Anal. Chem. 1983, 55,671-677
671
Laser Microprobe Mass Analysis of Asbestos Fiber Surfaces for Organic Compounds J. K. De Waele, E. F. Vansant, P. Van Espen, and F. C. Adams" Department of Chemistty, Universitaire Instelling Antwerpen (U.I.A.), Universiteitsplein 1, 8-26 70 Wilrlk, Belgium
Laser microprobe ma88 analysis was applied for the analysis of organic impurities at the surface of asbestos fibers. Benzo[a]pyrene was adsorbed from the gas phase and from a benzene solution, benzidine from aqueous solution and N ,Ndimethylaniline from benzene. By use of laser desorption conditions these compounds, and in the case of N,N-dimethylaniline Its oxidation products, such as Methyl Violet, could easily be detected. I t appeared that the asbestos samples used contain organic contaminants at the surface.
Asbestos is becoming more and more of a concern to scientists, the construction industry, and the general public because there is reason to believe that exposure to the fiber, under some circumstances, is associated with lung cancer, asbestosis, and other diseases (1). From the biochemical and environmental standpoint, fine respirable asbestos fibers of high surface area have considerable capacity to adsorb other substances. They may therefore selectively adsorb the constituents of living cells and thereby upset the delicate balance of cell reproducibility, but they may also adsorb pollutants from the atmosphere to be transported to the lung and deposited in the tissue ( 2 ) . Because of the hypothesis that the toxicity of the asbestos fibers is correlated with the adsorption power of pollutants (343, the intention of this paper is to characterize the surface of asbestos. Therefore, organic impurities were adsorbed onto the asbestos surface by using a gas adsorption process (7) or through equilibration of the fibers with a benzene or water solution. A rapidly increasing number of publications illustrate the usefulness of laser microprobe mass spectrometry in single particle analysis (8-11). The primary utility of the method in this application appears to lie in the areas of quick routine qualitative and semiquantitative analysis of individual particles of micrometer size for minor and in some conditions trace level inorganic constituents (12,13). At the same time there is evidence that by use of the technique as a variable ionization source for mass spectrometry, information can be gained regarding the presence of molecular inorganic and organic species present in the microscopical objects (14). In this paper we want to report preliminary results which show that laser microprobe mass analysis (LAMMA) can detect sensitively chemical impurities present a t the asbestos fiber surface. EXPERIMENTAL SECTION
The LAMMA Instrument. For a full description of the LAMMA-500instrument (Leybold-Heraeus,GmbH, K6h,FRG) we refer to other publications (12,15, 16). The most attractive features of the LAMMA instrument are spatial resolution to 1pm, high collection efficiencies (10-50%), and ease of operation and simplicity. On the other hand, the mass resolution of 700-800 is marginally adequate for inorganic analysis and for the characterization of fingerprint organic spectra. The laser energy on the sample can be varied with a set of attenuating filters. It is monitored by an energy meter. Laser 0003-2700/83/0355-067 1$01.50/0
energy is adjusted either to provide complete vaporization of a micrometer size sample of the fiber or else to give the lowest energy which provides a mass spectrum. Adsorbed impurities onto the fiber surface can be detected in this laser desorption (LD) operation mode (17). In the LD mode a rather small number of ions are formed belonging to a limited number of ion species that are in most cases directly related to the molecular structure of the material analyzed,often with a high yield of the parent molecular ions (M+),protonated (M + l)+, or cationized species (M + Na)+ or (M + K)+ (17-19)-When laser desorptionis applied to asbestos, the fibers remain intact and the characteristic mass spectrum of the material vanishes to an insignificant intensity. Reagents: benzo[a]pyrene (BaP), 99+% , Gold Label, Aldrich-Europe, Belgium; benzene, 99+ % , spectrophotometricgrade, Gold Label, Aldrich-Europe,Belgium; benzidine dihydrochloride, >99, Fluka AG, Buchs, FRG; NjN-dimethylaniline(DMA),99%, Aldrich-Europe,Belgium; hydrogen chloride (0.1 N), Titrisol, Pro Analysis, Merck, FRG; Methyl Violet 2B, certified (Basic Violet l), Aldrich-Europe, Belgium. Materials. UICC (Union Internationale Contre le Cancer) standard asbestos of crocidolite and amosite from South Africa, Canadian (Type B) and Rhodesian (Zimbabwe)(type A) chrysotile, and Finnish anthophyllite were used (20-22). The UICC has sponsored the preparation and distribution of standard reference samples of each class of asbestos, and these samples have been employed in inhalation, ingestion, inoculation, and in vitro experiments. Other applications are in identification of asbestos type in aerosols and tissues, standardization of conventional fiber counting, development and calibration of techniques and instruments for fiber assessment, and studies on influence of fiber size and shape on inhalation and retention (23). All filtrations were carried out by means of Schleicher& Schiill filter paper, no. 589l Schwarzband, 4 110 mm. For introduction of asbestos into the LAMMA instrument for analysis, a standard copper electron microscopy grid, coated with a Formvar film, is brought into contact with the fibers. A number of them stick to the grid coating. Fibers large enough to be detectable with the optical microscope can be selected for individual analysis. RESULTS AND DISCUSSION Adsorption of Benzo[a Ipyrene on Crocidolite. Procedure. Benzo[a]pyrene (BaP) was adsorbed onto the UICC standard of crocidolite using two methods: Adsorption from a Benzene Solution. To a 30-mg crocidolite sample, 20 mL of a M BaP solution in benzene is added. The suspension is refluxed at a temperature of 60 "C for 15 h. The asbestos is fiitered over a Schleicher & Schiill filter and rinsed twice with a pure benzene solution. Gas-Phase Adsorption. Gas-phase adsorption is performed with an apparatus for vapor-phase adsorption of polycyclic organic matter, designed by Korfmacher et al. (24), and Miguel and Natusch (7). Approximately 30 mg of asbestos is weighted into an expanded-bed adsorption tub. The expanded-bed is heated to a temperature of ca. 150 "C. The BaP vapor in nitrogen gas is passed over the material until equilibrium adsorption is obtained as indicated by fluorescence measurements of the effluent. The adsorption bed is then cooled rapidly (7). Whereas a t high laser energy above 1p J , pure BaP gives rise to a fragmentation pattern, at lower energy the spectrum 0 1983 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983
Figure 1. Positive laser mass spectra of benzo[a]pyrene (BaP)adsorbed from solution onto separate crocidolite fibers for three laser energies.
is considerably simpler: the fragmentation peaks become negligible and the molecular ion peak becomes more intense. Below 0.35 p J , the latter becomes the only detectable mass spectral component. The negative mass spectrum, even when collected at very low laser energy, provides an intense fragmentation pattern C,H- with n even for the most abundant peaks. The intensity of mass peaks around the molecular mass of BaP is much lower than that for the positive spectra and consists of major peaks at m l e = 251 and 253. The positive spectrum of the solution doped crocidolite samples shows the presence of the characteristic BaP desorption spectrum a t low laser energy. Figure l gives three examples. At low laser energy the molecular ions at m l e = 252 and 253 are the only features of the mass spectrum. They are detected on more than 90% of the fibers analyzed. Their intensity is maximum around 0.5 pJ. At still higher energy the intensity starts leveling off and the inorganic cluster ions of crocidolite become the dominant peaks in the spectrum. The BaP doped crocidolite mass spectrum does not correspond entirely with that obtained from pure crocidolite fibers at a similar laser energy. The Mg+ and Fe+ elemental ions are considerably decreased in intensity sometimes even entirely absent from the spectra taken at low energy. Figure 2a,b illustrates this peculiar behavior which is presently unexplained. When subsequent laser shots of comparable energy are directed at the same location of a fiber, the organic fragments gradually disappear from the spectra and at the same time the intensity of the crocidolite substrate increases but remains distorted compared to an undoped sample. Figure 2c shows a typical example of a spectrum taken after removal of most of the adsorbent through two previous laser shots. These observations indicate that LAMMA can be used as a rather crude depth probing device. Negative spectra taken in similar conditions do not lead to a ready detection of the adsorbent. The spectra correspond closely with those obtained for pure crocidolite. Gas-phase adsorption provided a bulk concentration of 580 pg g-l of BaP as determined with gas chromatography. LAMMA analysis of the doped samples indicates an inhomogeneous distribution on the fibers. Indeed, a fraction of them give rise to the BaP molecular ions in the positive spectrum, another fraction not (ca. 25% and 75%, respectively). In the latter case several other peaks appear consistently in the spectra. Figure 3 shows a characteristic example of each of the two typical LD spectra obtained. It can be assumed that the mass spectral components at m l e = 348,
Figure 2. Positive laser mass spectrum of an untreated crockloliie fiber (a),a fiber doped with BaP from solutlon (b), and a fiber which was subjected to two previous laser shots (c).
-
0.74 P J
contaminant
150
200
250
300
350
Loo
m/e
Flgure 3. Typlcal positlve laser desorption (LD) spectra of gas-phase
BaP doped crocidolite: BaP containing fiber (a); fiber containing an accldental surface impurity (b).
360, and 376 f 1result from an organic surface contaminant present at the UICC crocidolite before the adsorption experiment, which inhibits further adsorption of the polyaromatic compound. Separate experiments proved the presence of the impurity on crocidolite when the material was stored into closed polyethylene bags of the same origin as the one used for the adsorption experiment. It has been reported previously that the UICC asbestos standards are often contaminated by storage in polyethylene bags (25, 26). Bulk analysis has proved the presence of 3,3’,5,5’-tetra-tert-butyldiphenoquinone (molecular mass = 408) resulting from the presence of antioxidants in the POlythene packing material (25). The absence of the molecular ion at m l e = 408 suggests the presence of other compounds in this particular sample. We are presently working on the elucidation of the component and several other contaminants in asbestos samples. Results will be published elsewhere. Adsorption of Benzidine on UICC Asbestos. Benzidine was adsorbed onto the five UICC standards from an aqueous solution. This compound has been previously adsorbed onto clay minerals such as montmorillonite (271 and sepiolite and laponite (28). It gives rise to a good adsorption and to intense
ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983
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Table I. Intensity Ratios of Different Elemental Eon Peaks t u "Na for Benzidine Treated and Untreated Anthophylite Fiber Fibers relative intensity to 23Na laser energy, IJ.J 24Mg 27A1 pure treated pure treated pure treated
39K
Wi
I
0.37 0.45 0.68 0.88 1.14
0.39 0.45 0.63 0.92 1.13
20 20 50 50 30
50 10 20
3 1.1
7
1.3
5
0.8
9
5 0.3 1.3 0.1 0.09
benzi40Ca 56Fe dine m/e pure treated pure treated = 184 ~
pure
treated pure
