Conversely, organic compounds that undergo a partial decomposition beneath their actual melting point may not be easily detectable by visual observations. The ion chamber technique described here would appear to provide a sensitive means whereby organic compounds, which undergo partial decomposition before melting, can be monitored. It would appear that methyl-, dimethyl-, and di-n-propylmalonic acids would fall into this class of compound. Compounds which would appear to decompose ajter melting are exemplified hy diethyl- and benzylmalonic acids. Thus, organoparticulate analysis, using an ion chamber detector technique, would appear to be a very sensitive and useful technique to monitor the thermal stabilities of organic compounds. I t would seem to offer distinct advantages over conventional techniques, such as melting point determination, TGA, mass spectrometry, infrared and UV spectrometry, color and visual changes, etc., which have been used previously to study the thermal decompositions of organic compounds. Other types of organic compounds are now under investigation and will he the subjects of future publications in this area. LITERATURE CITED (1)
Figure 8. Scanning electron microscopy photograph (2500 magnifi-
cation) of dimethylmalonic acid particulates collected on a glass fiber disk the same as that obtained using melting point or other visual change methods to evaluate thermal stability (refer to Table 111).The melting point procedure, however, does not take into account the possibility of these organic compounds thermally decomposing ajter they have melted.
C. 0. Doyle. "Evalmtion of Experimental Polymers", WADD Tech. Rep. 60-283, U.S.A.F.. Wright-Patterson AFB, Ohio, May 1960.
(2) J. 0. 8. Smith and 0.C. Phillips. Micrchem. J., in press. (3) E. H. Rodd, "Chemistry 01 Carbon Compounds", VoI. 18. Elsevier Publishing Co.. New York. N.Y.. 1952, p 962. (4) G. F. Skala. J. Rech. Atmos., 189 (1966). (5) C. E. Murphy and C. 0.Doyle. Appl. Poiym. Symp., 2, 77 (1966). (6) G. F. Skala, Ami. Chem.. 35,702(1963). (7) E. Stenhagen, S. Abrahamsson. and F. W. McLafferty, "Atlas 01 Mass Spectral Data", Val. 1, Interscience Publishers, New York, N.Y. 1969. (8) A. I. Kitaigorodskii. "Organic Chemical Crystallography". Consultants Bureau Publishers. New Y a k . N.Y.. 1961.
(11) F. W. van Luik, Jr.. and R. E. Rippsre. And. Chem., 34, 1617(1962)
RECEIVEDfor review July 16, 1975. Accepted October 6, 1975.
Study of Lead(l1)-Manganese(l1) Energy Transfer in Sodium Chloride Pellets Richard 0. Delumyea' and George H. Schenk'
Department of Chemistry, Wayne State Univenky, Detroit, Mich. 48202
The solid state lead(I1)-manganese(l1) energy transfer system has been studied in sodium chloride pellets from a quantitative viewpoint. Lead(ll) and manganese(l1) are quickly coprecipitatedfrom a saturated sodium chloride solution by adding ethanol. After filtration and drying, the sodlum chlorlde matrlx Is compressed into a crystalline pellet using standard pellet techniques. The excitation maxima at 275 and 303 nm are those of lead(ll) and the broad emlssion maxlma at 610 nm Is that of manganese(l1). The coprecipltation of lead(ll) In an excess of manganese(l1) is repro-
'
Present address, Department of Chemistry, University of Michigan, Ann Arbor, Mich. 48104
ducible enough to yield a reproducible analytical lumlnescence curve. A speclal pellet holder was devised for the Turner filter fluorometer to permlt routine fluorometric measurement of traces of lead(l1).
Fluorescence and phosphorescence are used routinely in qualitative and quantitative determinations of organic compounds. Few purely inorganic systems, however, luminesce in solution and hence inorganic luminescence measurements are based largely on the use of fluorescent organic chelating complexing agents ( I ) . There are exceptions, of course; for example Bozhevlo'nov and Solov'ev (2) developed a luminescent method for determining lead in ANALYTICAL CHEMISTRY, VOL. 48. NO, 1, JANUARY
1976
95
cooled concentrated hydrochloric acid samples. Thallium(1) can also be measured in chloride solutions for both qualitative ( 1 ) and quantitative ( 3 )purposes. In the solid state, however, several inorganic species can be characterized by luminescence methods. The best known example is of course the uranyl ion. However, an even more promising selective approach exists for the solid state. This involves determining a metal ion by measuring the increase it causes in the luminescence of a second species; Le., solid-state energy transfer. This has been qualitatively demonstrated for lead (11) by measuring the enhancement of zinc sulfide luminescence ( 4 ) . In ionic matrices, such as sodium chloride, several metal ions luminesce upon ultraviolet excitation. Preliminary investigations showed that emission from lead(I1) in sodium chloride occurs in a region where the crystal has an intrinsic luminescence. In addition, the emission bands from the lead(I1) occur at 320 and 360 nm which is close to the excitation source emission. To be suitable for filter fluorometric analysis, expensive and low transmittance interference filters would have to be used; therefore alternate systems were sought. A promising system for a quantitative application of the above general approach is that which results in the orangered emission from certain natural-occurring Halite minerals. In 1949, Murata and Smith (5) investigated the origin of this luminescence by preparing synthetic halite samples with a manganese(I1) content similar to that of natural samples. The luminescence was found to arise from the manganese(I1) ion in the sample, but synthetic samples containing only this ion did not emit. Further work led to the conclusion that the lead(I1) ion had to be present to “sensitize” the manganese. The symbol NaCl(Pb:Mn), where NaCl represents the matrix, lead the sensitizer, and manganese the emitter, has been adopted for such systems. An energy transfer system such as NaCl(Pb:Mn) overcomes the problems involved with monitoring the NaCl(Pb) system. The emission monitored will usually be far removed from both the excitation maxima and the intrinsic luminescence of the matrix; thus simple sharp cut filters can be employed. In addition, the acceptor may be chosen to obtain a broad, intense emission band. In most cases, the emission intensity of the acceptor exceeds that of the sensitizer. The theoretical mechanisms of solid state luminescence in general and energy transfer in specific have been investigated by others. A brief review of current mechanistic theories as they relate to the system NaCl(Pb:Mn) will be presented since several observations can be explained by reference to the theory. Four methods exist for the transfer of energy from the absorbing species, or Sensitizer ( S ) to the emitter, or Acceptor (A). These are: 1) radiative transfer, where an emission band of S overlaps an absorption band of A; 2) charge transport, where a positive hole migrates in the crystal; 3) motion of excitons as in organic systems, and 4) resonance transfer. Theories explaining energy transfer in the NaCl(Pb:Mn) system have been proposed by Butler (61, Mott and Gurney ( 7 ) ,Dexter ( 8 ) ,and Schulman et al. (9). The last two are generally accepted. The two last authors state that the specific mechanism for energy transfer is one of resonance transfer. Dexter divides resonance transfer into five steps: 1) absorption of energy (E,) by S; 2) relaxation of the lattice surrounding S to reduce E , to E1 (Stokes shift); 3) transfer of energy ( E l ) to A; 4) relaxation of A to or near its ground state and relaxation of the lattice around A to reduce E1 to E2 (Stokes shift); and 5 ) radiant emission of energy (E2). Step three 96
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
will occur if the electric dipoles of S and A overlap, or if the dipole of S overlaps the quadrupole of A. In the NaCl(Pb:Mn) system, dipole-quadrupole overlap does occur in step three of the energy transfer (8, IO). Dexter (8) derived an expression for the probability of energy transfer in the NaCl(Pb:Mn) and similar systems. In such cases, the probability, PSA,of energy transfer in this dipole-quadrupole system is inversely proportional to the eighth power of the distance, R , between the donor and acceptor:
where a = 1.266, Qs = transition strength, c = velocity of light, n = index of refraction, sa = decay time of the acceptor, (g,’g,/g,g,’) = degeneracy term, (t/K1/*t,) = comparative matrix field effect of lattice corrected by dielectric of the matrix, and Sf[E]dE = integration over emission energies. Knowing that the lead(I1) and manganese(I1) ions occupy cationic sites in the lattice, Dexter calculated that in the NaCl(Pb:Mn) system, an excited lead(I1) ion could excite up to 100 lattice sites. These cationic sites may be occupied by sodium ions, other lead(I1) ions, or manganese(I1) ions. The three possible energy transfers (lead-crystal, leadlead, and lead-manganese) have been used (8, 11) successfully to interpret the excitation and emission spectra of NaCl(Pb) and NaCl(Pb:Mn). Furthermore, Schwartz and Vale (12)state that much of the luminescence occurs in regions of crystal defect where the concentrations of lead(I1) and manganese(I1) ions are abnormally high. The implication is that luminescence is more intense in imperfect crystals than in perfect crystals.
EXPERIMENTAL Apparatus. The Turner Model 210 “Spectro” absolute spectrofluorometer equipped with a 75-watt Xenon source, 10.0-mm quartz cuvettes, 10-nm bandwidths, and special pellet mountings, was used to obtain the fluorescence excitation and emission spectra. This spectrofluorometer has been previously described by Turner (13). The absorption spectra for interference studies were obtained on the Beckman Models DB and Acta V, and the Cary Model 14 spectrophotometers. Metal concentrations were determined using a Perkin-Elmer Model 303 atomic absorption spectrophotometer. The calibration curves and detection limits were obtained using a Turner Model 110 filter fluorometer with a low pressure mercury arc source emitting at 254 nm. The main primary filter used was the 7-54 narrow pass ultraviolet filter (230-420 nm); no attempt was made t o filter out lines such as the 366 nm that are weakly emitted by the source. T o eliminate reflected exciting radiation, a 2A sharp cut filter (37% T a t 415 nm) was used as a secondary filter, often in combination with a narrow pass filter such as a number 48 (peaks a t 460 nm) or number 58 (peaks at 525 nm). If the emission was too intense, a neutral density filter (ND 1%)was used to reduce emission to 1%of the original intensity. The following combinations were used with the indicated source intensity settings ( l X , 3X, etc.) on the Turner fluorometer: 20 ppb Pb(I1): 2A ( 1 X ) 0.02 ppm Pb(I1): 2A 58 (3X) 2-20 ppm Pb(I1): 2A 48 ( 1 X ) 2-40 ppm Pb(I1): 2A ND 1%(3X) Special Pellet Holder for the Fluorometer. The standard Turner pellet holder was found to be unacceptable since the mirror behind the sample appeared to reflect radiation from our mercury arc source. A special pellet holder was devised both to obtain excitation and emission spectra on the Turner spectrofluorometer,and to obtain calibration curves on the Turner Model 110 filter fluorometer. The pellet holder is shown in Figure 1. The numbers refer to the length of each dimension in millimeters. The holder was made of plain steel and painted black. Three screw holes were
+
+ +
14
34
lo
lU
iS6 i
Flgure 1. Diagram of the pellet holder designed and constructed to attach to the door of a Turner Model 110 filter fluorometer The numbers refer to the length of the line in mm. The plate in front has been distorted to show where the pellet is inserted but is usually at 90' to the back plate. The V is slightly removed from the back plate so that a pellet lays flat against the back plate
placed on the left side, permitting it to be attached to the standard Turner filter fluorometer door in place of the cuvette sample holder. The plate in front has been distorted in Figure 1 to show where the pellet is placed, but it is attached a t right angles to the back plate. The front plate is attached by screws (not shown) to the back plate. The purpose of the front plate is to prevent reflection of radiation from the source into the photomultiplier. The small V-shaped ledge is also screwed (not shown) to the back plate. This V ledge is about 5 to 6 mm wide and easily accommodates a standard sodium chloride pellet (P in Figure 1).T h e V ledge is slightly depressed where it joins the back plate so that a 1-mm thick sodium chloride pellet can be positioned flat against the back plate without falling forward. The angle of the V is such that it will support a 13-mm diameter sodium chloride pellet. Reagents. ACS certified reagent grade salts were used in all experiments. A 2.0-mg Mn/ml stock solution was prepared using manganese(I1) chloride dihydrate. A 1.0-mg Pbiml stock solution was prepared using anhydrous lead(I1) chloride. Saturated sodium chloride was prepared by adding 375 g of sodium chloride to 1 liter of distilled water a t 80 "C, and allowing it to cool. Procedu- es. Method I . This method was developed where dilution of the sample is necessary or when only a small volume of unknown sample is available. T o establish an analytical curve for lead(II), the 1.0 mg/ml stock solution was used to prepare standard lead(I1) solutions in the desired concentration range; e.g., 2-20 ppm Pb. Exactly 1 ml of each standard lead(I1) solution was pipetted into 23.0 ml of saturated sodium chloride solution in a 100-ml beaker. Exactly 1 ml of the stock manganese(I1) solution was then added to each beaker. After the standards were mixed, the lead unknown(s) and blank solution were similarly treated by substituting exactly 1 ml of lead unknown sample and 1 ml of distilled water, respectively, for the 1 ml of standard solution. After the above mixtures were prepared, the time for all subsequent steps was kept constant and reproducible for the standards, unknown sample, and blank. The times given below were not always used exactly but are representative. As each solution was magnetically stirred, 25.0 ml of 95% ethanol (not absolute) was added, and the stirring was continued for 2 min. The solutions were then allowed to stand for the same length of time (any given time between 2 and 5 min was satisfactory) to allow the sodium chloride precipitate and coprecipitated lead and manganese chlorides to settle. Each precipitate was then filtered using a 30-ml medium porosity sintered glass filtering crucible. All filtering crucibles were dried in a 110 "C oven for the same time, about 1 hr. A 250.0 f 0.4 mg portion of each dry precipitate was pressed into pellet form using a standard 13-mm potassium bromide die a t 8 to 10 tons pressure. To measure the luminescence of the standards and the unknown sample, the blank pellet was first mounted in the special pellet holder on the door of the Turner filter fluorometer, and the fluorometer was zeroed in. The blank pellet was replaced by each of the pellets containing a standard amount of lead(I1) in turn. The appropriate mercury source intensity, as listed above in the description of apparatus, was selected, as was the secondary filter combination necessary for the concentration range of lead being measured. The luminescence intensity of each standard lead pellet
0 1200 :
~
:
:
:+:
300
:
'
400
'
'
:
;
1
560
:
:
:
'
660
:
:
:,'ol
WAVELENGTH
(nrn)
Figure 2. Luminescence spectra taken with Turner Model 210 Spectro Absolute Recording Spectrophoto fluorometer, using 10-nm excitation and emission band widths Curve 1. Excitation spectrum of NaCI(Pb:Mn), emission wavelength of 605 nm. Curve 2. Emission spectrum of NaCI(Pb:Mn), excitation wavelength of 276 nm. Curve 3. Emission spectrum of a NaCI(Mn) blank, excitation wavelength of 276 nm
was then measured. An analytical curve was then constructed by plotting the luminescence intensity of each standard pellet vs. the concentration of the lead(I1) in the 1 ml of standard lead(I1) solution added to the saturated sodium chloride. (There was no point in using the final concentration of lead(I1) because it is precipitated by the addition of ethyl alcohol.) The luminescence intensity of the unknown lead(I1) sample(s) is read a t the same time as those of the standard lead(I1) solutions, and the concentration of the lead(I1) in the 1 ml of unknown sample added to the sodium chloride is read from the analytical curve. Normally all standards and unknowns were measured the same day that they were precipitated and pressed into pellets, but the pellets could be kept in the dark in a desiccator and read for several weeks afterwards. Method ZI. This method was developed where large amounts or volumes of the sample are available, or where a low concentration of lead(I1) is suspected. An 8.800 f 0.080 g sample of solid sodium chloride is weighed out for each sample, standard, and blank. Exactly 25.00 ml of unknown sample, lead(I1) standards, and distilled water (blank) is pipetted into 100-ml beakers, the solid sodium chloride is added, and the mixtures are dissolved by magnetic stirring. Exactly 1 ml of stock manganese(I1) solution is added to each beaker, including the blank. The times are kept constant and reproducible as for Method I, and 25.0 ml of 95% ethanol was added, and the stirring was continued for 2 min. The same steps as given in Method I are then followed. The analytical curve will involve the concentration of lead(11) in the 25.00 ml of standard or unknown sample.
RESULTS AND DISCUSSION Excitation and Emission Spectra. In a sodium chloride matrix, the corrected luminescence excitation and emission spectra of NaCl(Pb:Mn) are as shown in Figure 2. Two excitation maxima (curve 1) are shown, a t 275 and a t 303 nm. These have been interpreted ( 1 1 ) as follows: the band at 275 nm arises from a weak transition from ground state ('SO)lead to a triplet excited state (3P1)via a spin flip. The intense band at 303 nm arises from a strong transition from the same ground state to a singlet excited state ('Po) of lead. These same excitation bands were observed in the NaCl(Pb) system (II), but at 273 and 290 nm. A 260-nm band was also observed ( l l ) ,but this was attributed to lead(I1) ions entrapped in the lattice without an associated cation vacancy. The emission spectrum (curve 2 in Figure 2) exhibits three emission bands. Two bands are the result of lead emission: the peak at about 320 nm and the shoulder a t about 360 nm. These are also present in the emission specANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
* 97
Table I. Study of Reproducibility of Coprecipitation of Lead(I1) in Sodium Chloride Lead(l1) added,
Lead(I1) c o - p p t d , Clgb
ClP
10.
Recovery, %
9.89 21.54 49.72 74.12 144.8 485.9 969.8
98.9 20. 107.7 50. 99.4 100. 74.1 200. 72.4 500. 97.2 1000. 97.0 Av 92.4 i 10.8 (90%) QAmount calculated to be in 0.50 g of sodium chloride precipitate. b Actual quantity observed in 0.50-g sample of precipitate upon atomic absorption analysis. trum (11) of NaCl(Pb). The broad emission band centered a t 610 nm is the result of emission by the excited 4G state of manganese(I1) according to McClure (14) and Orgel ( 1 5 ) . A simple crystal field electronic designation for the emission is:
Thus, in the emission process, the excited electron in the 4G state loses energy by spin unpairing and “jumping up” ground state of mangato the eg sublevel, giving the nese(I1). It is interesting that the manganese emission is, strictly speaking, phosphorescence. This emission would ordinarily not be observed by exciting the manganese because the molar absorptivities of all accessible bands are less than 0.1 M-’ cm-l. Intense emission is only achieved in NaCl(Pb: Mn) by exciting lead(I1) which absorbs intensely and transfers a large amount of energy to manganese, which is then able to emit intensely. Instrumental Parameters. The use of a spectrofluorometer was rejected because of the weak intensity of the xenon arc source below 300 nm. A filter fluorometer with a low pressure mercury arc source was chosen instead for convenience and because of the high photon output at 254 nm relative to the xenon arc output at 273 or 303 nm (Figure 2). The primary filter selected was the standard (7-54) short wave silica filter; this filter transmits from about 230 to 420 nm and has a high transmittance (160%).Any reflected mercury lines above 254 nm were blocked by an appropriate secondary filter. Several different secondary filter arrangements were used, depending on the amount of lead present. These are listed in the Experimental section and were selected to absorb any reflected mercury lines from the source at 405 nm or below. Evaluation of Procedures. In most of the inorganic luminescence cited, work was done on carefully grown single crystals prepared from melts or saturated solutions, neither of which are analytically practical. An alternative method which has been used for qualitative identification of lead(11) is the addition of concentrated hydrochloric acid (16). During the preparation of this manuscript, Ryan et al. ( 1 7) reported on the use of a calcium oxide matrix to determine lead(I1) and bismuth(II1) via solid state luminescence. Their procedure involved ten separate steps, is time consuming and subject to several errors. The precipitation method developed here involved the addition of an organic solvent miscible with water but having a dielectric constant considerably less than water. Both acetone and 95%ethanol were found to be suitable, but 95% ethanol was selected because of the filterability of the resulting sodium chloride crystals. Not all of the manga98
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
nese(I1) is coprecipitated ( 5 ) ; attempts to use atomic absorption to find how much lead was coprecipitated were not always satisfactory because of the small amounts of lead involved and the large matrix effects of the sodium chloride present. The data for lead recovery are shown in Table I. Solutions containing known amounts of lead(II), were treated according to Method I. A weight of the dried precipitate (0.50 g) was dissolved in 10 ml of distilled water. Standards were prepared containing 0.50 g of sodium chloride to correct for matrix effects. Under optimum conditions, the detection limit was about 1 hg/ml. Thus, as shown in the table, samples containing less than 10 pg were not detected. The variation in the remaining samples is due to the problems of operating the atomic absorption spectrophotometer under conditions of such high alkali concentrations. The data in Table I indicate that the percentage of lead coprecipitated is reproducible over various concentration ranges. For example, over the 0-50 ppm (0-50 pg) lead analytical range, the recovery is high and consistent. The percentage of lead coprecipitated varies from a low of 98.9% (10 pilead) to a high of 107% (20 pg lead). Further, the intensity of luminescence for a series of ten replicate samples confirms this reproducibility. The ten samples containing 2 ppm of lead(I1) were treated according to Method I, and the luminescence intensity was recorded. A mean value of 25.0 instrumental scale divisions was obtained. with a standard deviation of 2.65, or a 90% confidence limit of 25.0 f 1.5; which is close to the accuracy of the dial reading of the Turner filter fluorometer. The effect of the uolurne of alcohol added on the amount of precipitate recovered from a saturated salt solution was also studied. Precipitation occurred when only 3 ml of alcohol was added to 25 ml of solution, but the luminescence intensity was only half that attained when 25 ml of alcohol was added. As the volume of alcohol was increased, the luminescence intensity approached a constant value, and the amount of sodium chloride recovered increased from 0.5 g (3 ml alcohol) to 3.0 g (25 ml alcohol) of a total of about 8.8 g sodium chloride present. The weight chosen for the pellets was 250 f 0.2 mg during the investigation of the method, This low tolerance can be raised to f l mg without deterioration in reproducibility. Analysis of the final weight of 27 pellets showed their weight to be 247.3 f 0.8 mg indicating a consistent loss in weight, probably during the transfer and pressing of the pellets. To illustrate that the method has an added flexibility in pellet weight, an unknown sample was treated according to Method I and the precipitate was made into three sets of three pellets weighing 150, 250, and 350 mg. The average emission intensities were 35, 47, and 58 relative units, respectively. Below 150 mg, pellets were too fragile and above 350, the pellet was not evenly pressed in a 13-mm die. Variations in the pressure applied did not alter the sample intensities in the range 8-10 tons pressure. The optimum amount of manganese(I1) found was 2.4 mg per 25 ml of saturated salt solution. This was found by treating samples containing 0.05 mg of lead(I1) with varying amounts of manganese(I1) in preparing the pellets. This concentration (1.7 X M ) is of the same order of M manganese(I1) concentration magnitude as the 8 X found for the system CaC0dPb:Mn) by Schulman et al. (16). As the concentration of manganese(I1) is increased above this value, the luminescence decreases to about 65% of the optimum luminescence. There are several possible reasons for the decrease in luminescence at higher manganese(I1) concentrations. It will be possible, if lead and manganese are close enough, for back transfer to occur before
Table 11. Interference by Metal Ions on a Standard Lead(11) Sample Metal ion
Oppm
4ppm
8ppm
2Oppm 4 0 p p m
Iron(111) 66 56 42 25 Mercury(I1) 58 56 45 37 Copper(I1) 39 44 40 40 57 46 Magnesium(I1) 55 56 Zinc( 11) 56 61 51 58 Cobalt(11) 62 69 50 55 Tin(11) 30 60 ca.80 ... a Luminescence of sample containing metal ion. taken on Turner filter fluorometer.
n p Pb/ml
Flgure 3. Plot of luminescence intensity vs. lead(l1) concentration in
ng/ml or parts-per-billion (A)Points obtained using an 8 sharp cut secondary filter: (0) Points obtained using a 2A
+ 48 secondary filter combination
relaxation of the lattice around manganese occurs. Another possible explanation is that of reabsorption of some of the emission by manganese(I1); this is possible because it does have a weak absorption band from 500 to 600 nm. Finally, the manganese may distort the lattice appreciably, as will be discussed later. Schulman, Ginther, and Klick (10) observed that the system NaCl(Pb) showed an 80% decrease in luminescence intensity in a 12-day period when excited a t 273 or 290 nm. A decrease in the excitation efficiency of lead(I1) such as this would substantially reduce emission in the NaCl(Pb: Mn) system, since lead must absorb before energy transfer can occur. In this work, however, samples were stable, within the limits of reproducibility of the filter fluorometer, for a t least three weeks. In addition, the final pellets used to construct an analytical curve are stable for several days when kept in a desiccator. The drying temperature did not affect luminescence intensity of samples. No difference was observed in the luminescence intensity of samples which were air-dried at room temperature and that of samples dried a t 110 or 150 "C. This is in agreement with Schulman et al. (9) who observed that only the 260-nm lead(I1) excitation band (not observed in NaCl(Pb:Mn)) was affected by heat. Analytical Curves for Lead(I1). The Turner filter fluorometer was used to evaluate the analytical curves obtained for lead(I1) by Methods I and 11. Excitation was primarily a t 254 nm, and the secondary filter combinations were chosen to correspond to the concentrations ranges listed in the Experimental section. Using Method I, the analytical curves obtained over a range spanning an order of magnitude of concentration of lead(I1) are essentially linear. For example, using the 2A 48 secondary filter combination, the analytical curve for lead(I1) is linear in the region from 2 to above 20 ppm. At concentrations higher than this, the curve becomes nonlinear and bends downward, beginning close to 30 ppm. Analytical curves a t lower concentrations of lead(I1) appear to exhibit a reproducible curvature. As shown in Figure 3, the slope of the curve increases continually in the 20to 200-ppb lead concentration range. Without modification, the detection limit for Method I is of the order of 1 ppb. This limit may be lowered by using Method I1 since
+
25 23 48 45
... ... ... Data
the volume of sample used is increased by a factor of 25. However, limited evaluation of this method yielded analytical curves which did not have a zero intercept. In the ppm range, the intercept was 20 Turner luminescence units from a curve which was drawn using 45 Turner units as the maximum intensity (9 ppm lead). Interferences. Many cations quench, exert an inner filter effect, or luminesce in fluid solution so that many potential interferences exist for this solid state method. Interferent cations at final concentrations of 4, 8, 20, and 40 ppm were added to a saturated sodium chloride solution containing 2 ppm of lead(II), and pellets were prepared using Method I. The resulting data are listed in Table 11. T o assist in the interpretation of the data, absorption spectra were also obtained for each of the interferent solutions. Three of the cations pose potentially serious problems. At concentrations in excess of lead(II), iron(II1) decreases the luminescence intensity of the standard NaCl(Pb:Mn) sample. The data indicate a semilogarithmic dependence of the decrease on the concentration of iron(111). Additional study confirmed this observation. Since iron(II1) has a significant absorption band in the region below 300 nm, it appears that the iron(II1) is not quenching luminescence, but rather absorbing some of the exciting radiation. The mercury(I1) ion has a similar, though less intense absorption band below 300 nm. In agreement with this observation, decrease in luminescence of standard NaCl(Pb:Mn) did not occur until a tenfold excess of mercury(I1) was added. By far the most significant interference was the tin(I1) ion. Small amounts of tin(I1) caused very large increases in the luminescence intensity. Three possible explanations for the large increase are: 1) tin(I1) may be acting as an energy transfer sensitizer for manganese(II), 2) tin(I1) may in some manner increase the efficiency of the lead-manganese energy transfer, and 3) tin(I1) itself may luminesce in sodium chloride matrix. Excitation and emission spectra were taken for samples of NaCl(Pb:Mn), NaCl(Pb:Sn), NaCl(Sn), NaCl(Sn:Mn), and NaCl(Sn,Pb:Mn) to try to resolve this question. The emission spectra of NaCl(Sn:Mn) exhibited a broad emission band centered at 530 nm but with a strong shoulder a t 610 nm, where manganese(I1) emits. The excitation spectra for this system and for NaCl(Sn) were obtained by setting the emission monochromator a t 485 nm, where manganese(I1) emission is negligible. The excitation bands for both the foregoing systems were a t 230 and 290 nm, as compared to the lead(I1) excitation bands a t 275 and 303 nm. This, together with the emission band of NaCl(Sn) at 530 nm, definitely indicated that the principal emission arises from tin(I1). When NaCl(Sn,Mn) is excited, both tin(I1) and manganese( 11) emit, but the manganese(I1) emission is not as intense. The effects of tin(I1) can be overcome through use of a xenon source and careful selection of the exciting ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
99
Table 111. Systems Exhibiting Sensitized Luminescence Matrix
Sensitizer
Acceptor
NaCl NaCl KC1 CaF, CaSiO, CaCO, MgdPO,), Ba,MgGe,O,
Lead( 11) Silver(I ) Lead(I1) Cerium(II1) Lead( 11) Lead(I1) Europium(111) Neodymium
Antimony(II1) Copper(1) Manganese(I1) Manganese(I1) Manganese(I1) Manganese(11) Manganese(11) Ytterbium
Reference
wavelength. When using a filter fluorometer, however, tin(I1) may be regarded as a serious potential interference. The remaining metal ions in Table I1 caused much less interference at comparable concentrations than the above three cations. In addition, anions such as fluoride, sulfate, carbonate, and phosphate caused variations of up to 20% in the luminescence intensity. The effects of these diverse ions on the luminescence intensity appear to be the result of distortion of the lattice rather than direct interference with the luminescence mechanism. (A luminescence mechanism does not explain the peaking of luminescence a t 4 ppm magnesium, zinc, and cobalt in Table 11.)An explanation of these effects has been developed and will be presented elsewhere. Advantages of Solid State Luminescence of Lead. Most methods for the fluorometric analysis of metals rely on the formation of a complex between a metal and a fluorescent ligand. No chelation technique has yet been devised for the fluorometric analysis of lead(II), however. A fluorometric method for lead(I1) in concentrated hydrochloric acid at -70 OC was developed by Bozhevlo'nov and Solov'ev (18).The technique requires special fluorometers constructed for low temperature work and the use of very chemically pure hydrochloric acid, platinum crucibles, and a quartz Dewar. The authors state that up to 10-fold excesses of tin(II), antimony(III), thallium(I), and nickel(I1) can be tolerated whereas 10-fold excesses of iron(II1) or copper(I1) cause quenching. Under optimum conditions, this technique can determine 5 X lo+% by weight lead(I1) in concentrated hydrochloric acid. Incorporation of the lead(I1) into a solid matrix eliminates the need to cool the sample, since the metal ion is locked into a solid lattice, reducing the losses due to nonradiative decay. In addition, such problems as oxygen quenching, important in solutions (especially at room temperature), are greatly reduced. As previously mentioned, Ryan et al. ( 1 7 ) have recently published work on the fluorometric determination of bismuth(II1) and lead(I1) in the solid state, using a calcium oxide matrix. It is of interest to compare our method with the above method. In addition to the complexity of the CaO(Pb) method, the following points can be made. The seven advantages listed by Ryan et al. apply to both methods. Both methods are sensitive, with detection limits of about 20 ng/ml. Reproducibility was comparable (&lo%) relative to the energy transfer system (&5%).The previous authors mention the following interferences: small amounts of bismuth(III), cobalt(II), nickel(II), and manganese(I1) as quenchers, excesses of silver(1) and cadmium(II), and above 10-fold excesses of iron(II1). They do not report the effect of anions on their system. We believe that two additional metal ions may cause severe problems in CaO(Pb) system. Thallium(1) and tin(I1) have been observed in our work, as well as elsewhere, to have characteristics similar to
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lead(I1) and bismuth(II1) in solid systems. The energy transfer system NaCl(Pb:Mn) has the following interferences: mercury(II), iron(III), nickel(II), and most severely, tin(I1). The sample in the CaO(Pb) method is pulverized and tamped into 5-mm holes in a 1-mm sheet of aluminum with a spatula. The amount which can be held in this arrangement will vary with crystal size, tamping pressure, etc. Use of the pellet holder and a KBr die will permit reproducible weights of sample to be monitored and should therefore be more accurate. The solid-state luminescence technique presented here, as well as that of the CaO(Pb) method, offers a new method for the quantitative analysis of metals not otherwise detectable by conventional fluorescence methods. Whether one uses an energy transfer acceptor or not is a choice to be made after careful comparison of the advantages with the disadvantages of close control of solution volumes, the need for reproducible coprecipitation procedures, and filtration. It is the belief of the authors that with sufficient study, multielement analysis can be performed on a single pellet. The experimenter is given wide latitude in choosing a system; for example, the choice of matrix, acceptor (or acceptors), excitation wavelength of various sensitizers, emission region being monitored-that of the sensitizer, acceptor, or other element. To illustrate this flexibility, some energy transfer systems which have been reported in the literature are given in Table 111. Some of these may prove analytically useful in the future. LITERATURE CITED C. E. White and R. J. Argauer, "Fluorescence Analysis", Marcel Dekker, New York, 1970. E. A. Bozhevlo'nov. and E. A. Solove'ev, Zh. Anal. Khim., 20, 1366 (1965). G. F. Kirkbright, T. S. West, and C. Woodward, Talanta, 12, 517 (1965). A. M. Gurvich, A. P. Nikiforova, and M. J. Tomback, Zh.Anal. Khim., 25, 1529 (1970). K. J. Murata and R. L. Smith, Am. Mineral., 31, 527 (1946). K. H. Butler, J. Nechochem. SOC.,95, 267 (1947). N. F. Mott and R. W. Gurney, "Electronic Processes in Ionic Crystals", Oxford Press, New York, 1940, p 207. D. L. Dexter, J. Chem. Phys., 21, 836 (1953). J. H. Schulman. E. Burstein. R. J. Ginther, M. White, and L. W. Evans, Phys. Rev., 78, 178 (1949). J. H. Schulman, R. J. Ginther. and C. C. Klick, J. Nectrochem. SOC., 97, 123 (1950). J. H. Schulrnan, R. J. Ginther, and C. C. Klick. Opt. SOC. Am. 40, 854 (1950). K. K. Schvartz and G. K. Vale, lzv. Akad. Nauk. SSR, Ser. Fir., 25, 333 (196 1). G. K. Turner, Science, 146, 183 (1964). D. S. McClure, "Electronic Spectra of Molecules and Ions in Crystals", Academic Press, New York, 1959, pp 149-151. L. E. Orgel, J. Chem. Phys., 23, 1958 (1955). J. H. Schulman, L. W. Evans, R. J. Ginther, and K. J. Murata. J. Am/. .. Phys., 18, 732 (1947). D. E. Ryan, E. J. Prine, J. Holzbecher, and R. E. Young, Anal. Lett., 6, 721 (1973). > - -, ETA. Bozhevlo'nov and E. A. Solov'ev, J. Anal. Chem. USSR, 20, 1366 (1965). A. Kh. Khalilov, E. Yu, Salaev. A. P. Mamedov, T. D. Alieva, and F. K. Isaev, Bull. SOC.Sci. SSR, Phys. Ser., 25, 325 (1961). C. B. Lushchik, N. E. Lushchik, and K. K. Schvartz, Opt. Spectros. USSR, 9, 113 (1960). S.C. Sen and H.N. Bose, Z. Phys., 201, 368 (1967). H.Dziergawa and H. Lange, Z. Phys., 140, 359 (1955). J. B. Merrill and J. H. Schulman, J. Opt. SOC.Am., 36, 471 (1946). T. R. J. Botden and F. A. Kroger, Physica, 14, 553 (1948). N. A. Gorbacheva, izv. Akad. Nauk. SSSR, Ser. Fiz. Astron. Akad. Nauk Est. SSR 11, 3 (1959). E. J. Sharp and J. E. Miller, J. Appi. Phys., 40, 4680 (1969).
RECEIVEDfor review October 7, 1974. Accepted September 12,1975. Presented in part at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, April 1972.
