Electron spectroscopy - Analytical Chemistry (ACS Publications)

ELECTRON SPECTROSCOPY David M. Hercules, Department of Chemistry, University of Georgia, Athens, Ga. 30601

a t the history of spectroscopic techniques currently used by chemists, one is struck by a common denominator in their development. The techniques usually have been devised by physicists t o study some fundamental property of matter, and subsequently it has been realized t h a t information of value t o chemists can be obtained from them. Development for practical use by chemists has followed. Infrared and ultraviolet spectrophotometry, nmr, mass spectrometry, and electron spin resonance serve as typical examples. A new technique, electron spectroscopy, appears to be following a similar pattern and offers the potential of becoming a widely used and highly valuable chemical tool. Electron spectroscopy is the technique of measuring the binding energies of electrons in molecules, by determining the energies of electrons ejected by the interactions of a molecule with a monoenergetic beam of X-rays or photons. It is convenient to distinguish between electron spectrometry in which the ionizing source is a high-energy XLOOKING

ray beam and t h a t in which the source is a beam of photons frome.g., a helium discharge. B y common usage the former is known as ESCA or electron spectroscopy for chemical analysis and the latter as photoelectron spectroscopy or PES. Both parts have applications t o chemical analysis but the instrumentation, information obtained, and interpretive procedures differ somewhat. A third related area of development is Auger spectroscopy, a subject discussed recently in these pages ( 1 ) . Historically, Siegbahn and coworkers utilized a double focusing iron-free magnetic spectrometer for high-resolution energy analysis of photoejected electrons. T h e basic theory of this instrument was described in 1946 (2) and a description of the double focusing instrument reported in 1956 ( 3 ) . T h e value of electron spectroscopy lies in the observation of chemical shifts initially reported for copper in 1957 ( 4 ) ,but the general utility was appreciated only as recently as 1964 (5-7). Utilization of electron spectroscopy for ionization potential

ANALYTICAL CHEMISTRY presents two articles on electron spectroscopy. Instrumentation a n d techniques have developed along t w o lines. I n one case, Siegbahn a n d coworkers i n Sweden developed an improved beta-ray spectrometer t o measure core electrons. For applications i n chemistry, he used a high-energy excitation source, X-rays, a n d called his technique Electron Spectroscopy f o r Chemical Analysis (ESCA). I n t h e other case, Turner a n d his colleagues i n England developed instrumentation t o study t h e ionization potentials of valence electrons, Turner used a beam of photons and low energy a n d called his technique Photoelectron Spectroscopy (PES). These t e r m s are now c o m m o n l y used interchangeably as they basically measure t h e same things. They are complementary techniques t h a t have great potential i n b o t h s t r u c t u r a l a n d analytical chemistry. For solid samples, t h e techniques are especially valuable i n studying surfaces. David M. Hercules i n his article which begins on t h i s page discusses electron spectroscopy i n general, b u t principally t h e ESCA techniques. The article by D. Betteridge a n d A. D. Baker of t h e University of Swansea, which begins on page 43 A, covers some of t h e analytical possibilities of photoelectron spectroscopy.

20A

ANALYTICAL CHEMISTRY, VOL. 42, NO, 1, JANUARY 1970

measurements was first reported by Turner in 1962 (8) with a description of instrumentation following in 1963 (9). Recently, Turner has reported instrumentation and studies using high resolution electron spectroscopy to determine the vibrational states of molecular ions (10). The basic processes common to all electron spectroscopic techniques are illustrated in block diagram form, shown in Figure 1. Ionizing radiation impinges on a sample causing ejection of an electron. The electron goes into a n electron monochromator, and the energy of the photoejected electron is determined. After energy resolution by the monochromator, a signal proportional to electron intensity is read out on the detector. The kind of information obtained depends upon the energy of the ionizing radiation and resolution of the monochromator. For example, if high-energy radiation such as Xrays or y-rays is used, core electrons can be ejected and their binding energies determined. Conversely, if low-energy excitation is used, such as vacuum ultraviolet radiation, valence shell electrons will be ejected permitting determination of ionization potentials. It is this qualitative flexibility coupled with a potential for quantitative measurement t h a t makes electron spectroscopy potentially attractive as a chemical technique. Qualitative information may be obtained t o permit distinction of various electrons within a molecule on the basis of their binding energies, as well as changes in their chemical environment, yielding structural information similar to infrared or nmr. The quantitative aspects of electron spectroscopy should be comparable t o X-ray fluorescence since similar excitation and detection techniques are used. THEORY OF ELECTRON SPECTROSCOPY

This section will give a brief introduction to the theory of electron spectroscopy as it relates to chemical applications. There are basically two types of processes responsible for emission of electrons from

REPORT FOR ANALYTICAL CHEMISTS Table I. Fundamental Processes in Electron Spectroscopy Electron Ejection

(1) Photoionization A hvl * A+* ee (2) Electron Bombardment A ele+ A+* e l e + eze

+

+

+

+

Electronic Relaxation (1) X-Ray Emission

A+*+A++ (2)

Auger Emission

A+*+

A++

Source of

hvz

Electron

3

+ e8e

atoms: electron ejection and electronic relaxation. These are summarized in Table I. Electron excitation may be induced by either photoionization or electron bombardment. The ionizing radiation used can vary from y-rays to ultraviolet photons requiring only t h a t i t be in excess of the binding energy of the electron. There are two fundamental processes for electron relaxation in atoms, X-ray emission (fluorescence) and Auger electron emission. As will be discussed further, these are competing processes.

Residual

Figure 1.

torr

Magnetic Field

r c cn C

Q,

c C

Photoelectron Spectrum

+

LI

K

n

P

L2

L3

u

-

Enerqy

Figure 5. Simulated X-ray fluorescence, X-ray absorption, and photoelectron spectra for same elements

How the theoretical considerations relate to electron spectroscopic measurements can be seen from Figure 6 . The energy levels of the sample are indicated assuming it to be an electrical insulator. The spectrometer is assumed t o be metallic, and its levels are given on the right-hand side of the diagram. The energy of the incident photon is shown on the far left-hand side of the figure. For photoejection of a core electron in the sample, the energy of the incident photon will be distributed among four processes. First, the binding energy, Eb, of the core electron relative to the Fermi level of the material in which i t is found. This is the parameter which electron spectroscopy ultimately measures. Second, the work function, +s, of the material, which is the energy necessary t o raise the electron from the Fermi level to the free electron level. Third, the recoil energy, E,, which results from the conservation of momentum in the photoejection process. Fourth, the kinetic energy, T,, of the electron in space after i t has left the sample. Recoil energy arises because of the necessity of appropriately distributing kinetic energy in the photoejection process. By application of the law of conservation of momentum it has been shown ( 7 ) t h a t recoil energy is generally unimportant for electron ejection from light atoms. If we use aluminum Ka as the exciting radiiation, recoil is unimportant in atoms heavier than lithium. For copper, K, recoil is unimportant in atoms heavier than sodium.

ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970

23A

Report for Analytical Chemists

f

Er

TS

Leva I

Electron Level

Fermi Level Valence

dependent on atomic number, photoelectron lines will generally be narrower than X-ray fluorescence lines involving comparable levels. For example, if one is measuring the electron binding energy in copper using aluminum K , excitation, the intrinsic width of the aluminum radiation and the intrinsic width of one copper level will determine the line width. Studying comparable levels by X-ray fluorescence requires the width of 2 copper levels 2 eV each. The net effect will be t h a t the photoelectron line can have widths of about 1.5 eV, whereas the comparable X-ray line will have widths on the order of 3 to 4 eV. INSTRUMENTATION

I 1 Eb

7

hv =

1

pzzzq fore Level

k p3 - 4~ev E,-0

E b + $sp+

ev

T s p t E,

Figure 6. Energy considerations in measurement of electron binding energies

= energy of exciting photon = binding energy of photoejected electron = work function of sample +,# = work function of spectrometer J, = kinetic energy of photoejected electrons in the vicinity of the sample T., = kinetic energy of the photoejected electron in the vicinity of the spectrometer E , = recoil energy hv Ea

As the free electron having energy, T,, traverses the space between the sample and the spectrometer, it will encounter a potential gradient due t o the difference in work functions between the sample and the spectrometer. Therefore, it will be accelerated (or decelerated) t o the kinetic energy T,, as i t reaches the free-electron level of the spectrometer material. Adding the work function of the spectrometer brings the electron t o the Fermi level of the spectrometer. It is clear then t h a t the kinetic energy measured by the spectrometer is T,, and t h a t the binding energy of the electron in the sample can be computed according tQ

24A

0

If one wishes to measure the absolute binding energy of an electron, it is important to know accurately the work function of tihe spectrometer. But if only relative measurements are desired (as will be the case for most chemical work), i t is only necessary t h a t the spectrometer work function remain constant. This has been shown to be true (7). According to Equation 1, since the spectrometer work function is constant and lthe recoil energy is negligible, the uncertainty in the measured binding energy will arise from either an uncertainty in the measured photon energy or the binding energy of the core electron itself. Because the natural line width of X-ray lines is inversely


The block diagram shown in Figure 1 indicates the tasks t h a t must be accomplished by electron spectrometers. They must produce intense ionizing radiation, irradiate the sample t o photoeject the electron in such a geometry that the electrons can be introduced into the monochromator, carry our energy resolution on the photoejected electrons, detect the electrons and then read out the signal in such a way to give a relative intensity as a function of electron energy. A number of possible instrumental systems can be generated depending upon selection of sources, monochromators and detectors. These components which are used or have potential use in electron spectroscopy are listed in Table 11. Sources

As indicated in Table 11, a variety of possible sources can be used. The ideal source provides a n intense, steady level of radiation in such a way that it can be focused conveniently on the sample. The ultimate precision of measurement is determined by the stability of source intensity. Essentially there are four types of sources t h a t provide the required energy: X-rays, ultraviolet photons, electrons, and y-rays. Although y-rays have not been used to date they are a possible ionizing source for photoejecting the core electrons of very heavy atoms like the transuranium elements. X-ray tubes used in electron


Table I I .

Components for Photoelectron Spectrometers

Sources Electron gun

Monochromators Retarding Field

7-Rays

Magnetic

X-rays

Electrostatic

Cu K a 8048 eV Cr K a 5415 eV

AI K a 1487 eV

Detectors

M g K a 1254 eV

GM-Tube Electron multiplier

Ultraviolet He resonance 21.3 eV (584 8)

Photographic

Kr resonance 10.3 eV (1201 A) H g resonance 4.89 eV (2537

A)

Readout Rate meter

Degaussing

Scaler

Helmholtz coils

Computerized system

Paramagnetic shielding

spectroscopy are of conventional design and consist of a heated cathode a t high negative potential and a water-cooled anode maintained a t ground potential. Cooling of Xray tubes is essential because of the high power dissipated. Operating conditions of 10 to 15 kV a t 20 t o 50 mA are not uncommon. Rotating anode sources have been used t o minimize effects of localized heating on the anode. It is also desirable t o design X-ray tubes in such a way bhat the anode is not in direct line with the cathode but t h a t the electron beam is focused by electrostatic deflection. This minimizes deposition of tungsten from the filament on the anode, a problem t h a t can be particularly acute when using X-rays from light elements. A variety of excitation energies is available from X-ray sources. Some typical examples for the K , lines of Cu, Cr, Al, and Mg are given Table 11. These elements give a range of energies khat will photoeject the core electrons of most elements of interest. The X-ray tube and sample compartments are usually separated by a thin window through which the exciting X-rays pass. T h e separation of source and sample compartments is essential t o avoid scattered electrons from the X-ray source entering the sample compartment. The use of a thin filter

can eliminate white radiation from the source as well as to separate X-ray lines in certain cases. Alternatively, one could use a bent crystal monochromator t o select truly monochromatic X-rays although this would drastically reduce the source intensity. The use of a bent crystal monochromator does appear attractive, however, from the standpoint of further reducing electron line widths. Ultraviolet sources are also convenient for electron spectroscopy, particularly in studies of low-energy photoionization processes. Such a source usually consists of a gas flowing through a thin tube a t about 1-Torr pressure and excitation either by electrical or microwave discharge. Excitation energies available are considerably lower than those from X-ray sources, as indicated in Table 11. Such sources are particularly useful for exciting valence shell electrons, reducing the background caused by higher energy sources. Also the line widths of uv photons are much smaller than those of Xrays and therefore, uv-excited electrons have narrower bandwidths. Electron gun sources have been used for a variety of studies. These direct a narrow beam of electrons onto a sample, collecting the scattered electrons with a Faraday cage. Electron gun sources have been used t o excite Auger

spectra in solid samples. Typical beam currents are 1 pL4, The particular advantage of electron gun sources is t h a t they provide a conveniently variable source of excitation which can be controlled in both energy and intensity. T h e beam is also readily focused. Although it is difficult t o excite discrete photoelectron lines in solid materials using electron sources, Auger electron emission does produce discrete lines. Electron excitation of gases also gives discrete lines. There are two major disadvantages to electron gun sources. First, significant scatter of the incident electrons results in high backgrounds, particularly when investigating solid samples. Second, many materials drcompose under electron bombardment which are stable under X-ray or uv excitation. Sample

As has been mentioned earlier, the physical form of the sample is not important if differential pumping techniques are available, gas, solid, and liquid samples having been run. However, because it is more convenient t o run solid and liquid samples, a practical restriction of low vapor pressure is often placed on the sample material. T o run samples having high vapor pressure a t room temperature, cryogenic probes have been used. Use of such probes permits a wide range of materials to be investigated. Another criterion for the sample is t h a t it must not decompose under high vacuum. This would restrict one from studying say inorganic carbonates t h a t easily evolve CO,. A significant problem does exist when attempting t o measure photoelectron spectra of solutions where freezing the solution may have significant effect on the result. This represents a technological problem still t o be overcome. For most samples, photoejected electrons will not emerge from a depth greater than 100 A. This means that electron spectroscopy is basically a surface technique and one must be careful t o ensure t h a t the surface is representative of the hulk of the material, if bulk prop-

ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1 9 7 0

*

25A


erties are to be measured. Small samples are required for electron spectra, g being adequate, and spectra possibly being obtained on as little as g. A potential problem exists in charging of the sample if i t is not in good electrical contact with ground. T o date adequate currents have been observed, even in insulators, to replace the electrons photoejected. Good electrical contact must be maintained between the spectrometer and the sample to ensure the interpretation given in Figure 6 is correct. Selecting the energy of the exciting radiation near to the binding energy of the photoelectron minimizes the energy resolution required by the monochromator. However, a t the same time it introduces the problem of getting the electrons out a finite depth in the sample, and the selection of excitation energy in any practical case will have to be a compromise between the two.

tively zero. The means for compensating earth field varies with the electron energy and the type of monochromator used, but usually is done by one or a combination of two techniques: a Helmholtz coil system or paramagnetic shielding material. Because it is necessary t o resolve electron energies t o O . l % , the residual magnetic field and fluctuations in the field must be held to a comparable value. Ideally one should reduce the earth’s magnetic field along tihe electron trajectory to below G. This means t h a t not only should the residual field be compensated to this value but the gradient along the trajectory should be a t this level, as well. Compensation for the vertical component of the earth’s field is far more crucial than for the horizontal field. The most widely used technique of field compensation is a series of Helmholtz coils, the basic construction of which has been described by Lee-J17hiting (11). Siegbahn and coworkers ( 7 ) have traditionally used two ciroular vertical coils and two sets of two square horizontal coils. For their location they have found this arrangement to be satisfactory. When using a magnetic spectrometer in an urban area, difficulties with the simple Helmholtz sys-

tem are encountered. We have used three sets of tihree independent Helmholtz coils along with a feedback system as indicated in Figure 7 . The vertical coils are 12 ft square with the center coil located approximately a t the center of the horizont’alplane of the spectrometer and the upper and lower coils approximately 4.5 ft above and below the spectrometer. The horizontal coils are ca. 9 ft square and are placed a t ca. 3-ft intervals. A set of nine independent power supplies is used so that the current through each coil may be varied independently. This was necessary because of the extreme gradients encountered in buildings containing structural steel. Even by use of this system, to minimize the field gradient along the electron trajectory, a small additional correction coil was necessary . Despite the elaborate Helmholtz coil system used, large fluctuations in field were observed in the 2-10 mG region depending upon the time Compensation for Earth’s of the day and the day of the week. Magnetic Field Fluctuations correlated with traffic Because electron paths are influalong city streets as well as a subenced by magnetic fields, it is necway station located some 600 m essary t o have the electron trajecaway. Field perturbations due t o tory determined only by the field traffic and subway trains, could be of the monochromator. This means compensated by use of a feedback t h a t $he earth’s magnetic field in loop as indicated in Figure 7 . The the vicinity of a n electron specfeedback network used a magnetrometer must be reduced t o effectometer probe placed a t the plane a t the spectrometer vhere the spectrometer contributed no net field HELMHOLTZ and feedback to the center coil of POWER MAGNETOMETER SUPPLY the vertical Helmholtz system. VERTICAL The use of paramagnetic mateHELMHOLTZ COILS rial such as p-metal or netic and conetic shielding is not permissible SPECTROMETER with magnetic spectrometers. Because these materials shield by cutting the force lines of the earth’s field, they also would perturb the spectrometer field and the required resolution would not be possible. However, if an electrostatic monochromator is used, it is possible to shield the instrument with paramagnetic materials or by combining a less highly controlled Helmholtz coil system with paramagnetic materials. It is this particuI lar consideration which most strongly suggests that electron RECORDER AMPLIFIER spectrometers built for use in chemical laboratories should utilize elecFigure 7. Block diagram of electron spectrometer system 26A

0


trostatic rather monochromators.

than

magnetic

Monochromators

Essentially three types of monochromators have been used in electron spectroscopy-retarding field, magnetic, and electrostatic. T h e latter two simply use different means of focusing an electron along a trajectory, while the retarding field spectrometer is quite different in principle. The principles of each are outlined in Figure 8. Retarding field spectrometers haye been reported by Al-Joboury and Turner (9) and by Frost et al. (12). The Turner instrument used a cylindrical geometry for the retarding electrode while the Frost instrument used spherical electrodes. The distance from the source t o collector in the cylindrical spectrometer was about 4 cm and the spherical one about 3 in. The basic idea of a retarding field -p ectrometer is t o have two grids, GI and G2 in Figure 8A between a source of electrons, S , and a collector, C. The grids are metallic screens and provide approximately 70% transmission, An increasing potential difference is applied between the grids t o retard electrons between the source and collector. When the potential difference is large enough, electrons of a given energy, e?, will be retarded and the signal a t the collector will be reduced. The collector signal, q, is amplified by a charge-sensitive amplifier, differentiated, and read out on a recorder, as the potential difference between the grids is scanned. The obvious disadvantage in a retarding field speotroineter is t h a t the initial signal is the sum of the signals from all electrons, which makes distinction between low energy electrons difficult. Background from scattered electrons is particularly bothersome. Retarding field spectrometers appear to be particularly good for low-energy electrons a t low background, such as for uv excitation of gaseous samples. They have large effective solid angles. Magnetic monochromators used in electron spectroscopy are largely of the double focusing variety adapted from the design of SiegQ

I

L-l

A ?

I

B

Split Coil Svstom

90

C Retordinq

M

SOURCE

Analyzer

Figure 8. Schematic diagrams of electron spectrometers

A. Retarding field spectrometer

= source C = collector G,,G2 = grids A € = voltage source S

el,ez

= electrons

of different energies q = charge from cup E. = voltage from charge-sensitive amplifier

B. Double focusing magnetic spectrometer C. Spherical electrostatic spectrometer and retarding field

bahn e t al. ( 2 3 ) . These are ironfree instruments generally manufactured from brass or aluminum, a 30-cm radius being adequate for most chemical work. Double focusing is provided by an inhomogeneous magnetic field produced by a set of four cylindrical coils placed about the electron trajectory as shown in Figure 8B. Also as shown, the critical angle between the source and the detector is qh or 254’56’. As originally derived, the instrument used two concentric solenoids for coils, but a coil system was devised t o permit better access t o the spectrometer sample chamber for chemical work ( 7 ’ ) . Siegbahn’s group has also described an electrostatic mono-

chromator ( 7 ) ’but the most recent design is one due to Helmer and Weichert (1.9)’ which is a modification of the spherical electron analyzer described by Percel (14). This type of monochromator consists of two concentric spheres with a potential difference between them as shown in Figure 8C. The design of Helmer and Weichert utilizes a retarding field to reduce the initial electron energy from ca. 1000 V t o 100 V. This permits a smaller monochromator t o be used since the energy resolution is smaller and also provides an increase in luminosity which more than compensates for the decrease in brightness caused by the reduction of electron kinetic energy.


27A


This is the monochromator design used by Varian in their commercial instrument. Again, a great advantage of electrostatic monochromators over magnetic monochromators is that a minimal Helmholtz system is required for the former. Detectors

Basically the detector used in electron spectroscopy must count electrons. To date four types have been used. I n the Siegbahn-type instruments, GM counters, electron multipliers, and photographic plates have been used while in the Turner-type instrument, charge cups have been used. The charge cup detector is dictated by the nature of the retarding field spectrometer (9, I $ ) , where the electrons passing through the grids accumulate charge on the cup. This type of detector has been shown in Figure 8A. The GM-type detector originally used by Siegbahn et al. was a proportional flow counter using a Formvar window and a postacceleration system for the detection of electrons below 2 keV ( 7 ) . Eledron multipliers have been used, both continuous-channel and the dynode types. The continuous channel multiplier counts electrons with high efficiency t o very low energies and, being small, can be placed readily in the focal plane of the spectrometer. If necessary, several channel detectors can be put over the exit slit t o improve sensitivity. Our instrument uses a dynode-type electron multiplier which is connected t o a charge sensitive amplifier and then to a counter, as indicated in Figure 7. This type of multiplier is quite sensitive, has low background, and is convenient t o use. Photographic detection has been used ( 7 ) with electron spectrography. It offers the advantage of integrating over fluctuations in the exciting radiation and is capable of detecting very low electron signals. However, for most scanning work relating t o chemistry electron multiplier detectors are more convenient. One place where photographic detection is particularly useful, is when an X-ray monochrometer is used to isolate the ex28A

citing X-ray line. I n this situation one encounters a drastic reduction in intensity, and a tracking method must be used to count the electron tracks on the emulsion of the photograph. An instrument to accomplish this has been described ( 7 )* Scan and Readout Systems

The scan and readout systems used in electron spectroscopy are basically of three types: continuous scan, incremental scan, and multichannel analyzers. In the continuous scan system, the field on the spectrometer is increased conitinuously as a function of time, and the signal from the detector is simultaneously monitored by a rate meter. The spectrometer current scan and the output from the rate meter are synchronized to allow accurate recording of spectra. Although the continuous scan method is valuable for roughly locating the positions of peaks, noise is sufficiently high t h a t it cannot be used in high resolution work. The incremental scan method increases the current through the spectrometer in a series of small steps, counting the signal from the detector during each increment. M7hen the increments are plotted as a function of field, 'a spectrum is produced. I n their early work, Siegbahn e t al. utilized a mechanical stepping system although a computerized stepping system such as t h a t shown in Figure 7 is preferred. The multichannel analyzer approach in a sense combines the two, scanning a large number of increments continuously between two

Table 111. Nitrogen (1s) Sulfur (1s) Chlorine (2p) Copper(1s) Iodine (4s) Europium (3d)

-

-2.0

-

-

CHEMICAL SHIFTS

The type of information obtainable from electron spectroscopy depends upon the mode of excitation used. For electron bombardment, largely Auger spectra result. If ultraviolet radiation is used, only outer electrons are excited and if soft X-rays are used, inner shell electrons are ejected. Because of space limitations, we will discuss only those applications associated with inner electron photoejection. Auger spectra have been discussed previously (1) and ultraviolet excited spectra largely give ionization

Chemical Shifts" of Oxidation States

Elementb

-2

limits of field. It is this approach which is probably the best suited t o chemical spectrometers. Some feel for the total system associated with electron specicrometers can be obtained from Figure 7. The regulated X-ray power supply determines the ultimate precision with which electron intensities can be measured. The probe in the center of rthe spectrometer feeds back to the Helmholtz coil power supply system through the magnetometer to keep the field of the spectrometer constant. Commands t o the computer which run the scan system are relayed through teletype. The current through the coils is read out by the digital voltmeter (DVM) and fed back to the computer. Likewise the computer programs a power supply to scan small lincrements over the range of the spectrum desired. This can be over a single photoelectron peak or over several. The laccumulated signals are continuously displayed on a television screen and when sufficient resolution is achieved, the spectrum is printed.

-1 *O"

*o *o

-

0

-

Oxidation State +2 +3 +4.5d +5.1

+l

-

-

-

-

*O

+0.7

+4.4

*o

-

+4

-

+5

+8.0

-

+4.5

-

+3.8

-

+7.1

+6

+7

$5.8

-

-

+9.5

-

- - - - - - - - - +5.3 - +6.5 - - *O +9.6 - - - -

All shifts given in electron volts. Measured relative to indicated oxidation state (*). Electrons measured given in parentheses. c Arbitrary zero for measurement, end nitrogen in NaNs. d Middle nitrogen in NaNa. 0


c

We announce the Schoeffel SD 3000 Spectrodensitometer. It is the state of the art in analysis and evaluation o f TLC, gel, column, paper chromatograms and electrophoresis strips. The leading commercially available system capable of true quantitative evaluation. It is clearly and demon-

matogram staining and can resolve overlapping zones otherwise invisible. The system can detect many substances, non-destructively , in sub-nanogram amounts. With the SD 3000, chromatography takes a giant step toward becoming an exact science With your help.

. . I

I f you would like technical data, reprints describing use of the SD 3000 in quantitation of steroids, estrogens, etc. andlor a prompt and convincing demonstration please write or 'phone. U S A .24Booker Street, Westwood, EUROPE Schoeffef Instrument GMBH, 2351 Trappenkamp, C

sey 07675 f2011664-7263 2,Deutschtand f043233 564

Circle no. 96 on Readers' Service Card


potentila1 data and the structures of excited ions. The chemical shift data for inner electrons appear t o be most applicable to analytical chemistry. Table I11 shows a compilation of photoelectron chemical shifts recorded to date for copper ( 1 5 ) )sulfur (16, 1 7 ) ) iodine (18, 1 9 ) , europium (18, 1 9 ) , chlorine ( 2 0 ) ,and nitrogen ( 7 ) . Shifts are reported relative to some indicated oxidation state. As can be seen from Table 111, the magnitude of the chemical shift varies from one element to another as well as from one oxidation state t o another for the same element. For example, the chemical shift between the +5 and $7 oxidation states of chlorine is 2.4 eV, whereas for iodine it is 1.2 eV. Likewise, the shift between copper 0 and +l is 0.7 eV, but the shift between copper +1 and +2 is 3.7 eV. I n addition to the chemical shifts reported in Table 111 for formal oxidation state changes, chemical shifts for different atoms in organic compounds are known. Extensive data have been obtained for organic nitrogen and sulfur compounds ( 7 ) , and the implication of these to analytical chemistry will be discussed in the next section. Model for Interpreting Oxidation State Changes

An atomic core electron is influenced by the attractive force of the nucleus and a repulsive force due to the outer electrons. The repulsive force is a net shielding effect between the nucleus and the core electrons. If a change in oxidation number occurs in the valence shell the shielding effect of the valence electrons on a core electron is reduced and the binding energy of the core electron increases. Therefore, in the most simple sense shifts in photoelectron peaks with oxidation state reflect the increase in binding energy of core electrons as valence electrons are removed. An increase in positive oxidation state will cause an increase in binding energy of a core electron, and a decrease in kinetic energy when that electron is photoejected. Reflecting this decrease, the magnetic or electrostatic field necessary to 30 A

focus the electron will be smaller; therefore, an increase in positive oxidation state will cause a shift to lower field for the photoejected electron. Because one usuallv nhotoejects core electrons, changing the electron density in the valence' shell will have similar efiects on K and L levels and these photoelectrons show approximately the same shift per given change in oxidation state. Also one would predict that electron shifts for higher degrees of ionization, such as +5 to +6 us. +1 to $2, would be larger as indicated in Table 111. The magnitude of the shift in photoelectron peak, expected on the basis of this simple model, can be computed readily ( 7 ) . If a charge, q , is removed from the valence shell of an atom the potential energy of the inner electrons will be changed by the amount

AE = q / r

(2)

where r is the radius of the orbital of the valence electron. AE is given in atomic units. For removing a unit charge from a distance of 1 A (two atomic units), the change in energy expected would be 0.5 av or 14 eV. Obviously this is much too large. The erroneous assumption in the simple electrostatic model was that the valence electron is removed to an infinite disbance from the core electron. This is not true in the case of ionic solids since the valence electron is moved only to some finite distance, on a counter-ion. A correction is necessary for this effect, comparable to the ?\ladelung lattice correction. Thus Equation 2 can be modified

Where a is the Madelung constant, generally on the order of 1.7. If we consider now t h a t the electron has moved only a distance of 3 A to a counter-ion, the change in energy predicted is ca. 6 eV. Although this number is still larger than most observed shifts, it is approaching an appropriate value. It has also been assumed above t h a t interactions between atomic electrons arise from the shielding effect of the


outer electrons, assuming no overlap of the atomic orbitals. T h a t this is not a valid assumption is well known. When one wishes to consider electron shifts for nonionic compounds (such as organic compounds), the problem becomes even more complex. I n a system having an easily polarized electron cloud, such as a-molecular orbitals, a perturbation a t some distance can affect the electron density around a given atom. For example, changes a t the p-position of an aromatic ring affect the electron density on thc other side of the ring. Therefore, any attempt to explain photoelectron shifts for complex organic molecules based on simple electrostatics will be inadequate. There are two approaches which have been tried to correlate electron shifts for organic molecules: electronegativity and molecular orbital calculations. Although few correlations have been done for each, it appears that molecular orbital calculations more accurately predict relative binding energies, probably reflecting the better overall utility of molecular orbital calculations in describing electron densities. Correlation of Chemical Shifts with Computations

Hartree-Fock Fyee Ion-Approximation. Attempts to correlate chemical shifts with numerical calculations have been tried by Siegbahn et al. ( 7 ) and Fadley et al. (19). Although neither permits quantitative comparison between calculated and obseryed shifts, both point out interesting qualitative considerations. Fadley et al. attempted t o account for shifts in core electron binding energies by two models. The first utilized an energy cycle t o split the calculated binding energies into a free-ion contribution and a contribution from the Madelung correction. It was found t h a t the Madelung correction contributed a significant portion of the binding energy shift and could be used to explain why simple calculations gave values too large. The second model was essentially a charge shell approximation for bonding electrons in a fixed geome-

Tomakeournewsmall X-Y Recorderactbig, just.plug in a couple of these. HP's new 7034A is as trim as you can make an 8% "x 11" X-Y recorder. But size is the only thing small about it. The frame has all the features and versatility of our big X-Y Recorder. Such as 1500 in/secz acceleration and 30 injsec slewing speed, to catch transients most X-Y recorders miss. Guarded circuits to reject ac and dc common-mode signals. Exclusive, silent electrostatic paper holddown to eliminate slippage. Disposable ink cartridge to eliminate mess and make color changes easy. And zero set/check for fast verification of zero position without removing or shorting the input signal. High dynamic performance is 11906

matched by the flexibility we've achieved with our unique plug-in concept: two may be used in each axis, and can be cascaded. With eight plug-ins to choose from, you can add to your measurement capabilities as the need arises. With the Time Base plug-in, you can capture X-T or Y-T data at Yz sec/in to 100 sec/in sweep rates. Expand low-level signals for detailed study with the DC Preamplifier. Suppress steady-state dc to reveal small-signals using the DC Offset plug-in. Plot single channel data at 50 points/sec with the Null Detector and accessory point plotter, PIot two channels independently with the Scanner.

Eliminate ac signal components over 50 Hz with the Filter plug-in. And for more run-of-the-mill recording jobs, try our low-cost DC Attenuator or DC Coupler. You'll be glad to know that the price on this new X-Y Recorder is also small: just $1195 for the basic instrument. Plug-in prices start at $25. For all the big details, contact your local HP field engineer. Or write to Hewlett-Packard, Palo Alto, California 94304; Europe: 1217 Meyrin-Geneva, Switzerland. __

H E WLE TT GRAPHIC

-77PAC KARD Le( RECORDERS

Circle No. 111 on Readers' Service Card


try with obher ions. It was found to give the correct qualitative direction of the chemical shift. Free-ion Hartree-Fock calculations were made for fluorine, chlorine, bromine, iodine, and europium in several oxidation states. Each of these predicted too large a value for the observed shift but less than t h a t predicted by the classical model. The electron oxidation shifts for fluorine were predicted between 10 and 25 eV, chlorine 10 t o 15 eV, bromine 9 t o 13 eV, iodine 8 to 13 eV, and europium ca. 20 eV. The magnitude of the Madelung correction to be applied in a given situation is complicated by several factors. First, the exact geometry must be known. Second, because photoelectrons are ejected from sights near the surface, t h t full three-dimensional Madelung correction is not completely appropriate. Therefore, an average between 0.5 and l of the maximum Madelung correction was considered. It was found that when 3 / 4 maximum correction was applied to K I 0 3 and KIOI, the fractional ionic characters and ionic change computed correlated well with measurements obtained from 310s~bauer spectroscopy. Other problems, which also need to be considered, are local charging of the sample which can shift the reference level, as well as electronic relaxation effects. This study also pointed out t h a t it is not appropriate t o use a common ion as a n internal standard when comparing chemical shifts of various salts. For example, in measuring shifts for K I , KIO3, and KI04, one might be tempted to use a potassium ion peak as an internal standard. However, the Madelung correction also can result in shifts of several eV in the location of the potassium line. Both free-ion Hartree-Fock calculations and charge-shell approximations can be used to determine general trends in binding energy shifts. Both predict a decreasing chemical shift going down a column in the periodic table, say from chlorine t o iodine. They predict accurately t h a t the chemical shift varies little from one electron to another throughout the core. GO32A

0

determine if two nitrogen atoms in a molecule will give separate photoelectron peaks. I n addition to the results shown in Figure 9 for nitrogen, Siegbahn and coworkers ( 7 ) have also obtained reasonable correlations for sulfur compounds and inorganic chlorine compounds. Correlation with CNDO Charges

Figure 9. Correlation between calculated charge and binding energy for nitrogen 1s electrons. After Siegbahn et al. (7)

ing across a row of the periodic table from left to righlt shows an increase in chemical shift of the same electron. Electronegativity Correlations. Siegbahn has done extensive correlations between shifts in electron peaks and the charges calculated from resonance theory ( 7 ) . The charge, q, on an atom may be defined as:

Q is the formal charge and I N is the partial ionic character summed over all bonds involving the atom. The partial ionic character is related t o the differences in electronegativity between the atoms forming a bond. For example, the amino nitrogen is calculated to have a charge of -0.42 while the nitro nitrogen is calculated to have a charge of +0.87. A typical correlation for nitrogen is shown in Figure 9. Although there is not a linear relationship between binding energy and calculated charge, most of the compounds fall on the line except for sodium azide ( I ) , potassium cyanide (W), sodium nitrite ( 1 5 ) , and yalero nitrite ( 1 6 ) . The correlation is good considering the approximate method used for the computions and the wide variety of structures considered. Thus electronegativity correlations provide a convenient first approximation t o

ANALYTICAL CHEMISTRY, VOL. 42, NO.

I, JANUARY 1970

Recently Hollander et al. (21) have published a correlation between the calculated nitrogen atom charge by CNDO methods and the measured photoelectron binding peaks for 19 nitrogen containing compounds. They observed two lines in their correlation, one characteristic of anions and the other characteristic of neutral molecules and cations. Whether the second line for cations is related to the greater positive lattice potential or an artifact in the CNDO method is yet unknown. A noteworthy feature is t h a t all points are well correlated and there are no large discrepancies, such as those found in the electronegativity correlation. CHEMICAL APPLICATIONS Structure Determination

Electron spectroscopy has great potential as a technique for chemical structure determinations, possibly comparable in scope to nmr and infrared. As one example, Figure 10 shows the carbon-1s spectrum for methyl trifluoroacetate. The photoelectron peaks are situated approximately under the carbon atoms of their origin, indicating it is possible to distinguish between the four types present. Figure 11 shows the photoelectron spectrum of the nitrogen-1s electrons from 2- (4nitrobenzene sulfonamido) pyridine. Again, the photoelectron peaks are located approximately under the nitrogens of origin. Not only is qualitative information available from these spectra but the ratio of numbers of atoms of a given type can be determined as indicated by the 1:1 ratios of the spectra. I n the nitrogen-Is spectrum of sodium azide, NaN3, two peaks are observed in a 2: 1 ratio corresponding t o the two identical end nitrogens and the center nitrogen.

N e w Coleman UV-VIS-NIR spectrophotometer. So good, we’ll let its charts do the talking. New Coleman Model EPS-3T Hitachi Spectrophotonieter is the first all-solid-state instrument of its kind. It’s a double-beam, ratio-recording instrument

that yields sophisticated data throughout the 170 t o 2600 m p range. It has 12 features not found un any other spectrophotometer in its price class. Here are some examples of its work:

The chart above shows the remarkable resolution of Coleman Model EPS-3T. Critical adjacent peaks on the complex benzene vapor profile are clearly delineated.

To demonstrate repeatability, the instrument overprints t h e same chart from the same sample. Note t h a t chart presentations a r e all linear, not logarithmic.

A significant mercury spike, perfectly centered a t 253.7 mp. Wavelength accuracy is not affected by changes in ambient temperature. Photometric accuracy is 0.37,T.

--

Above, the 100% T line of the Coleman Model EPS-3T a t maxim u m sensitivity. T h e line is demonstrably flatter than that of any similar spectrophotometer.

Model EPS-3T offers a wide choice of operating parameters. Nine scale modes. Six scanning rates, from 1.5 to 60 minutes. Any member of your staff can easily operate it.

You open the sample compartment and insert cells without touching any of the controls.With this safety shuttering feature, you leave operating parameters set, while you run test, after test, after test.

Sorry, there’s no room left for a picture of Model EPS-3T. But we’ve saved this, and a number of other pleasant surprises for you. Call Coleman for the name of the nearest dealer and a demonstration. Send for Bulletin A-306 Write to Coleman Instruments Division. The Perkin Elm er Corporation, 42 Madison S t . , Maywood, Illinois 60153

-

Circle No. 14

on Readers’ Service Card

PERKIN--ELMER

Report for Analytical Chemists =/120s F

I F - C

O

H

H

I1

I

I

-C-0-C-C-H

I H

I F

/ H

1500 W

U

5

0

0

500[L

z

z

5 1000

g

3

U

V 0

c z w

W +

z

500

0

CARBON 1s

I

I I

I

I

I

860

850

eV

KINETIC ENERGY

eY

* eV

I

285 BINDING ENERGY

290

1

I

410

400 BINDING ENERGY

Figure 11. Nitrogen 1s photoelectron spectrum for 2-(4-nitrobenzene sulfonamido) pyridine. After Siegbahn et al. (7)

Figure 10. Carbon 1s photoelectron spectrum for ethyl trifluoroacetate. After Siegbahn et al. (7)

398

I

I

1190 1195 KINETIC ENERGY

295

I

NITROGEN Is

400

404

402

406

4

I Precision of Meosuremenf

I

398

I 400

'

I 402

,

I 404 Binding Energy ( e V )

I

406

4

Figure 12. Correlation chart for nitrogen 1s electron binding energies and functional groups

= range of observed energies

1( )

= number of compounds used in correlation

Data used to establish correlations are largely due to Sieg bahn et al. (7)

To obtain some idea as t o the types of compounds between which electron spectroscopy will distinguish, Figure 12 shows the approximate ranges of photoelectron peaks observed for different organic functional groups containing nitrogen atoms ( 2 2 ) . The numbers in parentheses are the number of compounds for which data were available, but because of the small number of examples for many classes of 34A

compounds, these charts must be regarded as preliminary. Several features are noteworthy. For example, in Figure 12 ik is apparent t h a t electron spectroscopy will clearly distinguish between types of inorganic nitrogen compounds, such as nitrate, nitrite and cyanide. Further, compounds containing nitrogen-oxygen bonds tend t o have photoelectron peaks a t higher binding energies, ca. 405 eV,


than those containing nitrogennitrogen or nitrogen-carbon bonds, ca. 400 eV, and those containing charged nitrogen atoms in the middle region, ca. 402 eV. Therefore, simply knowing the location of a peak gives some idea as to the nature of the nitrogen atom present. An interesting phenomenon is observed in the correlation for nitro compounds where 19 examples were available. It appears, to a first approximation, t h a t aromatic nitro compounds bunch toward one end of the correlation, while aliphatic compounds bunch toward the other. It is quite reasonable that as more electron spectral data become available for larger numbers of nitrogen compounds, the broad correlations on the chart would break up into subsets. The correlation for sulfur-2s electrons shown in Figure 13 is similar to t h a t for nitrogen. Sulfur-oxygen compounds tend to bunch a t higher binding energies, ca. 165 eV, whereas sulfur-sulfur or sulfur carbon compounds generally tend to show lower binding energies, ca. 163 eV. It is interesting t o note that compounds containing both sulfur-sulfur and sulfur-oxygen bonds tend to pull peaks toward the middle-ie., increasing the binding energy of sulfur-sulfur bonds and decreasing the binding energy of sulfur-oxygen bonds. Again, although only a few compounds are available for correlation, it appears that electron spectroscopy may be a convenient technique for distinguishing between sulfhydryl and disulfide bonds, an item of no small interest to biochemists. Electron spectroscopy has already been applied to the solution of one structural problem. There had been some doubt about the structures of disulfide-dioxide, as t o whether they show the disulfoxide structure, A, or the thiolsulfinate structure, B, as indicated below.

0 0 R-S-S-R t t

0 R-S-S-R t

J(A) (B) 0 For structure A, one sulfur peak should be observed in the sulfuroxygen bond region. For B, two sulfur lines should be observed, one


I 1

162 I

t

I s=

I

so

Precision of Measurement

162

160

164 I

H I

166 I

1

168 I

I so;

so4=H ( 3 )

H

R S ( S 0 2 ) R (4)

I

RSSO~@

164 166 Binding Energy ( e V )

168

Figure 13. Correlation chart for sulfur 2s electron binding energies and functional groups

1( )

= range of observed energies = number of compounds used in correlation

Data used to establish correlations are largely due to Siegbahn et a/. (7)

in the sulfur-oxygen region and one near sulfur-sulfur. The observed photoelectron spectrum of cystine S-dioxide shows two lines indicating the validity of structure B.

tion potentials for a number of fluorinated benzenes and Dewar and Worley have measured the ionization potentials of 67 compounds ( 2 9 ) .

Determination of Ionization Potentials.

Surface Studies

Returning for a moment t o lowenergy (ultraviolet) excitation, considerable data have been reported indicating electron spectroscopy t o be useful for measuring ionization potentials, particularly below the 21.21-eV. limit of the helium resonance line. Al-Jobury and Turner have reported ionization potential data (24) for 48 compounds including small gaseous molecules aliphatic, alicyclic, olefinic, and aromatic hydrocarbons, as well as a number of alchohols and amines. Also, Radwan and Turner have published ionization potentials for ozone ( 2 5 ) , and Baker and Turner have presented ionization potential data for acetylene, diacetylene and their deuterated derivatives ( 2 6 ) . Baker, May, and Turner ( 2 7 ) have determined r 2 and r 3 ionization potentials for 21 monosubstituted benzenes and 23pdisubstituted benzenes. Clark and Frost (28) have published ioniza-

Electron spectrocsopy is largely a surface technique which constitutes both an asset and a liability. When doing structural studies, one must be certain t h a t the surface is representative of the bulk material. On the other hand, this is a particularly valuable asset when applied to surface chemistry. The average penetration of a photoelectron is probably not much greater than 20 t o 50 A-certainly 100 A a t the outside. This has been illustrated by Siegbahn et al. (7) when they deposited different numbers of multiple layers of a-iodo stearic acid and measured the intensity of the photoelectron peak from iodine. For depths of 1, 3, and 10 double layers, they observed signals in the ratio 1: 2.1 : 3.5. Although this indicates t h a t multiple layers do not give additive signals, it points out one of the great advantages of electron spectroscopy for studying surfaces: t h a t a sur-

face can be studied even though it is covered by more than one layer of material. I n the same study Siegbahn e t al. obtained iodine spectra of a surface layer containing one iodine atom per 10 A2-i.e., less than a monolayer. Therefore, sensitivity of electron spectroscopy is such that submonolayers on surfaces may be studied. It appears, on the basis of these preliminary studies, t h a t electron spectroscopy has great potential in surface chemistry. It should have a significant advantage over techniques such as low-energy electron defraction (LEED) because both the surface and the layer covering it can be viewed simultaneously. This should be particularly advantageous to catalyst studies, for the nature of changes occurring at active sights on the catalysts, as well as changes in the material being absorbed on the catalyst, can be determined simultaneously. To date no other instrumental technique is capable of providing this type of information on a wide variety of compounds. Quantitative Analysis

I n addition to its use as a qualitative technique in structural chemistry, electron spectroscopy also shows potential as a quantitative analytical tool. Because photoelectron spectra can be obtained for virtually any element, it is not inconceivable that electron spectroscopy could become a universally applicable technique for determining elemental ratios in a wide variety of samples. It shows particular promise for obtaining elemental ratios in organic compounds with much greater ease than can be done by either mass spectrometry or wet‘ chemical analysis. There are a number of factors which affect the recorded intensities of photoelectron lines. One of these is the photoelectron cross section given by p

= CZ4h3+ b

where the first term containing 2 4 is an absorption term and b represents a smaller scattering term. This effect tends to favor excitation of high 2 elements since, all other factors being equal, they will have


35A

New and recent books for analytical chemists from Wiley-Interscience X-RAY SPECTROCHEMICAL ANALYSIS Second Edition By L. S. BIRKS, Head, X-Ray Optics Branch, Naval Research Laboratory. Shows how X-ray spectra can be employed for quantitative chemical analysis and describes the principles and practices in the generation, dispersion, and detection of the characteristic X-ray lines. It emphasizes the use of X-ray wavelengths to distinguish the elements present and how the line intensities are used to perform quantitative analysis. The Second Edition stresses the role of computer programs for rapid analysis, and information is given on instrumentation, particularly concerning X-ray spectrometers. Volume II in the Wiley-lnterscience Chemical Analysis Series. CONTENTS: Simplified Fundamentals. Principles of X-Ray Generation, Diffraction, and Sbsorpticn. Excitation for X-Ray Analysis. Dispersion: Spectrometer Geometry and Crystal Properties. Detectors and Circuits. Energy Dispersion. Analysis, Precision, and Accuracy. Mathematical Methods for Quantitative Analysis. Applications and Specimen Preparation. Electron Probe Microanalysis. Appendix. 7969 743 pages $9.95

ANCILLARY TECHNIQUES FOR GAS CHROMATOGRAPHY Edited b y LESLIE S. ETTRE, Encyclopedia of lndustrial Chemical Analysis; and WILLIAM H. McFADDEN, lnternational Flavors a n d Fragrances. Here is a discussion of individual systems relating gas chromatography with the various ancillary methods. The book does not simply give a literature survey of all the techniques dealing with a particular interfaced system, but rather it summarizes the aspects of a particular system, focusses on its pros and cons, describes the most important Instrumental arrangements, and discusses what kind of information can be obtained through its use. CONTENTS: Principles: What are Ancillary Techniques? Microreaction Gas Chromatographic Techniques. Pyrolysis Gas Chromatography. Precolumn Reactions for Structure Determination. Gas Chromatography and Mass Spectroscopy. Gas Chromatography and Infrared and Raman Spectrometry. Gas Chromatography and Thin-Layer Chromatography. Chemical Identification of Gas Chromatographic Fractions. Special Iden1969 Approx. 416 pages $17.50 tification Detectors.

PROGRESS IN GAS CHROMATOGRAPHY Edited b y HOWARD PURNELL, University College, Wales. These articles cover well established areas such as preparative scale methods and effluent identification, which heretofore have not been discussed critically or in depth. Promising new areas of development are explored: adsorbent modification, pressure programming, liquid surface affects, physicochemical measurement, and theoretical “tailoring” of solvents. In addition, a very personal, and practical view of column theory leads to interesting and novel comparisons of column performance and gives much original guidance on practical matters. Volume 6 i n the Wiley-lnterscience Advances i n Analytical Chemistry and Instrumentation Series. CONTENTS: PART I : THE PHYSICO-CHEMICAL BACKGROUND OF GAS CHROMATOGRAPHY: SOLUTION, ADSORPTION, AND PARTITION. The Vaporisation of Liquids. Diffusion in Gases and Liquids. The Flow of Gases. PART II: CHROMATOGRAPHIC THEORY. Techniques of Chromatography. The Theoretical Plate Concept. Rate Theories of Chromatography. Experimental Tests of the Rate Theory. The Thermodynamics of Gas Chromatography. PART Ill: GAS CHRO36A

MATOGRAPHIC PRACTICE. Equipment and Materials. Detectors. Analytical Techniques. Non-Analytical Applications. 7968 392 pages $74.95

THE ANALYTICAL CHEMISTRY OF SULFUR AND ITS COMPOUNDS-Part

I

Edited b y J. H. KARCHMER, Esso Research and Engineering Company. This volume is concerned primarily with inorganic sulfur compounds, and provides numerous analytical procedures drawn from widely scattered sources and written by outstanding people in the field. The main emphasis is on the principles underlying the analytical methods. Thus, the analytical chemist is provided with the means of devising his own method to solve his particular problem and to avoid the disastrous effects of possible interfering materials. Information covers scope, limitations, lower limit of sensitivity, accuracy, and reproducibility of selected analytical procedures. Volume 29 i n the Wiley-lnterscience Chemical Analysis Series. 1970 Approx. 528 pages $35.00

CHEMICAL APPLICATIONS OF SPECTROSCOPY, Second Completely Revised and Augmented Edition-Volume 9, Part 1 Edited b y W. WEST. Techniques of Organic Chemistry Editor: ARNOLD WEISSBERGER. A completely rewritten and updated presentation of the theory of spectra of organic compounds. This part is essentially devoted to the electronic absorption and emission spectra of organic compounds. CONTENTS: Introductory Survey of Molecular Spectra (W. West). General Theory of Electronic Spectra (A. B. F. Duncan). Interpretation of Electronic Absorption Spectra (F. A. Make?, R. S. Becker, and D. R. Scott). Fluorescence and Phosphorescence Spectroscopy (R. S. Becker). 7968 486pages $76.95

MODERN SEPARATION METHODS OF MACROMOLECULES AND PARTICLES Edited b y THE0 GERRITSEN, University of Wisconsin Medical Center. Describes some of the newest techniques for separating macromolecules and particles of biological importance. The macromolecules range from polypeptides through histones and serumproteins to ribosomal RNA. The particles range from viruses through subcellular organelle to lymphocytes and erythrocytes. Volume 2 in the Wiley-lnterscience Progress in Separation and Purification Series. CONTENTS: Large Pore “Disc” Electrophoresis. Differential Elution of Trapped Macromolecules. The Application of FreeFlow Electrophoresis to the Separation of Macromolecules and Particles of Biological Importance. The Use of Gradients of Colloidal Silica for the Separation of Cells and Cellular Particles. Equilibrium Density Gradient Separation and Analysis of Lymphocyte Populations. Partition in Polymer Two-Phase Systems-Some Recent Results. Factors in the Partition of Blood Cells i n Aqueous Dextran-Polyethylene Glycol TwoPhase Systems. Partition of Macromolecules in the Polyethylene Glycol-Dextran Phase System. The Separation of Lymphocyte Populations on Glass Bead Columns. Gel Filtration on Agarose Gels. On the Theoretical Aspects of Gel Chromatography. Separations Based on Size and Conformations. 1969 250 pages $74.95


ADVANCES IN ANALYTICAL CHEMISTRY AND INSTRUMENTATION-Volume 7 Edited b y CHARLES N. REILLEY, University of North Carolina a n d FRED W. McLAFFERTY, Purdue University. The latest volume in a continuing series dealing with significant new research, techniques, and developments as well as the status of important but more classical ideas. Written by outstanding workers i n the field. CONTENTS: Ultramicroanalysis with the Microscope (Walter C. McCrone): Recent Advances in Rdman Spectroscopy (J. C. Evans); The Production and Radioessay of Tritium Labeled Compounds (J. K. Lee and F. Schmidt-Bleek); Application of Kinetics to Automated Quantitative Analysis (Harry L. Purdue). 1968 229 pages $12.95

INTRODUCTION TO MASS SPECTROMETRY: Instrumentation and Techniques B y JOHN ROBOZ, Air Reduction Company, lnc. Presents an introduction to the theory, design, and operation of the various types of mass spectrometers. Shows how to select commercial mass spectrometer systems and components by discussing required instrument performance in many fields of applications. "This very comprehensive volume has quite adequately fulfilled the objectives stated by the author. . . . There are 503 literature references; the most recent is 1968, plus a list of 84 books and reviews arranged by categories. The book should . . . , b e valuable in any mass-spectrometry laboratory."-Glenn P. Happ, Eastman Kodak Company, in Journal of the Optical 1968 539 pages $22.50 Society of America.

MASS SPECTROMETRY IN SCIENCE AND TECHNOLOGY B y FREDERICK A. WHITE, Rensselaer Polytechnic Institute. In this interdisciplinary book, the author points u p the vital role of mass spectrometry i n nuclear and reactor physics, semiconductors and plasmas, chemistry, geology, environmental science, the physics and chemistry of surfaces, materials research, and space-related studies. The vast potential of mass spectrometry and stable isotopic tracers in medicine and biology is discussed, and the use of ion beams for fabricating semiconductor devices is presented. The latest instrumental developments relating to ion sources, ion detectors, and mass resolving systems are outlined and illustrated. An important information source, this volume brings a perspective to the subject that is of value to those engaged in research in the physical and life sciences and to engineers who are concerned with product monitoring and process control. 1968 352 pages $15.95

chemistry, the editors have compiled material concerning the historical development, general aspects, and methods of investigation of their subject. In the following three volumes, they deal further with the major types of carbonium ions, with the classical-nonclassical ion problem, and with the diversity of these ion types. The work of the leading scientists in the field has been compiled in this book to provide a unified presentation. Different papers with opposing views of controversial topics have also been included to give readers the chance to compare alternative interpretations. As a result, the material i n the book is a combination of invaluable information about the known facets of the field, and of stimulating discussions of less-known areas. A volume i n the Reactive Intermediates in Organic Chemistry series. 1968 462 pages $79.95 Volume II will be available in 1970. Approx. 464 pages $20.00

ION EXCHANGE AND SOLVENT EXTRACTION OF METAL COMPLEXES B y Y. MARCUS and A. S. KERTES, both at the Department of lnorganic Chemistry, The Hebrew University of Jerusalem. This comprehensive work treats solutions of electrolytes in aqueous and non-aqueous solvents and complex formation in solution as a general background. On this basis, it discusses ion exchangers and their role in complex chemistry, and solvent extraction of acids and metal complexes with various types of solvents, singly and i n mixtures. The book concludes with a comparative review of several illustrative systems, from a unified view of the two met ods discussed. CONTENTS: Preface; Aqueous Solutions of Electrolytes; Nonaqueous Solutions of Eelctroiytes; Complex Formation in Solution: Ion Exchangers; Cation Exchange of Complexes: Anion Exchange of Complexes; Principles of Solvent Extraction; Extraction by Compound Formation; Extraction by Solvation; Extraction by Ion-pairing: Synergistic Extraction; Application of Distribution Methods: Appendices; Author Index; Subject Index. 1969 1037 pages $44.95

ULTRA MICRO WEIGHT DETERMINATION IN CONTROLLED ENVIRONMENTS Edited b y SUMNER P. WOLSKY a n d EDWARD J. ZDANUK, both at P. R. Mallory & Co. lnc., Burlington, Mass. This book is the first centralized source of information on Ultra Micro mass determination through the use of the conventional and quartz crystal oscillator microbalances. The book focuses on theory and design; applications; and commercially available equipment. This organization provides an orderly introduction to those unfamiliar with the field. 1969 511 pages $19.95

CARBONIUM IONS-Volume I, General Aspects and Methods of Investigation Edited b y GEORGE A. OLAH, Case Western Reserve University; and PAUL VON R. SCHLEYER, Princeton University. In this first volume of their monograph on carbonium ion

-_--_----_--___-_______-___----

I

SAVE MONEY! Enclose your check and Wiley pays all delivery and handling charges. Gentlemen: Please send me each of the books I have checked below under the following and understand that Wiley-lnterscience pays all delivery and terms. I enclose $handling charges. If I am not satisfied with the book(s) I may return the order within 10 days for a full refund. 17 Bill me. I will pay handling and postage. Same IO-day privilege applies. Birks: X-RAY 471 07525-6 $9.95 0 Reilly/McLafferty: Ettre/McFadden: TECHNIQUES 471 24670-0 $17.50 ANALYTICAL 471 71534-4 $12.95 Purnell: PROGRESS 470 70238-9 $14.95 0 Roboz: SPECTROMETRY 470 72840-3 $22.50 Karchmer: SULFUR, Pt. I 471 45857-0 $35.00 0 White: SPECTROMETRY 471 94065-8 $15.95 West: TECHNIQUE OF ORGANIC CHEMISTRY, Olah/Schleyer: IONS, V. 1 470 65330-2 $19.95 V. 9, 2nd Ed., Pt. 1 470 93160-4 $16.95 0 Marcus/Kertes: METAL 471 56858-9 $44.95 Gerritsen: SEPARATION 471 29695-3 $14.95 0 Wolsky/Zdanuk: WEIGHT 471 95964-2 $19.95

To: WILEY-INTERSCIENCE, Dept. 211 a division of JOHN WILEY & SONS, Inc. 605 Third Avenue, New York, N.Y. 10016 In Canada: 22 Worcester Road, Rexdale, Ontario NAME ADDRESS COMPANY AFFILIATION CITY

I

I 1,

YOUR POSITION STATE

ZIP on Readers' Service Card ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970

Circle No. 58

37A

Report

NOSIR. THERE’S NOTHING SIMPLZ A B o v t RUNNING THIS LAB.

WHAT TEsrs TO RUN WHEN WHO CETS FNY RAISES.

IT’S

DECISIONS, DECISIONS, DECISIONS.

SOMEnMES INEN HAVETO AR0lWXTf COFFEE BRLAKss

AND THE B U T W S W A R E IS w. WRCHASIWG MclSlONS! PYRE%. ONW CORNING MAKES WE’% LABWARE. WHAT’S GOOO? WHATS BAD? NO OWSION WERE

Do YOU KNOW MY SON,

THE WREX”

No m a t t e r whether you know the salesman o r not, you c a n k n o w t h a t every PYREX b r a n d flask, beaker, bottle, dish, condenser, pipet, buret, cylinder is m a d e b y Corning. That’s c o m f o r t i n g , isn’t it? Available i n quality a n d q u a n t i t y from your Corning dealer.

SALESMAN?

CORNING L A B O R A T O R Y PRODUCTS

Makers of PYREX& Labware Circle 38A

NO. 51 on

Readers’ Service Card


higher absorption coefficients for the exciting X-ray lines. ilnother factor of importance in recording intensities is the kinetic energy of the electron emerging from the sample. I n general, the greater the kinetic energy, the more likely the electron to emerge unperturbed by the sample. When one considers a given type of core electron (such as 2s electrons) of a number of different elements, this effect will tend to favor the elements of low atomic number since the binding energies of their electrons will be lowest. Another factor which will be somewhat important is the detector efficiency, although with electron multipliers this is much less a problem than with the earlier GAI detectors. I n general, detector efficiency will show slight favoritism toward higher energy electrons again favoring elements of low atoiiiic number. Based on the factors ahove, it is apparent that a system will have to be well calibrated for excitation with a given X-ray line. This sort of calibration should not be difficult to perform, and once a spectrometer system is calihrated, recalibration should not be necessary. Vsing calibrations, such as those suggested, Siegbahn e t a l . ( 7 ) have performed quantitative analyses on a numlier of samples. They qualititatively deterniined the carbonto-chlorine-to-sulfur ratios in a number of organic compounds, as well as the carhon, nitrogen, oxygen. and sulfur ratios in a variety of amino acids and insulin. With the exception of oxygen analyses the results were quite good. generally to within is%. Sieghahn e t a l . ( 7 ) have also analyzed brass samples containing zinc 10-5056, copper 50-90%, tin c a . 0.7% and lead cn. 0.8%. Again, the relative precision was approximately 5 % . Because Siegbahii e t nl. were not primarily interested in electron spectroscopy as a n analytical tool, their calibrations were somewhat crude and do not represent the ultimate achievahle precision which could be expected from the technique. It is not inconceivable that with better calibrations precision 1 x 1 0 ~1% should he ohtainable.

Advantages and Disadvantages of Electron Spectroscopy as Analytical Technique

At this point after extolling the virtues of electron spectroscopy for several pages, it might be well t o consider what the pros and cons of the technique are for the analytical chemist. T h a t it has much potential in both qualitative and quantitative analysis is evident and t h a t the potential applications t o chemistry haye not yet, t o use a pun, "scratched the surface." Before anyone undertakes a program in electron spectroscopy, however, there are a number of factors which should be known, some of which have been mentioned before but which will be summarized here. Electron spectrometers require residual magnetic fields of ca. 10-4 G and a vacuum of better than 10-5 Torr. The technique is good for solid, liquid, and gaseous samples. Solid samples, both amorphous and crystalline, can be placed on tape or put into pellets, similar to the use of XBR in the infrared. Gas samples can be studied by using differential pumping techniques, the sample compartment serving as the containers. Liquid samples, assuming they have reasonable vapor pressure a t room temperature, must be frozen on a cold finger t o reduce vapor pressure. Elcctroii spectroscopy t o date has not been effective in studying solutions because of the high vacuum necessary-even using differential pumping techniques t o keep the solvent out of the main vacuum chamber ; any studies done on solutions under these conditions would be of questionable validity, with the solvent boiling from the surface of the solution. If Vacuum pump oil containing carbon is used, unless precautions are taken t o minimize diffusion of the pump oil into the main vacuum system, one obtains hydrocarbon coatings on all samples. This can be both an advantage and a disadvantage. The carbon-1s photoelectron line can be used as a standard; however, i t can also be annoying when studying surfaces or doing quantitative analysis. The technique is both qualitative and quantitative. Usually photoelectron lines of adjacent elements of the periodic table are far apart

so t h a t overlap between lines from similar electrons of nearest neighbors does not occur. For example, the nitrogen Is line is a t ca. 400 eV, carbon a t 290 eV, and oxygen at 530 eV. A similar situation is found for silicon, phosphorus, and sulfur. Electron spectroscopy is useful for almost every element in the periodic table. It is a particularly valuable technique for determining one element in the presence of many others. For example, Siegbahn has observed a cobalt signal from vitamin B12 where cobalt is one in 180 atoms. However, electron spectroscopy under current conditions of resolution is not good for distinguishing one atom only slightly different from say 20 others. I n this case the photoelectron peak from the one atom in question would appear only as a slight shoulder on a much higher peak and would probably be lost in the noise. The size of the sample required can be quite small. Photoelectron spectra are obtainable from micrograms of material, and Siegbahn has reported spectra from samples as small as g. It is not inconceivable t h a t electron spectroscopy could do a chemical analysis on a fingerprint. I n addition to its sensitivity, the technique should be, with reasonable advances in calibration, capable of 1% precision. Electron spectroscopy is generally a nondestructive technique. However, some organic compounds do decompose under bombardment from X-rays. For example, p nitroaniline has been shown t o decompose as have some chlorinated anilines. When an electron gun source is used for excitation, generally sample decomposition will be greater. For the determination of trace quantitlies of one element in the presence of others, electron spectroscopy should be sensitive t o the tenths of a per cent range. ACKNOWLEDGMENTS

The author wishes t o acknowledge the valuable assistance of John Jack, William Swartz, and Anthony Waraksa in setting up our electron spectroscopy program. I n

/

S&S now offers the Scientific Community three outstanding new products. Each one already a winner in its field. 1. "Tear-Proof" Folded Filter Papers, Exceptional high wet-strength; fast. Highly resistant to corrosive solutions and organic solvents. In different densities, and sizes from 12.5 up to 32 cm. Additional information available, Bulletin 6142. 2. Bac-T-Kote" Membrane Coated Filters. For clarification and sterilization of liquids such as drugs, wines, beverages. Three clarification grades, one sterilization grade. Exceptional ease of handling in press operations. Additional information, Bulletin S144. 3. TLC "ReadyPlates." Composed of cellulose and silica gel powders. The sub strata is glass. Plate size 10x20 cm, in packages of 50; 20x20 cm, in packages of 25. Also available are polyamid acetylated and ion exchange plates. Additional information, Bulletin #51, For free literature, fill out and mail coupon below, checking corresponding number.

Schleicher 8, Schuell, Inc.

Keene, New Hampshire 03431

Gentlemen: Please send free literature on Items circled below. 1. "Tear-Proof" 2. Bac.T.Kote a. TLC Folded Membrane "Ready.Plates" Filter Papers Coated Papers Name Address CltY State

7,"

AC-701

Circle No. 95

on Readers' Service Card


39A

You can cut costs two ways with the new Beckman F u t u r a e l e c trodes equipped with the exclusive Keeper CableTM.Here's how: You save 15% on replacement costs of electrodes because you don't have to replace the cable. And, if your cable-length requirements change, you change cable length without changing the electrode. Either way. . . both ways. . , you save and keep on saving. FuturaSeries electrodes are available in a wide selection of glass, reference and combination models. For full information, write for Data File 1319, Scientific Instruments Division, Beckman Instruments, Inc., 2500 Harbor Boulevard, Fullerton, C a l i f o r n i a 92634.

addition, I would like t o acknowledge contributions from Stig Hagstrom, Carl Nordling, Ragnar Nordberg, Anders Fahlman, .and Kai Siegbahn. Support of our photoelectron spectroscopy research by the United States Energy Commission under Contract AT(30-1)-905 to the Laboratory for Nuclear Science cat M I T is gratefully acknowledged.

INC.

This work was supported in part through funds provided by the United States Atomic Energy Connmission under Contract NO. AT(30-1)-!305.

BIBLIOGRAPHY (1) L. A. Harris, Anal. Chem. 40, 14, 24A (1968). (2) N. Svartholm and K. Sicgbahn, Arkiu. Mat. Astron. Fys., 33A, 21 (1946). (3) X. Siegbahn and K. Edvarson, Nucl. Phys., 1. 137 (1956). (4) C. Nodling, E. Sokolowski, and K. Sieebahn. Arlizw. Fus.. " ,13.483 (1958). ( 5 ) S. Hagstrorn, C. Nordling,' and K. Siegbahn, Phys. Lett., 9,235 (1964). (6),C., *dl&g, Hagstrom, and ,?; SilegDahn, L. I-tLyszK, 178, 483, qSY (1964). (7) K. Siegbahn et a!,. ESCA "Atomic, Molecular and Solid State Structure Studied by Mcans of Electron SpecI

S.

(26) C. Baker and D. W. Turner, Chem Comms, 1967,797. (27) A. D. Baker, D. P. May, 'and D. W. Turner, J . Chem. SOC., 1968B,22. (28) I. D. Clark and D. C. Frost, J . Amer. Chem. Soc.. 89,244 (1967). (29) M. J. S. Dewar and S. D. Worley, J . Chem. Phys., 50,654 (1969).

,

I

S.

~~

troscopy: Almquist and Wiksells, Upp-

Futura'": the DOUBLE-VALUE electrode

4 R I R ~1qfi7 ----, ( 8 ) D. W. Turner and M. I. AI-Joboury, J . Chem. Phys., 37,3007 (1962). (9) M. I. AlJoboury and D. W. Turner, J . Chem. Soc.. 1963.5141. (10) D. W. Turner, Proc. Roy. Soc., (London) A307,15 (1968). (11) G. E: LyWhiting, "Uniform Mag-

netic Fxelds, At. Energy Can., Ltd., Chalk Rivcr Report CRT-673, Chalk River, Ont., 1957. (12) D. C. Frost, C. A. MeDowell, and D. A. Vroom, Proc. Roy. Soc., (London) A296,566 (1967). (13) J. C. Helmer and N. H. Weichert,

Appl. Phys. Letters, 13,266 (1968). (14) E. M. Purcell., Phvs. . Rev.., 54.. 818 ' ( i w . D. M. Hercules graduated from Ju(15) C. Nordling, E. Sokolowski, and K. niata College with a B.S. degree in Siegbahn, Ark. Fys., 13,483 (1958). (16) S. Hagstrom, C. Nordling, and K. chemistry in 1954; be received his Siegbahn, Z. Physzk, 178,439 (1964). Ph.D. from Massachusetts Insti(17) S. angstrom, C. Nordling, and K. . tute .of Technology in analytical Siegbahn, Phys. Lett., 9,235 (1964). (18) C. S. Fadley, S. B. M. Hagstrom, chemistry in 1957 after studying J. M. Hollander, M. P. Klein, and D. A. with L. B. Rogers. After faculty Shirley, Science, 157, 1571 (1967). at Lehigh University, association (19) C. S. Fadley, S. B. M. Hagstrom, M. P. Klein, and D. A. Shirley, J . Juniata, and MIT, he became asChem. Phus.. (1968). " ,48.3779 . sociate professor a t t h e University (20) A. Fahlman, R. Carlsjon, and K. Siegbahn, Ark. Kemi, 25,301 (1966). of Georgia in September 1969. Dr. (21) J. M. Hollander, D. N. HendrickHercules' research interests include son, and W. J. Jolly, J . Chem. Phys., trace analysis by fluorescence and 49,3315 (1968). phosphorescence, relationships be(22) D. M. Hercules, unpublished studies. (23) G. Atelson, K. Hamrin, A. Fahlman, tween luminescence and molecular C. Nordling, and B. J. Lindberg, Specstructure, chemiluminescence, electrochzm. Acta., 23A.. 2015 (1967). troluminescence, the chemistry of (24) M. I. AI-Jaboury and D. W. Turner, J . Chem. Sac., 1964,4434. molecules in electronically excited (25l.T. N. Radwan and D. W. Turner, states, and electron spectroscopy. zbzd., 1966A, 85. ~~

~~

Circle NO. 82 on Reader$' Sewice Card

40 A

ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JAMUARY 1970

~~

Electron spectroscopy - Analytical Chemistry (ACS Publications)

Recommend Documents