at the rate of 24-200 mass units in 0.13 second, resolution is poor and signal clipping is excessive. The illustrated mass spectra were obtained with an instrument equipped with an electron multiplier so that limitations imposed by time constants were primarily in the recording galvanometers. Frequency response of 10002000 C.P.S. is satisfactory for mass spectral scans to 300 mass units at resolution of 300-400. If a n electron multiplier is not used, a typical d.c. amplifier operating at high gain will have a time constant of 0.1-0.01 second, and compromise of sensitivity, resolution, or scan rate will be required. The precision limitations shown by the duplicate scans of HV6 are partly caused by natural statistical error. If a n ion peak is traversed in a millisecond, then for a current of ampere approximately 100 ions will be collected, which may give a 10% counting error. Such a limitation shows that one should not use a higher scan rate than is necessary or use a higher resolution than is required. For most combined GC-mass spectrometry, scan rates of 1-4 seconds and mass resolution of 300 are adequate. Special systems may require extensions of these limits but compromises of quality, sensitivity, and precision will again be necessary. The possibility of obtaining fast-scan high-resolution mass spectra is also made impractical by this statistical
limitation. As resolution is increased, a longer scan time is required if the recording system does not have a faster response and to go from resolution 300 to 10,000 would require scan times of about 30-60 seconds. However, even if a tape recording system with a frequency response of about 5 x lo4 c.p.s. is used to store the inass spectral data, the time required to scan across a mass peak when a resolution of 10,000 is used would be about to 10-5 second. The reasonable statistical limit of sensitivity would be about 10-l2 to ampere of ion current. At the present time, the integrating properties of photographic detection (I6) seem preferabIe and will probably permit two or three orders of magnitude higher sensitivity. Use of a slower scan rate would be practical only in a few cases where the chromatographic peak was eluted very slowly. LITERATURE CITED
(7) Lindemann, L. P., Annis, J. L. .4SAL. CHEM. 32, 1742 (1960). (8) McFadden, R. H., Teranishi, R., Suture 200, 329 (1963). 19) ~, McFadden. R. H.. Teranishi. R.. Black, D. R:, Day, J.’C., J . Food Sei: 28, 478 (1963). (10) Ryhage, R., ANAL.CHEM.36, 759 (1964). (11) Selke, E., Scholfield, C. R., Evans, C. D.. Dutton. H. J.. J . Am. Oil Chemists’ SOC.38, 614 (1961). (12) Teranishi, R., Buttery, R. G., Lundin, R. E., LIcFadden, W. H., Xon, T. R., Proc. Am. SOC. Brewing Chemists 1963, 52. 3 ) Teranishi, R., Buttery, R. G., McFadden. R. H.. Mon. T. R., Wasserman, Janet, AsAL. CHEY. 36, 1509 (1964). 4) Teranishi, R., Corse, J . W.,MeFadden, w. H., Black, D. R., Morgan, A. I., Jr., J . Food Sei. 28, 316 (1963). 5) Taradi, P. F., Ettre, Kitty, ANAL. CHEM.34, 1417 (1962). 6) Watson, J. T., Biemann, K., Ibid., 36, 1135 (1964).
VV. H. MCFADDEN Western Regional Research Laboratory U. S.Department of Agriculture 800 Buchanan St. Albany, Calif. 94710 E. A. DAY
Nature 200, 435 (1963). (3) Dorsey, J. A., Hunt, R. H., O’Neal, ?*I.J.. AXAL.CHEM.35. 511 (1963). (4) Gohlke, R. S.,Ibid., 3 i , 535 (1959). (5) Ibid., 34, 1332 (1952). (6) Henneberg, D., Schomberg, G., “Gas Chromatography, 1952,” 31.van Sweag, ed., p. 191, Butterworth, Washington, D. C., 1962.
Department of Food Science and Technology Oregon State University Corvallis, Ore. REFERENCE t o a company or product name does not imply approval or recommendation of the product by the U. S. Department of Agriculture to the exclusion of others that may be suitable.
Trimethylphenylammonium Iodide as a Quantitative Precipitant for Gold SIR: A rapid, selective semimicro gravimetric method for the determination of gold was needed because the classic reduction methods (4, 7 ) are frequently inaccurate or time consuming, and spectrophotometric and titrimetric methods (2, 8 ) usually depend upon prior separations. Trimethylpheriylammonium iodide, first utilized by Pass and Ward ( 5 ) for a volumetric cadmium method, was later refined by the American Zinc Company ( 3 ) , and further utilized by Burkhalter and Solarek (I) for gravimetric analysis of bismuth. The precipitation of gold in the proposed method depends upon the formation of an insoluble salt of the quaternary cation and a tetraiodo anion complex formed when A h ( I I I )reacts with excess iodide ions. The compound i. stoichiometric and has the formula [(CH3)3(C6Hs)S]+ A h I a - . The brown-black flocculent precipitate is easily filtered and contains 23.42y0 gold by weight.
EXPERIMENTAL
Reagents. Precipitating solution : 40 grams of potassium iodide and 25 grams of trimethylphenylammonium iodide (Eastman Organic Chemicals, reagent 4423) are dissolved in 1 liter of water. Reagent wash solution: 1 part of the precipitating solution is diluted with 4 parts of water. Toluene wash solution: 1 liter of toluene is mixed with 25 ml. of ethanol. Buffer and complexing solution: 150 grams of sodium citrate dihydrate and 200 grams of anhydrous sodium acetate are dissolved in warm water and diluted to 1 liter. Gold standard solution: 3.9800 grams of 99.999yG gold (Western Gold and Platinum Corp., Belinont, Calif.) is weighed, carefully dissolved in 20 ml. of aqua regia, and diluted to 1 liter. Procedure. h weighed sample containing 5 to 50 mg. of gold or, for evaluation purposes, an aliquot of the standard solution is pipetted into a 250-ml. beaker. Five milliliters of
aqua regia are added, and t h e solution is evaporated to a sirup on t h e steam bath being careful not to go to dryness. Three milliliters of concentrated hydrochloric acid are added and the sample is again evaporated to a sirup which should finally contain -1 ml. of concentrated acid necessary to obtain proper pH. The solution is diluted with 25 nil. of distilled water. An ice cube made from distilled water (-40 cm.3) is added so that, with occasional stirring, the temperature of the solution will be maintained below 10” C. throughout the precipitation process. Finally, 15 ml. of the buffer solution is added, followed by 30 nil. of the precipitating solution. Precipitation occurs immediately and is complete within 30 minutes. The total volume should not exceed 120 ml. The ice cube melts in approximately 30 minutes, and the precipitate is filtered immediately on a weighed gooch crucible and washed sparingly with 5 to 10 nil. of reagent wash solution. The precipitate is now washed more thoroughly with 50 ml. of the toluene wash solution and air-dried to constant VOL. 36, NO. 12, NOVEMBER 1964
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weight in 10 minutes by the suction used in the filtration. The precipitate can also be dried a t 40’ C. without decomposition. However, higher temperatures tend to decompose the compound. The precipitate is nonhygroscopic, and a constant weight is easily obtained. The gravimetric factor for metallic gold is 0.2342. Destructioq of the precipitate and recovery of the gold may be accomplished by fuming with a mixture of nitric and sulfuric acids. RESULTS A N D DISCUSSION
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Gold in quantities of 5 to 50 mg. was determined in 20 samples without impurit,ies, with a mean relative error of 1.4%. Blanks containing various metal impurities were run with no precipitate formation noted. Table I shows the results obtained with known quantities of gold doped with various metal impurit,ies. Small amounts of nitrates do not affect the precipitation of gold in the strongly buffered solution (pH 4.4 to 4.8.) This p H must be maintained so that elements such as Cu(II), Fe(III), and Sn (IV) remain complexed by the sodium citrat’e. The solution must be kept cold to minimize solubility and prevent reduction of the gold by the citrat,e ion. Many elements, including Co(II), Cr(III), Ga(II1) In (111), M n ( I I ) , Mo(VI)! S i ( I I ) , Zn(II), and the alkaline earths form weak iodo complexes (6) and would not interfere with the gold precipitation, even without the sodium citrate, provided hydrolysis of these metals is prevented. The stable complexes of I3i(III), C d ( I I ) , Hg(II), P b ( I I ) , Pd(II), Pt(IS’), and Tl(1) cause interference even in the presence of sodium citrate. Silver can be separated as the chloride by filtration, after the initial dissolution of the sample, but care should be taken to maintain a small volume. LITERATURE CITED
(1) Burkhalter, T. ’ S., Solarek, J. F., ANAL.CHEM.2 5 , 1125-6 (1953). ( 2 ) Chow, A , , Beamish, F. E., Talanta
10,833-90(1963).
( 3 ) Haslip, L. H., American Zinc Co. of Illinois, E. St. Louis, Ill., unpublished
reports.
( 4 ) Hillebrand,
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W.F., Lundell, G . E. F., Bright, H. h., Hoffman, J. I., “.4pplied Inorganic Analysis,” 2nd ed., pp. 366-7, Wiley) New York, 1953. (5) Pass, h., Ward, A . M., Analyst 58, 667 (1933). ( 6 ) Kingbom, h.,“Complexat,ion in Analvtical Chemistry,” pp. 6-12, Interscience, Sew York, 1963. ( 7 ) \‘ogel, A . I . , “Quantitative Inorganic Analysis, Including Elementary Instrumental Analyeis,” 3rd ed., p p . 513-14, Riley, S e w York, 1961. (8) kyelcher, F. J., “The iinalytical Cse; of Ethylenediaminetetraacetic. Arid, p.248, \‘an Kostrand, Princeton, N. J., 1958.
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