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Table I. Nitrogen Determination at the Part-per-Million Level Max. Extracdeviation tion Kjelfrom Sample dah1 Coulometric Mean mean 1.6 0.2 1 . 6 1.5, 1.8, 1.5, 1 . 4 Naphtha A 1.5 0.1 1 . 2 1 . 6 , 1.4, 1 . 4 B
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B C Heavy gas oil A Lube oil I5/8" HYDROGEN)[
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1.4 1.4 0.7 0.2 0.16"
1.6, 1 . 2 , 1 . 5 1.2, 1 . 6 0 . 6 , O .8 0.4,0.5 0.23, 0.22,O. 34, 0.30,0.13,0.13 0.24" 0 . 3 9 , 0 . 2 4 , 0 . 2 5 , 0 . 3 1 5.2 5.0,5.3,5.4 2 . 0 2.1, 1 . 9 , 2.0, 2.0 1.1 1.0,1.2
1.4 1.4 0.7 0.5 0.23
0.2 0.2 0.1 0.1 ,0.11
0.29 5.2 2.0 1.1
0.10 0.2 0.1 0.1
Average of 12 determinations
'OUARTZTUBE
I
Table I shows typical results obtained with various lownitrogen samples. In all cases, the maximum deviation from the mean is within 0.2 ppm, and the results compare favorably with those obtained by a Kjeldahl method using a modified sulfuric acid extraction (2).
The Kjeldahl values for light gas oil A and B are the average of 12 determinations. Generally, the accuracy of the extraction Kjeldahl method for a single run is about 1 ppm. However, if the sample contains large amounts of olefins and easily sulfonated aromatics, the digestion procedure can be extremely tedious and the accuracy can decrease to 4-5 ppm. The coulometric method is not affected by the degree of olefinicity. Since the new equipment can be operated either as a programmed or a constant temperature unit, it can be used for samples containing less than 1 ppm nitrogen or as much as 5 nitrogen.
(2) 0. I. Milner, R. J. Zahner, L. S. Hepner, and W. A. Cowell, ANAL.CHEM.,30,1528 (1958).
RECEIVED for review November 2, 1970. Accepted January 7, 1971.
Figure 1. Hydrocracking hydrogenation tube RESULTS
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Syringe Procedure for Transfer of Nanogram Quantities of Mercury Vapor for Flameless Atomic Absorption Spectrophotometry Michael P. Stainton Fisheries Research Board of Canada, Freshwater Institute, 501 Unicersity Crescent, Winnipeg 19, Manitoba
DETERMINATION OF MERCURY by flameless atomic absorption spectrophotometry is well known (1-6). Mercury in solution is reduced to the metal, partitioned with air and transferred, in a n air stream, to a flow-through cuvette in the spectrophotometer. High sensitivity is possible [ E = 4.1 X lo6 (5)] but usually the equilibrated vapor i s diluted with carrier gas and sensitivity is lost. It has been shown (5) that with one dynamic system only 7 % of available mercury may be present in the cuvette at the time of measurement.
The method described allows transfer of mercury vapor, in equilibrium with reducing solution, to the cuvette. By using a small volume cuvette, a small sample can be used. Sensitivity is good a t 0.2 pg per liter of mercury in liquid solution. Precision is excellent with a relative standard deviation of 1 at the 20 pg per liter level. Instrument output is between 40 and 60 samples per hour with n o special apparatus requirements except an atomic absorption spectrophotometer.
(1) N. S. Poluektov, R. A. Vitkun, and Yu. V. Zelyukova, Z h . Anal. Khim.,19, 937 (1964). (2) M. S . Dill, Bull. Y-1572, Union Carbide Corporation-Nuclear Division Y-12 Plant, Oak Ridge, Tenn., 1967. (3) H. Brandenberger and H. Bader, At. Absorption Newslett., 6 (3), 101 (1967). (4) W. R. Hatch and W. L. Ott, ANAL.CHEM., 40,2085 (1968). ( 5 ) J. F. Uthe, F. A. J. Armstrong, and M. P. Stainton, J . Fish Res. Ed. Can., 27, 805 (1970). (6) G. Wobeser, N. 0. Nielson, R. H. Dunlop, and F. M. Atton, ibid., p 830.
Apparatus. ATOMICABSORPTION SPECTROPHOTOMETER. A Perkin-Elmer Model 403 was used for this work. Either automatic readout of absorbance o r chart readout is essential. Either a mercury hollow cathode lamp o r a mercury vapor lamp may be used. The latter requires its own power source but gives a better signal to noise ratio. CUVETTE. Borosilicate glass, 15 cm long 7 mm o.d., 5 mm i.d., with silica end windows 1 mm thickness attached with epoxy cement was used. Approximate volume was 3 cc (See Figure 1).
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EXPERIMENTAL
'ANALYTICAL CHEMISTRY, VOL. 43,
NO. 4, APRIL 1971
625
1.5 rnm. O.D. Polyethylene tubing
\Holder
Plastic needle coupling
for absorption cell
4 -Vapor
Disposable plastic syringe \
Figure 1. Cuvette and mounting arrangement for injection of mercury vapor 2 E. ?
LGCK V 3 J N T INGS ’ SCLCERED l O S E TO NOSE
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Figure 2. Double Luer-Lok connecting collar
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Figure 3. Mercury calibration curve for the range 4-20 pg/l. CUVETTE HOLDER. The holder shown in Figure 1 mounts the cuvette on the burner head and eliminates slray light passage. SYRINGES.Ten-cc polypropylene disposable syringes with 0.2-cc marking intervals, Luer tip and cap, and one 2-cc Cornwall continuous dispensing syringe were used. SYRINGECOUPLING.Two Luer-Lok needles were used with the capillaries removed and soldered nose to nose (See Figure 2). VORTEXMIXER. MICROBURET.A microburet of 0.2-cc total volume was used. Reagents. The reductant consisted of water (about 600 CC), HLSO4(S.G 1.84) (100 cc), NaCl ( 5 grams), (NH20H)2.HzS04 (10 grams), and SnS04 (20 grams). This was prepared in the order given, cooled, and diluted t o 1000 cc. MERCURIC CHLORIDE STANDARD A. HgCL (0.1354 grams) 626
ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
was dissolved in water and diluted t o 100 cc (1.00 cc = 1.00 mg Hg). The solution was stable for 1 month. MERCURICCHLORIDE STANDARD B. One cc of standard solution A was diluted t o 100 cc using lNHzSO4. (1.00 cc = 10.0 pg Hg.) The solution was stable for 1 day. The KM,O4 solution used was 6 % w/v in water. The HzOz used was 3 0 x wjv reagent grade. Procedure. The procedure described is for determination of mercury in tissue digests. Samples were digested by the method of Uthe et al. ( 5 ) using concentrated sulfuric acid and 6 % potassium permanganate. Digested samples contained a suspension of hydrated manganese oxides in about 20 cc of liquid, which was cleared with dropwise addition of 30% H202. Volume was made up to 25 cc with distilled water. Standards were prepared by adding 0, 0,100, 0.200, 0.300, 0.400, and 0.500 pg of mercury to 30-cc Kjeldahl flasks using a microburet. Standards were treated as samples using 4 cc of concentrated H2SO4and 15 cc of 6 % KM,04. The resulting suspension was cleared with 3 0 z H202. When it was cool, it was diluted to 25 cc with distilled water. The cuvette was mounted in its holder, on the top of the burner assembly of the spectrophotometer. By using the burner position controls, th: cuvette was aligned for minimum absorbance at 2537 A. The instrument was zeroed. After a series of samples and standards have been prepared as above, 2.0 cc of sample liquid were drawn into a syringe. With the 2.0-cc Cornwall syringe fitted with the double LuerLok coupling, 2.0 cc of reductant were injected into the syringe containing the sample. Six cc of air were drawn into the syringe and the cap was replaced over the tip of the syringe. The syringes were taken one a t a time, the tips touched to a vortex mixer for 10 seconds and the airvapor mixtureainjected into the cuvette. Maximum absorbance at 2537 A was recorded, and the same syringe was used to withdraw the vapor from the cuvette, zeroing the apparatus. RESULTS AND DISCUSSION
Figure 3 shows a calibration curve from 4 to 20 pg per liter mercury in liquid showing deviation from Beer’s law. The actual strip chart readout of a series of standards appears in Figure 4. While this is not the normal readout mode,
Table I. Precision Data for Mercury Standards Level Hg pg per liter in liquid phase 0 4 8 12 16 20 Absorbance Mean(7 = 6) 0.004 0.142 0.273 0.377 0.479 0.557 Standard deviation 0.001 0.003 0.003 0.004 0.003 0.003 Table 11. Per Cent of Available Mercury Presented for Absorbance Measurement Level Hg Moles Hg +/l. in Moles Hg in in 3-cc Available mercury liquid phase syringe cuvette measured, 4 8 12 16 20
3.98 X lo-" 7.96 x 11.94 X 15.92 X 19.90 X
0.72 X lo-" 1.38 x 1.90 X 2.42 X 2.81 X lo-"
18.0 17.3 15.9 15.2 14.1
the figures give some indication of the precision obtainable. Results from the digital readout of absorbance for a series of six replicates; from 0-20 Fg/liter, mercury in liquid, appear in Table I. Figure 5 shows the increase in absorbance measured, with varied agitation time. Partitioning appears to be complete after 10 seconds of agitation on the vortex mixer. Samples agitated and left standing for 15 minutes showed no significant change in absorbance. It is believed that the rate of injection of the vapor is fast enough to displace air from the cuvette without mixing, hence the high sensitivity. Absorbance rises rapidly to a maximum, remains constant for 5 seconds, and drops exponentially with a half life of 6 minutes. Losses of mercury from the cuvette are from diffusion through the exit port
01 2
5 1 0 I5 20 25 Aglfoflon Time - s e c o n d s
Figure 5. Increase in absorbance with agitation time and possibly from oxidation of the metal with oxygen generated by the peroxide in the reaction mixture. By using the molar absorptivity of Uthe et af. (5), the quantities of mercury presented for analysis can be calculated and appear in Table 11. At the 4 pg/liter level, 18z of available mercury is in the cuvette. The apparent decrease in efficiency at higher concentrations is to be expected, as Uthe et fil. (5) point out that mercury vapor does not obey Beer's law, there being curvature toward the concentration axis. The method when applied to the analysis of mercury in Northern Pike gave standard deviations of ~ k 0 . 0 3at the 0.5 ppm level and 10.68 a t the 10.0 ppm level. ACKNOWLEDGMENT
The author thanks F. A. J. Armstrong, C. Tam, and D. Metner for critical comments and analytical assistance.
RECEIVED for review October 16, 1970. Accepted December 21, 1970.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
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