Cart

Measurement and Results - Poor calibration for Ca, Mg, Fe, Zn, and K using ICP-MS

 
I am seeking technical help for custom made standards that have Ca, Mg, Fe, Zn, and K. The problem is that I am getting poor calibration for Fe, Zn, K, and Mg. My calibration curve starts from 0, 50, 200 up to 3000 ppb, respectively. When I inject 1000 ppb or above then the lower standards such 50 and 200 ppb comes out 30 ppb and 150 ppb while higher concentration standards turn out to be the same as injected. For example: The standards 0, 50, 200, and 500 ppb make a curve and 1000, 2000, 3000 ppb standards make another line which ends up generating a poor calibration curve. I always make new curve standards, the cones are cleaned, and I've replaced the sample probe (autosampler) and changed the PFA tubing, yet I still cannot fix the problem.
Sort By    Oldest  |  Newest
Responses
admin Total posts: 529
All of the elements in the standard are compatible and stable in HNO3 and/or HCl, which we assume would be the matrix that you are using to create your calibration standards. This observation, and the fact that some of the elements (Ca, B, P, Na) are fine, whereas other are not (Fe, Zn, K, Mg) indicates that the chemistry and preparation of the standards is not the issue. The comment “The standards 0, 50, 200, and 500 ppb make a curve and 1000, 2000, 3000 ppb standards make another line which ends up generating a poor calibration curve.” suggests that entire calibration range from 0-3000 ppb is not linear. Is that the case for the Fe, Zn, K, Mg regression lines? If the calibration curve is not linear, then the regression line constructed from it could easily be suitable for a part of the calibration range (the high values in your case), and not be suitable for the low values. The R2 parameter describing how close the data are to the fitted regression line should indicate if the regression line is appropriate. If the R2 values for all the regression lines are close to 1 (>0.99) then something else is occurring, in which case we’d suggest directly comparing  the raw data for all calibration points to those returned when you re-run the individual standards.
Posted: 09/05/18 22:41:11

1 Item

Show per page