GUIDELINES FOR MICROWAVE ACID DIGESTION
Contents
1. therefore the target is to bring the solution to the highest temperature in the shortest time first step and to hold this temperature until the digestion is complete second step For difficult samples requiring temperatures higher than 200 C a high pressure rotor MDR 1000 6S should be used while for easier samples requiring temperature below 200 C a medium pressure rotor MPR 300 12S is enough As a matter of fact when samples such as sand 1g are to be digested with 4mL HF and ImL HNO a suitable program would be Step Time minutes Power Watt 1 10 650 2 10 350 The above program refers to the simultaneous digestion of 6 samples A library of nearly 400 sample preparation methods is available from your local Milestone distributor Acid digestion guidelines Page 92. 10 minutes and to remain between 165 170 C for other 10 minutes US EPA METHOD 3051 Sample amount 0 5g Reagent 10mL HNO Acid leaching method Microwave digestion time 10 minutes Temperature to reach 170 C within 5 5 minutes and to remain between 170 180 C for the balance of 10 minutes US EPA METHOD 3052 Sample amount up to 1 0g Basic reagents 9mL HNO and 3mL HF Alternative combinations include HCI for Erg BA SB Fe and Al determination H O for organic samples and H O to slow down exothermal reaction Goal is total sample decomposition Microwave digestion time 15 minutes Temperature to reach 180 C within 5 minutes and to remain between 175 185 C for 10 minutes Acid digestion guidelines Page 8 Ethos D User Manual Rev 0 2001 MICROWAVE ACID DIGESTION OF INORGANIC SAMPLES Inorganic samples group includes metals alloys oxides ores rocks slags ceramics etc CHEMISTRY Sample preparation procedures are strongly dependent on the chemical nature of the material to be digested and on the elements to be determined The microwave acid s choice is often the same as used with conventional methods MICROWAVE PROGRAM Most inorganic samples show non exothermal reactions and do not produce large amount of gases The sample amount does not affect the ratio temperature pressure as much as with organic samples The digestion efficiency strongly depends on the digestion temperature
3. 25 C Acid digestion guidelines Page 1 ETHOS D User Manual Rev 0 2001 HYDROGEN PEROXIDE Hydrogen peroxide is an oxidizing agent 2H O gt 2H O O added to nitric acid it reduces the nitrous vapors and it accelerates the digestion of organic samples by raising the temperature A typical mixture ratio is HNO H O 4 1 volume volume HYDROCHLORIC ACID Hydrochloric acid has the following properties boiling point of azeotropic mixture with H O with 20 4 HCI is 110 C available with 38 concentration it dissolves salts of weak acids carbonates phosphates and most metals are soluble with the exception of AgCI HgCl and TiCI excess of HCl improves the solubility of AgCl converted into AgCl strong complex nature widely used for iron based alloys because of its ability to hold large amounts of chloro complex in solution other complexes formed are Ag I Au II Hg ID Ga IID TI ID Sn IV Fe ID and Fe ID it does not dissolve oxides of Al Be Cr Ti Zr Sn and Sb sulphates of Ba and Pb group II fluorides SiO TiO and ZrO 250 30 25 200 G 20 L 150 Rn p 2 fr T 15 5 Q Ww 100 uw F 10 50 5 0 T T T IF 0 0 300 600 900 1200 Time seconds The graphic above represents the temperature and pressure profile of hydrochloric acid up on microwave heating Notice that pressure has been controlled at 25bar resulting in a temperature of 205 C Acid digest
4. ETHOS D User Manual Rev 0 2001 GUIDELINES FOR MICROWAVE ACID DIGESTION INTRODUCTION TO THE REAGENTS MOST COMMONLY USED IN MICROWAVE DIGESTION The acids used in microwave digestion may be classified in two main groups non oxidizing acids such as hydrochloric acid hydrofluoric acid phosphoric acid diluted sulfuric acid and diluted perchloric acid oxidizing acids such as nitric acid hot concentrated perchloric acid concentrated sulfuric acid and hydrogen peroxide NITRIC ACID Nitric acid has the following properties boiling point is 120 C at 65 concentration poor oxidizing strength at concentrations less than 2M oxidizing strength increases with concentration and reaction temperature most common acid for oxidation of organic matrices with this typical reaction CH x 2HNO gt CO g 2NO 2H 0 it dissolves most metals forming soluble nitrates exceptions are Au and Pt non oxidized and Al B Cr Ti and Zr passivated these metals require acid mixtures or diluted nitric acid often mixed with H O HCI and H SO available in high purity for trace level analysis 250 30 25 200 G 20 L 150 p 2 2 vw n 100 uw 10 50 5 0 T T T i 0 0 300 600 900 1200 Time seconds The above graphic represents the temperature and pressure profile of nitric acid up on microwave heating Notice that pressure has been controlled at 25bar resulting in a temperature of 2
5. d as well as 1 2mL of hydrogen peroxide Fat content 275 Acid digestion guidelines Page 6 Ethos D User Manual Rev 0 2001 BASIC PROGRAM FOR 3 SAMPLES Step Time minutes Power Watt 1 9 250 2 3 600 3 2 300 BASIC PROGRAM FOR 6 SAMPLES Step Time minutes Power Watt 1 5 250 2 5 500 3 5 650 BASIC PROGRAM FOR 12 SAMPLES Step Time minutes Power Watt 1 8 250 2 8 500 3 8 750 Acid digestion guidelines Page 7 Ethos D User Manual Rev 0 2001 MICROWAVE ACID DIGESTION OF ENVIRONMENTAL SAMPLES A number of microwave digestion methods for environmental samples have been approved by the US EPA Environmental Protection Agency US EPA 3015 SW 846 Update II Microwave assisted acid digestion of aqueous samples US EPA 3051 SW 846 Update II Microwave assisted acid digestion of sediments slugs soils and oils US EPA 3052 SW 846 Update III Microwave assisted acid digestion of siliceous and organically based matrices A brief explanation of the methods is given here more detailed information are available from your Milestone local supplier up on request US EPA METHOD 3015 Sample amount 45mL Reagents 2 5mL HNO and 2 5mL HCI when analyzing the solution with FLAA or ICP 5mL HNO only when analyzing the solution with FLAA GFAA ICP or ICP MS Microwave digestion time 20 minutes Temperature to reach 160 C within
6. est organic samples according to the following reaction ORG HNO gt NOx CO HO Metals are converted into soluble nitrates available for analysis When operating the ETHOS with ATC 400CE Automatic Temperature Control device set the digestion temperature at 140 C or higher for samples rich in carbohydrates wheat sugar etc at 150 C or higher for samples rich in proteins serum albumin etc and at 170 C or higher for fatty samples cheese butter vegetable oil etc Those temperature represent minimum target digestion temperature for the above matrices The sample amount directly influence the pressure generated inside the vessel as sample is decomposed into CO gas Therefore some limitation in regards to the sample amount may apply Follow the scheme below Organic samples Fat content lt 75 Weight lt 0 5g Weight lt ig Weight lt 0 25g Weight lt 0 5g MDR 1000 6S MDR 1000 6 Note that the above sample amounts do refer to organic matter Therefore the moisture content of the sample should not be considered for instance if a sample contains about 80 of moisture up to 5 gram could be digested When operating the ETHOS without ATC 400CE Automatic Temperature Control follow the basic programs recommended by Milestone Microwave power and heating time strictly depend on the number of samples being prepared simultaneously To organic samples 4 8mL of nitric acid depending on sample amount should be adde
7. ion guidelines Page 2 ETHOS D User Manual Rev 0 2001 HYDROFLUORIC ACID Hydrochloric acid has the following properties ACID DIGESTION e Boiling point is 108 C at 40 concentration e non oxidizing strong complex nature used in digestion of minerals ores soils rocks and even vegetables containing silicates e major use is the decomposition of silicates according with this reaction SiO 6HF gt H SiF 2H 0 often used in combination with HNO or HCIO EVAPORATION CONCENTRATION Following dissolution many analyses require removal of HF to prevent equipment damage or to resolubilize insoluble fluorides e many analytes such as As B Se Sb Hg and Cr may volatilize COMPLEXATION Alternative approach to remove HF from the solution by addition of Boric acid the following reactions take place H 3BO 3HF gt HBF OH 2H O and HBF OH HF gt HBF H20 10 50 times excess Boric acid enhances reaction rate 250 30 25 200 G 20 L 150 p 2 2 N Ww 100 uw 10 50 dg 0 T T T T 0 0 300 600 900 1200 Time seconds The above graphic represents the temperature and pressure profile of hydrofluoric acid up on microwave heating Notice that pressure has been controlled at 25bar resulting in a temperature of 240 C Acid digestion guidelines Page 3 ETHOS D User Manual Rev 0 2001 SULFURIC ACID Sulfuric acid has the following propertie
8. s boiling point is 340 C at 98 concentration exceeding the maximum working temperature of TFM Teflon vessels e careful reaction monitoring is required to prevent vessel damages e it destroys organics by dehydrating action many sulfates are insoluble Ba Sr Pb 350 10 300 8 250 F T W 200 ra 2 o z 5 amp 150 E T4 KG 100 2 50 4 0 to 0 150 300 450 600 Time seconds The above graphic represents the temperature and pressure profile of sulfuric acid up on microwave heating Notice that temperature has been controlled at 300 C for 1 minute only without any pressure increase It is clear that the use of concentrated sulfuric acid in the MDR rotors poses significant problems because of its high boiling point 300 C are critical for TFM Teflon vessels and simply too high for PFA Teflon vessels they melt down at such temperature It is therefore advisable to use sulfuric acid only with a strict temperature control Acid digestion guidelines Page 4 ETHOS D User Manual Rev 0 2001 PERCHLORIC ACID Perchloric acid has the following properties boiling point is 203 C at 72 concentration hot and concentrated is the strongest oxidizing acid rapid sometimes explosive reaction with organic matrices often mixed with nitric acid for a controllable digestion of organic matrices all perchlorate are soluble with the exception of KCIO perchloric acid decompose
9. s at 245 C in a closed microwave vessel developing gaseous by products and a tremendous excess pressure WARNING Special care has to be taken when the use of perchloric acid is required Do not use at all perchloric acid with organic samples It may be possible to use perchloric acid with inorganic samples when the temperature does not exceed 200 C and when the amount of perchloric acid is below 20 volume volume of the total solution volume AQUA REGIA Aqua regia properties are the following made up by hydrochloric acid and nitric acid in a 3 1 volume volume mixture it produces NOCI nitrosyl chloride which decomposes in NO and Cl up on heating it dissolves precious metals it must be freshly prepared and used immediately otherwise it evolves chlorine gas overpressurizing and venting the vessel 250 30 I 25 200 F i Ek 20 amp 150 w fon 2 45 g 100 10 50 Ls 0 T T T IF 0 0 300 600 900 1200 Time seconds The above graphic represents the temperature and pressure profile of agua regia up on microwave heating Notice that pressure has been controlled at 25bar resulting in a temperature of 200 C Acid digestion guidelines Page 5 Pressure bar Ethos D User Manual Rev 0 2001 MICROWAVE ACID DIGESTION OF ORGANIC SAMPLES Organic samples group includes food feed tissues botanicals biological etc Nitric acid is the most common oxidizing agent used to dig
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