[mbckenny]

Hi,

I am wondering why are different wavelengths used depending on concentrations in flame-AAS?

For a Pb AAS for example, the wavelength 217.0 nm is useful for concentrations between 2-20ppm but 261.4 nm is useful for concentrations between 80-800ppm.

Thanks in advance

[Pradeep]

I am also operating AAS. But, not experience this. Where you got these information? That will be helpful for me also.

[Train]

Perkin Elmer has a great manual of AA methods and other information.  It's called "Analytical Methods for Atomic Absorption Spectroscopy"  You may be able to find a copy on their website or by contacting them.

I think the main reason the concentration range changes with wavelength is because the extinction coefficient changes with wavelength.

[mbckenny]

I am also operating AAS. But, not experience this. Where you got these information? That will be helpful for me also.

I am doing an experiment on my undergrad course where we analyse the concentration of lead in brass using flame-AAS. The information is in a printed experiment manual which I am unable to upload but a question I must answer in the report is;

'In order to select a suitable operating wavelength for measuring absorbances in AAS, the approximate working concentration needs to be known in advance. Why are different wavelengths used depending on the concentration?'

Cheers,

Mike

[marquis]

There are a number of issues with the specific wavelength chosen.

In AAS, you will see information like "linear range".  This is the ppm where a straight linear curve can be used.  It seems like the 217 nm value is much better for low sensitivity.  So it would be your choice for low detection limits.  On the other hand, you would use the higher wavelength for concentrated samples, so as much dilution isn't needed.

If you know the other elements present, you can also choose the wavelength based on interfering lines from the other elements.  You would choose a line that wouldn't be present, say from emission of the interfering elements.

If you are using a multielement lamp, the lines may be chosen to avoid the other element(s) used in the lamp,

The 261 nm line you mentioned surprises me.  The two common lines in general use for lead are 283 nm and 217 nm.  Did the text give any information on why that line was chosen?

[Pradeep]

The information I got about this topic is also completely agree with marquis's post.

[mbckenny]

Hi,

@marquis

ok, I know that the lead made up roughly 4% of the brass and I diluted my sample to around 2-20 ppm fitting in with the 217nm wavelength.

The other elements were Copper and Zinc and a cathode lamp was used.

Thanks for your answer now though, I think I understand choosing wavelengths is based on the concentration detection limits in compounds?

The 283nm line was also in the text, I just gave 217 and 261 nm to show an example of my query.

Cheers,
Mike

[marquis]

The most important reason is lack of interference from other elements. 

Concentration is one reason to choose a specific line.

You can also play some games with the burner head to change the linear range.  If you take the burner head and rotate it, so that instead of being parallel and just under the beam, it is at an angle.  Most burner heads have several angles available for this.  It makes the alignment a bit of a pain, though.

Good luck.

[Furanone]

If it like other forms of spectroscopy such as FTIR, then it is likely due to there being different wavelength absorptions with different sensitivities. One wavelength may absorb really well and be good for using on very trace amounts, but on too large amounts the signal would be saturated to instrumental limits. While a wavelength for the same element with low sensitivity would be good for high amounts, but it would give too weak a signal for trace amounts.