Use of Sample Masking Reagents

Simple calculations showed that EDTA does not complex metal ions such as magnesium(II) and calcium(II) at pH 4 but it does complex many other metal cations. Therefore, experiments were performed in which ETDA was added to the metal ion sample and the column was eluted with ethylenediammonium tartrate as before [14]. The amount of EDTA used was more than enough to complex the metal ions present, but an unduly high concentration of EDTA was avoided. The results obtained show that conditions can easily be established whereby magnesium and the alkaline earth cation peaks are hardly affected but metal ions that form stable EDTA complexes at about pH 4 are rapidly eluted. Because EDTA is added only to the sample and not to the eluent, it moves rapidly through the column and appears as part of the pseudo peak . 

Samples containing a large excess of iron(III) give extremely wide pseudo peaks when the ethylenediammonium tartrate is used. This excess of iron(III) will totally obscure the magnesium peak while calcium and strontium appear on the tail of the pseudo peak . 

The very weak complexing of iron(II) by tartrate suggested that iron(II) might be determined quantitatively by cation chromatography. This was proved to be true by Fritz and Sevenich [14], who determined iron(II) in the presence of iro-n(lll) and several other metal ions. Total iron in solution was determined after a prehminary reduction to iron(ll) with ascorbic acid. 

Several other complexing reagents are effective masking agents when added to the sample but not to the mobile phase. Nitrilotriacetic acid (NTA) masks a 10- to 100-fold excess of aluminum] 111), copper]II), nickel]II) or iron]III) when magne-sium]II), calcium and manganese] ) are to be determined by IC. Analysis of rare- 

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