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Anal. Chem. 1900, 60,742-746
Comparison of Microwave Drying and Conventional Drying Techniques for Reference Materials E. S. Beary Center for Analytical Chemistry, National Bureau of Standards, Gaithersburg, Maryland 20899
A microwave drylng oven was evaluated for potentlal use as an alternate method of sample pretreatment In an analytical laboratory. The hlghgredskn drylng data achleved suggests that this Instrument Is well-sUned to lndwtrlal appllcatlons such as quallty control and merlts further study. However, differences In conventlonal drylng and mlcrowave drylng make the current lmplementatlon of the microwave drylng oven unfeasible In a reference laboratory, especially when hlghly accurate measurements are requlred on a wide range of materlals.
One of the prerequisites for the accurate analysis of reference materials is the assurance that the analyzed sample is the same in all laboratories under differing environmental conditions. Thus, the analytical laboratories a t NBS are particularly interested in precise and reproducible methods of sample drying. Variation in the amount of absorbed water is one of the most common sources of nonreproducible sample weights. Biological and botanical samples as well as minerals and clays are particularly sensitive to changes in atmospheric conditions, which can translate into variable starting weights. It cannot be assumed that samples of the same material from different containers have equivalent amounts of adsorbed (or occluded) water. Drying, defined as the removal of water and other volatiles from the sample, must yield the same composition regardless of the starting moisture content and sample history. At NBS, the certification process of Standard Reference Materials (SRMs) begins with an extensive evaluation of the material for homogeneity and stability. Part of this evaluation includes establishing the best means of achieving a reproducible starting or equilibrium weight. Equilibrium weight is defined as the plateau of the drying curve when weight loss is plotted versus time. Variable equilibrium weights would result in imprecise and possibly biased analytical data. A drying study is a critical process for a reference laboratory which must be assured of the reproducibility of assays as well as trace element determinations. The traditional methods of sample drying are conventional ovens, vacuum ovens, and desiccators a t fixed humidity. The use of these devices for sample pretreatment results in easily reproduced methods and reproducible sample composition. Often, more than one drying method will yield identical results. For some materials, such as clays, a constant but different weight can be achieved when the oven temperature is changed, indicating that there are different temperaturedependent volatile components or degrees of hydration present in the sample. The recommended pretreatment is often the method(s) that is (are) least aggressive and yields a sample of stable composition that can be easily reproduced. Drying methods that chemically alter the sample must be avoided. Microwave ovens as well as several styles of infrared drying instruments are now commercially available for the purpose of rapid sample drying. The rapid drying instruments have gained in popularity in processing plants that monitor sample moisture as a measure of quality control. Infrared drying is based upon the same principle as conventional drying, namely
the temperature dependence of the vapor pressure of volatile components. Microwave drying provides an alternative to traditional drying methods. Theoretically, only polar molecules are affected by microwave radiation. Therefore, any polar molecule in a given matrix would be affected, including water. The magnetron produces a pulsing electromagnetic field at 2450 MHz in which the polar molecules in the sample first align with the field, and when the field direction changes, the molecules are forced to realign. This alignment/realignment results in the rapid oscillation of the molecule and eventual volatilization due to the heat of friction. Bound polar molecules are more difficult to volatilize because the fixed bond inhibits rotation ( I ) . For microwave drying, the theoretical mechanism does not address phenomena such as conductive heating and molecular disturbances of molecules surrounding polar molecules due to the high and concentrated energy introduced. In this work a microwave drying oven was evaluated as an alternate method of sample pretreatment in an analytical laboratory. Comparative measurements were made between microwave drylng and conventional drying techniques. In this regard, the study is part of an on-going evaluation of all aspects of sample preparation processes. SRMs representing diverse matrix types, ores, clays, and biological materials, were surveyed.
EXPERIMENTAL SECTION Apparatus. The microwave oven used in this study was a CEM AVC-80 drying oven. It is equipped with a special top loading electronic balance that is mounted with only the balance stem and pan protruding through the oven cavity floor. At the most sensitive setting this balance has an 8-g capacity with resolution to 0.1 mg. It is shielded from air currents with a cover that is transparent to microwave radiation. One hundred percent power is equal to about 600 W of microwave energy at a frequency of 2450 MHz. Since the wattage at 100% power varies between ovens, each oven can be calibrated by measuring the temperature change ("C) in 1 L of water (2). The duration of the duty cycle in this oven is 1s. The duty cycle is the percentage of time that the oven operates at full power. For example, at an 80% power setting, the duty cycle is 80% and the oven is on for 0.8 s and off for 0.2 s. The interaction of the control of the microwave power and the high resolution of the balance output can produce high-precision results. Two types of pads are used in this microwave drying system. One is a fiberglass pad that is not moisture sensitive and maintains a constant weight during the drying process. The second type of pad is used in addition to the fiberglass pad when even heating of the sample at a given temperature is desired. These 3 mm thick circular pads, Thermapads, are made of a proprietary material and are rated at different temperatures by the manufacturer. The pad absorbs the microwave radiation and becomes thermally hot, and the sample is primarily heated by conduction. The conventional oven used was a traditional air convection oven, and the temperature could be read with a mercury thermometer and controlled to f O . l "C. The vacuum oven was operated at room temperature at a vacuum pressure of about 30 Pa (0.2 Torr). For the study of effects of laboratory environmental variation, some of the dried samples were rehydrated in a desiccator at a fixed humidity of 95%. A more rapid rehydration/equilibration
This article not subject to US. Copyright. Published 1988 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 8, APRIL 15, 1988
743
was achieved by use of a modified, controlled temperature and humidity chamber which is commercially available (3). This rapid equilibration apparatus exposes the sample to dynamic changes and weight differences can be read in relative humidity (RH), directly from a digital balance. Procedure. Samples weighing 2 or 3 g each, of both the inorganic and organic SRMs, were studied. For microwave drying, the material was spread in a thin layer on a fiberglass pad so that a large and uniform sample area was exposed to the microwave radiation. For some materials a thin sample layer was placed between two fiberglass pads to promote the drying process. A Thermapad was used as a heat source to volatilize the water of hydration from certain salts. To serve as a heat source, the pad must be preconditioned. This was accomplished by exposing the Thermapad to one predetermined drying cycle in the microwave oven. The pad absorbs the microwave energy and reaches a given temperature; theoretically, to the temperature rated by the manufacturer. The sample was then placed onto the preheated pad and the drying cycle was repeated. Under these conditions, the microwave oven is essentially operating as a thermal oven in that the sample is thermally heated. The specific parameters for drying each material were established,which included microwave energy in percent power and duration of drying time in seconds. This microwave oven permits us to dry a sample to a constant weight at a given power and aids in establishing the drying time. In general, the highest power and shortest drying time that did not cause sample degradation resulted in the most efficient drying.
Table I. Percent Weight Loss in Milk Powder and Coal SRMs
RESULTS AND DISCUSSION The following drying experiments were done in this study: the microwave oven was compared with the vacuum oven for drying nonfat milk powder and coal; the microwave oven was compared with the conventional oven to dry clays; the microwave oven using conductive heating was compared with a conventional oven to dry copper ore. In addition to these direct comparative studies, experiments were also done in which samples were rehydrated and redried under varying conditions to determine if initial equilibrium weights could be achieved. Microwave Oven versus Vacuum Oven. Non-Fat Milk Powder, SRM 1549, and Coal (Bituminous), SRM 163213,were studied to compare microwave oven drying to vacuum oven drying. Data for desiccator drying of SRM 1549 were also obtained. Vacuum oven drying is often used for sample pretreatment when heating is undesirable. Some materials, such as botanicals and biologicals, are not normally recommended for drying in a conventional oven because of the possibility of thermal degradation of the sample and loss of volatile elements. The vacuum oven is usually recommended in these instances since it is a less aggressive procedure and can remove the moisture at room temperature. For the purposes of identifying a valid sample pretreatment procedure, it may be assumed that an equilibrium weight is achieved when replicates of the same sample yield the same percent loss on drying with good precision. The chance of obtaining good precision on a part of the drying curve other than the plateau is remote. Analysis of the dried sample verifies the stable composition. The results from this drying study are summaried in Table I. The nonfat milk powder lost about 2.0% in the vacuum oven and in the desiccator. The standard deviation for both methods is typical and generally acceptable. The losses on microwave drying, however, were about 40% greater relative to the other methods and the standard deviation was also greater, indicating that varying amounts of occluded water (and perhaps other volatiles) are lost in the microwave drying. This SRM was the only material tested that did not reach constant weight in the microwave oven. The testing was stopped when weight gain, due to sample degradation (burning), was noted. The standard deviation for SRM 1632b is more representative of the precision that can be obtained in microwave
drying. With most sample types this precision indicates that an equilibrium weight was reached. However, the average weight loss (1.03%) was lower than the average weight loss (1.26%) resulting from vacuum drying. Microwave Oven versus Conventional Oven. In Table 11, the data from the microwave drying of certain clay SRMs are compared with those for thermal drying. The precision achieved for replicate samples demonstrates that an equilibrium weight or end point was reached in each case. These equilibrium weights differ from each other in all cases except for the Dolomitic Limestone. Within experimental error, the end points for the limestone samples appear to be the same with perhaps a slightly higher loss of volatiles a t the higher oven temperature. Any of the methods described in this section satisfy the goal of reaching a reproducible weight. All of these samples are inorganic and therefore should represent a nearly ideal case for comparing these methods in terms of accuracy and precision since no volatile component other than water is known to be present. Microwave Drying a n d Rehydration versus Thermal Drying. Two conventional techniques were found to yield equivalent losses on drying Wheat Flour (SRM 1567a) and Rice Flour (SRM 1568a) samples. These were vacuum oven drying for 24 h at room temperature and conventional oven drying at 85 O C for 24 h. These techniques are the recommended methods of pretreatment of these SRMs. The conventional oven was chosen as the comparative technique for these flour samples. The drying procedures investigated for flour SRMs involved both microwave and conventional oven drying followed by rehydration a t 95% RH overnight and redrying using both oven types. In traditional drying experiments samples are rehydrated after initial drying to test the drying procedure. Only those procedures that produce a sample of stable and constant weight after rehydration and redrying are recommended for the pretreatment of SRMs. The microwave drying conditions necessary to obtain an equilibrium weight were established. These conditions, reduced power (20%) and extended drying time (20 min), were necessary to prevent sample charring. The results of this work are presented in Table 111. There was some variability in the weight lost in the initial microwave drying of both flours. Microwave drying alone yielded results lower than conventional drying by 1-3% absolute. Further drying of the microwave-dried samples in the conventional oven a t 85 O C for
vacuum ovenD
desiccator*
microwave ovenC
milk powder, SRM 1549
2.00 1.92 2.01
2.00 1.99 2.05
2.91 2.63 3.02
av
1.98 0.05 2.53
2.01 0.03 1.49
2.85 0.20 7.02
S
% RSD
coal, SRM 1632b
1.27 1.26 1.22
0.98 1.08 1.04
1.24 1.28 1.28
av S
% RSD
1.26 0.02 1.59
1.03 0.07 6.80
oVacuum oven: milk powder, 48 h, room temperature, 30 Pa; coal, 24 h, room temperature, 30 Pa. *Desiccator: milk powder, 40 h over Mg(C10&. CMicrowaveoven: milk powder, 8.00 min, 100% Dower. 1 Dad; coal, 8.00 min. 100% Dower, 2 Dads.
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 8, APRIL 15, 1988
Table 11. Percent Weight Loss in Clays and Limestone conventional ovena condA condB Brick clay SRM 679 av S
% RSD
2.74 2.76 2.77 2.76 0.02 0.72
av S
%
RSD
av S
%
RSD
0.72 0.70 0.68 0.70 0.02 2.86
Plastic Clay SRM 98b av S
% RSD
1.40 1.32 1.32 1.35 0.05 3.70
microwave oven cond Z 0.43 0.43 0.43
microwave oven cond Z
1.46 1.44 1.49 1.46 0.03 2.05
0.86 0.84 0.85 0.85 0.01 1.18
"Conventional oven: Condition A, 105 "C for 2 h; condition B, 140 "C for 2 h; condition C, 110 "C for 2 h. *Microwave Oven: condition X, 100% power for 2.00 min, two pads; condition Y, 100% power for 5.00 min, two pads; conditions Z, 90% power for 5.00 min, one Dad. Table 111. Percent Weight Loss in Flour SRMs sample
drying method A" Bb total
rice 1
8.6
2 3
7.6
wheat 1 2 3 4
7.9 8.3
1.2 9.3 2.6
9.8 9.3 10.2
2.0 9.8 1.8 9.4
9.9 9.8 10.1 9.4
Rice FlourV
Wheat Flour
-5'
tt
Time Figure 1. Percent weight change in flour samples: a, microwave drying at 20% power for 20 min; b, oven drying at 85 "C for 24 h; c, 9 5 % RH for 16 h.
0.16 0.12 0.17 0.15 0.03 20.0
0.88 0.77 0.85 0.83 0.06 7.23
conventional oven cond C condB
1
microwave oven cond Y
0.17 0.18 0.16 0.17 0.01 5.88
0.13 0.12 0.12 0.12 0.01 8.33
conventional oven cond C condB Flint Clay SRM 97b
2.20 2.22 2.21 2.21 0.01 0.45
3.12 3.10 3.16 3.13 0.03 0.96
conventional oven cond C condB Dolomitic Limestone SRM 88b
microwave ovenb cond X
Cc
drying method A" Bb total
-9.5
7.7
1.2
8.9
-13.8
12.4
1.0
13.4
-7.1
5.5
1.7
7.2
-1.0
"A, microwave oven: 20% power, 20 min. b B , conventional oven: 85 "C, 24 h. 'C, rehvdrated overnight at 95% RH.
24 h resulted in a total percent weight loss that was reasonably constant at about 10%. These findings are in agreement with a study by Davis and Lai ( 4 ) on the microwave drying of various flours. They observed that samples dried at reduced power (30%) reached an equilibrium weight which was also 1-3% (absolute) lower than conventional oven drying. The flour samples were then rehydrated overnight at 95% RH. Rice flour 1and wheat flour 4 gained back less than the total initial weight loss. This kind of weight change is typical of cyclic dehydration/hydration of botanical samples (3). However, rice flour 3 gained back more weight (13.8%) when exposed to high humidity than lost from the original sample
(10.2%). This behavior is atypical and may indicate some thermal degradation leading to the loss of occluded water or volatile components caused by the initial drying procedure. Despite the large difference in reabsorbed moisture (9.5% for rice flour 1and 13.8% for rice flour 3), both rice flour samples lost 90% of the reabsorbed water gained when dried in the microwave oven. The remaining 10% was lost when the samples were subsequently dried in the conventional oven. Figure 1 depicts the changes in weight (percent) during the processing of two flour samples. It demonstrates that in spite of the differences in intermediate weight losses after rehydration, the samples did return to a constant dried weight. Studiea involving conventional oven drying of flours show that all reabsorbed water is lost in conventional drying. Since all the reabsorbed moisture is "free" water, it is difficult to explain why only 90% is volatilized in microwave drying. A possible explanation is that while all the water molecules are affected by the microwave radiation, the relatively short treatment time does not permit complete diffusion under the static air conditions. In addition, a fiberglass cover pad used under these circumstances could aggravate this problem by trapping the water vapors.
Accelerated Environmental Apparatus and Microwave Drying versus Conventional Drying. Buffalo River Sediment, SRM 2704, is a material currently undergoing stability studies and certification processing. Rehydration/ drying experiments were done with this inorganic matrix material comparable to the above experiments done on the organic matrix flours. Replicate samples of the river sediment lost an average of 0.63% in microwave drying and an average of 0.80% in oven drying with precisions of 0.01% absolute for both techniques. This precision suggests that an equilibrium weight was reached in each case. However, the microwave-dried samples lost less moisture by 0.17% absolute (-20% relative), which was surprising based on the results of the clay/limestone experiment. Subsequent oven drying a t 110 "C for 2 h removed additional moisture by 0.32% absolute, greater than the expected value by 0.17% absolute (0.80%-0.63%). Since subsequent oven drying of the microwave dried sample involves the transfer of the sample from the fiberglass pad to a glass weighing bottle, there is ample time for moisture pickup. Dried samples are more moisture sensitive than undried samples, and this phenomenon can account for some of the difference. A 10-g sample of this undried material was then subjected to large changes in RH (40%-80%-40%) at 37 "C using the accelerated environmental apparatus. This humidity chamber was modified to directly measure changes in sample weight with a digital balance (3). This RH cycle (40%-80%-40%) was repeated three times. At the end of the treatment, the sample gained 0.06% in weight. The sample was then split into two portions. Three grams of this pretreated sample was
ANALYTICAL CHEMISTRY, VOL. 60, NO. 8, APRIL 15, 1988
Table IV. Percent Weight Change In River Sediment rehydrate in sample
air, 1 h
2A
+0.38
2B
+0.64
microwave dried
rehydrate in air, 1 h
wave dried
-0.34 -0.49
+0.37 +0.33
-0.34
0.9
Sample A
745
Sample B
micro-
-0.25
dried in the microwave oven (80% power, 4 min) and the remaining 7 g was dried in the conventional oven (110 "C, 2 h). These split samples lost 0.84% and 0.86%, respectively. The expected weight loss was 0.87% (the 0.06% gained in pretreatment plus the 0.81% weight loss of an untreated sample when oven dried at 110 "C). These data are in good agreement. To ensure that an equilibrium weight has been obtained in the microwave oven, the sample was rehydrated by leaving the sample exposed to environmental room conditions (35% RH, 21.7 OC, 744.5 mmHg) on the microwave balance pan for 1 h and redried. The rehydration/drying process was repeated twice. This sample is represented as sample A in Table IV. It is evident that an equilibrium weight was reached. A second 10-g sample of the river sediment was first dried in a conventional oven a t 110 "C for 2 h and lost 0.80%. This dried sample was placed in the accelerated environmental apparatus and exposed to cyclic changes in RH (80%-40%-80%) at 37 "C, as sample A. At the end of the treatment, the sample gained 0.78%, essentially all of the lost moisture. This sample was split into two portions and approximately 3 g was dried in the microwave oven and the remaining 7 g was dried in the conventional oven. The microwave drying yielded a weight loss of 0.67 % while the sample dried in the conventional oven lost 0.81%. The thermally dried sample behaved as expected while the results of microwave drying were low by 0.14% absolute. In addition, it became obvious that an equilibrium weight was not reached in the microwave drying of this particular sample. The sample was subjected to the same rehydration procedure on the microwave balance pan as sample A and is represented as sample B in Table IV. Sample degradation of this mostly inorganic matrix seems unlikely and therefore cannot explain the results found for sample B. At least some of these effects could be due to environmental conditions. The environmental room conditions during the microwave drying of sample B were RH 48%, 21.4 "C, and 741.4 mmHg. These room conditions were significantly different from those when sample A was tested (35% RH,21.7 "C, 744.5 mmHg). As previously stated, samples are spread in a thin layer on fiberglass pads and a large surface area is exposed to the air. The microwave oven operates at room temperature and atmospheric pressure; therefore large fluctuations in RH could have a significant affect on the sample with the large surface area exposed. These environmental effects are absent in a conventional oven operated at fixed temperatures. In addition, samples dried in the conventional oven are dried in weighing bottles and the surface is quite small in comparison to the total sample. When a constant weight is reached in microwave drying, it is assumed that an end point is reached. In this study, sample A reached an end point while sample B did not. Figure 2 depicts the weight changes of the dried and rehydrated river sediment samples. It can be seen that sample A resulted in good agreement between the two drying techniques after rehydration. However, the disagreement between the techniques in sample B indicates that microwave drying affects sample stability under these conditions. Since the portion of sample B dried in the conventional oven behaved "normally", it can be assumed that the sample had not degraded. Perhaps the previously established microwave drying conditions were no
O."Ui Y 1
.i
0.1 -0.1
Time Figure 2. Percent weight change in river sediment: a, microwave drying at 80% power for 4 min; b, oven drying at 110 "C for 2 h; c, 95% RH for 16 h.
Table V. Percent Water Lost from Hydrated Salts % loss
sample
molecules of water lost
microwave oven
theoretical value
cond A"
CuS04.5H20
4H20
Na2HP04-7H20
cond Bb 7Hz0
(1) 28.8 (2) 28.7
28.9
(1) 47.3 (2) 47.1
47.0
" Condition A, 100% power, 25.00 min, Thermapad. *Condition B, 100% power, 6.00 min, Thermapad. longer optimum for a sample pretreated in this manner. It is also possible that the moisture in sample B was in a different form and was removed less efficiently under the conditions cited. Regardless of the explanation, microwave drying was not successful in achieving an equilibrium weight for this SRM under all conditions tested. Microwave Oven with Thermapad versus Conventional Oven. A Thermapad can be used in microwave drying when heating a sample to a constant temperature is desired. It was demonstrated that the water of hydration could be quantitatively removed from samples of Na2HP04.7H20 (Table V). To effect these results, the Thermapad rated at 110 "C was pretreated by exposure to microwave radiation under the conditions determined suitable for drying the salt sample. The sample was then placed on the prepared Thermapad (now serving as a thermal heat source, as a conventional oven) and dried under the conditions described in Table V. All the hydrated water was removed from the Na2HP04-7H20,which is in agreement with the theoretical calculations for 110 "C. A copper ore SRM became the subject of extensive study since analytical experiments confirmed a changing Cu assay. This copper ore was predominately CuS, but had partially and variably oxidized to hydrated CuSO, over a period of time. The drying conditions recommended for the CuS (conventional oven at 105 "C for 5 h) did not result in a constant or equilibrium weight on these oxidized samples. It was determined that the hydrated water associated with the CuSO, was causing the imprecision. This copper ore sample was then dried in the microwave oven without the Thermapad at 80% power for 90 s. The microwave drying of replicate samples from the same bottle yielded drying data with high precision (about 3% relative standard deviation, RSD) (5). As expected, since oxidation was variable, the between-bottle precision was not as good. The effect of microwave drying on CuSO4.5H20was studied directly to estimate its effect on CuSO, in the ore. After 11/2
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 8, APRIL 15, 1988
h, the drying was incomplete and the results were erratic. Therefore, a Thermapad rated at 110 OC was tried. A 10-g sample was then placed on a pretreated Thermapad and dried under the conditions described in Table IV for the CuS04. Eighty percent of the hydrated water was removed from the sample. These experimental values are in excellent agreement with the theoretical calculations at 110 "C. (Four of the five water molecules are removed from the CuS04.5H20at 110 "C. The fifth molecule of water is removed at 150 "C.) Since the Thermapad was necessary to dry the "pure" CuS04-5Hz0,but not the copper sulfate in the copper ore, it was assumed that the ore absorbed the microwave radiation resulting in controlled sample heating similar to the Thermapad. The microwave energy is absorbed by the major constituents in a sample, and these species may act as a constant heat source for drying other species in the sample. These effects are presently being studied.
CONCLUSIONS In nearly every matrix studied, excellent precision was obtained in microwave drying. This high precision demonstrates that microwave drying is a viable means of quality control in processing plants and is indeed used in that context. However, there are disadvantages to this method as an alternative to traditional drying methods in a reference laboratory. Microwave drying nearly always differs from traditional techniques and therefore could not be used as an alternative to traditional drying for certified materials. In addition, pretreatment required for reference materials must involve the use of readily available equipment. Many laboratories do not have access to microwave drying ovens. Even when the same percent weight loss as conventional drying is achieved, there is no assurance that the composition of the sample is exactly the same as with conventional drying. Compositional changes can be caused by the unique mecha-
nism of microwave drying. Based on the results of both the flour SRMs and the inorganic river sediment, sample stability after exposure to high RH is questionable. This instability is not necessarily indicative of altered composition. In a reference laboratory nonroutine samples of a wide variety of matrices are studied. Therefore, the time involved establishing the drying parameters becomes significant and must be considered as part of the total drying time. Under these circumstances there was no time advantage of microwave drying over conventional methods in the materials studied. A t this time, for the reasons stated, traditional drying (oven or vacuum oven) methods must be recommended for reference materials.
ACKNOWLEDGMENT The author thanks Lawrence A. Machlan, who provided the vacuum oven drying data for the coal SRM, and Judith Arey of CEM Corp. for her help with the microwave drying of the clay, coal, and milk powder. Registry No. CuS04-5H20,7758-99-8; Na2HP04.7Hz0, 7782-85-6.
LITERATURE CITED (1) Copson, David A. Microwave Heating; AUI: Westport, CT, 1975. (2) Operation and Service Manual Automatic Volatllity Computer Model ACV-80; CEM Corp.: Indian Trial, NC, 1982; Section 3-4. (3) Moody, J. R.; Beary, E. S. Anal. Cbem. 1987, 59, 1481-1483. (4) Davis, A. B.; Lai, C. S. CerealChem. 1984. 6 1 , 1-4. (5) Beary, E. S.; Brletic, K. A,; Paulsen, P. J.; Moody, J. R. Ana/yst (London) 1987, 112, 441-444.
RECEIVED for review August 18, 1987. Accepted December 14, 1987. Certain commercial equipment, instruments, or materials are identified in this report to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the materials or equipment identified are necessarily the best for the purpose.
