New Standardless Method of Pb Analysis by Electrothermal Atomic Absorption Spectroscopy in Air Aerosols: Comparison with the Official Method

C L I N I O L O C A T E L L I , P I E R L U I G I R E S C H I G L I A N , G I A N C A R L O T O R S I , * F R A N C E S C O F A G I O L I , N I C O L A R O S S I , and D O R A M E L U C C I Department of Chemistry "'G. Ciamician ", University of Bologna, Via F. Selmi 2, 1-40126 Bologna, Italy (C.L., P.R., G.T., N.R., D.M.); and Department of Chemistr3; University of Ferrara, Via L. Borsari 46, 1-44100 Ferrara, Italy (F.F.)

A standardless analysis method with electrothermal atomization/ atomic absorption spectroscopy (ETA-AAS) measurements is pro- posed and applied to the determination of Pb in an urban environ- ment (Bologna, Italy) and compared with results obtained in the same place and time, with the method dictated by Italian law, which is derived by European Community Directives. It was found that the difference previously observed between the two methods is due to the low efficiency of the 0.45-p~m-pore-size paper filters, dictated by the official method. The uncovering of the error source validates the proposed method and highlights its accuracy. Moreover, the two methods can be conveniently combined to give two series of data relevant to pollutants associated with low- and high-dimension par- ticles. This capability gives a much more precise picture of the haz- ard of a pollutant.

Index Headings: Standardless analysis; Atomic absorption; Pb mea- surements; Urban environment.

I N T R O D U C T I O N

In previous papers ~ 3 it has been shown that, with a special atomizer and power supply, it is possible to obtain standardless analysis in electrothermal atomization/atom- ic absorption spectroscopy (ETA-AAS). By standardless analysis, we mean that a quantitative measurement can be made directly from an absorbance measurement through a constant K, independently measured, specific for a given absorption line of an element, using the equa- tion

KNo A 0 - (1)

sc

where Ao is the absorbance of the plateau of an absor- bance vs. time curve, K (cm 2 atom 1) is a spectroscopic constant obtained under well-defined experimental con- ditions,2-4 Sc (cm 2) is the atomizer cross section, which must be constant, and No (atom) is the number of atoms present in the atomizer at the beginning of the atomiza- tion process. The type of experimental curves from which Ao can be obtained is shown in Fig. 1.

A standardless analysis method is particularly useful and, in practice, necessary when aerosol components are determined with a direct instrumental analysis as pro- posed by us, 3 because, to the best of our knowledge, no external standards are available, nor can they be prepared with sufficiently high accuracy and precision.

We have shown that, by controlling experimental con-

Received 1 February 1995; accepted 11 March 1996. * Author to whom correspondence should be sent.

ditions, standardless analysis is possible for Pb associated with particulate matter in air by measuring the absor- bance A0 (in Eq. 1) obtained by using a device (see Fig. 2) which in a first step is operated as an electrostatic filter and, subsequently, as an atomizer in ETA-AAS measure- ments. In this way, we obtain No and, if the volume of air passed through the device in the accumulation step is known, we can obtain the level of Pb in the aerosol sam- pied.2. 3

Validation of the proposed method is based on a com- parison with the official method of Pb analysis in the atmosphere, 5 practically the same in Europe and America. In the official method, analysis is based on accumulating, on a 0.45-pore-pore-size filter, the particulate matter of the aerosol with a facial velocity between 33 and 55 cm s followed by total dissolution of the captured material to- gether with the filter and then quantitation of the relevant analyte. Such a comparison has already been reported, 3 and a positive error of about 20% was found for the new method. Such a large systematic error is a clear indication of a difference in accuracy of the two methods. Hence, further work is needed to clarify the reason for the dis- crepancies.

In this paper, we report measurements done with both methods in parallel in order to make a rigorous and mean- ingful comparison. It is demonstrated that, at least for Pb associated with particulate matter in the urban environ- ment of Bologna, the data obtained via filter collection/ analysis as proposed by the official method have a neg- ative systematic error up to 27%, thus giving support to our claim that the accuracy of the standardless method is good.

We obviously conclude that the method here proposed is preferable and, furthermore, that when the particle size distribution is unknown or insufficiently known, a com- bination of the two methods can separate two f rac t ions-- coarse and fine particles. In this way, both the total and the fine fraction can be monitored with a much better definition of the risk, since the fine fraction is much more dangerous than the coarse. 6

EXPERIMENTAL

Instrumentation not described here has been described in previous papers. ~-3 For the reader's convenience, the electrostatic filter and associated equipment are presented in Fig. 2. Air is drawn with a pump at a known flow rate (1-2 cm 3 s -j) through a graphite tube at the heart of the furnace device (Fig. 2A). For the accumulation of parti-

Volume 50, Number 12, 1996 0003-7028/96/5012-158552.00/0 APPLIED SPECTROSCOPY 1585 © 1996 Society for Applied Spectroscopy

0.22

LU ¢O

0

m <

0

0 '

I I I I ~ !

2 3 4 5

TIME (sec)

. L = zz

A

B

a

I,LI O

0.23

o

I I 1 I I

0 1 2 3 4 5

TIME (see)

Fie. 1. Experimental absorbance vs. time curves. (a) Pb from solution 2 p~L, 30 ppb; (b) Pb captured from air.

C

FIG. 2. Devices for metal determination in aerosols: (A) disassembled furnace; (B) electrostatic sampler for particles; (C) device for position- ing the furnace.

cles in the graphite tube, the furnace is inserted in the sampler, which is a Teflon ® cup with a tungsten needle at the center. At the same time, a large voltage is applied between the tungsten needle and the furnace (Fig. 2B). 7 It has been demonstrated that the capture efficiency of this device in capturing air particulate matter can easily be maintained at 100% for all particles in the range stud- ied (0 .02 ->3 ~m7.8). The particles are deposited in a small central section of the graphite tubeY The furnace is then inserted in the housing (Fig. 2C), positioned in such a way that the optical beam of the hollow cathode source passes inside the graphite tube. From this point, the usual steps employed for an ETA-AAS measurement are fol- lowed. ~,2 It must be remembered that the power supply is a special one in which heating rates on the order of 8000 °K s-t can be obtained, and that the atomizer, with respect to commercial types, is longer (3.6 mm) with a smaller internal diameter (3.25 mm) and, most important, with no central hole for sample injection. 1,2 As a result of these design features, it is possible to have all atoms in the optical beam simultaneously, this being the necessary condition for obtaining A 0. The evidence that a constant value of A0 has been attained can be deduced f rom the plateau in the curve of Fig. 1 and f rom its constancy when the heating rate is decreased.

The measurements reported herein were made by set-

ting up two lines of instruments outlined in the block diagram of Fig. 3. In line A, standard instruments for collection of particles on a paper filter (Sartorius leadless filters 0.45-p~m pore size or other similar filters) are shown. In line A' , downstream of the paper filter, part of the air was directed to an electrostatic filter. In this way, particles that might have escaped collection by the paper filter are captured by the electrostatic filter.

Line B is essentially an electrostatic filter directly sam- pling the air near the point where the paper filter is lo- cated. The filter holder was of the standard type (type PF 20002, Zambelli , Milan, Italy) and was tested in order to make sure that air could not bypass the paper filters. The pumps for the electrostatic filters were type Wisa No. 113.005.600.0 (Germany). The power supplies were high-voltage Model PS 325 from SRS (CA, U.S.A.). Oth- er instrumentation, like volumetric gas meter, flow me- ters, and stop watches, were normal laboratory instru- ments, calibrated for these kinds of measurements.

The analysis of lead present in the solutions, obtained f rom the destruction of the paper filters, was performed in three different laboratories with three different tech- niques: in one lab, we used the standardless method; j,z in another, the analysis was made by normal ETA-AAS measurements with a Perk in-Elmer 1100-B Spectrometer equipped with a deuterium background corrector, an

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Pump

Pump

<1 Flowmeter

Flowmeter

A I <

[ t A'

Electrostatic filter

0.45 ,urn filter

LINE A

I High-voltage power supply I LINE B

Pump

FIG. 3.

< Flowmeter Electrostatic filter [

I High-voltage power supply I Block diagram of the instrumentation setup for parallel measurements.

HGA-500 graphite furnace, and autosampler AS-40; and in a third, we employed the differential pulse anodic stripping vol tammetry (DPASV) technique, carried out with a Multipolarograph A M E L Mod 433 (Milan, Italy), using a stationary mercury electrode as the working elec- trode, with Ag/AgC1, KCl-saturated, used as a reference electrode and a platinum wire used as an auxiliary elec- trode.

The standard solutions were prepared daily from con- centrated stock solutions (1000 mg/L, BDH, England).

R E S U L T S A N D D I S C U S S I O N

Accuracy and Precision of the Data. The accuracy of the data obtained with the standardless method is re- lated to the accuracy in measuring the constant K of Eq. 1. As already explained, this has been measured from solutions of known concentration of Pb under the same experimental conditions (0.2 nm bandwidth and 7 mA current of a Perk in-Elmer Pb Intensitron hollow cathode lamp at the 283.3 nm line) as for aerosol measurements. In both cases 3 the absorbance vs. t ime curves show well- defined plateaus f rom which A0 can be readily measured. During the period of the study, the value of K (1.08 ± 0.07) × 10 ~3 cm 2 atom-1 was not significantly different from previously reported values. 2

With respect to blank values, blank signals were not detected for aerosol measurements if the electrostatic fil- ters were cleaned by firing them about two days before their use. In the case of measurement of K from solution samples, the slopes of the A vs. concentration plots, even if blanks are present, do not alter the calculated value of K. Blanks can originate from several sources, including the graphite tubes of the atomization device. In the case of aqueous solutions, our experiments led us to suspect that Pb is mobilized from within the graphite material, thus giving blank signals. This mechanism does not op- erate in the case of atomization of particulate matter where only a small residual quantity of water may be present.

The accuracy of the data relevant to the analysis of the solutions obtained by dissolving the filters should be very high because we used lead-free filters and 65% suprapure nitric acid (Merck, Germany). A control for the presence of blanks revealed no significant concentration of Pb.

The precision of the data obtained from solutions, gen- erally three measurements with each technique, was around 9%, while the precision of the data derived from the measurements with the electrostatic filters was about 6%. This result is derived by the standard deviation of the slope of the calibration plot with aqueous solutions from which K is obtained as reported.

Efficiency of Particles Captured with Paper Filters of 0.45-1xm Pore Size. It has already been shown that the capture efficiency of the electrostatic filters used here is practically 100% for a large spectrum of particle di- ameters (0 .02 ->3 l, zm) . 7'8 The efficiency of paper filters used for collection of aerosols has not been investigated as far as we know. It is generally assumed, however, that this type of filter has an efficiency of practically 100% for 0 .3-~m particles, as required by the Italian law. 5 Paper filters similar to those used in this work (Millipore MF-MA) have been tested and were found to comply with the requirements of the law. 9

Three data sets can be obtained with the experimental setup of Fig. 3, namely, Pb~, which relates to the quantity of Pb captured on the paper filter; Pb 2, which reflects the Pb captured downstream of the paper filter; and, finally, Pb3, relevant to Pb captured directly from air. Pb2 and Pb 3 are obtained with electrostatic filters. A typical set of data is shown in Fig. 4. Here Pb2 and Pb3 are plotted as a function of the time. The horizontal lines indicate the time and the intervals in which measurements have been made.

Pbl is obtained from the analysis of the paper filter solution. Pb2 and Pb3 are derived f rom the integration of data like those shown in Fig. 4, relevant to the same time interval. The different intervals of times for the data ob- tained with the paper filters and the electrostatic filters

APPLIED S P E C T R O S C O P Y 1587

0.3

A

0,2

E

0.1

0.0

/ (Pb3)

~,,,~,~,~, pg/m 3

I ' I ' I ' I ' I

10 12 14 16 18

hours FIG. 4. Level of Pb concentration with time, determined by electrostatic filter measurements (Pb 2 and Pb3). The parallel lines are the average values compared with Pb~. April 3, 1995; windowsill of the Analytical Laboratory (University of Bologna).

are due to the need to maintain the absorbance data in the linear range of the calibration plot. It is clear that if the measurements are referred to the same time interval and to the same place, when the data are normalized, we should have

Pbl + Pb2 = Pb3 (2)

and that the fraction of Pb not collected by paper filter is

PbJPb3 or PbJ(Pbl + Pb2). (3)

The results are summarized in Table I. The data for mass balance (Eq. 2) are less numerous than those ob- tained from Eq. 3, because in this case we have two data for each run and, moreover, PbJPb3 can be obtained in- dependently of Pb, in much less time. The data relevant to Pb2/Pb3 are not homogeneous, because the time inter- val is different; however, this fact should not give very different results except for a small variation on the stan- dard deviation, because the values obtained from data

TABLE I. Experimental verification of Eqs. 2 and 3.

Escaped fraction [PbJPb 3 or Pb2/(Pb ~ + Pb2)] 0.27 ± 0.05 (22)" Mass balance [Pb3/(Pbl + Pb2)] 0.99 ± 0.11 (6)"

" N u m b e r of measurements .

derived from measurements like those given in Fig. 4 are already average values.

The quantity escaping collection by the filters was found to be around 27%. This percentage is practically independent of the filter origin and of the location chosen, while it showed dependence on weather conditions. Three filters of different origins were examined (Whatman, Sar- torius, and Sartorius leadless), and no significant differ- ences in results were found. Moreover, the same capture efficiency was obtained even after a filter operated con- tinuously for 24 hours. The same results were obtained by analyzing the content of the downstream air of the lines set up on the windowsill of our lab facing, at the height of about two and half meters, a courtyard used as a parking lot. The building is in Bologna's historic center, which allows limited traffic, with an average value around 0.15 txg/m 3 in the 9 -18 hour interval. A second site, the monitoring station of the Presidio Multizonale di Prevenzione USL 28, is located near the sidewalk of Via Vizzani, a one-way street with suburban traffic in a densely populated area, with an average level of Pb (over the month of May 1995) of 0.12 txg/m 3. We used the same experimental setup and filters employed in our lab, except for the pump and volumetric gas meter. The data were obtained in good weather conditions.

1588 Volume 50, Number 12, 1996

We have observed that the capture efficiency of the filters increases when the relative humidity of the atmo- sphere increases. The capture efficiency decreases when the temperature increases or the capture is done near a point source (for instance, a running car). Both obser- vations agree that the particles tend to form aggregates and that humidity favors this process. For this reason, the data given in Table I are to be thought of as an upper limit rather than an average value. A systematic study of the effects of environmental variables on the capture ef- ficiency of the filters is beyond the aims of the present paper. More extensive measurements singling out the in- fluence of local temperature and relative humidity will be the objects of a further study.

C O N C L U S I O N

From what has been presented above, it is concluded that the efficiency of 0.45-1xm paper filters used for Pb measurement in rather common urban environments has a systematic negative error that can be as high as 27% in dry and mild weather conditions. The systematic error previously reported 3 is therefore considered to be due not to the electrostatic filters method but to the type of paper filters used. This fact, together with the high capture ef- ficiency of electrostatic filters and the expected value of Pb3/Pb~ + Pb2, strongly suggests that the new standard- less method is valid. A confirmation of the present find- ings will be sought by using a particles counter to check the efficiency of the paper filters.

A final suggestion, derived from the experiments pre- sented in this paper, can be the use of the setup scheme of line A in Fig. 3 for capturing and measuring a fraction of the particles above a certain value on the paper filter and, at the same time, examining the fraction of an an- alyte associated with fine particles on the electrostatic filter. This method generates a much more informative picture of the potential hazards of pollutants associated with particulate matter.

ACKNOWLEDGMENTS

The authors wish to thank C. W. McLeod for the revision of the language and Dino Franchini for helpful assistance in the measurements done at PMP, USL 28, Bologna, Italy. The work was partially supported by the University of Bologna.

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3. G. Torsi, R Reschiglian, M. T. Lippolis, and A. Toschi, Microchem. J. 53, 637 (1996).

4. G. Torsi, Spectrochim. Acta gOB, 707 (1985). 5. D.P.R. No. 203, 24 Maggio 1988, Gazzetta Ufficiale della Repub-

blica Italiana No. 140, 16/06/1988. 6. J. H. Vincent, Analyst 119, 19 (1994). 7. G. Torsi and E Palmisano, Spectrochim. Acta 41B, 257 (1986). 8. G. Tarroni, C. C. Lombardi, C. Melandri, M. Formignani, T. De

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9. B. Y. H. Liu, Y. H. Pui, and K. L. Rubow, "Characteristics of Air Sampling Filter Media", in Aerosol in the Mining and Industrial Work Environments, V. A. Marple and B. Y. H. Liu, Eds. (Ann Arbor Science, Ann Arbor, 1983), Vol. 3, p. 989.

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