Comparison of Gas Chromatographic Method and pH-Alkalinity Method for Determination of Total Carbon Dioxide in Sea Water SIR: Swinnerton, Linnenbom, and Cheek (2) determined dissolved gases in aqueous solutions by gas chromatography. They used an all-glass sample chamber in which dissolved gases were stripped from solution by an inert carrier ga.. We recently extended their method to determine total carbon dioxide in sea water, and compared our results with those of the widely used pH-alkalinity method (1). Apparatus and procedure were essentially the same as those described by Swinnerton, Linnenborn, and Cheek ( 2 ) . However, we used only one silica gel column to resolve the carbon dioxide chromatogram. To convert dissolved bicarbonate and carbonate ions in sea water into carbon dioxide, a 3-ml. sea water sample was mixed with 2 ml. of O.lLITHC1 in the sample chamber, followed by stripping of the gas formed. One- to 3niA11per liter solutions of sodium carbonate were used as standards. Reproducibility of the chromatographic method was good, with a relative standard deviation of +0,7% for five replicate det.erminations. To further test the validity of the chromatographic method, we compared it with the pH-alkalinity method (1) a t various salinities. Low salinity samples for this test were prepared by diluting sea water with distilled water. The two methods agreed well within a
Table 1. Comparison of Two Methods of Total Carbon Dioxide Determination
Sample number 1 2 3 4 5
6 7 8
9
Salinity, p.p.t. 2 0 5 2 10 3 15 5 21 1 26 4 31 3 33 8 36 4
Total carbon dioxide, mM per liter Gas pHchroalkamatolinity graphic Differmethod method ence 019 020 001 045 041 002 0 76 0 80 0 04 1 10 1 12 0 02 1 44 1 42 0 02 1 75 1 76 0 01 2 05 2 01 0 04 2 19 2 16 0 03 2 33 2 31 0 02
maximum difference of 0.04m.V per liter (Table I). The pH-alkalinity method relies on measurement of pH, alkalinity, salinity, inorganic boron concentration, and temperature of sea water (1). Furthermore, we need to know dissociation constants of carbonic acid and boric acid in sea water to calculate the total carbon dioxide. Cnder ideal conditions relative standard deviation obtainable from this method is about +1y0. The chromatographic method offers a direct and rapid means of determining total carbon dioxide in sea water. I t takes not more than 5 minutes to analyze a sample. Further tests show
that the method is reliable aboard ship, and it in applicable to rain, river, and lake waters. Therefore, the modified method of Swinnerton, Linnenbom, and Cheek ( 2 ) should prove more useful than the pH-alkalinity method in routine oceanogrsphic, limnological, and water pollution work>. ACKNOWLEDGMENT
The authors thank Magdalena Catalfomo for completing a portion of the laboratory work, and J. P. Sullivar., c‘. S. S a v a l Oceanographic Office, and J. W.Swinnerton, U.S. Naval Research Eaboratory. for their technical assistance. LITERATURE CITED
(l),Harvey, H. IT., “Chemistry and Fertility of Sea-Waters,’! p. 172, Cambridge Univ. Press, Cambridge, Mass., 1960. ( 2 ) Sw-innerton, J. W., Linnenbom, V. J., Cheek, C. H., AXAL. CHEM.34, 483 (1962). KILHOP ~ R K GEORGEH. KENNEDY HI-GHH. I > O B S O S Department of Oceanography Oregon State University Corvallis, Ore. 97331 Work supported by Xational Science Foundation grants GP-622 and GP-2232, and by the Ofice of Naval Research contract Xonr 1286(lo), Project, ?;R 083- 102.
L n d iu m
Enca psula tion Technique for Introducing Weighed Samples in Gas Chromatography SIR: Obtaining known amounts of representative samples of volatile liquids for gas chromatography by injection techniques is difficult (4). Variations in volume injected are a source of error (8). I3ecause these variations depend on the depth and time of insertion of the needle into the preheat zone, selective vaporization may be a significant part of the error. Select,ive va1)orization is also likely when samples are stored before analysis. As a result, the accuracy of the analysis is uncertain because of sampling errors. Encapsulation ensures introduct,ion of a known amount of a representative , even after prolonged storage. Errors arising from variations in volume and manipulation of the syringe are avoided. Because thr entire sample within the capsule (tntcrs the gas chromatography in1686
ANALYTICAL CHEMISTRY
strument, selective vaporization is eliminated. Weighing the encapsulated sample eliminates the need to add a reference compound. Therefore, accuracy of gas chromatography may be determined without the uncertainty that sampling errors may be significant. To enbure representative sampling, an indium encapsulation technique originally developed for mass spectrometry ( 2 ) has been adapted for gas chromatography ( 7 ) . Indium encapsulation is faster and easier (3, 6 ) than glasb encapsulation ( I ) ; it avoids the flame sealing and the larger vaporization chamber usually required with glass encapsulation. In testing the technique, encapsulated samples were repeatedly weighed to determine weighing accuracy and the effectiveneqs of retaining volatiles during 4 weeks of storage. The rela-
tionship between weight and detector response wab established and applied to the gas chromatography analyses of encapsulated samples. Weighed samples were analyzed to determine the effectiveness of the approach in eliminating the need for a reference compound and retaining voltatile components. EXPERIMENTAL
The crimping device and a typical capsule are shown in Figure 1. The crimper consists of two jaws Lvelded to tweezers. The upper jaw is brass, the lower i3 iron. A hampie is encapsulated I tared secin the following manner. : tion is positioned against the aligning stop on the lower jaw of the crimper so that the ends of the tubing are under the two projections in the upper jaw. The magnet is placed on the tubing to hold the position qhown b y the dashed outline. Khile held on the crimper jaw.
