Chloride matrix spike issues - Dionex ICS-2100

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Hi everyone,

I'm currently running a Dionex ICS-2100 IC for anions. I'm having an issue with my matrix spikes for chloride. Our calibration range is 5 to 200 for chloride. My matrix spikes are 25 ppm. My recoveries, however, are bad, typically around 65-75%. I work for an accredited lab and our SOP allowable recoveries are 80-120% for the data to be valid. Usually with a failling matrix spike, we reprep and rerun the spikes, and if those fail we report the data as having possible matrix interferences.

Now I've begun to experiment with the spikes, trying different amounts and trying different ways of spiking. Where we are there is a lot of road salt in the winter, and so my samples typically need a 5x dilution. I have tried diluting the sample and then spiking on top of the diluted sample. I have tried spiking on the straight sample and then diluting that. Each time the recoveries are terrible.

By now your first question to me is probably "Have you checked your standards?" The answer to this is yes. I use stock standards that are purchased (one as a calibration and one as a second source) and I use these same stock standards to make all of my calibration standards and to spike for my other 6 analytes. The spikes for the other analytes always have beatiful recoveries and I never have an issue with my calibration standards. The only analyte giving me trouble is chloride.

My main question is has anyone else ever experienced issues with chloride matrix spikes and does anyone have any advice on what I can try? I appreciate any thoughts you all have on the subject.
chrome gal wrote:
Hi everyone,

I'm currently running a Dionex ICS-2100 IC for anions. I'm having an issue with my matrix spikes for chloride. Our calibration range is 5 to 200 for chloride. My matrix spikes are 25 ppm. My recoveries, however, are bad, typically around 65-75%. I work for an accredited lab and our SOP allowable recoveries are 80-120% for the data to be valid. Usually with a failling matrix spike, we reprep and rerun the spikes, and if those fail we report the data as having possible matrix interferences.

Now I've begun to experiment with the spikes, trying different amounts and trying different ways of spiking. Where we are there is a lot of road salt in the winter, and so my samples typically need a 5x dilution. I have tried diluting the sample and then spiking on top of the diluted sample. I have tried spiking on the straight sample and then diluting that. Each time the recoveries are terrible.

By now your first question to me is probably "Have you checked your standards?" The answer to this is yes. I use stock standards that are purchased (one as a calibration and one as a second source) and I use these same stock standards to make all of my calibration standards and to spike for my other 6 analytes. The spikes for the other analytes always have beatiful recoveries and I never have an issue with my calibration standards. The only analyte giving me trouble is chloride.

My main question is has anyone else ever experienced issues with chloride matrix spikes and does anyone have any advice on what I can try? I appreciate any thoughts you all have on the subject.


How much of the standard are you spiking into the sample to make the matrix spike?

We had such problems with our spikes once. I noticed that if a sample had no hit recovery would be near 100%, but the more the sample had of the analyte the worse the recovery would be. What it turned out to be was we were not allowing for the dilution factor caused by spiking the sample.

We spike 1.25ml of standard into 10ml final volume. Which calculates our correctly when there is no analyte in the sample. But when it is present you have to figure the fact that you only used 8.75ml of sample and not 10ml. Without allowing for that, you will subtract more analyte than you should from the total when figuring recoveries.

sample = 10ppm CL = 100mg/10ml
MS = 10ppm CL + 5ppm(spiked) = 15ppm(in theory)
MS = 87.5mgCL(in 8.75 ml) + 50mg(from spike standard) = 137.5mg = 13.75ppm as analyzed.
Recovery= ((13.75ppm-10ppm)/5ppm)*100= 75% recovery
But you actually recovered 100%

If your sample contained 12ppm and you recover 100% the calculated recovery is even worse if you do not take into account a new sample dilution of 8.75ml sample/10ml total volume.

Maybe you are correcting for this, but just saying it took us a while to figure out where we were figuring it wrong, even our LIMS was miscalculating it until we added in the correct initial sample volume versus final sample volume calculation. Most Spike recovery calculations take into account only using a few microliters of spike standard per several milliliters of samples, maybe even a liter so the dilution factor is negligible.
The past is there to guide us into the future, not to dwell in.
James_Ball,

Thanks for your reply. It turns out that we were not correcting for volume. When we did, the spikes worked pefectly. Thanks so much for your help!
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