Atomic Absorption (AA) Spectrometry

Determination of Copper in Brass by Atomic Absorption

Reading:

pp. 479-483 in Harris text

Objective:

Introduction:

This experiment will be an introduction to atomic absorption (AA) spectrophotometry.  Atomic absorption is a very powerful technique for the quantitative determination of individual elements, and it’s very simple to use and understand.  The diagram below shows the basic design of the AA instrument.

AAS Diagram

Since we are interested in absorption of light by atoms (atomic absorption), it is important that the sample be "atomized".  This is the purpose of the flame.  The liquid sample is sucked into the flame where the high temperature breaks the liquid up into small droplets and creates individual atoms.  Some samples are harder to atomize than others, so the choice of fuel and oxidant can be varied to change the flame temperature.  An air/acetylene flame is the most popular, and will work fine for our purposes.

A beam of light shines through the flame, and this light can be absorbed by individual atoms in the flame (hence the name atomic absorption).  Each element absorbs its own characteristic wavelength (color) of light, so we need the monochromator to isolate the particular wavelength of interest for the element we’re trying to determine.  The detector is a photomultiplier tube (PMT) which converts the stream of photons into an amplified electrical signal which can be easily measured.

The light source used in an AA instrument is unique.  It is called a hollow cathode lamp (HCL).  A hollow cathode lamp is an argon-filled tube containing a hollow metal cylinder made of the element we are determining, copper in our case.  This cylinder is the hollow cathode, and it is held at a negative electrical potential.  As current is passed through the cylinder, argon is ionized.  The positively charged argon ions are accelerated toward the copper cathode.  When they collide with the cathode they can actually knock copper atoms from the solid cathode into the gas phase.  Many of these copper atoms are electronically excited.  When they relax back to their ground states they emit radiation.  Now this isn't just any old radiation.  Much of this radiation is at exactly the same wavelength we need for absorption by copper atoms in our sample in the flame.  It should make sense to you that the wavelength of light emitted by excited copper atoms in the HCL is the exact wavelength which will be absorbed by ground-state copper atoms in the flame.  (Think about it.)

The picture below shows our Varian AA 240 spectrometer with both the flame and hollow cathode lamp operating.  This instrument can hold four separate lamps (the bottom is position #1), and a mirror directs the light from the selected lamp through the flame.  Note the red glow of the hollow cathode lamp when it is turned on.

Atomic Absorbtion Spectrometer with Labels

I think you'll be amazed at how easy this instrument is to operate, and the analysis time is VERY fast.  The atomic absorption spectrophotometer was introduced in the 1950's and its popularity spread in dramatic fashion.  Yes, there are some disadvantages.  Irreproducibility in the flame limits our precision, our sample is consumed rather quickly so we need a lot of it, and the element-specific HCL pretty much limits us to looking at one element at a time.  But for laboratories which are interested in routine determinations of a single metal, or a small number of metals, the relatively inexpensive AA is very often the method of choice.