Good evening

Maybe it's a bad calibration of your AAS

Also, the error read is not so great,

Me personally I always use the method of internal standards to recalibrate my measurementsAdd your answer

Greetings

Good evening

Maybe it's a bad calibration of your AAS

Also, the error read is not so great,

Me personally I always use the method of internal standards to recalibrate my measurementsAdd your answer

Greetings

In addition to calibration errors, AAS measurements are susceptible to chemical (matrix) and ionisation interferences.  These effects  interferences may vary between measurement occasopns.  Consult the manual with the instrument or look for one on line.

I should also have said that spikes may help and that I agree about bridging standards

thank you everyone. The problem is I didn't run the AAS, I put the sample into certified laboratory so for any reasons I can't blame the "AAS" step, yet, I know the procedure that they did.

so sir,ma'am, there is nothing can happen with the sample during storage , isn't it?

I have electroplating sample, I try to remove copper (II) with electrocoagulation using Fe and Al electrode.

first I check the initial level of copper which is 11,3455 ppm

then I used the sample into  first optimation (4 electrode combinations)

all of the 4 combinations gave the AAS result below 11,3455 ppm.

but then in the next month I run the 2nd optimation (4  variations of process time)

but in this case I didn't check the initial level because I used the 11,3455 ppm as my initial level for 2nd optimation. 

but the result for this 2nd optimation is above 11,3455 ppm  and below 14.1960ppm which gave me negative % Removal (% Efficiency).

so I repeat the 2nd optimation (4 variations of process time) and I in this  2nd trial I check the initial level and gave me 14, 1960 ppm. The 4 variations gave me result under 14.1960 and above 11.3455 ppm.  This make me think that the result of the 4 variations of process time from the  first trial of 2nd optimation  was not wrong because the result is appropriate with the newest initial and after electrocoagulation level of 2nd trial. 

eventhough the electrode contains copper, I think this is not the problem as the result of newest initial level ( which means I didn't run the electrocoagulation process) is aroung 14 ppm just same as the result of the variation of time.

I'm sorry sir, ma'am If my explanation is hard to understand because of my english  It's okay if you don't wanna bother yourself to understand my explanation. I realize my english is bad. but at least pleast answer this

"so sir,ma'am, there is nothing can happen with the sample (copper (II) ) during storage , isn't it?"

Thank You Very Much.