1429
Anal. Chem. 1900, 60,7429-7433
High-Resolution Mass Spectrometric Determination of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans Using an Alternative Lockmass System Vince Y. Taguchi,* Eric J. Reiner, David T. Wang, Otto Meresz, and Bernard Hallas]
Ontario Ministry of the Environment, P.O.Box 213,Rexdale, Ontario, Canada M 9 W 5Ll
A hlgh-resolution mass spectrometric selected Ion monltorlng (HRMSSIM) methodology for the analysls of all tetra- through octachlorlnated dlbenzo-p-dloxlns (CI,DD, x = 4-8) and dC benzofurans (Cl,DF, x = 4-8) utlllzlng an alternative lockmass system has been developed at the Mlnlstry of the Envlronment (MOE). Incorporatlon of the MOE lockmass system gives an enhancement In senslttvlty, easler control of the lockmass concentratlon In the Ion source, and greater resolutlon between the lockmass and the sample Ion signals than does the conventlonal perfluorokerosene (PFK) lockmass system. I n addition, the method can be adapted to any sector lnstrmenl having a slmllar lockmass faclllty and to any HRMS-SIM analysls wlth the approprlate choke of reference standards. A comparlson of the MOE and PFK systems Is reported.
T h e study of polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF), in particular the C14-DD congeners, by gas chromatography/ high-resolution mass spectrometry (GC/HRMS) has attracted great interest in the past decade (1-11). Recent studies (12-14)have shown that the bioaccumulation of PCDD and PCDF increases with increasing chlorine substitution. Also, the acute lethality of some C1,- and C16-DD and -DF congeners, which have LDm values only 1 order of magnitude greater than t h a t of 2,3,7,8-C14DD (the most toxic congener), is of great concern. These two observations indicate t h a t the monitoring of all PCDD and PCDF congeners is important. The difficulties encountered with environmental samples also make selectivity (mass resolution) and sensitivity important considerations for the method of choice. The presence of considerably higher concentrations of biological, environmental, and industrial interferences predicates that efficient sample preparation be carried out prior to the instrumental analysis. Some interferences, such as polychlorinated biphenyls (PCBs), are difficult t o remove completely and oftentimes are present in the prepared sample extract in concentrations considerably higher than that of the analyte. Capillary column high-resolution gas chromatography/highresolution mass spectrometry (HRGC/HRMS) is one method that can give high degrees of both gas chromatographic and mass spectrometric selectivity in PCDD and PCDF analysis. A high-resolution mass spectrometric selected ion monitoring (HRMS-SIM) methodology has been developed a t the Ministry of the Environment, utilizing a VG-ZAB mass spectrometer a t 12 000 resolution (RP) and employing nonP F K lockmasses. To determine exact masses in the HRMS-SIM mode, simultaneous introduction of a reference compound, typically PFK, is required. The lockmass level must be set, as low as practically possible because of supPresent address: HALTECH, 26 Morrow Ave., Toronto, ON, Canada M6R 252. 0003-2700/88/0360-1429$0 1.50/0
pression of the sample ion current. The majority of the ion current in the P F K mass spectrum (97% (15))is carried by ions outside the range of interest for PCDD and PCDF analysis ( m / z 300-500), and this causes suppression of the sample ion current. Green et al. (16) have shown that the suppression or space charge effects due to He in the ion source in GC/MS analysis are essentially eliminated because of its high ionization energy and because reasonable ionization efficiency of the analyte can be maintained if lower electron energies are used. Because the ionization energy of P F K is lower than that of He, the space charge effects observed from P F K may be significant, even though its concentration in the ion source is considerably lower than that of He. T o decrease this suppression, other reference compounds such as diiodoethylene (17) and diiodobenzene (18)have been used as lockmasses in 2,3,7,8-C14DDanalyses. When these two compounds were used as lockmasses, a resulting enhancement in sensitivity was observed. However, for these two liquids as well as PFK or perfluorotributylamine, control of the lockmass level in the ion source is quite difficult with a conventional heated liquid inlet system. For these reasons, in our laboratory we utilize a mixture of the solid aromatic hydrocarbons coronene, tetraphenylcyclopentadiene, and decacyclene as lockmasses.
EXPERIMENTAL SECTION The MOE method may be used to analyze a wide variety of sample matrices such as sediments, soils, fish, clams and other biological samples, waters, effluents, air, fly ash, etc. The sample extract preparation and cleanup procedures have been reported in detail elsewhere (19, 20) and will not be discussed here. HRGC/HRMS analysis was accomplished by using a Varian Vista 6000 gas chromatograph coupled to a VG-ZAB-2F double-focusing reversed-geometry mass spectrometer. The data was acquired with a VG 11/250 data system that uses a DEC PDP 11/24 minicomputer. Two types of analyses can be carried out. The "all-congener" analysis involves the monitoring of all C14DDand C14DFto C1,DD and C1,DF congeners by using a 30-m SPB-5 or DB-5 0.25-mm i.d. 0.25-pm bonded-phase capillary column. For this analysis, the column is initially set at 120 "C and the temperature subsequently held for 1 min. The temperature is then programmed at 30 "C/min to 220 "C, then at 5 "C/min to 320 "C, and finally at 30 "C/min to 350 "C where it is held for 5 min. The "isomer-specific" analysis for 2,3,7,8-C14DDinvplves the monitoring of C4DD isomers by using a 60-m SP-2331,0.25-mm id., 0.2-pm-filmcapillary column. The temperature is set initially at 120 "C, held for 1 min, and then programmed at 15 "C/min to 220 "C, followed by 3 "C/min to 250 "C where it is held for 30 min. Samples were injected through an on-column injector that was held initially at 80 "C, then programmed at 180 "C/min to 350 "C for the all-congener analysis (250 "C for the isomerLspecific analysis), and held for 19 min (46 min for the isomer-specific analysis). The head pressure on the capillary column (30-m DB-5) was approximately 10 psi. The transfer line from the gas chromatograph to the mass spectrometer was held at 280 "C, and the mass spectrometer ion source was held at 270 "C. 0 1988 American Chemical Society
1430
ANALYTICAL CHEMISTRY, VOL. 60, NO. 14, JULY 15, 1988
Table I. Parameters for Selected Ion Monitoring Approximate Group Times for "All-Congener" Method Using a 30-mCapillary Column and IO-psi He Head Pressure: 03:OO to 11:35 rnin group 1 11:40 to 14:lO rnin group 2 14:15 to 16:55 rnin group 3 17:OO to 19:lO rnin group 4 19:15 to 23:OO rnin group 5
Table 11. Parameters for Selected Ion Monitoring of TCDDb group 1:
'3C,zC14DD C14DD-COCl
Ions Scanned with: PFK lockmasses group 1: PFK" Cl4DD
group 2:
group 3:
group 4:
group 5:
318.9792 319.8965 321.8936 323.8906 13Cl&1,DD 331.9368 333.9338 303.9016 Cl4DF 305.8987 307.8957 330.9792 PFK" 337.8627 ClSDF 339.8597 341.8568 353.8576 C1,DD 355.8546 357.8517 380.9760 PFK" 387.8186 C&DD 389.8156 391.8127 371.8237 C&DF 373.8207 375.8178 404.9760 PFK" 405.7847 C17DF 407.7818 409.7788 421.7796 C1,DD 423.7 767 425.7737 454.9728 PFK" 457.7377 CleDD 459.7348 461.7318 13C12ClsDD 469.7780 471.7750 441.7428 ClsDF 443.7398 445.7369
coronene" CldDD
MOE lockmasses coronene" C1,DF
300.0939 303.9016 305.8987 307.8957 319.8965 C14DD 321.8936 323.8906 331.9368 '3C1zC14DD 333.9338 370.1722 TPCPDb 337.8627 C1,DF 339.8597 341.8568 353.8576 ClSDD 355.8546 357.8517 370.1722 TPCPDb 371.8237 ClGDF 373.8207 375.8178 387.8186 CI6DD 389.8156 391.8127 decacyclene" 450.1409 405.7847 CLDF 407.7818 409.7788 421.7796 423.7767 425.7737 decacyclene" 450.1409 457.7377 ClsDD 459.7348 461.7318 13ClzClsDD 469.7780 471.7750 441.7428 ClsDF 443.7398 445.7369
" Lockmass. Tetraphenylcyclopentadiene (lockmass). The dwell time for each ion in a group (cycle) was 50 ms, and the delay time between ions was 15 ms. The masses of the three most abundant isotopic molecular ions of the C1,DD and C14DF to C1,DD and C&DF series, as well as the two most abundant molecular ions for each of the internal standards ('%,,Cl,DD and 13C12Cl&D),and the lockmasses were monitored at 12 000 RP in five groups. The magnet was set for each group, and the accelerating voltage was then switched for both the PFK and MOE lockmass systems as shown in Table I. The windows for masses monitored at 12 OOO R P are 25 millimass units (mmu) a t m / z 300 and 42 mmu a t mlz 500. Because the accelerating voltage is scanned over a small range, the differences in sensitivities among the ions in each group are negligible. In order to obtain the correct isotopic ratios in the magnet current control mode (as opposed to the magnet field control mode), it was observed that the lockmass had to be monitored first for each group and the remaining masses in that group had to be monitored in a cyclic manner in order of increasing mass. To achieve better quantitation of some real (dirty) samples, the corresponding 13C-labeleddioxin was used to spike each congener group. Also, for dirty samples, the ions resulting from the loss of COCl as well as the molecular ion can be monitored. The exact masses for the C1,DD isomers are shown in Table 11.
300.0939 319.8965 321.8936 323.8906 331.9368 333.9338 256.9328 258.9298 260.9269
a Lockmass. bFor the "isomer-specific" method, the ions listed above are monitored from 6:OO-33:OO rnin by using a 60-m SP-2331 capillary column with IO-psi He head pressure. Monitoring of the COCl loss is optional, depending on the interferences present in the sample. The dwell time and delay time were 50 and 15 ms, respectively.
Table 111. Isotopic Ratios for Molecular Ions congener
M+/(M + 2)'
(M + 4)+/(M + 2)'
c14
c17
0.77 0.62 0.51 0.44
0.49 0.65 0.81 0.98
congener
(M + 2)+/(M + 4)'
(M + 6)+/(M + 4)'
c1,
0.88
0.65
c15
c1,
The lockmass calibration was accomplished by using either PFK or a 1:1:3 mixture of coronene, tetraphenylcyclopentadiene,and decacyclene (MOE mixture). For the PFK method, PFK was introduced into the ion source through the heated liquid inlet system. For the MOE method, the MOE mixture was introduced into the ion source via the solid insertion probe. The mixture was prepared by making a 1:1:3 slurry of the above solid hydrocarbons in dichloromethane. The slurry was allowed to dry a t room temperature. This mixture was then placed into a melting point capillary to a height of about 3-4 mm. A 10-pL aliquot of dichloromethane was added, and the mixture was stirred and allowed to dry overnight. The instrument was tuned to 12000 R P by using m/z 380.9760 for the PFK method and m/z 370.1722 for the MOE method. For both methods, the digital scanner (accelerating voltage) was calibrated by using PFK. For the MOE method, the PFK was introduced after the tuning and was pumped out after the calibration procedure was completed. The criteria for positive PCDD and PCDF identification are correct retention time windows for each group of congeners as determined by a standard flyash sample, correct accurate mass, correct isotopic ratios (within =!=0.15%see Table 111), and a signal-to-noise (S/N) response greater than 5:1 for all three ions monitored. An additional requirement for the isomer-specific analysis is that the retention time of the particular congener is within A2 s of the isotopically labeled standard. The concentrations of the various congeners were calculated by using external standards to determine the response factors for the particular congeners. Internal standards labeled with 13C and/or 37Cl were used to determine recoveries, and injection standards such as 13C12C16DFwere used to monitor instrument performance. An external standard was injected initially and in every third injection thereafter. The concentration for each congener was then determined by using the external standard run immediately before or after the sample. This type of normalization corrects for possible drift in instrument sensitivity.
RESULTS AND DISCUSSION Mass spectrometrists have long been aware of the fact that calibration standards like PFK can cause suppression of the sample ion current in trace analysis. This effect is particularly noticeable when the majority of the ion current from the calibration standard is outside the range of interest. For this
ANALYTICAL CHEMISTRY, VOL. 60, NO. 14, JULY 15, 1988
100'
Table IV. Statistical Data for MOE System"
901
401I
2Bl 1 flhl!].
430
10
,
386
, , ,,,,,, ,11 , ,,,, , ,
320
340
11,
,
,
,
,
,
360 388 400
428
,
,
,d/
440
,
460
,
,
,
480
,
,
RRSS
560
i
40
X , area counts
S
S, '70 of X
area/pg
390.33 292.34 201.53 582.88 342.00 653.14 384.52 705.13 621.91 690.23 459.88 404.35
73.74 61.50 40.87 105.30 62.01 106.88 66.23 94.01 119.73 101.18 66.59 54.17
18.89 21.03 20.28 18.06 18.13 16.36 17.22 13.33 19.25 14.66 14.48 13.40
3.90 2.92
Cl4DF Cl4DD '3C12C14DD C1,DF C1,DD C&DF C&DD ClTDF C1,DD ClsDF ClsDD 13C12C1sDD
38
50
1431
2.83
3.89 2.28
4.35 2.56 3.52 3.11 3.45 2.30 2.38
"Injections: 100 pg of C14DF and 2,3,7,8-C14DD;150 pg of Cl,DF, Cl,DD, C16DF, and C1,DD; 200 pg of Cl,DF, Cl,DD, C18DF, C1,DD; 75 pg of 1,2,3,4-13C12-C14DD(corrected for 95% isotopic purity); 170 pg of '3C12-C18DD(corrected for 85% isotopic purity). S = standard deviation: X = mean of five runs. Table V. Statistical Data for PFK System"
1
I 150
I
/ /
20
Figure 1. Mass spectrum of the MOE lockmass mixture in the range of interest (top) and over the range m l z 50-500 (bottom).
reason, highly aromatic hydrocarbons that usually give the molecular ion as the base peak and show little or no fragmentation were chosen as the mass calibration standards. They are also mass-sufficient, as opposed to PFK, PCDD, and PCDF, which are mass-deficient. Figure 1 (top) shows the mass spectrum of coronene, tetraphenylcyclopentadiene, and decacyclene (MOE mixture) in the range of interest (mlz 300-500) for PCDD and PCDF analysis. As expected, only the molecular ions for each of these compounds are observed. Figure 1 (bottom) contains the complete mass spectrum ( m / z 50-500) of the MOE mixture and shows that a considerably smaller portion of the m / z 300-500 region is taken up with mass calibrant ion signals compared to that observed for the P F K system ( 1 5 ) . Also, over the range m / z 50-500, the majority of the ion current for the MOE system is inside the range of interest. There are far fewer fragments a t masses less than m / z 300 as compared t o the P F K spectrum. The HRMS-SIM data for a series of five replicate injections of C1,DD and C1,DF to C l a D and ClsDF standards have been determined for both the MOE and PFK lockmass systems and are shown in Tables IV and V, respectively. Both systems exhibit similar trends: an increase in chlorine substitution gives rise to a decrease in signal strength, and signals obtained from the furans are stronger than those obtained from dioxins. The differences in response factors may, in part, be due to the differences in ionization efficiencies of the various analytes (21). Waddell et al. (22) conclude that, for the C14 congeners, the differences in the response factors of the furans versus those of the dioxins can be explained by the stabilizing effect of the biphenyl linkage present in the furans as opposed to the ether linkage present in the dioxins. In the same study, Waddell e t al. (22) report that with an approach previously
X , area counts
S
S , 70of X
area/pg
155.31 99.01 59.33 223.55 112.26 212.96 115.06 213.16 159.77 211.60 131.87 117.71
8.54 6.66 4.79 15.64 9.84 18.24 8.24 18.01 11.95 16.99 11.83 6.83
5.50 6.72 8.08 7.00 8.87 8.57 7.16 8.45 7.48 8.03 8.97 5.80
1.55 0.99 0.83 1.49 0.75 1.42 0.77 1.07 0.80 1.06 0.66 0.69
Cl4DF C1,DD 13C12C1,DD ClSDF ClSDD C&DF C&DD C17DF C17DD ClsDF ClsDD 13C12C18DD
"Injections: 100 pg of C1,DF and 2,3,7,8-C14DD;150 pg of C15DF, Cl,DD, Cl,DF, and C1,DD; 200 pg of Cl,DF, Cl,DD, C18DF, and ClsDD; 75 pg of 1,2,3,4-13C12-C14DD (corrected for 95% isotopic purity); 170 pg of 13C12-C18DD (corrected for 85% isotopic Duritv). S = standard deviation: X = mean of five runs. Table VI. Comparison of PFK and MOE Lockmass Systems
MOE lockmass (area/pg)
PFK lockmass (area/pg)
enhancementa (MOE/PFK)
3.90 2.92 2.83 3.89 2.28 4.35 2.56 3.52 3.11 3.45 2.30 2.38
1.55 0.99 0.83 1.49 0.75 1.42 0.77 1.07 0.80 1.06 0.66 0.69
2.6 3.0 3.4 2.6 2.9 3.1 3.3 3.3 3.9 3.3 3.5 3.4
Cl4DF C1,DD l3ClzCl4DD C15DF C1,DD '&DF C&DD C17DF C1,DD ClsDF ClsDD l3ClzC1,DD
" X = 3.19: S = 0.38. described by Sauter e t al. (23),the relative response factors for various analytes in the ion source are dependent on a number of variables. These variables include the rate of total ion production (I),the fraction of total ionization in the ion source (F),the efficiency to which an ion is extracted from the source and focused onto the analyzer ( E ) ,the analyzer transmission efficiency ( T ) , and the current gain a t the electron multiplier (G). For isomeric ions, the relative response factors are dependent on only the first two parameters ( I and
0.
1432
ANALYTICAL CHEMISTRY, VOL. 60, NO. 14, JULY 15, 1988
The major difference between the PFK and MOE lockmass systems is an increase in sensitivity observed when the MOE lockmass system is used. The area counts obtained per picogram injected are summarized and compared in Table VI for both methods. When MOE lockmasses are used as mass calibration standards, an enhancement in sensitivity by a factor of approximately 3 is observed for all congeners of dioxins and furans monitored. If the MOE lockmass level decreases, no enhancement in sensitivity is observed. On the other hand, for the PFK system, a reduction of the PFK concentration in the ion source increases sensitivity. With the lowest level of P F K concentration in the ion source that enables the lockmass to be observed a t 0.01 mV (minimum value to enable calibration), the MOE system is still a factor of 3 times more sensitive than the PFK system. The lack of fluctuation in sensitivity observed with changing lockmass concentrations under the MOE method indicates that the MOE lockmass mixture does not cause suppression of the sample ion current in the ion source as PFK does. Also, the P F K method necessitates a constant suppression of ionization in order to maintain a constant sensitivity. Kuehl et al. (18) have also reported an overall reduction in base-line noise and an increase of signal to noise of a factor of 2-3 times when the molecular ion of diiodobenzene was used as a lockmass in 2,3,7,8-C14DDanalysis. The major advantage of a solid lockmass system over a liquid one such as diiodobenzene is that control of the lockmass level in the ion source is much easier. The solid insertion probe heater may be used to keep the concentration of the MOE mixture in the ion source relatively constant. Typical probe temperatures range between 80 and 120 "C. Table VI1 shows the data for a series of five replicate injections of low concentrations of C14DD and C1,DF t o ClsDD and C1,DF standards. The signal-to-noise ratio for all congeners of dioxins and furans was about 30:l. Also, a t these concentrations, the average relative standard deviation in the signal strengths, without correction for drift, is about 6%. This is considerably lower than the 13-20% values quoted for the identical standards injected a t 100-200 pg levels (see Table IV). The large relative standard deviations are due to drifts in signal strengths that are probably caused by drifts in the magnetic field and/or Y-focus supplies. At the end of a run. we have found that any drift in signal strength can be corrected by adjusting the Y focus. As a result, our present protocol calls for a standard to be run every third injection in order to compensate for this drift. Reproducibility and sensitivity are also good over a wide range of concentrations. The dynamic range over 4 orders of magnitude (1-10000 pg of 1,2,7,8-C14DDinjected) is linear (correlation coefficient = 1.0000). There are two disadvantages associated with this method. The first is that the retention time of earliest eluting C&DF congener is very close to that of the last eluting C14DD. The group times are set so that the C1,DF congener may not be detected, and therefore a low value for the total concentration of C1,DF may be reported. For essentially all types of samples, this error is considerably less than the average relative standard deviation of the method (15%). The second disadvantage involves the lack of correction for relative response factors for each congener in each group. If 2,3,7,8-C14DDand 2,3,7,8-C14DF are used as standards, the relative response factors range from 0.60 to 1.65 for the C1,DD (10) and from 0.40 to 2.10 for the C1,DF (22). By use of the above two congeners. which have response factors in the middle of the ranges for each group, the errors involved by neglecting the use of relative response factors can be minimized.
0
'
',
,
:,-;,: 23:30
,-,'
,",
24:OB
24:38
25:BB
25:38
26:OO
Figure 3. Low-resolution mass chromatograms monitoring the three most abundant molecular ions of CI,DD for a sludge sample (top) and high-resolution mass chromatograms of the same sample (bottom).
A number of examples using the MOE method are shown below for a variety of samples. Example 1 (Figure 2) shows that the HRMS-SIM mass chromatogram a t 12 000 R P for the m / z 322 molecular ion of 2,3,7,8-C14DDinjected a t a concentration of 5 pg gives a signal-to-noise ratio of 30:l. Example 2a (Figure 3, top) shows the low-resolution mass spectrometry/selected ion monitoring mass chromatograms (determined on a Finnigan 4500 mass spectrometer) for the m / z 320, 322, and 324 molecular ions of CllDD for a sludge sample. The m / z 322 mass chromatogram is partially masked, while the m / z 324 ion is almost totally lost among the interferences. By use of the MOE HRMS/SIM "isomer-specific"
ANALYTICAL CHEMISTRY, VOL. 60, NO. 14, JULY 15, 1988
M0:B
Table VII. Statistical Data for MOE System at Low Concentrationsn
X,area counts
S
17.01 14.71 11.04 23.65 15.28 24.65 15.28 22.87 22.15 21.82 15.31 14.56
1.74 1.06 0.97 1.03 0.49 0.93 0.85 1.50
CllDF Cl4DD '3C,,C14DD C15DF C15DD C1,DF C&DD C1,DF CI,DD ClsDF ClsDD 13C12C1sDD
X
S, 90 o f
area f p g
10.19 7.22 8.80 4.36 3.25 3.76 5.52 6.56 2.61 5.63 6.99 6.33
0.58
1.23 1.07 0.92
3.42 2.94 3.10 3.15 2.02 3.29 2.04 2.29
04 108,C
,
a
,
,
,
,
,
,
,
,
,
,
,
,
,
1
\
!BB 0 1, 1, 8
1
2.22
2.18
n
1.53
1.70
11.
4, 1; 3
m n
1, 1, 8
50 :
50
"Injections: 5 p g o f C1,DF a n d 2,3,7,8-C14DD;7.5 p g o f C15DF, Cl,DD, C16DF, a n d C1,DD; 10 p g o f Cl,DF, Cl,DD, ClsDF, a n d ClsDD; 3.75 p g o f 1,2,3,4-13C,2-C14DD(corrected f o r 95% isotopic purity); 8.5 p g o f l3Cl2-Cl8DD(corrected f o r 85% isotopic p u r i t y ) . S = standard deviation; X = mean o f five runs.
lBBI,B 4 , 1, 3
1433
,
,
,
,
,
,
,
'
8:00
8.30
9:BB
9,30
10:00
18.30
11 BO
Figure 5. High-resolution mass chromatograms of the three most abundant molecular ions of the CI,DF congeners present in a fish sample. Each trace has been magnified 50 times and shifted upward for clarity.
sample when the "all-congener'' method is used. Note that the concentration of isomers present a t both low and high concentrations can be determined in the same run. Registry No. Cl,DD, 41903-57-5; Cl,DD, 36088-22-9; Cl,DD, 34465-46-8; Cl,DD, 37871-00-4; ClgDD, 3268-87-9; CldDF, 55722-27-5; ClSDF, 30402-15-4; Cl,DF, 55684-94-1; Cl,DF, 38998-75-3; Cl,DF, 39001-02-0; TPCPD, 15570-45-3; coronene, 191-07-1; decacyclene, 191-48-0.
LITERATURE CITED
84
,
,
1 B y 4, 1; 3
'
A
l00lG 5, 1, 3
3
0
I!
1\ , ,
, ,
,
100 H 5,'1, 3
1
50
,
,
,
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,
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,
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,
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h
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0
'
21 00 21 30 22 08 22 30 23 00 Figure 4. High-resolution mass chromatograms of the three most abundant molecular ions of the CI,DF (top) and CI,DF (bottom) congeners present in a sediment sample. 19 30
20 BO
20 30
method as shown in Example 2b (Figure 3, bottom), a value of 200 ppt was determined for 2,3,7,8-C14DD. Example 3 (Figure 4) shows typical HRMS-SIM mass chromatograms obtained by the "all-congener" method for the C&DF and C1,DF congeners present in a sediment sample. Note the signal t o noise and signal strength. Each C1,DF congener was determined to be present at 6 ppt, and the C1,DF congener was determined to be present a t 13 ppt. Example 4 (Figure 5) shows the the HRMS-SIM mass chromatograms for the C14DFisomers present in a typical fish
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RECEIVED for review September 16, 1987. Accepted March 7 , 1988.
