The Chemical Analysis of Argonne Premium Coal Samples

Edited by Curtis A. Palmer
U.S. Geological Survey Bulletin 2144

Coal ash by inductively coupled plasma-atomic emission spectrometry and inductively coupled plasma-mass spectrometry

By Allen L. Meier, Frederick E. Lichte, Paul H. Briggs, and John H. Bullock, Jr.

PRINCIPLE

In coal ash, 58 major, minor, and trace elements are determined by a combination of inductively coupled plasma-atomic emission spectrometry (ICP-AES) and inductively coupled plasma-mass spectrometry (ICP-MS) using two decomposition techniques. A multi-acid decomposition (a mixture of hydrochloric, nitric, perchloric, and hydrofluoric acids) is used to determine 31 elements (Crock and others, 1983), the remaining elements are determined in coal ash following a sodium peroxide sinter decomposition technique (modification of Borsier and Garcia, 1983). The ICP-AES is standardized with a digested coal ash reference standard and a series of multi-element solution standards (Lichte, Golightly, and Lamothe, 1987). Calibration for each element determined by the ICP-MS is made by using the average intensity of five blanks taken through the entire procedure(s) and the intensities acquired on a solution of a glass standard (PP-93) containing a known concentration of each element (Lichte, Meier, and Crock, 1987).

INTERFERENCES

ICP-AES interferences may result from spectral interferences, background shifts, and matrix effects (Thompson and Walsh, 1983). Interelement correction factors and background corrections are applied using the proprietary data system software (Thermo Jarrell Ash, 1988). It is common to not report an affected element due to the extraordinary interference of the affecting element. Matrix effects can generally be negated by proper matching of standard and sample matrices.

ICP-MS interferences come from matrix effects, instrumental drift, and isobaric overlap of some elemental isotopes and molecular ions formed in the plasma resulting in suppression or enhancement of measured ion intensity. A glass standard is used so samples and standards are matrix matched. Internal standards are added to compensate for matrix effects and instrumental drift. The standard solution is run at 15 sample intervals, drift is calculated, and correction applied between standards. The isotopes measured are selected to minimize isobaric overlap from other elements and molecular species that might be present.

SCOPE

Analysis by ICP-AES and ICP-MS for major, minor, and trace elements is useful for a variety of coal and geochemical investigations. The elements analyzed and their reporting limits are shown in tables 26 and 27. Twelve to twenty samples can be prepared daily for each decomposition technique.

APPARATUS

• Thermo Jarrell Ash, Model 1160 Plasma Atomcomp simultaneous Inductively Coupled Plasma Atomic Emission Spectrometer
• Inductively Coupled Plasma Mass Spectrometer, Sciex Elan 250
• Hotplate with aluminum heating block
• 30-mL Teflon vessels with caps (Savillex)
• Muffle furnace
• Graphite crucibles, ACGC-315 (A C Technologies Inc)
• 4-oz plastic disposable specimen jars with screw caps
• 13x100 mm disposable polypropylene test tubes with caps (ICP-AES)
• 17x100 mm disposable polypropylene test tubes with caps (ICP-MS)

REAGENTS

• Deionized (DI) water
• Hydrochloric acid (HCl), conc reagent grade (37%)
• Nitric acid (HNO₃), conc reagent grade (70 %)
• Perchloric acid (HClO₄), conc reagent grade (70%)
• Hydrofluoric acid (HF), conc reagent grade (48%)
• Sodium peroxide (Na₂O₂), ground to minus 80-mesh (<180 m m)
• 1% HNO3: Dilute 10 mL conc HNO₃ to 1000 mL with DI water
• 15% HNO3: Dilute 150 mL conc HNO₃ to 1000 mL with DI water
• Hydrogen peroxide (H₂O₂), solution (30%)

Internal standards

• In-Lu internal standard 40/1000 µg/mL solution: Dissolve 1.1371 g lutetium oxide, Lu₂O₃, in a minimum volume of HNO₃. Add 40 mL 1000 mg/mL commercial Indium standard. Dilute to 1000 mL with 1% HNO₃.

Oxide correction solutions

• Ba and Ce oxides standard: Prepare a solution to contain 1 mg/mL of each element,
  2.5 mg/mL Lu, and 1.5% sodium peroxide. To a 100 mL volumetric flask, add 1.5 g sodium peroxide, 25 mL DI water, 25 mL 15% HNO₃, 0.625 mL 400 mg/mL Lu solution, 0.1 mL 1000 mg/mL Ba, 0.1 mL 1000 mg/mL Ce, and dilute to volume with 1% HNO₃.
• Gd and Sm oxides standard: Prepare a solution to contain 1 mg/mL of each element, 2.5 mg/mL Lu, and 1.5% sodium peroxide. To a 100 mL volumetric flask, add 1.5 g sodium peroxide, 25 mL DI water, 25 mL 15% HNO₃, 0.625 mL 400 mg/mL Lu solution, 0.1 mL 1000 mg/mL Gd, 0.1 mL 1000 mg/mL Sm, and dilute to volume with 1% HNO₃.
• Eu, Nd, and Pr oxides standard: Prepare a solution to contain 1 mg/mL of each element, 2.5 mg/mL Lu, and 1.5% sodium peroxide. To a 100 mL volumetric flask, add 1.5 g sodium peroxide, 25 mL DI water, 25 mL 15% HNO₃, 0.625 mL 400 mg/mL Lu solution, 0.1 mL 1000 mg/mL Eu, 0.1 mL 1000 mg/mL Nd, 0.1 mL 1000 mg/mL Pr, and dilute to volume with 1% HNO₃.
• Ta oxide standard: Prepare a solution to contain 1 mg/mL Ta and 1.5 mg/mL Lu. To a 100 mL volumetric flask, add 25 mL DI water, 25 mL 15% HNO₃, 0.625 mL
  200 mg/mL Lu solution, 0.1 mL 1000 mg/mL Ta, and dilute to volume with 1% HNO₃.
• Nb and Mo oxides standard: Prepare a solution to contain 1 mg/mL of each element, and 1.5 mg/mL Lu. To a 100 mL volumetric flask, add 25 mL DI water, 25 mL 15% HNO₃, 0.625 mL 200 mg/mL Lu solution, 0.1 mL 1000 mg/mL Nb, 0.1 mL 1000 mg/mL Mo, and dilute to volume with 1% HNO₃.

Calibration standard

PP-93: In house glass standard material containing all elements used for calibration.

SAFETY PRECAUTIONS

All laboratory personnel must wear safety glasses, a lab coat or apron, and gloves when working in the laboratory. All digestions and flux (sodium peroxide) preparations must be performed in a chemical fume hood (digestions using perchloric acid are handled in a perchloric acid hood); the latter is washed down after each days use. All personnel must read the CHP and MSDS for each procedure. Calcium glucaonte gel should be available in labs where HF is in use.

PROCEDURE

The instrument operating parameters are shown in tables 28 and 29.

Multi-acid digestion:

1. Weigh 0.2 g sample into a Teflon vessel. Standard coal ash and duplicates are taken through the procedure as well as two samples of PP-93 (in-house glass standard material used for calibration).
2. Add 0.1 mL Lu internal standard (1000 mg/mL).
3. Rinse sample from side walls of the Teflon vessel with a minimum of DI water.
4. Slowly add 3 mL conc HCl.
5. Add 2 mL conc HNO₃. Allow any reaction to subside.
6. Add 1 mL HClO₄ and 2 mL HF.
7. Place Teflon vessels on double aluminum heating block preset at 110° C and heat to incipient dryness. Raise temperature to 160° C for 1 hour. Remove Teflon vessels from hot plate.
8. Add 1 mL HClO₄ and take to dryness at 160° C.
9. Remove vessels from hot plate and allow to cool.
10. Add 1 mL HNO₃ and 1 drop H₂O₂ and heat at 110° C for 5 minutes (samples high in Mn may require more H₂O₂).
11. Cool Teflon vessels and add 19 mL 1% HNO₃, cap and allow to sit overnight.
12. Use the sample solution in the Teflon vessel directly for ICP-AES.
13. For ICP-MS, take 2 mL sample solution in the Teflon vessel and dilute to 8 mL with 1% HNO₃.
14. Wash Teflon vessels with soap and water, rinse with DI water and dry at 100° C.

Sinter method:

1. Weigh 0.1 g sample into a graphite crucible. Standard coal ash and duplicates are taken through the procedure as well as two samples of PP-93 (in-house glass standard material used for calibration).
2. Add 0.5 g sodium peroxide (dry Na₂O₂). Mix sample and Na₂O₂ thoroughly (keep under a heat lamp until placed into muffle furnace).
3. Heat in a preheated 450° C muffle furnace for 30 min.
4. Remove crucibles and allow to cool.
5. Place the crucible in a 4 oz specimen jar and add 20 mL DI water. Cap the jar and swirl a few times (process may be halted at this time until ready for analysis).
6. Add 0.2 mL Lu internal standard solution (1000 mg/mL) to each jar.
7. Add 20 mL 15% HNO₃. Let stand until reaction has stopped (approximately 30 min) and then mix thoroughly.
8. Use the solution directly in the specimen jar for both ICP-AES and ICP-MS.
9. Clean the graphite crucibles by soaking in 5% HCl overnight. Remove from the acid, rinse with water, soak for 30 min in 1% sodium hydroxide solution, rinse with water, and soak overnight in 1% HNO₃. Rinse with DI water and dry at 100° C. If a white residue appears on the crucible, repeat cleaning procedure.

CALCULATIONS

For the multi-acid decomposition, a 0.200 g sample is diluted to 20 mL.  The dilution factor = 100.

Concentration (ppm) = sample volume (mL) ´ ICP-AES reading (ppm)
                                     sample weight (g)

For ICP-MS, a 2 mL aliquot is diluted to 8 mL before analysis. The dilution factor = 400.

Concentration (ppm) = Sample volume (mL) ´ ICP-MS reading (ppm)
                                     sample weight (g)

For the sinter decomposition, a 0.100 g sample is diluted to 40 mL.  The dilution factor = 400.

Concentration (ppm) = Sample volume (mL) ´ ICP-AES reading (ppm)
                                     sample weight (g)

Concentration (ppm) = Sample volume (mL) ´ ICP-MS reading (ppm)
                                     sample weight (g)

ASSIGNMENT OF UNCERTAINTY

The analytical results for selected reference materials duplicate samples, and method blanks are summarized in tables 30 and 31.

BIBLIOGRAPHY

Borsier, M. and Garcia, M., 1983, Analyse automatique d'echantillons geologiques par plasma ICP: Spectrochimica Acta, v. 38B, nos. 1/2, p. 123-127.

Crock, J.G., Lichte, F.E., and Briggs, P.H., 1983, Determination of elements in National Bureau of Standards geological reference materials SRM 278 obsidian and SRM 688 basalt by inductively coupled plasma-atomic emission spectroscopy: Geostandards Newsletter, v. 7, no. 2, p. 335-340.

Kane J., 1990, Written communication to the editor, Collaborative trial values (percent ash = 6.32): U.S. Geological Survey, Reston, Va.

Lichte, F.E., Golightly, D., and Lamothe, P.J., Inductively coupled plasma-atomic emission spectrometry, in Baedecker, P., ed., Methods for Geochemical Analysis: U.S. Geological Survey Bulletin 1770, p. B1-B10.

Lichte, F.E., Meier, Allen L., and Crock, James G., 1987, Determination of the rare earth elements in geological materials by inductively coupled plasma mass spectrometry: Analytical Chemistry, v. 59, no. 8, p. 1150-1157.

National Institute of Standards and Technology, 1985 and 1993, Certificate of analysis: U.S. Department of Commerce, Gaithersburg, Md.

Potts, P.J., Tindle, A.G., and Webb, P.C., 1992, Geochemical reference materials compositions: CRC Press Inc., Boca Raton, Fla., 313 p.

Thermo Jarrell Ash Corporation, 1988, ICAPtm61 Operator's Manual.

Thompson, M. and Walsh, J.N., 1983, A handbook of inductively coupled plasma spectrometry, p. 16-36.

Source: pubs.usgs.gov/bul/b2144/coal_ash.htm