ANALYTICAL CHEMISTRY
388 The developing tank is a borosilicate lass jar, with handles, OIJtained from A. H. Thomas Co., Philajelphia, Pa. (Catalog No. 6289-A,outside diameter 30.3 cm., inside diameter 29.0 cm., hei h t 60 cm.). The handles are 2.5 cm. from the top of the jar a n f a r e formed by indentations of the wall of the jar. These serve as supports for the frame, which is suspended on them by the ears, so that no portion of the frame touches the developing liquid. The top is an ordinary piece of plate glass 12.625 inchw square, which is sealed to the jar with a heavy lubricant to give an air-tight seal. For the chromatogram, paper strips are marked a t 4.5 and 8.5 cm. from the end to be immersed in the developing liquid. The sample to be chromatographed is applied at thr 8.5-cm. mark and allowed to dry. The frame is suspended from ring stands by its handles outside the tank and each strip is pierced first a t the lower or 4.5-em. mark, stretched taut, and then pierced by the top pin. With all the strips in position, the frame assen~blyis lowered into the tank until supported by the ears. Care must be taken to prcvent rontaniination of the strips by the tape while b h g mounted. The glass lid is set in position. When the solvent front has advanced sufficiently, the strips may be pulled off the pins selectively, or the entire frame may be removed from the t,ank and t h p strips dried, fiprsyetl, and developed while mounted.
I11 practicr, this aipparatua has greatly facilitated chromatographic procedures. I t is durable and permits the handling of large numbers of samples during the solvent separation phase of paper chromatography.
LITERATURE ClTED
Atkinsun, H. F., Nat7m, 162, 858 (1948). Consden, R., Gordon, A . H., and Martin. A . J P., Riochern. J , 38, 224 (1944). Hcftniann, Erich, Science, 111, 571 (1960). Loiigeneoker, W. H., ANAL.Cmm., 21, 1402 (1949). Imngenecker, W.H., Science, 107, 23 (1948). hfa, R. hi., and Fontaine, T. D., Ibid., 110, 232 (1949). Rockland, L. R., and Dunn, 31. S.,Ibid., 109, 559-40 (1949). Steward, F. C., Stepka, W., and Thompson,J. F., Ibid., 107, 451 (1948). Killiams, R. J., and Kirby, H., Ibid., 107, 481-3 (1948). Winsten, W.A., Ibid., 107, 605 (1948). Wolfson, W.Q., Cohn. C.. and Devaney, W. A., Ibid., 109, 541-3 (1949).
Improved 5-Mg. Rider for Ains worth Microchemical Balances LAWRENCE E. BROWN Southern Regional Research Laboratory, Kew Orleans, La.
RREGCLABITIES in the seating of the 5-nig. wire riders, customarily furnished with older Ainsworth microchemical balances Types F D and F D I , arc an important source of error. The magnitude of the seating error can be reduced by using a 0.5 mg. rider, but a sacrifice of convenience is entailed by the lower capacity of the beam. .4 5-nig. rider with a low seating error has been constructed of aluminum foil The essential feature of thiu rider is that it has a thin, btraight bcaring edge and can be brought to rest in the bottvni of the notch of the balance beam with ease and without undue nianipulation of the rider carrier. The rider is made of pure aluminum foil, 0.0036 to 0.004 inch thick. After a 2-inch (5-cm.) square of the foil is covered on both sides with cellulose tape, it is placed on a smooth hard surface and the eye of the rider is cut with a small sharp cork borer (No. 1) by a single light tap. The foil is then flattened between two pieces of plate glass and the irregularities in the circumference of the eye are removed ~ i t ah 3-mm. conical grinding wheel.
A-
The legs sliould be about 1-mm. wide and the angle with the bearing edge about looo. The cellulose tape is removed by soaking the formed rider in acetone. The legs are trimmed in length until the rider weighs about 5.1 mg. The final adjustment to exactly 5 mg. is made by dissolving some of the aluminum by immersion in 0.1 iV sodium hydroxide, which attacks microscopic burrs and imperfections preferentially. After final cleaning and drying, the rider is flattened by pressing between pieces of plate glass. I t is then ready for use.
Table I .
Standard Deviations of Observations Observed for Pan hrrestment and Rider,
Rider
+ Y
vu
Calculated for Rider Alone,
u* Y
5-mg. wire rider 1.9 1.7 5-mg. foil rider 1.3 1.0 0.9 0.5-mg. wire rider 1.2 a (x u ) = s observed for pan arrestment and rider. (z) = 8 observed for pan arrestment alone = 0.8 y. It = 30 observations in all cases. b y 8 calculated for rider seating alone a8 square root of differences of squares of (z g) and ( L ) .
+
5.5
m m.i 0.5
Figure 1.
o ~
0.5
+
mm.io.2
,ALUMINUM
;,lF
-
FOIL
m 2m o
2
Foil Rider
The coated foil is then placed on a hard smooth surface, such as a piece of late glass. The cutting edge of a rib-backed razor blade is re8uced to a length of 4.8 .t 0.2 mm. and used to cut the cross bar or bearing edge of the rider. The cutting edge of the blade is placed symmetrically about 0.5 mm. from the eye and the cut is made with a single tap on the back of the blade (Figure 1). Similarly, sharp blades are used to cut the insides and outsides of the legs, the outside shoulders, and the outside of the eye arc. The most critical points are the inside corners.
The performance of the foil rider compared with that of the riders supplied by the manufacturer, using an Ainsivorth Type FDI microchpmical balance, is indicated by the data given in Table I. The reading error on the optical lever scale was assumed to bc ncgligible. The standard deviation reported for the 5-mg. wire rider is for the best series of observations attained by the author after considerable experimentation in rider seating. Unless Ftiict care is used in seating, a much larger standard dcviatjoii occurs. The foil rider is adequately seated by a vertical drop of it from thc c;trrier Further manipulation, such as touching or rocking the seated ridrr with the tip of the carrier, results in l o w r precision. The difference found between the standard deviations of the 5-mg. foil and wire riders is statistically significant a t the 95% level. The other standard deviations observed are coilsidered typical mid indicate that the 5-mg. foil rider has ~1 seating error comparable to that of a conventional 0.5-mg. rider. RECEIVED July 6, 1950
