Food

Elemental Analysis Manual: Section 4.5A Supplemental Information on In-house Method Validation

<< Elemental Analysis Manual (EAM) for Food and Related Products  

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

Table of Contents

4.5A.1 ANALYTICAL LIMITS

4.5A.2 REFERENCE MATERIAL RESULTS

4.5A.3 FORTIFICATION RECOVERY RESULTS

4.5A.4 CONCLUSION

GLOSSARY


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.

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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 Table 1. Estimate of Analytical Limits

Analytical LimitsMercury
Number of MBKs10
Average conc. Found (µg/L)0.0013
Standard deviation (µg/L)0.00222
ASDL (µg/L)0.0085
ASQL (µg/L)0.067
LOD (µg/kg) for 0.5 g anal. portion0.86
LOQ (µg/kg) for 0.5 g anal. portion6.7

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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 HgRM
Reference MaterialaReference Value (mg/kg)Found (mg/kg)Recovery (%)
NIST 1566b Oyster Tissue0.0371 ± 0.00130.03390
NIST 2976 Mussel Tissue0.0610 ± 0.00360.062102
NIST 2977 Mussel Tissue0.101 ± 0.0040.09998
NIST RM 50 Albacore Tuna0.95 ± 0.10.948100
NIST 1641d Natural Water1.590 ± 0.0181.64103
Summary
mean:99
minimum:90
maximum:103

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

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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 Productsa

 Fortification recoveryLevel Added
ProductTotal Hg
(mg/kg)
Inorganic Hgb
(%)
Organic Hgc
(%)
Hg
(µg)
Hg
(mg/kg)
Trout, fresh0.01499950.51
Catfish, fresh0.05898900.51
Tuna, canned0.06100980.050.1
Tilapia, fresh0.06797950.51
Orange roughy, fresh0.149102990.51
Swordfish (A), fresh0.1991091030.250.5
Tuna, fresh0.3651151040.51
Bluefish, fresh0.3721011010.51
Grouper, fresh0.4721051000.51
Swordfish (B), fresh1.771141090.51
Shark, fresh1.8590not determined0.51
Summary
mean:10399 
minimum:9090 
maximum:115109 

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 recoveryaLevel Added
BatchMBK#1 MBK#2Inorganic Hgb(%)Organic Hgc(%)Hg (µg)Hg (µg/L)
10.0038 0.003399950.510
20.0003 0.0002103990.510
3-0.0020 -0.00191061020.510
40.0025 0.00391011000.12
50.0017 0.0009101960.051
Summary
mean: 0.0013 10298  
minimum: -0.0020 9995  
maximum: 0.0039 106101  

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.

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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.


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. 

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