ICP-AES系列学习6:Compaitibility and Precision Issues

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2005/03/23

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6. Compaitibility 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 quarts as structured or better yet crystalline silica).

Equation 6.1:
H+1 + F-1 HF (Ka = 8.9 x 10-4)

It follows that solutions containing HF that are neutralized with a base to eliminate HF will not attack silicates provided that the HO-1 concentration is not too high (i.e., the pH is not above 8). This is why organic amines such as triethanol amine are so good at eliminating HF attack simply through neutralization of the HF as opposed to NaOH, which will attack silicates if high enough in concentration.

There is a general misunderstanding that the addition of boric acid will eliminate HF attack, allowing the analyst to use glass introduction components. It is common practice to react HF with boric acid (typically, 1 gram of boric acid is added for every 1 mL of 49 % HF) to form the mono-fluoroboric acid. Unfortunately, fluoroboric acid will attack glass (including concentric nebulizers) and the attack of silicates, in general, is not greatly altered. The formation of the fluoroboric acid will diminish the tendency to form insoluble fluorides such as CaF2 which is why it was originally added.

Glass Introduction Systems:
Glass introduction systems are generally preferred by analysts because they are less expensive, have shorter washout times, and give better precision than plastic. This is why many analysts opt to use all-glass introductions provided the HF content is < 100 ppm. Quartz is less reactive than glass and is sometimes used if the analyst is concerned with making low level B measurements in a trace HF matrix.

Our laboratory uses a Type C glass concentric nebulizer at an Ar flow of ~ 0.75 L/min, a pressure of 30-35 PSI, and a sample introduction rate of 0.7 mL/min (manufactured by Precision Glassblowing). The spray chamber is an all glass cyclonic and the torch is made of quartz. A typical measurement precision is between 0.2 and 0.5 % RSD and the washout times are excellent for all elements, including B and Hg ( Hg takes ~ 75 seconds of rinse with 10 % (v/v) HNO3). Trace levels of HF are easily tolerated even when elements such as Si and B are measured.

Recommendations:
HF concentrations ≥ 0.1 % will attack both glass and quartz and cause considerable problems for the analyst attempting to determine Si, B, or Na. It is necessary to either switch to an HF-resistant introduction system or neutralize the HF with a base. Our laboratory introduces 1000 to 20000 礸/mL solutions of all the 'HF' elements using the neutralization (triethanol amine) option with the addition of H4EDTA when required for chemical stabilization, while other laboratories get excellent results using the HF-resistant (plastic) introduction systems. The PFA concentric nebulizer is popular with a PFA or PEEK spray chamber and Al2O3 (inner tube) torch. I would suggest checking with your instrument manufacturer for power supply and gas flow compatibility before investing in an HF resistant system.

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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.

Suspended Solids

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 祄 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 祄 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.

Closing Remarks

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.