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Photodiode Array Detectors in UV-VIS Spectroscopy: But I Theoretical aspects of photodiode array detection are presented in this month's
INSTRUMENTATION.
In
next month's issue, the second article in this two-part series will cover photodiode array detection in spectroelectrochemistry, high-performance liquid chromatography, stopped-flow kinetics, and other analytical chemistry applications. The properties of UV and visible light and the responses of many compounds to these wavelengths have long been considered a useful tool both in identification and in quantitative analysis of compounds. Molecular absorption UV-visible (UV-VIS) spectrophotometry requires detectors with high-UV to near-IR (NIR) response, large dynamic range, linear response, low noise, and temporal as well as thermal stability. The photodiode array used as a UV-VIS spectrophotometric detector meets these requirements with the added advantage over classical systems of acquiring the entire UV-VIS spectrum simultaneously. Because spectral acquisition is a parallel process, many of the concerns of UV-VIS spectrophotometer users, such as sample photodegradation, scanning nonlinearities, and difficulties in observing transient phenomena, have been minimized if not eliminated. 0003-2700/85/A357-1057$01.50/0 © 1985 American Chemical Society
Conventional UV-VIS spectrometers Complete UV-VIS spectrophotometric information can be obtained either by scanning across the spectral region of interest or by simultaneously monitoring this region in its entirety. Conventional scanning monochromator systems use a continuous source (deuterium or tungsten lamp) and a dispersive element (a grating or a prism). This dispersive element is mechanically rotated to vary the wavelength of
light passed through a fixed exit slit. The resulting monochromatic light passes through the sample cell and is detected by a photomultiplier tube. Because sampling is limited to a very narrow wavelength range, data points are acquired at different times. The optical information is acquired inefficiently and sometimes inaccurately (Figure 1). Attempts to use multiple photomultiplier tubes as parallel detectors have been accomplished with mini-photo-
Figure 1. Conventional spectrometer optical configuration ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985 · 1057 A
multiplier tubes in predetermined po sitions (corresponding to spectral re gions of interest) across the focal plane of a polychromator, each with its associated readout electronics. Even though the photomultiplier tube-exit slit combination is limited to the measurement of one spectral resolution element at a time, its wide linear dynamic range and sensitivity, in combination with the fact that it converts light intensity directly to an electronic signal, have traditionally made it the detection system of choice for the majority of spectrochemical measurements. Now, however, the lin ear photodiode array (LPDA) has pro vided a logical alternative. With an LPDA spectrophotometer, all wavelengths in the 190-1100-nm region can be acquired simultaneously in milliseconds or less. Monochromat ic light and scanning mechanisms are no longer required. The array detector provides a new dimension to UV-VIS spectroscopy in that spectral acquisi tion time is no longer the limiting step in most chemical analyses. To gain a comparable system response time to that of an LPDA spectrophotometer system, a conventional spectrophoto meter would produce a signal severely degraded by noise. Rapid UV-VIS de tection has allowed spectroscopists to characterize transient intermediates in kinetics experiments and in electro chemical reactions and to resolve over lapping chromatographic peaks spec trally. The LPDA is also referred to in the literature as a PDA or DA, which can be a linear array or a two-dimen sional array; an OMA (optical multi channel analyzer, Princeton Applied Research); and a PCD (plasma-coup led device) linear image sensor (Hamamatsu). Multiplex techniques
Two instrumentation approaches to simultaneous spectrophotometric de tection that have evolved in the past two decades are multiplex and multi channel techniques. In the former, en coded information (in the frequency domain) is received by a single detec tor, and transform methods based on either Hadamard or Fourier math ematics are used to convert data into spectral information. Two factors can increase the S/N ratio improvement from multiplexing in Fourier trans form spectroscopy for detector-noiselimited situations. First, the signal level is raised by a factor of Ν because of simultaneous observation of all Ν resolution ele ments. Second, because each element is viewed for an iV-fold longer time, there is in some cases a square root of Ν improvement in the S/N ratio for a spectrum. However, in the photonshot-noise-limited UV-VIS region this
Figure 2. Common-path interferometer (a) Optics of the Fourier transform spectrometer: S, light source; BS, beam splitter; M1, M2, M3, plane mirrors; L, lens; D, self-scanning photodiode array, (b) Interferometer optics equivalent to the spectrom eter optics in (a): S1, S2, virtual sources; L, lens; I, distance between two corresponding points of the vir tual sources; f, focal length. Adapted from Reference 5 with permission of the Optical Society of America
iV-fold higher signal level may actually decrease the S/N ratio (Felgett's dis advantage) (2). Measuring all the spectral intervals at the same time in the UV-VIS region may become a dis advantage because the photon shot noise at each spectral interval contrib utes to noise, causing a small signal to be buried in the noise from a large sig nal. In the IR region, where Fourier transform techniques are. commonly used, the detector itself is nearly al ways the dominant noise source. Sev eral researchers have shown that mul tiplex spectrometers based on the Fourier transform can improve the S/N ratio of IR measurements, but it is of limited utility in UV-VIS mea surements (2). Hirschfeld points out that a second-order Felgett's advan tage (the distributive Felgett advan tage) may exist in the UV-VIS region (2). The use of Fourier transform techniques in the UV-VIS region has been justified by some workers based on considerations of wavelength regis tration and resolution, optical throughput, or requirements for mul tichannel detection capability.
1058 A · ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
Multichannel techniques
During the past decade the rapid re finement of semiconductor-based technology and the related develop ment of powerful microprocessor sys tems have led to the evolution of a number of multichannel detector sys tems. As state-of-the-art TV cameras have matured, their use as spectrometric parallel detectors has been demon strated, and they have gradually gained acceptance among spectrosco pists. These electronic image sensors, which are available in a variety of types and spatial geometries, can be broken down into three basic ele ments: a radiation-sensing element, a charge storage element, and a readout system. The photographic plate, multiple photomultiplier tubes, the image dis sector, the vidicon, the charge-coupled device, the charge injection device, and the LPDA are examples of optical image detectors. The merits and prob lems associated with each of these de vices have been adequately discussed in the literature (3, 4). The most recent development in
Figure 3. Diagram of the LPDA showing reverse-biased p-n junction with diffused p-type bars in the η-type silicon substrate
electronic image sensors is the LPDA, a device that combines all three of these elements (radiation sensor, stor age element, and readout system) into a single integrated circuit. This detec tor is a large-scale integrated circuit fabricated on a single monolithic sili con crystal. The device consists of an array of diodes (also called elements, channels, and pixels), each acting as a light-to-charge transducer and a stor age device. The LPDA is better suited to UV-VIS spectrometry than are most other optical image sensors be cause of its large quantum efficiency (about 40-80%) throughout the U V VIS-NIR range (190-1100 nm); large dynamic range; and geometric, radio metric, and electronic stability. The array itself is quite tolerant of high temperature, humidity, vibration, and electrical and magnetic fields. It is not adversely affected by light (except when levels are sufficiently high to heat the silicon wafer to more than 100 °C).
system are stability against distur bance, ease of optical adjustment, and large optical throughput. Limitations of the system are resolving power (limited by the number of elements in the array) and, potentially, the dy namic range of the array. This type of multiplex-multichannel UV-VIS spectrometer is available commercial ly, and investigations of its merits are just beginning. The LPDA
Currently available self-scanned LPDAs have anywhere from 128 to 4096 elements per array. Arrays de signed by Reticon for spectroscopic applications are spaced on 25-μπι cen ters (0.001 in.) and have apertures of 2.5 mm, yielding an aspect ratio of 100:1. This corresponds to the typical aspect ratio of a conventional spec trometer or polychromator slit. The
integrated circuit package also con tains the necessary scanning circuitry for readout of the array. Address con trol circuitry is associated with each individual sensor or diode. A reversebiased p - n junction diode serves as each photosensitive element, and ad dressing (readout) is done by a shift register. LPDA geometry is defined by dif fused p-type bars in an η-type silicon substrate (Figure 3). The η-type and p-type surfaces are both photosensi tive. At the beginning of the measur ing cycle, the capacitive diode element is fully charged. The charge stored on the reverse-biased p - n junction as well as between the p-type strips can then be discharged between scans (read out). This discharge occurs because of photon-generated charge carriers (light falling on the diode) and ther mally generated charge carriers (dark current). Charges generated by light incident on the areas between diodes or p-regions divide proportionally be tween adjacent diodes to produce the response function shown in Figure 4. This response function must be con sidered when defining the spectral res olution of the array. Readout is accomplished by using two TTL level signals, a start pulse, and a clock. Each photodiode is con nected to the output line of a field ef fect transistor (FET) switch, which is controlled by a single bit that is shift ed through the shift register. When the FET switch is addressed, the di ode is charged up to its full reversebias potential (6). The circuitry can accomplish the recharging of each di ode in less than a microsecond with the multiplexer switching between ele ments occurring at a clock rate of 250 kHz (4 ^s/diode) to 2 MHz (0.5 Ms/diode). This clock rate is user selectable to match the A/D converter limita tions; the upper limit, which is compo-
Combined multiplex and multichannel techniques
Okamoto and co-workers (5) have reported the use of a triangle com mon-path interferometer with source doubling and an LPDA for use in the 300-ηπι-1.2-μΐη region (Figure 2). The radiation passes through the sample into a common-path interferometer, which forms the spatially resolved in terference pattern. This pattern is de tected by the LPDA. An associated computer system reconstructs the spectrum from the digital interferogram by fast Fourier transform (FFT). A significant feature of this interfero meter system is the absence of slits, apertures, or moving mechanical parts. The reported advantages of the
Flgure 4. Aperture response of the LPDA, showing the response of a single diode to an optical image whose size is equal to the diode spacing (25 μητι) A, incident radiation on one diode set on 25-μπ centers; B, actual diode (p-sillcon), 13-μιη width; C, n-silicon matrix region that is illuminated, 6-μιη width. Diode 2 integrates the energy In the A region as 52% from the p-sllicon area and 18% of each of the adjacent n-slllcon areas for a total of 88%
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Figure 5. Sequential readout process of the LPDA
nent limited, is 2 MHz. Once a scan is initiated by a start pulse the entire array must be scanned sequentially. Each diode is fully re charged so that there is no readout lag or incomplete readout of the stored signal with a single scan. The integra tion time, which is the same for all di odes, is the total time each diode inte grates light before it is read out again. This integration time is controlled by the time between start pulses, with the minimum time being the length of time required to read out the entire array and the maximum time being when diode saturation (caused by a combination of signal integration and dark current) occurs. The signal level necessary to establish reverse bias on the diode is a measure of the total light intensity and the dark current integrated over the time between scans of the array, provided that the capacitor was not fully discharged (the saturated diode condition). The analog signal from the common output line of the detector is run through an amplifier sample and hold system, which reduces noise; the sig nal is then digitized and transmitted via a suitable interface to a microcom puter. For many applications, con tinuous acquisition and storage of LPDA spectra are impractical, as the number of spectra acquired quickly exceeds the memory capacity of the associated computer system. To pre vent diode saturation during the inter im period, the associated computer system periodically initiates a read cy cle of the array without digitization (periodic refreshing of spectral infor mation prior to spectral acquisition). The LPDA requires a finite time (Δί) to scan each of the η successive diode elements. Because the LPDA readout is a sequential process the first diode in the array is read out at a different time than the last diode.
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However, the time interval between reading out the first element or the last element in successive scans is con stant. The radiation intensity record ed by the nth diode in the array corre sponds to a sampling period which dif fers by n-At from that for the first diode (Figure 5). Although for many purposes, such as HPLC detection, this sampling error may be negligible, there are applications, such as fast re action kinetics, in which this error can introduce temporal distortion. LPDA readout modes
Various readout methods for the LPDA include real-time readout, the variable integration technique (7), di ode grouping, and "skipping" diodes. Real-time readout occurs when all the diodes in the array are read out after one integration period and the spec tral data are stored in computer mem ory as a spectrum covering all Ν di odes and are immediately displayed. In the variable integration time mode, multiple analyses are per formed in which different signal inte gration times are used for successive spectra, which are then added into the same computer memory file. Maxi mum use of the available A/D range is achieved by using integration times that are inversely proportional to the corresponding signal levels in the successive spectra. This readout great ly increases the practical single-diode dynamic range. In diode grouping, adjacent diodes are electronically summed after digiti zation into predefined groups. This re sults in a loss of spectral resolution but a gain in S/N ratio. Additionally, grouping results in a virtual decrease in the bandwidth of the spectrophoto meter. However, the grouping tech nique is limited because once the bandwidth surpasses the natural bandwidth of a chromophore, a reduc-
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Figure β. Benzene absorbance spectrum showing reduction in absorbance as the grouped diode bandwidth exceeds the bandwidth of the chromophore (a) At full array resolution, no grouping; 2.1069 AU at 253.7 nm. (b) Grouping adjacent diodes to produce a spectrum at one-half the array resolution; 2.0467 AU at 253.7 nm. (c) Grouping of four adjacent diodes to produce a spectrum at one-quarter the array resolution; 1.9460 AU at 252.5 nm. Calibration is from di ode to diode so there is no reading at intermediate grouped diodes
tion in absorbance results (Figure 6). Skipping diodes is a mode in which diodes that measure a portion of the spectrum that contains no significant information are read (recharged) but not digitized. Skipped diodes are read at the rate of 0.5 MS (or other high speed rate). The ability to skip diodes can be important in the study of rapid time-dependent phenomena such as rapid-mixing kinetics. In addition to a temporal gain, there is also a conser vation in digital memory equal to the
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number of diodes skipped. However, unlike the diode grouping technique, there is no S/N gain with the skippeddiodes readout technique (Figure 7). Some commercial units also use dis crete diodes manufactured by Hamamat.su (35 diodes on 1-mm centers) and United Detector Technology (2-12 diodes on 0.075-in. centers). Un like the LPDAs from Reticon, Hamamatsu, and other manufacturers, each of the discrete diodes is read in a par allel fashion on individual output
lines. The characteristics of the discrete diode arrangement are similar to the LPDA in that both are based on silicon technology. The small number of diodes in these arrays, however, limits their use to the study of broadband phenomena with a limited spectral coverage. S / N improvements with an LPDA
Figure 7. Absorbance spectrum of benzene using a skip mode that digitizes only even-numbered diodes; 2.014 AU at 253.7 nm
Figure 8. Dark-current and fixed-pattern noise (a) 10-ms integration time, showing dark current only, (b) 100-ms integration time, showing dark current and fixed pattern, (c) 1-s exposure, showing fixed pattern only
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Random noise in the LPDA consists of four components: the photon noise of the incident light, the shot noise of the dark current, the noise from the preamplifier, and the reset noise of the diodes. Although the random noise can be decreased somewhat by smoothing (8), the analyst can choose experimental parameters to decrease certain contributing noise sources. Fixed-pattern effects in the LPDA are caused by a combination of the diode-to-diode dark-charge variations, overall spatial variations of the detector, and coupling of the clock phases into the video lines (reset or readout noise). If the array temperature and the integration time (or readout) are kept constant, the fixed-pattern effect is constant and may be digitally subtracted from the spectrum. Typical values for fixed-pattern noise are 1-2% of the saturation full-scale value (A/D maximum). The dark current is a temperature-dependent noise source and can be halved for each 7 °C decrease in Reticon array temperature (9 °C decrease for Hamamatsu arrays); thus, it can be reduced to an insignificant level by cooling the array. Fluctuations in the detector temperature cause proportional fluctuations in the dark current. Effective elimination of these fluctuations requires thermostating to within 0.001 °C (7). If digital subtraction is used, the fluctuations may show up as low-frequency noise. This effect is negligible, however, when the darkcurrent signal is below the signal level of the readout noise, which is the limiting noise factor in the LPDA (7). Many commercially available units achieve the required temperature stability with a Peltier cooler. The net effect of dark current for uncooled or insufficiently cooled arrays is high noise in low-light-level situations or in the case of strongly absorbing samples. Dark current at long integration times or for uncooled arrays can rapidly reduce the maximum measurable signal level by reducing the signal range of the A/D. This is because the spectral signal at longer integration times is sitting on a large dark-current level, and little or no dynamic range is left to integrate the signal photons linearly (9) (Figure 8). Individual samples of the same array type can differ from each other by 1% to 10% in their overall dark charge,
depending on system specifications and cooling. Within the array itself, diode-to-diode dark-charge variations are typically in the 1-8% range, again depending on system specifications and cooling. For long exposure times, even with cooled arrays, the dark-cur rent charge is in competition with the photon-generated signal for discharg ing the reverse-biased diodes. For short integration times essentially only the fixed-pattern noise is ob served, whereas at intermediate inte gration times the diode-to-diode darkcurrent variations are superimposed on the fixed-pattern noise. For long integration times only the fixed-pat tern noise is present on top of the saturated dark current (Figure 8). The optimum strategy for obtaining the largest signal (using the full A/D range) is to increase the time between readouts until the signal is just under saturation (integrate the signal onchip). On-chip integration improves the S/N ratio proportionally to the ex posure time. Ensemble averaging (the classical form of signal averaging) or averaging the signal in memory will improve the S/N ratio only in propor tion to the square root of the accumu lated exposure time. LDR Generally the larger the diode size, the greater the dynamic range; the narrower the diode size, the greater the resolution. Thus the diode dimen sions that match those of the spec trometer slit are the best compromise for maximum resolution and dynamic range. Photodiode array detectors have a limited dynamic range relative to the photomultiplier tube. As an ex ample, the Reticon " S " series of LPDAs, which was designed for spec troscopic applications, has a dynamic range of about 10,000 for a single inte gration time (7). The photomultiplier tube exceeds this value by about two orders of magnitude. There are a number of ways to de fine the linear dynamic range of the LPDA system. The detector dynamic range is generally lower than the sin gle-diode range (intrascenic dynamic range) because of the veiling-glare phenomenon introduced by the inter nally reflected light from the window, the silicon wafer, and the metallic components of the spectrograph. The single-diode dynamic range is, howev er, often set by manufacturers to be equal to the dynamic range of the A/D converter because the readout noise is often arbitrarily set equal to one count. Intraspectral dynamic range is the ratio of the largest and smallest in put signals that can be simultaneously measured in one scan. Thus, unless a variable integration technique is used, the intraspectral dynamic range de-
Figure 9 . LPDA spectrophotometer showing reverse-optics arrangement Reproduced with permission from Tracor Northern company literature
fines the sensitivity of the device. The useful dynamic range of the LPDA can be improved by ensemble averaging (accumulation of successive spectra into one file in the computer memory). This technique is limited only by the memory dynamic range (determined by the word size of the computer) of the system. However, the flicker (1/f) noise associated with the fixed-pattern signal limits the S/N ratio improvement that can be real ized with the memory accumulation or ensemble averaging to something less than the theoretical value. The charge-integrating capability of the array (on-chip signal averaging or ontarget signal integration) enhances the signal and averages the noise. Ulti mately, however, stray light and dark current will limit the dynamic range. Characteristics of the LPDA spectrophotometer An LPDA spectrophotometer re quires a reverse-optical arrangement (Figure 9). White light is passed through the sample and into the polychromator. The polychromator then disperses light across the LPDA, which replaces the exit slit of a con ventional spectrophotometer. The ar ray is located in the focal plane of a polychromator so that each diode cor responds to a particular wavelength resolution element of the UV-VIS spectrum. The polychromator used must have a flat field or focal plane, so that the entrance slit is in focus for all diodes along the detector array. The array, consisting of Ν diodes, parti
1068 A · ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
tions the spectrum across it into Ν wavelength increments. The LPDA operates in the chargeintegrating mode and hence integrates the incident radiation intensity simul taneously in each of these wavelength increments. The minimum interval is determined by the time required to read all the photodiode information. Commercial units read out and digi tize the diode information at rates from 4 MS to 28 MS per diode. In cases where very low radiation is encoun tered, the elapsed time between each reading (the integration time) can be increased to improve the S/N ratio. The ability of this device to measure the intensity of several hundred wave length increments in a millisecond time scale shifts the problem in spec troscopy from one of rapidly acquiring the signal to one of having sufficient data storage and display rates compa rable to the array readout time. The array provides an almost ideal sensor for the digital acquisition of spectra, as the array itself, by its presence in the focal plane of the spectrometer, digitizes the spectrum into discrete in tervals. Parameters that govern the overall performance of diode-array-based spectrophotometers are photometric range and precision, spectral resolu tion, sensitivity, S/N in the spectrom eter system, and stray-energy radi ation. Unlike scanning spectrometers, whose wavelength accuracy is me chanically limited, the LPDA spec trometer is limited only by geometric constraints of the detector itself, by
vibration and thermal expansion of the optical components, and by the stability of the source. Wavelength accuracy is equivalent to the diode spacing (typically 0.025 mm) multiplied by the linear dispersion of the spectrograph. Its geometric registration and, therefore, its wavelength accuracy and precision are greater than that of any mechanically scanned spectrometer. These features are essential for various digital data-processing techniques, including multicomponent analysis. The LPDA system is vulnerable to the adverse effects of stray radiant energy. In the LPDA spectrophotometer, not all of the stray radiation inside the polychromator can be eliminated with a narrow-bandpass filter, as is the case with a single-wavelength nonscanning spectrometer. Spectral order overlap for a given grating setting may be eliminated by placing a bandpass filter directly in front of the array. Some commercial units use a concave holographic grating to decrease stray radiation. Because the spatial distribution of the stray light can be accurately measured, software correction techniques may be used to extend the photometric linearity. Talmi has shown that the polychromator and not the LPDA window is the predominant source of stray light in their LPDA system (10). Spatial resolution of the LPDA spectrophotometer is one channel wide. Although spectral resolution is equal to the product of the reciprocal linear dispersion of the polychromator (nm/mm) and the width of the diode (typically 0.025 mm), the actual spectral resolution of the LPDA is worse by approximately a factor of two. This discrepancy comes from the aperture response function of the array (Figure 4). The analyst must carefully evaluate the requirements for resolution and spectral coverage. For a given array, spectral coverage is always gained at the expense of resolution.
Figure 10. Isometric plot of a spectrochromatogram of zimeldine and metabolites separated by reversed-phase HPLC I, zimeldine; II, norzimeldine; III, zimeldine N-oxide; IV, desmethyl zimeldine; IX, zimeldine hydroxylamine. Adapted with permission from Reference 15
time-varying data is especially suitable for chemometric techniques of data reduction because of the parallel nature of the data collection, which gives more orthogonality or dimensionality in the data sets (13). Derivative spectra generally exhibit greater differences than the original absorbance spectra and have the advantage of being independent of the absolute absorbance level. The second and higher order derivative spectra selectively enhance the amplitudes of sharper features and suppress broad bands (14). Additionally the wavelength can be manipulated digitally in a number of ways to enhance selectivity and sensitivity. If all the absorbances of all wavelength resolution elements are summed, a total absorbance chroma-
togram or spectrum is obtained that can be useful for detecting impurities (15). Further useful manipulations include coaddition of spectra for improved S/N; smoothing of spectra and chromatograms; linear and logarithmic scaling for convenience of presentation; multiwavelength plotting of chromatograms; and isometric threedimensional display of absorbance, wavelength, and time (15) (Figure 10). An alternative graphical presentation proposed recently for chromatographic data (or other time-dependent phenomena) is to reduce the A, t, and wavelength data to a contour plot, comprising a series of concentric isoabsorptive lines mapped in the wavelength and time plane. Nonhomogeneity of a chromatographic peak can sometimes be interpreted from
Data processing
One intrinsic advantage of the LPDA is that spectral data are directly available in digitized form for storage, retrieval, and software manipulation by a number of digital algorithms, such as log base 10 (absorbance) for direct comparison with standards; arithmetic operations on spectra; spectral derivatives both in the time and spectral domain for enhanced qualitative characterization and resolution of spectral and chromatographic peaks (11, 12); and spectral deconvolution by the principles of overdetermination (the number of wavelength resolution elements exceeds the number of absorbing components) and factor analysis. LPDA detection of
Figure 11. Contour plot of zimeldine and metabolites separated by HPLC I, zimeldine; II, norzimeldine; III, zimeldine N-oxide; IV, desmethyl zimeldine; IX, zimeldine hydroxylamine. Adapted with permission from Reference 15
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the contour plot by asymmetry in the shape of the contour lines (IS) (Figure 11). To implement a significant real time data-processing system for LPDA systems two factors must be addressed. First, the instrument must be able to distinguish spectral data from background; second, the data ac quisition rate should exceed that specified by the Nyquist frequency of the signal being measured. These fac tors affect the data storage require ments and data acquisition tasks for the system. Commercial systems may handle these two constraints differ ently. The analyst must carefully match the requirements of the experi ment with the data storage rate, ca pacity, and sampling rate of the com mercial instrument.
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References (1) Hirschfeld, T. J. Appl. Spectrosc. 1976, 30,68-69. (2) Boutilier, G. D.; Pollard, B. D.; Winefordner, J. D. Spectrochim. Acta 1978, 33B, 401-15. (3) Talmi, Y. Anal. Chem. 1975,47,65870 A. (4) Talmi, Y. Anal. Chem. 1975,47, 699709 A. (5) Okamoto, T.; Kawata, S.; Minami, S. Appl. Opt. 1984,23, 269-73. (6) EG&G-Reticon, "S-Series Solid State Line Scanners 128, 512, and 1024 Ele ments" specification sheet, 1978. (7) Talmi, Y.; Simpson, R. W. Appl. Opt. 1980,9, 1401-14. (8) Savitzky, Α.; Golay, M. J. Anal. Chem. 1964,36, 1627-30. (9) Horlick, G. Appl. Spectrosc. 1976,30, 113—23
(10) Talmi, Y. Appl. Spectrosc. 1982,36, 1-18. (11) Milano, M. J.; Lam, S.; Grushka, E. J. Chromatogr. 1976,125, 315-21. (12) FeU, A. F.; Scott, H. P.; Gill, R.; Mof fat, A. C. Chromatographia 1983,16, 69-78. (13) Kowalski, B. R. Trends Anal. Chem. 1981,1, 71-74. (14) Fell, A. F. UV Spectrom. Group Bull. 1980 8 5 (15) Fell,' A. F.; Clark, B. J.; Scott, H. P. J. Pharm. Biomed. Anal. 1984,1, 557-72.
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Dianna G. Jones is the manager of the Optical Analytical Systems and Applications Group at Tracer North ern. She received a B.S. from Duke University and a Ph.D. from Florida State University. Her interests are in the area of optoelectronic image de vices used in UV-VIS spectroscopy.
LEEMAN LABS INC. 600 Suffolk St., L o w e l l , MA 01854, (617) 454-4442 Circle 128 for ICP Seminar Information. Circle 129 for Product Information. ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985 · 1073 A
