Magnesium and calcium determination in desalted crude oil by direct sampling graphite furnace atomic absorption spectrometry

Direct sampling graphite furnace atomic absorption spectrometry (DS-GF AAS) was proposed for Mg and Ca determination in desalted crude oil. A Zeeman-effect background correction system with variable magnetic field was used and measurements were carried out with low and high sensitivity. Pyrolysis and atomization temperatures, calibration with aqueous standards and the effect of sample mass to be introduced into the atomizer were evaluated. The use of chemical modifiers was not necessary for both elements. For Mg determination, two sensitivity conditions (high and low, respectively) were used: i) main absorption line (285.2 nm), 2-field mode (0.8 T) and gas stop flow during the atomization and ii) secondary absorption line (202.6 nm), 2-field mode (0.8 T) and gas stop flow during the atomization. For Ca determination, three low-sensitivity conditions using the main absorption line (422.7 nm) and variable magnetic field were used: i) 3-field dynamic mode (0.6-0.8 T) and gas stop flow during the atomization, ii) 3-field dynamic mode (0.4-0.8 T) and gas stop flow during the atomization and iii) 3-field dynamic mode (0.6-0.8 T) and gas flow during the atomization. The characteristic mass and limit of quantification for the highest sensitivity condition were 0.3 pg and 1.2 ng g(-1) for Mg and 4.8 pg and 0.1 mu g g(-1) for Ca, respectively. For all sensitivity conditions, good linearity (R-2 above 0.997) and relative standard deviation below 14% were achieved. Accuracy for DS-GF AAS method was evaluated by results comparison with those obtained by GF AAS after microwave-assisted digestion, which were in agreement. Additionally, recovery tests were carried out and recoveries for both analytes ranged from 92 to 107%. The use of DS-GF AAS provides some advantages such as simplicity, low risk of contamination and up to twelve measurements can be performed per hour.