Analytical information
The AS15 column uses a dilute KOH eluant that is made by the eluant generator. For silica analysis alone (without other anions) the eluant KOH concentration remains constant. Silica is detected with a spectrophotometer using a modified molybdate yellow method.
Note: This method was developed with the AS15 column in mind. We now have installed an AS18. The only difference should be in the retention time of the silica peak.
Sample Preparation
All samples should be kept refrigerated and in the dark until they are analyzed, which should be done as quickly as possible. Samples must be free of high molecular weight organics (tannins, 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.
All natural samples need to be filtered for particulates through a ≤0.45 μm filter. Some samples must also be filtered to remove high-molecular weight organics and transition metals such as iron. Particulates, organics, and transition metals can damage the column, and particulate damage is irreversible. Refer to the following table:
| 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 Si stock solution in a 100 ml volumetric flask.
| Ion | Stock concentration, ppm | ml used | Standard/1, ppm |
| Siaq | 1000 | 2.5 | 25* |
| * 53.5 ppm as SiO2, 85.6 ppm as H4SiO4 | |||
Transfer Standard/1 to a clean 125 ml plastic bottle. Transfer appropriate numbers of 5 ml aliquots of DI water and Standard/1 to four other bottles. The numbers of 5 ml aliquots are as follows:
| 5 ml Aliquots of | Standard/1 | Standard/2 | Standard/5 | Standard/10 | Standard/20 |
| DI water | - | 5 | 8 | 9 | 9 |
| Standard/1 | - | 5 | 2 | 1 | 1 from standard/2 |
| Component | Resulting concentrations, ppm | ||||
| Si aq | 25 | 12.5 | 5 | 2.5 | 1.25 |
| SiO2 aq | 53.5 | 26.75 | 10.7 | 5.35 | 2.675 |
| H4SiO4 aq | 85.6 | 42.8 | 17.12 | 8.56 | 4.28 |
The result will be 60 ml of Standard/1, 45 ml of Standard/2, and 50 ml of the others. Pour ~4 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.
Post column reagent (PCR) preparation
Fill a 1000 ml plastic volumetric flask ~80% full with DI water. Add in the following order:
| Reagent | Add amount | To get |
| 1 M Na2MoO4 stock solution | 20 ml | 20 mM |
| High-purity concentrated (~70%) HNO3 | 9 ml | 9 mM |
| 150 g/liter stock solution of Na lauryl sulfate | 9.5 ml | 0.14% |
| DI water | To fill flask to volume | 1000 ml |
Gently mix the solution, trying not to make foam. Transfer it to an 1 liter plastic PCR pressure vessel bottle, put the bottle in the vessel, and put on the top.
Instrument setup
The instrument is set up as usual, except for the additional items:
- Change the plumbing so that the line from the column goes into the mixing T rather than to the suppressor.
- Turn on the post-column reaction coil heater to 90°C.
- Turn on the UV-visible detector. It takes about 5 minutes to warm up.
- Pressurize the PCR vessel to 25 psi.
- Run the samples as usual using one of the silica methods.
The silica is separated and is eluted from the column at about 4.4 minutes. It is mixed with the PCR solution in the mixing T, starting the reaction that forms a silico-molybdate complex that is yellow, absorbing strongly in the blue. This reaction is slow, so the heated reaction coil is used to speed the reaction up. The reacted solution enters the detector and the signal is measured by absorbance at 410 nm. Phosphate also forms a yellow molybdate complex. However, because phosphate elutes so long after silica (broadening the peak), and because phosphate is usually low in abundance, and because efficient conversion to the yellow complex occurs at different pH than the silica complex, it should not pose any significant interference.
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Schenectady, NY 12308 U.S.A. |