438
Anal. Chem. 1980, 52, 438-444
samples during a n elapsed time of 2 h and 6 h, respectively, for the polychromator and sequential monochromator systems. These time estimates reaffirm a well known fact, namely, that polychromators possess obvious advantages for the routine simultaneous determination of the same set of elements in large numbers of samples of similar composition. However, as stated in the introduction, scanning monochromators offer attractive features for the determination of a far broader range of elements in samples of widely varying composition.
ACKNOWLEDGMENT T h e authors gratefully acknowledge helpful discussions with R. N. Kniseley and W. J. Haas, Jr., during the course of this study, and t h e assistance of M. Tschetter in developing the NaOH fusion procedure. This research was supported by the US. Department of Energy, Contract No. W-7405-eng-82.
(4) Fassel, V. A. Anal. Chem. 1979, 51 1290 A. (5) Barnes, R. M.; CRC Grit. Rev. Anal. Chem. 1978, 7 , 203. (6) Spillman, R. W.; Malmstadt, H. V. Anal. Chem. 1976, 4 8 , 303. (7) Johnson, D. J.; Plankey, F. W.; Winefordner. J. D. Anal. Chem. 1975, 4 7 , 1739. (8) Kawaguchi, H.; Okada, M.; Ito, T.; Muzuike, A. Anal. Chim. Acta 1977, 9 5 , 145. (9) Boumans, P. W. J. M.; Van Gool, G. H.; Jansen, J. A. J. Analyst(London) 1976, 101, 585. (IO) Olson, K. W.; Haas, W. J., Jr.; Fassel, V. A. Anal. Chem. 1977, 4 9 , 632. (11) Winge R. K.; Peterson, V. J.; Fassel, V. A. Appl. Spectrosc. 1979, 33,
206. (12) Roldan, R. Rev. Sci. Instrum. 1969, 4 0 , 1388. (13) Haas, W. J., Jr.; Butler, C. C.; Kniseley. R. N.; Fassel, V. A. Abstracts of the 1977 Pinsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Payer No. 409. (14) Larson, G. F.; Fassel, V. A.; Winge, R. K.; Kniseley. R. N. Appl. Spectrosc. 1976, 30, 384. (15) Winge, R. K.; Fassel. V. A.; Kniseley, R. N.; DeKalb, E.; Haas, W. J., Jr. Spectrochim. Acta, Part B 1977, 32,327. (16) Myers. A. T.; Havens, R. G.; Conner, J. J.; Conklin. N. M.; Rose, H. J., Jr. U . S . Geol. Surv., Prof. P a p . 1013.
LITERATURE CITED (1) Fassel, V. A. Science 1978. 202, 183. (2) Greenfiekl, S.;Jones, I. L.; McGeachin, H. M.; Smnh, P.B. Anal. Chem. Acta 1975, 7 4 , 225. (3) Boumans, P. W. J. M.; De Boer, F. J. Proc. Anal. Div. Chem. Soc. 1975, 12, 140.
RECEIVED for review May 22, 1979. Accepted November 30, 1979. This work was supported by the U.S. Environmental Protection Agency (Interagency Agreement 1AG-EPA-78-DX0146).
Automation of Microsampling Cup Atomic Absorption Spectrometry D. G. Pachuta’ and L. J. Cline Love* Department of Chemistry, Seton Hall University, South Orange, New Jersey 07079
A digidally controlled automatic sampler for microsampling cups and a programmable analog integrator have been developed and tested. Both are key components of the automated system described In thls work. Performance tests using lead as a reference anaiyie show equivalent method variances exist between the automatic sampler and an optimized manual Injector. This automatic sampler is also capable of reliably maintaining precise and rapid cup transfers. Cycle times are 22 s which is an advantage over manual inJectlonmethods. Further reductions in the cycle time of this device are possible. Simulated transient signals applied to the integrator show that it is capable of better than 0.1 % measurement precision. Area calculations Indicate that integration errors are less than 1 %. For best results, peak profiles and areas of real signals must be slmuttaneously recorded. The automation approach taken should result in faster and more efficient trace metal determinations.
The microsampling cup or “Delves” cup approach to sample atomization possesses many of the advantages commonly found in other micro-Atomic Absorption Spectrometry (AAS) techniques. These include minimal sample size requirements, improved sensitivities, and lower detection limits of several key trace metals ( 1 - 4 ) . In addition, this technique is well suited for off-line batch processing schemes, has a fast meal Present address: Department of Chemistry, Kean College of New Jersey, Union, N.J. 07083.
0003-2700/80/0352-0438$01 .OO/O
surement cycle (15-45 s) and is amenable to complete automation. Several applications involving the direct determination of metals in some solid samples have also been reported (5-13). Because of these abilities, highly rapid trace analyses can be performed. An inherent disadvantage of currently available nonflame devices is their relatively slow measurement cycle which typically requires 1 to 2 min for completion. Only the carbon graphite micro-boat ( 1 5 ) and L’vov platform (16) cup (14), systems are capable of batch-processing operations. In many cases then, dissolved samples must be reproducibly positioned within the atomization device a n d undergo separate dry ash-atomize-workhead cooldown cycles. While automatic samplers have minimized errors associated with sample introduction, the major time consuming aspects of these techniques remain. T h e automatic sample injector for microsampling cups which is described in this work was not only designed to eliminate errors associated with manual cup injection, but also to substantially reduce overall analysis times. This is possible because the atomization device, (air/acetylene or NzO/ acetylene flame) always remains “on”, therefore avoiding many of the time consuming steps required in classical nonflame systems. T h e chief functions of this digitally controlled automatic sampler are to maintain a precise and rapid transfer of sample cups from a multiple position tray to the flame, and then to return the cups to their original tray position. Other essential functions currently performed by the automatic sampler include interaction with the system integrator and chart drive control of a strip chart recorder. While these 8 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 3, MARCH 1980
439
b
S
P
h
c
c TO
flUMt
r
,
~tL[
d
L
2
I ,
Figure 1. a-c. Schematic diagram of the automatic sampler. A = slotted 36-position sample tray; B = 10' stepper motor; C = vertical lead screw; D = horizontal lead screw: E = parallel guide shafts: F = flexible coupler; G = loop assembly vertical support: H = pin-vise loop supports; I = triangular loop: J = loop assembly; K, L = reversible lead screw motors; M, N, 0, P = limit switches; Q , R = upper assembly end stops: S = microsampling cup; T = cup lifting platform; U = vertical lifting shaft; V = shaft support; W = main frame: X = adjustable locating pins. (Dust cover assembly not shown for clarity.)
operations are taking place, off-line batch processing schemes can be carried out to further increase method efficiency. A precision analog integrator was also developed for obtaining peak areas of transient type signals. This module was necessary because the microsampling cup method has been reported t o be susceptible to analyte vaporization rate interferences relating to the sample matrix or physical differences between sample cups (17). These problems are most often encountered when an air-acetylene flame is used (18). By measuring peak areas, these vaporization rate interferences can often be effectively eliminated, linear dynamic ranges increased, a n d matrix matching of standards made easier (17-19). T h e relative merits of peak height or peak area measurements, however, depend on a number of sample and instrument variables a n d both elicit important information about the measurement system. Because of these factors, the integrator module measures both peak profiles and peak areas simultaneously on each sample. This information can be sent to a dual pen recorder, or multiplexed in Binary Coded Decimal (BCD) form to a variety of peripheral devices for data logging a n d / o r data reduction. In an earlier approach to automating the micro-sampling cup system, Aldous and co-workers utilized a special photon counting system which required an on-line minicomputer for data acquisition and peak area calculation (17). While this
system did improve the performance, speed, and reliability of this technique, it did not incorporate an automatic sampling device or hydrogen lamp background correction system and could not be readily adapted to conventional AAS instrumentation. The design strategy used in the present work was to develop highly versatile modules which would be compatible with most commercial AA instruments. They are also capable of being interfaced with or controlled by other components (i.e. programmable calculator). Both simulated and real transient signals from lead standards were used to determine the precision and accuracy of the analog integrator. A manual cup injector was used for comparison purposes in order to evaluate the cycle times and reliability of the automatic sampler. The results of these and other performance tests will be discussed.
EXPERIMENTAL Automatic Sampler Design. The automatic sampling device for the microsampling cup technique is shown in Figure 1, a-c. To outline its operation, three basic mechanical movements were needed to transfer cups from a sample tray out to the flame and then return them back t o the same tray position. The first was a precise 10' rotation of the 36-position stainless steel sample tray. This was accomplished with a torque reduced stepper motor (Ledex Model 1690-027). The second motion was vertical and facilitated the transfer of cups between the sample tray and
440
ANALYTICAL CHEMISTRY, VOL. 52, NO. 3, MARCH 1980
Table I. Operating Sequence of t h e Automatic Sampler
Shift sample tray. Lift cup above tray. Return loop t o sample tray area. Lower cup into loop. Transfer loop t o flame. Send “start cycle” command t o integrator. Activate chart drive for preset time ( i f in “record peak or profile” mode). Activate chart drive upon receiving “return” command from integrator (if in “record only peak height” mode). Return loop t o sample tray area upon receiving “end cycle” command from integrator. Send “reset” command t o integrator. Deactivate chart drive ( i f in “record only peak height” mode). Lift cup from loop after preset cooldown time. Transi‘er loop outside sample tray area. Lower cup into tray. Automatic sampler now returns t o Step 1 5’
53
52
c-3
*
54
CF-E
-3 ‘ 9 C M h T E i 92-2 ii
Figure 2. Simplified circuit diagram of the automatic sampler. E-1,2 = optically isolated control lines to/from the integrator module. E-3 = optically isolated external start/stop option. M-1 = stepper motor; M-2,3 = lead screw motors. RLY 1-5 = optically isolated DPDT mechanical relays. RLY-6 = optically isolated triac for recorder chart drive control. Control and timing = digital logic circuit for relay control. Feedback-hold = digital logic control for sequencing operations. S 1-4 = SPDT limit switches on lead screws. 7490 - 7442 = decade counter/decoder driver circuit
cup-holding loop assembly. A 6-inch lead screw mechanism (PIC Model X1-10) powered by a 700 rpm (28-V dc) reversible motor (Hughes Model 4085) carried out this function. The third movement was horizontal involving the transfer of cups between the sample tray and flame area. This was accomplished with a 12-inch custom built lead screw mechanism powered by a 700 rpm (28-V dc) reversible motor (Hughes Model 4085). All mechanical motions were required to be rapid, essentially vibration free and have very smooth acceleration/deceleration properties. From Figure 1, a-c, it can be seen that the loop assembly rode on two parallel guide shafts. A flexible coupler attached the upper loop assembly to the lower lead screw mechanism. Beside reducing vibrational pickup on the loop assembly, this flexible coupler also served to ensure the smooth acceleration/deceleration of this loop assembly and the precise terminal positioning of a cup a t both ends of horizontal travel. The entire sampling program consisted of 9 major steps which are listed in Table I. Control of this sampling program was accomplished by a hard-wired Transistor-Transistor-Logic (TTL) digital logic circuit. A simplified diagram of this circuit is shown in Figure 2. Execution of this nine-step cyclic program by the logic circuit centered around a decade counter-decoder/driver (7490-7442) system which was driven by a feedback-hold network. In this circuit design, only one output line of the decoder/driver (7442) could be in a low logic state a t any given time. This low
Figure 3. Signal processing circuitry of the transient signal integrator. Output points (OUT 1-4) can be sent to a 4.5-digit panel meter with BCD outputs and/or to a recorder. Output points (OUT 1,3) can be multiplexed to the BCD outputs under external control. All amplifiers were Analog Devices Type 184J, 233L, 260K, or 506LH. See Ref. 19 for details
logic output from the decoder/driver was typically used to close an optically isolated Double-Pole-Double-Throw (DPDT) mechanical relay (RLY1-6) which in turn caused the correct motor (Ml-3) to operate in the desired direction. To signal the completion of a discrete mechanical operation a limit switch (S1-4) on either the horizontal or vertical lead screw assembly would typically be triggered. This would generate a single pulse in the feedback-hold network which connected to the input of the decade counter, causing it to increment to the next step. By using a decade counter, the entire circuit would automatically reset a t the end of nine counts or steps so that the entire process was continually repeated. Transient Signal Integrator Design. A simplified circuit diagram of the analog integrator module is given in Figure 3. The signal processing operations start when a novel sample and hold amplifier (OA-2A-B) (29)obtains a representative (low-pass filtered) base-line value and precisely holds this value throughout the integration period. At OA-3 the polarity of this held base-line voltage is inverted so that it can be subtracted out from the original signal at OA-4. Therefore, when utilizing a background correction system, only the net atomic absorption signal is integrated at OA-5. The function of OA-8-9 is to provide a high gain signal detection circuit. This circuit allows the integration amplifier (OA-5) to “turn on” via logic control of FET Switch F4 only when the net signal exceeds a preset threshold value. This circuit is required to prevent base-line noise from being integrated and therefore increase the signal-to-noise ratios for peak area measurements. At the end of the integration period, F E T switch F-6 is closed momentarily which results in the peak area signal being displayed on a strip chart recorder (via buffer amplifier OA-6) in the form of a bar graph. The final peak area value is also displayed as a latched 4.5-bit digital number on the module’s front panel meter. Control of the integrator measurement cycle is carried out by a hardwired digital logic circuit. Provisions were also made for the external programming of all integrator functions in the event that this mode became desirable in future applications. The integrator measurement cycle could be started by a digital signal received from the automatic sampler. At this point the integrator would also send a “hold” command to the automatic sampler which prevented it from withdrawing a cup from the flame until a measurement cycle was complete. Both of these functions were carried out via optically isolated control lines. Alternatively, a measurement cycle could also be started by a microswitch mounted on a manual cup injector or by a front panel toggle switch. Apparatus. The atomic absorption instrument used for system evaluation tests was a Varian Model 1200 equipped with a BC-6 background corrector. Both the burner mount and burner positioner support rails had to be modified to accommodate a Boling triple-slot burner head (Perkin-Elmer)fitted with a ceramic absorption tube (Trace Metal Instruments, Delmar, N.Y.). The
ANALYTICAL CHEMISTRY, VOL. 52, NO. 3, MARCH 1980
burner-nebulizer was equipped with a Varian dual input port sidearm, through which both acetylene and pressurized air entered the burner chamber. At the nebulizer input port, an auxiliary air pressure connection was made. The Boling triple-slot burner head was mated to the Varian burner chamber without modification. To accommodate the gas flow rates required for the Boling burner-microsampling cup apparatus, a modular gas box (Perkin-Elmer Model 303) was used. This was necessary because the Varian instrument was incapable of providing an adequate air flow rate for this type of burner head. A critical requirement of this burner system was that deionized water had to be aspirated a t all times when the burner was fitted with a ceramic absorption tube. Without water aspiration, the burner slots on the Boling burner head would overly expand, thereby choking off gas flows through them resulting in a burner flashback. With proper water aspriation, however, the burner system is quite stable. The manual cup injector used was a commercial unit (Perkin-Elmer) fitted with a Single-Pole-Double-Throw (SPDT) microswitch. This switch would trigger the start of a measurement cycle when a cup was injected into an air-acetylene flame. A pin-vise type of loop support and triangular nichrome cup holding loops (Trace Metal Instruments, Delmar, N.Y.) replaced the corresponding Perkin-Elmer assembly. The microsampling cups used throughout all series of experiments were either the solid nickel, solid nichrome, or axial holed nickel types (Trace Metal Instruments, Delmar, N.Y.). A dual pen strip chart recorder (Linear Instruments Mcdel132) was modified so that its chart drive mechanism could be externally controlled by the automatic sampler. This interaction was carried out via an optically isolated triac (Monsanto-MSR16O-B). Aqueous lead standards were used throughout this work and measurements were made at the 283.3-nm Pb resonance line. The 1200 instrument settings were essentially those recommended by the vendor, including background correction mode and minimal damping of the absorbance signal sent to the integrator module. No special instrument conditions were employed. Reagents. The solutions and standards used were prepared from ACS reagent grade materials and deionized water which was preanalyzed and found to contain insignificant amounts of lead. Working aqueous lead standards were prepared by diluting a stock standard of loo0 gg mL-’ Pb2+(from Pb(NO& salt in 1% “OB) with 1% H N 0 3 to maintain acidic conditions. Transfer of microliter volumes was made with Eppendorf pipets equipped with disposable plastic tips. Procedure. The first phase of system evaluation involved the application of simulated transient atomic absorption signals to the analog integrator module. These pulsed signals were of precisely known magnitude and time duration, simulating absorbance signals ranging from 0.01 to 1.0 A.U. and time duration from 0.050 to 20 s. From these tests the performance characteristics of all circuit components such as the sample-and-hold amplifier (SHA),signal detection (SDC) and integration systems could be established. The second phase of testing primarily involved the use of a modified manual cup injector, commercially obtained nichrome, solid and axially holed nickel microsampling cups, aqueous lead standards, and the analog integrator module. These conditions were chosen to further evaluate the integration circuitry in addition to establishing reference points for various measurement system parameters such as “within” and “between cup” precision, linear dynamic ranges, analysis speed, and overall method reliability. Once these reference parameters were established, the third phase of system evaluation tests was carried out. These tests involved the automatic sampling device in conjunction with the same instrument conditions and lead standards used in the second phase. Design modifications were carried out on the automatic sampling device until it met or exceeded the operating criteria established with the modified manual cup injector which was considered t o be at or near a performance limit with respect to measurement precision or analysis speed.
RESULTS AND DISCUSSION Transient Signal Integrator P e r f o r m a n c e . Data obtained from t h e first phase of system testing demonstrated that t h e integrator analog circuitry was inherently capable of
_
_
441
~
Table 11. Evaluation of Integrator Circuit Performance by Application of 0.050- and 10.0.Second Simulated
Transient Signal Pulses simulated signal (absorb-
set
ance)
I“
0.01
0.02 0.05 0.1 0.2 0.3 0.8 1.0
0.01 0.02
IIb
0.05
0.1 0.2 0.4 0.8 1.0
least
actual input signal, mV 0.967 1.994 4.936 9.833 19.75 39.52 78.32 99.18 0.967 1.994 4.936 9.833 19.75 39.52 ’78.32 99.18
ave. area
output,‘ V
squares predicted output,
0.106 0.219 0.546 1.090 2.192 4.398 8.711
10.89 0.0992 0.20511 0.5074 1.014
2.029 4.073 8.056 10.20
7\
RSD, 7
0.47 0.22 0.12 0.11 0.09 0.05 0.07 0.04
0.107
0.220 0.545 1.085
2.159 4.359 8.639 10.94 0.0994 0.2051 0.5077 1.011 2.031 4.065 8.05.5 10.20
0.09 0.06 0.05 0.06 0.10 0.06 0.06 0.11
set
linear regression coefficientsd
correlation coefficient
I I1
y - 11.ox - 0.0099 y = 1 0 . 3 . ~- 0.00091
1.00 100
0 . 0 5 0 s signals impressed on simulated base-line levels
of 0.05 A . U . ; integrator conditions O A l gain x 100, OA6 time constant = 0.015 s, signal detection circuit-
enabled. 10.0-s signals impressed on simulated baseline levels of 0.05 A.U.; integrator conditions: O A l gain x 10, OA6 time constant 1.0 s, signal detection circuitenabled. Based on 10 replicates. Where y =~ OX6 area output ( V ) , .Y = amplitude of simulated signal pulse (A.U.).
performing highly precise and accurate integration over the entire range of anticipated transient signal lifetimes. Typical relative standard deviations (RSDs) for replicate test signal area measurements ranged between 0.5 and 0.03%. In most cases the inherent circuit data precision was better than 0.1 % RSD, even when t h e shortest (0.050 s) signal lifetimes were applied. An example of this inherent circuit precision is given in Table 11. Response was linear, with correlation coefficients of 1.00 for both d a t a sets shown. T h e area measurements obtained (in volts) were within 1% of the values expected from linear regression calculations. These performance tests involving simulated transient signals showed that inherent circuit errors were minimal. Therefore, any substantial degree of nonlinearity or y-intercept found in an analytical calibration curve should not be related t o t h e integrator’s analog circuitry. For t h e conditions employed during t h e second phase of system evaluation tests, it was generally observed that when using randomly chosen cups of similar past history t h a t average “within cup” RSDs for peak height and area measurements were quite similar. “Within cup” precision refers t o the degree of precision that can be expected from replicate measurements taken with the same microsampling cup. I n addition, it was found that axial holed nickel cups gave better overall precision than either solid nickel or nichrome types of cups. The range of overall RSDs for axial holed nickel cups was between 1.5 and 4.5%,whereas for other types of cups, a range of 3.5 to 7.0% was typical. These “ideal condition” tests demonstrated that t h e inherent precision of t h e integrator analog circuitry (0.1-0.03%) will be a minor variance
442
ANALYTICAL CHEMISTRY, VOL.
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MARCH 1980
Table 111. Results of Model 11, One-way Classification ANOVA Tests for Estimation of “Within-Cup” and “Between-Cup’’ Variability Factors t o Overall RSD % totalh variance between cup factor 7~
seta
dfb
dfc
between groups
groups
moded
9
110
PA PH PA PH PA PH PA PH
1
within
2
9
90
3
9
80
4
9
80
RSD 7’‘ within 4.0 4.7 4.0
4.6 3.2 2.7 3.7 3.9
model IIf ANOVA
RSD %g total
NS
3.0
*** NS *** NS *** NS ***
6.4
47
4.0
6.0 3.2 4.6 3.7 6.4
44
65
63
Selected batch of 1 0 randomly chosen axial-holed N i cupsjset; 40 ng Pbicup. Degrees of freedom between groups ( 1 0 cups 1). Degrees of freedom within groups = (10 cups) (no. replicate runs - - 1). PA = peak area, PH = peak height. Average RSD %5 within groups = within c u p reproducibility. f NS = not significant, * * * = significant at 99.9% confidence level. g Total RSD % for all measurements taken. ti Calculated % of total variance contributed by physical differences between cups. 7
~
-
h
--
5 Pb
~
r“
Figure 4. Typical calibration curve at 283 nm for aqueous Pb standards in axial-holed Ni cups. 0 Peak area (av of 10 replicates) 0 Peak
TIM[
:MIY]
Flgure 5. Dual pen recorder tracing of replicate Pb standards (40 ng). Effect of cup history on sensitivity: cups 1-10 = “new” batch, cups
height (av of 10 replicates)
11-20 “older” batch. All cups axial-holed Ni type
factor in any real analysis using the automated microsampling cup system. An equivalence of “within cup” precision for peak height a n d area measurements was achieved only when the integrator’s SDC was enabled and the SHA input was low pass filtered. This indicated that base-line noise factors can make a significant contribution to peak area precision even under relatively ideal conditions. I t also suggests that electronic integrators lacking similar noise discrimination capabilities may not be optimal for processing transient AAS signal pulses. Figure 4 demonstrates a typical calibration curve for aqueous lead standards when both peak height and area measurements were taken simultaneously. I t was found that peak area measurements consistently exhibited longer linear dynamic ranges and curved more gradually than peak height measurements. Both calibration curves also had negligible y-intercepts. From these observations, it appears that because of their sharp rise and decay characteristics, the relative amount of signal not integrated by this threshold approach was negligible. Subsequent tests regarding the effect of SDC threshold levels on peak area measurements showed that for typical transient signals, this setting was not critical and could be easily optimized by running blank cups and adjusting the threshold until no residual area measurement was recorded. T h e most significant series of tests run during the second phase of system evaluation involved a set of relatively new a n d previously used holed nickel cups. Each set of cups contained the same amount of lead standard (40 ng). Peak height and area values for this series are shown in Figure 5 . From these data it can be seen that peak area measurements
were independent of cup history b u t peak heights for the “older” cups were significantly lower. It was also observed that for some sets of cups with similar prior histories that significant differences often existed for “within cup” and “between cup” precision. This “between cup” precision refers to the degree of precision that can be expected from replicate measurements using different, randomly chosen microsampling cups. The statistical test used to establish this difference was a Model 11, one-way classification analysis of variance (ANOVA) (20) in which the microsampling cups were considered to be a random treatment effect. An example of these data is given in Table 111. For these sets of cups, it was consistently found that peak height measurements exhibited a significant “between cup” variance factor which accounted for between 40 to 65% of the overall RSD found. Corresponding peak area measurements, however, showed no significant “between cup” variability. Other workers have experienced similar “loss of sensitivity” as it was called and significant “between cup” variability ranging between 10 and 30% (1,21-23). If only peak height measurements are recorded, which has been typical for many reported applications of this method, then these factors can represent a major source of error if undetected. Previous attempts to minimize these error sources have included either tedious “sensitivity matching” routines or schemes where the same microsampling cup was used for all sample measurements. This second approach, while effective, negates the batch processing capability of this technique. It is apparent, from peak area measurements taken on these samples, that a primary source of “sensitivity loss” or “between cup” variability is physical differences between cups which
ANALYTICAL CHEMISTRY, VOL. 52, NO. 3, MARCH 1980
443
Table IV. Comparison of “Within Cup” Precision Typically Achieved with the Manual and Automatic Sample Injectors manual injector
automatic sampler
__-
av.d dfb within RSD 70 within groups mode‘ within setn groups mode‘ dfb
set’
Flgure 8. Example of alternative dual pen recording mode
only affect analyte vaporization rates and not atomizationcollection efficiency. These data confirm those which have been previously documented by Aldous et al. ( 1 3 , and further illustrate that for the microsampling cup method both peak profile and area measurements should be simultaneously measured on each sample. Examples of the two forms of recorder readout available with this automated system are shown in Figures 5 a n d 6. Peak profile data can be used to continuously monitor the physical condition of sample cups and the status of various system parameters such as sample atomization rates. Peak area data can be used to compensate for vaporization rate interferences along with providing information related to atomization-collection efficiency and atom residence times. This dual measurement capability is currently lacking from most commercial atomic absorption instruments or integrators which monitor either peak height or area data but cannot display both simultaneously. Exceptions primarily involve specialized types of programmable calculators or microcomputer based systems. Many commercial instruments then lack sufficient versatility and/or are not truly optimized for processing the types of signals generated by this atomization device. T h e analog integrator module developed in this work specifically overcomes these problems and is also compatible with the automatic sampler designed for microsampling cups. Automatic Sampler Evaluation. Results obtained during the third phase of system evaluation tests showed that the automatic sampling device could maintain rapid and reliable transfer of all types of commercial microsampling cups. Statistical comparisons utilizing F-tests consistently found no significant difference in “within cup” precision for data taken with a modified manual cup injector or the present design of the automatic sampler. Table IV illustrates some of these data which were taken under similar instrument conditions and identical concentrations of lead standards, using both types of injectors. These data indicate that the automatic sampler was capable of maintaining the critical sample cup-absorption tube geometry. For both types of injectors it was found that if a pin-vise loop support clamp were not used, loop heights with respect to the absorption tube orifice could be easily changed during normal operation. Such variability in the sample cup-absorption tube geometry caused sudden and dramatic changes in analytical sensitivity which resulted in poor “within cup” precision and generally unreliable results. However, with the pin-vise mechanism, these sudden changes in analytical sensitivity were not observed and very sturdy geometries could be maintained for long periods of time. Alignment of the automatic sampling device and optimization of sample cup-absorption tube geometry were found to be straightforward and not much more difficult or time consuming than procedures used with manual cup injectors. Realignment was required only when a nichrome triangular loop was replaced. One of the critical features of the microsampling cup method which requires systematic monitoring is the physical condition of the cup holding loops. Strict timetables for routine replacement of these loops should be adhered to. For the conditions employed in this work, it was found that about
1
90
2
90
3
90
PA PH PA PH PA PH
2.2 2.6
d
90
2.0 1.9
5
110
3.6
6
90
4.2
PA PH PA PH PA PH
av.‘* RSD %
within 2.3 1. ‘7 3.2 2.7 2.1
3.2
Set = batch of 1 0 randomly chosen axial-holed cups. Degrees of freedom within groups :- 1 0 cups x ( n o . replicate runs - 1). ‘ PA = peak area, PH = peak height. Average RSD 5% within groups (within c u p s ) ; 4 0 n g of Pb ( f r o m aqueous standards) used throughout all test series.
1000 lead analyses could typically be carried out with a given nichrome triangular loop. If replacement of this loop was not made a t this time, it was often observed that microsampling cups could partially weld to these loops. This welding phenomenon was more prevalent when older nichrome loops and nickel cups were being used and can be experienced with both types of injectors. T h e consequences of partial welding for manual injectors is that loop distortion can occur during the manual removal of a welded cup. For the automatic sampling device, welding result,s in an improper exchange of a sample cup between the loop and vertical lift platform after a welded cup returns from the flame. By adhering to a normal maintenance scheme for the replacement of various system components such as loops and sample cups, these welding problems were eliminated. With the present design of the automatic sampling device, one complete transfer cycle can be performed in 22 s excluding atomization and “cup cooldown” time. When aqueous lead standards were analyzed, about 15 s of atomization time and 6 s of “cup cooldown” time were required. Therefore, the total cycle time for aqueous lead standards (which produced a relatively long duration signal) was 43 s. These automatic cycle times compare favorably to those achieved with a manual injection device. Precise quantitative comparisons of cycle times between these two devices are difficult because manual injection times were operator dependent. It is expected that if a small number of samples were being analyzed by a skilled operator, then cycle times for the manual injection system would be slightly faster. For large volume work, however, the automatic sampling device should result in a significant reduction in overall cycle time. In addition to the rapid cycle times, another advantage of automated sample injection is that while one tray of samples is being analyzed, an operator is free to prepare the next batch of samples off-time or evaluate the data generated. This is not the case for manual injection procedures. Work is currently in progress to further decrease cycle times for the automatic sampling device and t o evaluate other materials such as ceramic coated molybdenum for use as sample cups and loop components. These materials should allow the automated microsampling cup system to be compatible with a nitrous oxide-acetylene flame.
ACKNOWLEDGMENT T h e authors thank Andrew Mangini, Department of Chemistry, Machine Shop, for his excellent craftsmanship and aid in the construction of the automatic sampler. Thanks also to Galen W. Ewing for his creative suggestions and discussions.
444
Anal. Chem. 1980, 5 2 , 444-448
LITERATURE CITED
(13) (14) (15) (16) (17) (18) (19)
(1) Delves, H. T. Analyst(London) 1970, 9 5 , 431 ( 2 ) Kerber, J. D.; Fernandez, F. J. At. Absorpt. Newsl. 1971, 10, 78. (3) Joselow, M. M.; Bogden, J. D. At. Absorpt. News/. 1972, 1 1 . 127. (4) Mitchell, D. G.; Ward, A. F.; Kahl, M. Anal. Chim. Acta, 1975, 76, 456. (5) Kahn, H. L.; Fernandez, F. J.; Slavin, S. At. Absorpt. Newsl., 1972, 1 7 , 42. (6) Mitchell, D. G.;Maines, I. S.; Aldous, K. M. Abstracts, 15th Eastern Analytical Symposium, New York, Nov. 1973, No. 6. (7) Joselow, M. M.; Bogden, J. D. A m . J . Public Health, 1974, 6 4 , 238. (8) Law, 0. W.; Li, K. L. Analyst(London), 1975, 700, 430. (9) Pachuta, D. G.; Love, L. J. C. Abstracts, 27th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1976, No. 80. (10) Favretto, L. A t . Absurpf. News/.. 1976, 15. 98. (11) Favretto. G. L.; Pertoidi, M. G.; Favretto, L. At. Absorpt. Newsl., 1977, 16, 4. (12) Mitchell. D. G.; Mills, W. N.; Ward, A. F.; Aldous, K. M. Anal. Chim. Acta 1977, 9 0 , 275.
(20) (21) (22) (23)
Carter, G. F. B r . J . I n d . Med. 1978, 3 5 , 235. Matousek, J. P. A m . Lab. 1971, 3(6),45. Hwang, J. Y.; Thomas, G. P. A m . Lab. 1974, 6(11), 42. Slavin, W.; Manning, D. C. Anal. Chem. 1979, 5 1 , 261. Aldous, K. M.; Mitchell, D. G.; Ryan, F. J. Anal. Chem. 1973, 45, 1990. Ward, A. F.; Mitchell, D. G.; Aldous, K. M. Anal. Chem. 1975, 4 7 , 1656. Pachuta, D. G. Ph.D. Thesis, Seton Hall University, South Orange, N.J., 1977. Sokal, R . R.; Rohlff, F. J. "Biometry'; W. H. Freeman: San Francisco, Calif.. 1969; Chapter 8. Fernandez, F. J.; Kahn, K. L. At. Absurpt. News/. 1971, 10, 1 Oisen, E. D.; Jatlow, P. I. Clin. Chem. 1972, 18, 1312. Joselow, M. M.; Singh. N. P. At. Absorpt. News/. 1973, 12, 128.
RECEIVED August 14, 1979. Accepted December 13, 1979. Research support provided by the State of New Jersey under provisions of the Independent Colleges and Universities Utilization Act is gratefully acknowledged.
Determination of Lead in Urban Air Particulates by Microsampling Cup Atomic Absorption Spectrometry D. G. Pachuta' and L. J. Cline Love" Department of Chemistry, Seton Hall University, South Orange, New Jersey 07079
A rapid, automated method for the determination of urban lead partlculates has been developed. The air samples employed in this work were collected with high volume samplers using cellulose filters. Sample preparation involved punching out 0.50 cm2 disks from these filters, placing them in nichrome microsampling cups followed by dlrect injectlon into an airacetylene flame. Synthetic standards were comprised of blank disks spiked with lead. Calibration curves were based on peak area measurements. Data obtained at the 261.4-nm line were In good agreement with established X-ray fluorescence and conventional flame atomic absorptlon spectrometry techniques. Correlation coefficlents between the mlcrosampling cup method and these two technlques were 0.965 and 0.996, respectlvely. Repllcate precision typically ranged from 6 to 15% RSD. Between-day consistency tests for ambient lead levels demonstrated a variabiltly of fO.l wg m-3 or less. Automated cycle times were 33 s per sample disk.
must undergo separate dry ash-atomize-workhead cooldown cycles. This greatly increases overall analysis times even when automated sample injectors are used. The microsampling cup or "Delves Cup" approach to sample atomization is not subject to these disadvantages. Several direct applications have been reported for lead in various types of solid samples (4-13) including cellulose matrices (11, 13). This micro-atomization method is also capable of off-line batch processing operations, possesses an inherently fast measurement cycle ranging from 15 to 45 s and has been automated (14-16). Mitchell and co-workers have previously discussed the use of a microsampling cup method for the determination of lead particulates collected on Fiberglas media (10). In their report a special photon counting-minicomputer system (15) was used to calculate peak areas and perform quality control tests on the data. Synthetic standards were prepared by pipetting aqueous lead solutions onto blank Fiberglas disks that had been previously coated with gelatin and activated charcoal. In this work a versatile measurement system, which included a novel automatic sample injector ( 1 6 ) , was used to determine lead particulates collected with high volume samplers employing cellulose filters. T h e microsampling cup method that was developed required minimal sample preparation and with automation large numbers of air samples could be processed in a rapid and efficient manner. By using cellulose filters a n d measuring peak areas, synthetic lead standards were easily prepared and calibration schemes were greatly simplified. Data obtained with this method were compared against established X R F (1) and conventional flame AAS procedures (3). The results of these and other evaluation tests are discussed.
Routine monitoring programs for airborne lead particulates typically require the collection of many samples. This means that in addition to being sufficiently precise and accurate, the analytical methodology selected should preferably be simple, rapid, a n d amenable to automation. T o varying degrees, several methods involving X-ray fluorescence (XRF) ( 1 , 2 )and atomic absorption spectrometry (AAS) (3-8) meet these criteria. A disadvantage of X R F methods is that the instrumentation required is quite expensive and unavailable to many laboratories. AAS instruments are more readily available and well suited for lead determinations over a wide range of ambient concentrations a n d air volume requirements. Unfortunately, conventional flame and a majority of nonflame AAS methods are subject to tedious sample dissolution requirements. An additional disadvantage for nonflame devices which do not possess batch processing capabilities is that each sample
EXPERIMENTAL Sample Collection. The air particulate samples used throughout this work were collected during an earlier environmental study of the greater Newark, N.J., area which took place from 1973 to 1975 ( 1 7 ) . Composite 24-h samples were obtained with calibrated high volume air samplers (Precision Scientific Model 63083) equipped with 24-h timers. Cellulose based papers (Schleicher and Scheull No. 589 Green Ribbon) in 20.3 X 25.4
'Present address: Department of Chemistry, Kean College of New Jersey, Union, N.J. 07083. 0003-2700/80/0352-0444$01 .OO/O
c
1980 American Chemical Society
