ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
mol and 0.166 X and 0.191 X each of PA and a-KG, respectively.
LITERATURE CITED
mol for 5 X lo-'' mol
DISCUSSION The present method is the first HPLC method involving the separation of native a-keto acids and the post-column derivatization. As to the HPLC-fluorescence detection of keto acids, only one report has been published by Grushka et al. (15) who used 4-bromomethyl-7-methoxycoumarin(Br-Mmc), a fluorogenic reagent for carboxylic acid. The HPLC system described here enabled fluorometric determination of keto acids at the picomole level, which is at least 1000-fold, 10-fold, and 5-fold more sensitive than the 2,4-dinitrophenylhydrazone (IO), the Br-Mmc (1.9, and the quinoxalone (11) methods, respectively. The fluorogenic NMN reaction used is not always specific for a-keto acids; however, the present HPLC method which combined the anion-exchange chromatography and the NMN reaction is selective for keto acids. The high sensitivity of the method may permit the direct injection of samples such as urine and deproteinized blood for the determination of a-keto acids. Determination of activities of transaminases by the present method is being studied in our laboratory.
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(1) Friedemann, T. E.; Haugen, G. E. J . Biol. Chem. 1943, 147, 415-42. (2) Friedemann, T. E. "Methods in Enzymology", Colowick, S. P., Kaplan, N. 0.Eds.; Academic Press: New York, 1957; Vol. 111, pp 414-18. (3) Kubowitz, F.; OR,P. Biochem. Z. 1943, 314, 94-117. (4) Ochoa, S,; Mehler, A. H.; Kornberg, A . J . Biol. Chem. 1946, 174, 979-1000. ( 5 ) Marbach, E. P.; Weil, M. H. Clin. Chem. ( Winston-Salem, N.C.) 1987, 13, 314-25. (6) Greengard, P. Nature (London) 1956, 178, 632-34. (7) Lowry, 0. H.: Roberts, N. R.; Lewis, C. J. Biol. Chem. 1958, 220,879-92. (8) Langenbeck, U.; Hoinowski, A.; Mantel, K.; MWing. H.--U. J. C k m t o g r . 1977, 143, 39-50. (9) Hayashi, T.; Sugiura, T.; Terada, H.; Kawai, S.; Ohno, T. J . Chromatogr. 1976, 118, 403-408. (IO) Terada, H.; Hayashi, T.; Kawai, S.;Ohno, T. J . Chromatogr. 1977, 130, 281-86. ( 1 1 ) Liao, J. C.: Hoffman, N. E.; Barboriak, J. J.; Roth, D. A. Clin. Chem. ( Winston-Salem, N.C.) 1977, 23, 802-805. (12) Spikner. J. E.; Towne, J . C. Anal. Chem. 1962, 34, 1468-71. (13) Mizutani, S.;Wakuri, Y.; Yoshida, N.; Nakajima, T.; Tamura, 2 . Chem. Pharm. Bull. 1989, 17, 2340-48. (14) Takeda, M.; Kinoshita, T.; Tsuji, A. Anal. Biochem. 1976, 72, 184-90. (15) Grushka, E.; Lam, S . ; Chassin, J. Anal. Chem. 1978, 50, 1398-99. (16) Nakamura, H.; Tamura, 2. J . Chromatogr. 1979, 168, 481-87. (17) Nakamura, H.; Tamura, 2 . Anal. Chem. 1976, 50, 2047-51.
RECEIVED for review February 28, 1979. Accepted June 1, 1979.
Multiwavelength Detection for Liquid Chromatography with a Repeat-Scanning Ultraviolet-Visible Spectrophotometer Koichi Saitoh and Nobuo Suzuki" Department of Chemistry, Faculty of Science, Tohoku University, Sendai, 980,Japan
A repeat-scanning spectrophotometer was designed to scan the 200-800 nm spectral range In 375 ms with a repetition rate of 2 Hz, or in 750 ms with a repetition rate of 1 Hz. The flow cell used for chromatographic experiment had a sample path of 10 mm and a volume of 8 kL. The spectrophotometer was interfaced to a small computer to perform multiwavelength detection. The simultaneous recording of chromatograms at different monitoring wavelengths, and instantaneous recording of absorption spectra were performed. The capability of multiwavelength detection has been demonstrated with an experiment on the gel chromatography of benroylacetone and its Be(I1) and Cr(II1) chelates.
High performance liquid chromatography (HPLC) currently offers efficient separation and reduced analysis time for a variety of compounds. In order to take advantage of these capabilities of liquid chromatography (LC), there is an urgent need for a detector which make possible instantaneously the confirmation of the chemical species eluted from a separation column. Although different types of detectors based on a variety of principles have been introduced into LC, the ultraviolet (UV) or visible light absorption detector is still the most widely used. Most of the commonly used detectors are based on light absorptivity measurement and operate at single and fixed wavelength. Such a fixed wavelength instrument is limited to detecting components which have noticeable absorption at the monitoring wavelength. In addition, it does not furnish necessary information on the identity of the 0003-2700/79/0351-1683$01 .OO/O
compound. The introduction of an adjustable wavelength device would solve the former limitation. However, it is, in practice, troublesome to adjust the monitoring wavelength a t every turn to ensure detection with optimum sensitivity for individual components eluted. A commercial type of recording spectrophotometer has been used for recording the absorption spectra of components in eluate. However, so long as a conventional recording spectrophotometer is used, the continuous elution process has to be temporarily interrupted while the spectrum is being recorded because of the restricted recording speed of the spectrophotometer, and this is the so-called stop-scan technique. In recent years, some types of multiwavelength absorption detectors (MWD) for instantaneous recording of the spectra of components have been introduced into LC systems. Bylina et al. ( I ) employed a cathode-ray tube (CRT) whose phosphor screen was located in the focal plane of a monochromator. The luminous spot, whose deflection was electronically controlled on the screen of the CRT, was a movable light source. Denton et al. ( 2 ) used an oscillating mirror rapid scanning spectrometer. Their MWD exhibited rapid scanning capability up to 4.25-kHz scanning rate and 218-Hz repetition rate. However, the light source (xenon arc lamp) would not be practical, and the flow cell was relatively large (87 pL). The use of multichannel integrated photosensors, such as the photodiode array (3)and silicon target vidicon ( 4 , 5 )were also reported. These devices are readily adapted to computer processing techniques. They are, however, less sensitive than photomultiplier tube, and a compromise has to be made between the spectral resolution and the desired wavelength 0 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
@.-$M H $4(
Fbure 1. Optical diagram of repeat-scanning sPectroPhotometer. ( w ) 30-W tungsten lamp; (D) 30-W deuterium lamp; (SS) Light source diffraction selector; (SL,,SL,) entrance and exit slits; (M,-M,) mirrws; (a) grating; (WC) wavelength cam; (BS) beam splitter; (SFC, RFC) sample and reference flow cells; (PMT) photomultiplier tube; (EC) encoder; (C) electric clutch; (EM) electric motor; (MH) manual handle
range because of the restricted numbers of the photosensing elements integrated. Most of the MWDs reported to date were excellent in rapid scanning capability. However they do not fulfill the essential requirement of a scanning cycle and wide spectral range covering from UV to visible regions with high resolution. In addition, these devices are still expensive. The present authors have designed a new MWD for LC consisting of a repeat-scanning UV-visible spectrophotometer and a small computer for data processing. The spectrophotometer can scan repeatedly the entire wavelength range (200-800 nm) with a slewing rate of either 0.8 or 1.6 nm/ms. Two light sources, a deuterium and a tungsten lamp, were automatically alternated in the desired wavelength regions. A high resolution of up to 1nm is achieved, and the small flow cell with 8-pL volume is acceptable. This paper describes the design of the above MWD and the feasibility of the MWD is demonstrated with a preliminary experiment on the gel chromatographic separation of benzoylacetone and its Be(I1) and Cr(1II) chelates.
INSTRUMENTATION Optical System. The spectrophotometer is a dual beam optical instrument capable of operating between 200 and 800 nm. The basic design of the optical system is illustrated in Figure 1. The monochromator employed in this system has a Czerny-Turner optical arrangement with a focal length of 250 mm. A 30-W deuterium (D) and a 30-W tungsten (W) lamps are used as light sources. Radiations from the former and the latter light sources are automatically selected to be effective in the spectral regions that are shorter and longer than 340 nm, respectively, with the aid of a source selector (SS). The SS is a disk on which a mirror and filters are attached, and rotates in conjunction with a wavelength drive mechanism in the system. The wavelength of the monochromator depends on the angular deflection of a grating (GR) with 1200 lines/mm, and this angular deflection is controlled by the revolution of a wavelength cam (WC) which is driven either automatically or manually. The electric clutch (C) placed among the WC, an electric motor (EM), and a manual handle (MH) is used to ensure the operation in an automatically/manually driven mode. When the monochromator is driven by the EM, the WC is rotated a t a rate of 60 or 120 rpm, and this controls the rotation of the GR in such a manner that the monochromator scans repeatedly the 200-800 nm spectral range for each period of 375 ms with a repetition rate of 2 Hz, or for each period of 750 ms with a repetition rate of 1 Hz. The bandwidth of beam from the monochromator is adjustable stepwise to be 1,2, or 4 nm by adjusting the exit
slit (SL,). A beam splitter (BS) of a static type is employed to divide the exit beam of the monochromator into the sample and reference components. The flow cell used in the spectrophotometer is made of polyfluoroethylene except the window made of quartz. The cell has a volume and path length of 8 pL and 10 nm, respectively. The detection of the sample and reference beam intensity is provided by a couple of photomultiplier tubes (PMT, Hamamatsu TV R446). A shaft encoder (EC) is directly connected with the wavelength drive mechanism and generates a number of pulses linear to the movement of the GR. These pulses, when fed back to an electronic counter, enable a computer to monitor the wavelength, as will be described in more detail later. When the wavelength drive is connected with the MH, the present instrument operates as a manually adjustable single wavelength detector for LC. Data Processing System. For the purpose of performing on-line data processing, a small computer (JEOL JEC-5) with 4096 words of 16-bit was employed, with the following peripheral equipment: ASR teletype, high-speed papertape reader and puncher, magnetic drum memory with a capacity of 8192 words of 16-bit, and JEOL standard interface unit for 16-bit digital data input (DDI) and output (DDO) (8 channels were available for each function). Principal home-made additions to the system included (1) a data acquisition and display subsystem for conditioning signals, for reading the wavelength of the monochromator, and for displaying processed data; (2) a remote operator’s console with command input keys for controlling the functions of the data processing; and (3) instrument interfacing. A diagram of the system components is shown in Figure 2, and some of the components are detailed below. As was mentioned above, a shaft encoder (EC) was coupled to the wavelength drive on the monochromator. The EC was calibrated so as to generate a “wavelength pulse” for every 1-nm change in the monochromator within the wavelength range from 200 to 800 nm, and also a “scan pulse” for every beginning of the scanning. The former pulse was applied to a electronic pulse counter whose result was reset to zero synchronously with the latter pulse. The result, N , of the counter is expressed as: N = X - 199, at a wavelength of Mnm) in an integer. The wavelength data thus processed were delivered to the computer through one of the DDI channels of the standard interface unit. The PMTs operated in such a manner that their sensitivity was independent of wavelength, by use of automatic control of the high voltage applied to each PMT. The signals from the PMTs were processed by current-voltage transducers and then by a log-ratio transducer. The voltage output of the latter transducer was proportional to absorbance, and this signal was converted into a digital mode. An analog-to-digital conversion unit consisted of a variable-gain amplifier, a sample-and-hold amplifier (Analog Devices AC-582), an analog-to-digital converter (ADC) with 10-bit resolution (Burr-Brown ADC80AG-10),and their associated electronics. The A/D conversion was carried out synchronously with every “wavelength pulse” from the EC placed at the monochromator. Therefore, the spectral resolution in the A/D conversion process was 1 nm. The output of the ADC was connected with one of the computer DDI channels. Digital-to-analog converters (DACs) were employed for the purpose of interfacing analog display equipment, such as an oscilloscope, pen recorder, and X-Y recorder, to the computer. The computer offered its output data to six channels of DDO in the standard interface unit. Four DACs with 10-bit resolution (Analog Devices DAC-lOz-1) and two DACs with 8-bit resolution (Analog Devices AD-559) were connected with the
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
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optical s y s t e m
and
--1
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hold
t
/EC, I
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I
wavelength pulse
’
pulse (reset& counter scan pulse
command input keys
JEC-5 CPU (4 K-words)
I
8FCI
DDO
magnetic drum (8 Kword)
to le typewriter
paper-tape reader
paper4 ape puncher
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I
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pen u p l Q m
Flgure 2. Block diagram of repeat-scanning spectrophotometer system
respective DDO channels. Each DAC had its analog output voltage from 0 to 10 V applicable to the input of an analog equipment. In our currently used system, a Matsushita VP-654 2-pen recorder, a Matsushita CT-130A oscilloscope with a 130-mm screen and Rika Denki BW-133 X-Y recorder with an effective chart size of 250 mm X 180 mm were emmployed as the analog equipment. A relay contact output built in the computer was available for control of the X-Y recorder pen up/down. The remote operator console included key switches and their associated electronics. When a operator pressed one of the key switches, a pulse was delivered to an appointed channel of the process control interrupt input (PCI) of the computer. The commands for controlling experimental functions during a experiment were introduced into the computer through the key switches. Data Acquisition. The spectrophotometer software package used was our original program that acquired, displayed, and stored experimental data. In addition, the program included various data processing routines for making the spectrophotometer applicable to a MWD for LC use. Prior to beginning a chromatographic experiment, the operator specified through the teletype the wavelengths a t which elution should be monitored and the desired wavelength range in which absorption spectra should be recorded or filed. These operational parameters could be altered a t arbitrary times during the experiment, according to the commands given through the operator console. During the chromatographic experiment, the spectrophotometer scanned continuously the 2W800 nm spectral range with a repetition rate of 2 or 1Hz. In each scanning cycle, the absorbance data at the specified wavelengths were extracted, and then presented on the independent data channels of the 2-pen recorder. Thus, the chromatograms at different wavelengths are simultaneously monitored. The spectrum of the desired wavelength range could also be extracted from the whole spectrum in each scanning period and be displayed on the oscilloscope. The spectra thus obtained could be filed in the drum memory or hard copied with the X-Y recorder, according to the commands given through the command input keys on the operator console a t arbitrary time intervals. In the above data pro-
200
300
400 500 600 W A V E L E N G T H ( nrn 1
700
800
Figure 3. Effect of base-line correction routine. (A) Crude base-line spectrum; (B) corrected base line. Both sample and reference flow cells are filled with water. Scanning rate, 0.8 nm ms-’; slit, 1 nm
cessing routine, the spectral data are handled with a resolution up to 1 nm. After completion of the chromatographic experiment, one could plot the stored data in the drum memory as the individual spectra or the “3-D chromatogram”, that is a three-dimensional presentation of absorbance, wavelength, and time. The computer program included a base-line correction routine for spectral data. At the beginning of each experiment, a base-line spectrum was recorded and stored on the core memory. This base-line spectrum was subtracted from all other spectra delivered from the spectrophotometer. The effect of the base-line correction routine is demonstrated in Figure 3. Both sample and reference flow cells were filled with water in this demonstration.
EXPERIMENTAL Chromatographic System. A Kyowa Seimitsu Model KHU-26 chemically inert piston pump and a Model KU-1 plunger-type pulsation damper (Mitaka, Tokyo, Japan) were used to deliver eluant to two Kyowa Seimitsu 5 mm X 730 mm glass columns being connected in series. Each column was packed with Fracogel PGM 2000 (E. Merck) with a 32-46 l m particle size distribution. The columns were thermostated at 25 “C. Sample was introduced into the column with a Chromatronix Model CSVA automatic cheminert sample injection valve having a sample
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300
3 50 4 00 W A V E L E N G T H , nm
450
Figure 4. 3-D chromatogram for separation of Cr(bzac),( l ) ,Be(bazc), (2), and Hbzac (3). Fractogel PGM-2000, 2 X (730 mm X 5 mm); solvent, benzene; flow rate, 0.32 mL min-’; scanning rate, 0.8 nm ms-’; slit, 1 nm
chamber with 20-pL capacity. The reference flow cell of the spectrophotometer was placed between a solvent reservoir and the pump, and the sample flow cell followed the column used. Interconnections of all components were made using 0.5-mm i.d. Teflon tubing and Kyowa Seimitsu tube-end-fittings. Reagents. Benzene, prior to being used as an eluant, was purified by distillation, and then degassed by stirring under reduced pressure. Commercially available benzoylacetone (Hbzac) was recrystallized from ethanol. Bis(benzoy1acetonato) beryllium(I1) (Be(bzacj2)and tris(benzoylacetonato)chromium(III) ( C r ( b ~ a c )were ~ ) prepared from Hbzac and nitrate salts of corresponding metal ions, and purified by recrystallization. Analysis of Be(bzac)z. Found: C, 73.42; H, 5.93%. Calcd: C, 73.09; H, 5.48%. Analysis of Cr(bzac),. Found: C, 68.09;H, 5.44%. Calcd: C, 67.28; H, 5.28%. A 20-pL portion of p-dioxane solution of these compounds mixture was applied to the column. A 21.6-pg amount of Hbzac, 9.8 pg of Be(bzac)z,and 30.4 pg of Cr(bzac), were taken into the column. Elution was carried out at a solvent flow rate of 0.32 mL/min. Detection. In this experiment, dual-wavelength monitoring at 320 and 370 nm was recorded with a Matsushita PV-654 2-pen recorder. The repetition rate of the spectral scanning was set to be 1 Hz.
RESULTS The gel chromatography of metal complexes is one of our research interests (6-8). Figure 4 is a 3-D chromatogram of Cr(bzac)3, Be(bzac)z, and Hbzac recorded with the present MWD system. The third axis, which is displaced in the figure a t a 45’ angle, is time. The spectrophotometer scanned the 200-800 nm spectral range every second during the experiment, and the spectra displayed on the oscilloscope were renewed at every second. The model compounds, Hbzac and its metal chelates, showed no absorption band beyond the range of 450 nm; hence the spectra in the range of 280-450 nm were extracted and hard copied with the X-Y recorder at about 30-s intervals. The scanning time for the range from
i
1.01 I
0
a
2
30
60
ELUTION T I M E ,
90 MIN
Figure 5. Simultaneous recording of chromatograms for separation of Cr(bzac), (l),Be(bzac), (2), and Hbzac (3), at 320 nm (a),and 370 nm (b). Other conditions as in Figure 4
280 to 450 nm was 212.5 ms. In this period, the mobile phase flowing through the optical cell was only 1.13 pL. This value is less than 15% of the cell volume. The scanning rate of the spectrophotometer (1.25 nm/ms) is high enough to get a reliable spectrum, provided that the flow rate of an eluant is not so high. The observed spectra for the elution bands 1, 2, and 3 shown in Figure 4 coincided with the known spectra for Cr(hzac)3, Be(bzac)z, and Hbzac, respectively. Thus, identification of the components could be easily and instantaneously carried out at the column outlet. Figures 5a and 5b represent the chromatograms recorded simultaneously at 320 and 370 nm, respectively. The monitoring at 320 nm allowed detection of all the components eluted from the column, whereas the monitoring at 370 nm gave the selective response for C r ( b ~ a c ) ~ .
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
ACKNOWLEDGMENT The authors thank the Japan Spectroscopic Co. for technical assistance. LITERATURE CITED (1) Bylina, Andrzej; Sybilska, Danuta; Grabowski, Zbigniew R.; Koszewski, JBzef. J . Chromatogr. 1973, 83, 357-62. (2) Denton. Mark S.;DeAngelis. Thomas P.; Yacynych, Alexander M.; Heineman, William R,; Gilbert, T, W, ~ ~Chem, ~ 1976, l 48,, 20-24, (3) Milano, M. J.; Lam, S.;Grushka, Eli. J . Chromatcgr. 1976, 125, 315-26.
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(4) McDowell, Alan E.; Pardue. Harry L. Anal. Chem. 1976, 48, 1815-17. (5) McDowell, Alan E.;Pardue. Harry L. Anal. Chem. 1977, 49, 1171-76. (6) Saitoh, Koichi; Suzuki, Nobuo. J . Chromatogr. 1975, 109, 333-39. (7) Suzuki, Nobuo; Witoh, Koichi; Shibukawa, Masami. J . Chromatogr. 1977,
138, 79-87. (8) Saitoh, Koichi: Suzuki, Nobuo. Bull. Chem. Soc. Jpn. 1978, 51, 116-20.
RECEIVED for review January 3,1979. Accepted May 23,1979. The authors thank the Ministry of Education for a Scientific Research Grant.
Electro-Optical Ion Detector for Capillary Column Gas Chromatography/Negative Ion Mass Spectrometry Bjorn Hedfjall and Ragnar Ryhage" Laboratory for Mass Spectrometry, Karolinska Institute,
S-104 0 1 Stockholm,
An LKB 2091 GWMS instrument has been equipped with a microchannel plate electron multiplier array and a P-20 phosphor coated glass fiber optical window. The image obtained on the window is detected by a photodiode array controlled by a computer via an interface. The detector system has a true parallel spectrum measurement for each scan; and the time that the sample is exposed, as compared to the time it is actually measured, Is approximately the same. The ratio of the highest to the lowest simultaneously detectable masses is 1.08. The advantage of using an electro-opticalion detector In capillary column work along wlth the sensltlvity of the system and other data are discussed. The instrument was run In a negative chemical ionization mode with ammonia as reagent gas.
Electro-optical ion detectors (EOID) for mass spectrometers are still in the experimental stage. Only a few papers presenting mass spectral data from this type of detector have been published (1-6). The advantage of using a mass spectrometer with EOID as an integrating detector is that all ions in the collected spectra are detected simultaneously. An EOID system includes a channel electron multiplier array (CEMA). The CEMA has been carefully tested on mass spectrometers where organic ions in the mass range m / z 31-614 were studied. The minimum detectable ion current and the maximum obtainable resolution were determined (5, 7). The CEMA used in mass spectrometry applications is equipped with a phosphor screen and an optical fiber glass window. Phosphor is coated directly onto the window which is used to guide the light image out of the vacuum system. In papers published on EOID systems, an optical lens and a vidicon camera have been used to detect the light image on the window ( 2 ) . Other systems also include an optical glass fiber to format the image directly on the vidicon camera ( 4 ) or via an optical lens system to the camera ( 1 , 3, 5 ) . Some authors are planning for future development to use a photodiode array instead of a vidicon camera ( 4 ) . None of the above references describe the computer and the interface used for data collection. Therefore no conclusion can be drawn as to whether or not the spectra are collected with a long or short time interval between each spectrum. A long time interval 0003-2700/79/0351-1687$01 .OO/O
Sweden
will result in a loss of information from the sample. Nor is there any mention if, or how, the integration time, the CEMA, or the phosphor screen voltage are to be changed to record weak and strong peaks in the same spectrum. The present paper describes an EOID system used in capillary column GC/MS work, where the EOID is equipped with a photodiode array.
INSTRUMENTATION GC/MS Instrument. The single focusing mass spectrometer used in this experiment was an LKB 2091 instrument equipped with a capillary column inlet, a one-stage jet separator, a heated inlet system and an electronic mass marker. The instrument operates in both electron impact and chemical ionization modes and in both cases either negative or positive ions can be detected. A glass column 25 m X 0.3 mm i.d. coated with SP-lo00 was used at a temperature of 190 "C. The carrier gas flow rate was 5 mL He/min and a make-up gas at 15 mL He/min. The accelerating voltage was kept at -3.5 kV, the electron voltage a t 250 eV, and the total electron emission current at 0.5 mA. EOID System. Figure 1 shows the principles involved in transferring data to the computer from the CEMA connected to the mass spectrometer. The CEMA is angled 45" to obtain an approximate focal plane over the detected mass range. It consists of many individual channels placed in parallel, to comprise an active area of 5 cm2 with a diameter of 25 mm and a thickness of about 1mm. In our work, two CEMA plates are used and they are placed in close proximity to each other with channel bias angles of 8"/8" front/back. A voltage of maximum 2 kV can be maintained across the channel plates. The optical window consists of glass fibers having a diameter of 6 pm. The inside of the window is coated with aluminum and P-20 phosphor for a viewing of the CEMA output. The distance between the second CEMA and the phosphor screen is determined to give an optimum resolution at 5 kV. The total spatial resolution of the CEMAs is 16 lpmm-'. The CEMAs were delivered by Galileo Electro-optics Corp., Sturbridge, Mass. The coupling between the vacuum window and the photodiode array is made of rigid optical glass fibers (5 mm in height X 25.5 mm width X 10 mm length) with a fiber diameter of 7-8 pm. The fibers are black coated and have a hexagonal transverse section. The optical glass fiber was delivered by Volpi AG, Urdorf, Switzerland. 0 1979 American Chemical Society
