CHROMATOGRAM
I
~
7
STRIP CHART RECORDER
I
A
1
1
Figure 1. Schematic of experimental setup
detection electronics permits the reduction of elimination of unwanted light since the use of a window may exclude or reduce the effect of phosphorescence and scattered laser light; and d ) the use of time-resolved as well as wavelengthresolved detection may permit the analysis of a mixture of fluorescent species without the need for their separation. Much work remains, such as the demonstration of linearity with aflatoxin concentration; however, the present preliminary results encourage us to belief that aflatoxins can be detected and quantified a t the ultra-trace level with relatively simple equipment. Laser fluorescence analysis has already been used in the gas phase to detect l0-l8-gram samples of fluorescent species (8) and its use in condensed media, such as high pressure liquid chromatography (9, IO), should be expected to grow as convenient UV laser sources are developed.
ACKNOWLEDGMENT We warmly thank Larry M. Seitz, U.S.Grain Marketing Research Center, US.Department of Agriculture, Manhattan, KS, who called this analytical problem to our attention and who provided us with samples of aflatoxins, obtained from USDA-ARS Southern Regional Research Center, New Orleans, LA.
LITERATURE CITED (1) L. A. Goldblatt, "Aflatoxin, Scientific Background, Control and Implications", Academic Press, New York, 1969. (2) M. Enomoto and M. Saito, Ann. Rev. Microobiol.. 26, 279 (1972). (3) J. L. Ayres and R. 0.Sinnhuber, J. Am. OilChem. SOC.,43, 423 (1966). (4) W. A. Pons, Jr., J. A. Robertson, and L. A. Goldblatt, J. Am. Oil Chem. SOC.,43, 665 (1966). (5) W. A. Pons, Jr.. A. F. Cucullu, A. 0. Franz, Jr.. and L. A. Goldblatt, J. Am. Oil Chem. SOC., 45, 694 (1968). (6) J. A. Robertson, W. A. Pons, Jr., and L. A. Goldblatt, J. Agric. Food Chem., 15, 798 (1967). (7) A. J. Pesce, C.-G. Rosen. and T. L. Pasby, "Fluorescence Spectroscopy", Marcel Dekker Inc.. New York. 1971. (8) R . N. Zare and P. J. Dagdigian, Science, 185, 739 (1974). (9) C. D. Scott, Science, 186, 226 (1974). (10) "Advances in Chromatography-1974", A. Zlatkis and L. S. Ettre. Ed., Elsevier Scientific Publishing Co., Amsterdam, The Netherlands, 1974.
PLATE
Michael R. Berman' Richard N. Zare2
DISPLACEMENT
Figure 2. Laser fluorescence chromatogram scans
( a ) Separation of B1 and GI aflatoxin peaks: and ( b )detection of aflatoxin B1 at lowest level attempted. Only an expanded portion of the plate is shown. For a solvent development length of 9.5 cm, the B1 and G1 spot maxima are separated by about 1 cm
source is many times brighter than a conventional UV light source during the time the laser is on; b) the laser light source is coherent and, hence, may be focused on a spot rather than illuminating the entire chromatogram; c) gated
Department of Chemistry Columbia University New York, NY 10027 RECEIVEDfor review December 31, 1974. Accepted February 26, 1975. This work was supported by the National Science Foundation. l Undergraduate Senior, State U n i v e r s i t y of N e w York a t S t o n y Brook, S t o n y Brook, NY 11794. * To w h o m correspondence should be addressed.
Auger Parameter in Electron Spectroscopy for the Identification of Chemical Species Sir: X-Ray photoelectron spectroscopy offers two principal kinds of useful information: a semiquantitative estimate of the relative number of atoms of different elements in the layers near the surface, and information on the identity of the chemical species. For the latter, the principal spectral feature used has been the chemical shift in kinetic energy or binding energy. Early thoughts about the Auger lines led to the conclu-
sion that the chemical shifts should be similar and in the same direction as those of photoelectron lines. Observations on some Auger lines ( I ) led to the contrary conclusion that Auger chemical shifts are abnormally large for metaloxide pairs with core-type Auger lines (Auger processes with final vacancies in core-type orbitals). Earlier work had also shown that large Auger chemical shifts may also happen between pairs of nonconducting-salts (2). The effect ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
1201
Table 1. Chemical Shifts and the Auger Parameter (A1 K a Radiation) Element
Chemical form
EB(PW
Is
Ne Na
Ne(de Na(sY Na1 NaBr NaCl NaSCN Na, SO3 NaNO2 Na,S04 NaNO, NaF Na,SiF, Na(g)'
863.2 865.7 1071.7 1071.4 1071.5 1071.4 1071.1 1071.1 1071.4 1 0 7 1.O 1071.2 1 0 7 1.O 1071.5 1076.8 2P3/2
Mg A1
Zn
Ga Ge As
In Sn Sb
Te 1202
194.8 188.4 579.3 576.2 575.7 575.3 575.2 575.1 574.8 574.4 574.4 573.2 572.8 569.7
8.9
6.4
-0.3 -0.2 -0.3 -0.6 -0.6 -0.3 -0.7 -0.5 -0.7 -0.2 5.1
2.8 3.4 3.7 3.5 3.6 4.2 4.2 4.4 5.4 6.3 14.7
3.1 3.6 4 .O 4.1 4.2 4.5 4.9 4.9 6.1 6.5 9.6
0.9
7.4
6.5
2.6
6.7
4.1
3.9
7 .O
3.1
-0.5
2.3
2.8
u23L23
L3M23M2 3
217.1 205.8 251.0 249.8
972.09 965.48 57.09 55.5g
4.7
11.3
6.6
-0.3
1.2
1.5
CaC 1, CaF,
348.1 347.7
292.1 289.1
153.6' 150.2'
-0.4
3 .O
3.4
2p3/2
L3M45IY5
0.0
3.6
3.6
0.9 3.4
5.3 15.2
4.4 11.8
0.5
5.5
5 .O
3.0 3.6
7.2 9.2
4.2 5.6
7.1
10.6
3.5
2.8
5.5
2.7
0.1
4 .O
3.9
0.9
4.9
4.0
1.9
6.4
4.5
2.4
6.6
4.2
4.7
9.8
5.1
CU CUC1 Zn ZnF, zn(g)" Gaj Ga(ox)h Gej GeO, NazGeF, As'
Sej
Na2Se03
Cd
818.2 809.3 994.2 991.4 990.8 990.5 990.7 990.6 990.0 990.0 989.8 988.8 987.9 979.5
2P3/2
932.4 932.4 1021.7 1022.6 1025.1 1116.8 1117.3 1217.4 1220.4 1 2 2 1 .o 1323.0 1330.1 3 % ~
Ag
2.5
KL23L23
241.5 246.2 292.6 292.3
KAsF, Se
-Aa
A r (s)d Adde KI KF
WSzk
cu
A E (Auger)
Na,S,O,'
Mg MgF, A1 Al(ox)hi
S
Ca
A~B(PE)
1185.8 1178.4 1393.1 1386.4 1615.7 1608.7 2115.0 2112.7
sij si (0x1~
K
ab
49.8 50.7 72.8 75.4 99.4 103.3 162.8 162.3
Si
Ar
E( Aug er)c
Ag AgzSO4 Cd CdF, Inj InF, Sn NaSnF, Sbj KSbF, Te"
55.5 58.3
918.8 915.2 992.2 986.9 977.0 1068.3 1062.8 1145.1 1137.9 1135.9 1224.8 1214.2
749.0' 742.5' 979.3' 975.2' 228.5 225.4 791.2 788.4
364.6 361.0 527.3 522.9 515.5 698.5 693.5 875.9 871.7 870.3 1061.2 1057.7
L3M45GS
1306.9 1301.4
3dw2
W~45"4S
368.0 368.1 404.7 405.6 444.1 446.0 484.8 487.2 528.0 532.7 573.2
358.4 354.4 383.9 379.0 410.3 403.9 437.6 4 3 1 .O 464.4 454.6 491.7
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
875.8 873.1 239.8' 235.9' 302.0' 298.0g 367.8' 363.3g 435.8' 5 3 1 .68 505.8g 500.7g 578.3g
Table I. (Continued) Element
Chemical form
EBWP
Na, Te04 Xe(s)d xe(de
Xe
576.6 670.0 674.1
485.7 545.0 535.0
4r7/2
h15N67N 67
33 .O 36.1
WS,k Na,WO,'
W
41712
Pt
Pt K,P tC 1,'
E( A ug er)c
1728.0 1722.2 M4N6
AEB(PE)
,b
575.7" 728.4g 722.5g 274.4 271.7
4E(Auger)
-4a
3.4
6 .O
2.6
4.1
10.0
5.9
3.1
5.8
2.7
ri67
2040.7 625.1 2035.4 622.0 2.2 5.3 3.1 0 Photoelectron binding energy. Samples were prepared in a nitrogen atmosphere and examined without access to air unless noted otherwise. Non-metallic samples were ground to a powder and mounted on Scotch Brand tape. A correction for static charge was made, using the adventitious carbon CIS line set equal to 284.6 eV, the value it assumes on inert metallic specimens. A Varian IEE spectrophotometer was used. Its voltage scale was set to give the Au 4f7 2 line at 83.8 eV and the Cu 2p3 2 line at 932.4 eV. Auger parameter, a , equals the difference in kinetic energies, E(Auger) - E(photoe1ectron). The most intense Auger line in the group was used in all cases except that M4N45N45was used instead of the M ~ N ~ B peak. N ~ s Ion-implanted in iron. e Reference ( 3 ) .The work function for iron was added to all vacuum level kinetic energies to make the values comparable with the solid phase. Fermi-level referenced data. f Reference ( 4 ) .The work function for sodium was added to the vacuum level kinetic energies t o make data comparable with the solid phase data. g Add -1000 or -2000. Oxidized in air at a temperature sufficient to generate metal oxide peaks more intense than the element lines. Element peaks still measurable. Vapor-deposited in vacuo. S ( L M M ) line used as reference connecting Cr X-ray spectrum to that of A1 K n . With the latter, the C Is line = 284.6 eV was used as the charge reference. Na(KLL) and O(KLL) lines used as references to connect Cr spectrum to A1 spectrum. The C Is line at. 284.6 was used as the absolute reference. Reference ( 5 ) .Work function added to vacuum level kinetic energies. Photoelectron data were only for the 3d line. The value for 2p3 2 inserted was calculated assuming LEB is the same for the 3d and 2p3 2 lines. T e powder used with oxide coat, elemental and oxide lines both observable. K(LMM) and C l ( L M M ) Auger lines used as references to connect Cr X-ray spectrum to the A1 X-ray spectrum. The C Is line was used as the absolute reference.
71 .O 73.2
(I
was ascribed to stabilization due to polarization energy ( I ) or extra atomic relaxation energy in screening the doublehole final state in the Auger transition. The effect has now been explored further and shown t o be quite general. In 'Table I are shown data for elements exhibiting the effect with KLL, LMM, and MNN lines. ( M ~ N ~ R are N ~ used s rather than MjN45N4j because the lines are sharper.) Of the elements with core-type Auger transitions that are energetically possible with A1Ka radiation, the effect has not been demonstrated for the first series of transition elements (L3M24M23) and the rare earths (M4N4gN45)apparently because of shake-up processes and/ or multiplet splittings that eliminate single dominant sharp lines. Data for K, Ca, Cs, and Ba are incomplete because of lack of data for the metals (the vacuum in our instrument is not sufficient for the task). Data for iodine do not show the effect. With those exceptions, data demonstrate the effect to be general. A few experiments with a chromium anode demonstrate the effect beyond the energy limitations of the AlKcv radiation (Table I) for Al, Si, S, W, and Pt. I t was noted early (2) that the differences between the kinetic energy of an Auger line and a photoelectron line gives an easily measurable value with a significant range among sodium salts. We have adopted this as a parameter characteristic of a chemical state and call it the Auger parameter, a . It has one very practical advantage-it requires no static charge correction and, in general is easily measurable to 10.1 eV. As shown in the table, the larger Auger chemical shift leads to values for AN that are a major fraction of A E (Auger) and much more accurately measurable. The Auger parameter is proposed as a parameter uniquely characteristic of a chemical and physical state. The Auger parameter is defined as the kinetic energy of an Auger electron minus that of a photoelectron from the same element. 01
= E(A)
- E(PE)
(1)
Analysis of this equation in terms of theoretical treatments by Gelius ( 6 ) and Shirley ( 7 ) discloses it to be composed
mainly of terms involving energies of the orbitals in the ground state, intra-atomic relaxation energies, extra-atomic relaxation energies (polarization energies), and electronelectron interaction terms. When two states of the same element are compared in order to develop an equation for the chemical shift in a , intra-atomic relaxation and electronelectron coupling are believed to change only minimally and can be ignored. The Sa for a KLL Auger-1s photoelectron pair is then given approximately by S a = -2Ac(K)
+ 2Ac(L) + 2ARea(K+)- ARea(L+L+)(2)
where Sc is the change in the orbital energy in the neutral is the change ground state of the shells indicated, Mea"(+) in (extra-atomic) polarization energy of the K+ state, and ARea(L+L+)is the similar term for the double-hole final Auger state. The latter should be four times the former, ( I ) , and Sc(K) Sc(L), so that Sa g -2ARea(K+), or the change in a is substantially due to the change in polarization energy, stabilizing the ionic state. As demonstrated in Table I, the range in N among insulating solids indicates a t o he significantly different among insulators. I t is believed this results from differences in gegen-ion polarizability and inter-atomic distances.
LITERATURE CITED (1) C. D. Wagner and P. Biloen, Surf. Sci. 35, 82 (1973). (2) C. D. Wagner, Anal. Chem., 44, 967 (1972). (3) K . Siegbahn, C. Nordling, G. Johansson, J. Hedman, P. F. Heden. K. Hamrin, U. Gelius, T. Bergmark. L. 0. Werme, R. Manne, and Y. Baer, "ESCA
Applied to Free Molecules", North-Holland Publishing Co.. Amsterdam, 1969.
(4) S . P. Kowalczyk, R. A. Pollak, F. R. McNeely, L. Ley, Phys. Rev., Sect. B, 8, 3583 (1973). (5) S. Aksela and H. Aksela, Phys. Lett., 48A, 19 (1974). (6) U. Gelius. Phys. Scripta, 9, 133 (1974). (7) D. A. Shirley, Chem. Phys. Lett., 17, 312 (1974).
and D. A . Shirley,
C. D. Wagner Shell Development Company P.O. Box 481 Houston, T X 77001
RECEIVEDfor review December 6, 1974. Accepted January 27, 1975. ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
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