nential. The length L in this system is the length of the dye cell. During one round trip in the laser cavity, the light passes through the dye cell twice and is reflected once off each end mirror. The intensity of the beam 1 may now be expressed as:
I = I o R I R z exp(2aL) (2) where R1 and R2 represent the reflectivities of the two mirrors. If the beam must also pass through an absorbing medium with transmittance T , the resulting intensity after one round trip is:
I
=
I,R,R,T2 exp(2aL)
(3)
Setting R equal to the geometric mean of the two reflectivities (RIR2)l and y = -In RT, Equation 3 becomes:
r)]
(4)
- r)]
(5)
I = I o exp[frXaL After n round trips, the intensity will be: I
=
I o exp[%z(cuL
The ratio of the intensity of the laser when an absorber is present inside the cavity, I , to that when no absorber is present, I',is:
where y' = -In R since T will be 1 when no absorber is present. Equation 6 can be simplified to: I' 1 (7, logF = 2n logT Note that log (Z'/Z) is the apparent absorbance A , and log ( 1 / T )is the actual absorbance of the sample A , which was inserted inside the cavity. Thus Equation 7 becomes:
A , - = 2nA.
(8)
According to Equation 8, the apparent absorbance should be linearly related to the actual absorbance, the amount of enhancement should be equal to twice the number of round trips for the laser pulse in the cavity, and the enhancement should increase as the cavity length is decreased. Laser pumped dye lasers have pulse durations in the range of 5 to 20 nsec (14, 15). This would allow about 15 round trips for a 10-nsec pulse in a 10-cm cavity and, hence, a predicted enhancement of about 30, which is in the range of the observed enhancements. Equation 8 can only be considered as providing qualitative agreement with the observed behavior of the enhancement. In particular, the important time dependent characteristics of the dye laser have not been considered in its derivation. The dye laser is homogeneously broadened with all modes in rapid communication with each other. In addition, the pulse duration of the dye laser is a complicated function of many parameters such as the shape, intensity, and duration of the pump (ruby laser) pulse, the various lifetimes of the excited states of the dye, and the nature of the solvent used for the dye. A detailed study of the time dependent characteristics of the dye laser as a function of wavelength when an intra-cavity absorber is present would be necessary in order t o draw more definitive conclusions. Received for review June 20, 1973. Accepted August 24, 1973. Financial support by the University of Alberta and the National Research Council of Canada is gratefully acknowledged.
(14) 6. 6. McFarland, Appl. Phys. Lett.. 10, 208 (1967) (15) M. Bass and J. I. Steinfeld, / € E € J. Quantum Electron., 4 , 53 ( 1 968).
Surface Analysis of Platinum/Tin Alloys by X-Ray Photoelectron Spectroscopy Robert Bouwman and Paul Biloen Koninklijke/Shell-Laboratorium, Amsterdam (Shell Research B. V . ) , The Nethedands
X-Ray photoelectron spectroscopy (X-PS, also named ESCA) comprises irradiating a solid to be studied with monochromatic X-rays of energy hv ( e . g . Mg Ka, hv = 1253.6 eV or A1 K u , hv = 1486.6 eV) and monitoring the energy distribution of the electrons emitted from the solid (1, 2). The electron energy distribution is characterized by well defined peaks, the major proportion of which can be attributed t o inner shell ionizations of the atoms of the solid. The electrons have a characteristic kinetic energy, E, equal to E = hv - I - e&, where Z is the ionization energy of the core electron measured relative to the Fermi level and & is the work func-
tion of the spectrometer entrance. Depending on the X-ray source used (see above), E is between 0 and 1480 eV. This range of E values corresponds to a mean free path of inelastic scattering of the order of 10 A (3, 4 ) . It is thus evident that X-PS essentially belongs to the group of surface analysis techniques. Surprisingly, however, X-PS has found little application in this field so far. In most cases, it is used exclusively to monitor changes in the charge distribution on atoms by measuring the chemical shift S E of the characteristic electron lines, caused by a shift in the ionization energy, AI, of the core electron. We have made an X-PS study of platinum/tin alloys whose surface composition, as recent Auger spectroscopic work ( 5 ) has shown, can be changed considerably under
et a/., "ESCA-Atomic, Molecular and Solid State Structure Studied by means of Electron Spectroscopy," Almqvist and Wiksell, Uppsala, 1967. (2) w. N. Delgass. T. R. Hughes, and C. S. Fadley. Catal. Rev., 4, 179 (1970).
(3) J. C. Tracy, Nato Summer School on Electron Spectroscopy, Ghent, Belgium, 1972. (4) C. R. Brundle in "Surface and Defect Properties of Solids," Vol. I , The Chemical Society. Burlington House, London, 1972, p 171 (5) R. Bouwman, L. H. Toneman. and A. A. Holscher, Surface Sci.. 35. 8 (1973).
(1) K. Siegbahn
136
A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 1, J A N U A R Y 1974
ELECTRON
Table I. Peak Areas (Arbitrary Units) and Average Surface Compositions as Measured in the Photoelectron Spectra of Oxidized and Reduced PtSn and PtaSn
PtSn reduced PtSn oxidized PtsSn reduced Pt3Sn oxidized
I
INTENSITY, ARBITRARY UNITS
REDUCED PtSn
Sn 3d5,2
Pt 4f712
Sn/Pt
Average surface comp. (at % Sn)
2.92 4.30 1.24 3.04
1 .oo 0.064 1 .oo 0.64
2.9 66.5 1.2 4.7
53 96 32 65 METAL
Table II. Binding Energies as Derived from Line Positions in the Photoelectron Spectra of Reduced Platinum, Tin, and Platinum/Tin Alloys
1
80
78
76
74
72
490
488
486
484
482
System
Pt3Sn PtSn
/ Sn3ds
486 6 4857 485 5
a Reference materials gold 417 per 2p3 2 932 2 eV
2
2
I Pt4f7
2
71 8 71 5 72.0
BINDING ENERGY,eV
Figure 1. X-Ray photoelectron spectra of reduced PtSn
A/
414.8 f 0 2 4142f02 4135f02
83 8 eV. graphlte 1s 284 3 eV cop-
I , Sn 3d5
PtSn oxidized PtsSn reduced Pt&n oxidized
..
I
P! 4 f ~ , ~ OXIDE
Table I l l . Change of the Binding Energies as Derived from the Line Positions in the Photoelectron Spectra of Oxidized and Reduced PtSn and Pt3Sn PtSn reduced
ipti
1
Line positions, eVa
Pt and Sn metals
m
480 ISnl
2
485.5 486.4 485.7 486.7
I, Pt 4f7
Pt 4 f
,,* OXIDE
,.
METIL
..
2
72.0 75.1 71.5 71.1
OXIDE
1
80
490
1
,
I
78
76
74
72
70 I P I I
488
486
404
482
480 1%)
811r3ING ENERGY e i
the influence of heating in vacuo, oxidation, and reduction. The photoelectron spectra of two alloys, PtSn and PtsSn, both stable intermetallic compounds ( 6 ) , were measured with a Varian IEE-15 spectrometer using a n aluminum anode as X-ray source. Powdered alloy samples were mounted by pressing them into the grooves of a stainless-steel cylinder. This assembly was then introduced into a specially designed flow cell which permits gas admission and heating a t a predetermined temperature. The sample ,holder, the spectrometer entrance. and the flow cell were equipped with ball valves and evacuable sluices, so that the samples could be transferred from the cell into the spectrometer without exposure to the atmosphere. Treatment was performed with either hydrogen or oxygen under 1 bar pressure and a t a temperature of 550 and 500 “C, respectively. Figures 1 and 2 show spectra of the alloys after treatment. For both alloys, the oxidation results in a marked decrease of the platinum signal, measured relative to the tin signal. The changes induced by reduction are opposite (Table I). These phenomena are reversible. The line intensity ratios were converted into “average surface compositions” with the aid of elemental sensitivity data obtained by Wagner (7). [The term “average surface composition” as here used is the composition averaged over the topmost atomic layers within the escape depth of the relevant photoelectrons. It should be borne in mind that this is a weighted value because of the expo(6) M. Hansen, “Constitution of Binary Alloys,” McGraw-Hill, New York. N . Y . . 1958, p 1142. ( 7 ) C . D. Wagner,Ana/. Chem.. 4 4 , 1050 ( 1 9 7 2 ) .
Figure 2. X-Ray photoelectron spectra of oxidized PtSn Vertical display of upper spectrum enlarged with respect to lower spectrum and to those of Figure 1
nential attenuation of the photoelectron signal with increasing depth of the solid. A fuller explanation is given elsewhere (8).] The values are given in the last column of Table I. The line positions of the platinum and tin peaks converted into binding energies are set out in Tables I1 and 111. It can be seen from Table I t h a t after oxidation the surface is considerably enriched with tin. After reduction. the average surface composition is much closer to that of the bulk but still richer in tin. These surface enrichment effects are qualitatively in agreement with Auger spectroscopic results ( 5 ) . They can be understood on the basis of the interaction between the atoms at the gas-solid interface, as discussed below. The driving force for surface enrichment is. in general. the reduction in surface free energy, the latter being determined by the mutual interactions between substrate atoms and interactions between substrate atoms and adatoms. Large mutual interactions between substrate atoms are reflected by a large heat of sublimation or a high surface energy. Consequently, surface enrichment with the component having the smallest heat of sublimation is favored. Substrate atom/ad-atom interactions give rise to surface enrichment of that substrate component which has the strongest interaction with the ad-atoms. The extent of this interaction is adequately described by the heat of adsorption. A comparison of the heats of ad(8) R Bouwman and P Biloen, Surface SCJ in press
ANALYTICAL CHEMISTRY, VOL.
46, NO. 1 ,
JANUARY
1974
137
sorption of 0 2 and Hz on Pt and Sn discloses that tin is likely to be enriched in the surface by oxygen chemisorption, whereas platinum is enriched by hydrogen chemisorption. The combined impact of both types of interaction gives rise to the expectation of tin enrichment in the surface by oxygen chemisorption at elevated temperature. For hydrogen chemisorption, such a prediction is not possible; but our experimental results show that the surface is enriched with tin also in this case, if to a smaller extent. A detailed discussion of the chemisorption-induced surface enrichment phenomena of the platinum/tin system has been published elsewhere ( 5 ) . The high-energy resolution attained in X-PS permits the observation of chemical shifts in the line position, AE Alloying as such causes the line position of the Sn 3d5/2 and Pt 4f7/2 peaks to shift as is shown in Table 11. It is interesting to note that the Sn 3d5,2 peak is found a t an ionization energy which is about 1 eV less for the alloys than for pure tin, whereas the Pt 4f712 peak is subject to much smaller effects. The excess negative charge a t the tin atoms in the alloy, indicated by the line shifts of the 3d line, is not in agreement with Knight shift and Mossbauer data (9) on Pt/Sn alloys which suggest a transfer of the Sn 5s electron to the Pt 5d band. From Table 111, it is seen that oxidation of PtSn causes a shift of approximately 1 eV of the Kanekar, K R P Mallikarjuna Rao, and V Udaya Shankar Rao, Phys Lett 19,95 (1965)
(9) C R
Sn 3d5,2 line and of approximately 3 eV of the Pt 4f7/2 line. The latter value is in close agreement with the finding of Kim et a / . (10) for the Pt/PtOz system. Oxidation of PtsSn causes a AE value of the Sn 3d line similar to that in PtSn but the Pt 4f line remains at about the same position. The above-mentioned results suggest that platinum is thoroughly oxidized when present in a diluted form in a tin-oxide matrix (see surface concentration in Table I). Our results prove that the surface enrichment effects studied by AES are detectable by X-PS as well. For this particular alloy system, the kinetic energy of the relevant photoelectrons (-1000; -1400 eV) is much greater than that of the relevant Auger electrons (-400; -250 eV). As the mean free path for inelastic scattering increases with the kinetic energy in the above energy range, X-PS probes the alloy to a greater depth than AES. This is reflected in the average surface concentrations (see Table I) which lie closer to the corresponding bulk values for the reduced alloys studied by X-PS than by AES. The difference in probing depth of the two techniques on one and the same alloy system provides the opportunity to study the concentration profile between the tin-enriched surface and the underlying bulk in more detail (8). Received for review May 14, 1973. Accepted August 27. 1973. (10) K S. Kim, N. 6296 (1971)
Winograd, and R E. Davis, J Amer Chem S o c , 93,
Simple, Sensitive Spectrophotofluorometric Method for Hydrazine in Plasma Stanley Vickers and
E. K. Stuart
Merck lnstitute for Therapeutic Research, West Point, Pa. 19486
The use of hydrazine and its derivatives as sources of energy and as chemotherapeutic tools has prompted interest in their toxicity and metabolism, and has generated methodology for their measurement in plant and animal tissues. Watt and Chrisp ( I ) developed a spectrophotometric analytical assay based on the observation of Pesez and Petit ( 2 ) that hydrazine and dimethylaminobenzaldehyde (DMAB) produced a yellow colored derivative. Investigators utilizing the DMAB procedure have obtained information on the dose-blood level relationship of hydrazine in rats ( 3 ) and dogs (4, 5 ) a t the microgram level. This report shows how DMAB may be used to measure nanogram quantities of hydrazine in plasma. Dimethylaminobenzalazine, which is the chromophore utilized in the above spectrophotometric assay, also fluoresces intensely in chloroform solutions saturated with trichloroacetic acid. The application of this fluorometric method enabled levels of hydrazine to be measured in (1) G. W. Watt and J. D. Chrisp, Anal. Chem., 24, 2006 (1952). (2) M . Pesez and A . Petit, Bull. SOC. Chlm. France, 1947, 122. (3) 8. A . Reynolds and A . A . Thomas, Amer. lnd. Hyg. Ass. J . , 26, 527 (1965). (4) H . McKennis, J r . , J . H. Weatherby, and L. 8 . Witkin, J. Pharmacol. ~ x p Ther., . 114, 385 ( 1955). (5) F. B. Smith and D. A . Clark, Toxicol. Appi. Pharmacoi.. 21, 186 (1972).
138
monkey plasma after administration of a much smaller dose than previously required by the colorimetric method.
EXPERIMENTAL Apparatus. Fluorescence was measured in an Aminco-Bowman spectrophotofluorometer using quartz cuvettes (10 mm lightpath) at an excitation wavelength of 466 nm and an emission wavelength of 546 nm (the instrument was calibrated by the use of quinine sulfate). Reagents. Hydrazine hydrate (99-lOO%, Matheson. Coleman & Bell), trichloroacetic acid (Baker Analyzed), chloroform (Fisher Analytical), p-dimethylaminobenzaldehyde (Fisher Certified), and anhydrous magnesium sulfate (Baker Analyzed) were used in this method. Authentic dimethylaminobenzalazine was prepared by mixing a solution of hydrazine hydrate (1.6 g) in ethanol ( 4 ml) with a solution of p-dimethylaminobenzaldehyde ( 5 g) in hot ethanol (20 ml). The yellow precipitate was filtered and recrystallized from dimethylformamide. Dimethylaminobenzalazine was identified by mass spectral data: mp 274" [lit. (6) 264-266 "C]. Anal. Calcd for C I ~ H Z ~ N C,~73.45; H, i.48; N , 19.05. Found: : C, 73.52; H, 7.85; N, 18.98. Procedure. Plasma (1 ml) was mixed with aqueous 10% trichloroacetic acid ( 3 ml) and centrifuged (15 min). The supernatant was removed as completely as possible and shaken with chloroform ( 2 ml) for 15 min; the aqueous layer was removed as completely as possible and mixed with 0.4% ethanolic p-dimethyl(6) P. R .
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, N O . 1, J A N U A R Y 1974
Wood, Anal. Chem., 25, 1879 (1953).
