Combined Thin-Layer Chromatography/Flameless Atomic Absorption Method for the Identification of Inorganic Ions and Organometallic Complexes Haleem J. Issaq' and Edward W. Barr NCI Frederick Cancer Research Center, P.O. Box B, Frederick, Md. 2 170 1
Thin-layer chromatography (TLC) is an excellent microseparation method but it is unfortunately a method of little value with regard t o the identification of the separated substances. On the other hand, spectroscopic methods are extremely useful €or t h e elucidation of the structure of substances but these methods require relatively pure sample material. Chromatographic a n d spectroscopic methods, however, complement each other ( 1 ) . T h e use of combined techniques for t h e analysis of samples is useful and, in most cases, time-saving. Numerous gas chromatography/mass spectrometry (GC/MS) coupling systems have been described ( 2 ) .Recently, the coupling of liquid chromatography/mass spectrometry (LC/MS) was reported (3, 4 ) . T h e combination of atomic absorption spectrometry (AAS) a n d GC t o determine the mercury content of various food stuffs (for example, methylmercury and other organomercurials) was employed with total levels of mercury in the same sample measured by AAS ( 5 ) .Inorganic mercury is then taken as the difference between levels of total and organic mercury. Although the use of TLC/MS and TLC/IR have been reported ( I ) , the combination of TLC/AAS was not reported. This paper reports the combination of thin-layer chromatography with flameless atomic absorption spectrometry (FAAS) to identify an inorganic compound in an impure organometallic complex and to determine the recovery a n d purity of organometallic samples.
EXPERIMENTAL Reagents and Materials. Silica gel 60 precoated TLC plates (EM Labs, Inc., Elmsford, N.Y.) were used directly with no required preactivation. All solvents were previously distilled in glass (Burdick and Jackson, Muskegon, Mich.). Tellurium diethyldithiocarbamate solutions were made up in chloroform at a concentration of 1.716 mg/ml and quantitatively spotted with 10 g1 Drummond microcap. Tellurium diethyldithiocarbamate was purchased from R. T. Vanderbilt, New York, N.Y. The plate was developed in hexane-benzene-acetone (65:1012). Apparatus. A.A. A Perkin-Elmer Model 403 atomic absorption spectrometer with a strip chart recorder, a Te hollow cathode lamp (Westinghouse) and a Perkin-Elmer HGA-2000 hollow graphite atomizer with modified graphite tubes (6) was used. Eppendorf microliter pipets with diposable plastic tips were employed to introduce the sample into the graphite tube furnace. The polypropylene tips were thoroughly washed with distilled deionized water before use to prevent any contamination. TLC. A Camag Eluchrom Automatic Elution System (Camag Inc., New Berlin, Wis.) was used to elute the spots from the plate. Standard TLC developing tanks were used. Method. Freshly prepared solution, 10 gl, were spotted on silica gel plates which were then developed with a hexane-benzene-acetone mixture (65:10:12).Following development, the plates were air-dried at room temperature. Two spots at RI'Sof 0.7 and 0.6 were located visually on the plate. A circle perimeter of 2.5-cm diameter was scored through the plate adsorbent coating and centrally about each of the two observed spots, using a special milling device which is provided as an accessory to the Eluchrom system. After a plate was scored, it was placed on the Eluchrom unit and an elution head was placed over each scored circle. The elution heads were tightly clamped on the plate, forming a Teflon (elution head)-glass (TLC plate) seal. Elution was carried out at the slowest flow setting (0.1 ml/min) and the spot eluant was collected into a clean test tube. Methanol, 0.25 ml, was required to quantitatively elute each spot. The eluants were then tested by flameless atomic absorption for the presence of tellurium.
0.75
0.5
Lower Spot
!i
4 P
n
a
0.25
Upper Spot 0.0
'
Figure 1. FAAS test
r * 4 -
Time
of
eluants from two TLC spots
The spectrometer was operated at the 214.2-nm absorption line with a lamp current of 10 mA and a 1-mm slit width. A 5-p1 aliquot of the eluant was introduced into the graphite tube furnace. The sample was dried at 100 "C for 30 s, charred at 400 "C for 30 s, and atomized at 2000 "C for 6 s. The graphite furnace was operated with a nitrogen purge and a water flow of approximately 1.5 and 3.0 l./min, respectively. Deuterium background correction was employed.
RESULTS AND DISCUSSION The objective of this study was t o combine TLC with FAAS t o determine inorganic ions and organometallic compounds qualitatively and quantitatively. The separation of inorganic ions and organometallic compounds by TLC has been reported (7,8). Tellurium diethyldithiocarbamate (TDDC) was chosen as a model example in this study. Previous experience with this compound showed that TLC yields two spots on silica gel at Rf's 0.6 and 0.7, respectively (9).One of these spots was t h e TDDC, but i t was not possible t o visually determine which since both were the same color (yellow) and size. Elution of the spots with methanol using the Eluchrom unit was easily achieved (10) with 95% recovery of the TDDC compound based on FAAS measurements of tellurium. Five-kl aliquots of each of the eluants of the two spots were tested b y FAAS (Figure 1).It is clear that tellurium is present only in t h e TDDC spot. T h e purity of t h e compound was determined based on the tellurium results. The eluants may also be tested by other techniques t o further ascertain t h e identity of each spot. For our purposes, t h e TLC/FAAS method gave the required results.
LITERATURE CITED (1) G. Szekely in "Pharmaceutical Applications of Thin Layer Chrornatography", K. Macek, Ed., Elsevier Publishing Co. Amsterdam, 1972, pp 101-111.
(2)G. W. A. 1971.
Milne, "Maw Spectrometry", Wiley-lnterscience. New York,
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(3) P. J. Arpino, 8. G. Dawkins, and F. W. Mclafferty, J. Chrornetogr. Sci., 12, 574 (1974). (4) P. R. Jones and S. K. Yang, Anal. Chern., 47, 1000 (1975). (5) G. Westoo, Acta Chern. Scand., 22, 2277 (1968). (6) H. J. lssaq and W. L. Zielinski, Jr., Anal. Chern., 47, 2281 (1975). (7) J. G. Kirchner, "Thin-Layer Chromatography", lnterscience Publishers, New York, N.Y., 1967. (8) "Thin-Layer Chromatography", E. Stahl, Ed., 26 ed., Springer-Verlag, New York, N.Y. 1969, pp 664-667 and pp 837-853.
(9) H.J. Issaq, unpublished results. (10) H. J. lssaq and E. W. Barr, unpublished results.
RECEIVEDfor review August 9, 1976. Accepted September 24, 1976. Research eponsored by the National Cancer Institute under 'Ontract No. N01-C0-25423 with Litton Bionetics$ Inc.
Amalgam Electrode Designed for Precise Electromotive Force Measurements Hitoshi Ohtaki," Makoto Tsurumi, and Takayoshi Kawai Department of Electronic Chemistry, Tokyo Institute of Technology, 0-okayama, Meguro, Tokyo 152, Japan
In these decades a remarkable improvement has been achieved in ion selective electrodes. However, metal amalgam electrodes are still favorable for the precise electromotive force (emf') measurement of metal ions in solution because of their high accuracy and selectivity. The amalgam electrode may be disadvantageous in its handling and preparation and its instability caused by oxygen in air and dissolved in solution. Aladjoff's apparatus (1)is not suitable for storing the amalgam in the vessel after one has used it once. A usual J-type electrode described by Holloway and Reilley ( 2 )is too simple in shape t o prevent the contact of the amalgam phase with air during the preparation of the electrode. Therefore, from a practical point of view, an amalgam electrode which is easy t o handle without lowering the precision is required by analytical chemists who are interested in the behavior of ions in solution. The present electrode is designed on t h e basis of t h e following idea: a metal amalgam of a relatively low metal concentration is prepared by electrolysis of a solution containing relevant metal ions, and the amalgam is transferred t o the tip of the electrode without contact with any stopcock or air. The apparatus depicted in Figure 1consists of three parts, A, B, and C. A is the amalgam electrode which will be set in a measuring vessel. B is a chamber in which the metal amalgam is prepared by electrolysis in combination with a half-cell C. First of all, Nz gas is introduced from d and the mercury is placed in the chamber i under the nitrogen atmosphere. T h e tip c is stoppered with a glass cap. T h e three-way stopcock e is turned so that Nz gas flows from d t o i through a guide tube j . A slightly acid solution containing metal ions is then introduced from g t o i. The same solution is also poured from n into the half-cell C. The solutions in both compartments contact with each other at k,and 1 is closed with a pinchcock. During electrolysis of the solution by using a Pt foil as t h e anode and the mercury pool as the cathode (terminal h ) ,Nz gas is continuously bubbled from d t o m and m' through j . When a desired amount of the metal amalgam is prepared, C is removed from B (this procedure is not absolutely necessary), and g is closed. The amalgam thus prepared can be stored for a t least one month in this apparatus under a nitrogen atmosphere. The electrode A is connected t o an empty measuring vessel in which Nz gas is filled. The gas streams from a gas inlet of the vessel to a (the cover is taken out) through the hole b and the tip c . a is then connected with f and the amalgam in B is transferred to c through j by pressing the mercury phase with Nz gas. B can be removed from A , and a test solution is iptroduced t o the measuring vessel. When one makes amalgam from solid metal and mercury in B , part C is unnecessary t o set. 190
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U
h
C
L
Figure 1. Amalgam electrode. Explanations of the symbols are given in the text
We used the electrode in combination with Kawai's reference half-cell ( 3 )for measuring emf's of a cell containing Cd2+, Pb2+, or In3+ in ionic media of 1 M and 4 M NaC104. The measurements were carried out within f O . O 1 mV in each case. The potential reached a constant value within 10 min and remained unchanged for several hours a t each point of the measurements.
LITERATURE CITED (1) I. Aladjoff, Acta Chern. Scand., 23, 1625 (1969). (2) J. H. Holloway and C. N. Reilley, Anal. Chern., 32, 249 (1960). (3) H. Tsukuda, T. Kawai, M. Maeda, and H. Ohtaki. Bull. Chern. SOC.Jpn., 48, 691 (1975).
RECEIVED,for review August 23, 1976. Accepted October 4, 1976.
