Internal standard addition calibration: Determination of calcium and magnesium by atomic absorption spectrometry

•The internal standard addition (ISA) calibration method is proposed for the first time.•ISA combines the principles of internal standardization and standard additions.•Ca and Mg were determined in CRMs and biodiesel samples by ISA.•Sr and Mn were employed as internal standards for Ca and Mg, respectively.•The precision of ISA is comparable to other classical calibrations.

Internal standard addition (ISA) is a new calibration method that combines the principles of internal standardization and standard additions. The current work demonstrated the effectiveness of ISA for the determination of Ca and Mg in biodiesel samples and certified reference materials (CRMs) by flame atomic absorption spectrometry. Manganese and Sr were selected as internal standards for Mg and Ca, respectively. Results for Ca and Mg in CRMs using ISA were in agreement with certified values at the 95% confidence level (t-test). The relative standard deviations (n = 12) were ca. 6% for both analytes. For comparison purposes, Ca and Mg were also determined by the traditional methods of external standard calibration (ES), standard additions (SA) and internal standardization (IS). Recoveries obtained with ISA (Ca: 93–109%; Mg: 100–106%) were similar to those found with IS (Ca: 100–112%; Mg: 98–105%), but significantly better than ES (Ca: 219–291%; Mg: 111–120%) and SA (Ca: 97–127%; Mg: 106–128%). Results for Ca and Mg determined in biodiesel and CRMs using ISA were more accurate and more precise than those obtained with ES, SA and IS.