Sample Preparation Guides

Background

Currently liquids are the most common physical form for sample introduction to modern flame and plasma atomic spectroscopic analytical instrumentation (ICP and AAS). However this has not always been the case.

From the mid 19^th century when the first "emission spectroscope" was introduced (1851 - Masson) to well into the mid 20^th century when AAS was introduced (1954 - Walsh), the introduction of samples as solids was more common. During this period, the use of an a-c arc, a d-c arc, and a-c spark were the common methods of excitation. Flames increased in popularity with the introduction of flame photometry, which relied almost entirely upon the introduction of liquids. Improvements in the design of pneumatic nebulizers have supported the popularity of liquids during the introduction of AAS in the 1950's and later, ICP-OES instrumentation in the early 1970's. Liquids are more suited to current ICP and AAS instrumentation due to our better understanding of liquid sample introduction systems and our ability to introduce liquids in a more repeatable manner than can be accomplished for solids. In addition, unless you want to keep switching introduction systems, it's preferable to convert the samples that are solids to liquids than to convert the samples that are liquids to solids.

One of the problems with the direct analysis of solids is homogeneity with respect to the size of the excitation area. The physical and chemical forms of the solid sample may also present problems. The process of getting the sample to the excitation source presents difficulty as well. Upon the conversion of the solid to a liquid, the elimination of homogeneity, physical form and physical transport problems associated with the solid are overcome.

However, the conversion of solids to liquids presents the analyst with a new and different set of problems. Many, if not most, sample types start out as solids in part or total where a great deal of time and effort is invested into converting them into a liquid form. Even the sample types that would be described as liquids require chemical treatments for stability and compatibility purposes associated with the measurement technique(s) employed. Sample preparation chemistry has consequently become a popular topic where the term preparation chemistry now refers to "the conversion of solids to liquids suitable for introduction into modern ICP and AAS instrumentation."

The chemistry of the elements in aqueous media is of interest as well as the chemistry required to bring them into solution. We're not only concerned with the destruction of the solid, but also the preparation of a solution that is stable and compatible with the chemical components of the solution and the physical components of the introduction system. My previous guides, Trace Analysis: A Guide for Attaining Reliable Measurements and ICP Operations: A Guide for Using ICP-OES and ICP-MS, have dealt with the theory and basics of the common sample preparation techniques. This Guide is intended to provide specific and highly detailed information about certain elements in regards to sample preparation.

Topics

When one considers the subject of "sample preparation solution chemistry," there are many topics that could be presented. The general plan for this guide is to present specific information about each element. My hope is that this information will be useful to the analyst facing the task of preparing a sample solution for analysis, i.e. practical information. The many topics that could be presented will be limited to those that are of common interest to users of inorganic standards and, in some instances, standards provided only through Inorganic Ventures. Each of the elements will be discussed, with the exception of those few (H, N, O inert gases, etc.) that are not commonly measured using ICP or AAS.

The information presented for each of the elements will include:

1. Preparation and solution chemistry of samples containing the element. Preparation methods will be given for common chemical forms and for several sample types.
2. Chemical incompatibilities and tendencies.
3. Hydrolytic stability data and effect of ligands.
4. Preferred matrices.

The elements will be presented in order of increasing periodic group and atomic number as follows:

• Group 1 — Li, Na, K, Rb, Cs
• Group 2 — Be, Mg, Ca, Sr, Ba
• Group 3 — Sc, Y, La, the lanthanides, Th, and U
• Group 4 — Ti, Zr, Hf
• Group 5 — V, Nb, Ta
• Group 6 — Cr, Mo, W
• Group 7 — Mn, Re
• Group 8 — Re, Ru, Os
• Group 9 — Co, Rh, Ir
• Group 10 — Ni, Pd, Pt
• Group 11 — Cu, Ag, Au
• Group 12 — Zn, Cd, Hg
• Group 13 — B, Al, Ga, In, Tl
• Group 14 — Si, Ge, Sn, Pb
• Group 15 — P, As, Sb, Bi
• Group 16 — S, Se, Te
• Group 17 — F, Cl, Br, I

Resources

If you're not familiar with basic sample preparation and associated techniques, then the following articles may be of help:

• Introduction to Sample Preparation
• Container Material Properties
• Stability of Elements at ppb Concentration Levels
• Contamination From Reagents
• Acid Digestion of Inorganic Samples
• Acid Digestion of Organic Samples
• Sample Preparation by Fusion
• Ashing Procedures
• Element and Matrix Compatibility
• Elemental Analysis of Zeolites

References

The following references were very useful in preparing this Guide:

• Gorsuch, T.T. The Destruction of Organic Matter; Pergamon Press: Elmsford, NY, 1970.
• Introduction to Microwave Sample Preparation, Theory and Practice; Kingston, H. M., Jassie, L. B., Eds.; American Chemical Society: Washington D.C., 1988.
• A Handbook of Decomposition Methods in Analytical Chemistry; Bock, Rudolf, Ed.; Halsted Press, Div. Wiley & Sons: New York, 1979; translated by Ian L. Marr.
• Mizuike, A. Enrichment Techniques for Inorganic Trace Analysis; Springer-Verlag: New York - 1983.
• CRC Handbook of Chemistry and Physics; Lide, D. R., Ed.; CRC Press: Boca Raton, FL.
• Encyclopedia of Analytical Science; Townshend, A., Ed.; Academic Press: New York, 1995, Vols. 1-10.
• Inorganic Ventures' Analytical Periodic Table; Information on elemental compatibility, stability, sample preparation, preferred lines and spectral interferences for ICP-OES and ICP-MS; Inorganic Ventures, Inc.: 2001-2013.
• Sampling and Sample Preparation; Stoeppler, M., Ed.; Springer Publishing: New York, 1994.
• Trace Analysis: A Structured Approach to Obtaining Reliable Results; Prichard, E., MacKay, G. M., Points, J., Eds.; The Royal Society of Chemistry: Cambridge, U.K., 1996.
• Guidelines for Achieving Quality in Trace Analysis; Sargent, M., Mackay, G., Eds.; The Royal Society of Chemistry: Cambridge, U.K., 1995.
• Smith, Roy-Keith Handbook of Environmental Analysis; Genium Publishing: U.S.A., 1994.