Nickel determination by GFAAS

Hello, I have some question related with Nickel determination by GFAAS (Agilent 240z AA). I have followed the default method but the peak shape is not look good. As you can find in the picture below.

Standard 2= 5 ppb and Standard 6= 50 ppb

My question is:

1. Are they still acceptable if r value for calibration curve is in the limit ?

2. Are there any solution to this if the peak shape is not acceptable ?

Moreover, I have tried to use the modifier (Pd 500 ppm), the peak shape look better but the r value seem lower and start running out of limit with following calibration curve. Please kindly find the picture:

Thank you in advance for your help.

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