Fiaure 1. Vacu" um-distillation assembly
-eomm
UI
4
D
--3
This assenibly presents certain advantages over Xober's original tube. With the old design, it was difficult to evaporate the solvent without contaminating the second bulb with droplets of the distilland. Recovery of the distillate without contamination was also difficult, as it was necessary to cut the glass tubing between the bulbs. Accurate determination of yields is also simpler with the new design. Transfer of the distillate into another flask becomes an easy matter. The drip-tip joints and the direction in which they are arranged prevent contamination of the distillate by vacuum grease (Dow Corning high vacuum silicone grease, stable to over 200' C., is suitahle for lubricating the joints). The apparatus is equally suitable for vacuum sublimation. The diameters of the bulbs may he varied to accommodate larger or smaller samples and, by reducing the length of tubing, a larger number of bulb unite may be used. By attaching appropriate standard-taper condensers or condensation traps in that part of the assembly outside of the oven, materiak of high vapor pressure may be vacuum-distilled and collected.
The author is indebted to Frederick Highhouse, N I H Instrument Section, for advice in designing this assembly. The heating oven was designed by Grant C. Riggle, N I H Instrument Section, and by Ronald B. Ross of this laboratory in modification of a design kindly supplied by R. K. F. hlanske.
Robert E. Davis and J. M. McCrea, Petroleum ond Chemical Research Loboratory, The M. W. Kellogg Co., Jersey City,
introduction is an important S factor influencing precision of gas chromatographic analyses (8). The AMPLE
system developed in the writers' laboratory permits rapid introduction of small, reproducible volumes of liquid samples into a gas chromatographic column. It is an adaptation of a standard mass spectrometer liquid inlet system to gas chromatography, and
common method of injecting liquid samples with a hypodermic or micrometer syringe and serum bottle cap was introduced by Ray ( 5 ) , hut is difficult to control for small samples and results in an effective sample chamber of indefinite volume. A special needlesyringe device developed by Adams ( I ) allows introduction of controlled small volumes, but is rather difficult to manipulate and necessitates venting the column before injecting samples. More elaborate systems for sample introduction, utiliiing bypass systems with a bellows crusher (3) or cold trap ($I), involve time-consuming operations and hence are unsuitable for routine use. Bypass systems are desirable, however, as they permit introduction of the sample in a reproducible manner (8). A bypass-type liquid sample inlet 1 1 14
ANALYllCAL CHEMISTRY
STOPCOCK POSITIONS
'6
N.J.
utilizes an evacuated ehamber with mercury - sealed orifice, appropriate means for bypassing and sweeping out the sample chamber with carrier gas, and capillary pipets for measuring and transferring the liquid samples. The system is readily adaptable to most
SOURCE
gas chi oniatographs, including commercial apparatus equipped with gassampling systems. Self-filling, constant-volume, capillary mirropipets (4) are commonly used t o introduce liquid samples into mass spectrometers. These pipets are now usually used with commercially available mercury-sealed orifice systems (Precision Capillary Dipper Co., Baton Rouge, La.) rather than the mercurycovered sintered-glass d i s h originally proposed (4). The orifice allom all components of wide boiling mixtures to enter the system during the brief introduction period without the fractionation that frequently occurs when such mixtures pass through sinteredglass disks. The figure shows a schematic diagram of a mercury-sealed orifice liquid inlet system adapted to gas chromatographic usc. Glass stopcocks, if used, must be lubricated with a suitable hydrocarbon-insoluble grease when hydrocarbons are run (7). If the system is adapted to gas-sampling values on standard gas chromatographic apparatus, the chamber between stopcocks A and C replaces the usual gas sample chamber. The expansion chamber should be as flow-through as possible, to minimize pockets of temporary holdup. To introduce a sample. the chamber is first evacuated, using any convenient Lacuum pump with stopcocks A , B. and C in positions 1, 1, and 4, respect ively. .4slight residual pressure merely rcsults in a n air peak, and usually docs not interfere with liquid sample introduction or analysis of the resulting chromatogram. A pipet is filled by capillary action by touching the tip of the pipet to the surface of the sample liquid. Stopcock C is then turned t o poition 1 and liquid is introduced through the orifice by removing the plug and rapidly placing the dipper against the orifice. Mercury flows through the dipper and orifice and forces the sample quantitatively into the evacuated system. The plug is replaced. Then stopcocks A and B arc simultaneously turned to positions 4 and 2, respectively, sweeping the w n p l e into the column with carrier gas.
Sample size is determined by dipper volume and the number of times the dipper is emptied into the system. As these factors are varied readily, sample size may be controlled convenirntly and reproducibly. Sample chamber volume (between stopcocks A and C) is normally about 10 ml., which limits the amount of any component that can be vzporized at room temperature. For example, only about 1 11. of liquid n-octane will xxporize in 10 ml. a t 20" C. However, normal size samples of most liquids boiling below 100" C. can be completely v:iporized with the mercury-sealed inlet >\-stern operated a t room temperature. Small amounts of remaining liquids are
vaporized a s sample is swept into the column. In this case resulting peak heights are not as repeatable a s those obtained after complete vaporization. Modifications to tile SJ stem are necessary for complete vaporization of high boiling liquids. Suitable valves (3) are needed to replace greased stopcocks, and electrical heating n indings are necessary to hold temperature of the system above ambient. Barring toxicity hazard, there iq no objection to operating a mercury-sealed orifice a t temperatures up to say 50" C., where I apor pressure of mercury is only 0.013 mm. However, gdliuni iq preferable for sealing purposes, in view of its extremely low vapor pressure and the resultant lack of toxic vapor. An objection to use of gallium in this system is the large inventory of an expensive metal: most mass spectrometer laboratorieq w i t c h to a fritted-glass disk inlet system which requires a much smaller inventory when snitching from mercury to gallium as sealant. -4fritted disk inlet can equally well replace the orifice inlet in applications to gas chromatography when gallium is involved. Repeatability of sample introduction is illustrated by Table I. Samples of 2-methylbutane introduced a t room temperature n-ith n single filling of a 3-6-11. pipet gave a n essentially constant peak height reading with a standard deL iation of 1.27,. Samples of two 3.6-11. dippers full of % , 2 . 4 trimethylpentme introduced a t room temperature, n ith the column operated at elevated temperature, gave peak heights with a somewhat larger standard deviation of 2.2%. As these estimates of standard deviation include errors of fluctuation of the gas chromatograph a s \vel1 as the sampling error, sampling error may be smaller than the numbers given.
Table I.
Repeatability of Sample Introduction Peak Height, Cni. 2,2,4-
2-Methyl- Trimethylbutane pentane 24.96 24.60 25.14 24.41 24.73 24.81 24.22 24.77 Av. 24.71 Std. dev. 0.29 Std. dev. as % 1.2
24.70 24 74 24.39 24.97 24.30 23.26 24.24 23.90 24 31 0.54 2.2
Because peak height is proportional to the amount of component present (2), this liquid sampling system can be used for calibration and determination of minor components of a sample,
without having to use the internal normalization procedure based on total peak areas. The liquid inlet system described has prored very satisfactory in routine use. More than 200 gas chroniatograms have been taken on liquid hydrocarbon samples using it. Tenney and Harris (6) have recently described another type of system for micropipet sample introduction. LITERATURE CITED
(1) ildams,
iY. G., Ethyl Corp, Baton
Rouge, La., unpublished paper.
(2) Callenr. A . B.. Cvetanovic. R. J.. Can. J . Chem. 33, 125G (1955). Dimhat, AI., Porter, P. E., Stross, F. H., A x . 4 ~CHEM.28,290 (1956) Purdy, K. M.,Harris, R. J., Zbid., 22, 1337 (1950). Ray, N. H., J . A p p l . Chem. 4, 21 (1954). Tenney H hI., Harris, R. t J , ASAL. CHEM.,29, 317 (1957). Van de CraatB, F., Anal. Chim. Acta 14, 136 (1956).
Efficient Removal of Dissolved Oxygen in Polarography Karl Koyama and C. E. Michelson,' Hanford Atomic Products Operation, General Electric Co., Richland, Wash.
must usually be D removed from a solution prior to ISSOLVED OXYGEN
polarographic analysis, and this is nearly always done by bubbling a stream of inert gas through the solution. In some polarographic cells, the gas passes into the solution through a capillary or small-bore tubing. I n the LaitinenBurdett cell [ANAL.CHEM.22, 833-5 (1950)], the gas enters through a fritted-glass disk, whereas in the MeitesMeites cell [AKAL. CHEM. 23, 1194 (1951)] it passes through a fritted-glass cylinder. The time required for coiiiplete deaeration is greatly decreased with either type of cell, and, as stated by Meites and PIIeites, oxygen could not be detected after 1 minute. Experiments with sintered-glass gas dispersion cylinders of three different porosities and with capillaries of 0.1and 1.6-rn1n. inside diameter confirmed the statement that complete deaeration could he secured within 1 minute by using a coarse-porosity fritted cylinder to disperse the gas stream. .4lthough the medium-porosity fritted cylinder deaerates a t x-irtually the same rate, complete deaeration may requirc up to 2 minutes when an extra-coarse porosity cylinder is used. With either capillary, deaeration was only 50 to 70% complete after 1 minute. B y carrying the experiment further, the deaeration effected in 1 minute with 1 Present address, General Engineering Laboratories, General Electric Co., Schenectady, N. Y. VOL. 2 9 ,
NO. 7, JULY 1957
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