Analysis of Propellants by Infrared Spectroscopy - ACS Publications

Analysis of Propellants by Infrared Spectroscopy FR.iNK PRISTERA Picatinny iirsenal, Dover, 5. J.

;in investigation into the use of infrared spectroscopy in the analysis of propellants was undertaken because the conventional methods are involved, lengthy, and not specific, require considerable sample, and for qualitative purposes require extensive knowledge of the many possible ingredients used together with their analytical properties. The inFestigation was limited to the ether-soluble ingredients, as t h e determination of the ether-insoluble ingredients by conventional methods is rather simple. During the investigation the infrared spectrograms of 24 common ether-soluble ingredients of propellants were prepared. A qualitative infrared method was developed and used in the analysis of unknown propellants. The method is 'faster and more specific, and requires less sample, less experience, and less specialized knowledge than conventional methods. Quantitative infrared methods for analyzing mixtures of several ingredients have been tested on synthetic mixtures, and found to be about as accurate, faster, and more specific than ronventional methods.

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ingredients consist essentially of nitrocellulose, inorganic salts and a very few other substances such as graphite and potassium sulfate. The qualitative and quantitative determination of the ether-insoluble ingredients, by standard chemical methods ( 2 , 3 , 5 ,6,9,f f - f 4 18, f Q ) , is in general simple, rapid, and specific.

S THE past, propellants were comparatively simple mixtures

( 7 , 8, 10) consisting essentially of nitrocellulose with one or very few other ingredients such as nitroglycerin, dinitrotoluene, and diphenylamine. Today, however, propellants are usually very complex mixtures, consisting in some cases of as many as 10 ingredients. I n addition, the variety of ingredients used today is much greater than in the past. I n general, the ingredients of propellants are divided into two groups, soluble and insoluble in ethyl ether. The ether-insoluble

Figure 1. Spectrogram of Representative Propellant 20 % 5%

Nitroglycerin (NG) Dinitrotoluene (DNT) Diphenylamine (DPA)

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Figure 2.

Infrared Spectra of Propellant Ingredients Nitroglycerin 99+%, 2 '3% solution

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Figure 3.

Infrared Spectra of Propellant Ingredients

Metriol trinitrate (methyl trimethylol methane trinitrate) 99+%, 2 9% solution

gether with their analytical properties. Even with the use of chromatography and streak tests (15, 18, 22, 23) the analysis is still time-consuming, tedious and difficult, and requires considerable sample because the streak tests are not applicable to substances such as triacetin and petroleum jelly, are difficult t o apply in others, such as the phthalates, and even when they apply, the tests are generally indicative of classes and not of individual compounds. Thus if nitroglycerin, dibutyl phthalate, and ethyl centralite were present. the streak tests would indicate the presence of a nitrate, a phthalate, and a centralite. Therefore, in order to ascertain the exact nature of the ingredients present, it would be necessary to isolate a suffi-

The ether-soluble matter, especially in the case of double-base powders (containing a liquid nitrate such as nitroglycerin) is a complex mixture consisting generally of a liquid organic nitrate, with additional ingredients such as gelatinizers, plasticizers, stabilizers, and flash reducers. Assuming that the nature of the ingredients is known, as is the case in control work, the quantitative determination of such ingredients generally requires several days, more than 10 grams of sample, and in many cases use of chromatographic methods that are difficult to control ( I I , f 4 , 19). The qualitative determination of the ether-soluble ingredients by conventional chemical methods is a formidable task requiring extensive knowledge of the many possible ingredients used to-

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Figure 4.


Diethylene glycol dinitrate (DEGN) 99+%, 5 % solution

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Triethylene glycol dinitrate (TEGN) W+%, 5 % solution

cient quantity of each of the ingredients in a sufficiently pure form so that by a combination of chemical and physical data, the particular nitrate, phthalate, and centralite may be ascertained. As infrared spectroscopy is known ( 1 ) to have been effectively used in the analysis of multicomponent mixtures of organic compounds, an investigation was undertaken to determine the feasibility of using infrared spectroscopy in the tieterminntion of the ether-soluble ingredients of propellants.

ancc scale in addition to the transmittance scale, so that absorb a w e measurements could be made directly instead of being calculated from the transmittance readings. The instrument was calibrated using ammonia. and carbon dioxide ( $ I ) , and was found to be within 0.02 micron throughout the region between 2 and 1; microns. The absorbance measurements were all made on a single 0.10-mm. cell, thereby simplifying the calculations by eliminating the exact thickness of the cell from consideration. The cells were filled and cleaned using an infrared cell filler (201, consisting of an eye dropper with a rubber ring a t the tapered end. Rock salt optics were used throughout. The solvents used were all treated with anhydrous sodium sulfate and sodium chloride to minimize any solvent action on the sodium chloride cell. The spectrograms of the ingredients were prepared in solutioll form in suitable solvents ($0, 24).

INSTRUXIENTATION A S D TECHNIQUES

A Perkin-Elmer doublebeam infrared spectrophotometer (Model 21) was used. The instrumrnt was fitted n-ith iln absorb-

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Dimethyl phthalate, E. K. white label, 5 % solution

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Diethyl phthalate, E. K . white label. .57c solution

RESULTS

The infrared spectra of 21 coininon ether-soluble ingredients of propellants have been investigated and are recorded in Figures 2 to 2 5 . -1qualitative infrared method of determining the ethersoluble ingredients of propellant has been tvorked out. The method, with a little experience, requires less than half an hour t o analyze a sample qualitatively. This, of course, is niuch less than required by conventional methods. The infrared method has been applied to unknown samples and the findings have been eonfirnied b y standard conventjonal tests and also by the fact that quantitative analyses of these samples based on the qualitative findings accounted. within experimental error, for 10070 of the samples. Figure 1 gives the spectrogram of a representative propellnnt. including hands assignments. In addition to the

Figure 8.

qualitative. cluantitative methods of anal-sis w r e worked out covering two different mixtures of ingredients such as may tie found in propellants. Iflien the niethocls were applied to known syiithetir mixtures of the ingredients, the results listed in Tables I a n d I1 were obtained. The results show the methods to be

Table 1.

F-alues Obtained o n Synthetic SampIes Anal: r;ed by Infrared Method ; i I'oiinil

h-itroglj-cerin. SC Dirthylphthalate, % 2-Sitrodii,lien3lamine, 570

Added 72.7 23.1


Dibut,l phthalate, E. K. white label, 5 % solution

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Figure 9.


Diootyl phthalate 99+%

Carbide & Carbon Chemicals Corp., 5 % solution

carbon tetrachloride and the spectrogram of the carbon tetra chloride solution was obtained in the regions 2 to 12 and 14 to 15 microns using a 0.1-mm. cell. (Nitroglycerin, when present in considerable quantities, will not completely dissolve in carbon tetrachloride; however, after the mixture has been shaken and the two phases allowed to separate, the carbon tetrachloride solution will have enough nitroglycerin in solution so that i t may be easily detected.) The ingredients present were ascertained by examination of the spectrogram obtained and comparison of it Kith those of the ingredients. Where the spectrum in the 12- to 14-micron region ww desired to elucidate the composition of tho sample, the spectrum was obtained just as described above, ['Ycept that nitromethane was used as the solvent. Quantitative Methods. NETHOD A (for ether extract consisting of about 70% nitroglycerin, 2 5 W diethyl phthalate, and 5% 2-nitrodiphenylamine). Ethylene chloride solutions of the sample wsre accurately prepared corresponding to the followirig concentrations and measured at the peaks of the bands occurring

accurate, in addition to being faster and more specific than conventional chemical methods. EXPERKMENTAL METHODS

Qualitative Method. The ether-soluble matter from approximately 1 gram of propellant was diluted to about 1 gram with Table 11. Values Obtained on Synthetic Sample Analyzed by Infrared Method B Found Added Nitroglycerin, % Triacetin % Ethyl cen'trslite % Dinitrotoluene, '%

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Figure 11.

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Monophen? lurethan, melting point 52" C., 5 % solution

3I)out the following wave lengths, using B 0.1-mm. cell and the c~.ll-inand cell-out technique: For 2-nitrodiphenylamine, 2 grams of ethylene chloride solution containing 0.4 gram of sample, measured a t peak of 6.65mimon band. For diethyl phthalate, 2 grams of ethylene chloride solution cmtaining 0.1 gram of sample, measured a t peak of 5.77-microri Imnd. For nitroglycerin, 4 grams of ethylene chloride solution containing 0.025 gram of sample, measured a t peak of 6.01-micron I)and. The absorbance of the cell and the absorptivities of the solvent :md ingredients had previously been determined a t the same three Points using the same spectrometer conditions ( 1 7 ) . I n the Cas(' of the ingredients, the absorptivities were determined using ai: closely as practical the same concentr:ltionqni: in thr solutions of the sample ( 4 )

The percentage - composition was calculated from thv a1)ove data. M m H o D B (for ether extracts consisting of approxi~n:itt~Iy 55% nitroglycerin, 20% triacetin, 20y0 ethyl centralite, and 5% dinitrotoluene). The infrared method used for this mixture was thr same as Method -4, except that (1) chloroform was used as the solvent, (2) the bands used were: nitroglycerin 7.75 microns, triacetin 5 7 2 microns, centralite 6.67 microns, and dinitrotoluene 6.55 microns, and (3) the ether extract was diluted so that the ingredirnts had ar)orouimatelv the followine concentrations when mGnsured: nitroglycerin 5%; triacetin l.O%, centralite 5%, and dinitrotoluenc 5%.

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DISCUSSIOK

rn using the qualitative method of analysis, occasionally, it rvas

possible to determine only the class in which a minor ingredient belonged, such as a phthalate or a urea. but there v some doubt

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Diphenylurethan, melting point 72' C., 59" solution

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N-Ethyl-rV-phenylurethan, E. K. white label, 15% solution

as to the specific compound. Sucli cases occurred when the concentration of a minor ingredient was very small or occasionally if the major ingredients masked the useful bands of a minor ingredient. I n such cases a preliminary, even partial, separation by chromatography (16, 16, 22, 23) followed by infrared analysis removed all doubts as to the exact nature of the niinor ingredient. I n Figures 2 to 15, 17 to 19, and 21 to 25 spectrograms of the carbon tetrachloride solution u-ere obtained in the regions 2 to 12, 14,and 15 microns; in methyl nitrite a t 12 to 14 microns. The spectrograms of the ingredients were prepared in solution form in suitable solvents for the following reasons: (1) Some of the ingredients were solids. (2) many of the liquid ingredients

mere sufficiently polar and so strongly absorbing in the infrared that even with the thinnest obtainab!e film some of the major bands would not be resolved, and ( 3 ) solutions are necessary for quantitative work and very desirable for the best qualitative work. TVhenever possible, a 5y0 solution in a 0.10-mm. cell corresponding to a 0.005-mm, sample thickness was used because under those conditions the prominent and useful bands of the ingredients appeared well resolved. In the qualitative analysis by the infrared method described under “Experimental Methods” the ingredients of a sample are ascertained by evamination of the spectrogram of the sample. An examination of the spectrograrnr of the ingredients of pro-

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Figure 14.

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a-Dimethyldiphenylurea (methyl oentralite), melting point 121’ C., 5 % solution

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s-Diethj Idiphen? lurea (ethyl centralite), molting point 72' C., 59'0 solution

pellant. d l C J i 5 ' S that the various chsses (nitrates, phthalates, ureas, etc 1 have certain common bands characteristic of the various groups. In addition, the spectrograms of the individual members of each class possess specific characteristics which can be used to differentiate the various members of each class. In the elamination of the infrared spectrogram of a sample for the purpose of ascertaining its constituents, experience and a working knowledge of the shape and position of the various bands are great assets: however, the following spectral characteristics were found very valuable: Nitrates are characterized by two sharp bands a t about 6 and 8

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microns. and a broad band at about 12 mic,rotis. TIP glycol nitrates such as diethj-lene glycol dinitrate ( D E G S ) and triethylene glycol dinitrate (TEGX), having an ether group, possess a band a t about 8.8 microns characteristic of this group. The individual nitrates of the glycol and nonglycol series are distinguishable from each other by presence of certain characteristic bands or by difference in shape or exact position of bands; thus, the 7.8-micron band of nitroglycerin has a shoulder, whereas that of nietriol trinitrate does not. The tip of the 6-micron band in diethylene glycol dinitrate has a shoulder, whereas that of triethylene glycol dinitrate does not. Furthermore, the peak of

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Figure 16.


uns-Diphenylurea, melting point 187' C., 2 70 solution In chloroform. 2 to 8.0, 8.35 to 12.0 microns I n methyl nitrite, 8.0 to 8.35, 12.0 to 15.0 microns

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Figure 17.

Infrared Spectra of Propellant Spectra

Diphenylamine. melting point 53' C., 5 % solution

the C-0-C band in diethylene is a t about 8.75 microns, whereas that of triethylene glycol dinitrate is a t about 8.85 microns. Phthalates are characterized by fairly sharp bands a t about 5.8, 8.9, 9.35, and 9.65 microns and a broad band a t about 7.8 microns. The individual phthalates can be distinguished as follows: Dimethyl phthalate has a very sharp band a t 7.0 microns and also bands a t 8.4 and a t 10.35 microns. Diethyl phthalate has a very sharp band a t 7.3 microns and another band a t 9.9 microns. Dioctyl phthalate and dibutyl phthalate both have bands a t about 6.85 and 7.25 microns; however, dibutyl phthalate has a doublet a t 10.5 microns, whereas dioctyl phthalate has a

single band at 10.5 microns. The ratio of absorbance at the 3.4micron region (C-13 stretching frequency) over the absorbance a t the 5.8-micron region (carbonyl stretching frequency) increases in going from dimethyl phthalate to dioctyl phthalate, and this can serve as a means of identification. This can be used only when the unknown is a single ingredient. Ureas are all characterized by a strong band a t 6.0 microns characteristic of the urea carbonyl and also a sharp band a t about 6.7 microns. The individual ureas can be identified as follon-s: Ethyl centralite possesses doublets a t about 7.25 and 7.8 microns. Methyl centralite possesses a small but sharp band a t 7.0 microns

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Figure 18.

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Infrared Spectra of Propellant Spectra

Nitrosdiphenylamine, melting point 66' C., 5 % solution

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Figure 19. Infrared Spectra of Propellant Ingredients 2-hitrodiphenylamine, melting point 75' C., 5 % solution

at 8.7 microns. Diphenyl urethane possesses a strong, very sharp band a t 7.3 microns. Tartrates are characterized by a band a t 2.9 microns characteristic of the hydroxyl group and another a t 5.7 microns characteristic of the ester carbonyl group. Diethyl and dibutyl tartrates also possess a strong broad band a t 8.0 microns and two .ivell-defined bands a t 8.9 and 9.2 microns. Diethyl tartrate is distinguished from dibutyl tartrate by the fact that it possesses a very sharp band a t 7.3 microns and a fairly strong band a t 9.8 microns. Nitro aromatics such as DST (2,4-dinitrotoluene) and 2-nitro-

ani1 a strong band a t 7.4 microns.

as-Diphenyl urea possesses a eak band a t about 3.0 microns characteristic of theS-H stretching frequency and also a strong band a t 7.25 microns. Urethanes usually have complex spectra and not many common (-lass characteristic bands; however, all have a band a t about 5.8 and a fairly sharp strong band at about 13 microns. The individual members can be recognized as follows: o-Tolyl urethane possesses a band a t 2.9 microns and a doublet a t about 1:3.1 microns. hfonophenyl urethane possesses a doublet a t ahout 2.9 microns and an unresolved doublet a t about 13.1 niicrons. N-Phenyl-N-ethyl urethane possesses a strong band

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Z,&Dinitrotoluene, melting point 70' C., 5 % solution I n chloroform, 2.0 t o 7.75, 8.75 t o 12.1 microns I n benzene, 7.75 to 8.75, 12.1 t o 14.5 microns

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Figure 21.

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Diethyl tartrate, E. K . white label, 570 solutiou

diphenylaniine are characterized by bands at 6.25, 6 . 5 , and i . 4 characteristic of the aromatic nitro group. 2-Sitrodiphenylamine is distinguishable from DNT by the fact that it possesses a band at 3.0 microns characteristic of the X-H group.

Col. C. IT.Clark, and 4.J. Clear, C. J. Bain, and Robert Frye of Picatinny Arsenal for help rendered in the publication of this report. LITERATURE CITED

ACKNOWLEDGMENT

(1) ANAL.CHEV..January 1949.

The author wishes to acknowledge the assistance of George Clift, who carried out much of the experimental work, and of B. T. Federoff, who gave valuable assistance in connection with references. -1ppreciation is further expressed to Col. C. R. Duttuii,

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(2) (3)

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January 1950, and January 1951. Reviews on Infrared Spectroscopy.. Ara, -4.P., “Tratado de Explosiros,” 1st ed., pp. 763-71, La Habana, Cuba, Cultural, S. A , , 1945. Becker, F., and Hunold, G . .i.,z. ges. ,ychzcss-U . SpTengstoffw. 28,233-7,284-6, 372-6 (1933).

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Figure 22.


Dibutyl tartrate, E. K . white label, S 7 0 solution

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Figure 23.


Diphenyl formamide, melting point 73’ C., 5 % solution

(4) Brown, T. L., and Bernstein, R. B., A N ~ LCHEM., . 23, 673-4 (1951). 15)Brunswig, H., “Das Rauchlose Pulver,” pp. 311-26, 36S-T8, Berlin, W. de Gruyter & Co., 1926. (6) Clift, G. D., and Federoff, B. T., “Manual for Explosives Laboratories,” Vol. I, Chap. XII, pp. 11-14, T-ol. 11, pp. 5.65.11, Philadelphia, Lefax Society, 1942. ( 7 ) Daris, T. L., “Chemistry of Powder and Explosives,” Tol. 11, pp. 292-5, X e w York, John Wiley 8. Sons, 1943. ( 8 ) Hayes, T. J., “Elements of Ordnance,” S e w l-ork, John Wiley ‘3 Sons, 1938.

(9) Kast, H., and Mete, L., “Chemische Gntersuchung der Sprengund Ziindstoffe,” pp. 269-303, Braunschweig, Friedr. Vieweg und Sohn, 1944. (10) Marshall, A., ”Explosives,” Vol. I , Philadelphia, P. Blakiston’s Sons & Co. 1917. (11) Office of Chief of Ordnance, Washington 25, D. C., Joint Army-Iiavy Specification, JAN-P-381. ‘12) Ibid., JAN-P-668. (13) I bid.,JAN-P-715. (14) Office of Chief of Ordnance, Washington 2 5 , D . C . , llilitary Specification, MIL-P-10557 (Ord).

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Figure 24.

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Phthalide, melting point 74’ C., 2‘70 solution

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Glycerol triacetate (triacetin), E. K. white label, 5 % solution

Office of Scientific Research and Development (OSRD), KO. 1837, “Chromatographic Investigations of Smokeless Powders and Related Substances,” No. I. Ovenston, T. C., A n a l u s t , 74,344-51 (1949). Philpotts, A. R . , Thain, W., and Smith, P. G., Ax.41..CHEM., 23,

(15)

(t6) (17)

268-72 (1951).

(18) Picatinny Arsenal Tentative Specification, PXS-1197 (available through Office of Chief of Ordnance, Washington 25, D. C.).

( % l Y )l b i d . PXS-1250 (Revision 1 ) . (20) Pristera, F., A p p l . Spectroscopy, 6, No. 3,29-44 (1952).

(21)

Pristera, F., Perkin-Elmer Instrument ,Vews,

2 , KO. 2

(Winter

1951).

(22) Schroeder, W. .I.,Keilin, B., and Lemmon, R. M . , Ind. Eng. Chem., 43,939-46, (1951). ( 2 3 ) Schroeder, W. A , , Malmberg, E. W., Fong, L. L., Trueblood,

K. N., Landerl, J. D., and Hoerger, E., Ibid.; 41, 2818-27 1949).

(24) Torkington,

P., and Thompson, H. W,, Trans. Faradall

Soc., 41,

184-6 (1945).

IIPCEIYED June 5 , 1952.

Accepted January 7, 1953.

Extraction of Metal Thiocyanate Complexes with Butyl Phosphate Iron Thiocynate LABEN MELNICKl AND HENRY FREISER C’niversity of Pittsburgh, Pittsburgh 13, Pa.

H. F. BEEGHLY Jones & Laughlin Steel Corp., Pittsburgh 7, Pa.

P

REVIOUS work by Aven and Freiser ( 1 )indicated that butyl phosphate is a useful solvent for the extraction of iron(II1)

thioryanate. This is important in steel analysis, since iron interferes Kith the determination of various elements in steel and must be removed. Aven and Freiser found that with butyl phosphate, a commercially available, nonvolatile, nonflammable solvent, rapid extractions may be made without the inherent difficulties of the Rothe ether method. The time required per extraction and the cost of the method are both less where butyl phosphate is used instead of ether. Butyl phosphate may be readily recovered. Also, butyl phosphate is better for extracting iron(II1) thiocyanate than ether ( 3 ) . Precision obtained by 1

Present address, Jones & Laughlin Steel Corp , Pittsburgh 7, P a .

use of t h e two methods is comparable. This paper is a report on a more detailed study of various conditions affecting extraction of iron( 111)thiocyanate with butyl phosphate. REAGENTS AND APPARATUS

Unless otherwise stated, all reagents are C.P. or reagent grade. .411 p H measurements were made with a Beckman p H meter. Sodium thiocyanate. Stock solutions were standardized with silver nitrate. Ammonium hydroxide, filtered. 30% Hydrogen peroxide. Sodium hvdroxide. Sitric accd. Tributyl phosphate obtained from Commercial Solvents Corp. This was used without further Durification. Stock iron solutions prepared with Kational Bureau of Stand-

Analysis of Propellants by Infrared Spectroscopy - ACS Publications

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