Graphite furnace technique

The flow injection AAS system with online preconcentration will challenge the position of the graphite furnace technique, because it yields comparable sensitivity at much lower cost by using simpler apparatus and separation mode. The method offers unusual advantages when matrices with high salt content (e.g., seawater) are analysed, because the matrix components do not reach the nebuliser. 

The preponderance of work on multielement analysis in seawaters has been carried out using the graphite furnace technique, as this has the additional sensitivity over the direct technique that is required in seawater analysis. 

Inductively coupled plasma atomic emission spectrometry has proved to be an excellent technique for the direct analysis of soil extracts because it is precise, accurate and not time-consuming, the level of matrix interference being very low. Of course, the graphite furnace technique yields better detection limits than the inductively coupled plasma procedure. 

Atomic absorption spectrometry (AAS) is a relatively new analytical technique among the spectroscopic methods. As described in the previous chapters, AAS gives high sensitivity, precision and accuracy along with experimental convenience and a wide instrumental availability. Therefore, this technique has been extensively employed for the analysis of marine samples. However, the elemental contents of marine samples are generally very low, and suitable preconcentration procedures are required. Recent development of graphite-furnace techniques and gas generation techniques has extended the applicability of AAS to marine analysis. The determination of Cd, Cu, Ni, Pb, Hg, As, Sb, Se, Sn and Te has become much easier as a result of the development of these techniques. 

The few articles currently available regarding trace analysis without preconcentration, use in general the graphite furnace technique [102,120, 138] with sample sizes of the order of microliters, and deal with the elements Sb [47, 83], Pb and Bi [48-50], As, Sb, Bi, Sn, Cd, Pb [10, 57, 116] as well as Al, Cr, Sn [6, 62], Co, and Mg [104]. Alkaline earths can be determined directly with the flame method [122, 147], Further techniques of atomic absorption by flame use concentration methods, for example for the determination of small concentrations of tin [17], Te [26], Co, Pb, and Bi [104], and W [106]. From the analytical viewpoint, it is only useful to remove the iron matrix. The extraction of the elements to be determined from the matrix always carries with it the danger of losses and therefore results showing concentrations that are too low. 

The graphite furnace technique for atomic absorption spectrophotometry permits molybdenum determination at the ng level with small sample volumes. This technique is, however, prone to matrix interferences when used for molybdenum analysis. The boiling point of molybdenum is about 4600°C, about 1800°C above the maximum temperature obtained in the graphite furnace. Therefore, molybdenum atomization can only take place by a mechanism which includes the formation of a compound that is volatile at 2800°C. Molybdenum atoms will only be produced if the volatile molybdenum compound dissociates at this temperature. Any ions or compounds which affect the complicated atomization mechanism will alter the sensitivity of the method. This crucial fact is the primary reason for the extreme matrix sensivity of the method. 

Atomic absorption methods are the most frequently used for the determination of trace metals in a wide range of materials. This review is written at a point in time when most analysts still view the graphite furnace technique as an accessory to flame atomic absorption. Actually the two techniques are quite separate and independent. This review is thus written with the understanding that atomic absorption is two techniques, flame AAS and furnace AAS. This position will not be defended here but such a defense has been published (Slavin, 1986). Thus we treat the two techniques in separate chapters. 

The so-called Delves cup method was applied in many laboratories for PbB determination before the graphite furnace technique reached its highly developed present stage. The method was published by Delves (1970) and at that time it was one of the first reliable micro methods requiring only very small amounts of blood. It has been adopted by several authors for the analysis of capillary blood as well as for lead determinations on disks punched from filter paper previously spotted with a drop of blood (Cernik and Sayers, 1971 Cernik, 1974 Delves, 1977). 

The basic concepts of atomic absorption spectrometry were published first by Walsh in 1955, this can be regarded as the actual birth year of the technique. At the same time Alkemade and Milatz designed an atomic absorption spectrometer in which flames were employed both as a radiation source and an atomizer. The commercial manufacture of atomic absorption instruments, however, did not start until ten years later. Since then the development of atomic absorption spectrometry has been very fast, and atomic absorption (AA) instruments very quickly became common. The inventions of dinitrogen oxide as oxidant and electrothermal atomization methods have both significantly expanded the utilization field of atomic absorption spectrometry. These techniques increased the number of measurable elements and lowered detection limits. Todays graphite furnace technique is based on the studies of King at the beginning of the twentieth century. 

Non-specific background absorption occurs to a larger extent in the graphite furnace technique than with flame AAS. For this reason, it is urgently recommended that automatic background compensation be used. Whenever a new method is developed, the background absorption of typical... 

Increasingly, due to their superior intrinsic sensitivity, the AAS currently available are capable of implementing the graphite furnace techniques. Available suppliers of this equipment are listed in Appendix 1. 

Graphite furnace techniques are about one order of magnitude more sensitive than direct injection techniques. Thus lead can be determined down to 50 pg/1 by direct AAS and down to 5 pg/1 using the graphite furnace modification of the technique. 

The Zeeman technique, although difficult to establish, has an intrinsic sensitivity perhaps five times greater than that of the graphite furnace technique, e.g., a 1 pg/1 detection limit for lead. 

Increasingly, due to their superior sensitivity, AAS instruments can implement graphite furnace techniques. 

Detection limits are based on 98% confidence level (3 standard deviations). The values for the graphite furnace technique are referred to sample aliquots of 50 pL. 

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