Anal. Chem. 1981, 53, 1175-1179 Dautartas, M. F.; Manri, K. R.; Evans, J. F. J. Electroanal. Chem. 1980, 110, 379. Dautartas, M. F.; Evans, J. F. J. Electroanal. Chem. 1980, 109, 301. Stargardt, J. F.; Hawkridge, F. M.; Landrum, ti. L. Anal. Chem. 1978, 50. ... 930. . Heineman, W. R.; Yacynych, H. J.; Alexander, M. Anal. Chem. 1980, 52,345. Kaufman, F. B.; Engier, E. M. J. Am. Chem. SOC.1979, 101, 547. Kaufman, F. 8.; Schroeder, A. H.; Engier. E. M.; Dramer, S. R.; Chambers, J. Q. J . Am. Chem. SOC. 1980, 102, 483. Yacynych, A. M.; Kuwana, T. Anal. Chem. 11978, 50, 640. Betteiheim, A.; Chan, R. J. H.; Kuwana, T. J. Electroanal. Chem. 1980, 110, 93. Van De Mark, M. R.; Miller, L. L. J. Am. Chem. SOC. 1978, 100, 3223. Miller, L. L.; Van De Mark, M. R. J. Electroeanal. Chem. 1978, 88, A37
(39) Miiier, L. L.; Van De Mairk, M. R. J. Am. Chern. SOC.1978, 100, 639. (40) Kerr, J. B.; Miller, L. L. J. Nectroanal. Chem. 1979, 101, 263. (41) Kerr, J. B.; Miller, L. L.; Van De Mark, M. 13. J. Am. Chem. SOC. 1980, 102, 3383. (42) De Grand, C.; Miller, L. L. J. Am. Chem. SOC.1980, 102, 5728. (43) Lewis, N. S.; Bocarsiy, A. 8.; Wrighton, M. S. J. Phys. Chem. 1980, 8 4 , 2033. (44) Wrighton, M. S.; Austin, R. G.; Bocarsly, A. El.; Bolts, J. M.; Haas, 0.; Legg, K. D.; Nadjo, L.; Paiazzotto, M. C. J. Eir,,troanal. Chem. 1978, 87, 429. (45) Wrighton, M. S.; Paiazzotto, M. C.; Bocarsiy, ,A. B.; Bolts, J. M.; Fischer, A. B.; Nadjo. L. J. Am. Chem. SOC.1978, 100, 7264. (46) Wrlghton, M. S.; Austin, R. G.; Bocarsly, A. El.; Bolts, J. M.; Haas, 0.; Legg, K. D.; Nadjo, L.; Palazzotto, M. C. J. Am. Chem. SOC. 1978, 100, 1602. (47) Bolts, J. M.; Wrighton, M. S. J. Am. Chem. SOC. 1978, 100, 5257. (48) Bolts, J. M.; Bocarsiy, A,. B.; Palazzotto, M. C.; Walton. E. G.; Lewis, N. S.; Wrlghton, M. S. J. Am. Chem. SOC.1979, 101, 1378. (49) Bolts, J. M.; Wrighton, M. S. J. Am. Chem. SOC.1979, 101, 6179. (50) Flscher, A. B.; Kinney, J. B.; Staley, R. H.; Wrighton, M. S. J. Am. Chem. SOC.1979, 101, 7863. (51) Bocarsly, A. B.; Walton, E. G.; Bradley, M. G.; Wrighton, M. S. J. Electroanal. Chem. 1979, 100, 283. (52) Bookbinder, D. C.; Lewls, N. S.; Bradley, M. G.; Bocarsiy, A. B.; Wrighton, M. S. J. Am. Chem. SOC.1979, 101, 7721. (53) Bocarsiv. A. B.: Waltori. E. G.:. Wrighton. . M. S. J. Am. Chem. Soc. 1980, ib2, 3390. (54) Ghosh, P. K.; Splro, T. G. J. Am. Chem. SOC. 1980, 102, 5543.
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(55) Hill, J. M.; Royce, D. G.; Fadley, C. S.; Wagner, L. F.; Grunthaner, F. J. Chem. Phys. Lett. 1978, 44, 225. (56) . . Fadlev, C. S. J. Electron Spectrosc. Relet. Phenom. 1974, 5 , 725 and references cited. (57) Mehta, M.; Fadiey, C. S. Chem. Phys. Lett. 1977, 46, 225. (58) Wagner, L. F.; Hussain, 2.; Fadiey, C. S.; b i r d , R. J. Solid State Com21. 453. .mun. .. W??. .- . . (59) Ansell, R. 0.;Di‘ckinson, T.; Povey, A. F.; Sherwood, P. M. A. J. Electron Spectrosc. Relet. Phenom. 1977, 1 1 , 301. (60) Mason, D. C.; Mintz, D. M.; Kuppermann, A. Rev. Scl. Instrum. 1977, 48, 926. (61) Clark, D. T.; Dilks, A.; Shuttleworth, D.; Thomas, H. R. J. Electron Spectrosc. Relet. Phenom. 1978, 14, 247. (62) Fraser, W. A.; Fiorio, J. V.; Deigass, W. N.; Robertson, W. D. Surf. Sci. 1973, 36, 661. (63) Reilman, R. F.; Mesezane, A.; Manson, S. T. J. Nectron Spectrosc. Reliit. Phenom. 1978, 8 , 389. (64) Cooper, J.; Zare, R. N. J. Chem. Phys. 1988, 48, 942. (65) Hail, J. L.; Slegel, M. W. J. Chem. Phys. 1988, 48, 943. (66) Scofiekl, J. H. Lawrence Livermore Laboratory Report No. UCRL51326, Jan 1973. (67) Carter, W. J.; Schweitrer, G. K.; Carison, T. A. J. Nectron. Spectrosc. Relet. Phenom. 1974, 5 , 827. (68) Penn, D. R. J. Electron Spectrosc. Relat. Phenom. 1978, 9 , 29. (69) Fischer, A. B.; Wrighton, M. S.; Umafia, M.; Murray, R. W. J. Am. Chem. SOC.1979, 101, 3442. (70) Moses, P. R.; Wier, L. M.; Lennox, J. C.; Finklea, H. 0.; Lenhard, J. R.; Murray, R. W. Anal. Chem. 1978, 50, 576. (71) Lapeyre, G. J.; Smith, R. J.; Anderson, J. J. Vac. Scl. Techno/. 1977, 14, 384. (72) Smith, T. W.; Kuder, J. E.; Wycheck, D. J . Polym. Scl. 1978, 14, 2433. (73) Roiison, D. R.; Umana, M.; Burgmayer, P.; Murray, R. W. Inorg. Chem., in press. (74) Wiiiman, K. W., University of North Carolina, unpublished results, 1980.
.
RECEIVED for review January 28, 1981. Accepted April 13, 1981. This research was supported in part by a grant from the National Science Foundation. The XPS equipment was purchased with the assistance of grants from the National Science Foundation and the North Carolina Board of Science and Technology.
Microprocessor-Based Data Acquisition System for Chemiluminescence Measurements D. F. Marlno and J. E). Ingle, Jr.” Department of Chemistty, Oregon State University, Corvallis, Oregon 9733 1
An automated discrete sampling chemilluminescence (CL) measurement system based on a KIM 6502 microcomputer Is described. This system allows for different modes of data acquisition and calculation, dark signal correction, and injection of the last reagent. The performance of the measurement system is evaluated with three different chemical systems to determine the effect oni signal to noise ratio (S/N) of peak height vs. peak area data acquisition with an anaiog-to-digital converter (A/D), peak area data acquisition with a voltage to frequency converter (V/F) and up/down counter (UDC), and peak area data acquisition with a photon counter (PC) and UDC. There is no significant difference between peak height and peak area S/N at 25 OC. A factor of 5 improvement In S/N is obtained under dark shot noise limited conditions using a cooled (211 K) photomultiplier tube with PC peak area detection as opposed to V/F peak area detection. Dark current shot noise is limiting near the detection limit, while sample flicker noise Is limiting at higher anaiyte concentrations.
Solution chemiluminescence (CL) meawrements are often made with a discrete sampling design (I--5)in which the CL
reaction is initiated by injection of the last reagent with a resultant peak shaped signal vs. time profile. The discrete sampling CL photometer used in our laboratory has been described in detail (1-4). It consists of a sample module containing a standard l-cm cell which is temperature controlled with provision for stirring. Analyte and reagent solutions are added with Eppendorf pipets and the reaction is initiated by injection of the final reagent with an electronic pulse activated pneumatic syringe. The CL radiation a t all wavelengths is collected by the photomultiplier tube (PMT) detector and the instantaneous CL signal is displayed on a recorder after current-to-voltage conversion and filtering. A great deal of manual interaction was required in the interpretation of these peak shapes. Only peak height data could be conveniently acquired from a recorder tracing, and the data for the individual runs had to be averaged and statistics worked out “off-line” with a calculator, increasing the total time of analysis and restricting the options of the operator t~ the most rudimentary of data acquisition techniques. Although analog peak height and area circuitry was constructed in the past (I), problems with dark current base line drift prevent routine use of this circuitry. In addition, manual activation of the circuitry and “off-line” data processing were still necessary ( I ) . To realize fully the potential of the discrete
0003-2700/81/0353-1175$01.25/00 1981 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 8, JULY 1981
sampling method, we incorporated a microprocessor control and data acquisition system into the simple system described above to allow the operator the versatility of software modification of the CL experiment and to permit instant “on-line” CL data processing. The remainder of this paper will focus on the basic design of the KIM 6502 microprocessor system, associated peripherals, and software that make up the KCLOS versatile data acquisition system. This system is capable of acquiring and processing (1) analog peak height and peak area data with a 12-bit A/D, (2) analog peak area data with a voltage-to-frequency converter (V/F) and three-byte up/down counter (UDC), and (3) photon counting (PC) peak area data with a modified Ortec PC unit and UDC. Data obtained from three typical CL chemical systems with KCLOS will then be presented and discussed in terms of the relative merits of peak height vs. peak area detection and analog peak area vs. PC peak area detection and the limiting noise sources.
EXPERIMENTAL SECTION Instrumental Methods. The heart of the developed control system is the single-boardKIM microcomputer produced by MOS Technology (Norristown,PA). The features and operation of this computer are documented in detail in several manuals (6) and will be reviewed in brief here. The KIM is based on the 6502 eight-bit microprocessor and is equipped with (1) 14 1/0 bits that are bidirectional and individually programmable, (2) a cassette tape interface for program and/or data storage, (3) a 20-mA current loop Teletype (TTY) interface, (4) 1K static RAM, (5) an interval timer, (6) a 2K ROM operating system, and (7) a hexidecimal keypad and display. The KIM is, in fact, designed for the home hobbyist and has three principal limitations. First, the TTY/CRT interface hardware is inadequate for use with a TTY, since the TTY side of the interface consists of reed relay switches,that provide a great deal of contact “bounce” which the KIM interface simply cannot tolerate. The TTY interface works with CRTs and electronic printers and keyboards. Second, the onboard 1K RAM and 1/0 capabilities of the KIM are usually insufficient for serious experimental applications. Third, no provision is made on the KIM board for the addition of ROM or EPROM containing important operating programs (BASIC). This requires the operator to load these very long programs from cassette tape, a tedious affair. Fortunately, one may overcome all of these limitations by the addition of two external (off the main KIM board) devices: (1) an opto-isolated TTY interface, and (2) a commercially available 8K memory, I/O, and EPROM expansion board. These devices and peripheral modules to be described were constructed by using printed circuit technology. All resistors are W with 5% tolerance and all capacitors are 50-V ceramic disks unless otherwise noted. All peripheral modules were encased separately with dedicated power supplies. The opto-isolated l T Y interface board is based on the isolation of both the l T Y send and receive 20-mA lines with TI 111optical isolators. The TTY interface board works reliably up to 1200 BAUD and details of the design are available from the authors. Expansion of the KIMs memory and 1/0 capability was accomplished with a commercial 8K RAM expansion board, Memory Plus, purchased from the Computerist (S. Chelmsford, MD), and documented in detail in their publication (7). Briefly, the features of this board include (1)8K of static RAM, (2) a 6522 versatile interface adaptor (VIA) chip which provides 16 more bidirectional 1/0lines, two 16-bit timers, a serial shift register, and versatile handshaking and interrupt capabilities, and (3) sockets and address decoding for 8K of EPROM (2K X 8, Intel 2716 or equivalent). The EPROM was loaded with an 8K floating point BASIC program by Microsoft supplied by Johnson Computer (8) (Medina, OH). The unbuffered address lines of the KIM were also buffered with Tri-State buffers for direct memory access (DMA) applications, not used for this work, and is discussed elsewhere (9). The KIM, opto-isolated TTY interface, and Memory Plus boards were securely mounted in an aluminum case and provided with a small fan for cooling. The power supply requirements for this KIM microcomputer system were 8 V unregulated dc at ca. 3 A, 12 V dc at ca. 500 mA, and 5 V dc at ca. 1A and were supplied
CLEAR
0C3
-
EC2
DATA EC 1
I
DATA
J PULSE TRAIN IN
@
DOWN/UP
BNC
Flgure 1. Three-byte multiplexed updown counter: IC1-IC6, 74193 binary up/down counter; IC7-IC12, 74126 TRI-STATE buffer; IC13IC15, 7408 AND gate; IC16, 7404 Inverter; C1 22 pF electrolytic.
by a KL Power Supply Model 512 (Montgomeryville, PA). For communication with the KIM, an inexpensive keyboard printer module was constructed from a commercial ASCII keyboard, a commercial 32 character per line electrostatic printer and printer drive board, and a UART (Universal asynchronous receiver and transmitter). This device was connected to the KIM 20-mA serial TTY interface and is described in detail elsewhere (10). Several peripheral modules were constructed for data acquisition with the KIM. For the most part, these modules operate in conjunction with the KIM 1/0 ports. The analog-to-digital converter (A/D) is based on an Analog Devices (Norwood, MA) AD7501 one of eight analog multiplexer and an Analog Devices AD 574 8- or 12-bit A/D. The device is capable of (1) selecting one of eight analog channels for digitization, (2) performing an 8-bit conversion in 16 p s or a 12-bit conversion in 25 p s , and (3) transferring the resulting data into the KIM using eight 1/0lines and internal multiplexing to deliver the high eight and low four bits of the conversion in two transfers. A total of 15 1/0lines are required to operate the module: eight for the multiplexed data, one for the 8/12 mode control, one for start conversion, one for the end of conversion signal handshake, one for the analog multiplexer enable, and three for analog channel selection control. Either a unipolar 0 to +10 V range or a bipolar -10 to +10 V range are switch selectable. More detail on the hardware and software necessary to use this device and manipulate the raw data into a form compatible with the KIM is available from the authors. A 24-bit counter board was constructed for counting experiments in which the KIM controls the counting time for a pulse train directed to 6 cascaded up/down binary counters. The counter board can be used in conjunction with photon counting circuitry or with a voltage-to-frequencyconverter (V/F). For all analog measurements, a Teledyne Philbrick (Dedham,MA) Model 7405 V/F converter was used which provides 100 kHz/V up to 10 V. The use of six up/down binary counters provides a large count capacity (P3= 8.39 X lo6) and the ability to perform a hardware subtraction of “dark” counts from “signal” counts, creating a net signal counter. Finally, with 24 bib of data to move into the KIM, some method of 1/0line minimization is necessary if all of the KIM 1/0 lines are not to be tied up. The module pictured in Figure 1was constructed to fulfillthese purposes and is a three-byte multiplexed up/down counter (UDC). Careful analysis of Figure 1will reveal that a total of 14 I/O lines are required for the UDC: eight for data, three for the Buss
ANALYTICAL CHEMISTRY, VOL. 53, NO. 8, JULY 1981
1177
-1
h
L- --
C 5A
FIRST STAGE ECL COUNTER
- --- - - ORTEC9315 S C A L E R A "
:IRCUIT
TTL
To
UDC
E C L l T T L CONVERTER
Flgure 2. ECL/TTL conversion circuit: " I C I , IC2, MC10105 OR gate; IC3, MC10198 ECL monostable; IC4, MC1039 ECL/TTL converter; "IC5A, MC10131 ECL D flip-flop; 131, R2, R3, 1.5 kQ; R4, 400 Q 10-turn pot; R5, R6, 1 kQ; C1, 47 pF ceramic; D1, 1N4305; "Ortec's designation.
Control logic which will be described later, cine for the down/up control, one for the gate control, and one for clearing the binary counters. Thus the KIM can control the direction of the count, gate the pulse train into the UDC, and clear all of the counters by using 1/0 bits in conjunction with IC13-IC16 and the clear pins of the 74193 counters (IC1-IC6). Once the KIM has gated a pulse train and allowed a count to accumulate, the data are transferred into the! KIM through eight 1/0bits by employing TRISTATE multiplexing of the data lines. IC7-IC12 are under the control of three KIM 1/0 bits, Buss Control (BC) 1,2, and 3. When BC1 is high, IC11 and IC12 are enabled and data bits 0-7 are presented to the KIM 1/0 port for transfer into memory. When BC2 is high, IC9 and IC10 are enabled and bits 8-15 are presented, and when BC3 is high, IC7 and IC8 are enabled and bits 16-23 are presented. With a machine language program, a 24-bit number can be transferred to KIM memory in 48 ~ s . As mentioned, the UDC is very useful for data acquisition in counting experiments such as photon counting (PC). Unfortunately, most commercial photon counting units are not designed to interface directly with external transistoptransistoplogic (?n) counting circuitry since they employ ECL (emitter coupled logic) devices for the f i i t few stages of PC due to tht?rapid rate of arrival of photons at high count rates. Since the Ortec (Oak Ridge, TN) Model 9315 photon counter in our laboratory consisted of separate preamplifier, amplifier/discriminator, and ECL count/display modules, it was felt that minor modification of the ECL count/display module would allow one to use the Ortec essentially as a pulse amplifier and discriminator, while the ECL photon count was converted to ?TI,levels and directed to the UDC. Since the UDC is under KIM control, this process would essentially provide one with a computer-controlled photon counter. Figure 2 illustrates thie modifications made to the Ortec count/display module, along with the device constructed to adapt the rapidly responding ECL signals to slower, TTL compatible signals. This ECL/TTL converter module was carefully constructed on a ground pliine PC board and placed in a case grounded aluminum box. Power for this device was acquired via regulation of the Ortec +12 and -12 V supplies to +5 V and -5.2 V with an LM340T-5 and an LM320T-5.2, respectively. The output of IC2 in the Ortec 9315 scaler A circuit (11)was tapped and carefully brought out on 50-0 coax cable to the adaptor unit, consisting of an MC10198 ECL monostable (6C3) and an MC1039 ECL/TTL level convertem (IC4). The pulse width of IC3 is adjusted to 100 ns minimum with R4 in order to ensure TTL compatability. IC4 then converts the -0.9 V ECL signals to +5
Flgure 3. Block diagram of three modes of chemllurninescence data
acquisition.
-
M times
M times
Inject, collect N points in T ms of CL signal
S h i f t in net peak area d a t a
Process data ond print net peak heights, areas, mean peak height, area; u peak height, area
Process data and print peak areas, mean peak area, v peak a r e a
Flgure 4. Software flow diagram for different data acquisition modes.
V TTL signals, and the -1.75 V ECL signals to 0 V TTL signals for output to the UDC. For KIM control purposes, the maximum photoelectric count rate with the UDC is 10 MHz. This is more than adequate for investigations requiring PC detection in the first place-Le., low light level experiments with count rates of less than about lo5 photoelectrons/s. Figure 3 is a block diagram of the complete microcomputerbased CL instrument which illustrates the three modes of op-
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 8,JULY 1981
eration. Figure 4 shows a simplified flow chart of the software. More detailed flow diagrams and listings are available from the authors. Using a single operating program, the investigator has a choice of taking analog data with the 12-bit A/D or of performing a counting experiment using either a V/F and standard “analog” PMT equipment with the UDC previously described or the modified PC apparatus described earlier in conjunction with the UDC. In both types of experiments the KIM, on operator command after the necessary reagents are added to the cell, (1)takes dark current data for a time T, (2) injects the final CL reagent, (3) takes CL signal data for a time T, and (4) performs all calculations with the data after M runs have been made providing the operator with instant feedback on reproducibility,calibration curve linearity, etc. The programs construction is that of a BASIC “skeleton” for interaction and sophisticated computation, with an extensive machine language subroutine network for 1 / 0 control and data acquisition. The two are connected by BASIC USR, PEEK, and POKE commands. The KCLOS program occupies about 6K of RAM leaving about 2K for data storage and manipulation. This allows room for over 500 data points per run in the A/D mode (each data point corresponds to a double precision dark and signal value). In the A/D mode, the peak height is found by searching for the largest signal value while peak areas are found by simply summing the N points collected over time T. The net peak height and area are found by subtracting the mean value and sum of the N dark current points from the maximum signal point and the area, respectively. In the counting mode, the 3-byte number proportional to the net CL peak area is transferred from the UDC to RAM after each run. Subtraction of the dark current area is performed by counting down during acquisition of the dark current signal and counting up during the CL peak. The “on-line” calculation of means and standard deviations and dark current subtraction results in a reduction in total measurement and data workup time to about 20% of the time needed with the manual system. Furthermore, incorporation of a microprocessor also makes it easy to utilize further data reduction techniques (e.g., least-squares construction of calibration curves) through simple software changes and to incorporate increased automation by addition of new hardware. Measurement Procedures. For evaluation of the KCLOS versatile data acquisition system, the humic acid (HA)-Mn04-, Cr(V1)-lophine, and Co(I1)-lophine CL systems (3,12,13) were employed. It was felt that the peak shapes obtained with these CL systems are representative of the different types of peaks commonly obtained in discrete sampling CL, Le., a fast (30 s) peak with a blank, respectively. Each system was studied by using the analog mode of KCLOS (with the A/D converter), and the S/Ns and detection limits obtained were compared for peak height and peak area data for two different cutoff frequency settings of the CL noise filter. A critical comparison between peak height and area data acquisition has not been made by any investigator to date. In addition, the HA CL system was studied by using the V/F and photon counting peak area data acquisition mode of KCLOS. Studies were conducted with a 1P28 PMT at ambient temperature (293 K) and at a low temperature (211 K) in order to evaluate the S/N and detection limit benefits of cooling the PMT to reduce the dark noise for a CL system whose detection limit is limited by dark noise. The CL instrumental configurations were as illustrated in the discussion of KCLOS (Figure 3). A 1P28 PMT was used throughout at a potential of -680 V for the analog mode study and at a potential of -lo00 V for the V/F-photon counting study. All CL measurement parameters (concentration, mixing orders, etc.) were exactly as previously described (3, 12, 13). The general procedure followed for the analog data acquisition study was as follows. Five runs were first made without opening the PMT shutter to evaluate the standard deviation of the difference between two dark runs for M = 5. Following this, five blank runs were made to establish the blank signal level (if any) and the standard deviation between blank runs. Finally, standards were run at or near their detection limits and the average net CL peak height and area calculated, along with the standard deviation of the peak heights and areas for M = 5.
Table I. Comparison of Data Acquisition Modes for Humic Acid Chemiluminescence
mode
AID 0.5 Hza
1 Hza
S / N height 7.0 S / N area 3.5
PC
VIF 1.0 3.0
1.0
3.0
sb
sb
sb
sb
5.3 3.6 0.76
4.6 5.5 5.5 height, 0.57 mg/L c1 area, 1.1 1.1 0.88 0.73 0.73 mg/L a Cutoff frequency. b Integration time. c,
6.1
0.66
Table 11. Comparison of Peak Height and Peak Area Calculation of Lophine CL Signal with Co(I1) and Cr(V1) 0.2 pg/L Co(11) S / N height S / N area c, height, pg/L c,
area. U ~ I L
0.5 pg/L
Cr( VI)
8.7
1.1
6.5 0.045 0.061
1.6 0.91 0.63
Table 111. Comparison of V/F Analog and Photon Counting Data Acquisition of Humic Acid CL Data with and without Cooling of the PMT 293 K
VIF PC
211 K
S/N
c , , mg/L
1.5
2.6 2.2
1.8
S/N 4.2 19
c,, mg/L 0.95 0.21
The procedure followed for the V/F and photon counting study was the same as outlined above, with the exclusion of any peak height data. Cooling of the PMT was accomplished with a liquid Nz cooled housing in a manner previously described (14).The temperature of the PMT was accurately determined with an LX5600 (National Semiconductor) solid-state temperature transducer in conjunction with a digital voltmeter as previously described (14). Desired temperatures were reached and stabilized by varying the flow rate of the Nz cooling gas passing through the liquid Nz and PMT housing.
RESULTS AND DISCUSSION Acquisition Mode Studies. Table I presents the results of a comparison of A/D, V/F, and photon counting data acquisition for a 2.0 mg/L HA sample. A/D data are for a lOO-point, 1s acquisition as a function of the noise fiiter cutoff frequency while V/F data are for a 0.5-Hz noise filter cutoff frequency as a function of gate time. A 1.0-s gate time is sufficient to allow the great majority of the HA peak to be counted, while a 3.0-5 gate time allows one to look a t virtually all of the peak, including the “tail” region where little signal is available. As can be seen, there is no significant difference in detection limits or S/Ns between A/D peak height or area calculation, or the mode of data acquisition for peak area calculation. Table I1 presents A/D data for a 0.2 pg/L Co(I1) sample and a 0.5 pg/L Cr(V1) determined by lophine CL. A total of 100 points were collected in both cases with 10 and 1s data acquisition times for Co(I1) and Cr(VI), respectively. The data indicate that for both slow and fast peaks with blanks, peak height and peak area calculation yield essentially the same detection limits. A significant detection limit improvement is seen for an HA CL determination using a cooled PMT. Table I11 presents a S/N comparison between V / F and PC peak integral determination a t 293 K and 211 K for a 2.5 mg/L HA sample,
Anal. Chem. 1981, 53, 1179-1182 2 0-
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//SLopEso09
