Patent Application
20190291070
The present invention relates to microwave assisted digestion and related sample-preparation reactions carried out under high temperature and pressure. Microwave assisted acid digestion is a sample preparation technique in which various samples (soil is particularly exemplary) are added in small amounts (e.g., 15 g) to acid-resistant, microwave transparent, pressure vessels along with an appropriate amount (e.g., 55 mL) of one or more strong mineral acids. In that regard, nitric acid (HNO3) can be handled in glass, but hydrofluoric acid (HF) must be handled in fluoropolymer vessels.
When heated, such combinations tend to generate very high pressures, most of which come from carbonates (CO3−2) or carbon-based compounds. Acid combinations (nitric plus hydrochloric) can lead to greater pressures than single acids (nitric alone), acids without samples can reach 12 atmospheres (atm), and total pressures can reach 30 atm with samples.
Because the relevant samples are present in small concentration (usually parts per million, and sometimes parts per billion), the vessels must be scrupulously clean prior to each use. Because fluorinated hydrocarbon polymers tend to be porous (and thus absorbent), this cleaning step is time-consuming, requires a certain amount of expertise, and takes up laboratory space. Additionally, relevant regulatory schemes (e.g., US EPA Nos. 3051A and 3546) demand scrupulously clean reaction vessels.
As an additional problem, when a sample with an unknown amount of the relevant element(s) is used, the amount of the element may be very high, and can essentially spoil the vessel for a smaller content sample even after thorough cleaning. For example, if digestion is carried out on a sample that carries 500 ppm of a heavy metal, the vessel will generally thereafter be unsuitable for testing where the expected level would be 50 ppm of that metal.
Vessel liners have been used previously (
A similar set of considerations applies to using microwave extraction as a sample preparation technique to isolate analytes from a sample matrix and into a solvent from which they can be identified.
Merely inserting a flexible liner is unsatisfactory because such liners do not conform to the inner surfaces of a reaction vessel. As a result, measuring the temperature of the vessel's outer surface fails to provide an accurate measurement of the temperature of the acid or solvent inside. This in turn precludes such liner use in regulated digestion methods such as US EPA No. 3051A or extraction methods such as EPA No. 3546.
In one aspect, the invention is a combination for microwave assisted acid digestion and related sample-preparation reactions, that includes a microwave transparent pressure-resistant reaction vessel, a flexible film fluoropolymer liner inside the reaction vessel, the flexible film liner having a size and shape that substantially conforms to the inner walls of the reaction vessel, a pressure-relief closure on the reaction vessel and the flexible film liner, and an infrared temperature detector that operates in wavelengths (frequencies) to which both the reaction vessel and the flexible liner are transparent, so that an exact fit and conductive heating to the outside of the reaction vessel are not required.
In another aspect the invention is a method for digestion in strong mineral acids that includes the steps of adding a strong mineral acid and a digestion sample to a microwave transparent flexible film fluoropolymer liner that is chemically resistant (inert) to the strong mineral acid even at elevated temperatures inside of a closed microwave transparent pressure resistant reaction vessel, heating the strong mineral acid and the digestion sample with microwaves, and measuring the temperature of the strong mineral acid and the digestion sample using an infrared detector that measures wavelengths (frequencies) to which both the flexible film liner and pressure resistant vessel are transparent.
In another aspect the invention is a method for microwave assisted extraction that includes the steps of adding an organic solvent and an extraction sample to a microwave transparent flexible film fluoropolymer liner that is chemically resistant (inert) to the organic solvent even at elevated temperatures inside of a closed microwave transparent pressure resistant reaction vessel heating the organic solvent and the extraction sample with microwaves and measuring the temperature of the organic solvent and the extraction sample using an infrared detector that measures wavelengths (frequencies) to which both the flexible film liner and pressure resistant vessel are transparent.
In another aspect the invention is a method of assembling a disposable liner and a pressure resistant reaction vessel prior to carrying out a digestion reaction. The method includes the steps of spreading the mouth of a flexible film fluoropolymer liner, inserting the flexible film fluoropolymer liner with its mouth spread into a microwave transparent pressure resistant reaction vessel, adding a digestion acid and a digestion sample to the flexible film fluoropolymer liner in the reaction vessel, inserting a floating plug into the spread mouth of the flexible film fluoropolymer liner in the mouth of the reaction vessel, and securing the floating plug with a vessel closure.
In another aspect the invention is an assembly tool for the combination of a flexible film fluoropolymer liner and a microwave transparent pressure resistant reaction vessel. The assembly tool has a tool stem with a tapered truncated frustoconical nose on one end, and a sliding coaxial sleeve on the tool stem. The coaxial sleeve has a plurality of tapered fingers adjacent the nose of the tool stem, so that placing the liner on the fingers and moving the fingers coaxially with the stem spreads the mouth of the liner.
The foregoing and other objects and advantages of the invention and the manner in which the same are accomplished will become clearer based on the followed detailed description taken in conjunction with the accompanying drawings.
The invention is the combination of (i) a flexible fluoropolymer liner that can be inserted into the vessel, and that will withstand the expected temperatures of microwave assisted digestion (at least 175° C., often 200° C., and sometimes 250° C.); used in conjunction with (ii) a particular temperature measurement arrangement.
In a first aspect, the invention is a combination for microwave assisted acid digestion broadly indicated at 20 (
A pressure relief closure 22 is on the reaction vessel and the flexible film liner 27. An infrared temperature detector (37,
The contents of U.S. Pat. Nos. 6,927,371 and 8,852,533 are incorporated entirely herein by reference.
As set forth in U.S. Pat. No. 8,852,533, the floating plug is held in place by the reaction of the closure 22 (which can also be referred to as a lid) which is fixed to the vessel 21 using, in the illustrated embodiment, a threaded relationship including the male threads 28 on the reaction vessel 21. When the pressure inside the vessel 21, and in the invention inside the liner 27, exceeds a mechanically defined set point, the top 38 of the lid 22 can flex to allow the floating plug to move slightly and release gas pressure through the opening 29 while the digestion reaction continues. In some embodiments the flexibility of the lid portion 38 defines the pressure release set point, while in other embodiments (e.g. U.S. Pat. No. 6,927,371), the vessel system 20 is clamped inside of a frame, and a bolt or other clamp bears down (to an amount desired by the user) on the flexible portion 38 to define the pressure exerted and thus the pressure at which gas will escape.
Because the liner 27 must come into contact with strong mineral acids and the digestion sample, the liner material is selected from polymers that can be formed into the liner and that can resist the strong mineral acids, the high digestion temperatures, and accommodation of the accident high temperatures. In exemplary embodiments, the liner 27 is made out of materials such as perfluoroalkoxy (PFA) fluorinated ethylene propylene (FEP) or polyvinylidene difluoride (PVDF). Although polytetrafluoroethylene (Teflon®) has a number of the desired benefits, it cannot be formed into films for the liner at costs as advantageously as PFA or FEP.
PFA (Perfluoroalkoxy) is a true melt-processable fluoropolymer. In service it is considered interchangeable with PTFE in terms of its chemical service and temperature and pressure characteristics. PTFE has the highest permeation performance of the fluoropolymers, even greater than paste extruded PTFE. PFA also provides the smoothest and least wettable finish of all of the fluoropolymers. The trade-off, however, is cost, with PFA being more expensive than PTFE.
FEP (Fluorinated Ethylene Propylene) is another melt-processable fluoropolymer. It does not have the almost universal chemical resistance of PTFE and PFA and its maximum operating temperature in service is 150° C. Where the chemical performance and temperature range above 150° C. are not issues, FEP provides a less expensive alternative to PFA. FEP can be conveniently made into sheets.
PVDF (Polyvinylidene difluoride) is an engineering fluoropolymer and has a more limited range of chemicals performance and an upper temperature limit of 120° C. Although replaced in many applications by the improved performance of PTFE resins, PVDF remains a material of choice for low temperature halogen applications (e.g., bromine and chlorine).
As will be described again in detail with respect to
The advantages of rotating the vessels 20 on the turntable 30 include the ability to carry out a number of digestions at the same time in the same cavity, while also accommodating the standing node nature of microwaves in cavities of the size and shape into which the turntable 30 and the vessels 20 will fit.
Although the use of the word “disposable” is essentially the user's choice based upon their budget, the liners of the invention are currently expected to retail in the US$1 range. In exchange for this price point, the user (many of whom are testing laboratories that conduct dozens or hundreds of tests each day) can avoid the time and labor costs of vessel cleaning
The invention includes the use of an infrared detector for which both the liner and the vessel are transparent to the detector frequencies. For example, fluorinated polymers are transparent within the region (approximately) of 1000 nm-7.50 micron. Thus, the detector measures infrared (IR) radiation within some or all of the 1000 nm-7.50 μm range
Because of that, the disposable liner does not need to provide a heat conductive pathway from the hot acid to the external surface of the vessel to give a proper temperature measurement. In fact, because the infrared detector is sensitive to wavelengths to which the vessel and the liner are transparent, the temperature measurement can be more accurate than conventional systems.
In another aspect the invention is a method for digestion in strong mineral acids that includes the steps of adding a strong mineral acid and a digestion sample to a microwave transparent flexible film fluoropolymer liner (e.g., 27) that is chemically resistant or inert to the strong mineral acid selected even at elevated temperatures and positioned inside of a closed microwave transparent pressure resistant reaction vessel (e.g. 21).
The strong mineral acid and the digestion sample are heated in the vessel and liner with microwave radiation, and measuring the temperature of the strong mineral acid and the digestion sample using a IR detector (e.g., 37,
The method can further comprise the steps of opening the reaction vessel, typically after cooling to near ambient temperature (to allow the interior pressure to subside), removing the acid and the digestive sample from the liner 27 and removing the liner 27 from the vessel 21, and thereafter adding a new liner 27 to the reaction vessel without any intervening step or otherwise cleaning the reaction vessel 21.
The invention further comprises the steps of repeating the entire digestion process with microwaves and temperature measurement for a new sample in the new liner.
As set forth previously, the strong mineral acids are typically selected from the group consisting of nitric (HNO3), sulfuric (H2SO4), hydrofluoric (HF), and hydrochloric (HCl), as well as mixtures of two or more of these acids.
In order to work with these acids, and is set forth with respect to the structural aspects, the liner is typically formed of PFA, FEP or PVDF. The vessel that holds the liner can be selected from these polymers, or additionally from PTFE, or from any other polymer that otherwise can withstand the expected pressure and temperatures, and the corrosive aspects of the strong mineral acids.
In the method, the pressure resistant vessel is closed with a fixed force and vented without otherwise opening the reaction vessel when the pressure inside the vessel exceeds the applied fixed force.
The invention is particularly useful with EPA Method 3051A (https://www.epa.gov/sites/production/files/2015-12/documents/3051a.pdf; accessed Mar. 5, 2018), which is incorporated entirely herein by reference.
In particular, EPA Method 3051A at § 6.1.1 requires that a microwave decomposition system sense the temperature to within ±2.5° C., automatically adjust the microwave field output power within 2 seconds of sensing, and use temperature sensors that are accurate to ±2° C.
EPA Method 3051A at § 9.2.4 (“Preparation of reference samples for specific determinative methods”) lists various post-digestion testing methods.
In another embodiment, the invention is a method for microwave assisted extraction, of which US EPA 3546 is exemplary (https://www.epa.gov/hw-sw846/sw-846-test-method-3546-microwave-extraction; accessed Mar. 23, 2018). Microwave extraction uses microwave energy to produce elevated temperature and pressure conditions (i.e., 100-115° C. and 50-175 psi in the extraction context).
Microwave extraction is applicable to, inter alia, the extraction of semivolatile organic compounds, organophosphorus pesticides, organochlorine pesticides, chlorinated herbicides, phenoxyacid herbicides, substituted phenols, polychlorinated biphenyls (PCBs), polychlorinated dibenzodioxins (PCDDs; “dioxins”), and polychlorinated dibenzofurans (PCDFs), which may then be analyzed by a variety of procedures (e.g., chromatography). Microwave extraction can also be applicable for the extraction of additional types of analytes that the skilled person can incorporate without undue experimentation.
Typically, microwave extraction is carried out on solid matrices containing from 50 to 10,000 microgram per kilogram (μg/kg) of semivolatile organic compounds, 250 to 2,500 μg/kg of organophosphorus pesticides, 10 to 5,000 μg/kg of organochlorine pesticides and chlorinated herbicides, 50 to 2,500 μg/kg of substituted phenols, 100 to 5,000 μg/kg of phenoxyacid herbicides, 1 to 5,000 μg/kg of PCBs, or and 10 to 6000 ng/kg of PCDDs/PCDFs.
Microwave extraction frequently uses a solvent mixture of hexane and acetone (1:1) for matrices such as soil and sand. Other organic solvents or solvent systems are well understood in the extraction arena and can be selected by the skilled person without undue experimentation. Hexane is a water-immiscible solvent and acetone is a water-miscible solvent. The water-miscible solvent helps facilitate the extraction of wet solids by allowing the mixed solvent to penetrate the layer of water on the surface of the solid particles. The water immiscible solvent extracts organic compounds with similar polarities.
In addition to its convenience advantages, the use of the liner helps the user meet relevant regulatory requirements. Both EPA 3051A and 3546 require stringent control of vessels in order to avoid false positive or otherwise erroneous results (e. g., “the analyst should demonstrate that all parts of the equipment are interference-free; § 9.3 of EPA 3546; see also § 4.1 of EPA 3051A).
The use of the liner according to the invention also simplifies post-reaction waste management (§ 15.0 of EPA 3546; §§ 14 and 15 of EPA 3051A).
In the extraction embodiment the invention includes the steps of adding an organic solvent and an extraction sample to a microwave transparent flexible film fluoropolymer liner that is chemically resistant (inert) to the organic solvent even at elevated temperatures inside of a closed microwave transparent pressure resistant reaction vessel, heating the organic solvent and the extraction sample with microwaves, and measuring the temperature of the organic solvent and the extraction sample using an infrared detector that measures wavelengths (frequencies) to which both the flexible film liner and pressure resistant vessel are transparent.
In the extraction embodiment the method can also include the steps of closing the pressure resistant vessel with a fixed force and venting the vessel without otherwise opening the reaction vessel when the pressure inside the vessel exceeds the fixed applied force.
The extraction method can further include the post-extraction steps of opening the reaction vessel, removing the organic solvent and the extraction sample from the liner and removing the liner from the vessel, and thereafter adding a new liner to the reaction vessel without an intervening step of otherwise cleaning the reaction vessel. The liner thus enables carrying out a new microwave assisted extraction on a new sample, in the new liner.
As in the other embodiments, the melt-formable fluoropolymer for the liner is selected from the group consisting of perfluoroalkoxy, fluorinated ethylene propylene and polyvinylidene difluoride.
For a number reasons, the liner 27 is most effective with the floating plug 25 when the mouth of the liner 27 is wider than the remainder of the liner 27. Liners can be manufactured with a wider mouth, but doing so adds a fairly significant cost increase. Accordingly, the method of the invention further comprises spreading the mouth of the flexible film fluoropolymer liner 27, inserting the flexible film fluoropolymer liner 27 with its mouth spread into the microwave transparent pressure resistant vessel 21, adding the digestion acid and the digestion sample to the liner 27 in the reaction vessel 21, inserting the floating plug 25 into the spread mouth 42 of the liner 27 in the mouth 24 of the reaction vessel 21, and then securing the floating plug 25 with vessel closure 22.
In this aspect, the invention further comprises an assembly tool for the combination of the liner 27 and the reaction vessel 21. As illustrated in
In the illustrated embodiment, the assembly tool further includes a tool base 44 with the tool stem 46 mounted perpendicularly on the tool base 44 with the tapered truncated frustoconical nose 47 on the opposite end of the tool stem 46 from the tool base 44. A finger base 45 parallel to the tool base 44 carries the sliding coaxial sleeve 48 perpendicularly on the finger base 45 so that moving the finger base 45 relative to the tool base 44 spreads the tapered tool fingers 50 and, when a liner 27 is on the fingers 50, the fingers 50 spread the mouth of the liner.
Although as set forth herein the invention makes unique use of particular optical relationships, the general optical relationships between and among wavelengths of electromagnetic radiation, optical (or other) materials, and detector ranges are well understood in the art and can be selected by the skilled person without undue experimentation.
Microwaves are propagated into the cavity from the source 34, of the most common of which are magnetrons, but can also include klystrons, and IMPATT diodes. The operation of any one or more of these is well understood to the skilled person.
The diode 37 can be positioned inside or outside of the cavity 36. If inside, the diode 37 must be positioned or shielded (e.g., a microwave choke) to avoid interference from the microwaves in the cavity 36. If outside, the diode 37 must have a free and IR-transparent optical path to the vessel 21. These are straightforward design choices that can be carried out by the skilled person, and without undue experimentation.
In many cases, a waveguide 35 is used to bridge the position of the source 34 and the cavity 36 and to help propagate the microwaves in an intended matter.
Because EPA Test No. 3051A and other similar tests require accurate temperature measurement and feedback within defined time intervals, both the source 34 and the detector 37 operate in conjunction with a processor 40, to which the source 34 and the detector 37 are connected through the respective communication lines 53 and 54
The liner and vessel system of the present invention avoids the need for, and the cost and effort of obtaining, these close tolerances.
In the drawings and specification there has been set forth a preferred embodiment of the invention, and although specific terms have been employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being defined in the claims.
