EXPERIMENT 2: DETERMINATION OF ASH CONTENT OBJECTIVE To determine mineral content in tea leaves and coffee powder.
Introduction 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. The most widely used methods are based on the fact that minerals are not destroyed by heating, and that they have a low volatility compared to other food components. The three main types of analytical procedure used to determine the ash content of foods are based on this principle: dry ashing, wet ashing and low temperature plasma dry ashing. The method chosen for a particular analysis depends on the reason for carrying out the analysis, the type of food analyzed and the equipment available. Ashing may also be used as the first step in preparing samples for analysis of specific minerals, by atomic spectroscopy or the various traditional methods described below. Ash contents of fresh foods rarely exceed 5%, although some processed foods can have ash contents as high as 12%, e.g., dried beef. 1.
Dry ashing for the majority of the samples,
Dry ashing is incineration at high temperature (525°C or higher) accomplished in a muffle furnace. Crucible selection becomes critical in ashing because type depends on the specific use. Quartz crucibles are resistant to acids and halogens, but not alkali. Porcelain crucibles resemble quartz crucibles in their properties but will crack with rapid temperature changes. These crucibles usually used because they are relatively inexpensive. Steel crucibles are resistant to both acids and alkalis and are inexpensive, but they are composed of chromium and nickel, which are possible sources of contamination. Platinum crucibles are inert are the best crucibles but they are currently far too expensive for routine use for large number of samples. The advantages of conventional dry ashing are that it is a safe method, it requires no added reagents or blank subtraction, and little attention is needed once ignition begins. A large number of crucibles can be handled at once, and the resultant ash can be used for analyses like individual elements, acid-insoluble ash, and water-soluble and insoluble ash. The disadvantages are the length of time required (12 - 18 h, or overnight) and expensive equipment. There will be a loss of the volatile elements and interactions between mineral components and crucibles. Volatile elements at risk of being lost include arsenic, boron, cadmium, chromium, copper, iron, lead, mercury, nickel, phosphorus, vanadium and zinc. 2. Wet ashing for samples with high fat content (meats and meat products) as a preparation preparati on of elemental analysis, Wet ashing is a procedure for oxidizing organic substances by using acids and oxidizing agents or their combinations. Minerals are solubilised without volatilization. Wet ashing is preferable to dry ashing for specific elemental analysis. The oxidation time is short and requires a hood, hot plate, long tongs and safety equipment. Nitric and perchloric acid acids are preferable, but a special perchloric acid hood is necessary. The disadvantages of wet ashing are that it takes almost
constant operator attention, use of corrosive reagents and only small numbers can be handled at any one time. 3. Low-temperature Low-temper ature plasma dry ashing for preparation of samples when volatile elemental analyses are conducted.
Apparatus Crucible (or similar porcelain or metal dishes) Muffled furnace Sample Hot plate
Procedure 1. Dry a representative sample (weigh accurately to nearest mg ~ 3 – 5 g of samples) in a crucible in an oven at 130°C overnight. Char the sample on an electric hot plate or over a low f lame in a fume cupboard until it has ceased smokin smoking. g. 2. Place the above crucibles cruci bles (containing charred sample) in a cold muffle oven and bring the temperature to 550°C. 3. Ignite the the sample sample 12- 18 h (or overnight) at 550°C. 4. Turn off the muffle furnace and wait wait to open until the temperature has dropped to at least 250°C, preferably lower. Open door carefully to avoid losing ash that may be light and fluffy. 5. Use safety tongs to quickly quickly transfer the crucibles to a dessicator with a porcelain plate and desiccant. Cover crucibles, close the dessicator, and all ow crucibles to cool cool prior prior to weighing.
