lection process. Visual inspection of the spectra for those compounds that deviated the most from the expected relationships indicated that these spectra were generally less intense than those obtained for the compounds that showed the expected relationships. This was most noticeable for guanosine because it yields the least intense spectra of the four nucleosides. Since adenosine and cytidine yield the most intense spectra, the results of analyses involving these nucleosides came closest to the expected results. While the results reported here cannot be considered conclusive because of the limited data set studied and the limited resolution of the data collection process, we feel that they are sufficiently encouraging to warrant further studies with a larger and better defined data set, and that we have demonstrated a useful approach for the factor analysis of complex data sets.
LITERATURE CITED (1) R. J. Rummei, “Applied Factor Analysis”, Northwestern University Press, Evanston, Ill., 1970. (2) R. W. Rozett and E. M. Petersen, Anal. Chem., 47, 1301 (1975).
R. W. Rozett and E. M. Petersen, Anal. Chem., 47, 2377 (1975). R. W. Rozett and E. M. Petersen, Anal. Chem., 48, 817 (1976). J. 8.Justice, Jr., and T. L. Isenhour, Anal. Chem., 47, 2286 (1975). J. L. Wiebers and J. A. Shaplro, Biochemistry, 16, 1044 (1977). D. R. Burgard, S. P. Perone, and J. L. Wiebers, Blochemisfry, 18, 1051
.- . .
i,i w, n,
(8) J. A. McCloskey In “Basic Principles in Nuclelc Acid Chemistry”, Vol. I, P.O.P. Ts’o, Ed., Academic Press, New York, N.Y., 1974, Chapter 3.
D. R. Burgard S. P. Perone* Department of Chemistry Purdue University West Lafayette, Indiana 47907
J. L. Wiebers Department of Biological Sciences Purdue University West Lafayette, Indiana 47907 RECEIVED for review April 25,1977. Accepted June 6,1977. This work supported by NSF grant No. MPS74-12762, the Office of Naval Research, and NSF grant No. PCM76-21554.
Atomic Spectrochemical Measurements with a Fourier Transform Spectrometer Sir: In a preliminary report, we discussed the potential application of Fourier transform spectrochemical instrumentation to atomic spectral measurements (1). Since that time we have further developed our experimental system in order to improve our measurement capability in the visible and ultraviolet spectral regions. The Michelson interferometer that we have designed and built for application to Fourier transform spectroscopy has three optical inputs; a He-Ne laser, a white light (tungsten bulb) source, and the spectral signal of interest. Laser fringe referencing is used to sequence digitization and to control the velocity of the moving mirror using a phase-locked loop. The mirror drive system consists of an electromechanically driven mirror supported by an air bearing. The mirror movement is repetitive and both the scan rate and length can easily be set or altered. With control signals derived from the white light interferogram and the laser fringes, signal interferograms can be precisely time averaged. This system uses a unique “pretrigger” approach that allows acquisition of any desired number of signal interferogram data points both before and after the zero path difference position. We did not have this last feature when the data were acquired for Ref. 1. This meant that only “one sided” interferograms could be acquired with no points acquired before the central fringe of the signal interferogram. This made it essentially impossible to reliably phase correct spectra. With our present ability to acquire full double sided interferograms,i.e., an equal number of points on either side of the eero path difference position, excellent spectra can be calculated without using any phase correction procedures, i.e., by calculating the amplitude spectrum. As will be seen, this makes a considerable improvement in the quality of the spectra that can be measured with our system. In addition the interferometer is now interfaced to a larger computer (PDP 11/10) which is presently capable of 4 k transforms (floating point FORTRAN). This has allowed up to more fully use the designed resolution capability of the interferometer. Our previous maximum transform size was 812 points. The flame (Air-CzHz)emission spectrum of Li, K, Rb, and Cs as measured using our Fourier transform spectrometer with 1446
ANALYTICAL CHEMISTRY, VOL. 49, NO. 9, AUGUST 1977
four different sampling rates is shown in Figure 1. A Si photodiode was used as the detector. These spectra were calculated from 512-point double-sided interferograms (Le,, 256 points each side of zero path difference) and 50 scans were taken of each interferogram. The solution used to obtain the spectra shown in Figure 1 contained about 250 ppm of each element. In addition to illustrating the marked improvement in spectrum quality arising from the acquisition and processing of double-sided interferograms, this series of spectra also provides an excellent illustration of the utility of aliasing in providing optimized resolution and spectral coverage with a limited transform size. Since we use the standard He-Ne reference laser, the basic sampling interval for the interferometer system is 0.6328 pm. This means that the shortest wavelength of light that can be properly sampled without aliasing is 1.266 pm (7901cm-l). In order to measure the major emission lines of Cs, Rb, K, and Li in the near-IR (See Table IV of Ref. 1)without aliasing, a bandwidth of 15 803 cm-l is necessary. This can be achieved with our measurement system by frequency doubling the laser fringe signal using phaselocked loop techniques ( 2 )to provide a sampling interval of 0.3164 pm. However, with a 512-point transform limit the potassium and rubidium doublets cannot be resolved. This is shown in Figure la. The numbers (6,7,8) shown below the spectral peaks refer to spectral regions associated with the i-4 sampling rate (see Table I11 of Ref. 1). The arrqws indicate the direction of increasing wavenumbers. Using a 0.6328-pm sampling interval (Figure lb), resolution is doubled and the spectral information is aliased. With a sampling interval of 1.266 pm (Figure IC),the potassium doublet is resolved and the cesium lines have aliased into the potassium-rubidium region and in the last spectrum (Figure Id) measured with the 2.532-pm sampling interval, all lines are well resolved and all three regions overlap via aliasing. It is clear from this series of spectra that aliasing can be used to advantage in optimizing spectral coverage and resolution when making Fourier transform spectrochemical measurements.
1
I
15,803 c
I
I
K
(bl
7
8
l
-7
I
'
-6
11,852 cm
1
7901 cm-l
K
1i(0
I
393
37
I
15,803 cm-'
13,827 cm-' 9 8 7 6 cm-' 6+(Cesium)
1 AI
11,852 cm-' 11,852 cm-' Ma 382 9 3
1
K 766. A
(d)
K 769.8
R b 780.0
+,
7-
,---.+L.
26300
26100
..-----
L . "a __ 25900
25700
s-.y 25500
+ - 4 . r .
* ,
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~
__
.
25100
WAVENUMBERS
Figure 2. (a)Spectrum of a Mg-ACCa hollow cathode lamp from 3 1 604 to 23703 cm-' (316.4 to 421.9 nm) as measured with Fourier transform spectrometer. (b) Expanded plot of 26300 to 25 000 cm-' region. The wavelengths are given in nm
Cs 094 3
15,803 cm-' 11,852 cm" 11,852 cm-'
394 4 0 I
-8 (lithium) (Potassium, Rubidium) -6(Cesium)
13,827 cm-' 13,827 cm-' 9 8 7 6 cm-l
Figure 1. Flame emission spectrum of Cs, Rb, K, and Li measured with sampling intervals of (a) 0.3164 pm, (b) 0.6328 pm, (c) 1.2656 pm, and (d) 2.5312 pm. The wavelengths in Flgure I d are given In nm
The spectrum shown in Figure I d should be compared to that shown in Figure 6 of Ref. 1to note the improvement in quality as a result of measuring a double sided interferogram. It should also be pointed out that since the spectrum shown in Figure 6 of Ref. 1was calculated from a 512-point one-sided interferogram (2.532-pm sampling interval), it has a theoretical resolution of 8 cm-' as compared to 16 cm-l (both triangularly apodized) for the spectrum shown in Figure Id. At 770.0 nm, a resolution of 16 cm-l corresponds to 0.95 nm. These data clearly indicate that acquisition and processing of double-sided interferograms significantly improve the spectral quality of line emission spectra measured with a Fourier transform spectrometer. The phase correction procedures utilized in mid-IR Fourier transform spectroscopy are designed primarily for absorption spectra measured using broad band sources and are not particularly effective with line emission spectra. During a recent visit to our department, J. C. Polanyi (3) indicated that he had experienced similar difficulties in phase correcting infrared line emission spectra (infrared chemiluminescence) measured with a Fourier transform spectrometer, as has Griffiths (4)in the measurement of CO emission spectra. Finally, one might think that the phase correction procedures utilized in Fourier
transform NMR (5, 6 ) might be applicable to the interferograms shown in Ref. 1. We made some attempt to use these procedures but found that each aliased region required a different phase angle making it next to impossible to phase correct a spectrum such as that shown in Figure 6 of Ref. 1. Some results obtained using the larger data system are shown in Figure 2. The source was a Mg-Al-Ca hollow cathode lamp (Ne filler gas) and the detector was a 1P28 photomultiplier tube. The spectrum shown in Figure 2 was calculated from a 4096-point double-sided interferogram (Gaussian apodization) sampled at 0.6328-pm intervals. Fifty interferograms were time averaged resulting in an observation time of about 50 s. The spectral bandpass of the system was limited with a Corning 7-59 filter primarily to 31 604-23 703 cm-l (316.4 to 421.9 nm) [region 4 of Table I in Ref. 11 with some aliasing in from region 3 possible. The theoretical resolution was about 8 cm-l which is equivalent to 0.1 nm in the center of this region. While the complete 4096-point spectrum is too detailed to adequately show on a journal page, we have included a plot of 3000 points of the spectrum in Figure 2a to give the reader a feel for the spectral range simultaneously measured using region 4. It should be noted that the Ca 422.67 nm line is actually aliased into this region from region 3 (See Table I in Ref. 1). The region from 26300 to 25000 cm-l (621 points) is shown expanded in Figure 2a. Note that the two magnesium lines at 382.93 and 383.23 nm (Ah = 0.3 nm) are resolved to the baseline. The calculated wavelengths of these lines taken from four independently measured spectra are listed in Table I along with the literature values. These data indicate that both precise and accurate wavelength measurements can be ANALYTICAL CHEMISTRY, VOL. 49, NO. 9, AUGUST 1977
1447
analysis capability of our system (both qualitative and quantitative) utilizing flame, dc arc, and ICP sources.
Table I. Line Wavelengths Calculated from Spectra Measured with Fourier Transform Spectrometer Experimental values, nm Literature values, nm 1 2 3 4 Mg Mg Mg A1 A1 Ca Ca
382.93 383.23 383.83 394.40 396.15 393.37 396.85
382.88 383.16 383.76 394.33 396.10 393.31 396.77
382.88 383.16 383.76 394.36 396.10 393.31 396.77
382.88 383.16 383.79 394.36 396.10 393.31 396.80
LITERATURE CITED
382.88 383.16 383.76 394.33 396.07 393.31 396.77
made with our present Fourier transform spectrometer in this spectral region. At most, only one point jitter (0.03 nm) is observed and all wavelengths are accurate to 0.07 nm or better. Further studies on the application of Fourier transform spectrochemical instrumentation to atomic spectrochemical measurements are continuing in our laboratory. These studies include the assessment of the simultaneous multielement
(1) Gary Horlick and W. K. Yuen, Anal. Chem., 47, 775A (1975). (2) H. V. Malmstadt, C. G. Enke, S. R. Crouch, and G. Horllck, “Optimization of Electronic Measurements”, W. A. Benlamln, Menlo Park, Calif., 1974, p 67. (3) J. C. Poianyl, Departmentof Chemistry, University of Toronto, Toronto, Ontario, private communication. (4) P. R. Griffkhs, Pittsburgh Conferenceon Anamlcal Chemistryand Applied Spectroscopy, Cleveland, Ohio, Paper No. 440 (1977). (5) “Nicolet 1080 System for FT-NMR”, Nicolet InsWument Corporation, 5225 Verona Road, Madison, Wisc. 53711. (6)J. W. Cooper, “The Minicomputer In the Laboratory”, John Wlley, New York, 1977, p 301.
W. K. Yuen Gary Horlick* Department of Chemistry University of Alberta Edmonton, Alberta, Canada T6G 2G2 RECEIVED for review March 22,1977. Accepted May 11,1977.
Stoichiometry of Nitrogen Dioxide Determination in Triethanolamine Trapping Solution Sir: Nitrogen dioxide may be determined in air by a variation of the Saltzman method (1) in which the trapping solution is 0.1 N triethanolamine (TEA) in water (2). Several workers have proposed differing stoichiometries for the formation of nitrite ion in this trapping solution (2,3) based on the results of analyses of prepared atmospheres containing NO2 in low concentrations. However, the mechanism for the reaction has not been established. The absence of mechanistic hypotheses for this important reaction renders the elucidation of a trapping mechanism at relatively high NO2 concentrations important in providing a starting point for investigation of the stoichiometry a t low NOz concentrations. The present investigation was conducted on the reaction of gram quantities of NOz with equivalent amounts of TEA. In aqueous solution, equimolar amounts of nitrite and triethanolammonium nitrate were formed. When the reaction was run in methylene chloride with the exclusion of water, a precipitate having properties consistent with formulation as a nitroso ammonium salt could be observed. On the basis of these results, the following scheme is proposed for the reaction of equivalent amounts of NOz and TEA in an aqueous solution: 2 N 0 2 = N 2 O4 N204 + (HOCH2CH2)3N (HOCH2CH2)3NNO*NOi
I
+
H20
+
-
(HOCH2CH2)3NNO*
(HOCH2Cl$)3NH*NO;
NO; HN02
It
This reaction path requires a stoichiometric factor of 0.5 for the conversion of gaseous NO2 to nitrite ion.
RESULTS AND DISCUSSION Reaction of a slight excess of TEA in distilled water with a measured amount of NO2 yielded 96% of the theoretical amount of nitrite based on the proposed reaction scheme. In addition, 83% of the theoretical amount of recrystalized I1 could be recovered from the aqueous solution. Since no characterization of I1 could be found in the literature, its 1448
ANALYTICAL CHEMISTRY, VOL. 49, NO. 9, AUGUST 1977
structure was established by satisfactory elemental analysis, molecular weight determination by freezing point depression, NMR (Figure l),IR (Figure 2), and UV (Figure 3). When TEA was reacted with NO2at -5 “C in CH2Clzunder nitrogen with the exclusion of water, a white precipitate I, different from 11, formed immediately on addition of NOz. Compound I melted over a range (60-67 “C) with decomposition and vigorous evolution of gas. The qualitative UV spectrum obtained in methanol from freshly isolated I was different from the spectra of 11, NO2, or NaNOz in methanol (Figure 3). Gradual decomposition with evolution of gas occurred during warming to room temperature under both vacuum and a nitrogen atmosphere. Attempts to isolate I for elemental analysis were unsuccessful; however, its behavior is consistent with formulation as a nitroso ammonium salt. Under neutral or alkaline conditions the nitroso salt would be expected to yield nitrite on hydrolysis ( 4 , 5). Although isolation of I followed by reaction with water gave a nitrite yield of only 54%, immediate reaction with water in situ resulted in a nitrite yield of 86% based on the proposed reaction scheme. Formation of I1 and appearance of nitrogen dioxide above the solid when I was exposed to moist air can be accounted for by hydrolysis of I to yield nitrous acid which subsequently decomposes to nitric oxide and nitrogen dioxide. Also, in accordance with the results expected -for a nitroso ammonium intermediate, compound I1 could be recovered without recrystallization in 97% yield when the NO2 addition was run in methylene chloride in the presence of one equivalent of water. The formation of I from NOz is in accord with a number of observations in the literature. Spectroscopic evidence indicates that nitrogen dioxide in equilibrium with dinitrogen tetroxide may exist to some extent as a tautomer that can be formulated as nitrosyl nitrate (6). Consistent with this Tiding, dinitrogen tetroxide appears to heterolyze in solution to NO’ and NO3- (7, 81, which in the presence of TEA would be expected to lead to I. Reactions of primary and secondary amines with potential NO’ donors appear from kinetic considerations to proceed
