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:

1. Increasing the relative amount of HF through a shift in the equilibrium of equation 6.1 below and;
2. 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 µg/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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