Flame Spectroscopy The important role of small amounts of elements in physical, chemical, and biological systems has emerged as methods of analysis have increased in sensitivity. Much of this progress may be attributed to the demands of materials science and biological research and to the availability of modern instrumentation. The success achieved in trace element analysis has resulted from the contribution of a wide variety of instrumental techniques involving widely different principles and capabilities. In terms of the amount of information obtained in a given analysis, these trace methods can be conveniently classified into single and multielement methods. A single-element method is optimized to determine a given element with high accuracy and precision. Multielement methods are particularly valuable for survey analyses where simultaneous information on a large number of elements is desired. Obviously, for comprehensive coverage a compromise in operating conditions is often necessary to encompass the different behavior of some elements. The tremendous popularity of the various flame techniques, Le., atomic emission (AE), atomic absorption (AA), and atomic fluorescence (AF), for the solution of a wide variety of trace analytical problems can be related to the speed, high precision and accuracy, simplicity, reasonable sensitivity, and relatively low cost of these techniques. Flame spectrometry, however, has developed primarily as a single-element method. This paper will discuss the multielement potential of each of the three flame techniques and the problems which must be overcome before multielement flame spectrometry becomes a practical reality. In addition, the various approaches currently under study to achieve multielement capability in flame spectrometry will be reviewed.
Flame Cells Table I shows a comparison of the desirable characteristics of flames for multielement emission, absorption, and fluorescence. 712 A
Secondary reaction zone
lnterconal zone Primary reaction zone Stainless steel
Beckman oxyacetylene burner
U
Figure 1. Fuel-rich oxyacetylene flame source for multielement atomic emission
Atomic Emission. With conventional single-element flame emission, 44 elements can be determined with flame sources a t concentrations of less than 1wg/ml ( I ) . This does not mean that 44 elements can be simultaneously determined a t these concentrations by flame emission. These detection limits are obtained by optimization of conditions for each element according to its spectrochemical properties. These spectrochemical properties include: €he excitation potential of the analytical line; the ionization potential of the element; and the freeatom population of the element for a given flame in the zone sampled by the spectrometer. Since these properties vary from element to element and since the flame is not uniform, single-element determinations are conventionally carried out by optimi-
ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
zation of the flame zone sampled by the spectrometer, Le., height, and by altering the chemical and physical environment by optimization of the fuel-to-oxidant ratio. In simultaneous multielement analysis, individual optimization is not possible, and a compromise must be reached. The ideal flame cell for simultaneous multielement flame emission analysis should provide optimum excitation conditions for a wide variety of elements in as small a geometrical region as possible so that this entire region can be viewed by the spectrometer a t one time. Boumans and DeBoer ( 2 )studied the premixed nitrous oxide-acetylene flame as a potential excitation source for simultaneous multielement analysis. They evaluated this flame with a variety of burners and concluded that shielding with either nitrogen or argon is necessary for simultaneous multielement determinations. Shielding produces a large temperature gradient in this flame ( 3 ) ,and Boumans and DeBoer conclude that this provides the required wide range of excitation conditions over a small flame volume. Another flame which has not been studied in terms of simultaneous multielement analysis is the fuel-rich oxyacetylene flame (Figure 1).Fassel and coworkers (4-6) showed that this flame is capable of exciting 43 elements a t less than 1pg/ml concentrations by conventional single-element flame emission. The region of optimum emission is the interconal zone. In a fuel-rich oxyacetylene flame, this region provides a reducing atmosphere necessary for the production of a large free-atom population of analyte, particularly for those elements that have a tendency to form refractory oxides. Further work is necessary to determine the actual potential of this flame for simultaneous multielement analysis. Atomic Fluorescence. In atomic fluorescence the flame functions only to provide a means of producing free atoms from the analyte. In addition to being efficient in producing a large free-atom population, the flame used for atomic fluorescence should ideally
Kenneth W. Busch and George H. Morrison
Report
Department of Chemistry Cornell University Ithaca. N.Y. 14850
Multielement flame spectrometric methods are easily amenable to automation and have many have a low spectral radiance, i.e., background. Whereas the influence of flame background on the detection limit in atomic fluorescence is most severe with an unmodulated source and dc detection, the presence of intense flame background is detrimental even for systems employing modulation. Although the unmodulated flame background is not amplified directly with ac detection, its presence results in noise at the output of the amplifier. This reduces the signal-tonoise ratio and adversely affects the resulting detection limits. The ideal flame cell for simultaneous multielement analysis by atomic fluorescence would be a low background flame which is capable of producing a large free-atom population for a wide variety of elements in a small flame region. In addition, the flame gases should have a low quenching cross section. A low background flame commonly employed for single-element determinations by atomic fluorescence is the turbulent hydrogen/argon/entrainedair flame (7).This flame is actually a hydrogen-air flame where the argon serves as an aspirating gas which has a low quenching cross section. Although this flame has been used in atomic fluorescence studies involving single-element determinations of volatile elements, the low flame temperature and nonreducing environment are likely to lead to severe interferences (8) in multielement determinations. A promising flame for simultaneous multielement atomic fluorescence is the separated nitrous oxide-acetylene flame reported by Kirkbright and West (9) (Figure 2 ) . Flame separation allows observation of the redueing interconal zone of the flame without interference from the radiation emitted by the secondary reaction zone. Thus, the observed background radiation for the hottest flame region, Le., just above the primary reaction zone, is lower in certain wavelengthregions than if a conventional unseparated nitrous oxide-acetylene flame were used. The suitability of this flame will depend on its ability to simultaneously produce large free-atom pop-
potential applications to samples in the clinical, metallurgical, and environmental fields
ulations for a wide variety of elements. Finally, Slevin et al. (10)described the use of an air-acetylene flame in conjunction with a specialized MBker burner for use in atomic fluorescence. This burner provides an outer shielding flame which surrounds an inner flame into which analyte is introduced. The outer flame causes a more uniform temperature gradient across the inner flame, resulting in more uniform atom distributions compared with a standard Mkker burner. It was concluded that the sheathed MBker burner is superior to the standard MBker burner for atomic fluorescence. An air-acetylene flame was used because of its relatively low spectral radiance. The ability of this flame and burner combination to efficiently produce a large free-atom
k'i
Secondary reaction zone
Silica tube
Interconal region
I/CN
emission
+
Burner head
Figure 2. Separated nitrous oxideacetylene flame source with circular slot burner for atomic fluorescence. Flame separation can also b e achieved without silica tube by inert gas shielding of flame for atomic emission
population for a wide variety of elements needs to be evaluated before its suitability for multielement analysis is demonstrated. Atomic Absorption. The flame requirements for simultaneous multielement flame atomic absorption are the production of a large freeatom population for a wide variety of elements in a small flame region. The shielded nitrous oxide-acetylene flame described by Boumans and DeBoer (2) seems most promising in this respect. Primary Sources for AA and AF
Table 11shows a comparison of primary light sources for multielement absorption and fluorescence. Atomic Absorption. Flame absorption is conventionally carried out by use of the principle originally discussed by Walsh (11).This entails the determination of the absorption at the line center by using a narrowline source emitting the given resonance line of the element, whose emission line profile is less than the absorption line profile of the analyte in the flame. Although single-element hollow cathode discharge tubes are readily available for conventional single-element atomic absorption, their use in multielement atomic absorption is limited by the optical requirements of the atomic absorption experiment. Thus, some form of opticalarrangement must be found to combine the separate radiation beams produced by each lamp into a single beam prior to passage through the flame. One approach to this problem was described by Mavrodineanu and Hughes (12)who used the principle of reverse optics to produce a single beam from several hollow cathode lamps. The lamps were arranged along the focal plane of a grating spectrograph at positions corresponding to the wavelength of the desired resonance line. Radiation from each lamp after striking the grating was recombined into a single beam which emerged from the entrance slit and was subsequently passed through the flame. Their arrangement is shown schematically in Figure 3.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 8 , JULY 1973
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41 Versatile, ‘Go Anywhere’ Industrial pH Transmitters
Table I. Flame Characteristics Desirable for Multielement Flame Spectrometry Technique
Characteristics
Atomic emission
Atomic absorption
Atomic fluorescence
Good atomization efficiency for wide variety of elements Long residence time in optical path High temperature for good excitation efficiency Large variation in excitation conditions over small flame region to allow simultaneous excitation and observation of many elements Good atomization efficiency for wide variety of elements Long residence time in optical path Good atomization efficiency for wide variety of elements Long residence time in optical path Low background spectral radiance Low concentration of quenchers Low scattering of exciting radiation
Potential flames Shielded N20-CzH2 Fuel-rich oxyacetylene
N20-C2H2
Separated NzO-C~HZ Flame sheathed airacetylene
Table I I. Primary Light Sources for Multielement Flame Spectrometry Technique Atomic absorption
Approach
Limitations
Single-element hollow cathode lamps Multielement hollow cathode lamps Tandem hollow cathode lamps Time-resolved spark
Complicated optical arrangement required Number of elements which can be incorporated in given lamp limited Limited to three lamps by light losses Less sensitive & less reproducible than hollow cathode lamp Low spectral radiance at short wavelengths & less sensitive than hollow cathode lamp Number of elements which can be incorporated into given lamp I imited Multiple microwave power supplies required
Continuum
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Atomic fluorescence
Multielement electrodeless discharge tubes Banks of singleelement electrodeless discharge tubes Banks of pulsed hollow cathode lamps Tunable laser Continuum
An alternative approach to the primary source problem of simultaneous multielement analysis by atomic absorption is the production of a light source which simultaneously radiates resonance lines of a variety of elements, i.e., multielement light sources. Massman (13)studied the factors involved in the production of multielement hollow cathode lamps. Concentric rings of metals retained in a copper or steel sheath have pro-
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No serious limitations except system may become unwieldy for large number of lamps Very expensive at present Low spectral radiance at shorter wavelengths
duced lamps with up to four elements. Since the sputtering rates and melting points of the elements are parameters affecting the production of multielement hollow cathode lamps, it seems unlikely that this approach will yield lamps having a large enough number of elements for true multielement analysis. Strasheim and Butler (14, 15) made specially designed hollow cathode lamps which allow tandem
Lens
Flame
!
v
Lens
V
Direct reader
cal plane
Hollow
cathode lamps Figure 3. Optical arrangement f o r synthesis o f multicomponent radiation beam f r o m s i n g l e - e l e m e n t hollow cathode lamps
Spectrophotometers for Water Analysis Figure 4. Tandem hollow cathode l a m p s f o r p r o d u c t i o n o f multicomponent r a d i a t i o n beam
mounting (Figure 4). This system, however, is limited to three lamps by light losses. If each lamp were a fourelement multielement lamp, a maximum of twelve elements could be done simultaneously with a tandem arrangement. Strasheim and Human (16)described the use of a time-resolved spark as a primary light source for multielement atomic absorption. A rotating graphite disk electrode was used to introduce solutions containing copper, zinc, calcium, and magnesium into the spark gap. Results indicate, however, that the hollow cathode provides higher sensitivity and better precision. Further work will be required to determine whether this system is capable of functioning with more than four elements. A final approach to the problem of light sources for simultaneous multielement atomic absorption is the use of the continuum source. Various workers ( 17-20) demonstrated that a continuum source can be used for atomic absorption in conjunction with a monochromator having sufficient resolving power. In general, however, the detection limits are poorer with a continuum source as compared to a hollow cathode. Most workers have used a 150-W xenon arc lamp for these experiments. An additional problem with this source is the
low spectral radiance produced in the far ultraviolet, Le., less than 250 nm. Nevertheless, a continuum source offers one possibility for simultaneous multielement atomic absorption. Atomic Fluorescence. In atomic fluorescence the narrowness of the emission line of the source is less important (except from the point of view of scatter) since the detector does not view the source directly. What is important, however, is that the source be unreversed and have a high radiance over the absorption line width. Although sources possessing a high radiance are more difficult to produce, the optical arrangement of the atomic fluorescence experiment is more suited to multielement analysis. Specifically, banks of lamps may be arranged around the flame without the necessity for complicated optical arrangements or multielement lamps. The most common light source for atomic fluorescence in the past has been the electrodeless discharge tube. Although some work was done to produce multielement electrodeless discharge tubes (22-25),the number of elements is limited. Using a bank of single-element electrodeless discharge tubes would be prohibitively expensive unless one microwave power supply could be used to run them all. The most promising prospect for
You can't f i n d spectrophotometers any easier to use-especially for water and wastewater analysis. The new D R / 2 Model 2504 is available in a handy carrying case; Model 2582 IS a handsome laboratory instrument. Both offer the advantages of dialing any of various light wavelengths instead of selecting from a set of individual color filters for each colorimetric test. DRI 2 instruments utilize a circular variable interference filter with a wave. length range of 400 t o 700 nanometers (nm). Special wavelength dial positions are also marked for suspended solids testing (infrared light) and for measurement of color in water. Electronics are all solid-state with transis. torized voltage regulation. The carrying case model can be operated on either eight standard flashlight batteries or 1151220 V, 5 0 / 6 0 Hz; the laboratory model operates on either 115 or 220 Volt power. Each control is at your fingertips on the instrument panel. The spectrophotometer chassis also lifts out of i t s case to facilitate battery change or inspection. IT'S 2 SPECTROPHOTOMETERS IN 1 The instruments also perform well for general laboratory use. For analysts who choose to mix their own reagents and calibrate the spectrophotometer on their particular sample, Hach provides special blank meter scales. The instrument may also b? f i t t e d with an optional cell adapter which accepts commonly used 1 0 cm. sample cells. New Handbook For Water Analysis A Hach procedures handbook of simplified water and wastewater analysis is provided with each instrument. Titrations and colorimetric determinations are detailed-more than 70 tests in all. Most procedures are based on Standard Methods but have been carefully simplified so anyone can perform without previous training
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ANAL.YTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
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Table Ill. Detection Systems Used for Multielement Flame Spectrometry Approach
Rotating filters
Advantages Temporal Low cost Large solid angle collected by detector
Disadvantages
Wide bandpass may lead to interferences Lack of versatility in programming for different spectral lines
Scanning monochromator
Easily programmed for different spectral lines Narrow bandpass reduces spectral interferences Wide wavelength range
Some form of wavelength control necessary Scan speed limited by electronic response time Small measurement time for any given resolution element Adversely affected by source drift
Sequentially programmed monochromator
Easily programmed for different spectral lines Narrow bandpass reduces spectral interferences Permits optimization of flame & source for each element Wide wavelength range
Some form of wavelength control necessary Adversely affected by source drift
Image-dissecting photomultiplier
Electronic scanning; no moving parts High scan speed possible
Scan speed limited by electronic response time Small measurement time for any given resolution element Adversely affected by source drift Wavelength range limited by monochromator dispersion Compromise between wavelength range & resolution High cost
Direct-reading spectrometers
Vidicon detectors
716 A
Spatial High cost Simultaneous integrated measurement reduces Measurement of line & influence of source drift background difficult Wide wavelength coverage Not easily programmed for different spectral lines Alignment critical Limited number of wavelengths can be monitored Simultaneous integration of large numbers of wavelengths possible Background subtraction possible Storage Used in conjunction with Conventional table-top monochromator without exit slit Easily changed to monitor different spectral lines
High cost Wavelength range limited by monochromator dispersion Compromise between resolution & wavelength coverage Less sensitive than photomuitiplier
ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
light sources for multielement atomic fluorescence is the hollow cathode lamp. Operated in its conventional mode these lamps do not possess a high enough radiance for satisfactory atomic fluorescence. Recent work (26-28) showed that the radiance of these lamps may be increased sufficiently by operation in the pulsed mode. Power supplies capable of simultaneously operating banks of these lamps could easily be designed. Mult ie le ment Detect io n Systems In conventional flame spectrometry the radiation of analytical interest is present along with unwanted radiation from a variety of sources. To separate the desired analytical radiation from the unwanted radiation, a monochromator is used. The nature of the unwanted radiation differs in flame emission, absorption, and fluorescence; hence, the requirements placed on the monochromator differ. Nevertheless, use of a conventional monochromator restricts flame spectrometry to single-element determinations. Table I11 classifies the various approaches which have been taken to achieve multielement detection. To measure intensities at different wavelengths, a multichannel device is necessary. Multichannel devices may be divided into two major classes: temporal multichannel devices and spatial multichannel devices. Temporal devices employ a single detector, where each channel is separated from the previous one in time. Spatial devices employ multiple detectors, where each channel is separated in space.
Temporal Multichannel Devices Scanning Spectrometers. A single-channel spectrometer is one where, a t any instant, light from just one resolution element is being detected and recorded. The most common example of a single-channel spectrometer is a monochromator, where an exit slit blocks all of the dispersed radiation except that within a given resolution element. An obvious approach to using this type of system for temporal multichannel detection is to provide for some form of spectral scanning. A variety of spectral scanning systems were developed (29, 30) for various purposes. These systems are based on moving the detector if the spectrum is stationary (31) or scanning the spectrum past a fixed exit slit either by oscillating the dispersing element (32) or oscillating an auxiliary mirror (33). There are several limitations to the use of spectral scanning systems for multielement analysis. One of the
Axis of rotation
-
\
Image of graticule on slit aperture
7
Photomultiplier
Lens
t]
Mirror
Slit aperture
.
Condenser lens
Figure 5. Electrooptical system for wavelength tracking for scanning monochromators
reasons for the development of simultaneous multielement techniques in analytical chemistry comes from the need to analyze limited amounts of sample for many elements. Under these conditions single-element techniques generally consume too much sample and time to permit the sequential determination of many elements. The first limitation of spectral scanning is that it is not truly simultaneous. Such systems approach simultaneity as the scan speed is increased. However, as the scan speed is increased, the time constant of the electronics must be decreased. The maximum scan speed is then determined by the response time of the electronics. Secondly, the use of spectral scanning will be less precise because of the short sampling time for any given resolution element. Finally, spectral scanning will be adversely affected by long-term drift in the signal as the scan proceeds. A vital factor in the use of a scanning system for multielement analysis is the development of a highly reproducible means for accurately determining the wavelength being passed by the exit slit of the spectrometer a t any instant. Dawson et al. ( 3 4 ) describe a system which accomplishes this (Figure 5 ) . With their instrument, spectral scanning is obtained by an oscillating diffraction grating. The wavelength setting is determined electrooptically by a mirror attached to the reverse side of the grating. The mirror is arranged to project an image of a finely ruled graticule on a slit. A photomultiplier placed behind the slit produces a reference pulse train as the image of the graticule passes across the slit. This reference pulse train is used to gate the analytical signal into the appropriate integrator. The system, which scans at a rate of 1800 nm/sec, was used to determine Na, K, Ca, and Mg in clinical sam-
ples with a precision of 3%. Sodium, potassium, and calcium were determined by flame emission, and magnesium by atomic absorption. An airacetylene flame was used. Malmstadt and Cordos (35)designed an extremely flexible, modular automated instrument for atomic fluorescence and atomic emission which is also adaptable for atomic absorption. This instrument, which operates on a rapid sequential basis, overcomes the limitations of conventional scanning systems described earlier. The heart of the system is the programmable monochromator (36).This monochromator, controlled by digital logic, can be made to slew between wavelengths of analytical interest and remain at a given analytical wavelength until the rest of the system has been optimized for that particular element and until sufficient signal has been sampled, before proceeding to the next wavelength. Eight elements can be determined in less than 1min with this system by atomic fluorescence. An average time of 3 sec is required for wavelength setting and optimization of flame conditions and primary light source for a given element, whereas 1-2 sec is required for the measurement step. This system has been used to determine Zn, Cd, Fe, Mn, Mg, Cu, Cr, and Ca by atomic fluorescence in a variety of NBS alloys. It was also used to determine Cu, Zn, and Mg in urine and Zn, Fe, Mn, Mg, and Ca in NBS orchard leaves. Scanning Detectors. An approach which has not yet been applied to multielement flame analysis involves the use of image-dissector photomultipliers (37). Figure 6 shows a schematic diagram of this device. The image dissector consists of a photocathode and an electron imaging section separated from a conventional dynode chain electron multiplier by a plate
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
717 A
Electron image on aperture plate
\
Anode
I
I
I
chain
power 1
Display
1
Figure 6. Image-dissector photomultiplier
with a slit aperture. The electron imaging section forms an electron image on the aperture plate from the light image incident on the photocathode. This electron image is swept across the face of the plate by electromagnetic sweep coils surrounding the tube. Only the portion of the electron image which passes through the slit in the aperture plate can reach the first dynode and be amplified by the dynode chain. Thus, instead of mechanical scanning, the image dissector utilizes electronic scanning. Harber and Sonnek (38)described an electronic scanning spectrometer using an image-dissector photomultiplier. The spectrometer utilizes a 12.7-cm Czerny-Turner mount, f/ 4.5, with a reciprocal linear dispersion of 131 &'mm. A spectral range of 250 nm was covered a t a scan rate of either 100 or 1000 scans/sec. Such a system could profitably be used in conjunction with signal averaging techniques. Filter Photometers. Another approach to temporal multichannel measurement for multielement analysis employs a filter wheel instead of a scanning monochromator. Mitchell and Johansson (26) described a system for atomic fluorescence. Four hollow cathode lamps are appropriately focused on the flame. A filter wheel is placed between the flame and a single photomultiplier. Four interference filters are sequentially rotated into position in front of the photomultiplier by a synchronous motor operating a t 1Hz. Trigger pulses derived from the rotation of the wheel by a magnet-coil device control the electronic system. The 718 A
photomultiplier signal is amplified and appropriately gated to one of four integrators, prior to readout with a digital volt meter. The instrument was used for the determination of Ag, Cu, Fe, and Mg by use of an air-hydrogen flame. Dagnall et al. (39)used a Technicon Corp. prototype (AFS-6) multielement atomic fluorescence filter photometer based on the design originally proposed by Mitchell and Johansson (26) to determine Cu, Fe, Mg, Mn, Ni, and Zn in aluminum alloys. The accuracy of the results compared with the published values of the British Chemical Standards was good, and the precision generally in the 1-570 range. Dagnall et al. (40) also used this same instrument in the analysis of soil extracts for Ca, Cu, Mg, Mn, and Zn. The analysis must be carried out a t two dilutions to ensure that each element is determined in a linear region of the calibration curve. Spatial Multichannel Devices
A spatial multichannel device is one where, a t any instant, light from several resolution elements is being detected by multiple detectors and recorded. Such systems are potentially more efficient than single-channel systems. Multiple Slit-Multiple Detector Systems. This approach is exemplified in the familiar direct-reading spectrometer, where selected spectral lines are isolated from the dispersed radiation by exit slits positioned along the focal plane of the spectrometer and detected by photomultipliers positioned behind each slit.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
The most obvious problem associated with direct-reading spectrometers is the positioning and alignment of the exit slits a t the proper point on the focal plane. Even with the most careful alignment of the exit slits, environmental factors such as changes in the temperature of the spectrometer and the refractive index of the air are apt to result in misalignment. In addition, because of the problems associated with'slit alignment, the spectrometer is not conveniently changed to monitor new wavelengths, Le., new elements. Multichannel direct readers are severely limited in the number and location of the wavelengths a t which intensities can be measured simultaneously. The system becomes unwieldy and expensive if the number of separate detectors increases beyond 10 or 20, whereas the measurement of line and background intensities is impossible. Scanning methods employing vibrating slits or quartz plates (41-46) have been used for background correction. Laqua and coworkers (47, 48) used glass fiber optics to simultaneously measure adjacent wavelengths. The use of glass restricts this approach to the visible region of the spectrum. Finally, the optical speed of directreading spectrometers is generally slow owing to the long focal length required to obtain the necessary high dispersion to allow detector placement. This adversely affects the use of these systems in conjunction with flames, which are relatively low-intensity sources compared with arcs and sparks. TO alleviate some of the limitations of direct-reading spectrometry, Golightly et al. (49) described the use of image-dissecting photomultiplier tubes. Since these tubes have an internal slit, the need for spectrometer exit slits is eliminated. This factor, combined with the scanning nature of the tube, alleviates the critical alignment problems conventionally associated with direct readers. As a result of reduced alignment problems, the instrument may be more conveniently programmed for different sets of spectral lines. The use of these tubes also permits the determination of background and dark current contributions to measured line intensities. Unfortunately, these tubes and their ancillary electronics are, at present, too costly to permit utilization of banks of these detectors indirectreading spectrometers. The direct-reader approach was used by Vallee and Margoshes (50) in 1956 to determine simultaneously Na, K, Ca, and Sr by flame emission; however, since that time little has appeared in the literature using this approach (12, 15, 16).
I
I
P-type semiconductor /
I
I
1
Sweep coiI\l[ Focus coil/)
I
\ I I
Signal out
\
Scanning electron beam I
I
Figure 7. Silicon diode vidicon t u b e detector for rnultielement analysis
Multichannel Detectors. The first and perhaps most widely used multichannel detector has been the photographic plate. Among the limitations of photographic detection are low efficiency (
