Chemistry Reference
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chromatography. About 500 sample injections could be performed before breakthrough
of humic substances occurred as indicated by ultraviolet measurements made on the
eluate at 225nm.
Most of the recoveries are not significantly different from 100%. For sulphate,
recoveries tend to be about 10% low at high concentrations of humic acid while an
opposite tendency can be noted for nitrate. One reason for this behaviour might be the
competition between sulphate and nitrate and other ions present in the humic acid for the
binding sites of the humic substances.
12.2.4
Mixtures of organic and inorganic onions (bromate, chloride, bromide,
nitrite, nitrate, hypophosphite (HPO
2
2−
), selenite, selenate, sulphate,
phosphate, pyrophosphate, arsenate, chromate, α-hydroxybutyrate, butyrate,
formate, acetate, glycolate, gluconate, valerate, α-hydroxy valerate, pyruvate,
monochloroacetate, dichloroacetate, trifluoroacetate, galactonurate,
gluconurate, α keto glutarate, oxalate, fuma rate, phthalate, oxalacetate,
citrate, isocitrate, cis aconitate, trans aconitate, succinate, maleate, malonate,
quinate, tartrate, hexane sulphonate, octane sulphonate, octane sulphate,
decane sulphonate, dodecane sulphonate and dodecane sulphate
Dionex Corporation [24] have issued a technical note covering the application of anion
exchange chromatography to the determination of the very wide range of anions, quoted
above, in non saline waters.
Mobile phase ion chromatography is a form of reversed phase ion pair chromatography
in which chemically suppressed conductivity detection is used. As in reversed phase high
performance liquid chromatography, gradient elution is accomplished by varying the
percentage of organic solvent in the eluant during the run.
Although the ionic strength of the eluant may remain the same throughout the run, the
background conductivity can decrease due to the changing dielectric constant. These
baseline changes can be compensated either by chemical means or by computer baseline
subtraction. Often, mobile phase ion chromatography is used to elute ions which are very
strongly retained. When this is the case, the baseline conductivity change can be
minimised without affecting the separation by adding very weakly retained ions directly
to the stronger eluant (higher percentage of organic solvent). Using gradient mobile phase
ion chromatography, the separation of five alkyl sulphonates and sulphates is shown in
Fig. 12.16. The AMMS-mobile phase ion chromatography suppressor (P/N 37106
Dionex Inc) should be used.
The analytical conditions recommended by Dionex for analysing mixtures of anions
listed under the heading to this section are given below.
Although many columns and eluants can be used for gradient elution, the conditions
used in Figs. 12.17 and 12.18 are recommended as a good starting point. The HPIC-
ASSA (5µ) separator with sodium hydroxide
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