Sample Preparation Basics

Part I.  The Chemistry of the Elements in Aqueous Media-Preparing Aqueous Blends of the Elements from Single Element CRMs

You have a shelf full of single element solutions and reagents and need to make a multi-element blend (MEB). Some of the questions you will be asking yourself are:

1. What matrix should I use?
2. Should the matrix be dictated by the sample preparation method? What about the stability of the standard solution(s) in this matrix? 
3. Is there an order of additions of the water, matrix and individual element solutions that must be followed?
4. Are there interelement or matrix incompatibilities I need to avoid?
5. Will the blend be stable? Should all of the elements be in one solution or must I split into two or more solutions?
6. What container material do I use to prepare and store these blends and how do I clean them?
7. Do I pipet or weigh the individual element aliquots? How do I calculate the uncertainty of the final blend(s) and how will this impact the uncertainty of the final result?

These are some of the questions a trace analyst must be able to answer. A working knowledge of the wet chemistry of the elements is critical in the preparation of samples and chemical standard solutions for analysis using ICPs that are stable. The chemistry of different element combinations relates to interelement precipitation, individual element hydrolysis, chemisorption on container walls, contamination via reaction wall apparatus, and reaction with the sample container and ICP sample introduction system. Chemical stability of analytical solutions is necessary for accuracy as well as compatibility of the analytical solutions to the container and ICP introduction system. In support of typical sample preparation approaches, the chemical compatibility of the elements is presented as charts in the form of periodic tables (ref). Although helpful, these charts lack the detail necessary for the analyst to prepare complex multi-element blends (MEBs). There are approximately 4.7 x10²¹ different combinations of the most common 72 elements at a single concentration. If we take into consideration different concentrations, oxidation states and matrices, an accurate calculation of the number of possibilities cannot be calculated without defined restrictions. Over the past 30 years at Inorganic Ventures, we have studied the stability of many different multi-element combinations, concentrations, container materials and matrices. Part I will present the chemistry and a basic procedure for the preparation of MEBs.

1. What matrix should I use?

Most analysts prefer nitric acid (HNO₃) matrices. For ICP-MS measurements there are fewer mass interferences than with the other common mineral acids. Nitric acid is compatible with all of the elements, with the possible exception of osmium (Os). Some of the elements require other reagents to remain in solution (HF, HCl, tartaric acid), but can be mixed with nitric acid in the preparation of multi-element blends. In addition, HNO₃ is very popular in acid digestion sample preparations in part due to its oxidizing properties at moderate to high concentrations, temperatures and pressures.
The elements that are stable/soluble and commonly diluted in aqueous/HNO₃ are shaded in red below along with those that are compatible with HNO₃ but are shaded in white if other solution aids are required:

1. Os should never be mixed with HNO₃ due to the formation of the very volatile and toxic OsO₄.
2. Cl is oxidized to molecular Cl₂ which is volatile and adsorbs on plastic.
3. Br and I are oxidized to molecular Br₂ and I₂ which adsorb onto plastic.
4. Dilutions of Hg and Au in HNO₃ below 100 ppm should be stored in borosilicate glass due to Hg⁺² adsorption on plastic.
5. Not soluble above concentrations of 1000 µg/mL.
6. Trace levels of HCl or Cl- will form AgCl↓, which will photoreduce to Ag⁰.
   F denotes that the element can be diluted in HNO₃ if complexed with F^-.
   Cl denotes that the element can be diluted in HNO₃ if complexed with Cl-.
   HF denotes that the element should have excess HF present when diluted with HNO₃
   T denotes that the tartaric acid complex can be diluted in HNO₃.

The use of hydrochloric acid (HCl) is the next most popular acid matrix. HCl is the most compatible of the common mineral acids with large multi-element mixtures but results in more mass interferences with ICP-MS and is more corrosive to electronic circuit boards than HNO₃. The elements that can be diluted in HCl are shaded in blue below:

1. Concentrated (35 percent) HCl will keep up to 100 µg/mL of Ag⁺ in solution as the Ag(Cl)^X-(X-1) complex. For more dilute solutions, the HCl can be lowered such that 10% HCl will keep up to 10 µg/mL Ag in solution.
   NOTE:  The Ag(Cl)^X-(X-1) complex is photosensitive and will reduce to Ag⁰ when exposed to light. HNO₃ solutions of Ag⁺ are not photosensitive.
2. Parts-per-billion (ppb) dilutions of Hg⁺² in HCl are more stable to adsorption on the container walls than are dilutions in HNO₃.
   F denotes that the element is more stable to hydrolysis if complexed with F^-. In the case of Si and Ge the fluoride complex is generally considered a necessity.

Dilutions in ‘water only’ at pH 7 are not as common for most elements but may be required to prevent chemical reactions of some of the compounds containing the element. Please note that solutions at pH 7 may support biological growth and, therefore, the long-term stability should be questioned. Those elements that may have an advantage to being diluted in ‘water only’ at pH 7 are shaded in yellow below:

Hydrofluoric acid (HF) requires the use of HF-resistant introduction systems. Modern HF-resistant introduction systems are more expensive than glass, but have comparable washout times and measurement precision. There are times when the use of HF offers a major advantage over other reagents.
Those elements where an HF matrix may be optimal are shaded in green below:

1.   HF is used for Si₃N₄ preparations and other nitrides.

Sulfuric acid (H₂SO₄) is commonly used in preparations and, therefore, added to standards in combination with other acids. Elements that either benefit or comfortably tolerate the presence of H₂SO₄ are shaded in orange below:

1. Dilutions of Hg and Au in H₂SO₄ below 100 ppm should be stored in borosilicate glass due to adsorption on plastic.
2. Trace levels of HCl or Cl^- will form AgCl↓, which will photoreduce to Ag⁰.
   F denotes that the element can be diluted in H₂SO₄ if complexed with F^-.
   Cl denotes that the element can be diluted in H₂SO₄ if complexed with Cl^-.
   HF denotes that the element should have excess HF present when diluted with H₂SO₄.
   T denotes that the tartaric acid complex can be diluted in H₂SO₄.

Phosphoric acid (H₃PO₄) is not commonly used in preparations since it attacks glass, quartz, porcelain and Pt containers at elevated temperatures (greater than 100 °C). However, the presence of H₃PO₄ will not adversely affect any of the elements at low µg/mL levels.

2. Should the matrix be dictated by the sample preparation method? What about the stability of the standard solution(s) in this matrix?

Most analysts attempt to match the matrix of their samples and calibration standards to avoid matrix interferences. Analysts may choose one of the following situations:

Situation (a) - Match the acid content of your calibration standards and samples in both the type of acid used and the concentration of the acid.  
Situation (b) - Match the elemental matrix components of your calibration standards and samples to the greatest extent possible. In this situation, the analyst who knows the composition of the sample has this capability.
Situation (c) - With unknown sample matrices, matching is not possible and is most accurately dealt with using the technique of standard additions. However, this approach is slow compared to the calibration curve technique with the use of internal standardization.
Situation (d) - The use of internal standardization is very effective in many cases but may introduce, or not correct for, all errors. This statement does not apply to isotope dilution ICP-MS, which is considered to be a primary analytical technique.

3. Is there an order of additions of the water, matrix and individual element solutions that must be followed?

The general rule is to add all of the acid and as much water as possible to the preparation container followed by each element aliquot addition with gentle mixing between aliquots. The goal is to avoid hydrolysis and interelemental interactions that may occur at localized high concentrations of the elements or localized pHs of ˜7 (unmixed solution).

4. Are there interelement or matrix incompatibilities I need to avoid?

There are a great number of incompatibilities that are complicated by matrix components, concentration, spectral effects, oxidation state and laboratory apparatuses, such as introduction system, mixing flask, preparation equipment.  For more information consult our Interactive Periodic Table using this link - http://www.inorganicventures.com/periodic-table

5. Will the blend be stable? Should all of the elements be in one solution or must I split into two or more solutions?

This is a question that you may need to call or email us about. This question is the focus of our Chemical Stability/Homogeneity group at IV. We are in the process of writing a program that will be made available through our website that will answer this and many other questions. It will be available in late 2015 or early 2016.

6. What container material do I use to prepare and store these blends and how do I clean them?

Most blends are best stored in low-density polyethylene (LDPE). We have found that LDPE is the cleanest of the plastics including PFA and PTFE. There are, however, some general rules to follow that are detailed in the following link: http://www.inorganicventures.com/container-material-properties

7. Do I pipet or weigh the individual element aliquots? How do I calculate the uncertainty of the final blend(s) and how will this impact the uncertainty of the final result?

The uncertainty of the blend you prepare is the square root of the calculated sum of the squares of the relative standard deviations of the individual uncertainties.

If you are preparing a dilution gravimetrically by taking the weight of the aliquot and the weight of the final solution, and these weights are uncertain to 1 part in 10,000, then the relative uncertainty of the individual elements is roughly equal to the relative uncertainty of the single element CRM from which it was made. Please note that this is not the case if you are taking volume aliquots and the uncertainty may be significantly greater than the single element CRM. Please see the following article on error budgets: http://www.inorganicventures.com/understanding-error-budgets