New procedure for intensity and detection limit determination in spectral trace analysis: application for trace mercury by TXRF

A new procedure for the identification and quantification of spectral signals based on the method of maximum likelihood is presented. It is particularly appropriate for the analysis of signal interference or cases of a small signal in a sizeable noisy background. The formulation yields precise signal intensity values and has advantages in its estimation of detection limits over established signal processing methods. The method developed is applicable to spectroscopic techniques where net counts are proportional to acquisition time, such as particle induced x‐ray emission (PIXE) and other x‐ray fluorescence spectroscopic methods, Mössbauer spectroscopy, some cases of α, β and γ spectroscopy, etc. The proposed method was applied in two experimental instances to the quantification of traces of the XRF mercury signal in the presence of a large, strongly interfering gold signal: (i) In liquid solutions assayed by means of total reflection x‐ray fluorescence (TXRF) spectrometry, where mercury amalgamated on a gold‐plated thin film sputtered on pure quartz reflectors was irradiated with a molybdenum anode x‐ray tube; detection limits achieved by the method were 100 ppb, significantly lower than obtained by standard net‐area processing procedures; (ii) solid sample XRF spectra analyzed with the proposed method yielded consistent and reliable signal intensities and evidence of detection limits almost an order of magnitude smaller than achieved by standard net‐area processing procedures. Copyright © 2002 John Wiley & Sons, Ltd.