Evaluation of a water-jacketed spray chamber using time-of-flight inductively coupled plasma mass spectrometry (TOF-ICP-MS)

Abstract

The application of a water-jacketed spray chamber was evaluated at several temperatures between 4 and 30 °C using a time-of-flight inductively coupled plasma mass spectrometry (TOF-ICP-MS) instrument. As expected when the spray chamber was operated at lower temperatures, less water vapour was transported to the ICP. However, sensitivity (signal per unit concentration) was unchanged when the nebuliser gas flow was optimized at each of the different temperatures, and the expected lowering of the degree of formation of oxide polyatomic ions was not achieved. The effect of lowering the temperature of the water that controlled the temperature of the spray chamber apparently did lower the rate of introduction of water vapour into the ICP. This decrease in water vapour introduced into the ICP resulted in (at the same value of nebuliser gas flow) an apparent increase in plasma temperature. This in turn resulted in a decrease in sensitivity, which was regained by increasing nebuliser gas flow (lower plasma temperature). The maximum sensitivity was comparable at all spray chamber temperatures studied, with higher nebuliser gas flows being required with lower spray chamber temperatures. Other parameters of interest, the background, oxide formation, doubly charged ion formation and detection limits, were comparable at all spray chamber temperatures. This study emphasizes the importance of making multidimensional studies of the ICP-MS. The figure of merit behaviour of the system was comparable to that obtained with first generation quadrupole ICP-MS instruments. With increasing nebuliser gas flow (0.5 to 0.9 l min−1) very striking changes were observed in several parameters. The background decreased from ca. 700 counts s−1 to less than 10 counts s−1; the sensitivity (M counts s−1 ppm−1 abundance−1) varied from very low to a high of ca. 3 and then decreased; the formation of polyatomic oxide ions (using as an example CeO+/Ce+) was approximately 30% at the maximum sensitivity; and the detection limit for Ce reached a minimum of less than 0.01 ppb at an intermediate value of the nebuliser gas flow.