Anal Bioanal Chem (2012) 404:2105–2111 DOI 10.1007/s00216-012-6306-y

TECHNICAL NOTE

A calculation mode for the correct GC/ECNI-MS-SIM determination of polychlorinated terphenyls in the presence of high excesses of polychlorinated biphenyls Natalie Rosenfelder & Walter Vetter

Received: 27 June 2012 / Accepted: 26 July 2012 / Published online: 4 September 2012 # Springer-Verlag 2012

Abstract Polychlorinated terphenyls (PCTs) are a class of persistent organic pollutants difficult to analyze by gas chromatography with mass spectrometry operated in the selected ion monitoring mode (GC/MS-SIM) in environmental samples due to the retention time and mass range overlap with polychlorinated biphenyls (PCBs). To overcome these drawbacks, we developed and evaluated a mathematical calculation algorithm which allows to detail the interference of PCT congeners in GC/electron capture negative ion (ECNI)-MS-SIM chromatograms by PCBs. The calculation takes advantage of the abundance and ratio of two suitable isotope peaks of the molecular ion of PCTs. With the help of this method, we detected at least 63 tetra- to nonachlorinated terphenyls in the blubber of a harbour porpoise (Phocoena phocoena) from the North Sea. The interference of these peaks by PCBs ranged from >100 to 0 %. The novel calculation method used in combination with GC/ ECNI-MS-SIM is suitable to analyze PCTs in environmental and food samples. However, it can also be applied to GC/EI-MS measurements. Keywords Gas Chromatography-Mass Spectrometry . Organic compounds/trace organic compounds . Polychlorinated compounds . Mathematical correction

Electronic supplementary material The online version of this article (doi:10.1007/s00216-012-6306-y) contains supplementary material, which is available to authorized users. N. Rosenfelder : W. Vetter (*) Institute of Food Chemistry, University of Hohenheim, Garbenstr. 28, 70593 Stuttgart, Germany e-mail: [email protected]

Introduction Polychlorinated terphenyls (PCTs) are a class of industrial organohalogen compounds produced and used in parallel with polychlorinated biphenyls (PCBs) [1]. Chemically, they emerge from three directly connected phenyl units. Based on biphenyl, the third ring can be attached in ortho-, meta- or para-position (Fig. 1). PCTs are produced by chlorination of the terphenyl backbones. Chlorine substituents can be on all rings in all positions, and in either case, the chemical formula is C18H14-xClx. This leads to a theoretical variety of more than 8,000 congeners, which is higher than for most other organohalogen multicomponent mixtures [2]. PCT producers were located in the USA, Japan and in European countries (Germany, Italy and France) [3]. Reported production volumes (between the 1950s and 1972) were ∼57,000 t in the USA [3] and 2,700 t in Japan [4]. Only little information is available on the PCT production and use in Europe. Reports list 4,000 t in one plant in France and 2,500 t in Italy [5]. It was estimated that the PCT production amounted to about 5 % of the PCBs [1]. Reports on the legal status of PCTs are scarce. Due to the same physicochemical properties, PCTs have substituted PCBs when these were stepwise banned [4]. Since the mid-1980s, no PCT production is known to occur anywhere, but PCTs may be found in old existing equipment [5]. In accordance with EU Directive 96/59/EC, the definition of PCBs also covers PCTs [5]. Therefore, analysis on PCBs should also include PCTs. This is in contrast to North America where the term PCBs does not cover PCTs [5]. Compared to PCBs, information about the concentrations and the fate of PCTs in the environment is very limited [3, 6–9]. The reasons for this lack in knowledge are diverse: Little is known about the actual variety of congeners in

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a

procedures were reported in the original papers [14, 15]. The protocols are shown in the Electronic supplemenatry material of this manuscript. Reference standards (individual PCB congeners) were purchased from Dr. Ehrenstorfer (Augsburg, Germany) and LGC Promochem (Wesel, Germany).

b c

Fig. 1 Backbone of polychlorinated terphenyls based on biphenyl with an additional phenyl unit in a para position, b meta position and c ortho position. Chlorine substituents can be on all rings in all positions. Currently, none of the PCTs present in environmental samples is structurally known

technical PCT mixtures and residues in the environment, and only a few individual PCT standards have been synthesized [10–12]. Most importantly, however, both the GC elution profiles and the mass ranges of the molecular ions of PCTs severely overlap with those of PCBs. It was mentioned that PCT results from one study are difficult to compare with those of other studies [3]. Consequently, errors of 30–40 % were reported to occur in gas chromatography–mass spectrometry (GC/MS) selected ion monitoring (SIM) quantification of PCTs [8]. Hence, the use of GC/MS-SIM was discouraged due to the introduction of relatively large uncertainties associated with this technique in the analysis of this class of compounds [8, 11]. Instead, the perchlorination of terphenyl has been proposed in order to simplify the analysis [3, 13]. Certainly, the high sensitivity of GC/MS-SIM would make this method ideal for the quantification of individual PCTs. In this study, we aimed at developing a novel GC/ electron capture negative ion (ECNI)-MS-SIM method which allows for the accurate determination of PCTs in the presence of PCBs in environmental samples. A mathematical algorithm is used to exactly eliminate the interference of PCBs from the ions chosen for PCT quantification. The method takes into account the presence of two orders of magnitude higher abundant PCB concentrations which overlap with PCTs.

Materials and methods Samples, chemicals and sample clean up Blubber sample extracts of two marine mammals were taken from previous studies. The harbour porpoise (Phocoena phocoena) sample was from the German North Sea coast (1989), and the monk seal (Monachus monachus) sample from Mauritania (Africa, 1997). The samples, chemicals and clean up

Gas chromatography with electron capture negative ion analysis Analyses were run with a 7890/5975C GC/MS system (Agilent Technologies, Waldbronn, Germany). Transfer line and ion source temperature were set to 300 and 150 °C. Methane 5.5 (Air Liquide, Bopfingen, Germany) was used as the reagent gas at a flow rate of 40 mL/min. One microliter of sample extracts was injected by means of an Agilent 7673 GC/SFC automatic injector operated in pulsed splitless mode. An HP-5MS column (30 m length, 0.25 mm internal diameter and 0.25 μm film thickness) was installed in the GC. The flow of the carrier gas (helium 5.0) was set to 1.2 mL/min. The GC oven program started at 50 °C (hold time 2 min), then at 10 °C/min to 300 °C (hold time 29 min). In the full scan mode, m/z 50–800 was recorded after a solvent delay of 8 min. Determination of PCTs was performed in the selected ion monitoring (SIM) mode. Calculation of interferences was based on the following low mass ions (a), high mass ions (b) and control ions: (1) from 8– 19 min: m/z 334, 336a, 338b (triCTs) 368a, 370b, 372 (tetraCTs), (2) from 19–21.5 min: m/z 368a, 370b, 372 (tetraCTs), 404a, 406b, 408 (pentaCTs), (3) 21.5–22.5 min: m/z 404a, 406b, 408 (pentaCTs), 438a, 440b, 442 (hexaCTs), (4) 22.5–24 min: m/z 438a, 440b, 442 (hexaCTs), 472a, 474b, 476 (heptaCTs), (5) 24–28 min: 472a, 474b, 476 (heptaCTs), 506a, 508b, 510 (octaCTs), (6) 28–50 min: 538a, 540b, 542 (nonaCTs), 572a, 574b, 576 (decaCTs). Peak identification and calculation mode Areas of the low mass ions (a), high mass ions (b) used for the calculations in the different time windows were transferred into an Excel sheet. In the first instance, the ratio between low and high mass ion was automatically calculated. The theoretical ratio of any given PCT and PCB is generally smaller for PCTs (Table 1). Thus, these two values represent the minimum value (PCT not interfered by a PCB) and maximum (no PCT, only peak originating from a PCB) ratios that can be found. Peaks whose ratio of quantification to verification ion was outside the theoretical ratio of PCTs and PCBs were excluded because in this case, it indicates the presence of an interference from another compound class for which the result cannot be corrected. In addition, for tri- to octaCTs, it was checked that the third ion measured (control ion, see above) was not higher abundant than 30 % of the confirmation ion. Peaks which did not fulfill

Results and discussions

heptaCTs

octaCTs e

d

pentaCTs

hexaCTs c

tetraCTs

Problems associated with the co-existence of PCTs and PCBs in samples

b

19.9672 17.2871 IRPCB

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these relations will be marked with an asterisk. All peaks identified in each mass range were summarized in one table, listed with increasing retention time. Peaks appearing at identical retention times (±0.05 min) in different mass ranges were listed as one compound, i.e. the one with the highest mass detected, while lower mass was interpreted as fragment ion of the compound with the higher mass.

a

10.5563

c

2.2981c b

3.0382b a

2.0345a

IRPCT and IRPCB ions were used as quantification ions after correction according to the calculation algorithm. Values to be inserted in Eq. 5 for the calculation of tetraCTs, pentaCTs, hexaCTs, heptaCTs and octaCTs

1.8473d

7.1490

d

1.5440e

5.406e

508 (0.23 %) 508 (16.78 %) 474 (0.09 %) 370 (0.02 %) 370 (17.22 %)

406 (7.09 %)

406 (0.01 %)

440 (10.59 %)

440 (0.03 %)

474 (13.92 %)

498

506 (1.26 %)

506

506 (25.91 %)

464 430

472 (0.67 %) 438 (24.34 %)

436 394

404 (0.10 %) 404 (21.54 %)

402 360

368 (0.35 %) 368 (35.03 %)

Low mass used in IR (abundance) High mass used in IR (abundance) IRPCT

368

438 (0.30 %)

470

511

472 (25.72 %)

494

516 476

502 468

482 441

426 434

446 406

392 400

371 376

Highest mass (>0.003 %) Base peak

358 366 Lowest mass (monoisotopic)

411

460

decaCB hexaCTs heptaCBs pentaCTs hexaCBs tetraCTs

Table 1 Masses, isotope abundances and isotope ratios (IR) of lower to higher mass of PCTs and PCBs

octaCBs

heptaCTs

nonaCBs

octaCTs

A calculation mode for the GC/ECNI-MS determination of PCTs

PCT isomers span over the same retention time range as PCBs bearing two more chlorine substituents. Unfortunately, the mass range of the isotope patterns of the molecular ions of these co-eluting groups of PCTs and PCBs generally shows some overlap (Table 1). As an example, M− (or M+) of tetraCTs (m/z 366) is 8 u higher than M− of hexaCBs at m/z 358, and the base ion of the ion cluster of tetraCTs [M+2]− at m/z 368 corresponds with [M+10]− of PCBs (Table 1). The [M+10]− of PCBs at m/z 368 contributes only very little to the hexaCB isotope pattern. Nevertheless, due to the much higher concentrations of PCBs in environmental samples [8], the impact of hexaCBs on m/z 368 is still immense. As a consequence, the ion traces m/z 360 (only hexaCBs) and m/z 368 look very similar (Fig. 2a, b). Thus, by means of m/z 368 only PCBs were detected in the sample. Theoretical calculation verified that the interference of a tetraCT by hexaCBs for m/z 368 is 1.2, 10.7 and 54.5 %, in case that the tetraCT/hexaCB ratio is 1:1, 1:10 and 1:100, respectively. The latter scenario, i.e. PCTs/PCBs ∼1:100 is rather realistic for environmental samples. These figures fit well with the error associated with the classic GC/ ECNI-MS-SIM determination of PCTs [8]. By contrast, the peak pattern of m/z 370 and m/z 372 (only tetraCTs) looks different (Fig. 2). Still, the contribution of hexaCBs to m/z 370 (Fig. 2c; minor isotope peak of hexaCBs) cannot be ignored, while m/z 372 is not formed by hexaCBs and thus represents tetraCTs (or traces of [M−Cl]− fragment ions of pentaCTs) (Table 1). However, quantitation of tetraCTs by m/z 372 cannot be recommended due to two reasons. First, its ion abundance contributes only 3.9 % to M− of tetraCTs so that both the sensitivity and reproducibility are low. Second, GC/ECNI-MS quantitations should take advantage of a verification ion which could be only m/z 374 which is only 0.35 % of M− (Table S1, supporting materials). Therefore, quantitation of tetraCTs by means of m/z 368 or m/z 370 would be favorable. Comparison of Fig. 2a (only hexaCBs) with Fig. 2d (only tetraCTs) illustrates that the individual GC isomer profiles of both compound classes are different. Thus, when m/z 368

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N. Rosenfelder, W. Vetter

Irel

a

PCB 153

19.6

20.0

20.4

Irel

20.8

21.2

PCB 153

21.6 [min]

b

Developing a calculation algorithm of the interference of a given PCT by a PCB The interference of a PCT by a PCB peak can be calculated by determining both the area and the (measured) ratio of the selected lower mass ion (example, m/z 368 in the case of tetraCTs and hexaCBs) and the 2 u higher mass (example, m/z 370). The following basics can be listed: 1. The abundance of the lower mass ion is the sum of the contributions of the PCT (a) and the PCB (b) 2. The abundance of the higher mass is the sum of the contributions of the PCT (c) and the PCB (d). This leads to Eq. 1:

19.6

20.0

20.4

Irel

20.8

21.2

21.6 [min]

PCB 153

c

19.6

20.0

20.4

20.8

21.2

Irel

21.6 [min]

d KT2 KT3 KT1 19.6

*

* *

*

20.0

20.4

20.8

21.2

21.6 [min]

Fig. 2 GC/ECNI-MS-SIM ion traces of the blubber extract of a harbour porpoise (P. phocoena) from the North Sea. a m/z 360 (base ion of hexachlorobiphenyls), b m/z 368 (base ion of tetrachloroterphenyls), c m/z 370 and d m/z 372 (only response to tetraCTs or [M− Cl]− from pentaCTs). Peaks labeled arise from the dominant PCB congener in marine mammals (PCB 153) and three tetrachloro PCTs chosen as key congeners (KT1, KT2 and KT3) representative for PCT residues in different samples. Peaks marked with an asterisk do not originate from PCTs

R ¼ ða þ bÞ=ðc þ dÞ with R

being the measured ratio of the abundance of lower to higher mass a and c being the proportion of lower and higher mass of any PCT b and d being the proportion of lower and higher mass of any PCB In Eq. 1, only R can be determined. In order to solve the problem with four unknowns, these unknowns have to be subsequently substituted by knowns. In the present case, the isotope ratio (IR) of lower to higher mass is known for the PCT (IRPCT) and the PCB (IRPCB; Table 1). Thus, “a” can be substituted with “IRPCT·c” and “b” can be substituted with “IRPCB·d”. Insertion into Eq. 1 leads to Eq. 2: R ¼ ðIRPCT  c þ IRPCB  d Þ=ðc þ d Þ

ð2Þ

Equation 2 only contains two unknowns, which are related to each other. Consequently, Eq. 2 has to be resolved for c and d. In a first step, multiplication with (c+d) leads to Eq. 3: R  c þ R  d ¼ ðIRPCT  c þ IRPCB  d Þ

ð3Þ

sorted for d and c, respectively, leads to Eq. 4: cðR  IRPCT Þ ¼ ðIRPCB  RÞd

and m/z 370 are used for quantitation, the interference of each PCT peak by PCBs will be different and may range from zero (no interference) to >100 % (in case of the interference of a minor tetraCT peak with a major hexaCB). Therefore, any simple determination of these ions in the SIM mode is associated with a high error [8]. The same problems were also valid for pentaCTs (interfered by heptaCBs), hexaCTs (interfered by octaCBs), heptaCTs (interfered by nonaCBs) and octaCTs (interfered by PCB 209). In such cases, the interference of each PCT congener by PCBs must be calculated as shown in the following section.

ð1Þ

ð4Þ

resolved for c leads to Eq. 5: c ¼ ðIRPCB  RÞd=ðR  IRPCT Þ

ð5Þ

Since c and d represent the proportions of the peak by the PCT (c) and the PCB (d), Eq. 5 provides the contribution of the PCT to the higher mass ion. Likewise, the contribution of the PCB results from the total peak area minus the contribution of the PCT. Measuring the total peak areas of the lower and higher mass, calculation of R and insertion into Eq. 5 (with d0area of the higher recorded mass−c) will

A calculation mode for the GC/ECNI-MS determination of PCTs

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Table 2 GC retention times, areas of the lower and higher isotopic mass used for the calculation, ratio of the two ions and area of the higher mass ion actually originating from a PCT and contribution of the PCT (in percentage) to a given peak No. per Cl degree (key congener) TetraCTs T1 T2 T3 T4 T5 T6 Total area (% of area) PentaCTs P1 P2 P3 P4 P5 P6 P7 P8 P9

tR (min)a

Peak area of lower massb

19.857 20.074 20.294 20.404 20.919 21.017

m/z 368 9,250 8,910 34,200 265,000 63,500 853,000

20.521 21.171 21.319 21.407 21.832 22.107 22.216 22.26 22.348

m/z 404 5,770 58,200 255,000 90,100 34,700 31,600 519,000 48,800 14,400

21.884 22.157 22.821 22.875 22.969 23.494

m/z 438 68,600 13,300 16,800 281,000 421,000 129,000

23.462 23.596 23.65

Hp4 Hp5 Hp6 Hp7 Hp8 Hp9 Hp9 Hp10 Hp11 Hp12 Hp13 Hp14 Hp15 Hp16 Hp17 Hp18

Total area (% of area) HexaCTs H1 H2 H3 H4 H5 H6 Total area (% of area) HeptaCTs Hp1 Hp2 Hp3

Peak area of higher massb m/z 370 4,170 534 2,820 16,400 29,400 56,900 110,000 m/z 406 2,860 8,430 16,400 7,620 2,760 2,190 29,300 5,340 3,950

m/z 472 153,000 118,000 5,500

78,900 m/z 440 7,990 4,790 5,280 33,100 50,100 17,000 118,000 m/z 474 21,900 21,700 3,160

25.212 25.559 25.631 25.807 25.862

10,100 29,500 18,200 302,000 212,000

25.932 26.082 26.158 26.234 26.321 26.394 26.527 26.815 27.418 27.484 27.73

227,000 113000 370,000 37,000 478,000 985,000 792,000 454,000 480,000 347,000 1,290,000

Ratio R (lowerto-higher mass)

2.22 16.7 12.1 16.2 2.15 15.0

2.02 6.90 15.5 11.8 12.6 14.4 17.7 9.14 3.65

Area contribution of PCT to higher massb m/z 370 4,120 21.5 959 1,170 29,200 8,570 44,000 (28 %) m/z 406 3,030 6,510 4,330 3,660 1,200 716 3,880 3,420 3,810

Percent contribution of peak to higher mass peakc (%)

99 4 34 7 99 15

106c 77 26 48 44 33 13 64 96

6.99 5.46 1.74

30,600 (40 %) m/z 440 1,910 4,510 4,720 8,240 13,100 6,090 38,600 (33 %) m/z 474 646 6,910 3,230

3 32 102c

5,240 15,500 9,070 157,000 114,000

1.92 1.91 2.01 1.92 1.86

5,160 15,300 8,790 1,550 114,000

99 99 97 99 100

118,000 60,500 198,000 19,700 260,000 527,000 426,000 247,000 257,000 187,000 649,000

1.94 1.87 1.87 1.88 1.84 1.87 1.86 1.84 1.87 1.85 1.99

115,500 60,300 197,000 19,600 260,000 525,000 425,000 247,000 255,000 187,000 631,000

98 100 100 99 100 100 100 100 100 100 97

8.58 2.78 3.18 8.50 8.39 7.60

24 94 89 25 26 36

2110

N. Rosenfelder, W. Vetter

Table 2 (continued) No. per Cl degree (key congener) Total area (% of area) OctaCTs O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12 O13 O14 O15 O16 O17 O18 O19 O20 NonaCTs N1 N2 N3 N4

tR (min)a

24.96 25.824 26.281 26.374 26.445 26.617 26.702 26.899 26.985 27.07 27.134 27.22 27.352 27.515 27.598 27.678 27.775 27.852 27.924 27.99 28.805 28.897 29.683 29.767

Peak area of lower massb

Peak area of higher massb

m/z 506 1,738,000 9,400 16,200 6,900 22,400 55,000 29,400 28,600 40,400 71,000 240,900 208,300 200,700 121,000 58,000

3,300,000 m/z 508 334,000 6,580 9,250 4,980 14,500 33,700 18,600 20,200 25,800 45,500 153,000 132,000 131,000 78,100 36,800

73,900 199,000 108,000 204,000 49,000 m/z 540 17,500 25,200 18,100 36,700

52,600 129,000 69,700 125,000 34,300 m/z 542 12,800 19,600 13,500 26,600

Ratio R (lowerto-higher mass)

5.21 1.43 1.75 1.38 1.54 1.63 1.58 1.42 1.56 1.56 1.58 1.58 1.54 1.55 1.57 1.40 1.54 1.55 1.63 1.43 1.37 1.29 1.34 1.38

Area contribution of PCT to higher massb 3,078,000 (93 %) m/z 508 17,400 6,780 8,750 5,190 14,600 32,900 18,500 20,900 25,700 45,400 151,000 131,000 131,000 77,900 36,500 54,500 129,000 69,600 123,000 35,400 m/z 542 10,100 23,500 12,700 19,800

a

Retention time of PCB 153 (20.53 min) and of PCB 209 (24.96 min)

b

Areas rounded to three significant numbers. Calculations were performed before rounding the values

Percent contribution of peak to higher mass peakc (%)

5 103c 95 104c 100 98 99 103c 99 100 99 99 100 100 99 104c 100 100 98 103c 79 120c 94 74

c

Values >100 % indicate the limit of the calculation mode which is depending on the exact determination of the ratio of the two isotopic peaks. Generally, the quality of the calculation was accepted when the deviation was less than 10 %

provide the proportions of PCTs and PCBs in a given peak. Using this method, any PCT peak interfered by PCBs with two orders of magnitude excess or more can be determined. Determination of PCT congeners in blubber of a harbour porpoise from the German North Sea By means of Eq. 5, we determined the tetra- to octaCTs (interfered by hexaCBs to decaCB) and nonaCTs (no interference by PCBs) in the blubber extract of a harbour porpoise from the North Sea. For this reason, all peaks giving response to the quantitation and verification ion were transferred into an Excel sheet, and the proportion of PCT and PCB was determined. Results for tetraCTs (m/z 368 and m/z 370) are listed in Table 2. Six peaks (T1–T6) were detected, and the contribution of the tetraCTs to the respective peaks ranged from 4 %

(i.e. heavily interfered) to 99 % (i.e. no interference). The three dominant tetraCTs in the sample (T1, T5 and T6) contributed 99, 99 and 15 % to the respective peaks. As can be seen from these measurements, only 28 % of the area of m/z 370 originated from tetraCTs, and the overdetermination of PCTs would be 3.5-fold the corrected value (Table 2). In the same way, we identified nine pentaCTs (P1–P9) by means of the quantification ion m/z 406 (Table 2). Interference of individual pentaCTs by heptaCBs was up to 87 %. Noteworthy, only 40 % of the peak area of m/z 406 originated from pentaCTs. Six peaks detected by means of the quantification ion m/z 438 (partly) originated from hexaCTs. The contribution of the hexaCTs to the peak areas ranged from 24 to 94 % (Table 2). The area determined by m/z 438 without correction would have erroneously suggested a threefold higher PCT amount.

A calculation mode for the GC/ECNI-MS determination of PCTs

We also detected 18 heptaCTs (Hp1–18) by means of m/z 472 from which only Hp1 and Hp2 were interfered by PCBs (Table 2). This is in agreement with the fact that maximally three nonaCBs (i.e. PCB 206, PCB 207 and PCB 208) can be present in samples. As a consequence, 93 % of the peak area detected by means of m/z 472 actually originated from heptaCTs. Noteworthy, Hp4–Hp18 eluted after PCB 209 from the GC column. We also observed some overlap of PCTs with different degree of chlorination (Table 2). For instance, 20 octaCTs were detected which widely overlapped with heptaCTs. The first eluted octaCT, i.e. O1, was interfered by PCB 209 and eluted prior to the fourth heptaCT from the column (Table 2). As can be seen from the data of O2 to O20 (which were not interfered by PCBs), the calculations were in good agreement with the theory: the ratio of the higher to lower mass should match the theoretical value of 1.554 (Table 2). The deviation from this value (see the last column in Table 2), differed only by ±7 % from the theoretical ratio. Finally, we detected four nonaCTs in this sample. In this case, the calculated contribution of nonaCTs in the harbour porpoise (which reflects the deviation from the correct ratio of the ions monitored) ranged from 74 to 120 %. Deviations of more than 10 % were considered high and inacceptable. Altogether, 63 PCT congeners (6 tetraCTs, 9 pentaCTs, 6 hexaCTs, 18 heptaCTs, 20 octaCTs and 4 nonaCTs) were identified in the harbour porpoise sample, and each PCT congener could be corrected for the interference by PCBs by means of the new calculation algorithm. The number of PCTs could even be higher if some PCTs were coeluting. As anticipated, most severe interferences were observed for tetra- to hexaCTs because the interferents—hexa- to octaCBs—usually represent the dominant PCB congeners in environmental samples. In contrast, the interference of heptaCTs (by only three nonaCBs) and octaCTs (by PCB 209) was minor. Based on a hypothetical equal response factor of individual PCTs, the main contribution to the PCT pollution of the harbour porpoise sample originated from heptaCTs. We also conducted a thorough analysis of a monk seal sample from Mauritania. In this sample, we detected 41 congeners, and tetraCTs contributed most to the PCT content. Compared to the harbour porpoise, this sample contained three additional tetraCTs and two additional hexaCTs. Seven of the nine pentaCTs and all further PCTs detected in the monk seal were also detected in the harbour porpoise sample. Although many different PCTs were detected in the marine mammals, the congener number was small compared to the theoretical congener number of >8,000 and the peaks present in technical PCT mixtures.

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Conclusions We conclude that our calculation method suited to determine PCTs is more reliable than without correction for interferences by PCBs. Moreover, the calculation mode can also be adopted to GC/EI-MS-SIM measurements based on the analysis of the molecular ions of PCTs. In combination with the use of suitable internal and external standards, the present method can be used in the future to analyse PCTs in more samples from different locations and different trophic levels. Still some drawbacks in the PCT analysis have to be overcome in the future. Above all, there is a need for individual PCT standards [11]. Due to the different pattern observed in the samples, the selection of indicator congeners similarly to PCB analysis would be an important step forward. The method and data provided in the present study may serve as suitable starting points for studies on the environmental fate and food chain enrichment of PCTs.

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