Anal. Chem. 1982, 5 4 , 135-136
135
Ion Current !Surfaces and the Determination of Tetrachlorodiibenzo-p-dioxin by Tandem Mass Spectrometry Sir: If reactions downstream of the ion source of a mass spectrometer containing tandem analyzers are monitored selectively, the ion cuirrents transmitted are much less degraded by “chemical noise” than those for fragmentations within the source (I),and the mass spectxometer can be used in this manner for rapid and sensitive determination of components of complex mixtures (tandem mass spectrometry or MS/MS)(I-3). Chess and Gross have rlecently reported the direct determination lby MS/MS of tetrachlorodibenzo-pdioxin (TCDD) with picogram detectioln limits in environmental samples, using a double focusing mass spectrometer of conventional geometry (source/elect,ric sector/magnetic sector/collector), but they did not interpret the shapes of the mass spectral peaks that they obtained (4). The ion current transmitted by the collector slit of a double focusing mass spectrometer may be recorded as a function of the independent variables accelerating voltage (V),electric sector voltage ( E ) ,and magnetic induction (B). Interpretation of the four-dimensional information is facilitated by constructing from it a three-dimennronal ion current surface (5, 6). For instance, the ion current may be displayed as a function of the parameters In (IpI) and In (Ipl), each of which is derived from V, E , and B , and the resultant ion current surface can accommodate all the changes in V, E and B required to display the peaks of interest. The symbols p and p are defied to be p = ( Vo/V)(E/Eo)and p = (EO/E)inB,where Voand Eo are reference values of V and E a t which the main beam is transmitted by the electric sector and l i is~the~ mass of the singly charged ions transmitted by the magnetic sector when V = V, and E = Eo. We wish to show that the results of Chess and Gross are readily interpreted in terms of such an ion current surface. For a mass spectrometer having the same geometry as that used by Chess and Gross, Table I shows the calculated p and p values of the centers of the metastable peaks for loss of the elements of COCL from M+. ions of TCDD in the field-free regions preceding the electric sector (1st FFR) and magnetic sector (2nd FFR). Figure 1 is our schematic contour map of part of the (In ( I p I ) , In $I), ion current) surface in the region
D
la)
~
lb) I(
264.
262.
260
-
258 -
256.
0 790
0 800
0 810
Flgure 1. Schematic contour map of the ion current surface in the vicinity of the 1st FFR metastable peaks for COCI. loss from M+. ions of TCDD. The broken lines show the relationshlp between p and p for (a) a V,Escari at p = 0.803 and (b) a scan generating a constant neutral spectrum for losses of co35c~..
of the cluster of 1st FFR metastable peaks. In generating the figure, we have assumed that the component peaks have Gaussian profiles in both the p and p directions and that their (arbitrarily chosen) ratios of height to width in the p and p directions are uniform for all the components. Surface features due to reactions occurring outside the 1st FFR have been omitted. Scanning modes of the mass spectrometer correspond to cross-sections of the ion current surface. For example, a V
Table I. Metastable Peaks for Losses of‘ COCl. from M i -Tons of TCDDa Mi. ion
neutral lost
co
c1 c035c1 35
c035a
c035c1
co c035c1 cow1 co co 37c1 co c037c1 co37c1 3 5 c 1
35c1
37c1
c037c1 c037c1 c037c1
co
37c1
calcd daughter ion abundance 100.0 12.4 98.1 12.1 32.4 4.0 3.7 0.4 32.4 4.0 31.8 3.9 10.5 1.3 1.2 0.1
1st FFR P
0.8032 0.8038 0.8044 0.8050 0.8056 0.806% 0.8068 0.8073 0.7982 0.7988 0.7994 0.80 0 1. 0.8 0 0 7 0.8013 0.8019 0.8025
2nd FFR P
P
256.9 257.9 258.9 259.9 260.9 261.9 262.9 263.9 256.9 257.9 258.9 259.9 260.9 261.9 262.9 263.9
206.4 207.3 208.3 209.3 210.2 211.2 212.1 213.1 205.1 206.0 207.0 208.0 208.9 209.9 210.8 211.8
Dissociations in the, first field-free region (1st FFR) and the second field-free region (2nd FFR) of a mass spectrometer containing an electric sector followed by a magnetic sector. Values of P and P are calculated for peak centers. For 2nd FFR metastable peaks, P = 1.0. a
0003-2700/82/0354-0135$01.25/0 0 1981 American Chemical Society
Anal. Chem. 1982, 54, 136-137
136 P
320*257
322459
il
h
I
I
256
260
268
262
264
Figure 2. Schematic constant neutral spectrum for losses of C035CIin the 1st FFR.
scan corresponds to a section with a plane of constant p and to a horizontal line in Figure 1. The resultant profile should show two broad and possibly overlapping peaks, having heights in the ratio of the natural abundances of 37Cland 35Cl. Chess and Gross (4, 7) used a V,E scan (a linked scan of V and E at constant B with V I E held constant), which corresponds to a section of the ion current surface with a plane of constant p and to a vertical line in Figure 1. The profile obtained from a VJ3 scan at p = 0.803 (Figure 1) will be influenced by instrumental factors and by the distribution of kinetic energy released in the fragmentation, but given a small range of w , and peaks that are sufficiently broad and flat-topped in the p direction, it should approximate the profile from a constant neutral spectrum (8-11) for losses of C035Cl. from the isotopic M+. ions. Peak heights in this latter profile should correspond to the distribution of natural isotopes for CllH40C13,provided that there is no interference from other reactions such as loss of the elements of CHOCl from the M+. ions, from ion fragmentations of other components of the mixture, or from reactions occurring outside the 1st FFR. Our simulation of the constant neutral spectrum for losses of C035Cl., based on the same assumptions as for Figure 1,is shown in Figure 2. It agrees well with the profiles obtained by Chess and Gross over the range of p from 256.5 to 259.5 p for which comparison is possible ( 4 ) and with that
obtained for losses of C1- from M+. ions of 2,3,4,6-tetrachlorophenol (7). Note that the senses of the x axes of the figures of Chess and Gross and our Figure 2 are opposite. An ion current surface contains peaks for all the ion reactions in a mass spectrometer possessing tandem analyzers, whether the ion currents originate from a single compound or from a mixture in the ion source. High selectivity in MS/MS necessitates a judicious choice of those regions of the surface that contain peaks peculiar to the component or components of interest. Appropriate cross-sections through the distinctive peaks can then be used for multiple ion monitoring to validate an assignment and to increase the specificity of the analysis. The concept of an ion current surface facilitates the understanding and planning of MS/MS experiments.
LITERATURE CITED Kondrat, R. W.; Cooks, R. G. Anal. Chem. 1978, 50, 81 A-92 A. Yost, R. A.; Enke, C. G. Anal. Chem. 1979, 51, 1251 A-I264 A. McLafferty, F. W. Acc. Chem. Res. 1980, 13, 33-39. Chess, E. K.; Gross, M. L. Anal. Chem. 1980, 52, 2057-2061. Lacey, M. J.; Macdonald, C. G. Org. Mass Spectrom. 1977, 12,
587-594. Lacey, M. J.; Macdonaid, C. 0.; Donchl, K. F.; Derrick, P. J. Org. Mass Spectrom., in press. Gross, M. L. Unlversity of Nebraska, personal cornmunicatlon. Lacey, M. J.; Macdonald, C. G. Anal. Chem. 1979, 51, 691-695. Zakett, D.; Schoen, A. E.; Kondrat, R. W.; Cooks, R. G. J. Am. Chem. SOC. 1979, 101, 6781-6783. Haddon, W. F. Org. Mass Spectrom. 1980, 15, 539-543. Shushan, B.; Boyd, R. Anal. Chem. 1981, 53, 421-427.
Michael J. Lacey* Colin G . Macdonald CSIRO Division of Entomology P.O. Box 1700 Canberra City 2601, A.C.T. Australia RECEIVEDfor review June 16,1981. Accepted September 28, 1981.
Exchange of Comments on Matrix Interferences in Graphite Furnace Atomic Absorption Spectrometry by Capacitive Discharge Heating Sir: Chakrabarti et al. ( 1 ) have presented details of their promising new technique of graphite furnace atomic absorp-
tion by capacitive discharge heating. Particularly intriguing is the possibility of thereby developing an essentially
Table I. Reported Values for U.S. Geological Survey Marine Mud MAG-1 Ni
Pb
amt of Pb, PPm 28.0 27.2 26.2 23.8 20.4b
a
Key:
techniquea GFAA
Mass spec. GFAA FAA
Comp. spec.
ref 5 6 7 8 2
amt of Ni, Ppm 70.2 70 60 56 53.8 53.5 53 53 50.7 50 50 48.3 48 45 41 30
technique Comp. spec. XRF FAA FAA XRF FAA NAA NAA
Em. spec. Em. spec. NAA XRF
Em. spec. XRF
Em. spec. NAA
Mn
ref 2 9 10 11 12 8 13 14 15 9 16 10 17 18 9 19
amt of Mn, PPm 1020 880 850 850 770 770 770 7 15 695 690 670 660 610
technique Comp. spec. XRF XRF FAA
Color Em. spec. FAA FAA
Em. spec. NAA FAA FAA NAA
ref 2 12 9 20 15 9 9 21 17 19 8 11 22
Color, colorimetric; Comp. spec., computerized emission spectrometry; Em. spec., other emission spectrometry;
FAA, flame atomic absorption; GFAA, graphite furnace atomic absorption; Mass spec., mass spectrometry; NAA, neutron Values taken by Chakrabarti et al. (1)as “most probable”. activation analysis; XRF, X-ray fluorescence. 0003-2700/82/0354-0136$01.25/00 I981 American Chemical Society
