Compatibility and Precision Issues

High Dissolved Solids

For conventional fixed cross-flow and concentric nebulizers, high dissolved solids may be a problem. The problem lies in the 'salting out' of the matrix component(s) in the nebulizer. This occurs in the nebulizer at the point where the solution goes from a liquid to a mist, resulting in a temperature drop and reduced solubility. If the solution component is well below its solubility limit then a conventional nebulizer will not experience salting out. Therefore, the question is: "What is 'high'?"

The answer is relative to the solubility of the matrix. If you are aspirating a 0.7 % solution of B as boric acid then salting out will occur. A 4 % solution of Cu as the nitrate or chloride will not salt out. Salting out is indicated by poor precision and a gradual loss of signal. The analyst has several options - he/she can:

1. Dilute the sample.
2. Humidify the sample Ar stream.
3. Use one of the high solids or high pressure concentric nebulizers mentioned in part 5 of this series.
4. Increase the solubility of the culprit.

Our laboratory uses option 1 or 4 in order to retain the excellent characteristics of the type C concentric glass nebulizer. The addition of TEA is made to high boric acid solutions. This greatly increases the boric acid solubility and eliminates salting out. Other matrices are best dealt with through dilution, where the lowest concentration of the matrix metal that can be tolerated by a type C concentric - in our experience - is 10000 ppm.

Was this helpful?  Tell a Friend | Let us know