Evaluation of ultrasonic nebulization for the analysis of transient samples: A theoretical model and practical considerations

Three spray chambers designed for use with ultrasonic nebulization have been evaluated for the introduction of transient samples into an inductively coupled plasma. The spray chambers were (1) a conventional Fassel type, (2) a commercially available CETAC spray chamber, and (3) a novel spray chamber design that was constructed in-house. All have different volumes and modes of central-channel gas introduction, and consequently different washout times. Interestingly, they can all be described by the same model for both their washin and washout times. This model was able to predict the time response of the three spray chambers based upon the convolution of a Gaussian and an exponential curve. In particular, the experimental washout times can be successfully fit to an exponential-decay model that pertains to an ideal mixing chamber. It was found that the washin time (Gaussian response) was limited mainly by the nebulization process itself and partly by dispersion of the sample aerosol as dictated by the length of the desolvation assembly and by the length of tubing that carried the dry aerosol to the plasma source. It was determined that the spray chamber washout time is the factor that limits instrument response and that the desolvation system utilized in the study caused no appreciable peak tailing but did slighty broaden the Gaussian peak width caused by a laminar flow profile and by the nebulization event. The newly designed spray chamber offered washout times that are 1.6 and 5.2 times faster than the CETAC and Fassel spray chambers, respectively. The practical limitations of ultrasonic nebulization for the analysis of transient samples is discussed with a focus on spray-chamber design and based upon the simple model that was developed for the response curves of transient samples.