Characterization of Barium Sulphate as a TLC Material for the Separation of Plant Carboxylic Acids H.S. Rathore*/H.A. Khan Chemistry Section, Z. H. College of Engineering and Technology, Aligarh Muslim University, Aligarh 202 001, India Key Words Thin layer chromatography Barium sulphate/calcium sulphate Carboxylic acids Citrus fruit Summary A mixture of barium sulphate and calcium sulphate has been shown to be a good TLC material for the detection and the quantification of a variety of carboxylic acids known to be plant growth regulators and carboxylic acids of importance in the citrus fruit industry. Introduction TLC is used for the qualitative and quantitative analysis of a wide variety of compounds [1] and the potential of numerous coatings has been studied. Our previous work [2, 3, 4, 6, 6] showed that glass plates coated with calcium Sulphate can be used to separate organic acids occurring in plant tissues. Barium sulphate is more insoluble than cal- cium sulphate [7]; it is a non-toxic, cheap and chemically inert material that can also be used as an adsorbent. Barium sulphate has been used in TLC for the separation of sulpha drugs [8] and pharmaceuticals [9]. This paper reports the separation of some carboxylic acids of importance to the fruit growing industry on coatings of barium sulphate alone and mixed with calcium sulphate. The solutions used were prepared by the procedure report- ed previously [6]. Plates were prepared as follows -- A slurry of the various coatings in distilled water was applied to the glass plates with the applicator to give a film thickness of 0.75 mm (or 1.00 mm for quantitative determinations). Layers of 0.25 mm were found to be suitable for the separation of small amounts while thick layers (0.75 mm) were suitable for large amounts. The plates were first allowed to dry at room temperature and then in an oven at 110~ for 1 hour. The composition of the coatings is shown in Table I. Test solutions were spotted onto the plates with a fine ca- pillary. The plates were kept at 30 ~ (room temperature) for 15 minutes to remove solvent and then developed. For tailing, the front limit (RI) and the rear limit (RT) were measured while for compact spots Rf values were cal- culated in the usual way. Quantitative separation of /3-naphthaleneacetic acid from oxalic acid A known volume of both acids was spotted on plates coat- ed with coating B. The plate was developed with ethyl ace- tate and the acids were located with 1% ethanolic alkaline bromophenol blue. The acids were separated again on a fresh plate and the previously indicated portions of the coating were removed with a spatula. /~-naphthaleneacetic acid was eluted with 25 ml of hot 2-propanol from the material scratched off the plate while the oxalic acid was removed by 25 ml of hot distilled water. Both the solutions were made up to 50 ml in 50 % 2-propanol by adding water or propanoI and then titrating with standard alkali and phe- nolphthalein indicator. Experimental A Stahl apparatus with a universal applicator made in India was used to coat glass plates (20 x 4 cm). The plates were developed in glass jars (25 x 5 cm). Barium sulphate was from GSC, India. Calcium sulphate dihydrate, carbon tetrachloride, chloroform, ethyl acetate, 2-propanol, sodium hydroxide and phenolphthalein were from Merck, India. Bromophenol blue and the carboxylic acids were from Sigma, U.S.A. All other reagents were of analytical grade. Results Most important separations achieved on different coatings in common solvents are: NTAA (2-4)from CA (0), IAA (0), MEA (O), MOA (0), OA (0) and TA (0) on coating C in carbon tetrachloride; ClA (3-8) from CA (0), MEA (0), MOA (0), OA (0) and TA (0) on coating A in chloroform; MOA (0), MEA (0), CA (0), OA (0) and TA (0) from ClA (3--6), NTAA (0.62) and NXAA (0.65) on coating B in chloroform; ClA (0), IAA (0), NTAA (0) and NXAA (0) from CA (I), MEA (I), MA (I), MOA (I), OA (I), PAA (I), 432 Chrornatographia Vol. 23, No. 6. June 1987 Originals 0009-5893/87/6 0432-03 ~ 03.00/0 9 1987 Friedr. Vieweg & Sohn Verlagsgesellschaft mbH