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Anal. Chem. 1981, 53, 2148-2149
Determinatlon of Trace Amounts of Cerium in Paint by Inductlvely Coupled Plasma Atomic Emission Spectrometry Kit L. Wong NL ChernicaldNL Industries, Inc., P.O. Box 700, Hightstown, New Jersey 08520
Cerium occurs as a contaminant or as a tag element in many commercial materials. Although a number of analysis methods are known for cerium, each method still has drawbacks. The major problems which frequently arise when performing a cerium analysis include: (a) insufficient sensitivity, (b) serious matrix effects, and/or (c) tedious and time-consuming sample pretreatments. Any or all of these factors can lead to poor accuracy and reproducibility. Thus, there still exists a need to develop a single, rapid and accurate method for analyzing ultratrace to macroscopic amounts of cerium. Inductively coupled plasma/optical emission spectrometry (ICP-OES) has been reported to offer a comparatively lower detection limit for cerium ( I , 2). The detection limits of several methods are compared to that of ICP-OES in Table I. Butler et al. (2) have clearly demonstrated ultratrace detection of Ce in low and high alloy steels. Because commercial paint samples represent a complex but entirely different matrix than alloy steels, this study was undertaken to test the application of ICP-OES and to study the effects of certain instrumental factors, foreign ions, solvents, etc. when performing cerium analyses on a commercial paint sample. EXPERIMENTAL SECTION The principal components of the instrumentation and the numerical values of some operating parameters employed in the present study are listed in Table 11. Approximately 10.0 g of a yellow traffic paint type mixture was heated slowly on a hot plate to dryness. The dried sample was transferred to a furnace and ashed at 450 “C for 6 h to destroy organic material. The sample was then dissolved in an acid mixture containing 10 mL of “Os, 10 mL of HCl and a few drops of HzOz. The resulting solution was diluted to 100 mL with distilled water and filtered; a 10-mL aliquot was pipetted and diluted to 100 mL for the analyses. The dilution step was employed to minimize the salt content in the sample. In preparing the reference solution, 1%v/v of “OB, HC1 and a few drops of HzOz,and proportional amounts of Si, Pb, and Cr were added to the pure Ce solutions to match the overall composition of the paint sample. R E S U L T S AND DISCUSSION Chemical a n d Ionization Interferences. Several workers ( 3 , 4 )have reported the determination of Ce by flame atomic absorption and flame atomic emission. In the review of these studies, several problems are seen to arise, and also, these two techniques have limited applicability for paint analysis. The four major problems with flame atomic absorption and flame atomic emission are (a) poor sensitivity, (b) considerable ionization interference (-60%), (c) tendency of cerium to form a stable cerium monoxide, and (d) serious interelement interferences. The possibility of stable compound formation interference in toroidal ICP is virtually negligible; not only is the temperature in the plasma observation zone high enough to achieve complete atomization but also the time the aerosol takes to travel through the “tunnel” in the plasma before it reaches the observation zone is relatively long. Thus, all compounds are likely to atomize completely ( I ) . T o study the effects of the major elemental components of the paint on Ce, we added 400 ppm of Si, Pb, Cr, and Na individually to the reference solution. The intensities of these solutions are compared with that of the pure Ce solution. The results are shown in Table 111. The data indicate that the
Table I. Comparison of Detection Limits methods
detection limit, ppm
ICP-OES ( 1) flame atomic emission (3) flame atomic absorption ( 4 ) colorimetry ( 5 ) amperometry ( 5 ) neutron activation ( 5 ) mass spectrometry (6) optical emission spectrographic ( 7 ) (Stallwood Jet)
0.0004 28 30 0.25 500 0.005 0.001 5
Table 11. Specification of the Components of the Instrumentation and the Operating Parameters monochromator
ISA, Model J Y 38P, 1 m focal length, 2400 grooves/mm halographic grating
plasma HF generator
Plasma Therm, Model HFS 1500D,27 MHz
forward power reflected power plasma argon flow rate auxiliary plasma
1.0 kW
4 w 25 L/min flow is turned off for normal operation 1 L/min is used for starting plasma 1 5 mm above load coil stainless steel nebulizer 0.9 L/min
-
argon flow rate vertical observn zone nebulizer aerosol carrier argon flow rate sample uptake rate data acquisition system
2.0 mL/min spectra recorded on a strip chart recorder
Table 111. Effect of the Foreign Ions (400 ppm Each) on Cerium (10ppm) solutions solutions % error Ce Ce
+ Na + Pb
-1 0
Ce t Cr Ce t Si
% error
0
-3
interelement interferences are small. The interelement interferences can be eliminated completely if so desired, by matching the major components for both sample and reference solutions. Larson et al. (8)observed that ionization interference was negligible in their study. A similar result was observed for Ce during the present investigation. Although the ionization potential of Ce is relatively low, the ionization interference is insignificant if the plasma region observed by the viewing field of the spectrometer is optimized. T r a n s p o r t Interference. Transport interference is due to the fluctuation of aerosol carrier argon flow rate. This fluctuation in flow rate is due to the changes in the physical properties of the solution, such as the surface tension, viscosity, droplet size, etc. Since the sample flow rate is relatively small for ICP compared with that of flame atomic absorption, a small fluctuation in the sample uptake rate may greatly affect the intensity of the analyte. This could be, in principle, a
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Anal. Chem. 1981, 53. 2149-2152
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Table IV. Effect of Various Solvents on Cerium Analysis (10%v/v on 20 ppm of Cerium) re1 net exptl vs. solutions intens actual value, % distilled watera HNO, HC1 "30, H,PO, H,SO, ",OH ethanol a
81 68 68 66 55 58
100
27 77
84 84 81 68
72 33 95
Standard solution.
major disadvantage of ICP over FAA. Several different solvents were used in this work for determining the degree of thle transport interference. The results are shown in Table IV. The data suggest that the solvent composition of the sample should be matched as closely as possible to that of reference solutions. Nebulizer Effects. T'hree of the most popular nebulizers, stainless steel, cross-flow, and concentric glass nebulizer, were compared for their capability of efficient aerosol generation. By optimization of the operational conditions for each nebulizer, all three devices produced similar detection limits, with stainless steel being the best. The stainless steel nebulizer was used in the paint analysis because this device can tolerate a relatively higher salt content in the sample. The other two nebulizers were clogged partially or completely a few minutes
after starting the analysis. These nebulizers require frequent cleaning to restore the original flow rate of the aerosol and necessitate restandardization of the ICP. The present study establishes a relatively simple and rapiid analytical procedure for determining the ultratrace amount of cerium in paint samples by using ICP-OES. The determined paint concentration of Ce was 0.032% 0.001 which is within the range of the manufacturer's specification. The data indicate good agreement for both direct determinatioln and standard addition methods. The calibration curve extended over a broad smalytical dynamic range for Ce (0.2-700 ppm). This implies that both macroamounts and ultratrace amounts of Ce can be analyzed for in a single reference solution.
*
LITERATURE CITED (1) Boumans, P. W. J. M.;de Boer, F. J. Spectrochim.Acta, Part B 1975, 308 309-340. (2) Butler, Constance C.; Knlseley, R. N.; Fassel, V. A. Anal. Chern. 1975, 47, 825-829. (3) Kniseley, Richard N.; Butler, Constance C.; Fassel, V. A. Anal. Chern. 1969. 41. 1494-1496. (4) Thomas, P. E. "Resonance Lines", Cary Instruments: Monrovia, CA, Vol. 1 (I), p 6. (5) Yoe, J. H.; Koch, H. J. "Trace Analysis", 1st ed.; Wlley: New York, 1957; p 626. (6) Roboz, J. "Introduction to Mass Spectrometry, Instrumentation and Technlques", 1st ed.; Wlley: New York, 1988; p 390. (7) "Seml-Quantitative Spectrochemical Analysis"; Spex Industries, Ino.: Metuchan. NJ. 1964. (8) Larson, G: F.;'Fassel, V. A.; Scott, R. H.; Kniseley, R. N. Anal. Chem. 1975, 47, 238-243
RECEIVED for review April 24, 1981. Accepted July 8, 1981.
Polyatornic Interferences in High-Resolution Secondary Ion Mass Spectra of Biological Tissues Margaret S. Burns Depatfments of Ophthalmology and Biochemistry, Albeti Einstein College of Medlcine/Montefiore Hosptial and Medical Center, 11 1 East 210th St., Bronx, New York 10467
Formation of polyatomic species during ion bombardment is a complicating factor in secondary ion mass spectrometric analysis of materials (1,2).Biological specimens are almost entirely C, H, N, and 0 and it is expected that there will be many hydrocarbon polyatomic species which can interfere with quantitative determination of elements of interest in physiological experiments ( 3 , 4 ) . In the present work high mass resolution spectra of tissue preparations are examined to assess the importance of these potential interferences.
EXPERIMENTAL SECTION Eyes were enucleated from cats, anesthetized with Nembutal, or from toads subsequent to guillotining. The eyes were quickfrozen in an isoperitane slush cooled in liquid nitrogen and lyophilized by use of a chemisorption vacuum system (5). The samples weire held at -30 "C until dry. Samples of retina and choroid a few millimeters square were cut and directly embedded in Spurr's law-viscosityresin. Ten micrometer sections were cut with a dry glass knife, pressure mounted on a boron-doped silicon wafer (Monwanto, Inc., Dedham, MA) and overcoated with 500 A of 99.99% Au to provide conductivity. The Cameca IMS-3F (Charles Evans and Associates, San Mateo, CA) was operated with either positively charged cesium primary ion beam or with positively charged oxygen. Negative secondary ions were collected under cesium bombardment and positive ions with oxygen. Oxygen bombardment of 5.5 keV was used with a 250 pm X 250 /bm raster and 500 nA beam current. 0003-2700/81/0353-2149$01.25/0
High mass resolution spectra of selected areas were taken by use of a 50 pm diameter field aperture. Cesium bombardment was at 14.5 keV and 100 nALbeam current and the same geometry as with oxygen.
RESULTS AND DISCUSSION The type of biological tissue examined in this study is not crucial because we are examining ions that are prevalent a t high concentrations in all tissues. The retina and choroid provide a convenient preparation because they consist of different anatomical structures with known differences in chemical composition (Figure 1). For example, the nuclear areas are largely deoxyribonucleic acid and have a higher I' content than other cell structures. It has been shown that the choroid and pigmented structures are rich in calcium intensity (6). With oxygen bombardment the positive secondary ions arle predominantly the alkaline elements (Figure 2). The high mass resolution spectrum at 23+, 24') 35+, 39+, and 40' from the region of cat choroid is typical of the spectra obtained from all tiseue areas (Figure 3). Sodium, 23') i13 always a single peak. Magnesium, 24+, is always a doublet and sometimes contaiins a poorly resolved shoulder on the C2 peak. This is probably NaH'. A mass resolution of 10000 is necessary to separate these signals at 10% valley definition (Table I). The relative height of Mg+ to Cz+varies depending 0 1981 American Chemical Society
