Analytical Applications of Flame OES

Flame OES can be used to determine the concentrations of elements in samples. The sample usually must be in solution form. Generally, one element is determined at a time if using an AAS system in emission mode. Multichannel instruments are available for the simultaneous determination of two or more elements. Detection limits can be very low as seen in Appendix 7.1, Table Al. Detection limits for the alkali metals are in the ppt concentration range when ionization suppression is used. One part per trillion in an aqueous solution is 1 pg of analyte per mL of solution or 1 x g/mL. Most elements have detection limits in the high ppb to low ppm range. 

An important factor in quantitative analysis by flame OES is the solvent used for the samples and standards. When water is the solvent, the process of atomization is endothermic and relatively slow. If the solvent is organic, the reactions in the flame are exothermic and atomization is rapid. Other things being equal, more free atoms are liberated and 

If we aspirate an unknown sample solution and measure an intensity of 1626 counts, we can use the equation to calculate the Li concentration in the unknown. Inserting 1626 as y in the equation and solving for x, the concentration x is calculated to be 5.88 ppm Li in the unknown solution. 

If we subtracted the blank first, the calculation would be as shown in Table 7.5. Now in order to use the blank-corrected calibration curve, we have to subtract the background intensity from our sample intensity. The corrected intensity is shown in Table 7.5. Inserting y = 1176 into the equation shown and solving for x gives us 5.88 ppm Li. As expected, the results are the same since the background was accounted for in both approaches (and assumed to be the same in all samples and standards). 

Suppose we need to determine Li in human serum to monitor an Li-based antidepressant medication. We would not expect the background from a serum sample to be the same as the background from an aqueous standard solution. We might make our calibration standards in lithium-free semm, but if we only have one sample to run, that is not efficient in terms of cost or time. This is a good example of when to use the MSA. In MSA, the calibration curve is made up in the sample itself. 

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