Analysis of minerals

• Gravimetric Analysis
• Colorimetric analysis
• Titration methods Atomic absorption spectroscopy
  • EDTA compleximetric titration
  • Redox reactions
  • Precipitation titrations
• Ion-selective electrodes
• Atomic spectroscopy
  • Atomic absorption spectroscopy
  • Atomic emission spectroscopy

The major physicochemical characteristics of minerals that are used to distinguish them from the surrounding matrix are: their low volatility; their ability to react with specific chemical reagents to give measurable changes; and their unique electromagnetic spectra. The most effective means of determining the type and concentration of specific minerals in foods is to use atomic absorption or emission spectroscopy. Instruments based on this principle can be used to quantify the entire range of minerals in foods, often to concentrations as low as a few ppm. For these reasons they have largely replaced traditional methods of mineral analysis in institutions that can afford to purchase and maintain one, or that routinely analyze large numbers of samples. Institutions that do not have the resources or sample throughput to warrant purchasing an atomic spectroscopy instrument rely on more traditional methods that require chemicals and equipment commonly found in food laboratories. Many of the minerals of importance to food scientists can be measured using one of these traditional methods.

Gravimetric Analysis

The element to be analyzed is precipitated from solution by adding a reagent that reacts with it to form an insoluble complex with a known chemical formula. The precipitate is separated from the solution by filtration, rinsed, dried and weighed. The amount of mineral present in the original sample is determined from a knowledge of the chemical formula of the precipitate. For example, the amount of chloride in a solution can be determined by adding excess silver ions to form an insoluble silver chloride precipitate, because it is known that Cl is 24.74% of AgCl. Gravimetric procedures are only suitable for large food samples, which have relatively high concentrations of the mineral being analyzed. They are not suitable for analysis of trace elements because balances are not sensitive enough to accurately weigh the small amount of precipitate formed.

Colorimetric analysis

These methods rely on a change in color of a reagent when it reacts with a specific mineral in solution which can be quantified by measuring the absorbance of the solution at a specific wavelength using a spectrophotometer. Colorimetric methods are used to determine the concentration of a wide variety of different minerals. Vandate is often used as a colorimetric reagent because it changes color when it reacts with minerals. For example, the phosphorous content of a sample can be determined by adding a vandate-molybdate reagent to the sample. This forms a colored complex (yellow-orange) with the phosphorous which can be quantified by measuring the absorbance of the solution at 420nm, and comparing with a calibration curve. Different reagents are also available to colorimetrically determine the concentration of other minerals.

Titration methods

EDTA compleximetric titration

EDTA is a chemical reagent that forms strong complexes with multivalent metallic ions. The disodium salt of EDTA is usually used because it is available in high purity: Na₂H₂Y. The complexes formed by metal ions and EDTA can be represented by the following equations:

The calcium content of foods is often determined by this method. An ashed food sample is diluted in water and then made alkaline (pH 12.5 to 13). An indicator that can form a colored complex with EDTA is then added to the solution, and the solution is titrated with EDTA. The EDTA-indicator complex is chosen to be much weaker than the EDTA-mineral complex. Consequently, as long as multivalent ions remain in the solution the EDTA forms a strong complex with them and does not react with the indicator. However, once all the mineral ions have been complexed, any additional EDTA reacts with the indicator and forms a colored complex that is used to determine the end-point of the reaction. The calcium content of a food sample is determined by comparing the volume of EDTA required to titrate it to the end-point with a calibration curve prepared for a series of solutions of known calcium concentration. If there is a mixture of different multivalent metallic ions present in a food there could be some problems in determining the concentration of a specific type of ion. It is often possible to remove interfering ions by passing the solution containing the sample through an ion-exchange column prior to analysis.

Redox reactions

Many analytical procedures are based on coupled reduction-oxidation (redox) reactions.Reduction is the gain of electrons from atoms or molecules, whereas oxidation is the removal of electrons. Any molecular species that gains electrons during the course of a reaction is said to be reduced, whereas any molecular species that gains electrons is said to be oxidized, whether or not oxygen is involved. Electrons cannot be created or destroyed in ordinary chemical reactions and so any oxidation reaction is accompanied by a reduction reaction. These coupled reactions are called redoxreactions:

Analysts often design a coupled reaction system so that one of the half-reactions leads to a measurable change in the system that can be conveniently used as an end-point, e.g., a color change. Thus one of the coupled reactions usually involves the mineral being analyzed (e.g., X = analyte), whereas the other involves an indicator (e.g., Y = indicator).

For example, permanganate ion (MnO₄^-) is a deep purple color (oxidized form), while the mangenous ion (Mn²⁺) is a pale pink color (reduced form). Thus permanganate titrations can be used as an indicator of many redox reactions:

The calcium or iron content of foods can be determined by titration with a solution of potassium permanganate, the end point corresponding to the first change of the solution from pale pink to purple. The calcium or iron content is determined from the volume of permanganate solution of known molarity that is required to reach the end-point. For iron the reaction is:

Potassium permanganate is titrated into the aqueous solution of ashed food. While there is Fe²⁺ remaining in the food the MnO₄^- is converted to Mn²⁺ that leads to a pale pink solution. Once all of the Fe²⁺ has been converted to Fe³⁺ then the MnO₄^- remains in solution and leads to the formation of a purple color, which is the end-point.

Precipitation titrations

When at least one product of a titration reaction is an insoluble precipitate, it is referred to as a precipitation titration. A titrimetric method commonly used in the food industry is the Mohr method for chloride analysis. Silver nitrate is titrated into an aqueous solution containing the sample to be analyzed and a chromate indicator.

AgNO₃ + NaCl --> AgCl(s) + NaNO₃

The interaction between silver and chloride is much stronger than that between silver and chromate. The silver ion therefore reacts with the chloride ion to form AgCl, until all of the chloride ion is exhausted. Any further addition of silver nitrate leads to the formation of silver chromate, which is an insoluble orange colored solid.

The end point of the reaction is the first hint of an orange color. The volume of silver nitrate solution (of known molarity) required to reach the endpoint is determined, and thus the concentration of chloride in solution can be calculated.

Ion-selective electrodes

The mineral content of many foods can be determined using ion-selective electrodes. These devices work on the same principle as pH meters, but the composition of the glass electrode is different so that it is sensitive to specific types of ion (rather than H⁺). Special glass electrodes are commercially available to determine the concentration of K⁺, Na⁺, NH₄⁺, Li⁺, Ca²⁺ and Rb⁺ in aqueous solution. Two electrodes are dipped into an aqueous solution containing the dissolved mineral: a reference electrode and a ion-selective electrode. The voltage across the electrodes depends on the concentration of the mineral in solution and is measured at extremely low current to prevent alterations in ion concentration. The concentration of a specific mineral is determined from a calibration curve of voltage versus the logarithm of concentration. The major advantages of this method are its simplicity, speed and ease of use. The technique has been used to determine the salt concentration of butter, cheese and meat, the calcium concentration of milk and the CO₂ concentration of soft drinks. In principle, an ion selective electrode is only sensitive to one type of ion, however, there is often interference from other types of ions. This problem can often be reduced by adjusting pH, complexing or precipitating the interfering ions.

Atomic spectroscopy

The determination of mineral type and concentration by atomic spectroscopy is more sensitive, specific, and quicker than traditional wet chemistry methods. For this reason it has largely replaced traditional methods in laboratories that can afford it or that routinely analyze for minerals.
Atomic spectroscopy is used to provide information about the type and concentration of minerals in foods. The type of minerals is determined by measuring the position of the peaks in the emission or absorption spectra. The concentration of mineral components is determined by measuring the intensity of a spectral line known to correspond to the particular element of interest. The reduction in intensity of an electromagnetic wave that travels through a sample is used to determine the absorbance: A = -log(I/Io). The Beer-Lambert law can then be used to relate the absorbance to the concentration of atoms in the sample: A = a.b.c, where A is absorbance, a is extinction cofficient, b is sample pathlength and c is concentration of absorbing species. In practice, there are often deviations from the above equation and so it is often necessary to prepare a calibration curve using a series of standards of known concentration prepared using the same reagents as used to prepare the sample. It is also important to run a blank to take into account any impurities in the reagents that might interfere with the analysis.
Prior to making atomic spectroscopy measurements a food sample is usually ashed. The resulting ash is dissolved in a suitable solvent, such as water or dilute HCl, before injecting it into the instrument. Sometimes it is possible to analyze a sample without ashing it first. For example, vegetables oils can be analyzed by dissolving them in acetone or ethanol and injecting them directly into the instrument.

Atomic absorption spectroscopy

Atomic absorption spectroscopy (AAS) is an analytical method that is based on the absorption of UV-visible radiation by free atoms in the gaseous state. The food sample to be analyzed is normally ashed and then dissolved in an aqueous solution. This solution is placed in the instrument where it is heated to vaporize and atomize the minerals. A beam of radiation is passed through the atomized sample, and the absorption of radiation is measured at specific wavelengths corresponding to the mineral of interest. Information about the type and concentration of minerals present is obtained by measuring the location and intensity of the peaks in the absorption spectra.

Atomic emission spectroscopy

Atomic emission spectroscopy (AES) is different from AAS, because it utilizes the emission of radiation by a sample, rather than the absorption. For this reason samples usually have to be heated to a higher temperature so that a greater proportion of the atoms are in an excited state (although care must be taken to ensure that ionization does not occur because the spectra from ionized atoms is different from that of non-ionized atoms). There are a number of ways that the energy can be supplied to a sample, including heat, light, electricity and radio waves.