Instrumentation Dianna G. Jones Tracw N~them 2551 west Beitline Highway Middleton. Wis. 53562-2697
PhotodiodeAmw Detectors in W-VIS Speetmicopy: Rut I Theoretical aspects of photodiode array detection are presented in this month b INSTRUMENTATION. In next month5 issue, the second article in this two-part series will couer 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 clagsid 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 ohserving transient phenomena, have heen minimized if not eliminated. 00052700185/A357-1057SOl.SO10 @ 1985 IAmerican chemical Society
Conventional UV-VIS spectrometers Complete W-VIS spectrophotometric information can be obtained either hy scanning across the spectral region of interest or by simultaneously monitoring this region in its entirety. Conventional scanning monochromator systems use a continuous source (dew terium or tungsten lamp) and a dispersive element (a grating or a prism). This dispersive element is mechanically rotated to vary the wavelength of
I
MonochromaHc lght
I I slit
light psssed 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 narraw 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-
I
Figure 1. Conventional spectrometer optical configuration
ANALYTICAL CHEMISTRY. VOL.
57. NO. 9. AUGUST 1985
1057A
multiplier tubes in predetermined positions (correspondingto spectral regions 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 a t a time, ita 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 linear photodiode array (LPDA) has provided a logical alternative. With an LPDA spectrophotometer, all wavelengths in the 190-1100-nm region can he acquired simultaneously in milliseconds or less. Monochromatic light and scanning mechanisms are no longer required. The array detector provides a new dimension to UV-VIS spectroscopy in that spectral acquisition time is no longer the limiting step in most chemical analyses. To gain a comparahle system response time to that of an LPDA spectrophotometer system, a conventional spectrophotometer would produce a signal severely degraded by noise. Rapid W-VIS detection has allowed spectroscopists to characterize transient intermediate in kinetics experiments and in electrochemical reactions and to resolve overlapping chromatographicpeaks spectrally. The LPDA is &o referred to in the literature as a PDA or DA, which can be a linear array or a two-dimensional array; an OMA (optical multichannel analyzer, Princeton Applied Research); and a PCD (plasma-coupled device) linear image sensor (Hamamatau).
Multiplex techniques Two instrumentation approaches to simultaneousspectrophotometricdetection that have evolved in the past two decades are multiplex and multichannel techniques. In the former, encoded information (in the frequency domain) is received by a single detector, and transform methods based on either Hadamard or Fourier mathematics are used to convert data into spectral information. Two factors can increase the S/N ratio improvement from multiplexing in Fourier transform spectroscopy for detector-noiselimited situations. First, the signal level is raised by a factor of N because of simultaneous observation of all N resolution elements. Second, hecause each element is viewed for an N-fold longer time, there is in some cases a square mt of N improvement in the S/N ratio for a spectrum. However, in the photonshot-noise-limited UV-VIS region this 10581,
E
N-fold higher signal level may actually decreasethe S/N ratio (Felgett’s disadvantage) (I).Measuring all the spectral intervals at the same time in the UV-VIS region may hecome a disadvantage because the photon shot noise at each spectral interval contributes to noise, causing a small signal to be buried in the noise from a large signal.
In the IR region, where Fourier transform techniques are commonly used, the detector itself is nearly always the dominant noise source. Several researchers have shown that multiplex spectrometers based on the Fourier transform can improve the S/N ratio of IR measurements,but it is of limited utility in W-VIS measmments (2).Hirschfeld points out that a seeond-order Felgett’s advantage (the distributive Felgett advantage) may exist in the W-VIS region (I).The use of Fourier transform techniques in the UV-VIS region has heen justified by some workers based on considerations of wavelength registration and resolution, optical throughput, or requirements for multichannel detection capability.
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, A W S T 1985
Multichannel techniques During the past decade the rapid refinement of semiconductor-based technology and the related development of powerful microprocessor systems have led to the evolution of a number of multichannel detector systems. As state-of-the-art TV cameras have matured, their use as spectrometric parallel detectors has been demonstrated, and they have gradually gained acceptance among spectroscopists. These electronic image sensors, which are available in a variety of types and spatial geometries, can be broken down into three basic elements: a radiation-sensing element, a charge storage element, and a readout system. The photographic plate, multiple photomultiplier tubes, the image dissector, the vidicon, the charge-coupled device, the charge injection device, and the LPDA are examples of optical image detectors. The merits and problems associated with each of these devices have heen adequately discussed in the literature (3.4). The most recent development in
Ten years of researchstand behind these LC injectors. And Rheodyne'snot doneyet. You can expect Rheodyne always to be designing and testing new LC sample injectors. Doing just that, weLe acquired a lot of valuable knowledge that we can x s s on to you. Both in the form of better-designed sample njectors-and in the form of expert advice on the most ?ffectiveway to use them. Our aim has always been to make injectors more Jersatile and easier to use, while maximizing the precision and resolutionof your liquid chromatograph.Rheodyne's Model 7125, pictured in the foreground below, is an excellent example of this philosophy It injects sample either
from a loop that's completely filled (to maximize precision) or from a loop that's partially filled using a syringe (to save time).The 7125 offers unexcelled versailitx ease of use, and reliability Expert advice on how to use sample injectors is available to you in Rheodyne'sTechnical Note No. 5. It tells how to achieve high precision while avoiding sample wastage and cross contamination.Ask your Rheodyne dealer for free copies of all ourTechnica1 Notes and our catalog. Or contact Rheodyne, Inc.,PO. Box 996, Cotati. California 94928, U.S.A.Phone (707) 664-9050.
I Photon in
& ' .,.:...
Figure 3. Diagram of the LPDA showing reversebiased p-n junction with diffused ptype bars in the n-type silicon substrate
electronic image sensors is the LPDA, a device that combines all three of these elements (radiation sensor, storage element, and readout system) into a single integrated circuit. This detector is a large-scale integrated circuit fabricated on a single monolithic silicon 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 storage device. The LPDA is better suited to UV-VIS spectrometry than are m a t other optical image sensors hecause of its large quantum efficiency (about 4&80%) throughout the UVVIS-NIR range (190-1100 nm); large dynamic range; and geometric, radiometric, and electronic stability. The array itself is quite tolerant of high temperature, humidity, vibration, and electrical and magnetic fields. It is not adversely affected hy light (except when levels are sufficiently high to heat the silicon wafer to more than 100 "C). Combined multlplex and multichannel techniques Okamoto and co-workers ( 5 ) have reported the use of a triangle common-path interferometer with source doubling and an LPDA for use in the 300-nm-1.2-rm region (Figure 2). The radiation passes through the sample into a common-pathinterferometer, which forms the spatially resolved interference pattern. This pattern is detected by the LPDA. An associated computer system reconstructs the spectrum from the digital interferogram hy fast Fourier transform (FFT). A significant feature of this interferometer system is the absence of slits, apertures, or moving mechanical parts. The reported advantagesof the loBOA
system are stability against disturbance, ease of optical adjustment, and large optical throughput. Limitations of the system are resolving power (limited hy the number of elements in the array) and, potentially, the dynamic range of the array. This type of multiplex-multichannel W-VIS spectrometer is available commercially, and investigations of its menta ere just heginning.
The LPDA Currently available self-scanned L P D h have anywhere from 128 to 4096elements per array. Arrays designed hy Reticon for spectroscopic applications are spaced on 25-rm centers (0.001 in.) and have apertures of 2.5 mm, yielding an aspect ratio of 1W1. This correspondsto the typical aspect ratio of a conventional spectrometer or polychromator slit. The
integrated circuit package also contains the ncoessary scanningcircuitry for readout of the array. Address control circuitry is associated with ea& individual sensor or diode. A reversebiased p-n junction diode serves as each photosensitive element, and addressinn (readout) is done h r a shift registe; LPDA geometry is defied hy diffused ptype bars in an n-type silicon substrate (Fnure 3). The n - t m and ptype surfaces are both p h o k n s i tive. At the heginning 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 (readout). This discharge occurs hecause of photon-generatedcharge carriers (light falling on the diode) and thermally 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 he considered when defining the spectral resolution of the array. Readout is accomplished by using two TIZ level signals, a start pulse, and a clock. Each photodiode is connected to the output line of a field effect transistor (FET) switch, which is controlled by a single bit that is shifted through the shift register. When the FET switch is addressed, the diode is charged up to ita full reversebias potential (6).The circuitry can accomplish the recharging of each diode in less than a microsecond with the multiplexer switching between elementa occurring at a clock rate of 250 kHz (4 @/diode) to 2 MHz (0.5d d i ode).This clock rate is user selectable to match the AID converter h i l a tions; the upper limit, which is compo-
Floun 4. Aperture response of the LFOA, showing tha response of a single diode t an optical image whose size is equal to the diode spacing (25 pm) A. lnoldent radiationon one dlode M I on 2 5 r m cenhm; 6. &maldiode (p.allim).13-rm widn: C. rwillmmatrlx replon msl is illunlnaW.bpn wldlh. Dl& 2 lntepratea Uw uwr~yIn mS A ngbn a8 52% hanth, p.alllm m a d 18% of edch ofths ~ r d l i c o n e f e a IcfaloIaI 8 M 88%
ANALYTICAL CI-EMISTRY. VOL. 57, NO. 9, A W S T 1985
EDXRF WITH THE IBM PC/AT
I I I
3 Specfirme 5000 Nondestructive elemental analysis IBM PC/AT workstation Wide range of analysis software Automation to single button operation Access to extensive IBM PC software library Broad application for research, quality or process control environments
345 East Middlefield Ro& IBM
PC and IBM PCYAT are
0
Mountain View, California 94043
(415)967-0350
registered trademarks of International Business Machines Corporation CIRCLE 206 ON READER SERVICE CARD
us unionawn STOPPED FLOW
. .....,
Union Giken apparatus achieves a dead time as short as 'A millisecond for measurement of fast reactions Gas pressure is directly applied tc the surface of the reactants in each reservoir, mixing and flow bein( controlled from the end of t h c system by a high.response stor valve. The RA-401 STOPPEr FLOW SPECTROMETER is tht ideal match for fast reactions.
The RA451 DATA PROCESSOR gener ates a simulated reaction curve fro" the measured results and automaticall, calculates the rate constant. rlIOYCT8ONOF C I T Q C * S O Y S 5
191
"~
IIOlrn
5nnn
The R A 4 1 5 RAPID SCAN ATTACC MENT monitors the change in absori tion swnrum during the reaction.
Other attachments are available fc T-jump, P-jump, fluorescence an tlash photolysis. ConDCt m.lole .Y-rt.gsnr
for moa dew,:
ATAGO BUSSAN CO., LTD 7-23 Schome Shimbashi. Mlnam-ku. Tokio 105 Japan Telex: 28411 Cable: TAGOA TOKYC Phone: (031 432-8741
CIRCLE 3 ON READER SERVICE CARC
1002A
.
..
Flgure 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 he scanned sequentially. Each diode is fully recharged so that there is no readout lag or incomplete readout of the stored signal with a single scan. The integration time, which is the same for all diodes, is the total time each diode integrates light before it is read out again. This integration time is controlled hy 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 necewry 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 signal is then digitized and transmitted via a suitable interface to a microcomputer. For many applications, continuous 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 prevent diode saturation during the interim period, the associated computer system periodically initiates a read cycle of the array without digitization (periodic refreshing of spectral information prior to spectral acquisition). The LPDA requires a fmite time (At) to scan each of t h e n 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.
ANALYTICAL CHEMISTRY, VOL. 57, NO, 9. AUOUST 1985
However, the time interval between reading out the first element or the last element in successive scans is constant. The radiation intensity recorded by the nth diode in the array corresponds to a sampling period which differs 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 reaction 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 (3,diode grouping, and "skipping" diodes. Real-time readout occurs when all the diodes in the array are read out after one integration period and the spectral data are stored in computer memory as a spectrum covering all N diodes and are immediately displayed. In the variable integration time mode, multiple analyses are performed in which different signal integration times are used for succegsive spectra, which are then added into the same computer memory file. Maximum use of the available A/D range is achieved by using integration times that are inversely proportional to the correspondingsignal levels in the succegsive spectra. This readout greatly increases the practical single-diode dynamic range. In diode grouping, adjacent diodea are electronically summed after digitization into predefined groups. This results 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 spectrophotometer. However, the grouping technique is limited because once the bandwidth surpasses the natural bandwidth of a chromophore, a reduc-
. ,. . , . ...
1
..
...
__..
RESEARCH GRADE PERFORMANCE IN THREE NEW CONFIGURATIONS uiu1-u
I
1'1-111:
experience, customer support, value.. .when you buy a Digilab FTS spectrometer, you are getting one of the industry's finest instruments. For literature, send the coupon today to Bio-Rad, Digilab Division, 237 Putnam Avenue, Cambridge, MA 02139. Or call TOLL FREE (800) 2254248.
When you buy one of the three new FTS spectrometers shown above, you are buying a spectrometer that is discriminatingenough to resolve even minor spectral components, sensitive enough to detect trace concentrations and fast enough for r-r/IR and kinetics. {esearch made oerformance. auulicatio
I
I Amlications:
' Please send me
a brochure L_______ _ _ _ _on _ the _ _(check _ _ _one): _ _FTS _ _50: _ _ _ _ _FTS _ _60: _ - - - - - FTS 80: Digilab Division
~
CIRCLE 18
ON READER SERVICE CARD
~
~
LNSPEC'SCatalogs :'s Like Having . . .
1
rnromatograpn Stockroom I
On Your Shelf cl Detectors 0 Pumps 0 Packings 0 Valves 0 TLC Plates 0 Vials 0 Solvents 0 Standards 0 Tubing 0 Syringes lour source for thousands of :hromatography products are in Snspec's LC 8 TLC catalog and sc catalog. Wspec will quickly ship the nstrumentation & supplies you zhoose from manufacturers such 3s Altex. J. T. Baker, Brownlee, Hamilton, J 8 W, Kratos, L.D.C.. S.G.E.,Upchurch, Valco. Whatman...hundreds of supplier! of high quality separation products. Contact us now for your catalogs and see for yourself all the new state-of-the-art products Anspec offers to make your work in chromatography easier than ever.
CALL TOLL FREE: 800-521-1 720 800-482-3452 in Michigan 800-237-1017 in Canada
P.O. Box 7730UAnnArbr.Ml4810 :Ircls No. 1 for LC. No. 2 l a M:
1OMA
l--
I
-
Flgulv 6. Benzene absorbance spectrum showing reduction in absorbance as tha ~ o u p e ddiode bandwldih exceed8 me banbwldm of the chromophore (a) A I lull m y rssolution. M oaplng: 2,1060 AU a 253.7 nm. (b) Oaplnp SdPCa dlodg 10 poduce a s p e w a -If the M Snrollllon; ~ 2.0467 AU a 253.7 rm. (e) e a p l n p ot tour adlaca*dl0d.r IO produce a specturn at onsgusrta the M ~ Yreso11*Ion; 1.9460 AU a 252.5 nm. Callbrawn Is hom dlod.IO dloda w mSre It no wading a1 Intsmadlat.proupsd dlodg
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 r a k of 0.5 ps (or other highspeed rate). The ability to skip diodes can be important in the study of rapid time-dependent phenomena such as rapidmixing kinetics. In addition to a temporal gain, there is also a comer. vation in digital memory equal to the
ANALYTICAL CHEMISTRY, VOL. 57. NO. 9, AUGUST 1985
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 disCrete diodes manufactured by Hamamatsu (35 diodes on 1-mm centers) and United Detector Technology (2-12 diodes on 0.075-in. centers). Unlike the LPDAa from Reticon. Hamamatsu, and other manufacturers, each of the discrete diodes is read in a parallel fashion on individual output
Accurate fluid density measurement without hydrometers and pycnometers, Met tier/Paar Digitai Density Meters All the cumbersome procedures you're accustomed to, all the time spent and money lost on measuring density, specific gravity or concentration, can now be things of the past. With Mettler/Paar digital density meters, you can obtain accurate results-quickly, easily, and reliably. No hydrometer breakage,time-consuming pycnometer measurements or weighing procedures to be concerned with. Think how much more effectively you can tighten manufacturingprocedures, lower production costs and improve product quality. Unlimited appiicatlon Mettler/Paar density meters are used everywhere. Whether for research, quality or process control
-in petrochemical, electronics, food and beverage, general chemical, and pharmaceutical industries.There are several models to choose from to fit your specific requirements. For example, in the lab, the DMA46 offers 4-place precision and simple operation. It has a built-in thermostat and microprocessor to calculate and display density, specific gravity, concentration and other density related values. And for work outside the lab, take the portable DMA35 anywhere you need on-the-spot determinations. For in-line measurement and control, choose from a variety of DPR process and control systems.
Meeting your needs Models are available with precision from 10-3g/~m3 to 1 0 - 6 g / ~ mand 3 CIRCLE 141 ON READER SERVICE CARD
The MettleriPaar DMA35 is the only totally portable, hand-held density meter.
are well-suited for many ASTM, AOAC and brewing industry methods. Results can be obtained in percent alcohol, Brix or API values. Prices range from about $1,000 for portable meters to $4,000 and up for lab and process systems. For more information, contact Mettler today. Instruments are also available for titration, thermal analysis and reaction calorimetry. Mettler Instrument Corporation, Box 71, Hightstown, NJ 08520. Or call: (609) 448-3000.
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 broadhand phenomena with a limited spectral coverage.
'. Absorbance spectrum of benzene mbered diodes: 2.014 AU at 253.7
I-
IOWA
I
+ # -,.-,:-
ANALYTICAL CHEMISTRY. VOL. 57, NO. 9, AUGUST 1885
SIN improvements with an LPDA 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 the analyst can choose smoothing (8, 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 4 %of the saturation full-scale value ( A D maximum). The dark current is a temperatwe-dependent noise source and can be halved for each 7 O C decrease in Reticon array temperature (9 "C decrease for Hamamatsu arrays); thus, it can be reduced to an insignificant level by m l i n g the array. Fluctuations in the detedor temperature cause proportional fluctuations in the dark current. Effective elimination of these fluctuations requires thermostating to within 0.001 OC (7). If digital subtraction is used, the fluctuations may show up as low-frequency noise. Thii 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 commerciallyavailable units achieve the required temperature stability with a Peltier m l e r . The net effect of dark current for unmled or insufficiently cooled arrays is high noise in low-light-levelsituations 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 aignal photons linearly (9)(Figure 8).
Unlock the productivity of the Perkin-Elmercomputers in your lab. If one of your analytical instruments is controlled by a Perkin-ElmerSeries 7000 microcomputer, you now have one of the finest ProfessionalComputers in the world.
The keys to productivity Perkin-Elmeroffers an exciting variety of software for the P-E Series 7000. Many packages are available to make your work easier, more satisfying and more productive. For example: y & with PECOWRITER/ 7000, our easy to learn and use word processing package, or with LEX-6ETM. another word processing program which includes a spelling dictionarv and a mass mailina database. Communicate with other computers and online databases using TERM/7000. And &qu&
data from a wide variety of experlments and instruments with DAQ/7000. Unleash your creativity Our listof general purposesoftware isconstantlygrowing.Youcan d y &with Supercomp-TwentyTM spreadsheet. Orqanize information with our database management package. And graDh data with our curve-fitting statistics program. For your own special purposes, create programs in either BASIC or FORTRAN 77 Access our huge library Whether you're choosing from the most extensive collection of analytical software available, or using your P-E Series 7000 as a sophisticated laboratory tool, you
have all the power and flexibility required. All our programs and computers are truly chemist-friendly Selze the opportunity Perkin-Elmersoftware will dynamically ~ increase the productivity of your people and your instruments. To discuss the opportunities touched on here, call your Perkin-Elmer representativetoday. For literature, dial 1-800-762-4000. Perkin-Elmer Corp.. Main Ave. (MS-12) Norwalk. CT06856 U.S A , Tel: 12031 . . 762-1000. Telex 965-954. Booeiseener6 Per.. 1 E i e r 8 C 3 Gmun Pos'laci 1'20 7770 -CDC' -13ei Feaeri Pez-o c o l ' J e r ~ a,r .-e .C7551 81' Perkin-ElmerLtd , Post Office Lane, Beaconsfield. Bucks HP9 1QA. England. Tel: Beaconsfield (049 46) 6161 L E X 661sa reg8rlered Irademarh ol ACE M~CroSyIlem5Lld Supercomp Twenty .s a reg8Slered trademalh 01 Access Technology 1°C
PERKIN-ELMER The science &computer company. Where solutions come first. CIRCLE 167 ON READER SERVICE CARD
dependingon system specifications and cooling. Within the array itself, diode-to-diode dark-charge variations are typically in the 1 4 % range, again depending on system specifications and cooling. For long exposure times, even with cooled arrays, the dark-current charge is in competitionwith the photon-generated signal for diseharging the r e v e r s e - b i d diodes. For short integration times essentially only the fixed-pattern noise is observed, whereas at intermediate integration times the diode-to-diode darkcurrent variations are superimpcwl on the fixed-pattern noise. For long integration times only the fued-pattern noise is present on top of the saturated dark current (Figure 8). The optimum strategy for obtaining the b e s t 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 improvea the S/N ratio proportionally to the exposure time. Ensemble averaging (the classical form of signal averaging) or averaging the signal in memory will improve the S/N ratio only in proportion to the square root of the accumulated 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 dimensions that match those of the spectrometer slit are the beat compromise for maximum resolution and dynamic range. Photodiode array detectors have a limited dynamic range relative to the photomultiplier tube. As an example, the Reticon “S” series of LPDAs, which was designed for s p troseopic applications, has a dynamic range of about 10,OOO for a single integration time (7). The photomultiplier tube exceeds this value by about two orders of magnitude. There are a number of ways to def i e the liienr dynamic range of the LPDA system. The detector dynamic range is generally lower than the single-diode range (mtrascenic dynamic range) because of the veiling-glare phenomenon introduced by the internaUy reflected light from the window, the silicon wafer, and the metallic componentsof the spectrograph. The single-diode dynamic range is, however, often set by manufacturers to be equal to the dynamic range of the A D converter bacause the rendout noise is often arbitrarily set equal to one count. Intraspectral dynamic range is the ratio of the largest and smallest input signals that can be simultaneously measured in one scan. Thus, unless a variable integration technique is wed, the intraspectral dynamic range de1068A
fines the sensitivity of the device. The useful dynamic range of the LPDA can be improved by ensemble averaging (accumulationof successive spectra into one fde 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 (1lO noise associated with the fmed-pattarn signal limits the S/N ratio improvement that can be realized with the memory accumulationor ensemble averaging to something Less. than the theoretical value. The charge-integrating capability of the array (on-chip signal averaging or ontarget signal integration) enhancea the signal and averages the noise. Ultimately, however, stray light and dark current will limit the dynamic range.
Charactdstics ol the LPDA 8pectrophotometef An LPDA spectrophotometerrequires a reverse-optical arrangement (Figure 9). White light is passed through the sample and into the polychromator. The polychromator then disperses light a u m the LPDA, which replacea the exit slit of a conventional spectrophotometer.The array is located in the focal plane of a polychromator so that each diode corresponds to a particular wavelength resolution element of the W-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 detedor array. The array, consisting of N diodes, parti-
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AWiUST 1085
tions the spectrum across it into N wavelength increments. The LPDA operates in the chargeintegrating mode and hence integrates the incident radiation intensity simultaneously in each of theae wavelength incrementa. The minimum interval is determined by the time required to read all the photodiode information. Commercial units read out and digitize the diode information at r a t a from 4 pa to 28 ps per diode. In eases where very low radiation is encountered, the elapsed time between each reading (the integration time) can be i n e r e a d to improve the S/N ratio. The ability of this device to measure the intensity of several hundred wavelength incrementa in a millisecond time scale shifts the problem in spectmscopy from one of rapidly acquiring the signal to one of having sufficient data storage and display rates comparable 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 intervals. Parameters that govern the overall performance of diode-arraybased spectrophotometersare photometric range and precision, spectral resolution, sensitivity, S/N in the spectrometer system, and stray-enerpy radiation. Unlike scanning spectrometers, whose wavelength accuracy is mechanically limited, the LPDA spectrometer is limited only by geometric constraints of the detector itself, by
0 Yes! I'd like to know
0 Yes! Please send me an Application Repolt for my
*
more about XRF and the TEFA Monitor.
1
Title/Position Name Firm Name City
State
Zip
Phone Mail to: TEFA Monitor, EGBG ORTEC, 100 Midland Road, Oak Ridge, TN 37830
EGzG ORTEC w e put x-ray fluorescence to work.
CIRCLE 50 ON READER SERVICE CARD
I
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 effecta of stray radiant energy. In the LPDA spectrophotometer, not all of the stray radiation inside the polychromator can he 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 he 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. 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 ( I I , 1 2 ) ;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 1070,.
Figure 10. Isometric plot of a spectrochromatogram of zimeldlne and metabolites separated by reversed-phase HPLC I. rimeldine: 11. nonimeldine:111. Ameldine KoxMs: IV. depmemyl zimeldine: IX, rimeldinsh~oaylamlne. Adapted vim permission horn Reler15
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 (23).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 sharDer features and sumress broad .. bands (14). Additionally the wavelength can be manipulated digitally in a numberof 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 he 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 he interpreted from
Time (min)
Figure 11. Contour plot of zimeldine and metabolites separated by HPLC I, zimeMins;ii. nwzimeidlne:111. zlmeldlne N-oxide: IV. desmemyi zimeldine; IX, zimeldinehydroxylamine. Adaptad wim p~rmlsslmfrom Reference 15
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
adding a single shelf7
pi
When you manage a library these days, you’re well aware that space is precious. And, additional shelves for backfiles volumes are expensive. Well, the American Chemical Society knows your concern. That’s why all the Society’s 2 1 primary publications in the field of chemistry are available in microfilm editions-including complete volumes back to 1879.
If you are setting up a microfilm system, expanding or changing one, or just want to discuss the possibilities-an ACS Sales Representative is ready to work with you. Just give us a call at 800-424-6747, or write to the Sales Office, American Chemical Society, 1155 Sixteenth Street, N.W., Washington, D.C, 20036. Start Saving Space in Your Chemical Reference Files NOW!
AMERICAN CHEMICAL SOCIETY PUBLICATIONS Chemical publisliers since 1879. Accounts of Chemical Research Chemical & Engineering News Environmental Science & Technology Inorganic Chemistry Journal of Medicinal Chemistry The Journal of Organic Chemistry The Journal of Physical Chemistry
Analytical Chemistry Biochemistry CHEMTECH Chemical Reviews Langmuir Macromolecules Organometallics
I&EC Fundamentals I&EC Process Design & Development I&EC Product Research & Development Journal of Agricultural & Food Chemistry Journal of the American Chemical Society Journal of Chemical Information & Computer Sciences Journal of Chemical & Engineering Data
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
1071A
Weatment and Disposal of Pesticide Wastes Raymond F. Krueger and James N. Seiber, Eds. Outlines the problems of pesticide waste disposal with special emphasis on developing better disposal strategies Examines the chemical, biological and physical methods of disposal, the basic principles underlying each of these methods, and the analytical and modeling approaches applicable to waste disposal conditions. Also looks at the EPA regulatory viewpoint on safe disposal Includes some novel approaches that have potential for wide scale use in the future ACS Symposium Series No 259 373 pages ( 1984) Clothbound US & Canada 564.95 Exoort 577.95
BiOregUlatOK : Chemlstry and Uses Robert L. Ory and Faik R. Rittlg, Eds. Reports the latest research on the "miracle compounds" of agriculture-bioregulators Covers the chemistry of these compounds and their applications and uses in improving crop yields and/or quality of various crops Discusses synthetic bioregulators and their effects on major cereals, oil seeds, legumes, citrus, tree fruits, nuts, and more Looks at bioregulator applications for resistance to insects, fungi and microbial toxins, temperature extremes, and environmental stress Also covers research on natural bioregulators present in certain crops ACS Symposium Series No. 257 296 pages ( 1984) Clothbound Export 553.95 US & Canada 544.95
Pesticide Synthesis Through Rational Approaches Philip 5. Magee, Gustave K. Kohn, and Jullus J. Menn, Eds. Investigates the different methods used to design and synthesize new agricultural chemicals Discusses successful strategies using a biorational rather than empirical approach to design and discover new pesticide chemicals Looks at the next generation of pesticides as well as tools and techniques that will optimize their biological activity Includes sections on
1072A
guided and serendipitous discovery, biochemical and physical design-assisted synthesis, and computer-assistedsynthesis and QSAR. ACS Symposium Series No 255 35 I pages ( I 984) Clothbound US & Canada 554.95 Export 565.95
Dermal Exposure Related to Pesticide Use Discussion of Risk Assessment Rlchard C. Honeycutt, Gunter Zweig, and Nancy Ragsdale, Eds. Confronts the issue of a serious chemical risk-pesticide exposure Reports data vital for assessing and regulating farm worker safety Looks at dermal absorption, field studies of exposure methodology, exposure assessment and protection, predicting exposure levels, and interpretation of data ACS Svmoosium Series No 273 544 cages ( 1985)Clothbound US & Canada 579.95 Export 595.95
Advances in Pesticide Formulation Technology Herbert B. Scher, Ed. Assesses new developments in pesticide formulations research Looks at the most active areas of research and new products concepts Examines the physical chemical principles of pesticide formulation science as well as rhe pesticide chemical structure-biological activity relationship ACS Symposium Series No 254 264 pages 1 1984) Clothbound Export 553.95 US & Canada 544.95
The Chemlstry of Allelopathy Biochemical Interactions Among Plants Alonzo C. Thompson, Ed. Reports on the rapidly growing field of allelopathy, stressing the biological and chemical techniques Assesses the allelopathic effect in weeds, crops, and aquatic plants, and studies the chemistry of allelopathic agents and the synthesis of cooperative efforts of biologists and chemists in solving the problems related to this topic and in using this natural phenomena to control weeds ACS Symposium Series No. 268 47 1 pages ( 1 984) Clothbound Export 595.95 US & Canada 579.95
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
Bioregulators for Pest Control Paul A. Hedln, Ed. Looks at new concepts and trends in pesticide chemistry and unifies approaches to the control of plants, insects, and diseases Highlights the mechanisms of herbicides, fungicides, and endogenous plant hormones Covers advances in bioregulators, both natural and synthetic Will add much to the elucidation and adoption of novel chemical compounds for the control of pests ACS Symposium Series No 276 570 pages ( I 985) Clothbound US & Canada 589.95 Export 5 107.95
The Agrochemicals Handbook Provides comprehensive data and information on a wide variety of products Lists trade, chemical and approved names as well as manufacturers of active components of agricultural products Covers toxic effects, chemical and physical properties, precautionary measures, and degradation metabolism of active components Includes herbicides, fungicides, insecticides, nematicides, acaricides, rodenticides, and more Special feature1 Regular updates with announcements and introduction by agrochemical manufacturers of new active ingredients, new pesticidal formulations. and new uses of existing products (Supplements will be shipped as soon as they are available J Published by the Royal Society of Chemistry I ,000 pages Pillar-postbinder ( I 983) US & Canada 5248.00 (includesSupplements 1-5)
Replacement Supplements 520.00 each
Order from: American Chemical Society Distribution Office Dept 124 1 155 Sixteenth Street, NW Washington, DC 20036 To charge your books by phone, call toll free (800)4296747 and use vour VISA, Mastercard, or American Express.
the contour plot by asymmetry in the shape of the contour lines (15) (Figure 11).
I
To implement a signficant realtime 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 acquisition rate should exceed that specified by the Nyquist frequency of the signal being measured. These factors affect the data storage requirements and data acquisition tasks for the system. Commercial systems may handle these two constraints differently. The analyst must carefully match the requirements of the experiment with the data storage rate, capacity, and sampling rate of the commercial instrument.
If you haven’t looked at
LEEMAN you haven’t looked at
ICP
(1)Hir8chfeld.T. J. Appl. Spectmc. 1976, 30,6249. (2) Boutilier,G. D.;Poll+, B. D.; Winefordner, J. D. Spectrochim. Acta 197s. 338,401-15. (3) Talmi,Y. AM^. Chem. 1975.47.658IO A. (4) Talmi, Y. AM^. Chem. 1975.47,699I09 A. (5) Okamoto, T.; Kawnta, S.; Minami, S. A pl. 0 t. 1984.24 269-73. (6)!XXblRet‘ icon. “S-SeriesSolid State Line Scanners 128,512,and 1024 Elements” specificationsheet,1978. (7) Talmi, Y.;Simpson. R. W.Appl. Opt. 1980.9,1401-14. (8) Savitzky,A,; Golay. M.J. AM^. Chem 1964.36, 1627-30. (9) Horlick. G. Appl. Speetmc. 1976.30. 113-23. (10)Talmi, Y.Appl. Spectrosc. 1982.36, 1-1s _ ~ _
In three short years, we’ve brought a state-of-the-art ICP spectrometer from our imaginations to some of the most prestigious analytical chemistry benches in
Y .
(11)Milano. M.J.; Lam, S.;Grushka, E.J. Chromatogr. 1976,125.31&21. (12) Fell, A. F.; Smtt, H. P.;Gill,R; Moffat, A. C. Chmmatographia 1983.16.
”-.
-‘’
i‘
\
.
-.
CO-?P
the field.
(13)Kowdski. B.R. Trends AM^. Chem. 1981,l.71-74. (14) Fell, A. F.UVSpectrom. Group Bcrll. 1980.8. 5. (15) Fell, A.,F.; Clark, B. J.; Smn,H. P.J. Pharm. Biomed. AM^. 1984,l.551-72.
We did it through innovative design, technical craftsmanship, hard work, and commitment to our customers.
PLASMA SPEC features high resolution, benchtop size, versatility and ease of operation. Performance, reliability, service, price, and customer satisfaction are the elements in our success. Let us demonstrate it to you. Call or write for information on our free, hands-on, local seminars. Dianna G. Jones is the manager of the Optical Analytical Systems and Applications Group at Tracor Northern. She receiued a B.S. from Duke University and a Ph.D. from Florida State Uniuersity. Her interests are in the area of optoelectronic image devices used in UV-VIS spectroscopy.
I LEEMAN LABS INC.
600 Suffolk St.. Lowell. MA 01854. 16171 4544442 Ckcls 128 tu W Semiw Infmnatlon. Ckcls 129 for &&I Inramatim. ANALYTICAL CHEMISTRY.
voc.
57. NO.
B, AWVST isss
ioni
