ASTM D7740 Standard Practice for Optimization, Calibration, and Validation of Atomic Absorption Spectrometry for Metal Analysis of Petroleum Products and Lubricants
7. Apparatus
7.1 A simple schematic representation of AAS is shown in Fig. 1.
7.2 The basic AAS instrument consists of a suitable light source emitting a light spectrum directed at the atomizer through single or double beam optics. The light emitted by the source is obtained from the same excited atoms that are measured in the atomizer. The light leaving the atomizer passes through a simple monochromator to a detector. The measured intensity is electronically converted into analytical concentration of the element being measured. Quantitative measurements in AAS are based on Beer's Law. However, for most elements, particularly at high concentrations, the relationship between concentration and absorbance deviates from Beer's Law and is not linear. Usually two or more calibration standards spanning the sample concentration and a blank are used for preparing the calibration curve. After initial calibration, a check standard at mid range of calibration should be analyzed.
7.3 The ground state atom absorbs the light energy of a specific wavelength as it enters the excited state. As the number of atoms in the light path increase, the amount of the light absorbed also increases. By measuring the light absorbed, a quantitative determination of the amount of the analyte present can be calculated.
7.4 Two types of AAS instruments use either single beam or double beam. In the first type, the light source emits a spectrum specific to the element of which it is made, which is focused through the sample cell into the monochromator. The light source is electronically modulated to differentiate between the light from the source and the emission from the sample cell. In a double beamAAspectrometer, the light from the source lamp is divided into a sample beam which is focused through the sample cell, and a reference beam which is directed around the sample cell. In a double beam system, the readout represents the ratio of the sample and the reference beams. Therefore, fluctuations in the source intensity do not become fluctuations in the instrument readout, and the baseline is much more stable. Both types use the light sources that emit element specific spectra.
7.5 In AAS, the sample solution whether aqueous or non-aqueous, is vaporized into a flame, and the elements are atomized at high temperatures. The elemental concentration is determined by absorption of the analyte atoms of a characteristic wavelength emitted from a light source, typically a hollow cathode lamp which consists of a tungsten anode and a cylindrical cathode made of the analyte metal, encased in a gas-tight chamber. Usually a separate lamp is needed for each element; however, multi-element lamps are in quite common use. The detector is usually a photomultiplier tube. A monochromator separates the elemental lines and the light source is modulated to discriminate against the continuum light emitted by the atomization source.
7.6 Burner System - A dual option burner system consists of both a flow spoiler and an impact bead for optimal operation under different analytical conditions. Equivalent precision is obtained with the air-acetylene flame using the flow spoiler or the impact bead. However, for nitrous oxide–acetylene flame, noticeably poorer precision is obtained when using impact bead.
7.7 Flame Sources:
7.7.1 Usually, AAS instruments use flame as the atomization source. An air-acetylene flame is used for most elements; the nitrous oxide-acetylene flame reaches higher temperature (2300°C for air-C 2 H 2 versus 2955°C for N2O-C2H2), and is used for atomizing the more refractory oxide forming metals. Flame conditions used in AAS are summarized in Table 4.
7.7.2 Out of several possible combinations (Table 4), air-acetylene and nitrous oxide-acetylene are the most commonly used flames as atomization sources in AAS. Over 30 elements can be determined with the air-acetylene flame. The nitrous oxide-acetylene flame is the hottest of the flames used and produces a maximum temperature of 3000°C. It can atomize refractory elements such as aluminum, silicon, vanadium, and titanium, and others, all forming highly refractory oxide molecules in the flame. Although nitrous oxide-acetylene flame can be used for the determination of over 65 elements, in practice it is used only where air-acetylene flame is ineffective.
7.8 Hollow Cathode Lamps:
7.8.1 A typical hollow cathode lamp consists of a quartz envelope containing a cathode, made of the element to be determined and a suitable anode. The sealed envelope is filled with an inert gas such as argon or neon at a low pressure. When a high voltage (up to 600 volts), is applied across the electrodes, positively charged gas ions bombard the cathode and dislodge atoms of the element used in the cathode. These atoms are subsequently excited and the spectrum of the chemical element is emitted. Hollow cathode lamps are preferred as the light sources because they generate a very narrow line, about one tenth of the elemental absorption line width. Usually these lamps are stable and can be used for several thousand determinations. By combining two or more elements of interest into one cathode, multi-element hollow cathode lamps are produced. For chemical elements which do not have close resonance lines and which are metallurgically compatible, multi-element hollow cathode lamps save the analyst considerable time not having to switch the lamps and recalibrate the instrument for the determination of multiple elements in the same sample.
7.8.2 Failure of hollow cathode lamps occur when the fill gas is gradually captured on the inner surfaces of the lamp, and finally, the lamp can no longer be lighted. Higher lamp current accelerates the gas depletion and cathode sputtering and should be avoided. It is a compromise between obtaining good sensitivity for the elements being determined and prolonging the lamp life.
7.8.3 Although hollow cathode lamps are an excellent, bright, and stable line source for most elements, for some volatile elements, where low intensity and short lamp life time is a problem, electrode-less discharge lamps can be used. The latter are typically more intense than hollow cathode lamps, and thus offer better precision and lower detection limits for some elements.
7.9 Nebulizers:
7.9.1 Liquid sample is introduced into a burner through the nebulizer by the venturi action of the nebulizer oxidant. In its passage through the nebulizer, the liquid stream is broken into a droplet spray. During nebulization some liquids are broken into a finer mist than others. For example, MIBK is more efficiently converted into a fine droplet size than water. The nebulizer draws the solution up a tube of narrow diameter or capillary. High-viscosity fluids flow through the capillaries at a slower rate than the low-viscosity fluids. Hence, it is important to keep the viscosities of the samples and standards solutions similar to avoid the possibility of physical interference problems.
7.9.2 Nebulizer capillaries readily become clogged by particulate material and they sometimes corrode. It is very important to keep the particulate materials out of the nebulizers even though it may require a time-consuming filtration step.
7.10 Monochromators - A monochromator isolates a single atomic resonance line from the line spectrum emitted by the hollow cathode lamp, excluding all other wavelengths. A typical resolution in AAS for this discrimination is 0.1 nm band-pass. The light emitted by the spectral source is focused onto a narrow entrance slit. From this the light diverges until it reaches the first mirror where it is collimated into a parallel beam and directed towards the grating.
7.11 Detectors - A photomultiplier is used as a detector device in AAS because of its sensitivity over the range of wavelength used in AAS. The photomultiplier produces an electrical signal which is proportional to the intensity of the light at the wavelength which has been isolated by the monochromator. This electrical signal is then amplified and is used to provide a quantitative measure of absorption.
7.12 Readouts - The readout system of an AAS consists of a way to convert the electrical signal from the photomultiplier to a meter, a digital display, or a graphic printout. All modern instruments are capable of directly converting the signal to a metal concentration after inputting the sample weight taken for analysis, and a previously prepared calibration curve.
8. Reagents and Materials
8.1 Purity of Reagents - Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all reagents conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society where such specifications are available. Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination.
8.2 Base Oil - U.S.P. white oil, or a lubricating base oil that is free of analytes, and having a viscosity at room temperature as close as possible to that of the samples to be analyzed.
8.3 Organometallic Standards - Multi-element standards, containing 0.0500 mass % of each element, can be prepared from the individual concentrates. Refer to Practice D4307 for a procedure for preparation of multi-component liquid blends. When preparing multi-element standards, be certain that proper mixing is achieved. An ultrasonic bath is recommended. Most laboratories use commercially available stock standards in either single or mixed element formats and at varying concentrations.
NOTE 1 - Secondary standards such as those prepared from petroleum additives, for example, can be used in place of those described. If the use of such secondary standards does not affect the analytical results by more than the repeatability of this test method.
8.4 Dilution Solvent - A solvent that is free of analytes and is capable of completely dissolving all standards and samples. Mixed xylenes, kerosine, and ortho-xylene have been successfully used as dilution solvents.
