854
Anal. Chem. 1984, 56,854-856
f1.9% to =k5.2%. The recovery studies for the same concentration range showed values of 93-102% with an average of 96%. These results are in accord with previous studies of coupled enzyme systems using the pCOz electrode (14). Registry No. MTX, 59-05-2; dihydrofolate reductase, 900203-3.
100
80 21
t .->
LITERATURE CITED
3
2 60 0
5 C
40 hp
20
0
6
12 18 Methotrexate, pg/L
24
30
Flgure 4. Calibration curve for methotrexate determination at pH 6.2 and 37 O C with 2.5 X M P-NADPH and 10 units L-' dihydrofolate reductase.
the range of clinical interest (12, 13). Precision and Recovery Studies. Table I shows both the within-run precision and recovery of methotrexate added to synthetic laboratory samples. In the concentration range of 1.5 to 12.0 pg L-I methotrexate, the precision ranged from
(1) Stryer, Lubert "Biochemistry", 2nd ed.; W. H. Freeman & Co.: San Francisco, CA, 1981; pp 526-528. (2) Falk, L. C.; Clark, D. R.; Kalman, S. M.; Long, T. F.. Clin. Chem. (Winston-Salem, N.C.) 1976, 22, 785-788. (3) Brown, L. F.; Johnson, G. F.; Witte, D. L.; Feld, R. D., Clln. Chem. (Winston-Salem, N . C . ) 1980, 26, 335-338. (4) Buice, R. G.; Evans, W. E.; Karas, J., Nicolas, C. A,; Sidhu, P.; Straughn, A. B.; Meyer, M. C.; Crom, W. R . Clln. Chem. (Winston-Sa/em, N . C . ) 1980, 26, 1902-1904. (5) Lankelma, J.; Poppe, H. J . Chromatogr. 1978, 149, 587-598. (6) AI-Bassam, M. N.; O'Sullivan, M. J.; Bridges, J. W.; Marks, V. Clin. Chem. (Winston-Salem, N . C . ) 1979, 25, 1448-1452. (7) Wannlund, J.; Azari, J.; Levine, L.; DeLuca, M. Biochem. Slophys. Res. Commun. 1980, 96, 440-446. (8) Ferrua, B.; Milano, G.; Ly, B.; Guennec, J. Y.; Masseyeff, R. J . Immuno/. Methods 1983, 60, 257-268. (9) Kinkade, J. M., Jr.; Vogler, W. R.; Dayton, P. G. 8iOChem. Med. 1974, 10, 337-350. (10) Przybylski, M.; Preiss; Dannebaum, R.; Fischer, J. Biomed. Mass Spectrom. 1982, 9 , 22-32. (11) Pontremoli, S.;Grazi, E. Methods Enzymol. 1986, 9 , 137-141. (12) Davis. J. E.; Solskv. R. L.: Gierina. L.: Malhotra, S. Anal. Chem. 1983, 55, 202R-214R. (13) Scheufler, Eckhard Clin. Chlm. Acta 1981, 1 1 1 , 113-116. (14) Hassan, S. S. M.; Rechnitz, G. A. Anal. Chem. 1982, 54, 303-307.
RECEIVED for review October 21, 1983. Accepted December 5 , 1983. We are grateful for the support of NIH Grant GM25308.
Electrochemical Probe for Simultaneous Extraction and Identification of Elements in Metal Alloys Daniel Mario Alperin, Victor Idoyaga Vargas, and Hector Carminatti* Instituto de Investigaciones Bioquimicas "Fundacion Campomar" and Facultad de Ciencias Eractas y Naturales, Universidad Nacional de Buenos Aires, Obligado 2490 (1428) Buenos Aires, Argentina A method based on well-known electrochemical procedures is currently used for the extraction and identification of metal alloys (1-3). In this method, the elements are passed to the ionic state with a dc source by using two electrodes, one applied to the metal and the other to a wet paper which is in contact with the sample. After the extraction, the paper is submitted to detection procedures. Here we describe a probe in which the electrodes are placed differently from those of the methods mentioned above. In this probe both electrodes are in contact with the paper. The paper of the probe could be used for the simultaneous extraction and identification of ions, requiring only a small area of the sample (e.g., 1 mm2). Principle of the Method. The scheme describing the functioning procedure of the probe is illustrated in Figure 1. Two platinum electrodes contact one side of the paper, whereas the other side contacts the metal. The electric resistance across the thickness of the paper is lower than across its length, thus permitting the generation of anodic and cathodic areas on the metal surface. Due to the low resistance across the thickness of the paper, the electrodes may be placed on either side of the paper without noticeable differences in the detection procedure. As shown in Figure 2, the electrodes are placed to self-retain the paper strip.
Ions liberated from the metal are attracted to the cathodic electrode passing through the paper and when the latter is chemically sensitized, they interact with the reagents as described below. Alternatively, an unsensitized paper could be used. After extraction, unsensitized paper may be submitted to conventional identification procedures (4,5).
EXPERIMENTAL SECTION The construction diagram of the probe is illustrated in Figure 2. A pencil-sized acrylic square bar holds two electrodes made of platinum sheet 0.25 mm thick. The paper must be inserted between the electrodes as shown in Figure 2. The electrodes must be positioned symmetrically with respect to the center of the bar and should be shorter than the acrylic support to avoid contact with the sample. Platinum electrodes are silver soldered to copper wires and fixed to the acrylic body with plastic screws and/or glued with a latex based adhesive. Dimensions are not critical and depend on the availability of materials or special needs. A dc current source was constructed with four 9-V batteries disposed so as to obtain 36 V. The source was connected to the probe terminals in series with a push button switch. Polarity of the current applied to the probe is indifferent. The paper used in all cases is Whatman No. 3 MM. Unless stated otherwise, our description of the method applies to the simultaneous identification of nickel, iron, and chromium in steel alloys (Table I) and gold, copper, nickel, chromium, and iron in
0003-2700/84/0356-0854$01.50/00 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984 .
_____-__.__-
Table I. Detection of Elements in Steel Alloys' % composition material type number
Fe Bal. Bal. Bal. Bal. Bal. Bal.
Ni
W5
__
Cr
iron
detection __________ nickel
chromium
5 0 A 645 5 18 202 8 18 304 17 10 316 12 19 317 2 12 414 0 0 100 Iron 0 0 100 Cr 0 0 100 Ni ' The percent composition of the materials listed was obtained as described under Experimental Section. Detection refers to the present method and positive ( + ) indicates the presence of the characteristic color band in the paper matrix. Bal. means balance to about 100%. urn E l e c t r o d e s
f
1
*'
1
(Liberated eiernents
Metal s u r f a c e
Scheme describing the functioning principle of the method. For explanations see text.
Flgure 1.
R 2;R E A C T I V E
Z O N E
Table 11. Detection of Elements in Different Alloys results of % detection, composition color of material elements (nominal) each band AU 100 violet gold green copper cu 100 gold, 1 8 K Au 75 violet cu 25 green 90 pink Chromel P Ni Cr 10 yellow 55 green Constantan cu Ni 45 pink Monel Ni 66 pink cu 31.5 green C, Mn, Si Bal. Cunife Ni 20 pink cu 60 green 20 brown Fe ' The colors indicated correspond to separated bands, See Experimental Seclion for details. Bal. means balance to about 100%. the sample. Current is then applied to the probe for about 5-30 s. Care should be taken so that the temperature is maintained over 15 "C during or after the test. Lower temperatures do not yield reproducible results.
RESULTS AND DISCUSSION
Flgure 2. Construction diagram of the probe. R, and R, correspond to the electric resistances indicated in Fig. 1. See text for details.
other metal alloys (Table 11). For this purpose, the paper is sensitized by immersing for a few seconds in an oversaturated solution of diacetyldioxime (lo%, w/v) in ethanol 96'70,drying in air, and cutting into strips. Plastic and glass materials should be used. Reagent paper remains stable for many years if stored dried in a closed dark container. The steel alloys described in Table I were analyzed by using a Model 9200 Texas Nuclear X-ray fluorescencespectrometer from Ramsey Engineering Co. The alloys listed in Table I1 were obtained from the following sources: solid gold and 18K gold, Central Bank of Argentina; copper, Riedel-de Haen, Hannover, Federal Republic of Germany; Monel, Foster Wheeler, Ltd., Berkshire, England; Chromel P and Cunife, General Electric Co., Schenectady, Ny; and Constantan from Siemens A.G., Munich, Federal Republic of Germany. The detection procedure is carried out as follows. The probe with the paper previously humidified with drinking water (total dissolved solids = 200 mg/L) or a 0.2 g/L sodium sulfate solution is placed in contact with a polished surface of the sample. It should be noticed that only the paper must be in contact with
Color bands could be observed: pink, if nickel is present; yellow, if chromium is detected; and both color bands if both elements are present. Iron appears as a brown band only in nickel alloys, but not in chromium nickel steels. These observations are illustrated in Table I. This phenomenon has not been futher studied because it is beyond the scope of the present work. However, the following tentative interpretation may be given. Due to the high resistance of the electrolyte across the thickness of the paper (R, in Figure 1)and the very low resistance of the metal, the potential difference generated between the anodic and cathodic zones of the metal is much lower than the source. It appears that the over-voltage needed to dissolve the iron from the chromium-containing alloys listed in Table I is not reached under the conditions of the experiments (6). If the voltage of the source is increased to about 72 V, the iron dissolution takes place and a brown band appears in the test paper (data not shown). Using the same procedure as described for steel alloys with the only difference that an oversaturated at (25 "C) solution of sodium chloride has been used as electrolyte, we have tested other metal alloys. The results are shown in Table 11. It should be mentioned that the diacetyldioxime sensitized paper was used in all the assays. To test the sensitivity of the method, the following experiment was made: An alloy containing 3.5% of Ni was
856
Anal.
chem. 1804, 56. 858-859
1 !.)
j ..I
j:: ~~
Flpure 3. Paper strip immediately after application of the detection procedure for 10 s to A 645 steel. The nickel band (a) appears pink In the wiainal and band (b). corresponding to iron. was dark brown. Cobm remain pwnmnenlly stable. The distance on the paper between the pointers from (a)to (b) corresponds approximately lo R, and ltm thickness of Me paper to R , as indicated in Figures 1 and 2.
tested hy this method, yielding a full color development in 10 s. However, alloys with lower contents of nickel were not analyzed by the present procedure. A paper strip obtained immediately after applying the detection procedure on A 645 steel is shown in Figure 3. These findings suggest that the method allows rapid and inexpensive large scale control tests of steel and other alloys. Since the method requires small areas of the sample, proximal mnes in the same sample and thin matinge muld be identified. Furthermore, samples located in arena of difficult access, e.g., pipes, covered with coatings as thick as 5-20 cm can he analyzed through a small hole. To sum up, this method allm the extraction, reaction, and separation of nickel and chromium in a few seconds from
different steel alloys and gold, copper, nickel, chromium, and iron from other alloys. At least for nickel, a comparable sensitivity to that of the standard methods was obtained. All these promsea ormr in the paper matrix of the probe. It must be mentioned that the probe itself is small, portable, versatile, and inexpensive. In addition, other elements may be detected by this procedure using a paper specifically sensitized. ACKNOWLEDGMENT We thank Luis F. Leloir, the other members of the Instituto de Investigaciones Bioquimicas "Fundacion Campomar" and Alberto Meschini from Universidad Nacional del Sur (Argentina) for helpful discussions and criticism. D.M.A. is a Fellow of the Consejo Nacional de Inveatigaciones Cientificas y Tecnicas (Argentina). H.C. and V.I.V. are Career Investigators a t the same institution. Registry No. Fe, 7439-89-6; Ni, 1440-02-0;Cr, 7440-47-3; Au, 7440-57-5; Cu, 7440-50-8; Chrome1 P, 12605-72-0; Constantan, 12605-79-7;Monel, 11105-19-4; Cunife, 39451-89-3; steel, 1259769-2; stainless steel, 12597-6&1; diacetyl dioxime, 95-45-4. LITERATURE C I T E D (1) RDurdve tu Ebardyik Spot Test A b y Cast. 811. 1945, 4 , 8. (2) Butler. J. A. V. '"ElacQical Rsnomena at Inlerlau~sin Chsm(rby. physics and B W . McMIllan Publishing CC.: New Y a k . 1951: p
123.
(3) Minlusd. C. L.: Meda. J.: Pas& R. LEMITAn. 1978. 2 . 15. (4) "Annual Book 01 ASTM Standards". R. 12; Chsmlcal Amtysb ot Metab; SampHng and ~nabsbof MI &ring Ores: ASTM: mladec ma. PA. 1981
REYXNEDfor review September 22,1983. Accepted December 9, 1983.
High-Pressure Acid Dissolution of Refractory Alumina for Trace Element Determination Henry A. Fonerl
. ..
Ceramics Deoartment. The Houldsworth School of ADDlied Science, The University of Leeds, Leeds LS2 GT, Great Britain High-purity refractory aluminas have important uses in the electronics and ceramics industries. Doped single-crystal aluminas such as ruby and sapphire are used in the construction of lasers. For electronics and electrical purposes sintered IreerystaUized" (i.e., polycrystalline) alumina is often used. Single-crystal alumina (mp 2050 "C) has been fused during fabrication while the other types have also been heated to very high temperatures. Above 1200 OC alumina converts into the a structural form. The physical, electrical, and manufacturing properties of alumina are all much affected hy the levels of impurities present, and hence the determination of trace elements in these materials is of great practical importance. The extreme hardness of the material (9 on the Mho d e ) makes it difficult to grind without introducing extraneous elements (2-3). It is impossible to dissolve a-alumina by a simple acid treatment and the necessity of using alkaline fluxes to dissolve the samples for analysis is a source of contamination. De arc spectrographic methods have also been used to analyze alu'Preaent addreea: The Geological Survey of Israel, 30 Malkhe Yisrael St., Jerusalem 95 501, Israel.
minas ( 3 , 4 ) , hut not very successfully (5). Acid solutions are convenient for analysis hy modern instrumental methods and. using high-purity acids, it is psaible to achieve very low blanks. A number of authors have attempted to dissolve a-alumina in acids a t normal pressures (6-9). This author has tried to dissolve finely ground fused alumina (-Zoo mesh) by these methods with very little success. By use of Mendlina's phosphoric/sulfuric acid mixture (3, the sample was dissolved with great difficulty (3 to 4 h) and with significant loss of material due to humping. Since, thermodynamically, a-alumina should be readily Boluble in acid, attempts have been made to achieve solution by increasing the reaction temperature (and hence pressure). Two principal approaches have been described in the literature: (a) the use of singlechamber sealed pressure vessels and (h) the use of double-chamber pressure vessels comprising a sealed tube containing the sample, with a compensating outer pressure to prevent the inner tube from bursting. Single-Chamber Pressure Vessels. The use of PTFElined bombs to decompose refractory materials and rocks is well-known (10-12) and this type of apparatus has also been used with alumina (13). A small (3.5 mL) platinum-lined 0 1984 Amdcan ChsmlcaI Socler,
