Flame AAS

Flame AAS methods have been applied to analyses of metals in ambient air [1—8]. If sufficiently large samples are taken, a wide range of elements can be determined. 

Several methods are in general use for introduction of a material into the flame. These include solution aspiration, gas (hydride) evolution and entrainment into the flame, and direct introduction of solid substrates. All three have been used for flame-AAS analysis of trace elements from air sampling however, most work is carried out by use of the first two methods. In each case a suitable solution is first obtained by the dissolution of the elements of interest from the sample. Sample dissolution will be discussed in greater detail in a following section. 

The topic of interferences in AAS analyses is dealt with in Chapter 3 of this book. In general it is wise to take the precaution of checking for sample matrix interferences using the method of standard additions, to make general use of background correction techniques unless proven unnecessary, and to match closely the matrices of samples and standards. These precautions will limit the likelihood of errors due to a variety of potential interferences. 

Solution aspiration rates, fuel and oxidant mixtures, gas flow rates, burner choice, matrix effects and interelement interferences must all be taken into account when using flame AAS. While optimal choices for the above parameters vary from instrument to instrument, recommendations which afford reasonable starting points for operation have been published by both the Intersociety Committee for Methods of Air Sampling and Analysis (ISC) [7] and the National Institute of Occupational Safety and Health (NIOSH) [8]. These recommendations were the result of ISC efforts supported by both the United States Environmental Protection Agency and NIOSH. 

1 Flame AAS. Table 4 lists various reasons for low sensitivity and poor precision in FAAS. 

Incorrect standard solution Particles greater than 10 pm Incorrect chemistry (no ionization buffer) 

Partially blocked nebulizer Partially blocked burner Burner mis-aligned Dirty spray chamber Moisture trap in air supply full 

Incorrect standard reference material, incorrect standard preparation, and incorrect sample preparation can cause incorrect results. In addition, if background correction is not employed when required, it will give rise to erroneous results. 

In FAAS a continuous flow of aerosol reaches the flame where it produces a steady signal which is proportional to the analyte concentration in the measurement solution. With an aspiration rate of 5-10 mL/min the sample consumption is typically of the order of 1-2 mL per determination. This sample consumption can be reduced to values around 0.1 mL when flow injection (FI) is used for sample introduction [4]. In this technique a continuous flow of a carrier, such as water or a dilute acid, into which microliter volumes of the measurement solution are injected is transported to the nebulizer, producing a transient instead of a steady-state signal [5]. Among die 

Schematic design of a mixing chamber burner for AAS (Courtesy of Perkin-Elmer). 

FAAS is a fast technique which is easy to automate, particularly when FI techniques are used, and is hence well suited for routine applications. However, FAAS is not directly applicable to trace element determination, particularly in tissue samples, which are often significantly diluted in the acid digestion procedure. Because of its simplicity, FAAS is nevertheless often the detector of choice for various preconcentration procedures. It is particularly attractive in combination with online preconcentration and separation using FI techniques (see Sec. 5.2 and 5.3). 

A well-designed mixing chamber burner with an impact system produces an aerosol with a high percentage of fine droplets which can be completely volatilized and atomized in the laminar flames of low burning velocity used in FAAS. For this reason spectral interferences are rarely observed in FAAS. 

Among the nonspectral interferences transport interferences in the nebulizer are relatively common in the analysis of body fluids. This is certainly no problem when 10- or 20-fold diluted serum is used for the determination of the electrolytes. If, however, undiluted or only slightly diluted body fluids are aspirated directly, the viscosity of these liquids can impair aspiration rate and nebulization efficiency relative to the reference solutions used. If the sample solution cannot be diluted sufficiently to avoid this interference, a frequently used alternative is matrix-matched standards, i.e., reference solutions with a viscosity close to that of the samples. Another alternative is to use the method of additions, which can perfectly correct for this interference. This calibration technique, however, is labor-intensive and time consuming, and is restricted to the linear part of the calibration curve. Viscosity of the sample solutions is much less of a problem when FI techniques are used for sample introduction. This is because samples are not aspirated but pumped to the nebulizer, because much smaller sample volumes are used, and because the sample is always in a carrier solution which supports nebulization and removes all potential residues in the nebulizer-bumer system. 

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For the bones the preferenee has been given to atomie-absorption speetrometry with flame and graphite furnaee atomization beeause of a strong effeet of ealeium and phosphorous on the analytieal signals of mieroelements under determination in DCP-ai e AFS. It has been shown that In the presenee of lanthanum ehloride no interferenee effeets were observed in flame AAS for Ca, Mg and Sr. FTA AAS has been used to determine Mn and Li in bones. RSD for FAAS determination of Ca, Mg, Sr were 3-6 %, as for Li and Mn -10-12%. 

The main advantages of electrothermal atomisers are that (a) very small samples (as low as 0.5 pL) can be analysed (b) often very little or no sample preparation is needed, in fact certain solid samples can be analysed without prior dissolution (c) there is enhanced sensitivity, particularly with elements with a short-wavelength resonance line in practice there is an improvement of between 102- and 103-fold in the detection limits for furnace AAS compared with flame AAS. 

Method abbreviations D-AT-FAAS (derivative flame AAS with atom trapping), ETAAS (electrothermal AAS), GC (gas chromatography), HGAAS (hydride generation AAS), HR-ICP-MS (high resolution inductively coupled plasma mass spectrometry), ICP-AES (inductively coupled plasma atomic emission spectrometry), ICP-MS (inductively coupled plasma mass spectrometry), TXRF (total reflection X-ray fluorescence spectrometry), Q-ICP-MS (quadrapole inductively coupled plasma mass spectrometry)... 

NIOSH. 1994. NIOSH Manual of Analytical Methods, 4th edition. Methods 7082 (Lead by Flame AAS), 7105 (Lead by HGAAS), 7505 (Lead Sulfide), 8003 (Lead in blood and urine), 9100 (Lead in Surface Wipe Samples), U.S. Department of Health and Human Services, Centers for Disease Control, National Institute for Occupational Safety and Health. 

A combination of hydride generation with GFAAS was proposed for signal enhancement, instead of flame AAS. It was found that older tubes had better sensitivity than new ones31. 

Currently a balance seems to have been reached in the use of the various techniques for the determination of metals at trace levels. In its modern form AAS remains important and competitive where small ranges of elements need to be determined in samples. Sensitivities obtainable by flame AAS are often similar to those for ICP-AES and where graphite furnace volatilization is used they are not infrequently superior (Table 8.4). 

Preetha CR, Biju VM, Rao TP. On-line solid phase extraction preconcentration of ultratrace amounts of zinc in fractionated soil samples for determination by flow injection flame AAS. At. Spectrosc. 2003 24 118-124. 

Flame AAS can be used to measure about 70 elements, with detection limits (in solution) ranging from several ppm down to a few ppb (and these can be enhanced for some elements by using a flameless source). Both sensitivity and detection limits (as defined fully in Section 13.4) are a function of flame temperature and alignment, etc. The precision of measurements (precision meaning reproducibility between repeat measurements) is of the order of 1-2% for flame AA, although it can be reduced to <0.5% with care. The accuracy is a complicated function of flame condition, calibration procedure, matching of standards to sample, etc. 

One important factor which limits the performance of flame AAS is interference, both spectral and chemical. Spectral interference occurs where emission lines from two elements in the sample overlap. Despite the huge number of possible emission lines in typical multielement samples, it is rarely a problem in AA, unless molecular species (with broad emission bands) are present in the flame (in which case, a higher temperature might decompose the interfering molecule). If spectral interference does occur (e.g., A1 at 308.215 nm, V at 308.211 nm) it is easily avoided by selecting a second (but perhaps less sensitive) line for each element. 

Flame AA utilizes a large flame as the atomizer. A photograph of this atomizer was first shown in Figure 7.20 and is reproduced here in Figure 9.2. The sample (a solution) is drawn into the flame by a vacuum mechanism that will be described. The atomization occurs immediately. The light beam for the... 

Flame AA is the oldest of the three techniques and has been widely used for many years. The instruments are relatively inexpensive and methods have been well tested and are well understood. 

TABLE 9.1 Listing Showing Which Oxidant Is Recommended for the Various Metals and Nonmetals Analyzed by Flame AA... 

The burner used for flame AA is a premix burner. It is called that because all the components of the flame (fuel, oxidant, and sample solution) are premixed, as they take a common path to the flame. The fuel and oxidant originate from pressurized sources, such as compressed gas cylinders, and their flow to the burner is controlled at an optimum rate by flow control mechanisms that are part of the overall instrument unit. 

The optical path for flame AA is arranged in this order light source, flame (sample container), monochromator, and detector. Compared to UV-VIS molecular spectrometry, the sample container and monochromator are switched. The reason for this is that the flame is, of necessity, positioned in an open area of the instrument surrounded by room light. Hence, the light from the room can leak to the detector and therefore must be eliminated. In addition, flame emissions must be eliminated. Placing the monochromator between the flame and the detector accomplishes both. However, flame emissions that are the... 

Interferences can be a problem in the application of flame AA. Interferences can be caused by chemical sources (chemical components present in the sample matrix that affect the chemistry of the analyte in the flame) or spectral sources (substances present in the flame other than the analyte that absorb the same wavelength as the analyte). 

Besides flame AA and graphite furnace AA, there is a third atomic spectroscopic technique that enjoys widespread use. It is called inductively coupled plasma spectroscopy. Unlike flame AA and graphite furnace AA, the ICP technique measures the emissions from an atomization/ionization/excitation source rather than the absorption of a light beam passing through an atomizer. 

The sample is drawn from its container with a peristaltic pump to a nebulizer. There are several designs for the nebulizer, but it performs the same function as the flame AA nebulizer, converting the sample solution into fine droplets (with the larger droplets flowing to a drain) that flow with the argon to the torch. The emissions are measured by the spectrometer at a particular zone in the torch, often called the viewing zone or analytical zone, as shown in Figure 9.19. 

The advantage of ICP is that the emissions are of such intensity that it is usually more sensitive than flame AA (but less sensitive than graphite furnace AA). In addition, the concentration range over which the emission intensity is linear is broader. These two advantages, coupled with the possibility of simultaneous multielement analysis offered by the direct reader polychromator design, make ICP a very powerful technique. The only real disadvantage is that the instruments are more expensive. See Workplace Scene 9.3. 

Element Flame AA Graphite Furnace AA ICP Cold Vapor Hg Hydride... 

Flame AA Light absorbed by atoms in a flame is measured A well-established technique that remains very popular for a wide variety of samples and analytes... 

Experiment 30 Verifying Optimum Instrument Parameters for Flame AA... 

Does Beer s law apply in the case of flame AA Explain. 

What is meant by matrix matching Why is this important in a flame AA experiment ... 

What are three safety issues with regard to flame AA and how are they dealt with ... 

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