copper ( K d = 3), cadmium (Kd = 2), iron (Ka < 1). zinc (Kd < l), gallium (Kd < I), bismuth ( K d < l), ( K d < 1) on the separation of scandium from pl studied. These elements were selected to incluue munu- LU tetravalent elements having Kd values in the range between 1000 (strongly adsorbed) and < 1 (weakly or not adsorbed), These studies showed that 100 pg of elements having Kd > 1000 can be tolerated in the determination of scandium. For copper, which has a Kd value of 3 , l mg can he tolerated. For those elements having Kd of 2 or less, 100 mg did not interfere. Although the method applies specifically to the determination of scandium in plutonium, it can he readily adapted to determining other strong cations, such as rare earths, or
scandium in other materials that are not adsorbed on cation rate and measure ACKNOWLEDGMENT
The authors are indebted to Dr. C. F. Metz, who supervised this work, for valuable suggestions, and to Mi!is H. L. Barker L-"-, -P"n.re.-.oJ for performing some of the separations and : ments.
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RECEIVED for review November 15,1968_ 14ccepted January 8, ^. 1969. The Los Alamos mentmnc Iaboratory IS ooeratea unaer the auspices of the Atomic Energy CommissioiR.
Infrared Spectra of Compounds Separated by Thin Layer Chromatography Using a Potassium Bromide Micropellet Technique W. J. de Klein Laboratory of Organic Chemistry, State University of Leiden, Leiden
COMBINATION of thin layer chromatography (TLC) and infra1mertmrmnu ;q In intprprtinolnrl nrnmirino eutpn.in Ir r, (IR) ,__., ._.______l.... r____..l... _.__..I._ n of both techniques. The separation of the components of a mixture hv TLC and examination of the resultine mots hv IR ~, is one of the most useful ways of identifying the unknown compounds. Several procedures have been described in the literature. These include scraping of the sample from the support,
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from the support material ( I ) or followed hy packing the suooort .. into a Pasteur tvoe .. Dioet . . and elution of the ComDound into the tip of the pipet, evaporation of the solvent and dissolution of the compound in a solvent which is suitable for IR measurement (2); and scraping the support surrounding the spot from the chromatoplate, building a dam of KBr powder onto the tip of the spot, and eluting the compound directly onto the KBr powder (3). Experience has shown that the method of Sturn, Parkhurst, and Skinner ( I ) is not directly suited to the infrared experiment for it was developed as a combination with ultraviolet spectroscopy which permits a much larger dilution. The method of McCoy and Fiehig (2) is rather time consuming ( 4 M 5 minutes) and suffers from the restriction that spectra can only he measured in a solvent. Rice's method (3) is a rather promising technique but is limited by the skill of the individual worker for considerable loss occurs when the elution solvent spills from the support on to the glass plate. A method is now described which is rapid and generally applicable for it uses the KBr technique and gives relatively high recovery. Garner and Packer ( 4 ) have described a similar technique employing Harshaw's Wick-Stick.
Figure 1. The shape of the KBr wall around the tip of the spot depends somewhat on the size of the spot. The upper spot is small and requires a relatively larger envelope EWERIMENTAL
Glass plates (5 x 20 cm) were coated with a 250-p layer of either silica gel (Merck HR), or silica gel containing an inorganic fluorescent indicator and gypsum (Merck G F 254), or aluminium oxide (Merck G F 254). The plates were air dried and heated as described. They were then stored in an air-tight box. Glass plates coated with cellulose (Merck, Fertigplatten F, thickness 80 p ) were used in some experiments. One to fifty micrograms of a compound in solution were applied to the plates with a micropipet (Camag AG, Muttenz, Schweiz). The plates were developed in the usual manner in covered chromatographic tanks. The walls of the tanks were not paper covered for vapor saturation. Location of separated components was done hy irradiation with ultraviolet radiation at 254 or 350 mp. The spots were outlined hy scratching the support. After evaporation of the elution solvent, the TLC support was removed around the spot. The glass around the spot was tarefullv, cleaned with tissue oaoer or wash leather. A wall of (1) P. A. Sturn, R. M. Parkhurst, and W. A. Skinner, ANAL. GEM., ~~~.~~ KBr powder was built around ihe tip of the spot and an open 38, 1244 (1966). space of 1-2 mm left between the spot and the KBr wall (2) R. N. McCoy and E. C. Fiebig, ibid., 37, 593 (1965). (Figure 1). This wall can easily he built hy lightly tapping a (3) D. D. Rice, ibid., 39, 1906 (1967). (4) H. R. Gamer and H. Packer, Appl. Spectrosc., 22, 122 (1967). Pasteur pipet, containing finely divided KBr powder, with the ~
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VOL. 41, NO. 4, APRIL 1969
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Table I. Dependence of Recovery and Contamination on Type of Compound and Solid Support Amount of material Ordinate Contamination expansionb around 1100 cm-lc Compound Type of support brought upon plate (pg) % Recoverya o-Nitrophenol Silica gel 20 Medium 3 x Some o-Nitrophenol Aluminium oxide 25 None 10 x Some o-Nitrophenol Cellulose 25 Low 6 x Considerable p-Nitrobenzaldehyde Silica gel 40 High l x None l x p-Nitrobenzaldehyde Silica gel 20 High None l x p-Nitrobenzaldehyde Silica gel 4 High Hardly detectable p-Nitrobenzaldehyde Aluminium oxide 4 High l x Hardly detectable p-Nitrobenzaldehyde Cellulose 4 Low 5 x Hardly detectable Benzophenone Silica gel 10 High I x Hardly detectable l x Benzophenone Silica gel 5 High Some Benzophenone Silica gel 2 High 2 x Considerable Benzophenone Aluminium oxide 10 High l x Not detectable I x Not detectable Benzophenone Aluminium oxide 5 High Not detectable 5 Medium 2,5 x Benzophenone Cellulose Phenyl acetic acid Silica gel 20 Medium 3 x Considerable 10 x Hardly detectable Phenyl acetic acid Aluminium oxide 20 None 20 Low 3 x Phenyl acetic acid Cellulose Some a High recovery 100-70%, medium 70-40%, low 4 0 4 % . b Ordinate expansion was used when yo T of strongest band was less than 45%. c When contaminated interpretation of spectra was most difficult around 1100 cm-I.
capillary end pointing away from the spot. The wall can be adjusted with a micro spatula if necessary. Care should be taken not to contaminate the KBr with the support material. A solvent in which the compound is readily soluble was now added to the spot. It is best to use an a-polar solvent, alcohols or other strongly complexing solvents should not be applied. The compound was concentrated into the tip of the spot by moving the solvent slowly over the length of the spot. It is essential that the solvent spill over the edges of the spot that lie within the semicircle of KBr. The solvent now spreads rapidly over the glass surface and is sucked up by the KBr powder. Because the solvent carries the compound into solution, the compound is thus transferred from the spot to the KBr powder. When the solvent reaches the powder, no additional solvent is added in order to avoid spilling the solvent beyond the KBr powder. Elution volumes normally used were 100-15Opl. The solvent was allowed to evaporate and the KBr powder was collected and transferred to a micro KBr disk (5). In this experiment a lead disk of 1 mm thickness with a bore of 1.5 mm i.d. was used. Five milligrams of KBr are needed to fill this disk. The pellets were pressed in an ordinary commercial KBr press at 8-9 tons pressure under vacuum. The IR spectra were obtained in the usual manner using a KBr lens beam condenser and a beam attenuator. The spectra were taken on a Perkin-Elmer 225 or Hitachi EPIG 2. RESULTS AND DISCUSSION Experimental results are given in Table I. The percentage recovery was estimated from the intensities of the bands of the IR spectra. As can be seen from the table the direct elution technique is sensitive to: (a) the type of compound to be eluted, compounds that form strong hydrogen bonds show a poor recovery; (b) the support material used, different support materials show different recoveries; (c) contamination of the spectrum of the eluted compound with that of the support material. This contamination is both dependent on the support material used and the compound that is eluted. (5) W. J. de Klein and K. Ulbert, ANAL.CHEM.,41, 682 (1969).
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ANALYTICAL CHEMISTRY
From a point of easy recovery silica gel can best be used in this technique. On the other hand spectra of compounds that have been eluted from silica gel generally show more contamination with the solid support than those eluted from aluminium oxide or cellulose. All three supports show their strongest absorption around 1100 cm-1. The absorption of the 1100 cm-1 band of silica gel is much stronger than that of aluminium oxide or cellulose. This is one reason why spectra show contamination by silica gel quicker than by aluminium oxide or cellulose. Comparing this technique with that mentioned by Rice (3), the same amount of contamination was always found. This indicates that contamination mainly results from the difficulty of thoroughly cleaning the glass plate around the spot and from (colloidal) solution of the support into the solvent used. Contamination partly imposes on this technique the lower limit of amount of compound that can be treated. This limit ranges from 1-2 pg for relatively nonpolar compounds to 10-20 pg for strongly hydrogen-bonded compounds. Compounds that contain -OH and -COOH groups that can readily hydrogen-bond to the solid support suffer from a low percentage recovery. A slight improvement can be effected when the thickness of the solid support layer is reduced from 0.25 mm to 0.1 mm. However, generally the amount of material to be separated on the plate also has to be reduced in this case, thus giving no overall gain in amount of material that can eventually be brought into the KBr pellet. The solvent that is used to elute the compound on to the KBr powder should be relatively a-polar and dry for strongly polar solvents--e.g., alcohols, DMSO, and DMF-cause strong contamination of the spectrum with solid support, especially with silica gel. In our experiments chloroform, ether, and acetone gave good results. Silica gel G F 254 and aluminium oxide G F 254 contain 10-13z gypsum as binder. The presence of gypsum in this amount had no effect on the quality of the spectra. RECEIVED for review October 18, 1968. Accepted December 6, 1968.
