Anal. Chem. 1982, 5 4 , 891-895
Sensitized Room Temperature Phosphorescence in Liquid Solutions; with 1,4=Dibromonaphthaleneand Biacetyl as Acceptors J. J. Donkerbroek, C. Gooijer,* N. H. Velthorst, and R. W. Frel Department of General and Analytlcal Chemistry of the Free University, de Boelelaan 1083, 1081 HV Amsterdam, The Netherlands
The potentlal In analytlcal chemistry of sensltlzed phosphorescence In Ilquld solutions at room temperature (RTPL) Is examined. After excltatlon (before radlatlonless decay Is effective) the analyte molecule transfers Its trlplet-state energy to an acceptor molecule whlch subsequently emlts phosphorescence. 1,4-Dlbromonaphthalene and blacetyl are lnvestlgated as acceptors In solvents frequently used In liquid chromatography. Three points are consldered: (I) the excitation wavelength ranges for the analytes, llmlted by the acceptors, (11) the mlnlmum trlplet-state energles of the analyte, necessary to make the energy transfer process dlffuslon controlled, and (Ill) the phosphorescence efflclencles of the acceptors at room temperature In varlous solvents. More favorable excltatlon properties make blacetyl more widely appllcable than 1,Q-dlbromonaphthalene.Llmlts of detectlon obtalned for a number of substltuted benzophenones and blphenyls In acetonltrile/watet (1:l) are on the order of IO-'
M.
Many organic compounds, having a low fluorescence yield because of an efficient radiationless deactivation via intersystem crossing, can be sensitively detected by phosphorimetry. Unfortunately, intense phosphorescence signals seem to be only obtainable if the quenching of the emitting triplet state by oxygen and impurities is prevented which is not realized for (ordinary) liquid solutions. For this reason the application of phosphorescence in analytical chemistry has been limited to rigid glassy solutions at 77 K and to analytes in the adsorbed state at room temperature (RTP). Recently, Cline Love and co-workers (I) introduced the method of micelle-stabilized room-temperature phosphorescence (MS-RTP). It would be very attractive if phosphorescence could also be applied to liquid solutions at room temperature (RTPL) in nonmicellar systems. Such a technique would be particularly interesting asi a detection method in dynamic systems (flow injection, liquid chromatography, etc.). In a previous study (2) we have shown that in some cases, especially for nonflexible molecules, RTPL is quite sensitive, e.g., the limit of detection (LOD) of 1,4-dibromonaphthalene (Br,N) in n-hexane at room temperature is 5.4 X M. Flexible compounds that do emit strong phosphorescence in frozen solutions generally produce RTPL signals which are too weak to be of analytical importance. However, it was concluded that sensitized RTPL, in contrast to direct RTPI,, has a considerable potential in analytical chemistry. In this imethod the analyte is detected indirectly via RTPL of an acceptor. After excitation the analyte acts as an energy donor (D) according to D(TJ + A(%) D(S& + A(TJ (1) where A is the acceptor; So and Tl are the electronic ground states and the lowest triplet states, respectively. Sensitized RTPL is expected to have an excellent potential when the
above reaction is diffusion controlled (2). This condition i s fulfilled if the triplet state of the acceptor is lower than that of the donor (3). Under these circumstances the energy transfer reaction frequently competes successfully with other deactivation pathways of D(T1). Therefore the acceptor must have not only a high phosphorescence quantum efficiency iin liquid solutions but also a low triplet-state energy. Furthermore it should be realized that the sensitized RTPL method can only be applied at an excitation wavelength a t which the analyte is excited while the acceptor is not. This implies that the absorption spectrum of the acceptor plays an important role. Especially the last point prompted us to examine biacetyl (BIAC) (well-known as a RTPL emitter ( 4 ) ) as an acceptor since its molar absorptivity is very low over a large wavelength region. In the present paper the acceptors BrzN and BIAC are compared in a number of solvents often applied in liquid chromatography; the solvents were used without extreme purification (see Experimental Section). Furthermore the sensitivity of the method is demonstrated by the determination of the LOD's of some benzophenones and biphenyls in acetonitrilelwater.
THEORY In accordance to Backstrom et al. ( 4 ) and Birks ( 5 ) ,the intensity of sensitized RTPL of a donor-acceptor couple is given by the product of the rate of light absorption by the donor (labD), its efficiency of intersystem crossing (OiScD), the efficiency of the energy transfer from the donor to the acceptor (O,""), and the phosphorescence efficiency of the acceptor (0,"). Additionally it should be realized that for uncorrected spectra the photomultiplier output, P(sens), depends on both the excitation wavelength A,, (via the source and the excitation monochromator) and the emission wavelength A,, (via the emission monochromator and the photomultiplier), so that P(sens) = kIo,e,(2.3eD[D] l)diacDOtDAOpASpA
(2)
In eq 2 k is an instrumental constant independent from A,, and A,, IabsDis approximated as Io,,,(2.3~D[D]1) which is allowed for low optical densities of D; lo,ex denotes the intensity of the light source at A,, cD is the molar absorptivity of D at that wavelength, [Dl is the donor concentration, and 1 is the optical pathlength. SPA is a dimensionless parameter connected to the wavelength dependence of the detection system. Equation 2 only applies if the acceptor does not absorb radiation at hex. In the simplified diagram of Figure 1 where vibronic transitions are not considered, the deactivation pathways of the donor molecule in the liquid phase after excitation to its S1 state are visualized. Naturally, we are primarily interested in weak fluorescing molecules with a high intersystem crossing efficiency given by (5)
eiscD=:
kisP kfD + knfD kiscD
0003-2700/82/0354-08~1$01.25/0 0 1982 American Chemlcai Society
+
(3)
892
ANALYTICAL CHEMISTRY, VOL. 54, NO. 6, MAY 1982
Flgure 1. Energy dlagram for a model system in sensitized phosphorescence. I,: Is the rate of light absorption by the donor. k?, k,?, and kIsGD are the rate constants in s-' of the intramolecular deactivation of the donor vla fluorescence, internal conversion, and intersystem crossing, respectively. kpDand kPAare the phosphorescence rate constants of the donor and the acceptor, while kn: and k,: are the overall rate constants of intra- and intermolecular nonradiative deactivation in s-'. k,[A is the apparent rate constant of the energy transfer reaction in s- .
11
In media where external heavy atoms are absent, BiecD is mainly determined by intramolecular rate constants. Therefore we have considered it as a constant. After having arrived in the TI state, in the absence of acceptor the donor is rapidly deactivated either via emission of phosphorescence or via intra- and intermolecular radiationless decay. The associated triplet lifetime, rOD,defined as (6) T~~
= (kPD
+ kn?)-l
(4)
ranges from to s (4, 7-9),being much shorter than in rigid solutions where triplet lifetimes are found from 10 s. The difference is caused by k,: which is much to higher in the liquid state. Sensitized RTPL will be important if there is a real chance that in the presence of the acceptor energy transfer according to eq 1occurs within rOD. In this case the efficiency of energy transfer BtDA, which can be defined as (2)
distinctly deviates from zero. Finally, the radiation observed is determined by the phosphorescence efficiency of the acceptor BPA which is equal to ( 5 )
e,* =
kPA kpA kn:
+
For a given donor (the analyte), P(sens) depends on the choice of the acceptor. As already noted in the introduction three points have to be considered: (i) The SI state energy of the acceptor, see Figure 1,which limits the excitation wavelength region for the donor. In practice this means that it is not always possible to choose A,, corresponding to the maximum of I0,,,cD. (ii) The TI state energy of the acceptor, which determines the value of kt and thus of BtDA; see eq 5. If the TI state of the acceptor lies below the T1 state of the donor, k , has the value of a diffusion controlled process (3). In the reversed situation etDAwill be negligible. (iii) The influence of the solvent on the phosphorescence efficiency of the acceptor in the liquid state. These points will be discussed in the following sections for the two acceptors chosen.
EXPERIMENTAL SECTION Apparatus. The RTPL experiments were performed on a Perkin-Elmer MPF-44 fluorescence spectrophotometer (Norwalk, CT) provided with two Hamamatsu photomultiplier tubes (type R777-01-HA),a differential corrected spectra unit (DCSU-2),and
I 250
300
350
-
\
400 WuWQngth
450
(nm
Flgure 2. Absorption spectra of Br,N (1) and BIAC ( 2 ) in hexane.
a XBO 150-W xenon lamp (GRAM, Munchen, G.F.R.). Details of the experimental setup have been given before (2). The phosphoroscope was not used; the cell was placed in the fluorescence cell holder. In sensitized RTPL experiments 4.5 mL of acceptor solution (for concentrations see Tables I1 and IV) was deoxygenated with nitrogen gas during 2-3 min and the background signal at the emission maximum of the acceptor was registered (the excitation wavelength depends on the donor; excitation and emission spectral band-passes used are 8 nm). Subsequently a 9 WLstandard to M) was injected solution of the donor (in the range and the RTPL signal was measured. If necessary the background signal was electronically suppressed. The limits of detection were based on a signal to noise ratio of 3. In the experiments, constant spectral band-passes of 8 nm were used. This implies that the phosphorescence intensity observed corresponds to a fraction of the total number of photons entering the emission monochromator. This fraction is higher for a narrow phosphorescence band than for a broad one. Chemicals. n-Hexane HPLC grade (Baker, Deventer, The Netherlands), acetonitrile p.a. (Baker),and methanol p.a. (Baker) were used without further purification. Dichloromethane p.a. (Baker) was doubly distilled. Before use the water was distilled and subsequently filtered through an ultrafiltration system (Millipore, Utrecht, The Netherlands). 1,4-Dibromonaphthalene (Eastman, Rochester, NY) was two times sublimated. Biacetyl 98% (Merck, Amsterdam, The Netherlands) was distilled at reduced pressure. 4,4-Dibromobiphenyl (Fluka, Buchs, Switzerland) was recrystallized twice from ethanol; 4-bromobiphenyl (Chemical Procurement Laboratories, New York) was used without further purification. The benzophenones were gifts from the medicinal chemistry department of the Free University; they were used without further purification.
RESULTS AND DISCUSSION The Excitation Range Allowed by the Acceptors. The absorption spectra of BrzN and BIAC both in hexane are depicted in Figure 2; because of the large difference between the molar absorptivities, they are plotted on a logarithmic scale. The influence of the solvents under consideration on the absorption spectrum of Br,N appears to be negligible in view of the experimental accuracy. The molar absorptivity at 300 nm amounts to 7.9 X IO3 M-l cm-l; it rapidly falls off with increasing wavelengths. From this we conclude that BrzN can be used as an acceptor at donor excitation wavelengths larger than 330 nm. At smaller wavelengths simultaneous excitation of the acceptor provides serious problems. The absorption of BIAC in hexane extends to wavelengths up to about 460 nm. At first sight this implies that the excitation range permitted by the acceptor is small. However, as will be obvious from Figure 2, molar absorptivity is very small over the whole wavelength region above 220 nm (in more polar solvents the c values in the visible part of the spectrum
ANALYTICAL CHEMISTRY, VOL. 54, NO. 6, MAY 1982
are even lower) (10). Thus the question arises whether it is possible to use a IBIAC concentration which is on the one hand low enough to make its excitation negligible and on the other hand high enough to guarantee a reasonable value of etDA. The Choice aif the Acceptor Concentration. The dependence of etDi4on [A] is given in eq 5. Of course the maximum value attainable for etDAis equal to one, which is only realized if [A] limits to infinity. Therefore it is more realistic to require that @?A 1 0.5. The acceptor concentration needed to make OtDA = 0.5 depends on both k, and ~ 0 For ~ . illustration, assuming for kt the relatively small value of lo9 M-l s-l (3),for a Bhort-lived triplet with roD= 1ws, [A] must exceed M whereas for a long-lived triplet with roD= 1 ms, [A] must be larger than lo4 M. The above considerations M imply that acceptor concentrations lower than 1.0 X should not be used because then donors with triplet lifetimes shorter than 10 I.LS may not be detected sensitively. The T, State ]Energies of the Acceptors. As mentioned before it is well-known from the literature (3)that the energy transfer according to eq 1 in liquid solutions is, generally, diffusion controlled if the Tl-state energy of A is lower than the TI-state energy of D. The TI-state energy is equal to the energy of the 0-0 band in the phosphorescence spectrum. For Br2N in ethanol at 77 K we have found this band at 495 nm (820.2X lo3 cm-l) which is close to the value in EPA (ether, isopentane, alcohol, 5521, as reported by Mttrchetti and Kearns (11),Le., 492 nm ( ~ 2 0 . 3 X lo3 cm-'). For BIAC a T1-state energy of 19.7 X lo3 cm-l (corresponding to 507 nm) has been reported (12,13). From these results it is obvious that RIAC is somewhat more favorable as a triplet acceptor than Rr2N. In fact their difference in ?',-state energies of 500 cm'-l implies that the energy transfer from Br2N, acting as the donor, to BIAC, acting as the acceptor, will be diffusion controlled. In the subsequent section this reaction is applied to estimate the ratio of the OpASpA values for BIAC and Br2N. The Phosphorescence Efficiencies of BIAC and Br,N in Various Solvents. The solvent dependence of the phosphorescence efficiencies of BIAC and Br2N is examined by determining the relative values of their OPA values in a number of solvents. Furthermore, in order to compare the values two acceptors quantitatively, the ratio of their OpASpA is calculated; see leq 2. The solvent dependence of BPBuC is simply obtained by measuring the relative intensities of the phosphorescence maximum at 520 nm and the fluorescence maximum at 460 nm of M BIAC solutions. As will be obvious from Figure 2 the effect of selfaabsorption on the fluorescence spectrum will be of minor importance since at 460 nm the molar absorptivity is only 10 M-' cm-l. The associated photomultiplier outputs Ppand Pf are, respectively, compared to eq 2
(7) and
pl. = kIabsBIACO B I A C s BIAC f
f
(8)
so that their ratio PplPfis given by
PP -P f
BIACg B I A C S BIAC p P B B l A C S BIAC
4sc
(9)
f
where 8$'uc is the fluorescence efficiency. Equation 9 can be rewritten (see Figure 1 and eq 3) as
In eq 10 to a good approximation only
BPBIAc
is solvent de-
893
-.-
Table I. Ratio of Intensities of Phosphorescence and Fluorescence for BIAC in a Number of Solventsa solvent PpPf n-hexane 12.0 acetonitrile 28.5 acetonitrile/water 12.7 (v:v = 1:l)
solvent methanol methanol/water (V:V = 1:l) dichloromethane
PpIPf 5.3 4.9
19.7 x l o 3 cm-' ( ~ 5 0 7nm)
t
+
t
t
t
t t
tt,very good; t , good; -, not recommendable.
Combination of eq 2 (where D is BrzN and A is BIAC) with eq 11 reveals that LOD(sens)
P(d
can be assessed from eq 5. In our experiments the concentration of BIAC is M. 7BrzN0 in hexane has been previously extimated at 1.7 ms (2);the solvent dependence of OpBrZN indicates (2) that the triplet lifetime of BrzN in acetonitrile,acetonitrile/water, methanol, and methanol/water is on the order of 104s and in dichloromethane of lod s. Since kt will be larger than lo9 M-' s-l, in n-hexane OtBr2N-B1AC will be larger than 0.99, while in acetonitrile, acetonitrile/water, methanol, and methanol/water it will be larger than 0.90. Therefore, measwement of LOD(dir)/LOD(sens) in the above solvents excluding dichloromethane reflects the difference in emission properties with an accuracy of about 10%. From Table I1 we conclude that the only solvent in which BrzN has more favorable emission properties than BIAC is n-hexane; in methanol and methanol/water BrzN and BIAC are similar whereas in acetonitrile and acetonitrile/water BIAC is the favorable one. The Sensitivity of the Sensitized RTPL Method. To discuss the sensitivity inherent to the sensitized RTPL method and its usefulness in analytical chemistry, one must choose the more versatile of the two acceptors. This can be done on the basis of Table I11 which summarizes the results of the preceding sections. The most striking difference between BrzN and BIAC is that BrzN can only be applied to detect analyte molecules absorbing strongly at wavelengths above 330 nm, whereas for BIAC such a restriction does not exist, because of ita low molar absorptivity. The T1-state energies of the two acceptors are almost the same; BIAC is a little more favorable. Furthermore in many solvents BrzN and BIAC have similar phosphorescence emission properties. So, BIAC is more generally applicable as acceptor than BrzN especially because of its excitation properties. Many compounds of analytical interest have maximum t values at wavelengths shorter than 330 nm where BrzN cannot be used. In a previous paper (2)for benzophenone in hexane a LOD value of 4.0 X lo-' M was reported by using sensitized RTPL with BrzN as acceptor. This value is not a good indication for the sensitivity of the sensitized RTPL method because, at the excitation wavelength applied, the molar absorptivity of benzophenone is only about 100. More favorable LOD values can be achieved by means of sensitized RTPL with BIAC as acceptor. This is shown in Table IV where data OtBrlN-BIAc
compound benzophenone 4-bromobenzophenone
+
+
Table IV. Limits of Detection (LOD) in mol/L Obtained via Sensitized RTPL, with BIAC as Acceptor in Acetonitrile/Water (v:v = 1 :1)
4-methyl benzophenone 2-chlorobenzophenone 4,4'-dibromobiphenyl 4-bromobiphenyl
[BIAC],M 5 x 10-3 5 x 10-3 10-4 5X
5 x 10-3 10-4 10 -4 10-4
&XI
nm
330
LOD,M 3.2 X
280
1.9 x 10-7
280 330 280
3.2 X 2.7 X
280 280 280 288 290
2.9 X
1.9 X 5.4
X
5.1 X 3.6 X
2.4
X
lo-' lo-' lo-' lo-' lo-' 10.' lo-'
obtained for some substituted benzophenones and biphenyls in acetonitrile/water (v:v = l:l),a solvent frequently used in liquid chromatography, are collected. The excitation wavelengths 280 nm (for the benzophenones), 288 nm (for 4,4'dibromobiphenyl), and 290 nm (for 4-bromobiphenyl) corresee eq 2. Within this context spond to the maximum of it should be realized that in our experimental setup where a Xe lamp is used as a light source, Io,,x falls off rapidly with decreasing wavelengths. The importance of the proper choice of the excitation wavelength is demonstrated for benzophenone and 4-bromobenzophenone. Changing the wavelength from 330 to 280 nm leads to an improvement of the LOD value by a factor of about 15. An additional point is the optimal choice of the acceptor concentration. In Table IV for benzophenone and 4-bromobenzophenone, two acceptor concentrations were tested, Le., 5.0 X M and 1.0 X M. The former concentration guarantees an efficient energy transfer even for donors with triplet lifetimes of 0.1 p s . The latter is the minimum concentration as discussed above. For M,the sensitivity is found to be con[BIAC] = 1.0 X siderably larger. This must be attributed to a decrease of the noise level as a result of the 50 times lower background signal. Apparently it should not be excluded that even lower LOD values can be obtained by optimizing the BIAC concentration. To estimate the sensitivity of sensitized RTPL as a detection method in continuous flow systems, one must translate the LOD values in Table IV into absolute quantities of substances. Taking as an injection volume the reasonable value of 100 FL (in liquid chromatography injection volumes of 20 pL are not unusual), it is calculated from Table IV that the LOD values are on the order of 1 ng. This point will be elaborated on in a subsequent paper.
CONCLUSIONS In conclusion we can say that sensitized room temperature phosphorescence in liquid solutions is a feasible approach to detect many analytes, which do not fluoresce, and hence it can be considered as complementary to fluorescence detection. In principle, the method can be used for all compounds, which exhibit phosphorescence in rigid solutions at 77 K, since the main condition to be fulfilled is an efficient triplet formation. Sensitized RTPL is an indirect emission method, which implies that only the excitation properties of the analyte (the donor) play a role. All compounds are detected at one relatively long emission wavelength of the acceptor, so that effects of stray light and fluorescence background are reduced. Biacetyl has more favorable acceptor properties than 1,4dibromonaphthalene. It can be applied in many solvents, frequently used in liquid chromatography, to detect a n a l y k s with triplet-state energies higher than 19.7 X lo3 cm-I, with M. a limit of detection on the order of In the near future results, obtained with this approach will be reported for dynamic (continuous flow) systems. Besides
Anal. Chem. 1982, 5 4 , 895-897
we are searching for acceptor systems which are still more favorable than biacetyl, in order to extend the application potential for sensitized RTPL.
LITERATURE CITED (1) Skriiec, M.; Cline Love, L. J. Anal. Chem. 1960, 52, 754-759. (2) Donkerbroek, J. J.; Elzas, J. J.; Gooijer, C.; Frei, R. W.; Velthorst, N. H. Talanta 19111, 28, 717-723. (3) Turro. N. J. "Modern Molecular Photochemistry", 1st ed.; The Benjamln/Cumminga Publishing Co., Inc.: Menlo Park, CA, 1978;Chapter 9
895
(6) Parker, C. A.; Joyce, T. A. Trans. Faraday SOC. 1969, 6 5 , 2823-2829. (7) Bonner, R. EL; DeArmond, M. K.; Wahl, G. H. J . Am. Chem. Soc. 1972, 94,988-989. (8) Bonnier, J. M.; Jardon, P. J . Chim. Phys. 1972, 68, 432-435. (9) Favaro, G. J . Chem. Soc., Perkin Trans. 2 1976, 8 , 669-874. (IO) Forster, L. S J . Am. Chem. SOC. 1955, 7 7 , 1417-1421. (11) Marchetti, A. P.; Kearns, D. R. J . Am. Chem. SOC. 1967, 89, 768-777. (12) Sandros, K.; Backstrom, H. J. Acta Chem. Scand. 1962, Y6, 958-968. (13) Birks, J. B. "Photaphysics of Aromatic Molecules", 1st ed.; Wiley: London, 1970;Chapter 11.
(4) Backstrom, H L. J.; Sandros, K. Acta Chem. Scand. 1960, 14,
48-62.
( 5 ) Birks, J. 8. "Organic Molecular Ph&Jphysic$", 1st e&; Wiiey: London, 1974;Vol. 2, Chapter 3.
RECEIVED
for review August 3, 1981. Accepted January 15,
1982.
DibenzyIiammonium and Sodium Di benzyIdithiocarbamates as Precipitants for Preconcentration of Trace Elements in Water for Analysis by Energy Dispersive X-ray Fluorescence Robert V. Moore
U.S.Environmental Protection Agency, Environmental Research Laboratory, 900 Atlantic Drive, NW, Atlanta, Georgia 303 18
Preclpltatlon with combined dlbenzylammonhm dibenzyldithiocarbamate anld sodlum dlbenzyldithlocarbamate at pH 5.8 can be used to separate 22 trace elements from water. Membrane filtration of the precipitate yielded a thin sample, suHable for analysls by energy disperslve X-ray fluorescence spectrometry. Allkalls9alkaline earths, lanthanides, and helides were not preclpltated, permlttlng a Glean separation Of trace elements from the macro constituents of drinking water and drlnklng water supplies. Methods are glven for preparation of reagents of higher purity than previously described.
Until the environmental effects of trace elements in water are more fully established, analytical chemists need to perform analyses a t the lowest attainable detection limits. The proposed Water Quality Criteria ( I ) for protecting freshwater aquatic life set concentration limits for mme elements that are below the capability of conventional analytical techniques without preconcen tration of the water sample. Furthermore, energy dispersive X-ray spectrometry (EDXS) is a promising multielement analytical technique that cannot be applied to the trace elements of writer without preconcentration and transformation to a solid film. This paper describes the use of salts of dibenzyldithiocarbamate as precipitants, a method first suggested by Linder et al. (2) in 1978, for 22 elements found in water and their subsequent determination by EDXS. The role of precipitation techniques in EDXS has been discussed in a recent review article on preconcentration methods by Leyden and Wegscheider ( 3 ) . EXPERIMENTAL SECTIQN Apparatus and Operating Conditions. EDXS analysis was performed with an Clrtec, Inc., tube-excited fluorescence analyzer (TEFA). 'Two X-ray irradiation conditions were used to assure the determination of all precipitated elements. A molydenum target tube equippeid with a molybdenum filter operated at 200 pA and 35 k V was used in procedure 1, where a tungsten target tube with a terbium filter operated at 200 pA and 50 kV wm used in procedure 2. The countringtime range was 800-1000 live-time seconds. The detector was lithim-drifted silicon with a 12.5 mmz
active area, The resolution was 165 eV at full width half-maximum at 10ps shaping time and loo0 counts/s on the 5.9 keV manganese Ka peak (%Fesource). The window was 0.088 mm thick beryllium. Filtrations were made by using water aspiration to provide vacuum. The filters were 25 mm diameter, 0.4 pm porosity Nuclepore polycarbonate membranes. The polycarbonate filter rested on a 0.45 wm porosity Metricel filter. This arrangement produced an even distribution of the precipitate on the polycarbonate filter, eliminated the grid effect of the filter support, but did not increase the time needed for filtration. To minimize background, we mounted the filter with the residue on ultrathin polyester film so that only the Film and filter were subject to X-ray irradiation. Reagents. Dibenzylammonium dibenzyldithiocarbamate (DDDC) and sodium dibenzyldithiocarbamate (NaDDC) are not available commercially but are readily prepared in the laboratory. The preparation procedure for DDDC as described by Linder et al. (2) and Haase and R. Wolfenstein ( 4 ) was modified for this work. The following amended method gives a much better yield and a product of greater purity in a shorter preparation time. Dissolve 42 mL of dibenzylamine (bp 176 "C, 12 mm) in 50 mI, of reagent-gradeanhydrous diethyl ether in a 250-mL beaker. Cool below 5 "C. With stirring, add a solution of 6 mL of reagent-grade carbon disulfide in 15 mL of diethyl ether. The solution must be added slowly so that the temperature does not rise above 5 "C. Continue stirring, allowing evaporation to occur, and maintain temperature below 5 "C. In 1-2 h crystallization will start. Keep stirring to keep the crystals small and to prevent caking. When crystallization ceases, filter and wash several times with diethyl ether. Dry under vacuum. The 70% yield of crystals are faintly yellow and have an 83-84 "C melting point. To prepare NaDDC, dissolve 38 mL of dibenzylamine in 50 mL of reagent-grade acetone in a 250-mL beaker. Cool below 10 "C. Slowly add, with constant stirring, a solution of 12 mL of carbon disulfide in 13 mL of acetone. Do not let the temperature rise above 10 "C. Dissolve 4.4 g of reagent grade sodium hydroxide in 20 mL of water. Cool below 5 OC. Add the sodium hydroxide solution slowly to the acetone solution, keeping the temperature below 10 "C, and stir for 15 miti. Evaporate the acetone and water under vacuum. After 38-40 mL have been removed, add 50 mL of anhydrous diethyl ether. Continue evaporation under vacuum until 40-50 mL of liquid has been removed. Repeat the ether evaporation step at least three more times. This will assure the removal of the water and acetone. Crystallizationwill begin during
This article not subject to US. Copyright. Published 1982 by the American Chemical Society
