Computer-Controlled Atomic Absorption Spectrometer for Measurement of Transient Atom. Populations K. M. Aldous, D. G . Mitchell, and F. J. Ryan New York State Department of Health, Division of Laboratories and Research, New Scotland Avenue, Albany, N. Y. 12201
An instrument for the measurement of transient atom populations such as those produced by microsampling atomization systems is described. It is built up from a dc-operated special line source, atomization unit, photodetector, photon counter and on-line computer. The system measures integrated absorption signals and compensates for drift in lamp intensity and the position in time of the absorption peak. The advantages of integrated absorption measurements are discussed, and results obtained using the Delves cup atomization unit are reported.
During the last few years, several atomization techniques producing transient atomic populations have been introduced into routine atomic absorption spectrometry. Among these are the carbon rod ( I ) , Massmann furnace ( 2 ) , L'vov furnace ( 3 ) , tantalum strip (4, sampling boat ( 5 ) , and Delves cup (6) atomization techniques. These newer techniques are useful because they frequently have much higher atomization efficiencies than conventional pneumatic nebulizer-burner systems. This efficiency gives excellent detection limits for many elements and permits analyses using small sample volumes, typically 1-100 MI. However, these transient atomization techniques are invariably used with standard atomic absorption spectrometers, which were originally designed to measure the stable absorption signals produced by nebulizer-burners, and the signal-processing and readout circuitry used in these instruments is not optimal for transient signal detection. They measure peak absorbance rather than integrated absorbance and thus have some important disadvantages: a ) Peak absorbance measurements are much more susceptible to variations in physical factors, such as cup thickness in the Delves procedure, than integrated absorbance measurements. b) With peak absorbance measurements, choice of time constant is critical. A low time constant will give rapid signal response but with a noisy readout. Conversely, a high time constant will give a smooth but distorted curve. c) With present instruments, peak absorbances must be measured on a chart recorder. Digital display and printer readouts cannot be used, since they must be reserved for the measurement of stable signals. d) With recorder readout systems, it is often difficult to isolate an atomic absorption peak from a preceding, nonspecific absorption peak arising from smoke scatter.
To avoid these disadvantages, we have developed a fully automated atomic absorption spectrometer capable of accurately measuring both peak and integrated absorbances simultaneously. THEORETICAL CONSIDERATIONS Consider the process of atom transfer through the atom cell (the atom cell is the volume in which absorption of radiation occurs). The equation for atom balance within the cell is:
dh'ldt = E - L
(1)
where N is the atom concentration in the cell at time t, and E and L are the number of atoms entering and leaving the cell per unit of time, respectively. Atomization can occur in three stages: buildup, equilibrium, and decay. During the build-up stage, with thermal atomization processes such as the Delves cup and graphite cuvette procedures, E will approximate a linear function of time t. Thus:
E = At (2) The total number of atoms Q entering the cell during the build-up time tl is given by:
Q
=
$"E dt
=
1"At dt
(3)
Integrating and combining with Equation 2 gives:
(4) The rate of loss of atoms is given by:
L = N/t2 where t2 is the average residence time in the cell. Equation 1may be rewritten as: d N - 2Qt
dt t,* which can be integrated (7) to:
(5)
N
t2
Also, the maximum atom concentration in the cell Nmax during the atomization cycle is given: (1) M. D. Amos, P. A . Bennett, K. G . Brodie, P. W. Y. Lung, and J. P. Matousek, Ana/. Chem.. 43, 211 (1971). ( 2 ) H. Massmann, Spectrochim. Acta, 238,215 (1968). (3) 6.V. L'vov, Spectrochim. Acta. 17, 761 (1961). (4) J . Y. Hwang, P. A . Ullucci, S. B. Smith, and A . L. Malenfant, Anal. Chem.. 43, 1319 (1971). (5) H. L. Kahn and J. S. Sebestyen, At. Absorption Newslett., 9, 33 (1970). (6) H. T. Delves, Analyst (London), 95, 431 (1970).
1990
(7) B. V. L'vov, Proceedings, 2nd International Atomic Absorption Spectroscopy Conference, Sheffield, England, July 1969.
A N A L Y T I C A L CHEMISTRY, VOL. 45, NO. 12, OCTOBER 1973
Equation 7 describes the kinetics of atom buildup in the cell, the basic assumption in this equation being that Q a t . This should be approximately true for rapid transient atomization processes which do not approach equilibrium. After time tl, atomization from the sample ceases, and the atom population decays exponentially to zero, according to:
N
=
N,,,
X exp
(it) -
(9)
Figure 1 shows the variation of atom concentration within the cell with time for various atomization parameters. As can be seen, the peak height variation as a result of changes in the atomization time or average residence time of the atoms within the cell is quite large. With transient atomization techniques, the total number of atoms entering the cell (assumed to be proportional to the analyte in the sample) can be measured in a t least two ways: by measuring peak absorbance proportional to the maximum atom concentration PImax,and by measuring integrated absorbance, proportional to integrated atom concentration SO"N dt. Almost all experimental work to date is based on peak absorbance measurements, but these give accurate estimates of analyte concentration only if all of the following conditions are met: a ) Q is constant, requiring a constant degree of atomization; b) t 2 is constant, requiring a stable gas flow pattern and a reproducible chemical environment in the atom cell; and c) tl is constant, requiring a constant rate of sample heating and atom release from the matrix. The first two conditions are likely to be met by most atomizers and many sample matrices. However, the third is much more doubtful, since tl is likely to be affected by a number of physical factors, such as the thickness of the nickel cups for Delves cup atomizers, the stability of heating achieved with the carbon rod atomizer and the L'vov and Massmann furnaces, and the amount of inert matrix material which can physically impede the atomization process. These will change the atomization time tl, causing large changes in peak absorbance readings, particularly when tl < t 2 . Integrated absorbance, however, is proportional to SO"N dt and hence is proportional to Q t 2 . Integrated absorbance measurements will therefore be directly related to an analyte concentration, provided that Q and t 2 are constant; and these parameters are independent of variations in the rate of release of atoms into the atom cell. Integrated absorbance data should therefore be more precise and interference-free than peak height data.
EXPERIMENTAL Instrument Design. Figure 2 shows a block diagram of the components of the prototype instrumental system. The optical layout is t h a t of a typical atomic absorption spectrometer, with all optical components mounted on a Zeiss-type triangular-section optical rail (no. 22-6011, Ealing Corporation, Cambridge, Mass.). Hollow cathode lamp radiation is rendered parallel, using a quartz lens with a focal length of 5 cm (no. 01-100037, VarianTechtron, Walnut Creek, Calif.), and is passed through the absorption tube to be refocused by a similar lens onto the entrance slit of the monochromator. A light baffle with a circular aperture 1 cm in diameter is placed a t each end of the absorption tube to ensure that all the radiation passes through the tube and to'reduce the amount of emission reaching the photodetector from the flame and the glowing silica walls. An aluminum heat screen is also positioned in front of the monochromator to minimize thermal wavelength drift.
'/t2
Variation of atom concentration N in the cell with time
Figure 1.
t. 0 = total number of atoms entering cell; t, = atomization build-up time, average residence time in cell
t2 =
MONOCHROMATOR ABSORPTION TUBE
M I c R~;S
wI
TI
n
I D/A CHART RECORDER
-i 1
PHOTON COUNTER
INTERFACE
K COMPUTER
Schematic diagram of photon-counting, on-line computer, atomic absorption spectrometer Figure 2.
The microatomization system used was a Delves cup unit (Perkin-Elmer, Norwalk, Conn.), comprising a triple-slot burner, absorption tube, and sample insertion device. This system was selected because it gave promising results when used with a conventional atomic absorption instrument for blood lead analyses. The triple-slot burner, which supports the silica absorption tube, was modified to make a gas-tight fit with a Techtron AA5 burnernebulizer unit. The latter fits rigidly onto the optical rail, and minimal movement between the absorption tube and the arm which inserts the sample cups into the flame was ensured by a t taching the microsampling unit to the burner-nebulizer base. This allowed precise vertical and lateral adjustment of the absorption tube in order to optimize absorption signals. A 0.25-meter, f 3.6 monochromator (Model no. 82-410, JarrelAsh, Waltham, Mass.) with fixed 100-j~Mslits was used, with the photomultiplier tube (Model 62568, Gencom/EMI, Plainview, N.Y.) operated in the single photoelectron pulse-counting mode. A radio-frequency-shielded and refrigerated photomultiplier housing was employed (Model TE104, Products for Research, Danvers, Mass.). This shielding prevents the injection of unwanted signals from laboratory equipment; and thermoelectric cooling of the photomultiplier tube to -30 "C reduces the dark count from ca
A N A L Y T I C A L C H E M I S T R Y , VOL. 4 5 , NO. 12, OCTOBER 1973
1991
A TRIGGER
B TRIGGER
CHOPPER SAMPLE TIME
CLOCK
‘TEST’
ARIlHMITIC
CONTROL
INPUT 8 DECADE
ISBIT WORD +PARALLEL OUTPUT TO COMPUTER
COUNTER 0
1
store
! ‘ - - - I f
Does c
Area
-
0
7
‘N’PRISETCOUNTIR
Figure 3.
Block diagram of the photon-counting unit
I 200 to ca. 5 counts per second. (In atomic absorption, where lamp radiation is directly focused onto the detector, lamp intensity is ca. 1 X 106 counts per second, and refrigeration is unnecessary. However, in applications such as atomic and molecular fluorescence, refrigeration will give improved measurements a t low light intensity.) The output of the photomultiplier tube is fed to a combined amplifier and pulse-height discriminator unit (Model 1120, SSR Instruments, Santa Monica, Calif.j . After amplification and pulse shaping, the resulting 1-volt, 10-nsec pulses are fed into an eightdecade BCD photon-pulse counter (Model 1110, SSR Instruments). A power supply (Model 1106) provides stabilized high voltage for the photomultiplier tube and power for the amplifier unit. A block diagram of the counter is shown in Figure 3. In this application only the data channel is used, with the counting period set at 30 msec. The measurement process is initiated when the operator inserts a sample in the flame. This activates a microswitch which generates a programmed delay loop of ca. 1 sec and a series of counter encode commands. The counter records the number of photons detected during the counting period: then the data are read out through the arithmetic unit and output register to an on-line laboratory minicomputer (Model 2114B, HewlettPackard, Cupertino, Calif.). The output register simultaneously updates a digital-to-analog converter with a chart recorder output. The measurement-readout cycle is repeated 200 times with each sample, and the data are stored in memory for subsequent processing. Data Handling. With the Delves cup system, a number of data handling problems must be solved: a ) Unashed biological material gives a large nonspecific absorption peak before the atomic absorption peak, and the former peak must not be read out. Accordingly, the minicomputer is programmed to sift the count-rate data a t the completion of each measurement cycle in order to locate the trough between the two peaks or the change of slope if no trough is present. After atomization is complete, the data are sifted again in order to calculate the background radiation count rate lo. The count rates over the atomization peak are converted to absorbances using the 10value, and the integrated absorbance is calculated. Also, the position of the trough between the peaks or the change of slope (the start of the atomic absorption signal) varies with time because of variations in sample heating rates. b) Lamp radiation intensity (particularly with lead hollow cathode lamps) varies with time, and compensation should be made for this. c) Samples and sample cups are occasionally grossly contaminated by metals from air particulates, sweat, and other sources. These cause gross positive errors which should be rejected. d ) Microsampling procedures are inherently less precise than macro procedures. Stringent quality controls should be maintained, and an adequate number of replicates should be analyzed. To meet all of these requirements, even the most sophisticated methods of manual data handling would be too slow and unreliable, since they require a delay between data collection and processing. The following on-line automated procedure seems optimal (Figure 4). An initial calibration procedure is carried out, and a correlation coefficient and a least-squares line of best fit are computed. If the correlation coefficient exceeds a preselected minimum value (typically 0.98), calibration is complete. The line equation and corre1992
1-
P O S i tion
I n p u t Sample d
Cali
Subroutine ‘ ‘ ~ a t a ’ ’
Print
Figure 4.
Lead Conc.
Basic computer program flow diagram
D = delay time. N = number of readings per measurement, C = number of calibration standards, R = correlation coefficient
lation coefficient are printed out, and the operator can proceed with the analysis. Otherwise, an error message “Recalibrate” is printed out, the fault is diagnosed, and the calibration is repeated. Replicate samples are then analyzed. There is very little extra work involved in carrying out, say, triplicate analyses rather than single analyses, since the time-consuming measurement and computational steps are automated. The mean signal levels are computed, and signal values >2070 above that range are rejected. Such values are well outside the range of statistical variation that could be expected for uncontaminated samples. Finally, the corrected mean signal and metal levels are computed and printed out.
DISCUSSION AND RESULTS Cup Quality Effects. The effects of variances in cup quality were evaluated using new nickel cups from Perkin-Elmer for lead analyses. After initial burning off to remove trace lead contamination, cups were stored in a desiccated, airtight cabinet, since particulate matter would soon accumulate inside the cup if it were left uncovered in the laboratory. ( A set of 30 cups left out overnight gave contamination levels equivalent to 10 ng of lead.) Most cups could be used about 30 times before signs of deterioration were obvious, but for acid digestates the number of analyses per cup was reduced. Cup deterioration can be detected by observing the trough position relative to the start of the measurement cycle, thin cups showing a shorter elapsed time than new cups. Deterioration also shows as a gradual change in sensitivity when peak measurements are being used and explains the necessity to grade cups as “sensitive” 6r “insensitive” before use (6, 8, 9). For integrated absorbance measurements, grading of cups is not necessary, since the only parameter which changes is the atomization rate. (8) R E Ediger and R L Coleman At Absorptfon Newsiett 1‘1, 33 (1972) (9) F J Fernandez and H L Kahn, At Absorption Newsiett 10, 1 (19711
A N A L Y T I C A L CHEMISTRY, VOL. 45, NO. 12, OCTOBER 1973
Table I. Comparison of Peak Absorbance and Integrated Absorbance Measurements for the Determination of Lead in Potable Water Sample No. 1 2 3 4
Recorder traces for the determination of 50 pg/l. lead in potable water samples
Figure 5.
However, after extensive usage the cups develop pinholes in the bases, with consequent loss of sample and erratic results. For routine operation, the condition of each cup should be checked after 30 runs, and it should be replaced before 50 analyses have been made. The silica absorption tubes must be checked frequently for cracks and replaced after about 100 hr operation. Occasional losses in sensitivity have been traced to tube deterioration, particularly of the inner surface. Daily cleaning to remove powder deposits will reduce the rate of sensitivity loss. Sample Matrix Effects. To illustrate the effects of sample matrix on peak absorbance and integrated absorbance measurements, a series of potable water samples, spiked to contain 50 pg/l. lead, were analyzed. Samples (200 pl) were dispensed into cups, dried a t 140 "C for 10 minutes and determined at 217 nm. Cups were not graded and had previously been used for these analyses. Several cups had slight deposits on the inner surface from nonvolatile water solids. Some typical recorder traces obtained for these samples are shown in Figure 5, and corresponding peak absorbance and integrated absorbance results are shown in Table I. Integrated absorbance measurements give a precise estimate of lead levels, despite the variation in signal shape. This can be attributed to the changes in rate of atomization, which cause profound changes in peak height values. Provided the whole analyte is vaporized out of the cup and the same amount of atomization occurs, integrating over the absorption peak will give a more precise measure of the total quantity of analyte. This simple example indicates the necessity of integrating absorption signals from transient atomization devices where the peak absorbance is considerably affected by several physical parameters.
CONCLUSION The instrumental system described in this paper has several major advantages for microsampling techniques:
5 6 7
a 9 10
Mean Std dev % Re1 std dev
Integrated absorbance, arbitrary units 9.7 10.8 10.3 9.4 9.9 9.1 9.8 10.6 10.0 10.4 10.0
0.5
5
Peak absorbance 0.41 0.38 0.31 0.62 0.19 0.18 0.18 0.44 0.25 0.31 0.33 0.14 43
a ) The system measures integrated absorbances. Theoretically this should always give equal or better precision than peak absorbances, and for lead determinations in potable water samples it gives greatly improved precision. Comparable performance with conventional instruments can only be obtained, if ever, by very careful standardization of sample cups, cup treatment, and cup history prior to analysis. b) The system is fully automated and substantially removes error sources arising from operator bias. c) The system has extensive, built-in quality control checks, which are vital for microsampling techniques. The system has been successfully used for routine blood lead analyses with the Delves cup technique (10). We propose to use it for the determination of other trace metals with alternative atomization techniques.
ACKNOWLEDGMENT We wish to thank Thomas Moran for his help in the interfacing of the photon counter and associated electronics to the computer. Received for review November 22, 1972. Accepted May 17, 1973. This work was supported in part by General Research Support Grant 5 SO1 RRO5649-04 from the National Institutes of Health. (10)
D. G. Mitchell, K. M . Aldous, and F. J. Ryan, unpublished work, Albany, N. Y . , 1972.
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