conversion to the thiocarbamate was achieved. This reaction was followed by observing the complete disappearance of the thiol proton. The NMR spectrum of the derivative shows a downfield shift of 0.55 p.p.m. for the group to This group appears as a triplet at 3.18 P.P.m. ( J = 6.5 C.P.S.) coupled with the methylene group alpha to the carbonyl which also is a triplet at 2.70
p.p.m. The NH peak of the derivative is broad and appears a t about 10 p.p.m. EXPERIMENTAL
were recorded as mately 10% deuteriochloroform or dimethylsu~foxide-&solutions on a Varian Associates A-60 NMR spectrometer, Peak positions are given in parts per million (p.p.m.) downfield from internal tetramethylsilane a t 0.
LITERATURE CITED
( 1 ) Goodlett, V. W., ANAL. CHEM.37, 431 (1965). ( 2 ) Marcus, S. H., ILlitler, S. I., J . Phys. Chem. 68, 331 (1964). PETERE. BUTLER
WOLFGANG H. AIUELLER ESSOResearch and Engineering Co. Linden, N. J.
Improved Spectral Measurements in Spark Source Mass Spectrometry Using Transmittance Areas SIR: One of the factors affecting quantitative results in spark source mass spectrography is the measurement of spectral intensities as recorded on a photographic plate. Preliminary results in a previous study (3) indicated that improved precision was obtained through the use of planimeter-measured transmittance peak areas rather than the more conventional absorbance peak height data. This study represents a more comprehensive investigation of the use of transmittance areas in the evaluation of mass spectrographic data. The determination of transmittance areas was greatly facilitated by the use of an electronic peak integrator installed as an auxiliary output on a conventional recording densitometer.
Figure 1.
Schematic diagram of the electronic peak integrator
EXPERIMENTAL
A Kuclide GRAF-2 spark source mass spectrograph and a Jarrell-Ash Model 23-100 recording microphotometer were used in this study. The experimental and instrumental parameters have been previously described ( 3 ) . These parameters are not of primary importance here because absorbances at the peak maxima were compared with integrated areas of the same peaks in transmittance units, both sets of data being obtained from the same photographic record in every instance. The Ilford &-I1 emulsion was used. Integrated Area Measurement. Transmittance areas were determined from the photographic data using a rapid, convenient electronic integrator system coupled to the densitometer. A schematic diagram of the integrator, designed and constructed to our speoifications by the DeVar Division of Consolidated Electrodynamics Corp., is presented in Figure 1. The modular unit consists entirely of solid-state components, each module performing a given function. The operation and description of the integrator is best accomplished by considering a peak being scanned by the densitometer. The background on one side of the peak is scanned with the external switch controlling the integrator in 1408
ANALYTICAL CHEMISTRY
the closed position. This background signal is introduced into two parallel systems, an adder-subtractor module, and an adjustable lag module. During this time, the adjustable lag determines the average value of the background and equal signals are being received by the two terminals of the addersubtractor. The signals being equal, there is no output from the latter module. As the scan reaches the vicinity of the peak to be integrated, the external switch is opened, thus interrupting the input to the lag module. Under this condition, the lag unit maintains an output equal to the average background. As the incoming peak signal changes, the signal received by the positive side of the addersubtractor increases, producing a difference in the two signals received by the latter. The value of this difference is continuously integrated, converted to square-wave pulses, and displayed on an electronic counter. Integration ends when the external switch is closed and the system immediately begins sampling to establish a new background average. The unit, as used in this laboratory, was set to average the background over 8 period of five time constants, the latter having been adjusted to 1 second.
The amplifier gain was set to produce 6OOO counts per minute per 10-mv. signal. Any further increase in the signal per unit area may be readily obtained by increasing the gain or by reducing the rate of the densitometer scan. Areas determined electronically were linear with planimeter-measured areas. Extensive usage of the integrator unit has demonstrated that the times of initiation and termination of integration are not critical provided, of course, that the times selected are not within the interval during which the peak signal is being received. The presence of an extraneous, unresolved line or band near that to be measured may, however, preclude accurate sampling of the background level. In instances where the peak is superposed on a sloping background, the area can be reproducibly measured by scanning from both directions and taking an average of the two results. To evaluate the precision with which an area could be determined, five replicate integrations of photographic data representing a wide range of intensity levels were run by two operators. The standard deviation calculated from these data was, in every instance, equal to or less than dr,where N
isotope lines, the W1*2 measurements for each exposure recorded were applied to their respective characteristic curves to determine relative intensities. Each intensity was subsequently plotted against the corresponding exposure level as determined by the charge monitor system of the mass spectrograph. The improved reliability of the transmittance area data over the entire intensity range, and particularly at the low intensity level, is readily apparent. An extension of the range at high intensities, of considerable practical significance in quantitative analysis, also results from the use of transmittance areas. I n contrast, the intensity area data are subject to uncertainty in measuring half-intensity peak widths particularly when the peak height has effectively reached the saturation condition evidenced at approximately the 3 x 10-10 coulomb exposure level by the curve derived from absorbance data. The prominence of this problem is significantly reduced through the use of transmittance area measurements. The extended range, of course, is a consequence of the increase in line width which occurs even after the emulsion has been saturated at the center of the line. In addition to these advantages, extensive use of transmittance areas has shown that emulsion response curves are more reproducible from plate to plate. For example, curves fitted by the method of least squares exhibited a relative standard deviation of 10.5% for 12 plates run on one matrix as compared to 18.4% for
Amp1 ificalion focfor 3
1 I
I
5 I
I
50
IO
I 103
I I50
20
30
40
I
I
I
I
I
200 250 Areo , cwnls
I
xx,
I
350
I
400
Figure 2. Effect of amplification on the transmittance area measurement error for a low intensity (8870 T) peak
represents the average transmittance area in counts as determined by the integrator. This inference of the applicability of counting statistics was further investigated by determining the effect of increasing the integrator gain on the measurement precision. Figure 2 presents the results for a single low intensity (88y0transmittance) photographic peak. Amplification of the integrator readout by increasing the gain validates the applicability of counting statistics as demonstrated by the fact that the observed measurement reproducibility was always better than the d@relationship.
bance, and intensity areas derived as. suggested by Owens and Giardino (6). All data were obtained from the same exposures on a single photographic plate and were treated in a consistent manner in the application of the Churchill method. After determining the preliminary and characteristiccurves for each type of measurement using the singly-charged tungsten-182 and -183
RESULTS AND DISCUSSION
Consideration of the determination of integrated absorption intensities by measurement of areas in fractional absorption units, as proposed by Bourgin ( I ) and discussed by Wilson and Wells (7), suggests that a linear relationship exists between peak height absorbance and transmittance areas over a large range. The experimental validity of this for photographically recorded data has been verified in this laboratory by plotting transmittance areas us. absorbance for the same exposures over a large intensity range. Linearity was observed over the entire range for which absorbance could be reliably measured. Further justification for the use of transmittance areas is given in Figure 3 which presents mass spectral data in the form of intensity us. exposure plots for tungsten obtained through the Churchill method of plate calibration (2) for three types of measurements: transmittance areas, peak height absor-
I
I
I
0-1 I
Id0 Monitor exposure
, Coulombs
I 10-8
Figure 3. Comparison of emulsion response curves for W182using three types of measurement 0 Transmittance area
A Peak height absorbance
m
Intensity area
VOL 38, NO. 10, SEPTEMBER 1966
1409
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b. Peak height absorbance
't
I
0.J 10'2
I
1041 IO4 Monitor elpowre, Cculombs
Figure 4.
1I 10-9
Probability emulsion r e s p w e curves for carbon impurity in W a.
Using transmittance oreas
b Using peak height absorbarm
the same plates using absorbance measurements. The application of transmittance area measurements using an alternative method of plate calibration proposed by McCrea (4, 6 ) is presented in Figure 4a. The corresponding absorbance messurements are included in Figure 4b for comparison. To apply this method, meas and peak heights coincident with emulsion saturation were determined as a function of mass by running a graded series of exposures in approximately (113)1/~increments for a group of el+ ments covering the mass range 7-209. m e n the area, N , of a spectral peak did not change by an amount greater than 3(N)'/Z between two consecutive exposure levels, it was taken as the saturation area. Areas determined in this manner were found to be directly pro1410
e
ANALYTICAL CHEMISTRY
portional to (mass) l12. Saturation absorbances were readily determined by densitometer examination of the same exposure series. These values closely approximated the inverse (mass)*/* function given by Owens ( 6 ) . Division of the absorbance or area measured for a particular exposure by the respective saturation value for that mass produces the probability parameter which is plotted against the corresponding monitor exposure level in the figure. Again, the improved consistency of the transmittance area data relative to the absorbance data is apparent from the curves. Figure 4 demonstrates a difficulty which has frequently been encountered when absorbance measurements are used and also shows the alleviation of this problem through the use of transmittance areas. To calculate the con-
centration of the carbon impurity in the tungsten, the carbon curve must be parallel to the matrix curve as is the case for area measurements (Figure 4 ~ ) . A change in the emulsion response as a function of mass, with a concomitant change in the saturation absorbance value used to compute the probability parameter, is effectively the cause of the deviation from parallelism observed in Figure 4b. Considering the inverse (mass) l/z effect for absorbance measurements, the emulsion response function approaches exponential proportions at low masses. Thus, a small change in the response characteristics of a phot+ plate for any reason produces a striking deviation from normal behavior at the low masses with essentially no effect at high masses. The direct proportionality between response and for transmittance area measurements tends to offset this difficulty. In addition, localized emulsion variations have a less deleterious effect on integrated area measurements because larger portions of the emulsion are involved in a given measurement. Applications to Other Spectral Techniques. The improved consistency and the extension of emulsion calibration curves derived from transmittance areas have also been observed for optical emission data. Treatment of replicate exposures run on a series of standards using trsnsmittance area and absorbance measurements produced matching analytical curves. An analysis of the overall variance observed for the two measurement methods indicated that a significant improvement in reproducibility at the 90% confidence level was obtained through the use of transmittance areas. I n this particular instance, a factor of two reduction in the overall variance was observed. The possibilities associated with scale expansion techniques which are possible with area measurements are very attractive. The potential advantages derive from the expected decrease in measurement error suggested by Figure 2 and the increased ability to discern small changes in line intensity. However, improved reproducibility can be obtained only when the photographic measurement error is a significant contributor to the total error of the ansr lytical method. The ability to measure to 1% will not reduce a +1@-20~o variation which may be inherent in the emission source. I n addition to the potential advantages discussed above, the use of transmittance area measurements in infrared and ultraviolet-visible absorption spectrophotometry allso offers a mode of readout which exhibits a reduced dependence on the spectral band pass of the instrument when resolution from adjacent absorption bands is not a prob-
lem (7). Experiments conducted in this laboratory have shown that t r a n s mittance area measurements allow the use of a spectral band pass four to five times greater than that required for highest sensitivity when peak height measurements are used.
(2) Churchill, J. R., IND.ENQ. CHEM., ANAL.ED. 16, 653 (1944). ( 3 ) Harrington, W. L., Skogerboe, R. K., Morrison, G. H., ANAL. CHEM.37, 1480 (1965). ( 4 ) McCrea, J. M., Proc. XI1 Ann. Conf.
LITERATURE CITED
21, 1016 (1965). ( 6 ) Owens, E. B., Giardino, N. A., ANAL. CHEM.35, 1173 (1963).
( 1 ) Bourgin, D. G., Phys. Rev. 29, 794 (1927).
M a s Soectrv. and Allied ToDics, - . Paper 92; Mohtreal, 1964. ( 5 ) McCrea, J. M., Spectrochim. Acta
(7) Wilson, E. B., Jr., Wells, A. J., J . Chem. Phys. 14, 578 (1946).
R. K. SKOGERBOE W. L. HARRINGTON~ G . H. MORRISON Department of Chemistry Cornell University Ithaca, N. Y. WORK supported by National Science Foundation research grant (GB-3324). ’Present address: E. I. DuPont de Nemours and Co., Wilmington, Del.
Spectrophotometric Determination of Coordinated Pyridine SIR: The pyridinium ion is a weak acid withpK = 5.18 at 25OC. (4)and has a strong absorption peak at 255 mp with molar absorptivity, B = 5350. These two properties of the ion were first used for the quantitative determination of pyridine in air samples by Hofmann (8), and in phosphoric acid extracts of hydrocarbons by LeRosen and Wiley (3). However, the pyridine content of metal complexes has usually been obtained through the microanalysis of the nitrogen content. Here it is shown that coordinated pyridine can be quantitatively determined by a combination of steamdistillation and spectrophotometric measurement. EXPERIMENTAL
Reagents. The chemicals used were of analytical reagent grade. Pyridine was dried over potassium hydroxide and distilled immediately before use. Preparation of Compounds. The vyridine complexes of Mn, Co, Ni. Cu, Zn, and Ccl were prepared by well established methods (7). The mercury complex was prepared as described in the literature (6). Analysis. The uitrogen content was determined by the Micro Dumas method in the Microanalytical Lahoratory of the Chemistry Department. Spectral Measurements. -4bsorb&rice measurementq were made on a Beckman Model 13)u Spectrophotometer in a room niaiiitained a t 25” C. using matched 1-cm. and 10-cm. silica cells. The molar absorptivity, e, of the pyridinium ion in 0.lX sulfuric acid was determined for a series of solutions with concentration ranging from 4 X 10-6M t o 1.4 X 10-‘M and found to be 5346 f 31 in agreement with the reported value ( 5 ) . -Procedure. In the analvsis of w r i dine in a complex, 10 ml. 6f a sohiion ?oiitaining about 2.5 to 5 mg. of the complex was treated with 20 ml. of 2 N sodium hydroxide solution and steamdistilled. .ibout 150 ml. of distillate was coilected in a flask containing 25 ml. of I N sulfuric acid. The distillate was diluted to 250 rnl. in a standard flask
Table I.
Nitrogen and Pyridine Determination of Pyridine Complexes
Compoundsa MnPd(NCS)r COP~(NCS)Z NiP,(NCS)Z CUPZ( NCS)z ZnPZ(NCS)z CdPz(NCS)z HgPd CrZO,)
Nitrogen microanalyses, % N Calcd. Found Difference, %
Pyridine analyses, yo pyridine Calcd. Found Difference, %
17.24 17.10 17.11 16.58 16.49 14.49 4.87
64.90 64.37 64.41 46.82 46.56 40.90 27.52
17.17 16.80 17.16 16.54 16.40 14.66 4.90
-0.4 -1.8 +0.3 -0.2 -0.5 $1.2
+0.6
Mean = 0 . 7 a
P
=
65.2 64.5 64.0 46.3 46.1 40.9 27.6
+0.5 +0.2 -0.6 -1.1 -1.0 . . .
-0.3 Mean = 0 . 5
pyridine.
and the absorbance of the solution measured in matched l-cm. silica cells a t 255 mp. For the blank solution, the same procedure of distillation was employed except that 10 ml. of water was used in place of the sample solution. Several steamdistillations of pyridine from solutions containing known amounts of pyridine were carried out under the same experimental conditions. The recovery of pyridine was found to be quantitative. RESULTS AND DISCUSSION
The results from the steamdistillation of the pyridine complexes are shown in Table I. The mean deviation of the percentage difference between the calculated and the experimental values for the pyridine determination is close to 0.5%, a value comparable to that obtained in the nitrogen analysis by the Micro Dumas method. One attractive feature which recommends the proposed method is that it can be performed rapidly with the usual laboratory facilities. Furthermore it gives the pyridine content rather than the total nitrogen content, and hence is suitable for the determination of pyridine in the presence of other nitrogen-containing compounds, provided these do not absorb in the same region of the spectrum. Should the distillate contain other species absorbing in the same region of
the spectrum as the pyridinium ion, the method would have to be slightly modified to include measurements a t more than one wavelength and the use of a set of simultaneous equations tor calculating the concentrations of the component species (I). ACKNOWLEDGMENT
The author thanks A. K. Kiang for his interest and Mrs. H. K. Tong for the microanalyses. LITERATURE CITED
(1) Bauman, R. P., “Absorption Spectroscopy,” p. 403, Wiley, New York, 1962. (2) Hofmann, E., Arch. Hyy. Bakterzol. 128, 169 (1942). (3) LeRosen, H. D., Wiley, J. T., ANAL. CHEM.21, 1175 (1949). (4) Murman, R. K., Basolo, F., J. Am. C L m . SOC.77, 3484 (1955). (5) Podall. H. E.. ANAL.CHEM.29. 1423 . (1957). ’ (6) Spacu, G., Dick, J., Z. Anal. Chem. 76, 273 (1929). (7) Vogel, A. I., “A Textbook of Quantita-
tive Inorganic Analysis,” 3rd ed., pp. 132, 493, 497, 526, 529, 533, Longmans, Green and Co., London, 1962. K. P.
..\NO
Chemistry De artment, University of gingapore, Singapore VOL 38, NO. 10, SEPTEMBER 1966
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