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PhotodiodeArray Detectorsfor LC PDA detection may become as important to LC as mass spectrometric detection is to GC The growing importance of photodiode array (PDA) detectors for liquid chromatography (LC) was apparent recently when an analytical instrument manufacturer introduced a new LC system a t the Pittsburgh Conference, only to withdraw it a month later. An officer of the firm confided that one of the reasons for the withdrawal was that “our instrument was based on a scanning monochromator instead of a diode array. There’s no doubt about the fact that diode arrays are the name of the game nowadays in LC detection.” I t is becoming increasingly clear that the PDA detector may one day be as important for LC as the mass spectrometer currently is for gas chromatography. A number of companies either have PDA units for sale or on the drawing boards; some of the units already available commercially are described below.
and look for other’bits of information. If there are sample components that do not respond at 254 nm, you are plumb out of luck. “With a PDA detector such as ours,” Kushner continues, “you can go hack and pull data from other wavelengths out of the file. Instead of doing a lot of repetitive work a t different individual wavelengths, the information is all there in computer memory. There’s a tremendous savings in time and effort.” In a recent Pittsburgh Conference presentation, Dianna G. Jones of Tracor Northern pointed out that PDA detection also makes it possible to evaluate a chromatographic peak for purity by software and data manipulation rather than hy refinement and iteration of the chromatographic separation. The idea that data manipulation can be a cost-effective substitute
for higher chromatographic resolution is a theme that Bruce Kowalski of the University of Washington has long championed (1). In fact, some of the chemometrics curve resolution software developed by Kowalski and by Infometrix, the company he helped found, is available as an option with Hewlett-Packard‘s PDA LC detector. Where does the PDA spectrometer fit in with other optical spectrometric techniques? As pointed out in “Multichannel Image Detectors” (21, spectrometric information can he obtained either hy scanning across the spectral region of interest or hy simultaneously monitoring all wavelengths in the spectrum (Figure 1). The primary advantage of simultaneous detection is an improvement in S/N or a reduction in the observation time required for the measurement. Simultaneous spectrometric detection can be further
Advantages of PDAs According to Arthur S. Kushner of LDC/Milton Roy, the PDA detector will never replace the single-wavelength and variable-wavelength LC detector. Cheaper and inherently more sensitive than the PDAs, conventional LC detectors will always he needed for some applications. Nevertheless, the PDAs provide capabilities that are not available with single- or variable-wavelength LC detectors. For instance, with conventional variable-wavelength detectors the eluent flow must he stopped to trap a peak in the flow cell while a UVArIS spectrum is obtained. The much faster PDA detectors, some of which can acquire a complete spectrum in as little as 0.01 s, have no trouble obtaining spectra in real time, without any disturbance of the eluent flow whatsoever. With a single-wavelength detector, explains Kushner, “After you run your chromatogram at, say, 254 nm, everything is finished. You can’t go back 836A
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=Scanning Spectrometer ISeauential)
;irnultaneous ipectrometric Detection
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1 Solielate Imagers
(PDAs. Charge-Coupled )evices ck-”plnje-”--
Figure 1. Solid-state image detectors such a s PDAs represent one of a number of multichannel techniaues
ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983
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The Power of Parallel Spectral Detection RAMAN Spectroscopy, Chemical Kinetics, Laser-induced Fluorescence, Plasma Diagnostics, HPLC and more benefit from this technique. Parallel detection has been proven to be a powerful ally to the optical spectroscopist ever since the first researcher removed the exit slit on a monochromator and replaced it with photographic film. The serious limitations of using film were quickly realized, and soon, television cameras replaced film in most applications.
Tracor Northern's TN-6112 detector has 1024 detecting elements positioned precisely on 25um centers. When positioned at the focal plane of a fixed grating spectrograph, the wavelength calibration becomes non-varying -making specifications such as wavelength accuracy and repeatability -obsolete.
DARSS SYSTEM OVERCOMES LIMITATIONS In 1977, Tracor Northern developed the DARSS (Diode Array Rapid Scan Spectrometer) which immediately overcame the difficulties associated with photographic film, vidicon and SIT tubes. This revolutionary development gave researchers the ability to acquire a complete spectrum at rates as fast as 5.16 milliseconds, with no bloom or lag.
SPEED OF ACQUISITION NOT ONLY BENEFIT The real power of rapid parallel detection extends beyond the speed of acquisition. Not only is laboratory time shortened, but sample degradation during the test process is minimized. However, speed is not the only advantage gained when converting from classical single wavelength detection techniques to rapid parallel detection. For example, parallel spectral detection insures precise wavelength calibration capabilities.
Similarly, lamp stability (a critical factor in a scanning system) is no longer critical because all increments of the optical spectrum are sensed simultaneously. There are no moving parts with the Tracor Northern DARSS system -so there is no mechanical wear, no vibration, no adjustments, no lead screw variables, and most importantly, all optical reflections and refractions are identical for all measurements.
SYSTEM MAY BE MATCHED TO SPECIFIC APPLICATIONS One of the most outstanding characteristics of the Tracor Northern DARSS System of parallel detection is its ability to be configured to specific applications. To illustrate this point, there are currently eleven (11) different detectors available -each fulfilling a particular function so the demands of a particular application may dictate the detector. For example, if an application involves nanosecond kinetics with high resolution -the TN-6132 detector will meet these demands. However, if the resolution requirement is not as critical, but the gating speed must be high, the TN-6133 detector is an economical yet effective detector. In fact, this detector can gate at speeds as fast as 5 nanoseconds for high speed application. In contrast, the TN-6132 detector has a 25 millimeter gatable image intensifier rather than 18mm on the TN-6133.. .and can be gated as fast as 10 nanoseconds. The TN-6120 Series of detectors are cost effective and non-gatable, but feature the highest gain of all detectors. These are particularly useful in applications for fluorescence, chemiluminescence and other such low level applications.
TN-1710 ANALYZER Complementing the outstanding parallel detection system of the DARSS, is the equally superior perfor-
mance of the TN-1710 Multichannel Analyzer. This analyzer features modular versatility with the power of an LSI-11 computer. Over 50 plug-in modules give ROM-based software versatility.
Recently developed is the TN-1710-54 Time Histogram Module ideal for plasma etch endpoint detection as well as HPLC and other applications where a time history of any group of spectral features is desired.
TN-6200 DARSS COMPUTER INTERFACE (DCI) For those researchers wishing to transfer data to larger laboratory computers, but still utilize the power of the DARSS parallel detection system, the TN-6200 DARSS Computer Interface is ideal. This interface is designed for rapid parallel transfer of spectral data directly from the DARSS detector to other computers for storage and further processing. A singularly important feature of the TN-6200 is that it can be programmed so that during the acquisition process, most functions are controlled by the DCI, thus relieving the computer of most on-line control functions.
l b a ~ oNorthern r TRACOR NORTHERN 2551 WEST BELTLINE HIGHWAY MIDDLETON, WISCONSIN 53562 (608) 831-6511 TRACOR EUROPA B.V. P.O. BOX 333 3720 AH BILTHOVEN THE NETHERLANDS (030) 780855 CIRCLE 208 ON READER SERVICE CARD
ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983
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--Shutter
Lens
System
Flow’
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Figure 2. Optical system or a PDA LC derecror. noapreu irom Hewlett-Packard company literature with permission
subdivided into multiplex techniques such as Fourier transform, where encoded information is received by a single detector, and multichannel parallel systems, in which spectral information is either spatially or temporally dispersed. Multichannel parallel systems include direct readers, in which a series of photomultiplier tubes is positioned along the focal plane of the spectrometer and optoelectronic image devices (OIDs), a group that includes electron readout devices, such as vidicons and intensified vidicons, as well as the solid-state imagers-PDAs, charge-coupled devices (CCDs), and charge-injection devices.
PDA Deslgn A PDA is a series of light-sensitive elements etched onto a silicon chip. The elements work in parallel to simultaneously monitor a range of wavelengths spread across the face of the chip by a dispersing element such as a holographic diffraction grating. Incident photons generate a charge that is stored on individual diodes. The accumulated charges are then switched sequentially by shift registers to form the detector output. The optical system of a PDA LC detector is typically of very simple design (Figure 2). Source radiation is focused through a flow cell a t the end of the LC column, and the emerging radiation is dispersed by a grating onto the linear PDA. In the setup shown, a 838 A
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three-nosition shutter. used for calihration or dark current compensation, is the only moving part. Why has the PDA been universally adapted for LC detection instead of some of the other OIDS that presumably might he equally useful for simultaneous multichannel spectroscopy? According to Yair Talmi of Princeton Instruments, Inc., silicon vidicons (SVs) generally have poor UV response and are much more difficult and expensive to operate than PDAs. In addition, because of discharge lag (incomplete target readout) and blooming (cross talk due to overspill of signal charge between diode elements), SVs are not suited to measurements of transient snectrometric phenomena. Charee-counled devices (CCDs) are fast enough fir LC detection, and their noise level and dark charge level are very low, making these detectors excellent for astronomical ohservations ( 4 ) . In fact, a CCD has been developed for the Space Telescope, scheduled to he launched in 1986, and a camera with a CCD detector was recently used for the first sighting of Halley’s comet in its current approach to Earth (5).However, commercially available CCDs do not respond in the UV region, and their signal collection aspect ratio is very poor. Other OIDs have similar problems that make them less desirable than PDAs for LC detection.
ANALYTICAL CHEMISTRY, VOL. 55. NO. 8. JULY 1983
“For optical spectroscopic LC detection, diode array detectors will hecome very important, particularly in the research market,” says Ron Majors of Varian, an authority on LC detectors. “But right now they’re very expensive. By the time you have a working photodiode array detector system, the cost is $15,000 to $20,000 or more, compared to, say, $6000 to $9OOO for a conventional scanning UV/visible LC detector. You’re paying more for the photodiode array detector than you are for the rest of the system, in some cases.” Nevertheless, Yair Talmi, whose :ompany, Princeton Instruments, Inc., dpecializes in multichannel-image-detector-based research systems, feels that the higher price of the new PDA LC detectors is justified by their higher performance. “All the manufacturers are going that route,” explains Talmi. “Considering the price/performance ratio, it’s the only game in town that makes sense for that application.”
Commercial Detectors A number of PDA detectors for chromatography have recently been introduced. The one thing that seems certain is that many more will soon he available. The following is a description of some of the current offerings. Hewlett-Paekard offers a dedicated PDA LC detector, the H P 1040A, in addition to two general-purpose PDA UVNIS spectrometers that can he wed for LC detection. the 8450A and the 8451A. The 1040A LC detector. introduced by the company ahout a year ago, includes a dedicated computer for system control and for interface with the user. The detector, which can obtain spectra in as little as 10 ms, can he connected to any existing LC. HP’s Stephan A. George stresses that the 1040A can he used with conventional LC columns, hut that “it’s also fast enough for future trends, such as microhore.” George says the 4.5-pL flow cell has been optimized for the best possible sensitivity/resolution trade-off. “The software product is the best available,” George continues. “The software is menu-driven, so that a turnkey approach can he used. But the user also has access to the data file structure, so he can write his own BASIC software to adapt the instrument to his experiment.” LDC/Milton Roy’s CMX-50 PDA LC detector, introduced a t the 1982 Pittsburgh Conference, is designed to operate in conjunction with a Digital Equipment Corporation PDP/ll com-
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ANALYTICAL CHEMISTRY. VOL. 55, NO. 8. J U Y 1983
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There’s no doubt about the fact that diode arrays are the name of the game nowadays
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puter. ‘TheCMX-50 is available as either a UVNIS or a UV-only system. According to LDC/Milton Roy’s Art Kushner, “We offer what I believe to be the most complete software package in the field in terms of data processing: ratio calculations, retrieval of different spectra, ete. In addition, the instrument can collect more data on a continuous basis than any of its competitors-six scans per second for an extended period of time, though 90 minutes is about as lone” as anvone would want to go.” LKB. Data from LKB’s model 2140 Rapid Spectral Detector is collected, stored, and processed in a dedicated data unit. This unit includes a variety of control and postrun calculation functions (background subtraction, ratio calculations), as well as output for a digital printer-plotter and bidirectional computer communication with an IBM PC. According to LKBs David Weber, the instrument’s advantages include: * the ability to simultaneously monitor up to four wavelengths, permitting multicomponent detection in complex samples (multichannel chromatograms); capability for “wavelength buncbing,” the integration of all signals between two preset wavelengths for greater accuracy and lower detection limits; comparison of absorption spectra with spectra in a user-generated library for positive identification of sample components; and the ability to display three-dimensional chromatoerams (time vs. wavelength vs. absorgance).’ PhiliodPve Unieam. The PU 4021 Miltichannel UVNIS Detector has a number of modes of operation tailored to the varying requirements of the analyst. A programmed wavelength facility makes it possible for the operator to select up to nine different wavelengths to be monitored during a chromatographic run. A spectral storage facility may be used to store spectra either manually or automatically during the chromatogram. According to Bill Monahan of Sargent-Welch Scientific, U.S. distributor for Philips/Pye Unicam, “You will be able to interface the 4021 to a variety
8 4 2 A * ANALYTICAL CHEMISTRY, VOL. 55, NO. 8. JULY 1983
of different systems, so you won’t be locked into a particular data system or a particular pump. We have a very powerful detector, and it will be readily compatible with just about any system available out there. The instrument has a number of measuring and programming capabilities that make it pretty powerful; so if you don’t go into a separate data system, it’s still a very capable instrument.” Shimadzu’s SPD-M1A LC detector is capable of three-dimensional output and features double-beam optics. According to Steven Cubbedge of Shimadzu, “The instrument has two source lamps, one deuterium and one tungsten. The aim is to get high illumination over the entire UV/visible range so we can achieve sensitivities similar to a standard LC UV detector.” Cubbedge says the double-beam system also provides a very stable spectral baseline. “We have gone to a complete turnkey system,” explains Cubbedge, “with a dedicated microprocessor for controlling the optics and for data processing. No other computer is required, but an IEEE interface to connect the unit to a larger computer is currently being developed.” Traeor Northern’s model 6050 Multipurpose Spectrophotometer is a modular UVNIS system. Modules are available to adapt the system for a number of different applications, including LC detection. “We don’t have a turnkey system right now that you can just plug in, turn on, and get LC data,” explains Tracor’s Don Landon, “though we have one unit that is close to it. We consider our system to he primarily research oriented.” According to Bob Compton of Tracor, “The kind of people that buy the 6050 are those who are doing HPLC along with other kinds of spectroscopy. We can configure the instrument to do exactly what the user wants. For a multipurpose system that includes HPLC capability, yon can’t beat what it bas to provide.” Stuart A. Borman References (1) Borman, Stuart A. Anal. Chem. 1982, 54,1379-80 A. (2) Talmi, Yair. Ed. “Multichannel h a z e Detectors”; American Chemical Socieiy: Washington, D.C., 1979. (Talmi is currently working on a new book, tentativeIv titled ”Imaee Detectors in Soectroscocy,” schedulea for publieationbythe American Chemical Society in late 1983.) (3) Talmi, Y.; Simpsun, R. W. AppL Opt. 1980,19 (91,1401-14. (4) Kristian, Jerome; Blouke, Morley. Sci. Am. October 1982,247,6&74. (5) Eberhart,J. Science News, 1982,122 (181,277.
