First, you need to be sure that the solutions (blank and food sample) are not mixed up (AAS is a sequential technique). Was any reference material used to validate the results? If these are "yes" then the cause may be contamination of the reagents themselves or external contamination that has happened only to the blank. It is a pity to waste time on a thorough investigation. You should repeat the analysis, this time with more care.

please elaborate a little; you might have mixed some tubes while preparing your samples.

-what is in your blank? it may contain an acid that can cause precipitation (resulting salt).

-very unlikely but hydride generation might be occurring if there is, somehow HBr- in your solution.

-did you employ background correction?

-can you verify that the lamp's voltage was steady and the beam is alligned?

-however AAS is quite a straightforward method, it extremely relies on the operator's attention and experience.

I gather it wasn't you running the AAS Instrument. Why not ask the operator what the procedure they used for blank correction was (if any) and if you could see the raw data?

Erhan Akkaya has made some good points. Have you also considered:

• It is possible that the blank reading was 0.418 mg/L and the sample concentration also read 0.418 mg/L but has had the blank subtracted.
• Another option is the matrix of your sample was sufficiently different to the calibration solutions that it was effectively negative in comparison to the calibration set.
• What is the lower limit of detection (LOD) for the method? If the blank reads up to 0.5 mg/L with no Pb in it then 0.418 mg/L is effectively "not detected".

As to how to treat it in the discussion, I think you are not able to claim anything other than "there was less than a measurable amount of Pb in the samples", or words similar to that effect.

I would agree with Jonca. I would also suggest that your analyst should be cautioned to pay more attention to the data they report. This appears to be an easily avoidable mistake.