Atomic Absorption Determination of Iron in Cereal

READING (FROM HARRIS TEXT):

• • 549-554 (An Overview, Flames)

  • 560-561 (Hollow Cathode Lamp)

  • 818-819 (Dissolving Inorganic Materials with Acids)

  • 822-823 (Liquid Extraction Techniques, note Figure 28-12 is the same instrument you'll be using)

OBJECTIVES:

• • To determine the mass percentage of iron a sample of breakfast cereal

  • To become familiar with the operation of an atomic absorption spectrometer

  • To become familiar with the principles of microwave digestion

This experiment will be an introduction to atomic absorption (AA) spectrophotometry, which we'll be using to measure the amount of iron in a sample of breakfast cereal. 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.

Since we are interested in absorption of light by atoms (i.e., atomic absorption), it is important that the sample be "atomized". The liquid sample is first converted to an aerosol by a flowing stream of gas - a process called nebulization. The aerosol then enters the burner where the high temperature of the flame evaporates the solvent 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 with a temperature of 2400-2700 K 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 with a hollow metal cylinder containing the element we are determining, iron 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 iron cathode. When they collide with the cathode they can actually knock iron atoms from the solid cathode into the gas phase. Many of these iron 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 iron atoms in our sample in the flame. It should make sense to you that the wavelength of light emitted by excited iron atoms in the HCL is the exact wavelength which will be absorbed by ground-state iron atoms in the flame. The figure below shows the whole process.

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.

I think you'll be impressed with 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.

MICROWAVE DIGESTION

In order to introduce the cereal into the instrument, it will have to be dissolved in aqueous solution. Many types of samples can be digested (decomposed and dissolved) in nitric acid with heating. Traditionally this has been done on a hotplate in a fume hood. However, digestion can instead be carried out in a much safer and more efficient fashion using a laboratory microwave oven. These instruments have become popular for digestion, extraction, and organic synthesis, and working with multiple samples is much easier than when using hotplates. While some applications have used conventional kitchen microwave ovens, laboratory-grade instruments offer sophisticated temperature sensing and programming, and are much safer due to proper venting, vapor sensing, and pressure relief.

Microwave systems heat by exciting the rotation of dipoles within a liquid. Compounds such as water and other polar liquids absorb microwave radiation rapidly, subjecting the sample to rapid heating and elevated pressures.  Heating is very efficient since the energy is transferred directly to the liquid instead of through a hot plate and beaker as is in the traditional method.  Samples digest or dissolve in a short period of time.

We will be performing acid digestions of the cereal samples in a simple-to-use microwave instrument called the MARS 6, a very popular instrument manufactured by CEM. A picture of the unit is shown below, and more information can be found on their website.  Sample temperature is measured by an infrared sensor which allows us to program the instrument to heat and cool the sample to specific temperatures over defined time periods.

REFERENCES

Skoog, D.A.;  West, D.M.;  Holler, F.J.;  Fundamentals of Analytical Chemistry (7th ed.)  Orlando:  Saunders, 1996, pp 863-864.

Cresswell, S.L,; Haswell, S.J.; "Microwave Ovens - Out of the Kitchen," Journal of Chemical Education, 2001 78(7) 900-904.

MARS 6 Microwave Reaction System Operation Manual,  CEM Corporation, 2011.