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Anal. Chem. 1982, 5 4 , 2372-2375
In our work (14), we find that each of these sytems will give exponential gradients. Our studies indicate that stirred chambers give exponential behavior over a wider range of concentrations than do gradient systems using the various sizes and lengths of tubing. Many of the objections that others have to stirred chambers may be overcome by using small (100 pL) dilution chambers. ACKNOWLEDGMENT The skillful technical assistance of Darla Higgs is gratefully acknowledged. LITERATURE CITED (1) (2) (3) (4)
Betteridge, D. Anal. Chem. 1978, 5 0 , 832A-846A. Ruzicka, J.; Hansen, E. H. Anal. Chim. Acta 1980, 114, 19-44. Ranger, C. B. Anal. Chem. 1981, 53, 20A-32A. Stewart, K. K. Talanta 1981,,,28, 789-797. ( 5 ) Ruzicka, J.; Hansen, E. H. Flow Injection Analysis"; Wiley: New York, 1981. (6) Pardue, H. L.; Fields, B. Anal. Chlm. Acta 1981, 124, 39-63. (7) Ruzicka, J.; Hansen, E. H.; Mosbaek, H. Anal. Chim. Acta 1977, 92, 235-249. (8) Nagy, G.; Toth, K.; Pungor, E. Anal. Chem. 1975, 4 7 , 1460-1462. (9) Nagy, G.;Feher, Z.; Toth, K.; Pungor, E. Anal. Chim. Acta 1977, 9 1 , 87-96. (IO) Nagy, G.;Feher, 2.; Toth, K.; Pungor, E. Anal. Chlm. Acta 1977, 9 1 , 97-106. (11) Nagy, G.;Feher, Z.; Toth, K.; Pungor, E. Anal. Chlm. Acta 1978, 100, 181-191. (12) Astrom, 0.Anal. Chim. Acta 1979, 105, 67-75. (13) Ramsing, A. U.; Ruzicka, J.; Hansen, E. H. Anal. Chlm. Acta 1981, 129,1-17. (14) Stewart, K. K.; Rosenfeld, A. G. J . Autom. Chem. 1981, 3 , 30-32.
(15) Horvai, G.;Toth, K.; Pungor, E. Anal. Chim. Acta 1976, 82, 45-54. (16) Pardue, H. L.; Fields, B. Anal. Chlm. Acta 1981, 124, 65-79. (17) Pungor, E.; Feher, 2.; Nagy, G.; Toth, K.; Horvai, G.; Gratzl, M. Anal. Chim. Acta 1979, 109, 1-24. (18) Nagy, G.;Feher, 2.; Pungor, E. Anal. Chim. Acta 1970, 52,47-54. (19) Brown, J. F.; Stewart, K. K.; Higgs, D. J . Autom. Chem. 1981, 3 , 182-186. (20) Stewart, K. K. Anal. Chem. 1977, 4 9 , 2125. (21) Wolf, W. R.; Stewart, K. K. Anal. Chem. 1979, 5 1 , 1201-1205. (22) Mundie, C. M.; Cheshlre, M. V.; Anderson, H. A.; Inkson, R. H. E. Anal. Biochem. 1978, 7 1 , 604-607. (23) Hudson, G.J.; John, P. M. V.; Bailey, B. S.; Southgate, D. A. T. J . Sci. Food Agric. 1976, 2 7 , 681-687. (24) Vratny, P.; Ouhrabkova. J. J . Chromatogr. 1980, 191, 313-317. (25) Blakeney, A. B.; Mutton, L. L. J . Scl. FoodAgrlc. 1980, 3 1 , 889-897. (26) Kraml, M. Clln. Chlm. Acta 1966, 13, 442-448. (27) Howe, J. C.; Beecher, G. R. J . Nutr. 1981, 111, 708-720. (28) Stewart, K. K.; Brown, J. F.; Golden, 6. M. Anal. Chim. Acta 1980, 114. 119-127.
RECEIVED for review February 17, 1982. Resubmitted and accepted August 23, 1982. A preliminary report of this work was presented at the 1981 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 9-13, Atlantic City, NJ, by K. K. Stewart and A. G. Rosenfeld, paper number 824. This work was supported in part by an Interagency Reimbursable Agreement No. 2Y01-HB60041-05 from the National Heart, Lung, and Blood Institute, NIH. Mention of trademark or proprietary products does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not imply their approval to the exclusion of other products that may also be suitable.
CORRESPONDENCE Soft Negative Ionization of Nonvolatile Molecules by Introduction of Liquid Solutions into a Chemical Ionization Mass Spectrometer Sir: Over the last 6 years our efforts have been directed toward interfacing a high-performance liquid chromatograph (HPLC) to a quadrupole mass spectrometer by nebulizing a constant fraction of the liquid effluent from the chromatograph into a high-pressure chemical ionization (CI) source. This method has been called direct liquid introduction (DLI) (1). Initial efforts to develop the analytical potential of the DLI method were restrained by experimental difficulties such as instabilities of the vacuum (2),clogging of the interface (3), or improper HPLC packings and solvents ( 4 ) . These inconveniences being now less commonplace, it becomes easier to test the performance of the DLI method by applying it to the separation and detection of known nonvolatile organic molecules as also currently done in other laboratories (5-9). The performance characteristics of this combined liquid chromatography/mass spectrometry (LC/MS) interface are significantly improved when the droplets from the nebulizer are allowed to drift through a heated zone (8-10), sometimes referred to as the desolvation chamber, prior to introduction into the CI source. Since solute ions may arise in part from the liquid solution (11-16), a complete desolvation to dryness of the liquid droplets within the desolvation chamber should be avoided. By use of an appropriate geometry for the desolvation chamber, the droplets are accelerated to sonic velocities. This approach is comparable to that followed by Vestal et al. (12,13,16) who have shown that rapid thermal vaporization of high speed droplets assists the ionization of
nonvolatile molecules when neutral solutions are nebulized in the presence of an external ionization source or produces gas-phase solute ions directly when electrolytic solutions are nebulized and vaporized. The operating principle of our high-speed DLI device and preliminary results obtained for different fragile molecules have been previously reported (10). This correspondence describes in detail the device used in these experiments. Also included are the mass spectra obtained for vitamin B12and the antibiotic erythromycin A. They show the capability of handling polar molecules of molecular weight over m / z 1000 and the occurrence of electron capture chemical ionization (17) under DLI LC/MS conditions. EXPERIMENTAL SECTION General Equipment. A Waters Associates, Inc. (Milford,MA), Model 6000A solvent delivery system and Model U6K injector were used with a Merck (Darmstadt,GFR) reversed-phase column (4.6 mm X 25 cm) packed with 10-gm Lichrosorb RP 18. Mass spectra were recorded on a Nermag Model R-10-10-C quadrupole mass spectrometer,equipped with a Nermag LC/MS interface and Model SIDAR l l l B data processing system. No liquid nitrogen cryopump ( 2 ) was used in this study. The instrument has an operable mass range up to 1500 amu and is fitted with a conversion dynode electron multiplier detector for recording of negative ions. Primary ionization of the solvent vapors was accomplished by a 70-eV beam of electrons from a heated rhenium ribbon. The standard metallic cage around the rhenium ribbon
0003-2700/82/0354-2372$01.25/00 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
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A
5 L-
cm t
J
Flgure 1. Schematic diagram of the LC/MS transfer line of droplets: (1) Vespei end of the D L I probe, (2) black painted copper radiator, (3) stainless steel desolvation chamber, (4) preevaporatlon zone, (5) heated divergent cone, (6) 30-W electrical heater, (7) alumina insulator, (8) C I
source block. was replaced by a high transmission metallic grid with the same geometry. The modification increases the filament lifetime to several weeks when pure water is the CI reagent gas. LC/MS Interface. The new interface includes an integrated liquid effluent splitter and diaphragm nebulizer, hereafter referred to as the DLI probe, and (5 droplet transfer line placed between the DLI probe and the CI source. The DLI probe has been described elsewhere (3)and utilizes a similar nickel diaphragm with a ca. 2-pm pinhole orifice as the nebulizer. It was used without modifications,but no cooling water was circulated through the DLI probe. The L,C/MS interface wm operated in the split mode, nebulizing 10 pL/min of solution from the full HPLC effluent. The DLI probe is removable through a vacuum lock. The droplet transfer line (Figure 1) comprises a black painted copper radiator, a heated desolvation chamber, and an alumina insulating spacer. The assembly is permanently screwed to the CI source block. The radiator (3 cm 0.d.) fits around the end of the desolvation chamber to which the tip of the DLI probe butts tightly during a LC/MS run. It keeps the diaphragm in the DLI probe tip at ca. 60 "C when the drift cone in the desolvation chamber is at high temperatures; nebulizcxtion by the diaphragm would otherwise be unstable (3). The radiator replaces the cooling water through the DLI probe. The desolvation chamber is a stainless steel block (15 mm X 46 mm), machined to accommodate a cartridge heater and the droplet drift zone. The shape of the drift zone is sharply converging at the entrance arid then diverging at a low angle in the remaining portion. The droplets are concentrated and accelerated during their flight through this nozzle. The alumina spacer acts as a thermal barrier between the desolvation chamber and the CI source, so that both zones can be held at different temperatures. Typically, the temperatures of the chamber and the CI source block are 200 "C and 150 "C, respectively. In addition, the spacer insulates the chamber electrically from the CI source. Being already insulated from the DLI probe by the Vespel cone at the probe tip (3),the desolvation chamber can be floated at any voltage in the range il kV by an external voltage supply. A focusing effect on the electrically charged droplets generated by the nebuliz,er, or a Townsend discharge under conditions similar to that described by Hunt et al. (18),has been observedl on a similar experimental setup, and the results will be reportled later. In this study, both the de.solvation chamber and the CI source were held a t 0 V and at different temperatures. Reagent Solution and Samples. Vitamin Blz was purchased from Merck, and erythromycin A was obtained from the French "Hopital Necker", Paris. Careful preparation of the HPLC mobile phase is essential for trouble-free LC/MS runs. Acetonitrile wm HPLC grade purchased from Merck. Deionized water was distilled on 800 "C quartz heaters under a stream of filtered air, resulting in complete appogenic water, and finally degassed by sonication. In this study, a 50:50 (v:v) acetonitrilewater solution was prepared and then filtered through a 0.5-wm Millipore filter, and buffered at pH 5 by a volatile trimethylamine-formic acid buffer. A new solution was prepared and used on the same day. LC/MS Operations. The eluent was pumped a t a flow rate
1353
1259
1
1278 ~
,,,I,,)I!,
,,,ill,
,,,
1260
,,
1,
1297 ,1 ,-,
1300
,,,~ ,
1371 ,,,,
,,, ,,,
I,
,, ,,,,
,-
1160
Flgure 2. (A) Structure of vitamin B,. Molecular weight is 1354.57 amu. (B) Negative ion mass spectrum of vitamin B,, under D L I LCIMS.
of 1.3 mL/min through the chromatographic system and 10 wL/min of column effluent was nebulized by the DLI probe. The pressure of solvent vapors in the CI source was 0.2 torr as monitored by a thermocouple gauge connected to the source block. Ten micrograms of vitamin Blz and 5 pg of erythromycin A were injected onto the HPLC column during separate runs and eluted with K' values of ca. 3 and 4, respectively. The amount of solute transferred to the CI source was 92 ng and 46 ng, respectively. Mass spectra were collected and processed in the usual mode of operation for the DLI LC/MS technique (3, 10). RESULTS AND DISCUSSION Vitamin Biz. Cobalamins, including cyanocobalamin (vitamin Blz) have often been used to test the potential of new analytical methods such as HPLC during the total synthesis of vitamin B,, (19) and several soft ionization techniques for mass spectrometry (20-23). Field desorption (20), fast atom bombardment (22),and laser desorption (23) have produced molecular species for vitamin BIZunder positive or negative ionization. However, the reported mass spectra are generally dominated by fragment ions. The LC/MS negative ion spectrum of 92 ng of vitamin Blz into the CI source (Figure 2) shows an abundant (M - H)-ion a t m / z 1353 and a weak satellite ion a t mlz 1371 corresponding to (M - H + HzO)-. Other fragment ions in the m / z 1200-1300 mass range are observed but have not been interpreted. There are no significant peaks below mlz 1100. Vitamin BIZis the highest molecular weight molecule that we have analyzed by LC/MS. In contrast, the FAB negative ion spectrum reported by Barber et al. (22) shows a fragment ion a t m / z 1327 corresponding to (M - HCN)-- as the more
2374
ANALYTICAL CHEMISTRY, VOL.
k EUYTHROMYCINE
54,
NO.
NOVEMBER 1982
13,
OH
NEQ. MODE
I
n4
,630
I
B
.
~
I i
733
I 715
j
I
Figure 3. (A) Structure of erythromycin A. Molecular weight is 733.46 amu. (B) Negative Ion mass spectrum of erythromycln A, under experimental conditions identical with that In Flgure 2.
abundant ion in the molecular weight region, and requiring 20 pg of sample into the ion source. Consequently, FAB ionization appears as a higher energy and less sensitive technique for the analysis of vitamin B12than DLI LC/MS. We have been unable to record a good positive ion spectrum of vitamin B12,only fragment ions a t the low mass end were observed. This contrasts with Vestal's results announced at a recent conference (16): a good positive ion spectrum of vitamin BIZwas obtained by thermospray LC/MS (12,13), but the negative ion analysis was not attempted. Thus more work is needed to evaluate both systems and the difference of results may reflect a temporary situation; however, it is interesting to observe that these two LC/MS methods come to comparable performance. They were developed independently and have different origins, but their continuous evolution during the recent years may end up in comparable devices using the same operating principles. Erythromycin A. This antibiotic is not considered difficult for a mass spectrometric determination and has been well studied (24). Although it is not volatile enough to be analyzed by gas chromatography, it shows a weak but visible molecular ion under electron impact, and a base peak ion corresponding to MH+ under conventional isobutane CI with solid probe introduction; the molecular ion M+. is almost the only ion found in the positive ion FD spectra recorded at different temperatures of the anode emitter. Equally good positive ion LC/MS spectra have been reported for erythromycin A using a DLI interface (9) and for erythromycin B (a dehydroxy analogue of erythromycin A) with a moving belt interface (25). However, negative ion spectra were not given. This particular example of application also illustrates the interest of independent temperature settings of the desolvation chamber and the CI source, which constitutes a major improvement over a previously described DLI LC/MS system (8, 9). The positive ion spectrum of erythromycin A is identical with that obtained by Kenyon et al. (9) and shows a base peak ion a t mlz 734 corresponding to MH+. The negative ion spectrum (Figure 3) shows a molecular ion a t m / z 733 corresponding to M-. resulting from ionization by electron cap-
ture. The base peak ion a t m / z 630 is not visible in any previously reported spectra of this antibiotic. This unusual fragment ion is tentatively explained by the adduct of an acetonitrile molecule from the CI gas to the (M - 144) ion. Similar acetonitrile adducts to protonated molecules or fragment ions have been frequently observed in our laboratories in spectra of samples in an acetonitrile solution. To the best of our knowledge, it is the second example of a molecule showing an abundant molecular anion under DLI LC/MS. Kenyon et al. had previously recorded a M-- ion in the negative ion spectrum of dicophol, a chlorinated insecticide, and observed a strong dependence of the peak abundance with the temperature of the CI source walls, as frequently found for samples ionized by electron capture chemical ionization (17). A dramatic reduction of the molecular peak abundance was observed when the source temperature was varied from 150 OC to 200 "C. The situation was identical when we ran erythromycin A. However, the LC/MS analysis could be optimized by raising the desolvation temperature to 200 OC, which assists the vaporization of liquid droplets, and keeping the CI source walls at ca. 150 OC, which minimizes thermal fragmentation of the molecular anion. General Discussion. The simple device described in this work is an inexpensive modification to a conventional chemical ionization mass spectrometer, unlike the dedicated LC/MS instrument used by Vestal et al. (12,13). It greatly improves the overall performance characteristics of the DLI LC/MS system for recording the negative ion spectra of high molecular weight polar molecules. We still lack an explanation for the experimental observation that sometimes no spectrum is obtained in the positive ion mode whereas abundant molecular species are seen under the negative ion mode. The optimum temperatures of the desolvation chamber and the CI source are to date determined experimentally when running a given molecule (10). When running an unknown mixture by LC/ MS, a low CI source block temperature ca. 100-150 "C is selected, and the desolvation chamber is set a t ca. 200-250 "C. The predominance of either (M - H)- or M-- ions from different samples analyzed under identical experimental conditions cannot be predicted reliably. Data on the relative electron affiiity and acidity of complex molecules are generally not available, which may be confusing when running unknown samples. On the other hand, the behavior of known molecules under the conditions of the DLI method increases our knowledge on negative chemical ionization mechanisms and solvent effects on organic ions in the gas phase. Finally, the question of the ionization mechanism is becoming of increasing importance, although direct experimental proofs are still missing. We currently assume that droplets are electrically charged during nebulization of HPLC effluents and concentrated during their flight through the nozzle. Ions are believed to be released from the droplets when they reach the ion source. Alternatively, the droplets could be fully desolvated before they reach the CI source. In our experiments, an external ionization source provided by a beam of electrons was needed to record sample ion currents. These speculations are comparable to those suggested by different authors who have obtained good results when analyzing solutions of nonvolatile molecules by mass spectrometry, using apparently different experimental systems, but which could utilize the same ionization mechanisms (11-16).
CONCLUSION The preliminary results of this study and the work done by using related methods encourage us to apply the described DLI LC/MS system to electrolytic solutions and solutions of preformed ions, including cationated molecules and quaternary ammonium salts, to further elucidate the ionization mechanisms and to determine the performance limits of the method.
Anal. Chem. 1982, 5 4 , 2375-2376
LITERATURE CITED (1) Arpino, P. J.; Krien, P. Proceedings of the 26th Annual Conference on (2) (3) (4) (5) (6) (7) (8)
(9) (10) (11) (12) (13) (14) (15) (16) (17) (18)
Mass Spectrometry aind Allied Topics, St. Louis, MO, 1978; pp 426-428. Arpino, P. J.; Guiochon. G.; Krien, P.; Devant, G. J. Chromatogr. 1979, 185,529-547. Arpino, P. J.; Krien, P.; Vajta, S.; Devant, G. J. Chromatogr. 1981, 203, 117-130. Mauchamp, B.; Krien, F’. J. Chromatogr. 1982, 236, 17-24. Henion, J. D.; Wachs, 1’. Anal. Chem. 1981, 53, 1963-1965. Evans, N.; Williamson, J. E. Blomed. Mass Spectrom. 1981, 8 , 316-321. Schaefer, K. H.; Levssn, K. J. Chromatogr. 1981, 206, 245-252. Melera, A. Adv. Mass Spectrom. 1980, 88,1597-1615. Kenyon, C. N.; Meleria, A.; Erni, F. J . Anal. Toxlcol. 1981, 6 , 216-230. Dedieu, M.; Juin, C.; Arpino, P. J.; Bounine. J. P.; Guiochon, G. J . Chromatogr. 1982, 25 I , 203-213. Arpino, P. J.; Guiochon, G. J. Chromatogr. 1982, 251, 153-164. Blakley, C. R.; Carmody, J. J.; Vestal, M. J. A m . Chem. SOC.1980, 102,5931-5933. Blackley, C. R.; Carmody, J. J.; Vestal, M. ,417al. Chem. 1980, 52, 1636-1641. Irlbarne, J. V.; Thom$on, B. A. J . Chnm. Phys. 1976, 64, 2287-2294. Tsuchiya, M.; Taira, T. Int. J. Mass Spectrom. Ion Phys. 1980, 34, 351-359. Blackley, C. R.; Vestal, IM. L. presented at the 30th Annual Conference on Mass Sepectrometry and Allied Topics, Honolulu, HI, 1982; abstract paper MPA 11. Hunt, D. F.; Crow, F. W. Anal. Chem. 1978, 50, 1781-1784. Hunt, D. F.; McEwen, C. N.; Harvey, T. M. Anal. Chem. 1975, 47, 1730-1734.
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Woodward, R. B. Pure Appl. Chem. 1973, 33, 145-177. Schulten, H. R. Int. J. Mass Spectrom. Ion Phys. 1979, 32,97-283. McFarlane, R. D.; Torgerson, D. F. Science, 1976, 191, 920-922. Barber, M.; Bordoli, R. S.; Sedgwick, R. D.; Tyler, A. N. Biomed. Mass Spectrom. 1981, 8 , 492-495. (23) Graham, S. W.; Dowd, P.; Hercules, D. M. Anal. Chem. 1982, 54, 649-654. (24) Rinehart, K. L., Jr.; Cook, J. C., Jr.; Maurer, K. H.; Rapp, U. J. Antibiot. 1974, 27, 1-13. (25) Arpino, P. J.; Guiochon, G. Anal. Chem. 1979, 5 1 , 682A-701A. (19) (20) (21) (22)
M. Dedieu* C. Juin Nermag Corp. 49 quai du Halage 92500 Rueil-Malmaison, France P. J. Arpino G. Guiochon Ecole Polytechnique Laboratoire de Chimie Analytique Physique 91128 Palaiseau, France
RECEIVED for review May 20,1982. Accepted August 16,1982. Financial support for this work was received from the French “Ministgre de l’lndustrie et de 1’Equipement” under contract NO. 79-2-35-059.
Capillary Gas Chromatography/Mass Spectrometry with a Microwave Discharge Interface for Determination of Radioactive-Carbon-Containing Compounds Sir: Minor compounds in complex organic matrices represent an analytical challenge to organic mass spectroscopists, both in their detection and in structure elucidation. In biochemistry and pharmacology it is often possible to employ radiolabeled precursors when characterizing unknown metabolites. Gas chromatography with detection of radioactivity has long utilized nuclear decay t o permit very specific compound localization in clomplex chromatograms (1, 2). The design of combustion chiambers and radiochemical detectors significantly degrades the chromatographic resolution attainable with capillary columns (3-5). The amount of 14C normally employed in metabolism experiments suggests that mass spectrometric detection of 14C should compete with radiochemical detection--i.e., 1000 dpm equals approximately 100 pg of 14C. The limitation of mass detection is interference from 13C,2H, l8O, etc., which contribute to a significant and variable M + 2 background for most organic molecules. Quantitative degradation of organic molecules to di- or tristomic products simplifies heavy nuclide detection. We have exploited a low-pressure microwave-induced plasma interface to convert all carbon-containing compounds in a capillary GC effluent to CO and C 0 2 which can be measured in a conventional mass spectrometer without loss of chromatographic resolution and with a detection limit of 300-500 dpm. EXPERIMENTAL SECTION A Varian 1400 gas chromatograph equipped with a capillary splitter inlet set at 400:l (Supelco) was used with a HewlettPackard 50 m X 0.3 mm 11.d.SE 54 coated fused silica capillary column. The end of the column was extended into a 1/4 in. Swagelok “T”where its 1 mL/min helium flow was mixed with 0.1 mL/min of oxygen (UHP Matheson) and then passed through a 1/4 in. o.d., 4 mm id., 10 cm long quartz tube into a microwave
discharge cavity. Connections to the quartz tube were made with Vespel SP22 (40% graphite) ferrules and Swagelok fittings. The polyimide coating on that part of the capillary column which extended into the make-up “T” was burned off prior to assembly to minimize background. The microwave cavity was of a novel design (details are available upon request) to perform under a relatively high-pressure oxygen-rich environment and was powered by a 2450-MHz Raytheon Microtherm microwave power supply. The quartz tube was coupled to a 1 m section of 0.030 in. i.d. stainless steel capillary tubing which ran concentrically through the direct introduction probe into the mass spectrometer ion source. The interface was heated to approximately 230 OC up to the discharge cavity and operated at ambient temperature from that point on. A mixture of 514 ppm butane in helium was obtained from Matheson. The [l-14C]palmiticacid (50 mCi/mmol, New England Nuclear) was methylated with diazomethane in ether and compared for purity with unlabeled methyl palmitate and palmitic acid using conventional GC/MS techniques. A Finnigan Model 1015 quadrupole mass spectrometer with Extranuclear SpectrEl electronics and differential pumping was used to obtain mass spectra data recorded with a Ribermag SADR data system or Finnigan PROMIM. RESULTS AND DISCUSSION Because of the low operating pressures in the discharge tube, ca. 1 torr, no make-up gas was needed to ensure high chromatographic resolution. Only 0.1 mL/min of O2 was needed to combust completely any analyte introduced. This complete combustion is demonstrated in Figure 1where approximately 1 mL/min of butane/He mixture was introduced via the make-up gas line and its spectrum analyzed both without and with the discharge. No ions representing intact butane were observed after the discharge was turned on; the summed intensities of major product ions approximately equal
This article not subject to U.S. Copyright. Publlshed 1982 by the American Chemical Society
