Essentially everyone who is utilizing an inductively coupled plasma has observed and studied the effects that plasma power and flow parameters and sample matrix have on analyte emission intensities. In analyzing the results of such experiments it is very important to appreciate the significant effect that the location and extent of the observation zone can have on the meaning and interpretation of the acquired data. For example, a short study carried out in our laboratory on the effect of plasma power on analyte emission intensities is shown in Fig. 1. Upon looking at this series of spectra it can be seen that the intensities of the Call and SrII lines are directly and strongly dependent on plasma power. The intensity of the Cal line, on the other hand, is seen not to be strongly dependent on power. In addition, its intensity appears to decrease as power is increased. For the measurements illustrated in Fig. 1 the plasma was imaged with a lens onto the entrance slit of a photodiode array spectrometer. The image formed in the plane of the entrance slit was a factor of 2 smaller than the object (plasma). One of the characteristics of this measurement system is such that a relatively narrow spatial region of the plasma (about 1 mm in height) is observed. This spatial window was set so as to observe the plasma emission about 20 mm above the load coil, which, in our system, is the region of strong ion line emission for species such as Ca and Sr.
G. Horlick and M. W. Blades, "Clarification of Some Analyte Emission Characteristics of the Inductively Coupled Plasma Using Emission Spatial Profiles," Appl. Spectrosc. 34, 229-233 (1980)