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 t h a t 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 t h a t 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. I n 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 t h e 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
1 1 15
a medium- or coarse-porosity fritted cylinder was found to be more efficient than that effected in 45 minutes with a capillary tube. With either capillary, deaeration was only 90 to 95% complete after 45 minutes.
The experiments were performed in both air-saturated 2 M nitric acid and 0.1M potassium chloride, the inert gas flowing a t 150 cc. per minute through the gas-dispersion tube being tested into 25 ml. of the electrolyte contained
in a beaker-type cell. The time required for deaeration was essentially the same with either solution. Although helium was used throughout the experiment, deaeration can be effected equally well with other oxygen-free gases
Precise-Volume Fraction Collector D. N. Eggenberger and E. F. Cavanaugh, Research Division, Armour and Co., Chicago, 111.
FRACTION
COLLECTOR
friction-free rotation. The third point is a roller, 3/4 inch in diameter and 1/2 inch long, mounted on the 4-r.p.m. motor shaft. A center post keeps the wheel in place. The temperature of the column jacket can be changed automatically during a run, using two preset thermoregulators in the column jacket and the circuit shown in Figure 3. The tube a t which the temperature change is desired is equipped with a short metal sleeve. Through a pair of light spring fingers which touch each tube as it passes, the sleeve actuates the latching relay. switching to the other regulator. A latching relay, although more expensive than a self-energizing ordinary relay. has the advantage that should a power interruption occur, the temperature will go to the correct value when power is resumed. A push button energizes the unlatch coil, restoring the original temperature.
was needed
L ! to deliver small closely reproduced
volumes of aqueous solution from a chromatographic column. Siphons [Schrmn, E., Bigwood, E. J., ANAL. CHEM. 25, 1424 (1963)], producing volume variations on the order of 1% with aqueous solutions, did not meet the 0.1% reproducibility desired. Dropcount [Stein, W. A., Moore, s., J. Biol. Chem. 176, 337 j1948)] and timing methods produce volume variations with changes in surface tension. The system described here has been d(~1ircring1 ml. sample. nith a maximum variation in volume of + 0.0008 ml.
Figure
1.
Volume-measuring schematic Q Bakelite_ Block
The volume-measuring device (Figure 1) is a glass tube with a small rubber or Tygon drain pinched shut by a simple clamp, which can be held open by an energized solenoid. When the solution connects a sealed-in contact with an adjustable upper contact, the drain opens. The simple trip circuit, a t the left end of Figure 2, including the 5823 gas relay tube, relay, and capacitor, has been used previously (R. Q. Rlackmell, Northwestern University, Chicago, Ill., private communication). The 1-megohm potentiometer adjusts the sensitivity to allow for variations in conductance of the solution being collected. When the motor starts the turntable, the microswitch is operated by a finger nhich is pushed by each tube as it moves past. The microsnitch in turn operates relay 2 to keep the motor running until the finger slips off the tube. stopping the turntable in position for the next sample. The time-delay relays are gmperite or similar types and can be changed to suit various applications. If for any reason the measuring tube should overfill, the circuit is “self-jamming” ; the drain circuit is activated, the solenoid is inactivated because l l X 5 is open, and the table continues t o rotate. If the motor operates more than 10 seconds, the 115C10 relay, in parallel with it, opens up the tripping and solenoid circuits and stops the motor, thus breaking the jam. After another 10 seconds the circuit is ready for the next cycle. The turntable bottom plate rests a t three points equally spaced around the circumference. Two of the supports are 3/,,-inch ball bearings, providing nearly 1 1 16
ANALYTICAL CHEMISTRY
I
Figure 2.
Control circuit
All relays and microswitch are shown in their normally unoperated positions. R i and 5823 tube constitute tripping circuit. 1 15C5 delay relay holds drain solenoid open 5 seconds. 1 1 5 N 0 3 0 relay starts turntable drive motor 30 seconds after tripping occurs. 115CIO relay “de-jams” circuit (see text) d
o
Reset button
lllvoc
-
1
Figure 3.
1
l
j
ldchilg relay
Temperature control and switching circuit
I meg
