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Anal. Chem. 1989, 6 1 , 1530-1533
Supersonic Jet Spectrometry of Chemical Species Laser Ablated from Organic Polymers T o t a r o Imasaka, Kouji Tashiro, a n d Nobuhiko Ishibashi*
Faculty of Engineering, Kyushu University, Hakozaki, Fukuoka 812, Japan
Laser abiation/supersonic jet spectrometry Is applied to several organk polymers. For sample ablation, a first dye laser beam (337.5 nm) is focused onto the polymer surface. A second synchronized tunable dye laser excites ablated chemkai species in a supersonic jet. A sharp spectral feature (7.3 cm-’) is observed in the excitation spectrum for the polystyrene sample, which is assigned to origlnate from a styrene monomer. The signal is much weaker for acrylonltrUe/butadlene/styrenecopolymer (ABS resin), and no peak Is observed for polycarbonate containing no styrene segments. The abiated chemical species is localized to 2.4 mm (4 ps) in the jet pulse. Sufficient rotational and translatlonal cooiings provide good selectivity and sensltivlty, by spectral narrowing and sample focusing in the jet pulse.
Various methods have been developed for analysis of nonvolatile macromolecules such as organic polymers. Recently laser microprobe mass analysis (LAMMA or LA/MS) gained its importance because of its capability for microanalysis of the polymer surface (1,Z). Performance has further been improved by using a Fourier transform technique; it allows direct determination of the chemical formula for ejected ions, because of its ultrahigh mass resolution (3-5). However, this method still has some difficulties in the analysis of samples containing many chemical species with similar structures, such as isomers. Laser ablation (desorption) supersonic jet (LA-SSJ) spectrometry has recently been developed for analysis of nonvolatile or thermally labile molecules. This method is applied to spectroscopic studies of metal clusters such as Cuz (61, Moz (7),A13 (8),etc., inorganic molecules such as Ga,As, (9),Sicz (IO),etc., and organic molecules such as naphthalene (11). Furthermore, it is also used for determination of biological molecules such as amino acids, peptides, and tryptophan analogues (12-1 7). The observed line width is so narrow that even rotational isomers can be resolved. Some other molecules such as insulin have been measured by supersonic jet spectrometry, but no advantage of spectral selectivity has been demonstrated (18-27). In this study we first apply laser ablation supersonic jet spectrometry to organic polymers. The present approach provides a great advantage for analysis of constituents in a polymer material, because of its good selectivity, given by a narrow line width due to rotational cooling, and high sensitivity, given by sample focusing in the jet pulse due to translational cooling. The process of monomer ablation from a polymer surface is also discussed in this study. EXPERIMENTAL SECTION Apparatus. A block diagram of the experimental apparatus is shown in Figure 1. A structure of the nozzle is essentially identical with the one reported (6) (whose structure is schematically illustrated in Figure 5 ) . A rotating polymer rod sample
* Author to whom correspondence should be addressed.
is located 6.5 mm away from an orifice of the pulsed nozzle (0.8 mm i.d.). After an argon gas is injected into a nozzle throat (1.5 mm i-d.),a first dye laser for sample ablation (Lambda Physik, EMGlOQMSC,FL2002, p-terphenyl, 2 mJ) is fired and focused by a lens (30 cm focal length) onto the polymer surface through a hole (2.5 mm i.d.). The ablated chemical species is entrained into a stream of argon and is succeediily expanded into a vacuum from the nozzle throat. The distance between the sample and the outlet of the nozzle throat is 6.5 mm. The molecule in the supersonic jet is excited by a second dye laser (Quantel, YG581C, TDL50, rhodamine 6G, UVX-2, DCC-3,3 mJ). The laser beam is focused by a lens (30 cm focal length) at 15 mm away from the point of sample ablation. Fluorescence from the molecule in the jet is collected by a lens (10 cm focal length) and measured by a monochromator (Jasco,CT-100) equipped with a photomultiplier (Hamamatsu, R928). The signal is measured by a boxcar integrator (NF Circuit Design Block, BX-530A) and is displayed by a strip chart recorder. The vacuum system used is described in detail elsewhere (28). Sample. Polystyrene beads supplied from Wako Pure Chemical Industries (n = 1600-1800) were melted in the test tube (10 mm id.), and a brass or aluminum rod (8 mm diameter) was inserted coaxially to fix as a holder. After polystyrene was cooled, the test tube was removed. The polystyrene rod was re-formed by sandpaper to a rod diameter of 10 mm. Other polymers such as acrylonitrile/butadiene/styrene copolymer (ABS resin) and polycarbonate were purchased as 10-mm rods from Sanko Plastic Co. The sample rod was rotated and translated through a hole (10 mm i.d.) located just below the nozzle throat to expose a new surface by using a screw combined with a speed control motor (Oriental Motor, 21J 3RGA-A2). The rotating speed was changed from 0.03 to 6 rpm by using two speed-reduction gears (Oriental Motor, 2GA lOXL, 10 times; 2GA 60K, 30 times). At this speed range the sample lasted from 13 min to 40 h, depending on the rotating speed used. The reagent of styrene monomer was obtained from Wako Pure Chemical Industries, which was used without further purification. RESULTS A N D DISCUSSION Wavelength of Ablation Laser. For recording an absorption spectrum of polystyrene, polymer beads were melted on a microscope cover glass to form a thin film (thickness, 1 mm). Absorbance of the polystyrene sample increased with decreasing the wavelength, gradually from 400 nm and more rapidly from 300 nm. This is due to light absorption by benzene rings in polystyrene. In the present study the wavelength of the dye laser for ablation is adjusted to 337.5 nm, which is the shortest wavelength available for the excimer-laser-pumped dye laser. Threshold for ablation depends on the wavelength of the exciting laser. It is reported that the signal intensity increases by a factor of 50 by changing the wavelength from 355 nm to 266 nm for tryptophan, which has a strong absorption band at 266 nm (29). Probably, more efficient sample ablation might be achieved by using the fourth harmonic of a NdYAG laser (266 nm) or a C 0 2laser (10.6 Hm), but the present wavelength (337.5 nm) is used because of its convenience in this study. Polystyrene. A supersonic jet excitation spectrum measured for a sample of polystyrene is shown in Figure 2A. The sharp spectral feature is similar to that observed for a styrene monomer, as shown in Figure 2B, measured by conventional
0003-2700/89/0361-1530$01.50/0@ 1989 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 14, JULY 15, 1989
Monochromator
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I
A Dye Laser
Pulse Nozzle
Lens
ABS
1
Sample Rod
Prism
d'
Figure 1. Block diagram of experimental apparatus.
PC -
I
0 287.0 287.5 286.5 Wavelength (nm)
Figure 4. Supersonic jet excitation spectrum for polystyrene (PS), acrylonitrile/butadiene/styrene copolymer (ABS), and polycarbonate (PC).
i 282
284
286
288
Wavelength (nm) Figure 2. Supersonic jet excitation spectrum: (A) chemical species ejected from polystyrene surface by laser vaporization; (6) styrene monomer introduced by conventional pulsed supersonic jet nozzle. The fluorescence wavelength is adjusted to 295.9 nm (slit width, 2.4 nm).
300 Wavelength (nm)
!90
310
Figure 3. Supersonic jet fluorescence spectrum: (A) polystyrene; (6) styrene monomer. The excitation wavelength is adjusted to the largest peak in Figure 2.
supersonic jet spectrometry. The present result clearly shows that styrene monomers are ablated from the surface of polystyrene. This is consistent with reported efficient ablation of the styrene monomer from polystyrene by ArF laser (193 nm) and KrF laser (248 nm) excitations (30, 31). This assignment is further confirmed by measuring the fluorescence spectrum, as shown in Figure 3. Peak-to-peak correspondence between the spectra for styrene ablated from polystyrene and styrene monomer is excellent. It is reported that other chemical species such as CsH6and C4Hzare also ejected from the polystyrene surface by ArF laser and KrF laser excitations (30,31). The fragments might be detected by changing the wavelengths for excitation and fluorescence detection, though such experimental works have not been performed in this study because of limited tunability of the laser. Other Polymers. Fluorescence excitation spectra for three polymers, including polystyrene, are shown in Figure 4. Polystyrene consists of styrene segments, so that it gives the
largest signal. The ABS resin, partly consisting of styrene segments, gives a smaller signal. On the other hand no signal peak is observed at all for polycarbonate, containing no styrene segments. Thus present spectrometric technique is useful to discuss chemical constituents of the polymer material without any pretreatment. Sensitivity. The signal intensity was recorded by stopping sample rotation for polystyrene. The signal intensity decreased to zero in 50 s, and the depth of the hole reached l mm. The repetition rate of the ablation laser is 20 Hz, so that a 1-pm sample is ablated every shot on the average. The sample ablated by a single laser shot was calculated to 1 ng, assuming a beam diameter of 50 pm a t the polymer surface. The signal-to-noise ratio is about 5 in the excitation spectrum, then the detection limit is estimated to a subnanogram range. This is similar to the values (0.056-1 ng) reported for carbobenzoxy-derivatized peptides measured by laser ablation supersonic jet/multiphoton ionization mass spectrometry (LA-SSJ/MPI-MS); this work is performed under incomplete rovibronic cooling and a wavelength-fixed laser (266 nm) with a large pulse energy (- 10 mJ) is used for multiphoton ionization (32). The present detection limit is poorer than picogram and femtogram detection limits obtained for phenylthiohydantoin and protoporphyrin IX dimethyl ester by nonjet LA-MPI-MS (33-35). Needless to say, the present method has a distinct advantage over nonjet LA-MPI-MS with respect to spectral selectivity, given by efficient rotational cooling as shown in the following section. Rotational Cooling. The signal peak for 0-0 transition of styrene was more carefully measured for the sample of polystyrene. The spectral line was slightly split to two peaks and an overall line width was 7.3 cm-', which was contrast to a line width of 3.6 cm-' observed for the styrene monomer measured by using a conventional supersonic jet nozzle. Because of a rather poor signal-to-noise ratio, quantitative discussion is difficult, but a slightly broader line width observed for polystyrene might be due to inefficient rotational cooling caused by the structure of the nozzle throat or by the elevated temperature with laser heating. However, it should be emphasized that the spectrum for polystyrene is completely resolved and rovibronic cooling is sufficient for demonstration of spectral selectivity in supersonic jet spectrometry. Translational Cooling. In laser ablation, the sample is immediately heated (lo-* K s-l) in the nanosecond range (35, 36). Then, it may be possible to prepare the sample molecule to a well-localized region in a jet pulse. If so, it is quite useful to focus chemical species to a limited region to enhance the signal intensity. Figure 5 shows dependence of the time be-
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1989
Delay Time (US) Figure 5. Dependence of time between vaporization and excitation lasers on fluorescence intensity. The separation ( x ) between the
vaporization and excitation lasers is 15 mm. tween ablation and excitation laser pulses on the fluorescence intensity. The signal increases rapidly after 25 ys from the ablation laser pulse, and it gradually decreases in 4 ys. The average velocity of the jet is calculated to 600 m/s (15 mm/25 p s ) , corresponding to a pulse width of 2.4 mm (600 m/s X 4 ys) in the jet. The mach number, M , in supersonic jet expansion is calculated by
M = A(X/D)2/3
(1) where A (=3.26) is constant, X is the distance between the nozzle and the sample detection position, and D is the nozzle diameter (37). The ratio of the molecular velocity in the jet, u , and the most probable velocity, umP, in a Maxwell-Boltzmann's distribution corresponding to M = 0 is given by
u/umP = M[r/(3 + ( 3 / 2 ) ( ~- 1)WI1/*
(2)
= (3kTo/7?2)'/'
(3)
UmP
where y is the ratio of the specific heats a t constant pressure and temperature (y = Cp/C, = 5/3). Assuming To = 300 K and X = 15 mm, the mach number is calculated to M = 18. The final velocity of the molecule a t the sample detection position under present experimental conditions ( M = 18) is calculated to 800 m/s. The velocity of the molecule immediately after jet expansion is much lower, so that the average velocity is estimated to be slightly less than 800 m/s. This value is consistent with the average velocity of 600 m / s observed. The translational temperature is simply estimated by the following equation in this study, instead of a more complicated one (eq 6 in ref 37) f ( u ) = ( r n / 2 ~ k T ) exp(-mu2/2kT) ~/~
(4)
where m is the mass of the molecule, k is the Boltzmann constant, and T i s the temperature. From the observed curve in Figure 5, the translational temperature was well-fitted to 25 K. On the other hand, the theoretical translational temperature, Ts, calculated from the nozzle parameter etc. is given by (37) Ts/TO = [ l (7- 1)W/2]-' (5)
+
The estimated value from present experimental conditions is 3 K, which is obtained by assuming M = 18 and To = 300 K. It is difficult to discuss discrepancy quantitatively due to rather poor accuracy in ihe experimental data. However, it is possible to ascribe it to sample spread immediately after laser ablation, which is considered to be 1-2 mm (1.7-3.3 p s in the time domain) and is not negligible in comparison with 4 ws. Temperature rise in the nozzle throat by laser heating may also increase To,increasing Ts. However, if one explains
Pulse Energy (mJ)
Flgure 6. Dependence of pulse energy for sample vaporization on fluorescence intensity: (A) tightly focused; (B) poorly focused. Two
independent results, circle and triangle, are presented in the same graph. this discrepancy only with this temperature rise, the temperature in the nozzle throat (To)should exceed 3000 K. This is not reasonable, since heated chemical species by laser radiation (6000 K (31), 350-450 K (36))is rapidly cooled by collision with Ar. This rather high translational temperature may be also attributed to an imperfect nozzle structure. Nonsymmetric distribution of the styrene molecule, shown in Figure 5, may support this discussion; the tail after 27 p s implies presence of the dead volume in the nozzle throat. Signal Enhancement. In conventional pulsed supersonic jet spectrometry, the typical pulse width is 1 ms (38). The sample molecule can be more efficiently detected by localizing it in spatial and time domains. Sheath flow gas dynamic focusing to a central core in the jet has been demonstrated, and a signal enhancement factor of 30 is achieved (39). As shown in Figure 5, the sample molecule is focused into 4 ys; then an enhancement factor of 250 is achieved in the present study. The laser ablation technique has a distinct advantage over a gas-flowing pulsed supersonic jet technique with respect to sensitivity, when the sample amount is limited. Present results are consistent to a reported value of -5 p s obtained by LA-SSJ placing the sample outside the nozzle for jet expansion (40). It is noted that the unused sample remains on the rod surface. Therefore, very toxic chemicals doped in a polymer are quite safely measured and easily recovered. This is a practical but important advantage, when supersonic jet spectrometry is applied to hazardous substances such as dioxines and polychlorinated biphenyls. Threshold Energy for Vaporization. Figure 6 shows dependence of the pulse energy on the signal intensity. Two experiments are performed by changing the beam focusing condition. When the ablation laser is tightly focused onto the polymer surface, threshold for signal appearance decreases to almost zero. In this case the observed signal was stronger, and the spectrum could be measured in a better signal-to-noise ratio. When the laser beam is poorly focused, the signal appears from 0.5 mJ. In this case the signal intensity was weak. These results imply that a threshold energy is necessary for sample ablation. This fact is consistent with the result obtained by nonjet LA-MPI-MS (30). Beam Diameter at Sample Surface. In our preliminary study the sample rod was rotated fast enough to irradiate a new polymer surface. However, it is noticed in later works that the signal intensity increases with decreasing the rotating speed. Figure 7 shows dependence of the beam separation between successive laser shots. The signal intensity increases with decreasing the separation gradually from 140 pm and
ANALYTICAL CHEMISTRY, VOL. 61, NO. 14, JULY 15, 1989
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Vaporization
Laser
L 150 50 100
0
Separation y (urn)
Flgure 7. Dependence o f separation @ ) between successive laser
shots on
fluorescence
intensity.
more rapidly from 50 pm. In this experiment the laser with a beam diameter of 6 mm is focused onto the polymer surface by the lens with a focal length of 30 cm. Then, the diameter at a beam waist is calculated to 20 pm, by assuming a Gaussian beam. Apparently, the actual beam diameter for a nonGaussian beam, inevitable for the excimer-laser-pumped dye laser, is much larger than the above value by a factor of 2 or 3. Then, the beam diameter of the ablation laser at the polymer surface is estimated to be 40-60 pm, which coincides with the separation, at which the signal rapidly increases. This fact implies that sample ablation becomes more efficient by superposition of successive laser shots. Thus the signal increase may be attributed to temperature rise and acceleration of the vaporization rate by laser heating with previous laser shots. The fluence of the ablation laser is calculated to 100 J/cm2 at present experimental conditions (2 mJ, 50 pm diameter). This fluence is much larger than a value of 20 mJ/cm2 used for ablation of polystyrene by KrF laser excitation (31). It might be ascribed to difference in molar absorptivities at 337.5 nm and 248 nm. Process of Sample Ablation. Under strong laser radiation chemical species are ablated from the polymer surface through two possible mechanisms: (1)a thermal process due to surface temperature rise and (2) a photochemical process through multiphoton excitation. The experimental result shown in Figure 7 may be discussed by separating it to two parts: regions (1)above and (2) below 50 pm. Region 1 is characterized by rapid signal increase with decreasing beam separation. By the previous laser shot the temperature of the polymer surface increases. Generated heat cannot be dissipated in 50 ms (20 Hz), and the polymer surface is kept partially melted or softened until the next laser shot. It is reported that a local pressure of IO7 Pa (100 atm) is generated immediately after laser pulse at the solid surface (41). Thus efficient evaporation or microexplosion may occur from the partially melted polymer surface to form monomers. Therefore, this is ascribed to the thermal process. In region 2, the signal intensity decreases with increasing the separation but does not reach zero. In this case (>140 pm) ejection of the styrene monomer is achieved by a single laser shot. Two possibilities might be still considered; one possibility is generation of monomers through a thermal effect by a single laser shot and another possibility is photochemical decomposition by multiphoton excitation. It is, unfortunately, difficult to specify one of these possibilities from the present data. However, the rotating speed of the sample rod is adjusted low enough to enhance the signal intensity (e50pm), and the major process concerned
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in this study is the thermal process rather than the photochemical one. Photofragment spectroscopy of polymer material has been studied by nonjet LA-MPI-MS (30,31). Because of the poor spectral selectivity, it is difficult to determine the chemical structure of the product molecule since so many isomers are possible. Because of this reason, a photochemical process in laser ablation has not been clarified in detail. The present LA-SSJ technique may allow assignment of the chemical structure and provide information about ablation and fragmentation mechanisms.
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RECEIVED for review August 30, 1988. Revised April 6, 1989. Accepted April 14, 1989. This research is supported by Grant-in-Aid for Scientific Research from the Ministry of Education of Japan and by Nissan and Kurata Foundations.
