NOTE Zimmerman and Nielsen: Measurement of strontium-to-calcium ratios in otoliths of anadromous salmonids 



715 



o 



5 

 4 

 3 

 2 



1 

 



5 - 

 4 



3 - 

 2 



1 

 

 5 



4 - 

 3 



2 

 1 

 



5 - 

 4 



3 



2 



1 



- 



5 



4 



3 



2 



1 







7 Lim 

 •- 



20 Mm 



B 



Sample sequence at same location 



Figure 2 



Repeat measures of atomic Sr/Ca with (A) varying beam diameters, common beam 

 current (50 nA), and (B) varying beam currents, common beam diameter (10 jim). 

 Open circles represent low Sr/Ca regions and solid circles represent high Sr/Ca 

 regions. 



for each beam-current configuration ranged from 99 ppm to 

 290 ppm in the low Sr/Ca region and from 312 ppm to 844 

 ppm in tiie high Sr/Ca region (Fig. 3B). 



Discussion 



In our experiment, for measuring Sr/Ca ratios in otoUths 

 with WD-EM analysis, the TAP crystal was the best choice 

 for the measurement of Sr because it provided the advan- 

 tage of higher count rates and higher resolution of Sn Use of 

 the PET crystal to measure Sr has not been reported in the 

 literature. In fact, crystal choice is frequently not reported, 

 making it difficult to know whether TAP or PET crystals 



have been used to measure Sr. Personal communication 

 with several researchers confirmed that the TAP crystal 

 is commonly used to measure Sr and has been cited as the 

 crystal used (Kalish, 1990; Thresher et al., 1994). Given the 

 different results possible with the use of different crystals to 

 measure Sr, reporting the crystals used should be included 

 in papers reporting Sr/Ca ratios measured on WD-EMs. 

 Note that even though measurements of Sr with the TAP 

 crystal appeared to be the best method for otoliths, the TAP 

 crystal should not be used to measure Sr in materials con- 

 taining Si because of analytical interference from Si. 



Variation of beam diameter has very little effect on 

 the limit of detection of Sr, even at extremely high beam 

 current densities. Rather, variation in beam current had 



