Fluoride and phosphate calibration issues
Posted: Thu Aug 04, 2016 7:46 am
For last couple of years we were doing calibration for anions in range of 0,1 mg/l to 8 mg/l for fluoride, nitrite and phosphate and also 1 mg/l to 8 mg/l for chloride, nitrate and sulfate.
Till this year we had no problems with calibration, curves were linear for all of the anions and recoveries were always in the specified range (90-110%)- we had loop 10 uL. In June this year we couldn't meet criteria for linearity (>0.99) for some anions so our maintenance recommended to us to change the loop to 25ul for better sensitivity. We changed the loop and done the calibration again. Coefficient of correlation for fluoride was 0.96, and recoveries were poor. Calibration curve was set up from zero, otherwise it would go from negative peak. We set the quadratic form of curve and we saw that for phosphate and for fluoride there is no linearity (it isn't a straight line it's a curve) - in the 2015 it was a strait line in this range for both of the anions. So we tried to analyse CRM's in range of linearity (phostphate from 2,5mg/l-5 mg/l and 4-8 mg/l for fluoride) with more points (phosphate wasn't detected below 2,5 mg/l and fluoride below 1,0 mg/l). For phosphate we determined linearity between 2,5 mg/l and 3,5 mg/l and for fluorides between 4mg/l and 7 mg/L.
For other anions linearity changed a bit but coefficient of correlation is still above 0.99 - recoveries are between 90-95% for concetration of CRM that is in the middle of the calibration curve (90% is minimum).
Column that we use is for about 2000 to 2500 samples, we analysed less than 1000 samples. All of the samples are filtered trough filter with pores 0.25 um and diluted if conductivity exceeds 1000 uS/cm.
Our maintenance suggested to change the suppressor, column and precolumn but with no guarantee that it will solve the problem.
Did anyone had similar problems, what was the solution? If anyone has some idea what could be the problem, please help.
We are using Metrohm 761 Compact IC, column Thermo Scientific ionpack AS14 4mm, Dionex ASRS Ultra 2 - 4 mm, Metrohm 813 Compact autosampler.
Till this year we had no problems with calibration, curves were linear for all of the anions and recoveries were always in the specified range (90-110%)- we had loop 10 uL. In June this year we couldn't meet criteria for linearity (>0.99) for some anions so our maintenance recommended to us to change the loop to 25ul for better sensitivity. We changed the loop and done the calibration again. Coefficient of correlation for fluoride was 0.96, and recoveries were poor. Calibration curve was set up from zero, otherwise it would go from negative peak. We set the quadratic form of curve and we saw that for phosphate and for fluoride there is no linearity (it isn't a straight line it's a curve) - in the 2015 it was a strait line in this range for both of the anions. So we tried to analyse CRM's in range of linearity (phostphate from 2,5mg/l-5 mg/l and 4-8 mg/l for fluoride) with more points (phosphate wasn't detected below 2,5 mg/l and fluoride below 1,0 mg/l). For phosphate we determined linearity between 2,5 mg/l and 3,5 mg/l and for fluorides between 4mg/l and 7 mg/L.
For other anions linearity changed a bit but coefficient of correlation is still above 0.99 - recoveries are between 90-95% for concetration of CRM that is in the middle of the calibration curve (90% is minimum).
Column that we use is for about 2000 to 2500 samples, we analysed less than 1000 samples. All of the samples are filtered trough filter with pores 0.25 um and diluted if conductivity exceeds 1000 uS/cm.
Our maintenance suggested to change the suppressor, column and precolumn but with no guarantee that it will solve the problem.
Did anyone had similar problems, what was the solution? If anyone has some idea what could be the problem, please help.
We are using Metrohm 761 Compact IC, column Thermo Scientific ionpack AS14 4mm, Dionex ASRS Ultra 2 - 4 mm, Metrohm 813 Compact autosampler.