analyte interacts with the membrane at the first position and a second analyte interacts with the membrane at the first and second positions. In another example, a molecular mass of the first analyte is greater than a molecular mass of the second analyte and the second thickness is greater than the first thickness. In a further example, the first analyte has a first diffusion coefficient and the second analyte has a second diffusion coefficient, and a distance between the first and second positions is selected to facilitate the second analyte interacting with the membrane at the second position. In one or more embodiments, an average time for the first analyte to traverse the membrane at the first position is similar to the average time for the second analyte to traverse the membrane at the second position. For instance, the first and second thicknesses and the first and second positions are selected to prevent undue broadening or lateral spread of the mass spectroscopy peaks associated with the first and second analytes.
In one specific example of membrane may have a (i) concave curvilinear, (ii) linearly-tapered or (iii) plano-concave geometric contour from the first to the second positions along the variable thickness region. These geometric contours facilitate the passage of larger molecules to pass through at the first position and smaller molecules to pass through both the first and second positions along the input face of the membrane. Finally, after passing through the membrane, the analytes of interest may be detected, at block 18, by a mass spectrometer.
The geometry of membrane 300 includes a variable thickness region VR from a first position P1 to a second position P2. That is, the membrane 300 is selected such that it is thin at the first position P1 where the gas chromatography (GC) effluent vapor stream contacts the membrane 300, i.e., upon deposition or output from a GC capillary tube or column 410. Low volatility molecules 113 are depleted from the carrier gas near this initial point of contact due to typically high membrane-air partition coefficients. These are the molecules which require a very thin membrane for producing a fast response (molecules similar to Fentanyl). The variable thickness region VR of the membrane 300, is positioned on the porous support 310, and becomes gradually or incrementally thicker as the distance increase from the first position P1 to the second position P2. This facilitates an increase in the total membrane area and allows the passage of smaller molecules 112 having a more moderate vapor pressure such as Sarin molecules 112. Sarin typically has higher membrane diffusion coefficients and can tolerate a thicker membrane region. Molecules 111 such as HCN or some other very volatile vapor, or gas have a low partition coefficient so more area is needed before depleting the vapor stream.
The housing 104 may include a column or conduit 124 configured to distribute the carrier gas 114 uniformly over the input face or face surface 116 of the membrane 300. In one embodiment, the housing 104 includes an output column or nozzle 124 which complements the geometric shape of the face surface 116 to evenly distribute the input flow of carrier gas 114 over the face surface 116 of the membrane 300. In this embodiment, an output face of the membrane 300 engages a porous support surface 310 such that the analytes may flow through an aperture of the support surface 310 and into the mass spectrometer 250.
In one method, the membrane 300 may be fabricated by silicone imprint molding. A high precision mold is prepared by grinding operations typically used for the production of high precision optical lenses such as those made from BK7 glass. The mold is laid flat and a thin stainless steel ring is clamped around the perimeter of a glass mold to create a form. A silicone mold release such as Dow Corning—Molykote 316 is applied to the assembly and a polymeric mixture of Dow Corning Sylgard 184 is used to make the membrane itself. A base and an activator are mixed in the ratio of 10:1 before pouring the forms. The membrane 300 may be air dried and baked at 100 C for 1 hr. to complete the cure. Other post cure operations such as rinsing with toluene may also be used to remove uncured base or mold release agents.
In
In the described embodiment, a filament produces electrons to ionize the carrier gas and analytes. The resulting ions are extracted from the ionization region with lenses for mass analysis. The mass spectrometer detector can be any of a variety of types including a magnetic sector, a time of flight (TOF), a quadrupole filter, or an ion trap.
In this design, the membrane 300 is truncated at the point 311 by an impermeable region of the support material below the membrane. No analyte or carrier gas can flow through this impermeable region of the support. The radial dimension to stop flow into the MS (where 310 stops and 311 starts) is chosen because at the point where the ratio of target analyte to carrier gas 114 falls below a desired threshold. The position of 311 can be moved inward as a design decision. This, for example, could occur at half the radius depicted in
The membrane 300 depicted in
Note that the
Although the membranes disclosed above in
In another embodiment, the shape of the curve showing membrane thickness in
With respect to the membrane support 310, this can be constructed using meshes or frits. But in cases where the membrane is self-supporting, the area of this aperture can be defined by a mechanical shutter similar to a camera iris to dynamically adjust the range of chemicals entering the mass spectrometer. In another example, concentric regions of the support can be provided that are blocked like shown in reference number 311 and then open again as you move further out. This can advantageously help tune for low- and high-volatility, without the need for mid-volatility.
In yet another embodiment, a temperature gradient rather than a change of thickness may be used to create the desired behavior since the membrane air partition coefficients and membrane analyte diffusion coefficients both vary with temperature. Generally the membrane air partition coefficient will decrease with increasing temperature while the polymer analyte diffusion coefficient will increase with increasing temperature. This means that a hot region positioned at the column exit will favor low volatility (and typically larger) analyte by providing fast diffusion coefficients. A cooler region further out will benefit higher volatility analyte by increasing their membrane air partition coefficients relative to the hotter region in the center of the device. The cooler region also reduces the carrier gas flow into the MS. In a similar manner, rather than using varying thickness to achieve the gradient in partitioning behavior, a polymer blend with appropriate material properties that vary along the membrane may be used.
In
In the described embodiment, the housing 506 defines a lateral or circular channel 520 which is in fluid communication with the input nozzle 504 at one end and a ring-shaped plenum 524 at the other end. Moreover, the ring-shaped plenum 524 is in fluid communication with the exhaust conduit 502 of the housing 506 for exhausting carrier air 114 away from the mass spectrometer 250. As shown in
It should be understood that various changes and modifications to the embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present disclosure and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.
Although several embodiments of the disclosure have been disclosed in the foregoing specification, it is understood by those skilled in the art that many modifications and other embodiments of the disclosure will come to mind to which the disclosure pertains, having the benefit of the teaching presented in the foregoing description and associated drawings. It is thus understood that the disclosure is not limited to the specific embodiments disclosed herein above, and that many modifications and other embodiments are intended to be included within the scope of the appended claims. Moreover, although specific terms are employed herein, as well as in the claims which follow, they are used only in a generic and descriptive sense, and not for the purposes of limiting the present disclosure, nor the claims which follow.
This application is a non-provisional application filed under relevant portions of 35 U.S.C. 111 and 37 CFR 1.53, and claims the priority and benefit of, U.S. Provisional Patent Application No. 63/008,862, filed on Apr. 13, 2020. The entire contents of such application is hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 63008862 | Apr 2020 | US |
