Capsule-Based Microwave Digestion

focal point BY GuY LI~G~RE AND ERIC D. SALIN*

DEPARTMENT OF CHEMISTRY l MCGiLL UNIVERSITY,

801 SHERBROOKE STREET WEST, MONTREAL, QUEBEC H3A 2K6, CANADA

Capsule-Based Microwave Digestion

O V E R V I E W OF D I G E S T I O N S Y S T E M S

S amples can be found in solid, liquid, or gaseous form. While one might argue that solids are

the most ubiquitous type of sample analyzed, it would be difficult to de- fend the convenience of handling liquids. Indeed, most of our analytical instrumentation, f rom the electro- chemical through chromatographic to the spectroscopic, requires samples in liquid form. After a decade of effort in our laboratory develop- ing solid-sample introduction methods for inductively coupled plasma emission and mass spectrometry, we have come to appreciate the convenience of liquid samples. As we ap- proached parts per billion detection limits in solid samples using electro- thermal vaporizat ion) we encoun- tered inhomogeneity problems in our solid samples and standards. When tackling difficult " rea l " solid-sample types, 2 we were forced to use methodologies such as standard additions and internal standardization, which are exceedingly cumbersome with solid samples. Indeed, it was our ex-

perience with solid-sample analyses that led us to search for better ways to convert solid samples into a liquid format.

Automated liquid-handling equip- ment can now provide on-line dilu- tions, standard additions, and internal-standards preparation. In the solid domain these techniques still resist automation and probably will continue to do so for the foreseeable future. Therefore, despite the disadvantage of dilution, which unavoid- ably occurs during the digestion of a solid sample, liquid samples and their a t t endant p r epa ra t i on procedures will be with us for many years.

The conversion of a solid sample to a liquid format is still a c lumsy process . The re are a n u m b e r of methodologies available; however, two stand out for comparison pur- poses: fusion and liquid digestion. Fundamentally, these methods are quite similar. Both involve adding the sample to a reagent that will convert the sample to a soluble form. In fusion techniques, the sample and reagent are raised to a high temperature, resulting in a molten-salt form which can provide a variety of desired environments, including oxi-

dizing, reducing, acidic, or basic. The cooled salt solution can then be easily dissolved. This technique is qui te l abor - in t ens ive and suffers from contamination problems if very low level analysis is desired. When one performs liquid digestions, sol- vents with similar chemistries are used; however, the temperatures and pressures are maintained so that the solvent stays primarily in the liquid form. Of the two techniques, liquid digestion is the most common and will be the focus of our further discussion.

L iqu id d iges t ions can be performed at atmospheric pressure in procedures often called "open-beaker" techniques. However, the at- tainable temperatures are often lim- ited by the ambient pressure, one atmosphere, which constrains the boiling-point temperature. Since the rate of reaction is fundamentally related to the temperature, this condition poses a limit on the time required to digest a sample. A faster reaction can be obtained by sealing the sample and solvent in a pressure-resistant bomb and heating the ensemble. While providing a faster digestion time, the bomb introduces an ele-

14A Volume 49, Number 4, 1995

ment of handling hazard because of the potential explosion risk, particularly with powerful oxidizing reagents, and the escape of dangerous fumes . Open beakers , of course , have the same fume problems but do not become pressured, so that they reduce the likelihood of explosion.

Bombs (or at least their interiors) must be made of chemically resistant material and must be able to with- stand high pressure. A typical example would be a screw-top stainless steel bomb with a Teflon ® liner These bombs were placed in hot fur- naces and often took more than twenty minutes to reach their desired t empera tu re . The hea t ing p rocess was, in larger part, accomplished through heat exchange with the atmosphere of the furnace. The heat exchange was inefficient and, consequently, the process was slow. At the end of the heating process, the bomb would be cooled to nearly room temperature and then carefully, manually depressurized by having its top unscrewed.

Over the last ten years, with the development of home-appliance mni- crowave ovens, there has been a rapid development of microwave bomb- digestion systems. The bombs are constructed of material which is microwave-transparent, thereby allowing the available energy to be sup- plied directly to the sample and solvent rather than indirectly through the bomb, then the liner, and finally in the sample and solvent. Micro- wave-heated samples can reach their desired temperatures in one or two minutes, at least an order of magni- tude faster than classical furnace/ bomb systems. The result has been the rapid growth of microwave-digestion technology which, with a few exceptions, consists of a some- what "hardened" and cosmetically altered home appliance and a simple bomb. While these systems offer more rapid heating and, in some cases, temperature monitoring, they are like the classical bomb and require extensive sample handling including: (1) sample preparation, (2) sample weighing, (3) transport to the bomb, (4) sealing of the bomb, (5)

Pressure Temperature Gauc e Transducer

(~) ? Digestion Tube

III / Output I t m'~r~'E~r~r~ I !,np, ut o,vo

~ - J o , n t ~1 ~ _ ~ ~ ~''/ I~ - T.Jo~int~ I . _ L J [ Mi owave [

Sample I Oven I P~istaltic Collector Waveguide Waveguide Pump

Attenuator Attenuator

Pump Tube

Slurry

FI6. I. Original "stopped flow'-tube-based microwave digestion system.

placement of the bomb in the oven, (6) removal of the bomb from the oven, (7) removal of the liquid sample quantitatively, and (8) cleaning of the bomb. Only a few of these steps can reasonably be automated. Traditional microwave bombs are the last evolutionary step in a beaker-digestion paradigm.

A new approach to microwave digestion was first suggested and used for blood samples by Burguera et al. 3 in 1986. Their approach involved pumping the sample through a tube coiled inside a microwave oven. No attempt was made to operate at pressures higher than atmospheric. This arrangement can be called a "f low- ing s t ream" system. These first ex- periments were critical because they demonstrated that microwave energy could be coupled into narrow-bore tubing. In 19884 we first reported a h igh -p res su re tube a r r angemen t , 4 which was later desc r ibed as a "s topped f low" system. 5 The system is illustrated in Fig. 1.

This tube system differs from others in that it uses valves in the sample-transport tubing to contain the sample and solvent within the microwave oven. A pressurized flowing stream system was first described by Haswell and Barclay 6 in 1992 and later commercial ized by CEM Cor- poration. The CEM system uses an HPLC pump to develop higher pressures, narrow-bore tubing, and a re-

stricting orifice at the exit. This configuration allows temperatures great- er than 120°C to be reached.

Both types of tube configurations provide numerous advantages over the traditional bomb arrangement by eliminating several of the steps mentioned above: (4) sealing the bomb, (5) placing the bomb in the oven, (6) removing the bomb from the oven, (7) removing the liquid sample quantitatively, and (8) cleaning the bomb. These labor reductions do come with a price; however, the sample must be slurried for pumping into the system. The effectiveness of the technique depends dramatically on the ability to generate a good, stable slurry. Some samples slurry well; some, such as geo log ica l s , s ink like a stone; while others, like bovine liver, creep up container walls. The sample must remain uniformly slurried during the pumping process or nonho- mogeneity effects will be evident. The alternative, quantitative solid- sample transfer to the digestion tube, would be attractive but is very difficult to implement because of the myriad of physical effects, f rom stat- ic e lec t r ic i ty to cap i l l a ry act ion, which can affect the various transfer mechanisms that one might envision.

A second disadvantage of tube- based systems is that the narrow- bore tubes cannot readily be cleaned. In addition to potential memory effects, many sample types will leave

APPLIED SPECTROSCOPY 15A

focal point

a thin layer of undigested material (e.g., protein from biological samples 5) on the inside of the tube. These layers of undigested material may cause no immediate problem; however their intermediate-term effects are difficult to predict if dissim- ilar sample types that employ different digestion chemistries are run. The long-term effect will be a reduction of the inner-tube diameter, with the obvious potential for clogging.

Narrow-bore tube designs have another characteristic which may be undesirable under certain circumstances. Samples which evolve gases during the digestion process inevi- tably lead to the formation of bubbles in the tubing. These bubbles then effectively segment the sample slurry into sections. I f a section con- tains a small amount of solvent but a relatively large amount of sample (e.g., a large particle), reagent deple- tion can occur with a resulting in- complete digestion for that small segment of the sample. The potential for errors of this type can be mini- mized by using a very fine powder, but this approach then increases the labor involved in the sample-preparation step.

In the beginning of our work with tubing sys tems , 4,5 we were concerned about nonuniform distribu- tion of microwave energy inside the oven cavity. Initial heating tests re- vealed that there were indeed dis- tinctly different energy zones in the oven cavity. When a digestion tube was placed in the system and loaded with sample, bubbles formed, allowing the motion of the solution to be monitored. We observed that the formation of bubbles and nonuniform heating cause continual motion of the sample solution in the oven chamber. This motion ensures that the various segments of solvent and sample receive approximately the same amount of energy and reach a similar temperature.

After the development of our first prototype, 4,5 we became concerned about several facets of the design. We were uncomfortable with the need for a slurried sample because of the reasons mentioned above, as well

as the additional labor involved with grinding the sample finely. Our most serious concern, however, was the physical longevity and robustness of the system. A microwave digestion system is obviously going to be most valuable when it is used to digest samples that take long periods of time to process at room temperature. Many of these samples (e.g., geolog- icals, ceramics) are physically very hard. As they (or any sample) are pumped through the system, they must go through a valve of some type. Trapping of small amounts of material in the valves is almost impossible to avoid. This trapped material will then lead to two problems. Immediately, one will have memory effects. Over the longer term, one will have degradation of the valve. The exposed portion of the valves must be made of a material which is impervious to hot acids, and these materials are all relatively soft. In a laboratory handling a variety of samples, one would expect that valves might degrade quite quickly. Our concerns about the longevity of the valves and the labor involved with slurry-sample preparation eventually led us to a radically different design.

CAPSULE-BASED MICROWAVE DIGESTION: A NEW DIRECTION

The Capsule Concept: An Over- view. The fundamental change in approach came with the recognition that powders are, in themselves, a difficult type of sample to process. The slurry-generation step is just one of many trials in handling a powdered sample. However, if a sample can be handled in an encapsulated form until it is in the digestion solvent, one can prevent degradation of and losses in valves and losses in handling. Once in the solvent, the sample carrier (the capsule) is bro- ken down and releases the powdered sample into the solvent, where it is attacked by the reagents. As long as the sample material has sufficient homogenei ty when it is placed in a capsule, which may require some

grinding, it is far better to handle the sample in an encapsulated than in a powder form.

Figure 2 presents a symbolic comparison of the steps involved with the three possible approaches for high-temperature microwave digestion: c a p s u l e - b a s e d (left) , s lur ry- based (middle) , and t radi t ional bomb-based (right). Each step in the digestion process which can easily be automated is symbolized by a robot arm, and each step that is nor- mally done manually is symbolized by the human figure. Note that the time involved with the steps, automated or manual, is not quantified in this figure. We will compare the systems after describing how the capsule-based system works. A number of the advantages will emerge during the course of the discussion.

The ope ra t ion of the capsu le - based microwave digestion system proceeds in several steps, as illustrated in Fig. 3. First (Fig. 3A), the encapsulated sample is pushed into the digestion tube by a " squeegee" , which is a flexible plastic rod with a soft Teflon ® (or other inert material) plug on the end. Solvent, usually an acid, is then pumped into the digestion tube so that the capsule is im- mersed in the digestion solvent (Fig. 3B). Valves at either end of the tube are then closed, sealing the system, and microwave energy is applied (Fig. 3C). From this point on, temperature must be monitored to ensure that the sample is maintained at the digestion temperature for the specified length of time. Pressure is also m o n i t o r e d to ensure that des ign spec i f ica t ions are not exceeded . When the sample/solvent mixture has completed its digestion sequence or an overpressure is reached, the sample is cooled and vented. The digestion mixture is cooled by a room- temperature water flow through a tube wrapped around the digestion tube (Fig. 3D). When the mixture has cooled, the system is vented through a computer-controlled valve system and the cooling-water tube is blown free of water. I f the sample digestion has run the specified time at the required temperature, the large


Sample

Encapsulate

Weigh

Digest ((Q ~ i i ; i i ~ ! i i ~ ! ! i i i i i i ~ i ; ! i i i i ¸ i i i i ! v ¸ i~i~i i~i i i~ i~i l l

I _

Sample

Grind

Transfer and Weigh

Digest

/ =nsfer Transfer id Wash and Wash

~ 1 ~ Sample Weigh

~1~ 'x'~ Digest

I I \ ons or " ~ / / / and Wash

FIG. 2. Symbolic depiction of steps involved with three microwave digestion schemes. The columns represent, from left to right, capsule- based microwave digestion, tube-based microwave digestion, and traditional bomb-based microwave digestion. Each human figure symbolizes a significant labor-consuming step. Each robot arm figure symbolizes an easily automatable step.

valves at the ends of the tubes are opened and the sample/solvent mixture, presumably digested, is pushed out of the digestion tube by the squeegee system (Fig. 3E). The end of the squeegee is manufactured to the diameter of the tube and forms a gas-tight seal. If the mixture has not been heated at the required temperature as long as specified, then gases are vented after the tube is cooled, and microwave energy is reapplied to achieve the specified heating time.

A Closer Look. The Capsule. The capsule could be made in a variety of ways, but our approach has been to use a traditional pharmaceutical- type design; however, the materials used for medicinal applications are completely inappropriate for elemen- tal analysis because of their high inorganic content, so we have selected polyacrylamide as our capsule ma-

terial. This material is used for elec- trophoresis and is available in very clean form. After digestion of a capsule there are no detectable levels of any inorganic material with the use of ICP-MS. Other encapsulating materials, such as nylon, might be used, and it would also be possible to use binders and to press samples into tablet form. The capsule we use is similar in design to the standard medical 00 size with a 2 -mL volume. We have l eng thened the capsu le body so that the dimensions are now 9 m m by 35 mm. This size allows over a gram of sample, depending on its density, to be placed inside. The p o l y a c r y l a m i d e capsu le we ighs roughly 70 rag.

Encapsulation of the sample provides enormous advantages for the busy laboratory. These advantages fall into two categories, marking and

handling. Capsules can be both col- o r - coded and bar -coded , m a k i n g them easy to identify for both humans and automated systems. One can imagine any number of possibil- ities for the bar coding, including having special instructions encoded with ink-jet or laser writers. The main advantages, however, lie in the domain of handling. Not only are capsules trivial for humans to handle (they can be washed if necessary), but they are ideal for automated handling. Robots can easily pick up capsules because the capsules are sufficiently robust to resist crushing. The pharmaceutical industry has already developed extensive machinery for capsule manufacture and movement . A controlled manufacturing process should produce capsules with sub- mill igram weight deviations, making weighing (taring) of the capsule un-


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m INSERT CAPSULE m ADD REAGENTS

Ig APPLY ENERGY COOL AND VENT

m REMOVE SOLUTION

FIe. 3. Steps in capsule-based microwave digestion.(A) The system with capsule pushed in by squeegee.(B) The system after reagent addition.(C) The arrangement when the flange valves are closed and the microwave energy applied.(D) The arrangement when the tube is cooled and then vented simultaneously on both sides to reduce pressure.(E) The arrangement when the squeegee is used to remove the liquid digest from the tube.

A. Flange ValveOpen

I

B. Flange Valve Closed

C. Open Flange Assembly with Capsule and Squeegee

D. Flange Assembly Sealed for Pressurization

FIG. 4. Flange valve.(A and B) Simple flange valve in open and closed position.(C) Actual flange assembly used in open configuration illustrating insertion of capsule past flange valve by squeegee.(D) Sealed flange valve.

necessary. Once the sample is prepared so that sufficient homogenei ty is achieved, robots or automated machinery could handle sample load- ing, weighing, and transport to the digestion system.

Capsules also offer other potential for labor reduction. Internal standards could be impregnated into the capsule body. Standard additions, matrix matching, buffering, and other methodologies can be implemented by adding a second, previously prepared "me thod" capsule to be digested along with the sample capsule. Under certain circumstances, particularly with liquid samples, the sample can be collected on site and directly loaded into the capsule, further reducing labor and its attendant hazards of contamination and error.

Valves. The search for a new valv-

F

Ve

Pump

live

Pressure Senso

Flange Valve

Microwave

FIG. 5. Complete capsule-based microwave digestion system. The pneumatic actuator is used to rapidly change the position of the vent valve during the venting process. Venting is implemented simultaneously from both sides. Pressure is monitored continuously by the in-line pressure system.

ing approach was triggered by the problems expected with convention- al valves in narrow-bore tube-based stopped-flow systems. The present design of the capsule system uses two types of valves. Large flange valves are used as the main valves. The flange valve, illustrated in Fig. 4, has two aspects which make it particularly attractive. The first advantage lies in an unencumbered en- trance to the tube that allows a capsule to be inserted without any ob- struction (Fig. 4C). The ball valve is the only common type of valve which allows similar access. The second important feature, which is not available in any other valve, is the access provided to the surfaces when the valve is opened. When opened, the flange valve provides c o m p l e t e access to all su r faces , thereby allowing them to be washed free of any particles or liquids which might cause valve degradation or memory effects.

The re is a c o m p u t e r - c o n t r o l l e d secondary valve system, as illustrated in Fig. 5. This system uses a very

fast, electronically controlled, pneumatically actuated valve (for speed and power). This system utilizes small-diameter tubing and can be opened and closed in approximately 10 ms, allowing the system to be depressurized in small increments. The reasons are discussed below in the sect ion conce rn ing the d iges t ion tube. The secondary valve system is also used for introduction of liquids both during the initialization of the system and later if necessary.

Squeegee. In addition to introduc- ing the capsule and removing the digested mixture, the squeegee provides one other important function: cleaning. The edges of the squeegee provide a physical cleaning action which sweeps out particles and strips out any remaining liquid. In bomb systems, this action must be carried out manually by an operator, where- as it is not implemented at all in narrow-bore systems.

The Digestion Tube. The tube is " U " shaped with the bottom of the tube lower than the top, as illustrated in Fig. 3A-3E. In essence, it is a


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tube-shaped cleanable bomb. At this time, the tube is made of 0.5-in. ( 2 7 . 7 - m m ) - o u t e r - d i a m e t e r , a/8-i n. ( 9 . 5 - m m ) - i n n e r - d i a m e t e r Teflon ® PFA (perfluoroalkoxy) tubing with a working temperature of 200°C. PFA tubing has almost ideal chemical resistance for the various acids used in digestion work. One could wish for a higher resistance to pressure; how- evel; the material has been excellent for the prototype capsule system. We have experimented with glass; the interface between the glass tube and metal valves, however, proved to be a weak point.

The tube is sufficiently wide to ensure that its performance is quite different from that of a narrow-bore tube. Perhaps the most interesting as- pect is the ability to vent the sample/ solvent mixture while it is pressurized. This procedure is impossible in narrow-bore tubing because the bubbles will s imply push the liquid out the venting orifice. In the wide-bore tube, the solution actually boils. I f the venting is done gradually, as allowed by the fast secondary valve system, boiling is controlled and the solution can be brought gradually down to atmospheric pressure. While venting affords protection against overpressure, it also allows the addition of more reagent or a new reagent as part of a digestion sequence. This flexibility suggests that one might return to many of the powerful chemical digestion systems used in the p a s t - - f o r example, nitric acid followed by peroxide.

System Control. There are a number of parameters which must be monitored for both analytical and safety considerations, particularly digestion-tube temperature and pressure. Digestion tube temperature is monitored by means of an infrared sensor located underneath the digestion tube. Thermocouples were less satisfactory for this task because of a tendency to either heat or arc in the microwave field when placed against the outside of the tube. Thermocou- ples do ope ra te p rope r ly when placed inside the tube in a solution; however, inside placement blocks capsule insertion. Magnetron tern-

perature is a monitor of reflected power, while cooling-water temperature (monitored on the outlet side) is an indicator of the cooling process. Both are monitored by thermocouples. Pressure monitoring is critical to avoid overpressurizing of the system. Pressure is monitored with a Teflon®-covered sensor placed in line with the vent tubing. The digestion system can be controlled by a var ie ty o f m e c h a n i s m s , all o f which are under computer control. These include (1) microwave power, (2) gas release through the secondary valve system, and (3) flow of cooling water. All these capabilities are unde r digital control. Tempera- tures and pressure are monitored by a 12-bit analog-to=digital converter.

Microwave power is controlled in a traditional " cook ing" microwave arrangement by cycling the main power tube on and off at full power. To create an arrangement like those of commercia l microwave bomb systems, we have modified the power electronics so that we can run a shorter duty cycle (on-of t ) than is allowed by kitchen microwave ovens.

L a b o r . Capsule-based microwave digestion was developed to minimize labor. Figure 2 illustrates the steps that are involved in capsule-based, slurry-based, and bomb-based tech- nologies. All offer rapid heating, so that there is no fundamental " t ime" performance advantage to any of the techniques. The first column illustrates a capsule-based digestion sequence. Human labor is involved only at the first step, which involves placing the sample in a capsule. One could imagine this step being done in the field (homogeneity allowing) or being done by a robot. However, p o w d e r - h a n d l i n g t e c h n o l o g y does not yet seem to be up to dealing with the wide variety of samples that must be analyzed by many laboratories. If grinding is involved, it will be only that amount required to achieve homogeneity.

I f we consider the eight s t e p s - - ( l ) preparing the sample, (2) weighing the sample, (3) transporting the sample to the bomb, (4) sealing the

bomb, (5) placing the bomb in the oven, (6) removing the bomb from the oven, (7) removing the liquid sample quantitatively, and (8) cleaning the bomb- -capsu le -based microwave digestion requires labor only at step 1. Slurry-based microwave digestion requires finer grinding and then transfer of the ground material to a weighing vessel. In Fig. 2, we have illustrated this procedure as two labor steps. Traditional microwave bomb-based digestion (Fig. 2, right) does not require extensive handling (usual ly on ly mi ld gr inding) , al- though, like all the others, it may require drying or other similar steps. Consequently, we have lumped the sample preparation (step 1) and the weighing (step 2) into a single labor step. Labor becomes a larger factor when we consider transferring the weighed sample to the bomb, sealing the bomb, and transferring it to the microwave (steps 3, 4, and 5). These steps are not easily automated. Sim- ilarly, removal of the bomb, quantitative transfer of the digest, and cleaning of the bomb are labor-con- sumptive. We have separated the last sequence into two significant labor steps, which may require a cooling interval and depressurization (step 6) and which can usually be performed in a single step of moderate length (steps 7 and 8).

Obse rva t ions . Since the digestion tube is made of semi-transparent PFA, it is possible to observe the system operation visually in ways that would not be possible with an opaque material. The digestion process is quite interesting to watch. As microwave power is applied, the capsule pops open well before the capsule body is digested. This re- sponse is probably due to heating of the gases inside the capsule. The rate of heating is related to the ionic strength of the solvent. Water heats very slowly in comparison to the rate for an acid solution. As the capsule pops open, the solid sample is dis- tributed throughout the solvent, with some sample at the solution surface and a small amount deposited slight- ly above the solution surface. These particles are deposited by the bub-


bles that arise as the solvent boils. One might be concerned about di- gesting these particles, and that concern leads to another observation about the system.

One must appreciate that heating in this system, as in all microwave bomb systems, is from the solution outward. The valves do not rise ap- preciably above room temperature, and the upper portions of the tube are just warm to the touch. This set- up provides an enormous benefit with respect to the engineering of high-pressure fittings. It also means that the system is not at thermal equilibrium, so that pressure cannot be used as an indirect measure of temperature. This approach has the considerable advantage that vapor- ized solvent is continually being condensed on the upper portions of the tube wall and washed down over any particles deposited on tube wall surfaces. Unlike in the narrow-bore tube systems that we have observed, the entire solut ion is cons tant ly mixed by boiling. Boiling under con- stant pressure is not a problem; how- eves; venting, when necessary, must be done with some care to avoid sample loss. This loss can be pre- vented by first cooling and then using small, fast pressure drops to avoid having the solution boil up through the secondary vent valves as pressure drops. The pneumatically actuated valve system allows this venting to take place without diffi- culty. Furthermore, it offers the secondary benefit of acting as a computer-controlled emergency pressure relief in the event of a massive overpressure which could not wait for a cooling cycle. In this case, the vent system would have' to be cleaned by flushing with water.

The entire system is controlled by an interpretive language called Mi- cro 2, which was developed in our laboratory. Typical exper imenta l data are presented in Fig. 6. Note that four different parameters are monitored: pressure, digestion-tube temperature, magnetron temperature, and cooling-water temperature. At time zero, microwave power is applied and we observe a steady in-

.=

Q O-

E

200

1 O0

Bovine Liver Digested in 10 ml HNO 3

0 ~

0 500 1000 Time (sec)

Percent Power On

1500 2000

Tube Temperature Tube Pressure Magnetron Temperature Cooling Temperature Power On/Of f

FIe. 6. Experimental data from a bovine liver NIST 1577 sample. The blue trace is the temperature as monitored by the infrared sensor. The red trace is the pressure as monitored by the in-line sensor. The green trace is the magnetron temperature as monitored by a thermocouple. The brown trace is the temperature of the cooling water measured with a thermocouple. The black trace is a measure of the power applied. See the text for a detailed discussion.

crease in temperature as the software controls microwave energy to produce a predefined temperature ramp. The black trace at the bottom is a histogram representation of the times during which microwave energy was applied. During the first 200 s we observe a linear temperature rise, but a marked pressure increase occurs after about 100 s as the bovine liver begins to digest and starts releasing carbon dioxide. The increase is quite rapid, and at about 250 s the software notes that a pressure threshold has been reached. At this time all microwave power is cut off and the cooling water flow is started. We observe a drop in both tube pressure and temperature while the cooling- water temperature rises and drops as the solution is cooled. At about 500 s the vent threshold is reached (70°C), and the system is vented over a short time to atmospheric

pressure. The heating process then begins again; however, the starting temperature is higher and the system now plateaus at our designated max- imum temperature of 180°C. Once again the pressure rises, but much more slowly, and the cycle contin- ues. There are additional subtle fea- tures of the trace in Fig. 6. First, each time the sample is cooled, it drops to a lower and lower pressure, suggesting that there is less carbon dioxide in the tube atmosphere. Oth- er gaseous components, like water vapor, condense out on cooling.

T H O U G H T S ON T H E F U T U R E

Capsule-based microwave digestion is very attractive because of the labor reduction and additional safety provided by unattended operation; however, the system's performance is dictated by the same chemical and


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phys ica l laws which gove rn o ther m i c r o w a v e sys tems . D iges t ions in this sys t em take ju s t as long as they w o u l d with any o ther sy s t em oper- a t ing at the same tempera tu re . Be t te r m a t e r i a l s e x i s t a n d s h o u l d a l l o w h igher t empera tu re s and consequen t - ly even fas ter d iges t ion t imes . Stil l , the sys tem, as conf igured , is a ser ial a r rangement . Our o b s e r v a t i o n s o f " o n " vs. " o f f " t ime o f the m a g n e - t ron d e m o n s t r a t e qui te c lea r ly that we have far more p o w e r than is re- qu i red for a s ing le - tube a r rangement . Our e s t ima te is that a s ingle 7 0 0 - W a p p l i a n c e - g r a d e magne t ron can heat at leas t five tubes s imul t aneous ly . I f such a mu l t i p l e - t ube a r r a n g e m e n t is a d o p t e d , t hen o n e m u s t c o n s i d e r whe the r the sys tem shou ld be de- s igned to run in a ba tch mode , wi th all s amples o f a p p r o x i m a t e l y the

same type, or whe the r the sys tem should run a synchronous ly . In o rde r to apprec i a t e the poss ib i l i t i e s , con- s ider an a p p r o a c h in which ind iv id- ual tubes are in r a c k - m o u n t e d mod- ules, each with its o w n smal l magne t ron and sensors . These modu le s cou ld be s e r v i c e d by a s ingle rai l- m o u n t e d robot . Such an a r r angemen t appears to be ve ry cos t -e f fec t ive s ince d iges t ion t imes are long in c o m p a r i s o n to s amp le - in t roduc t i on or r emova l t imes.

Wi th m o d e r n ins t rumenta t ion (for example , I C P - A E S ) , d i s so lved sam- p les can of ten be run in less than a m i n u t e . S a m p l e p r e p a r a t i o n o f t e n c o n s u m e s an o r d e r - o f - m a g n i t u d e more t ime in the fo rm of l abor and may invo lve f rom hours to days o f wai t ing . C a p s u l e - b a s e d m i c r o w a v e d iges t ion m a y shif t ana lys i s into a

new pa rad igm, wh ich wil l resul t in s igni f icant l abo r r educ t ion for many s a m p l e types whi le pu t t ing more c h e m i s t r y and f lex ib i l i ty into the an- a lys t s ' hands.

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Capsule-Based Microwave Digestion

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