@
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Figure 2. Various types of blades for the suction scrapper device (A) Regular razor blade: (B) Scoring blade. The width of the cutting edge is 2 rnrn. (C) Stripping blade. The cutting edges are 3 rnrn wide: the intervening spaces measure 7 mrn. This blade gives up to 19 channels in a 20 X 20-cm plate. (D) Stripping blade. Each cutting edge is 4 rnrn wide: up to ten channels, 1.5 crn in width, can be formed in a 20 X 20-crn plate
Figure 1. Top view of the supporting guide (left) and cross section (right)
the supporting unit and parallel to its base. The adjustable screw serves also to immobilize the guide a t any point of the slit. A vertical flange (H) placed a t the top of the longer arm prevents tilting and ensures that this arm of the guide will form a 90' angle with the base of the chromatographic plate at all times. A 30-cm metric ruler in the movable bar (E) is used to determine the position of the guide when in operation. An additional marking (0) is provided on the same bar to facilitate alignment of the chromatographic plate with the origin of the ruler. A second ruler attached to the long arm of the sliding guide, 20 cm in length, permits the exact positioning of pipets or microsyringes for the application of samples.
OPERATION In using the guiding support, we have found that it is more convenient to score the finishing line first. The movable bar E is displaced toward the short arm of the sliding guide while the chromatographic plate is being inserted with its bottom parallel to the longer arm of the sliding guide. Before releasing the movable bar, the bottom of the plate is aligned with the origin of the 30-cm ruler (i.e., marking 0 on the bar). The sliding bar is moved so that the right border of its long arm coincides with a marking on the ruler, equal to the distance of the line of origin from the bottom of the plate plus the desired length of the run. For example, if the line of origin is to be a t 2 cm from the bottom of the plate and a 10-cm run is needed, the sliding guide is moved to the 12-cm mark on the ruler. In most cases, however, allowance has to be made for the scoring instrument and therefore the actual displacement of the guide is less than 12 cm. The adjustable screw F is then tightened and scoring is performed with a pencil, a stainless steel pen (with some dust formation), or, preferably, with the suction scrapper described ( I ) using a modified blade. The modified blades (Figure 2) can be made in the laboratory with a narrow glass-cutting stone. After scoring, the thin layer plate is removed, rotated horizontally 90' and placed again in the guiding support, with its left border a t the origin of the 30-cm ruler. Narrow strips of adsorbent are now removed with the suction scrapper. Since the adsorbent is immediately sucked off into the elution unit of the device, the possibility of dust being inhaled is minimized. The sliding guide is advanced in either direction as required, using the longer ruler to determine its position. Tightening the adjustable screw F prevents its displacement during stripping. An entire 20 X 20-cm plate can be prepared in a minute or less by this procedure. The sam1726
ANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
ples are applied in the middle of each channel by positioning the micropipet against the border of the sliding guide, a t the selected line of origin. Determination of Rf's on the developed plate is accomplished by simply reading the distance travelled by the compounds on the 20-cm ruler.
RESULTS AND DISCUSSION The supporting guide described facilitates and expedites several steps during thin layer chromatography. Sharp and clean strips are consistently obtained with no delay. The use of spotting or scoring templates becomes unnecessary. Since the design of the blades for the suction scrapper can be varied and the sliding guide can be stopped a t any position, the width of strips and intervening channels can be changed as needed for any of the multiple spot applicators currently available (2-5). Although we have used (6) the supporting guide in conjuction with the suction scrapper, other vacuum devices (7-9) could probably also perform well. ACKNOWLEDGMENT We thank Pansi Warren for excellent secretarial assistance. LITERATURE CITED (1) 0. Sudilovsky and P. H. Hinderaker, Anal. Biochem., 45, 525 (1972). (2) S. Sarnuels, J. Chromatogr., 32, 751 (1968). (3) J. S. Chahl and C. C. Kratzing, Clin. Chim. Acta, 26, 177 (1969). (4) D. Shapcott and 6. Lernieux, J. Chromatogr., 70, 174 (1972). (5) A. F. Rosenthai and M. G. Vargas, Clin. Chem., 18, 1425 (1972). (6) J. F. Tornashefski, Jr., R. J. Barrios, and 0. Sudilovsky, Anal. Biochem., 60, 589 (1974). (7) C. J. Clernett, Anal. Chem., 43, 490 (1971). (8) J. W. Fairbairn and S.El-Masry, J. Pharm. Pharmacol,, 19, 93s (1967). (9) J. Y. Cheng. J. Fish. Res. Bd. Can., 30, 460 (1973).
RECEIVEDfor review March 20, 1975. Accepted May 15, 1975. CORRECTION Absolute Determination of Phosgene: Pulsed Flow Coulometry
In this article by H. B. Singh, Daniel Lillian, and Alan Appleby, Anal. Chem., 47, 860 (1975), on page 863, column 2, the last five lines of the Discussion section, immediately before the conclusion section, should read (CC13COC1, COC12,HCOC1, CC14, Clz, HC1, etc.). These results lend support to the observations of Noweir et al. (18) that the importance of phosgene decay by the gas phase phosgene-water reaction has been overemphasized in the literature.
