Table 111. Panama Canal Zone-Atmospheric Site Time Albrook Tower Top 0930-1030
Albrook Tower Bottom Fort Sherman
1145-1245 1400-1500 0930-1030 1145-1 245 1400-1500 0930-1030 1145-1 245
H a s Samples ppb H2S 0.07 0.21 0.26
0 0.11 0.32 0.06 0.29
Some of these materials were trapped in the 1.ONNaOH, giving a fluorescence nearly equal to that of the FMA in the Turner filter fluorometer. The analyzed samples therefore showed a higher fluorescence than the FMA without Sa-. This problem was overcome by measuring the fluorescence of the sample prior to the addition of FMA, and subtracting the preanalyzed fluorescence from the final value. Possible alternatives which were not investigated include the use of different wavelengths and/or narrower pass bands for both excitation and fluorescence. Field Sampling. On 31 October 1968, the method described above was applied to the measurement of atmospheric H2S at previosly established sites (11) in the Panama Canal Zone. Air was passed at 2 l./min through 10 ml of 1.ON NaOH for one hour using 5 % KHCO, prefilters. The samples were analyzed the following day. Results are shown in Table 111. The NO, level was about 1 ppb, so no attempt was made to correct for any possible error from this source. The measured values are 10 to 100 times lower than the values predicted by Junge (12). However, the SOz concentrations obtained during the same sampling period were also much lower than Junge’s values.
No interference was noted for Mn2+, Ni2+, Co2+, Cuzf, Cd2+, Pbz+, Fez+, Fe3+, K f , and Na+ at a 1000-fold mole excess. This is not in agreement with Griinert, Ballschmiter, and Tolg (5). Air containing NO2 and SO2 was drawn through a bubbler containing 2 X lO-7M S in 1 .ONNa0J-I. At concentrations greater than 10 ppb these gases tended to react with the Sachanging it to a form that resists analysis. Sixty liters of gas with 100 ppb of these contaminants would eliminate all of the Sa- present. Prefilters impregnated with 5 % K H C 0 3 as described by Pate, Lodge, and Neary (10) eliminated the SOz interference while allowing H2S to pass at concentrations as low as 1 ppb. Lower H2S concentrations were not checked. However, NO2 could not be removed from the air prior to reaction with the Sa-. Anti-oxidants such as glucose, ascorbic acid, and fructose, were added to the Sa- solution, but they did not inhibit the NOa interference. Sulfamic acid also did not help. Thus, NO2remains the major interferent. Hydrogen sulfide determinations made recently in the Panama Canal region showed an enhancement of fluorescence levels caused by large amounts of organic materials in the sampled air (as evidenced by gas chromatographic data).
RECEIVED for review March 3, 1969. Accepted August 15, 1969. Presented in part at the 10th Conference on Methods in Air Pollution and Industrial Hygiene Studies, San Francisco, Calif., February 19-21, 1969. The Panama experiments were supported by the U. S. Army Research Office Department of Defense. The National Center for Atmospheric Research is sponsored by the National Science Foundation.
(10) J . B. Pate, J. P. Lodge, Jr., and M. P. Neary, Anal. Chirn. Acra, 28, 341 (1963).
(11) J. P. Lodge, Jr., and J. B. Pate, Sci., 153, No. 3734,408 (1966). (12) C. E. Junge, “Air Chemistry and Radioactivity,” Academic Press, New York, N. Y., 1963.
ACKNOWLEDGMENT
The authors acknowledge the helpful assistance of John B. Pate, Arthur F. Wartburg, and Miles D. LaHue.
X-Ray Emission Analysis of Paints by Thin Film Method J. D. McGinness, R. W. Scott, and J. S. Mortensen‘ Analytical Research Department, Sherwin- Williams Research Center, Chicago, 111. 60628
QUANTITATIVE X-ray emission analysis of paints is seriously complicated by the extreme variation in the absorption coefficient of the matrix due to the wide variety of compounds which may be used for the pigmentation. Methods for determination of specific elements in complex systems by means of matrix matching, dilution, and thin film-controlled mass have been reported (1-6). However, none of these techniques alone have been found adequate when applied to the analysis of paint. 1 Deceased. (1) E. L. Gunn, ANAL.CHEM., 29, 184 (1957). (2) Ibid.,33,921 (1961). (3) B. J. Mitchell, ibid.,p 917. (4) H. A. Liebhafsky, H. G. Pfeiffer, E. H. Winslow, and P. D. Zemany, “X-Ray Absorption and Emission in Analytical Chemistry,” John Wiley and Sons, New York, 1960. (5) S. A. Bartkiewicz, and E. A. Hammatt, ANAL. CHEM., 36, 833,
(1964). (6) M. L. Salmon, “Advances in X-Ray Analysis,” Vol. 5, W. M. Mueller, Ed., Plenum Press, New York, 1962, p 389. 1858
In the present study the several techniques for solving the matrix problem have been combined. The liquid paint sample is diluted in an internally standardized varnish matrix and then analyzed in the form of a dried thin film for several pigment elements in this single sample preparation. Preliminary comments on this approach were reported in 1965 (7); the final procedure and some initial results were reported by the authors in 1966 (8). This paper constitutes a report of the results obtained by this procedure and the conclusion that the procedure essentially eliminates matrix or interelement effects in the analysis of paints by X-ray emission.
(7) J. D. McGinness, P. J. Secrest, and D. J. Tessari, “Standard Methods of Chemical Analysis,” 6th ed., Vol. 111, Part B, F. J. Welcher, Ed., D. Van Nostrand Co., New York, Chap. 54, 1966. (8) J. D. McGinness, R. W. Scott, and J. S. Mortenson, Paper No. 157 (unpublished), Fifth National Meeting of the Society
for Applied Spectroscopy, June 1966, Chicago, Ill.
ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969
EXPERIMENTAL Instrumentation. A Norelco Universal Vacuum spectrometer equipped with a scintillation detector, a flow proportional counter, lithium fluoride and ethylene diamine ditartrate analyzing crystals, and a tungsten target FA60 X-ray tube was utilized. A full-wave rectified power supply operating at 50 kV and 40 mA was used to operate the FA60 X-ray tube. A Norelco Mark I electronic circuit panel equipped with a pulse height analyzer was used to record all data. Reagents and Materials. An internal standard stock solution was prepared which consisted of a quick drying alkyd resin in methyl ethyl ketone to which was added 0.1 % copper in the form of copper naphthenate (Nuodex Products, Elizabeth, N. J.) The nonvolatile content of the stock solution was approximately 60%. A Gardner Ultra applicator (Gardner Laboratory, Inc., Bethesda, Md.) was used to prepare uniformly thin films of the prepared standards and unknown samples. Aluminum foil (J. T. Baker Chemical Co., Phillipsburg, N. J.) in the form of sheets 6 in. X 6 in. X 0.001 in. was used as the substrate in most cases. One-quarter mil Mylar film (Cadillac Plastic and Chemical Co., Chicago, Ill.) can also be used as a substrate. Procedure. Standards and unknowns were prepared for analysis by a sixty-fold dilution of the samples in an internally standardized varnish. This was done by first weighing accurately about 30 grams of the standard varnish into clean 1/4-pint paint cans or 2-ounce glass jars. To this varnish was added the proper weight of standard or unknown paint. A series of calibration standards was prepared from single of titapigment containing paints to contain 0 to 0.8 wt nium, calcium, and lead. For analysis of unknowns approximately l / l gram of paint, accurately weighed, is taken. Several 6-mm diameter polished borosilicate glass beads are added, and the containers are closed and shaken for 15 minutes on a paint shaker. The diluted samples were then prepared as thin films on aluminum foil by use of the Gardner Ultra applicator set to produce a 1-mil wet film, This device provides a means of drawing a liquid material down over a substrate with a constant clearance set between the l/s-in. flat blade edge of the device and the substrate. A relatively uniform thin film results, depending on the flatness of the substrate, which is about 4 inches wide. However, as later discussed, the need for careful control of film thickness uniformity is minimized by use of the Cu internal standard. The wet films were dried at 105 "C for ten minutes. A 1.25-in. disk was cut from each standard film and then inserted into the X-ray spectrometer. Ratios of the intensity of the K a line of Ti and Ca cs. the intensity of the Ka second order line of the Cu internal standard and of the PbLPl and LP3 line intensity cs. CuKa first order line intensity were determined. The intensity ratios for the standards were plotted against the known weight per cent concentrations of Ti, Ca, and Pb in the standards. By means of these plots, the intensity ratio obtained on the unknown samples was then converted to element concentrations.
z
RESULTS Standard working curves were obtained for Ti, Ca, and Pb which were linear in the 0 to 0.8z element concentration range. Quantitative data obtained with several different paint systems and pigments are presented in Table I. Because most of the matrix effects appear to have been eliminated, it appears that an element may be determined in the presence or absence of other elements by reference to a single calibration curve. Standards may be prepared to contain multiple pigment elements or single pigment elements, either as dispersions of actual insoluble pigments or, for most paints,
Table I. Quantitative Results Element % Ekment Sample or paint matrix sought Chemical X-Ray Alkyd resin Ti 11.2 11.7 TiO2, Si02 Ti 4.5 Lacquer 4.7 TiOz, BaSO4, ZnS Ti 7.4 6.9 Pigment 7.4 Ti02-CaS04a 7.5 Ti 14.4 15.1 Enamel 14.4 TiOz 15.2 A. Silica encapsulated TiOna Ti 17.6 18.9 B. Silica encapsulated TiOp Ti 17.6 17.6 Ti Alkyd resin 13.7 13.0 TiOt, Sios, CaC03, Talc, Pb, & Ca driers Pb Alkyd resin 5.2 5.5 PbCrO4, PbrO4, Fe203, mica 2.1 Pbb Asphalt 2.4 PbO, Pb, Al, Carbon black Pb Alkyd resin 10.9 10.6 Pb304, ZnCrO4, Fe203, talc 11.4 11.4 Ca Alkyd resin 5.7 5.4 CaC03, TiOz, Talc, Pb, & Ca driers Pigments were completely dispersed in the copper containing stock solution and resulted in a very low-pigment content "paint" which was then analyzed without further dilution. * Note presence of lead both as oxide and as lead metal.
Table 11. Matrix Effects in Pigments Intensity C/S element Ka Element sought" Light matrix Heavy matrix Titanium 3,480 1,060 Antimony 6,260 630 Iron 22,270 10,240 a Analyzed as 1 concentration of oxide in siliceous extender' Sios,Mg, Ca, light matrix; or in basiclead chromate, Pb0.PbCr04, heavy matrix.
as solutions of organic compounds of the element. Of considerable advantage is the fact that the dried standard films may be retained for later reference. Given a wet sample of paint (assuming, of course, that the X-ray equipment is on and the proper working curves have been prepared) one should be able to determine major pigment elements on the total sample basis in not much more than one hour, well before such standard characteristics as nonvolatile matter may be determined. The differences between duplicate determinations should not exceed 7.8 of the amount present more than one time in twenty. The difference between the average of duplicate determinations and the chemical value or the true value should not exceed 10% of the amount present more than one time in twenty.
z
DISCUSSION The data in Table I1 from the authors' laboratory illustrate the seriousness of the matrix effect in paints. These data were obtained on pigment powders (smaller than 325 mesh) at infinite thickness. Figure 1 perhaps even more graphically illustrates the situation.
ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969
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t N O N L I N E A R DETECTOR RESPONSE
CHROMATE M A T R I X
0
I
I
1
I
I
I
I
I
I
I
10
20
30
40
50
60
70
80
90
100
0
IO
20
30
50
40 PERCENT
PERCENT
8,
1102
w
I
0.2
0.4
PERCENT
0
0.2 PERCENT
100
which the internal standard provided. Copper was chosen on the basis of its position on the periodic chart, its detectability by more than one X-ray detector, and the fact that it is not a common pigment element. Zirconium soaps have been used as the internal standard when copper is present in the paint sample being analyzed. Figure 4 contains the data for a titanium containing paint diluted and internally standardized, but drawn down at different thicknesses. As can be seen, the one mil draw down provided the best line, and as expected the ten mil draw down was too thick, absorption effects being evident. Data are not shown, but several different dilutions were also tried. Considering sufficient dilution to minimize the matrix effect, while at the same time not diluting beyond a point of maintaining enough sensitivity to provide adequate precision for semiquantitative analysis, it was concluded that a 60-fold dilution was optimum for the systems investigated. The
0
0.4
0.6
0.8
T I IN D I L U T E
06
08
SAMPLE
Figure 4. Internal standard thickness comparison
11 IN DILUTE S A M P L E
Figure 3. Internal standard thickness control 0
90
Figure 2. 1-60 dilution in LiC03-starch
For dry pigment powders, the matrix dilution technique outlined by Gunn ( I ) was found useful in reducing the absorption effects. The pigment sample was diluted in a 1 :1 mixture of starch:lithium carbonate, mixed by hand in a mortar and pestle, and then pressed under a 10-ton load into a 11/4-in. diameter pellet or briquet. The usual portion is 100 mg of pigment per six grams of diluent. The quantitation of a given element is much improved, as illustrated for Ti in Figure 2, but a different standard is still required for each paint. Although paint films are applied “thick” in practice by the paint customer, it was found that uniform thin paint fiims of less than a few microns effective thickness (with respect to heavier inorganic pigment elements) could be prepared for purposes of X-ray analysis by dilution of the paint with a varnish prior to preparation of the film. Because it was difficult to reproduce specific film thicknesses, it was also decided to include an internal standard in the varnish. To a degree then, all three of the classical approaches to eliminating matrix effects were used-light element dilution, thin film, and internal standardization. Figure 3 illustrates the spread of data which resulted without the internal standard, as well as the resultant straight line
1860
80
Ti02
Figure 1. Matrix effect on titanium
-
70
60
ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, N O V E M B E R 1969
0 = 1 mil
*
drawdown
= 3 mil drawdown = 10 mil drawdown
It appears that this thin-film method may be good enough to replace many of the wet chemical methods now employed for analysis of pigments separated from paint. Indeed, the method looks so promising that, given a dry pigment powder mixture, it would appear that making a paint out of the sample prior to analysis might be proper. This was done for some of the examples given in Table I. However, in doing this, special attention needs to be given to redispersing the pigments, as agglomerates of undispersed pigments which are much greater than one micron in diameter would be expected to have as severely reduced X-ray intensities as those observed for iron and zinc particles of only 8-micron size (9). When in doubt about the particle size ranges involved or the degree of dispersion, the technique of microscopic examination is recommended (9). If particle sizes are too large, then it would appear critical that standards be prepared from pigments of similar particle size to those observed in the unknowns.
working curves and the results shown in Table I were developed on this basis. Although the method is described as thin film method, one mil (or one-half mil dry film thickness) is not really thin in the trLle sense of the word. However, on the basis that the paint is diluted 60 times with respect to heavier pigment elements, it is, with regard to X-ray absorption, below infinite thickness and approaching a true thin film. Fortunately, for the X-ray spectroscopist in the paint industry, most of the pigments used in paints are chosen or manufactured to have particles of 0.2-micron size. According to some recent work of Gunn (9) particles of this size should provide X-ray intensities which approximate the intensity of the pigment elements in solution. The fact that standards prepared here from insoluble pigment dispersions fell on the same working curve as standards prepared from soluble metal organics tends to verify this. Also fortunate is the fact that when larger particle size pigments or extenders are used in paints, they are usually compounds of light elements such as Mg, Al, Si, and Ca. Again based on Gunn’s recent work and his experience with Si02 particles suspended in liquid hydrocarbons, these materials would not be expected to produce X-ray intensities which deviated from those expected for solution standards until the particle sizes ranged well above the onemicron size considered to be limiting for suspensions of metallic particles of iron and zinc (9).
The authors acknowledge the assistance of the SherwinWilliams Resin Research Dept. in preparation of a special fast drying varnish vehicle and the valuable background information developed by Roger L. Harper during the period that he was assigned to the Analytical Research Dept. as an X-ray spectroscopist.
(9) E. L. Gunn, “Advances in X-Ray Analysis,” Vol. 11, J. B. Newkirk, G. R. Mallett, and H. G. Pfeiffer, Eds., Plenum Press, New York, 1967, p 164.
RECEIVED for review June 6, 1969. Accepted September 5 , 1969.
ACKNOWLEDGMENT
___
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Diffusion-Thermal Ionization Source for Mass Spectrometric Assay of Trace Metals W. G . Myers and F. A. White Department of Nuclear Science, Rensselaer Polytechnic Institute, Troy, N . Y. 12181
THEREARE many types of mass spectrometer ionization sources, none of which can be considered to be ideal. For quantitative mass spectrometric analysis of solids and liquids, the surface or thermal ionization source has distinct and important advantages over several other types, in addition to a high sensitivity. Although multiple-filament thermal ionization sources (I-3) are in wide use today, the single-filament source, especiallythe “V” configuration (4,offers many advantages [i.e., simplicity, superior geometric and ion optical properties to provide a higher transmission (5, 6), lower background emission ( I , 6), less warpage, etc.] to warrant its use for general analytical purposes. However, the usefulness of single-filament thermal ionization sources is severely limited by the fact that the rate of evaporation of a sample and the temperature of the ionizing surface cannot be varied inde(1) M. G. Inghram and W. A. Chupka, Rea. Sci. Instr., 24, 518
(1953). (2) H. Patterson and H. W. Wilson, J. Sci. Instr., 39, 84 (1962). (3) W. E. Duffy and R. E. Carr, Fourteenth Anriiial Conf. Mass Spectry. & Allied Topics, May 22-27 1966, p 54. Rea.. (4) . , F. A. White, T. L. Collins, and F. M. Rourke,. Phvs. . 101, 1786 (1956). ( 5 ) L. A. Dietz. Rev. Sei. Instr.. 30. 235 (1959). (6j C. M. Stevens “Analysis of Essential Nuclear Reactor Materials,” c. J. Rodden, Ed., U. S . AEC, 1964, pp 1023-4.
pendently. The chemical form of the sample used on this type of ionization source is obviously very important. Consequently, in the analysis of most elements, a single-filament thermal ionization source is usually constrained to operate well below its potential efficiency, in a temperature range which will provide a reasonable degree of ionization, without an excessively rapid depletion of the sample via evaporation. A new technique has been conceived for thermally ionizing small quantities of the chemical elements on single-filaments, and for minimizing the production of spurious ions that give rise to errors in isotopic ratio measurements. The concept of this new single-filament source is to utilize a diffusion mechanism in addition to thermal surface ionization, in order to obtain a higher ionization efficiency and a mass spectrum which is not obscured by impurities or superimposed molecular spectra. The sample, together with its isotopic spike, is encapsulated within an appropriate filament that is made of a refractory metal of high work function. The filament is then operated at a high temperature, which causes the sample atoms to diffuse to the exterior surface where a fraction becomes ionized by thermal ionization. This approach provides a number of advantages while still retaining those previously cited for single-filament sources. These include : prompt loss of the sample in the form of neutral vapor is minimized; the diffusion time required for the sample atoms to reach the
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