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Anal. Chern. 1983, 55,2000-2002
maining anions. Proceeding along these lines, it is found that the number of ways of reacting away the ions is A!R!. The number of physically different recipes, however, is only AlR!/2 because each process of sequentially forming A + R - 1 salts can be exactly reversed so that the last salt formed is the first and the first, last. The number of recipes is further reduced when some ions considerably outweigh others. For example, if the cation quantities a, b, c , ... and anion quantities r, s, t , ... are such that a 1 r + s, then a scheme which reacts away ions in the order a - r , a - s, ... will be equivalent to an otherwise similar one in which the order starts a - s, a - r. In general, the number of recipes is reduced whenever the sum of the quantities of a lesser number of ions of one charge type equais or exceeds the sum of a greater number of ions of the other type; each relationship such as a 1 r + s t or a b 1 r + s -tt , for example, takes its toll in the reduction. The number of recipes for a seawater with four cations and four anions is nominally 288 (4!4!/2) but this is reduced to only
+
+
104 (by an actual count) because of the preponderance of Na and C1 ions. Available from the author are short computer programs in interactive BASIC for the proration of ion quantities and the reacting away of ions. Input for the latter program consists of ion symbols and quantities; salt formulas and the respective quantities are obtained as output. Each order of ion input has the potential of producing a different recipe. Another available program permutes the ions and calculates all possible recipes containing up to and including four cations and four anions.
Frederick M. H o r n a c k Department of Chemical and Physical Sciences University of North Carolina Wilmington, North Carolina 28406
RECEIVED for review April 11,1983. Accepted July 11, 1982.
Fluorescence Spectrometry with Photodissociative Excitation of Ammonia and Hydrazine Sir: Standard fluorescence techniques are limited for analytical purposes by the relatively small number of substances which emit light. Even among those that do fluoresce, relatively broad and possibly overlapped emissions limit the usefulness of such methods. In general, only fluorescence from diatomic and triatomic molecules and radicals may be dispersed sufficently to be rotationally and vibrationally analyzed. Although bound state absorption spectra can be simplified by seeding the molecule in a supersonic expansion, this technique will not be useful when the electronic transition is between the ground state and a dissociative level, or where the density of vibrational states in the upper level is very high. In the past 15 years, a large body of work has shown that small excited fragments are produced when polyatomic molecules are dissociated by vacuum ultraviolet (VUV) photons. Okabe ( I ) and Ashfold, Simons, and MacPherson (2) have published comprehensive reviews which include this work. Most important to the experiments described here, the pattern of the fluorescence from the photodissociatively excited molecule is ideosyncratic, depending on both the identity of the parent molecule and the energy of the light impinging. Clearly, such a situation was to be expected, as the emission pattern is a measure of the distribution of excess energy among the degrees of freedom of the fragments, and this in turn is determined by the potential energy surface of the dissociative state of the parent molecule accessed by absorption of the VUV (vacuum-ultraviolet) photon. Indeed, the experiments mentioned above were undertaken in an attempt to explore these excited state surfaces and their effect upon photodissociation. Thus, the nascent quantum state distribution of the fragments is a measure of a particular excited state of the parent molecule reached by excitation a t a particular wavelength and the pattern of emission is a signature of both the identity of the parent molecule and the energy of the exciting light. There is a clear analogy to mass spectometry, where one is concerned with the fragmentation patterns of molecules and how such patterns are affected by changes in the ionizer energy.
In this paper we demonstate how these dissociative fluorescence patterns can be used to analyze mixtures of ammonia and hydrazine.
EXPERIMENTAL SECTION A block diagram of the experimental apparatus is shown in Figure 1. Briefly it consists of a Kr resonance lamp for excitation, a vacuum system through which the gas under study may flow slowly, and a fluorescence detection system. The latter is composed of a 0.6-m Jobin-Yvon HRS 2 monochromator equipped with an 1800 line/mm grating and having an effective aperture of f/4.9. Dispersed light was measured by a thermoelectrically cooled EM1 9789 photomultiplier, whose dark count rate was less than 2 counts/s. Light pulses were counted after being transformed to standard logic pulses in a 100-MHzOrtec discriminator. To measure the emitted spectra, we fed the signal into a multichannel analyzer, whose channel advance was driven synchronously with the monochromator’s scanning motor. Since ammonia and hydrazine produce excited NH radicals that emit in different spectral regions, mixtures could be analyzed by fixing the monochromator at the appropriate wavelength and simply counting the signal in a constant time period. Ammonia used in this work was Matheson electronic grade, 99.998% pure. Anhydrous, 97% purity hydrazine, acquired from Matheson, Coleman and Bell, was further purified by bulb-to-bulb distillation. It proved important to flow the gaseous samples through the cell in order to prevent the buildup of secondary products, which themselves could be photolyzed to produce excited fragments. The Pyrex fluorescence cell was equipped with a Wood’s horn so as to suppress scattered light from the lamp, and the interior of the cell wm coated with black Teflon. The side of the cell facing the lamp was a hemisphere with an O-ring groove on its end. Thus the lamp housing could be joined directly to the cell. The lamp itself projected slightly inside the cell and was sealed by a single MgF plate glued onto its end. This arrangement maximized the solid angle for irradiation, while holding the number of lossy elements between source and sample to a minimum. The design of the microwave excited lamp was similar to those described by McNesby and Okabe (3). The gas pressure was measured by a noninverted ionization gauge which had been reproducibly calibrated in its upper ranges against a 10-torr Baratron capacitance manometer. The ultimate
0003-2700/83/0355-2000$01.50/0 0 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 12, OCTOBER 1983
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Figure 1. Block dlagram d the experimental apparatus. a
Flgure 3. Total intergrated emisslon from ammonia as a function of pressure. The monochromator slits were centered at 324 nm with a band-pass of 0.3 nm. The collection time per point was 5 min. 5000
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Flgure 4, Total Integrated photodissociative excitation signal from hydrazine. The mo&chromator was centered at 336 nm and the
I 1
band-pass was set at 0.6 nm.
324 0
338
o 337 o
W a v e l e n g t h (nm)
Flgure 2. Photodissociative excitation spectrum of NH fragments from
the photolysis of (a) ammonia (NH(c'II-a'A) emission following 123 nm photolysis of ammonla)and (b) hydrazlne (NH(A311-X32) emisslon
following 123-nm photolysis of hydrazine). achievable vacuum in the system was 4
X
lo4
torr.
RESULTS AND DISCUSSION We chose to investigate ammonia-hydrazine mixtures because it has long been known that photolysis of ammonia in the VUV leads to formation of NH(clII) radicals, while excitation of hydrazine leads to creation of NH(A3rI) fragmenta. The ammonia photolysis has been investigated by Becker and Welge (4),Masanet, Gilles, and Vermeil ( 5 ) , Alberti and Douglas (6), and Quinton and Simons (7). The hydrazine
photolysis has been studied by Becker and Welge (4). To establish the signature of each parent molecule in our apparatus, we recorded an emission spectrum for pure samples. Examples of such spectra may be seen in Figure 2a, for alA ammonia, and Figure 2b, for hydrazine. The c'n emission can be resolved by our monochromator, and the fluorescence parameterized as being a Boltzmann distribution, in which the excited state has a population distribution of 3000 K. I t should be emphasized that this is just a convenient description of the rotational distribution of the excited state and does not mean that a thermal equilibrium with this temperature exists. The fluorescence spectrum of NH (A3n X327 from hydrazine shows the prominent Q bands of the triplet system for the (0,O) and (1,l)vibrational bands. Both the ratio of the vibrational bands for hydrazine and the rotational distribution for ammonia may be used as signatures in analysis. Studies were done on the intensity of the signal as a function of the pressure of the pure gas. Since the fluorescence regions
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Anal. Chem. 1903, 55,2002-2005
NH3+N2H4
MIXTURE a
PRESSURE=Bmicrons
324.0
336.0
b
"3
PRESSURE=I 6 microns
I 324.0
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Flgure 5. (a) Photodissociative excitation spectrum of a mixture of hydrazine and ammonia (1: 1). (b) Photodissociative excttation spectrum of pure ammonia.
for these gases are mutually exclusive, we simply opened the slits of the monochromator to pass the fluorescence from each gas and counted the total signal. Background signal could be measured by moving the monochromator to an adjacent position, where there was no emission. For ammonia the monochromator band-pass was set to 0.3 nm, centered at 324 nm, while for hydrazine we used a 0.6 nm band-pass centered at 336 nm. Figures 3 and 4 show the measured signals as a function of pressure. These plots show that there is a linear dependence of the signal on pressure in the region below 10 pm. Above this pressure the curves deviate from a linear behavior because of inelastic collisions between the excited radicals and the parent molecules. The parent molecules are always in excess because of the low photon intensity from the lamp. The actual point a t which this deviation will occur depends on the quenching cross sections between the excited fragments and the components of the ambient gas. In general, working at a pressure where the cross section would have to be a few times gas kinetic to compete with the radiative deexcitation should ensure that collisions play little or no role. Thus, pressures below 0.01 torr should be safe. At such pressures the time between collisions is about 10 ps, which is much longer than the radiative lifetime of most allowed transitions. T o show that samples could be analyzed when they were mixed, we prepared a 1:1 mixture. Figure 5 shows the observed emission spectrum from this mixture and from a reference ammonia sample. The clear signature of the hydrazine can be seen in the upper spectrum obtained by photodisso-
ciative excitation of the mixture and is not to be found in the lower excitation spectrum of the ammonia reference. The total integrated intensity of the emission in the ammonia and hydrazine emission regions of the spectrum is linear with both the pressure and the mole fraction up to the pressures of tens of micrometers. The actual sensitivity depends on the flux of photons from the W V source and the transparency of the lamp window. Although this is variable from day to day, a reference sample of some pure gas can be used to establish the sensitivity of the system at any time. Use of multichannel plate detectors would substantially increase the data collection rate of signatures. We have demonstrated for one case the method of photodissociative excitation. For reasons outlined above, different parent molecules should have unique fragment fluorescence signatures and therefore application of this method could be quite general. This emission signal is characteristic of the nascent distribution of the photofragments. Collisional processes would, of course, scramble the original pattern. Thus, such experiments must be done at pressures low enough that the fluorescent lifetime is much shorter than the mean time between collisions. This condition is satisfied for all but the highest pressures used in this work. It would be especially useful where other methods of analysis have narrowed the number of possible components, leaving the choice among a small number of similar molecules, or where there are known to be only one or two components in a mixture to be analyzed. Registry No. NH,, 7664-41-7; NzH4,302-01-2. LITERATURE CITED (1) Okabe, H. "Photochemistry of Small Molecules"; Wiley: New York, 1978. (2) Ashfold, M. N. R.; MacPherson, M. T.; Sirnons, J. P. In "Topics in Current Chemistry"; Springer Verlag: West Berlin, 1979; VoI. 86, p 1. (3) McNesby, J. R.; Okabe, H. In "Advances in Photochernlstry"; Noyes, W . A., Pitts, J. N.,Eds.; Interscience: New York, 1985; Vol. 3, p 157. (4) Becker, K. H; Welge, K. H. Z.Naturforscb., A 1963, 18A, 600. (5) Masanet, J.; Gilles, A.; Vermeil, C. J . Pbotochern. 1974/75, 3 . 417. (6) Alberti, F.; Douglas, A. Chem. Pbys. 1978, 3 4 , 399. (7) Quinton, A. M.; Sirnons. J. P. J . Chem. SOC.,faraday Trans. 2 1978, 78, 1261.
'
Present address: Department of Chemlstry, University of the District of Columbia, Washington, DC.
Joshua B. Halpern* Edmund B. Koker' William M. Jackson
Laser Chemistry Division Howard University Washington, D.C. 20059
RECEIVED for review May 13, 1983. Accepted July 11, 1983. The authors gratefully acknowledge the support of the National Aeronautics and Space Administration under Grant No. NSG 7378.
Laser Desorption Chemical I onization Mass S pectr ometry / Mass Spectrometry Sir: The first application of lasers to mass spectrometry was reported in 1963 (I) shortly after the first observation of laser action. Now, 20 years later, much research has been done in this area (2) and a second generation laser microprobe mass spectrometer (Leybold-Heraeus, LAMMA 1000) has been
commerciallyavailable for some time ( 3 , 4 ) . Two reviews have covered the principles of the technique (5)and its application to structural analysis (6). Cotter (7) has shown the benefits of operating the ion source in the chemical ionization (GI) mode to take advantage of the
0003-2700/83/0355-2002$01.50/00 1983 Arnerlcan Chemical Society
