Anal. Chem. 1984, 56,1519-1521
A
LAMP
Figure 3. Two ways to spatially homogenize the light beams from the lamp: (A) diffuse-reflectance integrating sphere; (B) “scrambled” op-
tical fiber. spectrally structured background signals differ by a far larger factor. The analytical consequences of these biases are proportional to the ratio of the time integrated error signal to that of the integrated analyte atom signal. Spatially isothermal, tubular, atomizers having lengths which are large relative to their diameters will be least affected because any radial concentration gradients initially formed will disappear in a time which is short relative to the total duration of the absorption signal. On the other hand, open “filament type” atomizers represent a “worst case” scenario. Detection of these biases in actual practice will be quite difficult. Unless the bias is both in a negative direction and large enough relative to the analyte atom signal to produce a negative net “absorbance” response, the bias will generally go undetected during routine analyses of *unknowns”. It should be realized, too, that errors of this sort are not compensated for by using the popular “standard additions” technique. The writer must stress that the values listed in Table I are calculated errors based on hypothetical concentration gradients. Truly realistic estimates of actual analytical biases must wait until quantitative time-resolved data on the radial concentration gradients of all of the light absorbing species formed in real atomizers atomizing real samples becomes available. The optical apparatus developed by Holcombe et al. ( 4 ) would be ideally suited for obtaining the necessary data. This writer’s sole experience with an analytical bias unambiguously attributable to imperfect light beam coincidence was a base line shift that occurred when the light baffle plate of a Varian Techtron Model 90 atomizer workhead had not been perfectly aligned with the atomizer tube. The base line shift occurred during the time that the analyte volatilized and consequently caused difficulties in obtaining accurate signal integrals. The shift was caused by lateral movement of the atomizer tube, the side of which was inadvertently serving as a field stop (because one electrode rod post of this atomizer is fixed while the other “gives” to prevent graphite breakage due to thermal expansion, there is substantial lateral move-
1519
ment of the tube when the system is heated). In this instance the moving graphite behaved as a radially asymmetric, nonatomic absorber (or in other words, like a knife edge). An analogous situation occurs with Massmann-type furnaces when sampling “probes” are used to create nearly isothermal atomization conditions (IO). Noncoincidence of SB and RB light beams may be caused by optical components other than the source lamp(s). Imperfections in either the quality or alignment of mirrors, lenses, beam combiners, polarizers, etc., may cause significant RB/SB noncoincidence, especially if the components move during an individual instrument cycle. For this reason even Zeeman background corrected instruments may not be absolutely immune to this problem. A spectrometer’s relative sensitivity to errors of this sort may be gaged by slowly passing a knife edge vertically through the light path at the point normally occupied by an atomizer. The writer can suggest several ways to reduce the magnitude of errors traced to noncoincident light beams. The simplest of these is to deliberately not sharply focus the light beams on the center of the atomizer-the usual practice. Doing this has the effect of broadening the beams which reduces the slopes of the emission intensity gradients across the atomizer. Unfortunately, in small-tube graphite furnace applications, doing so also causes a loss in total light throughput, increasing noise. Another solution is to restrict light measurements to only the central portions of the light beams. This can be done by placing a field stop at the atomizer or by simply limiting the height of the entrance slit of the monochromator. However, doing so will also reduce the total light intensity that the spectrometer has to work with. Two other possible fixes require the use of additional optical components at an additional focal point situated between the hollow cathode lamp and the atomizer. They both serve the purpose of rendering the light beams spatially homogeneous. The first approach (Figure 3A) utilizes a diffuse reflectance integrating sphere; the other (Figure 3B) uses a length of “image-scrambling” fiber optic light pipe.
LITERATURE CITED (1) Smith, S. B.; Hieftje, G. M. Appl. Spectrosc. W83, 37, 419. (2) Siemer, D. D. Appl. Spectrosc. 1983, 37, 552. (3) Willard, H. H.; Merritt, L. L.; Dean, J. A. “Instrumental Methods of Analysis”, 5th ed.; D. Van Nostrand: Princeton, NJ, 1974; p 372. (4) Holcombe, J. A.; Rayson, G. D.; Akerlind Spectrochim. Acta, Part 6 1982, 378, 319. (5) Rayson, G. D.; Holcombe, J. A. Spec*rochim. Acta, Part 6 1983, 386,987. (6) Holcombe, J. A.; Rayson, G. D. Prog. Anal. At. Spectrosc. 1983, 6, 225. (7) Siemer, D. D.; Lundberg, E.; Frech, W. Appl. Spectrosc., in press. (8) Anderson, R. G.; Johnson, H. N.; West, T. S. Anal. Chim. Acta 1971, 57, 281. (9) Koirtyohann, S. R.; Plckett, E. E. Anal. Chem. 1965, 37, 601. (10) Manning, D. D.; Slavln, W.; Myers, S. Anal. Chem. 1979, 57, 2375.
Darryl D. Siemer Westinghouse Idaho Nuclear Co., Inc. P.O. Box 4000 Idaho Falls, Idaho 83403
RECEIVED for review January 6,1984. Accepted March 9,1984.
Secondary Ion Mass Spectrometric Analysis of Polymer and Coal Surfaces for Trace Inorganic Elements Sir: Applications of secondary ion mass spectrometry (SIMS) to the study of organic polymeric surfaces have increased rapidly in the past few years. Particular emphasis 0003-2700/84/0356-1519$01.50/0
has understandably been placed on the production and detection of secondary ion molecular fragments, which are characteristic of the polymeric structure. Experience gained 0 1984 American Chemical Society
1520
ANALYTICAL CHEMISTRY, VOL. 56,
NO. 8,
JULY 1984
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Flgure 1. (A) Positive SIMS spectrum of a PTFE surface normally , ' primary ion beam. (b) Positive mounted and analyzed with a 20-nA 0 SIMS spectrum of PTFE analyzed under specimen isolation (SI) conditions.
with SIMS analysis of involatile organic molecules (1) has shown that very low primary ion fluxes can be used with some polymers to yield SIMS spectra containing significant quantities of monomer ions, or ions closely related to the hydrocarbon backbone (2-4). More recently yet, SIMS studies of the organic constituents of coals have been undertaken using similar conditions (51, but because of the heterogeneity of most coals, little progress has been made in relating observed spectra to definite organic structures. The detection of inorganic constituents within or on a primarily organic matrix can also be an important aspect of the materials characterization. In the case of polymers, the trace elements detected indicate the type or efficiency of catalyst used, as well as the presence of elements at the surface which may cause adhesion difficulties. Trace inorganic constituents within coal are important indicators of the biological or geologic processes responsible for formation of particular macerals (6). By use of SIMS, the detection of inorganic constituents in such organic matrices is limited by the strong background contribution from molecular ions, chiefly those from organic species. In fact, organic and polymer matrices give rise to higher backgrounds in their SIMS spectra than do most metals and inorganic materials. A method for the suppression of molecular ions in SIMS spectra of minerals (3, and metals and semiconductors (8) has recently been described. This method, specimen isolation (SI),which results in molecular ion suppression of factors of IOw3or better, allows the surface to charge to a fixed potential, from which the secondary ions entering the analyzer have kinetic energies of several hundred volts, Such potentials are maintained stably, even on an insulating surface, using a charged aperture set immediately above the surface to confine the charge within a fixed area (8). The application of the SI technique has now been extended to the SIMS analysis of some highly insulating organic polymer and coal surfaces. Specimens of poly(tetrafluoroethy1ene) (PTFE), polystyrene, and a Western Canadian bituminous coal were analyzed in a Cameca IMS-3 F ion microscope using the SI procedure described earlier (7,8). The specimens were mounted in a normal Cameca holder with, in some cases, a 1-mm PTFE electrical isolator placed between the specimen and the holder surface. The size of the illumination hole in the holder is critical to the surface potential developed during ion bombardment (8),and in these cases a 5-mm opening was used. The primary beam used was a mass-filtered 8-keV
Mass Number ldaltons)
-
Flgure 3.
Positive SIMS spectrum of a bituminous coal surface mounted under SI conditions.
100-nA beam of ISO- ions which was focused to a 4 0 - ~ mspot in the center of the area exposed by the illumination hole. Secondary ions were extracted for a 150 hm field with all other secondary ion beam diaphragms wide open to maximize the counting rate. For comparison, SIMS spectra of the organic surfaces were taken in a "normal" mode with gold-coated surfaces in contact with the holder and an 02+primary ion beam. Figure 1A shows a bar graph logarithmatic plot of positive secondary ion intensities from 0 to 100 daltons taken during a 100-s scan of a gold-coated PTFE surface. Above a fairly constant background, the more prominent peaks at 31,89, and 93 daltons are believed to result from CF+, CF2+,and CF,+ and are in accordance with earlier studies by Briggs and Wooton (4). By contrast, use of the SI technique on an uncoated Teflon surface results in virtual disappearance of all molecular ions, leaving only ions due to C+, F+,and the inorganic impurities. In Figure 1B the dramatic disappearance of the background enables the detection of minor impurities in the PTFE matrix, such as iron, titanium, calcium, silicon, and aluminum. A similar SI/SIMS study of uncoated polystyrene of low purity (see Figure 2) shows that 18 constituent inorganic trace elements including silver and a number of transition metals can be detected in a 3-min scan. On the basis of sputtering rate data obtained by profilimetry measurements, the ion beam would have penetrated 200 nm of polymer during that period. Figure 3 shows the SI spectrum for a 3-min analysis (equivalent of 190 nm thickness sampled) for a mid-volatile bituminous coal surface. The areas chosen for analysis appear
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Anal. Chem. 1984, 56, 1521-1524
featureless in an optical microscope and had a medium reflectance value typical of vitrane. Of particular interest is the clear appearance of scandium, ytterium, and elements of the lanthanide series. These trace elements, and others, such as vanadium have not been previously detected on a microscopic scale in coals. The extent of molecular ion suppression for these organic compounds is considerably greater than that found earlier in SI-SIMS spectra typical of metals (8)or metal oxides (7). For example, in the SIMS analysis of PTFE the SI-induced suppression of the major molecular fragment CF+ (mle 31) has been found to be suppressed by greater than a factor of 1 X lo5 compared to a SIMS spectrum taken with normal mounting. The best suppression factors for FeO+ and SiO+ in spectra of these metals is 1 X lo4 (8). This suggests that the kinetic energy distributions of these typical organic molecular ions is, in general, considerably narrower than those for metal oxide or metal cluster ions. The implication of the above is that SI-SIMS achieves its highest degree of effectiveness in the analysis of organic substrates. With background molecular ions reduced by over 5 orders of magnitude throughout most of the mass range, it is indeed possible to predict detection sensitivities in the ng/g for many elements. The detection of trace inorganic elements within an organic matrix would be much more difficult and time consuming by spectroscopic or activation techniques, both of which lack the microscopic analytical capabilities of SIMS. Work has begun on quantitation of certain elements in a coal matrix using an ion implantation technique (9). The other advantage of the SI-SIMS technique to polymers resides in the complete absence of charging effects on any surface, no matter how insulating. Alternative methods for charge compensation, such as metallic surface coatings and electron flood guns, are of limited use particularly where depth profiling causes a change in charge compensation requirements.
Registry No. PTFE, 9002-84-0; Li, 7439-93-2; B, 7440-42-8; 0, 7782-44-7; Ca, 7440-70-2; Na, 7440-23-5; Mg, 7439-95-4; Al, 7429-90-5;Si, 7440-21-3;S, 7704-34-9;C1,7782-50-5;K, 7440-09-7; Ti, 7440-32-6; Cr, 7440-41-3; V, 7440-62-2; Mn, 7439-96-5; Fe, 7439-89-6; Ag, 7440-22-4; Ba, 7440-39-3; Ce, 7440-45-1;P, 772314-0; Sc, 7440-20-2; Co, 7440-48-4; Ni, 7440-02-0; Cu, 7440-50-8; Sr, 7440-24-6; Zr, 7440-67-7; La, 7439-91-0; Pr, 7440-10-0; Nd, 7440-00-8; Y, 7440-65-5; polystyrene, 9003-53-6. LITERATURE CITED Benninghoven, A.; Jaspers, D.; Sichtermann, W. Appl. Apby. 1976, 1 1 , 35-37. Gardeiia, J. A.; Hercules, D. M. Anal. Chem. 1980, 52, 226-232. Campana, J. E.; Decorp., J. J.; Coiton, R. J. Appl. Surf. Sci. 1981, 8 , 337-341. Briggs, D.; Wooton, A. B. S I A , Surf. Interface Anal. 1982, 4 , 109-1 15. McIntyre, N. S.;Chauvin, W. J.; Martin, R. R.; McPhee, d. A. Scanning Electron Microsc. 1883, 1 1 1 , 1115-1127. McIntyre, N. S.; Martin, R. R.; Chauvln, W. J.; Winder, G. C.; Brown, J. R.; McPhee, J. A., submitted to fuel. Metson, J. 6.; Bancroft, G. M.; McIntyre, N. S.; Chauvin, W. J. S I A , Surf. Interface Anal. 1983, 4 , 181-185. McIntyre, N. S.; Fichter, D.; Robinsgn, W. 6.; Metson, J. 6.; Chauvin. W. J., submitted to S I A , Surf. Interface Anal. Ramseyer, G. 0.; Morrison, G. H. Anal. Chem. 1983, 55, 1963-1970.
N. S . McIntyre* W. J. Chauvin Surface Science Western Laboratory Faculty of Science University of Western Ontario London, Ontario N6A 3K7, Canada
R. R. Martin Department of Chemistry University of Western Ontario London, Ontario N6A 5B7, Canada
RECEIVED for review December 8,1983. Accepted March 26, 1984.
AIDS FOR ANALYTICAL CHEMISTS Liquid Chromatography of Cephalosporin C on Substituted Polystyrene Resins Daniel Sacco* and Edith Dellacherie Laboratoire de Chimie-Physique Macromol6culaire. C.N.R.S.-E.R.A. France The extraction of biological compounds from fermentation liquors usually involves many difficulties due to the presence of related contaminants and various mineral salts. Of the various purification methods, chromatographic techniques have become prominent (I), owing to the great variety of chromatographic media that are commercially available. Thus in the particular case of cephalosporin C, a P-lactam from which many semisynthetic antibiotics are produced, much work has been done to improve its separation by liquid chromatography. Because cephalosporin C is markedly hydrophobic at low pH, it can be purified moderately well by chromatography on activated charcoal columns ( 2 , 3 ) .The use of other nonpolar adsorbents, such as Amberlite XAD type resins (4-6),has made it possible to improve significantly the quality of these separations based on adsorption. Salto et al. (3, proceeding from the work of Pietrzyk et al. (8-11), determined what
23, ENSIC-1, rue Grandville, 54042 Nancy Cedex,
phenomena underlie the interactions between this compound, several of its derivatives, and macroporous styrene-divinylbenzene resins (XAD-4). In addition, cephalosporin C has been purified by combined use of anion-exchange and cation-exchange resins, though this method necessitates long and complicated procedures (12-14). More recently, a new analytical-scale separation procedure has been developed only for derivatives of cephalosporin C, using reversed-phase high-performance liquid chromatography (15). Commercial resins bearing CIS (16, 17) chains were chosen. Generally speaking, one can note that the main feature of the various adsorbents used for the separation of cephalosporin C from the contaminants in its biosynthesis medium is their relatively poor selectivity. Here we describe a study of styrene-divinylbenzene resins bearing immobilized ligands with characteristics intended to
0003-2700/84/0356-1521$01.50/00 1984 American Chemical Society
