Reagents for enhanced detection of low volatility analytes

Abstract

The use of volatilization reagents is disclosed for improved detection of inorganic oxidizers such as, but not limited to, chlorates and perchlorates. Detection methods are disclosed whereby a reagent can transfer a proton to the anion (i.e., chlorate, perchlorate, etc.) of an inorganic salt analyte, forming an acid (i.e., chloric acid, perchloric acid) that is easier to detect by a mechanism whereby the acidified reagent is more easily vaporized, and hence, more easily detected. Concurrently, the anion of the acid forms a new salt with the cation released from the salt that was acidified. The reagents can also include acidic salts or cation-donators, more generally. In some embodiments, hydrated reagents or co-reagents that can release water can be employed.

Claims

1. A swipe for detection of an analyte, comprising: a substrate configured to collect a sample for analysis, and a proton-containing acidic reagent associated with the substrate and configured to react with an analyte, if present in the sample, to transfer a proton to the analyte so as to generate a higher vapor pressure analog of the analyte, wherein the acidic reagent comprises a polymeric acid, and a thermally labile hydrate that can provide an additional source of water upon heating, thereby facilitating reaction of the proton-containing acidic reagent with the analyte.

2. The swipe of claim 1, wherein the thermally labile hydrate comprises sodium thiosulfate pentahydrate.

3. The swipe of claim 1, wherein the analyte is an ionic component of an explosive material.

4. The swipe of claim 3, wherein said ionic component is selected from the group consisting of salts of chlorate, perchlorate, permanganate, dichromate, and osmium tetraoxide.

5. The swipe of claim 1, wherein the acidic reagent has a pKa of less than 2.5.

6. The swipe of claim 5, wherein said acidic reagent comprises a polymeric organic acid.

7. The swipe of claim 1, wherein the acidic reagent is a polymeric organic acid.

8. The swipe of claim 1, wherein the acidic reagent comprises a hydrated acidification reagent.

9. The swipe of claim 8, wherein the hydrated acidification reagent comprises a hydrated solid state acid.

10. The swipe of claim 9, wherein said hydrated solid state acid comprises a hydrated sulfonate—or sulfate-containing acid.

11. The swipe of claim 1, wherein said acidic reagent exists in the solid or liquid phase.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The appended drawings have been included herein so that the above-recited features, advantages and objects of the invention will become clear and can be understood in detail. These drawings form a part of the specification. It is to be noted, however, that the appended drawings should not be considered to limit the scope of the invention.

(2) FIG. 1 is a flow diagram depicting various steps in a method according to an embodiment of the present invention for detection of an analyte molecule of interest in a sample,

(3) FIG. 2 illustrates an acidic volatilization reagent scheme according to the invention for detection of an analyte salt. The acidic volatilization reagent exchanges its cation (H+) and anion (A−) with the oxidizer analyte salt to create an acid with the same anion as the analyte salt. The volatility of this acid increases the availability of the anion A− for detection in negative ionization mode.

(4) FIG. 3 illustrates representative net reactions according to some embodiments of the invention. Reactions (a) through (f) show volatilization of chlorate and perchlorate salts using: hydrochloric acid (a, b), sulfuric acid (c, d) and sodium bisulfate (e, f). In each case, a more volatile acid, chloric (HClO.sub.3) or perchloric (HClO.sub.4) acid is generated from the relatively involatile analyte salt. Reaction (g) shows how sodium thiosulfate pentahydrate (Na.sub.2S.sub.2O.sub.3.5H.sub.2O) liberates water molecules in the presence of applied heat (Δq). Water molecules liberated by reaction (g) help facilitate reactions of type (e) and (f).

(5) FIGS. 4A-4B illustrate the chemical structures of the polymeric acids Nafion® (FIG. 4A) and polystyrene sulfonic acid (FIG. 4B).

(6) FIGS. 5A-5B illustrates a 3D model of anhydrous (5A) and hydrated (5B) sodium bisulfate, respectively.

(7) FIG. 6 schematically illustrates that proton transfer between two solids is a relatively inefficient process.

(8) FIGS. 7A-7C illustrate the transfer of a proton from a hydrated acidic salt (sodium bisulfate monohydrate) to solid potassium chlorate via protonation of a nearby free and mobile water molecule.

(9) FIG. 8 schematically depicts a swipe according to an embodiment of the present invention.

(10) FIG. 9 is a graph of normalized mass spectrometer signal (Y) vs. time (X) obtained by introduction of liquid sulfuric acid (20 μL, 10%) onto a 5 μg sample of dried potassium perchlorate in the desorber unit of a TD APCI mass spectrometer at 150° C.

(11) FIG. 10 is a graph of thermal desorption temperature (Y axis) vs. experimentally measured background subtracted normalized mass spectrometer signal (X axis) for detection of chlorate and perchlorate salts (left traces), commonly detected explosive materials (right traces), and oxidizer salts treated with a dilute acidic reagent (red points on the right).

(12) FIG. 11 is a graph of experimentally measured background subtracted normalized mass spectrometer signal (Y axis) vs. pH (X axis) of the acidic volatilization reagent sulfuric acid (H.sub.2SO.sub.4) for detection of potassium perchlorate.

(13) FIG. 12 is a graph showing experimentally measured background subtracted normalized mass spectrometer signal (Y axis) for bare potassium perchlorate at 250° C. and for potassium perchlorate exposed to a variety of different acidic volatilizing reagents at 150° C.

(14) FIGS. 13A-13D are graphs of negative ion TD APCI mass spectra all at a thermal desorption temperature of 150° C. FIG. 13A is a graph of the blank background spectrum. FIG. 13B is a graph of 0.5 μg KClO.sub.3(s). FIG. 13C is a graph of the background subtracted 150 μg NaHSO.sub.4.H.sub.2O(s). FIG. 13D is a graph of the background subtracted of 0.5 μg KClO.sub.3(s) combined with 150 μg NaHSO.sub.4.H.sub.2O(s).

(15) FIG. 14 is a graph of the background subtracted mass spectrum (ion intensity v. mass to charge ratio) for enhanced detection of 5 μg potassium chlorate (KCl0.sub.3) using 73 μg of Nafion™ as an acidification reagent. The appearance of the ions ClO.sub.3.sup.−, ClO.sub.2.sup.− and ClO.sup.− indicates the presence of an inorganic oxidizer salt containing chlorate anions. The thermal desorption temperature was 150° C.

(16) FIG. 15 is a graph of the background subtracted mass spectrum (ion intensity v. mass to charge ratio) for enhanced detection of 0.5 μg potassium chlorate (KCl0.sub.3) using 900 μg polystyrene sulfonic acid as an acidification reagent. The appearance of the ions ClO.sub.3.sup.−, ClO.sub.2.sup.− and ClO.sup.− indicates the presence of an inorganic oxidizer salt containing chlorate anions. The thermal desorption temperature was 150° C.

DETAILED DESCRIPTION

(17) Certain exemplary embodiments will now be described to provide an overall understanding of the principles of the structure, function, manufacture, and use of the devices and methods disclosed herein. One or more examples of these embodiments are illustrated in the accompanying drawings. Those skilled in the art will understand that the devices and methods specifically described herein and illustrated in the accompanying drawings are non-limiting exemplary embodiments and that the scope of the present invention is defined solely by the claims. The features illustrated or described in connection with one exemplary embodiment can be combined with the features of other embodiments. Such modifications and variations are intended to be included within the scope of the present invention.

(18) All publications, patents and patent applications cited herein, whether supra or infra, are hereby incorporated by reference in their entirety. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural references unless the content clearly dictates otherwise. The terms used in this invention adhere to standard definitions generally accepted by those having ordinary skill in the art. In case any further explanation might be needed, some terms have been further elucidated below.

(19) The term “about,” as used herein, refers to variations in a numerical quantity that can occur, for example, through measuring or handling procedures in the real world; through inadvertent error in these procedures; through differences in the manufacture, source, or purity of compositions or reagents; and the like. Typically, the term “about” as used herein means greater or lesser than the value or range of values stated by 1/10 of the stated values, e.g., ±10%. For instance, a concentration value of about 30% can mean a concentration between 27% and 33%. The term “about” also refers to variations that would be recognized by one skilled in the art as being equivalent so long as such variations do not encompass known values practiced by the prior art. Each value or range of values preceded by the term “about” is also intended to encompass the embodiment of the stated absolute value or range of values. Whether or not modified by the term “about,” quantitative values recited in the claims include equivalents to the recited values, e.g., variations in the numerical quantity of such values that can occur, but would be recognized to be equivalents by a person skilled in the art.

(20) The terms “polymeric acids,” “polymeric organic acids,” and polymeric sulfonic acids” are used herein consistent with their standard use in the art. A polymeric acid refers to a polymer having one or acidic functional groups. In certain preferred embodiments, the majority, or more than 75 percent, or all of the repeat units include an acid moiety capable of donating a proton. The repeat units needn't all be identical. The repeating subunits of the polymer can be bonded together to form a linear, a branched or a cyclic structure.

(21) The term “polymeric organic acid” as used herein, is intended to encompass molecules (i) having one or more repeat units that are covalently linked together, (ii) largely comprising elements that occur in natural materials, namely carbon, hydrogen, oxygen, and nitrogen, and (iii) can donate a proton to another molecule. In some embodiments, the polymeric organic acids of the invention have at least some carbon atoms in their backbone. In certain preferred embodiments, the majority, or more than 75 percent, or all of the repeat units include an acid moiety capable of donating a proton. The repeat units needn't all be identical. The repeating subunits of the polymer can be bonded together to form a linear, a branched or a cyclic structure.

(22) The term “polymeric inorganic acid” is typically used to describe analogs of polymeric organic acids having atomic backbones completely free of carbon, such as polysiloxanes (silicon-oxygen backbones), polyphosphates (phosphorous and oxygen backbones), and polysilanes (all silicon backbones). In some instance, such molecules can have an equivalent effect in inducing evaporation of low volatility analytes. In certain preferred embodiments, the majority, or more than 75 percent, or all of the repeat units include an acid moiety capable of donating a proton. The repeat units needn't all be identical. The repeating subunits of the polymer can be bonded together to form a linear, a branched or a cyclic structure.

(23) The term “polymeric sulfonic acid” as used herein, is intended to encompass molecules (i) having one or more repeat units that are covalently linked together, (ii) include sulfur and (iii) can donate a proton to another molecule. Similarly to polymeric organic acids, in certain preferred embodiments the majority, or more than 75 percent, or all of the repeat units include an acid moiety capable of donating a proton. Again, the repeat units needn't all be identical and the repeating subunits of the polymer can be bonded together to form a linear, a branched or a cyclic structure.

(24) In some embodiments, the molecular weights of the polymeric organic acids and polymeric sulfonic acids useful herein can range from between 1,000 and 1,000,000 Da, or between 10,000 and 200,000 Da, or between 50,000 and 150,000 DA. In certain embodiments, a variant of polystyrene sulfonic acid having a molecular weight of about 75,000 Da was useful. However, it is understood to those skilled in the art that in certain aspects of this invention is the polymer's ability to act as an acid, that is to say, to donate a proton. Thus by extension any polymer, independent of its molecular weight, which exhibits such properties, is intended to be encompassed.

(25) In some embodiments, the number of repeat units in the polymeric organic acids and polymeric sulfonic acids useful herein can range from about 50 to about 1000 repeat units or between about 100 and about 700 repeat units or between about 200 and about 600 repeat units. In certain embodiments a variant of polystyrene sulfonic acid having about 400 repeat units was useful.

(26) In general, the pKa's of the acidic evaporative reagents (or polymeric acids) useful herein are less than 2.5, or less than 2, preferably less than 0, or in some instances more preferably less than −2. In some embodiments, sulfonic acids, such as benzene sulfonic acid (pKa=−2.8), polystyrene sulfonic acid (pKa=−3), substituted polystyrene sulfonic acids (pKa=−3 to −6) and Nafion™ a perfluorinated sulfonic acid (pKa=−6) have been useful.

(27) In summary, the key aspects of the acidic evaporative reagents are their ability to convert strong oxidizers (chlorate, perchlorate) into their acidic form (chloric acid, perchloric acid), while having both no discernable vapor pressure and chemical stability to >150° C., thus not off-gassing any unwanted vapors into the analysis instrument, thereby allowing the analysis instrument to receive only the acidified oxidizers.

(28) The term “swipe” is used herein in its general sense to mean a vehicle for collection of a sample. Typically in the context of IMS, the swipe is a substrate including a least one of paper, fabric, cloth, fibrous matte, gauze, cellulose, cotton, flax, linen, synthetic fibers and blends of such materials. However, other materials such as ceramic or semiconductor materials can also be used as “swipes” depending upon the analysis scheme.

(29) Mass spectrometry is an analytical process for identifying a compound or compounds in a sample by assessing the molecular weight, chemical composition and structural information based on the mass-to-charge ratio of charged particles. Mass spectrometry is widely considered to have the best specificity of any technique applicable to a broad class of explosive compounds. In general, a sample undergoes ionization to form charged particles as ions; these charged particles are then passed through electric and/or magnetic fields to separate them according to their mass-to-charge ratio. The terms “mass spectrometry” and “spectrometry” are used herein to encompass techniques that produce a spectrum or spectra of the masses of molecules present in a sample. Mass spectrometry includes, but is not limited to, ion mobility spectrometry (IMS), differential mobility spectrometry (DMS), field asymmetric ion mobility spectrometry (FAIMS), and mass spectrometry (MS), all of which rely upon ionization of the analyte or a complex that includes the analyte. The analysis performed in spectrometry is typically referred to as “mass/charge” analysis, a method of characterizing the ions detected by a spectrometer in terms of their mass-to-charge ratio. The abbreviation m/z is used to denote the quantity formed by dividing the mass number of an ion by its charge number. It has long been called the mass-to-charge ratio although m is not the ionic mass nor is z a multiple or the elementary (electronic) charge, e. Thus, for example, for the ion C.sub.7H.sub.7.sup.2+, m/z equals 45.5.

(30) The ionization process can be performed by a wide variety of techniques, depending on the phase (solid, liquid, gas) of the sample and the efficiency of the target analyte(s) in question. Some examples of ion sources can include electron ionization, glow discharge ionization, resonant ionization, field desorption, fast atom bombardment, thermospray, desorption/ionization on silicon, atmospheric pressure chemical ionization, spark ionization, inductively coupled plasma ionization, secondary ionization by sputtering ion beams off the target's surface, and thermal ionization.

(31) Ambient-pressure ionization, collision-induced ionization, and atmospheric-pressure chemical ionization refer to a characterization techniques in which picogram to microgram quantities of an analyte can be analyzed. The process generally refers to a chemical sample that is introduced into an ionization region as either a solid, liquid, or gas. In the ionization region, the analyte is in contact with other gases and ions that are part of the ionization region. Additional ions are produced through the collision of the analyte molecules with ions within the ionization reagent that are present in the ion source, electro-magnetic device. Inside the ion source, the ionization reagent is present in large excess compared to the analyte. Electrons and/or ions entering the source will preferentially ionize the ionization reagent. Collisions with other ionization reagent molecules will induce further ionization, creating positive and/or negative ions of the analyte. The ions are drawn into the spectrometer by either a carrier gas or focused into a beam by an electromagnet, then separated into individual beams based on the mass/charge ratio of the ions. The ion beams are separated in a mass spectrometer and collected either sequentially in a single detector or simultaneously in a set of multiple detectors to yield isotopic ratios. Highly accurate results require that sample cross-contamination be minimized.

(32) The traditional methods for explosives detection usually involve wiping the ambient surface with a special material wipe followed by thermal desorption/gas phase ionization of the explosive compounds in the presence of an ionization reagent. The performance of a detection approach such as this depends, in part, on the efficiency with which the explosive compound is transferred from the swipe into the ionization region of the analysis instrument during the desorption step. It is therefore desirable to maximize this efficiency for explosive compounds which have low vapor pressures.

(33) The terms “desorption,” “desorb” and “desorbing” as used herein refer to technology of increasing the volatility of molecules, for example target analytes, such that they can be removed (separated) from the solid. Thermal desorption is not incineration, but uses heat and a flow of inert gas to extract volatile and semi-volatile organics retained in a sample matrix or on a sorbent bed. The volatilized compounds are then either collected or thermally destroyed.

(34) In certain embodiments, the reagents of the present invention are low volatility compounds. The terms “low volatility” and “low vapor pressure” as used herein are intended to describe compositions that do not readily evaporate or sublimate at room temperature (e.g., at about 25° C.). Typically such low volatility compositions are solids or viscous liquids and have a vapor pressure at room temperature of less than 1 Torr, or more typically less that 10.sup.−1 Torr. In some preferred embodiments, the low volatility reagents of the present invention can have a vapor pressure at room temperature of or less that 10.sup.−2 Torr or, more preferably, less that 10.sup.−3 Torr.

(35) With reference to flow chart of FIG. 1, in some embodiments, a method for detection of an analyte molecule potentially present in a sample, e.g., inorganic oxidizers such as chlorates and perchlorates, is disclosed that includes treating the sample with an evaporative reagent to form a higher vapor pressure analog of the analyte if present in the sample (step 1). The treated sample can then be subjected to mass spectroscopy to detect the analyte, if present in the sample (step 2). The evaporative reagent can be an acidic volatilization reagent. For example, in some embodiments, the acidic volatilization reagent can be a polymeric organic acid, such, a polymeric sulfonic acid. In some embodiments, the acidic volatilization reagent can be a hydrated solid state acidification reagent, such as, potassium bisulfate monohydrate.

(36) FIG. 2 provides a reagent scheme for detection of an oxidizer in accordance with the present teachings, where an acidic volatilization reagent induces a salt metathesis reaction to increase the availability of an originating salt's anion for detection in a negative ionization mode. The increased vapor pressure of the newly formed acid results in significantly increased detection sensitivity and the ability to perform the detection at lower temperatures. Compounds that can act as acidic volatilization reagents for such enhanced detection include, without limitation, strong acids, weak acids, and acidic salts. An acidic salt can dissociate to release a weak acid via thermal decomposition, or by dissociative solvation in the presence of water. The weak acid released from the acidic salt can then participate in the salt methathesis reaction shown in FIG. 2.

(37) In some instances, the use of a liquid reagent may not be preferable. Additionally, the use of a strong acid (even one that is substantially diluted) may not be desirable or allowed in certain environments. In such instances, the use of solid acidic salts (e.g. sodium bisulfate, NaHSO.sub.4) can be the preferred method of providing an acidic reagent. These solids may be applied to existing swipes or introduced during manufacturing of chemically treated swipes. The hydrated form of sodium bisulfate easily vaporizes at low temperatures, liberating the weak acid HSO.sub.4.sup.− which is effective at participating in salt metathesis reactions. The presence of water typically increases the effectiveness of such reactions, and that too can be thermally released from a solid salt. Sodium bisulfate can exist as a monohydrate and can essentially supply its own water. To supply even more water to assist the reaction, a solid compound (e.g. sodium thiosulfate pentahydrate, Na.sub.2S.sub.2O.sub.3.5H.sub.2O) can be introduced to the swipe material alongside the acidic volatilization reagent. Numerous such hydrates exist, and when choosing one, three characteristics should be considered. First, the salt of the hydrate should not interfere with the desired salt methathesis reaction. More specifically, it should not compete with the analyte for protons. Secondly, the hydrate should melt and release its hydrated water molecules at temperatures easily attainable in a thermal desorber (i.e., <200° C. or more preferably <150° C.). Lastly, the hydrate should contain as many associated water molecules as possible for the sake of efficiently.

(38) FIG. 3 outlines reactions that are representative of some embodiments of the present invention. More specifically, reactions (a) through (f) show volatilization of chlorate and perchlorate salts using: hydrochloric acid (a, b), sulfuric acid (c, d) and sodium bisulfate (e, f). In each case, a more volatile acid, chloric (HClO.sub.3) or perchloric (HClO.sub.4) acid is generated from the relatively involatile analyte salt. Reaction (g) shows how sodium thiosulfate pentahydrate (Na.sub.2S.sub.2O.sub.3.5H.sub.2O) liberates water molecules in the presence of applied heat (Δq). Water molecules liberated by reaction (g) help facilitate reactions of type (e) and (f).

(39) The term “hydrate” as used herein is intended to encompass compositions in which there are one or more water molecules associated with at least some of the reagent or co-reagent molecules.

(40) A series of acidic (or other) reagents may be utilized by introduction simultaneously or at discrete times and temperatures to induce selective reactions with expected oxidizers (e.g. reagent A for perchlorate, reagent B for chlorate, reagent C for hydrogen peroxide, reagent D for TATP, reagent E for HMTD). This invention will allow introduction of multiple reagents, if required for more selective detection.

(41) In some embodiments, polymeric organic acids are employed as evaporative reagents for enhanced detection of an analyte molecule present in a sample, for example, for enhanced detection of inorganic oxidizers, such as chlorates and perchlorates. In some embodiments, the polymeric organic acid can have a molecular weight, for example, in a range between 1,000 and 1,000,000 Da, or between 10,000 and 200,000 Da, or between 50,000 and 150,000 DA. Further, in some embodiments, the number of repeating units of the backbone of the polymeric organic acid can range from about 50 to about 1000 repeat units or between about 100 and about 700 repeat units or between about 200 and about 600 repeat units.

(42) Some examples of suitable acidic evaporative reagents include, without limitation, perfluorinated sulfonic acids (e.g., such as but not limited to Nafion™) and polymeric sulfonic acids (e.g., such as but not limited to polystyrene sulfonic acid).

(43) Without being bound to any particular theory, polymeric organic acids, such as perfluorinated sulfonic acids, can act as effective evaporative agents in accordance with some embodiments of the present teachings because (1) their sulfonic functional groups render them extremely acidic, e.g., a pKa of less than 2.5, or less than 2, preferably less than 0, or in some instances more preferably less than −2. (2) these polymeric acids are known to be highly hygroscopic, and typically have water molecules absorbed on their surface that can aid in proton transfer, and (3) these polymeric organic acids can be tailored to release their protons without contributing other materials to the gas phase. As such, in some embodiments, the polymeric organic acids are selected that can enhance the detection of analyte molecules of interest in a very “clean” fashion, i.e., without generating unwanted byproducts. By way of example, this feature can be valuable for explosive trace detection (ETD) as many ETD equipment can manifest strong spectral signals from even small contributions of nuisance chemical byproducts to the gas phase.

(44) In some embodiments, the polymeric organic acids are employed in a solid phase while in other embodiments the polymeric organic acids are employed in liquid phase for enhanced detection of an analyst in a sample.

(45) As discussed above, the intended goal of using acidification reagents for enhanced detection is to efficiently convert inorganic oxidizer salts into their detectable volatile analogs. In some cases, one unintended consequence of this reagent chemistry can be desorption of the acidification reagent itself (or byproducts thereof) during the thermal desorption process in an ETD. This phenomenon has been observed with sodium bisulfate in both the solid and solution state (See, e.g., FIG. 13C). The appearance of new peaks in the IMS or MS is undesirable, as any new peak adds to the ‘spectral clutter’ and could potentially be confused for threat compounds in some circumstances.

(46) An idealized acidification reagent would convert the anions of inorganic oxidizer salts into their high vapor pressure acidic analogs and make no other contribution to the detectable ensemble of molecules in the gas phase. This could be accomplished by using an acid in which the counter anion (negatively charged component that remains after donating a proton) has a sufficiently low vapor pressure at the thermal desorption temperature. Another strategy involves tethering the counter anion to the substrate of the ETD swipe used for sample gathering.

(47) One advantage polymeric acids according to the present teachings is that the use of such polymeric acids can allow effectively employing both of the above strategies to minimize, and preferably inhibit, contribution to observed spectral clutter. As discussed in more detail below, Applicants have demonstrated the ability of these materials to ‘turn on’ the acidic enhancement mechanism with little to no contribution to observed spectral clutter.

(48) By way of example, FIGS. 4A and 4B show the structures of two polymeric sulfonic acids, namely Nafion™ and polystyrene sulfonic acid, that are suitable for enhanced detection of analytes of interest, and particularly chlorates and perchlorates in accordance with the present teachings. These acids have polymeric backbones that can have arbitrary lengths, from a few subunits to a bulk polymer. Each polymer can have numerous acidic groups branching from the side. While in some embodiments sulfonic acids are used, in other embodiments other acidic functional groups could be utilized. One advantage of these acids is that the sulfonic acid group, which remains behind after the proton (H.sup.+) has left the acidic group, is covalently attached to the relatively massive and nonvolatile polymer backbone. This inhibits the ability of the counter anion to enter the gas phase and introduce unwanted spectral clutter.

(49) As discussed above, in some embodiments, solid-state acidification reagents can be used for enhanced detection of analytes of interest. In some such embodiments, the presence of microscopic amounts of free excess water (such as water released from a hydrated crystal upon thermal desorption) can catalyze proton transfer from the acidic reagent to the analyte molecule of interest. Without being bound to any particular theory, the water in hydrate compounds can retain its intact molecular identity. While such water molecules participate in the formation of a crystal lattice, they can be reversibly added or liberated from the crystal under proper conditions.

(50) By way of example, the presence of hydrated or absorbed water on/in a solid-state acidification reagent can help facilitate the proton transfer mechanism between an acidic reagent and a target chlorate or perchlorate salt. Both inorganic and organic solid state acids can exist as hydrates. One such example of an inorganic acid that can exist as a hydrate is sodium bisulfate, shown in FIGS. 5A and 5B, in its anhydrous and hydrated states, respectively.

(51) Chemistry in liquid and gas phase, which are inherently mobile phases of matter, typically proceeds more efficiently than solid-state chemistry. For example, as shown schematically in FIG. 6, a solid-state transfer of a proton between the bisulfate anion of anhydrous sodium bisulfate and the chlorate anion of potassium chlorate (a two-body reaction mechanism) can be a relatively inefficient process.

(52) The presence of water greatly enhances the efficiency of proton transfer, because water molecules are highly mobile themselves (in either the liquid or gas phases), and individual water molecules are able to accept extra protons. The protonated water molecules are referred to as hydronium ions. Hydronium ions on their own are quite mobile, and can also rapidly transfer their ‘extra’ proton to neighboring water molecules.

(53) A solid hydrate (such as sodium bisulfate) can shed water molecules when heated. Thus, in a typical thermal desorption environment, like those found in commercial IMS based ETD equipment, heated hydrates and hygroscopic acids can be surrounded by copious amounts of ‘free’ water molecules. These free water molecules are in a good position to get protonated by an acidifying reagent and efficiently transfer that proton to a solid potassium chlorate surface some distance away.

(54) By way of example, FIGS. 7A-7C schematically show how a single water molecule can promote the transfer of the proton between the two solids. While these figures show a single water molecule assisting the proton transfer mechanism, more water molecules (released from other hydrated crystals or from the ambient environment) could also participate in the proton transfer. In these situations, the newly formed hydronium cation would simply transfer its ‘extra’ proton to a neighboring water molecule in the direction of the potassium chlorate. Water vapor or liquid is an effective conductor of protons, and acts to catalyze the proton transfer reaction.

(55) More specifically, FIG. 7A depicts that the sodium bisulfate transfers a proton from a hydronium cation to a nearby water molecule. The hydronium ion is highly mobile and can readily move between the two solid surfaces to react with the chlorate anion of potassium chlorate, as shown in FIG. 7B. FIG. 7C shows that the hydronium cation donates a proton to the chlorate anion to form chloric acid. The generated chloric acid is highly volatile and hence can be easily detected.

(56) In some embodiments, swipes for use in detection of analytes of interest, such as, chlorates and perchlorates, are disclosed, which are impregnated with one or more evaporative reagents according to the present teachings. By way of example, FIG. 8 schematically depicts a swipe 10 (also referred to as smear, wipe) according to one embodiment, which includes a substrate 12. The substrate 12 can be formed of a variety of different materials, such as paper, fabric, cloth, fibers, glass, or synthetic material. By of example, in some embodiments, the substrate is fabricated of polyester, muslin, or cotton. The substrate can be preferably formed of a material that can be resistant to chemical degradation during testing in the approximate pH range of 0.1 through 14 to avoid reacting or decomposing. Further information regarding suitable substrates can be found in published U.S. patent application having publication no. 2014/0030816 entitled “Reagent Impregnated Swipe For Chemical Detection,” which is herein incorporated by reference in its entirety.

(57) One or more evaporative reagents 14 according to the present teachings, such as those disclosed herein (e.g., polymeric sulfonic acids) is deposited on, embedded in, or otherwise associated with, the substrate 10. The reagents 14 can be associated with the substrate via a variety of different physical and/or chemical mechanisms, such as physical entrainment, non-covalent and/or covalent bonds. In some embodiments, one or more polymeric organic acids can be associated with the substrate by tethering the polymer backbone to the substrate, e.g. via covalent bonds. In some such embodiments, one or more linkers may be employed to tether the polymeric acid to the substrate.

(58) In some embodiments, the swipe 10 can be heat resistant, absorbent and/or chemically resistant at elevated temperatures and can have hydrophilic properties for wetting when using fluid reagents. While in some embodiments, the swipe has a sheet-like structure, in other embodiments, it can have a three-dimensional structure.

(59) Commercial applications include use in all industries involved in chemical detection including but not limited to explosives detection, chemical warfare detection, homeland security, and toxic industrial chemical and pollution monitoring.

EXAMPLES

(60) This invention has been reduced to practice for detection of potassium perchlorate, sodium perchlorate, potassium chlorate, and sodium chlorate via API mass spectrometry. As described earlier, in negative-ion-mode atmospheric pressure chemical ionization, the vaporization (and hence ionization) efficiency of ‘bare’ oxidizer salts is extremely limited. The examples provided below are only for illustrative purposes, and are not intended to necessarily illustrate the optimal ways of practicing the invention and/or optimal results that may be obtained.

Example 1

(61) FIG. 9 illustrates enhancement in observed mass spectrometer ion signal upon addition of an acidic volatizing reagent to an inorganic oxidizer salt. More specifically, FIG. 9 is a graph of normalized mass spectrometer signal (Y) vs. time (X) obtained by introduction of liquid sulfuric acid (20 μL, 10%) onto a 5 μg sample of dried potassium perchlorate in the desorber unit of a TD APCI mass spectrometer at 150° C. In this instance, the mass spectrometer was in MRM mode, measuring the 9983 Da loss channel. Note the dramatic increase in signal at 0.14 minutes when the acidic volatilizing reagent was pre-mixed with the potassium perchlorate sample. In the absence of the acidic volatilizing reagent, no signal was observed from potassium perchlorate at such a low temperature.

Example 2

(62) In another example, in order to increase the amount of free perchlorate anion available for detection, an acidic reagent known to protonate the perchlorate anion (.sup.35ClO.sub.4.sup.−) was added to a dried potassium perchlorate sample. In these experiments, a liquid reagent, namely 5 μL of 0.01% sulfuric acid (CAS#7664-93-9), was added over a previously dried 5-μg sample of potassium perchlorate on an inert silicon wafer surface. The silicon wafer was then transferred to a thermal desorption unit on a TD APCI source for detection via mass spectrometry. The experimental data presented in FIG. 10 shows the benefits of acidic reagents in terms of both increased mass spectrometer signal and enabling thermal desorption to take place at lower temperatures.

(63) More specifically, FIG. 10 is a graph of thermal desorption temperature (Y axis) vs. experimentally measured background subtracted normalized mass spectrometer signal (X axis) for detection of chlorate and perchlorate salts (left traces), commonly detected explosive materials (right traces), and oxidizer salts treated with a dilute acidic reagent (red points on the right). Increased detection sensitivity is denoted by larger values on the X axis. This figure demonstrates how the presence of an acidic volatilization reagent increases the signal level. The diagonal red arrow in the center of the figure illustrates a 10.sup.4 factor increase in observed mass spectrometer signal at decreased thermal desorption temperature as a result of treating chlorate and perchlorate salts with dilute sulfuric acid. As shown in FIG. 10, the net result of using the acidic volatilization reagent is increased sensitivity (on the order of a factor of 10.sup.4 for the dilute sulfuric acid solution used) at a modest thermal desorption temperature of 150° C. It is important to note that this thermal desorption temperature is also near the optimal value for the other more conventionally detected explosives (TATP, PETN, and RDX) included in FIG. 10. This demonstrates that the acidic volatilization reagents can operate in a temperature regime that favors current detection methods and instrumentation.

Example 3

(64) In order to determine the minimum acidity (maximum pH) at which the acidic volatilization mechanism was still functional, a series of signal vs. pH measurements were carried out. FIG. 11 shows the results of an experiment conducted in order to determine the maximum acceptable pH for the acidic volatilization reagent sulfuric acid (H.sub.2SO.sub.4). FIG. 11 is a graph of experimentally measured background subtracted normalized mass spectrometer signal (Y axis) vs. pH (X axis) of the acidic volatilization reagent sulfuric acid (H.sub.2SO.sub.4) for detection of potassium perchlorate. The numbers next to the data points indicate the percent solution (v/v) of sulfuric acid corresponding to each measurement. A 0.01% (pH 2.74) sulfuric acid solution still produces an acceptable amount of signal enhancement for detection of the perchlorate anion. As shown in FIG. 11, the ion signal generated by acid volatilization of potassium perchlorate (KClO.sub.4) begins to drop off after a pH of 2 is reached. An acceptable result (in terms of enhancement of the normalized signal compared to bare potassium perchlorate) is still achieved at a pH of 2.74, which is equivalent to a 0.01% sulfuric acid solution.

Example 4

(65) FIG. 12 presents an overview comparing background subtracted normalized mass spectrometer signal for bare potassium perchlorate solid in a TD APCI source and potassium perchlorate treated with a variety of acidic volatilizing reagents. In FIG. 12, experimentally measured background subtracted normalized mass spectrometer signal (Y axis) is shown for bare potassium perchlorate at 250° C. and for potassium perchlorate exposed to a variety of different acidic volatilizing reagents at 150° C. All of the acidic volatilizing agents were presented to the solid potassium perchlorate in liquid form in noted concentrations (v/v) except for the sodium bisulfate data point denoted as solid (s).

Example 5

(66) FIGS. 13A-13D show mass spectra of sodium bisulfate monohydrate enhancing the detection of potassium chlorate. It is clear that the blank background spectrum (FIG. 13A.) and the spectrum of potassium chlorate by itself (FIG. 13B) are markedly similar. Sodium bisulfate monohydrate produces a spectrum of its own (FIG. C). The combination of potassium chlorate and sodium bisulfate monohydrate produces new and unique signals that indicate the presence of chlorate anions (ClO.sub.3.sup.−, m/z=83, 85) as well as chlorate breakdown products (chlorite, ClO.sub.2.sup.−, at m/z=67, 69 and hypochlorite, ClO.sup.−, at m/z=51, 53).

Example 6

(67) It was observed that polymeric acids can be highly effective at enhancing trace detection of inorganic salts with a thermal desorption atmospheric pressure chemical ionization (TD APCI) mass spectrometer. More specifically, both Nafion™ and polystyrene sulfonic acid were observed to enhance detection of chlorates and perchlorates from the milligram/microgram detectable range down to the microgram/nanogram detectable range, an increase in detection on the order of three orders of magnitude.

(68) FIGS. 14 and 15 present representative mass spectra showing detection of potassium chlorate (KClO.sub.3) with both Nafion™ and polystyrene sulfonic acid, respectively. It is clear that these polymeric acids are strong enough to enable the acidification/volatilization, and their hygroscopic property ensures that they carry enough absorbed water to enable the reaction to proceed in an efficient manner. The relative lack of clutter in these mass spectra indicates that these polymeric acids are enabling the acidification/volatilization mechanism without introducing unwanted by-products to the gas phase ion population.

(69) One skilled in the art will appreciate further features and advantages of the invention based on the above-described embodiments. Accordingly, the invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims. All patents, publications and references cited herein (including the following listed references) are expressly incorporated herein by reference in their entirety.