Determination of Lead Alkyls by Gas Chromatography with Electron Capture Detector
SIR:An important use of the electron capture detector is that of analyzing lead alkyls. This detector has the unique property of being very seniitive to lead alkyls and halogenated scavengers and essentially insensitive to all other hydrocarbon materials found in gasoline. Lovelock and Lipsky were first to suggest the potentialities of the electron capture detector (3) and Lovelock and Zlatkis suggested its use for analyzing lead alkyls (4). They were followed by Dawson ( 2 ) who improved the technique using temperature programming with a special terminus scrubbing column. The scrubber column was used to remove the lead scavengers which seriously interfere with the analysis. Barrall and Ballinger ( 1 ) used a 1, 2, 3tris (Z-cyanoethoxy) propane (TCEP) column without a scrubber. They were able to separate all five lead alkyls but were forced to use a long analysis time to elute the lead scavengers. This article describes the use of the Barrall-Ballinger type column operated isothermally and with a silver nitrate scrubber column. The five lead alkyls were completely separated in 10 minutes with good quantitative recovery. The overall accuracy of the method is about 4%.
isomers of lead are separated according to increasing molecular weight. The trace represents a standard mixture containing 6 25 mole % of lLIerPb, 25 mole % Me3EtPb, 37.5 mole % hfenEt2Pb, 25 mole % hfeEtaPb, and 6.25 mole % Et4Pb. Thiq mixture was diluted with spectrograde hexane t o 4 ml./gal. as Et4Pb, the legal maximum. The resulting solution was then diluted 1:200 with the same solvent so that the sample would not exceed the linearity range. Operating conditions are: column, 72" C.; injector, 95' C.; detector, 150' C.; carrier, IZ'? a t flow rate, 27 ml./min.; cell potential, 22 volts; attenuation, 10X4X. Calculations. The relative electron affinity of t h e individual lead alkyls was calculated based upon t h e percentage coinposition given in the manufacturer's specifications. T h e amount of each in t h e standard mixture, in micrograms (pg.), was related to its peak area. The area of each peak was determined using .a Disc Chart Integrator. The sensitivity is expressed as Disc Units per microgram a t 1X-1X attenuation. From this, the concentration of lead additives in any gasoline sample can
all
Table
1.
Relative Electron Capture Sensitivity
Disc
unitslun. IX-IkY
22 volts Lead alkyl Tetramethyllead ( MeaPb) 465,000 Trimethvlethvllead (MerEtPb) 463,000 Dimethildiethyllead ( ?vIe2Et2Pb)
505,000
Methyltriethyllead (MeEtdPb) 470,000 Tetraethyllead (EtaPb) 245,000
be calculated. Tablt. I relates the electron afhities in Disc Units per microgrnni of the five lead additiT P'. RESULTS AND DISCUSSION
The standard deviation on replicate injections was calculated from area measurements. Ten consecutive injections of 0.4 111. of the antiknock motor mix showed a relative standard deviation from 3.9 to 4.45% for IvIeaEtPb, ;\Ie2Et2Pb, and MeEt3Pb. I n the case
Me2 Et2P b
EXPERIMENTAL
Apparatus. A Kilkens Aerograph H y - F I Model A-610-B with electron capture detector was used in this qtudy. Two standard accessories, a detector heater and voltage control, Model A-630, were also employed. T h e output of t h e chromatograph was recorded on a I m d s & S o r t h r u p Fpeedomau H l - l W recorder. A Disc C h a r t Integrator, Model 207, wds used for area determinations. Method. The analytical method presented here consists of separating the lead alkyls on a 10-foot X '/pinch stainless steel column of 10% T C E P on * o / l ~ ~Chromosorb W,H M D S treated. The scrubber section, a 6-inch X l/8inch stainless steel column composed of 20% Carbomax 400 (saturated AgSOa) on 30/m Chromosorb W precoated with 8% KOH, is attached between the analvtical column and the detector. The separating efficiency of this technique is shown in Figure 1. T h e alkyl 1980
ANALYTICAL CHEMISTRY
Figure 1.
Sample: 4.0 cc. per gal. as EtrPb diluted 1 :200,0.4pl. injection
of 1LIedPb and Et4Pb the relative standard deviation WE,S higher than 8 and 147,, respectively. This larger error is due to the difficulty in measuring the area under a small peak. The relative standard deviation for hIeaPb and Et4Pbusing higher concentrations of the individual compound:; was 3.5 and 3.65%. The sensitivities of all compounds n hich have electron affinity properties L-ary mith the conditions of analysis as: column temperature, cletector temperature, flow rate, voltage applied across the cell, and the cl2anliness of the source. For this reascn, it is advisable to calibrate the instrument frequently with known standards.
The scrubber section serves a very important function. It adsorbs the column material and thereby maintains the full sensitivity of the detector. It also removes the halogenated lead scavengers by reacting with silver nitrate in the packing. Since these scavengers elute a t approximately twice the retention time of ETdPb, time of analysis may be considerably shortened by their removal. If the analysis of these lead scavengers is important, i t may be included with the lead analysis by simply using a scrubber without silver nitrate. The possibility of chemical interchange in the column was investigated by injecting a mixture of Me4Pb and
Et4Pb into the chromatograph. S o intermediate compounds were detected. LITERATURE CITED
(1) Barrall, E. M., Ballinger, P., 144th
Meeting ACS, Los Angeles, Calif., March 1963, Abstracts of Papers, p. 18B. ( 2 ) Dawson, H. J., Jr., ANAL. CHEW35, 542 11963). (3) Ldvelock, J. E., Lipsky, S. R., J . Am. Chem. SOC.82, 431 (1960). (4) Lovelock, J. E., Zlatkis, A., ANAL. CHEM.33, 1958 (1961). ERXESTJ. BONELLI HAROLD HARTMANN Wilkens Instrument & Research, Inc. P. 0. Box 313 Walnut Creek, Calif.
A Photometric Method for Trace Mercury Determination Using a Beckman DlJ Spectrophotometer SIR: The photometisic determination of submicrogram amounts of mercury based on the absorption of the mercury resonance line at 2537 A. has been reported b y Ballard arid Thornton ( 2 ) . Subsequently, use of t GE ultraviolet intensity meter ( 3 ) and a method of correcting for organic: substances ( I ) \$ere published as refinements of the original method. Thi? apparent simplicity and high sejisitivity of the niethod recommended its use in the determination of trace amounts of mercury in various materials including those used by the photographic industry. V h e n the GE intensity meter was difficult to obtain, a n alternative measuring system was sought. I n the references cited, the Beckman DU spectrophotometer apparently was not used for the mercury determination but only in obtaining a correction for organic compounds present. This note describes modifications made to a DU to
adapt i t to mercury determinations using a narrow (0.02- to 0.05-mm.) slit. Figure 1 shows a schematic diagram of the apparatus. The photocell compartment is separated from the monochromator by the mercury absorption cell which is supported a t its ends b y two receiving plates. A blue filter C.S. (Corning Glass Works catalog KO. 7-54) is permanently mounted over the phototube aperture as a protection from exposure to room light. The whole sybtem is tied together with three long assembly screws. A 4-watt. Sola regulated, U-shaped germicidal lamp (GE catalog No. G4T4/1) is used as a mercury light source. A movable electric heating thimble completes the assembly. Compressed air is used t o flush the absorption cell betv-een samples. The cell is carefully painted and wrapped with opaque tape to prevent interferences from overhead fluorescent lights. The extraction procedure outlined by Ballard and Thornton (2) was generally
5lercury L i g h t S o u r c e Q u a r t z Prism
satisfactory. S n analytical curve was obtained allowing ready detection of 0.1 pg. of Hg. Approximate per cent absorption figures mere as follows: blank pad-4%, 0.1 pg.-30%, 0.2 pg.48%, and 0.3 pg.-GO%. A 50-gram quantity of KBr dissolved in water or a 1OO-nd. volume of 47% K&03 solution can be extracted and the mercury concentration determined in the p.p.b. range. Extraction efficiency was investigated using radioactive HgZo3isotope as a tracer. Quantitative separation (>%70) with these solutions was obtained using 12- to 20-mg. pads in the extractor. Tracer experiments also revealed erratic mercury extraction from urine samples with or without wet ashing. -4 convenient separation procedure for volumes u p t o 5 ml. of such acidified samples is to precipitate the mercury directly in the sample tube by adding cadmium acetate solution and saturating with HnS. After centrifuging, the decanted liquid contains less than 2% of the added tracer concentration. The residue may then be dried below 100OC. in preparation for the final measurement. LITERATURE CITED
Phototube
(1) Ballard, A. E., Stewart, D. W., Kamm, W. O., Zuehlke, C. W., ANAL. CHEM.26,921 (1954). (2) Ballard, A. E., Thornton, C. D. W., IND. ENG. CHEM.,AXAL.ED. 13, 893 Beckman DU
Absorption
Spectrophotometer
Figure 1,
/
Schematic of apparatus
V-Sample
Tube
(1941).
(3) Zuehlke, C. W., Ballard, A. E., ANAL.
CHEM.22,953 (1950). RALPHE. MANSELL ELWOOD J. HUNEMORDER Chemical Physics Research Laboratory The Dow Chemical Co. Midland, Mich. VOL 35, NO. 12, NOVEMBER 1963
1981
