Anal. Chem. 1994,66, 1897-1901
Silica-Fiber Microextraction for Laser Desorption Ion Trap Mass Spectrometry M. E. Cisper, W. L. Earl, N. S. Nogar, and P. H. Hemberger' Chemical Science and Technology Division, Los Alamos National Laboratoty, MS G740, Los Alamos, New Mexico 87545
We have coupled sample collection by solid-phase microextraction on disposable fused silica optical fibers with analysis by laser desorption ion trap mass spectrometry for rapid screening of organic contaminants in complex matrices. Because the silica-fiber probe serves as both the sampling medium and the sample support for laser desorption, traditional methods of sample preparation are eliminatedwith the expected gains in speed and simplicity. Pyrene was the benchmark compound in these experiments but we show that the technique is also applicable to other polycyclic aromatic hydrocarbons (PAHs) and semivolatile compounds, laser dyes, pesticides, and peptides. Derivatizing the silica fiber improves the analyte collectionefficiency, and firing the laser during a ring electrode rf ramp promotes dependable trapping of laser-desorbed ions. We describe a method for combining solid-phase microextraction (SPME) with laser desorption/ionization in an ion trap mass spectrometer (LITMS) for rapid analysis of organics in complex samples. Solid-phase microextraction is a quick, solvent-free extraction technique using disposable fused silica fibers to facilitate analysis of aqueous solutions. It has found application in such diverse areas as the analysis of volatile and semivolatile organic compounds'-" and caffeine in beverage^.^ Both uncoated fibers and chemically modified fibers have been used successfully. In the latter case, the chemically modified fibers have a stationary phase similar to that used in fused silica GC columns.6 Upon exposure of the fiber to a solution, analytes from the aqueous phase begin partitioning to the stationary phase on the fiber. Under the proper conditions (agitation or stirring of the solution), equilibrium extraction of analytes can occur within minutes.' In a typical analysis, thermal desorption from the exposed fiber takes place in the injection port of a gas chromatograph (or on-column), and the desorbed compounds are subsequently analyzed by chromatography4 or gas chromatography/mass spectrometr~.~,* Sample introduction for gas chromatographic analysis using a laser-coupled optical fiber placed inside a capillary GC (1) Arthur, C. L.; Pawliszyn, J. Anal. Chem. 1990, 62, 2145-8. (2) Arthur, C. L.; Killam, L. M.; Buchholz, K. D.; Pawliszyn, J.; Berg, J. R. Anal. Chem. 1992, 64, 1960-6. (3) Arthur, C. L.; Killam, L. M.; Motlagh, S.; Lim, M.; Potter, D. W.; Pawliszyn, J. Environ. Sci. Technol. 1992, 26, 979-83. (4) Louch, D.; Motlagh, S.; Pawliszyn, J. Anal. Chem. 1992, 64, 1187-99. ( 5 ) Hawthorne, S. B.; Miller, D. J.; Pawliszyn, J.; Arthur, C. L. J . Chromatogr. 1992, 603, 185-91. (6) Pawliszyn, J . In Solvent-free extracrion of enuironmcnral samples; Clement, R. E., Siu, K. W. M., Hill, H. H., Jr., Eds.; Lewis: Chelsea, MI. 1992; pp 253-77. (7) Arthur, C. L.; Pratt, K.; Motlagh, S.; Pawliszyn, J.; Belardi, R. P. J . High Resolut. Chromatogr. 1992, 15, 7 4 1 4 . (8) Potter, D. W.; Pawliszyn, J. J . Chromatogr. 1992, 625, 247-55. 0003-2700/94/0366-1897$04.50/0 0 1994 American Chemical Society
column has also been de~cribed.~ The sample was desorbed into the column when light was transmitted through the fiber end upon which a sample had been deposited. Excellent chromatographic separation due to the rapid injection of analyte into the chromatographic flow was reported. Laser desorption mass spectrometry (LDMS) has been applied to a range of challenging analytical problems including peptide sequencing'" and analysis of complex liquid or solid samples.' 1-15 In laser desorption, a high-intensity laser pulse ( I 2 106 W/cm2) interacts with a sample surface to initiate desorption of material from the fixed substrate; dissociation and ionization are among the many processes that may occur in the plasma that is formed above the irradiated surface. The integration of laser desorption and quadrupole ion trap technology can allow laser desorption within the ion trap volume itself.16J7 Recently, such instrumentation was used for the rapid determination of both organic and inorganic ions in complex mixtures through both positive and negative ion detection.15 Direct interrogation of environmental samples by LDMS can be hindered by background or matrix interferences and sample introduction limitations. Selectivity or separation of the analytes can be achieved by chemical extraction but such techniques take time and often generate hazardous solvent waste. In this paper, we report on the combination of laser desorption ion trap mass spectrometry and solid phase microextraction on fused silica fibers, in a method suitable for rapid contaminant screening of environmental samples. The method also has application as a general screening tool for a variety of compounds, perhaps limited only by fiber coating selectivity or fiber surface technology. Direct laser desorption of microextracted material provides numerous advantages as a screening technology. Among these are rapid sample turnaround (from sampling to analysis in 5 minor so), little or no sample preparation, greatly reduced waste generation and disposal, vis-a-vis time- and solvent-intensive extraction methods, and inexpensive and easily prepared (9) Pawliszyn, J.; Liu, S. Anal. Chem. 1987, 59, 1475-8. (10) Yang, L. C.; Wilkins, C. L. Org. Mass Spectrom. 1989, 24,409-14. (1 1) Land, D. P.; Pettiette-Hall, C. L.; Hemminger, J. C.; McIver, R. T,. Jr. Ace. Chem. Res. 1991, 21.42-7. (12) Bjarnason, A.; DesEnfants, R. E., 11; Barr, M. E.; Dahl, L. F.Organometallics 1990, 9, 657-61. (13) Young, C. E.; Pellin, M. J.; Calaway, W. F.; Joergensen, B.; Schweitzer, E. L.; Gruen, D. M. Nucl. Inrrrum. Methods Phys. Res. 1987, 827, 119-29. (14) Muller, J. F.; Berthc, C.; Magar, J. M. Fresenius' Z . Anal. Chem. 1981,308, 312-20. (15) Alexander, M. L.;Hemberger,P. H.;Cisper, M. E.;Nogar,N. S. Anal. Chem. 1993, 65, 1609-14. (16) Hcller, D. N.; Lys, I.; Cotter, R. J.; Uy, 0.M. Anal. Chem. 1989,61,1083-6. (17) Glish, G. L.; Gocringer, D. E.; Asano, K. G.; McLuckey, S. A. Int. J . Mass Specrrom. Ion Processes 1989, 94, 15-24.
Ana!ytical Chemistry. Voi. 66, No. 11, June 1, 1994
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sampling material. The method also offers the convenience of being both a sample collection and sample introduction technique. Fused silica fiber technology may also provide a means to safely transport or archive samples. The generation of laser-desorbed ions requires some attention to trapping considerations. Ions created outside the ion trap or at the interior electrode surfaces of the trap meet a substantial effective potential barrier to the trap interior in the form of the applied ring electrode radio frequency (rf) voltage.I8J9 The potential barrier inside the trap may prove insurmountable for the injection of low-energy ions while higher energy ions may pass through the barrier completely. Although Debye shielding20 and ion-buffer gas collisions21 may increase the probability of trapping, the former by altering the electric field to which the ion is exposed and the latter through damping of ion motion and reduction of kinetic energy,22other measures are sometimes required to improve trapping efficiency. We have found that the coordinated timing of laser desorption with a rising rf potential on the ring electrode significantlyincreases the probability of trapping ions desorbed from a fiber probe in the Finnigan ITMS. Enhanced sensitivity for laser desorbed compounds using this technique was initially observed with trimethylphenylammonium iodide on a stainless steel probe tip and was reported recently.18 It is theorized that laser desorbed ions, formed near the surface of the ring electrode, are able to enter the trapping well when the rf potential is low; their escape is prevented by the subsequent increase in rf and a concomitant deeper trapping well. This laser ablation/rf technique proved to be essential for the detection of material collected by fiber microextraction. A filtered noise field (Teledyne MEC/Hitachi) applied during laser desorption can also improve the detection of selected ions by ejecting unwanted background ions (i.e., reducing chemical noise) We report here on studies of microextracted pyrene and other PAHs and peptides. Microextracted pesticides, laser dyes, ferrocene, trimethylphenylammonium iodide, and p dichlorobenzenewere also detected by laser desorption of fused silica fibers. We also describe the use of surface derivatization of the glass fiber to improve sample uptake and chemical specificity of the microextraction process. .23924
EXPERIMENTAL SECTION The laser/ion trap configuration used for these experiments has been outlined in detail p r e v i o u ~ l y and ~ ~ - is~ ~only summarized here. A XeCl excimer laser (15-ns pulse duration, 308 nm) was used to supply energy (13 mJ/pulse) for laser desorption. The beam was focused (30 cm lens) through a 25" hole in the ring electrode of a Finnigan ion trap mass spectrometer as described previously. (18) Eiden, G. C.; Cisper, M. E.; Alexander, M. L.; Hemberger, P. H.; Nogar, N. S . J . Am. SOC.Mass Spectrom. 1993, 4, 706-709. (19) Major, F. G.; Dehmelt, H. G. Phys. Reu. 1968, 170; 91-107. (20) Beu, S. C.; Hendrickson, C. L.; Vartanian, V. H.; Laude, D. A. J. Znt. J. Mass Spectrom. Zon Processes 1992, 113, 59-79. (21) Hemberger, P. H.; Nogar, N. S.; Williams, J. D.; Cooks, R. G.; Syka, J. E. P. Chem. Phys. Lett. 1992, 191,405-10. (22) Reiser, H. P.; Julian, R. K., Jr.; Cooks, R. G. Znt. J. Muss Spectrom. Zon Processes 1992, 121, 49-63. (23) Garrett,A. W.;Cisper, M. E.;Nogar,N. S.;Hemberger,P. H. Rapid Commun. Mass Spectrom. 1994, 8, 174-178. (24) Goeringer, D. E.; Asano, K. G.; McLuckey, S. A.; Hoekman, D.; Stiller, S. W. Anal. Chem. 1994,66, 313-318.
1898 Analytical Chemistry, Vol. 66, No. 11, June 1, 1994
I
I End Cap
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I
electrode
Holder
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Figure 1. Fiber probeking electrode schematic, cut-away view. The laser beam (308 nm, 15 ns, 1 3 mJ pulses) enters from the left and is focused at the fiber surface. The laser power at the surface is estimated to be 1 3 X IO6 W/cm2 (145 mJ/cm2).
A)
Scan Function RF voltage
Fire Laser
timef zero
Time
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Ring Electrode RF Voltage, "near" T=O
Vmax
- 500 psec time zero
laser fires
Figure2. Schematic of scan function, laser/rf timing. For most efficient trapping, and highest signal, the laser was fired during the rise of the rf trapping potential.
The disposable fiber was supported by means of a centered ~ 1 . mm 1 diameter hole drilled in a 304L stainless steel tip; Figure 1 shows a schematic view of the fiber probe/ring electrode assembly. This support allowed insertion of the fused silica fiber through a gate valve and into a hole on the ring electrode opposed to the beam entrance. Fiber length (21-23 mm) was controlled to ensure that the tip was approximately flush with the inner surface of the ring electrode. Material extracted on the fiber tip surface was desorbed by the impinging laser beam. Helium was used as a buffer gas at an uncorrected gauge pressure of (1-2) X Torr for all experiments. Laser power at the desorption site was estimated to be 5 3 X lo6 W/cm2 with an approximate spot size of 1 X 2 mm. Figure 2 depicts a schematic view of the scan function used in this work. The laser was triggered by a digital delay generator (SRS DG 535) which was triggered synchronously with the scan function at a zero crossing of the rf generator as the rf began to rise. If the synchronized trigger corresponding to the initia'l rf rise is called to, the laser was fired 40-70 ps later at either a random
or selected phase angle of the ring electrode rf. The rf potential was found to rise with a =175 ps average time constant, reaching Vmax 4 0 0 ps after desorption. The ions were stored for 10-1 5 ms after desorption before the analytical ramp. The signal was found to depend on both the rf amplitude and phase for material desorbed from both the fiber probe and the stainless steel probe tip; details are reported e l ~ e w h e r eTwo .~~ to ten microscans were averaged for each scan. Preparation of the optical fiber as it is conventionally used for thermal desorption in a GC has been described by other auth0rs.l Briefly, the sampler used in our experiments consists of a fused silica fiber having a core diameter of 0.6 mm and a total diameter with cladding and jacket of 1.04 mm (3M Power-Core Optical Fiber, Part No. FT-600-UMT). In preparation for these experiments, the fiber was cut to length and the jacket material was stripped from one-half to threequarters of the fiber’s length using a wire stripper. (Jacket material left on one end of the fiber provided a snug fit in the hole of the stainless steel tip.) Acetone was used to strip the (proprietary) cladding from the core. The fiber tip was lightly wet sanded with emery paper and the exposed fiber core was cleaned using methanol. Total fiber probe preparation took less than 5 min. Several laser desorption mass spectra of “blank” fibers prepared in this manner were acquired to check for possible interferences. In general, the absolute signal levels were small, 5 5 0 counts full scale, and the distribution of signal across the spectrum fluctuated from shot to shot. In some instances, small signals due to residual cladding or surface material were sometimes observed on “blank” fiber scans, but after microextraction analyte ions were usually significantly greater than background ions. A chemical modifier was applied in some instances to significantly improve sample uptake and signal stability. Fibers were prepared with a chemically derivatized surface by reaction with trimethylsilyl chloride. The exposed glass surface was first cleaned by dipping in 30% H202 solution followed by a rinse in 3 M HCl. They were then dried at -60 OC for 16 h. The dried fibers were then dipped in a 10% (v/v) solution of trimethylsilyl chloride in dry methanol. They were again dried at 60 OC for about 2 h and the reaction with the silylating agent was repeated by dipping the fiber in the 10% trimethylsilyl chloride solution again. Pyrene and anthracene solutions were prepared, at concentrations ranging from 1 ppb to 10 ppm in methanol, from Supelco standards (4-0082 and 4-0076). Weighed soil samples were spiked with the two PAH solutions to achieve the desired concentration in soil. The peptide leucine-enkephalin (Sigma) was dissolved in a 1:l mixture of HPLC H20 and methanol and combined with a solution containing the matrix compound, 2,5-dihydroxybenzoic acid or DHB (Aldrich), which was similarly dissolved in 1:l HzO/methanol. The two solutions were mixed to obtain 1.4 X M leucine-enkephalin and 1.7 X lo-* M DHB. Sample solutions were placed in small vials. The prepared fiber, fitted in the stainless steel tip, was dipped in solution to an estimated depth of 0.3-1 cm with minimal stirring or agitation. To screen spiked soil samples, 1 mL of methanol was added to the vial containing 0.5 g of soil (5-50 ppm PAH (25) Eiden, G. C.; Garrett, A. W.; Cisper, M. E.; Nogar, N . S.; Hemberger, P. H. Submitted for publication in Int. J . Mass Specrrom. Ion Processes.
concentration in soil) and thevial was shaken. Fiber exposures for the soil sampling experiments were limited to 3 min.
RESULTS AND DISCUSSION Initial experiments were aimed at characterizing response and suitability of the method. Trimethylphenylammonium iodide, a compound used in our laboratory to characterize laser desorption on a stainless steel probe, was easily detected on a fiber probe, even without synchronization of the rf rise with the laser firing. Desorption occurred at steady-state rf conditions on the ring electrode (Le., the ring electrode voltage was held constant at 380 V&pk when the laser fired). Efficient trapping and persistent detection of other microextracted compounds, such as pyrene, anthracene, and p-dichlorobenzene, were not observed with desorption during steady-state rf conditions. With proper coordination of the laser firing and the ring electrode rf step,18microextracted material was reliably detected with acceptable signal levels. Subsequent experiments were performed by exposing fibers to solutions containing compounds known to absorb at the laser wavelength we were using, 308 nm. Among these compounds were Coumarin 500, a laser dye (MW 257), ferrocene (MW 186), and p-dichlorobenzene (MW 146). In all cases, the parent ion or the protonated parent ion was observed, along with meaningful fragment ions. In some cases, such as the leucine-enkephalin spectrum discussed later and the malathion spectrum, parent-adduct ions were detected. Polycyclic aromatic hydrocarbons were selected for more controlled study after these confirmatory experiments. Concentrations and microextraction exposure times were the parameters of interest. Desorption of fiber probes exposed to 1 to 10 ppm solutions of pyrene generally resulted in a pyrene signal at m / z 202 which persisted for up to a hundred laser shots, Figure 3a. Typically, irradiation of a fresh sample produced peak signals of several hundred counts; after a hundred laser shots, the peaksignal fell to less than one hundred counts per shot. This is consistent with typical surface depletion rates for the laser intensity used. A rough estimate of sample size suggests that the amount of sample delivered to the ITMS is sufficient for screening and, in some cases, for more detailed analysis. Because fused silica has been used extensively for microcolumn chromatography and in quartz crystal microbalances, its surface properties are reasonably well characterized. For a smooth quartz surface, the maximum adsorbed material is typically a monolayer or b i l a ~ e r . ~Even ~ . ~for ~ a roughened surface, the extent of adsorption is unlikely to average greater than one to five layers, though it will clearly be dependent on the analyte, solvent, temperature, and other factors. This would give a maximum surface coverage 5 3 x 10-8 g/cm2 for molecules having an average mass of 100 a.u. Since the area of our fiber tip is 3 X cm2, the total sample size is 5 1 x 10-10 g, and since the ion signal is reproducible for 1100 shots, the amount desorbed per shot is
