Chromatography Forum

Mark, did you try ortho phosphate instead of pyro(?), that didn´t work?

Kostas, almost same question: Phosphate didn´t remove enough Fe ions?, or did you think it was appropriate to use the "hammer" (EDTA) right away?
Now, if I understood past discussions (and refs.) on this correctly, passivation with HNO3 is more complicated than just removing Fe ions.

I have tried to use orthophosphate as a masking/removal agent for iron contamination in other contexts, and it didn't work. So, for the phosphopeptide studies, I just skipped to the stronger chelator. I chose pyrophosphate over EDTA because at 214nm, pyrophosphate is transparent.

At Dionex, we factory-test our Acclaim columns for metal activity, so it is important to keep our manufacturing and test equipment clean. We clean/passivate our stuff monthly. It took almost a year before everything was reliably clean.

Passivation with HNO3 not only removes iron from the surface, but also builds a protective layer of chromium oxide (and for 316-SST, molybdenum oxide). Halide ions, especially at low pH, dissolve that oxide layer, which is why they are considered corrosive.

By the way, I have found that often it is the UV detector cell that becomes contaminated, and it needs a good cleaning too. I'm not sure if that is iron, or something else, but HNO3 does get rid of it.

Mark, what species of pyrophosphate is suitable to use? Its tetrasodium salt?

Hans,

The "hammer" would probably work in most cases, but I won't claim that I have investigated all the alternative possibilities. We have though injected phosphoric acid before our phosphopeptides and that approach seem to work in the past... but we haven't done a systematic study...

Now... about the phosphopeptide work we have also (home) built a metal free HPLc system. So, I would suggest that if people are going to work in a regular basis on the analysis of very chelating compounds it might be useful to buy a metal free system, otherwise, a normal or sliglty modified system should work after the appropriate passivation...

Whether you can get away with using a stainless steel system for doing ion analysis depends on a lot of different factors including:

1). How frequently you will be doing ion analysis. If you will be doing such analysis frequently or for extended periods, you will probably be better off staying clear of trying to get away with using a stainless steel instrument considering the factors below.

2). Whether or not any of your analytes have chelating properties. As mentioned previously, you may have problems with certain chelating analytes if you work with a stainless steel system. Phosphate falls into this category as do most of the divalent carboxylic acids. Of course, if you're after cation analysis you definitely would be better off staying clear of stainless steel instrumentation.

3). Whether or not you will be using a wide variety of different pH eluents on your instrument. Contrary to what you might hear from some who post on this forum, it's not enough just to passivate your instrument. How long the passivation lasts is a function of your eluent (chloride containing eluents can remove the passivation layer) and basic eluents can convert the passivation layer from oxides to hydroxides in much the same way that base treatment converts Si-O-Si in fused silica to silanols. Once the surface layer has been converted to the hydroxide form it can be stripped away by treatment with dilute acid even if the acid is not corrosive. If this happens you will need to repassivate in order to prevent corrosion. If you work exclusively under acidic conditions and work with noncorrosive eluent components (e.g. don't use things like chloride in your eluent system) you might be OK. Likewise, if you work exclusively under neutral or basic conditions you will probably be OK with stainless steel provided you don't have any chelating analytes. But if you frequently switch back and forth between basic or neutral and acidic eluents or use chloride containing eluents you may need to passivate fairly often in order to operate safely.

4). Whether or not you are planning on using a suppressor. Although there's no problem per se with using a suppressor in a stainless steel system, the corrosion products will tend to concentrate in your suppressor and will necessitate more frequent cleaning of the suppressor in order to remove corrosion products. Accordingly, it's generally better to stay clear of using a stainless steel system under these conditions.