ANALYTICAL CHEMISTRY, VOL. 51, NO. 8, JULY 1979
1325
I
-
LtO
Z L3
a
w-
I
0
Figure 1. Scatter diagram of solvent-induced chemical shifts for the NMR probes: (1) "F in 3-fluoropyridine; vs. ( 2 ) 15N in pyridine
40,
v .
1.0
0
1
TT"
Figure 2. Comparison of A615N (pyridine) with the Kamlet-Taft T * parameter. ( 0 )Nonhydrogen bonding solvents; (0)hydrogen bond donors. Numbered points refer to the solvents in Table I
If the probe response in a specified reference state is designated as (Rp)o, then the shift in the property becomes: AI?, = R, - (R,)o. This general form can now be re-stated as Equation 2 to describe the particular function for the 'jNpyridine NMR shift in a given solvent compared to the 15N chemical shift in gaseous pyridine.
A6 = 6, - ( 6 J o = br*
+u
CY
(2)
The final multiple regression in Equation 3 was established from the data in Table I by iterative procedures and is shown by the plot in Figure 3 as well.
+
+
A6 = 5 . 9 2 ( ~ * 3 . 3 ~ ~ )1.5
T r * + act Figure 3. Multiple regression for A6 15N(pyridine)as a function of the Kamlet-Taft parameters, T * and a
(3)
The linear correlation coefficient is 0.993 including all eleven solvents. As a predictive equation, the maximum uncertainty in the calculated A6 is f1.3 (SD). The uncertainty in the coefficient of a is f0.05 (SD); and the nonzero intercept of Equation 3 arises from the condition that A6 = 0 for the gas phase reference state whereas the T * scale goes to zero for cyclohexane. Duthaler and Roberts have concluded that the solvent shielding effects found in the 15N-NMR spectrum of pyridine are determined largely by hydrogen bonding from the HBD solvents ( 3 ) . The weighting coefficient of CY in Equation 3 is consistent with that conclusion when one considers the relative importance of the two parameters for the stronger hydrogen
bond donors. However, for the weaker HBD solvents (Le., dichloromethane and chloroform) as well as the nonhydrogen bonding solvents, the dominant mechanism for the medium effect clearly has its source in probe-solvent dipolar interactions within the cybotactic region ( I O ) . Because of the magnitude of the slope for the regression line in Figure 3, the measured 15N-NMRchemical shifts for pyridine should provide an alternate route to the evaluation of a-scale values for the weaker HBD solvents. This is potentially significant since it has been shown by Taft and Kamlet ( 5 ) that not only are their experimental uncertainties in the a-values for the weak HBD solvents much larger (=k20%) than for the stronger HBD solvents but also the solvatochromic comparison method appears to be close to its lower limit of resolution for HBD influences when applied to the weaker donors. On the other hand, the uncertainty in the T * values is reported to be f O . l l kK based upon forty-seven regressions derived from many solvatochromic indicators (6) and that uncertainity is uniform over the total range of the T* scale. For the empirical analysis of medium effects upon reaction kinetics, it appears that the Kamlet-Taft scales will have broad applications. Therefore, alternative methods for evaluating the solvent hydrogen bonding parameters need to be examined in order to increase the reliability of both a and 0 values over the total ranges of those scales.
LITERATURE CITED Kolling, 0. Anal. Chem. 1977, 49,591. Kolling, 0.Anal. Chem. 1978, 50, 212. Duthaler, R.; Roberts, J. D. J . Am. Chem. SOC. 1978, 700, 4969. Karnlet, M.; Taft, R. W. J . Am. Chem. SOC.1978, 98,377. Taft, R.; Kamlet, M. J . Am. Chem. SOC. 1976, 98,2886. Karnlet. M.; Abboud. J.; Taft, R. J . Am. Chem. SOC.1977, 99,6027. Abboud, J.; Karnlet, M.; Taft, R. J . Am. Chem. SOC. 1977, 99,8325. Giarn, C.;Lyle, J. J . Am. Chem. SOC.1973, 95,3235. Figueras, J. J. Am. Chem. SOC.1971, 93,3255. (IO) Knauer, 6 . ;Napier, J. J . Am. Chem. SOC. 1976, 98, 4395. (1) (2) (3) (4) (5) (6) (7) (8) (9)
Orland W. Kolling Chemistry Department Southwestern College Winfield, Kansas 67156
RECEIVED for review January 8,1979. Accepted April 4,1979.
Radiative Auger Effect in X-ray Fluorescence Analysis Sir: In 1969 Aberg and Utriainen (I) found evidence for the existence of a K-L2 radiative Auger transition. They 0003-2700/79/0351-1325$01 .OO/O
observed a broad X-ray emission structure on the low-energy side of the K a lines of Mg, Al, Si, and S. 0 1979 American
Chemical Society
1326
ANALYTICAL CHEMISTRY, VOL. 51, NO. 8, JULY 1979
Table I. Comparison of Experimental Data for the Silver K Spectrum ref'. 8
1.
this work
Ag K n intensity - 1 . 2 x 106Q 1.54 x 10: backgrounds intensity between - 5000 92000 K L,L, - 200 eV and K L,L,
-
+
200eV
backgroundipeak IR'4Ej1Ka(%)
4 . 2 x 10 ' 0.28 = 0 . 0 6
6.0 x 1 0
c0.01
Approximate value obtained froin Figure 2 in ref. 8 . As proposed in ref, 8. ~~
~
~~
In this work, an attempt is made to resolve this inconsistency in the literature and to determine the importance of the RA effect in X-ray analysis techniques.
EXPERIMENTAL
0.0lt
,
,
10
20
,
,
,
LO
50 60 ATOMIC NUMBER Figure 1. Compilation of some literature data on the intensity of the radiative Auger structure, relative to the Ka intensity as a function of the atomic number. (0)Scofield (6)theoretical calculation, (A) Aberg (7)experimental, (+) Presser (8)experimental 30
X-ray spectra were obtained with an X-ray fluorescence spectrometer (Kevex 810 excitation system) consisting of a high power tungsten anode X-ray tube, a secondary target and filter assembly to provide irradiation facilities with Ti, Ge, Mo, Ag, Sn, and Nd K-radiation. a 30 mm2 X 3 mm Si(Li) detector (160 eV resolution at 5.895 keV), associated electronics for signal processing, and a conventional multichannel analyzer for data acquisition. The spectra are stored on a 7-track magnetic tape unit for off-line calculations with a PDP 11/45 computer.
RESULTS The effect is due to an alternative decay mode of a Kelectron vacancy. The energy of the radiative Auger photon is given by:
ha = E(K-LiLj) - E,i,(L) with hw: energy of the RA-photon; E(K-LiLj): K-LiLjAuger transition energy; and Ekh(L): kinetic energy of the emitted L-electron. The kinetic energy of the emitted electron varies between zero and E(K-LiLj);hence ho is continuous with a maximum energy equal to E(K-LiLj). Since the radiative Auger transition is a fundamental rearrangement process, competitive to K X-ray emission, it should be observed with equal relative intensity irrespective of the creation mode of the K vacancy. This has been verified through the excitation with photons and electrons by Siivola (2) and with protons by Richard ( 3 ) . T h e presence of the RA structure could be of importance for the analytical application of energy-dispersive X-ray techniques such as X-ray fluorescence (XRF) and protoninduced X-ray emission (PIXE). In these methods, spectrum deconvolution using nonlinear least-squares fitting is frequently employed ( 4 , 5 ) . This requires the knowledge of all contributing spectral components for an accurate fitting function definition. The presence of the RA structure could also give rise to the analytical misinterpretation of the X-ray spectral data, by erroneous assignation of K or L emission intensity of an element to the RA structure. The importance of the effect is, of course, directly related to its relative intensity. However, from available literature information, it is not clear to what extent the RA transition interferes in energy-dispersive X-ray spectrometry. Figure 1shows a compilation of literature data on the intensity of the RA structure relative to the K a intensity for various elements; whereas the theoretical calculations of Scofield (6) and the experimental results of Aberg (7) indicate a decrease in intensity of the RA band for higher atomic number elements, the opposite tendency is experimentally found by Presser (8). If the results obtained by Presser were correct, the RA effect might be of considerable importance in X-ray spectrometric analysis of samples containing higher atomic number elements.
High intensity spectra were measured with the energydispersive spectrometer to cover the range of elements which are important in X-ray analysis and for which the RA effect was expected to be detectable. For the low 2 elements (aluminum to chlorine), the RA structure does not show up in the spectra: the maximum of the RA band is situated too close to the Kcu line and it overlaps with the peak. For sulfur, the difference between the S Ka-energy and the K-L2 L3 energy is only 204 eV, the fwhm of the S KO-line being 140 eV. The first elements for which the RA effect could be observed were potassium and calcium. The low-energy tail of the KO peak, caused by secondary detector effects (9) is rather intense for the low energetic K radiation of these elements and blurs the RA structure, as can be seen in Figure 2. Although quantization of this structure is not accurately possible, its intensity relative to the KO line is of the order of 0.1%. The radiative Auger structure is more easily detectable for the transition elements titanium to zinc. Figure 3 shows the spectrum obtained from a thin manganese sample excited with Mo K radiation. The Mn K a peak contains 8.0 X lo6 counts. The RA band shows up as a broad structure between the tail of the K a peak and the KP escape peak. The Mn K-L2L3 energy is indicated on the figure and corresponds closely to the maximum of the observed structure. Again the relative intensity can be estimated a t 0.1% or less. T o obtain information on the radiative Auger effect for higher atomic number elements, experiments comparable to those of Presser (8) were carried out. A high intensity silver K spectrum was obtained by exciting a 0.13-mm thick high purity silver foil with neodymium K radiation. In contrast with the results of Presser, who used 2.5-MeV protons for the excitation of a thin silver deposit on a 15 pg/cm2 carbon backing and who measured a RAE to Kcu intensity ratio of 0.28 & 0.05%, no detectable structure was observed a t the K-LL band energy. An upper limit for the relative intensity of the RA effect could be established a t 0.01 %. The results of both experiments are summarized in Table I. The structure observed by Presser in the silver K spectrum could not have been caused by a radiative Auger transition, since
ANALYTICAL CHEMISTRY, VOL. 51, NO. 8, JULY 1979
* 1327
Figure 3. High intensity Mn K X-ray spectrum (thin Mn sample excited with Mo K X-rays). The RA structure shows up between the tail of the M n Kcu peak and the Mn K@escape peak
Id
I1
I
I
30
35
,iI
L5
2800 Cs E I K - L I L , I
24 42 KeV
> 0, a N
b
t
E, keV
Figure 2. High intensity Ca K X-ray spectrum obtained with a Si(Li) detector through excitation of a thin Ca sample (45 pg/cm2 CaF, on 4 pm thick Mylar foil) with Ge K X-rays. The Ca K L b 3 Auger energy is indicated
-
2LOC
~.
Table I L Comparison of the Energies of the Peaks Found in the Structure of the 133BaEC Spectrum with the Energies of the Tin K a X-rays experimental!y determined energy, keV first peak second peak
25.036 i 0.006 25.263 I 0.006
'
K energies of tin, keV
25.044 ( K a , ) 25.271
(KC,)
in that case it should also have been observed in our experiment. One possible explanation could be the presence of low energy y radiation, resulting from Coulomb excitation of the target material by 2.5-MeV protons. A highly intense spectrum of a '33Ba-source was recorded to further investigate the RA transitions in higher Z elements. The same experiment was also carried out by Presser. The energy region around 25 keV of the spectrum is given in Figure 4. The observed structure was subjected to careful spectral analysis. The full lines in Figure 4 are the results of a nonlinear least-squares fit of the data with two gaussians having a fixed intensity ratio of 1:2 and with a straight line as an estimate for the background radiation. As appears from Table 11, the energies of those two peaks correspond within the experimental uncertainty with the Kal and K a , energies of tin. Energy calibration of the spectrum was performed internally using the cesium K lines. The peaks in our spectrum are most probably the result of X-ray fluorescence
2000 J
24 0
Sn K "2
S"
2 5 . W KeV
25.271 KeV
K4,
I
25 0
I
26 0 E , keV
Figure 4. Part of '33Baspectrum between 24 and 26 keV. The peak structure is analyzed with a nonlinear least-squares fit with two gaussians. The full lines indicate the results of the fit of tin present close to the detector in the cryostat. The tin
K absorption jump energy (29.20 keV) is indeed very close to the cesium K a l energy (30.97 keV) so that the excitation of tin should be very efficient. The Cs K-LLenergies are also indicated in Figure 4. No RA transition could be detected in the spectrum. I t is our belief that the fluorescence of tin near the detector was mistakenly interpreted by Presser as a RA structure.
CONCLUSIONS The radiative Auger effect can be observed with an energy dispersive X-ray spectrometer, equipped with a Si(Li)-detector, for the elements potassium to zinc. The RA structure for these elements has a relative intensity, in accordance with theoretical predictions, of the order of 0.1%. Therefore, in most practical analytical situations, the RA effect will not
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 8, JULY 1979
cause any appreciable int,erference. The experimental evidence obtained for silver and cesium indicates a decrease in RA intensity with increasing atomic number, as predicted by theory. The presence of the RA effect should be recognized when using accurate nonlinear least-squares fitting techniques for spectrum deconvolution. This can be done using a previously described method ( I O ) , which incorporates all deviations of the peak shape from the gaussian profile.
LITERATURE CITED (1) T. Aberg and J. Utriainen, Phys. Rev. Lett., 22, 1346 (1969). (2) J. Siivola. J. Utriainen, M. Linkoaho, G. Graeffe, and T. Aberg, Phys. Lett. A , 32,438 (1970). (3) P. Richard, J. Oltjen, K. A. Jamison, R. L. Kauffman, C . W. Woods, and J. M. Hall, Phys. Lett. A , 54, 169 (1975). (4) P. Van Espen, H. Nullens, and F. Adams, Nucl. Instrum. Methods, 142, 243 (1977).
(5) H. C. Kaufmann. K.R. Akselson, and W. J. Courtney, Nucl. Instrum. Methods. 142. 251 (19771. , , (6) J. H. Scofield, Phys. Rev. A , 96, 1041 (1974). (7) T. Aberg, Phys. Rev. A , 4, 1735 (1971). (8) G. Presser, Phys. Rev. Lett. A , 56, 273 (1976). (9) F. S. Goulding, Nucl. Instrum. Methods, 142, 213 (1977). (10) P. Van Espen. H. Nullens, and F. Adams, Nucl. Insbum. Methods, 145, 579 (1977).
P. Van Espen H. Nullens F. Adams* Department of Chemistry University of Antwerpen (U.I.A.) 2610-Wilrijk, Belgium RECEIVED for review February 2 , 1979. Accepted March 7 , 1979.
Thin-Layer Cell for Routine Applications Sir. It has been well demonstrated in the literature that thin-layer electrochemical (TLE) techniques offer a number of advantages compared to conventional electrochemical methods (1-10). First of all the total electrolysis solution is a layer about cm in thickness and, thus, the total volume needed in the experiment can be very small and, consequently, the total amount of electroactive substance very low. Furthermore, most of the electrochemical techniques can be applied to the thin-layer electrode cells with the added advantage that the equations describing electrochemical response a t a working electrode are very simple. In the literature, a variety of electrode and cell designs for T L E have been described ( 8 ) ,Le., micrometer base electrode systems ( I , Z ) , “capillary” wire electrodes ( 3 , 4 ) ,dip-type TLE (2, I O ) , thin metal-film sandwich electrodes ( 5 ) ,a conducting glass cell (6, I O ) , a gold wire electrode and chamber TLE (9). In general, the thin-layer electrodes and cell designs mentioned are either expensive and difficult to construct or require unusual manipulative techniques. Furthermore, although some of the T L E cells are specifically designed to use only a small amount of electroactive substance (volumes on the order of 1 pL),ohmic polarization also limits the applicability to routine use because of its distortion effects on electrochemical response curves ( 5 , 10). These limitations become very pronounced with poorly conducting or dilute electrolytes and high reactant concentrations. One of the ways to minimize this problem was described by Hubbard ( 8 ) ,which used the porous-boundary (porous Vycor glass) separator with micrometer based electrodes, and Tom and Hubbard (10) using the same material mounted concentrically around a Pt rod electrode which also acts as a separator with respect to a concentric auxiliary electrode. However, these cells are difficult and expensive to construct. Also, diffusion and adsorption of the electroactive species in the porous glass has been found to be a problem (11). We report here a very simple and inexpensive wire T L electrode and cell system which can be practically applied for routine T L E experiments which is made possible by the use of the new membrane material, Nafion (DuPont), as a separator. Nafion is a cation-exchange membrane which is permeable to cations and impermeable to anions and any large neutral species. The thin-layer cell configuration including an enlarged view of the thin-layer electrode itself is given in Figure 1. The thin-layer cell consists of an outside cylinder (a) made
n
A
il!!
‘ 0‘ Thin-layer cell. (a)Teflon tubing i d 1.6 mm, (b) gold wire, (c) Nafion membrane, (d) reference electrode, (e) solution outside of thin-layer electrode,(f) auxiliary electrode (gold wire),and (9) expanded view of the tip of TLE Figure 1.
of Teflon tubing, which contains the thin-layer electrode assembly, the reference electrode (d), and the auxiliary electrode (gold wire in this case) (f). The thin-layer electrode assembly (8) consists of a gold wire of 0.5-mm o.d. (b) surrounded by Nafion membrane tubing (c). The space between the gold wire and Nafion membrane is the actual thin-layer cavity which contains electrolysis solution (the average solution thickness is approximately 2.5 X cm). In this case, the reference electrode (d) was also made with a Nafion membrane. The bottom of the NaFon tubing was sealed and filled with calomel and saturated KCl solution with electric contact being made by Pt wire. Volume of the thin-layer electrode assembly can be varied anywhere from 0.25-1.0 pL, and is adjusted by varying the height of the solution in the cell. As an example 2-mm height corresponds t o a volume of 0.65 pL f 5% error, based on 10 independent fillings measured by chronocoulometry. The total volume of the total thin-layer cell is approximately 15-20 pL depending again on length of the Nafion and Teflon tube. The lower end of the T L E assembly extends below the bottom of the Teflon cell body so it can be easily and separately filled with the electroactive solution. It is most important to note that the TLE assembly can contain reactant solution while the auxiliary electrode
