ASTM D7740 Standard Practice for Optimization, Calibration, and Validation of Atomic Absorption Spectrometry for Metal Analysis of Petroleum Products and Lubricants
4. Summary of Practice
4.1 An Atomic Absorption Spectrometer (AAS) is used to determine the metal composition of various liquid matrices. Although usually AAS is done using a flame to atomize the metals, graphite furnace (GF-AAS) or cold vapor (CV-AAS) may also be used for metals at very low levels of concentration or some elements not amenable to flame atomization. This practice summarizes the protocols to be followed during calibration and verification of the instrument performance.
5. Significance and Use
5.1 Accurate elemental analysis of petroleum products and lubricants is necessary for the determination of chemical properties, which are used to establish compliance with commercial and regulatory specifications.
5.2 Atomic Absorption Spectrometry (AAS) is one of the most widely used analytical techniques in the oil industry for elemental analysis. There are at least twelve Standard Test Methods published by ASTM D02 Committee on Petroleum Products and Lubricants for such analysis. See Table 1.
5.3 The advantage of using an AAS analysis include good sensitivity for most metals, relative freedom from interferences, and ability to calibrate the instrument based on elemental standards irrespective of their elemental chemical forms. Thus, the technique has been a method of choice in most of the oil industry laboratories. In many laboratories, AAS has been superseded by a superior ICP-AES technique (see Practice D7260).
5.4 Some of the ASTM AAS Standard Test Methods have also been issued by other standard writing bodies as technically equivalent standards. See Table 2.
6. Interferences
6.1 Although over 70 elements can be determined by AAS usually with a precision of 1 - 3 % and with detection limits of the order of sub-mg/kg levels, and with little or no atomic spectral interference. However, there are several types of interferences possible: chemical, ionization, matrix, emission, spectral, and background absorption interferences. Since these interferences are well-defined, it is easy to eliminate or compensate for them. See Table 3.
6.1.1 Chemical Interferences - If the sample for analysis contains a thermally stable compound with the analyte that is not totally decomposed by the energy of the flame, a chemical interference exists. They can normally be overcome or controlled by using a higher temperature flame or addition of a releasing agent to the sample and standard solutions.
6.1.2 Ionization Interferences - When the flame has enough energy to cause the removal of an electron from the atom, creating an ion, ionization interference can occur. They can be controlled by addition of an excess of an easily ionized element to both samples and standards. Normally alkali metals which have very low ionization potentials are used.
6.1.3 Matrix Interferences - These can cause either a suppression or enhancement of the analyte signal. Matrix interferences occur when the physical characteristics - viscosity, burning characteristics, surface tension - of the sample and standard differ considerably. To compensate for the matrix interferences, the matrix components in the sample and standard should be matched as closely as possible. Matrix interferences can also be controlled by diluting the sample solution until the effect of dissolved salts or acids is negligible. Sometimes, the method of standard addition is used to overcome this interference. See 6.2.
6.1.4 Emission Interferences - At high analyte concentrations, the atomic absorption analysis for highly emissive elements sometimes exhibits poor analytical precision, if the emission signal falls within the spectral bandpass being used. This interference can be compensated for by decreasing the slit width, increasing the lamp current, diluting the sample, and/or using a cooler flame.
6.1.5 Spectral Interferences - When an absorbing wavelength of an element present in the sample but not being determined falls within the bandwidth of the absorption line of the element of interest a spectral interference can occur. An interference by other atoms can occur when there is a sufficient overlapping between radiation and emitted by the excited atoms and other absorbing atoms. Usually the bandwidth is much wider than the width of the emission and absorption lines. Thus, interferences by other atoms are fortunately quite limited in AAS. The interference can result in erroneously high results. This can be overcome by using a smaller slit or selecting an alternate wavelength.
6.1.6 Background Absorption Interferences - There are two causes of background absorption: light scattering by particles in the flame and molecular absorption of light from the lamp by molecules in the flame. This interference cannot be corrected with standard addition method. The most common way to compensate for background absorption is to use a background corrector which utilizes a continuum source.
6.2 Standard Addition Method - One way of dealing with some of the interferences in the AAS methods is to use a technique called standard addition. IUPAC rule defines this technique as "Analyte Addition Method", however, the phrase "standard addition method" is well known and is widely used by the practitioners of AAS; hence, there is no need to adopt the IUPAC rule. This technique takes longer time than the direct analysis, but when only a few samples need to be analyzed, or when the samples differ from each other in the matrix, or when the samples suffer from unidentified matrix interferences this method can be used. The method of standard addition is carried out by: (1) dividing the sample into several (at least four) aliquots, (2) adding to all but the first aliquot increasing amount of analyte, (3) diluting all to the same final volume, and (4) measuring the absorbance, and (5) plotting the absorbance against the amount of analyte added. The amount of the analyte present in the sample is obtained by extrapolation beyond the zero addition. The method of standard addition may be less accurate than direct comparison; but when matrix interferences are encountered, it is necessary to use standard addition.
6.3 Chemical Suppressants - In some cases, ionization suppressors or other chemical reagents are added to the sample and standard solutions to suppress such interferences. Examples include: Test Method D3237 (lead in gasoline) uses iodine solution in toluene, Test Method D3831 (manganese in gasoline) uses bromine solution, and Test Method D4628 (additive elements in lubricating oils) uses potassium salt as ionization suppressant.
