Page 33 - AAS & AES & FES 01082016_Neat

wavelength selectors. The detectors are the same as those used for AAS. Generally a

photomultiplier tube is used as the detector. Readout devices also are identical to those used in

AAS. Analog meters, digital meters, and computer-controlled devices are encountered most
often.

The burners that are used for FES can be either total-consumption burners or premix
burners. In the past total-consumption burners were used widely. Because of the erratic flames

that result from total-consumption burners, premix burners are presently used for most analyses.

The flame gases can be any of those described for AAS. As the temperature increases the
number of excited atoms and the intensity of the emitted radiation increases.

A flame that is too hot can result in a decreased signal owing to ionization of the analyte
element. Low-temperature flames can be used for easily excited group IA elements. An air-

acetylene flame can be used for the assay of the bulk of the elements. Nitrous oxide-acetylene
flames can be used for refractory compounds and less easily excited elements.

The interferences that are encountered with FES are the same as those encountered in

other techniques that utilize flames as sources of atoms. Chemical, spectral, and ionization
interferences are essentially identical to those described for AAS. Spectral interferences are more

likely to be encountered in FES than in AAS because the bandpass of the monochromator, rather
than the relatively narrow emitted bandwidth of a line source, determines the wavelength range

of the measured signal.

In AAS, radiation from the source was chopped and the detector was tuned to respond
only to radiation at the chopping frequency. In that way some steady state interferences

emanating from the flame could be eliminated. Of course, that is not possible with FES.
Generally FES is more sensitive to fluctuations of the flame than is AAS.

Difficulties associated with fluctuations in the intensity of measured emissive line that

result from fluctuating flame conditions or a fluctuating rate of aspiration into the flame often
can be overcome by use of the Internal-standard method. A known constant concentration of a

second element is added to each standard solution of the analyte. The element of known
concentration is the internal standard.

The Intensities of both the analyte line and the line of the internal standard are measured.
A working curve is prepared by plotting the ratio of the measured Intensity of the analyte line to

the intensity of the line from the internal standard. Because both elements were subject to the

same environmental conditions, the assumption is made that the effects on the two elements are
identical. Consequently, the resultant working curve compensates for the problems associated

with erratic flames and rates of aspiration.

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