Atmospheric Tracer Technology -- Time Integrated Sampling
Our integrated sampling system is best described by explaining the sampling and analysis process. The system works like this:
1. Prior to the experiment, the sampling locations are selected. A tag with a bar coded number is placed at each site
and the site latitude and longitude are collected and loaded into the Automated Tracer Gas Analysis System (ATGAS).
2. Just before each tracer release, a Programmable Integrating Gas Sampler (PIGS) is taken to each sampling site.
The PIGS consists of 12 microprocessor controlled air pumps in a weather proof waxed cardboard box.
(The cardboard was selected to minimize cost and reduce theft and vandalism of the PIGS.) Placed inside the PIGS
is a smaller box (called a cartridge) which contains 12 Tedlar bags. Both the PIGS and the cartridge have a
unique number on a bar code label. A hand held computer records the location number, the PIGS number, and the
cartridge number and then downloads into the PIGS processor memory the start time and the sample time per bag. The time per
bag may vary from a few minutes to multiple hours. Each bag may have a different sampling time and pauses can also
be incorporated.
3. After all the PIGS are placed and programmed, the hand held computers are brought to the laboratory and
the information they have collected is up-loaded into the ATGAS system.
4. At the programmed start time, the PIGS each collect 12 sequential air samples by pumping tracer laden air into the
bags. The start time is usually selected to be at or near the beginning of the tracer release time. The total run time for the
PIGS is the sum of all the bag sampling times and between bag pauses. For example, if the PIGS were programmed to sample 1
hour per bag, it would sample for a total of 12 hours. The sampling parameters are adjusted to put about 500 to 1,000 ml
of air in each bag.
5. After the PIGS have completed their sampling, the cartridges are collected and brought to the
laboratory. Each cartridge is in turn connected to the ATGAS for analysis. The ATGAS consists of a gas
chromatograph (GC) with an ECD detector, custom autosampler, and controlling computer. The results from
all 4 ATGASs are downloaded to one PC. The bar code label on the cartridge is read and the ATGAS matches
the samples with the correct location, start time, and sampling time. It then analyzes each sample for tracer
concentration and the results are stored with the correct location and time information. The result is a set
of time averaged concentrations that can easily be compared with model predictions.
The system is designed to handle a large number of samples efficiently and minimize errors in handling and
tracking the samples. All data flow electronically and discrepancies are automatically detected. The system was
designed for SF6, but has been successfully used in the past with several other gases.
6. A complete quality control (QC) program is implemented for the system including analysis checks before and after every
sample, control samples, blanks, and a double review of all data.
7. After all results have gone through a quality control review, each cartridge is cleaned using a
special cleaning apparatus designed to clean 6 cartridges simultaneously. Cartridges are then analyzed
via the ATGAS system to ensure that each bag within the cartridge is clean. Cartridges showing signs of
contamination are re-cleaned and re-analyzed. The cleaned cartridges can then be reused as needed to complete
a project consisting of several tests.
Specifications for PIGS
- Size: 24 in. x 16 in. x 13 in. (61 cm x 41 cm x 33 cm)
- Mounting: May be set on the ground or hung on brackets attached to poles.
- Number of Samples: 12 per PIGS (if more are needed, multiple PIGS may be placed at a single location and programmed to run sequentially).
- Sample Time per Bag: May be programmed from a few minutes to several hours. Each bag may sample for different amounts of times.
- Number of PIGS Available at FRD: Approximately 100
- Number of Cartridges: Over 600 (enough to do 6 tests with all PIGS before re-use of cartridges is required).
- Power Requirements: One D-cell battery powers a PIGS for 5 sets of samples.
ATGAS SF6 Measurement Characteristics
Measurement characteristics of SF6 are based on the limit of detection
and the limit of quantitation. The instrument limit of detection (ILOD) and instrument limit of
quantitation (ILOQ) are calculated values determined on each ATGAS prior to
project commencement. These results are the concentration limitations for
each instrument and represent a signal that is 3 and 10 times, respectively,
the standard deviation of a replicated laboratory control standard.
The method limit of detection (MLOD) and method limit of quantitation (MLOQ)
are calculated values determined for the entire project upon its
termination. These results are the concentration limitations for the entire
method and represent a signal that is 3 and 10 times, respectively, the
standard deviation of a replicated field control standard and includes both
the instrument limitations as well as the method limitations.
Instrument and method limitations will fluctuate slightly due to changing
field and project requirements and aging of sample bags and tubing. The results
below are averages based upon past projects.
- ATGAS limit of detection: 1 ppt
- ATGAS limit of quantitation: 4 ppt
- Method limit of detection: 3 ppt
- Method limit of quantitation: 7 ppt
- Maximum concentration: 1 ppm (but may be increased using a smaller sample size)
(ppt = parts per trillion by volume)(ppm = parts per million by volume)
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