A Modulated Sample Introduction System for Atomic Emission Spectrometry

A modulated sample-introduction device for use in flame emission spectrometry has been constructed and evaluated. With this device, the flow of aerosol to the excitation source is modulated by the application, at a specific frequency, of a pressure pulse to the nebulizer chamber; the use of frequency-selective detection then permits efficient signal recovery. If source background remains at a constant level, it is not detected and the signal-to-background ratio (S/B) is consequently improved. Because the technique provides a decrease in the instrument 1/f and flame-background flicker noise, detection limits are improved. These features are especially prominent when the analyte line of interest is in a spectral region of high source background. Modulation frequencies of 8 to 20 Hz have been explored and have resulted in good depth of modulation (>99%) and signal-pulse separation. The construction and operation of the device will be described and its performance for several elements evaluated in terms of detection limits, S/B enhancement, and working curve linearity. Also, the extension of the technique to selective spectralline modulation schemes and to inductively coupled plasma emission spectrometry will be considered.