Mass Spectrometric Determination of Oxygen in Water Samples ISRAEL DOSTROVSKY
AND
F. S. KLEIN
W e i z m a n n I n s t i t u t e of Science, Rehovoth, Israel
from the walls of the vessel, stopcock C is turned so as to connect a small trap, T , which is attached to C. The trap is immersed in liquid air and all the material condensed in it by opening stopcock B. After pumping out, the apparatus is ready for the next sample, which may be introduced while the reaction products are still frozen in T . While the equilibration of the fresh sample is taking place, with stopcock B off, sampling stopcock C is turned to connect T with the apparatus and the contents of the trap are allowed to warm up. As soon as the rapid evolution of gas ceases (as indicated by the manometer), C is turned through 90" and the carbon dioxide is introduced into the mass spectrometer sampling line through D. The water in T may be recovered if required, but in any case T is replaced by a clean dry trap in readiness for the next sample.
WIDELY used procedure for the mass spectrometric iso-
A topic determination of ox)-gen in water samples is that of
Cohn and Urey ( 3 ) . Carbon dioxide is equilibrated with a liquid water sample and is then introduced into the mass spectrometer. From the ratio of mass peaks 44 and 46 (C0l6Ol6and COIBolB) and the equilibrium constant for the carbon dioxide-water exchange reaction, the atom percentage of 0I8in the sample may be calculated. One disadvantage of this method is the length of time required for the completion of the equilibration process, from 4 to 12 hours. The authors have modified the original method of Cohn and Urey by carrying out the equilibration reaction on a heated platinum wire placed in a gaseous mixture of the two components. The equilibrium under these conditions is complete within a few minutes. The apparatus used for the equilibration is shown on Figure 1, and was constructed as an integral part of the gas inlet system of the authors' mass spectrometer.
This procedure permits a complete analysis to be made in under 15 minutes and a large number of samples can be treated in rapid
succession. In Table I are collected some of the results obtained by this procedure, compared with results obtained on the same samples by the original procedure of Cohn and Urey.
The equilibrium vessel, A , consists of a cylindrical borosilicate glass bulb of 100-ml. capacity fitted with a vacuum stopcock, B, a t its lower end and carrying a re-entrant thimble a t the other. Around this thimble, and supported by small platinum hooks projecting from it, is arranged a platinum wire, 25 em. long and 0.2 mm. in diameter, in a manner analogous to that used in some early incandescent lamps. A hairpin bend a t the bottom of the platinum loop is provided to ensure that no liquid water remains in the stopcock opening. The only other component of the system which needs special mention is the sample-introducing and -removing stopcock, C, which m-as made from a vacuum T-shape hollow-plug stopcock provided with a dosing cavity. A 10/30 standard taper socket was sealed to it. To minimize memory effects, the lengths of tubing between the components B, C, and D were kept as short as possible.
To vacuum line
To COSreservoir
cm 0 I
2
3 4 5
Table I.
Mass Spectrometric Determination of Samples of Water
Ratio Peaks 44/46, Dostrovsky Sample Expt. and No. No. Klein Mean A 1 22.4 2 23.1 3 22.6 22.7 B 1 36.7 2 36.7 3 36.2 36.5 C 1 62.7 2 64.0 63.2 D 1 220 2 221 3 217 4 224 220
3.77
Ratio Peaks 44/46, Cohn and Urey 23,l 21.8 22.6 35.4 36.1 35.9 62.0 63.6
(1.0)
214 210 221
Enrichment 10.56 6.57
0 l 8
in
Mean
Enrichment
22.5
10.42
35.8
6.55
62.8
3.71
215
ii
(1.0)
Procedure. The apparatus is evacuated thoroughly through stopcock D and the equilibrium vessel is baked for a few minutes by heating the platinum wire to red heat. With the heating current off, liquid air is introduced into the thimble. The sample of water (20 mg.) is then introduced into the dosing cavity of C, with the aid of a micrometer syringe. This quantity of water is sufficient to fill the cavity almost completely. Stopcock C is then turned so as to expose the cavity to the vacuum, whereupon the water distills onto the cooled thimble. Stopcock B is then turned off and purified carbon dioxide from a storage flask is introduced through D until a predetermined pressure is indicated on the manometer, M . The quantity of carbon dioxide introduced was equivalent to about 1.5 ml. a t normal temperature and pressure. The gas is introduced into A by opening B momentarily and the liquid air remaining in the thimble is removed by blowing some air through it. The filament is then switched on and the current regulated so as to maintain the wire at bright red heat, varying the current with changes of pressure in A . Two minutes after all traces of liquid water have disappeared
Figure 1. Apparatus
I n the course of the equilibration reaction a certain dilution of the sample by the carbon dioxide oxygen atoms occurs. The concentIration of 0l8 in the water sample may be calculated readily from the observed ratio of peaks 44 and 46 by means of Equation 5 . The relevant equilibrium constants in the equilibration reaction are:
414
415
V O L U M E 2 4 , NO. 2, F E B R U A R Y 1 9 5 2 Table 11. Sample Sequence Sample A Normal water Limitinz value for normal waterSample B Normal water
Ratio Peaks
Ratio Peaks 44/46
Apparent Dilution by Previous Sample, % ’
22.4 189
1.9
220 36.7 198,5
2.2
and by definition
(3) (assuming equal sensitivity of spectrometer to C02’0 and C016018 molecules). From consideration of material balance it follows that:
NO - Xe)= 2b(n. - no) (4) where No and .V6 are the atom fractions of 0 1 8 in the water sample before and after equilibration, respectively; no and n, are the corresponding quantities for carbon dioxide. a and b are the number of millimqles of water and carbon dioxide, respectively, taken for analysis. Ro and R, are the ratios of mass peaks 44 to 46 for carbon dioxide before and after equilibrium, respectively. From Equations 1 and 3 it follows readily that
1 2(R 1/21 Introducing these values into Equation 4 we obtain:
and from Equations 2 and 3 similarly n
=
+
1
1
In using Equation 5 , for K the value of 2.005 is taken for
equilibration by the authors’ method when the temperature is near 1000” K., and 2.076 for the room temperature equilibration by the method of Cohn and Urey. The equation used here, even without the dilution-correcting term, differs from the equation given for this analysis by Bentley ( I , 2 ) which contains algebraic errors. The enrichments (columns 5 and 8 of Table I ) were obtained by dividing the atomic fraction calculated from Equation 5 by the value of the concentration of normal water obtained experimentally in a similar way. This procedure was adopted to minimize the effect of any systematic errors on the value of the enrichment. The apparatus possesses a small memory effect. If a normal water sample be analyzed immediately after a concentrated one, the peak ratio will be found somewhat lower than normal. This effect is probably due to traces of water or carbon dioxide absorbed on the surface of the glass and possibly also due to exchange with the glass oxygen atoms. The magnitude of this effect is illustrated in Table 11. It was found convenient to wipe out memory effect in switching between samples of widely different isotopic composition by flushing the apparatus with the new sample once or twice before admitting the equilibrated gas to the mass spectrometer. In general, the work is planned in such a manner as to avoid measuring samples of extreme concentrat,ion difference in succession. By careful work, giving more time to baking out and pumping, it is possible to reduce the memory effect \vel1 below the figures in Table 11. LITERATURE CITED
(1) hrnstein a n d Bentley, Quart. Rev. Che7n. Soc.. 4, 172 (19.50). (2) Bentley, .VucZeonics, 2 (2), 18 (1948); Cold S p r i n g Harbor S y m p . Quant. Bid., 13, 11 (1948). (3) C o h n and Urey, J . Am. ChenL. Soc., 60, 679 (1938). RECEIVED
I l a r c h 14, 1951.
Rapid Determination of Organic Acids in Cured Tobacco FORREST G. HOUSTON AMI JAMES L. HAMILTON K e n t u c k y Agricultural Experiment Station, Lexington, Ky.
HE usual method for determining organic acids in plant Tmaterial by the technique of Pucher, Vickery, and Wakeman ( 8 ) is time-consuming and subject to considerable experimental error. Isherwood ( 1 ) has used partition chromatography to determine organic acids in apple tissues. The organic acids are extracted from a tissue sample with an appropriate solvent and the extract is concentrated to a small volume. h aliquot is then placed a t the top of a specially prepared column of silica gel and the acids are washed down the column with a mixture of 1-butanol and chloroform. An external indicator is used to determine when an acid is emerging from the column, and it also indicates when the acid has been completely washed through. h the different organic acids travel down the column a t different rates, it is possible to collect them in separate fractions and thus determine the amounts of each acid by a simple titration procedure. Certain modifications of Isherwood’s technique provide a simple and accurate method for the rapid determination of oxalic, malic, and citric acids in dry plant materials such as cured tobacco leaves. EXPERIMENTAL
By placing the acidified sample of tissue a t the top of the column, the extraction and partitioning are made to occur simultaneously, thus eliminating the necessity for concentrating a
large volume of extract. This also makes possible the use of a smaller sample of material for analysis. Sharpness of the separations depends upon selection of a proper size of sample and judicious manipulation of the two solvent mixtures used.
For cured tobacco the two solvent mixtures that gave best separations over a wide range of total organic acid concentrations were 15 and 30% (v./v.) 1-butanol in chloroform. The solvents were saturated with 0.5 N sulfuric acid, stored in large reservoirs, and delivered t o the chromatograph columns under about 5 pounds of air pressure. Delivery of the solvents through two-way pressure stopcocks attached to the columns permits ready interchange of solvents on a column and simultaneous analysis of several samples. The setup used in this laboratory is designed to accommodate six columns. The column is a borosilicate glass tube 13 mm. in internal diameter and 300 mm. long with indentations near the bottom to support a perforated aluminum disk which is covered with filter paper and a small plug of cotton. A capillary tube drawn to a fine tip and provided Lyith a grooved stopcock is used to control the flow of external indicator from a large reservoir. A small funnel about 50 mm. in diameter and with a stem about 150 mm. long is bent near the base of the cone, so that the major portion of the stem is horizontal and its tip is turned down. The funnel is plugged at the bottom of the cone with a small twist of glass wool, which provides an efficient chamber for mixing the external indicator and effluent solvent. The funnel is placed under the column, so that it receives the effluent. The color of the emulsion is noted as it travels the horizontal portion of the stem. By properly fitting the glass wool, a good emulsion can be obtained
