Food

Elemental Analysis Manual: Section 4.5B Supplemental Information on Method Performance

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

Version 1 (June 2008)
Author: John Cheng

Table of Contents

4.5B.1 MERCURY QUALITY ASSURANCE PROGRAM (MQAP)

4.5B.2 PROFICIENCY RESULTS

4.5B.3 INTERLABORATORY PRECISION AND HORRAT

4.5B.4 CONCLUSION 

GLOSSARY


Interlaboratory performance of EAM Method 4.5 was estimated from analytical data and results from laboratory proficiency testing. Although the design and purpose of proficiency testing is different than for method performance studies, the fundamental requirements and conditions for reproducibility were met and the data and results were suitable for determining interlaboratory method performance.

4.5B.1 MERCURY QUALITY ASSURANCE PROGRAM (MQAP)

The MQAP is a proficiency testing program organized and managed by the Canadian Food Inspection Agency for assessing laboratory performance. Participation in the program is voluntary, free of charge, open to all organizations, and participant information is held confidential. Each dispatched round consists of several test materials that are sent to enrolled labs for analysis. Analytical instructions are provided along with a report form for reporting results. An extensive report summarizing results, descriptions of analytical methodologies used, and performance scores (participants are identified by code only) is distributed to all participants upon conclusion.

In 2004, four FDA laboratories participated in MQAP series 352-355. MQAP series 352-355 consisted of 4 canned seafood samples to be analyzed for total mercury. Each can was identified only by a sample number with no additional information. The accompanied paperwork instructed participants to analyze each sample in triplicate on 2 different days and to report all six total mercury values. All FDA labs followed procedures in EAM Method 4.5 draft A using identical mercury analyzers (CETAC Technologies Quick Trace model M-7500) and microwave digestion systems from CEM Corporation. FDA participation was coordinated through the Division of Field Science (DFS). A copy of the MQAP 352-355 final report1 and the 4 laboratory codes used to identify FDA participants in the report was provided by DFS.

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4.5B.2 PROFICIENCY RESULTS

To compare interlaboratory results, the MQAP used assigned values based on consensus, i.e., the mean, corrected for outliers, of all reported results for each sample1. FDA results, individually and collectively, were in good agreement with assigned values (4.5B Table 1). Individual FDA lab results and the interlaboratory mean were within 1 standard deviation of the assigned values for all 4 samples. However, the ratio of FDA mean to assign value was slightly greater than 100% for all samples. This apparent difference is likely due to a lowering influence on the consensus value from participants that used open beaker acid digestion during sample preparation, a process that is susceptible to mercury loss due to volatilization. Also, assigned values in comparison studies are not intended to be used in the same context as true or certified values (reference materials) so a small difference is not a good indicator of error. A graphical comparison of means and variances (4.5B Figure 1) between FDA interlaboratory mean and the MQAP assigned value does not appear statistically or significantly different.

4.5B Table 1. FDA Mercury Results for MQAP Samples 352-355a

 Hg (mg/kg)
FDA Lab352353354354
10.368 ± 0.0270.316 ± 0.0090.202 ± 0.0130.610 ± 0.012
20.407 ± 0.0120.333 ± 0.0080.222 ± 0.0040.654 ± 0.018
30.402 ± 0.0410.317 ± 0.0230.190 ± 0.0390.638 ± 0.047
40.398 ± 0.0070.316 ± 0.0060.206 ± 0.0060.616 ± 0.027
Mean0.3940.3210.2050.63
Assigned valueb0.377 ± 0.048 (61)0.302 ± 0.043 (62)0.198 ± 0.032 (62)0.576 ± 0.094 (62)
FDA Mean
assigned value
104%106%104%109%

a FDA results are mean ± 1 standard deviation; n=6.
b Corrected mean ± 1 standard deviation of all MQAP participants in mg/kg. Number of data points is in parenthesis.

EAM Figure 4.5B: bar graph comparison of FDA interlaboratory mean to MQAP assigned value in order of increasing sample concentration 

4.5B Figure 1. FDA MQAP 352-355. Comparison of FDA interlaboratory mean to MQAP assigned value in order of increasing sample concentration.

4.5B Table 2 shows the MQAP series 352-355 z-scores for FDA participants. Individual z-scores were calculated as follows:

EAM Equation 4.5B.1: Equation to calculate individual z-scores

where

xm = laboratory mean
xc = MQAP assigned value
σ = standard deviation of assigned value

All FDA z-scores were acceptable, i.e., |z| ≤ 2, indicating satisfactory performance.

4.5B Table 2. FDA z-scores for MQAP 352-355

 Sample ID
FDA Lab352353354355
1-0.1880.3360.1330.365
20.6190.7320.7490.831
30.5090.359-0.2330.661
40.4210.3420.2560.427

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4.5B.3 INTERLABORATORY PRECISION AND HORRAT

Interlaboratory precision was estimated by adding the intra-laboratory variance and variance of the interlaboratory mean. The intra-laboratory variance was estimated from the pooled intra-laboratory standard deviations (average variance). The observed interlaboratory precision ranged from 5 to 12% relative standard deviation and the HORRAT ranged from 0.3 to 0.6 (4.5B Table 3).

4.5B Table 3. FDA interlaboratory precision for MQAP series 352-355 Mean and stdev (n = 6) in mg/kg.

 Sample #352
FDA Labmeanstdevpooled stdeva
10.3680.0270.0255
20.4070.012
30.4020.041
40.3980.007
FDA interlab0.3940.018 
Interlab precision 
stdevb0.031 
RSD7.9 
HORRATc0.4 
 Sample #353
FDA Labmeanstdevpooled stdeva
10.3160.0090.0133
20.3330.008
30.3170.023
40.3160.006
FDA interlab0.3210.008 
Interlab precision 
stdevb0.157 
RSD4.9 
HORRATc0.3 
 Sample #354
FDA Labmeanstdevpooled stdeva
10.2020.0130.0209
20.2220.004
30.190.039
40.2060.006
FDA interlab0.2050.013 
Interlab precision 
stdevb0.0247 
RSD12 
HORRATc0.6 
 Sample #355
FDA Labmeanstdevpooled stdeva
10.610.0120.0292
20.6540.018
30.6380.047
40.6160.02
FDA interlab0.3940.018 
Interlab precision 
stdevb0.0355 
RSD5.6 
HORRATc0.3 

a EAM Equa tion 4.5B.2: Equation for pooled standard deviation where sk = standard deviation of mean for laboratory k (k = 1 to 4)
b EAM Equation 4.5B.3: Equation for pooled standard deviation where sr = pooled intra-laboratory standard deviation and sd = standard deviation of interlaboratory mean
c Reference 2

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4.5B.4 CONCLUSION

The results from MQAP proficiency testing indicate satisfactory performance among the FDA laboratories using EAM Method 4.5 draft A. FDA interlaboratory results are in good agreement, within statistical uncertainty, with overall consensus values. The estimated interlaboratory precision ranged from 5 to 12% relative standard deviation for seafood samples containing approximately 0.2 to 0.6 mg/kg total mercury.

REFERENCES

  1. Mercury Quality Assurance Program Series 352-355 Final Report, December 27, 2004. Canadian Food Inspection Agency. 
  2. Official Methods of Analysis of AOAC INTERNATIONAL (2005) 18th Ed., AOAC INTERNATIONAL, Gaithersburg, MD, USA, Appendix D: Guidelines for Collaborative Study Procedures To Validate Characteristics of a Method of Analysis. AOAC disclaimer icon. 

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