Flameless Atomic Absorption spectrophotometry

The FAAS method offers similar detection limits to NAA and is suitable for the determination of low levels of lead. Equipment costs are reasonable and the instrumentation is commonplace in many analytical laboratories. A large number of metallic elements, over a wide concentration range, extending down to ultra-trace level, can be analyzed, thus making the technique versatile and useful for other forensic applications as well as FDR detection. Apart from cost, the main advantages are simplicity, speed of analysis, and "in house" operation. One disadvantage of FAAS is that it is not capable of simultaneous multielement analysis.

In FAAS the sample is electrically heated to a high temperature, thus breaking the chemical bonds and enabling individual atoms to float freely in the sample area. These ground state atoms are then capable of absorbing ultraviolet or visible radiation. The wavelength bands that each specific element can absorb are very narrow and different for every element. The desired element can be considered able to absorb only "resonance lines" whose wavelengths correspond to transitions from its ground state to some higher energy level.

Amplifier Spectrophotometer

Data Amplifier collection

Figure 16.2 Basic components of a flameless atomic absorption spectrophotometer.

Data Amplifier collection

Figure 16.2 Basic components of a flameless atomic absorption spectrophotometer.

A basic atomic absorption instrument includes a source of radiation, a system for placing sample atoms in the ground state, a monochromator to separate the resonance line of interest, and finally a detector to measure the decrease in signal from the source when absorbing atoms are placed in the light beam. The magnitude of the decrease in signal is dependent on the amount of the element of interest in the sample. Figure 16.2 illustrates the components of a basic flameless atomic absorption spectrophotometer.

FAAS is the most popular technique for the quantitative determination of elements associated with FDR: lead, antimony, barium, copper, and mercury. Other relevant elements have also been determined, and the use of FAAS for FDR detection is well documented in the literature.135-138

All bulk elemental analysis methods, such as NAA and FAAS, suffer from the serious disadvantage of lack of specificity in that the elements detected are not unique to FDR but also occur from occupational and environmental sources. Many surveys were carried out to determine background levels of lead, antimony, and barium on the hands of people not involved with firearms. Some surveys also included copper and mercury. Both general and occupational data were gathered and threshold levels established for each of the elements. The threshold level may be defined as the level above which the results may be significant and correlate to the discharge of a firearm. The best that could be stated was that the levels detected were consistent with the discharge of a firearm but could not be taken as conclusive proof of the presence of FDR.

A more definite method was sought resulting in the particle analysis method, which is claimed to conclusively identify FDR particles.139-144

0 0

Post a comment