Focus
Photodiode Array Detectors for 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 at 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." It 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.
for higher chromatographic resolution is a theme that Bruce Kowalski of the University of Washington has long championed (i). 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" (2), spectrometric information can be obtained either by scanning across the spectral region of interest or by 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
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 back and pull data from other wavelengths out of the file. Instead of doing a lot of repetitive work at 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 by refinement and iteration of the chromatographic separation. The idea that data manipulation can be a cost-effective substitute
A d v a n t a g e s of P D A s
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 be 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 be stopped to trap a peak in the flow cell while a UV/VIS 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
Spectrometry
Simultaneous Spectrometric Detection
Scanning Spectrometer (Sequential) Multichannel Parallel Detection
Multiplex Techniques (FT, etc.) Optoelectronic Image Detectors
Electron Readout Devices (SVs, Silicon-Intensified Target Vidicons)
Multiple PMTs (Direct Reader)
Solid-State Imagers (PDAs, Charge-Coupled Devices, Charge-Injection Devices)
Figure 1 . Solid-state i m a g e d e t e c t o r s s u c h as PDAs r e p r e s e n t o n e of a n u m b e r of multichannel techniques
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0003-2700/83/0351-836A$01.50/0 © 1983 American Chemical Society
Focus
Shutter
Photodiode Array
Grating
Figure 2. Optical system of a PDA LC detector. Adapted from 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 Design
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 at the end of the LC column, and the emerging radiation is dispersed by a grating onto the linear PDA. In the setup shown, a
three-position shutter, used for calibration 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 be 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 spectrometric phenomena. Charge-coupled devices (CCDs) are fast enough for LC detection, and their noise level and dark charge level are very low, making these detectors excellent for astronomical observations (4). In fact, a CCD has been developed for the Space Telescope, scheduled to be 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.
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"For optical spectroscopic LC detection, diode array detectors will become 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 $9000 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 company, Princeton Instruments, Inc., specializes 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 be available. The following is a description of some of the current offerings. Hewlett-Packard offers a dedicated PDA LC detector, the H P 1040A, in addition to two general-purpose PDA UV/VIS spectrometers that can be used for LC detection, the 8450A and the 8451 A. The 1040A LC detector, introduced by the company about 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 be connected to any existing LC. HP's Stephan A. George stresses that the 1040A can be used with conventional LC columns, but that "it's also fast enough for future trends, such as microbore." George says the 4.5-ML 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 be 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 at the 1982 Pittsburgh Conference, is designed to operate in conjunction with a Digital Equipment Corporation PDP/11 com-
An Expanding Experience. . .
Focus
There's no doubt about the fact that diode arrays are the name of the game nowadays in LC detection.
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puter. The CMX-50 is available as either a UV/VIS 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, etc. 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 long as anyone 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 LKB's 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 bunching," 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 chromatograms (time vs. wavelength vs. absorbance). Philips/Pye Unicam. The PU 4021 Multichannel UV/VIS 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
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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." Tracor Northern's model 6050 Multipurpose Spectrophotometer is a modular UV/VIS 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 be 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, you can't beat what it has to provide." Stuart A. Borman References (1) Borman, Stuart A. Anal. Chem. 1982, 54, 1379-80 A. (2) Talmi, Yair, Ed. "Multichannel Image Detectors"; American Chemical Society: Washington, D.C., 1979. (Talmi is currently working on a new book, tentatively titled "Image Detectors in Spectroscopy," scheduled for publication by the American Chemical Society in late 1983.) (3) Talmi, Y.; Simpson, R. W. Appl. Opt. 1980,19 (9), 1401-14. (4) Kristian, Jerome; Blouke, Morley. Sci. Am. October 1982, 247, 66-74. (5) Eberhart, J. Science News, 1982,122 (18), 277.
