US EPA Engineering and Analysis Division Brian Englert

Office of Science and Technology May 18, 2010

1

Development of Pooled Method Detection Limits (MDLs) and 

Minimum Levels of Quantitation (MLs) for EPA Method 1668C. 

May 18th, 2010 

Introduction

This document describes how EPA used MDL data from three (3) laboratories to produce pooled MDLs 

and MLs for EPA Method 1668C.

Method detection limit (MDL) data were received from three laboratories. LAB 1’s MDLs and LAB 2’s 

MDLs were from spikes of each congener into water at a concentration of 10 pg/L; Lab 3’s MDLs were 

from spikes of various congeners into water at concentrations of 50 to 150 pg/L. LAB 1’s MDLs were 

from analyses run in 2009; LAB 2’s MDLs were from analyses run in 2005; Lab 3's MDLs were from 

analyses run in 2006.

Table 2 of Method 1668 (Revisions A and B) list retention times for individual congeners and for 

coeluted congener groups (coelutions). Coelutions exist because the gas chromatographic (GC) column 

does not resolve all 209 congeners; in fact, and in spite of repeated attempts, no GC column resolves all 

209 congeners. Because of slight differences in GC columns, LAB 2’s and Lab 3's coelutions are 

different than LAB 1’s coelutions and those in Table 2 of Method 1668 (LAB 1’s coelutions and those in 

Method 1668 agree). The difference in LAB 2’s and Lab 3's coelutions necessitated deconvolution of 

LAB 2’s and Lab 3's MDLs in coelutions that differed from the coelutions in Table 2 of Method 1668.

The process of producing pooled MDLs and MLs involved three steps:

1) Deconvoluting LAB 2’s and Lab 3's MDLs in coelutions that did not match the coelutions in Table 2 

of Method 1668.

2) Combining the individual congener MDLs deconvoluted from LAB 2’s and Lab 3's coelutions into 

MDLs for the coelutions in Table 2 of Method 1668, and

3) Combining LAB 1, LAB 2, and Lab 3 individual MDLs and MDLs for the coelutions in Table 2 of 

Method 1668 to produce pooled MDLs and MLs for individual congeners and coelutions.

The MDLs and MLs calculated by these three steps are in Table 2 of Method 1668C.

Determination of Laboratory MDLs for individual Congeners

For MDLs determined for an individual congener (i.e., a congener not involved in a coelution), the 

laboratory-reported MDLs were not adjusted; however, all applicable calculations were reviewed to 

confirm that the MDL procedure was followed correctly.

Deconvolution of LAB 2’s and Lab 3's coelutions

LAB 1’s 2009 MDLs are listed in Table 1; LAB 2’s 2005 MDLs are listed in Table 2; and Lab 3's 2006 

MDLs are listed in Table 3. For all three labs, MDLs were reported for at least one group of two or more 

coeluting congeners. A spike of each of the 209 congeners at a constant concentration (e.g., 10 pg/L) 

results in combined spikes 2 to 6 times the concentration of the individual congeners (e.g., 20 - 60 pg/L 
Engineering and Analysis Division Brian Englert

Office of Science and Technology 202-566-0754

05/18/2010

2

for a spike of 10 pg/L for an individual congener) because 2 to 6 congeners elute from the GC as a single 

peak (see Table 2 of Method 1668 for the coelutions). MDLs for these coelutions would be expected to 

be 2 to 6 times higher than for an individual congener that elutes as a single peak. However, in reviewing 

MDLs from the three labs, it was noticed that the MDL for a coelution was not consistently higher by the 

multiple of the number of congeners in the coelution. Therefore, to establish individual MDLs for the 

LAB 2 and Lab 3 coeluting congeners that were inconsistent with Table 2 coelutions, and to not 

underestimate the MDL for an individual congener in a coelution, we did the following. The MDL for an 

individual congener in a coelution was estimated as the greater of the MDL for the coelution divided by 

the number of congeners in the coelution, or the mean of the MDLs for four individual congeners, two of 

which eluted most closely before, and two of which eluted most closely after, the coelution.

For example, for coeluted congeners 100/93, LAB 2’s MDL was 7.6 pg/L. Dividing by 2 to produce an 

estimate for an individual MDL for each congener yields 3.8 pg/L. However, the mean of the MDLs for 

congeners 94, 95, 84, and 89 (the 2 individual congeners that elute most closely before and the 2 

individual congeners that elute most closely after the coelution) was 5.7 pg/L. Therefore, we set the MDL 

for each congener (100 and 93) to 5.7 pg/L.

LAB 2 and Lab 3 coelutions that required deconvolution into individual congeners, and the choice of

deconvolution (divide by the number of congeners or the average of the 4 most closely eluted individual 

congeners) are listed in Tables 4 and 5, respectively.

Determination of Laboratory MDLs for Coelutions

Once MDLs were determined for each individual congener for LAB 2’s and Lab 3's coelutions 

inconsistent with the coelutions in Table 2 of Method 1668, the individual MDLs were added to form 

MDLs for the coelutions in Table 2 of Method 1668. LAB 1’s original coelutions already corresponded 

to the coelutions in Method 1668; therefore, LAB 1’s MDLs for the coelutions in Table 2 of Method 1668 

were used as received.

Calculation of pooled MDLs and MLs

Pooled MDLs were calculated for each individual congener and each coelution by first calculating a 

pooled standard deviation. If possible, this pooled standard deviation was calculated from the standard 

deviations in data from the three labs. In cases where an MDL was determined by deconvolution, the 

standard deviation was “back-calculated” by dividing the MDL by the 99th percentile t-statistic with the 

appropriate degrees of freedom (i.e., the number of replicates analyzed by LAB 2 minus 1 (7 - 1 = 6)). 

The pooled standard deviation was then calculated by weighting the labs' standard deviations by the 

number of replicates minus 1 (so LAB 1's standard deviation was weighted more because LAB 1

analyzed 9 replicates whereas LAB 2 and Lab 3 analyzed 7). This additional weighting resulted in a 

slight increase in the pooled MDL above lab MDLs that were identical.

To form minimum levels of quantitation (MLs), the pooled MDLs were multiplied by 3.18 and rounded 

to the number nearest to 1, 2, or 5 x 10 n, where n is 0 or an integer, per the ML procedure (see 68 FR 

11790). MDLs and MLs for other media and MLs in the final extract were calculated by taking into 

account the sample and extract volumes. The resulting MDLs and MLs were then inserted in Table 2 of 

Method 1668C.

MDLs for Future Coelutions Different from Those in Table 2 of Method 1668C
US EPA Engineering and Analysis Division Brian Englert

Office of Science and Technology May 18, 2010

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If, in the future, a laboratory has coelutions different from those in Table 2 of Method 1668C, the 

laboratory can use the information in this document to deconvolute its coelutions to obtain MDLs for 

individual congeners in a coelution, and reconstitute an MDL for a Method 1668 Table 2 coelution by 

adding the pertinent individual MDLs.
Engineering and Analysis Division Brian Englert

Office of Science and Technology 202-566-0754

05/18/2010

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Table 1 LAB 1's 2009 MDLs (pg/L)*

Congener MDL Congener MDL Congener MDL CongenerMDL

1 3.9 48 3.3 108/124 4.0 164 3.4

2 2.5 69/49 8.1 107 2.1 165 2.4

3 3.8 50/53 5.0 110/115 13.2 167 2.7

4 2.8 52 5.6 111 4.4 169 2.9

5 3.0 54 3.1 112 5.1 170 4.7

6 1.5 55 2.4 114 2.2 171/173 6.3

7 1.9 56 3.1 118 6.0 172 3.8

8 3.6 57 1.8 120 3.1 174 5.7

9 2.1 58 2.5 121 4.7 175 4.1

10 2.1 59/62/75 8.4 122 1.8 176 3.6

11 36.4 60 2.4 123 2.4 177 4.7

12/13 5.3 61/70/74/76 9.6 126 2.1 178 5.0

14 2.2 63 2.8 127 2.0 179 4.8

15 4.0 64 3.5 128/166 3.3 180/193 12.7

16 3.2 66 4.9 138/163/129/160 16.1 181 4.3

17 3.7 67 3.1 130 2.1 182 4.5

30/18 7.5 68 3.0 131 2.7 183/185 8.4

19 3.3 72 2.2 132 3.5 184 2.8

28/20 5.8 73 3.1 133 3.0 186 5.3

21/33 4.0 77 2.0 134/143 3.5 187 11.7

22 3.1 78 2.2 151/135/154 15.8 188 2.2

23 3.2 79 2.4 136 3.3 189 2.0

24 2.8 80 2.3 137 2.9 190 2.4

25 1.8 81 2.0 139/140 3.9 191 3.4

26/29 2.7 82 4.3 141 5.2 192 2.5

27 3.5 83/99 10.6 142 2.1 194 2.3

31 4.5 84 3.8 144 3.6 195 2.7

32 1.5 117/116/85 8.7 145 3.8 196 4.0

34 2.5 109/119/86/97/125/87 22.8 146 3.8 197/200 7.7

35 3.6 88/91 6.6 147/149 20.8 198/199 5.1

36 2.2 89 4.3 148 3.2 201 2.8

37 2.4 113/90/101 19.6 150 3.2 202 3.5

38 1.5 92 4.6 152 3.5 203 2.5

39 2.4 95/100/93/102/98 19.2 153/168 19.9 204 3.1

41/40/71 8.5 94 3.7 155 3.0 205 2.2

42 4.7 96 3.6 156/157 4.1 206 1.8

43 5.0 103 2.7 158 1.7 207 1.6

44/47/65 10.4 104 3.2 159 1.7 208 3.3

45/51 4.6 105 3.4 161 1.9 209 2.6

46 2.6 106 4.1 162 2.9

*From 9 replicate analyses of a spike of each of 209 congeners into water at a concentration of 10 pg/L.
US EPA Engineering and Analysis Division Brian Englert

Office of Science and Technology May 18, 2010

5

Table 2 LAB 2's 2005 MDLs (pg/L)*

Congener MDL Congener MDL Congener MDL Congener MDL

1 6.7 49/69 8.5 107 3.7 165 4.1

2 4.2 50/53 9.2 108/124 22.9 167 5.0

3 6.4 52 7.5 110/115 7.2 169 3.6

4 10.4 54 4.7 111 3.4 170 2.9

5 4.6 55 6.1 114 4.7 171/173 7.8

6 6.6 56 5.0 118 6.3 172 3.4

7 3.9 57 4.6 120 3.4 174 6.5

8 8.6 58 2.8 121 3.4 175 5.6

9 4.6 59/62/75 11.6 122 4.6 176 2.2

10 7.3 60 4.8 123 5.0 177 2.2

11 36.4 61/70/74/76 23.8 126 2.2 178 2.9

13/12 20.4 63 4.8 127 6.6 179 2.5

14 5.8 64 5.0 128/166 10.8 180/193 7.8

15 10.8 66 12.0 129/138/163 8.9 181 5.4

16 8.6 67 5.7 130 8.7 182 3.6

17 10.9 68 3.9 131 1.3 183/185 4.3

18/30 11.5 72 3.7 132 4.6 184 3.3

19 9.7 77 4.4 133 2.7 186 4.2

20/28 16.6 78 4.4 134/143 10.4 187 4.5

21/33 12.6 79 3.2 135/151 6.3 188 4.3

22 9.9 80 3.6 136 3.4 189 2.8

23 3.2 81 3.4 137/164 3.5 190 2.5

24 11.2 82 8.3 139/140 4.4 191 3.1

25 7.7 83 9.3 141 3.8 192 3.7

26/29 9.0 84 6.0 142 4.4 194 5.0

27 5.6 85/116/117 8.4 144 5.5 195 6.2

31 10.1 86/87/97/109/119/125 10.5 145 3.1 196 6.2

32 5.7 88/91 7.4 146 4.9 197/200 5.6

34 3.4 89 5.6 147/149 4.5 198/199 13.0

35 9.6 90/101/113 4.7 148 5.0 201 4.3

36 7.5 92 3.7 150 3.4 202 3.9

37 9.0 93/100 7.6 152 2.3 203 4.9

38 4.7 94 4.5 153/168 7.1 204 3.1

39 7.3 95 6.8 154 4.9 205 5.5

40/41/71 6.5 96 2.6 155 3.2 206 3.2

42 4.0 98/102 12.1 156/157 4.5 207 2.7

43/73 9.3 99/112 17.7 158 2.5 208 3.5

44/47/65 10.7 103 2.5 159 3.4 209 2.5

45/51 12.1 104 5.7 160 7.2

46 2.6 105 4.4 161 2.6

48 2.6 106 5.8 162 4.1

*From 7 replicate analyses of a spike of each of 209 congeners into water at a concentration of 10 pg/L.
Engineering and Analysis Division Brian Englert

Office of Science and Technology 202-566-0754

05/18/2010

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Table 3 Lab 3's 2006 MDLs (pg/L) *

Congener MDL Congener MDL Congener MDL Congener MDL

1 17.1 65/47/44 71.2 111 24.8 169 26.1

2 11.2 62/75/59 64.8 120 24.0 188 26.3

3 18.6 42 28.7 108/124 47.7 179 25.2

4 20.5 41/71/40 75.6 107/123/106 88.8 184 25.4

10 22.7 64 22.0 118 53.0 176 22.1

9 12.1 72 22.4 122 21.7 186 25.3

7 13.7 68 24.2 114 26.3 178 24.4

6 11.8 57 20.3 105 30.3 175 25.2

5 14.3 58 26.0 127 24.3 187 26.0

8 25.4 67 21.7 126 29.9 182 22.7

14 12.4 63 20.9 155 25.2 183/185/174 75.0

11 20.4 61/70/76/74 104.0 152 25.3 177 19.4

13/12 27.2 66 27.0 150 26.8 181 23.2

15 27.5 55 21.3 136 29.0 171/173 54.0

19 10.2 56 27.2 145 28.2 172 23.3

30/18 25.9 60 25.8 148 25.2 192 23.5

17 11.2 80 19.8 151/135/154 80.0 193/180 52.6

27/16/24 39.0 79 23.8 144 25.6 191 23.7

32 13.1 78 28.6 147/149 57.3 170 21.4

34 11.2 81 32.8 134/143 58.3 190 24.6

23 10.8 77 24.9 139/140 52.0 189 23.8

29/26 20.2 104 23.9 131 31.5 202 44.2

25 13.2 96 27.0 142 29.7 201 35.9

31 31.5 103 20.2 132 29.4 204 37.9

28/20/21/33 72.3 94 22.4 133 22.0 197/200 77.2

22 13.4 95 36.5 165 22.6 198/199 65.2

36 13.2 100/93/102/98 85.7 146/161 48.0 196 35.4

39 13.3 88/91 38.9 153/168 49.0 203 32.7

38 12.3 84 18.1 141 29.9 195 38.7

35 13.7 89 21.5 130 22.7 194 32.7

37 15.0 121 20.7 137/164 50.2 205 27.5

54 25.7 92 23.6 138/163/129/160 112.5 208 28.2

50/53 44.0 113/90/101 82.9 158 28.7 207 33.7

45/51 38.1 83/99/112 66.4 166/128 52.1 206 28.7

46 18.6 119/109/86/97/125/87 131.7 159 24.8 209 28.8

73/52/43 69.1 117/116/85 68.1 162 22.8

69/49 45.0 110/115 68.4 167 23.2

48 25.8 82 26.4 156/157 42.3

*From 7 replicate analyses of spikes into water of mono- thru tri- chloroCBs at 50 pg/L; tetra- thru hexachloroCBs at 100 pg/L and hepta- thru deca- chloroCBs at 150 pg/L.
US EPA Engineering and Analysis Division Brian Englert

Office of Science and Technology May 18, 2010

7

Table 4 Individual Test America-Knoxville MDLs deconvoluted from MDLs for coelutions that do not 

match coelutions in Table 2 of Method 1668A

MDL for an individual congener was determined as the larger of the MDL for the coelution divided by 

the number of congeners in the coelution, or the mean of the MDLs for four individual congeners, two of 

which eluted most closely before, and two of which eluted most closely after, the coelution

Coelution Congener numbers of congeners averaged

100,93 94,95,84,89 (means that MDLs for these 4 congeners were averaged)

102,98 /2 (means that the MDL for these congeners was divided by 2)

99,12 /2

151,135 145,148,154,144

137,164 141,130,160,158

138,163,129 141,130,160,158

Table 5 Individual Lab 3-Columbus MDLs deconvoluted from MDLs for coelutions that do not match 

coelutions in Table 2 of Method 1668A

MDL for an individual congener was determined as the larger of the MDL for the coelution divided by 

the number of congeners in the coelution, or the mean of the MDLs for four individual congeners, two of 

which eluted most closely before, and two of which eluted most closely after, the coelution

Coelution Congener numbers of congeners averaged

27,16,24 /3 (means that the MDL for these congeners was divided by 3)

28,20,21,33 /4

52,73,43 54,46,48,42 (means that MDLs for these 4 congeners were averaged)

95,100,93,102,98 94,95,84,89

83,99,112 121+92+82+111

107,123,106 111,120,118,122

146,161 133,165,141,130

137,164 141,130,158,159

183,185,174 /3