Anal. Chem. 1989. 67,914-917
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A, n m Flgure 5. Absorption spectra measured in the maxima of peaks in
Figure 3. values were measured. The results are shown together with pH values of the anodic and cathodic solutions in Figure 4. It follows from the comparison of Figure 2 and Figure 3 that both peaks in Figure 3 correspond to retained species. The peaks in Figure 3 were identified with the aid of absorption spectra in the Soret region. It can be seen from Figure 5 that spectra of both peaks are characteristic of horse myoglobin (10). Although the molar absorption coefficients of myoglobins are pH dependent, the difference between pH values of 6.85 and 7.35 is negligible (11). Thus the quantitative proportions of the two fractogram peaks agree with the well-known fact that myoglobin contains a higher amount of the basic form. The difference between the integral of the curve in Figure 2 and the sum of the integrals in Figure 3 can be explained either by the presence of low molecular ionic species of iron in the myoglobin preparate, which are separated from the channel through membranes via electrophoresis, or by changes in the molar absorption coefficient of myoglobin in the wide pH range in the absence of the electric field. In this case the pH gradient is formed by diffusion of H+ and OH- ions from electrolyte solutions into the channel through membranes and
myoglobin is spread across the channel. The molar absorption coefficients of myoglobin in the alkaline region are considerably higher than those in the neutral and acidic regions (11). Isoelectric focusing is a frequently used technique with an exceptional resolution; nevertheless it is relatively time-consuming and requires a high voltage and an efficient cooling. The addition of the solvent flow as the third separation-affecting factor can reduce these disadvantages. The dimension of the channel in the direction of the electric field (perpendicularly to the direction of the liquid flow) can be reduced to the millimeter range (in our case 5 mm). This permitted the decrease in the absolute voltage values while keeping a high field strength and resulted in a lower Joule heat production and a shorter focusing time. Moreover, the laminar flow of the solvent stabilizes pH gradient against convection. The successful separation of two components of myoglobin in relatively short time (- 1h) proved the viability of IEFFFF. Although IEFFFF was invented as an analytical technique, it can also be suggested as a continuous preparative technique (1).
LITERATURE CITED (1) Jan&, J.; Chmeik, J. Anal. Chem. 1984,5 6 , 2481-2484. (2) Jan&, J. Makromol. Chem., RapM Commun. 1982,3 , 887-889. (3)Chmeiik, J.; JanEa, J. J . Lip. Chromatogr. 1988,9 , 55-66. (4) Giddings, J. C. Sep. Sci. Techno/. 1983, 18, 765-773. (5) Chmellk, J.; Jan&, J. National Meeting of the Czechoslovak Chemical Society, Proceedings Section 1, 7; Banski Stiavnica, July 1984 p 38. (6) Semenov, S. N.; Kuznetsov, A. A.; Zolotarev, P. P. J . Chromatogr. 1988,364, 389-396. (7) Svensson, H. Acta Chem. S c a d . lS81, 15, 325-341. (8) Jan&, J.; Jahnovi, V. J . Lip. Chromatop. 1983,6 , 1559-1576. (9) Radola, B. J. Biochim. Blophys. Acta 1973,295, 412-428. (10) Sono, M.; Smlth, P. D.; Mc Cray, J. A.; Asakura, T. J . Bioi. Chem. 1978, 251, 1418-1426. (11) Sono, M.; Asakura, T. J . Bioi. Chem. 1976,251, 2684-2670.
J. Chmelik* M. Deml J. Janira Institute of Analytical Chemistry Czechoslovak Academy of Sciences 611 42 Brno. Czechoslovakia
TECHNICAL NOTES New Way To Mount Particulate Materlal for Laser Microprobe Mass Analysis R. A. Fletcher Center for Analytical Chemistry, National Institute of Standards and Technology, Building 222, Room A121, Gaithersburg, Maryland 20899
INTRODUCTION When particles are analyzed by a variety of microanalytical techniques, there is often a problem with transferring the particles of interest from the collector medium to the appropriate instrument-compatible support for analysis. This is especially true for mounting samples in the laser microprobe mass analyzer (LAMMA 500). The normal procedure for mounting particles is to first suspend them in a liquid and then evaporate an aliquot of this liquid onto a thin organic support film (such as collodian). Analysis is then performed by ablating the particles in or on the film, and at the same time puncturing and ablating part of the film. If the particles are initially caught on a membrane type filter, a small portion
of the filter can be cut, mounted on a support grid, and dissolved away leaving the insoluble particles. In both approaches, there is a question of particle loss in the transfer procedure and problems with dissolving particles and particle contamination from the solution. We wish to report a new way of mounting particles and microstructures on the LAMMA 500 which may also be useful for other microanalytical techniques. The mounting technique involves removing the top fiber layer from a high-purity quartz fiber filter that contains a collected particle sample. The small particles ( 1 to 10 pm diameter) remain attached to the fibers by van der Waals forces and possibly static charge. The particles are analyzed directly on the fiber by the laser microprobe.
This article not subject to US. Copyright. Published 1989 by the American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 8. APRIL 15. 1989 I
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Flgure 1. Quartz flbers Mntalning carbon particulate as viewed in the laser microprobe.
The LAMMA 500 is a time-of-flight mam spectrometer that utilizes a pulsed NdYAG laser to ablate and ionize the material of interest (1-3). The spatial resolution of the microprobe is on the order of 4 a m based on the spot size of the beam on the sample produced by using a 32X objective lens. The NdYag laser has a natural wavelength of 1060 nm, hut in the microprobe the fourth harmonic (226 nm) radiation is used. The laser microprobe has transmission geometry with respect to the laser source, the sample, and the time of flight (TOF) mass spectrometer. Advantages of this mounting scheme are 3-fold: first, the sample adheres to the fibers and is, therefore, not perturbed or contaminated after its collection on the filter; second, the quartz fibers are largely optically transparent to the (266 nm) radiation; third, the quartz fibers are ultrapure, and thus serve as a very clean support with little or no trace of carbon-containing compounds.
EXPERIMENTAL SECTION A commercial quartz fiber filter is heat treated to 500 O C for several hours to remove carbon compounds. Airborne particles are then collected on this carbon-free quartz fdter. The mounting technique is straightforward. The quartz fiber filter containing the particulate material is placed sample side up on a dean surface. A transmission electron micrcacupe (TEM) grid, that can be made of copper, nickel, gold, etc., is moved back and forth across the top of the particle-laden surface of the fiber filter. In this step, small (-2 to 10 pm diameter) quartz fibers cmrying the collected particles are transferred to the grid. The grid contains no film backing and thus contributes negligible background during an analysis of the particles adhering to the quartz fibers. Figure 1 shows several photomicrographs of a carbon soot sample collected on the quartz fiher filter mounted on a TEM grid, Notice that the particles are clearly visible. The laser beam is then directed to the obviously visible particles, ablating the fiber-mounted particles and particulate clusters for analysis in the time-of-flight
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 8, APRIL 15, 1989
O-+i
~
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i
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Figure 2. Summed spectrum of eight positive ion mass spectra of quartz fiber.
mass spectometer. In this example, probably only the top layers of the fiber filter are analyzed. To examine the fibers residing deeper in the filter, one must remove the appropriate layers of quartz fiber material. Due to the optical properties of the fibers, even small (optically invisible) particles may be analyzed by directing the laser beam upon the fiber surface.
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Summed spectrum of quartz fiber. Figure 3.
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negative ion mass spectra of
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RESULTS AND DISCUSSION There are several advantages of this simple collection and mounting technique. First, quartz filters, after heat treatment, are very clean with regard to most elemental contamination, especially carbon ( 4 ) . As an illustration, Figure 2 is the sum of eight positive ion spectra taken of the clean quartz fiber. This fiber filter had been heat-treated once, carefully wrapped in aluminum foil, and stored for 6 months before laser microprobe analysis. As can be seen in Figure 2, when the fiber is ablated by a high irradiance 226-nm laser beam, Si+, Siz+, and SiO’ are present in the positive ion spectral sums at m/z 28, 56, and 44, respectively. The irradiances are several times higher than necessary to ionize particle samples. There is a trace of Na+, Ca+, and K+ observed which appear to be universal contaminates on virtually all samples. The carbon content is not detectable in the positive ion spectra. This is advantageous for positive ion analysis on the laser microprobe. Quartz is largely transparent to the incident 266-nm radiation as supported by the fact that the optical lenses and windows in the microscope assembly of the spectrometer are made of this material. However, if subjected to sufficient radiant flux densities, even high-purity quartz will ablate and form ions in this laser microprobe (5). It takes approximately 1 x IO9 W/cm2 to generate a negative ion carbon cluster spectrum and about 3 times that irradiance to produce 0- and Si- from the pure quartz. In both cases the beam was focused onto the surfaces of the object. Figure 3 is the sum of 10 negative ion spectra taken of the quartz fiber. The dominant peak is 0- followed by SiOz- (mlz 60), Si03- ( m / z 76), Si204-, and Si205-at m / z 120 and 136. There appears to be a trace of carbon as evidenced by peaks at m / z 24,25 (C2- and CzH-) and at m / z 36 (C;). Note that the height of the major peak due to carbon ( m / z 24) in the summed spectrum is only a factor of 2 over the summed background. This level of carbon is insignificant compared to the amount found on a typical carbon particle sample such as an atmospheric particulate sample or a sample of heptane soot on quartz fiber (Figure 4). When an attached particle is analyzed, the laser beam is focused onto the particle, or onto the fiber surface. When the beam is focused upon. a clean quartz fiber a t irradiances on the order useful for carbon analysis, no ions and thus no spectra result. The above Si,O, spectra were obtained by focusing the beam into the fiber and not on the fiber surface.
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Figure 4. Single typical negative ion mass spectrum of soot generated from a controlled heptane burn. The soot particles were analyzed from quartz fibers.
Clearly Si particle analysis would be in question when using a quartz fiber mount. This “particle analysis from fiber” approach has been used in our study of various carbon soots and atmospheric carbon particles (6). The transfer efficiency for particles residing on the fiber may well be less than 100%. If we assume the particles are held to the fiber surface by van der Waals forces, the attachment force is proportional to the particle diameter. Mechanical forces required to remove these attached particles are proportional to the mass of the particle or diameter to the third power. Following the treatment of Ranade (9, the acceleration (in terms of gravitational acceleration, g) to move a 1O-pm particle is 4.5 x io4 g, a l-wm particle is 4.5 x IO6 g, and a 0.1-pm particle is 4.5 x lo8 g. For each order of magnitude decrease in particle size, there is an increase of 2 orders of magnitude in the acceleration required to remove the particle. From this we can see that the small particles are more likely to be retained on the fibers during the transfer process. However, we do see particles in excess of several micrometers in diameter retained on the transferred fiber. The above discussion refers to spherically shaped particles. Particles with large contact surfaces (to the fiber) will be held better than low contact surface particles. For example, other fibers, agglomerates, or platelets would have larger contact areas than spheres and thus be more tightly held on the fiber. Also, liquid “sticky” particles should remain fiber bound and thus have a better chance of transport. We are examining several microanalytical techniques for measuring the particle transfer efficiency.
Anal. Chem. 1989, 6 1 , 917-921
In summary, quartz fibers have been shown to be an excellent sample support for individual microscopic particulates. The fibers are derived from high-purity quartz filters often used to collect airborne particles. The small particles become attached to the quartz fibers in the filter collection process and the entire fibers are transferred into the instrument for analysis. It may be possible to use this fiber support for collecting organic liquids or adsorbed gases, then gently laser desorbing the species of interest from the surface as demonstrated for asbestos fibers (8,9).If the fibers were coated with silver or gold films before deposition of the sample, the optical properties of the film (absorption a t 266 nm) could possibly be used to facilitate laser desorption as demonstrated in the planar case (10).
ACKNOWLEDGMENT The author extends his thanks to G. Mulholland for the quartz fiber filters and L. Currie for suggesting LAMMS of carbon soot.
LITERATURE CITED (1) Hercules, D. M; Day, R. J.; Balasanmugam, K.; Dang, T. A.; Li, C. P. Anal. Chem. 1982, 5 4 , 280A.
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(2) Hillenkamp, F.; Unsold, E.; Kaufman, R.; Nitsche, R. Appl. Phys. 1975,
8 , 341. (3) Denoyer, E.; Van Grieken, R.; Adams, F.; Natusch, D. F. S. Anal. Chem. 1982, 5 4 , 28A. (4) Cadle, S. H.; Groblicki, P. J.; Stroup, D. P. Anal. Chem. 1980, 52, 2201. (5) Michiels, E.; Celis, A.; Gijbels, R. Microbeam Analysis San Francisco Press: San Francisco. CA, 1982; p 383. (6) Currie. L. A.; Fletcher, R. A.; Klouda, G. A. Nucl. Instrum. Methods Phys. Res., Sect. B 1987, 2 9 , 346. (7) Ranade, M. B. Adhesion and Removal of Fine Particles on Surfaces, AerosolSci. Technol. 1887, 7 , 161. (8) De Waele, J. K.; Vansant, E. F.; Van Espen, P.; Adams, F. C. Anal. Chem. 1983, 5 5 , 671. (9) De Waele, J. K.: Gijbels, J. J.; Vansant, E. F.; Adams, F. C. Anal. Chem. 1983, 5 5 , 2255. (IO) Fletcher, R. A.; Chabay, I . ; Weitz, D. A.; Chung, J. C. Chem. Fhys. Lett. 1984, 104, 615.
RECEIVED for review June 28,1988. Accepted December 27, 1988. Certain commercial equipment, instruments, or materials are identified in this report to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the NIST, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.
Estimating Contaminant Concentration from Uptake on a Diffusive Sampler Cecilia J. Tidwell Mine Safety Appliances Company, Pittsburgh, Pennsylvania 15320
Dwight W. Underhill* School of Public Health, University of South Carolina, Columbia, South Carolina 29208 INTRODUCTION Diffusive sampling is now important in a number of areas, including industrial hygiene and indoor air pollution. To find improved procedures for estimating analyte concentration from the analyte uptake by a diffusive sampler, the theory developed earlier ( I ) is applied here to data from controlled laboratory tests.
EXPERIMENTAL SECTION The sampling data were obtained with a commercially available diffusive sampler ( 2 ) ,the Mine Safety Appliances Co. organic vapor dosimeter. This diffusive sampler (Figure 1) contains a 0.38-cm air gap, which acts to rate limit the influent diffusive flux of analyte, a windshield, and two activated carbon-impregnated layers to trap the analyte which diffused across the air gap. Its case, and the porous windshield and separator, were constructed of polypropylene and polyethylene, respectively. The two activated carbon impregnated layers are separated by a thin porous polyethylene membrane. The backup layer is slightly smaller (6.9 X 0.74 cm) than the primary activated carbon layer (7.0 X 0.86 cm). This design is not unique, e.g. Du Pont (3) and 3M (4) have produced similar diffusive samplers containing two sorbent layers placed in series. The diffusive samplers were exposed in a dynamic system (Figure 2) in which the input air was maintained at a relative humidity of 74-82% (HydrodynamicsHydrometer Model 153050) at a temperature of 24-25 "C. The analyte was injected into this humidified air by an infusion pump from a lo-, 2 5 , or 50-mL syringe, through a 0.10 cm i.d. flexible Teflon needle into a heated injection port. The diluent air and contaminant vapor were further mixed in a manifold upstream of the badge exposure chamber. The infusion pump's syringe was weighed before and
* Corresponding author. 0003-2700/89/0361-0917$01.50/0
after each test. The reported concentrations of test reagent were calculated from the ratio of mass input to air flow; during each run a secondary check was made about once an hour with a flame ionization gas chromatograph-agreement was usually w i t h 5%. The effluent air flow from the badge exposure chamber was measured by a dry gas meter and maintained at 680 & 20 mL/s. In the exposure chamber (1.30 m long, 6.4 cm i.d., see Figure 3), this flow gave more than 60 air exchanges per minute. The average air velocity within the exposure chamber w a 0.43 m/s. According to the manufacturer (2),a minimum air movement of 0.17 m/s across the face of the diffusive sampler is needed to eliminate a stagnant zone depleted of contaminant. The exposure chamber contained a 1.17 m long removable tray with 16 slots, each 1.3 cm X 7.6 cm, for holding the bases of the diffusive samplers. The samplers were placed in two rows, 2.5 cm apart, starting 15 cm from the front of the chamber and ending 5 cm from the back of the chamber. This separation was needed to prevent starvation effects. The bottom of the tray was closed to increase the face velocity; a fine mesh screen at the chamber inlet minimized turbulence. The walls of the test system, the badge exposure chamber, the charcoal tube sampling manifold, the mixing chamber, the screening, and all wociated tubing were constructed entirely from stainless steel to reduce holdup and/or contamination effects. After exposure, the front and back layers of sorbent were placed in separate vials and, if stored for later analysis, placed in refrigeration at 5 "C. The activated carbon layers were desorbed separately with 2 mL of Spectro-quality carbon disulfide while being agitated for 30 min on a vibrator. Uptake on all samplers was determined with a Varian Model 3700 gas chromatograph (with automatic liquid sampler), coupled to a Supergrator-2 programmable integrator. The fraction of recovery of sorbed analyte was determined by using sorbent layers spiked with known quantities of analyte. In some cases 5-107'0 acetone was added to the carbon disulfide to increase recovery. Table I gives the 0 1989 American Chemical Society
