Measurement of P3* in Plant Material by the Use of Briquets d . J. \IhcKEUZIE AND L. A. DEAN Bureau of Plant Industry, Soils und .iprirulturul Engineering, U.S . Deparirnent of Agriculture, Beltwille, M d .
ROUTIKE quantitative procedure for the determination of P32 in plant material has been described by the writers ( 2 ) . This procedurr entailed the separation of the phosphorus from the plant material, its precipitation as magnesium ammonium phosphate hexahydrate, and its subsequent collection its a uniform layer on a specially prepared filtrr. -4faster routine quantit stive procedure became desirable when it was necessary to analyze ever-increasing numbers of vegetative samples from greenhouse and field experiments dcsigned to study the utilization of phosphorus from fertilizers labeled with P32. A procedure, therefore, has been established for the quantitative determination of the P32 activity of plant material by the use of briquets, thereby eliminating time-consuming c he mi c a 1 I precipitations and separations. The same degree of Figure 1- Form for Making Briquets Used to Measure precision as in the precipitaP32Activity in Plant tion technique ismaintained. Material
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tained by pressing the top surface of the briquet against a gwtrd plate, thus always keeping the top of the briquet a t a constant distance from the counter tube window. An opening 1 inch in diameter is provided in the guard plate. However, if it is nccc'ssary to reduce the counting rate, an aluminum filter may hc substituted. Sample changing is accomplished by lowering the briquvt away from the guard plate and locking the holder in this position. Tlic briquet holder is then removed from the counter mount and a new briquet is inserted. I\FINITE THICKNESS O F CO.MPRESSED PLANT MATERIAL FOR Pal
The infinite thickness of a radioactive material is that thichness of the material beyond which there is no increase in measurcd activity. To determine the actual amount of the plant material containing P3* which contributes to the measured activity of a
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FG-M
C O U N T E R TUBE
PHEPARATlON O F PLANT MATERIAL BRIQUETS FOR RADIOACTIVE MEASUREMENT
The samples of plant material containing P32are dried a t 65' C. and ground to pass through a 20-mesh screen. The form used for making the briquets (Figure 1)is of cylinderplunger type and is composed of plunger A , cylindrical form B, and bottom or base for the briquets, C (manufactured by Fred S. Carver, Inc., 345 Hudson St., New York 14, N. Y.). All parts are machined to fit very closely. To prepare a briquet, parts R and C of the form are assembled. Approximately 5 g r a m of the dried, ground plant material are then weighed and transferred to the form, after which part A is fitted into place. A pressure of 12,000 pounds per square inch is applied to the plant material for about 3 minutes by means of a laboratory hydraulic press. After the required time the pressure is released and the briquet is removed from the form. When C is removed the briquet is readily pushed out through the bottom of the form with the plunger, A . Briquets made under these conditions are cylindrical in shape, having a diameter of 1.125 inches and a height of approximately 0.25 inch (0.6 cm.). They are sturdy and easily reproduced and their dimensions are suitable for radioactive measurements. The briquetted plant material sample can be conveniently numbered with India ink. Contamination of laboratory equipment and samples was controlled by frequent cleaning with a vacuum cleaner.
GUAR0 P L A T E 1' D I A OPENING
REMOVABLE HOLDER
S A M P L E HOLDER AND COUNTER MOUNT
Figure 2. Support with Briquet in Position for Counting
briquet, increasing amounts of a radioactive plant sample were made into briquets and the activity was measured in the manner described above. The results of the experiment are shown in Figure 3. Constant activity was obtained with samples of 3 grams (thickness approximately 470 mg. per sq. cm.) or more. By using a t least 5 grams (thickness approximately 784 mg. per sq. cm.) of plant material as a sample all radioactive measurements can be safely made a t infinite thickness and sample thickness corrections are thereby avoided. To determine the relationship of concentration of P3* in compressed plant material to the measured activity, plant material containing Paz was diluted by intimate mixing with different
MEASUREMENT O F RADIOACTIVITY
The activity measurements of the briquetted plant material samples containing P32 are made by the use of a special holder, which is designed to fit counter mounts used in this laboratory. Figure 2 is a cross-sectional view, showing the holder with briquet in position for counting. Constant geometry is mxin489
ANALYTICAL CHEMISTRY
490
amounts of "nonradioactive" or check plant material. Five grams of each dilution were compressed into a briquet and the activities were measured. These activities when plotted against the per cent radioactive material in the sample (see Figure 4) gave a linear function. ,
ACTIVITIES OF BRIQUETS AND OF PRECIPITATES
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A comparison of the specific activity, P32/(P31 Pa2),of the fertilizer preparation with that of the plants grown on a soil receiving this fertilizer enables estimation of the proportion of the phosphorus contained in the plants derived from this fertilizer. Having established a reproducible method for the direct measurement of P32 in plant material, it remained necessary to compute a specific activity of the plant material which can be compared with that of the original fertilizer material. Because the specific activity of the fertilizer material is most conveniently measured by separating the phosphorus from the material and precipitating it as magnesium ammonium phosphate, the specific activity of the plant material should be measured under similar counting conditions. The ratio of the activities of plant samples measured as briquets to the activities of magnesium ammonium phosphate hexahydrate precipitates separated from these same samples, is a constant. Table I gives the radioactivity measurements of plant material samples both as briquets and as precipitates and the resulting relationship. The briquets were prepared as described above and the precipitates of the plant material were prepared from 2.00 grams of plant material as described previously (2). The factor for the conversion of briquet activities to that of the magnesium ammonium phosphate precipitate from 2.00 grams of plant material was 1.805. The amount, 2.00 grams of plant material, was chosen arbitrarily. Any convenient amount of plant material could have been rhosen for the determination of a subsequent conversion factor.
Table I.
Determination of Conversion Factor
(For conversion of briquet activity t o activity given by M ~ N H I P O ~ . B H Z O precipitate from 2.00 grams of same plant material) Activities' Magnesium ammonium phosphate ppt. from 2. 0 grams of plant Sample as Sample No. material briquet Factor Counts/sec. Counts/sec.
Corrected for background and radioactive decay. corrections were insignificant. a
0
I 5
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IO
Av. 1.805 Recovery time
I 15
I
20
p3' IN P L A N T M A T E R I A L (RELATIVE UNITS) 0
157 I
BRIQUET THICKNESS (MG./SQ.CM. ) 314 470 627 704 I I I I
940 I
Figure 4. Relation of Concentration of Pa2in Plant Material to Observed Activity of Briquets
It is shown in Figure 3 that the amount of plant material which contributes to the activity of a briquet is approximately 3.00 grams. The results of Table I indicate that the precipitation procedure is about 2.7 times as sensitive as the briquet procedure. The greater sensitivity of the precipitation procedure, however, is compensated for by the greater speed of the briquet procedure. INFLUENCE OF
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2
WEIGHT
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3 O F BRIQUET
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4
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5
I 6
(GRAMS)
Figure 3. Infinite Thickness of Briquets Prepared by Compressing Ground Plant Material Containing P3*
The specific activity of the plant material may now be calculated from the measured activity of the briquet and a t the same time be comparable with the specific activity of the original fertilizer material measured as a magnesium ammonium phosphate hexahydrate precipitate. The sppcific activity of a plant material sample may be expressed as: Specific activity =
measured briquet activity X 1.805 total phosphorus in 2.00 grams of sample
Total phosphorus in the plant material was determined by Mission's vanadomolybdate method (1, 3).
K40
ON BRIQUET ACTIVITIES
The naturally radioactive isotope of potassium, K40, can be expected to contribute some activity to the briquets, but it does not create any difficulty except when the P32 activity is low. If this occurs the radioactivity measurements should be made by a precipitation technique (2). Activities due to K40 in briquets of plant material have been measured and found to be about 0.1 count per second. Thus, with plant material of relatively high activities, the activity due to K40 becomes insignificant. Briquets of plant material from check treatments are, however, usually used in making background determinations. In this manner the correction for natural background and the activity due to the Ka are determined in one operation. The natural background activity using 2 inches of lead as a shield for the counter assembly employed in this laboratory is 0.25 count per second. LITERATURE CITED
(1) Koenig, R. A., and Johnson, C. R., I N D .ENG.CHEM.,ANAL.ED., 14, 155-6 (1942). (2) MacKenaie, A. J., and Dean, L. A . , ANAL.CHEM.,20, 559 (1948). (3) Mission, G., Chem.-Ztg., 32, 633 (1908). RECEIVED May 14, 1949.
