Evaluation of Silicon Vidicon Detector Sensitivity for Atomic Spectrometry Applications N. G. Howell‘ and G. H. Morrison* Department of Chemistry, Cornell University, lthaca, N. Y. 14853
A comparative evaluation of the sensitivity characterisficsof the ultraviolet sensitized sillcon vldlcon and silicon Intensified target vidicon tubes in the ultraviolet-visible wavelength region is described. Calculations of spectral responsivity detail the manufacturers’ estimates of the performance of the Image tubes relative to a common photomultiplier. Experimental measurements utilize a 0.5-m dual detection port spectrometer to provide both sliicon vldlcon and photomultiplier tube observation of the same spectroscoplc feature. The measurements consist of relative detectlvlty and detectability characteristics and flame emission detection ilmits for 23 elements. The detection limits using the vidicon tubes are compared with best photomultiplier literature values to evaluate their analytical performance.The silicon intensifiedtarget vidicon tube Is shown to provide detectionpower equlvaient to commonly used photomultipliers in the vislble spectral region.
Recent reviews have discussed the attributes of several new multichannel detector systems, and outlined their application to a variety of analytical techniques (1-6). State-of-the-art multichannel detectors such as vidicons, photodiode arrays, charge coupled devices, etc. are being evaluated for their present spectroscopic capabilities (7-11), although it is recognized that significant improvements will be forthcoming in this rapidly developing field. Previous reports from this and other laboratories have detailed the development and characterization of such systems employing silicon vidicon tubes for spectroscopic measurements (7-10, 12-18). These research efforts have shown that the combination of simultaneous multiwavelength detection and digital multichannel signal processing provides numerous analytical benefits. Vidicon tubes have been used for several different types of spectral measurements ( 4 ) ,but greater emphasis has recently been placed on developing atomic absorption (9)and emission (7, 8, 10, 17) systems capable of performing simultaneous multielement analyses. Important instrumental parameters and capabilities that have been evaluated include wavelength coverage (7), resolution (7,8,18), dynamic range (8,18),signal integration (7,181, and signal averaging (7,13). However, the spectral sensitivity characteristics of these silicon vidicon tubes used in atomic spectroscopy measurements have not been determined. Most of the previous studies have simply stated that these devices are less sensitive than standard photomultiplier tube (PMT) systems with little discussion about the magnitude (7-10). Two early reports did calculate the expected signal-to-noise ratio disparity between standard silicon vidicons and P M T units a t a specific wavelength (2, 19). Preliminary detection limit results have also been obtained in the atomic absorption (9,20) and flame emission (7, 10) modes, but direct comparison to well developed conventional systems could not be made. All of these data were presented for the standard-type ultraviolet sensitized (UV) I The Procter and Gamble Company, Winton Hill Technical Center, 6110 Center Hill Rd., Cincinnati, Ohio, 45224.
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ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
vidicon, with no information available on the more sensitive silicon intensified target (SIT) vidicon tube. This study provides a detailed evaluation of the detection capabilities of UV and S I T vidicon tubes in atomic spectroscopy applications. Differences in sensitivity between these tubes and a standard P M T are investigated for a variety of spectral line-to-background noise conditions in the ultraviolet-visible (200-800 nm) wavelength region. Sensitivity is evaluated in terms of responsivity, detectivity, detectability, and detection limits. The flame emission detection limits for 23 elements are compared to best reported P M T values. These sensitivity measurements were necessarily obtained as single wavelength data but can be extrapolated to multielement systems with proper regard to the limitations imposed by the dynamic range and wavelength coverage of the vdicon system. From these data, conclusions can be drawn as to the applicability of present silicon vidicon devices to many areas of spectroscopy.
EXPERIMENTAL Apparatus. The experimental facilities employed in this study are listed in Table I. The dual detector spectrometer system monitored radiation from either a mercury pen lamp, a magnesium and calcium hollow cathode lamp, or a nitrous oxide-acetylene flame. The constant intensity line sources were used for the direct sensitivity comparisons. The high temperature flame provided spectral background for the comparisons and was utilized as the source for flame emission detection limit studies. A block diagram of the instrumental layout is shown in Figure 1. The Supracil lens focused radiation from either of the sources onto the spectrometer entrance slit. A Jarrell-Ash 0.5-m Ebert mount monochromator equipped with the camera mount attachment was modified to permit both PMT and vidicon tube observation of the same spectral line. A first surface mirror provided the mechanism by which the same spectral source was monitored by either photodetector. The reflecting mirror, placed a t a 45O angle in front of the normal exit slit, was easily flipped in and out of the optical path with a lever mechanism. Thus, only about 3 s were required to switch between detectors. The vidicon tube mounting bracket was designed to slide along the optical axis, allowing proper focus and positional geometry to be obtained. A prime requisite for a rapid detector changeover was that a line centered on the exit slit would be reflected to near center of the vidicon tube. Thus, the same spectral feature could be monitored at optimum location in both the vidicon and PMT detection ports without a change in grating position. An 1180 grooves-per-millimeter grating blazed for 400 nm provided a 20-nm wavelength window across the 12.5-mm active face of either vidicon detector. The operational characteristics of the UV and SIT vidicon tubes and their control unit, the Optical Multichannel Analyzer (OMA), have been detailed in previous reports (3, 7, 12). Sensitivity comparison data were obtained in the real-time mode by enabling the real-time display switch on the OMA console. The desired peak or background channel was continuously monitored through the use of the system’s plot capabilities and the dual channel strip chart recorder. Detection limit data acquisition utilized the signal averaging function of the OMA. The PMT detection system used for the comparison studies was also operated in a continuous mode. The anode signal current was fed into a Keithley (Model 417) high speed picoammeter and monitored as a pen deflection on the recorder. Signal and background noise bandwidths were measured from the recorder output. Procedures. Detectiuity. The mercury pen lamp line source was positioned at the focal point of the spectrometer system. Six mercury
T a b l e I. E x p e r i m e n t a l Facilities Burner
Varian Techtron 5-cm high solids slot burner for nitrous oxide-acetylene.
External Optics
Plano-convex Supracil lens 5-cm diameter with 12.5-cm focal length stopped down to 2.6 cm. Polka dot beam splitter 1 inch by 1.5 inches, Instrumentation Laboratories.
Slits
Jarrell-Ash Model 82-092 0 - 4 0 0 ~dual unilateral entrance and exit slit assembly with straight jaws.
Spectrometer Jarrell-Ash Model 82-020 0.5-m scanning Ebert Mount monochromator with Model 82-016 camera attachment and a n 1180 grooves-per-millimeter grating blazed at 400 nm, providing a reciprocal linear dispersion of 1.6 nm/mm. Detectors
Ultraviolet sensitized silicon vidicon tube assembly Model 1205 F, Princeton Applied Research Corp. Silicon Intensified Target vidicon tube assembly Model 1205D, with option-01 scintillator screen, Princeton Applied Research Corp. RCA 1P28 Photomultiplier tube, Radio Corporation of America
Power supply High voltage dc supply, Model 412A, John Fluke Mfg. Co., Inc. Amplifier
Keithley Model 417 High speed picoammeter, Keithley Instruments, Inc.
OMA
Optical Multichannel Analyzer Model 1205A, Princeton Applied Research Corp.
Data Readout Tektronix Oscilloscope, Model 604 Monitor. Speedomax X/L 680 Dual Channel Strip Chart Recorder, Leeds and Northrup Co. Flow meters
Brooks Full-view Rotameters calibrated for nitrous oxide and acetylene, Brooks Instrument Co.
emission lines at 184.9,253.6,312.5,365.0,404.6, and 546.1 nm were sequentially monitored by both detection systems. Each spectral line was first observed on the vidicon tube. The intensity was adjusted, using a lens diaphragm and neutral density filters, to yield a real-time reading of 100 counts for the peak channel on the OMA. The entrance slit width was maintained a t 25 bm throughout. Once the intensity adjustment was completed, the vidicon system signal-to-background noise ratio (SBNR) was acquired. A light block was placed over the entire entrance slit and the resulting detector system noise band was obtained for 90 s. The signal magnitude was recorded for 20 s when the light block was removed and the line position optimized with respect to the channel being recorded. The display factor was generally changed by a factor of 10 to keep the signal response on the strip chart recorder scale. After two complete sets of vidicon signal and noise measurements, the mirror was moved to permit observation of the same spectral line intensity by the PMT system. All PMT measurements were obtained with the same slit width and a slit height of 10 mm. The RCA 1P28was operated at a nominal 800-V potential. The response time of the picoammeter had been set at 0.1 s making it equivalent to the OMA plot output. The PMT noise band was obtained by the recorder system for the same 90-9 time frame with the full slit light block over the entrance slit. The magnitude of the signal was then monitored, with proper scale factor changes on the picoammeter to facilitate the on-scale recording of the signal output. After two complete noise-signal observation sets were obtained for the PMT, the next mercury line was readied for observation with the vidicon detector. Both the SIT (with wavelength converting scintillator) and UV vidicons were compared against the 1P28-PMT system in separate experimental runs. The SBNR values at each line were measured using the recorder pen deflections. All recorder measurements were multiplied by the appropriate scale factor changes. The signal values were then divided by the detector system (no light) noise bandwidths for each SBNR. The noise bandwidths were estimates of the standard deviation of the observed pen fluctuations. The same standard deviation estimation procedure was used for each detector system trace. The width of the band between the sixth highest and sixth lowest voltage spike over the central 80-s noise region was divided by two and called the standard deviation. Detectability. The hollow cathode lamp was also focused at the monochromator entrance slit. The lamp was placed at a 90' angle to the optical axis with a polka dot array beam splitter positioned along the axis midway between the burner and the lens. This arrangement facilitated the simultaneous viewing of a constant intensity line source and superimposed spectral background radiation. Initially, the magnesium 285.2-nm line from the lamp was monitored by the SIT vidicon (at maximum photocathode potential) and its intensity was adjusted to provide a strong (400 count) signal in real-time. Background noise recording was obtained with a light block placed in front of the hollow cathode source. Thus any recorded noise would result from the flame source. After the lamp emission had stabilized, alternating measurements of the SBNR were obtained as the photocathode potentials of both PMT and SIT vidicon detectors were varied through their full available range. For magnesium only two
n
n
A BURNER
pMq-y-zLr ""..,
0
LENS
MONOCHROMATOR
BEAM SPLlTER
Figure 1. Block diagram of experimental facilities
background conditions were employed: one had no flame background, the other was a slightly fuel rich nitrous oxide-acetylene flame with the burner top positioned 3 mm below slit center. Similar SBNR data were acquired in the visible wavelength region as the calcium 422.7-nm line intensity was appropriately set and then viewed by both tubes throughout their photocathode range of 3-9 kV for the SIT vidicon and 0.5-1.0 kV for the RCA PMT. The calcium line was monitored in three conditions-without flame background, with an air-acetylene flame, and with a nitrous oxide-acetylene flame. The SBNR data were obtained in the same manner as described previously. The visible region flame backgrounds were acquired with the burner positioned 15 mm below slit center and slightly fuel-rich flame conditions. Detection Limits. Stock solutions for the detection limits study were prepared from high purity metals and salts in accordance with the methods described by Dean andRains (21). All dilutions were made with doubly distilled deionized water. Final detection limit measurements were performed on pure aqueous solutions containing only the species of interest. Barium and strontium values were obtained in the conventional manner using 10 fig/ml cesium as an ionization buffer for each (22-24). The spectrometer system used for the previous sensitivity comparisons was utilized for these measurements. Only the 5-cm slot burner was focused onto the entrance slit. Both the UV and SIT vidicons were utilized for flame emission detection limit data. The SIT was operated initially without modification and the data were reacquired after the placement of a terphenyl scintillator over its fiberoptic faceplate. ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
107
All data were taken from the OMA in its manual mode of operation ( 1 2 ) .The instrumental parameters of flame fuel-to-oxidantratio, burner position, slit width, and SIT photocathode voltage were optimized for each element. However, the majority of determinations were performed under similar conditions; an acetylene pressure of 8 1b/h2with a flow rate of 3800 cm3/min,nitrous oxide pressure of 30 l b h 2and a flow rate of 5500 cm3/min,the burner height at 10 mm below the center of the observation zone (3 mm for refractory elements), slit widths for 40 to 80 lm, and SIT photocathode voltages from 4.5 to the 9.0 kV maximum used for most elements. The method employed for the evaluation of individual detection limits was similar to the digital integrating photomultiplier technique of Boumans and de Boer ( 2 4 ) .Once the vidicon flame spectrometer system had been optimized, a solution containing a small but detectable amout of that element was aspirated into the flame. At this point 125 consecutive frame scans (4.1 s) of the flame spectrum were accumulated into memory. The flame background and a distilled water blank were removed by subtracting 125 frame scans of doubly distilled deionized water. The blank corrected final spectrum was then evaluated for the spectral line of interest. The intensity reading (in counts) was obtained by noting the peak height and subtracting an average background from 10 adjacent channels (12).This detectable signal measure was required for each of five separate runs of the pure analyte solution. A: average of these five intensities was used as the signal magnitude; I , at that analyte concentration (CL, pg/ml). The noise band estimate at that spectral location was found by again taking the difference between the peak channel employed for signal measurement, and the adjacent background; when only distilled water was aspirated into the flame. The standard deviation of those 20 noise values (AN,) was used as a measure of the noise bandwidth ( 2 4 ) .Estimated detection limits were easily calculated. The average signal for the detectable concentration was extrapolated to the signal ( S , ) theoretically obtainable at a concentration of 1 fig/ml ( S , = IJC,). The detection limit at the conventional,twice the standard deviation of the noise, confidence level (24) was then calculated by dividing the magnitude of the‘ noise bandwidth by the signal at 1 pg/ml ( 2 A N/ S , 1.
RESULTS AND DISCUSSION The term “detector sensitivity” is used as a general heading for the complete set of characterizing experiments described in this study. The common .usage of sensitivity in analytical chemistry conveys the ability to discern the difference between very small amounts of analyte species. Extrapolating this definition to the lower limit of instrumental observation, it reflects the smallest amount of substance which can be discerned as present (25). To properly evaluate the sensitivity of a detector for spectral measurements, a number of different characteristics must be considered. The detector should be evaluated in terms of its spectroradiometric parameters to determine its response capabilities for general spectroscopic applications. In addition, comparative atomic spectroscopy measurements are required to determine its sensitivity in performing chemical analyses. The spectroradiometric parameters, responsjvity and detectivity, consist of basic physical calculations and comparative evaluations predicated on data supplied by the manufacturer. The responsivity of a detection device refers to the magnitude of the change in output signal induced by a change in input radiant flux (26-28). Herein the quantity is calculated for each detector from the manufacturer’s data and displayed as a function of wavelength via spectral response curves. The influence of detector generated noise on the detection capability is displayed through the detectivity expression. Detectivity is defined as the responsivity divided by the rootmean-square (or standard deviation) of the no light detector generated noiseband (27,28). Our evaluation of this parameter does not strictly adhere to this definition because absolute measurements are not obtained. Instead, a practical determination of this important parameter is provided by comparative SBNR measurements for each detector under identical conditions of low spectral line intensity with no spectral background. 108
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
The atomic spectroscopy detection capabilities are then evaluated through comparative detectability and flame emission detection limit measurements. Detectability is used in this study as a quantitative measure of SBNR values for each detector under identical flame spectrometry conditions. The term covers the evaluation of detection capabilities where spectral background constitutes the major source of noise. Detection limit measurements in flame spectroscopy have been well defined in the literature (24-26). They are utilized as directly measurable evaluations of instrumental performance under specified spectroscopic conditions. The lower the reported detection limit concentration for a particular system, the better that instrument’s power of detection (26). Responsivity. A generally applicable estimate of the spectral response capabilities difference between the silicon vidicon tubes and a standard P M T can be obtained through a comparison of their absolute spectral response curves. Manufacturers’ data for the UV and SIT vidicons are provided in responsivity units of counts per photon, compatible with digital data format of the OMA (29,30). A spectral response profile for the RCA 1P28-PMT is given in amps per watt (31). Recently PMT detectors have been utilized with digital signal processing for spectroscopic applications (24). A comparative plot of the spectral response curves for both P M T and silicon equivalent format. Asvidicon tubes requires conversion tt~ suming the P M T signal conversion from amps to counts is made by the same amplification and digitization circuitry found in the OMA, an expression for the responsivity of the PMT can be derived in digital form. The analog anode signal current produced by a P M T is given as:
S, = Q , G d @ e where S, = signal response of the PMT, amperes; Qp = quantum efficiency of the photocathode; G , = gain of the dynode chain a t the applied voltage; A = area of the photocathode illuminated, cm2; @ = photon flux a t the detector, photons/cm2 s; e = charge on the electron, coulombs (2,31). Signal counts produced by the imagined PMT-OMA amplifier system within a certain time of observation is expressed as:
C, = S,D where C, = signal produced, counts; and D = digital circuitry amplification and conversion factor, counts s/coulom. The total number of photons striking the photocathode surface within a given observation period can be expressed as:
P = @At,
(3)
with P = the integral number of photons; and to = the time of observation, s. The resultant digital responsivity (R,) for a P M T is given by: (4) which has units of counts per photon. Numerical substitutions for the variables are made with regard to the specific P M T used in this report (RCA 1P28).The values for the wavelength dependent quantum efficiency (&,(A)) are taken from the RCA data book (32) where G , was also listed as 2.5 X 106.The constants D and to are specific to the OMA data circuitry (29). The digital conversion factor is given by D = 8.0 X 1O1O counts/A ((1count/2500 electrons) X (1electron/l.6 X C) X (3.2 X lop5 s for conversion)), and the observation time equivalent to a vidicon single frame scan period is given as t o = 3.28 X s. Thus the wavelength dependent responsivity expression for the RCA 1P28 reduces to
,
10000
\\\
3000 50001
0.3
I - ‘ \
\
IO00
500
.g
-
\
300-
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a0
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100-
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6
5
0005
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00031
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30-
\
\
\
o
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0.5
0.3
I 200
.OOOOl~
I
I
300
400
I
I
500 600 Wavelength, nm
I
700
011
I
800
Digital spectral response curves for PMT(RCA 1P28),SIT Vidicon, and UV Vidicon Tubes, using the amplification and digitization circuitry of the OMA Figure 2.
R,(M = Q,(A)(0.98).
(5)
This function, when plotted vs. wavelength, yields an absolute spectral response curve compatible with available UV and SIT vidicon data (29). For completeness, previously reported vidicon analog response equations (2,13) can be converted to digital form. The wavelength dependent UV vidicon responsivity @,(A)) is given by:
R,(W = 4u(A)GuteeD = Q,(A)(4.0 tr
x
(6)
where &,(A) = quantum efficiency of the UV tube target; G, = detector gain (1.0);t e = exposure time of a channel (3.28 X 10-2 s);and t , = signal readout time for a channel (3.2 X s). The equivalent SIT vidicon responsivity (R,( A)) expression reduces to:
R,(A) = QdM(0.60)
(7)
where the SIT vidicon tube gain is 1500;timing sequences are the same; and Qs(A) = S I T tube target quantum efficiency. This tube target quantum efficiency is dependent on the characteristics of the photocathode of the electrostatically focused intensifier section, the fiberoptic faceplate, and the scintillator screen when it is used. A plot of the three digital responsivity expressions 5,6, and 7 vs. wavelength yields the curves shown in Figure 2. Rather than calculate the complex S I T vidicon target quantum efficiency, the latest manufacturer’s plot was used (30). The graph (Figure 2 ) displays the P M T systems’ fairly substantial ultraviolet region responsivity advantage over both silicon vidicons. The majority of the reported difference between the 1P28-PMT and the SIT vidicon is attributable to the target quantum efficiency. The PMT’s photocathode (S-5 response) quantum efficiency decreases rapidly in the visible region where both image tubes reach their optimum spectral response. In fact, the responsivities of the RCA 1P28 and intensified vidicon are nearly equivalent over the visible wave-
I
200
I
300
I
I
I
400 500 600 Wavelength, n m
I
700
I
800
Flgure 3.
Comparison of responsivity and detectivity ratios
Rp/Rs (-),
RJR, (- -
- - -); SBNR-PMT/SBNR-UV Vid. (O), SBNR-PMT/SBNR-SIT
Vid ( 0 )
length range. The UV tube, with its target of high quantum efficiency, lacks the internal gain features of the other tested devices resulting in poorer spectral responsivity throughout the ultraviolet-visible region. A graphical representation of the ratio of the responsivity expressions provides a clear indication of the magnitude of these differences. The curves displayed in Figure 3 are the P M T expression 5 divided by either the UV vidicon Equation 6 or SIT vidicon Equation 7 . The solid curve corresponds to the P M T to SIT vidicon responsivity ratio (Rp/Rs), while the dashed curve provides the P M T to UV image tube ratio (Rp/R,). These calculated comparisons suggest an extremely large detector sensitivity difference between the UV vidicon and conventional P M T systems. Detectivity. While the responsivity data display the magnitude of the disparity in signal response for these devices, they provide no estimate of the signal strength required to be discriminated from the detector noiseband. A signal cannot be considered detected if it is masked by large detector generated background noise fluctuations. The most common sources of background noise in atomic spectroscopy applications are the excitation source, atomization reservoir, and detector system. The detectivity measurements evaluate the relative detection capability of either silicon vidicon with regard to the 1P28-PMT at the detector system noise-limited condition. To ensure accuracy in the comparative measurements of the spectroradiometric term, a dual detection port spectrometer was employed. The spectral line intensity monitored by either detector with this system differed only in the time a t which the observation was made and the collection and reflectance properties of the deflecting mirror. The constant intensity lamp reduced the error introduced by source drift between detector observations. The collection efficiency of the mirror was maximized by adjusting the mounting plate position to yield optimum signal strength a t the center of the silicon vidicon tube target. No correction was made for the small degree of imperfection in the reflectance of the first surface mirror. Any bias introduced by this arrangement would favor the P M T measurements. ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
109
500r
a
PMT=665L500r
b
/-il/..--- 3001 50
Intensifier Poteniial , K v
Figure 4. Signal-to-background-noise ratios for the SIT vidicon (a) At the 285.2-nm Mg line with no spectral background (-), with nitrous -): (b) At the 422.7-nm Ca line with oxide-acetylene flame background (no spectral background (-), with air-acetylene flame background (- - -), with nitrous oxide-acetylene flame background (- - - - -)
---
- -
Experimental detectivity ratios for the six mercury lines and two sets of PMT-vidicon observations are displayed in Figure 3 as separate points. The solid circles represent the SBNR values of the P M T divided by those for the SIT vidicon. Open circles correspond to similar ratios comparing the P M T to the UV vidicon tube. The detectivity and responsivity ratios for the 1P28 P M T and SIT vidicon are shown to be in close agreement. However, such is not the case for the data on the UV vidicon tube; detectvity ratios for the five measurable mercury lines display a detector sensitivity difference which is less than the vast amount forecast by the responsivity curves. If ultraviolet and visible spectral regions are separated a t 380 nm, the P M T system’s detectivity advantage in the ultraviolet region averages about a factor of 150 over the UV vidicon and 20 over the S I T tube. The visible wavelength advantage for the PMT reduces to a factor of 50 over the UV tube and the SIT vidicon exhibits an equivalence in detectivity. Detectability. Source and atom reservoir generated background noise predominate in most atomic spectroscopy applications. The effects of spectral background noise on the relative detector sensitivities are easily monitored on the same dual detector spectrometer system. For this evaluation two analytical lines commonly used in atomic spectrochemical analyses were monitored by both the SIT image vidicon and 1P28-PMT. The magnesium 285.2-nm line in the ultraviolet and calcium 422.7-nm line in the visible wavelength region were observed with varying levels of flame background superimposed as background noise. The SBNR values at each spectral condition were acquired over the range of available photocathode potentials with both detectors. The curves in Figure 4a illustrate the range of SBNR values obtained on the SIT vidicon for the magnesium line. The solid line represents the SBNR values acquired when no spectral background is present. The dotted line profiles the data found when a nitrous oxide-acetylene flame is superimposed on the same intensity magnesium line. The numbers listed at the curves’ end are the optimum SNBR values obtained with the P M T detector. The difference between the SIT vidicon and P M T SBNR detectability values decreases as the contribution of spectral background noise increases. The ultraviolet region detectability ratio (optimum SBNR for P M T divided by the optimum SBNR for SIT tube) falls from 17 where no spectral background is present to 7.8 when flame background is present. Note how the SIT vidicon’s SBNR improvement with in110
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
creasing photocathode voltage actually reaches a plateau before the most sensitive setting is reached. This behavior is vividly displayed in the visible wavelength data, Figure 4b. However, the devices have similar responsivity in this region and thus the optimum SBNR-PMT/SBNR-SIT vidicon ratios remain constant in going from no spectral background (the solid line) to an air-acetylene flame (the dashed line) medium intensity background, and finally the high intensity nitrous oxide-acetylene flame (the dotted line). At an intensifier voltage of 3.3 kV on the SIT vidicon, its spectral responsivity at 420 nm was equivalent to that of the UV vidicon. The SBNR values for the SIT vidicon at this low sensitivity did not display much of a change when spectral background noise was added. The noise did change the optimum P M T values by a large amount. If the SBNR values a t 3.3 kV (SBNR of about 4.0) are taken as the SBNR-UV vidicon data, then the detectability ratio of the SBNR-PMT/SBNR-UV vidicon is reduced from the no background factor of 53 to the medium value of 11and, finally, only 4.4 as the flame background intensity increases. This accounts for relative detector sensitivity variations of over an order of magnitude which depend solely upon the intensity of the spectral background. Detection Limits. The detection limit is one of the most popular as well as useful criteria employed to evaluate the capabilities of a particular analytical method or instrument (25).There is a large supply of data available in the literature on almost any element for a variety of analytical methods using several different instrumental configurations. In fact, this section of the detector sensitivity evaluation is based upon the direct comparison of vidicon system detection limits with best published values obtained with P M T detectors on similar instrumental components. Several analytical parameters and experimental conditions must be investigated if a meaningful comparison is to be made. First, there are three modes of atomic spectroscopic analysis to which the vidicon tube could be applied, i.e., absorption, emission, and fluorescence. Both atomic absorption and fluorescence measurements have benefited from the utilization of phase sensitive detection systems. Silicon vidicons, in their present development state, are not adaptable to modulated signal methods (33). Thus, a direct comparison with best absorption and fluorescence literature data would not be equitable. Atomic emission detection limits can be found for any number of spectroscopic excitation sources and instrumental configurations. Both continuous and phase sensitive detection systems have been used without an appreciable advantage displayed for either method (34).The atomic emission mode afforded the fairest evaluation measurements. The wellcharacterized nitrous oxide-acetylene flame was chosen as the excitation source because of its plausible multielement analysis capabilities ( 3 ) and large detection limit data base (22-24). The elements and corresponding analytical lines were chosen to span the ultraviolet-visible spectrum of atomic spectroscopic measurements. The burner system and spectrometer system used are similar to the components found in the P M T reports. Instrumental parameters were optimized for each element simulating the conditions employed for the P M T data (22-24). Throughout the study the OMA was set for a signal averaging time of 4.1 s (125 frame scans). The detection limits listed in Table I1 were obtained with three different silicon vidicon tube configurations. One set of data was acquired for the UV vidicon tube, another for the SIT vidicon tube, and the third utilized the SIT image vidicon after the scintillator was placed over its faceplate. Also listed in Table I1 are the corresponding P M T literature values for a nitrous oxide-acetylene flame. A comparison of the two sets of SIT tube data suggests that the application of the wavelength conveting screen does not degrade the visible region performance. Before a graphical matching of P M T and vid-
Table 11. Flame Emission Detection Limits Element Ag A1 Ba Bi Ca
co Cr
cu Fe In K Li Mg Mn Mo Na Ni Pb RP Sr Ti
v
W 24.
Line, nm 328.1 396.1 553.5 306.8 422.7 345.4 425.4b 324.7 372.0 451.1 766.5 670.8 285.2 403.1 390.2 589.0 352.5 405.8 780.0 460.7 399.8' 437.9 400.9
PMT' 0.002f 0.003f
0.001f 20.0d 0.0001f 0.03d 0.002f 0.01 d
UV vid. 0.3 0.05 0.02
0.01 0.001
5.0 0.001
0.0001
0.0004f
0.6 0.01 0.07 0.2 0.02
0.005f
SIT vid.a
0.002 0.01 0.006 0.06
0.00005d
0.03
0.00002d 0.005e 0.001f 0.1e 0.0005d 0.02f
0.00002 0.2 0.02 0.1
O.ld
0.8 0.1 0.002 0.2 0.5 3.0
0.008d 0.0002f 0.03f 0.007f 0.7f
0.0005 0.1
0.00001 0.003 0.07 0.0002
0.2 0.1 0.0002 0.06 0.02 1.0
SIT with scinLU 0.1 0.008 0.001 1.0 o.ao02 0.5 0.002 0.07 0.04 0.003 0.03 0.00001 0.07 0.002 0.04 0.00007 0.2 0.1 0.08 0.0001
0.06 0.02 0.9
Int. pot., kVg 9.0 4.5 9.0 7.0 5.0 9.0 7.9 9.0 7.5 7.5 9.0 9.0 9.0 9.0 4.8 5.6 9.0 8.0 9.0 7.3 5.0 5.5 4.7
Values are wg/ml. PMT value obtained at 359.4 nm. PMT value obtained at 365.4 nm. Reference 22. e Reference 23. f Reference SIT Vidicon Intensifier Potential.
I:
icon detection limits can be made, signal integration and averaging time difference among the values must be reconciled. The referenced detection limit values had signal integration times ranging from 15 s ( 2 4 ) and 10 s (22)to a signal output time constant of 3 s (23).Thessilicon vidicon-OMA detection system has both signal integration and averaging capabilities (7,13). These functions have been shown to improve SBNR values as the time of observation increases (7,18). The S I T vidicon could not employ the integration mode in the intense visible region of the nitrous oxide-acetylene flame spectrum. Thus the integration feature was not utilized in any of the detection data to maintain consistency in the data acquisition procedures. No doubt signal integration could have easily been employed in regions of low detector sensitivity and low background to minimize detector system amplifier noise ( 4 ) and correspondingly improve several vidicon detection limits. Intense flame background also limited the signal averaging time. The 5BCD memory capacity of the OMA allowed only 125 frame scan accumulations of high background spectra before the unit's memory range was exceeded. A recently developed computer interface built in this laboratory (15) could have acquired such data for hours. Where memory capacity was not a problem, extended accumulation times resulted in improved detection limit values. An illustration of the detection limit improvement is shown in Figure 5. The solid line on the log-log plot of detection limit vs. averaging time displays the values obtained for cobalt on the S I T vidicon (with scintillator) when shorter and longer accumulation times are employed. The line exhibits a least-squares best fit slope (-0.46) which approximates the anticipated inverse square root relation. The parallel dashed line established an averaging factor (af)which is used to correlate the 4.1-s vidicon detection limits to P M T values with differing integration times. The individual silicon vidicon detection limits (Table 11) were multiplied by the appropriate afvalue which corresponds to the experimentally verified averaging improvement for the comparable P M T detection limit integration time. Once this
correlation correction was applied, the adjusted vidicon detection limits were plotted vs. the PMT values. A log-log plot of the UV vidicon-PMT detection limit match is shown in Figure 6. The diagonal solid line marks the equivalence of data points for both systems. Most of these data fall about the dashed line which indicated a factor of ten advantage for the P M T systems. Considering only the responsivity and detectivity data, these flame emission values display significantly better UV vidicon sensitivity characteristics than were expected. The discussion of the visible region detectability ratios for the P M T and UV vidicon are supported by these detection limit observations. Bismuth, with an extremely intense spectral background, displays a situation where the tube's detection power is equivalent to the P M T even though a large responsivity disparity exists. A similar, even more impressive result is shown for bismuth with the SIT vidicon. The log-log detection limit comparison between the scintillator faced S I T tube and P M T in Figure 7, exposes a majority of equivalent values. Since experimental detection limits are considered accurate to a factor of two a t best ( 2 4 ) ,all except the six elements with emission lines below 380 nm display essentially equivalent flame emission detection capabilities. The poor ultraviolet region detection limit values may be partially attributable to the 400-nm blaze of the grating, but the results were predicted by both detectivity and detectability measurements. Comparisons for rybidium and potassium were not included in either UV or SIT vidicon graphs because of the extreme difference in the photocathode response characteristics for the P M T (RCA 1P21) used in their determination (22). The elemental symbols tagged with an asterisk (*) identify values that were obtained with the SIT vidicon where the intensifier section potential was reduced from its 9.0-kV maximum gain setting. The actual voltage settings are found in Table 11. The vidicon-OMA system has no means of biasing out intense continuum background signals, a feature found on many PMT-amplifier systems. This necessitates a reduction in detector gain whenever analyses are performed in ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
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- 2.0 - 1.5
--
- 1.0 .z -0.8 -0.6 I.?
- 0 . 5 c+ -0.4
.g
-0.3
5
a -0.2
0.3 0.5 Flgure 5.
1.0 3.0 5.0 IO Log Sipnal Averaging Time, sec
15 20
Detection limit improvement with signal averaging
Experimental values for cobalt (-),
averaging factor, at (-
0000l
- - - -) Flgure 7. PMT-SIT
U V Silicon Vidicon Detection Limits. p g / m l
Flgure 6. PMT-UV
vidicon detection limit comparison
0001 S I T Vidicon
001
01
I S c l n t i ~ ~ a t a Detection r)
Limits
01 , pQ/rn4
IO
vidicon detection limit comparison
above 380 nm. In fact, for cases with high spectral background noise, i.e., bismuth and molybdenum, even the UV vidicon can provide comparable detection capabilities. The data presented in this study confirm the observation that presently available silicon vidicon tubes are most capably applied to atomic or molecular emission spectrochemical analyses in the visible wavelength region. As source background fluctuation noise increases, detector responsivity requirements for comparable detection limits decrease. Continuous spatial multichannel devices like the S I T vidicon provide the means by which analytical chemists may realize the full potential of atomic spectroscopy methods through internal standard measurements, spectral stripping, and multielement analyses. Anticipated developments in device technology which increase dynamic range, enlarge target surface area, and improve ultraviolet region responsivity should make these systems the preferred atomic spectroscopy detectors.
high background locations. However, the curves in Figure 4b indicate that the reduction in gain does not result in a poorer SBNR if the spectral background's noise bandwidth is sufficiently large.
ACKNOWLEDGMENT The authors thank John D. Ganjei for valuable discussions and comments. Thanks are also extended t o Alain de Jesus
CONCLUSIONS
LITERATURE CITED
The comparative detector sensitivity data for these two silicon vidicon tubes proves especially useful in evaluating their applicability to atomic spectroscopy techniques. Most notably these image tube detectors' poor responsivity characteristics in the ultraviolet region display why the detectors are not well suited to atomic absorption or atomic fluorescence measurements. The signals monitored in atomic fluorescence are generally in the ultraviolet wavelengths, of low signal intensity, with minimal spectral background noise. This clearly represents the worst spectroscopic condition for vidicon devices in their present development status. A newly available, more sensitive vidicon, the intensified silicon intensified target (ISIT) vidicon tube also requires scintillator wavelength conversion to facilitate ultraviolet region response. Its resultant gain in ultraviolet region responsivity is about a factor of two over the SIT version (30).The new tube does, however, provide substantial improvement in the near infrared response. If the powerful multielement analysis capabilities of the fluorescence mode (6) are to be realized with multichannel detectors, manufacturers' design criteria must include substantial improvement in ultraviblet region detectvity. Our experience with flame emission measurements has shown the SIT vidicon to possess excellent detection powers for the determination of elements whose analytical lines lie
(1) R. E. Santini, M. J. Milano, and H. L. Pardue, Anal. Chem., 45, 915A (1973). (2) D. G. Mitchell, K. W. Jackson, and K. M. Aldous, Anal. Chem., 45, 1215A (1973). (3) K. W. Busch and G. H. Morrison, Anal. Chem., 45, 713A (1973). (4) Y. Talmi, Anal. Chem., 47, 658A (1975). (5) Y. Talmi, Anal. Chem., 47, 697A (1975). (6) J. D.Winefordner,J. J. Fitzgerald, and N. Omenetto. Appl. Spectrosc., 29, 369 (1975). (7) K. W. Busch, N. G. Howell, and G. H. Morrison, Anal. Chem., 46, 575 (1974). (8) M. J. Milano. H. L. Pardue, T. E. Cook, R. E. Santini, D. W. Margerum, and J. M. T. Raycheba, Anal. Chem., 46, 374 (1974). (9) K. W. Jackson, K. M. Aldous, and D. G. Mitchell, Spectrosc. Left., 6,315 (1973). (IO) D. 0.Knapp, N. Omenetto, L. P. Hart, F. W. Plankey,and J. D. Winefordner, Anal. C h h . Acta, 69, 455 (1974). (11) G. Horlick, Appl. Spectrosc., 30, 113 (1976). (12) K. W. Busch. N. G. Howell, and G. H. Morrison, Anal. Chem., 46, 1231 (1974). (13) K . W. Busch, N. G. Howell, and G. H. Morrison, Anal. Chem., 46, 2074 (1974). (14) N. G. Howell, J. D. Ganjei, and G. H. Morrison, Anal. Chem., 48, 319 (1976). 1151 . , J. D. Ganiei. N. G. Howell. J. R. Roth. and G. H. Morrison. Anal. Chem.. 48. 505 (1976). (16) T. E. Cook, M. J. Milano, and H. L. Pardue, Clin. Chem. ( Winston-Salem, N.C.), 20, 1422 (1974). (17) F. L. Fricke, 0.Rose, Jr., J. A. Caruso. Anal. Chem,, 47, 2018 (1975). (18) T. A. Nieman and C. G. Enke, Anal. Chem., 48,619 (1976). (19) G. G. Olson, Am. Lab., 57, (Feb. 1972). (20) K. M. Aldous, D. G. Mitchell, and K. W. Jackson, Anal. Chem., 47, 1034 (1975).
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for his assistance in obtaining the detection limit data.
(29) "Optical Multichannel Analyzer OMA Operating and Service Manual," MDL 1205A; 6/75, Princeton Applied Research Corp., Princeton, N.J., 1975. (30) D. E. Osten, lnd. Res., 17 (IO), 82 (Oct. 1975). (31) "Photomultiplier Manual," Manual PT-61, RCA Corp., Harrison, N.J., 1970. (32) "Photomultiplier Tubes Catalog," PIT-7008 12/7 1, RCA Corp., Harrison, N.J., 1971. (33) K.W. Jackson, K. M. Aldous, and D. G. Mitchell, Appl. Spectrosc., 28,569 (1974). (34) Y. Talmi, R. Crosmun, and N. R. Larson, Anal. Chem., 48, 326 (1976).
(21) J. A. Dean and T. C. Rains, "Standard Solutions for Flame spectrometry", in "Flame Emission and Atomic Absorption Spectrometry", Vol. 2, J. A. Dean and T. C. Rains, Ed., Marcel Dekker, New York, N.Y., 1971. (22) G. D. Christian and F. J. Feldman, Appl. Spectrosc., 25, 660 (1971). (23) E. E. Pickett and S. R. Koirtyohann, Spectrochim. Acta, Part B, 23, 673 (1968). (24) P. W. J. M. Boumans and F. J. De Boer, Spectrochim. Acta, Part B, 27,39 1 (1972). (25) G. H. Morrison and R. K. Skogerboe, "General Aspects of Trace Analysis", in "Trace Analysis: Physical Methods," G. H. Morrison, Ed., Interscience, New York, N.Y., 1965. (26) IUPAC, Information Bulletin-Number 27, "Nomenclature on Analytical Flame Spectroscopy and Associated Procedures" (1972). (27) C. S. Williams and 0. A. Becklund, "Optics: A Short Course for Engineers and Scientists," Wiley-lnterscience, New York, N.Y., 1972., (28) H. T. Betz and G. L. Johnson, "Spectroradiometric Principles, in "Analytical Emission Spectroscopy," Vol. 1, Part 1, E. L. Grove, Ed.. Marcel Dekker, New York, N.Y., 1971.
RECEIVEDfor review July 26,1976. Accepted September 23, 1976. This work was supported in part by the National Institutes of Health under Grant No. 5 R01 GM 19905-03 and by the National Science Foundatiop through the Cornell Materials Science Center.
Analytical Capabilities of the Selectively Modulated Interferometric Dispersive Spectrometer T. L. Chester' and J. D. Wlnefordner" Department of Chemistry, University of Florida, Gainesville, Fla. 326 1 1
A general signal-to-noise ratlo (S/N) behavlor model Is derived for the Selectively Modulated Interferometric Dispersive Spectrometer (SEMIDS) based on an evaluation of the instrument as a flame atomic emission analyzer. A comparison Is made to a slmllar model derived for a conventional dispersive spectrometer. It is shown that the S/N for SEMIDS strongly depends on the nature of the spectral background and that no significant S/N Improvement Is expected In SEMIDS (as compared to the dispersive spectrometer) for any realistic analytlcal measurement In the UV-vlslble spectral region. However, some Improvement may be realized In the Infrared spectral reglon.
The Selectively Modulated Interferometric Dispersive Spectrometer (SEMIDS) was first described by Dohi and Suzuki (1). It is essentially a Michelson interferometer in which the stationary reflecting mirror is replaced with a rotatable diffraction grating (see Figure 1).As a result, the multiplex nature of the interferometer is eliminated as interference occurs only for the Littrow wavelength of the grating. Slight oscillation of the remaining mirror results in interference modulation of the signal spectral component. Selective amplification of the ac signal component distinguishes it from the remaining dc background. Thus, no interferogram is produced, and no Fourier transform is required. The practical resolving power is nearly equal to the theoretical resolving power of the grating used and is independent of the entrance aperture area (within a limit). Thus, the luminosity-resolving power product for SEMIDS may be much greater than that of a conventional dispersive spectrometer with the same grating. Mechanical tolerances are much less severe for SEMIDS than for the Michelson interferometer (used as a Fourier transform spectrometer) and for the SISAM spectrometer (which is a Michelson interferometer employing gratings in place of both mirrors). Thus, use of SEMIDS is possible in the UV-visible spectral region where it has proved difficult for multiplex systems based on the Michelson interferometer, i.e., the Fourier transform spectrometer ( 2 ) ,and Present address, Procter & G a m b l e Co., M i a m i Valley L a b o r a tories, P.O. Box 39175, Cincinnati, Ohio 45247.
the SISAM spectrometer ( 3 , 4 ) .Investigation of SEMIDS is warranted by a potential Jacquinot (or throughput) advantage of lo2 to lo3compared to single slit dispersive spectrometers used at the same resolving power. SEMIDS was recently investigated for analytical utility in the UV-visible spectral region ( 5 ) .The preliminary conclusions given were that some potential analytical usefulness existed for SEMIDS in those situations where the measurement of faint light signals are of importance, namely, atomic emission, atomic fluorescence, and molecular luminescence. Subsequently, the present careful evaluation of SEMIDS as a flame emission detector has indicated that no signal-to-noise ratio (S/N) improvement results and, in fact, the detection limits obtained using SEMIDS are much inferior to those obtained by conventional spectrometric systems. In addition, considerations of separate spectral bandpasses for the signal and (background carried) noise components in atomic fluorescence spectrometry (AFS) indicate that no improvement in S/N should be expected from SEMIDS, SISAM, multiplexed systems, and other selective wavelength modulation methods for AFS (6). In order to understand the experimental failure of SEMIDS and to predict its behavior in other applications, a S/N behavior model was developed for SEMIDS and is presented in this paper. The model, evaluated for flame emission and experimentally verified, is generally applicable to other analytical spectrometric situations.
THEORETICAL CONSIDERATIONS Signals and noises (in terms of counts; count rates result in the same S/N expressions) for various spectrometric methods of analysis have recently been formulated by Winefordner et al. ( 7 ) .These will be followed to a large extent, but some expansion is necessary to properly describe SEMIDS. Noise Types. Only two types of noises were previously considered ( 7 ) ,photon (shot) noise and l/f (drift or flicker) noise. Photon (shot) noise, N,, in counts, is given by the square root of the number of the counts (due to radiation impinging on the detector) observed (assuming photon counting is used), N , = a ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
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