THE USE OF MICROWAVE TECHNOLOGY FOR DRY ASHING PROCEDURES. by S. E. Carr and C. R. Moser. CEM Corporation, Matthews, North Carolina.

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1 THE USE OF MICROWAVE TECHNOLOGY FOR DRY ASHING PROCEDURES by S. E. Carr and C. R. Moser CEM Corporation, Matthews, North Carolina March 1991 Presented at the 1991 Pittsburgh Conference and Exposition on Analytical Chemistry and Applied Spectroscopy. FR-82

2 ABSTRACT Dry ashing procedures can often take from several hours to overnight for completion. Typically, these analyses are performed using resistance muffle furnaces. A review of an alternative dry ashing technique which utilizes microwave instrumentation will be presented. Examples of microwave dry ashing procedures where sample analysis times are reduced to minutes will be shown. A variety of sample matrices will be analyzed by standard dry ashing techniques using a resistance muffle furnace and a microwave powered furnace. Comparisons between the two techniques will be made on the preparation and handling of each sample. Performance comparisons including accuracy, precision and speed of analysis will also be shown. The effectiveness of using a self-sealing quartz crucible for total volatiles in carbon filled polymers will be discussed and compared to an inert atmosphere technique. Data presented will show that analysis performed by rapid, microwave dry ashing techniques can be completed in minutes, with results comparable to resistance furnace techniques. -l-

3 INTRODUCTION In recent years the use of microwave technology in the field of analytical chemistry has grown in popularity. This growth has stemmed from the ability of microwave instrumentation to fulfill the needs of the analytical chemist. Recent advances in microwave technology have supplied the chemist with more simplistic methods of analysis and the ability to perform these analyses in a time frame which is conducive to plant operations. In short, microwave equipment has provided the chemist an efficient alternative to many standard analytical methods. Areas where microwave instrumentation has had the greatest impact include drying, acid digestions, and most recently dry ashing. RESULTS AND DISCUSSION The main objective of this study was to make comparisons between common dry ashing techniques using conventional furnaces, and modern, rapid ashing techniques utilizing microwave instrumentation. The performance of ashing techniques on several types of samples including paper, pet food, petro-chemical sludge, inorganic and carbon filled polymers, coal and coke were compared in this study. For the standard ashing techniques a conventional resistance heated muffle furnace and a tube furnace was used. For the microwave ashing techniques we used a microwave muffle furnace, model MAS-3 (Figure 1.). The microwave furnace chamber is heated by microwave absorbing silicon carbide elements. The furnace chamber is insulated with a ceramic housing which is typical in most muffle furnace designs. All the furnaces used in this comparison were equipped with a means of temperature feedback control. The microwave furnace is equipped with a built in exhaust system, which has two main purposes. It supplies air for the -2-

4 oxidation of sample materials along with removing any gases that may be produced during combustion of organic material. Table 1. shows a comparison of ash results obtained on a paper sample. Determining the ash content in paper is important to the paper industry as a process control test. The test parameters used were the same in both techniques, with the exception of the type of crucibles used and the actual ashing times. One gram samples of paper were placed in the furnaces two at a time and ashed at 8 o C. The analysis time for ashing in the conventional muffle furnace was 85 minutes; 6 minutes in the furnace and 25 minutes of crucible cooling time in a desiccator. The analysis time for the microwave technique was 35 minutes; 3 minutes in the microwave furnace and 5 minutes to cool. The ash values obtained by the two techniques compare quite well, and the precision of results is better for the microwave ashing technique. Sample containers utilized throughout this study were standard porcelain crucibles used with standard muffle furnace techniques, and quartz fiber crucibles (Figure 2.) which were specifically developed for the microwave furnace. Quartz fiber crucibles were used in the microwave furnace except for one sample that was liquid. Because the quartz crucibles are made of fibers they cannot be used in all furnace applications in particular to large volumes of liquids. The microwave muffle furnace is designed so that all types of crucibles, including metal, can be used in it. Tables 2. and 3. are results from samples of wet and dry pet foods. With these samples we selected materials that were representative of pet foods made from grains and meat byproducts. For the wet pet food, 1 gram samples and ashing temperatures of 6 o C. were used. Due to the high moisture content of the wet pet food, the samples that were ashed in the conventional muffle furnace had to be dried in a forced air oven for one hour before they could be ashed. This was done simply to avoid splattering and possible loss of sample. -3-

5 However, for samples that were ashed in the microwave furnace, the drying step was eliminated by placing quartz fiber disks above and below the sample to avoid splattering. The wet pet food sample (Table 2.) was ashed for 5 hours in the conventional muffle furnace according to the manufacturer's procedure, compared to an ashing time in the microwave furnace of 3 minutes. Again there was a time savings involved with cooling the crucibles. The results were very comparable for the two methods. The same procedures were followed on the dry pet food sample (Table 3.) as were performed with the wet sample. 5 hours in the conventional muffle and 3 minutes in the microwave furnace. Overall, for both the wet and dry pet food samples the microwave technique gave equivalent results, and was 1 times faster than the conventional method, in addition to eliminating the drying step. Table 4. is a petro-chemical sludge sample which was a mixture of water and organic solvents. An ash analysis of this type of waste material is important as it will direct the analyst to the better process of disposal. In the standard method 2 gram samples were weighed into porcelain crucibles and heated slowly over a bunsen burner for approximately 15 minutes. This was necessary to reduce the volume and to avoid boil-over or splattering of the sample. They were then placed in the conventional muffle furnace at 9 o C and ashed to constant weight, for approximately 45 minutes. For the microwave furnace method all parameters were the same as for the conventional method, except the preash step was eliminated. Because these samples were liquid, porcelain crucibles were used with the microwave method instead of the quartz fiber ones. The samples were covered in the porcelain crucibles with quartz fiber disks and placed directly into the microwave furnace at 5 o C. This temperature was held for 5 minutes and then ramped to 9 o C and held for an additional 1 minutes. The total time in the microwave furnace was 35 minutes. -4-

6 In both the microwave and the conventional techniques the crucibles had to be cooled in a desiccator for 25 minutes, after being removed from the furnace. Comparable data was obtained by the two techniques, but for this sample the conventional method produced values which were more consistant than the microwave technique. The microwave technique was 3% faster than the conventional method and the bunsen burner step was eliminated. Another sample analyzed was a Ti2 filled polyethylene. An ash determination is very important to the plastics compounding industry since it is a way to monitor the filler content in the end product of a mixing extrusion process. Like the petro-chemical sludge sample, the plastic had to be preashed over a bunsen burner for approximately 2 minutes, before it could be placed in the conventional furnace. For the microwave technique the samples were placed directly into the furnace in quartz fiber ashing crucibles. For both techniques, 5 gram samples were ashed at 8 o C. Table 5. shows the % ash values obtained by both methods. Again, the results were very consistant and the precision of the results are well within an acceptable tolerance level. There was considerable time savings of 8% with the microwave method, and the elimination of a flaming step. A high-density polyethylene filled with carbon black was also analyzed. The conventional method for determining the filler content of this sample is more complicated than for the inorganically filled polymers. This is because we are now measuring an organic filler of carbon black rather than an inorganic filler, like Ti2. For this reason, in the conventional method a tube furnace with a nitrogen purge of - 2 L/minute was used. In the conventional technique, one gram samples of filled polyethylene were weighed into combustion boats and placed in the tube furnace one at a time. The tube furnace was heated from room temperature to 6 o C in 2 minutes. The -5-

7 temperature was held at 6 o C for 2 additional minutes and then allowed to cool. The sample was not removed from the nitrogen-purged tube furnace until the temperature was below 1 o C In the microwave technique a different approach was taken. Instead of using a non-oxidizing atmosphere to insure against loss of the carbon black, a closed quartz vessel (Figure 3.) was used in the microwave furnace. This quartz vessel is designed so that when heating the sample, the vessel will open to release the volatiles, and then reseal so that ambient air is prevented from entering the vessel. The precision of the results (Table 6.) were better for the microwave technique than the standard method. The mean value for the microwave technique was.1% higher than by the standard method. The slight difference in results for the two techniques is not significant. The total time of analysis for the microwave technique was 12 minutes. 7 minutes to ash at 6 o C and 5 minutes to cool. This is compared to the 145 minutes in the tube furnace, 4 minutes for ashing and 15 minutes for cooling. The final two samples analyzed in this study were coal and coke samples. Unlike the filled polymers, the % ash is significant to.1% or even better, and is important for economic reasons. Both materials are typically analyzed using ASTM standard method D-3174, which is a ramping temperature procedure that takes 4 hours to complete. For the conventional method we used one gram samples of coke in porcelain crucibles. We followed the ASTM method and ramped the sample from ambient temperature to 5 o C in 1 hour, then ramped to 75 ' C in another hour and held it there for 2 more hours. Afterwards there was a cooling time of 25 minutes. This required a total time of 265 minutes. In the microwave technique we did not use a ramping procedure, but weighed the samples in quartz fiber crucibles and placed them -6-

8 directly in the microwave furnace at 75 o C. The total analysis time for the microwave technique was 65 minutes; 6 minutes to ash and 5 minutes to cool. The results (Table 7.) obtained by the microwave method fell directly in line with those obtained by ASTM method, with a mean value of.36 for the microwave method and.34 for the ASTM method. A two fold improvement in precision was obtained with the microwave method. For the coal sample a slightly different approach was taken. For two sets of samples the ASTM ramping method was followed in both the microwave and the conventional furnace. A third set of samples were ashed using a rapid microwave method. For this procedure one gram samples were weighed into quartz fiber crucibles and placed directly in the microwave muffle at 75 o C. The samples were ashed for 35 minutes then cooled in a desiccator for 1 minutes. The total ashing time for the microwave technqiue was 45 minutes versus 265 minutes for the standard method. Table 8. shows the mean and ranges of results were the same for all three methods. The rapid microwave method reduced the analysis time by 83%. Conclusions 1) 2) 3) 4) In conclusion this study showed that: Microwave ashing techniques significantly reduced the time required to ash a variety of sample types. Long and tedious preparation and handling steps used in standard practices can be eliminated when using microwave techniques. The accuracy and precision of the microwave results were in excellent agreement with those obtained by standard methods. The microwave methods are more simplistic and gives the laboratory chemist an efficient alternative to using standard methods. -7-

9 Jre fan duct (draws air through cavity to remove combustion by-products and cool the instrument) / 1 muffle furnace Thermocouple inserted through top chamber and into furnace cavity. Figure 1

10 Quartz Fiber Crucible Airflow around sample speeds combustion. Figure 2 : J L + no oxygen 111 Sample is heated Gas escapes Crucible reseais Figure 3

11 Table 1. Comparison of Total Ash for Fine Paper Microwave (%) Conventional ( Ash Values Mean: Range:.4.2 Total Analysis Time: 35 min. 85 min. Ashing Temperature: 8 o C 8 o C

12 Table 2. Comparison of Total Ash for Wet Pet Food Ash Values Microwave ( Mean: 1.34 Range:.4 Conventional (1) Total Analysis Time: 3 min. 385 min. Ashing Temperature: 6 C 6 C

13 Table 3. Comparison of Total Ash for Drv Pet Food Microwave ( Conventional ( Ash Values Mean: Range:.8.I3 Total Analysis Time: 3 min. 385 min. Ashing Temperature: 6 C 6 C

14 Table 4. Comparison of Total Ash for Petro-Chemical Sludge Ash Values Microwave ( Conventional ( Mean: Range:.25.I3 Total Analysis Time: 6 min. 85 min. Ashing Temperature: 9 C 9 C

15 Table Comparison of Ash for HDPE with Ti2 Filler Ash Values Microwave ( Conventional ( Mean: Range:.24.I7 Total Analysis Time: 15 min. 7 min. Ashing Temperature: 8 C 8 C

16 Table 6. Comparison of Total Ash for HDPE with Carbon Black Filler Ash Values Microwave ( Tube Furnace ( Mean: Range:.I4.21 Total Analysis Time: 12 min. 145 min. Ashing Temperature: 6 OC 6 C

17 Table 7. Comparison of Total Ash for Coke from Coal Ash Values Microwave ( Mean:.36 Range:.4 ASTM D Conventional ( Total Analysis Time: 65 min. 265 min. Ashing Temperature: 75 C 75 C

18 Table 8. Comparison of Total Ash for Coal Ash Values Microwave Microwave Conventional Rapid Method ASTM Method ASTM Method WI WI WI Mean: Range: Total Analysis Time: 45 min. 265 min. 265 min. Ashing Temperature: 75 C 75 C 75 C

BUREAU OF INDIAN STANDARDS DRAFT FOR COMMENTS ONLY. (Not to be reproduced without the permission of BIS or used as an Amendment)

Doc No. CED 50 (7780) BUREAU OF INDIAN STANDARDS DRAFT FOR COMMENTS ONLY (Not to be reproduced without the permission of BIS or used as an Amendment) Draft AMENDMENT NO. 1 TO IS 12235 (Part 17):2004 THERMOPLASTICS