Evaluation of a Cesium Primary Ion Source on an Ion Microprobe Mass Spectrometer Peter Williams* and R. K. Lewis’ Materials Research Laboratory, University of Illinois, Urbana, Illinois 6 180 1
Charles A. Evans, Jr. Materials Research Laboratory and School of Chemical Sciences, University of Illinois, Urbana, Illinois 6 180 1
P. R. Hanley General Ionex Corp., Ipswich, Massachusetts 0 1938
A commercially available Cs’ Ion source has been modlfied for use as the primary Ion source on an ion microprobe m a s spectrometer. The prlmary ion current Is stable to 1% over a I-h period. Up to 1000 h Operation is achleved on a single charge of cesium. Current densltles of 10 mA cm-2 or better are achleved In microfocused spot sizes ranglng from 8 pm to 30 pm. Secondary negative ion yields of electronegatlve specles under Cs’ Ion bombardment compare favorably with positlve ion yields of electroposltlve species under 02+Ion bombardment. Sputterlng rates of Si and GaAs are 2 and 5 tlmes faster respectlvely than 02’ sputtering rates for these substrates. These high Sputtering rates facllltate rapid depth proflling analyses. Detection limits have been determined under sample-limited depth profiling conditions for H, C, 0, F, and As in Si, and for S in GaAs.
-
The generation of sputtered ions of solid materials by energetic ion bombardment has become a versatile and sensitive technique for surface and thin film analysis ( I ) . Although noble gas primary ions can be used to bombard the sample, it has been known for many years that the yield of positive secondary ions is greatly enhanced by bombardment with electronegative species such as oxygen (2, 3). Duoplasmatron ion sources capable of producing intense oxygen ion beams are now standard on ion microprobes. It has long been known that cesium ion bombardment or the presence of cesium on an ion-bombarded surface leads to the enhancement of negative secondary ion yields ( 4 , 5 ) . Reliable cesium ion sources have been developed in recent years, for space thruster applications (6) and, more recently, as components of negative ion sources for particle accelerators (7, 8). A specially designed cesium ion source has recently been operated on an ion microprobe (Applied Research Laboratories IMMA) with very promising results (9). We have adapted a commercially available cesium ion source to our ion microprobe (AEI Scientific Apparatus, Manchester, England, Model IM20 (IO))in order to evaluate the performance of the source for surface and thin film analysis.
SOURCE DESCRIPTION The cesium ion source used in this study is based on a part of the “Hiconex” particle accelerator ion source (General Ionex Corp., Ipswich, Mass.). The source, shown schematically in Figure 1, is of the surface ionization type. Cesium metal is vaporized in the stainless steel reservoir which is maintained a t -260 “C, and diffuses via the feed tube to a porous ‘Present address, Department of Geological and Planetary Sciences, California Institute of Technology, Pasadena, Calif. 91125.
tungsten frit which is maintained at 1100-1200 “C. At this temperature, approximately 99% of the cesium evaporated from the outer surface of the frit is ionized (II), and the ionization efficiency is insensitive to small temperature changes. Heat conduction from the frit heater maintains the molybdenum feed tube at a temperature intermediate between those of the reservoir and frit. The frit and frit heater are held by a beam-forming electrode which ensures that the emitting surface is maintained concentric with the axis of the source housing. The source housing is machined from aluminum and has the approximate dimensions (8-inch diameter by 5 inches deep) of the standard duoplasmatron on the ion microprobe. The Freon cooling system of the duoplasmatron is adequate to cool the cesium source housing. The Freon serves as an electrically insulating heat transfer medium, not as a refrigerant. A solid stub projects from the reservoir. A hollow copper cooling block is clamped to the stub and is connected to the source housing by I/&. 0.d. copper tubing. Heat conduction through the copper tubing to the source housing aids in the temperature regulation of the reservoir. (The source is otherwise very well thermally isolated.) This arrangement ensures that the coolest part of the system is the reservoir which thereby controls the cesium vapor pressure. Freon may be circulated through the cooling block to facilitate rapid shut-down of the source. For operation on the ion microprobe, it is necessary that the source be floated at +15 to +20 kV relative to ground. Power requirements for the reservoir and frit heater are 40 W and 350 W, respectively. In these preliminary experiments, the source has been satisfactorily powered by unregulated “Variac”-controlled supplies.
ION MICROPROBE MODIFICATIONS Only one modification to the ion microprobe was required. The “Hiconex” source requires a negative ion suppressor electrode immediately following the ion extraction electrode to suppress negative ions which are sputtered fron apertures in the primary ion optical system. These sputtered negative ions can backstream up the Cs’ ion beam and rapidly damage the frit by sputtering. It was not convenient to install an additional electrode in the ion microprobe column because no spare electrical feedthroughs were available. Instead, the four primary beam steering plates, which were located in a suitable position to serve as a suppressor, were operated at -600 V relative to ground. This arrangement was entirely satisfactory in protecting the frit from damage. The additional lens action of the steering plate assembly introduced no detectable image aberrations when this arrangement was tested with the standard duoplasmatron ion source. ANALYTICAL CHEMISTRY, VOL. 49, NO. 9, AUGUST 1977
1399
SUPDRESSON FLATES
~~
B E A Y FORMhlG E.ECTR03E
EXTRAC-8ON ELECTRODE
CES UM RESESVOIR
. d - r
-
RESERVOIR THERMOCOUPLE
~C
POROUSIONIZER
-
-RESERVOIR HEITER
ob ~b
I
5'
IONIZER-^
Table I. Elemental Sensitivities under Cs+ Bombardment4 (mass analyzed counts/s/nA of Cs') Element (matrix) Count rate Si (Si) 9 x 105 As (GaAsP) i x 104 Asb (Si) 2 x lo6 P (GaAsP) 4 x lo4 Pb (Si) 3 x 105 H b (Si) 8 x 105
RESERVOIR COOLANT
hEATER 406
o-iav
Figure 1. Schematic of cesium ion gun as mounted on AEI IM20 ion
(Si)
7x
lo6
(Si) 4 x lo6 S b (GaAs) 4 x lo6 Normalized to 100 a t % concentration. Species present at < 1%concentration.
microprobe
OPERATION Typically, the source is loaded with a 5-g charge of cesium. Loading is accomplished in a glove bag containing argon. A sealed glass vial of cesium is heated outside the glove bag to melt the cesium. The vial is then transferred into the glove bag, the seal is broken, and the cesium poured into the reservoir. The reservoir seal is accomplished with a knife-edge flange of the "Conflat" design and a copper gasket. The large frit area allows rapid pump-out of argon trapped in the oven and transfer tube. The source was originally designed to operate on a horizontal beam line with the transfer tube horizontal and the copper gasket seal a t the top of the reservoir away from the molten cesium. Since the IM20 primary ion column is vertical, it is necessary for us to operate the source with the transfer tube vertical and the reservoir on its side, which allows the molten cesium to come into contact with the copper gasket. After several hundred hours of operation, some erosion of the gasket was noted. Copper was found to be plated onto the interior of the reservoir. Since the seal remained intact and since copper does not surface ionize at 1100 "C or poison the ionizer, this erosion poses no contamination problems. However, a new style reservoir is being designed with the gasket reoriented so that it does not contact the liquid cesium. This will eliminate erosion of the gasket. The source has proved to be remarkably resistant to degradation through exposure to the atmosphere. When the system was vented with argon, the source could be handled in air for an hour or so with no deleterious effect. In addition, after a vacuum accident which vented the system to the atmosphere for 24 h, a useable Cs' current could still be obtained after pump down. In the absence of such accidents, a single charge of cesium should last for over 1000 h. When the source was cleaned after several hundred hours of operation, it was estimated that most of the original cesium charge still remained in the reservoir. Contrary to our expectations, cesium contamination of the primary ion column was almost negligible. The major deleterious effect after several hundred hours of operation with a total Cs' current of 1-2 mA was the formation of sputtered metal films inside the extraction electrode assembly. These films eventually flaked off and could short the accelerating gap. When the standard duoplasmatron was reattached to the column and used to produce negative oxygen ions, small amounts of cesium on the source mounting flange initially caused excessive field electron emission at -20 kV. This problem disappeared after 1 day. PRIMARY ION BEAM PERFORMANCE A freshly cleaned and charged source will deliver -65 nA into a beam 20 pm in diameter (-20 mA cm-'). Over a period of several hundred hours, a current density of 10 mA cm-' is routinely attainable. These current densities compare
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1400
ANALYTICAL CHEMISTRY, VOL. 49, NO. 9, AUGUST 1977
favorably with duoplasmatron performance. Current density is roughly independent of beam size down to the smallest beam diameter used in our instrument (-8 pm). After a 1-h warmup period, current stability is typically better than 1% over an hour. The source is not subject to the high frequency oscillations that can occur in duoplasmatrons and the high thermal inertia of the structure precludes short-term drift. The filament for the frit heater is wound in a bifilar fashion to minimize the production of stray magnetic fields. The 35-A ac heater current produces no detectable 60-cycle modulation of the primary beam position a t spot sizes down t o 8 pm.
ANALYTICAL PERFORMANCE Sputtering Performance. When compared with oxygen ion bombardment, cesium ion bombardment at 20-keV impact energy sputters GaAs approximately five times faster and Si about twice as fast. These high sputtering rates result in a valuable reduction in the time required for depth profiling analyses. Cesium ions have a projected range in solid materials of about 60% of that of 02' ions of the same energy. Thus the extent in depth of ion-induced mixing is reduced. Examination in the scanning electron microscope revealed no roughening or cone formation in sputtered Si or GaAs samples. In our instrument, sample charging problems preclude the analysis of insulating samples with positive primary ions. Secondary Negative Ion Yields. Secondary negative ion count rates have been determined for matrix levels of As and P in GaAsP and Si in Si, for trace levels of H, C, 0, P, and As in Si, and for S in GaAs. These count rates are shown in Table I normalized to the respective atomic concentrations. For the matrix level species (Si, P, As) sensitivities were also determined under 0'' and Ar' bombardment and were found to be, respectively, two and three orders of magnitude lower than yields obtained using Cs' bombardment. For comparison, the yield of Fe' from iron under 02' bombardment is 8 X lo5 counts/sec/nA of Oz' (12). Detection Limits. Detection limits in secondary ion mass spectrometry are only infrequently dictated by ion yields. Most often they are dictated by background signals arising from interfering molecular ion species having the same nominal mass as the analytical species (12),from adsorption of residual vacuum species such as H, C, and 0, or from implantation of primary ion beam species. Under oxygen ion bombardment, a significant number of polymeric oxygencontaining secondary ion species are formed which interfere with heavier species. For example, detection of 32S-is severely hampered by 1 6 0 2 - , and 75As-detection in silicon is limited by 29Si30Si160-.Although high mass resolving power or energy discrimination (13) techniques can sometimes be used to reduce or eliminate the interference signal, the intensity sacrificed constitutes another limitation on detection capability. The use of Cs' ion bombardment results not only in high negative ion yields, but also in a significant improvement in the chemistry of the analytical process. The introduction
Table 11. Detection Limits for Cs+ Ion Bombardment (atoms/cm ) Species Signal limita Background Background (matrix) (background-free) levelb limitC 6 X 10l6 3 x 1015 As (Si) 1x 1 O I 6 4 x 1019 1 x 1017 0 (Si) 2 x 1017 4 x 10lS 1 x 1017 C (Si) 8 X 10” 4 x 1019 1 x 10l8 H (Si) 2 x 1017 2 x 1017 S (GaAs) F (Si) 2 x 1OI6 6 X 1OI6 a Concentration giving 100 counts (above background) when (30 X 30) pm2 X 100 A of sample is sputtered. Concentration which would give a count rate equivalent Concentration which would to the background signal B. give a count rate equivalent to 2&
18 -
H Implant
-
in SI
4l 2
of oxygen into the sample and the production of oxide polymer ions is avoided. Cesium has a high mass, is monoisotopic, and does not form intense polymeric species with the majority of samples. In most cases, the polymer species that do form are of sufficiently high mass that they do not interfere with the analytical species. Bulk detection limits are not particularly useful figures in SIMS because the major analytical applications of the technique come in sample-limited situations-in particular depth profiles. We have determined Cs’ ion bombardment detection limits for a number of elements using our routine depth profiling analysis conditions. These conditions constitute an extreme sample-limited situation. In routine operation, the focussed primary ion beam is rastered over a square region (100 X 100) Km2. The time taken to complete each “frame” of the raster may be varied, but is generally in the range 4-20 s. During this time, 20-200 A of sample may be sputtered, depending upon the primary beam current. An electronic aperturing technique is used to ensure that mass analyzed ions of the analytical species are counted only when the primary ion beam is within the central flat region (30 X 30) l m 2 of the sputtered crater (14). The ions counted during each frame are recorded on a digital printer. For the purposes of this article, we define a detection limit as that concentration which is detectable when a sample volume (100 x 100) bm2 x 100 A (corresponding to -3 x lo-’’ g of Si) is sputtered and 90% of the sputtered ions are rejected to ensure adequate depth resolution. Two types of limit can be distinguished. (1)Signal Limit. In the absence of appreciable background signal, the minimum acceptable count rate is taken to be 100 counts/frame (10% relative standard deviation). The signal limit is thus that concentration of the analytical species which produces 100 counts/frame under the depth profile conditions cited above. Species whose detection limits are of this type include As (in Si) and S (in GaAs). (2) Background Limit. A high background count rate may constitute a detection limit much higher than the signal limit discussed above. Species such as 0, C, and H are present in the residual vacuum and can adsorb on the sample surface to produce a background count rate which may mask the signals arising from intrinsic C, 0, and H in the sample. The background limit is conventionally defined as that concentration of the analytical species which produces a signal equivalent to twice the standard deviation of the background. In pulse-counting detection systems, the standard deviation of the background count rate B is A,so that the background limit to detection is 2&. Detection limits of the two types defined here are given in Table I1 for six elements: As, C, H, 0, F, and S. These limits were determined by performing depth profile analyses on ion-implanted samples. Such samples are well suited for these
01 0
1
I
BO
I60
1
I
240
1
I
1
I
1
320 400 480 560 Sputtering Time (sec)
640
720 800
Depth profile of 50-keV H ion im lant on Si. Projected range, 5920 A. Peak H concentration, 4 X 10R atom cm-3
Figure 2.
C Double Implant in SI
[ c l P = 2 x 10‘’
atom cm-3
Ap=1226%, 2377%
-
-
-
lo3;
io
240
400
480
,bo
Sputtering Time (sec)
Depth profile of C double implant (55 keV, 110 keV) in Si. Projected range, 1230 A, 2380 A. Peak C concentration, 2 X lo2’ atom cm-3 Figure 3.
determinations for the following reasons: (i) Peak concentrations of ion implanted species can be calculated from tabulated range statistics (15). (ii) Observation of the characteristic implant profile (approximately Gaussian) serves as confirmation that the desired analytical species is being monitored. (iii) Gross deviations from the expected Gaussian profile can generally be attributed to instrumental or background limitations. The calculated peak concentration then serves as an internal standard from which the concentration equivalent to the background level may be calculated. Depth profiles are shown in Figures 2-6 for five implanted species: H, C, 0, S, and F. Arsenic depth profiles and detection limits have been discussed in detail elsewhere (16). All of the profiles except that of fluorine exhibit a flat “tail” a t levels ranging from -50% of the peak implant signal for hydrogen to -1% of the peak for sulfur. We estimate that the electronic aperture technique is capable of discriminating by a factor of -lo3 against ions sputtered from the peak implant region exposed in the crater wall (14). We therefore attribute tails above a level of -0.1% of the peak count rate to background. The standard deviation of the count rate, B, measured in this region is used to derive a background deANALYTICAL CHEMISTRY, VOL. 49, NO. 9, AUGUST 1977 * 1401
Io7
1
I
1
I
0 Implant in Si [O],=9
x IO
20
atom cn
I
S Implant in GaAs
Rp= I l 4 0 i
.. .'*.
... ... .. .
IO2[
E
.'0e
L
cE
.*
0 3
5
c
0
IO2O
3 I
.....:. . ".. ..
**.E..
e ..
10:
o l9 I
I
I
120
60
I
180
: 3
240
Sputtering Time ( s e d
Figure 4. Depth profile of BO-keV 0 implant in Si. Projected range, 1140 A. Peak 0 concentration, 9 X 10'' atom I
I
I
8F2 Implant
In
I
SI
F Profile
[F] ~6.3 x IO1' atom Rp~1500%
lo5-
I
1
I
40
80
I20
I
I
200
240
I
I60
2
Sputtering Time ( 5 e c )
Figure 5. Depth profile of F implant in Si (150-keV BF2 implant). Projected range, 1500 A. Peak concentration, 6 X 10'' tection limit, 2 d . The fluorine profile differs from the other profiles in that it extends to very low concentration levels and is markedly non-Gaussian (i.e., not parabolic on the semi-log plot). The observed Gaussian shape of the boron profile for this sample (BF2+implant) indicates that the non-Gaussian shape of the fluorine profile is not an instrument-induced artifact. It is possible that the lowest F- signals accurately reflect implanted fluorine. However, in the absence of confirmation of the profile by an independent technique, we pessimistically designate the tail at -6 X 1OI6 atom cm-3 as background. A fluorine background could arise from the use of HF etchants and the ubiquitous use of Freon in the laboratory. The concentration equivalents of the background ion count rates are listed in Table I1 together with the background 1402
ANALYTICAL CHEMISTRY, VOL. 49, NO. 9, AUGUST 1977
1
I
0
40
I
1
I
I
80 120 160 200 Sputtering Time (sec)
I
240
I
, I1017
280
Figure 6. Depth profile of 270keV S implant in @As. Projected range, 2080 A. Peak concentration, 1.5 X lo'' atom crW3 detection limits. H and 0 are clearly present in significant amounts in the sample chamber (ion pumped, liquid nitrogen trapped, pressure 3 X lo-' Torr). Background signals are quite large for these species but the high ion yields achieved with Cs+ ion bombardment cause the background signals to have small relative standard deviations. The results of Table I1 indicate that a sample chamber pressure of -3 X Torr would be required to reduce the 0 and H background to levels equivalent to 1 ppm (6 X 10l6 atom ~ m - ~ ) . The carbon background limit is an order of magnitude lower than those of hydrogen and oxygen, probably reflecting the greater ability of the liquid nitrogen cooled surfaces in the sample chamber to pump ambient carbon-containing species. Intrinsic carbon in the silicon substrate probably contributes to the background level. Sulfur detection is signal limited at a level equivalent to the background level. That the background signal is due to the sputtering of 0; from adsorbed oxygen is demonstrated by its variation with sample chamber pressure and its inverse variation with sputtering rate. Arsenic is the only one of the species studied which is entirely signal limited. Because background signal was not a limiting factor, it was possible to improve the detection limit to 1 X 1OI6 atom cm-3 by increasing the sample volume consumed to a (250 X 250) pm2 crater (15). The cesium source is now in routine use for depth profile analyses of electronegative species. The high sensitivity of the system permits investigation of the behavior of dopant species implanted a t low concentrations into minimally damaged samples. Impurity effects can be studied a t natural levels without the necessity of doping the sample to artificially high levels for analysis by insensitive techniques. Finally, the system retains the advantages of SIMS: good depth resolution, high speed analyses, and the capability to obtain chemical concentration information from as-received samples.
ACKNOWLEDGMENT Ion implanted samples for this evaluation were kindly provided by D. Wolford, T. Sigmon, J. C. C. Tsai, K. Vaidyanathan, and D. Meyers. We thank R. Blattner for helpful discussion and V. Deline for experimental assistance. LITERATURE CITED C. A. Evans, Jr.,
Anal. Chem., 47, 818A (1975). (2) G. Slodzian and J.-F. Hennequln, C . R . Hebd. Seances Acad. Sci., Ser. 8,263, 1246 (1966). (3) C. A. Andersen, Int. J . Mass Spectrom. Ion Phys., 2 , 61 (1969). (4) V. Krohn, J . Appl. fhys., 33, 3523 (1963). (1)
(5) C. A. Andersen, Int. J . Mass Specfrom. Ion fhys., 3, 413 (1970). (6) R . G. Wilson and G. R. Brewer, “Ion Beams”, John Wiley, New York, N.Y.. 1973. (7) K. H. Purser, I€€€ Trans. Nucl. Sci., NS-20, 136 (1973). (8) R:Middleton, Nucl. Instrum. Methods, 122, 35 (1974). (9) H. Storms, Vailecitos Nuclear Center, Private communication. (10) A. E. Banner and 6. P. Stimpson, Vacuum, 24, 511 (1975). (11) Ref. 6, p 29. (12) P. Williams and C. A. Evans, Jr., Natl. Bur. Stand. (U.S.), Spec. Pub/., 427, 63 (1975). (13) R. F. K. Herzog, W. P. Poschenrieder, and F. G. Satkiewicz. Radiat. Eff., 18, 199 (1973).
(14) P. Williams and C. A. Evans, Jr., Int. J . Mass Spectrom. Ion fhys., 22, 327 (1976). (15) J. F. Gibbons, W. S. Johnson, and S. W. Mylroie, “Projected Range Statistics”, Halsted, New York, N.Y., 1975. (16) P. Williams and C. A. Evans, Jr., Appl. fhys. Lett., 30, 559 (1977).
RECEIVED for review March 28,1977. Accepted May 26,1977. This work was supported in part by National Science *Oundation Grants CHE-74-05745, and CHE-76-03694, and by General Ionex Corporation. DMR-76-010589
Effect of Matrix Composition on Line Intensity in the Determination of Light Elements in Organic Compounds by X-ray Fluorescence Spectrometry G. W. Tindall Tennessee Eastman Company, Division of Eastman Kodak Company, Kingsporf, Tennessee 37662
A method is described to calculate the effect of changes in organic composition on the x-ray fluorescent intensity of light elements. The calculation assumes that in these cases excitation is entirely by the monochromatic tube target Ilne. The limitations of this technique were explored using solutions of various elements in solvents. For the case of a llght element in an organic matrix, the technique enables standards prepared in one compositionto be used to determine the same element in any other known light element compositlon. This technique is useful in the determination of llght elements in organic compounds, especially polymers, where standards preparation is often difficult.
X-ray fluorescence spectrometry is useful for the determination of elements heavier than fluorine in organic chemicals,plastics, and fibers. In most cases, parts per million and greater concentrations of these elements can be determined rapidly and accurately. The major limitation of this technique is the need for standards. The fluorescent intensity measured for an element is a function of both its concentration in a sample and the mass absorption coefficient of that sample. Hence, standards must be prepared in the same organic composition as the samples to be analyzed. When a wide variety of organic compositions needs to be analyzed, it is not practical to prepare standards in this large number of compositions. If the effect of changes in organic composition on fluorescent intensity could be calculated, a standard for some element prepared in one composition could be used to determine this same element in any other composition. The equation governing fluorescent line intensity is given by:
spectrometer. (For Siemens spectrometer, this term equals 1.) K M , ~is, the mass absorption coefficient of the matrix for the wavelength of the analyte line being measured. This equation is valid for excitation by monochromatic x-rays (1). Since the output of the x-ray tube is not monochromatic, this equation is generally useless for calculating the effects of changes in composition on fluorescent line intensity. However, in certain cases this equation could be useful for calculating matrix effects. The tube most commonly used for the analysis of organic compounds is the chromium target tube. The target lines of this tube account for about 75% of the tube energy, and the energy contribution of the continum a t wavelengths longer than the Cr K a line is insignificant (2). The absorption edges of the matrix elements in an organic compound are all of longer wavelength than the Cr KCYline. For the analysis of elements lighter than vanadium, the absorption edge of the analyte is of longer wavelength than the Cr KCYline. In these cases where essentially all excitation results from the monochromatic Cr K a line, Equation 1 would be used to predict the effect of changes in composition on the fluorescent line intensity of an element. If the effect of composition changes can be calculated, Calibration standards prepared in one organic composition can be used to determine the same element in other organic compositions. As an example, suppose chlorine standards are prepared in methyl alcohol, and we wish to use these standards to determine chlorine in propyl alcohol. The fluorescent line intensity of the methyl alcohol standard is given by Equation 2. ICl, M e O H
=ACCl
Pel h C r ~
o
l
~
/JMeOH,hCr K~
+
(2)
P M e O H , hcl K~
The intensity of a propyl alcohol standard of the same concentration is given by Equation 3. where 11is the fluorescent line intensity for the analyte line measured. A is a constant for a given element and instrument conditions. CA is the concentration of analyte in the sample. k ~ , his~the ~ ) mass absorption coefficient of the analyte for the wavelength of the primary x-rays. M M , ~ is ~ , the mass absorption coefficient of the matrix for the wavelength of the primary x-rays. B is a term dependent on the geometry of the
‘Cl,
PrOH
pcl*h C r K a
=ACCl
PROH,h C r K a
+
(3)
P R O H , h c 1 K~
Combining these equations yields ‘Cl,
&OH
-
- IC,, MeOH
~ M H h, C r ’&OH,
hCr Ka
+ P M ~ O H ,ACI Kor
+ P R O H , hC1Ka
ANALYTICAL CHEMISTRY, VOL. 49, NO, 9, AUGUST 1977
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