Tandem Mass Spectrometric Determination of Eluting

Một phần của tài liệu current practice of gas chromatography mass spectrometry (Trang 112 - 118)

3. TANDEM MASS SPECTROMETRIC DETERMINATION OF

3.2. Tandem Mass Spectrometric Determination of Eluting

A total ion chromatogram obtained by MS–MS of 1àl of a solution (Table 1) containing 35 PCDDs and PCDFs is shown in Figure 2. This chromatogram is a merged total ion chromatogram as it is a display of the total ion count of each merged mass spectrum obtained as described above. Merging of the files has led to a contraction in the number of scans and a smoothed GC peak. The beginning and end of each of the five retention-time windows are represented by diamond- shaped indicators on the abscissa; the appropriate ion-preparation file was acti-

DioxinsandPCBsbyQuadrupoleIon-Trap101

Table 1 PCDD and PCDF Components of the 18 Peaks in the Total Ion Chromatogram Shown in Figure 2, and Their Concentrations

Peak Conc. Conc. Conc.

numbera Compound (pg/àl) Compound (pg/àl) Compound (pg/àl)

1 2,3,7,8-T4CDF 200 13C12-2,3,7,8-T4CDF 100

2 13C12-1,2,3,4-T4CDD 100

3 2,3,7,8-T4CDD 200 37Cl4-2,3,7,8-T4CDD 200 13C12-2,3,7,8-T4CDD 100

4 1,2,3,7,8-P5CDF 1000 13C12-1,2,3,7,8-P5CDF 100

5 2,3,4,7,8-P5CDF 1000 13C12-2,3,4,7,8-P5CDF 100

6 1,2,3,7,8-P5CDD 1000 13C12-1,2,3,7,8-P5CDD 100

7 1,2,3,4,7,8-H6CDF 1000 13C12-1,2,3,4,7,8-H6CDF 100 8 1,2,3,6,7,8-H6CDF 1000 13C12-1,2,3,6,7,8-H6CDF 100 9 2,3,4,6,7,8-H6CDF 1000 13C12-2,3,4,6,7,8-H6CDF 100 10 1,2,3,4,7,8-H6CDD 1000 13C12-1,2,3,4,7,8-H6CDD 100 11 1,2,3,6,7,8-H6CDD 1000 13C12-1,2,3,6,7,8-H6CDD 100 12 1,2,3,7,8,9-H6CDD 1000 13C12-1,2,3,7,8,9-H6CDD 100 13 1,2,3,7,8,9-H6CDF 1000 13C12-1,2,3,7,8,9-H6CDF 100 14 1,2,3,4,6,7,8-H7CDF 1000 13C12-1,2,3,4,6,7,8-H7CDF 100 15 1,2,3,4,6,7,8-H7CDD 1000 13C12-1,2,3,4,6,7,8-H7CDD 100 16 1,2,3,4,7,8,9-H7CDF 1000 13C12-1,2,3,4,7,8,9-H7CDF 100

17 O8CDD 2000 13C12-O8CDD 200

18 O8CDF 2000

aPeak number corresponds to the elution order as shown in Figure 2.

102 Plomley et al.

Figure 2 Total ion chromatogram obtained after the mass spectrum merging procedure for dioxins and furans contained in 1àl of solution described in Table 1. The diamond shapes reported on the abscissa indicate the retention-time windows during which succes- sive degrees of chlorinated compounds were eluted.

vated during each window. The peaks indicated with asterisks and identified as A through F in Figure 2 are due to impurities present in the sample.

Consider peaks 1, 2, and 3 observed during the first retention-time window of Figure 2. Peak 1 is due to 2,3,7,8-T4CDF and its coeluting13C-labeled isoto- pomer, peak 2 is due solely to13C12-1,2,3,4-T4CDD, while peak 3 is a composite not only of the native and13C-labeled compounds but of 37Cl4-2,3,7,8-T4CDD also, as reported in Table 1. For 2,3,7,8-T4CDF, the molecular ions M⫹•of m/z 304 and [M ⫹ 2]⫹• of m/z 306 were isolated simultaneously then dissociated using MFI with the first scan function of the first ion-preparation file. Fragment ion signal intensities arising from the loss of COCl•to yield [M⫺ COCl•]⫹of m/z 241 and m/z 243 were recorded. The fragment ions of m/z 241 and 243 arise as follows:

[C12H4Cl4O]⫹•

(m/z 304)

→[C11H4Cl3]⫹

(m/z 241)

⫹COCl• (4)

[C12H4Cl337ClO]⫹•

(m/z 306) →[C11H4Cl3]⫹

(m/z 241) ⫹CO37Cl• (5)

→[C11H4Cl237Cl]⫹

(m/z 243)

⫹ COCl• (6)

Dioxins and PCBs by Quadrupole Ion-Trap 103

Now as 2,3,7,8-T4CDF coelutes with the labeled isotopomer13C12-2,3,7,8- T4CDF, the second scan function of the first ion-preparation file is used for the ionization of 13C12-2,3,7,8-T4CDF and simultaneous isolation of the molecular ions M⫹•of m/z 316 and [M⫹ 2]⫹•of m/z 318; these isolated ion species are then dissociated using MFI. The fragment ion signal intensities arising from the loss of13COCl•to yield [M⫺13COCl•]⫹of m/z 252 and m/z 254 were recorded;

these fragment ions arise as follows:

[13C12H4Cl4O]⫹•

(m/z 316) →[13C11H4Cl3]⫹

(m/z 252) ⫹13COCl• (7)

[13C12H4Cl337ClO]⫹•

(m/z 318)

→[13C11H4Cl3]⫹

(m/z 252)

⫹13CO37Cl• (8)

→[13C11H4Cl237Cl]⫹

(m/z 254) ⫹13COCl• (9)

Peak 2 of Figure 2 corresponds to MS–MS of labeled13C12-1,2,3,4-T4CDD for which a third scan function is required in order to control ionization and isolation of the molecular ions M⫹•of m/z 332 and [M⫹2]⫹•of m/z 334. These isolated ion species were then dissociated and the fragment ion signal intensities arising from the loss of13COCl•to yield [M-13COCl•]⫹of m/z 268 and m/z 270 were recorded. The fragment ions of m/z 268 and 270 arise as follows:

[13C12H4Cl4O2]⫹•

(m/z 332)

→[13C11H4Cl3O]⫹

(m/z 268)

⫹13COCl• (10) [13C12H4Cl337ClO2]⫹•

(m/z 334)

→[13C11H4Cl3O]⫹

(m/z 268)

⫹ 13CO37Cl• (11)

→[13C11H4Cl237ClO]⫹

(m/z 270)

⫹13COCl• (12) Peak 3 of Figure 2 is composed of the fragment ion counts from three compounds, thus two additional scan functions were used in sequence for the MS–MS determination of the coeluting 2,3,7,8-T4CDD, 13C12-2,3,7,8-T4CDD, and37Cl4-2,3,7,8-T4CDD. The scan function for13C12-2,3,7,8-T4CDD is the same as that used for13C12-1,2,3,4-T4CDD of peak 2. For 2,3,7,8-T4CDD, the molecular ions M⫹•of m/z 320 and [M⫹2]⫹•of m/z 322 were isolated then dissociated and the fragment ion signal intensities arising from the loss of COCl•to yield [M ⫺ COCl•]⫹ of m/z 257 and m/z 259 were recorded. The fragment ions of m/z 257 and 259 arise as follows:

[C12H4Cl4O2]⫹•

(m/z 320)

→[C11H4Cl3O]⫹

(m/z 257)

⫹COCl• (13) [C12H4Cl337ClO2]⫹•

(m/z 322) →[C11H4Cl3O]⫹

(m/z 257) ⫹CO37Cl• (14)

→[C11H4Cl237ClO]⫹

(m/z 259)

⫹COCl• (15)

104 Plomley et al.

For37Cl4-2,3,7,8-T4CDD, the molecular ion M⫹•of m/z 328 was isolated then dissociated and the fragment ion signal intensity arising from the loss of CO37Cl•to yield [M-CO37Cl•]⫹of m/z 263 was recorded. The fragment ions of m/z 263 arise as follows:

[C12H437Cl4O2]⫹•

(m/z 328)

→[C11H437Cl3O]⫹

(m/z 263)

⫹CO37Cl• (16)

In the above discussion of peaks 1, 2, and 3 of Figure 2 where five different scan functions were required for MS–MS of six different tetrachlorinated dioxins and furans, only a single fragmentation channel was considered, that of the loss of COCl•. While COCl•is the major fragmentation channel, it is not the sole loss channel. For the dioxins [C12H8⫺xClxO2]⫹•, where x⫽4 to 8, the other fragmenta- tion channels involve the loss of Cl•, 2COCl•, and (CO)2Cl•, while for the furans, where again x⫽4 to 8, the other fragmentation channels involve the loss of Cl•, COCl2, and COCl3•.

Let us examine the three selected ion chromatograms shown in the un- merged file format in Figure 3(a). The top trace shows the product-ion chromato- gram for O8CDF, the middle trace for O8CDD, and the bottom for13C12⫺O8CDD.

O8CDD (middle) and13C12⫺O8CDD (bottom) coelute, while elution of the furan, O8CDF (top), overlaps the other two compounds. Yet the mass selectivity is not lost as is seen from the accompanying product-ion mass spectrum for each com- pound. Each product-ion mass spectrum shown in Figure 3(a) can be compared with the corresponding mass spectrum taken from the three selected ion chro- matograms (Fig. 3(b)) obtained after ion isolation and prior to CID. The mass spectra of Figure 3(b) show the isolated molecular [M ⫹ 2]⫹•and [M ⫹ 4]⫹• peaks. It should be noted that the abscissa of each trace in Figure 3(a) (CID) and Figure 3(b) (isolation) indicates both number of mass spectra and retention time.

The retention-time windows illustrated in parts (a) and (b) are very similar, how- ever the number of mass spectra/unit time in the former is twice that in the latter.

The acquisition rate during CID (Fig. 3(a)) is twice that during isolation (Fig.

3(b)) due to the smaller mass range scanned in the former.

Figure 3 Unmerged ion chromatograms for three coeluting octachlorinated compounds.

Each chromatogram is composed from the signal intensities of the ion species identified on the ordinate. (a) The fragment ion signal intensities were obtained from CID mass spectra and one such mass spectrum is displayed for each compound. (b) The isolated molecular ion signal intensities were obtained prior to CID and a mass spectrum showing the isolated molecular ions is displayed for each compound.

Dioxins and PCBs by Quadrupole Ion-Trap 105

(a)

(b)

106 Plomley et al.

Một phần của tài liệu current practice of gas chromatography mass spectrometry (Trang 112 - 118)

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