Methods for shot-to-shot normalization in laser ablation with an inductively coupled plasma time-of-flight mass spectrometer

Single-shot laser ablation (one laser pulse per analysis) combined with inductively coupled plasma mass spectrometry offers tremendous potential for the direct elemental analysis of solid samples with high spatial resolution. However, the precision of the method is usually limited by variations in the laser irradiance and in the specimen being sampled. In this study, two data analysis techniques were evaluated to improve single-shot measurement precision. Both methods exploit the simultaneous full-spectrum acquisition capability of a time-of-flight mass spectrometer. In the first approach, a normalization factor is computed from the summed signal generated by all sample constituents, the reasoning being that the summed spectrum should be proportional to the total mass of sample ablated. This scheme results in a greater than factor of two improvement in precision, moderately better than is possible with a single internal standard. The enhancement in measurement precision was found to be concentration dependent, with the greatest improvement (10-50 fold) experienced by high concentration elements. The second method correlates the attenuation of plasma matrix ions to analyte intensities. The attenuation technique demonstrated no statistically significant improvement in precision, limited by the relatively low signal-to-noise ratio of the attenuated signals.