Application of a Silicon-Target Vidicon Detector to Simultaneous Multielement Flame Spectrometry D. G. Mitchell, K. W.Jackson, and K. M. Aldous Division of Laboratories and Research, New York State Department of Health, New Scotland Ave., Albany, N . Y . 12201 Of the three analytical techniques, atomic emission (AE), atomic absorption (AA),and atomic fluorescence (AF) spectrometry, only in AE have multielement systems achieved a degree of popularity, with the longestablished photographic method of recording emission spectra and, more recently, the multiple slit-multiple detector (direct reader) type of instrument. Both of these systems leave much to be desired, however, because of the tedious measurement and interpretation ofthe photographic spectra and the alignment and geometrical difficulties of the direct reader. An extensive range of sophisticated instrumentation is now available for AA, which is simpler and more selective than AE and has become better established than the more recently developed AF spectrometry. A few AA and AF systems have been designed for the simultaneous analysis of more than two elements, but unfortunately, none is commercially available, and this is undoubtedly the most serious drawback of these two techniques. There are now a number of important potential applications for multielement analysis, such as the monitoring of elemental water pollutants, wear metals in lubricating oils, and toxic metals in biological materials. In principle, multichannel AF spectrometers are easier to design than AA instruments, since in the former case it is not necessary to direct all of the incident radiation along the same optical path, and so it is a simple matter to focus radiation from a number of hollow cathode lamps and/ or electrodeless discharge lamps into the atomization cell. There are several approaches to multielement analysis, and these have recently been reviewed by Busch and Morrison ( I ) . One approach involves the use of multichannel detectors such as linear photodiode or pho-
totransistor arrays ( 2 ) and television camera tubes. Busch and Morrison have suggested that the latter type is the most promising detector for the future. In such a device, dispersed radiation leaving a monochromator strikes a photosensitive surface to release electrons which develop a charge pattern across a target. The charge density a t each point on the target is a function of the incident radiation intensity, and the position is a function of wavelength. An electron beam searches the target and neutralizes each point of charge to produce a video signal. The photosensitive surface may be incorporated as part of the target, as in the silicon vidicon; or it may be a photocathode from which photoelectrons are accelerated toward a separate target, as in the image orthicon, the secondary electron conduction (SEC) vidicon, and the silicon intensifier target (SIT) vidicon. The first application of television tubes to atomic spectrometry was probably by Benn et al. ( 3 ) ,who in 1949 used an image orthicon for AE to examine low-intensity lines during a gaseous combustion reaction. This detector has a glass target, and the electrons in the search beam in excess of those required to neutralize the positive charge a t a particular point on the target are reflected into a fivestage secondary emission electron multiplier. The anode current of the multiplier then generates the output signal voltage. These workers observed that for the short exposure times needed to follow chemical reactions, the orthicon was about 50 times more sensitive than the photographic plate. The orthicon, however, exhibits poor signal-to-noise ( S / N ) characteristics ( 4 ) and has now been superseded by the vidicon, which has a simpler method of measuring the charge pattern, in that the video signal is the magnitude of the neutraliz-
A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 14, DECEMBER 1973
1215A
ing current. A vidicon tube with an antimony sulfide target has been suecessfully applied to molecular spectrometry (j), but this target does not show a linear response to photon flux, and a correction must be applied. The most useful vidicon tubes for the analytical chemist will probably be the silicon tube, the SIT tube, and the SEC tube, which has a KC1 target. The useful characteristics of these tubes are compared in Table I . Their relative merits have been described in detail elsewhere (6).Briefly, the SIT is useful for measuring low light intensities, since it provides a higher gain than the SEC vidicon. The SEC, however, has a much lower dark (leakage) current, permitting long integration by accumulating charge on the target without seriously affecting the S/Nratio. The SIT is more expensive than the SEC, but it is also far more robust and less likely to be damaged from overexposure. For AA, the silicon vidicon is probably adequate, and in this application the higher cost of the SIT and SEC vidicons would hardly be justified. However, for AE and AF, where low light levels must often be detected, an SIT or SEC tube with its higher gain would be preferable. Margoshes (7, 8) proposed the use of an SEC vidicon in the construction of a computerized AE spectrometer. This camera tube was to be used in conjunction with an echelle monochromator to simultaneously measure signal intensities a t up to 2048 wavelengths. The tube can be programmed (9, 10) such that instead of the electron search beam regularly sweeping the target, a number ofXY coordinates are stored in the computer memory and these direct the electron beam to a number ofpredetermined positions on the target surface. This random access system increases the dynamic range of the vidicon by a time integration technique. Areas of high illumination are read more frequently than weakly illuminated areas, thus permitting more charge to accumulate in the latter. The SEC vidicon described by Margoshes has a UV-transmitting sapphire window and is responsive at wavelengths as low as 165 nm. Such a system would almost certainly be the most suitable for emission work. (A detector of this type with random access programming is marketed by EMR Photoelectric, Princeton, N.J.; but like most commercially available vidicons it does not respond in the UV and would consequently be of little use for atomic spectrometry.) A silicon-target vidicon with good response down to about 200 nm is commercially available from SSR Instruments Co., Santa Monica, Calif. Although its S/Nratio is generally 1216A
Table Comparison
Of
l u b e Characteristics Silicon target
Dynamic range 103 Internal gain 1 Dark current, electrons sec-1 cm-I, 20°C
10'0
Quantum efficiency, 400 nm 50 Maximum integration time, seca 3 Relative cost 2
SIT
SEC
103
lo*
2 X lo3
lo2
10'0
103
15
12
3
104
10
7
Depends on temperature and area per resolution element. o
somewhat poorer than that attainable with a photomultiplier tube, the instrument can simultaneously detect signals from at least eight elements. A spectrometer incorporating this detector is described below. It is easily constructed, and the system is no more complex to set up than most single-element instruments. A large number of optical channels give it a great deal of flexibility, and it possesses the highly desirable feature that it can be used with atom cells producing transient signals. Application to Flame Spectrometry
During AE measurements, a highresolution detection system is required to isolate the spectral line from the often complex background emission spectrum. For the vidicon to be useful as a multielement detector, a spectral range across the target of a t least 150 nm is desirable. A monochromator with a sufficiently low linear dispersion to give this range has a relatively poor resolution, and during emission work it might be necessary t o use an echelle system, such as that of Margoshes, to obtain adequate resolution. The resolution required for AA and AF is much lower, and the vidicon should be better suited as a detector for these two methods. At the time of this writing, no results have been published for multielement AF spectrometry using a vidicon. An SIT, however, with its much improved sensitivity, should make the system feasible for the detection offluorescent signals. The principal difficulty during AA work is the multiplexing of sufficient suitable light sources along the same optical path, and this problem is not encountered during AF measurements when it is only necessary t o focus all the lamps at a point within the atomization cell. From this consideration AF is a highly attractive
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1 4 , DECEMBER 1973
method. It has already been shown that the vidicon should be feasible for multielement AA ( I ] ) , and this method possesses the advantage that an SIT is not required. Also, a variety of hollow cathode lamps are now available containing up to six elements; and if, say, eight elements are to be determined, it is often possible to detect all of them simultaneously by multiplexing two multielement lamps. Other disadvantages of both AA and AF would be common to any multielement instrument. A compromise in flame conditions and in the viewing position within the flame is always necessary, and the dynamic range presents a limitation during the simultaneous analysis of elements with widely differing concentrations. Design of a Multielement Atomic Spectrometer. To use a vidicon as a detector for AA, the arrangement need differ only slightly from the standard single-channel system. The vidicon tube is mounted on a suitably modified monochromator in place of the exit slit/photomultiplier assembly, and the usual detector electronics are replaced by the control console and an oscilloscope. A strip chart recorder can also be used if desired, although this is not essential. A typical instrumental arrangement for AA, which we have developed in our laboratory, is shown in Figure 1.The source is a multielement hollow cathode lamp (JarrellAsh, Waltham, Mass.), and a singleslot burner/nebulizer (Techtron, Palo Alto, Calif.) is used. A second lamp can be mounted in such a way that the radiation from both lamps is combined using a half-silvered quartz mirror. The lamp, lenses, burner/ nebulizer, and monochromator are mounted on an optical rail, and the system could easily be constructed in any analytical laboratory. This is a feature unrivaled by any other system reported to date. The detector shown in Figure 1 is the SSRI Model 1205 Optical Multichannel Analyzer (SSR Instruments Co.). This is a UV-responsive silicontarget vidicon, and its operation has been described in adequate detail elsewhere (I, 12, 13). The variation of spectral response with wavelength for this system is shown in Figure 2a. The quantum efficiency at 200 nm should be about 1070,similar t o that of a 1P 28 photomultiplier. One of the most sensitive photomultipliers at 200 nm is the R106 type, with a quantum efficiency of about 2570. This vidicon tube is suitable for AA, but for emission studies (AE or AF), an SIT should be more useful because it provides an electron gain of about 2000 over the silicon vidicon
i Figure 1. Multieiement atomic absorption spectrometer incorporating silicon vidicon detector A , SOUTCB;
e, burner: C, monochromator: 0 , vidicon tuhe: E , electronic consoie; F. oscilloscope display
(Table I). The SIT commonly uses a n S-20 photocathode with a fiber-optic faceplate, and the response of this system does not extend into the UV (Figure 2 b ) . It is not possible to provide UV response directly because a quartz fiber-optic faceplate is not available. An indirect method is thus required, and an SIT should soon he available in which a coating of a suitable fluorescent substance such as pterphenyl is used to convert radiation a t UV wavelengths into visible light. The quantum efficiency of the silicon vidicon (Figure 2a) is about 21% a t 253.7 nm; the UV-responsive SIT has a predicted quantum efficiency of only 1.0-1.5% a t this wavelength. Thus the electron gain of 2000 should result in a sensitivity increase of approximately 120. The dispersive system shown in Figure 1is a 0.25-m Ehert monochromator (Jarrell-Ash). The spectral response of the vidicon is dependent upon the accurate focusing ofradiation onto the surface of the target (or the faceplate, when an SIT is used). With only slight modification of this monochromator, the vidicon t.arget can be positioned in its focal plane. This is achieved by removing the exit slit assembly and machining off the flange which remains on the monochromator housing to leave an aperture approximately 30 mm in diameter. The vidicon tuhe is mounted over this aperture, and the focal plane can he adjusted to give a sharp image on the target by moving the adjustable entrance slit in and out. A diffraction grating blazed a t 190 nm and with 295 grooves/mm is suitable for AA and AF. This gives a linearly dispersed spectrum across the target covering a range of 168 nm. Thus, any elements with resonance lines from, say, 200-368 nm can he si-
\
.
.3".-
_"_
.YI,Y.l-lI
-I
spectral response with wavelength for UV-sensitive silicon vidicon showing percentage quantum efficiency. b. Respoi sivity of S-20 photocatl ode system used in SIT
L
t
200
400
600
800
mult aneously detected. The spectral rang,e could he further increased by usin(5 a grating of even lower dispersion, but this would result in a loss of resol ution. The grating described provider3 adequate resolution for most A A a,nd AF determinations, since, for exaniple, it enables the adjacent lines Ph 283.3 nm/Mg 285.2 nm and Cu 32423 nm/Ag 328.1 nm to he easily resol ved. Tf le same instrumental system COUlI1he used for AF, except that a circiilar burner head would be used with a series of high spectral output sour,ies irradiating the flame. The
IO00
number of elements to he simultaneously determined should he limited only by geometrical considerations in aligning the various light sources and by the spectral range provided by the monochromator. Measurement Technique. The electronic console of the SSRI Model 1205 vidicon contailis two shift register memories. A video signal may he observed in the real time mode, in which case it is updated a t the end of every electron search beam frame scan; or the accumulated data from a preset number of frame scans can he entered into one of the memory units.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 14, DECEMBER 1973
1217A
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 14, DECEMBER 1973
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During AA measurements, a timeaveraged signal is obtained if the accumulate mode is used, and this results in a better S/Nratio. Also, the memory capabilities of the instrument can be utilized to give a direct reading in terms of absorption. The pure solvent is first aspirated into the flame, and the accumulate preset control on the console is adjusted to count the data from, say, 100 frame scans. This information is stored in the first memory unit (memory A ) . The time to accumulate 100 such scans is approximately 4 sec. The analyte solution is then aspirated, and data for the same number of frame scans are stored in the second memory unit (memory B ) .A pushbutton operation then takes the difference (A - B ) ,and the resulting spectrum is the net absorption signal (Io - I t ) obtained simultaneously for each wavelength. This appears as a series of peaks on the oscilloscope screen. A cursor control permits the selection of any one of 500 vertical channels across the target area, and a digital readout is obtained in terms of intensity a t the channel selected. Typically, each absorption peak has a half-width of about five channels. The peak height may be read by selecting the channel a t the center of the peak, or the peak may be integrated by moving the cursor across the peak and summing the intensities for all the channels within the peak area. To obtain a signal in terms of percentage absorption, the net signal (A - B ) is divided by the signal obtained during aspiration of the solvent A : A - B 100A I - I 100 = % absorption I, The analog output, as displayed on the oscilloscope, can be plotted out on a strip chart recorder, ifdesired. The measurement technique for AF would be similar to that for AA, except that the analyte aspiration signal should be stored in memory A and the pure solvent aspiration signal in memory B. The difference (A - B ) would then be a spectrum in which peak height (or area) would be a measure of fluorescent intensity a t a particular wavelength and would be proportional to the analyte concentration. Typical Results for Multielement A A The photographs in Figure 3 are the spectra seen on an oscilloscope screen during the simultaneous determination of equal concentrations of the eight elements Zn, Cd, Ni, Co, Fe, Mn, Cu, and Ag in a n air-acety-
Figure 3. Oscilloscope displays during simultaneous multielement determination of eight elements by atomic absorption spectrometry Top, memory A ; center, memory 6 ;bottom, A -6
lene flame using the apparatus described above. The spectral range covered is 232.0 n m (Ni) to 328.1 nm (Ag), and the three spectra are, respectively, the accumulated contents of memory A , memory B , and the arithmetic difference A - B . Radiation from two multielement hollow cathode lamps was combined, using a half-silvered quartz mirror as a beam splitter. These results are only qualitative, but it is clear that the resolution of the system permits at least eight elements to be determined simultaneously, and probably many more if' suitable light sources can be multiplexed. Quantitative results from this system are now becoming available, and we hope to publish them together with sensitivity and S I N data in the near future. Results comparing single-element AA on the SSRI vidicon
The vapor-deposited, thin-film thermopile sensor at the heart of the Mettler scanning calorimeter is revolutionary for a commercial instrument. As a sensor it is fast and responsive-assures excellent heat contact with crucibles. It is ideally suited for accurate purity determinations, as well as heat of fusion, specific heat, and many more measurements. The modular TA 2000 DSC system offers everything you want in thermal analysis. It is more sensitive and reproducible than most comparable instruments. It offers 0.17 millicalories per second full scale sensitivity. And calorimetric reproducibility of *0.5%. Noise level is less than i 1 microcalorie per second. Temperature range covers -20 to 500"C, with precision of k 0 . l "C, and isothermal fluctuations of +0.002"(;. Extremely linear selectable heating and cooling rates in steps of 0.1 " C . Temperature programs can be controlled remotely. It is completely computer compatible. For your copy of the 8-page Mettler TA 2000 brochure, circle the number.
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 14, DECEMBER 1973
1219A
In liquid chromatography, success often depends on the column packing material and how well the column is packed. Perkin-Elmer brings you good news on both fronts. Eluent = trifluoro trichloroethane at 1.0 ml/mln. Detector = U.V. at 254 nm Sample = Nitrated Benzenes Peak No.
IO
VIT-X
@ i i w = 2,610,000
Compound Benzene Nitrobenzene p-dinitrobenzene m-dinitrobenzene o-dinitrobenzene 1,3,5-TNE
1 2 3 4 5 6
0
Polyst rene StsndYards
0.w
=
200,000
@.w
=
20,800
@Gw
=
2,100
100,000
Four 0.5-meter Vit-X columns in series. THF solvent
UNTREATED SILICA
10,000
I 0
1
IO
11
ORDINARY ADSORBENT
PEAK
d
1,000
A
I[@
100
SIL-x
u 40
0
I
20
0
MINUTES
Separation of nitrated benzenes on untreated silica, widely used commercial silica, and Sil-X-I column. Pressure drop on Sil-X-I is low (500 psi or 33 kg/cm’). Only SiI-X-I elutes all components and does it within 12 minutes.
1
I
IO 2 0 ~. MINUTES
1
30
alyticai speed (20 minutes), low.pressure drop (1000 psi, or 67 kg/cm*, across the entire four columns), and a linear dynamic range of 10’ solves analytical problems which have persisted for decades.
Figure 2.
New Sil-X@ Columns Give Better Separations, Narrower Peaks. The new Sil-X deactivated adsorption columns have high plate counts and low pack pressures. What this means to you is: Superb chromatograms (see figure 7). * No need to add water to solvents. Quick equilibration of columns. Reproducible retention data.
.
-
New Vit-X Columns Give Size Exclusion Runs in Minutes. Not Hours. Perkin-Elmer Vit-X packings are fully and permanently deactivated and of narrow pore size distributions, therefore: Vit-X can stand any pressure and resist any solvent . * Molecular size exclusion runs can be completed in minutes. * Veterans of gel permeation work will recognize a whole new opportunity. (see figure 2).
-
Linear and universal calibration plots are obtainable over a wide molecular weight range.
New Perkin-Elmer Columns Provide High Plate Counts, Low Back Pressure, Long Life. Perkin-Elmer’s new high-pressure packing technique produces columns which are packed very tightly and uniformly, with little or no segregation or migration of the particles in the column. What this means to you is: Very high plate counts. Low back pressure for greater analysis speed or more convenient pressure requirements. Several columns with different pore sizes can be connected in series, giving a large dynamic range for size exclusion. Long column life and great stability.
.-
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For information on all of our new developments, write to: Instrument Division, Perkin-Elmer Corporation, Main Avenue, Norwalk, Conn. 06856
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1220A
1
20
ELUTION VOLUME IN MILLILITERS
Four-component mix of polystyrenes on four Vit-X columns. This chrornatogram is likely to impress only specialists in polymer chemistry. They will understand that they are seeing a remarkable run. The combination of an-
Figure 1.
I
10
ANALYTICAL CHEMISTRY, VOL. 45, NO. 14, DECEMBER 1973
with those obtainable by conventional analysis on a Techtron AA-120 spectrophotometer have already been published ( l l ) ,and comparative sensitivities and S/Ndata are reproduced in Table 11. The adequate resolution of the vidicon system is mirrored in the sensitivities, which in most cases are comparable. The vidicon detection limits, however, average about a factor of seven poorer. The only reason why multielement results might differ from those by single-element analysis on the vidicon is that the spectral intensity within a multielement lamp varies considerably from element to element. For example, in Figure 3 the Ag line at 328.1 nm is far more intense than the Cd line a t 228.8 n m , although Cd has a higher sensitivity. It is thus possible that a slightly poorer S/Nratio will be obtained at these weaker lines. These peaks, however, appear weaker than they actually are, since the gain on the analog output was decreased to permit the larger peaks to appear fully on the screen. Signal/Noise Theory
In the following discussion, the effects of detector and source are considered in comparing theoretically the S/Ncharacteristics of ( a ) the multichannel AA spectrometer described above with ( b ) a single-channel dcoperated spectrometer using an R106 photomultiplier tube. ( a ) The signal current produced by a silicon vidicon may be expressed by: I, =
QeGA, ( N ) Y T
G = target gain, G = 1for a silicon vidicon A r = area of target scanned, cm2 N = incident light flux, photons cm-2 sec-l y = slope of the current vs. luminous flux curve, y = 1 for the silicon vidicon T = detector exposure time, sec Ta = active readout time to scan A r , sec For this system it is necessary only to consider source flicker noise and the video preamplifier noise, since dark current and shot noise do not significantly affect the detector performance. (In the case of the SIT and SEC vidicons, which possess considerable target gain, allowance has to be made for shot noise.) The preamplifier noise is expressed as an equivalent noise input current ( i n e q ) which is defined by: ne inep = - a m p s r m s (2) Ta where n = electron uncertainty per target element scanned in time Ta, sec. The noise due to source flicker is expressed (14) as a function of the signal current: i f = Z , ~ h f ' ' *a m p s r m s
where
x
= flicker factor, typically 10-2 .If = observation bandwidth
The overall noise then becomes: amps
(1
To where Q = quantum efficiency of the photosurface e = electron charge, coulombs
i
-4
(4)
=
( b ) The anode current of a photomultiplier tube is given by: la = GANQe
amps
Table II. Comparative Results by Single-Element AA: Vidicon vs. Techtron AA-120 Element and wavelength, nrn
Concn a t which RSD determined, PPm
Zn 213.9
0.5
Pb 217.0
10.0 1.0
cd 228. a Ni 232.0 Co 240.7 Fe 248.3 Mn 279.5 Pb 283.3 Mg 285.2 cu 324.8 Ag 328.1 Ag 338.3 Cr 357.9 Ca 422.7
Sensitivity," PPm
R S b Z
5.0 2.0 10.0 0.5 2.0
0.1
1.8
5.0 5.0 10.0 5.0
0.1 0.2 0.4 0.2
2.1
10.0
Techtron AA-120
Vidicon
0.03 0.5 0.05 0.4 0.4 0.3 0.1 0.9 0.02
10.0
(3)
3.5 1.1 2.6 1.9 2.7 5.2 1.8
5.0 2.0 2.6
4.3 1.7
Detectionb limit, ppm
0.01 0.4 0.04 0.3 0.3 0.3 0.09 0.5 0.01 0.07 0.1 0.2 0.1 0.08
Sensitivity," ppm
0.03 0.5 0.04 0.2 0.4 0.3 0.1 0.9 0.01 0.1 0.1 0.2
0.2 0.1
1% absor tion. 6 S/N = 2 1. Reprinted l o r n Ref. 21, p 310, by courtesy of Marcel Dekker, Inc.
RSD,%
Detectlonb limit, vvm
0.42 0.49 0.46 0.25 0.78 1.1 0.63 0.48 0.39 0.51 0.53 0.48 0.37 0.65
0.005 0.2 0.005 0.05 0.03 0.07 0.01 0.1 0.001 0.01 0.009 0.02 0.02 0.006
(5)
Want to automatically determine melting and boiling points without any possibility of misinterpreting results? Determine exact melting ranges of polymer films and fibers, liquid crystal preparations, and submicrogram quantities of organic compounds using thermal microscopy? Obtain digital dropping points of lubricants, food fats, paraffins and waxes? Softening points of fats, coal tar pitches, polymers and resins? The new generation of instruments, the modular Mettler FP system, will do them all faster, easier, more accurately . There's a separate Mettler FP furnace for each application. All provide high-precision, electronically controlled, linear (to f 0 . 1 " C ) heating and cooling with extremely fast response over the -20 to + 300°C range. Temperatures at which significant thermal events occur are digitally displayed. Accurate, reliable platinum resistance sensors, closely matched by Mettler, enable you to choose the furnace which precisely suits your needs. For detailed information, circle the number.
Z ,
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 14, DECEMBER 1973
1221 A
where G = dynode gain, typically lo6 A = area of the photocathode illuminated-i.e., slit size, cm2 N = incident light flux a t the slit, photons cm-2 sec-1 If it is assumed that source noise predominates when a photomultiplier is used, the overall noise may be expressed by:
i
=
= I , ~ h f " a~m p s r m s
i,
(6)
To compare the two systems, certain parameters must be normalized: the optical geometry between the source and monochromator must be the same, an equal photon flux per unit area must fall on both detectors a t the same wavelength, and an equal signal integration time must be considered. If a photon flux of 108 photons cm-2 sec-1 a t 300 nm is considered, the following values may be substituted in Equation 1: Q = 0.4, G = 1, A r = area of a single channel = 1.25 X 10-3 cm2, T = 32 X 10-3 sec, Ta = 32 X sec. Thus: 0 . 4 X 1.6 X lo-" X 1 X 1.25 x 1 0 - ~x po8Y x 32 X 10I, = amps 32 X = 8 X lo-''
amps
When we substitute in Equation 2, where n = 2500 electrons/channelframe (15),
--
1neo
2500
X
1.6
X
lo-"
a m p s rrns 32 X = 1.25 X l o - " a m p s rrns Also, in Equation 3:
i,
=
8
X
32 x = 4.4 x
X
amps rms a m p s rrns
If the values of ineq and if are substituted in Equation 4,if is clearly insignificant ( i = 1.25 X amps rms), and in this case the source noise does not contribute to the overall noise. Thus, for a single channel on the vidicon target,
Under our experimental conditions, however, the data from 100 scans are accumulated (total observation time, 3.2 sec).This gives a further S / N ratio enhancement of 1001/2-i.e., ( S / N ) v i d i c o n = 6.4. For the single-channel system a typical slit size is 1 cm x 100 gmi.e., A = cm2, and the quantum efficiency of an R106 photomultiplier at 300 nm is 0.15. Thus Equation 5 becomes: 1222 A
I , = 106 x 0.15 = 2.4 X
x 108 x X
1.6 lo-'
X
lo-''
amps
amps
If the total observation time is 3.2 sec ,
i
=
=
i, = 2 . 4 x
x
1.3 x IO-'' a m p s rrns
The S / N ratio of the single-channel system is thus:
Under these conditions the S/Nratio of the single-channel instrument should be approximately 30 times greater than that of the multichannel apparatus. In the above case, the main noise contribution in the multichannel system was preamplifier noise. However, as the source intensity increases and flicker remains constant, the source noise begins to contribute. For example, if the photon flux is 109 photons cm-2 sec-1, il = 4.4 x 10-12 amps rms. Substitution in Equation 4 now substantially alters the overall noise, and the S / N ratio for 100 frame scans becomes 61, which makes the S / N ratio of the single-channel instrument only three times greater than that of the multichannel system. In an S / N ratio comparison, the many experimental variables are difficult to quantitate, but three main points become apparent: As source intensity increases, vidicon preamplifier noise becomes less significant compared with source noise, and a better S/N ratio should be obtained; when preamplifier noise predominates, an S / N improvement should be obtained by accumulating the data from several scans or by integrating the charge on the target for a longer period before readout; with SIT and SEC vidicons, the sensitivity is greater than the silicon vidicon, but preamplifier noise should be the same. In the low light level region, where preamplifier noise is dominant, the S / N ratio for the SIT and SEC vidicons will always be greater than for the silicon vidicon. It can be concluded from these calculations that, using presently available light sources for multielement analysis, the silicon vidicon will always display a somewhat poorer S / N ratio than an instrument using a photomultiplier. This is supported by our experimental results. Possible Future Applications In the method for AA analysis described above, the vidicon detection
ANALYTICAL CHEMISTRY, VOL. 45, NO. 14, DECEMBER 1973
system is rapid, but the subsequent recording and interpretation of data are somewhat lengthy procedures. For a routine analysis instrument. it would be useful to have some form of on-line data storage or handling capability, especially when rapidly changing signals are being detected in a real-time scanning mode. A computerized system would be highly desirable. When measuring steady-state signals, the video signal produced by the scanning electron beam is usually digitized and stored in some binary format in a memory unit. These data can then be recalled after the completion of a measurement cycle to study the spectral intensities. Realtime data processing is not necessary here, and mass storage of data of this kind can be performed ideally by magnetic tape systems. Real-time data processing would be necessary, however, for some applications, such as locating the channel a t the center of a signal peak which covers several channels or measuring transient events. If 500 channels are available for data and each scan cycle has a duration of 32 msec (as in the SSRI instrument described above), the time required to scan each channel is about 64 psec, and fast AID conversion must be performed on the video signal to store all the data during each frame scan. In this case the speed of a digital computer is essential. A fully automated system incorporating a minicomputer should be able to perform two real-time functions: to store in real time the data from successive frame scans, so that transient signals may be simultaneously detected at several wavelengths and the signal shape with time determined after the measurement cycle; and to sum the data from several channels to obtain the total intensity over the emission profile, rather than measuring the peak height-i.e.. to perform real-time integration. Multielement determinations in both AA and AF could then be performed for atomization methods which produce only transient signals. With this unique capability for following the transient atomization of several elements simultaneously, the vidicon could be used as a detector for multielement analysis by the Delves cup technique (16) or by any of the nonflame atomization cells now commercially available. such as the Techtron carbon rod atomizer or the Perkin-Elmer HGA 2000 graphite Massmann-type furnace (PerkinElmer, Yorwalk, Conn.). These systems require only very small analyte samples compared with a flame. and with a vidicon. 10 or more elements could be determined in a sample of
only a few microliters. In the New York State Department of Health laboratories a Delves cup method for determining lead in fingerprick blood samples is now in routine use ( I 7). This method could be extended to the simultaneous analysis of other trace metals in the same 50-pl. blood samples. Other possible applications of television camera detectors in AA and AF using both flame and nonflame atom reservoirs are numerous, and the system would he especially useful in any case where a number of elements are now determined in the same sample by atomic spectrometry. Examples are potable waters and waste water samples, where measurement of up to 20 trace metals may be required ( I & ) , and used lubricating oils, where wear-metal concentrations are monitored as criteria of$he condition of the engine (19-22). Conclusions
Multichannel photoelectronic de tectors of the type described in this article have many potential uses in atomic spectrometry. A computerized multichannel AA or AF spectrometer incorporating such a detector would enable extremely rapid determination of a number ofelements. The simplicity of operation, precision, and selectivity of AA are combined with the versatility of‘methods such as emission spectrography, X-ray fluorescence, and spark source mass spectrometry. The instrumental arrangement is far less complicated than most multielement systems, requiring minimal operator skill; and it is certainly less expensive to construct and maintain than an X-ray fluorimeter or a mass spectrometer. The principal advantages of a spectrometer with a vidicon detector over previously designed multichannel AA or AF spectrometers are threefold and may be summarized as follows: it can be assembled from commercially available components without a major engineering effort; it has a large number of detector channels, allowing the simultaneous determination of at least 8 or 10 elements, as well as background correction procedures; and it is a true simultaneous analysis instrument, while most other systems detect signals sequentially, and it is thus suitable for observing transient signals. One disadvantage of the system is that a compromise in optical alignment, flame conditions, and type of flame (e.g.. air-acetylene or nitrous oxide-acetylene) is always necessary. Furthermore, when some elements are in excess over others, the difference in useful working range between elements is bound to impose a limitation. The latter disadvantage is com~
mon to most multielement techniques. Another disadvantage which might limit the method is the rather poor SIN ratio which has been encountered during AA work, but this is likely to be improved with integration. In spite of these limitations, we believe that vidicons and similar devices will become widely used in the construction of multielement atomic spectrometers and will greatly increase the value and versatility of AA and AF spectrometry. Acknowledgment
The authors are grateful to M. R. Zatzick (SSR Instruments Co.) for his valuable comments and criticisms during the preparation of the manuscript. References (1) K . W. Busch and G. H. Morrison,
Anal. Chem., 45,712A (1973). (2) G. Horlick and E. G. Codding, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, Paper No. 25, March 1973. ( 3 ) R. E. Benn, W. S. Foote, and C. T. Chase, J. Opt. SOC. Amer., 39,529 (1949). (4) S. A. Johnson, W. M. Fairbank, and A. J . Schawlow,Appl. Opt., 10,2259 (1971j. ( 5 ) R. E. Santini, M .J. Milano, H. L. Pardue, and D. W. Margerum, Anal. Chem., 44,826 (1972). (6) A. B. Laponsky and V . J . Santilli, “Recent Developments in Low Light Level Camera Tubes,” Reprint 48, Westinghouse Electric Corp., Horseheads, N.Y., 1971. (7) M. Margoshes, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, Paper No. 99, March 1970. (8) M. Margoshes, Opt. Spectra, 4 , 2 6 (19701. ~~~I
(9) T. Hirschfeld, U.S. Patent 3,728,029, April 17, 1973. (10) R. L. Vogenthaler and M. Margoshes, ibid.,3,728,576. (11) K . W. Jackson, K . M. Aldous, and D. G. Mitchell. Saectrosc. Lett.. 6. 315 (1973). (12) F . W. Karasek, Res./Develop., 23, 47 (19‘72). (13) Tivicon Image Tubes from Texas Instruments,’’Texas Instruments Inc., Dallas, Tex. (14) J . D. Winefordner and T. J. Vickers, Anal Chem , 36,1947 (1964). (15) Technical Data Available on SSRI Optical Multichannel Analyzer, SSR Instruments Co., Santa Monica, Calif. (16) H . T. Delves.Analwt (London). 95. 431 (1970). (17) D. G. Mitchell, K . M. Aldous, and F. J . Ryan,Anal. Chem., in press. (18) C. R. Parker, “Water Analysis by Atomic Absorption Spectroscopy,” Varian Techtron Pty, Ltd., Springvale, Australia, 1972. (19) J. A. Burrows, J. C. Heerdt, and J. B. Willis, Anal. Chem., 37,579 (1965). (20) E . A . MeansandD.Ratcliff,At.Absorpt. Newslett., 4, 174 (1965). (21) S.Sprague and W. Slavin, zbid.,p 367. (22) M. Gardels, D. Demers, and D. G. Mitchell, “Advances in Automated Analysis, Technicon International Congress, 1970,” p 513, Thurman Assoc., Miami, Fla., 1971. I
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