Fluoride detection advice please

Discussions about IC and related topics

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I just started as an analyst and have been running the IC (ICS 1100) with AG22 guard column, AS22 anion column, and ASRS 300 anion micromembrane suppressor for about a month now. We currently do fluorides via a fluoride electrode, but we would like to switch over to running them on the IC. The operator's manual and method 300.0 (EPA regulation handbook) are not very helpful in figuring out what needs to be done to run fluorides. I know that the water dip can interfere & that a more concentrated eluent would help that. I am currently using 0.9539 g sodium carbonate and 0.2352 g of sodium bicarbonate. Any suggestions for a better concentration to run fluorides? Also, method 300.0 mentioned that coelution of molecules similar in size to fluoride can cause interference. Is there any way to prevent this? (I also read somewhere that aluminum and iron can interfere also). Please help.
Hy by
Your mobile phase is too concentrated (9mM Na2CO3 / 2.8 mM NaHCO). Therefore all anions will elute earlier and more narrow to the water dip. Please check the manual from page 31 on. There are several application with Fluoride. If you make dilutions of samples (if possible) with your mobile phase you remove the negative peak and possible integration issues from the water dip. Metals like Al or Fe should be avoided or removel prior injection.
Best regards
Stefan Brand
Thermo Fisher Scientific
That was per 2 liters so my concentrations are only (4.5mM Na2CO3/1.4 mM NaHCO). Is that still too high? The manual says I can spike with 1.0 mL of 100X concentrated eluent. So would I be spiking with 450mM/140mM? Also, is there a way to remove iron and aluminum from samples? Or could I just add a quality control which has a high amount of aluminum/iron added to a low amount of fluoride and check my recovery? Thanks for the help.
Metals can be removed by using "On Guard Cartridges". You can find information about this and other cartidges here: http://www.dionex.com/en-us/products/accessories/reagents-accessories/onguard/onguard-ii-m/lp-73690.html.
What is the expected concentration of Fluoride? And what are the expected concentrations of Fe and Al? Are there any other ions present? What is the matrix? If you can dilute your sample then i recommend to do that with your mobile phase. This reduces the influence of the water dip.
Best regards
Stefan Brand
Thermo Fisher Scientific
Hi, I am using 0,5M CaCO3 solution as stock and diluting 50 times(20 mL to 1 L) for preperation of mobile phase for water samples.
You could also try concentration suggested epa5050. However these method is about burning wastes/coal in calorimeter bomb and absorbing a solution of 10 mL of mobile phase. Therefore you have to dilute at least five time (that usually works for coal to fit calibration curve)
However floride will be close to water and height will be less then half of chloride of same concentaration
Fluoride always elute before chloride and, depending on the column, sometimes close to the water dip.
You are useing calcium carbonate as an eluent. This is very special. Usually sodium carbonate is used. This might be a possible reson for the loss of fluoride as calcium fluoride has a low solubility.
Dr. Markus Laeubli
Manager Marketing Support IC
(retired)
Metrohm AG
9101 Herisau
Switzerland
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