long would have required 722,800 bytes to be completely stored, or almost exactly three times as much storage. Ninety per cent recognition for the time invested to check on the 240,240 possible orders was quite acceptable. The obvious extension of this technique would be to use a much larger data set to create the table described above. Clearly, most of the 240,240 entries were 0 for the 2800 spectra training set. The more members from which the table was generated, the more information the table would have contained. In addition, the table would always require the same amount of storage regardless of the number of spectra from which it was generated. Although generation of the table would be a time consuming process for larger data sets, once the table was generated, the search of the entries for the appropriate prediction would require no more time than that required in the present experiment. CONCLUSIONS The recognition results obtained using a Bayesian approach were considerably better than the 7% figure expected from random guessing. Bayesian approaches are not used very often, usually because of the difficulty of obtaining p ( A ( C ) , known as the a priori probability. However, in a situation such as the one described here, it is quite easy to estimate the a priori probabilities from the training set. Given that p (A( C) is obtainable, the Bayesian
approach can then be viewed merely as a formalization of common sense ( 6 ) .If the order 1, 2, 3, 4, 5 most frequently occurs when a compound belongs to class 1, then it is common sense to predict a spectrum giving that same order is of an acid. As was mentioned, the most likely next step for this type of approach would be to use very large training sets incorporating fewer restrictions. Also, the fourteen classes selected for this study would most likely not be the ones selected for a thorough investigation of a large data set. When using a table generated from a large data set, if the percentage of recognition still was near 90%, then one would have a quick and accurate means of predicting the type of compound without having to search large numbers of spectra. This study demonstrated the feasibility of employing a Bayesian approach for the classification of infrared spectra, but varying degrees of utility should be found when using discriminant functions generated from other types of data. ACKNOWLEDGMENT The authors are indebted to J. C. Marshall for assisting in the formation of the data set.
RECEIVEDfor review April 11, 1974. Accepted August 19, 1974. The financial support of the National Science Foundation is gratefully acknowledged.
Multielement, Nondispersive Atomic Fluorescence Spectrometry in the Time-Division Multiplexed Mode E. F. Palerrno,’ Akbar Montaser,2 and S. R. Crouch3 Department of Chemistry, Michigan State University, East Lansing, Mich. 48824
A new nondispersive multielement atomic fluorescence (AF) technique, which operates in the time-division multiplexed (TDM) mode, is capable of analyzing 4 to 8 elements in a flame in less than 3 seconds. The technique is rapid enough to allow multielement determinations on a transient atom population, such as that produced from a nonflame atomizer. The system employs computer-controlled pulsed hollow cathode lamps, a sheathed burner or nonflame atomizer, and a computer-controlled synchronous integrator as well as computerized data acqulsltlon and processing. The effects of sheath gas flow rates and burner position on AF signals and flame background are reported for the analysis of Hg, Cd, Zn, and Pb. Detection limits obtained with a 4-channel TDM-AF flame spectrometer are compared with those obtained with a conventional sequential, dispersive multielement AF system. The results Indicate that the TDM system can achieve excellent detection limits for certain elements. Detection limits are also reported for a 3-channel TDM, nonflame AF spectrometer and compared to those obtained by a single-element, dispersive technique. Present address, E. I. duPont de Nemours & Company, Experimental Station, Bldg. 269, Wilmington, Del. 19898. Present address, Ames Laboratory-USAEC and Department of Chemistry, Iowa State University, Ames, Iowa 50010. Author to whom requests for reprints should be addressed. 2154
In recent years, there has been an increasing demand in the analytical laboratory for instrumentation capable of performing multielement analysis. Several workers have shown that atomic fluorescence (AF) spectrometry is suitable for multielement determinations in the ppm or subppm concentration range ( 1 - 4 ) . The features of AF spectrometry that make it a useful technique for multielement analysis include the low detection limits which can be achieved for many elements, the specificity of resonance fluorescence, the ease with which an array of narrow line sources can be focused on an atomic vapor cell to excite atomic fluorescence, and the relative simplicity with which the fluorescence intensities of the various elements can be separated. Multielement AF systems have been described in which the fluorescence intensities of the emitting elements have been separated in time, with a rotating filter wheel ( I , 5 ) , with a scanning monochromator (2, 6-10), or with a se(1) (2) (3) (4) (5) (6)
(7)
D. G.
Mitchell and A. Johansson, Spectrocbim. Acta., Part E, 25 175 (1970). G. B. Marshall and T. S. West, Anal. Cbim. Acta, 51, 179 (1970). H.V. Malmstadt and E. Cordos, Amer. Lab., p 35, August (1972). E. Cordos and H. V. Malmstadt, Anal. Cbem., 45, 425 (1973). D. . Mitchell and A. Johansson. Spectrocbim Acta, Part E, 26 677 (1971). A. Fulton, K. C. Thompson, and T. S.West, Anal. Cbim. Acta, 51, 373 (1970). M. S. Cresser and T. S. West, Anal. Cbim. Acta., 51, 530 (1970).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 14, DECEMBER 1974
quentially programmed monochromator (3,4). The separation could also be made spatially with a direct reading spectrometer ( l l ) ,a photodiode array (12, 13),or a vidicon detector (14, 15). With time-separated AF systems, a popular approach to multielement analysis has been to use low duty-cycle pulsed hollow cathode lamps as sources and a synchronized wavelength sorting device (1, 3-5). With these systems, however, much time is lost because multiple pulses of fluorescence radiation are collected for one element, before the next element is excited. In the system described by Cordos and Malmstadt (3, 4 , 16, 1 7 ) , for example, one hollow cathode lamp is pulsed for a preset number of cycles, the monochromator is scanned to a new wavelength, the next lamp pulsed for a preset number of cycles, etc. Because of the specificity of AF spectrometry, it is possible to eliminate the wavelength dispersive device when narrow line sources are employed for excitation. Indeed several authors (18-24) have developed single channel nondispersive AF systems, which use either broadband filters or solar blind photomultiplier tubes to reduce scattered light and emission by the atomic vapor cell. With a nondispersive AF system and low duty cycle pulsed hollow cathode lamps, it is possible to interweave pulses from several other hollow cathode lamps during the OFF time of any one lamp, which reduces the analysis time for N elements by a factor of -1/N. The technique of transmitting data from several sources on the same line by the time interlacing of data is called time-division multiplexing (25, 26) and is commonly employed in communications systems. In this paper, a nondispersive, multielement AF technique, which operates by time-division multiplexing is introduced. The system is capable of analyzing 4 to 8 elements in a total analysis time of less than 3 seconds. Because of the interlacing of pulses from several hollow cathodes, this AF system is rapid enough to allow multielement analysis of a transient atomic vapor cloud, such as that produced from a nonflame atomizer. In the sections to follow, the principles of TDM systems are first introduced to give a perspective into the advantages and limitations of the technique. Then the instrumentation required for a multichannel AF spectrometer which operates in the TDM mode is described along with experimental parameters for operating the sources and atomizers. Detection limits obtained with the TDM-AF spectrometer are presented and compared to results obtained with single element AF techniques and other multielement AF methods. J. D. Norris and T. S. 'West. Anal. Chim. Acta., 55, 359 (1971). b i d , 59, 474 (1972). J. D. Norris, and T. S. West, Anal. Chem., 45, 226 (1973). D. W. Golightly, ,R. N. Kniseley, and V. A. Fassel, Spectrochim. Acta., P a r t 4 25, 451 (1970). G. H. Horlick and E. G. Codding. Anal. Chem., 45, 1490 (1973). P. W. J. M. Boumans and G. Brouwer, Spectrochim Acta., Par? 8,27, 247 (1972). M. Margoshes. Spectrochim. Acta., Part 6, 25 113 (1970). D. G. Mitchell, K. W. Jackson, and K. M. Aldous, Anal. Chem., 45, 1215A (1973). E. Cordos and H. V. Malmstadt, Anal. Chem., 44, 2407 (1972). /bid., 45, 27 (1973). T. S. West and X . K. Williams, Anal. Chem., 40, 335 (1968). P. L. Larkins, R. M. Lowe, J. V. Sullivan, and A. Walsh, Spectrochim. Acta., Part& 24, 187 (1969). T. J. Vickers and R. M. Vaught, Anal. Chem., 41, 1476 (1969). P. D. Warr. Talanta, 17, 543 (1970). T. J. Vickers, P. J. Slevin, V. I. Muscat, and L. T. Farias, Anal. Chem., 44, 930 (1972). R. C. Eiser and J. D. Winefordner, Appl. Spectrosc., 25, 345 (1971). P. L. Larkins, Spectrochim. Acta., Part B, 26, 477 (1971). P. F. Panter. "Modulation, Noise and Spectral Analysis," McGraw-Hill, New York, N.Y., 1965, pp 548-589. M. R. Aaron in "Modern Filter Theory and Design," G. C. Temes and S. K. Mitra, Ed., Wiley-lnterscience, New York. N.Y., 1973, pp 425-431.
TRANSMISSION
I c I - 0
D
MULTIPLEXER SWITCH
PATH
''J
DEMULTIPLEXER SWITCH
0
INFORMATION SOURCES
INFORMATION RECEIVERS
a)
B b)
Flgure 1. 4-channel time-division multiplex system a ) General diagram. 6) waveforms in the transmission path for pulse amplitude modulation
PRINCIPLES OF OPERATION Time Division Multiplexing. Time-division multiplexing is a common technique for the transmission of multiple channels of electrical information (25, 26). A general 4channel TDM system is illustrated in Figure la. The multiple channels of information are switched into a common transmission path by a multiplexer (selector switch) with each channel alloted a separate time slot. For analog electrical information sources, the multiplexing operation produces a transmitted signal that consists of interlaced pulse trains of identical repetition rate. Each pulse train is amplitude modulated by the sampled value of the information signal in one channel. Other pulse modulation schemes, such as pulse duration modulation and pulse code modulation can also be used, but pulse amplitude modulation is most common and easiest to implement. Figure l b shows a typical waveform observed in the transmission path for a pulse amplitude modulation (PAM) scheme. At the receiving end of the TDM system, a demultiplexer switch, synchronized to the multiplexer switch, separates the signals and sends them to separate information receivers. When PAM is employed, a low pass filter is generally employed a t the information reception site to extract the information signal from the modulated pulse train. TDM systems have several advantages when compared to the other common multiplex technique, the frequencydivision multiplex (FDM) method. First, the instrumentation for a TDM system is normally much simpler than that for an FDM system (25).Second, crosstalk among channels is usually rather easy to minimize with the TDM technique. The only requirement for negligible interchannel crosstalk is that the transmission system must have sufficient bandwidth and linearity in order to prevent the pulse waveforms from overlapping into adjacent time slots. Two common optical spectrometric techniques which use frequency-division multiplexing are Fourier Transform Spectrometry and Hadamard Transform Spectrometry. These techniques have the advantage of observing all spectral components simultaneously (multiplex advantage). However, the recovery of each information channel in an FDM system is much more complex than demultiplexing in a TDM system. Multichannel Atomic Fluorescence Spectrometry. For atomic fluorescence spectrometry, time-division multiplexing can be achieved in the optical domain by pulsing several hollow cathode lamps in a low duty cycle mode, but out of phase. Figure 2 shows a general block diagram of a computer-controlled-4-channel TDM atomic fluorescence
ANALYTICAL CHEMISTRY, VOL. 46, NO. 14, DECEMBER 1974
2155
Table I. Experimental System for Atomic Fluorescence Description and type
Component
Sources
Hollow cathode discharge lamps
Source power supply
Model EU-703 -70 hollow cathode power supply, modified (See Ref. 1 6 ) Premixed, air/H, modified (See Ref. 2 4 ) See Refs. 28 and 29
Burner Nonflame atomizers Lenses
One-inch d i a m e t e r , planoconvex and biconvex quartz lenses R 166 Solartblind
Photomultiplier tube Photomultiplier power supply Current -tovoltage converter Synchronous integrator Minicomputer
Model EU-42A high voltage power supply Model 427 c u r r e n t amplifier
P D P L a b 8,/e
I
I
I
i
I
I
I--___
FLAME OR NONFLAME ATOMIZCR
L---
-
SOLAR BLIND PMT
F i s h e r Scientific Co., Pittsburgh, Pa.
Esco Optics Products, 171 Oak Ridge, Oak Ridge, N.J. Hamamatsu Corp., Lake Success, N.Y. GCA McPherson, Acton, Mass. Keithley Instruments, Cleveland, Ohio
Digital Equipment Corporation, Maynard, Mass.
I
i
F i s h e r Scientific Co., Pittsburgh, Pa. GCA McPherson, Acton, Mass.
See Figure 5
MINICOMPUTER
I
I
Supplier
-
CURRENT TO VOLTAGE CONVERTER
1
-
I 1
I
SYNCHRONOUS INTEGRATOR
hold time of the integrator, an analog-to-digital converter (ADC) produces a digital output of the integrator. Demultiplexing of the information signals is accomplished by the minicomputer which directs the ADC outputs for each information channel into separate locations in core memory. The cycle of multiplexing, transmission, transduction, and demultiplexing is then repeated for a preselected number of times (usually 20) to improve the signal-to-noise ratio in each information channel. With the present system, 4 elements can readily be determined in less than 2 seconds and 8 elements in less than 3 seconds with flame atomization. With nonflame atomization, more time is required because it is necessary to program the heating of the atomizer to desolvate, ash, and atomize the sample.
Block diagram of computer-controlled multichannel AF spectrometer
INSTRUMENTATION
spectrometer. Each hollow cathode lamp is focused on a flame or nonflame atomic vapor cell. The minicomputer controls the circuitry for pulsing each hollow cathode lamp so that the ON and OFF times of all lamps can be controlled by software for optimization purposes. If a nonflame atomizer is utilized, the computer also controls the programmed heating of the atomizer (27). The fluorescence radiation from each element is transmitted through the same optical path, but separated in time. The optical signals thus appear as amplitude modulated, time-division multiplexed pulses of radiant energy. A solar blind photomultiplier tube (PMT) is used to transduce the radiant energy pulses into electrical current pulses. An analog integrator, synchronized to the time slot of each channel under computer control, integrates and holds the P M T output from each fluorescence pulse. During the
The TDM-AF spectrometer was operated under the control of a PDP lab 8/e minicomputer (Digital Equipment Corp., Maynard, Mass.) which contained a 10-bit ADC. For all interfacing, a minicomputer interface system (Model EU-gOlE, Heath Co., Benton Harbor, Mich.) was used with appropriate circuit cards. The software was written in assembly language (Macro-8). The remaining components of the system are described in Table I and below. Hollow Cathode Pulsing Circuitry. The circuitry for operating the hollow cathodes in a pulsed mode was nearly identical to that described by Cordos and Malmstadt (16, 17). However, the computer was utilized to control the ON and OFF times of each lamp rather than a hardware sequencing system. This information was transferred through four channels by the computer interface shown in Figure 3. The computer was programmed to provide input-output instructions 6311 to 6341 with a software-selected time pattern. With a PDP 8/e computer which includes a KA8-E external interface, octal 6 in bits 0-2 signifies an input-output transfer. The middle six-bits of the instruction word are device address bits, while bits 9-11 control the generation of input-output timing pulses (IOP's). Thus the I/O instruction 6311 indicates an 1/0 transfer to or from the device at address 31 and generates IOP1. The device select signals 31 to 34 were connected to the Data Latch Card (EU-800-FA, Heath Co.) inputs, and the corre-
Flgure 2.
(27) Akbar Montaser. Ph.D. Thesis, Michigan State University, East Lansing, Mich. 46824, 1974.
2156
ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 1 4 , DECEMBER 1974
TO HOLLOW CATHODE LAMPS
pEEp
ON
ON
+ .
u TIME
TIME
LAMP A
p---O F F TIME
-
6311 6151 -13
LAMP B
MI4
-1s
I
MIh
!Ill
I
DELAY
TIME
n 6321 6151
n
n
LAMP C
6331 6151
n
n
LAMP D
6341 6151
I
d
Figure 4.
- u
DEVICE SELECT IS
Figure 3. Computer interface
DATA LATCH CARD
for pulsed hollow cathodes
I
sponding Data Latch Card outputs provided ON control signals for the radiation source FET switches A, B, C, and D and the integrator, to be described later. The I/O instruction 6151 was used to turn the O N lamp OFF. In order to transfer this information, the Latch Card required two gating signals. The Device Select signals (31-34) were also connected to OR gate 1 and its output along with Device Select 15 was sent to OR gate 2. The output of OR gate 2, and IOPl were utilized as the gating signals for the Data Latch Card. This arrangement provided great versatility in optimizing the O N and OFF times, which was particularly important when measuring transient signals from nonflame atomizers. For both flame and nonflame atomization, a duty cycle of 1/16 was utilized. The ON times of the lamps were 2-10 msec for both systems. These times were often varied to account for different atomization rates for different elements with nonflame atomizers. The delay time between pulses for successive lamps was usually three times the ON time. A typical sequencing diagram for a 4-channel system is shown in Figure 4. Atomizers. Both flame and nonflame atomizers were employed. To reduce background emission for the nondispersive AF system, a separated air/Hz flame sheathed with argon was employed. The burner was similar to that designed originally by Larkins (24). Two stainless steel plates were fabricated to mount directly on a Jarrell-Ash pre-mixed burner. Fuel, oxidizer, and sample droplets pass through an inner circular array of holes in the top plate. Argon or nitrogen sheathing gas enters the bottom plate through Tygon tubing, mixes in the canal, and passes through an outer circular array of holes in the top plate. As the flow rate of the sheath gas increases, the flame separates into its primary cone and its secondary zone. The effect of the sheath gas is described in a later section. For nonflame atomization, either a platinum loop (28) or a graphite braid (29) atomizer above an Ar gas sheath (28) was employed. Computer control of the programmed heating of the atomizer was employed (27). A 4-111 sample was placed on the atomizer by syringe or by an automatic sampler (27).A low current was applied to the atomizer to vaporize the solvent. This was followed by a high current to atomize the sample. The lamps were operated only during the atomization step. Synchronous Integrator. To average the AF information present during each time slot, an integrator operated in synchronism with the firing of the hollow cathode lamps was employed. The integrator and its computer interface are shown in Figure 5. The output of a current-to-voltage converter (see Figure 2) was connected through a F E T switch to the OA voltage integrator. A FET switch was also employed to discharge the capacitor in the integrator feedback loop. The timing pulses from the computer, which cause each lamp to turn ON, also control the FET switch a t the input of the integrator. The sequence of operations for one cycle is shown in Figure 6. When the first lamp is turned ON, the integrator is also turned ON for a preset time (typically 2-10 msec). The lamp is then turned OFF by a command from the computer, and the integrator input is opened, which causes the integrated signal to be held for a preset time (typically twice the lamp ON time). During the integrator hold time, the ADC is enabled, and the resulting digital signal is stored in one location area in core memory. The integration capacitor is then discharged, and a program-controlled delay time (usual(28) S R. Goode, Akbar Montaser, and S. R. Crouch, Appl. Spectrosc., 27, 355 (1973). (29) Akbar Montaser, S. R. Goode, and S. R. Crouch, Anal. Chem., 46, 599 (1974).
Sequencing diagram for 4 pulsed hollow cathodes
DATA LATCH CARD
-
16 17
Figure 5.
27SE
FROM CURRENT
TO ADC
TO VOLTAGE CONVERTER
28ST
Synchronous integrator and its computer interface
LAMP
LAMP
A
B
ON
A + B+C+D
OFF
FET DISCHARGE
SWITCH INTEGATOR OUTPUT
ADC PERIOD
Figure 6.
ON
OFF
LAMP C
LAMP
n
n
n
D
n
n
7Mr-In
n
-n
n
Sequencing diagram for one complete operating cycle
ly three times the lamp ON time) is begun. Then lamp 2 and the integrator are turned ON, and the digitized signal for the second elel ment is stored in a second location area in core memory. The above process is repeated for the remaining lamps, which ends the first data cycle. Thus, for a 4-channel system, the result of one cycle is the storage of 4 digital representations of the AF information in separate core memory areas. The process is then repeated for a preset number of cycles (usually 20), which represents one experiment. Background subtraction was carried out by the computer. A separate background determination was run prior to the analysis on a water blank and the result stored in the computer. The computer then enters an integration routine, and the integrated AF signal for each lamp, after background subtraction, is printed on the teletype. Under program control, the entire process can be repeated for a preset number of experiments, after which average values and standard deviations are calculated. In the case of nonflame atomization, each experiment corresponds to a repetition of the processes of sample deposition onto the atomizer, desolvation, atomization, and acquisition of the AF information. Sequential, Dispersive AF System. For comparison of the nondispersive TDM-AF system to a sequentially-operated, dispersive AF system, a monochromator (Model EU-700, GCA McPherson, Acton, Mass.) was inserted into the system, and the hollow cathodes were pulsed in a manner similar to that described by Cordos and Malmstadt (16, 17). One hollow cathode was pulsed for a preset number of cycles, the monochromator was scanned to a new wavelength, the next lamp pulsed for preset number of cycles, etc.
RESULTS AND DISCUSSION Prior to obtaining analytical data for the TDM-AF system, a variety of preliminary studies were undertaken. Because the TDM system is a nondispersive system, atomizer parameters such as positioning with respect to the observa-
ANALYTICAL CHEMISTRY, VOL. 46, NO. 14, DECEMBER 1974
2157
m
c
3 K >
4
?,,
K
t a 4 >
3
cm
t z :
t
I
s
$
3 u
1
0
BURNER POSITION. INCHES
IO
10
30
FLOW RATE, L/MIN.
Figure 7. Lead fluorescence intensity and flame background as a
function of distance between top of burner and bottom of monochromator entrance slit 0 lead fluorescence, 0 flame background
Figure 9. Lead fluorescence intensity and flame background vs.
sheath gas flow rate in dispersive mode 0 lead fluorescence, Ar sheath gas; 0 lead fluorescence, N2 sheath gas; 0 flame background, Ar sheath gas
v)
t 3
>
a
E
i
d
I
*
6
BURNER POSITION, INCHES
Figure 8. Zinc fluorescence intensity and flame background vs.
burner position 0 zinc fluorescence, 0 flame background
tion window and sheath gas flow rates are critical for obtaining high signal-to-noise ratios. Since one of our primary objectives throughout this investigation was to compare the TDM approach to the sequential, dispersive approach, optimization studies and analytical data were obtained with both modes of operation. Atomizer Parameters. Flame Atomization. Among the parameters studied with flame atomization were the position of the burner with respect to the observation window, and the flow rate of sheath gas in both dispersive and TDM systems. In atomic fluorescence spectrometry, the major noise source at the limit of detection is either the flame background or dark noise (24), depending on the flame used and the wavelength of interest. With a nondispersive system, flame background is the major noise source. To reduce this noise, the tip of the primary cone should be lowered relative to the observation window. However, as the flame is lowered, a point is reached where the analyte fluorescence decreases even more drastically than does the noise 2158
due to the flame background. This is evident in Figure 7 where the fluorescence signal for an aqueous solution of 50 ppm lead and the flame background at 283.3 nm, (spectral bandpass of 2 nm) are plotted as a function of the distance between the top of the burner and the bottom of the monochromator entrance slit. In the case of zinc, the decrease in fluorescence signal is not as severe as is shown in Figure 8. The position profiles for cadmium and mercury were similar to that of zinc. A compromise burner position of 2 inches was chosen for multielement analysis on the basis of Figures 7 and 8. Profiles could not be determined for the nondispersive case because the flame background was too intense in the higher positions. Burner position profiles obtained in the dispersive mode dictated that the burner should be placed as close to the observation window as possible for maximum fluorescence. However, in the nondispersive mode, the burner should be positioned as far away as possible from the observation window to reduce the flame background. Therefore, a compromise had to be made. The burner was positioned at 2 and 5.25 inches below the observation window in the sequential, dispersive and TDM modes, respectively. The type of sheath gas has many effects on the measurement of the fluorescent intensity. Sheathing can reduce quenching by atmospheric nitrogen, constrict and control the height of the visible portion of the flame, cool the primary cone, increase the atomization efficiency, render the concentration of atoms in the flame more uniform, and separate the flame into its primary cone and secondary zone by preventing the unburned hydrogen from reacting with diffused atmospheric oxygen. The sheath gases that were employed for this study were argon and nitrogen, although argon was eventually used for all analytical data. To determine the effect of sheath gas flow rate on the fluorescence intensity and the background signals, data were taken at a burner position of 2 inches in the dispersive mode. The results for lead with both argon and nitrogen sheathing are shown in Figure 9. Note that the fluorescence intensity decreases in both cases with increasing sheath gas flow rate because the temperature decreases. Note also that the fluorescence intensity decreases more with nitrogen sheathing probably because of the enhanced temperature
ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 14, DECEMBER 1974
q LL
0 3
I.d
20
40
\
.
ARGON FLOW RATE L/MlN
\\'\
Figure 10. Hg, Zn, and Cd fluorescence intensity vs. flow rate in dispersive mode A Hg fluorescence, 0 Zn fluorescence,
Cd fluorescence
20
30
decrease and quenching. The background is significant at 283.3 nm, and also decreases with increasing sheath gas flow rate. Flow rate profiles for mercury, zinc, and cadmium are shown in Figure 10. The fluorescence intensity decreases in the case of mercury, while the AF signals for zinc and cadmium pass through a maximum with increasing sheath gas flow rates. The temperature decrease is not as important for these three elements as it is for lead. Although the flow rate profiles changed slightly as a function of burner position, the general shapes were as shown in Figures 9 and 10 for analytically useful flame heights. Flow rate profiles were also obtained in the nondispersive mode. However, measurements could not be made a t low flow rates, because the flame was not effectively separated, and, hence, the flame background was too great. The flow rate profile with nitrogen sheathing for cadmium is shown in Figure 11. Notice the tremendous decrease in background with increasing flow rate, whereas the flow rate has only a slight effect on the fluorescence signal. Similar results were obtained for Hg, Zn, and Pb. Argon sheathing generally proved to be slightly superior to nitrogen sheathing in all cases. Nonflame Atomization. When a filament-type nonflame atomizer is employed for multielement AF analysis, there are also a variety of parameters which must be optimized for nondispersive operation. The atomizer positioning is again critical, particularly for elements which atomize at high temperatures, because of background emission from the atomizer. A compromise must be reached in choosing the atomizer position because the AF signals decrease drastically with height above the atomizer due to condensation or chemical reactions of the analyte vapor. Among the other important parameters which were studied with nonflame atomizers were the sheath gas flow rates, the O N and OFF times of the lamps, and the temperatures for the desolvation and atomization steps. Since these parameters may vary considerably from element to element, compromise values must be chosen in a multielement analysis. Table I1 shows the compromise parameters chosen for the analysis of Zn, Cd, Pb, and Hg. Analytical Results. Flame Atomization. To study the feasibility of the TDM-AF technique for multielement analysis, analytical working curves and detection limits were obtained for several elements. The detection limit was defined as that concentration giving a signal-to-noise ratio (S/N) of 2. For flame atomization, results were also obtained in the sequential, dispersive mode for comparison purposes. As many of the parameters as possible were
40
NITROGEN FLOW RATE.L/MIN.
Figure 11. Cd fluorescence intensity and flame background vs. N2
flow rate in nondispersive mode Cd fluorescence, 0 flame background
Table 11. Parameters Chosen with Pt Loop Atomizer Sheath gas flow r a t e s 2 l./min 10 m m Vertical distance between atomizer and observation window O N t i m e s of lamps 2 msec Duty cycle of lamps 1/16 Delay t i m e between lamp pulses 6 msec Desolvation t e m p e r a t u r e 110°C Atomization t e m p e r a t u r e 1500°C
Table 111. Detection Limits Obtained in Sheathed Air/Hz Flame Detection limit, Peak
sequential
Detection
lamp current,
dispenive
limit, TDM
Element
mA
mode, u g / m l n
mode, wg/ml
Cd Zn Hg Pb
200 215 265 250
0.02b 0.5b 2.0'
20 '
0.02d
0.2d 2.06 5Od
Monochromator bandpass, 2 nm. b Ar flow rate, 20 l./min. No sheath gas employed. d Ar flow rate, 30 l./min.
maintained identical in the comparison. All lamps were operated with a 5 msec ON time and a duty cycle of 1/16. Five experiments were run for each element with 20 cycles of data obtained for each experiment. Burner positions were different in the two modes as discussed previously. One of the advantages of the sequential, dispersive mode is that it allows parameter changes while slewing the monochromator to the new wavelength. Hence, sheath gas flow rates were changed to optimum values for each element in this mode. For the TDM mode, all parameters must remain constant since lamp pulses are interlaced. The results of the comparison for a 4-channel system are shown in Table 111. For the sequential, dispersive system no sheath gas was employed for Hg and Pb analyses as Figures 9 and 10 show that optimum results are obtained without sheathing. For
ANALYTICAL C H E M I S T R Y , VOL. 46. NO. 14, DECEMBER 1974
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Table IV. Detection Limits Obtained with Nonflame Atomization Element
Atomizer
Cd Hg Zn Pb
Pt loop Pt loop Pt loop Pt loop GBA
Pb
& I
i
l
l
Zn fluorescence with TDM, nonflame system. Vertical axis, fluorescence signal arbitrary units; horizontal axis, 0.3 msec per division Figure 12. CRT display of data stored for
the TDM system, all results were obtained at an Ar flow rate of 30 l./min. Except for Pb, the detection limits in the TDM mode are as good or better than those obtained in the sequential, dispersive mode. The total analysis time for one 4-element experiment, averaging 20 cycles, was 1.6 sec in the TDM mode, while approximately 20 sec are required in the sequential dispersive mode for a 4-element analysis ( 3 ). It should be noted that our detection limits in the sequential, dispersive mode are not directly comparable to those reported earlier ( 3 ) , because a different flame was employed as well as a different integration system. Nonflame Atomization. With nonflame atomization because of the rapid atomization of the sample, a multielement analysis by the sequential, dispersive mode is not practical on one sample. However, because all lamp pulses are interlaced in the TDM mode and no monochromator is employed, multielement analysis is highly feasible. For the work reported here, three elements were determined in the TDM mode and results were compared to those obtained by a single-element, dispersive technique. In the TDM mode, digital data representing the AF signal and the time after the atomization step was begun were stored for each element in core memory. A display routine was used to allow oscilloscopic observations of the transient signals before integration of the data. For example, Figure 12 is a CRT display of the zinc AF data from one array. This AF peak was detected in the presence of mercury, cadmium, and lead with all four lamps operated in the time-division multiplex mode. Under keyboard control the data for the other three elements in their respective arrays can be displayed. The points represent the ADC conversions that were made during the integrator hold time. As can be seen, the peak is approximately Gaussian and the full width of the peak is about 1.5 seconds. During these 1.5 seconds, eighteen data points were taken. It can be seen now why the ON time of the lamps is important. For example, the data shown in Figure 12 were obtained with an O N time of 5 msec, and a total period time of 80 msec. For 18 cycles, this corresponds to 1.44 seconds. If the O N time is reduced to 2 msec, period time of 32 msec, at least 41 data points can be taken for each element during the 1.5 seconds that the atoms are in the observation window. The optimum O N time and number of cycles were determined for each element. The combination of O N time and cycle number, of course, determines the amount of time that the atomizer is at its highest temperature and hence the total operation time of the four lamps. 2160
Single -element dispersive mode, mlml
TDM mode, wlml
0.008" 0.5" 0.1" 5 0"
0.005 0.5 2.0
0.005*
...
Monochromator spectral bandpass, 4 nm. spectral bandpass, 2 nm.
...
* Monochromator
The actual time, relative to the start of atomization, that the atoms of each element were in the observation window varied according to the boiling point of the element. Mercury was observed earliest in time, followed by cadmium, zinc, and lead. However, there was an overlap of the peaks which would rule out the possibility of sequential operation. Working curves were determined for these four elements, Cd, Hg, Zn, and P b in the single-element dispersive mode. For these working curves, the atomizer was positioned directly below the entrance slit of the monochromator, and the radiation from the source was focused directly above the atomizer. The detection limits obtained with the Pt loop atomizer are shown in Table IV. For Zn, Cd, and Hg, the detection limits are lower than with flame atomization. This decrease in detection limit occurs because the background noise from the nonflame atomizer is less than the background noise from the flame atomizer. The detection limit for Pb, on the other hand, is higher than the flame detection limit because of the lower temperature of the Pt loop atomizer. The reproducibility of the fluorescence signal was 8-10% when a syringe was used to place the sample on the loop. This may be increased to 4-7% when an automatic sampler is employed (27). Analytical results were also obtained for P b with the graphite braid atomizer (GBA) in the single-element, dispersive mode. As can be seen from Table IV, the GBA detection limit for P b is very much improved because of the much higher temperature of the GBA. Detection limits were then obtained for Cd, Hg, and Zn in the multielement TDM mode with the Pt loop atomizer. These detection limits are also shown in Table IV. The detection limits for Cd and Hg are comparable to those obtained in the single-element, dispersive mode. However, the Zn detection limit is higher. This increase is due to the fact that the atomizer position for all elements had to be lowered relative to the observation window because of emission from the atomizer and scattering of primary source radiation. The scattering occurs because of the divergence of radiation from the pulsed hollow cathode lamps. As the loop is lowered, the temperature decreases drastically. Hence, the atomic population decreases. The nonflame TDM system may be used for multielement analysis. However, the detection limits obtained for certain elements with the Pt loop atomizer are inferior to those obtained for the single-element dispersive case. Work is now under way in these laboratories to use the higher temperature GBA in a multielement, TDM system.
CONCLUSIONS The nondispersive, time-division multiplexed, atomic fluorescence system described in this paper is the first technique which employs interlaced pulses and a single detector to determine various elements in a multiplexed
ANALYTICAL CHEMISTRY, VOL. 46, NO, 14, DECEMBER 1974
mode. The system can also be used in the sequential mode if desired. The principal advantage of the sequential mode of operation is that it allows the operator to change parameters between determinations to ensure that optimum conditions for each element are obtained. The multiplexing mode, on the other hand, has the advantage of a reduced analysis time which allows multielement analysis on a transient atomic vapor plume. However, a compromise has to be made as to the parameters chosen. The TDM method gave detection limits comparable to the sequential, dispersive method only for separated air-HZ and Ar-HZ flames. For other flames, the nondispersive method is inferior because the increase in energy throughput is accompanied by a greater increase in noise throughput.
T o take full advantage of the TDM system, a low background, yet high atomization efficiency atomizer should be employed. A good combination would seem to be a TDMAF system with a graphite filament nonflame atomizer. Work in this area is now under way in these laboratories.
ACKNOWLEDGMENT The authors thank Charles Hacker of the Michigan State University Department of Chemistry machine shop for construction of the lamp holder and burner.
RECEIVEDfor review February 20, 1974. Accepted August 12, 1974.
Precision of Atomic Absorption Spectrometric Measurements J. D. ingle, Jr. Department of Chemistry, Oregon State University, Corvallis, Ore. 9733 1
A detailed theoretical study of the factors which affect the relative precision of atomic absorption (AA) measurements is presented. The theory takes into account shot and flicker noise in various signal and background sources of radiation, readout noise, flame transmission noise, and sampling or atomization reproducibility. In many cases, the general equations can be simplified to limiting expressions if one type of imprecision is dominant. It is shown that the optimum range to make AA measurements depends on what factors limit the precision. Application of the theory to typical AA inslrument systems and also photon counting, non-flame, dualwavelength, and vidicon AA systems is also discussed.
The sources of noise in atomic absorption measurements have been discussed by a number of authors (1-6). Since noise often limits measurement precision, knowledge of the origins of the major sources of noise provides information that can be used for optimization of experimental variables. Knowledge of how the relative standard deviation in the absorbance (u*/A) or the signal-to-noise ratio (S/N) varies with analyte concentration ( c ) is important if standard and unknown solutions are to be adjusted to optimal concentration ranges. It is unfortunate that, of the great majority of abundant papers which deal with atomic absorption spectrometric analysis, only a small fraction present or discuss precision ( i e . , report standard deviations). If precision data are presented, they are often with reference to the detection limit rather than at concentrations or absorbances at which analyses are usually run. Finally, even if substantial precision data are presented, little attempt is made to identify the causes of the imprecision such as the type of noise that limits precision. Such information, if measurements were (1)J. D. Winefordner and T. J. Vickers. Anal. Chem., 36, 1947 (1964). (2)J. D. Winefordner and T. J. Vickers, Anal. Chem., 36, 1939 (1964). (3)M. L. Parsons, W. J. McCarthy, and J. D.Winefordner, J. Chem. Educ., 44, 214 (1967). (4)J D. Winefordner, V. Svoboda, and L. J. Cline, CRC Crit. Rev. Anal. Chem., August 1970. (5) W. Lang and R . Herrman, Optik, 20, 347 (1963). (6)J. D. Winefordner and C. Veillon. Anal. Chem. 37,416 (1965).
made under optimal conditions, would be useful to compare analyses of a given sample by two researchers or on two instruments. A number of workers (1-6) have dealt extensively with the types of noise and S/N near the detection limit and the dependence of the noise sources on various instrumental and chemical parameters. Such work has proved extremely valuable for comparison of AA to other flame techniques and for optimization of measurement conditions a t the limit of detection. The factors which influence the precision at concentrations significantly greater than the detection limit can be different from those factors a t the detection limit. Hence, optimization of experimental variables at the detection limit may not provide optimal conditions a t higher concentrations. The little work (7-11) which has been done to predict how U A / Avaries with A for AA measurements has referred for the most part to the older theories developed for molecular absorption measurements in which only reading errors or errors independent of T are considered, so that a minimum for o*/A is predicted at about 37% T . More recent discussions (12, J3) of molecular absorption have shown that the u*/A us. A curve predicted by the assumption of a constant error in T can be greatly in error if direct absorbance readout is used or if readout resolution is sufficient and noise dependent on T i s significant. Published u*/A us. A data (9, 14-18) for AA indicate that both the magnitude of UAIAand the shape of the UAIA curve depend on the element analyzed, even with the same instrument, and on the instrumental variables chosen for a (7)Walter Slavin, "Atomic Absorption Spectroscopy," Wiley-lnterscience. New York, N.Y., 19_68,pp 66-68. (8)Juan Ramierz-Munoz, "Atomic Absorption Spectroscopy," Elsevier Publishing Co., New York, N.Y., 1968,pp 167-259. (9)H. Khalifia, G.Syehla, and L. Erdey, Talanta, 12, (1965). (IO)J. Ramierez-Munoz, Microchem J., 12, 196 (1967). (11) I. Rubeska and B. Moldan, "Atomic Absorption Spectrophotometry," CRC Press, Cleveland, Ohio, 1969,pp 84-85. (12)J. D.Ingle. Jr., and S. R. Crouch, Anal. Chem., 44, 1375 (1972). (13)J. D.Ingle, Jr., Anal. Chem., 45, 861 (1973). (14)D. R . Weir and R . P. Kofluk, At. Absorption Newslett., 6, 24 (1967). (15)B. Meddings and H. Kaiser, At. Absorption Newslett., 6, 28 (1967). (16)J. T. H. Roos. Spectrochim. Acta, 248, 255 (1969). (17)J. T. H. Roos, Spectrochim. Acta, 258, 539 (1970). (18)J. T.H. Roos, Spectrochim. Acta, 288, 407 (1973).
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