Signal to noise ratios for continuum source atomic absorption spectrometry using a linear photodiode array to monitor sub-nanometre wavelength intervals

Abstract

The development and initial evaluation of a pulsed continuum source atomic absorption spectrometer using a linear photodiode array and weak pulsing of the source is discussed. A 256-pixel array, mounted in a high-dispersion monochromator, was used to monitor a narrow interval (0.4–0.8 nm) around the wavelength of interest, resulting in rapid (<10 ms) readout times. Experimental results confirmed the readout noise limitation of the array. The minimisation of readout lag, quantisation noise and relative analogue-to-digital (A/D) converter noise resulted in a readout noise limit that was twice the manufacturer's specifications. Pixel averaging, in conjunction with a large entrance slit width, reduced the absorbance noise. With flame atomisation, use of a linear photodiode array resulted in a factor of 3.9–6.2 improvement in the signal to noise ratio of five elements (zinc, cadmium, nickel, cobalt and iron) in comparison with photomultiplier tube detection. Lamp pulsing resulted in a 50% increase in integrated intensity and was limited by the power rating of the power supply. The combined effect of a pulsed source and photodiode array detection improved the signal to noise ratios by factors ranging from 5.8 to 8.5.