Many times in analysis, determination of specific ionic forms of elements are desired, particularly anions. In water samples, measurements of nitrate, ammonium or sulfate may be of interest. In addition, extractions of soils, rocks or plant tissue for some largely anionic elements such as nitrogen, phosphorus, chlorine, sulfur and boron are often more conveniently determined by chemical methods based on measurement of the anionic form of the element. Several instruments based on different technologies can be used to measure ions in aqueous samples.
- Ion chromatography is a separation technology that uses an anion exchange column to separate anions moving through the column, which are then measured by a detector system at the column outlet.
- Ion chromatography is performed using a DIONEX DX500 modular chromatography system with conductivity, electrochemical and absorbance detectors. Common anions are separated using a IonPac AS4A column and detected by suppressed conductivity. The SRS -II self-regenerating suppressor (DX500 system) enhances analyte sensitivity by providing superior suppression of the background carbonate eluent conductivity resulting in significant improvement in analyte detection limits.
- Routine measurements of nitrate, phosphate, sulfate, fluoride and chloride can be made in aqueous systems such as stream and pore waters. Detection limits for these constituents range from 20 to 200 ug/l (ppb). Nitrite and bromide can be included in the sample run for an additional cost.
- Working calibration standards are prepared each day from dilution of a 1000 ppm mixed stock solution. A three-point calibration is performed and an independent check QC sample is analyzed every 10 samples. Because of the long-term instability of nitrite in aqueous solution this ion is not included in the routine analysis. However, if required, nitrite standards can be made up on the day of analysis and this anion subsequently quantified for an additional cost.
- Specific ionic species in solution can be determined if required using ion chromatography as a separation technique, for instance, Fe(II) and Fe(III) in waters or extracts can be determined by ion chromatography with separation on IonPac CS5A column and absorbance detection following post column derivitization. Specific measurement of Cr (VI) can be made using an IonPac AS7 column with detection by UV/vis following postcolumn derivitization.
- For other elements e.g., arsenic and selenium, methods can be devised for separation of differing valence states on the IC, and eluent from the IC injected into the ICP-MS for detection at sub-ppb levels. Additionally, methods exist for determination of methylated forms of arsenic and mercury based on column retention and either IPC-MS or cold-vapor absorption detection See Metals Analysis page for more details.
Other Ion Analyses
Other dissolved ions are often determined potentiometrically; H+ (i.e., pH), and electrical conductivity (EC) are common potentiometric methods, described in the Other Services page. Other elements, particularly the halogens (chlorine, bromine), can be determined using ion-specific electrodes and may be a good analytical choice for large numbers of samples at higher (ppm+) levels requiring only a single element analysis.