Determination of Traces of Lead in Air by Atomic Absorption Spectrometry George8 Thilliez Central Research Loboratary, Etablissements Kuhlmann, 92 Lewllois-Perret, France Direct supply of air samples from the atmosphere to the burner of an atomic absorption spectrophotometer allows the rapid determination of certain elements such as lead. The method described has been in service in the tetraethyl- and tetramethyllead production installations of our Paimboeuf plant, since November 1965, with general satisfaction. I t allows continuous monitoring of the lead concentration in the environments where the organic derivatives of this element are manufactured or handled. It permits the detection of slight leaks in the apparatus, and improves safety. The limit of detection is 10- gram per cu meter. The sensitivity and accuracy are of the same order as those of discontinuous methods, which require from some hours to a day per analysis. This method is also applicable to determination of other elements in air, particularly mercury.
LEADIS A TOXIC ELEMeNr. In inorganic form, its hazard is limited by reason of the very low vapor pressure of industrial products and their high density, but this is not so in the case of organic compounds which are more or less volatile, particularly tetraethyllead and tetramethyllead. The presence of lead in air in plants manufacturing or simply handling such products is very dangerous, and necessitates a reliable method for monitoring the atmosphere and a specialized medical control. At present, the means of control (I) are based on the absorption of lead by iodine or one of its compounds. Air is aspirated by a pump from the different sampling points and bubbled through the absorbent solution. This solution is removed periodically and analyzed in the laboratory. Lead is determined by dithizonate colorimetry. These methods have stood the test of experience and have been useful, but they are inadequate because they are very slow. We therefore searched for a new technique that would give almost immediate results. We investigated atomic absorption methods because this procedure is readily applied to lead. By direct use of the air to be monitored in the supply to the flame, we can hope to liberate in the flame a sufficient quantity of atoms in the fundamental state to obtain a good absorption in the “cluster” of the hollow cathode lamp. Fuwa and Vallee (2) have described an extremely ingenious apparatus which improves the sensitivity of atomic absorption. They used an atomizing burner with straight injection, of the Beckman type, inclined on its axis, to channel the flame in a horizontal refractory tube of great length (up to 1 meter). The luminous cluster issuing from a hollow cathode lamp passes through this tube, in which it undergoes successive reflections, which further increase the optical distance. Thus, the cluster passes through the atomic “population” over a great length, and sensitivity is multiplied by a factor which can reach 100 compared with existing apparatus. This device has some disadvantages: (1) L. J. Snyder, 772 (1948).
W.R.Barnes, and I. V. Tokos, AN,
(2) K. Fuwa and E. L. Vallee, Zbid., 35,942 (1963).
Figure 1. Lead hollow cathode lamp
Flame-emission phenomena are often a problem, especially where unmodulated apparatus is used. Unstable readings are obtained. Tubing walls are quickly contaminated as soon as concentration of other elements is appreciable. The reflective power of the pipe is reduced. This causes reading errors. Tubing walls are very strongly heated, which can cause fixation of the element being monitored, followed by new conversion to the atomic state, resulting in delayed equilibrium. The aspiration of exterior air between burner and tube can draw unwanted elements into the flame, interfering considerably with the results. Zelyukova and Poluektov (3) have verified that the life of atoms of certain elements, including lead, allowed them to persist in the fundamental state even beyond the flame. Therefore, the sensitivity of lead determination can be considerably improved by using over a great length the space which contains free atoms. These observations have led us to perfect a new type of burner for the detection of lead in air. This burner can be used for other purposes and is the subject of a patent application (4). EXPERIMENTAL CENTER Source and Supply. The source is a hollow cylindrical cathode lamp of borosilicate glass (diameter 45 mm, length 120 mm), manufactured by our Research Center (Figure 1). The cathode is an aluminum cylinder 14 mm long and 7 mm in outside diameter, with a cylindrical cavity and with the same axis, 10 mm deep, 5 to 6 mm in diameter. A plate of fine lead is fitted in this cylinder.
(3) Y. V. Zelyukwa and B. S. Poluektov, Zh. Analif. Khim.,18, 435 (1963). (4) G. Thilliez (to Etablissements Kuhlmann), French Patent 21,967 (June 23, 1965). VOL 39, NO. 4, APRIL 1967
427
Pure air
CI
v6
'Io
-
to the atmosphere
Figure 3. Sampling circuits for fresh air and air coming from the two plants C, C,,CI, Ca. Compressors v6, Vt, VS,VQ,V,O. Bellows valves
c
1
3Hz-tNz
1
Air
Figure 2. Burner and silica combustion and measuring tube On both sides of the horizontal part of the tube, lenses and air outlet pipes for cooling a. Platinum cylinder b. O-ring This lamp is filled with neon gas, under a pressure of 10 mm of mercury. A quartz window attached to the glass with piceine allows the radiation to pass through it. The lamp is fitted with an apparatus producing a constant circuit which is well stabilized and regulated, usually to 5 ma. The voltage after starting is of the order of 220 volts. The operational life of our lamps exceeds 3000 hours. Near the lead wavelength (A 72833 A) there is a line whose origin is unknown (A = 2800 A) but whose advantages are specified below. The position of the lamp is adjustable in three planes. Optical System. The optical system consists of two quartz lenses of 60-mm focal length. The first, 20 mm in diameter, is placed in front of the window of the lamp; the second, 10 mm in diameter, is situated at the entrance of the spectrophotometer. Burner. The burner is shown in Figure 2. Two jets allow admission of gas. One of them (off-center) is connected with a pipe conducting the combustible gas, which may be hydrogen. However, because of the proximity of an ammonia installation, we use a synthetic gas mixture, 3 Hz N2, which gives excellent results. The other jet is connected with the tube for air admission by means of a metallic connection. The upper part is cooled by a water jacket. At the upper end of the pipe is placed a platinum cylinder, the bottom of which is perforated with 12 peripheral holes of 1-mm diameter and a central 1.5-mm hole. A throat allows the insertion of an O-ring joint. Combustion and Measuring Tube (Figure 2). This is an essential item for the monitoring process. It is made of transparent silica, and is T-shaped. The horizontal part has a length of 1050 mm and external and internal diameters of 13 and 10 mm. The perpendicular part, length 200 mm, external and internal diameters 24 and 21 mm, in connected via a leakproof O-ring to the burner.
+
This system allows elimination of four disadvantages which are inherent in long pathlength cells: 428
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ANALYTICAL CHEMISTRY
Since the flame is limited to the vertical part of the tube, its emission is suppressed and it is no longer necessary to modulate the receiver. The tubing walls are not contaminated in the horizontal part. Diameter increase of the vertical part overcomes disadvantages due to quartz overheating. External air aspiration is suppressed with the O-ring. Ventilation. Two double jets inject compressed air at the outlets of the tube,to avoid heating of the equipment, and also the projection of steam from the water of combustion which could condense on the lenses. Receiver. This is a Spectralux 1800, produced by the Safas Co., 5, rue Princesse Antoinette, Monaco-PrincipautC. This apparatus was modified to be more convenient for atomic absorption. The original grating, 700 lines per mm, was replaced by a Bausch & Lomb grating of 2160 lines, which considerably increases the power of resolution and (on using the maximum slit width of 1 mm) permits the isolation of the ray used. To this apparatus is connected a Graphispot recorder (GRT VAJ) of the Sefram CO., 74, rue de la FedCration, Paris 15e. Any sptectrophotometer of good resolution and sensitivity could be used. Auxiliary Apparatus. Pumping and Distribution Group. This apparatus is designed to conduct air from the installations to the measuring point and to calibrate the method, It is necessary to avoid the use of rubber and particularly PVC as tubing material, as these slowly absorb lead compounds. Nor must air be allowed to stagnate anywhere in the system. For these reasons, external and internal pipework is of metal or glass with metallic or Covar joints. All the valves are of the bellows-seal type. Air from the control points passes continually through the pipeline. The control panel for pumps and switches also includes the circuits and valves necessary for calibration. Installation Air and Pure Air System. Figure 3 shows the flow diagram for two control points (we could consider a much larger number of control points). Two installations manufacturing TEL and TML, respectively, are involved; in addition, one pure air connection to the apparatus is provided. Metallic tubing leads from the sampling points to the measuring station, and three compressors, CI, Cz, and CI, draw in the air constantly and discharge it into the atmosphere to avoid stagnation in the lines. The pure air line is arranged to pass through an activated charcoal absorber. A special compressor, C, whose rubber diaphragms are backed with Teflon sheets to eliminate any possibility of fixation of lead, and whose compressor chambers are cooled by water circulation, can draw in selected air from one of the three lines and deliver it to the burner (valves Va, V,, and V8).
Figure 4. Calibration circuits for air of known lead concentration and successive dilutions Bubbler filled with methyl compound placed in ice c, cl, cz, cg. Capillary tubes D1,D 2 , D,, D4. Rotameters V,, Vz, V,, V4, VS. Bellows valves C. Compressor
A metallic pot placed downstream of the compressor prevents vibration of the flame. A set of valves, V 9 lnd Vlo, allows the air to be vented to the atmosphere. First Calibration Procedure. Corresponding Circuits. Calibration by Means of an Atmosphere of Known Lead Concentration (Figure 4). A stream of compressed air is divided in two directions. On one side air is conducted with a very small flow via a capillary, c, and passes into a sintered glass absorber containing TML (in the presence of toluene) and placed in a bath at 0°C. On the other side the flow is large. A rotameter, D1,measures dl, the bubbling flow. The two brancnes meet again and a rotameter, D2, measures the total flow, dz. A small part of this air, which still has too high a concentration of lead, is collected and led off as required, by means of valves V l , V2,and V3in one of the three branches incorporating capillaries cl, c2, and c3, allowing a flow, d3, measured by rotameter D3. Valves V4 and V Sallow this air to be vented, or to be passed into tht: pure air stream supplying the burner, the flow of which, d4,is checked by rotameter D 4 . The final percentage of lead in air is equal to the percentage of air at the bubbler discharge, multiplied by the ratio:
di X d, d2 X d4
The blank (pure air) is determined by closing valve V4 and opening valve V S ,and the standardization points are obtained by reversing the positions of these valves and manipulating Vi, V2, and V3,choosing the capillary (or capillaries) determining the flow of lead-containing air. The theoretical leaa concentration calculated from the vapor pressure of TML (5) must not be taken as a safe standard and must be checked. Measurement of Lead Concentration in TML-Saturated Air Issuing from TML Sintered Glass Absorber. Air issuing from rotameter D Ihas to pass with the same flow, d1, into two sintered glass absorbers, serially mounted and containing 50 ml of an iodine-saturlted solution in CCl,. Pressure in the TML absorber is maintained constant. The main flow is rigorously controlled by an auxiliary water flow. After removal of iodine and CC14 by boiling the mixture with nitric acid, lead is finally determined by either atomic absorption or sulfate gravimetry. In the second absorber, no trace of lead was found. The experimental values are given in Table I. (5) D. R. Stull, Ind. Eng. Chem., 39,517 (1947).
Table I. Lead Found in Air Running through TML Absorber at O ° C Pb concn., Flow, Run no. Liters of air liters/hr mg/liter
Determined by Atomic Absorption 0.930 0.930 0.940 0.940 0.940
1 2 3 4 5
0.54 0.54 0.54 0.54 0.54
36.6 37.1 40.4 38.3 38.3
Determined Gravimetrically 6 7 8 9 10
0.920 0.890 0.815 0.810 0.815
0.71 0.71 0.71 0.71 0.71
37.4 39.6 37.9 35.7 35.4
Medium concentration, 38 mg/liter. Correct regulation of flows 4, d3, d4 provides checked air. Capillaries cI, CZ, and c3 used separately or in parallel and to obtain flows and lead concentration listed in Table 11. Table 11. Flows Used and Measured Lead Concentrations in Checked Air
(Flow di 0.71 ; d2 2800; d4 1200) Lead concn., Flows, liter/hr, d3 Capillaries used 10-6 g/cu meter 2.6 5.0 10.0 11.2 13.7
c1
c2 c3
+ c3 + c2 + Cl
c1
c3
21 40 80 90 110
Flows dl, d2,and d4 being definitively fixed, it is possible to choose, at the proper instant, the desired checking point and to plot the corresponding spot deviation. Once flows were fixed in accordance with values given in Table 11, we led checked air to bubble first into an iodine solution, then into an iodine monochloride solution. We prefer this second reagent because it absorbs lead better than the first; it allows a greater flow and necessitates a smaller quantity of reagent. Lead in iodine monochloride solutions is directly determined by classic atomic absorption. Results with the two methods are in good agreement. Second Calibration Procedure. Testing by Injection of Lead Organic Compound Solution (Figure 5). The first calibration method necessitates a relatively complicated apVOL. 39, NO. 4, APRIL 1967
b
429
figure 7. Lamp spectrum of active (2833 and inactive (2800 k)lines 0.
A)
speetnnnwitboutnalne
b. Spemum with flame, lead-free air c. Spectrum with flame,lead-polluted air
Table III. Lead Coneenbations of ISolutions and Ckcked Air by Testing by InjectionI of Solution Figure 5. Constant flow injection equip ment for small quantities of lead in organic solution paratus. We therefore developed a simpler and directly quantitative technique. The apparatus is tested by introducing an organic lead solution into the circuit at a low rate. The solution is evaporated in the air stream at a high flow rate, and lead passes into the flame. We have studied in this connection a drive system employing a threaded rod driven by a small electric motor, imparting a speed of 2 mm per minute to the rod. Different syringes could be fitted to this apparatus, the needles of which were pricked through a special rubber membrane, placed in a section of the pure air entry pipe. Tests carried out with 10- and 2500-pl syringes gave hourly rates of 0.022 and 5.5 ml. The solvents used were acetone, toluene, and pure gasoline with lead originating from TEL and TML.
All the results were absolutely identical for a given quantity of lead passing per unit of time into the installation; the passage of one of these solvents, without addition of lead, did not modify the deviation on the recorder, after the passage of air standardized by the first procedure.
Solution concn.,g/l Air conco., lo-' glcu meter
0.05 0.27 0.54 1.09 2.18 3.27 5.45 10.9 21.8 1
5
102040601M)200400
We checked with TML solutions in toluene. The piston stroke of our lO-pl syringe is 54.5 mm; since the piston speed is 2 mm per minute, solution flow is 0.022 ml per hour. Since the d, flow is 1200 liters per hour, we get lead in air concentrations given in Table 111. Figure 6 shows recordings obtained with concenirations varying between 1 and 100 X 10-1 g per cu meter. The corresponding plot is perfectly linear. A plot corresponding to higher values, although curved, is nevertheless usable. The two calibration procedures agree perfectly. Basis Line Verification. For good calibration, we must be sure that the basis line corresponds to lead-free air. The lamp spectrum allows this verification; it includes, besides the lead line, an inactive line. The two lines, of intensitie;l, and h, present a constant relationship 1,/12= K for each lamp (Figure 7,a). If air is lead-free, I,/& equals K when the flame is switched (Figure 7,b).
10.3711
100 x 10+q/rn'
4 Figure 6. Curve obtained by injection of organic lead solution,
10.230)
lead eooeeotrations in air varying between 1 X lod and 100 X 10-6 glcu meter
10.156)
Medium ramrdiug time for plnteau, 7 minutes Numbers in brseleia M ab.wrbanaa
10.0731
10.019' ~ld'g/rn'
(0.003) 3,,,/
/wL-W--
. 430
I
ANALYTICAL CHEMISTRY
ieral view of equipment nent for constant flow injection, recorder, spectrophotometer with equip-
$
Table IV. Results of Determination by Two Methods
I Operation of the Apparatus. Once the compressors are started, and the air flow is regular, one only has to heat the lower part of the tube to red heat with a Bunsen burner for example, in order to lighvthe flame. A safety arrangement using the ultraviolet emission of the flame operates an audible alarm to warn the works personnel in case of flame extinction or simply a change in the burning rate of the flame. Simultaneously, by closing the electromagnetic valves, the safety device stops the admission of the synthetic gas mixture. The standardization curve was definitivelytraced. We have only to check the basis line with purified external air and to verify one of the standards. During a complete service year, we have never found any deviation of the curve. RESCI TS, uisccssinu. A w ..\l'l'l IC.\TIOSS O F TECIIXIQUE
The apparatus permits the detection of concentrations of lead in the atmosphere as low as lo-' g per cu meter, which corresponds to spot displacements of the galvanometer recorder of the order of 2 mm. We can measure up to 5 X lo-' gram of lead per cu meter of air. For higher contents, sometimes found if interventions on installations are necessary, we must dilute with pure air. In Table IV, results obtained during different periods are listed, first with our apparatus (the mean value of the recording is calculated) and then by the iodine absorption method and dithizonate colorimetry or iodine monochloride absorption followed by atomic absorption with the solution. Figure 9 shows an hour recording in a TML fabrication installation. The technique described appears to meet the needs of factories handling organic lead derivatives. It leads, in addi-
(Concentrations in 1 0 - 6 g Pb/cu meter of air) Atomic Lead found absorption by absorption (calcd. on Iodine Manufacturing Control recordings moncinstallation time, given by Iodine chloride producing hours apparatus) TEL 11 9 11
TML
11 8 11 23
1 12 8
5
10 8 13 10 10 21 9 16
8 11 8 9
7
6 10 10 10 7 10 22 7 14 14 8 14
I2
60 48 32 ..
8 12 18 12 18 12 15 63' 53' 33 34 30
Intervention in installatii
tion, to increased safety of personnel and improvement in working conditions and in production efficiency. The total time during which masks must be worn is greatly reduced, and the detection of leakage, normally a difficult operation, is facilitated by the use of mobile sampling equipment. The fluctuations of the lead level during exploration of an area where levels are high permits localization of one or more leakage points. Figure 10 shows the recorded curve during repairs on a pipe of the plant. The increase of lead concentration is so VOL 39, NO. 4, APRIL 1967
431
100
70
60
Figure 9. Curves obtained in TML
- fabrication installation
Standardization point 40 X g/cu meter by air dilution b. Standardization point 40 X 10-8 g/cu meter by injection of lead organic compound solution c. Recorded curve during 1 hour of service a.
tion of lead in the organometallic state, but is applicable for any other element which is found in the atmosphere in gaseous form and is detectable by atomic absorption technique, its atoms having a sufficient lifetime. The method has been applied in particular to mercury.
i
DETERMINATION OF MERCURY IN AIR 4
- 100x 1G6g/m3 - 90 - 80
- 70
I
- 60 - 50 - 40 - 30
20
t
lo ' 0
Figure 10.
Curve during repairs on a valve
high that we note a complete saturation of the lead line. The ventilation of the installation is so efficient that after half an hour it is possible to return to the installation. Before the use of our apparatus, it was necessary to wait 5 hours. We notice for example the detection of a cyclic pollution caused by residual lead in a dead space between an autoclave closing valve and a charging funnel opening valve. Medical services require information on the average concentration at the various points, in order to know the magnitude of the hazards. The proposed method does not give this information, unless one apparatus is assigned to each installation building. It is possible, on each line connected to a sampling point, to provide a branch for allowing the air to bubble through an absorption solution. One can then determine directly each day, by atomic absorption with a standard apparatus, the lead retained by these absorbent solutions. All the control equipment can be integrated in a control room, greatly simplifying operations. This technique is not limited exclusively to the determina432
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ANALYTICAL CHEMISTRY
Apparatus for determining quantities of mercury in air has existed for a long time (6). It is based on the absorption of light emitted by a mercury vapor lamp. The adaptation of this equipment for use with the atomic absorption technique proposed for lead increases the sensitivity and selectivity. The hollow cathode mercury vapor lamp takes the place f! the lamp with lead, and emits rays of wavelength X = 2537 A. The burner is eliminated, as mercury is in the monatomic state, and the silica tube is replaced by a glass tube of the same length, into which the air being analyzed is passed. Zeroing is easy; it suffices to dilute the air which has passed through a vessel containing mercury at 0 "C.We have checked (by mercury absorption in a dilute iodine solution and measurement by atomic absorption of the mercury concentration of this solution) that this corresponds exactly with the figure calculated from the mercury vapor pressure at 0°C-i.e., 0.000186 mm of mercury. Zeroing then becomes simply a question of flow measurements. The threshold of detection is IO+ g per cu meter as in the case of lead. ACKNOWLEDGMENT
The author thanks in particular A. Akhal, J. P. Courtieu, P. Roussel, and C. Vo Thanh for technical assistance in this work in the Central Laboratory of Etablissements Kuhlmann. He also thanks the staff management engineers and workers of the plant where the apparatus was installed. RECEIVED August 29, 1966. Accepted December 9, 1966. Work carried out at the Central Research Laboratory, Etablissements Kuhlmann, 95, rue Danton, 92-LevaUois-Perret, (France), and at the Octel-Kuhlmann plant (subsidiary), 44Paimboeuf, France. ( 6 ) T. T. Woodson, R e a Sci.Instr., 10, 308 (1939).
