Question

9. How would you recommend to your technician to overcome the following problems that could arise in conventional dry ashing of various foods?

                                                        
a. You seem to be getting volatilization of phosphorus, when you want to later determine the phosphorus content.

b. You are getting incomplete combustion of a product high in sugar after a typical dry ashing procedure (i.e., the ash is dark colored, not white or pale gray).

c. The typical procedure takes too long for your purpose. You need to speed up the procedure, but you do not want to use the standard wet ashing procedure.

d. You have reason to believe the compound you want to measure after dry ashing may be reacting with the porcelain crucibles being used

Answer 1

9.a. (About volatilization of Phosphorous)

One of the main demerits of conventional dry ashing is the loss of elements such as As, B, Cd, Cr, Cu, Fe, Pb, Hg, Ni, P, V, and Zn. Among these elements, many are to be analysed after dry ashing as they are major components of nutrition. But there are several methods and alternatives that could help reduce or prevent the volatilization of important elements like Phosphorous. Phosphorous volatilization can be prevented by pre oxidation of the sample by oxoacids of Iodine (eg. Periodic acid ). Ashing aids such as Magnesium Nitrate when added to the sample can acclerate oxidation and thus prevent the volatilization loses of element to be analysed like Phosphorous. Lowering the temperature can also reduce volatilization losses. Alternatively, it is better to use wet ashing method wherever possible, since in wet ashing the losses of elements due to volatilization are minimum.

9.b. (About incomplete combustion of a sample high in sugar)

If the combustion is not complete after the initial normal heating process of the dry ashing method, the sample should be ashed again after adding some amount (drops) of water or nitric acid. If there is incomplete combustion even after re-ashing in this way, then there is another method of ashing. In this method, the ash formed after the first two ashing processes (ashing and reashing as said above) are made to form a suspension in water, which should then be filtered using an ashless filter paper. Then the dried filtrate and filter paper has to be re-ashed. Thus, complete combustion of the sample takes place.

9.c. (About ways to reduce the time required for dry ashing)

Organic compounds with high oxygen content can increass the speed of combustion, helping to obtain more CO2. Some of the substances that can be added to the sample to decrease ashing time are glycerine, alcohol etc. Addition of Hydrogen can increase the reaction chamber temperature, increasing combustion velocity, there by reducing the time required.

9.d. (Related to limitation of Porcelain Crucible)

Porcelain crucibles are the most used crucible for dry ashing process. This is because, porcelain crucibles are relatively inexpensive. Porcelain crucibles may also crack during rapid temperature elevation. But, there are several ashing products that may react with porcelain. To address this issue, we have numerous other options of crucible available viz. Quartz crucible, Steel crucible, platinum crucible etc. Quarts are resistant to acids but reactive to bases. Platinum is relatively inert, but are expensive. Steel crucibles are resistant to both acids and bases. Steel is also relatively cheap. Hence, an ideal option of crucible, wherever it is not advisable to use porcelain is Steel Crucibles.