I
CORRESPONDENCE
I
L
Detection of Lead in Air Filters at the Submicrogram Level In order to monitor air pollution with high accuracy, the detection of trace impurities must reach unusual sensitivities. A typical example is provided by the determination of lead quantities adsorbed on air filters which are commonly used for monitoring regions of low contamination. This is particularly the case for the Louvain-la-Neuve site, where the filters contain 10 to 100 nanograms of lead. In the ring oven technique of Weisz, the cations are eluted and concentrated along a thin ring on a piece of filter paper ( I ) . The technique presented in this paper is inspired by the ring oven procedure and applies it in one single dimension, thus avoiding the dispersion of the metallic ions on a circular perimeter, minimizing manipulation. The air monitoring filter is wrapped around the middle part of a thin strip of cellulose filter paper (Sartorius SM 11303; pore size, 1.2 km; size, about 100 mm by 2 mm), and maintained by cotton threads. If necessary, the air monitoring filter is previously exposed to nitric acid fumes in order to oxidize the metallic deposits. One end of the paper strip is dipped in an acid solution (0.1M "03). By capillary action, this solution migrates along the strip, imbibes the air monitoring filter, and elutes the metallic ions a t the opposite end of the strip (see Figure 1). This part is heated by a warm air stream which evaporates the solvent. A Teflon (Du Pont) plate grips the strip so that only the lower part is exposed to the warm air. The heated zone is thus well defined; the upper part of the strip is shielded by the plate and remains moist enough to allow the elution. After about one hour, all the soluble salts have been washed away from the filter into the elution strip, where they accumulate in the heated part and are concentrated into a very narrow zone (about 0.5-1 mm). There they may be identified and their amount estimated, either by the use of reactants giving specific colorations or by selective precipitations and elutions. These secondary elutions are made possible by a slight lowering of the Teflon shield. At the moment, only the determination of lead has been investigated. Its presence is revealed by the appearance of a red spot after the strip has been sprayed with a sodium rhodizonate solution. The complex is not stable in the presence of air and may be protected by a thin layer of plasticizer. Iron appears to interfere. As a remedy to this inconvenience, iron has been precipitated on the strip by means of (1) H . Weisz, "Microanalysis by the Ring Oven Technique," Pergarnon Press, Oxford, 1961.
2152
n2
Figure 1.
/
Schematic of the one-dimensional concentration tech-
nique (1) Tefion vessel containing the eluent ( 0 . 1 M " 0 3 ) . (2) paper strip, monitoring filter wound around the paper strip, ( 4 ) cotton threads, the elution part of the paper strip remains moist, (6) Teflon shield, warm air generator (e.g., hair dryer), (8) heated part of paper strip, zone where metallic ions are concentrated
(3) (5) (7) (9)
potassium hexacyanoferrate (11). Lead is unaffected by this reactant and may be further eluted by water. The method may be simultaneously applied on a large number of samples, which compensates for the rather long time that the procedure requires. Test samples treated in this way have shown that 10 nanograms of lead are still easily detected; blank samples yield negative results, provided reagents and water of very high purity are used. Rough estimation of the amounts of lead is possible by comparing the intensities of the red spots with those obtained from reference samples which have been simultaneously treated in an identical way. Further work with this very inexpensive device is in progress to improve the stability of the lead rhodizonate complex and to neutralize the interfering influence of iron. It is hoped that direct quantitative determinations by spectrophotometric techniques will be feasible. C. J-M. Ronneau N. M. Jacob D. J. Apers
Laboratoire de Chemie NuclCaire UniversitC de Louvain Louvain-la-Neuve, Belgium Received for review February 2, 1973. Accepted July 5, 1973. This work was performed under the auspices of the Centre National d'Etude de la Pollution Atmosphbrique par la Combustion.
A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 12, OCTOBER 1973
