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Anal. Chem. 1982, 54, 1850-1853
Quantitative Multimode Cavity Electron Spin Resonance Spectrometry by Two-step Spectral Integration Kunlo Nakano, Hlroshl Tadano, and Shlgeyuki Takahashi* D e p a ~ m e n tof Chemistry, College of Science, Rikkyo University, Nishi-Ikebukuro, Toshima-ku, Tokyo 17 1, Japan
Quantitative treatment of the electron spin resonance signal was studied by using a two-step Integral method. Normailring methods for two-step Integral wave helght under various measuring condltlons were examined, but no sultabie method for microwave power was found wlth a single cavity. Therefore, a comparison method with a multimode cavity was attempted for quantitatlve analysis. This method eliminated the effect of a difference In any of the measuring condltlons and moreover made it possible to compare preclsely the data of samples having different 0 values.
Quantitative analysis by using electron spin resonance
(ESR) has usually been carried out by measuring peak-to-peak heights or areas of ESR signals of derivative shape (1-13). However, in cases where the signal is not sharp and symmetrical, the measured amount is not in good proportion to the sample amount. Therefore, the present authors attempted a two-step integral method with an analog integrator of their own construction and acquired better results in linearity in the calibration curve and in sensitivity (14,15). Although applications of a two-step integral method have been seen (16-18), papers dealing with a normalizing method are few (18). The present authors also studied a normalizing method (19) and found that, for normalizing a two-step in, height should be divided by amtegral wave height ( h ) the plitude (“gain”) and modulation width and multiplied by sweep velocity. Further, it should be multiplied by input resistances and integrating capacitors and divided by the resistance ratios of output attenuators connected with the integrators. However, a proper normalizing method could not be found for microwave power. Moreover, signal intensities of samples having different Q values could not be compared with each other by using the usual single cavity. Hence, in the present work a comparison method by two-step integration using a multimode resonator, a so-called double cavity, has been attempted.
EXPERIMENTAL SECTION Apparatus. The ESR spectrometer and resonating cavity used were, respectively, a Model JES-ME-3X partially modified and a multimode cavity, Model JES-MCX-1(TE,,, mode), made by JEOL Co. Integration of the ESR signal was carried out by using a handmade integrator in which two operational amplifiers were employed (14). The original curve and the one-step integral and two-step integral curves are shown in Figure 1. In this figure, h is the wave height of the two-step integral curve. The multimode-cavity-resonatorused is shown in Figure 2. A sample and a standard are inserted into hole 1and hole 2. The phase of magnetic modulation applied to hole 1and hole 2 is either in the same phase or the antiphase, and the phase is selected by exchanging the connector. For improving precision of the measurements, quartz adaptors were fitted into both sample holes and the samples were fixed at definite positions. Mounting Method of Sample. The sample used was CuS04.5H20 powder (analytical grade) and its aqueous solution. When the sample was solid, as shown in Figure 3a, the sample (B) was put into a quartz sample tube (A) made by Wilmad Glass 0003-2700/82/0354-1850$01.25/0
Co. and then the sample was plugged lightly with cotton wool (C). In the case of a liquid sample, a quartz capillary (D)containing the liquid sample (E) was fixed in the center of the sample tube (A) with a plastic spacer (G), as shown in Figure 3b. When the Q value was depressed, the capillary (D)containing a Q value depressor (F) was inserted into the sample tube to a position just above the solid sample (B) as shown in Figure 3c.
RESULTS AND DISCUSSION Calculation of Signal Intensity Ratio. When the multimode-cavity-resonator shown in Figure 2 was used in practice, the integral wave height of a sample in hole 1differed slightly from that of the same sample in hole 2. Also, the wave heights of the same sample measured in the same phase and in the antiphase were different from each other. Therefore, some correction was required. When samples A and B are in hole 1 and in hole 2, respectively, the wave height (h,)measured in the same phase can be expressed by eq 1 and the wave height (ha)measured in the antiphase can be expressed by eq 2, where I A and I B
represent the signal intensities of samples A and B, a being the ratio of wave heighb measured in hole 2 and in hole 1and 0the ratio of wave heights measured in the antiphase and in the same phase. From the two equations, I A and I B are obtained as eq 3 and 4, and then the signal intensity ratio, IA/IB,
is obtained as eq 5.
(5) Now, the factors a and p must be determined. When the sample positions are interchanged, and the measured wave heights are represented as hi and h:, the signal intensity ratio can be expressed as eq 6. Then from eq 5 and 6, a and 0are
obtained as shown in eq 7 and 8.
=
H = h,‘h,
+ a) h,(R - a )
+ hahi
h,’h, - hah,‘
R = -I A IB
h,, ha,hi, and h,’ were measured under various conditions using samples A and B (50.83 mg and 21.21 mg, respectively, of copper sulfate powder). The values of a and p could be 0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982
1851
Table I. Results of Quantitative Analysis sample amt of CuSO4.5H,O, mg wt ration 102.30 4.673 71.16 3.251 2.404 52.63 1.810 39.62 1.463 32.02 1.021 22.35 9.58 0.438 Standard: CuSO4-5H,O21.89 mg.
-
sample hole in 2
sample hole in 1
calcd sample obtained ratiob obtained ratiob amt, mg 4.70 ? 0.07 4.68 f 0.08 102.5 3.28 f 0.06 71.6 3.27 i 0.04 52.8 2.43 i 0.02 2.41 f 0.04 1.80 f 0.02 1.81 i: 0.02 39.7 1.45 ? 0.01 1.46 i: 0.02 31.9 1.01 f 0.01 1.03 f 0.01 22.5 0.437 f 0.005 9.57 0.443 f 0.004 Each figure is the mean value of eight measurements.
calcd sample amt, mg 102.8 71.8 53.2 39.3 31.7 22.0 9.64
n Sample : C U S O ~ ' S H ~ O Mag,field : 3000i1000G Mod.width: 3.03G Micro.power : 6.0mW Sweep time
:hi".
Flgure 1. Original curve and one-step integral and two-step integral curves. h is two-step integral wave height. b
a
hole 1
hole 2
I
I
c
Flgure 3. Method of mounting sample: (a) powder sample, (b) solution sample, (c) powder sample and Q value depressor; (A) sample tube ($ = 4 mm), (B) CuS04.5H,0 powder, (C) cotton wool, (D) capillary sample tube (4 = 1 mm), (E) CuSO, solution, (F) Q value depressor, (G) plastic spacer, (H) stopper.
%Aee
connector
resonatlng c a v i t y
tube
Flgure 2. Multimode-cavlty-resonator TElos mode type.
calculated from eq 7 and 8, as R was considered to be equal to the weight ratio of A and B. The results were as follows: O( = 1.0373 f 0.0069, /3 == 1.0363 f 0.0083. These average values were employed a11 the cavity factors in the present experiments. Effect with Change in Microwave Plower. Variation in signal intensity with change in microwave power was examined, and the results, presented in Figure 4, show that the signal intensity ratio remained almost constant in spite of a wide variation in microwave power, although the integral wave heights changed very much. The mean value of the intensity ratio 2.397 0.026 was prsictically the same as the weight ratio of the samples 2.397. Effect with Change in Crystal Current. The effect of crystal current of microw,ave detector was examined. In the experiment, crystal current was changed by varying the position of the tuner while maintaining the microwave power constant. From the results, it can be seen that the change in crystal current scarcely affected the wave heights or their ratio. The mean value of Ihe ratio was 2.388 f 0.026, and this
*
i '
2
4
Power
6
8
(mW)
Flgure 4. Variation in signal intensity ratio with change in microwave power: (0)wave height measured In same phase, (A)wave height measured in antiphase, (0)ratio; standard = CuS046H,0 (21.21 mg), sample = CuSO4-5H,O (50.83 mg).
is considered in agreement with the ratio of the sample weights. Effect with Change in Modulation Width. Figure 5 shows the results of the variation in the integral wave heights and their ratio with change in modulation width. As shown in Figure 5, it is to be noted that, although h, and havaried with change in modulation width, the ratio was almost constant; and the mean value of the ratio 2.395 0.039 can be considered to be in agreement with the ratio of the sample weights. Quantitative Analysis for Solid Samples. An example of a quantitative analysis for copper sulfate powder is shown
*
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982
Table 11. a! and 0 Calculated under Existing Q Value Depressor depressor position depressor h,, cm ha, cm h,', cm hole 1 none 21.85 11.49 28.30 ethanol 26.14 10.70 25.81 water 18.90 1.82 20.13 1 M KCI 17.05 7.06 18.46 13.54 5.58 15.21 4 M KCl average
h i , cm -12.43 -11.44
-8.88 -8.11
-6.74
std dev
hole 2
none ethanol water 1 M KCI 4 M KC1
21.85 2 5.53 18.56 16.28 13.49
11.49 10.45 7.68 6.73 5.55
28.30 25.78 17.97 15.85 12.29
-12.44 -11.36 -7.90 -6.97 -5.43
average std dev
a!
P
1.0316 1.0401 1.0325 1.0298 1.0363 1.0341 0.0041 1.0316 1.0368 1.0303 1.0313 1.0364 1.0333 0.0030
1.0367 1.0370 1.0397 1.0374 1.0402 1.0382 0.0016 1.0367 1.0339 1.0372 1.0373 1.0372 1.0365 0.0014
PI
i
3.0
in
-D
n n
..
NO-
(1) ( 2 )
depressor Ethanol
0
'
I
3.03 Modulatlov wldth
10.0
6.06
(GI
Flgure 5. Variation In signal intensity ratio with change in modulation width: (0)h,, (A)ha, (0)ratlo; standard = CuS0,.5H20 (21.21 me), sample = CuS04.5H20 (50.83 mg).
in Table I. The standard used in this experiment was 21.89 mg and the samples were various amounts of CuSO4.5H20 powder as shown in the first column in the table. The measurements were carried out twice for each sample when the sample was either in hole 1or in hole 2. The obtained ratios were in agreement with their weight ratios. The equations of each series of the ratios obtained by the method of least squares can be given as follows: when the sample was in hole 1
Y = 0.04581X and when the sample was in hole 2
Y = 0.0459oX where Y is the ratio and X is the sample amount (mg). The weights of the standard calculated from both equations are 21.83 mg and 21.79 mg and agree well with the actual weight. These results indicated that this method can be used for quantitative analysis. Effect with Change in Q Value The effect on the signal intensity caused by decreasing the Q value of the sample was examined. Samples A and B were copper sulfate powder. Ethanol, water, or a potassium chloride solution was employed as the Q value depressor; it was put into a sample tube as shown in Figure 3c. Figure 6 shows the results when samples A and B were in hole 1and hole 2. In this figure, (1)is the case of the Q value depressor in hole 1and (2) is that in hole 2. From Figure 6,
.
(1) ( 2 ) Water
(1) ( 2 )
(1) ( 2 )
KC1 U M I
K C I (4N)
Figure 6. Variation in signal intensity ratio with change in Q value: (0) wave height measured in the same phase, (W) wave height measured in the antiphase, (0)signal intensity ratio; (1) Q value depressor in hole 1, (2) Q value depressor in hole 2. Samples A (CuS04.5H20, 50.83 mg) and B (CuS04.5H20,21.21 mg) were in hole 1 and hole 2,
respectively. Table 111. Relationship between Intensity Ratio and Concentrationa [CUSO, solution], M intens ratio intens ratio/concn 1.00 0.474 * 0.005 0.474 i. 0.005 0.80 0.382 i. 0.004 0.478 i. 0.005 0.60 0.292 i. 0.003 0.487 i. 0.005 0.40 0.197 f 0.004 0.492 ? 0.010 0.20 0.095 f 0.001 0.477 * 0.005 0.10 0.046 i. 0.004 0.460 i. 0.040 a The standard used is CuSO,.5H, 0 (9.58 mg). it is evident that the signal intensity ratio remains nearly constant, although the integral wave heights vary very much. When the sample positions were reversed, the same results were obtained. a and p calculated from the same data are shown in Table 11. These values of a and @ are considered to be nearly constant with change in Q value. Table I1 also suggests that the effect of the position of the Q value depressor is almost equal for hole 1 and hole 2. These facts indicate that quantitative measurements for samples having various Q values are possible by using a multimode cavity. Calibration Curve for Solutions. The standard was 9.58 mg of copper sulfate powder, and the samples used were copper sulfate solutions of various concentrations, as shown in Figure 3b. The same capillary tube was used for every sample solution. The relationship between the intensity ratio and the concentration is shown in Table 111. The calibration
Anal. Chem. 1982, 5 4 ,
curve had good linearity and could be used for quantitative analysis. In the Case of an Unknown a. Ideally, the measuring conditions of hole 1 and hole 2 of multimode cavity should be equal. However, hole 1and hole 2 were slightly different from each other in practice. Hence the factors a and 0 had to be determined. Although a and 0 are proper values for the cavity used, a is considered to vary with change in the shape and the shift of the position of the sample tube adaptors, even if the cavity is the same!. Accordingly, the value of a will be changed by exchanging or resetting the adaptors. The computation method in the case of an unknown a is as follows: Extract the square root of the product of eq 5 and eq 6 on each side of the equation, obtaining eq 81. As this equation
:=d-
(ha + ha/P)(h,' .- h,'/P) (ha - ha/P)(h,' + h,'/P)
(9)
does not include the factor a , the intensity ratio can be calculated without knowing the value of a,although it is necessary to do the measurements twice. That is, the signal intensity is the geometrical mean of the ratios obtained from eq 5 and 6 when the value of a is unknown.
CONCLUSION The two-step integral method using a multimode cavity has been found to be a good method for quantitative measurements of ESR signal. An intensity of an unknown sample can be decided as some-fold of a known amount of suitable standard substance, regardless of the measuring conditions and Q value of the sample. This makes it possible to compare signal intensities measured under different conditions at different times.
1853-1858
1853
For precise measurement, it is desirable that volumes of sample be not so large and amounts of standard and sample be not so different from each other. In such cases, measurements can be carried out within only a few percent error. Standards including various values of spin number are required for quantitative analysis over a wide range of sample amount. If attention is paid to the above, the present method will be useful for quantitative analysis using ESR. LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19)
Guilbault, G. G.; Meisel, T. Anal. Chem. 1969, 4 7 , 1100. Moyer, E. F.; McCarthy, W. J. Anal. Chim. Acta 1969, 4 8 , 79. Gullbault, G. G.; Moyer, E. S. Anal. Chem. 1970, 4 2 , 441. Meisel, T.; Guilbault, G. G. Anal. Chim. Acta 1970, 5 0 , 143. Fujiwara, S.; Tadano, H.; NakaJima,M. Bull. Chem. SOC.Jpn. 1970, 4 3 , 3023. Yamamoto, D.; Fukumoto, T.; Ikawa, N. Bull. Chem. SOC.Jpn. 1972, 45, 1403. Yamamoto, D.; Ikawa, N. Bull. Chem. SOC.Jpn. 1972, 45, 1405. Yamamoto, D.; Ozaki, F. Bull. Chem. SOC. Jpn. 1972, 4 5 , 1408. Bryson, W. G.; Hubbard, D. P.; Peake, B. M.; Simpson, J. Anal. Chim. Acta 1975, 77, 107. Warren, D. C.; Fltzgerald, J. M. Anal. Chem. 1977, 49, 250. Bryson, W. G.; Hubbard, D. P.; Peake, B. M.; Slmpson, J. Anal. Chim. Acta 1978, 96, 99. Bryson, W. G.; Hubbard, D. P.; Peake, B. M.; Simpson, J. Anal. Chim. Acta 1980, 116, 353. Sakane, Y.; Salto, K.; Matsumoto, K.; Osajima, Y. Bunsekl Kagaku 1981. ..- ., 30 .., 3023. - - --. Nakano, K.; Tadano, H.; Oshima, M. Nippon Kagaku Kaishi 1972, 2453. Nakano, K.; Tadano, H.; Suglmoto, S. BunsekiKagaku 1978, 27,256. Goldberg, Ira B.; Crown, H. R.; Robertson, W. M. Anal. Chem. 1977, 4 9 , 962. Goldberg, Ira B.; Crown, H. R. Anal. Chem. 1977, 4 9 , 1353. Goldberg, Ira B. J . Magn. Reson. 1978, 32,233. Nakano, K.; Tadano, H.; Sugimoto, S. Nlppon Kagaku Kaishi 1979, 885.
RECEIVED for review January 18,1982. Accepted June 1,1982.
Quantitative Room-Temperature Phosphorescence with Internal Standard and Standard Addition Techniques Malcolm W. Warren 111,' James P. Avery,* and Howard V. M a l m ~ t a d t " ~ University of Illinois, School of Chemical Sciences, 1209 W. California St., Urbana, Illinois 6 180 1
Greatly improved preclslon and accuracy are demonstrated by the methods of internal standard and standard addltlon wlth room-temperature phorrphorescence of adsorbed organic molecules. An Internal standard Is used to increase the preclsion of the measurement by a factor of I O , typlcally to 1-3 % from 10 to 20%. A two-point standard addition technlque greatly Improves the accuracy. For example, in a sample containing sodlum acetate the errors due to shifts In spectral distribution and intenslty are decreased from greater than 100% to less than 8%. Shifts in spectral intenslty and dlstributlon are accounied for wlth spectra calculated with factor analysis assumlng no previous knowledge of the spectral dlstrlbution or intensity. This allows measurement of anaiytes in "real" sample matrlces without the use of compllcated standards or a prevlous knowledge of ?he spectral distrlbutlon or Intenslty.
'The Dow Chemical Co., Analytical Laboratories, Building 1603, Midland, MI 48640. University of Colorado, Department of Electrical Engineering, Campus Box 425, Boulder, CO 80309. 3Pacific and Asia Christian University, P.O.Box YWAM, Kailua-Kona, HI 96740.
Room-temperature phosphorescence provides selective and sensitive detection of many organic compounds (1-3). Unfortunately, quantitative measurements have seldom been reported because of poor precision and accuracy. Quantitative measurements reported in the literature are for components in simple, well-defined sample matrices or use complicated sample preparation or preseparation methods. There are no reports of analyses made in sample matrices that change the spectral characteristics of the phosphore in an unknown manner. Also, there has not been a thorough evaluation of factors that affect accuracy in determining a specific analyte in complex matrices. The combination of internal standard and standard addition improves the precision for manual preparation of the sample on paper by a factor of 10, from about 10-20% to 1-3%, and provides accuracy to better than 8% ( 4 ) . The use of an internal standard decreases errors caused by nonreproducible sample handling and measurement parameters. Standard addition allows the measurement of components in a sample matrix that affects the excitationemission intensity and spectral distribution of the phosphorescence. Early papers on room-temperature phosphorescence indicated that it is free from the problems associated with
0003-2700/82/0354-1853$01.2!5/00 1982 American Chemical Society
