Ashing Methods and Techniques PDF

Methods and techniques in Ashing Introduction The ash content is a measure of the total amount of minerals present within a food, whereas the mineral content is a measure of the amount of specific inorganic components present within a food, such as Ca, Na, K and Cl. Etc… Reasons f...

Methods and techniques in Ashing Introduction The ash content is a measure of the total amount of minerals present within a food, whereas the mineral content is a measure of the amount of specific inorganic components present within a food, such as Ca, Na, K and Cl. Etc… Reasons for determination of ash & mineral content Nutritional labeling Quality Microbiological stability Nutrition Processing What is ash? Ash is the inorganic residue remaining after the water and organic matter have been removed by heating in the presence of oxidizing agents, which provides a measure of the total amount of minerals within a food. Analytical techniques for providing information about the total mineral content are based on the fact that the minerals (the analyte) can be distinguished from all the other components (the matrix) within a food in some measurable way. Methods of ashing Dry ashing Wet ashing Sample preparation Normally ,1-10g of samples are used in the analysis of ash content. Solid foods are finely ground and then carefully mixed to facilitate the choice of a representative sample. Samples that are high in moisture are often dried to prevent spattering during ashing Sample preparation High fat samples are usually defatted by solvent extraction, as this facilitates the release of the moisture and prevents spattering. contamination of samples by minerals in grinders, glassware or crucibles which come into contact with the sample during the analysis. For that reason , it is recommended to use deionized water when preparing samples. Samples are selected at random from large group of samples. Dry ashing Dry ashing procedures use a high temperature muffle furnace capable of maintaining temperatures of between 500 and 600 oC. Water and other volatile materials are vaporized and organic substances are burned in the presence of the oxygen in air to CO2, H2O and N2. Most minerals are converted to oxides, sulfates, phosphates, chlorides or silicates. Dry ashing Minerals are converted to oxides, sulfates, phosphates, chlorides or silicates. Most minerals have fairly low volatility at these high temperatures, some are volatile and may be partially lost, e.g., iron, lead and mercury. If an analysis is being carried out to determine the concentration of one of these substances then it is advisable to use an alternative ashing method that uses lower temperatures Procedure for ashing Take a finely weighed ground sample in a silica porcelain crucible. Take a weight crucible with sample. Keep the sample over the flame for churring. After churring,keep the sample in muffle furnace at 500-550 c for 3-4 hours. Carefully remove the silica crucible from muffle furnace. Keep crucible in desiccator for cooling, after cooling take a final weight. Ash calculation The food sample is weighed before and after ashing to determine the concentration of ash present. The ash content can be expressed on either a dry or wet basis. Preparation of ash solution Step 1: Pour the ash in conical flask. Step 2: Add 1:1 HCL and Water Step 3: Approximately add 50 to 60 ml Step 4: Keep water bath in 30 mins Step 5: Cool into room temperature Step 6: Filter through whatman no1 filter paper Step 7: Rinse with small amount of hot water Step 8: Make upto 100 ml /250 ml using water Step Temperature and Device time Drying Less than 110°C Hot air oven 1-2 hours Charring 280 °C is needed to achieve the complete destruction of organic matter (necessary when a voltammetric method of determination is to be used) Duration (decomposition and cooling): 20 min to 2 h Steel pressure digestion (12 samples) Advantages Almost no losses of analytes Minimal contamination Much safer than decomp. Using an HCLO4 containing mixture No difficulties associated with the use of H2SO4 Low consumption of acids Fast decomposition (microwave) → automation Disadvantages Expensive equipment Low sample weight → a sample has to be completely mixed Lower efficiency at t < 200 °C Resulting solution contains HNO3 → interference in hydride generation determination of as, se, sn etc. Open acid digestion Closed acid digestion Maximum temperature limited Maximum temperature :260- by the solutions boiling point 300 c Permits large sample weight Permits minimum quantity of samples High acid consumption Low acid consumption Loss of volatile elements Comparatively less (e.g.Hg,Pb) Contamination risk Less contamination risk l Digestion time taken high Microwave digestion 20 -60 mins Other sample prepration methods Microwave digestion Ultraviolet digestion (photolysis) Vapor phase acid digestion Microwave digestion Samples in digestion equipment heated by microwaves directly by the absorption of microwaves under closed condition. Once the set point temperature attains decomposition starts same rate. Nearly take 20-40 mins. Microwave digester Advantages Determine parameters Measurement is easy Contamination is less Sensor systems No man power and additional things Vapor phase acid digestion Recent tech.developed with in 40 years Mathusiewice-vapor phase attacking the dissolution &decomposition of inorganic and organic materials determination of trace elements(digestion -gasphase) HF &HNO3(half) acid vapor as digestion agent Zilbershtein-dissolve si and con impurities on a PTFE sheet.Residue and PTFE sheet placed in graphite electrode.spectroscopic analysis-failure  Sample placed in PTFE beaker.-perforated PTFE mounted. Kept above the level of liquid HF.(Closed system) HF vapor pass.sample have HCLO4&HNO3 Thomas and smythe explained vapor phase oxidation This tech introduced by wooley. Low temp to high temp (110-250) Device consist-airtight PTFE vessel with 2 concentric chambers. Ultraviolet digestion Liquids and slurries solids decomposed by UV lights in the presence of small amounts of H2O2 &HN03 or peroxothiosulphate (bev,waste water ,sewage treatment plants ) Digestion vessel keep in closet –UV lamp Digestion mechanism –conversion of OH- radicals from both water and hydrogenperoxide that is initialized by UV These radicals able oxide to CO2 and water. Not fully oxidized.partly like chlorinated phenols ,nitrophenols,hex.chl.benzene. Thanking you.