Evaluation - Atomic Emission Spectroscopy

Evaluation

Scale of Operation 

The scale of operations for atomic emission is ideal for the direct analysis of trace and ultratrace analytes in macro and meso samples. With appropriate dilutions, atomic emission also can be applied to major and minor analytes.

Accuracy 

When spectral and chemical interferences are insignificant, atomic emission is capable of producing quantitative results with accuracies of 1–5%. Ac- curacy in flame emission frequently is limited by chemical interferences. Because the higher temperature of a plasma source gives rise to more emission lines, accu- racy when using plasma emission often is limited by stray radiation from overlap- ping emission lines.

Precision 

For samples and standards in which the concentration of analyte ex- ceeds the detection limit by at least a factor of 50, the relative standard deviation for both flame and plasma emission is about 1–5%. Perhaps the most important factor affecting precision is the stability of the flame’s or plasma’s temperature. For exam- ple, in a 2500 K flame a temperature fluctuation of ±2.5 K gives a relative standard deviation of 1% in emission intensity. Significant improvements in precision may be realized when using internal standards.

Sensitivity 

Sensitivity in flame atomic emission is strongly influenced by the tem- perature of the excitation source and the composition of the sample matrix. Nor- mally, sensitivity is optimized by aspirating a standard solution and adjusting the flame’s composition and the height from which emission is monitored until the emission intensity is maximized. Chemical interferences, when present, decrease the sensitivity of the analysis. With plasma emission, sensitivity is less influenced by the sample matrix. In some cases, for example, a plasma calibration curve prepared using standards in a matrix of distilled water can be used for samples with more complex matrices.

Selectivity 

The selectivity of atomic emission is similar to that of atomic absorp- tion. Atomic emission has the further advantage of rapid sequential or simultane- ous analysis.

Time, Cost, and Equipment 

Sample throughput with atomic emission is very rapid when using automated systems capable of multielemental analysis. For exam- ple, sampling rates of 3000 determinations per hour have been achieved using an ICP with simultaneous analysis, and 300 determinations per hour with a sequential ICP. Flame emission is often accomplished using an atomic absorption spectrome- ter, which typically costs $10,000–50,000. Sequential ICPs range in price from $55,000 to $150,000, whereas an ICP capable of simultaneous multielemental analy- sis costs $80,000–200,000. Combination ICPs that are capable of both sequential and simultaneous analysis range in price from $150,000 to $300,000. The cost of Ar, which is consumed in significant quantities, cannot be overlooked when consider- ing the expense of operating an ICP.