776
Anal. Chem. 1985, 57,776-778
EION
Col Iis i o n a l ionization
limit
I
1
x”1
Photo ionization
I
Laser Xi2 excitation
ways. (a) The use of two-step laser excitation (5, 8) can increase the rate of ionization considerably and reduce the detection limits accordingly. In the case of Mg, for instance, we have found that the signal can be enhanced at least 2 orders of magnitude by using a second laser-excitation step. In addition, a second excitation makes it possible to reduce optical interferences substantially, which could be of great importance for investigating real samples. (b) For elements with low degrees of atomization in an ordinary air/acetylene flame, the number of free atoms in the flame, and consequently the LEI signal, can be considerably improved by means of a higher flame temperature, e.g., by using an N20/acetylene flame. It should be noted, however, that “improvements” of this kind may lead to a substantial increase in complexity of the analysis, and any of them should be used only when there is a definite need for it.
ACKNOWLEDGMENT Flgure 2. Excitation and ionization scheme for one-step laser enhanced ionization in flames. E, and E, denote the lower and upper laser level, respectively, while EIONrepresents the ionization limit. E, is the excess energy above the ionization limit when a second photon is absorbed in a photoionizationprocess.
of these cases (Co, Ga, and Pb) we still get comparable detection limits, while in other cases our limits are considerably higher, either due to unfavorable oscillator strength of available lines in our experiment or to high energy of the lower level of the laser transition (see Table I). In comparing the detection limits of LEI, for the elements studied here, with those of other standard methods of analysis, such as AAS in flame, flameless AAS, LIF, and ICP (see Table 11),we find that, when an optimal wavelength is used, one-step LEI has detection limits which are quite comparable with those of flameless AAS and usually superior to those of the other methods. In addition, LEI has in comparison with flameless AAS, the advantage of having the flame as an atomizer. Furthermore, it can be noted that even with rather unfavorable excitation lines, for instance, LEI can compete with AAS in flames. These observations, supporting those obtained earlier by the NBS group, demonstrate the power of the LEI method as a tool for trace-element analysis. The high sensitivity of the method has the consequence that preconcentration, frequently used in other methods, is here usually superfluous. Furthermore, we have shown elsewhere that LEI in flame can be conveniently combined with high pressure liquid chromatography (HPLC) (10). The work reported here is the first in a series of works we intend to perform in order to investigate the applicability of the LEI method for trace-element analysis under various experimental conditions. Besides using different dyes, which make other elements available for investigation as well as more optimal lines available for some of the elements studied here, our results can be improved particularly in the following two
The authors wish to thank Thomas Berglind for stimulating discussions. Regigtry No. Bi, 7440-69-9;Co, 7440-48-4; Cr, 7440-47-3;Fe, 7439-89-6; Ga, 7440-55-3; In, 7440-74-6; Mg, 7439-95-4; Mn, 7439-96-5; Na, 7440-23-5; Ni, 7440-02-0; Pb, 7439-92-1; Sn, 7440-31-5; T1, 7440-28-0;HzO, 7732-18-5.
LITERATURE CITED (1) Green, R . 6.; Keiler, R. A,; Schenck. P. K.; Travis, J. C.; Luther, G. C J . Am. Chem. SOC.1978, 98, 8517. (2) Turk, G. C.; Travis, J. C.; DeVoe, J. R. Anal. Chem. 1979, 51, 1890. (3) Turk, G. C.; Travis, J. C.; DeVoe, J. R.; O’Haver, T. C. Anal. Chem. 1978, 50,817. (4) Travls, J. C.; Turk, G. C.; Green, R. Anal. Chem. 1982, 54, 1006A. (5) Turk, G. C.; Mallard, W. G.; Schenck, P. K.; Smyth, K C. Anal Chem. 1979, 51,2408. (6) Berglind, T.; Rublnsztein, H.;RosBn, A. Institute of Physics, Chalrners University of Technology, GIPR-216, 1980, unpublished results. (7) Travis, J. C.; Schenck, P. K.: Turk, G. C.; Mallard, W. G. Anal. Chem. 1979, 51, 1516. (8) Axner, 0.; Berglind, T.; Heully, J. L.: Lindgren, I.; Rubinsztein-Dunlop, H. J. Appl. Phys. 1984, 55,3215. (9) Curran, F. M.; Lin, K. C.; Leroi, G. E.; Hund, P. M.; Crouch, S. R. Anal. Chem. 1983, 55,2382. (IO) Berglind, T.; Nilsson, S.; Rubinsztein-Duniop, H., submitted for publication in Anal. Chem. (11) Reader, J.; Corliss, C. H.; Wiese, W. L.; Martin, G. A. Natl. Stand. Ref. Data Ser. ( U S . , Natl. Bur. Stand.) NSRDS-NBS 68. (12) Striganov, A. R.; Sventitskii, N. S. NBS Monograph ( U S . ) 1968, no.
145. (13) Weeks, S. J.; Haraguchi, H.; Winefordner, J. 0. Anal. Chem. 1978, 50,360. (14) Fassel, V. A. Science 1978, 202, 183.
Ove Axner Ingvar Lindgren Ingemar Magnusson Halina Rubinsztein-Dunlop* Department of Physics Chalmers University of Technology/University of Gothenburg S-412 96 Goteborg, Sweden RECEIVED for review November 30, 1983. Resubmitted July 23,1984. Accepted November 5,1984. This work has been supported by the Swedish Natural Science Research Council.
Differentiation of Cationic from Neutral Transition-Metal Complexes Using Wires as Field Desorption Emitters Sir: Numerous types of emitters have been described for field desorption (FD) mass spectrometry ( I ) . Carbonaceous
microneedles (2-4) or microneedles formed from metal carbonyls (5), silicone whiskers (6),and cathodically reduced
0003-2700/85/0357-0776$01.50/0(E 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
560
460
480
580
570
777
590
m12
~~
693'
/
49+
TC I D M P E 13 PF6
680 530
Flgure
540
550
560
57C
m/i
Portions of the field desorption mass spectra of (a) Tc(DMPE),(CI),CI04 and (b) Tc(DMPE),PF, obtained from an unactivated wire: DMPE, bis( 1,2-dimethyIphosphino)ethane; DEPE, bis(l,2-diethy1phosphino)ethane. 1.
metal dendrites (7-9) have been used. All preparation procedures involve emitter "activation" which yields microneedles or dendrites attached to the wire. These procedures are time and labor intensive but are necessary to enhance local field strengths and affect field desorption. Rollgen and Giessmann suggested the use of unactivated tungsten wires as FD emitters for compounds with low appearance potentials (10). They reported ion emission for organic s a l h from unactivated 10-pm wires. This paper reports similar observations for F D mass spectra of cationic technetium complexes. Comparisons are made with cobalt and carbon emitters. The method has the added advantage of differentiating cationic from neutral transition metal complexes, as neutral complexes fail to yield ions from these unactivated wires. This distinction is often less evident from fast atom bombardment mass spectra (11, 12).
EXPERIMENTAL SECTION All spectra were obtained on a VG-ZAB-2F mass spectrometer. By use of the VG-2035 data system, mass spectra were recorded at 20-s exponential scans over a 1500 dalton range. Calibration was afforded using Ethaquad in the FAB mode (13). Both 10- and 25-pm tungsten wire were spot-welded onto standard Vacuum Generators or Vacumetric field desorption beads. The sample was transferred to the wire via a syringe. Desorption from the 10-wm wire was noted at ca. 18-20 mA. The best anode temperature of 25-pm wire emitters was ca. 120 mA and was established by using the desorption chemical ionization controller. Preparation, loading, and signal intensity favored the thicker wire so subsequent spectra were recorded from 25-fim
690
700
710
rn 12
Flgure 2. Portions of the field desorption mass spectra of (a) Tc(DEPE),(CI),CIO, and (b) Tc(DPrPE),(CI),CIO, obtained from an unactivated wire: DPrPE, bis( 1,2-dipropyIphosphino)ethane. The two chlorine isotope pattern is observed for both cations. unactivated wires. The field anode was held at a positive potential of 8 kV while the counterelectrode was at a negative potential of 4 kV.
RESULTS AND DISCUSSION Intedse and long-lived (ca. 5 min) ion emission was noted for each cationic transition-metal complex examined. Figure 1 shows a portion of the field desorption mass spectra of F~ T C ( D M P E ) ~ ( C ~ ) (top) ~ C ~ and O ~ of T C ( D M P E ) ~ P(bottom). These spectra were not averaged as is often necessary for weak or fluctuating FD signals (14) and represent standard FAB mass spectral scanning conditions. The absence of fragmentation contrasts with their FAB mass spectra (12) where significant fragmentation was observed. Their intensity and lack of fragmentation are due to the efficient desorption of the preformed cation under even low field strength conditions absence of field-enhanced fragmentation often obsing high field strength emitters. The unactivated wire affords desorption with minimal or no fragmentation. An increase in emitter current did not significantly enhance fragmentation. Figure 2 shows other examples, T c ( D E P E ) ~ ( C ~ ) , C(top) ~O~ and T c ( D P ~ P E ) ~ ( C ~ ) ,(bottom). C~O~ As for Tc(DMPE),(Cl),C104, both contain an isotope pattern which is consistent with the presence of two chlorines. Both also exhibit ion intensities which are similar to those obtained from activated or high field strength emitters. Attempts to ionize neutral transition-metal complexes such as Cr(ACAC)3, VO(ACAC)z, or structurally similar neutral technetium metal complexes from unactivated 10- or 25-pm
778
Anal. Chem. 1985, 57,778-780
1
I
I i
I
yield an intense molecular ion, Cr(ACAC)3+at m / z 349 and VO(ACAC)z+a t m / z 265. Some dimer is also seen for VO(ACAC)*a t m l z 530. The use of unactivated tungsten wires as field desorption emitters provides a rapid differentiation of cationic from neutral transition-metal complexes. It affords intense signals due to the intact cation with no fragmentation. The method avoids emitter activation and allows wires to be used for subsequent analyses.
Registry No. Tc(DMPE),(Cl)&lO,, 94481-35-3; Tc(DMPE),PFB, 89378-28-9;Tc(DEPE)~(C~)~CIO,, 94481-37-5;TC(DPrPE)2(C1)2C10,,94481-39-7; Cr(ACAC),, 21679-31-2; VO(ACAC)2, 3153-26-2;W, 7440-33-7;C, 7440-44-0. 350
340
360
LITERATURE CITED
mlz
1
r---s"
Beckey, H. D. "Principles of Field Ionization and Field Desorption Mass Spectrometry"; Pergamon Press: New York, 1977. Beckey, H. D.; Hilt, E.; Schulten, H.-R. J . Phys. E : Scl. Instrum. 1973, 6, 1043-1044. Linden, H. B.; Hilt, E.; Beckey, H. D. J . Phys. E : Sci. Instrum. 1978, 1 1 , 1033-1036. Beckey, H. D.; Schulten, H A . Angew. Chem., Int. Ed. Engl. 1975, 6,403-413. Lehmann, W. D.; Fischer, R. Anal. Chem. 1981, 5 3 , 743-747. Matsuo, T.; Matsuda, H.; Katakuse, I. Anal. Chem. 1979, 57,69-72. Bursey, M. M.; Rechsteiner, C. E.; Sammons, M. C.; Hinton, D. M.; Colpitts, T. S.;Tvaronas, K. M. J . Phys. E.: Sol. Instrum. 1976, 9 , 145-1 47. Semrav, G.;Heitbaum, J. Anal. Chem. 1979, 5 7 , 1998-2000. Goldenfeld, I. V.; Velth, H. J. Int. J . Mass Spectrom. Ion. Phys. 1981, 4 0 , 361-363. Rollgen, F. W.; Giessmann, U.; Heinen, H. J.; Reddy, S. J. I n t . J . Mass Spectrom. Ion Phys. 1977, 24 235-238. Cerny, R. L.; Sullivan, B. P.; Bursey, M. M.; Meyer, T. J. Anal. Chem. 1983, 55, 1954-1958. Unger, S . E. Anal. Chem. 1984, 5 6 , 363-368. DeStefano, A. J.; Keough, T. Anal. Chem. 1984, 56, 1846-1849. Snelling, C. R., Jr.; Cook, J. C.; Milberg, R. M.; Hemling, M. E.; Rinehart, K. L., Jr. Anal. Chem. 1984, 56,1474-1481. ~
i
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I. , -
260
270
rn 11
Figure 3. Field desorption mass spectra of (a) Cr(ACAC), and (b) VO(ACAC), using a high field strength activated emitter: ACAC,
acetylacetonate.
tungsten wires were unsuccessful. However, these species were efficiently ionized using either cobalt (7) or carbon emitters. Figure 3 shows portions of the FD mass spectra of Cr(ACACI3 (top) and VO(ACAC)2(bottom) using carbon emitters. Both
Steve E. Uneer I
The Squibb Institute for Medical Research P.O. Box 4000 Princeton, New Jersey 08540
RECEIVED for review October 22, 1984. Accepted December 3, 1984.
AIDS FOR ANALYTICAL CHEMISTS Modified Interface for Pyrolysis Gas Chromatography with Capillary Columns Robert S. Whiton and Stephen L. Morgan* Department of Chemistry, University of South Carolina, Columbia, South Carolina 29208 In analytical pyrolysis gas chromatography (PGC), the sample is thermally fragmented by a rapid temperature rise either in a heated chamber attached to the injection port or in the injection port of the GC. The volatile fragments are swept by carrier gas into the chromatographic column where they are separated and passed to a detector. Analytical pyrolysis is routinely used to fingerprint polymeric materials or to obtain structural information from nonvolatile or thermally labile compounds. To achieve an efficient chromatographic separation, the sample must be transferred to the column as a narrow plug. If the volume of the injection port is too large, the sample may be transferred to the column as an excessively wide band. This 0003-2700/85/0357-0778$0 1.50/0
problem has been well recognized in the field of pyrolysis GC ( I , 2). In capillary GC, the injection port volume problem is compounded by the small internal volume of the column and the low carrier gas flow. Chromatographic bands can be narrowed by cold trapping of pyrolysates on the column or in a precolumn cold trap ( 3 ) ,but subambient cooling is required. The normal capillary GC injection mode, split injection, uses a rapid carrier flow (50-100 mL/min) through the injection port to sweep the sample rapidly into the column. Only a small portion of carrier flow (1 mL/min) enters the column and the remainder is vented to atmosphere. Because only 1-2% of the sample enters the column, sensitivity is lowered. This lack of sensitivity may be compensated for by 0 1985 American Chemlcal Society
