xylose dehydration, the absorbance peaks resulting from glucose and xylose are further separated for a simultaneous determination.

The readings at 310 and 340 nm are on the sides of ab- sorption peaks. As a consequence, the readings are sensitive to small changes in wavelength. This is the primary reason why absorptivities a t 310 nm and 340 nm may differ between spectrophotometers. It is also a reason for uniformly setting the wavelength scale at each reading as recommended by the instrument manufacturer. With care, this is not a source of much error.

There are other instances in which the dehydration to fu- rans might be used to advantage. The absorption spectra in- dicate that an isolated arabinogalactan could be analyzed by absorbance measurements at 290 and 335 nm with no added NaC1. Analyses of other isolated hemicelluloses could be similarly approached.

Analyses of alkaline extracts are simplified by this proce- dure because the removal of inorganic salts or solvent which might be required for other analyses is unnecessary.

I t is estimated that four samples of wood or pulp can be

analyzed in about 90 min after weighing. About half of this is work time. The 30 min required for spectrophotometer readings will probably be reduced by the automatic instru- ments now entering the market. Single-sample analysis as described would cut the work time nearly in half. It is this saving in labor and time and the potential for automatic in- strumentation which makes spectrophotometry attractive.

ACKNOWLEDGMENT The author acknowledges the assistance of Jeanne Wip-

perman and Marilyn Effland who did the chromatographic analyses.

LITERATURE CITED (1) R. W. Scott and J. Green, Anal. Chem., 46, 594 (1974). (2) R. W. Scottand J. Green, Tappi, 55, (7), 1061 (1972). (3) J. F. Saeman, W. E. Moore, R. L. Mitchell, and M. A. Mlllett, Tappl, 37 (8),

RECEIVED for review June 7,1976. Accepted July 28, 1976.

336 (1954).

Simultaneous Determination of Manganese, Cobalt, and Copper with a Computer-Controlled Flameless Atomic Absorption Spectrophotometer

Erik Lundberg* and Gillls Johansson

Department of Analytical Chemistry, University of Umed, S-90 1 87 Umed, Sweden

A multisllt atomic absorption spectrometer for simultaneous determlnatlon of three metals has been developed for use with a graphlte furnace. Twenty complete measurements per second were made for each element durlng atomlratlon and these were stored in a mlnlcomputer memory. The transient atomlc absorption signals were corrected for furnace emission and nonspecific absorption. The noise and thus the detection limlts were somewhat higher compared with a Varlan AA6 spectrometer supplled with a CRA 83. The detection llmlts were 2, 20, and 8 pg for Mn, Co, and Cu, respectlvely. Relative standard deviations of 3-8 % were obtained for standard so- lutions. Varlatlon In absorbance with time was studied for the three elements and It was found that Mn had a much sharper curve shape than Co and Cu.

Flameless methods using the graphite furnace atomizer offer unique possibilities for determination of trace metals. The high absolute sensitivity and small sample sizes needed may offer great advantages in some applications. Solid sam- ples can be analyzed either as described by L’Vov (1) or by direct introduction of solids in the microgram range into a graphite tube as will be shown in a future paper. A flameless multielement technique should offer a number of advantages in addition to the obvious one of increasing the output from a given instrument and operator. These possibilities include the use of internal standards, determinations of quotients between elements, and determinations of several trace ele- ments in a unique sample, e.g., a solid biological microsam- ple.

A multielement nondispersive fluorescence system with a

graphite braid atomizer has been described (2) . Owing to the absence of a dispersing element, the background emission interfered for elements with medium or high atomization temperatures. This made the position of the atomizer very critical and many of the advantages of the graphite atomizers were lost. An electrically heated carbon cup has been used for sample introduction into a microwave plasma coupled to a vidicon detector (3 ) . The absolute sensitivity of this system was two to three orders of magnitude lower than that of a normal commercial graphite furnace spectrometer. The main reason seems to be the low sensitivity of the vidicon detector in the uv range compared with a photomultiplier tube.

Several reviews have summarized the development in multielement spectroscopic techniques (4-8) . The approaches include temporal devices in which the elements are deter- mined in a sequential mode, spatial devices with simultaneous determinations in a parallel mode, and multiplex devices like Fourier and Hadamard transform spectrometry (8). The multiplex mode is unsuitable for the present application be- cause of the unfavorable signal-to-noise ratio in the uv region. Multichannel detectors like TV-tubes and solid state diode arrays (6) have been used for multielement spectrometry. These detectors have at present much less sensitivity in uv than photomultiplier direct readers. The only detector type thus giving the desired sensitivity is the photomultiplier tube. Three different arrangements utilizing its advantages appear to be feasible. Namely, the direct reader using one photo- multiplier (9) for each element, the image dissector tube in combination with an echelle monochromator (10, 11) and a sequential multislit spectrometer (12). The latter solution should be simpler and cheaper than the other two, although it is less versatile than the image dissector spectrometer. The

1922 ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976

1 MINICOMPUTER

Flgure 1. Block diagram of computer-controlled multislit flameless AA spectrophotometer

S, . multielement hollow cathode lamp: S2, hydrogen lamp: E, beam splitter: L1, Lp, lenses: F, graphite furnace: PS, lamp power supply: IF, interface; CRT, 2- channel oscilloscope: R, strlp-chart recorder: TTY, teletype: E, concave mirror: T, photomultiplier tube: M, motor: P. photon coupled interruptor module: C, chopper with moving slits: A, adjustable fixed slits

A.

RING 1

UNO a

B .

Figure 2. ( A ) Chopper with three concentric slits and timing holes. The position of the ring readers are marked with x. (B) Three adjustable fixed slits

" J

design of the multislit spectrometer described in this work permits simultaneous registering of several transient ab- sorbance signals using a single photomultiplier tube.

EXPERIMENTAL Detector Optics. A 0.35-m f/6.8 Czerny-Turner monochromator

(model EU 700/E, Heath Co., Benton Harbor, Mich.) was modified by removing the exit folding mirror and the front cover plate, see Figure 1. The focal plane whic is almost flat lies 12.7 mm behind the plane of the front cover. After removing the baffle, a 50-mm portion of the spectrum is available and with the grating used, 1180 linedmm, this corresponds to about 100 nm. By turning the wavelength drive, any 100-nm portion of the spectrum can be utilized. A mask con- taining three 3 X 0.1 mm slits was mounted in the focal plane, see Figure 2, insert B. The slit positions could be adjusted over a distance of 2 mm. A circular disk containing three concentric slits, see Figure 2A, was mounted on the axis of a small dc motor. Both the fixed and the moveable slits were fastened to a monochromator attachment which could be screwed onto the monochromator frame. At the back of the attachment, there is an adjustable collimating concave mirror surface coated with magnesium fluoride (Spindler and Hoyer, f 160 mm, diam. 100 mm). The distance between the fixed slits and the mirror is twice as large as the distance between the mirror and the cathode of the photomultiplier tube (RCA 1P28A or Hamamatsu R106 UH for enhanced uv sensitivity). In this way, a diminished image of the fixed slits is formed on the cathode. The active surface of the cathode was 8 X 25 mm, and the tube was mounted in a horizontal position above the optical axis so that the image just filled up the cathode surface. The motor driven disk contained three concentric slits each covering an angle of 90°. The disk was placed 1 mm out of focus and to compensate for this and also for fine adjustments of the corresponding fixed slit, each concentric slit was made 1 mm wide. This arrangement ascertained that only light from one fixed slit a t a time reached the photomultiplier tube. In addition to the slits, the disk contained holes arranged in two concentric rings for timing the electronics. The positions of these holes when the motor was running were determined by two pairs of light emitting diodesand photodiodes (General Electric photon coupled interruptor module H13A1).

The parts described above were housed in the monochromator at- tachment which was painted black inside. A top cover could be re- moved for adjustments after closing an auxiliary shutter for the photomultiplier tube.

Timing and Electronics. Figure 3 shows the switching of the photomultiplier tube signal t o different channels and Figure 4 the timing pulses over three full turns of the motor driven disk. A pulse derived from the reader of ring 1 in Figure 2 is used as a reference marker. The first pulse shown in Figure 4 thus resets the logic in the

V O L l A O l w+-4

$ W I l C H I I SAMPLE SWllCHCS

AND

HOLD

Figure 3. Schematic diagram of the electronics for switching the photomultiplier tube signal to the mini-computer. x) All the other channels are identically wired

computer interface and turns on the hollow cathode lamp. When the first pulse from ring 2 arrives, it closes switch 1 so that the sample and hold amplifier collects the reading obtained from the photomultiplier tube. The sampling goes on until the next pulse arrives when switch 1 is opened and switch 10 is closed. The third pulse activates the an- alog-to-digital converter (Function Modules, Inc., model 161-12). The fourth pulse opens switch 10 and activates the computer sampling so that the first hotomultiplier reading is stored in the memory. I t also closes switch 2 which starts a similar sequence to transfer the next photomultiplier reading to the computer memory. During the first turn of the disk, the multielement hollow cathode lamp (Varian Techtron Cu, Mn, Co, Cr) is turned on and the atomic absorption reading is made for each of the three elements. When the next turn starts, the hollow cathode lamp is turned off and a hydrogen lamp (Varian Techtron 56-100024-00) is turned on. The background ab- sorption is read a t each wavelength and transferred to the computer via switches 4,5,6 and 13,14,15, respectively. Finally, during the third turn, both lamps are turned off and the emission from the furnace is measured a t each wavelength. Each complete measuring cycle requires three turns of the motor which runs a t about 3600 rpm. A complete cycle takes about 50 ms, thus giving 20 complete measurements per second and element.

One sample and hold amplifier is used for each channel in order to obtain an analog averaging. The resistance R and the capacitance C form a time constant of 0.3 ms. There are, in fact, channels for four elements but only three have been used in the following measure- ments. A two-channel oscilloscope was connected to the input of the

ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976 1923

RING I READER

RING 2 READER

SWITCWES 1 - 9

SWITCWIS 10-11)

n n i l nil 1 1 1 1 1 1 I P I ill 1 1 1 1 1 1 I I

A I D -CONVERSION n n I I I I I P COMPUrER SAMPLINQ n n n P I P I I I

W C - L A M P 1

W 2 - L A M P I I Flgure 4. Sequencing diagram for one complete measuring cycle. A high level indicates that the device is activated

W 0 2 a m a 0 v) m 4

Y

w a a

0.8

0.6

0.4

0.2

0

0.20 0.40 Co,Cu/ng

0.05 0.10 M n f n g

Figure 5. Analytical calibration curves for Mn, Go, and Cu. The wave- lengths used are listed in Table II

analog-to-digital converter. One channel shows the lamp sequence and the other the intensities of the nine channels.

Light Sources. A lamp holder for four hollow cathode lamps and one hydrogen lamp with a “polka dot” beam combiner and lens (Varian Techtron 02-100316-00 and 02-100408-00) was mounted on an optical bench. An additional lens (Varian Techtron) was used to focus the light onto the entrance slit of the monochromator. The power supply was switched by pulses from the computer interface. During the on-periods, a constant individually adjustable current was delivered to the lamps. The meters showed the peak value of the currents. The operating current for the multielement lamp was 20 mA and for the hydrogen lamp 35 mA.

Furnace. A Varian Techtron CRA-63 furnace with an adjustable holder was used. It was provided with a fiber-optic cable which transmitted ir-radiation to a photodiode as described earlier ( I3 ,14) . The output from the photodiode is compared to a voltage delivered by the computer via a digital-to-analog converter. If the actual tem- perature is lower than that corresponding to the value specified by the computer, a thyristor conducts and thereby delivers more power to the graphite tube. When the selected temperature has been reached, the power is reduced to the level necessary to keep the temperature constant. The transformer used is more powerful com- pared with that supplied by Varian to the CRA-63. It can deliver 10 kW and it is therefore possible to raise the temperature more quickly. The heating rate can be adjusted by varying the gain in the temper- ature control circuit.

Computer. An Alpha LSI minicomputer with 8K memory (Com- puter Automation, Inc.) and a teletype were connected via the stan-

dard input-output logic card to the interface module. A twelve-bit A D converter is used for the input from the photomultiplier as described above. Two sense-inputs on the input-output card are connected to the interface module conveying information about the position of the disk. A ten-bit D/A converter is also connected. It can be switched by the computer either to a strip-chart recorder for display of the tran- sient absorbance signals of the different elements or to the tempera- ture control unit to provide a set value. Eight more bits are used for the control of relay functions in the temperature control unit and for indicating lamps.

The unit called “interface” in Figure 1 is the communication center of the system. The timing information from the rotating disk is hardware-wired with standard TTL logic to control the switched lamp power supply, the sample and hold switches, and the data input to the computer. In addition to the control logic for these operations the interface module contains the A/D and D/A converters and some of the relays controlling the heating sequence program.

The program for communication with the interface is written in assembler language. It is necessary to program the input data trans- mission in a high-speed language since about 180 photomultiplier readings must be transmitted each second during atomization. The main program is written in BASIC. Upon a command from the teletype, the sequence is initiated and drying, ashing, and atomization are performed. The temperatures as well as the times for each step are specified in the program.

At the end of the ashing step the Io-values for each wavelength and for both lamps are measured, averaged during 3 s, and stored. During atomization, 180 valueds are transferred to the memory for a period of 3 s. After the atomization step, the emission signals from the fur- nace are subtracted from the atomic and background signals. Then log Zo/I is calculated for the respective signals, and background ab- sorbance is subtracted from the atomic absorbance. Upon a second command from the teletype, the transient absorbance signals from the three elements are fed to the strip-chart recorder, normally a t a rate of five values per second. After the recording, another run can be made immediately. For routine purposes, it is possible to make further calculations on the absorbance values and type the results in the form of concentrations or amounts on the teletype. Calibration curves can be stored. This has not been carried out since only 8K memory was available.

RESULTS Figure 5 shows calibration curves for Co, Cu, and Mn ob-

tained with standard solutions containing all three elements, and Table I shows the means and relative standard deviations. The 40 runs reported in Table I were made in succession. Two ~1 of the standard solution was pipetted into the furnace using a micropipet (Unimetric, 1-10 p l ) . The heating sequence started with drying at about 100 “C for 45 sand was followed by ashing at a controlled temperature of 600 “C for 60 s and atomization at a controlled temperature of 2200 “C for 3 s. I t

1924 ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976

h

Table I. Means and Standard Deviations for Standard Solutions Containing Mn, Co, and Cu

Mn

Amount, pg 20 50 IO 100

Number of analyses 10 10 10 10

Mean peak absorbance 0.161 0.401 0.556 0.138 Re1 std dev, % 5.3 3.6 4.1 3.2

c o

Amount, pg 100 200 300 400 Mean peak absorbance 0.165 0.285 0.399 0.495 Re1 std dev, % 8.1 5.6 5.5 3.1 Number of analyses 9 10 10 10

c u

Amount, pg 100 200 300 400 Mean peak absorbance 0.185 0.344 0.411 0.601 Re1 std dev, % 5.0 4.8 4.3 4.0 Number of analyses 10 10 10 10

Table 11. Comparison of Sensitivities and Detection Limits between Varian AA6 and the Multislit Spectrometer for Mn, Co, and Cu

Multislit Varian AA6 spectrometer

Element Mn Co Cu Mn Co Cu Wavelength, 8, 2195 24013248 2195 2401 3248 Sensitivitya, pg 0.9 3.0 3.0 0.5 2.6 2.3

Detection limit with 3.0 16.0 8.0 8.0 40.0 . . . Detection limit, pg 1.0 9.0 2.0 2.0 20.0 8.0

background correction, pg a 1% absorption.

can be seen that the calibration curve deviates from linearity no more than for a well-designed single-channel atomic ab- sorption spectrophotometer.

Such a curvature, especially a t high lamp currents, arises mainly from differences in line profile between the light source and the absorbing species. The good linearity thus proves that, in this application, it is permissible to place all the slits in a plane, although the focal plane is curved.

Table I1 shows the sensitivities and the detection limits, given as twice the blank values, with and without background compensation. Compared with corresponding measurements with the same furnace on a Varian Techtron AA6 instrument, it was found that this multislit spectrometer has somewhat better sensitivities but higher detection limits for the elements studied. The reason for the higher sensitivities is the higher furnace heating rate. The higher detection limits result from the increased noise level of this instrument. The time sharing, the higher noise and lower intensity of the multielement lamp, and light losses due to constructional compromises, all con- tribute to the higher noise level compared with a single-ele- ment system. When using the background corrector, there is a further increase in noise which affects the detection lim- its.

The noise is f O . O 1 absorbance unit for Mn and Cu and f0.02 for Co. The higher noise level for Co is due to the low lamp line intensity for this element (15) . Throughout, the peak absorbances were measured.

Signal Time Dependence. The multislit spectrometer

Mn

0.4

0 . 2 u m 2 0 U -

0.4 v) I / m 0.2 t a / w

0.6 f

0.5 1 .o 1.5

T I M E I S E C

Figure 6. Absorbance as a function of time for Mn, Co, and Cu

used here offers unique possibilities for a study of certain parameters in the graphite furnace technique. In the normal mode, one measurement is made on each element every 50th ms. By omitting the background and furnace emission mea- surements, this sampling rate can be increased three times.

Figure 6 shows the variation in absorbance with time when atomizing 2 ~1 of an aqueous standard solution containing 50 ppb Mn, 200 ppb Cu, and 200 ppb Co in 0.01 M " 0 3 . The furnace emission has been subtracted from the absorbance values but the background corrector was not used. The at- omization step started from an ashing temperature of 600 OC. The rate of temperature increase was about 1500 "C/s with a final temperature of 2200 "C. The Figure gives information about the highest ashing temperature that can be used without losses. This temperature must be below the temperature at which atomization just starts. The atomization process can be followed in detail and correlated with properties of the metal or its compounds. The temperatures at which the metals have a vapor pressure of 1 Torr are 1300 "C for Mn, 1630 "C for Cu, and 2056 "C for Co. Obviously, the atomization starts much earlier than at 1 Torr. All three curves relate to the same single atomization and the atomic vapors of the three elements must be affected to the same degree by losses from turbulence and gas expansion in th tube. Differences in curve shape must therefore reflect differences in the atomization rate or dif- fusional losses. The described multislit spectrometer should thus be suitable for studies of fundamental processes in the graphite tube.

Figure 6 also shows that the peak for Mn is sharper than for the other two elements, its half-width is around 300 ms. Such a sharp peak will be severely distorted by an ordinary labo- ratory recorder if recorded in real time as shown earlier by Posma et al. (15) .

A higher heating rate gives a higher atomic density in the graphite tube and thus improves the sensitivity. The change in peak height amounts to 35% for Cu, 29% for Co, and 21% for Mn when the heating rate is varied between 800 "CIS and 1740 OC/s. This shows that a reproducible heating rate is important for an accurate result when peak height measuring is used.

DISCUSSION The described system is comparable to a single-element

flameless atomic absorption spectrometer with respect to sensitivities and detection limits. The special advantage of

ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976 1925

simultaneous determinations of several elements will be useful for unique solid samples as well as for fundamental studies of atomization processes.

For general purposes, this technique has certain limitations and, therefore, the spectrometer will mainly be used for special applications. All absorbance measurements have a rather narrow operating range with adequate accuracy. When several elements are to be determined simultaneously, the variations in concentration between the elements are limited and ap- propriate dilution of a sample can be a problem. For a special application, this can be overcome by selecting resonance lines with suitable sensitivities. Besides, the computer can provide different scale expansions for the elements. The combinations of elements are limited by the availability of multielement hollow cathode lamps and by the portion of the spectrum available. With the grating used, this portion amounts to ap- proximately 100 nm. It is not possible to select wavelengths closer than 2 nm because of the design of the slits, As the number of elements increase, the mentioned limitations be- come more pronounced and, therefore, a practical upper limit of five elements is expected.

ACKNOWLEDGMENT The authors thank Svante Jonsson and Lars Lundmark for

help with the construction of the equipment used and Michael Sharp for linguistic revision of the manuscript.

LITERATURE CITED (1) B. V. L'Vov, Talanta, 23, 109 (1976). (2) E. F. Palermo, A. Montaser, and S. R. Crouch, Anal. Chem., 46, 2154

(3) F. L. Fricke, 0. Rose, Jr., and J. A. Caruso, Anal. Chem., 47, 2018

(4) K. W. Busch and G. H. Morrison, Anal. Chem., 45, 712A (1973). (5) R. E. Santini, M. J. Milano and H. L. Pardue, Anal. Chem., 45, 915A

(6) Y. Talmi, Anal. Chem., 47, 658A (1975). (7) Y. Talmi, Anal. Chem., 47, 699A (1975). (8) J. D. Winefordner, J. J. Fitzgeraid, and N. Omenetto, Appl. pectrosc., 29,

(9) R. Mavrodineanu and R. C. Hughes, Appl. Opt., 7, 1281 (1968).

(1974).

(1975).

(1973).

369 (1975).

(10) A. Danielsson and P. Lindblom, fhys. Scr., 5, 227 (1972). (1 1) A. Danlelsson, P. Lindbiom, and E. Soderman, Chem. Scr., 6, 5 (1974). (12) W. G. Elliott, Am. Lab., 2, 67 (1970). (13) G. Lundgren, L. Lundmark, and G. Johansson, Anal. Chem., 46, 1028

(14) G. Lundgren and L. Lundmark, Anal. Chem., in press. (15) F. D. Posma, H. C. Smit. and A. F. Rooze, Anal. Chem.. 47, 2087

(1974).

(1975).

RECEIVED for review March 17,1976. Accepted July 13,1976. This work was supported by grants from the Swedish Work Environment Fund.

Direct Analysis of Metals and Alloys by Atomic Absorption Spectrometry

D. S. Gough

Division of Chemical Physics, CSIRO, P.O. Box 160, Clayton, Victoria, Australia 3168

Metal samples can be analyzed by maklng atomic absorption measurements on atomlc vapors produced from the solld by cathodic sputtering in an argon glow dlscharge. The Pyrex sputtering chamber can be easily interchanged with the flame atomlzer of a conventional atomic absorptlon spectropho- tometer. A dual-modulatlon ampllfler provldes automatlc compensation for background absorptlon and for any variatlon in the Intensity of the atomic spectral lamp. Alloys of iron, aluminum, copper, and zlnc have been analyzed, and the fol- lowlng elements were determined: antimony, berylllum, cad- mlum, chromlum, copper, Iron, magnesium, manganese, molybdenum, nickel, lead, sllver, slllcon, titanium, vanadium, and zlnc. The time per analysis varles from about 2-3 min for brasses and Iron alloys to 5 min for zlnc and 10 for alumlnum alloys. The reproduciblllty depends on the matrlx sputtered and is typlcally fl% for Iron- and copper-base alloys, f2% for alumlnum-base, and f3 % for rlnc-base alloys. Detection limits are in the range 0.0003 to 0.04%.

Atomic absorption methods of chemical analysis are largely confined to the analysis of solutions. This paper reports progress in the development of methods for the direct analyses of metals and alloys in which some of the solid sample is converted into an atomic vapor by cathodic sputtering in an argon glGw discharge. The technique of cathodic sputtering has previously been used in the analyses of samples by ab- sorption (1-7), emission (8-12) and fluorescence spectroscopy (13). This paper is a sequel to an earlier paper in which the sputtering technique was used in conjunction with the fluo- rescence technique (13). In that paper it was reported that

some difficulties were encountered when sputtering aluminum alloys due to a tenacious oxide layer which forms on the sur- face of the metal. With the sputtering chamber then em- ployed, the atomic vapor concentration was too low to permit accurate absorption measurements. It was therefore necessary to use a high-intensity hollow-cathode lamp (14) in conjunc- tion with the fluorescence technique. This paper reports modifications to the design of the sputtering cell which enable aluminum alloys to be sputtered immediately, and which enable higher densities of atomic vapor to be produced so that a simple sputtering attachment can be added to a conventional atomic absorption spectrophotometer in the place of a flame atomizer. Under certain conditions, low levels of background absorption are observed. This background is attributed to agglomerates of atoms which absorb radiation in a broad band through the visible and ultraviolet spectrum. These agglom- erates are formed only when there is a high density of metal atoms in the presence of an inert gas, and are present in most of the work reported here.

All results in this paper were obtained using an amplifier which provides automatic correction for both background absorption and light-source fluctuations. The operation of the amplifier, and the method whereby background correction is achieved are discussed later in this paper,

EXPERIMENTAL Apparatus. A photograph of the apparatus is shown in Figure 1. Sputtering Cell. The sputtering chamber (Figure 2) consists of

a Pyrex tube approximately 15 cm long and 3.5 cm in diameter with quartz windows a t either end. I t has a 5-cm diameter flat in the middle of the top side on which an O-ring seals the specimen to the cell, thus enabling specimens to be rapidly interchanged. An anode and a pumping port are provided, and also a gas inlet tube fitting into a

1826 ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976