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Water-Based, Gravimetric Method for the Determination of Gas Sample Loop Volume

“Water-Based, Gravimetric Method for the Determination of Gas Sample Loop Volume” Analytical Chemistry 65 (1993): 2403-2406.

R.J. Wilke, * D.W.R. Wallace, and K.M. Johnson

Oceanographic and Atmospheric Sciences Division, Brookhaven National Laboratory, Upton, New York 11973

Gas sampling loops (GSLs) for gas chromatography valves (GCVs) are supplied with a nominal volume which suffices for many gas chromatographic determinations. The user need not calibrate them to further tolerances because the same loop is filled with standards of known concentrations to generate calibration curves which are subsequently applied to quantify unknown sample concentrations. However, gas sample loops are frequently used in calibration applications for which their volume must be known accurately. Examples include the following: (a) gas-phase analyses for which gas-phase standards are not available at multiple concentration levels. In these cases, calibration curves can be generated via injection of multiple aliquots of a known volume or separate injections of a series of different volumes; (b) calibration using gas-phase standards of analyses of a separate phase, e.g., purge-and-trap analysis of aqueous samples. A very exacting use of the GSL is required by the on-going effort to access the ocean’s role in climatic change by surveying the spatial and temporal oceanic distribution of total dissolved carbon dioxide (CT) during the multinational World Ocean Circulation Experiment-World Hydrographic Program (WOCE-WHP). The method chosen for the CT survey is continuous gas extraction of the CO2 liberated from acidified seawater followed by coulometric titration with a CO2 coulometer.1,2 Agreement has been reached on a goal of 1 part in 2000 for the accuracy and precision of the CT determination.2 The most convenient way of calibrating this method is to analyze known masses of CO2 from a GSL of known volume (V) filled with pure CO2 at some temperature (T) and pressure (P) and coulometrically titrating its content for comparison with the known amount.

For this purpose, we developed a reliable and straight forward means using water to calibrate the volume of GSLs to this required level of accuracy. Subesequently, the accuracy of the GSL volume determination was checked through the analysis of liquid-phase certified reference materials on the same apparatus. This paper describes the method used and our experimental results. The technique should prove relevant and useful for the many branches of analytical chemistry in which GSLs are employed.

EXPERIMENTAL SECTION

Figure 1 is a schematic of an automated arrangement of a GCV and GSLs, barometer, temperature sensors, process components, and coulometer for gas calibration developed and modified3 from an earlier system4 and suitable for the analysis of CT in seawater. Carbon dioxide originating from the GSLs or degassed from acidified seawater dispensed from the pipet into the fritted stripper follows the same pathway to the CO2 coulometer. In this way the combined error for the coulometric titration and losses from the process components (glassware, drying agents, etc.) can be determined. After the GSL is filled, the loop is purged with carrier gas and its content coulometrically titrated. The ratio (mass of CO2 injected (calculated))/(mass of CO2 determined (titrated)) is then defined as the gas calibration factor (GCF), which is used to correct subsequent titrations for small departures from theory. The coulometric titration has been found to be highly linear and after years of experimentation a consensus has been reached requiring a two-point calibration using an eight-port in-line GCV with two loops of 1/8 in. o.d. by 0.08 in. i.d. (hereafter, called the valve-loop pair).

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