Analytical information
The AS18 column uses a dilute KOH eluant that is made by the eluant generator, usually using a concentration gradient program that gradually increases the eluant concentration as the run progresses. This system can separate a wide variety of anions, including low molecular weight organic anions such as acetate, formate, propionate, and oxylate. The procedure here is for seven inorganic anions in natural waters: F-, Cl-, NO2-, Br-, NO3-, PO43-, SO42-. Low molecular weight organic anions (acetate, propionate, methylsulfonate, etc.) can also be analyzed by modifying this analytical method and adding the extra components to standards.
Sample Preparation
All samples should be kept refrigerated and in the dark until they are analyzed, which should be done quickly. Samples must be free of high molecular weight organics (tannins, detergents, etc.), particulates, and samples should not form precipitates in the alkaline eluant solutions. If samples precipitate anything prior to analysis, such as calcite, that might have a chemical component you want to analyze, you must put the precipitate back into solution before analysis. In doing so, do not to make the samples strongly acidic or alkaline; keep the pH between 2.5 and 11.
All natural samples need to be filtered for particulates through a 0.2m m filter, and some must be filtered for high-molecular weight organics and transition metals (e.g., iron). Particles and high concentrations of transition metals will damage the column, and particulate damage is irreversible.
| Sample filtration requirements. | |||
| Sample content | 0.2 μm particle filter $1.50 each*** | OnGuard P* filter $3.15 each | OnGuard H** filter $3.15 each |
| Normal, colorless, dilute samples. | Yes | No | No |
| Brown samples having dissolved tannins or other high molecular weight organic compounds. | Yes | Yes | No |
| Samples rich in transition metals (e.g., iron). | Yes | No | Yes |
| Brown samples rich in organics and rich in transition metals. | Yes | Yes | Yes |
| * Removes high molecular weight organics. | |||
| ** Removes transition metals and calcium. Solutions become acidic and may need to have evolved CO2 degassed in an ultrasonic cleaner. This can be done with the sample remaining in the sample bottle. | |||
| *** Or similar filtering in the field. | |||
Follow the instructions with the filters. Filters can be stacked on the end of the syringe and filtered in one step, with the particulate filter being the last. The filters are expensive so do not waste them.
Standard preparation
In general, standards should be similar in composition to the samples being analyzed. Since samples vary enormously, you may want to start with a generally useful standard. For this, dilute the following stock solutions in a 100 ml volumetric flask.
| Ion | Stock concentration, ppm | ml used | Standard/1, ppm |
| F- | 1000 | 0.2 | 2 |
| Cl- | 1000 | 10 | 100 |
| NO2- | 1000 | 0.1 | 1 |
| SO42- | 1000 | 10 | 100 |
| Br- | 1000 | 0.1 | 1 |
| NO3- | 1000 | 2 | 20 |
| PO43- | 3080* | 0.1 | 3.08 |
| * 1000 ppm as P, 3080 ppm as PO43-. | |||
Transfer Standard/1 to a clean 125 ml plastic bottle. Transfer appropriate numbers of 5.5 ml aliquots of DI water and Standard/1 to four other bottles. The numbers of 5.5 ml aliquots are as follows:
| Standard/1 | Standard/2 | Standard/5 | Standard/10 | Standard/20 | |
| 5.5 ml aliquots of DI water | - | 5 | 8 | 9 | 9 |
| 5.5 ml aliquots of Standard/1 | - | 5 | 2 | 1 | 1 from Standard/2 |
| Ion | Resulting concentrations, ppm | ||||
| F- | 2 | 1 | 0.4 | 0.2 | 0.1 |
| Cl- | 100 | 50 | 20 | 10 | 5 |
| NO2- | 1 | 0.5 | 0.2 | 0.1 | 0.05 |
| SO42- | 100 | 50 | 20 | 10 | 5 |
| Br- | 1 | 0.5 | 0.2 | 0.1 | 0.05 |
| NO3- | 20 | 10 | 4 | 2 | 1 |
| PO43- | 3.08 | 1.54 | 6.16 | 0.308 | 0.154 |
The result will be 56 ml of Standard/1, 49.5 ml of Standard/2, and 55 ml of the others. Pour ~3 ml of each standard into Dionex autosampler tubes, and press a black filter cap into the top of each using the filter cap tool. You should run standards at least at the beginning of the run and perhaps also within the run. Alternatively, you can periodically run a check sample and do corrections off line.
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