Instrumentation Ron Jenkins Philips Electronic Instruments. Inc. Mahwah. N.J. 07430
Following the discovery of X-rays in 1895 by Wilhelm Roentgen (I),most of the early application of X-rays was in the medical field. However, in the past 30 years or so, use of techniques based on the properties of X-rays has played an increasingly important part in materials characterization and analysis. X-rays are electromagnetic radiation and are manifested in two forms, continuous radiation and characteristic radiation. Continuous radiation is produced when a high-energy electron beam decelerates as it approaches the electron clouds that surround the atomic nucleus. Characteristic radiation is produced following the ejection of an inner orbital electron by highenergy particles and subsequent transition of atomic orbital electrons from states of high to low energy. In 1912 Moseley (2) showed that there is a simple relationship between the emission wavelengths and the atomic number of the excited element. Following the demonstration of the diffraction of X-rays by Max van Laue in 1912 (3),two major fields of materials analysis have developed. X-ray emission spectrometry is a technique for qualitative and quantitative elemental analysis by measurement of the wavelengths and intensities of Xrays from the excited elements. The second technique is X-ray diffraction, which is a means of materials characterization wherein the X-ray scattering characteristics of a substance are related to the molecular arrangement of the atoms making up the scatterer. This paper is specifically directed to the first of these methods, namely, X-ray emission spectrometry, in which primary X-rays are used to excite characteristic secondary radiation from the specimen being analyzed. The technique is therefore referred to as X-ray fluorescence spectrometry. 0003-2700/84/A351-1099$01.50/0 @ 1984 American Chemical Society
Transmiss.,.
I
Figure 1. Properties of X-rays
X-ray photons are characterized hy wavelength (A) and ener (E),which are related in the form A$) u 12.4lE (keV). X-ray fluorescence spectrometry uses either the diffracting power of a single crystal to isolate narrow wavelength bands or a proportional detector to isolate narrow energy bands from the polychromatic beam of characteristic radiation excited from the sample. The first of the methods is called wavelength-dispersive spectrometry, and the second, energy-dispersive spectrometry. Because of the known relationship between emission wavelength and atomic number, isolation of individual characteristic lines allows the unique identification of an element to be made, and elemental concentrations can be estimated from characteristic line intensities. Thus this technique is a means of materials characterization in terms of elemental composition.
Properties of X-rays When a beam of X-ray photons falls onto an absorber a number of different processes may occur, the two most important of which are photoelectric
absorption and scatter, as illustrated in Figure 1. In this example, a monochromatic beam of radiation of wavelength A,, and intensity Io is incident on an absorber of thickness I and density P. A certain fraction (Illo)of the radiation may pass through the ahsorber, the wavelength of the transmitted beam remaining unchanged. The intensity of the transmitted beam is given by I(X,) = I,.exp(-apx), where p is the mass absorption coefficient of the absorber for the wavelength b.The mass absorption coefficient term is made up of two parts, photoelectric absorption and scatter, of which the first is more important. Because photoelectric absorption is made up of absorption in the various atomic levels it is an atomic-numberdependent function. A plot of against A contains a number of discontinuities called absorption edges, a t wavelengths corresponding to the binding energies of the electrons in the various subshells. The absorption discontinuities are a major source of nonlinearity between X-ray intensity and composition in X-ray fluorescence spectrometry.
ANALYTICAL CHEMISTRY, VOL. 56, NO. 9. AUGUST 1984
IOWA
How toselect LC filters to maximize resolution. ~~
A column inlet filter is usually necessary to capture contaminating particles, yet this filter may easily impair resolution if not wisely selected. Rheodyne's Tech Note 6 reports experiments measuring how much filters of various sizes and flow geometry affect the resolution achieved by columns of several sizes. The'newer microbore columns and short 4.6-mm columns prove to be most sensitive to filter performance. If their resolution is to be preserved, an inlet filter with very little sample dispersion must be used. The Tech Note helps the reader select the optimum filter for his application: one with little enough sample dispersion to preserve resolution, yet large enough capacity to prevent a rapid rise in backpressure.
Send for Tech Note #6 For a copy free of charge contact Rheodyne, Inc., PO. Box 996, Cotati, California94928, U.S.A. Phone (707) 664-9050.
11OOA
Figure 2. Major landmarks in the development of wavelength and energy-dispersive spectrometers and the number of units sold WDS = wavelengtbdispersive 6pecbomer: EDS = energydisperslvespeclrometw
Scattering may also occur when an X-ray photon collides with the electrons of the absorbing element (Figure 1). Where this collision is elastic (i.e., no energy is lost in the collision process), the scatter is said to be coherent scatter (A = X, ). Under certain geometric conditions the coherently scattered wavelengths, which are exactly in or out of phase, may cancel each other out or add to one another. The addition of waves is called constructive interference, and this may give rise to diffraction maxima. A single crystal lattice consists of a regular arrangement of atoms. When a beam of radiation of wavelength X falls onto one of the sets of these atomic layers, in which the interatomic layer spacing is d, scattering will OCCUI a t an angle 0 in diffraction order n. The overall con. dition for reinforcement is that nX = 2d sin 8, this being a statement of Bragg's law. Thus, a single crystal of
ANALYTICAL CHEMISTRY, VOL. 56, NO. 9. AUGUST 1984
suitable interplanar spacing can be used as a means of dispersing a polychromatic beam of radiation into individual wavelengths so that the value of the wavelength can be measured and the intensity can be established.
X-ray Fluorescence Spectrometry X-ray fluorescence spectrometry is a well-established analytical technique and one of the most versatile methods for elemental analysis available today. Wavelength-dispersive spectrometers have been commercially available since the early 19508, and there are probably about 12 000 units in use worldwide today. Energy-dispersive spectrometer systems became available in the early 19709, and there are probably about 1000 stand-alone spectrometers in use, with perhaps slightly more than this number attached to scanning electron microscopes. X-ray fluorescence spectrome-
Those small LC columns neea a better filter. We're talking about those 1-mm or 2-mm microbore columns and short high-speedcolumns. Like larger conventional columns, they should have an inlet filter to protect them from particle contamination. Unlike conventional columns, they are highly sensitive to sample dispersion within this filter Filter sample dispersion that has little effect on the resolution of a conventional column can seriously reduce the resolution of a small column. To remedy this situation, Rheodyne made two low-dispersion filters designed especially to preserve the resolution of
small columns. The Rheodyne 7315 for microbore columns. And the Rheodyne 7335 for short high-speed columns. They're identical except for a tiny internal filter element. The accompanying chromatograms show the dramatic improvement in resolution that occurs when a new low-dispersion filter is used in place of a conventional filter For more about these new column inlet filters-plus our Tech Note 6 describing the experimental effect of filters on resolution-contact Rheodyne, Inc.. PO. Box 996, Cotati, California 94928, UEA.Phone (707) 664-9050.
I
.~ . LEFT Conventionalfiller With 1 mrn 2300 mm c o i m RIGHT Rheodyne 7315 lowdspersian lillerwilh 1 mrnx300 rnrn column ~~
~
.
'-
RHEOL,., THE LC CONNECTION COMPANY CIRCLE 178 ON READER SERVICE CARD
I I
ters were among the first analytical instruments to make full use of the potential of computers. The first computer-controlled spectrometers were produced as long ago as 1964. Today almost all spectrometers incorporate at least a microprocessor and in many cases also include a minicomputer. Figure 2 shows the major landmarks in the development of X-ray spectrometers ( 4 ) . The basis of the X-ray fluorescence technique lies in the relationship between the wavelength X (or energy E ) of the X-ray photons emitted by the sample element and the atomic number 2. This relationship has the form: El12.4 = l l h = K ( Z - s ) ~ in , which K and s are constants that depend on the spectral series of the emission line in question. When an atom is bombarded with high-energy particles (for example, X-ray photons), an inner orbital electron may be displaced, leaving the atom in an excited state. The atom can regain stability by rearrangement of the atomic electrons, and inner-shell vacancies may be filled with electrons from outer shells, leading to the emission of characteristic X-radiation. Not all vacancies result in the production of characteristic X-ray photons, because there is a competing internal rearrangement process known as the Auger effect. The ratio of the number of vacancies resulting in the production of characteristic X-ray photons to the total number of vacancies created in the excitation process is called the fluorescent yield, The fluorescent yield takes a value of around unity for the high-atomic-number elements to as little as 0.01 for the low-atomic-number elements, such as Na, Mg, and Al. For those vacancies giving rise to characteristic X-ray photons, a series of very simple selection rules can be used to define which electrons can be transferred. In general, a given element may give anywhere from three to 30 lines within the wavelength range of a typical commercially available spectrometer. Most commercially available X-ray spectrometers have a range from about 0.2 to 20 A (60-0.6 keV), which will allow measurement of the K series from F ( Z = 9) to Lu (2 = 71), and for the L series from Mn (2 = 25) to U (2 = 92). Other line series can occur from the M and N levels, but these have little use in analytical X-ray spectrometry. Although in principle almost any high-energy particle can be used to excite characteristic radiation from a specimen, an X-ray source offers a reasonable compromise between efficiency, stability, and cost, and almost all commercially available X-ray spectrometers use such an excitation source. 1102A
*
Instrumentationand Techniques for X-ray Spectrometry All conventional X-ray spectrometers comprise three basic parts-the primary source unit, the spectrometer itself, and the measuring electronics. The primary source unit consists of a very stable high-voltage generator, capable of providing up to around 3 kW of power at a potential of typically 60-80 kV, plus a sealed X-ray tube. The sealed X-ray tube has an anode of Cr, Rh, W, Ag, Au, or Mo. It delivers an intense source of continuous radiation, which then impinges on the analyzed specimen where characteristic radiation is generated. A portion of the characteristic fluorescence radiation is then collected by the actual spectrometer, in which the beam is passed via a collimator or slit onto the surface of an analyzing crystal. The crystal diffracts individual wavelengths in accordance with Bragg’s law. A single-channel,or sequential, X-ray spectrometer has one such channel, and this is generally provided with a number of crystals to select the appropriate wavelength range and dispersion conditions. A multichannel, or simultaneous, X-ray spectrometer has many single-channel spectrometers grouped around the sample, allowing the simultaneous measurement of up to 28 elements a t the same time. Although the sequential spectrometer has more versatility than the simultaneous system, it is slower. A photon detector, typically a gas flow or a scintillation counter, is then used to convert the diffracted characteristic photons into voltage pulses, which are integrated and displayed as a measure of the characteristic line intensity. Most modern wavelength-dispersive spectrometers are controlled in some way by a minicomputer or microprocessor, and by use of specimen changers are capable of very high specimen throughput. Once they are set up, the spectrometers will run virtually unattended for several hours. Like the wavelength-dispersive spectrometer, the energy-dispersive spectrometer consists of the three basic units-an excitation source, a spectrometer, and a detection system. In this case, however, the detector itself acts as the dispersion agent. The detector is generally a lithium-drifted silicon, Si(Li), detector, which is a proportional detector of high intrinsic resolution. The resolution of the detector is typically 160-180 eV, compared with 20-200 eV for the wavelength (crystal) dispersive system. The Si(Li) detector diode serves as a solidstate version of the gas flow detector in the wavelength-dispersive system, where the gas gain is unity. When an
ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUGUST 1984
X-ray photon is stopped by the detector, a cloud of ionization is generated in the form of electron-hole pairs. The number of electron-hole pairs created, Le., the total electric charge released, is proportional to the energy of the incident X-ray photon. The charge is swept from the diode by a high voltage applied across it. A preamplifier is responsible for collecting this charge on a feedback capacitor to produce a voltage pulse that is proportional to the original X-ray photon energy. Thus, when a range of photon energies is incident upon the detector, an equivalent range of voltage pulses is produced as a detector output. A multichannel analyzer is used to sort the arriving pulses a t its input to produce a histogram representation of the X-ray energy spectrum. The output from an energy-dispersive spectrometer is generally displayed on some sort of visual display unit. The operator is able to dynamically display the contents of the various channels as an energy spectrum, and provision is generally made to allow the operator to zoom in on portions of the spectrum of special interest, to overlay spectra, to subtract background, and so on, in a rather interactive manner. As in the case of the modern wavelength-dispersive systems, nearly all energy-dispersive spectrometers will incorporate some form of minicomputer that is available for spectral stripping, peak identification, quantitative analysis, and a host of other useful functions. Because all of the characteristic X-ray photons from the sample are directed onto the Si(Li) detector a t the same time, the photon flux a t the detector can be very high. The resolution of the Si(Li) detector and its associated electronics tends to deteriorate a t count rates in excess of about 50 000 counts/s, and it is necessary in actual analysis to limit the count rate to less than this value. Essentially two types of source unit are used to achieve this purpose. In the bremstrahlung source system a low-power (sometimes pulsed) tube is used. This is powered at around 500 W, compared with 3-4 kW used in a wavelength-dispersive system. The output from the source may be further modified by use of primary beam filters. In the second type, the secondary fluorescer system, a high-powered X-ray tube is used to excite characteristic radiation from a pure-element sample placed between the primary source and the sample to be analyzed. Thus, only characteristic radiation from the secondary fluorescer strikes the sample. This in turn allows selective excitation of certain elements from the sample. Both the sequential wavelength-dispersive spectrometer and the energydispersive spectrometer lend them-
Elementd d y s k made Convenient, Rapid, Nondestructive, Simultaneous, Qwtihtive, Productive, Sensitive, Easy, and Afforordable.. . bs' L
If you haven't considered enerhT dispersive x-ray fluorescence (EDXRF) because it was too cumbersome. different. o r expensive, call Tracor Xmy and find out ;ibout tlie Series i000 XRF System ;ind how it compares to .M,ICI' and other elemental analysis methods - iiicludiiig other XKF systems. The Tracor Xray i000 XKF System will ch;inge tlie way you think ahout XRF. Ant1 it just m y chmige tlie w y iou (lo elemental analysis. At Tracor X n y , EDXKF has coiiie clown to earth.
Tracor Xray 345 E. Middlelield Road Mountain View. CA 94043 (415) 967-0350 CIRCLE 195 ON READER SERVibc banu
ANALYTICAL GEMISTRY, VOL. 56, NO. 9. AUGUST 1984
1103A
ce. 2-4 RW,
selves admirably to the qualitative analysis of materials. Because the characteristic X-ray spectra are so simple, the actual process of allocating atomic numbers to the emission lines is a relatively simple process, and the chance of making a gross error is rather small. A further benefit of the X-ray emission spectrum for qualitative analysis is that because transitions do arise from inner orbitals, the effect of chemical combination, i.e., valence, is almost negligible. The great flexibility and range of the various types of X-ray fluorescence spectrometer, coupled with their high sensitivity and good inherent precision, make them ideal for quantitative analysis. Like all instrumental
lie at (xossovc.
. CrltlCal In wave-
Mspectral distribution of t b
methods of analysis, the high precision can be translated into high accuracy only if the various systematic errors in the analysis process are taken care of. The precision of a weIl.de. signed X-ray spectrometer is typically on the order of one-tenth of a percent or so, the major source of this random error being the X-ray source, i.e., the high-voltage generator plus the X-ray tube. In addition, there is a small count-time-dependent error arising from the statistics of the actual counting process. Systematic errors in quantitative X-ray spectrometry arise mainly from absorption- and specimen-related phenomena, including particle size and heterogeneity. Although these so-called matrix effects are somewhat complicated, many excellent methods have been developed over the past 20 years for handling them. The first step in any quantitative X-ray analysis is to ohtain a set of “pure” intensities, characteristic line intensities that are free of any instrumental effects. The box (at left) lists the five major steps involved in obtaining these intensities (5).It will he seen that somewhat different fadors may he involved in wavelength- and energy-dispersive methods, hut once the pure intensities are obtained, the method of conversion of intensities to elemental concentrations is the same for both methods. There are many methods for this intensity-to-concentration conversion process, and these are based on both fundamental and semiempirical approaches (6). The state of the art in X-ray fluorescence is that some standards are always required for quantitative analysis if the highest degree of accuracy is sought. In quantitative terms this accuracy corresponds to about one-tenth of a percent absolute. The advent of the minicomputer-controlled spectrometer has done much to enhance the application of the correction procedures, and today in most cases one is able to quantify most elements in the periodic table of atomic number nine (F)and upwards to an accuracy of a few tenths of a percent. The areas of application of the X-ray
1104A * ANALYTICAL CHEMISTRY, VOC. 56,
NO. 9, AUGUST 1984
One ai a time
Slow
&mps of 1-10
Relalively
60-91
fluorescence technique now cover almost all areas of inorganic analysis. Much research has been applied to the problem of extending the lower atomic number limit of the conventional X-ray spectrometer to below atomic number nine (F). Elements such as oxygen (Z = 8),nitrogen (Z = 7), and carbon (Z = 6)are difficult to analyze by most elemental analysis techniques, so the incentive to include these elements within the range of X-ray fluorescence is considerable. Three problems have combined to make this goal difficult to achieve. First, the fluorescence yield of the lower atomic numbers is small, and this in effect means that all other things being equal, copper (Z = 29), for example, will give about 20 times more K alpha photons than would carbon. Second, the X-ray penetration of the longer wavelengths is very small, on the order of a micrometer or so, and sample preparation techniques are extremely critical. Finally, the efficiency of the X-ray spectrometers has not been very good in the long-wavelength region. Much progress has been made in this third area, and recent developments in long-wavelength-efficient X-ray tubes (7) and layered synthetic microstructure crystals ( 8 )now allow the measurement of wavelengths down to about 50 A. With careful sample preparation, therefore, carbon, oxygen, and nitrogen can be measured, albeit with rather poor sensitivities. The analytical chemist has available a wide range of instruments for the qualitative and quantitative analysis of multielement samples, and in the choice of technique such factors as sensitivity, speed, accuracy, cost, range of applicability, and so on will be considered. Within the categories of X-ray spectrometers described, there is a wide diversity of instruments available, but as far as the analyst is concerned, they generally differ only in their speed, their cost, and in the number of elements measurable a t the same time. Table I lists the main characteristics of the four basic categories of X-ray fluorescence spectrometer along
Put Xray analysis in proper perspective. Look into Philips. To perform the kinds of complicated elemental analyses required in both science and industry today, Philips advanced X-ray spectrometer systems offer a number of significant advantages for even the most demanding applications. By incorporatingthe hands-on working experience of our customers into the product research conducted in our extensive laboratory facilities, we have developed the most advanced line of X-ray instrumentationcurrently available. These sophisticated analytical tools !!! provide unsurpassed speed, accuracy and reliability - as well as the ability to detect an extremely wide range of elements. Philips X-ray spectrometers have the sensitivity to reveal and analyze concentrationsfrom 100% to less than a part per million with the highest attainable k level of precision. Samples may be liquid, powder or solid - virtually eliminating the need for complicated preparation procedures. And the acceleratedoperating speeds of these systems enable them to handle as many as 1000 samples per day Our 80 years of internationalX-ray experience has helped us establish and maintain the highest tradition of innovation and excellence in the field. Today, more a* than half the spectrometer installationsin existence conssi of Pnilips eqL pment and %esupport lnem a witn the ndLsiry s mosi comprehensiveworldwide network of sales. training and service personnel. To get a better perspectiveon the process of X-ray spectrometry and how it can increase your analytical capabilities, look into Philips. Write or call: Philips Analytical, Lelyweg 1, 7602 EA Almelo. The Netherlands.Tel. (31) 5490-18291.Telex 36591 (In U.S.A.) PhiliDs Electronic Instruments. Inc.. Analytical X-ray Group, 85 Mcl .ee Drive. Mahwah, NJ 07430 Tel. (201) 529-3800. P
9
L’
i 1
CIRCLE 163
ON READER SERVICE CARD
PHIL I PS
with the relative costa. Single-channel wavelength-dispersive spectrometers are typically employed both for routine and nonroutine analysis of a wide range of products, including ferrous and nonferrous alloys, oils, slags and sinters, ores and minerals, thin films, and so on. These single-channel systems are very flexible, but relative to the multichannel spectrometers, they are somewhat slow. The multichannel wavelength-dispersive instruments are used solely for routine, high-throughput analyses where the great need is for fast,accurate analysis, but where flexibility is of no importance. Energydispersive spectrometers have the great advantage of being able to display information on all elements at the same time. A benefit here is that the data displayed not only show the elements that are present, hut also enable one to determine which elements are not present. This “negative” information is especially useful in, for example, qualitative powder diffractometry, where the object is to reconcile phases revealed by the diffraction method with elemental composition. Although energy-dispersive spectrometers are somewhat lacking in resolution compared with the wavelengthdispersive systems, they find great application in quality control, troubleshooting problems, and so on. They
have been particularly effective in alloy sorting and forensic science. Probably the biggest remaining problem in X-ray fluorescence spectrometry today is the need for fast, reproducible specimen preparation procedures, particularly for powder analysis. A modern multichannel wavelength-dispersive spectrometer is able to collect counts on up to 28 different elements, apply corrections, and print out concentrations, all in
