Table I\'. Determination of Compositionand the Calorific Valuea of a Typical Natural Gas 4, mole
Reference
Found
Standard derivation ( n = 17)
0.91 Carbon dioxide 0.92 0.016 Nitrogen 14.3 14.4 0.186 Methane 81.4 81.2 0.189 Ethane 2.78 0.02 5 2.81 0.40 0.38 0,009 Propane Butanes 0.14 0.15 0.004 Pentanes 0.046 0.002 0.035 Hexanes 0.003 0.023 0.019 Heptanes 0.005 0.015 0.009 Benzene 0.003 0.019 0.017 Toluene 0.001 0.004 0.006 Gross calorific value 18 8,355 8.361 at 25°C. kcal/m3 ideal, dry gas at 0°C and 1013 mbar aThe calorific value can be symbolically written as C = c a L ( x 2+ e , ) + a,[l - c(xi + t This means that the standard deviation of C is expressed as Since the methane content is not determined independently, the standard deviation is calculated as S , ~= where aL = calorific value of component i. a,. = calorific value of methane, x L = actual mole fraction of component i, c , = deviation of actual mole fraction of component i. and si = standard deviation of mole fraction of component i.
a
of the calorific values calculated from the reference and found compositions appearing in Tables I11 and IV. If the gas sample is contaminated with air during sampling, the analysis can be started a t an initial column temperature of -55 "C to obtain separation of oxygen from nitrogen. The initial column temperature should be kept constant for 6 minutes before the programmed operation is started. In view of the poor separation of oxygen from nitrogen, oxygen concentrations lower than 0.5% mole have to be determined separately using a column packed with Linde 5A molecular sieves. When no separation of oxygen from nitrogen is required, or when the oxygen concentration is negligible, the column temperature can be started at -10 "C as mentioned in the Experimental procedure. A higher initial temperature may lead to a non-linear response for the nitrogen peak a t a concentration higher than 15%mole. As helium is used as the carrier gas, it cannot be determined by this method. If helium is present in the natural gas, it has to be determined separately using a column with Linde 5A molecular sieves and a different carrier gas, e.g., nitrogen. Most Dutch natural gases contain helium in concentrations u p to 0.05% vol.
ACKNOWLEDGMENT The authors thank G. Strulik and D. Alten of BrigittaElwerath, Hannover, Germany, for their information on unpublished work.
LITERATURE CITED C2H6, and higher hydrocarbons) and to find the percentage of methane by subt,racting the sum of the minor components from one hundred. This method is accurate only if all the non-methane components have been determined precisely. If any suspicion arises, the methane content can be checked by gas chromatography, using volumetric detection according to JanAk (6, 7 ) . T h e standard deviation which can be reached in the determination of the individual components is shown in Table IV. The calculations yield a calorific value of 8358 with a standard deviation of 18 in 17 measurements (relative standard deviation = 0.22%). The high degree of accuracy is illustrated by a comparison
(1) "Selected values of physicai and thermodynamic properties of hydrocarbons and related compwnds.'' API Research Project 44, the National Bureau of Standards, 1": .ill ,.,;ton. D.C.. published in 1955 and 1962. (2) H. Sears, Aust. Mii?er. Gigelup. Lab. Bull., 6, 47 (1968). (3) A. Manjarrez and 0 Laciron d e Guevara. Rev. Inst. Mex. Petrol., 1 ( Z ) , 61 (1969) (Spanish). (4) F. van de Craats, "Gas Chromatography 1958," D. H. Desty, Ed.. Butterworths ScientificPublications. London, 1958. pp 248-64. (5) G. Strulik and D. Alten. Brigitta-Elwerath, Hannover, private communica-
tion. (6) J . Janak, Collect L . (7) J. Janak. Collect. Czc
RECEIVED
19, 684 (1954) (German). C wn. Cornrnun., 19, 700 (1954) (German). . wit. Curnmun.,
',
for review J;ily 1, 1974. Accepted October 4:
1974.
Determination of Individual and Total Lead Alkyls in Gasoline by a Simple Rapid Gas Chromatography/Atomic Absorption Spectrometry Technique D. T. Coker Esso Research Centre, Abingdon, Oxon, U.K.
A procedure is described, which uses a gas chromatographic attachment to an atomic absorption spectrometer, for the analysis of individual and total lead alkyls in leaded or unleaded gasoline. The attachment, which operates isothermally, does not use a complete chromatograph, and can be made from inexpensive components. The method is faster than most comparable technlques and has a through-put time of about 5 minutes per sample; no sample preparation is required and the precision of the method is comparable with the standard ASTMAP X-Ray fluorescence technique. 386
*
The detection limit is around 0.2 ppm lead for each alkyl and the method is suitable for trace lead levels in unleaded gasoline. With some modifications, this limit could probably be lowered to levels sufficiently low for pollution monitoring applications.
Lead alkyls from tetramethyl (TML) to tetraethyl lead
(TEL) are added to gasoline to improve its octane rating. The actual mixtures used vary widely and are usually ad-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, MARCH 1975
SECTION THRQ BURNER (SIDE VIEW1 COLUMN
0 RING SEALS
1"
~uN1ow THREADED INTO BURNER DODY
SILVER SOLDERED TO T
OUTLET
Figure 1. Layout of equipment
justed to give an even octane rating across the gasoline boiling range. The permitted levels of lead added are limited by legislation in most countries. The coming introduction of catalytic exhaust pollution control devices to car engines in the USA and Japan, however, will require unleaded gasoline as fuel, because lead acts as a catalyst poison, and a limit of around 20 ppm P b will be set. These considerations require that suitable analytical techniques be available to measure total lead in leaded gasoline, lead alkyl distribution in gasoline, and low level lead in unleaded gasoline. The lead alkyl distribution can also be required for many other reasons, such as: (i) T o help in identifying spillages and leakages. A recent survey of 30 gasolines from 8 UK manufacturers from 2 to 5 star grades showed that the lead alkyl distributions were different in almost all cases. This difference is due to manufacturers meeting different A factors in their blendsusually by adding various ratios of TML and T E L or occasionally only one of these. Some 25% of the gasolines surveyed also contained various intermediate alkyls between T M L and TEL. Also the total lead alkyls in gasoline spilled or leaked into soil deteriorate slowly, and a) the appearance of extra peaks due to partly halogenated lead alkyls, and b) the difference between the total lead as alkyls measured by this method and the total lead including inorganic forms, measured by one of the standard techniques, gives an indication of how long the gasoline has been in the soil. (ii) For predicting gasoline properties by compositional analysis (at present the subject of a study by ASTM and I P sub-panels). (iii) Lead is only effective as an anti-knock in a volatile form and total lead measurements may include some inorganic lead in aged or contaminated gasoline. (iv) Their effect on sensitivity (difference between the motor and research octane ratings). (v) For quality control on blending. (vi) Development projects and correlation programs. Standard methods at present in use for determining total lead can be summarized as: Wet chemical (ASTM D526/ IP96, D2547/IP248), Polarographic (ASTM D1269), Atomic Absorption (AAS) (ASTM D3237), and X-Ray Fluorescence (XRF) (ASTM D2599/IP228). XRF is probably the most popular of these methods as it is fairly quick, but the equipment is very expensive and its limit of detection is too high for trace lead in unleaded gasoline.
The AAS method requires sample preparation to overcome the effect of different responses from each alkyl by reacting with iodine. However, our experience has shown that if the gasoline has a high olefine content, this pretreatment can be ineffective. The satisfactory determination of individual lead alkyls is still an analytical problem, and only one standard method exists (IP 188/66T), which is very lengthy. Several approaches have been tried for the determination of individual alkyls involving a gas chromatographic separation of the alkyls from TML to TEL followed by detection of the lead using various techniques (1-6). In all these methods, the separation time is rather long (15 minutes or more), and many of them suffer from interferences. Recently a method ( 7 ) has been described using a GC separation with a flameless AAS detector which achieved separation of the P b alkyls. No separation time or precision data were given, and some solveht interference was present. None of these methods appear to be ideal for rapid routine use. In order to overcome these disadvantages, equipment and a method have been developed to determine individual lead alkyls with the following objectives in mind. (i) It should be a simple, inexpensive, "plug-in" attachment to a routine instrument. (ii) There should be no sample preparation involved. (iii) All the lead alkyls from TML to T E L should be completely resolved. (iv) It should take less time per analysis than XRF and have similar repeatability. (v) It should be sensitive enough to detect trace lead levels in unleaded gasoline. (vi) It must be interference-free and specific for lead. These objectives were met by designing a short high-efficiency GC column for isothermal operation, thus eliminating the need for a complete gas chromatograph. The alkyls from TML to TEL are eluted from the column in under 5 minutes, TML eluting in about 45 seconds. AAS was used for the detector in preference to FES as it is completely specific for lead, does not suffer from the flame band emission spectral interference of FES, and gives better precision. AA Spectrometers are also relatively inexpensive (compared with XRF Spectrometers) and are now one of the most common laboratory instruments. The main disadvantage with AAS is a more limited response range, compared with FES, but as the levels of lead in leaded gasoline to be determined do not differ too widely, the range over which the response is linear is not critical.
EXPERIMENTAL Apparatus. GC Attachment (see Figure 1 ) . COLUMN. 3 f t X lh-in. 0.d. (%,j in. i d . ) stainless steel packed with 10% PEG 20M (Carbowax) on 100/120 mesh Porasil C (Waters Associates). The packing is prepared by first dehydrating the support by heating a t 150 "C for 2 hours under vacuum. A solution of 10% PEG 20M in water-free methanol is added and stirred in, and the solvent is then removed by warming under vacuum in a rotating flask. The column is packed under vacuum using vibration to compact the packing, until no further settling occurs, then plugged. One and a half inches of free space are left a t the injection end. The column is then conditioned a t 250 "C for 2 hours. The use of spherical silica beads as support allows a uniform high column packing density which gives fast, high resolution separations from relatively short columns. A polar stationary phase is used because it is easier to coat onto Porasil, giving good peak shapes; and also, TML is retained sufficiently so that its peak is not too sharp, as the response of the AAS detector is best suited to broader peaks, while TEL is still eluted in a reasonable time in spite of the large difference in their boiling points.
ANALYTICAL CHEMISTRY, VOL. 47,
NO. 3, MARCH 1975
387
0 67 I O
'
a
Table I. Comparison of GC/AAS and XRF Results on Gasolines Blended to 0.60 g/l. Pba
I min
*I
I*
8 0 3
4.4s
sw
Pb ( g 4 I. ) as
T EL
05
,'
W
ic
-_
0.4
4 E
ul K
4
0.59 0.57 0.61 0.01 0.60 0. 58 0.57
0.01
CO.01
0.3
< 0.01
TW
0.01 0.01 0.01 0.57
