A N A L Y T I C A L CHEMISTRY, VOL. 51, NO. 4, APRIL
solvent was necessary. iZfter the graphite planchet is cleaned, it is placed on a glass plate on a hot plate, and heated to between 60-80 " C . T h e solution is then added, 50 pL a t a time, with the water quickly evaporating. Figure 1 shows the spectra of 250 pL of a solution containing 500 ppm Cr and 375 ppm K (from K,Cr,Oj), and 500 ppm As in nitric acid, loaded onto the graphik as described above. This preliminary evaluation of the holder in quantitative studies is promising. In summary, these graphite planchets seem to have many advantages as routine sample holders in XPS. They are simple to use, inexpensive, and, when treated carefully, virtually free of contaminants. They have proved useful in examining oxygen-containing compounds and seem to have potential in trace analysis by ESCA.
1979
583
LITERATURE CITED (1) G. E. Theriault, Thomas L. Batty, and M. J . B. Thomas, Anal. Chem., 47, 1492 (1975). (2) J. H. Scofield, J . Electron Specfrosc., 8, 129 (1976). (3) G. Johanssen, J. Hedmann, A. Berndtsson, M. Klasson. and R . Nelsson, J . Electron Soecfrosc.. 2. 295 (1973). (4) D. Briggs, V. S . Givson, and J K . Becconsall, J . Electron Specfrosc., 11, 343 (1977). (5) G. M. Bancroft, J . R. Brown, and W. S. Fyfe, Anal. Chem., 49, 1044 (1977).
RECEIVED for review September 8, 1978. Accepted October 31,1978. Acknowledgement is made to the Robert A. Welch Foundation and to the donors of the Petroleum Research Fund, administered by the American Chemical Society for the support of the research.
Blank Limitations in Laser Excited Solution Luminescence T. G. Matthews and F. E. Lytle" Department of Chemistiy, Purdue University, West Lafayette, Indiana 47907
In fluorimetry, the instrumental sensitivity is directly proportional to t h e intensity of the exciting radiation. This parameter, in turn, depends upon the source intensity, the throughput of the excitation optics, and, for a continuum source, the bandwidth of t h e monochromator. As a result, lasers have been demonstrated to be a very advantageous excitation source for trace luminescence analysis ( 1 - 4 ) . An increase in sensitivity does not, however, produce a corresponding improvement in the lower limit of detection because most practical samples are blank limited. Parker has outlined several contributing factors including elastic scattering of the exciting radiation, inelastic Raman scattering from the solvent, and fluorescence from the cuvette, the solvent, and any sample impurities ( 5 ) . T h e reduction of the total blank emission is therefore crucial to capitalizing on the laser as a source in studies involving either trace, high quantum yield or bulk, low quantum yield emitters in solution. This paper demonstrates t h a t a n improvement in the lower limit of detection can be achieved by a detailed consideration of the blank luminescence from both the cell and the solvent. A survey of high purity and spectral grade commercial solvents, including a complete range of polarity and several glass-forming mixtures, shows possible orders of magnitude variation in the blank. Simple purification procedures are found to be very effective in lowering reagent grade solvent emission, but none are competitive with high purity commerical solvents. EXPERIMENTAL The fluorimeter used for these experiments was constructed around a Phase-R Model K21K nitrogen laser providing 3-pJ, 4.5-ns pulses at a repetition rate of 28 Hz. Sample luminescence was focused through a Corning 0-52 filter (laser scatter attenuation XlOOO) into a Jarrell-Ash Model 82-405, 1/4-mmonochromator providing a 4-nm bandpass. Emission detection was achieved by an RCA 1P28B photomultiplier. A detailed description of this instrument will be published at a later date. Although the fluorimeter provided subnanosecond time resolution, all spectra were recorded at a time coincident with the peak of the laser pulse to more closely resemble the results expected with steady-state excitation. Data shown in the figures are corrected for the spectral response of the monochromatorphotomultipler combination. Unless otherwise stated, all solvents were tested as received from the manufacturer. Quinine bisulfate was purchased from 0003-2700/79/0351-0583$01 .OO/O
Eastman Kodak and recrystallized twice from ethanol. RESULTS AND DISCUSSION Cell Design. Scatter a n d fluorescence from the cuvette can be conveniently reduced well below solvent emission levels by designing a cell consisting of a 1-cm i.d. quartz tube epoxied to a 1.2-cm2plate cut from a Corning 7-54 filter. Exciting vertically through the bottom of the cell, the filter passes the laser beam with minimal scatter and fluorescence, and irradiates the solution without impinging upon the walls. An increased path length can also be achieved in this configuration for greater signal strength. Solvent Considerations. Solvent scatter and fluorescence present a far more complicated problem than cell design. In most cases, the total emission spectrum consists of temporally and spectrally sharp elastic and inelastic scattering bands (C-H, 0-H), and some degree of a spectrally broad, impurity fluorescence tail ( T ~< 2 ns). This is consistent with considerations of typical cross-sections for Rayleigh, vibrational Raman, and fluorescence processes. Despite the dramatically lower cross-sections for the scattering processes, t h e high number density of the solvent molecules causes both the Rayleigh and Raman signals to be comparable or greater in integrated intensity than the fluorescence emission from impurities in ultra-clean solvents. Although not the principal topic of this paper, it should be noted that a n instrument employing pulsed laser excitation is of obvious value in isolating long-lived sample emission from solvent impurity fluorescence and Raman scattering ( 2 ) . Naturally, for fluorophors with lifetimes of similar magnitude to those of solvent impurities or t h e laser pulse width, time resolution is of little value. For this latter case, the blank emission can be reduced only by solvent purification or a judicious choice of manufacturer. A survey of solvent emission (Figures 1-3) was taken including many very common solvents, a wide range of solvent polarity, and several manufacturers. Impurity fluorescence intensity varied by more than two orders of magnitude between spectral grade and high purity solvents. Only Burdick and Jackson ( B U ) high purity, distilled-in-glass solvents were found to have uniformily low backgrounds. Individual solvent comparisons between spectral grade and B&J high purity are 0 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 4, APRIL 1979
1
2.0
I
100 P P T
I
I
I
I00 PPT
00s
1
I 3 50
400
450
500
550
350
600
400
450
500
550
600
wavelength (nrn)
Wavelength (nrn)
Figure 3. Solvent luminescence, (- - - -) Spectral grade benzene. Flgure 1. Solvent luminescence, ( - - - -) Spectral grade benzene, E, (O-O-O) Spectral grade carbon tetrachloride, E, = 32.5 kcal/mol. = 34.5 kcal/mol. (O-O-O) B&J acetonitrile, 46.0 kcal/mol. (0-0-0) (O-O-O) BBJ chloroform, 39.1 kcal/mol. (O.-O--O) B&J carbon Distilled water, 63.1 kcal/mol. (O--O...O) Cyclohexane, 31.2 kcal/mol tetrachloride 12.0
10.0
8.0
PPB
6.0
4.0
QBS
2.0
2.0 1.6
--B
I .4
c
1.0
>r
1.2
.-
c
C v)
2 c -
0.8
0.6
0.2 0.15
0.4
0.1
0.2 0.I 0 350
400
450
500
550
600
0 350
400
450
500
550
600
Wavelength hrn)
Wavelength (nrn)
Figure 2. (O-O-O)
Figure 4. ass-forming solvents. (0-0-8) 10% USP grade ethanol, 90% B&J methanol. (O-O-O) 29% B&J propanol or butanol, 71% B&J ethyl ether. (O.-O--O) 50% B&J pentane, 50% B&J cyclopentane
shown for benzene (Figure 2) and carbon tetrachloride (Figure 3). Significant improvement was noted with B&J carbon tetrachloride and orders of magnitude of difference was noted with B&J benzene. In all cases, the luminescence intensity (450 nm, 0.1 M H,SO,) of a particular concentration of quinine bisulfate (QBS) is shown for comparison. It should be noted that residual fluorescence for the better solvents corresponds roughly to compounds having a concentration-quantum yield product of e.g., M and @f = 0.001. Figures 1,2, and 3 illustrate the extensive range of polarity available in high quality solvents. Applying the ET (25 OC) values developed by Reichardt and Dimroth (6) to classify solvent polarity, the largest gap between cyclohexane and
water was 7.6 kcal/mol (see Figure captions). Incorporating other B&J high purity solvents and a few binary water mixtures for gaps a t the polar end of the scale, separations of less than 2 kcal/mol could easily be achieved. Three weakly luminescent glass-forming solvent mixtures of low (1:1 pentane/cyclohexane), intermediate ( 2 5 propanol/ether), and high (1:9 ethanol/methanol) polarity are presented in Figure 4. The 1:9 alcohol mixture represents the least fluorescent combination of several mixtures of these two solvents which forms a stable glass (7). Solvent Purification. To ascertain the practicality of simple solvent purification procedures, a fractional distillation was attempted. Methyl cyclohexane was chosen for purification because it is used in several glass-forming solvent
Solvent luminescence, (- - - -) Spectral grade benzene. USP grade ethanol, E, = 51.9 kcal/mol. (O-O-O) B&J methanol, 55.5 kcal/mol. (O.-O...O) B&J benzene
ANALYTICAL CHEMISTRY, VOL. 51, NO. 4, APRIL 1979
585
high purity commercial solvents could be achieved. IO 0
CONCLUSION
'QE I
6 0
20
-
I0
m
c
0
x
=2
:
-
05 03
02
Solvent scatter and luminescence have been shown to be major factors controlling the limits of detection in the conventional fluorescence technique. Solvent luminescence can frequently be reduced by orders of magnitude with careful selection or purification. Temporal resolution would also be valuable for isolating any sample emission long lived in comparison to the fluorescent impurities of the solvent. Spectral resolution is of little value, however, unless the emission is greatly red-shifted from the excitation energy. In contrast, the interference caused by scattered emission can frequently be attenuated with either spectral or time resolution, because of its sharp spectral and fast temporal characteristics.
ACKNOWLEDGMENT We thank J. M. Harris for the initial work on the low cost cell design and the laser system.
01
LITERATURE CITED C
350
400
450
550
500
600
Wavelength ( n d
Figure 5. Methylcyclohexane purification. (0-0-0) (O-.O-.O) Purified
Unpurified.
mixtures but can only be purchased in less than spectral grade quality. T h e luminescence of the best of 4 equal fractions collected from a packed Vigreux column (-10 theoretical plates) is presented in Figure 5 . Although significant improvement was made, much more elaborate purification procedures would be necessary before results competitive with
(1) R. E. Brown, K. D. Legg, M. W . Wolf, and L. A. Singer, Anal. Chem., 46( 12). 1690 (1974). (2) F. E. Lytle and M. S. Kelsey, Anal. Chem., 46(7), 855 (1974). (3) R . N. Zare and M. R. Berman, Anal. Chem., 47(7), 1200 (1975). (4) J. H. Richardson and S. M. George, Anal. Chem., 50(4), 616 (1978). (5) C. A. Parker, "Photoluminescence of Solutions", Elsevier, New York, 1968. (6) C. Reichardt, and K. Dimroth, Fortschr. Chem., Forsch., 11, 1-69 (1968). (7) F. J. Smith, J. K . Smith, and S.P. McGlynn, Rev. Sci. Instrum., 33(12), 1367 (1962).
RECEIVED for review November 16, 1978. Accepted January 18, 1979. This research was supported in part through funds provided by the National Science Foundation under Grants MPS75-05907 and CHE77-24312.
Algorithm for the Determination of Decay Rate Constants by Reversal Current Chronopotentiometry Donald A. Tryk and Su-Moon Park" Department of Chemistry, The University of New Mexico, Albuquerque, New Mexico 8713 1
Current-reversal chronopotentiometry (CRCP) is a technique which can complement cyclic voltammetry by providing quantitative kinetic data for slow following reactions investigated qualitatively by cyclic voltammetry. CRCP has perhaps been under-utilized because of the difficulties of its data treatment. We wish to report a convenient algorithm for the computation of pseudo-first-order decay rate constants of electrogenerated species using the method of current-reversa1 chronopotentiometry. For the case where the forward electrolysis current, if, equals the reversal current, i,, the diffusion equation yields the analytical solution ( I ) . 2 erf
6= erf d
m
(1)
where t is the forward electrolysis time, T is the time elapsed between the current polarity switching time and the transition time for the electrogenerated species, and h is the pseudofirst-order decay rate constant. For the case in which i, # if, the solution is (2) (u
+ 1) erf 6 = erf d
m
(2)
where u = irjifi 0003-2700/79/0351-0585$01 O O / O
The usual method for the computation of k has been to read values of the dimensionless quantity ht from a graph or table of k t vs. the experimental dimensionless quantity, T / t (I). Herman has outlined a method for computer-generating such a table ( 3 ) . In order to increase the precision of h , the experiment may be run a t various values o f t to obtain a series of values of kt. These may be plotted against t , the slope of the line being k ( 1 ) . In terms of computer programming, however, a method requiring the use of a stored table and interpolation therefrom suffers from defects in storage economy, speed, and precision. Another approach is to recast Equation 1in an explicit form, giving the quantity t l r as a function of k r :
1 t / 7 = -[erf-1(2
kr
erf
\/G)I*1 ~
(3)
where e r r ' is the inverse of the error function. T h e inverse error function is readily implemented as a subroutine using the Newton method ( 4 ) , since the derivative of the error function is available ( 5 ) , and the error function itself is commonly available as a subroutine. The geometric form of Equation 3 is shown in Figure 1. At large values of t / 7 , k r 1979 American Chemical Soclety
