Elemental Analysis Manual: Section 4.5A Cold Vapor Atomic Absorption Spectrometric Determination of Total Mercury in Seafood Using Microwave Assisted Digestion

Appendix A - Supplemental Information on In-house Method Validation

Version 1 (June 2008)
Authors: John Cheng
Susan C. Hight

GLOSSARY

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The use of cold vapor atomic absorption spectrometry for mercury determination is well established and documented in literature¹. Microwave assisted nitric acid sample digestion in closed vessels is highly effective for decomposition of biological materials and contamination is low. EAM Method 4.5 was developed and validated in-house using a CEM Corporation MARS-5 microwave digestion system with XP-1500 Plus vessels and a CETAC Technologies Quick Trace model M-7500 mercury analyzer with an ASX-510 autosampler. Method performance was demonstrated by analyses of reference materials, seafood samples, portions of seafood samples fortified separately with inorganic and organic mercury, and method blanks fortified separately with inorganic and organic mercury².

4.5A.1 ANALYTICAL LIMITS

Analytical limits were estimated based on concentration measurements of unfortified method blanks (MBKs) from separate analytical batches. Results are summarized in 4.5A Table 1.

4.5A.2 REFERENCE MATERIAL RESULTS

Four lypholized seafood and one natural water reference materials with total mercury concentrations ranging from 0.0371 to 1.590 mg/kg were analyzed and the results presented in 4.5A Table 2. All results were from 1 analytical portion and were within the stated (certificate) concentration uncertainty except for Oyster Tissue (NIST 1566b) and Natural Water (NIST 1641d), which were in agreement with stated values within analytical uncertainty (±10%).

4.5A Table 2. Reference Material Results

	Total Hg		RM
Reference Materiala	Reference Value (mg/kg)	Found (mg/kg)	Recovery (%)
NIST 1566b Oyster Tissue	0.0371 ± 0.0013	0.033	90
NIST 2976 Mussel Tissue	0.0610 ± 0.0036	0.062	102
NIST 2977 Mussel Tissue	0.101 ± 0.004	0.099	98
NIST RM 50 Albacore Tuna	0.95 ± 0.1	0.948	100
NIST 1641d Natural Water	1.590 ± 0.018	1.64	103
Summary
mean:			99
minimum:			90
maximum:			103

^a NIST= National Institute of Standards and Technology; n=1.

4.5A.3 FORTIFICATION RECOVERY RESULTS

Eleven seafood products with total mercury concentrations of 0.014 to 1.85 mg/kg were analyzed unfortified and fortified separately with inorganic and organic mercury to assess recovery (4.5A Table 3). Relative fortification levels, expressed as the ratio of fortified to unfortified Hg concentration levels, ranged from 0.54 (Shark, fresh) to 71 (Trout, fresh). The combined average fortification recovery was 101% and the range was 90 to 115%.

4.5A Table 4 lists concentration measurements of duplicate method blanks and recoveries of method blanks fortified separately with inorganic and organic mercury in separate analytical batches. Fortification levels were 1, 2 and 10 µg/L. The combined average fortification recovery was 100% and the range was 95 to 106%.

4.5A Table 3.
Fortification Recovery of Organic and Inorganic Mercury in Seafood Products^a

		Fortification recovery		Level Added
Product	Total Hg (mg/kg)	Inorganic Hgb (%)	Organic Hgc (%)	Hg (µg)	Hg (mg/kg)
Trout, fresh	0.014	99	95	0.5	1
Catfish, fresh	0.058	98	90	0.5	1
Tuna, canned	0.06	100	98	0.05	0.1
Tilapia, fresh	0.067	97	95	0.5	1
Orange roughy, fresh	0.149	102	99	0.5	1
Swordfish (A), fresh	0.199	109	103	0.25	0.5
Tuna, fresh	0.365	115	104	0.5	1
Bluefish, fresh	0.372	101	101	0.5	1
Grouper, fresh	0.472	105	100	0.5	1
Swordfish (B), fresh	1.77	114	109	0.5	1
Shark, fresh	1.85	90	not determined	0.5	1
Summary
mean:		103	99
minimum:		90	90
maximum:		115	109

^a Values are the result from 1 analytical portion.
^b The source of inorganic mercury is inorganic mercury(II) ion in 7% (v/v) HCl solution.
^c The source of organic mercury is methylmercury(II) chloride dissolved in 7% (v/v) HCl solution.

4.5A Table 4.
Total Mercury and Fortification Recovery of Organic and Inorganic Mercury in Method Blanks

	Total Hg (µg/L)			Fortification recoverya		Level Added
Batch	MBK#1		MBK#2	Inorganic Hgb(%)	Organic Hgc(%)	Hg (µg)	Hg (µg/L)
1	0.0038		0.0033	99	95	0.5	10
2	0.0003		0.0002	103	99	0.5	10
3	-0.0020		-0.0019	106	102	0.5	10
4	0.0025		0.0039	101	100	0.1	2
5	0.0017		0.0009	101	96	0.05	1
Summary
mean:		0.0013		102	98
minimum:		-0.0020		99	95
maximum:		0.0039		106	101

^a Values are the result from 1 fortified method blank.
^b The source of inorganic mercury is inorganic mercury (II) ion in 7% (v/v) HCl solution.
^c The source of organic mercury is methyl mercury (II) chloride dissolved in 7% (v/v) HCl solution.

4.5A.4 CONCLUSION

EAM Method 4.5 is applicable for determination of total mercury in seafood. Following prescribed procedures and using specified equipment operated under recommended conditions, the method is capable of determining total mercury ≥6.7 µg/kg in seafood. Method accuracy was demonstrated by successful analyses of reference materials and acceptable fortification recovery results in seafood and method blanks.

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REFERENCES

1. Clevenger, W. L., Smith, B. W., and Winefordner, J. D. (1997) Trace Determination of Mercury: A Review, Crit. Rev. Anal. Chem. 27, 1-26. 
2. Hight, S. C., and Cheng, J. (2005) Determination of Total Mercury in Seafood by Cold Vapor-Atomic Absorption Spectroscopy after Microwave Decomposition, Food Chem. 91, 557-570.