FOOD ANALYSIS: TRACE ELEMENTS
Diagram
Two ways to read elements by light
Atomic absorption spectrometry (AAS)
Free atoms in a flame absorb light from a lamp at one exact wavelength. The light lost tells how much of the element is there.
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Inductively coupled plasma spectrometry (ICP)
An argon plasma near 10,000 K atomises and excites the sample, and the light given out is read. About 75 elements can be measured, many at once.
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The choice
AAS suits one element at a time at ordinary levels. ICP suits many elements at once, down to very low levels.
Figure 1: AAS measures light absorbed by free atoms; ICP measures light given out by atoms excited in a hot argon plasma. Drawn in our own style from the spectroscopy unit of the FTN Instrumental Techniques of Analysis notes, Unit 3, and from Cantle, Atomic Absorption Spectrometry.
Introduction
Atomic absorption spectrometry, written AAS, and inductively coupled plasma spectrometry, written ICP, are the two main instrumental methods for measuring trace elements in food, and both read elements by their light. The difference is the direction of the reading: AAS shines light from a lamp through free atoms in a flame and measures how much light the atoms absorb, while ICP heats the sample in an argon plasma near 10,000 K and measures the light the excited atoms give out. In this article ICP means the plasma emission method, ICP-AES or ICP-OES, as described in the FTN notes.
The FTN Food Analysis notes, Unit 6, describe AAS for calcium, iron, zinc and other minerals, and the FTN Instrumental Techniques of Analysis notes, Unit 3, set AAS beside flame photometry and the plasma. This article follows those two units, with Cantle, Atomic Absorption Spectrometry.
Quick comparison
Table 1: AAS compared with ICP
| Aspect | AAS | ICP |
|---|---|---|
| What is measured | Light absorbed by free atoms at one exact wavelength | Light emitted by excited atoms at their own wavelengths |
| Light source | A lamp that gives out light of the exact wavelength absorbed by one element, shining through the flame | No lamp: the plasma itself excites the atoms, and they give out the light |
| Atomiser | A flame, or a graphite furnace for very low levels; the heat breaks compounds into free atoms | An argon plasma near 10,000 K, made by passing argon through a radio-frequency coil |
| Elements at once | One element at a time, because the lamp serves one element | About 75 elements, many of them at the same time from a single sample |
| Detection limits | Higher in the flame; the graphite furnace is far more sensitive than the flame, because a few microlitres of sample are atomised in a small tube and the atoms stay in the light path longer | Lower than in flame methods, because the plasma is so hot and the excitation so complete |
| Sample handling | The mineral solution is sprayed into the flame | The sample is carried into the plasma as a fine mist from a nebuliser and spray chamber |
| Best for | Calcium, iron, zinc and other single elements in foods at ordinary levels | Multi-element analysis of foods, waters, soils and biological samples, and trace levels |
| Reading the result | The more atoms there are, the more light is absorbed, and the detector measures the loss, read against a calibration graph | The emitted light is sorted by a spectrometer and recorded, often by a solid-state detector array |
| Viewing | The light path passes through the flame | The plasma can be viewed from the side (radial view), which handles oils and salty solutions well, or along its axis (axial view), which gives better sensitivity |
| Limit | One element per lamp and per run, and chemical, ionisation and spectral interferences each need their own remedy | A dearer instrument that needs argon gas and a skilled operator |
Scientific explanation
Definition
Atomic absorption spectrometry is a method that measures an element by the light its free atoms absorb at a specific wavelength: the mineral solution is sprayed into a flame, the heat breaks the compounds into free atoms, a lamp giving light of the exact wavelength of one element shines through the flame, and the atoms absorb some of that light. Inductively coupled plasma spectrometry is a method in which the flame is replaced by an argon plasma, a hot, electrically conducting gas of argon ions and electrons, made by passing argon through a radio-frequency coil: the plasma reaches temperatures near 10,000 K, far hotter than any flame, so it atomises and excites almost every element, and the light given out is measured.
Working principle
In AAS, the mineral solution is sprayed into a flame, and the heat breaks the compounds into free atoms. A lamp that gives out light of the exact wavelength absorbed by one element shines through the flame, and the atoms absorb some of that light. The more atoms there are, the more light is absorbed, and the detector measures the loss. The result is read from a calibration graph made from standards, and the whole reading rests on the Beer-Lambert law. A graphite furnace can replace the flame when the element is present at very low levels: a few microlitres of sample are atomised in a small tube, and the atoms stay in the light path longer, so the method is far more sensitive than the flame. In ICP, the sample is carried into the plasma as a fine mist from a nebuliser and spray chamber. The plasma atomises and excites almost every element, including ones that form stable oxides in a flame, and about 75 elements can be measured, many at the same time from a single sample, with detection limits lower than in flame methods.
Apparatus
AAS needs a burner and flame or a graphite furnace, a means of spraying the sample into the flame, a hollow cathode lamp for the element, a monochromator to select the wavelength, and a detector. ICP needs an argon supply, a radio-frequency coil to make the plasma, a nebuliser and spray chamber to carry the sample in as a fine mist, a spectrometer to sort the emitted light, and a detector, often a solid-state detector array.
Measurement procedure
Both methods are run against standards. In AAS the sample is prepared by ashing the food and dissolving the ash in acid, the standards are run beside the samples, and the absorbance is read from the calibration graph. In the practical sessions for the FTN Food Analysis subject, students determine mineral elements through the flame photometer and the atomic absorption spectrophotometer, standardising and calibrating the instrument on the same day as the samples. In ICP the sample is also carried in as a solution, and the elements are read together from the one run.
Factors affecting readings
For AAS, the flame temperature, the choice of lamp, and interferences decide the reading: chemical interferences, ionisation, and spectral interferences each have their own remedies, and matching the standards to the sample matrix reduces interference from other elements present. For ICP, the plasma temperature and the viewing direction matter: the radial view handles difficult samples such as oils and salty solutions well, while the axial view gives better sensitivity. In both methods, running the standards beside the samples, not yesterday, keeps the calibration true.
Formulas and calculations
Both methods rest on one law, the Beer-Lambert law, which the FTN notes state as A = ebc, where A is the absorbance, e is the molar absorptivity, b is the path length, and c is the concentration. The law is the reason a calibration graph is a straight line.
Worked example, in the style of the books: take iron standards at 1, 2 and 4 mg per litre, giving absorbances of 0.10, 0.20 and 0.40. The line is straight, with a slope of 0.10 absorbance units per mg per litre. A sample giving an absorbance of 0.30 holds 0.30 divided by 0.10, which is 3 mg per litre. Dilution is then corrected in the same way as in the chromatography example: a sample diluted tenfold would hold ten times the read value.
Industrial applications
- AAS: measuring iron in flour, after the flour is burned to ash and dissolved in acid.
- AAS: measuring calcium, iron, zinc and other minerals in foods at ordinary levels.
- AAS with a graphite furnace: measuring a heavy metal present at very low levels, where the flame is not sensitive enough.
- ICP: multi-element analysis of foods, waters, soils and biological samples from a single sample.
- ICP: measuring trace lead in drinking water at very low levels, where flame methods cannot reach.
- ICP: surveying many elements at once in a food, instead of running one element at a time.
Advantages and limitations
- AAS, advantage: a simpler and cheaper instrument that measures many elements, and the graphite furnace reaches very low levels for one element at a time.
- AAS, limitation: one element at a time, because each element needs its own lamp, and chemical, ionisation and spectral interferences each need their own remedy.
- ICP, advantage: about 75 elements can be measured, many at the same time from a single sample, with lower detection limits than flame methods.
- ICP, limitation: a dearer instrument that needs a steady argon supply and a skilled operator, and it is more than a small laboratory needs for one element at a time.
FAQ
What is the main difference between AAS and ICP?
AAS measures light absorbed by free atoms in a flame, using a lamp for one element at a time. ICP measures light emitted by atoms excited in a hot argon plasma, and reads many elements at once.
What does ICP stand for in this article?
Inductively coupled plasma spectrometry, also written ICP-AES or ICP-OES: atomic emission spectrometry, or optical emission spectrometry, using an argon plasma.
Why is the graphite furnace more sensitive than the flame in AAS?
A few microlitres of sample are atomised in a small tube, and the atoms stay in the light path longer, so more light is absorbed.
How hot is the ICP plasma?
Near 10,000 K, far hotter than any flame, which is why it atomises and excites almost every element, including ones that form stable oxides in a flame.
Which method fits trace lead in drinking water?
AAS with a graphite furnace, or ICP: the flame photometer and flame AAS are not the tools for very low levels of a heavy metal.
References and further reading
- FTN subject: Food Analysis, Unit 6: Mineral and Vitamin Analysis; FTN subject: Instrumental Techniques of Analysis, Unit 3: Spectroscopic Techniques.
- J. E. Cantle (ed.), Atomic Absorption Spectrometry, Elsevier, 1986 (Google Books page read)
