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To reliably analyze target components in samples with good sensitivity, it is necessary to pretreat samples to minimize the influence of coexisting components and avoid clogging flow lines or column deterioration. The following describes some of the representative pretreatment methods for ion chromatography.
Filtration
This is the most common method used to prevent clogging columns and flow lines. It uses a disposable membrane filter with a 0.2 µm to 0.5 µm pore size. Various types of filters are available, but one designed for ion chromatography, which involves lower inorganic ion contamination levels, should be used. If samples contain large amounts of insolubles, precipitate the insolubles by centrifuge in advance to achieve more efficient filtering.
Ultrafiltration
Ultrafiltration is a convenient method for removing proteins and other organic substances that cannot be removed with a membrane filter. Filters are available for removing molecular weights ranging approximately from to 100,000. Disposable models are also available on the market. These filters can be generally divided into two types. One is pressurized with a syringe and the other is placed in a centrifuge.
Dilution
Normally either ion exchange water or mobile phase solution is used for dilution. Of course, the solution used for dilution must not contain any target components, so always measure a blank sample of the solution. Also, the dilution rate must be decided after considering the load to the column used and allowable quantitative range.
An effective way to avoid accidentally damaging the columns is to first measure a sample diluted 50 to 100 times and observe the results.
Dissolution and Extraction
Solid samples or organic solvents are difficult to analyze using ion chromatography, which does not ionize samples, without extracting or dissolving them into an aqueous solution. For solid samples with poor water solubility, first dissolve the sample in organic solvent, then extract them in water.
To measure unknown samples, test a mixture in advance to make sure they do not precipitate in the mobile phase when injected.
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Sample Solvents
Magnesium ions and calcium ions cause precipitation of hydroxides in basic aqueous solutions. This even occurs slowly in purified water that has not been pH-adjusted. As a result, the concentration of magnesium or calcium ions decrease over time. To prevent that, it is useful to prepare solutions using a dilute acidic aqueous solution. However, to prevent peak broadening, the acid concentration of the standard solution should probably be lower than the acid concentration of the mobile phase. In many cases, commercial standard solutions are 0.1- to 1-M nitric acid or hydrochloric acid solutions. Such solutions can probably be diluted with purified water without a problem, if the dilution rate is low.
Making the sample solvent acidic means that the competing H+ ions will inhibit the ions being measured from adsorbing to the container and instrument, which will provide the additional benefit of preventing contamination. To measure cations with high sensitivity, we recommend preparing samples in dilute aqueous acid solutions. (Nt)
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