Lecture 07; non spectral interferences by Dr. Salma Amir
- 1. Lecture No. 07 Course title: Atomic Spectroscopy Topic: Non-spectral interferences Course instructor: Dr. Salma Amir GFCW Peshawar
- 2. Interferences in AAS Interferences are physical or chemical processes that cause the signal from the analyte in the sample to be higher or lower than the signal from an equivalent standard. Interferences can therefore cause positive or negative errors in quantitative analysis.
- 3. Classes of interferences There are two major classes of interferences in AAS, 1. Spectral interferences and 2. Non-spectral interferences. Non-spectral interferences are those that affect the formation of analyte free atoms. Nonspectral interferences include chemical interference, ionization interference, and solvent effects (or matrix interference). Spectral interferences cause the amount of light absorbed to be erroneously high or low due to absorption or emission by a species other than the analyte atom.
- 4. Non-spectral interferences 1. Chemical interferences 2. Ionization interferences 3. Physical interferences (matrix interferences)
- 5. 1. Chemical Interferences A serious source of interference is chemical interference. Chemical interference occurs when some chemical component in the sample affects the atomization efficiency of the sample compared with the standard solution. The result is either an enhancement or a depression in the analyte signal from the sample compared with that from the standard.
- 6. Causes Due to flame: Chemical interference is a result of having insufficient energy in the flame or furnace to break the chemical bonds in molecules and form free atoms. Due to matrix of sample (anions): For example, a solution of calcium chloride, when atomized in an air–acetylene flame, decomposes to calcium atoms more readily than a solution of calcium phosphate. Calcium phosphate is more thermally stable than calcium chloride. A solution of calcium chloride containing 10 ppm Ca will give a higher absorbance than a solution of calcium phosphate containing 10 ppm Ca. If phosphate ion is added to a solution of calcium chloride, the absorbance due to Ca will decrease as the concentration of phosphate increases
- 7. Elimination of chemical interferences There are three ways of compensating for chemical interference. The first approach is to match the matrix of the standards and samples; that is, to have the same anion(s) present in the same concentrations in the working standards as in the samples being analyzed. This supposes that the samples have been thoroughly characterized and that their composition is known and constant. A second approach is to add another metal ion that forms an even more stable compound with the interfering anion than does the analyte ion. Such an ion is called a “releasing agent” because it frees the analyte from forming a compound with the anion and permits it to atomize.
- 8. For example, lanthanum forms a very thermally stable phosphate, more stable than calcium phosphate. To determine Ca in solutions that contain an unknown or variable amount of phosphate, such as those from biological samples, the analyst can add a large excess of lanthanum (as the chloride or nitrate salt) to all standards and samples. The lanthanum “ties up” the phosphate by forming lanthanum phosphate. If all of the phosphate is now present as lanthanum phosphate, this eliminates the dependence of the formation of Ca atoms on the phosphate concentration. The exact concentration of phosphate does not have to be measured; it is only necessary to add enough La to completely react with the phosphate in the solution to be analyzed. Usually 500–2000 ppm La is sufficient for most types of samples. The same amount of La must be added to all the solutions, including the blank. The releasing agent should be of the highest purity possible.
- 9. The third approach is to eliminate the chemical interference by switching to a higher-temperature flame, if possible. For example, when a nitrous oxide–acetylene flame is used, there is no chemical interference on Ca from phosphate, because the flame has sufficient energy to decompose the calcium phosphate molecules. Therefore, no lanthanum addition is required.
- 10. 2. Ionization interferences In AAS, the desired process in the atomizer should stop with the production of ground-state atoms. For some elements, the process continues to produce excited-state atoms and ions. If the flame is hot enough for significant excitation and ionization to occur, the absorbance signal is decreased because the population of ground-state atoms has decreased as a result of ionization and excitation. This is called ionization interference. Ionization interferences are commonly found for the easily ionized alkali metal and alkaline-earth elements, even in cool flames. Ionization interferences for other elements may occur in the hotter nitrous oxide–acetylene flame, but not in air–acetylene flames.
- 11. Elimination Adding an excess of a more easily ionized element to all standards and samples eliminates ionization interference. This addition creates a large number of free electrons in the flame. The free electrons are “captured” by the analyte ions, converting them back to atoms. The result is to “suppress” the ionization of the analyte. Elements often added as ionization suppressants are potassium, rubidium, and cesium. For example, in the AAS determination of sodium, it is common to add a large excess of potassium to all samples and standards. Potassium is more easily ionized than sodium. The potassium ionizes preferentially and the free electrons from the ionization of potassium suppress the ionization of sodium. The ionization suppression agent, also called an ionization buffer, must be added to all samples, standards, and blanks at the same concentration for accurate results Ionization can be reduced or eliminated by decreasing the temperature of the cell. For this purpose, a low temperature flame (air-propane) is often used.
- 12. 3. Physical (matrix) Interferences Other potential sources of interference are the sample matrix and the solvent used for making the sample solution. The sample matrix is anything in the sample other than the analyte. In some samples, the matrix is quite complex. Milk, for example, has a matrix that consists of an aqueous phase with suspended fat droplets and suspended micelles of milk protein, minerals, and other components of milk. The determination of calcium in milk presents matrix effects that are not found when determining calcium in drinking water.
- 13. Causes Sample solutions with high concentrations of salts other than the analyte may physically trap the analyte in particles that are slow to decompose, interfering in the vaporization step and causing interference. Differences in viscosity or surface tension between the standard solutions and the samples, or between different samples, will result in interference. Interference due to viscosity or surface tension occurs in the nebulization process for FAAS because different volumes of solution will be aspirated in a given period of time and nebulization efficiency will change as a result of the solvent characteristics. Metals in aqueous solutions invariably give lower absorbance readings than the same concentration of such metals in an organic solvent. This is due in part to enhanced nebulization efficiency of the organic solvent. In aqueous acidic solutions, higher acid concentrations often lead to higher solution viscosity and a decrease in absorbance due to decreased sample uptake.
- 14. The presence of matrix interference can be determined by comparing the slope of an external calibration curve with the slope of an MSA curve. If the slopes of the two calibrations are the same (parallel to each other), there is no matrix interference; if the slopes are different (not parallel), interference exists and must be corrected for. The solvent may interfere in the atomization process. If the solvent is an organic solvent, such as a ketone, alcohol, ether, or a hydrocarbon, the solvent not only evaporates rapidly but may also burn, thus increasing the flame temperature. The atomization process is more efficient in a hotter flame. More free atoms are produced and a higher absorbance signal is registered from solutions in organic solvents than from aqueous solutions, even though the metal concentration in the two solutions is equal.
- 15. Elimination Matching the solutions in the working standards to the sample solutions can compensate for matrix or solvent interferences. The type of solvent (water, toluene, methyl isobutyl ketone (MIBK), etc.), amount and type of acid (1% nitric, 5% HCl, 20% sodium chloride, etc.), and any added reagents such as releasing agents must be the same in calibration standards and samples for accurate results. Alternatively, the MSA may be used to compensate for matrix interferences. This calibration method uses the sample to calibrate in the presence of the interference. Properly used, MSA will correct for the solvent interference but care must be taken. The assumption made in using MSA is that whatever affects the rate of formation of free atoms in the sample will affect the rate of formation of free atoms from the added analyte spike in the same way. MSA will not correct for spectral interference or for ionization interference. MSA can correct for interferences that affect the slope of the curve but not for interferences that affect the intercept of the curve.