[shafaifer]

The LOD is defined as the minimum concentration of an analyte required, wherein the
analytical signal is significantly different from the blank signal (background signal).

Am I right in this:

• The lower LOD-value, the higher analyte concentration.

In an AAS study of the mass of calcium carbonate in potsherds (first dissolving the sherds in concentrated nitric acid, then measuring an absorbance-value, calculating a concentration for calcium  m_CaCO3(s) is obtained), what use is it to know what the minimum concentration of calcium is, wherein the
analytical signal is significantly different from the blank signal? If this cannot be answered, what use is it in general in analytic chemistry?

[mjc123]

Am I right in this:
•The lower LOD-value, the higher analyte concentration.

No, the statement is meaningless. Higher analyte concentration in what? Blank? Standard? Sample? You need to define more clearly what you mean.

what use is it to know what the minimum concentration of calcium is, wherein the analytical signal is significantly different from the blank signal?

This is important if you get a negative result, i.e. no discernible calcium signal. You can never say absolutely "there is no Ca in this sample" because no instrument is sensitive enough to show that. If your LOD is, say, 10 ppm (converted back to an equivalent concentration in the sample), then you can say "we can detect amounts greater than 10 ppm; we didn't detect anything; therefore the amount of Ca is no more than 10 ppm". That's why on certificates of analysis you find statements like "Ca: <10 ppm", not "Ca: zero".

If this cannot be answered, what use is it in general in analytic chemistry?

If you don't know the LOD, then you can't be quantitative about how much analyte can be present without it being detected. If you don't detect any Ca signal, then you perhaps know there's not huge amounts present, but you can't say definitely "there's no more than x ppm". Depending on the application, this can be a useless result.

[shafaifer]

Thank you very much,

No, the statement is meaningless. Higher analyte concentration in what? Blank? Standard? Sample? You need to define more clearly what you mean.

http://www.monzir-pal.net/Poisons%20and%20Drugs/Lecture%20material/Atomic%20Absorption%20Spectrometry.pdf

I have linked 49 pages about AAS. In chapter 6.4.1.3, page 413, line 7 - 9:

Potassium is more easily ionized than sodium. The potassium ionizes preferentially and the free electrons from the ionization of potassium suppress the ionization of sodium. The detection limit of the sodium determination thereby decreases.

In this example, Na is concerned. It seems to be that this is about the sample. The ionisation of sodium is suppressed upon addition of potassium which has a lower ionisation energy than sodium. By adding potassium, the ammount of free Na atoms is increased thus the detection limit for Na is lowered, right? So maybe I should state insted: The higher absorption-value (the more Na is absorbed), the lower LOD-value a considered analyte. 

[mjc123]

This is a very specific example. It is not true, to say in general "The lower LOD-value, the higher analyte concentration." - indeed, without a context, it is meaningless.

In the case of ionisation suppression, what you are doing is ensuring that a higher proportion of the analyte in the detector is in the detectable form (gaseous Na atoms), so that a given amount of analyte from the sample gives a higher signal (which means that the LOD will be lower). The concentration of Na in the sample does not change.
Perhaps a better formulation of your statement would be: The greater the ratio Absorption/Concentration, the lower the LOD.

Hope this is helpful.

[shafaifer]

Yes - it is. Thank you very much.

[Furanone]

The commercial lab I formerly worked at had a rule of thumb of at least for chromatography that the Limit of Detection for a peak was 5 times the noise of the baseline (defined as the std deviation of baseline), and Limit of Quantification for a peak was 10 times the noise of the baseline. Sometimes this wasn't always possible for extreme trace amounts so we would put as a sidenote with asterisk that it was en estimate.