Chlorofluorocarbon Measurements

Sample Collection

All samples were collected from depth using 10-L Niskin bottles. None of the Niskin bottles used showed a CFC contamination throughout the cruise. All bottles in use remained inside the CTD hanger between casts.

Both the Lamont-Doherty Earth Observatory and RSMAS analytical instruments were on board so each group sampled and analyzed every other station. Each system was capable of analyzing CFC-11, CFC-12, and CFC-113. The RSMAS system was also capable of analyzing CCl4. CFC sampling was conducted first at each station, according to WOCE protocol. This avoids contamination by air introduced at the top of the Niskin bottle as water was being removed. A water sample was collected directly from the Niskin bottle petcock using a 100 mL ground glass syringe which was fitted with a three-way stopcock that allowed flushing without removing the syringe from the petcock. Syringes were flushed several times and great care was taken to avoid contamination by air bubbles. Two duplicate samples were taken on most stations from random Niskin bottles, one duplicate was for same analytical system analysis, to calculate precision and the other was for cross analytical system comparison. Air samples, pumped into the system using an Air Cadet pump from a Dekoron air intake hose mounted high on the foremast were run when time permitted.

RSMAS Equipment and Technique

The RSMAS system analyzed 46 complete stations out of 88 for a total of 1357 samples on A20, and 42 complete stations out of 82 for a total of 1298 samples on A22. Halocarbon analyses were performed on a gas chromatograph (GC) equipped with an electron capture detector. Samples were introduced into the GC-EDC via a purge and dual trap system. The samples were purged with nitrogen and the compounds of interest were trapped on a main Porapack N trap held at ~ -15°C with a Vortec Tube cooler. After the sample had been purged and trapped for several minutes at high flow, the gas stream was stripped of any water vapor via a magnesium perchlorate trap prior to transfer to the main trap. The main trap was isolated and heated by direct resistance to 140°C. The desorbed contents of the main trap were back-flushed and transferred, with helium gas, over a short period of time, to a small volume focus trap in order to improve chromatographic peak shape. The focus trap was also Porapak N and is held at ~ −15°C with a Vortec Tube cooler. The focus trap was flash heated by direct resistance to 155°C to release the compounds of interest onto the analytical pre-column. The pre-column was the first 5 meters of a 60 m Gaspro capillary column with the main column consisting of the remaining 55 meters. The analytical pre-column was held in-line with the main analytical column for the first 2 min of the chromatographic run. After 2 min, all of the compounds of interest were on the main column and the pre-column was switched out of line and back-flushed with a relatively high flow of nitrogen gas. This prevented later eluting compounds from building up on the analytical column, eventually eluting and causing the detector baseline signal to increase.

The syringes were stored in a flow-through seawater bath and analyzed within 8-12 h after collection. Bath temperature was recorded every time a sample was analyzed for use in calculating the mass of water analyzed. Every 12 to 18 measurements were followed by a purge blank and a standard. The surface sample was held after the initial measurement and was sent through the process again in order to "restrip" it to determine the efficiency of the purging process.

Lamont-Doherty Earth Observatory Equipment and Technique

Water was transferred from the syringe into a purge and trap system interfaced to a Hewlett Packard 5890 gas chromatograph with an electron capture detector. The gas chromatography was carried out using a 40 inch x 1/8 inch diameter pre-column of porasil B, a 60 in. × 1/8 in. diameter main column of carbograph-1AC and a 4 in. × 1/8 in. diameter post column of molecular sieve 5A. The molecular sieve 5A column separated CFC-12 from nitrous oxide and was valved out of the gas stream before CFC-11 eluted from the main column. The combination of the pre-column and main column provided excellent separation of CFCs 11, 12, and 113 as well as separation of CFC-113 from methyl iodide. The gas chromatograph was calibrated against a known gas standard and concentrations are reported on the SIO98 scale. The precision of this technique was the larger of 1% or 0.01 pmol/kg.

Calibration

A gas phase standard, ALM35078, was used for calibration. The concentrations of the CFCs in this standard are reported on the SIO 1998 absolute calibration scale. Multiple calibration curves were run over the course of the cruise on each analytical system. Estimated accuracy is ± 2%. Precision for CFC-12, CFC-11, CFC-113 and CCl4 was less than 1%. Estimated limit of detection is 0.010 pM/kg for CFC-12 and CFC-113, and 0.005 pM/kg for CFC-11 and CCl4.

Technical Problems

In large part, sample collection and measurement were very successful. The integration of the computer software with the GC-EDC system hardware made the procedure almost completely automated. There were no incidents that caused significant instrument down time.

Processing of External Duplicates

External duplicates are defined as samples where RSMAS and Lamont both sampled the same station/bottle. The Lamont and Miami systems were compared throughout the A20 and A22 legs by running duplicate samples from stations on both systems.

For A20 the agreement between these duplicates was within the measurement error for CFC-11 and CFC-12. For CFC-11 and CFC-12 external duplicates, the CFC values were averaged if both RSMAS and Lamont samples had QB=2. If one of the lab's samples had a questionable (QB=3) or bad (QB=4) quality designation, then the other lab's sample was used for that CFC value for that particular station/bottle.

For A22 the Lamont CFC-11 and CFC-12 data were higher than the Miami data and were believed to be in error because the surface waters measured by the Lamont system were consistently supersaturated by about 10% and the Miami data were close to 100% saturation. A correction was applied to the Lamont data to bring it in line with the Miami data. All of the Lamont data for A22 was reduced by 5.5% and a further correction was applied to stations 38-82 to correct for a small leak that apparently developed in the Lamont system for these stations. Comparison of the duplicates run on the Lamont and Miami systems after the corrections yielded an error of the larger of 0.01 pmol/kg or 1.7 % for CFC-11 and CFC-12 and 0.01 pmol/kg for CFC-113.

Following the offset correction made by Lamont on their data, the CFC-11 and CFC-12 external duplicates (with QB=2) were compared to determine whether they were within 5% of each other. If the RSMAS and Lamont values were within 5% of each other, they were averaged. If they were not, then the value that most closely fit with the remainder of the station and surrounding stations was chosen. This was determined by examining plots of CFCs and Pressure.

For CFC113, there was a ~10% difference between the RSMAS and Lamont values. The external duplicates were not averaged. The CFC113 sample from the lab that sampled the remainder of the station was used instead. RSMAS sampled CCl4. Lamont did not analyze water samples for CCl4.

Last Modified: 2017-08-18 12:57 UTC