SAMPLE INPUT MODULATION AND LOCK-IN-AMPLIFIER DETECTION FOR SIGNAL-TO-NOISE RATIO IMPROVEMENT IN INDUCTIVELY-COUPLED PLASMA-ATOMIC EMISSION-SPECTROSCOPY

The effect of modulation of the sample atomization, accomplished by pu lsing the nebulizer pump, and lock-in-amplifier (LIA) detection of the modulated output signal of an ICP atomic emission spectrometer is stu died. The time constant of the nebulizer spray chamber allows a maximu m modulation frequency of approximately 2 Hz, but optimum performance for this system is found at a modulation frequency of approximately 1 Hz. A signal-to-noise (SNR) ratio improvement approaching a factor of three is found for the arrangement employed. It is proposed that more rapid modulation, achievable through the use of a nebulization system with a shorter time constant, should lead to even greater improvement in the SNR than was accomplished in this study. The dynamic range is i mproved, relative to the unmodulated system, as the detection limit is lowered without any loss of linearity at high concentrations. A linea r dynamic range of greater than 4 orders of magnitude is found for the modulated system.