Cadmium and lead levels assessment in chocolate samples

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josy-haddad's picture
Cadmium and lead levels assessment in chocolate samples

I am in the process of assessing the cadmium and lead levels in chocolate samples but having so many difficulties with the matrix and I am not sure whiether the problem is in the digestion or the spiking with standrads. After digestion all my samples are clear and when running standards with the matrix modifier on AAS I am getting a linear curve (r=1). However, aftert I spike the samples with 3 concentrations (low, mid, high), the values of absorbance are too high.  
My results are as follows for cadmium so far:
1) For the calibration curve run on AAS without chocolate samples: 
concentrations of 0.25ng/mL; 0.5ng/mL; 1ng/mL; 2ng/mL. curve is linear (r=1) 
LOD = 0.02 ng/mL and LOQ= 0.06 ng/mL
2) running microwave digestion with 7mL nitric acid and 1mL hydrogen peroxide
ramping 10 min till 200 and stand for 20mins (fan 1)
ramping 10 min till 25 and stand for 20mins (fan 3)
the obtained samples are spikes with standards 0.25ng/mL; 1ng/mL and 2ng/mL.
all obtained samples are clear and reconstituted till 25mL.
3) I am having problems in reading the digested samples on AAS the system is maxed out, values are not linear though the initial calibration curve was linear. Here's an example:
no standard addition: abs = 0.6363
standard 0.25ng/mL: abs = 2.1133
standard 1ng/mL: abs= 2.3856
standard 2ng/mL: abs= 2.4319

How should I proceed to optimize my method?