WO1998020341A1

WO1998020341A1 - Sample preparation for metals analysis using accelerated acid digestion (aad)

Info

Publication number: WO1998020341A1
Application number: PCT/US1997/020599
Authority: WO
Inventors: Joseph Lawrence Oliphant; Kenneth Alan Roberts; Douglas Wayne Later
Original assignee: Mountain States Analytical, Inc.
Priority date: 1996-11-08
Filing date: 1997-11-07
Publication date: 1998-05-14
Also published as: AU5435198A

Abstract

A method for the accelerated acid digestion of metals for an organic or inorganic solid or semi-solid matrix. A sample is placed in a digestion column (61) having a conventional heater (64). An aqueous acid solution (21) is placed in the column (61). The column (61) is then pressurized and heated to effect the digestion. The digestate is then transferred to a collection chamber (81) prior to metal analysis.

Description

SAMPLE PREPARATION FOR METALS ANALYSIS USING ACCELERATED ACID DIGESTION (AAD)

FIELD OF THE INVENTION This invention relates to the accelerated digestion of inorganic matrices, such as soils, for metals analysis. More particularly, this invention relates to a system and method for the digestion of metals from organic and inorganic solid or semi-solid matrices which uses low acid volumes, has rapid digestion times, is adapted for automation and minimizes impact to the environment.

BACKGROUND OF THE INVENTION AND DESCRIPTION OF PRIOR ART

In the field of analytical chemistry a major objective is to qualitatively determine, and preferably quantitatively measure, one or more analytes from a sample matrix of interest. The assaying of metals from ores, gangue, mill concentrates, smelter slags, and the like has long been of fundamental concern in the mining and smelting industry. With current emphasis in minimizing environmental pollution or damage, it is now imperative to know the metals content in various organic and inorganic matrices such as sediments, sludges, fly ash, air particulates, soil samples, plant and animal residues, foods, food products, pharmaceutical agents, etc. There are many techniques and instruments for the automated determination of metals contents, both qualitatively and quantitatively, once the metals in the sample are available in an acidified aqueous matrix for measurement. For example, techniques such as atomic absorption spectroscopy (AAS) in its various forms, e.g. flame, graphite furnace, hydride, cold vapor, etc; inductively coupled plasma spectroscopy (ICP) are readily accessible. However, there has not been the development of automated systems as a means for preparing a matrix or sample in such a manner that the metals content is made accessible for determination and measurement.

Also, most previous methods of digestion have involved the use of copious amounts of aqueous acid solutions which necessitated evaporation at elevated temperatures thereby releasing toxic fumes into the atmosphere.

Currently, metals digestion of said samples for environmental measurement are most frequently performed by techniques such as SW-846 Method 3050A which is a manual, labor intensive and slow procedure. This is sometimes referred to as the hot plate acid digestion method and can be considered to be a somewhat poorly stirred batch reactor that is open to the atmosphere. The advantages of this method are low price for equipment (just a hot plate with beakers and watch glasses) with simple operation. The disadvantages are due to the fact that it is open to the atmosphere and cannot be pressurized. The temperature cannot be raised above -95 C or the acid will boil causing loss of analytes . It is also very easy to lose the more volatile elements as water and acid are lost from the open system through evaporation.

Recently, EPA SW-846 Method 3051 for microwave assisted acid digestion of soils has been proposed. The automation of Method 3051 is hindered by the limited capacity of the microwave oven and the large amount of manual labor required both before and after the microwave step. Microwave systems are also expensive to purchase, maintain and operate. Method 3051 closes the system allowing higher temperature digestions due to the possibility of pressure increase. Heating is done by microwave energy so there are no thermal heat transfer barriers slowing heating of the sample, although the rate of sample cooling is still heat transfer limited. This method can still be considered a rather poorly stirred batch reactor, with the advantages of being closed during the digestion reaction. An improved microwave digestion system is shown in U.S. Patent 5,215,715 which uses a plug flow reactor with a flow restriction so that higher temperatures and pressures than atmospheric can be achieved. The sample passes through the system as a slurry with microwave energy being used to heat the slurry inside polytetrafluoroethylene (PTFE) tubing. This plug flow reactor has the advantages of better mixing than Method 3051 and also is easier to clean. There are however, disadvantages in using this system. For example, longer sample preparation times are needed to pulverize solid matrix samples, there is a potential for plugging of the flow path and the process uses large volumes of reagents (acid and water) to maintain the slurry flow. For constant density systems, the performance equations for batch and plug flow reactors are identical, with the total reaction time in the batch reactor replaced by the residence time in the plug flow reactor. However, there are safety problems associated with microwave use. Without careful calibration there are processing and safety considerations. For example, if pressure and temperature sensors are not properly calibrated there is the possibility of overheating and venting acid and analyte to the environment. Also, a long cool down period is required.

To sufficiently dissolve metals from a matrix by means of acid digestion requires a series of chemical reactions for each element to undergo a change in the oxidation state and dissolution from the inhomogeneous solid phase to a homogeneous acidified aqueous phase. Such chemical reactions can range from a very simple dissolving of the solid and diffusion of the analyte into a bulk liquid phase with low activation energies of a few kcal/mole to the breaking down of the solid matrix and reaction of the oxidizing acid with the element to increase its oxidation state, followed by dissolution and diffusion into the liquid phase. The more complex reactions can have high activation energies, on the order of 50 or more kcal/mole. Some samples may also have metal elements complexed with organic or inorganic molecules or combined with insoluble organic or inorganic phases . To release the metals into the liquid phase for quantitation, reactions have to take place with the organic molecules to facilitate the release of the metal. These can also be difficult reactions with high activation energies.

A study of the reaction kinetics for acid digestion of metals from an inhomogeneous solid inorganic or organic phase to an acidified water phase shows that pressure, in and of itself, affects the acid digestion rate only to a limited extent. However, the same does not hold true for temperature differences, particularly for substances having high activation energy values. For example, the rate of a low activation energy reaction, e.g. ~2 kcal/mole, (such as dissolving sodium chloride in water) , increases only by a factor of 3 when the temperature is increased from room temperature (25° C) to 175° C. On the other hand, the rate of a high activation energy reaction, e.g. -50 kcal/mole, (such as the oxidation of some metal complexes) increases by a factor of 10¹² or a trillion (million million) times when the temperature is increased from room temperature (25° C) to 175° C. Such increases in reaction kinetics are theoretically possible using the microwave Method 3051 and microwave method of U.S. Patent 5,215,715. A smaller, but significant, reaction rate increase over room temperature (about 10⁷) is possible using method 3050A at -95° C. Due to the disadvantages associated with labor, time and toxic vapor release of this method, there is little incentive to utilize this if the appropriate microwave technology is available. However, as noted above, even though the reaction rate using microwave methods is increased by a factor of about 10⁵ over that of Method 3050A, there are disadvantages associated with the use of such methods as noted above .

According to U.S. Patent 5,215,715, acid concentrations for the microwave digestion process may vary between about 5 and 30% with about 10% acid concentration being preferred. It is stated that digestion rates are not appreciably improved at higher concentrations using the digestion system disclosed. On the other hand, Method 3051 requires the use of concentrated nitric acid followed by dilution for analysis of metal content.

A process in which the acid concentration can vary would be desirable. In reactions that are as high as third order in acid concentration, the rate of reaction in going from about 3% total acid to 75% total acid is proportional to 25³ or 15,625 times. It would therefore be desirable to devise a method for the digestion of metals from soils or similar samples in an efficient and accelerated manner that produces results equal or superior to those of the prior art without the release of noxious or toxic vapors into the environment and without the use of microwave heating. With properly designed reaction vessels, heating can just as efficiently be done using convection or resistance heating. That is accomplished by the present invention.

An accelerated solvent extraction system utilizing higher temperatures and pressures for the organic solvent extraction of organic analytes is illustrated in PCT International Application WO 95/34360 published December 21, 1995. There is no mention in that application that it can be adapted or utilized for the digestion of metals from solid matrices. In fact, sand and diatomaceous earth are listed as being inert substances which do not contain extractable materials. The extraction process is directed to the use of non-aqueous organic solvents and there is no suggestion of using aqueous acids. Because of the corrosive nature of aqueous acids, regardless of concentration, the system disclosed in that application would not be suitable for use in the present invention.

OBJECTS AND BRIEF SUMMARY OF THE INVENTION It is a primary object of this invention to provide a system and method for the acid digestion of metals that are combined with solid matter into a homogeneous liquid acid digestate phase ready for introduction into a suitable apparatus for analysis.

It is also an object of this invention to provide a system and method for the acid digestion of metals that are combined with solid matter to produce a final digestate with approximately the same acid concentration as the other samples and standards so that reproducible sample introduction and nebulization in analytic instruments such as AA or ICP can be achieved. A still further object of this invention is to provide a system and method for the accelerated acid digestion of metals at high temperature utilizing convection or resistive heating.

A yet different object of this invention is to provide a system and method for the rapid acid digestion of metals using a variety of acids and additives at a range of acid concentrations at high temperatures to facilitate the sample preparation for analysis .

These and other objects are accomplished by means of a system wherein the digesting acid liquid flows, in plug flow, through a stationary solid particle phase sample contained in a heated digestion column. The column can be maintained under any desired temperature and pressure. Under low positive pressure from a compressed gas the digesting acid, e.g. aqua regia, enters an injection valve where a measured volume of acid is readied for injection. This aliquot of the digesting acid is then passed from the injection valve under pressure by a compressed gas, such as nitrogen, oxygen, air, carbon dioxide, helium, argon, neon and the like, into the digestion column and onto the solid sample. The composition of the compressed gas is not critical as long as it is functional and does not inhibit the digestion process . The gas used will most likely be inert. However, a gas which actually participates in the digestion process could also be used. The column can be heated by suitable conventional heating means and pressurized by a compressed gas to any desired temperature and pressure. The digestion may take place using either a static or dynamic, flow through mode. Preferably the static mode is used in which the column is closed at the bottom and maintained under pressure from gas entering the upper end of the column. If desired, both ends of the column may be closed in the static mode. In either mode, a measured amount of digesting acid is introduced into the column and is subsequently received in a collection vessel for analysis by any suitable means. In the static mode, the acid is added to the column and the digestion takes place at a desired temperature and pressure for a predetermined period of time. The digestate is then washed from the column using either compressed gas or additional acid. The digestion cycle can be repeated as many times as necessary to affect complete digestion of the metals from the sample using small amounts of acid in relatively short time periods. If desired, repeated digestion cycles can be carried out using different acids or concentrations of acids depending on the results desired and the metals being analyzed.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a schematic drawing of one embodiment of the system of the present invention.

DETAILED DESCRIPTION OF THE INVENTION The presently disclosed system and method are useful in the digestion of metals from a variety of organic or inorganic matrices, e.g. soils, particulates, ash, sands, ores, sediments and the like.

The process disclosed can be utilized for the digestion of a variety of metal ions which may be present in various oxide, salt or organic complex forms. The invention is limited only by functionality and may be utilized to digest alkali, alkaline earth, transitions and Groups IIB, IIIA, IVA, VA metals. Representative of these are metals selected from the group consisting of Ag, Au, Al , As, Ba, Be, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, Sb, Hg, Se, T, V and Zn.

Any of the strong inorganic acids or combination of acids and oxidizing agents may be used. Therefore, the term "acid solution" is inclusive of both acids and oxidizing agents. Representative acids include HCl, HN0₃, H₂S0₄, HC10₄, H₃P0₄, acetic, citric, etc. Exemplary of oxidizing agents are H₂0₂ and KMn0₄. Such acids, oxidizing agents_ and mixtures thereof may be used at any suitable strength or concentration. Particularly preferred are combinations of nitric and hydrochloric acid, e.g. aqua regia. Acids as dilute as 0.5% by volume up to full strength may be employed. Preferably, acid solutions will have a minimum acid concentration of about 3% by volume. However, it is not the percent concentration that is important since the only concentration limitation is that of functionality.

If desired, digestions can also be carried out in sequences such that one acid is used followed by another. Also, multiple columns (cells) can be utilized in series or in sequence depending on the sample, metal (s) to be digested, etc.. Because the system is closed and requires only small amounts of sample and acid, it is possible to adjust or adapt the operation according to the sample and the metals of interest .

A schematic of a preferred embodiment of the invention is shown in Fig. 1. However, it is to be emphasized that this is only one embodiment used to illustrate the invention and other acid delivery systems, control valves, sample loops, heating means, and the like which serve to perform the same function can be employed.

The system 10 comprises an acid reservoir and delivery system 20, a compressed gas source, injection valve and acid loop system 40 for delivering measured amounts of acid and maintaining and regulating pressure, a heated digestion column system 60 and a collection vessel 80 and interconnecting and operative parts as will now be described.

In the acid reservoir system 20 the digesting acid 21 is contained in a capped bottle 22 which can be maintained under a low, positive pressure via compressed gas from a gas source 50 regulated initially by a high pressure regulator 51, then passing through lines 52 and 53 to a low pressure regulator 54 and then entering bottle 22 through line 23 when an on/off isolation valve 24 is opened. Line 23 terminates above the surface of acid 21 in bottle 22. A line 25, also intercepted by an on/off valve 26, extends into and terminates near the bottom of bottle 22 such as to be below the surface of the acid 21. By means of the positive pressure exerted on the surface of the acid line 25 conveys acid from bottle 22 to the injection valve system 40 when valve 26 is open.

The injection valve system 40 incorporates a four way slider valve assembly 41 with a 5.0 ml acid loop 42 attached. One such assembly is a Rainin Slider Valve 201-56 available from Rainin Instrument, Co., Woburn, MA. In one valve position the valve is adapted to receive acid via line 25, under pressure from the acid reservoir system, to the valve assembly 41 which is in fluid communication with acid loop 42. The appearance of acid from waste line 43 indicates when loop 42 is filled with acid at which point the slider valve may be actuated or valve 26 may be closed. When filled, loop 42 will contain an exact volume of acid, e.g. 5 ml . When valve assembly 41 is repositioned, acid loop 42 is in fluid communication with lines 44 and 45. High pressure gas, e.g. oxygen or nitrogen, from a compressed gas container 50 or compressor enters through high pressure regulator 51 and line 44 to force the acid in loop 42 through line 45 and into the column 61 of the digestion system 60. Once the acid has entered the digestion system, valve 41 may be again repositioned such that loop 42 may again be filled with acid and pressurized gas may pass directly from line 44 to line 45 and into the digestion system to maintain the pressure in the digestion column as desired. The pressure in the digestion column 61 may be controlled by high pressure regulator 51 regulating the pressure of the compressed gas from source 50 entering lines 52 and 44 and passing via line 45 into the digestion column. The pressure facilitates the flow of acid to the column and the flow of digestate from the column into the collection vessel 80. Further, the pressure allows temperatures higher than the ambient boiling temperature of the acid to be used in the digestion column while retaining the acid in a liquid state and also prevents volatilization of volatile metal analytes. If necessary, other appropriate valve means may be installed along either lines 44 or 45 to regulate and/or maintain pressure to the digestion column.

The digestion system 60, is made up of a digestion column 61, capable of holding a small sample amount, e.g. 1-2 grams, and about 5-10 ml of digesting acid. The size of the digestion column, and other components of the system, are relative and larger or smaller components may be employed. Column 61 is accessible by removable caps 62 and 63 at either end. Wound around the column 60 is a heating coil or tape 64 which can rapidly heat and maintain the column and constituents in the column at temperatures of between about 25 and 200 °C. A valve 65 at the bottom of the column can be opened and closed as desired to enable digestate fluids to exit through line 66 which passes through a water cooling jacket 67 and exits tip 68 into a collection vessel 80. The interior of column 61 can be pressurized to pressures of between ambient and 500 psi by means of compressed gas entering through line 45.

The outlet end of the digestion column 61 is fitted with a valve 65 which can be opened to allow digestate or rinse solution to drain into a collection vessel 80 where it can be further processed and/or analyzed. Cooling jacket 67 serves as a condenser to liquify gaseous hot acids from the digestion column 61 to the liquid state thereby preventing compressed acid vapors from exiting tip 68 as a gas. In operation, the sample to be digested (e.g. sand, soil, ore) is placed in the digesting column 61 by removing either caps 62 or 63 and placing the sample in the column 61. Column 61 is then recapped and connected to the injection valve system 40 in a fluid tight relationship via line 45. The digestion column 61 and sample are heated by convection from the heating coil or tape means 64 to the desired temperature. While the digestion column is being heated, the acid loop 42 is filled via line 25 with acid 21 (e.g. aqua regia) from the acid supply reservoir 20 under low positive pressure exerted through line 23 as explained above. For purposes of illustration, 5 ml portions of acid are used. The injection valve 41 is placed in the mode that, under pressure from a compressed gas source from line 44, the acid in loop 42 is deposited into column 61. Following introduction of the acid, the injection valve 41 is placed in a mode such that column 61 is pressurized as determined by the high pressure regulator 51 via lines 44 and 45 to the desired pressure, e.g. about 200-500 psi.

During the initial acid introduction into the column 61 onto the sample, a valve 65 at the lower end of the digestion column is opened to the extent necessary to enable saturation of the sample with the digesting acid and also to bring about a partial pressure reduction in the digestion column as the acid in the acid loop is forced onto the sample. For example, depending on the composition of the sample, as the acid washes through the sample in the digestion column the pressure may need to be monitored to make sure the pressure does not exceed a predetermined range, i.e. 200-500 psi., due to the reaction between the acid and basic components in the sample, such as carbonates, oxides, hydroxides, etc. Depending upon the initial reaction, it may be desirable to allow the entire first acid aliquot from the loop to drain into the collection vessel 80 with valve 65 in an open position. In the alternative, it may be preferable to close valve 65 when the first drops of digestate appear at tip 68. Obviously, in order to retain any elevated pressure in column 61 during a sustained digestion period at an elevated temperature, it will be necessary to close valve 65. Therefore, the initial sustained digestion period at elevated pressures and temperatures in column 61 may be carried out using the initial acid aliquot or a subsequent aliquot.

In any event, in the static mode of operation, with valve 65 closed and a measured aliquot of acid from loop 42 being introduced into column 61 onto the sample, the column 61 is pressurized via lines 44 and 45 to the desired pressure and heated by coil 64 to the desired temperature and held for a predetermined period of time, i.e. 5 to 10 minutes, in a closed system. Valve 65 is then opened and the digestate is forced from column 61 through line 66 under pressure from the pressurizing gas entering column 61 through line 45. The digestate is fully liquified by passing through cooling jacket 67 and exits tip 68 for collection in vessel 80.

Valve 65 can then be closed and the cycle repeated as many times as necessary using the same or different acids, varying pressures, temperatures, etc. The digestate collected in vessel 80 can be analyzed after each collection or can be pooled and analyzed according to established procedures.

This process provides a better way of contacting the digesting acid reagents with the sample for reactions that approach equilibrium before being essentially complete. This is due to the fact that the freshest most unused acid reagent comes in contact with the most depleted and most reacted particles of the sample. Further, this system has the advantage of being a closed system so that pressure and temperature may be raised for more rapid digestion. Another advantage of this invention is that a large amount of sample can be reacted with a relatively small amount of acid providing the opportunity for much lower detection limits on soils and other solid samples.

It is important that the system, beginning at the acid reservoir 20 and continuing through the collection vessel 80, be constructed of acid resistant materials to prevent any destruction of apparatus components or sample contamination. No metal contact can be allowed in the flow system for even inert metals such as titanium may have impurities which could be digested and contaminate the sample. Glass or polymeric components capable of handling the temperature, pressure and corrosion challenges need to be utilized. Polymers such as fluorinated hydrocarbons, (e.g. Teflon , Tefzel, PEEK, etc.) may be utilized.

The acids used for the digestion can be selected to facilitate the digestion according to the sample and the metals to be determined or analyzed. The concentration of the acid can vary from as low as from less than 1%, e.g. 0.5%, to fully concentrated acids. As shown in the following examples, concentrations ranging from 3% to 75% are shown to be functional and the only limitation as to acid strength or combinations of acids is that of functionality. Lower concentrations may be all that are required if the metals to be analyzed are those which readily digest. For metals, such as Fe and Sb, which are more difficult, it may be desirable to employ more concentrated solutions or longer digestion periods. The amount or volume of acid used for digesting a sample can be kept at a minimum. Generally as little as 1 ml and not more than about 25 ml of acid/gram of sample will be required for any digestion cycle.

Temperature ranges are similarly flexible being limited only by the functional aspects of the invention. The temperature will preferably be sufficiently high to facilitate digestion of the metals from the sample. In that regard, temperatures in the range of about 25 to 200°C may be used with temperatures in the range of between about 95 and 175°C being preferred. Comparable to acid concentrations, temperatures for the more easily digested metals may be lower than temperatures required to extract the more difficult to digest metals. The pressures in the digestion column are not as critical as will be shown in the following examples. However, since the process preferably operates in a static or closed system, the pressures should be sufficient retain the digesting acid solution in a liquid state at the temperatures being employed. Further, pressures in the digestion column should be sufficient to discharge the digestate from the sample at the completion of the digestion cycle. Pressures in the range of about 10 to as high as 500 psi may be employed with pressures in the range of between about 50 and 200 psi being preferred.

EXAMPLES In the following examples an apparatus substantially as described in Fig. 1 was used. One end cap 62, of a glass digestion column 61 (size 15 cm long and 0.66 cm diameter) was removed and -0.5 gram of sample material was weighed into the column. The column 61 was placed in a tube heater made by wrapping heating tape around a small diameter pipe. A test of the heating capability was conducted by placing a thermometer inside the glass column and sliding the column inside the tube heaters. About 8 minutes was required to heat the column to >90% of the tube heater temperature .

While the dry sample in the column was heating, a 5 ml acid loop 42 in an injection valve 41 was filled with acid from a low pressure acid bottle. The injection valve was activated and acid forced into the digestion column onto the sample. The valve 65 at the bottom of the column 61 was opened until several drops of acid flowed into a 50 ml collection vessel 80. This procedure insures that the sample was saturated with acid. Valve 65 was then closed and the sample was allowed to digest at the predetermined temperature for five minutes under a predetermined pressure of a pressurizing gas entering column 61. Valve 65 was again opened and digestate was collected until pressurizing gas started coming into the collection vessel 80. The valve 65 was once more closed and the injection valve was placed in a position to refill loop 42 with acid. The procedure was then repeated making for a total of 10 is of acid used in the digestion.

The digestate collected in vessel 80 was then diluted to volume with deionized water readying the sample for analysis by ICP. The digested sample was removed from column 61 by removing the end caps and rinsing the column. The column and end caps were washed and dried in preparation for the next sample . Total cycle time for each sample, using the manual system described, was about 30 minutes. Metals analysis was performed using a Thermo

Jarrell Ash Model 61E ICP-AES calibrated and run as specified in EPA SW-846 Method 6010A.

Sample materials and reagents used were as follows. Nitric and hydrochloric acids were trace metals analysis grade. Deionized water was ASTM Type II showing no values above detection limits for reagent blanks. SRM Lot' #220 was a certified reference material purchased from Environmental Resource Associates identified as Inorganics Trace Metal Lot #220. The natural matrix certified reference material was a sludge amended soil manufactured by Resource Technology Corporation and designated as CRM005-050 Lot #J050.

Example 1 A blank sea sand matrix and that matrix spiked with a spiking solution were digested under identical conditions at 150°C at a pressure of 120 psi using nitrogen as the pressurizing gas. The digesting acid was 50% HC1, 25% HN0₃ and 25% water, Results of these digestions are shown in Table I .

TABLE I Blank and Matrix Spikes

lem. Blank A Blank B Ave . : Spike A % Rec. Spike B % Rec. Ave. %

(mg/kg) (mg/kg) Level Spike A Level Spike B Recovery (mg/kg) (mg/kg)

Ag 0.0 0.0 0.0 0.071 93.0 .0677 107.6 100.3

Al 253.6 226.5 240.1 3.56 113.3 3.38 125.0 119.1

As 0.0 0.0 0.0 3.56 98.6 3.38 105.6 102.1

Ba 76.7 41.8 59.3 3.56 89.2 3.38 89.8 89.5

Be 0.3 0.0 0.1 0.089 95.3 0.0846 99.0 97.1

Ca 54.8 39.9 47.3 1.78 91.5 1.69 95.7 93.6

Cd 0.0 0.0 0.0 0.089 106.1 0.0846 109.9 108.0

Co 0.0 0.0 0.0 0.840 100.0 0.846 105.6 102.8

Cr 1.4 2.0 1.7 0.356 100.2 3.38 104.3 102.2

Cu 1.1 1.3 1.2 0.445 100.3 0.423 105.3 102.8

Fe 525.5 540.0 532.8 1.78 80.0 1.69 96.6 88.3

K 60.6 45.8 53.2 17.8 98.1 16.9 104.8 101.4

Mg 19.0 15.3 17.2 3.56 100.3 3.38 107.0 103.6

Mn 2.9 2.2 2.6 0.890 99.5 0.846 105.4 102.4

Na 147.6 92.0 119.8 5.34 91.2 5.08 91.6 91.4

Ni 3.6 4.5 4.0 0.890 103.7 0.846 103.0 103.3

Pb 0.0 0.0 0.0 0.890 107.9 0.846 112.6 110.2

Sb 0.0 0.0 0.0 0.890 92.6 0.846 99.4 96.0

Se 0.0 0.0 0.0 3.56 95.5 3.38 103.3 99.4 l 0.0 0.0 0.0 3.56 98.3 3.38 105.5 101.9

V 1.2 1.3 1.2 0.890 98.0 0.846 103.1 100.6

Zn 57.8 31.6 44.7 0.890 80.7 0.846 85.2 83.0

Average % Recovery All Elements 100.0

The measured value of the elements in the blank are in units of mg/kg of sample. Positive results are seen for the elements Al, Ba, Ca, Fe, K, Mg, Na and Zn. The elements Cr, Cu, Mn, Ni and V also showed results that were just above the ICP detection limits. Duplication between blanks A and B are reasonable, although not excellent. Elements such as Al, Fe, and

Na, that had higher values could all be expected to be in the sea sand structural matrix. Other elements, such as Cr and Ni, that had measurable values just above the detection limit, showed good reproducibility between samples A and B verifying their inclusion within the sand matrix.

Spiking of the blank sea sand samples was done by adding 0.5 ml of a spiking solution to the dry blank in the digestion column before the column was placed in the heater tube and the digestion process started. The percent recovery of the elements in the spiking solution was calculated according to the formula:

% recovery = measured value/expected value x 100

Spike recoveries for elements not found in the blank were good. Spike recoveries for elements in the blank were more variable. However, based on the premise that this is a prototype of a developing system, results were considered reasonable and reproducible .

Example 2 To test the effects of temperature variation on the accelerated acid digestion of this invention, SRM lot #220 was digested in duplicate at three different temperatures, 25°C, 95°C and 150°C. All digestions were carried out under 120 psi N₂ pressure using the same 75% acid as in Example 1. Results are shown in Table II. TABLE II

Effect of Temperature on AAD

Element Certified 25°C 95°C 150°C

Value Average Average Average

(mg/kg) % Recovery % Recovery % Recovery

Ag 54 100.3 110.2 119.0

Al 4420 32.5 45.4 121.6

As 112 87.9 105.3 115.5

Ba 158 83.9 97.2 138.4

Be 99 99.9 106.0 114.7

Ca 2500 86.4 90.1 104.6

Cd 131 95.7 101.1 109.9

Co 60 93.3 99.9 108.9

Cr 187 85.6 101.4 114.9

Cu 144 107.1 110.1 121.4

Fe 10400 16.1 39.7 109.7

K 2310 73.5 86.2 132.0

Mg 2230 72.4 85.6 118.3

Mn 286 79.0 87.8 106.8

Na 656 87.7 93.3 120.0

Ni 106 100.6 113.6 117.5

Pb 208 93.4 99.8 111.4

Sb 56 32.7 111.8 181.9

Se 122 75.5 92.3 101.4

Tl 134 113.5 121.8 137.6

V 204 72.2 88.5 109.4

Zn 259 98.4 105.9 131.5

rage % Recovery

All Elements 81 951 20

To better explain the results it helps to divide the elements into two groups, those that are easy to digest and those that are more difficult to digest. The elements that are easier to digest will show good recoveries even under mild digestion conditions of temperature and acid concentration. Recoveries of these elements will not change greatly as the digestion conditions become more rigorous. On the other hand, the more difficult elements to digest will have low recoveries under mild digestion conditions and recoveries will increase significantly as conditions are made more rigorous.

From Table 2 it can be seen that elements in SRM #220 that are easier to- digest are Ag, As, Ba, Be, Ca, Cd, Co, Cr, Cu, K, Mg, Mn, Na, Ni , Pb, Se, Tl, V, and Zn. The more difficult element are Al , Fe and Sb. The more difficult elements come from the soil structural matrix and it requires more rigorous digestion conditions to break down and dissolve this matrix. However, Sb may be an exception. It has been found that when using Method 3050A, the longer the sample is digested the less Sb is found. One explanation may be that the certified values for SRM lot #220 are averages of values found in an interlaboratory round robin study. These laboratories would have used the open beaker digestion methods according to or at least similar to Method 3050A and therefore probably lost some of the Sb due to volatilization during digestion.

Using the closed digestion system disclosed herein should show significantly higher amounts of Sb.

Example 3 A comparison of how temperature affects recoveries can be shown by comparing two typically easy to digest elements with two more difficult to digest. In reference to Table 2 it can be seen that Ag and Be show a small percent increase as digestion temperature is raised. However, all recoveries are well within performance acceptance limits. On the other hand, elements Fe and Sb show very low recoveries at 25°C (room temperature) but are somewhat improved at 95°C and increase to high recoveries at 150°C. The stated performance acceptance limits for Sb are very broad, i.e. from 42% to 392% of the certified value. This indicates the unique difficulties the various laboratories have in analyzing for Sb.

Example 4 This example illustrates how high or complete recoveries can go using various means of rigorous digestions. For comparison with the accelerated acid digestion method disclosed herein Method 3050A and two other methods were used. A lithium tetraborate fusion was performed on duplicate samples and then analyzed using ICP. Also, a four acid (hydrochloric, nitric, perchloric, and hydrofluoric) digestion was done. The results of these digestions are shown in Table III.

TABLE III Attempts at Complete Digestion

Element Certified Method Four Acid Fusion 150°C AAD Value 3050A Digestion Digestion Digestion (mg/kg) % Recovery % Recovery % Recovery % Recovery

Ag 54 106 63 123 119

Al 4420 68 957 724 122

As 112 94 88 NA 116

Ba 158 97 407 NA 138

Be 99 95 81 97 115

Ca 2500 89 187 219 104

Cd 131 95 78 92 109

Co 60 89 81 106 109

Cr 187 99 86 NA 115

Cu 144 103 90 NA 121

Fe 10400 54 140 121 110

K 2310 94 1113 788 132

Mg 2230 89 94 115 118

Mn 286 88 126 129 107

Na 656 114 1890 1470 120

Ni 106 94 82 NA 118

Pb 208 93 87 NA 111

Sb 56 77 207 NA 182

Se 122 83 84 NA 101

Tl 134 97 89 NA 138

V 204 90 90 104 109

Zn 259 97 97 117 132

Average Recovery All Elements 91 120

Results of some elements analyzed by fusion are not available due to spectral interferences from some of the very high concentration elements such as Si and Li. For those elements that do give valid results, reasonable agreement is seen between the four acid digestion and the lithium metaborate fusion. For elements that are relatively easy to digest, the four acid digestion results are between about 80 and 100% of the certified values. For the fusion method, results are between about 90 and 120% of the certified values. Three elements, Ca, K and Na, that were originally classified as easy to digest, give very high results indicating that there must be significant sources of these elements in the sample matrix that were digested only under very intense conditions.

The difficult to digest elements, Fe and Sb, do not have rigorous digestion values significantly above the values obtained at the highest temperature (150°C) accelerated acid digestion. This shows that most of the Fe and Sb for this particular sample (SRM lot #220) are digested using the accelerated acid digestion method disclosed herein. It can therefore be concluded that the accelerated acid digestion method for SRM lot #220 at 150°C using 75% acid provides a nearly complete to complete digestion for most elements .

Example 5 Accelerated acid digestion (AAD) was carried out at 120 psi and 10 psi at 95°C to see if pressure had any effect on the digestion process other than to keep the aqueous acid from boiling at elevated temperatures . Duplicate samples were digested using one pressure just high enough to move the acid through the sample, e.g. about 10 psi and then compared with those obtained at 120 psi. Results are shown in Table IV.

TABLE IV

Effects of Pressure on AAD Digestion

ment Average % Recovery Average % Rec

(95°C, 120 psi) (95°C, 10 pj

Ag 110.20 107.50

Al 45.40 46.30

As 105.30 104.50

Ba 97.20 98.80

Be 106.00 106.90

Ca 90.10 88.80

Cd 101.10 102.40

Co 99.90 99.80

Cr 101.40 105.20

Cu 110.10 110.80

Fe 39.70 42.30

K 86.20 88.40

Mg 85.60 84.50

Mn 87.80 88.90

Na 93.30 93.30

Ni 113.60 105.60

Pb 99.80 102.10

Sb 111.80 110.30

Se 92.30 95.70

Tl 121.80 121.00

V 88.50 90.20

Zn 105.90 104.60

Average All Elements 95.14 95.36

From the above it can be seen that the results are essentially identical. From this the conclusion can be made that, for SRM lot #220 at least, pressure has a negligible effect on the digestion. This conclusion may not universally hold true. The soil particle surfaces of lot #220 are highly polar and are thus easily wetted by the aqueous acid digesting solution. A more hydrophobic sample would be more difficult to wet. For such a sample, raising the pressure may cause better contact between the acidic aqueous phase and the hydrophobic sample particles .

Example 6 The previous examples have been pressurized using

N₂ to pressurize the digestion column. To determine how a change in gas might affect the digestion, digestions were run at 150°C, using 75% acid at 120 psi using 0₂ and N₂ as pressurizing gases. Results are shown in Table V.

TABLE V

0₂ and N₂ as Pressurizing Gas

Element Average % Recovery Average % Recovery

N2 Gas 02 Gas

Ag 119.0 120.

Al 121.6 92.

As 115.5 118 6

Ba 138.4 151 5

Be 114.7 116.

Ca 104.6 101.

Cd 109.9 115.

Co 108.9 110 5

Cr 114.9 120.5

Cu 121.4 124

Fe 109.7 100 .6

K 132.0 115 2

Mg 118.3 110.5

Mn 106.8 108.3

Na 120.0 117

Ni 117.5 120

Pb 111.4 114

Sb 181.9 177

Se 101.4 105

Tl 137.6 133

V 109.4 107

Zn 131.5 137

Average

All Elements 120.3 119.1

From the above it is apparent that recoveries are almost identical when using either nitrogen or oxygen as the pressurizing gas. It can be concluded that, at least for these two gases, the gas used to pressurize the digestion column does not make any significant difference in the digestion. However, it is conceivable that stronger oxidizing gases may be found that have the potential for participating in and further assisting the acid digestion process. Example 7 For some digestions there are advantages to using more dilute acid concentrations. For example, dilute acids are safer to handle. Also, digestates from dilute acids may be introduced directly to AA or ICP apparatus for analysis without the need for further dilution. However, for more difficult to digest elements, stronger acid concentrations may be necessary. Two sets of samples of SRM lot #220 were digested at lower acid concentrations to illustrate how acid concentration affects the digestion. Previous digestions were made using 75% concentrated acid (50% HN0₃, 25% HC1 and 25% H₂0 by volume) . In this example the same 2 : 1 HN0₃ to HCl ratio was used but the acid was diluted to 15% (10% HN0₃, 5% HCl and 85% H₂0 by volume) and 3% (2% HN0₃, 1% HCl and 97% H₂0 by volume) . Each set was digested at 150°C at 120 psi using 0₂ as the pressurizing gas. The results of the digestion are given in Table VI .

TABLE VI

Acid Strength Variations

Element 3% Acid 15 % Acid 75% Acid

Average Average Average

% Recovery % Recovery % Recovery

Ag 91.5 116.7 119.5

Al 39.3 60.1 107.0

As 100.4 114.1 117.1

Ba 115.5 145.2 144.9

Be 111.1 114.2 115.4

Ca 93.0 94.9 103.0

Cd 108.8 108.8 112.7

Co 105.5 107.5 109.7

Cr 99.1 112.6 117.7

Cu 113.9 121.1 123.1

Fe 18.7 40.1 105.2

K 84.1 104.2 123.6

Mg 81.4 97.0 114.4

Mn 89.6 95.8 107.5

Na 105.0 116.0 118.8

Ni 110.1 117.5 118.9

Pb 106.1 110.7 112.7

Sb 46.1 84.7 179.9

Se 85.4 98.4 103.5

Tl 126.0 129.8 135.3

V 88.4 101.3 108.7

Zn 118.6 130.5 134.5

Average

All Elements 92.6 105.5 119.7

These results are similar to the percent recovery effected by temperature change. This suggests some type of equivalency of temperature and acid concentration for digestion effectiveness. The easy to digest elements, e.g. Ag and Be, show slightly increasing percent recoveries with increasing acid concentration, obtaining adequate recoveries, well within the performance acceptance limits, at all acid concentrations from 3% to 75%. The more difficult to digest elements, e.g. Fe and Sb, showed greatly increasing recoveries in going from low to high acid concentrations .

Example 8 Duplicate digestions were carried out with a natural matrix certified reference material, CRM005- 050 lot No J050. This material contains more fine particles than SRM lot #220 and all contaminants are at levels indigenous to the material . This provides a more rigorous test of the AAD digestion process. Digestions were run at 150°C, using 120 psi 0₂ as the pressuring gas and 75% acid. Results are given in Table VII.

TABLE VII Percent Recoveries of CR 005-050 Lot No. J050

Element Reference 150°C A 150°C B Average

Values LOT #J050 LOT J#050 % Recovery

(mg/kg) (mg/kg) (mg/kg)

Ag 36.3 40.5 36.2 105.6

Al 15333.0 15340.0 14700.0 98.0

As 6.9 5.7 4.4 73.3

Ba 852.9 1068.0 1152.0 130.1

Be 0.6 1.4 0.7 173.8

Ca 119477.0 111400.0 103600.0 90.0

Cd 13.7 14.9 13.3 102.9

Co 6.2 6.0 5.2 90.8

Cr 41.3 48.7 42.0 109.8

Cu 465.4 523.5 479.9 107.8

Fe 12650.0 12530.0 11660.0 95.6

K 6229.0 5821.0 5626.0 91.9

Mg 6706.0 6763.0 6306.0 97.4

Mn 172.4 179.6 167.1 100.6

Na 2488.0 2625.0 2622.0 105.4

Ni 26.0 35.7 29.4 125.2

Pb 89.2 98.1 86.3 103.4

Se 19.9 20.4 20.1 101.9

V 108.7 106.0 96.7 93.2

Zn 625.2 793.1 757.5 124.0

Average % Recovery Al l Elements 106.0

Excellent recoveries were obtained for most analytes ranging between about 90 and 110% of the reference value. The element Be had an anomolously high value, giving it an average value outside the prediction range for this material. However, all other analytes were well within the prediction range and most were within the tighter interval bounds.

From the above examples it may be seen that using the accelerated acid digestion system and process disclosed herein it is possible to obtain good reproducible digestion results for most metals. By adjusting acid concentrations, temperatures, or both, the intensity of the digestion can be adjusted over a wide range. This allows the user of this system to match the various digestion processes now in use. The above examples serve to illustrate one manner of using the invention and set forth the best modes presently contemplated for the system and method claimed. However, the invention is not to be limited to the specific disclosure and examples shown herein but may include any functional equivalents as determined by the scope of the appended claims .

Claims

1. A method for the accelerated acid digestion of metals from an organic, inorganic or combined organic/inorganic solid or semi-solid matrix sample comprising:

(a) placing the sample in a digestion column;

(b) introducing an aqueous acid solution into the column in an amount sufficient to contact said sample and digest metals therefrom;

(c) heating said column containing said sample and acid solution at an elevated temperature for a predetermined time to effect said digestion; and (d) removing said acid solution and digested metals from said digestion column.

2. A method according to claim 1 wherein said digestion column is maintained at a temperature of between about 25 and 200°C.

3. A method according to claim 1 wherein said acid is at a concentration of between about 0.5 and 100% by volume.

4. A method according to claim 3 wherein the volume to weight ratio of acid to sample is between about 1 to 25 mis of acid solution per gram of sample.

5. A method according to claim 4 wherein said acid in said acid solution is a member selected from the group consisting of nitric, hydrochloric, sulfuric, perchloric, phosphoric, acetic, and citric acids and mixtures thereof.

6. A method according to claim 5 wherein said acid solution also contains an oxidizing agent selected from the group consisting of hydrogen peroxide and potassium permanganate.

7. A method according to claim 5 wherein said acid is a mixture of nitric and hydrochloric acids.

8. A method according to claim 5 wherein said digestion column is pressurized at a pressure of between about 10 and 500 psi by a pressurizing gas during the digestion process.

9. A method according to claim 5 wherein steps (b) , (c) and (d) are repeated one or more times.

10. A method according to claim 5 wherein said predetermined period of time to effect digestion is between about 1 and 60 minutes.

11. A method according to claim 5 wherein said metal to be digested is a member selected from the group consisting of Ag, Au, Al , As, Ba, Be, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni , Pb, Sb, Hg, Se, T, V and Zn.

12. A method according to claim 5 wherein said sample is a member selected from the group consisting of sediments, sludges, fly ash, air particulates, soil samples, plant and animal residues, foods, food products and pharmaceutical agents .

13. A method for the accelerated acid digestion of metals from an organic, inorganic or combined organic/inorganic solid or semi-solid matrix sample comprising : (a) placing the sample in a digestion column;

(c) closing said column to the outside environment and pressurizing and heating said column containing said sample and acid solution at an elevated temperature and pressure for a predetermined time to effect said digestion; and

(d) opening the lower end of said column and removing said acid solution and digested metals therefrom.

14. A method according to claim 13 wherein said digestion column is maintained at a temperature of between about 25 and 200 °C by means of convective or resistive heating.

15. A method according to claim 13 wherein said acid is at a concentration of between about 0.5 and 100% by volume.

16. A method according to claim 15 wherein the volume to weight ratio of acid to sample is between about 1 to 25 mis of acid per gram of sample.

17. A method according to claim 16 wherein said acid is a member selected from the group consisting of nitric, hydrochloric, sulfuric, perchloric, phosphoric, acetic, and citric acids and mixtures thereof.

18. A method according to claim 17 wherein said acid solution also contains an oxidizing agent selected from the group consisting of hydrogen peroxide and potassium permanganate.

19. A method according to claim 17 wherein said acid is a mixture of nitric and hydrochloric acid.

20. A method according to claim 17 wherein said digestion column is pressurized at a pressure of between about 10 and 500 psi by a pressurizing gas during the digestion process.

21. A method according to claim 17 wherein steps (b) , (c) and (d) are repeated one or more times.

22. A method according to claim 17 wherein said predetermined period of time to effect digestion is between about 1 and 60 minutes.

23. A method according to claim 17 wherein said metal to be digested is a member selected from the group consisting of Ag, Au, Al, As, Ba, Be, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni , Pb, Sb, Hg, Se, T, V and Zn.

24. A method according to claim 17 wherein said sample is a member selected from the group consisting of sediments, sludges, fly ash, air particulates, soil samples, plant and animal residues, foods, food products and pharmaceutical agents .