1585
V O L U M E 23, NO. 11, N O V E M B E R 1 9 5 1 second flavonoid aglycone of unknown constitution. Quercetin obtained from buckwheat, by hydrolysis of rutin, was free of this impurity. Purified hesperidin contained a small amount of a second flavonoid glycoside. This could be largely removed by preliminary extraction of t.he relatively insoluble hesperidin with hot water.
California Fruit Growers Exchange, Research Department, Ontario, Calif. This investigation was supported in part by research grants from the Office of Naval Research (Project NR-059-226) and the Division of Research Grants and Fellowships of the National Institutes of Health, U. s. Public Health Service.
ACKNOWLEDGMENT
LITERATURE CITED
The authors wish to express their appreciation to the following individuals and organizations for the donation of some of the flavonoid samples used in this investigation: Charles H. Horton, Plant K-25, Oak Ridge, Tenn.: T . R. Seshadri, University of Delhi, Delhi, India; Joseph Pew, Forest Products Laborator! , Madison, Wis. ; Pharmacology Laboratory, Bureau of Agricultural and Industrial Chemistry, -4lbany 6, Calif.; and The
(1)’ Bate-Smith, E. C., “Biochemical Symposium KO.3,” Cambridge. Cambridge University Press, 1950. ( 2 ) Bate-Smith, E. C., and Westall, R. G., Biochim. Biophys. Acta.. 4, 427 (1950). (3) Wender, S. H., and Gage, T. B., Science, 109,287 (1949). RECEIVED February 1, 1951. Presented before the Division of Biological Chemistry a t the 118th Meeting of the AMERICAN CHEIIICAL SOCIETY,
Chicago, Ill.
X-Ray Photoelectron Spectrometer for Chemical Analysis RALPH G. STEINHARDT, JR., AND EARL J. SERFASS Lehigh University, Bethlehem, P a . Of all the many and diverse known types of charac; teristic spectra, the only one which has not been investigated previously from the analytical viewpoint is the x-ray photoelectron spectrum. Because of the low energies of x-ray photoelectrons, only electrons that originate close to the surface of an x-ray bombarded target contribute to the characteristic spectral edges. A n instrument is described which measures x-ray photoelectron intensities at energies from
I
F A beam of x-radiation is allowed to strike a solid target, two types of radiation will be emitted by the target: fluorescent x-rays and photoelectrons. Both types of radiation exhibit spectra that are characteristic of the material of which the target is composed. Although fluorescent x-rays have been used for chemical analysis ( I O ) , x-ray photoelectrons have not. This communication describes an instrument designed for the utilization of x-ray photoelectron spectra for chemical analysis. THEORY
The emission of photoelectrons and fluorescent x-rays through excitation by x-rays is governed by the equation:
T = hu,
- hu
-
wo
(1)
in R-hivh I’ = kinetic energy of photoelectron h = Planck’s constant L’. = frequency of exciting radiation v = frequency of fluorescent radiation w,, = work function of target material If u0 is such that the exciting radiation lies in the “soft” x-ray region (lo1*to 1019cycles per second), two conditions may be imposed on Equation 1:
(hu, - hv)>>w, hu,
hu
(2) (3)
The condition indicated in Equation 2 alters Equation 1 to:
T = h(u,
- u)
(4)
The condition indicated in Equation 3 implies that the energy of the emitted electron will be relatively small. Several important conclusions may be reached on the basis of the above:
6 to 17.5 k.e.v. and is shown to be capable of yielding qualitative and quantitative data for elements having atomic numbers greater than about 25. The instrument is a 180” magnetic deflection electron energy selector using a high-intensity x-ray tube for excitation and an ultra-thin window Geiger-Muller counter for detection. The potentialities of the instrument for performing atomic surface analyses would appear to be considerable.
If the frequency of the exciting monoenergetic radiation is maintained constant, the initial energy of the photoelectron is characteristic of the tarket material only. The energy of photoelectrons emitted under these conditions will be of the order of 0 to 17.5 k.e.v. Therefore only that portion of the target on or very close to the surface will emit electrons capable of escaping from the target. Furthermore, photoelectrons ejected from atoms lying beneath the surface will be partially absorbed and many of them will escape only after considerable energy loss; only those ejected from surface atoms will experience a virtually undetectable energy loss in escaping. Because of this absorption process, the low energy side of a characteristic photoelectron peak will exhibit an exponential rise in intensity while the high energy side will have an almost vertical descent. The energy defined by this vertical edge will be characterized by the chemical composition of the surface. The energy of the photoelectron will be practically unaffected by changes in the work function of the target, since wo is of the order of several electron volts. Thus it appears that n-ith an instrument capable of measuring the energies and corresponding intensities of x-ray photoelectrons it should be possible to perform a quantitative analysis of the nominal surface of a solid regardless of the physical characteristics of the surface. Furthermore, because the emitted electrons have their origin in the inner energy levels, the state of combination of an atom will usually not affect its x-ray photoelectron spectruni to any appreciable extent. The typical photoelectron spectrum is not as simple as might appear on the basis of Equation 4. The reasons for this are a9 follows: The incident radiation is not perfectly monoenergetic. The use of a crystal x-ray monochromator is not practicable because of the high intensity of incident radiation necessary to excite a sufficient number of photoelectrons to be measured precisely. The space required for a crystal monochromator would reduce seriously the intensity of the incident radiation through the operation of the inverse-square law. Instead, a Hull (11) filter is used, Further reduction of the contribution of the con-
ANALYTICAL CHEMISTRY
1586 tinuous sDectrum is effected by purely Kenmetric means. Tho
tensity of the exciting radiation, some form of intensity control must be provided.
electric vector (9, ld, 18, 19). 'As the plane of t h e electric vector is that defined by target, anode, and cathode, orientation of the spectrometer perpendicular to this plane will reduce the efficiency of the continuous spectrum in producing photoelectrons that will pass through the spectrometer. The contribution of the characaeteristic radiation is unaffected, as it is not polarized (81). Photoeleotrons may be ejected from any of several electronic energy levels. Aurrer electrons are Droduced bv the action of fluoresoent r a d i a h of the target bn its own itoms (f-4). However, the possibility, in a complex sample AB, of the excitation of photoelectrons from A by fluorescent radiation from B may usually be discounted (7, 8). Absorption and scattering of photoelectrons by the target result in broadening of the theoretically predicted peaks 17, 18). Absorption of the incident radiation depends upon t e mass ahsorption coefficients of the elements in the target. For targets composed of two or more elements, therefore, the slope of the low energy side of a particular peak will he a function of the
DESIGN AND CONSTRUCTION
h
of eiectrons ds their energies decrease. Electr& with energies less then about 6 k.e.v. are oompletely absorbed by the window.
Any x-my photoelectron spectrometer must consist of three hasic units:. (1) x-ray source, (2) electron energy selector, and (3) electron detector. In addition, a high vacuum system is needed t o produce a pressure in the energy Belector sufficiently low so that the mean free path of the photoelectron is long compared to the length of its path through the selector and to prevent serious attenuation of the electron heam. In the instrument which has been constructed, x-rays are produced in an x-ray tube, the intensity being regulated a t a preselected value by independent automatic regulation of both current and voltage. The x-rays are filtered by passage through a zirconium oxide filter and then are allowed to strike a sample. Photoelectrons are ejected from the sample and, under the influence of a controlled magnetic field, are deflected through an angle of 180'. They are detected by means of an ultra-thinwindow Geiger-Miiller counter, the pulses of which itre recorded on a scaler. By determining the counting rate at various intensities of the magnetic field, the x-ray photoelectron spectrum of the sample may he established. The present spectrometer differs from previous instruments (14, $0) mainly in the use of a counting tube for detection purposes. Previously photographic methods were used exclusively. This method of detection, while capable of yielding results of great precision in peak location, is not itpplioahle to analytical chemical problems for the fallowing reasons: Excessively long determination time (8 to 36 hours). Low signal-noise ratio inherent in open-ohamher camera. Densitometer must be employed for quantitative results. Continuous internal stsndardimiion is necessary for quantitntive results. The general appearance of thc instrument is shown in Figures
1and 2. In Figure 1 the spectrometer itself is located on top of the cabinet with the x-ray high-voltage transformer, filament transformer, high-voltage resistor, and Kenotron within t.he cabinet. Figure 2 shows the control panels, On the right is the
Figure 1. X-Ray Photoelectron spectrometer
The use of the spectrometer far quantitative analysis is further complicated by the fact that the number of characteristic photoelectrons emitted (for a given intensity and spectral distribution of incident radiation) is not simply proportional to the number of specific atoms in the target. The efficiency of production of photoelectrons increases spproyimately as the fifth power of the atomic number. Empirical calibration is necessitated by consideration of all the factors listed above. The intensity of the incident x-ray beam is a function of both tube voltage and current. As the number of photoelectrons emitted from any given target is strictly proportional to the in-
Figure 2.
Control Panels
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V O L U M E 23, N O . 1 1 , N O V E M B E R 1 9 5 1
ordinates of all charts are in counts per minute and the abscissas are in arbitrary units of magnet current meter reading. ( T o definite relation between magnet current and magnetic flux can be dranm a t present, for reasons which are given below.) The usual background was of the order of 12 to 15 counts per minute. Yo effect of the x-rays on background was noted and the counting tube was not photosensitive. Counting periods were such that the mean deviation did not exceed 5%. Plateaus were usually established before each spectrum deterniination and were from 50 to 100 volts in extent with slopes of 5 to 15%. The counter was opeiated a t about the middle of tht. plateau. 60
Figure 3. Schematic Diagram of Spectrometer Tube
s-r:iy control and on the left are the vacuum gage circuits, iiiiignet power supply, and scaler. The tube is a Machlett (Machlett Laboratories, Inc., Springdale, Conn.) Model il-2 equipped with a molybdenum target and beryllium windows. The current control circuit was adxpted with minor modifications from the circuit of LeMieus and Beeman (16) and is an inverse feedback stabilizer which continually npplies a correction to an impedance in series n-ith the primary of the x-ray filament transformer. The voltage control circuit is t)wed upon the same principle. The electron energy selectoi, (Figure 3 ) is a semicii,c.ular W J I ~ W tube (2.5 em. inside diameter) mounted between the pole faces of an iron electromagnet. The x-rays enter through tutw 9, impinging upon the sample B at an angle of 70". The elwtron beam, deflected by a magnetic field, is defined hy a ctircular opening at C and a detector slit a t D. Scatter is reduced hy circular openings a t E . All slits and baffles are beveled t o present a knife-edge to the approaching electrons in order to i ~ d u c escattering. The Geiger-Muller counter is located at F : i i i t I tlir high vacuum connection is at G. Plane of the ~ p e c 1 roiveter is perpendicular to line connecting x-ray anode and c.:itho.lc. Radius of curvature of the tube is about 7.8 em. and r,rsolution ( 1 5 ) is 0.014. Interior of the tube is coated with ro1loid:il graphite t o reduce secondary electron emission. The detection of x-ray photoelectrons with a Geiger-lliIc,ohol. The argon is dried by pussing through anhydrous magnesium perchlorate. Pressures of 7 . 5 to 9.0 em. of mercurJ- Lire rustomarily employed. In order to reduce pressure changes clue t o diffusion of gas through the window,,a , Miter gas reservoit, is connected t o the counter while the latter is in operation. r 7 l h e high vacuum systems consist of 3 D P I GF-25 three-stage i'ractionating diffusion punip (Distillation Products, Inc., 755 Ridge Road West, Rochester 13, S . Y.) hacked with a Ceilco 1Iegavac mechanical pump (Central Scientific Co., 1iOO Irving Park Road, Chicago 13, Ill.). h Pii,ani gage measures pressure on low vacuum side of diffusion pump, while a DPI VG-l.4 ionization gage measures pressure 011 high vacuum side. CRITICAL STUDY OF SPECTROJIETER OPERATION
In plotting spectra, no edges or peaks are drawn unless the?. are clearly defined and have been found to he reproducible. The
50
40 W I-
3
z
#JO
P z
2 0 0
2c
IC
0
I
05
Figure 4.
06 07 OB MAGNET CURRENT
I
I
0.9
ID
X-Ray Photoelectron Spectrum of Copper
The following spectra were obtained: 1. 2. 3. 4. 5. 6.
Copper (Figure 4) Zinc (Figure 5 ) Silver (Figure 6) Gold (Figure 7) Brass (Figure 8) silver-20 atoni % gold alloy (Figures 9 and 10) 80 atom
All metals used as target materials were at least 99.9% pure. The brass was a 67% copper-33% zinc alloy (by weight) and the silver-gold alloy was prepared in the laboratory to give an 80 atom % silver-20 atom % gold mixture. 111 spectra were obtained using an x-ray beam a t 50 kv.p. anti 10 ma. In the majority of cases the spectrometer pressure w ~ s from 2 X 10-6 to 4 X 10-6 mm. of mercury. The time required to make a single spectrum determination was from 2 to 4 hours. lIechanism assignments were based on the determination of flux inteiigity with a General Electric gauss meter and the measureneiit of the radius of curvature, p, of the spectrometer foll o ~ e dby a comparison with the theoretical value of H p . Beeausc of the low precision of the fluu meter, mechanism assign-
1588
ANALYTICAL CHEMISTRY
ments must be considered as tentative. [Because photoelectron energy is a difference function (see Equation 4), the resolution decreases rapidly a t high energies.] The spectrum of copper (Figure 4) clearly shows a double edge a t magnet current meter readings of 0.555 and 0.610. These are probably due to the interactions MoKa-CuKp and MoKaCuKa. The edge a t 0.795 is due to MoKor-CuL.
-
I
600
So0
-
400
-
W
t
3
z
f
edge and is not evident. Slight displacements of edge positions are noticeable on comparison of the brass spectrum with the spectra of copper and zinc. This is caused by the impossibility of obtaining precisely the same value of magnetic flus intensity for a given value of magnet current, owing to the low level a t which the magnet is operated. The spectrum of brass clearly shows the ability of the spectrometer to perform qualitative analyses. The difficulty of such an analysis with elements having atomic numbers as low as 30 and differing by only one unit is great, as the low counting rate might be expected to make peak definition inadequate for positive differentiation. In spite of the proximity of the main edges of gold and silver, a quantitative analysis of the 80 atom % silver-20 atom % ’ gold alloy was made. The results, although not of high precision, clearly indicate the potentialities of the s-ray photoelectron spectrometer. Comparing Figure 9 with Figures 6 and 7, the contributions of the gold photoelectrons on the low energy side of the major silver MoKa-AgLp peak again testify to the ability of the instrument to perform qualitative analysis. However, it is the heights of the MoKa-AgLp and MoKa-.4uM edges that are of importance in the quantitative analysis. Let
a Y a m -
= measured height of Ag peak in alloy HaU = measured height of Au peak in alloy H i b = measured height of Ag peak in pure Ag H i U = measured height of Au peak in pure Au H i K = measured height of Ag peak in pure Ag a t .4u peak position H i u = measured height of .4u peak in pure Au a t Ag peak position XAg = relative atomic concentration of Ag in alloy XAu = relative atomic concentration of Au in alloy H.kg
v)
t
a
0
200
-
100-
1100-
a4
05
0.6 a7 a8 MAGNET CURRENT
as
1.0
Figure 5 . X-Ray Photoelectron Spectrum of Zinc 900
The spectrum of zinc (Figure 5 ) is similar to the copper spectrum, as would be expected. The MoKa-ZnKp edge is clearly visible a t 0.575. The MoKa-ZnI
