Analytical Chem istry - DePauw University

Chapter 10 Spectroscopic Methods6351–5%. Accuracy frequently is limited by chemical interferences. Becausethe higher temperature of a plasma source gives rise to more emission lines,the accuracy of using plasma emission often is limited by stray radiationfrom overlapping emission lines.PrecisionFor samples and standards in which the analyte’s concentration exceeds thedetection limit by at least a factor of 50, the relative standard deviation forboth flame and plasma emission is about 1–5%. Perhaps the most importantfactor affecting precision is the stability of the flame’s or the plasma’stemperature. For example, in a 2500 K flame a temperature fluctuationof ±2.5 K gives a relative standard deviation of 1% in emission intensity.Significant improvements in precision may be realized when using internalstandards.SensitivitySensitivity is strongly influenced by the temperature of the excitation sourceand the composition of the sample matrix. Sensitivity is optimized by aspiratinga standard solution of analyte and maximizing the emission byadjusting the flame’s composition and the height from which we monitorthe emission. Chemical interferences, when present, decrease the sensitivityof the analysis. The sensitivity of plasma emission is less affected by thesample matrix. In some cases a calibration curve prepared using standardsin a matrix of distilled water can be used for samples with more complexmatrices.Se l e c t i v i t yThe selectivity of atomic emission is similar to that of atomic absorption.Atomic emission has the further advantage of rapid sequential or simultaneousanalysis.Tim e , Co s t, a n d Eq u i pm e n tSample throughput with atomic emission is very rapid when using automatedsystems capable of multielemental analysis. For example, samplingrates of 3000 determinations per hour have been achieved using a multichannelICP, and 300 determinations per hour with a sequential ICP. Flameemission is often accomplished using an atomic absorption spectrometer,which typically costs between $10,000–$50,000. Sequential ICP’s rangein price from $55,000–$150,000, while an ICP capable of simultaneousmultielemental analysis costs between $80,000–$200,000. CombinationICP’s that are capable of both sequential and simultaneous analysis rangein price from $150,000–$300,000. The cost of Ar, which is consumed insignificant quantities, can not be overlooked when considering the expenseof operating an ICP.