GB2259836A - Container for heating and ashing - Google Patents

Description

- - r C 1 4 t A Container for a Heatable Material The present invention

relates to a container for a heatable material for use in microwave ashing and analytical apparatuses.

In U.S. Patent 4,565,669 there are described apparatuses and processes f or ashing an ashable material by heating and ashing means, such as a silicon carbide, by means of microwave radiation, and ashing a sample to be analyzed, which may be resting on a support made of fused quartz fibers, by means of the heat generated in the ashing means. In such apparatuses the silicon carbide rests on a refractory material and the sample to be ashed is placed on a relatively thin quartz pad which is in contact with the silicon carbide. Such apparatus is positioned inside a computer controlled analytical apparatus, such as the MDS-81 microwave drying/ digesting system, manufactured by CEM Corporation, which is described in their bulletin entitled CEM Corporation Microwave Drying /Digestion System MDS-81 (laboratory microwave system), published in 1981.

Although the microwave ashing apparatus and process of the mentioned patent are useful in speeding ashing operations and analytical determinations dependent on them, the present disclosure is a further significant improvement. In the disclosed apparatuses and processes the sample to be ashed is in a furnace made of microwave transmissive (preferably essentially or completely microwave transparent) material, which is an open celled ceramic foam, preferably an open celled fused quartz foam. Such furnace material and the furnace structure help to 'maintain the ashing temperature uniform throughout the furnace cavity and additionally, such temperature is maintained at a desired level by a thermocouple control system, the probe of which is in the furnace cavity. More uniform heating of the ashable sample makes the ashing operation more consistent and more accurate. Furthermore, possible loss of sample material in air leaving the microwave apparatus is minimized and it has been found that it is usually unnecessary to employ a cover sheet of fused quartz fiber pad material to hold the ash in place and to prevent it from being - c 3 carried off in the exhaust air. Thus, the tare weight may be less when using the disclosed apparatus and therefore weighings can be more accurate. Various other advantages attend the present disclosure, including ease of use of the apparatus, ready removability and replaceability of the furnace door, improved burning of f of solvent from the ashable sample, which solvent accompanies any magnesium acetate ashing aid employed, accurate automatic control of ashing conditions, and quicker ashings.

Although various ashing apparatuses for analytical purposes have been described in detail in the literature, most of then. utilize muffle furnaces to generate heat and they employ crucibles to hold the samples to be ashed. So far as applicants know, before the filing of their U.S. patent 4,565,669, no other microwave ashing apparatuses and processes had been described in the literature. In U.S. patent 4,307,277 a microwave heating oven was disclosed for heating materials to high temperatures, as in producing a sintered ceramic. However, the heating ovens of that patent were not thermostatically controlled, did not employ applicants' open celled ceramic material for furnace walls and door, and were different from applicants' apparatus in various other important structural features. The various changes incorporated into the present disclosure are improvements over the structures and processes of U.S. patents 4,307,277 and 4,565,669 and contribute to the improved analytical results and quicker ashings that are obtained when employing the present disclosure.

Prior to the present disclosure quartz fiber discs had been disclosed as supports for samples to be ashed by heat generated by directing microwave energy onto microwave absorptive iz materials. In U.S. patent 4,565,669 a quartz fiber support pad and a cover of the same material were utilized to confine an ashable analytic sample to be analyzed during the ashing of such sample by heat generated by directing microwave radiation at microwave absorptive silicon carbide under such a support pad. U.S. patent 4,565,669 represents the closest art known to applicant with respect to the containers of the present invention for holding ashable material but it does not describe or suggest the containers of the invention and does not make them obvious (and the ashing process of the above patent does not result in the improved ashing that is obtainable with the present containers).

In accordance with this invention there is provided a container for ashable material, which material may be ashed by heat generated by microwave radiation of microwave absorptive elements in an ashing furnace, which container comprises a heat resistant, walled container which is microwave transmissive and porous, and is made of quartz microfibers which are held in walled container form.

The invention also provides a process for manufacturing such a container by shaping of a light weight, microwave transmissive and porous sheet of quartz microfibers to containers form and heating such sheet in such form, preferably after wetting and drying it, - 6 whereby a form retaining container results, which is resistant to ashing temperatures and other ashing conditions.

The containers are especially useful in conjunction with microwave ashing apparatuses as described herein, and which also form the subject matter of our copending Application No. 8929250.2. However, the invented containers also find use in other ashing applications, such as those conducted in conventional muffle furnaces, and in other heating operations, including fusions and dry ashings (wherein ash is produced for subsequent analyses, such as analyses for heavy metals).

The invention will be readily understood by reference to this specification, including the accompanying drawing, in which:

Fig. 1 is a front perspective view of the microwave ashing apparatus, with the chamber door open, with the furnace door removed and without any ashable sample in the furnace; Fig. 2 is an enlarged front perspective view like that of Fig. 1, with the furnace door in place, in almost closed position, with arrows indicating air flow into the chamber, into the furnace, out of the furnace and out of the chamber; Fig. 3 is an enlarged disassembled view of the ashing furnace assembly, with a base support and a protective screen under such support; Fig. 4 is a front perspective view corresponding to that of Fig. 1 but illustrating two containers of ashable material in the furnace; z k 1 7 - Fig. 5 is a rear perspective view of the exterior of the microwave ashing apparatus with a temperature control unit in place thereon; Fig. 6 is a schematic electrical circuit diagram of various elements of the microwave ashing apparatus; Fig. 7 is a front perspective view of a microwave ashing apparatus, with chamber door open and with furnace door removed to illustrate two containers in the furnace, and is similar to Fig. 4 but identifies additional features; Fig. 8 is a top front perspective view of a walled ashing container of the present invention; and Fig. 9 is a top front perspective view of an ashing container of the present invention having the side wall thereof formed about a mandrel.

In Fig. 1 ashing apparatus 11 includes a walled microwave retaining chamber like that of a CEM Corporation MDS-81 Microwave Drying/Digestion System which is defined by a bottom, two sides, a top, a rear, a front, and a door, which chamber wall is represented by numeral 13, shown applied to a side wall of the chamber. Door 15 is shown in open position so that ashing furnace 17 may be seen.

Such ashing furnace will be described in greater detail subsequently in the description of Fig. 3. Temperature controller 19 is connected to a thermocouple probe 21 in the furnace cavity by an electrical connector, not illustrated. The f low of air into the chamber, into the furnace cavity and out of the cavity and chamber will be described with reference to Fig. 2, as will be the operating and display panels of the "microwave 8 svsl - tem" nortlon of the ar)r)aratus, which panels are like those of,Che CEY! Ccrzoration MDS-81, mentioned above.

:n E2'1G. 2 air (or gas) f-low through the ashing apparacus is renresented bv the dotted arrows. Air enters the walled microwave rezaining chamber, designated by numeral 23), through grill coen-;ncs 25 and 27 in chamber side walls 29 and 31, which oDen-ngs are located near the bottom of the chamber, and passes upwardly and around furnace 17, cooling the exterior thereof, after i t passes out through suction opening or duct 33, from whence it is discharged the apparatus through an air exhaust -;ucz, -=s 11us --rated 'n 'E'1G. mre,;erablv to a fume hood or in -ble manner. In FIG. 2, furnace door 35, which is er "er:7t-SS Sk.:;sz-%nt-'a-l-lv tranezoidal in horizontal cross-section, with handle -crz-:cns cr finaer arios cut into the base of the tramezoid ( the.:r,nz ^-j;-: the door), is in niace in '-he furnace wall but the door ocennc is not comr)lete-"..,r closed, thereby allowing passage of air 4nto -he.':urnace cav-v (not shown in FIG. 2), as represented by arrows 37 and 39. Although the arrows. indicate the air f low under the door, air also enters the furnace cavity through the side clearances between the door and the furnace wall. Similarly, ai mav leave the furnace cavity through the top thereof, as reDresented bv arrows 41 and 43, and the upper portions of the side openings. Arrow 45 represents the passage of air and combustion products out of the furnace cavity through vertical bore 47, between thermocouple probe 21 and the wall of said bore in the upper part of the furnace 17. The gas exhausted from the cavitv zasses ou' 1Chrouch exhaust duct. 33 to a suitable hcod or other discharge means. 'I'hus, "here are created 1 passageways for the air or other gases through the furnace, and through the charuDer and --,rnace cavitv. it should be mentloned that air inlet ocenings 25 and 27 and exhaust duct opening 33 are shielded bv shielding material (not specifically shown) to prevent escaoe of microwave radiation from the microwave retaining chamber. The chamber walls and door are of a metal or metal alloy, such as aluminum or stainless steel, and may be coated with a radiation transmissive polymer, such as polytetrafluoroethylene. Alternatively, but not as desirably, the door may be glass lined and shielded to prevent any escape of radiation.

The temmerature controller 19 includes three control kevs and a disDlav. The kevs are marked S, increase and decrease (not so marked in FIG. 2), and the use thereof will be mentioned later in connection with a description of how the controller is programmed. The "microwave system" portion of the apparatus includes controls like those of the CEM MDS-81 laboratory microwave system. Such are an on-off switch 49, and control panels 51 and 53. Panel 51 includes- Prbgram01 l'eset, Enter, Stop and Start keys and panel 53 includes numerals 1, 2, 3, 4, 5, 6, 7, 8, 9 and 0 (none of which is soecifically illustrated). Display 55 is of alphanumeric type.

The ashing furnace 17, illustrated in FIG. 3f includes combinable and separable unitary upper and lower sections. Upper portion 57 is of a material of heat resistant and microwave transmissive properties, which is also of low thermal conductivity, preferably being an open celled fused quartz foam. A vertical bore or hole 58 allows passage through such upper Dortion of a thermocouple probe and connector, (neither being shown in this view). Ashing furnace 17 also includes a unitarv separable lower portion 59 of the same heat resistant material which contains a cavity therein which, together wit-h. a mazinc cavity in the upper furnace portion, forms the furnace cav-t-,,. Lower portion 59 includes a plurality of slots c-r grooves 5.51 the bottom thereof and other slots or grooves, such as t-ose illustrated at 63 and 65. Grooves 61 are for mes-. :t--on-ing -0-1ccr heating elements 62 and grooves 63 and 65 are for ocs. '.t-;on-;nc the heating elements 64 and 66, resDec41.--ivelv. S-m4--lar grooves, not visible in FIG. 3, are provided for position-inc frOnt elements 67 and back heating elements 68. Ce-'!-'ne heazing elements (not shown) may also be provided in upper the furnace, in suitable slots, grooves, channels o?_- ether means therein. The various heating elements are c-.c absorptive material that is capable of being heated tO an as'.-.;nc temperature by microwave radiation. A muchl preferred suCh material is silicon carbide, and Dreferablv the -,eaz-,nc elements are separated, with surfaces that are flusli with the furnace cavity interior walls. Furnace door 35. which is shown as being of trapezoidal horizontal cross- section (but -nav be o.E other suitable cross-section) matches in shame a corresmondinc wa-1.1 opening in the front of the upper furnace portion, and when _ft in place its interior and the upper and lower wall portion interiors define the furnace ca-vity. The door includes in the front face thereof a pair of grooves 69 which function as parts of a handle or gripping means to permit easy hand removal, closure or adjustment of the door position. The furnace is sunocrzed by a refractory block 71 which is under a minor Proportion of.: the furnace bottom surface. Such su-port allows c---culat-on of air or other gas under, -much of' the furnace bottom, thlereby facilitating ccolIng of it. Under the refractorv suzoort there is shown a separator, sue!. as a or screen, which. r,-,av be of temJerature j resistant plastic, metal or ether suit-able material. The function of the cloth or screen is to orevent scratching of the finish of the chamber intericr bv the refractorv suonort, which often has rough surfaces.

Because E.-_G. 4 is essentiallv the same as FIG. 1 except zhe furnace cavitv of FIG. 4 of a mair of 0 ashab le mate rial ( or ash oniv L'-.a t as-iec,- of F:G. will --e JescrIbed further herein. In FIG. 4 ashing furnace I d ---- secarable uiDoer and lower 57 and 59, z:-,cet--,er W-_th the heatinc elements 1-1-1ustrated in cf: w---'ch rear heatnc elements 69 are visible in FIG. 4, and parts de-fine the ashi-ing cavilLy when door 35 is in place.

in such cavitv are ocsJ,."k-oned two porous, walled containers 75 of qua.--- microfiber sheet material. In the containers are suitable charces of material 77 to be ashed (or they may contain the res,,.,-!t-.ng ash) Details of the ashlina orocedure will be described later in this smecification. In FIG. 5 ashing apparatus 11 is illustrated with temoerature controller 19 in position thereon, with a thermocouple (in the furnace cavity) connected to the controller. Numeral 79 J'den -;lies the oower cord to the ashing apparatus and louvers 81 and 83 are to Dermit airflow through an air jacket around the -ha- ---mber to '-e-lD czc-l the chamber exterior. Between the outer wall 35 - =.-d "-'ne chamber there Is -located a magnetron from which nicrowave radiation is directed Jinto the radiatlon retaining c.n.amber, the walls of which are of microwave reflective material, such as stainless steel or at.her suitable metal or alloy, which mav be coated with a paInt or polymeric protective coating. The macnetron is a standard part in microwave aDDaratuses of the present 4Cype and is concealed within walls thereof. Therefore, it is not illustrated in the present drawing. Neither is there shown any cooling fan for the magnetron, although such a fan is present in the aoDaratus. Number 87 indicates an opening in the apparatus for exhausting the air that is blown across the magnetron to cool (not illustrated) is Drovided within the apparatus exhaust air and combustion gases from the furnace and to create an f-lcw through the chamber and through the furnace. The such blower is desianated bv numeral 89 and the corresponding exhaust is identified by numeral 91. openings 112 corresoond to alr inlets 25 and 27 (in FIG. 2) and are for admission of air to the apparatus chamber (and furnace cavity). A receptacle 93 is provided for connection of the temperature controller cable 95. Electric power cord 97 is connected to controller 19 at 99. A fuse is provided at 101 and a power switch is indicated at 103. Thermocouple connector leads 105 and 107 are connected to a thermocouzle connector plug 109 and such leads or connector are/is also connected to a thermocouple (not illustrated in FIG. 5), which is preferably located in the top central portion of the furnace cavity. Such connector enters the ashing apparatus 11 at 110.

In F1G. 6 the relationshiD between the onerator kevboard (and alDhanumeric d4-smlav), -he microwave orocessor, the j - 1 L temperature controller, the thermocouple and the power contro' to the magnetron is illustrated. The operator keyboard controls the amount of mower employed and the time of heating, which are disDlaved in the alphanumeric display after thev are set '--V keyboard operation. The temperature controller controls ashing temperature and permissible variation of the temperazure (often +2 or +3C.) from that which is set. Thermocouule 114 inputs the temperature controller with the temperature in the furnace cavity and the controller operates the microwave --cwer control to switch the magnetron off when the temnerature is than set and to turn the magnetron back on when the falls below the set paint. More details about the the apparatus operator keyboard and temperature contr,:, '-. given subsequently.

In FIG. 7 a microwave ashing apparatus 111 cor.-2r---ses too, bottom, side and rear walls, all designated by numer-m-- 112. applied to a side wall, and door 115, which defilne a retaining chamber 118. Inside the chamber is a furnace 117. which includes top and bottom portions 119 and 121, and a furnace door, 123. Such furnace parts are made of microwave transmissive zcen celled quartz, which is of low thermal conductivity and is heatresistant, capable of being employed at very high temperatures without deterioration. Such a type material is ECCOFOAM R Q, preferably ECCOFOAM Q-G, which is described in a bulletin entltled ECCOFOAM Plastic and Ceramic Foams, of Emerson and Cumming, Canton, Massachusetts, dated March, 1980. Inside the furnace is a furnace cavity 125 and microwave absorptive material- 1^7 Is located ih grooves or slots (not shown) in the upper and lz;wer - 14 portions 119 and 121, with surfaces thereof even with the internal surfaces that define the furnace cavity. In the furnace cavity are illustrated two containers of the present invention, which are designated by numeral 129. Also shown in Fig. 7 are inlets 131 for air to enter the chamber, part of which air will pass through the furnace cavity, but most of which passes around the chamber 118 and serves to cool the walls thereof. Such air exits the chamber through outlet 133- A thermocouple 115 is located in the furnace cavity and is communicated by means of a connector (not illustrated) to temperature controller 137. Both the main microwave generating unit of apparatus and temperature controller 137 include controls and visual displays, which are readily apparent and therefore are not specifically numbered.

In Fig. 8 there is illustrated one of the containers of the present invention. Such container is of unitary construction, with bottom 139 and side wall 141 being made from the same sheet of porous unwoven quartz microfibers. The container illustrated had been made from a square portion of the fibrous material and includes seam lines like that shown at 143.

In Fig. 9 there is illustrated a step in the manufacture of container 129. As shown, the non-woven microfibrous quartz sheet has been formed about the base of cylindrical mandrel 145 and extra material has been trimmed off along top edge 147. A quartz monofilament 149 or an elastic band or similar restraining means holds the porous microfibrous quartz sheet tightly to the mandrel during the forming operation but is later removed, following normal manufacturing procedure. After shaping of the sheet, it is wetted, formed tightly around the mandrel, trimmed, 1 - removed from the mandrel and air dried, after which it is heated (,- Fired) to produce a form-retaining container. While air drying is preferred it may sometimes be omitted.

Although the container is illustrated as a short cylinder, other container shapes may also be produced, utilizing correspondingly shaped mandrels. Thus, containers of rectangular or square horizontal cross- sections may be produced. Although various shapes of containers may be made it will be preferred that such containers be relatively flat, usually being of a height/major horizontal dimension ratio less than 1:1 and preferably no more than 1:2, preferably being in the range of 1 to 2:55, e.g., about 1:5 or 3:10. While various sized of containers mav be employed, when such containers are flat and cylindrical it will normally be preferred for them to be from 2 to 10 cm in diameter, preferably 4 to 6 cm, and 0.5 to 4 cm high, preferably 1 to 2 cm high, and flat cylindrical containers are preferred.

The apparatus for the application of enough microwave energy to a sample of material can be ashed may be any such suitable microwave apparatus that can direct microwave radiation onto the heating elements in the furnace. As was indicated previously, a CEM Corporation MDS-81 system is useful but similar systems can also be employed, together with an internal furnace, temperature control and container for the ashable material. Preferably the system will incorporate a microprocessor, a digital computer and controls for regulating the application of microwave radiation to the elements to be heated. Thus, the - 16 microwave radiation may be applied for desired lengths of time and at different levels of radiation, if desired, but often the radiation level will be constant at the maximum design capacity.

Key elements of the microwave system utilized will be proper gas (air) f low through it f or cooling of the furnace, and no microwave load in the system except that in the furnace. Also, the furnace should be such as to allow exhaustion of combustion gases and inflow of fresh gas (air or suitable oxidizer).

It is noted that in some of the apparatuses referred to the microwave power range may be from 1 to 100% of full power (500 to 1, 500 watts in some instances) in 1% increments. Of course, lesser and greater powers may also be employed, for example up to several kilowatts, e.g., 0.3 to 5 or 0.4 to 2 kw., but 0.9 or 1 kw. will usually suffice. In the United States the frequency of microwave radiation employed will normally be 2. 45 gigahertz, and in Great Britain it is usually 0.896 gigahertz. Such frequency can be in the range from 0.3 to 50 gigahertz (or higher) and is preferably in the range of 0.8 to 3 gigahertz. The readouts of the described apparatuses have as many as 40 characters in their alphanumeric displays and in some instances may include audible tones for operator feedback. The operator controls include a keyboard of up to 20 keys for input.

One of the advantages of the present disclosure is that the described microwave apparatus may be employed for ashing or in other operations for which each such apparatus may have been primarily designed, such as moisture determinations, volatiles analyses and for the promotion of chemical reactions. Usually when the apparatuses are employed for ashings of materials they - 17 will be used at their highest power supply condition, which is often about 560 to 1,000 watts. Times of ashing may be adjusted, as desired, and usually ashing times will be from 2 to 20 minutes or 5 to 15 minutes, but the furnace may be pre-heated over periods from 5 minutes to 2 hours, usually 20 to 60 minutes.

The main material of construction of the furnace, which is inserted into the previously described microwave system and is a part of the present apparatus, is one which is heat resistant, of low thermal conductivity and transmissive of microwave radiation. It has been found tat such materials include ceramic, glass and quartz foams, with the quartz foams being highly preferred because they allow operations at higher temperatures, are of low thermal conductivities and are exceptionally transmissive of microwave radiation, being essentially or completely transparent to such radiation. Thus, it is considered that over 99% of the microwave radiation passes through the walls of the present furnaces unless it is absorbed by the microwave absorptive heating means in the furnace. of the quartz foams those which are open celled and fused are even more preferable. Such materials are available from Emerson and Cuming, of Canton, Massachusetts and are marketed under the registered trademark ECCOFOAM Q. Two forms of ECCOFOAM R Q are sold, ECCOFOAM QG and ECCOFOAM Q-R. The latter is heavier and stronger but for the purposes of the present disclosure it is preferred to employ the former. The characteristics of such fused quartz open celled foams are described in Technical Bulletin 6-2-12A, issued by such company. It is considered that fused foam materials of the types mentioned are useful in making the present furnaces, especially if - is they are of a density in the range of 0.3 to 0.8 g./c. cm., a modulus of rupture in the range of 10 to 50 kg./CM.2, and a thermal conductivity in the range of 0.5 to 1.5 BTU/hr./sq. ft./OF./in. Such materials should also be operative in the ashing applications of the present disclosure at suitable ashing temperatures, which more preferably are in the range of 800 to 1,OOOOC. The f oam quartz, which is essentially pure silicon dioxide, or foam ceramic should not decompose or deteriorate appreciably on subjection to such temperatures. When higher temperature ashings are to be undertaken an appropriate higher temperature material of construction will be employed, and the mentioned Eccofoams are preferred because they are stable at 1,6500C. for relatively short periods of time and are considered to be more stable at 1, 090 OC., to which they may be exposed for prolonged periods without adverse effects. The mentioned Eccofoam products are available in sheet form, said sheets measuring 30.5 x 45.7 x 7.6 cm. for Eccofoam QG and 30.5 x 45.7 x 11.4 cm. for Eccofoam Q-R. Such sheets or slabs are machined to desired shape, utilizing abrasive cutting and grinding techniques. Although Eccofoam can be cemented to itself and other materials such cementing will almost always be avoided in making the present furnaces because the cements are usually ineffective at higher temperatures or are degraded at such temperatures.

The ashing means is of a microwave absorptive material which does not have a Curie temperature below desired ashing temperatures and which is capable of being heated by microwave radiation to a temperature in the range of 400 or 5000C. to 1,6500C. or 1,7000C. Sometimes the ashing range can be even higher, being 1 k 1 limited bv the melting, sublimation or decomposition points of the ecuinment materials being emnloyed or of the ashable substance or its oxide(s), but normally the range of.600 to 1,000'C. is adequate and 800 to 950, 975 or 1,000C. is often more preferred. The ashing means is one which _Js stable at the intended use temi)eratures and is essentially or completely nonoxidizable at such a temperature. It should also be structurally sound at such use temperature, being resistant to disintegration, cracking and powdering. Although various materials are capable of absorbing microwave radiation and of being heated to temperatures in the ranges described, silicon carbide is the most useful and most preferred of such materials. Silicon carbide, in powder, granular or other small particulate form (wherein the effective diameters of the particles are usually up to 0.5 to 1 cm.) can be heated bv microwave radiation but generally in such a form it is not sufficiently effective to be employed as an ashing means for a variety of ashable materials such as may be encountered and for the analyses of which the present apparatus is intended. However, silicon carbide which is in continuous sintered or nonparticulate solid form is very satisfactory and has been employed successfully in analyses of various materials for ash contents. The continuous silicon carbide solid ashing means may be in various shapes

or forms to suitably fit in a furnace wall cavity, but regular parallelepipeds are preferred, such as flat prisms of rectangular crosssections. Suitable materials may be commercial "furnishing sticks", which may be used to true grinding wheels; of these those sold bv Norton Co. under the trademark CRYSTOLON, especially their Grade 37 C 220, whIch -'s bonded silicon carbide, are preferred, but other silicon car;--,-,de products may also be employed. 2-Pimong these are Norton Cz:'s. finishing sticks and silicon nitride banded silicon car--J;.des, designated W 137 and W 233. Even if- such products 7.,a-i physically deteriorate after many uses they are relatIve inexpensive, so scheduled periodic replacements, such. as about every 1,000 analyses, may be undertaken but so far applicants have not yet had to replace any Crystolon silicon carbide. Other microwave absorptive heat-lne eleTtents that may be used include ferrites, garnets and similar:-,,azer.-a-ls kncwn in the art.

The thermocouDle that is emD-!ove--'. to measure Che temperature in the furnace during microwave heazinc nneree: may ne any such suitable thermocouple whic.,i -'s the ashing temperature and is una-i:-2.,Eected any products and any other gases released f_ror. t.;-e during ashing thereof. It has been found that a Tvpe K thermocouDle (chromel-alumel) is satisli-ac-orv in t-he invented apparatuses. In use such thermocouple includes a solid sheath, which is electrically grounded to the chamber wall. lt has been found in practice that the thermccouDle ooeration and accuracv are not adversely affected by the microwave radiation. Tnstead of a thermocouple other temperature sensor devices can be used (with the temperature controls) to turn the magnetron power on and off, and thereby regulate the furnace temDerature. Such mav be infrared sensors, vapor pressure sensitIve switches, switches and exDansion sensi4Cive cauces, whIc'- all mav be appropriately located in the apparatus and connected to a 0 t - -1 - responsIve 1,,e.ricerature controller which can translate any signal received intc on-off' --:-,ijulses or instructions to the -.,,acnetr--n switch.

he::e-2eratu-e controller is an electronic -ns-r,-1n.ent of conventional design which onens and closes a magnetron electrical sunniv line in resDonse to electrical sianals from the "-her-.n,ocoumle. IIC will be discussed further when the programming thereof- is subsecuent1v described. However other forms of the conzro.-er mav be utilized with other temnerature sensing devices.

The ashable samnle should not be niaced directlv on the '-eatnc elenents or microwave transmissive wall mater4al of the evident, and there-fore a sunoort for the sannle is emc..-^ved. Such Sunoort should desirable be licht in weight and -O-Ist resist the high temperature of ashing. Also, ishould be 4 m crowave trans-m ssi_ve, preferably microwave transDarent or' essentially microwave transparent (usually transmitting over 95%- and preferably over 99% of such radiation), and it should not allow passage through it of the ashable sample dr the resulting ash. A suitable sucoort or container material for the ashable san.Dle is a cuartz microfiber (micron-sized) light weight filter material. The microfibrous cuartz sheet will preferably be one of a thickness in the range of 0.2 to 0.7 mm., of such porosity that the pressure drop across it is 1 to 5 mm. of mercury at 5 c-.n./sec. 'Lace velocity of air, resistant to high temperatures, such as up.. to 5006c., without any adverse effects, retentive of micron size z)a--tIcies, transmissive of microwave radiation, and of a weight in 2 the rance of 50 to 200 g./ri. Preferabiv the material will be of a thickness in t-he rance of 0.3 to 0.6 mm., of such porosity that - 22 the pressure drop across it is 2 to 4 mm. of mercury at 5 cm./sec. face velocity of air, resistant, although with some embrittlement, to high temperatures, up to 1,000C-, retentive of over 99% of micron size oarticles, transDarent to microwave radiation, and of 2 a weight in the range of 75 to 125 g./m. Such a container will normally weigh in the range of 0.2 to 0.6 g., preferably weighing 0.3 to 0.5 g.

A very suitable material of construction for the present containers is that sold bv Whatman Laboratory Products. Inc.r. Clifton, New Jersey, for use as air pollution.filters, under the name Whatman R Ultra-Pure OM-A Quartz Filters, which are described. in their publication No. 860-QM-AA. Accordina to such. Dublication, the described material is an ultra-pure quartzmicrofiber filter sheet which contains a small proportion (5%) of conventional borosilicate glass microfibers, which are in the sheet for papermaking purposes. Such publication does not describe or suggest the use of the mentioned material as a container, does not refer to ashing of analytical samples, and does not mention the use of microwave heating for ashing such samoles or for ashing other materials. According to the Watman publication the weight of the QM-A quartz filter is 85 g./m. 2 ' its thickness is 0.45 mm., it retains 99.999% of 0.6 micron particles at 5 cm./sec. face velocity of air, it is of a dry tensile strength, for a 1. 5 cm. wide strip, of 250 to 300 g., and it is capable of withstanding a maximum temperature of 5OCC.

To make the present containers a relatively simple process is employed, in which a non-woven sheet of the descr-Ibed ed, we n-4c--ofibrous cuartz is shao tted, formed, trimmed, removed from mandrel, air dried and fired. If the restraint and mandrel material(s) is/are sufficiently heat resistant the -may be conducted while the sheet material is held in mlace on the mandrel. Such heating is to a sufficientiv hig,h result in a form-retaining container, which temz)erature will normallv be at least 400C. but is preferably in the range of: 500 to 1,20CC. lleating time at the desired "curing",---emoerature will normally be in the range of 1 to 20 minutes, with rances 1 minutes and 5 to 12 minutes being preferred and mcre For example, a 10 minute heating period at about 800-900C.

often employed. It has been theorized that durinc ihe::ur--'nc operation the borosilicate glass component of the m--crc:cr2,-,s cuartz filter material is removed leaving a quartz fibers which are still porous and which are even resistant than the starting material.

.. 0 The described heating or firing of the conta-'ner be effected in various heating means, including ovens and furnace, but preferably is conducted.-...in-a microwave ashing furnace of the type in which the container is primarily intended to he employed. Preferably the heating will be to a temperature dt least as high as that to which the container will be subjected during ashing operations, but lower temperatures can also suffice. Moistening of the sheet material may be effected before shap"-,19, as well as after, and such moistening may be by spraying, roll application or immersion. It will usually be pre-fe- rable to limit the amount of moisture on the microporous cuartz material 1..oe---9 shaDed to that amount which is effective to -fac-;-'---.ate itS s..-j-'nc to desired container form, which amount will be that is sufficient to wet all such material. Drying before firing may be conducted on or off the mandrel, and -nav be bv hot air, radiant G1 heatIng or other means, in addition to-ambient air drving.

When a mandrel or at-her form Eor the microocrous sheet is not used during firing to form retaining configuration, as when a flaring dish shape is desired, the sheet may be formed to such a shape and during heating the outer edges thereof may be unsupported or may be supported, as by the upper walls of a larger cylinder. Various types of forms may be employed, including sleeves between which the desired container walls are held during heating, but for the manufacture of the preferred relatively short cylindrical containers a corresponding cylindrical mandrel, like that illusrated in FIG. 9, will preferably be utilized. Such mandrel mav be of anv suitable material, including various glasses, plastics, metals and allovs, such as copper, brass, steel and stainless steel, but if the mandrel is to be in place during firing it should also be heat resistant. If the heating of the shaDed sheet on the form is to be carried but in a microwave ashing apparatus, in which the presence of metals will often be avoided, the 'Lo= is desirably of a microwave transparent material, such as quartz, although various ceramics and glasses may also be employed under proper circumstances. Whichever firing procedure is f_ollowed, it will be satisfactory, providing that the container wall does not collapse or distort objectionably.

The heating or firing is preferably undertaken in a microwave ashing apparatus like that described in this aDnlication, which ooeration is convenient and puts the containers made to a test which almost duplicates actual use conditions.

i Heating in such apparatus will norrially be to the range olf ab-cut 800 to 1OCC., e.g., 850 or 950C., but mav be in the previcusly mentioned range of 500 to 1,200C. and can even be as low as 400C. or as high as 1, 600C. under some c..--cjms-l-ances.

It will be noted t-at In the -:--recoing -.ec-t-=t--on of firing temperatures many are in excess of "..he maximum temperature listed by the manufacturer of the quartz- filters, which is 500C.

Surprisingly, applIcant has found that such containers can be made to be shape-re tent ive by he-azing to temperatures close to or in excess of the -emDer-mz..-,re b-j the -.,ia,-iu.L:ac;---urer as tne -nax-- -nur.

tem,Derature to which s.'-.culd be ralsed. Durine sucl heating operation the sheet of -j-Z:-!"-er material converted to a '-c--,,A as'.-a---'e samples for microwave ashinc operatIons. Such permanent of the sheet material ---=.,Ies:D--ace at temneratures belew the melting point of quartz and the porous sheet does not. lose its porosity due to fusion. lt appears that the presence of the small proportion of borosilicate class microfibers in the quartz sheet is helDful in manufacturing the present containers but such is not considered to be essential -10r obtaining the desired result. 1 t is considered that other glasses may be substituted for the borosilicate glass or that such glasses may be omitted, and still, useful form-retaining containers for microwave ash analyses may be made, but it is preferred to utilize the present starting material, containing a small proportion, usually 1 to 10%, of borosilicate glass microfibers.

After heatinc is comnleted the container will. be removed from the source of heat and will be allowed to cool in air -I-- r--om - i6 - t e mp e -- a tu, r e Slow coolinc is favoured to relieve strains and to avoid excessive embrittlement. Cooling &times (to room temnerature) from 30 seconds to ten minutes are considered to be nroduce satisf_actory microwave ashing contalers.

A relativelv minor disadvantage of the quartz filter material mentioned is that i- accarently crystallizes and becomes brittle when subjected to elevated temperatures, such as those over about 500'C., for relatively long time. Nevertheless, it may be emoloved to hold the ashable sample and can be used repeatedly _z care is taken. it is estimated that between five and fifty ana-'vses can be run be-.'--re a new container of auartz filter should be Dut In service. Such items are relativelv inexpensive and ac--orcingi,i this "disadvantage" is not considered --c be sicnificant. A nreferred container for the ashable samDle;S illustrated in FIG's. 4 and 7-9.

Other containers of non-porous materials may be employed to hold ashable samples during ashings, such as crucibles made of quartz, borosilicate glass, ceramic, Dorclain and-platinum but uses of;these are normally limited to certain fusions and "dry asn4-ngs". For reasons which will be mentioned later, such containers are not as useful in normal microwave ashings as are suDDorts and containers made of the described quartz microfiber filter material.

Virtually all materials that can be ashed within the operating temperature range for the present apparatus can be sat-4s,,oac,.,-or-ly ashed in it. Among such materials there may be ment-oned svnthetic organic oolvmers, waste water sludges, act-ja-ed s'.Ludces, industrial wastes, river lake and stream bottom sediments, coals, foods, papers and building materials. often the ash contents of such materials are as low as less than 1% or 0.1%, but they can be higher, even 10% and more, and the invented apparatus will reproducibly and accurately ash such diverse materials and retain all the ash in the described porous containers.

To set up the illustrated and described apparatus and to operate it the following procedure should be followed:

1. If the thermocouple is not in place it should be inserted into the microwave retaining chamber, as illustrated in FIG's. 1, 2, 4 and 5, and as previously described in the ification. The solid thermocouple sheath should be properly sDec,. grounded to the chamber wall or other grounding location to prevent possible damage to the temperature controller.

2. Place the screen (73) and refractory block support (71) on the floor of the chamber.

3. Remove the top portion (57) of the ashing furnace and place it in the chamber under the thermocouple.

4. Align the hole in the top portion of the furnace with the thermocouple and raise such top portion upward so that the thermocouple is in the furnace cavity (23), which will be c-reated by installation of the furnace bottom section (59).

5. While holding up the top portion of the furnace, slide the bottom portion into the chamber and align it with t'portion.

6. Lower the top portion of the furnace onto the bottom porti.on. The thermocouple should extend into Che as.1L-iii, cavit-v aoDroximateiv 1 cm. but such distance may be ad-., 28 - desired, based on evaluations of analvtiLcal results, and may be within 0. 8 to 5 cm. from the too of t-he furnace cav--:--.,p,:re-ferabiv 0.8 to 3 cm. for the described furnace.

7. Place the door (35) in closed zesition en z-e ashinc furnace.

8. Install the temDerature controller on ton of the microwave retaining chamber and insert the thermocouple plug into the back of the controller, and connect t.ne temoerature controller cable to the microwave svs-k-e.m, as -:.'Llustrazed in ---'1G..

9. Insert the microwave svstem, and c--ntro-".'Ler Dower cord plugs (not illustrated) into the controller power switch to- ON Dcs-:---on 10. To minimize times reau:red zc '-e-=:: the ashable sampl-s, pre-heat the ashing furnace from rcc7, te.-.ioera-L-ure to the desired ashing temperature, which desired ashinc temDerature is set -Jnto the temperature controller as described separately below. Then, program the microwave system for 60 minutes of microwave heating and set the power at 100%. Depress the START key and allow the furnace to pre-heat. The furnace will usually achieve an operating temperature of about 950C. within 30 minutes or one of about 1,200C. within an hour. if it is desired to hold the furnace temperature longer than 60 minutes the microwave syst-em controls may be programmed for such longer time. Also, the ashing furnace temperature may be re-programmed according to the controller programming procedure to be desc--.fbed below.

11. Place the amount of samole.to be ashed -'n "-he container, or if several samDles are 'Co be ashed at the same ti-ne, place them in a plurality of containers.

and turn 29 - 12. Denress the S-LOP key, open the chamber door, remove the furnace dcc-- and Place container(s) of ashable samnle(s) i-n the ashing furnace cavity, using tongs. Replace the furnace door, cicsing it or leav.-ng it slightly ajar, 1-1 preferred, and then close the Cham-lner door.

13. Push the RESET button and demress the STAR'I" kev, which on the magnetron and starts heatIng of the sample(s).

m_er comnletion of ashinc, which usuallv takes about 10 minutes at tie desired temoerature, depress the S-LOP key, open zi-e chamber door and remove the furnace door (which can easily be Jene bv '-and des-,-e the hich nternal temnerature of the Emclov t-ongs to remove e container(s) of ash and Reolace the -furnace cool to roen. 1.-emDeral-ure Acer after removal of the container(s) to prevent heat damage to -.!.)e c-arber door. Then close the chamber door and deoress the START kev, to ma,:.nta-in the furnace at ashing temperature.

The following is a description of the procedure to be employed to program the temperature controller.

l. Insert the ther-mocouple plug into the controller. Depress the S key on the controller and 0 will appear on the controller disolav. Press the Increase key and hold it until 28 appears on the display. If 28 is overshot, press the Decrease key until 28 is reached.

2. Press the S key and C or F will appear.

a. If C appears and OC is the desired readout, mroceed to step 3.

:f: C amoears and F 4s the des-4----d read-out, press the D'lecrease kev and 'F- will appear, and then proceed to step 3.

C. If F aDmears and 1. is the desired read-out, oroceed to stem 3.

d. If F aDDears and C is-the desired read-out, press the Increase key and C will appear, after which proceed to step 3.

3. Press the S key and SP1H will appear momentarily. Press the Increase or Decrease key until the desired operating temperature set point appears. This sets the Upper temperature limit--, SP1H. The maximum operating temperature is designed into the controller circuitry. For examDle, it may be 1,20. VC. in some.ns-Lances or 1,65OcC. in others, depending on the construction of the aonaratus.

Press the S kev and SP1L will aiDnear momentarily. Then Dress the increase or Decrease key until 0 appears. This sets the!ower temDerature limit SP1L.

5. Press the S key and SP2H will appear momentarily. Then press the Increase key until 2499, the maximum value, appears. This sets the upper limit value, SP2H, which is not used in the program but is needed to make the unit operate properly.

6. Press the S key and SP2L will appear momentarily. Then press the Increase or Decrease key until 0 appears. This sets the lower limit value, SP2L, that also is not used in the program but is needed to make the unit operate properly.

7. Press the S key and HYS will appear momentarily. The.'. press the Increase or Decrease key until 1 appears. This sets the operating deadband for maximum operation precision.

8. Finally, press the S key and RUN will appear momentarilv. The actual temDerature of the ashing furnace win' t k 1 r 31 - then appear. Controller programming is now complete. Such programming must be completed within two minutes or the controller will exit the programming mode and it will be necessary to perform steps 1-8 again.

It will be noted that by following the immediately foregoing instructions (steps 1-8) for controller programming, upper and lower temperature limits are set into the controller program. Such set points may be identical, in which case when the measured temperature falls below a predetermined hysteresis value (which is usually 2 or 3 degrees) the magnetron will be turned on again, and it will be turned off when the measured temperature increases to about the same value above the set temperature.

The ashing apparatus of this disclosure is controlled by a combination of temperature controller and single chip type microprocessor. The microprocessor executes instructions from permanent storage in an internal EPROM. In operation the microprocessor receives commands and time data from an operator through the microwave instrument or system keyboard. The operator may view a response to most of the commands on the accompanying 20 digit alphanumeric display.

When the operator enters the time data on the keyboard, the data is stored in temporary RAM memory. once time has been entered for Stage 1 the microprocessor will allow entry of a Start command. When Start is pressed the microprocessor changes one of its output lines f rom high to low and begins to count down the time. This digital low is wired through a set of normally closed contacts in the temperature controller and then to the microwave solid state relay (SSR). This low turns on the SSR, which controls microwave nower. The SSR, in turn, then switches on the alternall:ing current (AC) to the microwave high voltage section and the magnetron generates microwave energy.

Microwave enercv directed into the furnace cavitv hea up the as.-1-ing 'furnace heating elements, which heat the furnace cavity and the sample to be ashed. The thermocouple senses the temDerature of the ashing furnace and the out-jut of the ther-ioc--ur)le is.--cr,-.nun-4calt--ed to the teTar)erature controller. which cont-:nua-".!v --omDares the measured lk'-'emDeratjre to thle set point temzer-zture, which!-as DreviousIv been entered. When the measured temcer-nture ecuals the set Do-int temoerature t.ne temcerature --oens -.-e normaliv closed contact and interruots the d-icit-=-' s'-naI z-'-.at had zrev--ouslv turned on --e SSR. Without thils sicnal z-.e -n-;------wave energy ceases and thle ashing -furnace holds at set point temDerature and slowl.7 beings to cool.

When the measured IkemDerature falls below a Dredeter-mined hysteresis value (usually 2 to 3 degrees), the controller closes the oDened contact and the SSR is- turned on. The microwave energy then raises the temcerature of the ashing furnace to the set point temoerature. Such, Drocesses continue until tne total heating time, as set bv the ooerator, has counted down to 0. The microorocessor changes the digital signal back to a high state and microwave heating and control cease. At any tIme during the countdown the ooerator may press the Stop key to stop the countdown and to halt the heating process, if that should be des,Lred- in %he above descriotion the temoerature controller is seoarate from the mi ------wave instrument (CE.M Xicrowave Drying/ ts 33 - Digestion System MDS-81) because the MDS-81 system was available "hardware' which could be used in conjunction with a less complex new controller. However, it is within the invention to integrate the temperature controller into the microwave instrument.

To ash an ashable sample in the present apparatus is a simple procedure. All that is required is to place the sample in a suitable container, of the type previously described, and insert it into the furnace cavity, close the furnace door and the chamber door, and press the Start button. After ashing temperature has been reached most samples will be completely ashed within about ten minutes but completion of ashing can be verified by weighing the ashed sample (in the container, after cooling) and reweighing after additional exposure to the ashing conditions. When the weight ceases to decrease completion of ashing is established, and so is the time needed to effect complete ashings, although one will usually employ additional time, say a 20% excess, to be sure. In such weighings the ashed sample and container should not be weighed hot but should be conditioned for weighing, as is known in the art, but such conditionings proceed very quickly with the support.

Normally the present apparatus and process are employed in analyzing materials for ash content. In such procedures the container is weighed without and with ashable sample content before ashing, and the container, with ash, is weighed after complete ashing. The percentage of ash in the original sample can then be readily calculated by dividing the ash weight by the sample weight and multiplying by 100. However. in some ashing operations it is common to employ a dispersing agent, such as magnesium acetate, which acts to prevent production of a vitreous or glass-like residue in the ashing container, which residue may contain some unashed sample. Without the use of such a dispersing agent false high or low readings for ash content could be obtained. When the dispersing agent is employed a blank run will normally be made to determine how much of the apparent ash weight is actually ashed dispersing agent, and such weight will be subtracted from the apparent ash weight to give the true ash weight.

Although various weights of samples and various numbers of containers of ashable samples may be employed in the ashing apparatuses of this disclosure, in a typical such apparatus, in which the furnace cavity is approximately 14 cm. x 14 cm. or about 200 sq. cm. in area, one will normally charge up to 4 or 5 porous, heat resistant and microwave transmissive containers of ashable sample, which containers will preferably be in short cylindrical form of base area of about 15 to 25 sq. cm. each, e.g., about 20 sq. cm., and with heights in the range of 0.8 to 2 cm., e.g., about 1 or 1.5 cm. Desirably, the weight of such containers will be as low as possible, usually being in the range of 0.2 to 1 g. each, preferably 0.3 toO.6 g., e.g., aboutO.4 or 0.5 g. The weight of ashable sample will normally be in the range of 1 to 10 g., preferably being in the range of 1.5 to 6 g., e.g., about 2 or 5 g. Ash contents may be high or low, up to a maximum of about 50% and to a minimum of 0.001% or even less. For materials like unfilled synthetic polymeric plastics and grain flours, such will usually comparatively low, normally being less than 5% and frequently being less than 1%, such as from 0.01 to 0.8%. For the i charges of ashable material ment-loned, with ash contents in the ranges recited, magnesium acetate dispersing agent is normally emDloyed, dissolved in ethanol (95%), so that the ethanol sclution is of a magnesium acetate concentration of about 15 g./1. Ab, c u t 3 ml. of the solution are drDoed onto the ashable samnle while iz is in the container and, if the container emnloved is Dor.ous and of light weight, heat resistant and microwave transmissive quartz microfibers, the soluz--cn will wet the entire ashable samiDle and will also wet the fibers of the container because of container porositv, microfibrous c,,iar.::= nature, and container design, anCi the alcohol flashes d,.,r.-:nc '-ea-.-.:ng in a "gentle" manner, nor carrying ash or sample out of the container, whereas when impermeable or convenz--'cnal 2ontainers are employed, such. as platinum crucibles, and combustion of the alcohol are often more violent, and somet-,-,,es cuantitles of samr)les are carried out of the container, -leading to false wash determinations.

The ashing t-emperature, as it is set for.the furnace cavity, is normal-lv in the range of 400 to 1,600C but such temperature should be chosen in light of the characteristics of the ashable samr)le and the microwave ashing apparatus. Many ashings and analyses are conductable below 1,200C. and a large number are conductable in the range of 600 to 1,000C., such as 950C. Thus, ashings of wheat and other grain flours may be effected at about 870 or 950C. and ashings of polvethylene and polypropylene may take D'Lace at about 5SO'C.

adjusted accordingly, but will normally be in the range of to minutes, prefera--'-,i 3 --c 15 minutes, e.g., about 1-0 minutes.

Ashinc Cimes mav be In some instances the ashing apparatus will be programmable so that the furnace temperature will be changed during the run. In such a situation sometimes the first heating.or ramp temperature may be comparatively low, e.g., about 1000C., to dry the sample, after which it may be increased to full ashing temperature.

k k The ashing container of the present invention provides special advantages to improve ease of analyses, speed and accuracyAlthough the ashing temperature in the microwave ashing apparatus may be in excess of the 5000C., maximum temperature specified by the filter manufacturer, it has been found that the container can be satisfactorily employed in high temperature ashing without deterioration sufficient to adversely affect the accuracy of the ash content determination. In fact, the same container can be used for a plurality of microwave ashing analyses, often more than 5 and up to 50, e.g., 10. With continued use it will be employable in the numbers of analyses mentioned without losing desired porosity for such ashing without breaking and without leaking sample or ash. In addition to the unexpected advantage of high temperature utility, the

containers of the present invention possess several other unexpected advantages and characteristics that make them ideal for microwave ashing and microwave ashing analyses. The microfibrous quartz material employed is porous, and allows air to pass through it without result-ing in loss of sample or ash. This is important because it promotes ignition and oxidation of the samDle (most- of the ash being in the form of oxides). When a dispersing agent, such as magnesium acetate in ethanol, is employed to treat the ashable sample before ashing, the porosity of the container imaterial (which is -,1a-:n- t-a--lned despite the high temperature heat--nc thereolf in the operation) is belie:Ted to contribute ta s-mooth flaming of the solvent rather than what resembles an exD!os.-ve combustion of' the solvent, which could carry awa-.:, scme c.L the samole. Such smooth flaming is believed to occur because the ethanol of the magnesium acetate solution spreads over the container due to the container's absorptive properties. The smooth _Flaming or combustion may also be partially attributable to "%he relatively low height of the container wall, which facilitates access of air to the sample and to the ethanol present. With L-he present containers such flaming can be effected in the furnace of the microwave aooaratus during the automated ashing operations whereas when ordinary non-porous crucibles of quartz, porcelain or platinum are employed in muffle furnaces or in microwave ashing furnaces, when suitable, it is usually desirable to remove the alcohol from the sample by flaming -it externally of the furnace before beginning the ashing operation.

In addition to being porous, the present containers are light in weight and are of low thermal conductivity. Because 1Chev Q are 'Light;Ln weiaht their weights are often significantly less than the sample weights and may even be-less than the ash weights, in some instances, which leads to more accurate weighings of the sample and ash." Furthermore, despite low thermal conductivity the lightweight and porous container cools faster when removed from the ashing furnace, so time is saved in cooling the container and ash before weighing, compared to when an ordinary crucible is employed. The containers, being thinner than ordinary crucibles and other containers, more readily transfer heat to ashable sa=les from external heat sources, such as microwave absorptive heating elements ad refractory muffle furnace walls.

Because the containers have side walls, they are superior to the flat sheet type support pads described in U.S. Patent 4,5565,669, and do not require cover pads to prevent loss of feathery ash into the exit air passing through the furnace and retaining chamber of the microwave ashing apparatus. The wall has the desired efEect of allowing access of oxidizing air to the sample while at the same time diminishing its velocity, so as to prevent any loss of ash from the container. However, as a safety measure, if it should be desired, a cover can be employed on the present containers, which may be made of the same material, shaped to suit, or may be of a more open porous material or screening, preferably of quartz filament or fibers.

The following examples illustrate, but do not limit the invention. Unless otherwise indicated, all parts are by weight and all temperatures are in C.

EXAMPLE 1

An official sample of wheat flour was analyzed for ash, utilizing the microwave ashing apparatus disclosure as described above, and the results obtained were compared to those that had resulted from standard analyses in which muffle furnace heating had been employed. Ten "experimental" runs were made, using either single containers of test sample or a plurality of such containers in the ashing apparatus at a time. The wheat flour employed was the standard, which was a check sample obtained from the American Association of Cereal Chemists. The ashing apparatus employed was a 1,000 watt CEM Corporation MDS-81 Microwave Drying/Digestion System unit, modified as described in the specification and employed in conjunction with a thermocouple, a temperature controller and a furnace of the types described previously herein. The materials of construction of the furnace were ECCOFOAM Q-G for the furnace body and door, and Norton Co. Crystallon Grade 37C220 silicon carbide for the beating elements. The base of the furnace is about 200 sq. cm., with the furnace cavity measuring about 14 cm. x -14 cm. in horizontal cross- section, and being about 5 cm. high. The thermocouple is of chromel- alumel type and the circuitry is that of FIG. 6.

The microwave ashing apparatus is set f or a temperature of 9500C. and is pre-heated to such temperature for about k hour. Then the chamber door and the furnace door are opened and;:, container of the sample is inserted into the furnace, using tongs. The container is made of quartz microf.iber sheet designatecl QM-A by the manufacturer, Whatman Laboratory Products Inc., and is in the shape of a flat cylinder 5 cm.. in diameter with a wall height of about 1.5 cm. it contains 2.1241 g. of: wheat flour sample and about 3 cc. of a 15 g./1. solution otE magnesium acetate in ethanal (95%), which had been drJDoed onto the sample so as to wet all of it, and the adjacent container bottom and wall. After insertion of the container o'L samDle the furnace door is replaced, in such position L-hat a passageway about 0.3 cm. wide is left, between the door and the furnace wall. Shortly after addition of the conta-ner and test- sample to the furnace the alcohol burns off withou t_ incident. --en af:ter charging of the furnace with the sample, the doors are ocene,-- and the container is removed, using tonQs, and -'s a'Z.-',2we,--; to cool in a desiccator, which takes about 60 seconds. The ccnta--'ner, w.'t- the ash and magnesium oxide residue (from the magnesium acetatel therein, is then weighed. Previously, tthe cent _s --:ie r- had been weighed empty and the equivalent magnesium oxide residue had been determined for the amount of magnesium acetate solution employed. The amount of ash was 0.0112 g. and the sample weight was 2.1241 g., so the percentage of ash in the sample was 0.527%.

The above ashing determination was repeated nine.times, for a total of ten such determinations. Results for these runs are given in Table 1, below.

1.

V A B!' U. -1 Run Code c j Wt. of Container t 0.5015 0.5014 O.cj22l 0. 53 9 0 0.4871) 0, 4940 0.5173 0.5060 0.5790 0. 4753 Ash t. Y190 (9.) $,,t. of Contalner 0.4893 0.4009 0. 5024 0.5668 0.5656 0.4732 0.4967 0. 4856 0.5567 0.4528 Wt. of Ash 4 M90 (g.) 0.0212 0.0206 0.0204 0.0222 0, D 2 2 2 0.0208 0, 0206 0.0204 0.0223 0.0225 Wt. of 1190 (4g.) 0.0100 0.0100 0.0100 0,0115 0.0115 0.0100 0.0100 0. 0 1 G 0 0.0115 0.0115 Wt. of Ash (9.) 0.0112 0.0106 0.0104 0.0107 0.0107 o.0108 0.0106 O.GI04 0. 0)08 0.0110 Wt. of Sample (g.) 2.1241 2.0392 2.0142 2. 0144 2.0305 2.0659 2.0378 2. 0276 2.0529 2.0426 Ash Content 0.527 0.520 0.516 0.531 0.527 0.523 0.520 0.513 0.526 0.539 (1, by weight) 0 43 As is seen from Table 1, the high determination is 0.539% and the low determination is 0.513%. The average is 0.524%. According to the American Association of Cereal Chemists, fifty-one analyses by muffle furnace ashing techniques, using an oven temperature of 871C. for one hour, yielded a high of 0.550% and a low of 0.504%, with an average of 0.530%. Thus, it appears that the microwave ashing apparatus yielded more consistent results and has been proven to be sufficiently accurate to be employed in replacement of the muffle furnace ashing procedure. EXAMPLE 2 Two additional wheat flour samples, identified as B and C, were subjected to microwave ashing, and ash contents of these samoles were determined and comoared to results obtained by standard muffle furnace analyses. The procedures followed were the sane as those of Example 1. For sample B three test runs were made and the ash content results were 0.508%, 0.512%, and 0.520%, giving an average of 0.513%. The ash content by standard muffle furnace analysis was 0.512%.

In three microwave ash analyses of Sample C the results were 0.724%, 0. 724% and 0.739%, giving an average of 0.729%. The standard analysis of the same sample resulted in a finding of an ash content of 0.730%.

A sample of polyethylene was analyzed for ash, using the apparatus and method described in Example 1 but omitting the magnesium acetate. Three samples of the same polyethylene were tested, with the ashing temperature being held at 550C. +3C. for ten minute periods. Ash contents of 0.008%, 0.008% and 0.006% were obtained, with the average being 0.007%. The various 1 44 weighings for the three runs made are given in TABLE 2, as are the ash contents TABLE 2-

Run Code Wt. of Container + Ash (g.) 0.6032 0.5972 0.58-14 Wt. of Container 0.6028 0.5968 0.5869 Wt. of Ash (g.) 0.0004 0.0004 0.0005 Wt. of Sample (g.) 5.0187 5.0082 8.0695 Ash Content (%, by weight) 0.008 0.008 0.006 In a manner described for the ash analvs-'s --A polyethylene three samples of polypropylene material were ana--,tjzed for ash content, using the apparatus and process oil: th.is invention. Ash contents determined were 0.024%, 0.025% and 0.024%, with the average being 0.024%. The various -4e:ch--'lngs are reported in TABLE 3, as are the ash contents. TABLE 3 R Run Code I U Y Wt. of Container + Ash (g.) 0.5984.'0.5939 0.5903 Wt. of Container (g.) 0.5972 0.5926 0.5884 Wt. of Ash (g.) 0.0012 0.0013 0.0019 Wt. of Sample (g.) 5.0103 5.1606 8.0431 Ash Content (%, by weight) 0.024 0.025 0.024 EXAMPLE 4

The described microwave ashing apparatuses and processes.are useful for performing microwave ash analyses of various other materials, including other foods and other synthetic organic polymers, various sludges and waterway sediments, papers, coals and building materials. The ash contents found in analyses of 1 - 45 such materials often range from less than 0.1% to 10% or more and such analyses are readily perfor-med and yield accurate results, compared to standard muffle furnace analyses. Of course, ashing times are subs 14- an tlally reduced, comnared to muffle furnace ashing times. Other ashing temoeratures are employable, ranging from 400 to 1,200'C., and temperatures as high as 1,600C. are feasible, with ashing times ranging from 5 to 20 minutes. To ash materials at such highest temperature some modifications of the apparatus and the m.'.cr--wave -:nstru.ment -,,,av be desirable.

out tese additlonal ana'L,.rses the aDoaratus size mav be c,ance--', the wattage -,iav be niodifled and the ashing procedures -may be alered. Thus, 4n some cases a 600 watt or 900 watt bas-.c unit (CEEM MES-81) is employed or such basic unit is replaced by other suitable instrument of such general type, as:nstead of employing the porous auartz microfiber container to hold L-he sample being ashed one may use a porcelain or quartz container or one made of other suitable material. In such cases the ashing procedure may be varied by ignition of the alcohol accompanying the magnesium acetate (when such is emDloved) external to the furnace (to avoid loss of sample due to sometimes violent ignitions in the furnace). Sometimes, even when the porous quartz microfiber container is used, if any alcohol present is not removed first by evaporation by low temperature heating, it may be considered desirable to remove the -furnace door during the initial heating of the sample so that the flaming of the alcohol can be -more controlled and so any loss of sample can be prevented.

46 - As was previously mentioned, air flow rates through the furnace -,iay be adjusted by opening or closing the furnace door.

Such flow rates depend on the degree of such opening and also depend on the total air flow rate through the chamber, which will usuall,.r be in the range of 1 to 5 cu. m./min. The total air flow and the various openings into and out of the furnace, in combination, will continue to supply air to the vicinity of the material being ashed so that as the products of combustion are removed thev are reolaced with fresh air. Despite the relatively large air flow through the chamber the ashing temperature is maintaInable the furnace because of the good insulating properties of the furnace wall and door and because of the relat-,ve'Lv m-A-lor proportion of gas that enters and leaves the Lurnace durinc ashing, with most of the air passing around the furnace.

Other variations of apparatus and process include employing other radiation absorptive materials instead of silicon carbide, e.g., ferrites, installing a quartz fiber safety screen over the ashing container, employing a radiation trahsparent turntable to provide even more uniform heating of the ashable sample, and varying the size and shape of the furnace to improve even heating and control of air flow through it. Although a turntable would promote more even heating of the microwave absorbent heating elements it has been found that furnace teMDeratures are essentially uniform throughout and multiple ashable samples are evenly ashed. This is attributable to the excellent insulating properties of the quartz foam (Eccofoam) - 47 furnace material. Accordingly, turntables and special radiation mixers are not needed, but are sometimes employed.

EXAMPLE 5

This example and Examples 6-8 relate to manufacture and analytical uses of ashing containers embodying the invention.

A 9 cm. x 9 cm. square of Whatman Ultra-Pure QM-A quartz filter, which is a non-woven sheet of quartz microfibers, is shaped about a substantially cylindrical glass form to a flat cylinder with a base about 6 cm. in diameter, and then the cylinder is wetted with about 3.0 g. of water which is applied by spraying it substantially evenly over the surfaces of the filter material. An elastic band is then applied to the cylinder wall, as illustrated in FIG. 9, to hold such wall in position. The application of water to the filter helps it to retain the cylindrical shape. Subsequently, the filter is trimmed and the elastic band is removed. Then the cylinder is removed, and is air dried and then is heated (or fired) in a muffle furnace for about ten minutes at about 8700C. to cure it, after which it is removed from the muffle furnace and allowed to cool in room temperature air. The result is a form-retaining, heat shaped, short cylindrical container, useful for microwave ashing of ashable materials, such as analytical specimens.

The container looks like that of FIG. 8 and those of FIG. 7. Although the container is form-retaining, even during use at elevated temperatures as a container for ashable material during microwave ashing thereof, it retains its desirable porosity.

- 11 8 - Alternatively, the container -,iav be fired in a microwave ashing furnace like that illustrated In FIG. 7, at a higher temperature, 950C., and the result Ls-"Lhe same. EXAMPLE 6 An ashing container in flat c.711ndrical form, essentially the same as that of Example 5 and FIG. 8, is made by wetting a 9 cm. x 9 cm. square of the same QM-A filter material with the same amount of water, forming it by means of a quartz mandrel, as shown in F1G. 9,;.n-k-o a flat cvl,nder, trimming such cylinder to desired 1.5 cra. heIght, and holdLng a sIde wall thereof to the mandrel b.j -means of a cuartz thread, also as illustrated in FIG. g. The shaced on the cuartz mandrel, is then sub4ected _= --ur--nc heatina to a temcerature of 950cC. for ten minutes in a -.i.-:crowave furnace, like ";..hat of FIG.

7, after which the heatinc -'s flat cylindrical container are removed from and allowed to cool in room temDerature air.

container is removed from the mandrel and is the thread in place or after removal thereof.

EXAMPLE 7

The container described in ExamDle 5, which weighs 0.50 g., has added to it, 2.01 g. of a check sample of wheat flour (from -s) and to the sample in the American Associat4on of Ceredl Chemist the container there is applied approximately 3 ml. of a 15 gJ1. ethanal (95%) solution of magnesium acetate, in such manner as to wet all the samole (and also to wet cart of the container). The container of test samole, wetted with the magnesium acetate solution, is placed in the -microwave ashing furnace of FIG. 7 and the mandrel and the microwave furnace After cooling, the ready for use with 1 after such aDoaratus furnace is brought to a temperature of 935C.

and heating at such temperature is continued -for ten minutes.

Such heating is then halted and the container of ash is removed.

The weight of flour ash and magnesium oxide is 0.02 g. and the we'ghr- of magnesium oxide (previously obtained experimentally for the volume of solution added) is 0.01% g. Thus, the cereal ash weighed 0.01 g., which corresponds to 0.05% of ash, which checks With results obtained by standard muffle furnace ashings (over a 90:,.,iinute per-Lod) of Che same sample.

in variations o.E this exoeriment containers produced by tne --rocedure described in ExamDle 5 as alternative and by the Drocedure Illustrated in ExamDle 6 are substituted and the results are the sa.-,ie. Furthermore, when a plurality of samples is ashed az the same in a of such containers in a microwave =shing aooaratus, such as illustrated in FIG. 7, accurate results -or each are also obtainable.

i 1 1 1 EXAMPLE 8 containers within the invention that are made from a microfibrous filter paper that does not contain borosilicate glass (which is present in the QM-A filter material) can also be made by the processes described, with suitable heating temperatures being employed in the range of 50 to 1, 0000C., such as 9500C., and will be satisfactory, even when only half the water is applied and when no water is applied beforehand (other suitable liquids, such as ethanol, may be substituted). Such containers are employable in microwave ashing apparatuses like those illustrated in FIG. 7 and accurate analytical results are obtainable, as is verifiable by comparison with standard muffle furnace analyses of the same test samples.

In addition, ash analyses of other materials, including other grain flours, synthetic organic polymeric plastics, such as polyethylene and polypropylene, stream sediments, waste water sludges, coal, milk powder and many. other ashable materials, are successfully performable using the described procedures and apparatuses. In such ashings the ashing temperature is varied within a 500 to 1,OOOOC. range and the ashing times are also varied, usually from 8 to 20 minutes, which will depend on the type of material being ashed and its ashing temperature. In all such instances satisfactory ashings and analyses are the results, which correspond with determinations made following standard muffle furnace procedures applied to the same test specimens. Such good results are also obtained when the cylinder is covered by a flat cylindrical cover of the QM-A filter material, but use of such cover is not necessary (although it may be considered to j 1 1 1 - 1, 51 - be a safety measure, to make sure that no ash is lost in the exhaust air).

Attention is drawn to our copending Application No. 8929250.2 from which this Application is divided and which has claims directed to the microwave ashing and analytical apparatuses described herein.

62- kims 1. A container f or a heatable material which is to be heated therein, which container is heat resistant during such heating operations, light in weight, porous and includes integral bottom and side wall portions made of quartz or borosilicate glass fibers or a mixture thereof, which are held together in walled and bottomed container form.

Claims (1)

1. 2. A container according to Claim 1 where the material to be heated is

   ashable and is to be ashed by heat in an ashing furnace, which container is heat resistant during ashing operations in which it is heated to a temperature up to 5000C., and the material for construction thereof is quartz microfibers.

   3. A container according to Claim 2 which container is suitable for use in an ashing furnace that is heated by microwave radiation of microwave absorptive elements thereof, which is microwave transmissive and porous.

   4. A container according to Claim 3 wherein the material of construction thereof is a non-woven thin sheet of quartz microf ibers, which sheet has ben heat cured to walled container form.

   5. A container according to Claim 4 wherein the material of construction is of a thickness in the range of 0.2 to 0.7 m.m., of such porosity that the pressure drop across it is 1 to 5 m.m. of mercury at 5 c.m.lsec. face velocity of air, resistant to high temperature, up to 5000C., retentive of micron size particles, transmissive of microwave radiation and of a weight in the range of 50 to 200 g.lin.2.

   6. A container according to Claim 3 which is of substantially flat cylindrical form, with the height/diameter ratio being in the range of 1:5 to 2:5 53 and with the weight of the container being in the range of 0.2 to 0.6 g.

   7. A process for manufacturing a container that is suitable for use as a container for ashable material to be ashed by heat from microwave radiation of microwave absorptive elements in an ashing furnace, which comprises shaping a light weight, microwave transmissive and porous sheet of quartz microfibers to container form and heating such sheet in such form, whereby a form retaining container results.