Anal. Chem. 1985, 57, 2441-2444 (19) Buffet, C. E.; Morris, M. D. Anal Chem. 1983, 55, 376-378. (20) Sepaniak, M. J.; Vargo, J. D.; Kettler, C . N.; Maskarinec, M. P. Anal. Chem. 1984, 56, 1252-1257. (21) Pang, T. J.; Morris, M. D. Anal. Chem. 1984, 56, 1467-1469. (22) Leach, R. A.; Harris, J. M. Anal. Chlm. Acta 1984, 764, 91-101. (23) Wellegehause, B.; Laepple, L.; Welling, H. Appl. Phys. 1975, 6 , 335-340. (24) Teschke, 0.; Whinnery, J. R.; Dienes, A. I€€€,/. Quantum. Nectron. 1976, QE-12, 513-515 (25) Sanders, D. A. Appl. Opt. 1984, 23, 30-35. (26) Nickolaisen, S.L.; Bialkowski, S. E. Anal. Chem. 1985, 57,758-762. (27) Twarowski, A. J.; Kliger, D. S. Chem. Phys. 1977, 20,253-258. (28) Weimer, W. A.; Dovichl, N. J. Appl. Opt., in press. (29) Weimer, W. A,; Dovichi, N. J. Appl. Spectrosc., in press. (30) Bird, R. B.; Stewart, W. E.; Lightfoot, E. N.; Chapman, T. W. "Lectures in Transport Phenomena";American Instltute of Chemical Engineers: New York, 1969.
2441
(31) Jackson, W. B.; Amer, N. M.; Boccara, A. C.; Fournier, D. App. Opt. 1981, 20, 1333-1344. (32) Weimer, W. A,; Dovichl, N. J., submitted for publication in J . Appl. Phys (33) Bevington, P. R. "Data Reduction and Error Analysis for the Physical Sciences"; McGraw-HIII: New York, 1969; Program 11-5. (34) Long, G. R.; Bialkowski, S. E. Anal. Chem. 1985, 57, 1079-1083 (35) Carter, C. A,; Harris, J. M. Appl. Spectrosc. 1983, 37, 166-172 (36) Nolan, T. G.;Dovichi, N. J. Anal. Chem., in press.
.
RECEIVED for review April 29, 1985. Accepted June 27, 1985. This work was funded by the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the National Science Foundation, Grant CHE-8415089.
Pulsed Laser Desorption for Resonance Ionization Mass Spectrometry N. S. Nogar* and R. C. Estler'
Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 C. M. Miller
Isotope and Nuclear Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
A pulsed Nd:YAG laser (1.06 pm) was used to desorb tantalum atoms from a sample fllament malntalned at a base temperature of 1200 OC; these atoms were subsequently detected by pulsed resonance loniratlon and tlme of flight mass spectrometry. Arrival tlme dlstrlbutlons were obtained by varying the tlme delay between the desorption laser and a probe (lonizatlon) laser. Ground-state tantalum atoms were found to have a most probable hydrodynamlc velocity of -1.0 X I O 5 cm/s for desorptlon pulse lntensltles of I O * W/ cm2. The thermal velocity dlstrlbution, however, was characterized by a width of =2.3 X I O 4 cm/s, and the Internal (electronic) excnation temperature was T < 2100 K. Overlap of the atom pulse and the probe laser pulse was excellent, with 510% effectlve duty cycle. Thls should Increase substantially the sensitivity posslble with resonance Ionization mass spectrometry.
Resonance ionization mass spectrometry (RIMS) is rapidly becoming an established field, with well-defined strengths, weaknesses, and areas of application. A significant fraction of published work has dealt with the use of conventional thermal (hot filament) sources for RIMS (1-18). These sources have a number of significant attributes, including reproducibility, stability, and a substantial literature of work in surface ionization upon which to draw (19). A significant drawback stems from the common use of pulsed lasers for RIMS analyses. The short pulses and relatively low repetition rates of most pulsed lasers, coupled with the constant evaporation of sample from thermal sources, results in a low (typically 110-4) effective duty cycle, and a substantial loss (nonuse) of analyte. This inefficiency can be a substantial burden when the sample is difficult to obtain or when sample size must be Permanent address: Department of Chemistry, Fort Lewis College, Durango, CO 81301.
minimized, as for radioactive materials (18). A number of solutions to this problem have been explored. One possibility is the use of continuous wave (CW) lasers for the resonance ionization process (14, 20). This is a viable alternative, exhibiting average detected currents as great as lo4 those of pulsed lasers, though it may be restricted to elements of rather low ionization potential (IP 5 8.8 eV) due to the limited spectral range available to CW lasers. A second alternative is a pulsed sample evaporation process. This has been demonstrated for laser ablation (21-23), for pulsed sputtering (24-28), and most recently for pulsed thermal sources (11). In the latter case, a thermal pulse is generated by supplying a current pulse to the resistively heated filament. When this pulse is supplied in addition to a continuous current which holds the sample at a temperature slightly below that needed for efficient evaporation, sample vapor pulse widths as narrow as 1 ms may be achieved (11). In this paper, we report on the use of an infrared laser to induce pulsed desorption (29-35) from a metal filament. Atom pulse durations as short as -3.5 ps duration are observed. Preliminary measurements suggest that the desorption process produces a nonthermal distribution of desorbed species with an internal (electronic) temperature somewhat lower than the hydrodynamic translational energy. EXPERIMENTAL SECTION Pulsed evaporation and ionization took place in the source region of a time-of-flight mass spectrometer described previously (7). Figure l a depicts a schematic of this region, while Figure l b describes the timing sequence used. Briefly, the Q-switch synch-out from the Nd3+:YAGdesorption laser (Quanta Ray/ Spectra Physics, DCR lA, Mountain View, CA) was used to master the timing sequence. The laser output, initially frequency doubled to provide a visible beam, was focused onto the sample filament with the aid of beam-steering prisms and a 250 mm focal length lens. Once alignment was completed, the frequency doubler was detuned, and residual 532-nm light removed with a dichroic mirror, leaving only (299%) the fundamental 1.06-pm beam. The 10-ns pulses from this laser typically arrived at the filament 100 ns
0003-2700/85/0357-2441$01.50/00 1985 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985
SAMPLE FiLAMENT
'
0-SWITCH SAMPLE PLUME IO 6 ~ m PULSE ARRIVAL
~
~
FLIGHT TUBE
I I)', (I _ _ _ _ _ _ _ _ I ; ', I I LOYE LASER
') /'
BOXCAR
k l 4 5 pr
GATE
I06pm
a
b
Flgure 1. (a) Schematic of the mass spectrometer source region, showing location of sample, heating and interrogation lasers, and ion extraction region. (b) Timing sequence for optical time-of-flight measurements.
after the Q-switch synch-out trigger. Beam size at the sample was approximately 0.12 cm in diameter, and pulse energies 535 mJ were used, producing intensities 5 3 X lo8 W cm-2. These values must be regarded as rough approximations, since the "doughnut" shaped output of the desorption laser produces a nonuniform intensity distribution. The sample was a 0.60 cm X 0.075 cm X 0.0025 cm tantalum filament, so that the 1.06-wm pulse overfilled it. The filament was maintained at a base temperature of -1200 "C by resistive heating. This was slightly below the temperature needed to produce a detectable resonance ionization signal without laser desorption. Pulses from an excimer-pumped dye-laser (17 ) were used to effect ionization. Tantalum ions were generated from ground-state tantalum atoms via a two-photon transition to the 4F3,2(43964 followed by ionization using a third photon cm-l) excited state of the same color (454.92 nm). The excimer-laser typically produced 100-mJ,15-11s pulses and was triggered after a variable delay (Berkeley Nucleonics Model 8010, Berkeley, CA). Dye laser pulses (10 ns, 2 mJ) were spatially filtered and loosely focused (0.07 cm diameter) through the ionization region. The laser beam propagated parallel to the long axis of the sample filament, at a distance of 1.6 cm. The extraction field was 110 V/cm, perpendicular to the paths of both laser beams. This was followed by a drift tube at -450 V; the flight path was 0.4 m. An ion lens between the extractor and flight tube maximized transmission of ions to the detector and minimized transmission variation due to ion velocity components perpendicular to the flight tube. Detection electronics consisted of a channel electron multiplier, a preamplifier, an amplifier, and a boxcar averager whose gate was set at a fixed delay (corresponding to the Ta+ flight time) after the excimer laser synch-out pulse. The boxcar gate width was adjusted so as to encompass only a few mass units about that of tantalum (181 u). An oscilloscope was used to observe simultaneously the arrival of the desorption and ionization laser pulses, the ion gignal, and the boxcar gate.
(In,
RESULTS AND DISCUSSION The pulsed desorption described in these experiments appears to be a quasi-thermal rather than a plasma-driven process. Initial evidence of this fact is provided by the absence of primary ions produced solely by laser impact as opposed to the presence of such ions in the well-known laser microprobe (33). In addition, the estimated intensity used for desorption in the current experiment, lo8 W/cm2, falls within the range observed for thermal desorption in previous experiments (34). Primary ions could be observed at somewhat higher intensities, c 5 X lo8 W cm-2, by pulsing the extraction field on several microseconds after the arrival of the infrared laser pulse. This allowed transit of primary ions to the extraction volume. At these higher intensities, time-dependent changes in metal atom yields and ablative craters could be observed. In current experiments, we observed only monatomic tantalum, and a relatively small amount (55%)of TaO, though our ionization method would not necessarily detect the presence of clusters or other molecules. Figure 2 shows the dependence of the detected Ta+ signal on the time delay between the desorption and interrogation
DELAY TIME (ps)
Figure 2. Temporal profile of ground-state tantalum atom pulse produced with 1 X 10' W/cm2 irradiation intensity. Distance from sample 1.6 cm. Error limits are typically f 1 to interrogation region was ion/pulse.
-
pulses. The data points represent a 1-2 min (750-1500 shot) average at each delay time. The curve is simply a smooth interpolation of the data and is not meant to indicate a theoretical fit. An analysis of the desorption process allows interpretation of the mean arrival time, as well as the distribution of arrival times. We wish to calculate the number density of atoms in the detection volume, which is a distance 1 from the target, as a function of time. In general, it must be recognized that faster atoms will arrive at the interrogation volume before slow ones and that atoms leave the surface at different times during the temperature transient. The flux density of atoms leaving the surface a t time T in the velocity range ij to ij do is given by (36, 37)
+
F ( u ) = T(r)G(s)ucos 0 do = T(T)F(u,O,$)U~ cos 0 sin 0 du d0 d$
(1)
where T ( T is ) the time evolution of atoms leaving the surface, G(ij) is the velocity distribution, F(u,0,4) is the (presumed isotropic) speed distribution, 0 is the polar angle defined with respect to the surface normal, and q5 is the azimuthal angle defined with respect to the dye laser axis (Figure 1). If t is the interrogation time and T is the time of emission from the surface, then v = l/(t - T ) and du = 1 dT/(t - r)', where 1 is the distance from the surface to the interrogation zone. The flux density can be converted to a time-dependent number density by dividing by the velocity, transforming coordinates, and integrating over all emission times (38) N(l,t,d,$) dl d0 d$ dt =
P sin 0 cos 0 dl d0 d$ dt
Jmm
T(T)F(l,t - ~ , d , $ ) dT (t - r)4
(2)
Here we have substituted the speed-dependent volume element on the left side, and F(l,t - T $ , + ) is obtained from the speed-dependent function by substituting the relationship shown above. In general, eq 2 cannot be reduced to a closed-form relationship between N and the functions T and F. In our case, several simplifying assumptions can be made. First, our thermal emission pulse is very brief compared to the time between emission and interrogation (see below) and may be treated as a delta function, so that N(l,t,O,$) dl d0 d$ d t = F(t7t70'4) P sin 0 cos 0 dl d0 d+ d t (3) t4
Next, we introduce a function E to represent the efficiency
ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985 2443
of signal generation. In general, this parameter will be a function of many variables, including laser intensity, position in the extraction field, and detector quantum efficiency. E will be a function of spatial coordinates, but not of time. Thus, the signal detected after a delay time between t and t + dt, and a distance between 1 and 1 - dl from the surface, will be S(Z,t,0,4) dl d t d0 d4 =
Since the analysis geometry is constant, geometric terms may be collected to yield WAVELENGTH (nm)
(5) where we have collected all spatial information into the constant K, Thus, F ( t ) may be obtained directly from Figure 2. Evaluation of the data then yields p, = 1.0 X lo6 cm/s for the hydrodynamic (center of mass) velocity, and Utherm = 2.3 x IO4 cm/s for the kinetic velocity (thermal width). Several features should be noted from Figure 2 and its analysis. The hydrodynamic and thermal velocities are obviously far from equilibrium, the former corresponding to a “temperature” (defined by u2rm/8k)of -8500 K; the latter to a “temperature” of -450 K. Other experiments were performed at varying desorption laser pulse energies, covering the range from the 1 X lo8 W cm-2 used to generate the data in Figure 2, to -4 X lo7W/cm2. The general trend observed is one of decreasing u,, and increasing uthern with decreasing 1.06-pm intensities. Although near-thermal distributions seem to be the norm, at least for physisorbed molecules (30),nonthermal distributions have also been previously observed (39,40). It is not yet possible to determine unequivocally the cause of our nonthermal velocity distributions. Several alternatives appear possible: nonequilibrium interactions on the surface, collisional interactions subsequent to desorption, and potential energy barriers to tantalum atom desorption. Possibilities for surface interactions include thermal focusing effects due to nonuniform illumination by the 1.06-pmbeam, and spallation driven by a reflected shock wave. On the other hand, gasphase collisions are also known to produce very narrow velocity distributions in free jet expansions (41) and may also cool the products of thermal desorption processes. We estimate that the number of laser desorbed neutral species will be only -lo5 per pulse in our equipment (see below), therefore, collisional cooling appears unlikely, although any degree of collisional cooling will depend on the details of the desorption process. In addition, potential barriers to desorption may be “selecting” the high velocity components of a thermal distribution of velocities. Since it is improbable that many adsorbed atoms and/or adsorbed background gas molecules exist at the 1200 “C filament surface, the cause of such a barrier is not readily apparent. Certainly, further study is needed as to which mechanism(s) are responsible for the observed velocity spreads in our experiments. The net analytical result of this nonthermal velocity distribution is that the atom pulse is temporally narrow, so that the effective sampling duty cycle for pulsed laser RIMS is greatly increased relative to continuous thermal desorption. The simplest way to calculate the overlap is to transform the temporal distribution of Figure 2 to a spatial distribution and then calculate the fraction of this spatial distribution that is overlapped by the 0.7 mm diameter probe beam. In order to simplify the calculation, we fit the observed distribution to a Gaussian form. For the data displayed in Figure 2, the spatial fwhm of the atom pulse is -3.2 mm a t the interrogation region. Thus, the temporal fraction addressed by the
Figure 3. Optical spectrum of laser desorbed tantalum, obtained at a 16-ps delay after the desorption pulse. Transitions originating in exclted states are labeled with an asterisk. (See ref 17.)
probe laser will be -0.1 of those passing through the volume swept out by the laser beam, assuming that the ground state atom flux is representative of the total flux of particles leaving the surface. This overlap is considerably greater than that reported previously (II), primarily because the optically generated thermal pulse in the sample filament is much shorter in duration than is the current modulated thermal pulse described previously. A rough calculation of the temporal profile of the thermal pulse (previouslytreated as a delta function) can be obtained from (42) T(t) = [ ~ / ( K c r ) ’ / ~0] l ~ R (r)/r1/’ tdr
+ To
(6)
where 6 = 0.2 is the optical absorptivity, K = 0.6 W cm-l K-l is the thermal conductivity, c = 3.0 J/cm3 is the heat capacity, Tois the base temperature, and R(t) is the temporal profile of the laser pulse. Evaluation of eq 5 suggests a peak temperature of -8000 K and an effective thermal pulse width of -120 ns. Although this is similar to our measured hydrodynamic temperature, the agreement is likely fortuitous, since the “constants” used in the calculation are all temperature dependent. The brief duration of this thermal pulse, relative to that obtainable with resistive heating, is due simply to the fact that a much smaller mass of material is heated, since the surface area addressed is cm2and the thermal penetration depth is cm. As a final observation, we note that the internal energy distribution in this experiment is not dramatically different from that observed previously (17)for a thermal source. Figure 3 shows a spectrum of laser-desorbed tantalum, taken with a 16 1 s delay between the desorption and probe pulses. This figure displays resonance ionization lines originating in both the ground and excited electronic states. An approximate electronic temperature may be determined by comparing the ratio of excited-state originating to ground-state originating line intensities in this work with those obtained previously (17)for a 2100 K thermal source. While there is considerable variation from line to line in the present data (effective temperatures ranging from 1500 to 2000 K), the relative excited state populations observed in this work are generally somewhat lower than those for the thermal source. It should also be pointed out that we have not characterized the internal energy distribution at widely varying delay times. The electronic temperature may vary in an irregular fashion over the pulse duration. A rough calculation of the total ionization probability for the pulsed-desorption/resonance ionization process yields 2 x lo4, with a geometric overlap of 5 x IO-’, a temporal overlap of lO-l, a partition function of 0.5, and an ionization/detection probability for this three-photon process of 10-l. This estimate in turn suggests that the total number of atoms removed from
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Anal. Chem. 7905, 57, 2444-2448
the surface by the laser is lo5per pulse. For sampling small deposits, this should allow both high sensitivity and reasonable statistical precision through repetitive sampling. Registry No. Ta, 7440-25-7. N
LITERATURE CITED (1) Bekov, G. I.; Letokhov, V. S . Appl. Phys. 8 1983, 30, 161-176. (2) Bekov, G. I.; Letokhov, V. S.;Radaev, V. N.; Baturin, G. N.; Egorov, A. S.;Kursky, A. N.; Narseyev, V. A. Nature (London) 1984, 372, 748-750. (3) Donohue, D. L.; Young, J. P.; Smith, D. H. Int. J . Mass Spectrom. Ion Phys . 1982, 43,293-307. (4) Donohue, D. L.; Young, J. P.; Smith, D. L. Int. J . Mass Spectrom. Ion Processes 1984, 56,307-319. (5) Donohue, D. L.; Smith, D. H.; Young, J. P.; McKown, H. S.;Pritchard, C. A. Anal. Chem. 1984, 56, 379-381. (6) Donohue, D. L.; Young, J. P.; Smith, D. H. Appl. Specfrosc. 1985, 39, 93-97. (7) Downey, S. W.; Nogar, N. S.;Miller, C. M. Int. J . Mass Spectrom. Ion Processes 1984, 61 337-345. (8) Downey, S. W.; Nogar, N. S.;Miller, C. M. Anal. Chem. 1984, 56, 827-828. (9) Fassett, J. D.; Travis, J. C.; Moore, L. J.; Lytle, F. E. Anal. Chem. 1983, 55, 785-770. (10) Fassett, J. D.; Moore, L. J.; Travis, J. C.; Lytle, F. E. Int. J . Mass Spectrom. Ion Processes 1983, 54,201-216. (1 1) Fassett, J. D.; Moore, L. J.; Shideler, R. W.; Travis, J. C. Anal. Chem. 1984, 56, 203-206. (12) Fassett, J. D.: Powell, L. J.; Moore, L. J. Anal. Chem. 1984, 56, 2228-2233. (13) Moore, L. J.; Fassett, J. D.; Travis, J. C. Anal. Chem. 1984, 56, 2770-2775. (14) kller, C . M.; Nogar, N. S. Anal. Chem. 1983, 55, 1606-1608. (15) Miller, C. M.; Nogar, N. S.;Gancarz, A. J.; Shields, W. R. Anal. Chem. 1982. 54. 2377-2378. (16) Moore, L.' J.; Fassett, J. D.; Travis, J. C. Anal. Chem. 1984, 56, 2770-2775. (17) Nogar, N. S.;Downey, S. W.; Mlller, C. M. Anal. Chem. 1985, 57, 1144-1147. (18) Young, J. P.; Donohue, D. L. Anal. Chem. 1983, 55,88-91. (19) Smith, D. H.; Walker, R. L.; Carter, J. A. Anal. Chem. 1982, 54, 827A-830A. ~
(20) Miller, C. M.; Nogar, N. S . Anal. Chem. 1983, 55,481-488. (21) Beekman, D. W.; Callcott, T. A.; Kramer, S.D.; Arakawa, E. T.; Hurst, G. S.;Nussbaum, E. Int. J . Mass. Spectrom. Ion. Phys. 1980, 34, 89-97. (22) Mayo, S.;Lucatorto, T. B.; Luther, G. G. Anal. Chem. 1982, 54, 553-556. (23) Williams, M. W.; Beckman, D. W.; Swan, J. B.; Arakawa, E. T. Anal. Chem. 1984, 56, 1348-1350. (24) Becker, C. H.; Gillen, K. T. Anal. Chem. W84, 56, 1671-1674. (25) Kimock, F. M.; Baxter, J. P.; Pappas, D. L.; Kobrin, P. H.; Winograd, N. Anal. Chem. 1984, 56, 2782-2791. (26) Kimock, F. M.; Baxter, J. P.; Winograd, N. Surf. Sci. 1983, 724, L41L48. (27) Parks, ?. E.; Schmitt, H. W.; Hurst, G. S.;Fairbank, W. M., Jr. Thin Solid Films 1983, 708,69-78. (28) Winograd, N.; Baxter, J. P.; Kimock, F. M. Chem. Phys. Lett. 1982, 68,581-584. (29) Burgess, D. R.; Hussla, I.; Stait, P. C.; Viswanathan, R.; Weitz, E. Rev. Sci. Instrum. 1984, 55, 1771-1776. (30) Burgess, D., Jr.; Viswanathan, R.; Hussla, I.; Stair, P. C.; Weitz, E. J . Chem. Phys. 1983, 79,5200-5202. (31) Hurst, G. S.;Payne, M. G.; Phillips, R. C.; Dabbs, J. W. T.; Lehmann, B. E. J . Appl. Phys. 1984, 55, 1278-1281. (32) Hall, R. 6.; DeSantolu, A. M. Surf. Sci. 1984, 737,421-441. (33) Cotter, R. J. Anal. Chem. 1984, 56,485A-504A. (34) Dittrich, K.; Wennrich, R. f r o g . Anal. At. Spectrosc. 1984, 7 , 139-198. (35) Ready, J. F. "Effects of High Power Laser Radiation"; Academic: New York, 1971. (36) Peliin, M. J.; Wright, R. B.; Gruen, D. M. J . Chem. Phys. 1981, 7 4 , 6448-6457. (37) Berry, R. S.;Rice, S. A.; Ross, J. "Physical Chemistry"; Wiley: New York, 1980. (38) Olstad, R. A.; Olander, D. R. J . Appl. Phys. 1975, 4 6 , 1499-1508. (39) Comsa, G.; David, R.; Schumacher, 8.-J. Surf. Sci. 1980, 95,L210L216. (40) Husinsky, W.; Bruckmuller, R.; Blum, P.; Viehbock; Benes, E. J . Appl. Phys. 1977, 48, 4734-4740. (41) Levy, D. H. Sclence 1981, 274, 283-269. (42) Cowin, J. P.; Auerbach, D. J.; Becker, C.; Wharton, L. Surf. Scl. i o n , 78,545-564.
RECEIVED for review March 22,1985. Accepted June 24,1985.
Structural Determination of Unsaturated Mycolic Acids by Fast Atom Bombardment and Tandem Mass Spectrometry Analyses of Their Amino Alcohol Derivatives Michel Riviere, Monique Cervilla, and Germain Puzo*
Centre de Recherche de Biochimie et de GBngtique Cellulaires du C.N.R.S., 118, route de Narbonne, 31062 Toulouse Cedex, France
The ethylenlc functlons of the mycollc acids Isolated from M. smegmails were transformed Into amlno alcohols. Thelr analyses by posltlve fast atom bombardment mass spectrom etry allows thelr molecular weight to be unamblguously established from their pseudomolecular Ions. Moreover their MIKE-CID (mass analyzed Ion kinetic energy colllslon induced dlssoclatlon) mass spectra permit the amlno groups borne by the aliphatic chaln and consequently the ethylenic functlons In the natlve molecule lnvestlgated to be located.
Mycolic acids are a-alkyl p-hydroxylated fatty acids of basic structure 1 which are present in the wall of mycobacteria, nocardia, and corynebacteria ( I ) . OH R~
I
- C H - CH- C O ~ H
I
Rl
1: R z , mer0 chain R, , O - ramified chain
Minnikin has shown that mycolic acid alkyl chain lengths, defined by the number of methylene units, permits the identification of some mycobacterial species (2). Daffe et al. recently noted that lipidic analysis allows identification of 22 species of mycobacteria among the 27 investigated (3). Knowledge of the exact structure is required to establish the metabolic pathway of the mycolic acids and also to resolve the problem of mycobacteria taxonomy. Their structures have been mainly elucidated using electron ionization (EI) mass spectrometry on mixtures of mycolic acids. By EI, methyl esters of mycolic acids (I) mainly given two rearranged fragment ions [R2CHO]+-and [R1CH2CO2CH3]+. allowing the identification of R2 and R1 and consequently the determination of the molecular weight. To establish their exact structures, Steck et al. proposed high-performance liquid chromatography (HPLC) for resolving mycolic acid mixtures isolated from M. smegmatis into molecular species (4). An a-mycolic fraction previously studied by Etemadi et al. (5) called B by Steck et al. (5) has been resolved by HPLC according to chain length. Electron ionization (EI) and chemical ionization (CI) mass spectrometry
0003-2700/85/0357-2444$01.50/00 1985 American Chemical Society
