Compatibility and Precision Issues
Section 5, Nebulizers, Spray Chambers and Torches, dealt with some of the basic nomenclature and types of introduction systems for liquid sample introduction to an ICP. This section will go into a little more depth concerning the most common compatibility issues facing the ICP operator.
Solutions Containing HF
The presence of HF causes the vast majority of compatibility problems between the sample matrix and the introduction system components. If you are preparing samples containing one or more of the following elements, then you are likely using HF in your sample preparation:
HF elements: Ti, Zr, Hf, Nb, Ta, Mo, W, Si, Ge, Sn, Sb
When HF Attacks
The introduction of solutions containing HF should be of concern to the instrument operator, especially if he/she is determining Si, B, or Na. When glass or quartz is exposed to HF, they are attacked to a degree depending upon the concentration of the HF and the type of glass or quartz. It is the HF molecule that does the attacking; not the fluoride anion (F-1). There is absolutely no attack by neutral solutions of F-1 upon any form of glass or quartz (note that there is water solubility of amorphic and crystalline forms of silica that is a function of the surface area, impurities, and structure).
The HF attack is enhanced by the presence of a strong acid, such as HNO3 or HCl, by:
- Increasing the relative amount of HF through a shift in the equilibrium of equation 6.1 below and;
- By adsorbing as the hydronium ion on the solid silica surface where it behaves as a catalyst (i.e., the reaction of HF with a solid silicate can be described by two equations that work in parallel).
In addition, the crystalline form of the silicate influences the rate of attack. The net result being that quartz is not attacked as readily as glass. (This is a generalization - please note that there are four 'production types' of quartz in addition to natural quartz where different solubility and contamination characteristics can be expected from each. It may be more appropriate to think of glass as amorphous silica and quartz as structured or better yet crystalline silica).
|H+1 + F-1 = HF (Ka = 8.9 x 10-4)|
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:
- Dilute the sample.
- Humidify the sample Ar stream.
- Use one of the high solids or high pressure concentric nebulizers mentioned in part 5 of this series.
- 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.
Samples containing suspended solids may cause a problem with the conventional fixed cross-flow or concentric nebulizers depending upon particle size. Solids that will pass through a 0.3 µm filter will not plug these nebulizers and will behave as if they are in solution with respect to the entire sample introduction process. Particles > 10 µm will not aspirate normally and are not likely to cause plugging. Many sample types have particulate that is easily visible to the naked eye and will cause difficulty with the cross-flow and concentric nebulizers. The Babington V-Groove, GMK Babington , Hildebrand dual grid, Ebdon slurry, Cone Spray, and Noordermer V-groove nebulizers are all popular choices. Other options include filtration to remove the solids and chemical treatments such as fusion, ashing, or acid digestion to dissolve the solids.
HF, high dissolved solids, and suspended solids are the most common compatibility issues facing the ICP analyst. The ways around these problems are often expensive, time consuming, and result in lowered detection limits, longer wash out times, and poorer precision. In extreme cases, alternate analytical measurement techniques are required. It is always best to consult with your instrument's manufacturer before switching introduction components outside the realm of those recommended/supplied by the manufacturer.