BIOCHEMICALLY IMPORTANT MINERAL ELEMENTS 1'^'^ 



certain marine worms). Still other elements, for example, selenium, are 

 found only in the tissues of plants or animals grown in certain restricted 

 localities. Some of these elements which are of interest for one reason 

 or another are discussed in more detail below. 



Ashing 



When it is desired to examine a biological sample for mineral elements, 

 the first step is to dry the sample, then burn it to remove organic matter, 

 and convert the mineral elements present into simple inorganic com- 

 pounds. From the chemical standpoint the process of burning or ashing 

 is essentially a very vigorous oxidation, which is carried out in the air 

 at a temperature of about 600-800°C. The organic substances present 

 are decomposed as the sample is heated and turn black on account of the 

 formation of free carbon. As the heating continues this carbon is oxi- 

 dized to carbon dioxide, which escapes. Disappearance of the black 

 color, therefore, indicates that the ashing is complete. The hydrogen in 

 the original organic matter is converted to water vapor, and the nitrogen 

 escapes in tlie form of nitrogen gas. 



The mineral elements are contained in biological materials partly in 

 complex organic combinations such as sulfur in methionine, phosphorus 

 in lecithin, iron in hemoglobin, etc. (see Table 8-2). As these organic 

 substances are destroyed during the ashing process, the mineral elements 

 in them combine with each other — metals with nonmetals — and fre- 

 quently also with oxygen to form inorganic salts such as the chlorides, 

 sulfates, phosphates and silicates of sodium, potassium, calcium, and 

 magnesium. The ash, then, consists largely of these salts. 



If the sample happens to contain relatively more metals than non- 

 metals, as in vegetables, fruits, milk, only a part of the metals present 

 can be converted into such salts because there will not be enough 

 nonmetals to go around. In this case the excess metals combine with 

 oxygen or carbon dioxide, which is always available from the burning 

 organic matter, to form oxides and carbonates. Sodium and potassium 

 form the carbonates, and calcium and magnesium the oxides, since their 

 carbonates are unstable at the high temperatures used. Such ash, 

 therefore, is strongly alkaline and, because of the carbonates present, 

 effervesces when dissolved in mineral acid. 



On the other hand, if the sample contains a larger amount of nonmetals 

 than metals, as in meats, cereals, eggs, the excess nonmetals will be 

 converted into the corresponding oxides, most of which are volatile, and 

 therefore escape {e.g., SO2) . Thus, 99 per cent of the sulfur in rice and in 

 corn meal is lost during burning. Silicon is an exception since its oxide, 

 SiOo, is very nonvolatile. This loss may be prevented by adding to the 

 sample before ashing a reagent which will shift the balance of metals 



