The Reston Groundwater Dating Laboratory

Analytical Procedures for Dissolved Gasses N2 / Ar

Preparation

Septum Bottles

The filled septum bottles are weighed. The total volume of water in samples and standards is determined by the difference between the full weight and the empty weight of the bottle. After the samples and standards are allowed to equilibrate to room temperature, 10-12 mL of water are removed from each bottle to create a headspace. This is accomplished by inserting a syringe needle, attached to a vacuum pump, into the stopper and withdrawing water. The headspace volume is determined from the difference in weight between the full bottle and the bottle with the headspace.

Analysis

Photo of N2 / Ar instrument

Samples and QA/QC standards are analyzed using a Hewlett Packard model 7890B GC (see schematic). Helium is used as the carrier gas. The headspace gas pressure is measured with a transducer. The gas is then introduced simultaneously into 2 sampling loops. One sampling loop is injected into an Alltech CTR-III column at 55°C to separate the Argon, Nitrogen, and Oxygen. These gases are quantified with a thermal conductivity detector (TCD). Chromatogram The second sampling loop is injected into an Alltech CTR-I column at 30°C to separate methane and carbon dioxide from the other constituents. After separation, the gas stream is passed through a Nickel methanizer converting the CO2 to methane and the two gases are quantified with a Flame Ionization Detector (FID). Minimum Reporting Levels (MRL) in milligrams per liter are summarized in the table below.

Analyte MRL mg/l
Precision +/- mg/l
Nitrogen 0.001 0.001
Argon 0.003 0.003
Carbon Dioxide 0.04 0.04
Methane 0.001 0.0005
Oxygen 0.002 0.002

 

Calibration

The GC is calibrated with four gravimetric gas standards and one NOAA air standard. The GC is calibrated at the beginning of each day. The calibration is checked again at the end of each day. Instrument drift is generally less than 1% for Ar, N2, and O2 and 1-2% for CH4 and CO2. The composition of the standard gases is given in the table below.

COMPOSITION OF STANDARDS (percent)

Standard

O2

CH4

CO2

N2

Ar + O2

Ar

AIR

20.946

-

-

78.084

21.88

0.934

C1 Std-3

-

4.95

10.007

85.043

-

-

C1 Std-4

-

1

.9995

98.0005

-

-

C3 Std-3

10.02

-

-

87.97

12.03

2.010

C3 Std-4

20.99

-

-

78.006

21.994

1.004

QA/QC

Standard samples for QA/QC control are prepared in the laboratory using three temperature controlled water baths set at 9.0, 16.0, and 24° C. Air is bubbled into the water. Water is equilibrated with air at these conditions for a minimum of 48 hours before sampling. Empty serum bottles with stoppers are weighed. The serum bottles are fillled by placing a sampling tube at the bottom of the serum bottle and allowing the water to over flow. About 500 mL of water is used to purge the bottle of air and water contacted by air. After the bottles are filled, the stoppers are inserted, and the bottles are stored in a refrigerator at about 4°C.

Standard deviations of the QA/QC water bath standards are typically ±0.3°C or less of the air / water equilibrium temperature. Three standards are analyzed daily.


Analysis of Ground-Water Samples for Helium by Gas Chromatography

Water samples for He analysis are collected in 150-mL septum bottles that are filled without headspace in the field using the same procedures as for N2/Ar samples. The samples are stored on ice in the field, and in a refrigerator at 4°C in the laboratory prior to analysis, to minimize expansion and possible sample loss, and to lower the rate of biological activity in the sample prior to analysis.  In the laboratory, after allowing the samples to come to room temperature, a 10-mL headspace is created by removing some of the water through the septum with a needle connected to a vacuum pump.  The water is allowed to equilibrate with this headspace at room temperature before analysis.  The headspace gas is injected into a gas chromatograph.

The concentrations of He are measured with a thermal conductivity detector (TCD).  The procedure is similar to that described by Sugisaki and others (1982).  The instrument is calibrated with five standards by injections of 1, 2, and 3 cm3 of a gravimetric standard gas containing 35.0 ppm volume per volume of He.  The concentrations in this standard are known to within ±1 percent.  The two other standards used were 2.0 and 3.0 cm3 of dry air.  The concentration of He is 5.24 ppm volume per volume of gas (Committee on Extension to the Standard Atmosphere, 1976).   The precision of the helium gas-chromatographic analysis is approximately 5 percent.