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AAS (Atomic Absorption Spectroscopy) is a quantitative analytical technique used to measure very small concentrations of ions in substances. The main idea is that the sample is atomised in a flame and then light is shot through the atomised sample and the absorbance is measured. enter image description here

However I don't get why they use an atomic emission lamp (a lamp made of only 1 element). What's wrong with using a normal incandescent lamp or some lamp that produces all the wavelengths at once (it would save you having to change the lamp for every test)? I've heard that multi-element lamps make the machine less sensitive due to noise but I don't understand how. If we tune the monochromator to the particular wavelength we want, it should be the same as with using an atomic emission lamp?

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Absorption bands are very small: smaller than the band which a typical monochromator is capable of isolating.

If a continuum light source was employed, the absorbed band would be much smaller than the band which would be isolated by the monochromator and which would be read by the detector. This implies a low SNR (signal-to-noise ratio), and a very poor resolution.

Using a single element lamp (or lamps with combined elements but with distinct emission frequencies) overrides the problem: a monochromator is well capable of isolating the different frequencies, and absorption, which occurs in a narrow band of frequencies, occurs in the same narrow band of light from the source: this implies a much higher SNR, and a much better resolution.

To make an easy to conceive example, let's say that the effect of absorption by the sample is the effect of a star shining in the sky: using a continuum light source implies watching it through the midday sky. A HCL lamp would be a midnight sky, in the metaphor.

Note that, aside from what textbooks say, continuum souce AAS (HR-CS AAS) does exist, but it requires, as expected, a sophisticated monochromator (high resolution monochromator, with a resolution of a few pm!)

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  • $\begingroup$ ah right, so in essence the monochromator isn't able to split the light well enough so we try to limit the wavelengths? $\endgroup$ – John Hon May 25 at 2:55
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    $\begingroup$ Exactly. We have a room full of each possible green fruit (a large frequency band), and we want to extimate by sight how many pears are being eaten over time. That's hard! HCL gives you a room with some pears to analyze :) $\endgroup$ – The_Vinz May 25 at 3:02
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One of my good acquaintances spent a considerable amount of time on continuum sources in atomic absorption spectrophotometry. It is very much possible, perhaps there is one commercial instrument as well by Jena (Germany). The source was a xenon arc lamp. In those cases, it is better to use a special monochromator, which is called an echelle monochromator. It is extremely high resolution one. This is one of the reasons why hollow cathode lamps are popular as stated in the answer by The_Vinz.

Now the alternative is atomic emission- you can detect all elements at once. High end atomic emission spectrometers use the same ultrahigh resolution echelle monochromators.

The punch line as stated above is that there is no simple/ "good enough" monochromator to isolate extremely narrow lines emitted or absorbed by the atoms. A hollow cathode lamp is an intelligent choice then.

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  • $\begingroup$ "commercial instrument by Jena (Germany)" => would that be Analytik Jena? (Jena is the town where Analytik Jena has their headquarters, but so does e.g. Zeiss and a bunch of other optics companies) $\endgroup$ – cbeleites supports Monica Jul 20 at 13:06
  • $\begingroup$ You are right, it must be Analytik Jena. $\endgroup$ – M. Farooq Jul 20 at 13:53

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