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 pulsing the nebulizer pump, and lock-in-amplifier (LIA) detection of the modulated output signal of an ICP atomic emission spectrometer is studied. The time constant of the nebulizer spray chamber allows a maximum 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 improved, relative to the unmodulated system, as the detection limit is lowered without any loss of linearity at high concentrations. A linear dynamic range of greater than 4 orders of magnitude is found for the modulated system.