What is the role of a monochromator in atomic absorption spectrometer?

Question

What is the role of a monochromator in an atomic absorption spectroscopy (AAS) instrument? What would happen if we run AAS analysis without a spectral sorting device?

Answer 1

This question has lingered on the site for a long time, which is a shame because it is a good question to ask. Atomic absorption spectroscopy (AAS) uses a hollow cathode lamp (HCL) as a source, and HCLs are line sources; therefore, if a line source is being used, why would an instrument need to be made more complex by adding a wavelength selector?

The first point to consider is what the source really looks like. The emission spectrum from HCLs have multiple lines. Here, for example, is the emission from a nickel HCL, taken from Spectrochemical Analysis by Atomic Absorption and Emission by L. Lajunen.

A wavelength selector is needed in order to ensure that only one of the emission lines is being used for the analysis.

Additionally, light can be scattered in the flame, which can introduce errors in the analysis if not eliminated. If you compare the block diagram of an AAS vs a typical visible spectrometer, you'll note that the wavelength selector is typically placed after the sample compartment for AAS to address these concerns about scattered light; however the issue is not that significant in visible spectroscopy so the wavelength selector can be placed before the sample holder.

This representative block diagram for AAS is taken from here.

A representative UV/Vis spectrometer block diagram is from this source

Bottom Line AAS needs a wavelength selector to ensure that the analytical line of interest is the only radiation hitting the detector and it minimizes the negative effects of radiation scattered in the flame. A good summary can be found here.

Answer 2

The answer by @bobthechemist is clear, concise and correct and I upvoted it. However, I think the first figure in bobthechemist’s answer does not fully convey the significant complexity of the emission spectra of hollow cathode lamps (HCLs) and the challenge thereby presented to the monochromator. Accordingly, this answer is an add-on to bobthechemist’s answer.

The spectral output of an HCL can be observed and acquired for various purposes by use of a suitable spectrometer, spectroscope or spectrograph. Herein, I use one of the latter instruments: a homemade echelle spectrograph. This instrument is shown below:

The input is via fiber optic cable and the nominal spectral range coverage is 400 to 700 nm, though the actual red end limit is around 744 nm.

Now consider the spectral outputs of HCLs having elements far apart in the periodic table. First, light from a lithium HCL, with neon fill gas, is used as the input to the spectrograph’s input fiber optic. The resulting echellogram is shown below:

The large majority of emission lines are due to the neon fill gas and a blue filter was used to attenuate the intense red neon emission lines. The crimson lithium emission near 670.8 nm, familiar to anyone who has ever performed a simple flame test for lithium, is used for analytical AAS purposes. For that purpose, everything else is unwanted and detrimental, so the monochromator in the AAS functions to reject as much of that unwanted light as is economically feasible. A typical monochromator bandpass would be around 0.2 nm. Note that the 670.8 nm region is relatively isolated from the surrounding neon lines and the lithium line, which is actually a close pair, is quite intense. These two factors contribute to lithium being easy to detect, with high sensitivity, via flame AAS.

Now consider the echellogram for a combination sodium and potassium HCL with neon fill gas:

This echellogram is not much more complex in appearance than that of lithium. In fact, neon emission dominates in both echellograms. Sodium, in particular, has a relatively simple emission spectrum, characterized by its famous yellow D lines. Note that the D lines are not close to neighboring neon or potassium lines and are intense. As for lithium, this makes AAS determination of sodium relatively sensitive.

For potassium, the preferred AAS determination lines are at 766.5 and 769.9 nm, as per the Agilent Technologies online document titled “AA Hollow Cathode Lamps - Recommended Operating Conditions”. These near-IR wavelengths are inaccessible with my spectrograph.

In the preceding echellograms, the analytically useful AAS wavelengths were relatively isolated and intense, making the monochromator’s task fairly easy to accomplish. But now consider the spectrum of uranium, acquired using a uranium HCL with argon fill gas:

Argon has a non-trivial emission spectrum, but uranium’s emission spectrum is far more complex: the echellogram is so dense with lines that it has even been investigated (see screenshot below) as a pseudo-continuum light source for AAS.

For whatever reason, the use of lab-constructed water-cooled high power uranium HCLs never caught on. But uranium has been determined by conventional AAS, e.g., at 591.5 nm in a fuel rich nitrous oxide-acetylene flame with added alkali for ionization suppression (@AChem, private communication.)

The next three figures show enlargements of the spectral region around 591.5 nm. First, the sodium and potassium HCL with neon fill gas:

The two neon atomic emission lines bracket the location where the uranium 591.5 nm line would be located.

The next figure shows the same enlarged spectral region for the uranium HCL with argon fill gas:

All the unlabeled lines are other uranium lines or argon lines. Two such lines are uncomfortably close to the desired uranium line.

Then this figure shows an equally weighted overlay of the previous two figures:

Now it is clear that AAS determination of uranium, at 591.5 nm, requires unrealistically narrow monochromator spectral bandpass because there are two neighboring lines that are simply too close. Consequently, uranium is not sensitively detected, via AAS, at 591.5 nm. This is probably for the best: aspirating uranium-containing solutions into a flame just does not seem wise.

Note: Echellograms can always be processed to yield spectra in conventional intensity versus wavelength (or wavenumber) format, but it is tedious and somewhat complicated. For present purposes, it was unnecessary, but conventional uranium HCL spectra have long been available. For example, the uranium emission spectrum, in the neighborhood of the 591.5 nm line, is on page 87 of a 240 page report from scientists at the Los Alamos National Laboratory (LANL):