Preparing Multi-Element Blends from Single Element CRMs
Sample Preparation Basics
Part I. The Chemistry of the Elements in Aqueous Media-Preparing Aqueous Blends of the Elements from Single Element CRMsYou 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:
- What matrix should I use?
- Should the matrix be dictated by the sample preparation method? What about the stability of the standard solution(s) in this matrix?
- Is there an order of additions of the water, matrix and individual element solutions that must be followed?
- Are there interelement or matrix incompatibilities I need to avoid?
- Will the blend be stable? Should all of the elements be in one solution or must I split into two or more solutions?
- What container material do I use to prepare and store these blends and how do I clean them?
- 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?
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:
- Os should never be mixed with HNO3 due to the formation of the very volatile and toxic OsO4.
- Cl is oxidized to molecular Cl2 which is volatile and adsorbs on plastic.
- Br and I are oxidized to molecular Br2 and I2 which adsorb onto plastic.
- Dilutions of Hg and Au in HNO3 below 100 ppm should be stored in borosilicate glass due to Hg+2 adsorption on plastic.
- Not soluble above concentrations of 1000 µg/mL.
- 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.
- 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.
- 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.
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:
- HF is used for Si3N4 preparations and other nitrides.
- Dilutions of Hg and Au in H2SO4 below 100 ppm should be stored in borosilicate glass due to adsorption on plastic.
- 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.
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