ICP Operations   • Introduction / Contents • About the Author • Other Guides and Articles Multi-element Standard Blends   1. Elemental and Matrix Compatibility • 2. Quality Issues • 3. Handling, Calculations, Preparation, and Storage Sample Introduction   • 4. Sample Introduction Systems • 5. Nebulizers, Spray Chambers, and Torches • 6. Compatibility and Precision Issues Performance Characteristics   • 7. Linearity and Detection Limits • 8. Spectral Interferences: Types, Avoidance, and Correction • 9. Key Instrumental Parameters Calibration Techniques   • 10. Calibration Curve and Standard Additions Techniques • 11. Internal Standardization and Isotope Dilution Problem Elements   • 12. Common Problems with Hg, Au, Si, Os, Na, and Ca • 13. Common Problems with Ag, As, S, Ba, Pb, and Cr Basic Calculations   • 14. Accuracy, Precision, Mean and Standard Deviation • 15. Significant Figures and Uncertainty • 16. Traceability

Questions around matrix and elemental compatibility are encountered by all of us on a regular basis. This section addresses this question. Only the most popular matrices will be presented here. Consult our Analytical Periodic Table for other matrices and more information on compatibilities. Technique and calculations will be discussed in part 3 of this guide.

Most analysts prefer nitric acid (HNO₃) matrices due to the solubility of the nitrates as well as its oxidizing ability and the relative freedom from chemical and spectral interferences as compared to acids containing Cl, S, F, or P. In addition, HNO₃ is very popular in acid digestion sample preparations.

The elements that are stable/soluble and commonly diluted in aqueous/HNO₃ are shaded in red below:

(1)  Os should never be mixed with HNO₃ due to the formation of the very volatile 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 volatile and it is corrosive to the instrument and it's electronics therefore, exposure should be kept to a minimum.

The elements that can be diluted in HCl are shaded in blue below:

(1)  Concentrated (35%) 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 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 at pH 7 are shaded in yellow below:

F  Denotes that Si is more stable to polymerization forming polysilicic acid when complexed with F^-.

Hydrofluoric acid (HF) requires the use of HF-resistant introduction systems. These systems are more expensive than glass, have longer washout times, and give a larger measurement precision. However, 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 ₃PO₄ will not adversely effect any of the elements at low µg/mL levels and below.

If your matrix is not listed above, consult our Analytical Periodic Table or contact us for assistance. 

 Part 2