It is concluded that a mixture of hydrofluoric and nitric acids can completely dissolve the sili- cate materials in soils without the need for perchloric or hydrochloric acids, thus minimizing reagent contamination. The calibration by stan- dard additions method requires further treatment of the analysis solutions, which increases the total analysis time and also the risk of contamination. Thus, external calibration is recommended pro- vided that matrix matched standard solutions are used. Calcium can be determined accurately with the nitrous oxide–acetylene flame without com- plete removal of silicon, while the removal of the silicon is necessary for the accurate determination in the air–acetylene flame. It is not necessary to remove the silicon to obtain accurate analyses for magnesium determination, both air–acetylene and nitrous oxide–acetylene flames can be used, but the former is twice as sensitive as the latter. Considering the problems associated with the op- eration and the lower precision, however, the use of fast-burning nitrous oxide–acetylene flame is not recommended for calcium or for magnesium determination. For the determination of stron- tium, it is concluded that the removal of silicon is not sufficient to perform accurate analysis in the air–acetylene flame despite the addition of lan- thanum as a releasing agent and potassium as an ionization suppressor. Thus, the nitrous oxide – acetylene flame should be used to eliminate the interferences (possibly due to aluminum and phosphates) on strontium.
It is thought that the indicative value of stron- tium in SRM 2710 reported by NIST is inaccu- rate. This hypothesis has been confirmed1.