absorption and counting geometry could be neglected. PROCEDURE. The samples weighing from 15 to 45 nig. were dissolved by a bisulfate fusion in porcelain crucibles and leached out with a 4y0 ammonium omlate solution. Additional solids n ere added to the diluted solution to total 0.8 gram per 100 ml. in every case. The samples were diluted in such a manner that counting rates in the range 300 to 10,000 counts per minute i\ere obtained. The counting was accomplished with an end-window Geiger counter.
Table 111.
0 20 0 50 1 00
8 9
10
RESULTS
The results obtained from the six samples of the one lot of commercially pure Nb206containing known amounts of Ta206 are given in Table I. The intercept obtained by the leastsquares method when the per cent of Ti1205 added was plotted against the counts per minute per milligram was 0.14. This value is the per cent Ta2O5 originally present in this lot of Kb20s as a n impurity. Adding this value (0.14%) to the known per cent T a 2 0 5added, another curve may be plotted of the true amount of TazOs as a function of activity. Using this new curve, all the other samples could be analyzed for the amount of Ta205. Figure 1 is a plot of these values and
Impurities added to Samples 8, 9,and 10
0
I00
200
300
400
500
600
700
COUNTS/ MINUTE/ MILLIGRAM
Figure 1.
0 50
5 00 10 00
0 25 0 50 1 00
dicate that the added elements do not interfere in the analysis. The good agreement between the tvio values obtained by independent methods leads to the conclusion that the results are fairly valid. The precision of values for the two or three samples made for each lot was also good, with a deviation from the average around 2%.
Tantalum found by Method 1
LITERATURE CITED
(1)
Eicholz, C. G., Nucleonics
10, 58
(1952).
was used as one method of obtaining the amount of Taz06 present in the other samples analyzed. The second method employed was more direct. It consisted of measuring the activity of the pure Ta20j, assuming it to be 100% Ta205,and then comparing it to the activities of all the NblOj samples. A comparison of these two methods is found in Table TI. The results reported in Table I1 in-
Kohn. Andre, C o m p t . r a d . 236, 1419 (1953). (3) Long, J. V. P., Snalyst 76, 644 (1951). (4) Ruben, S., Hussid, U. Z., Kanen, AI. D., J . Am. Chem. Soc. 61, 661 (1939). GILBERT HALVERSOS' 12) . ,
'
A L B E R T SHTASEL
Research Division Fansteel Metallurgical Corp. North Chicago, Ill. 1 Present address, Radiation Research Corp., West Palm Beach, Fla.
P-2-Thienyl-DL-alanine, Internal Standard for Automatic Determination of Amino Acids Frank
L.
Siege1 and M a r y K. Roach, Clayton Foundation Biochemical Institute, University of Texas, Austin, Tex.
automatic determination of amino T acids as described by Spackman, HE
Stein, and Moore (1) employs ninhydrin as the color-developing reagent. Ninhydrin reagent is stored in a dark bottle, under nitrogen, since this material is .susceptible t o osidation. A small leak in the system can markedly decrease the color yield of the ninhydrin reaction, with commensurate loss of analytical accuracy. Small losses in color yield may go undetected while valuable machine time is lost. The use of a n internal standard seemed indicated, and several compounds were screened for this purpose. Ideally, the standard should not be naturally occurring, should occupy a unique position on the amino acid elution pattern, should be recovered quantitatively from the column, and 1628
ANALYTICAL CHEMISTRY
should be of use on both the 150-cm. and 50-cm. columns of the instrument. -4 compound which is almost ideal by these criteria is P-%thienyl-~~alanine (Nutritional Biochemicals Corp., Cleveland, Ohio) ; this compound is now used routinely as an internal rtandard i n this laboratory. For both 30"-50" and 50" analyses on the 15O-cm. column p-%thienyl-~~-alanine emerges between leucine and tyrosine. On the 50-em. column analysis it emerges before the combined tyrosinephenylalanine peak. T o date the only compound we have found to overlap this standard is 3,4-dihydro~yphenylalanine (DOPA4); p-alanine and norleucine do not interfere. I n urine and in liver extracts there is a compound which emerges with P-2-thieriyl-u~-alanine and prei ents it
