AAS 55B, calibration


I am having calibration issues for AAS 55B for analysing Au.  242.8 nm is used.

The standards are:  Au only 1,2,4,10 and 20 ppm in 10% HCl and 5% HNO3.

We have two AAS instrument. For AAS - 1, 10 ppm is consistently lower than what used to be, absorbance is 0.380-0.390.  We use the same set of the standards and read on the other AAS (2), 10 ppm was read 5-7% higher than the 1st AAS instrument, absorbance is 0.405-0.415 , but the rest of the standards have got similar reading.

Could some one help me?

Many thanks 

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