May 15, 2014

POINT OF CONTACT

Principle investigator:
     Kirk L. Clawson
     NOAA Air Resources Laboratory Field Research Division
     1750 Foote Dr.
     Idaho Falls, ID 83402
     Kirk.Clawson@noaa.gov
     (208) 526-2742

README.TXT - Description of SF6 Tracer Bag Sampling and Data Files

Description of Equipment

Stationary time-integrating sampling of SF6 for PSB1 was performed using programmable 
bag samplers.  These samplers acquired time-sequenced air samples in bags that were 
subsequently analyzed for the concentration of the SF6 tracer.  The samplers collected 
12 samples by sequentially pumping air into each of 12 individual Tedlar® bags.  The 
integrated sampling time for each bag in the study was 10 minutes resulting in 12 
individual experiments within each of the five 2-h Intensive Observational Period (IOP) 
periods. The bag sampler housing is constructed from durable double-wall polypropylene 
manufactured by Mills Industries Inc. and measures 61 cm x 41 cm x 33 cm.  The other 
component of the bag sampler assembly is a cardboard sampler cartridge.  One hundred 
thirty five of the sampler boxes house a Motorola microprocessor (model MC68HC811E2). An 
additional 15 new samplers were built for PSB1 using Texas Instruments MPS-430 series 
microprocessors. All sampler boxes contain 12 microprocessor-controlled air pumps designed 
to start sequentially filling the bags at a time and duration specified for each bag.  The 
sampling period for each bag and the delay before each bag can be independently specified 
to create a sampling program customized for each situation.  The cartridge box contains 12 
Tedlar® bags. Prior to deployment, a sample cartridge was placed into each sampler box and 
connected by R-3603 tubing to the sampler pumps. The much more durable R-3603 tubing was 
used to replace old latex tubing prior to the experiment. The latex tubing was prone to 
degradation and cracking. With its cover in place, each sampler box and sampler cartridge 
assembly had a total mass of approximately 4 kg and was powered by a single D-cell battery. 
The microprocessor and air pump components of the sampler design have been used successfully 
in field experiments for many years and are known to be free of artifacts (e.g. Clawson et 
al. 2004, 2005).  The material used for the bag sampler housing represents a recently 
improved design that was extensively tested for reliability and potential sampling 
artifacts in 2007 and also found to be free of artifacts.

Description of Bag Sampling Grid

A total of 112 primary bag samplers were deployed on four sampling arcs during each IOP.  
For IOPs 1-3 these samplers were deployed on the 400, 800, 1600, and 3200 m arcs. For IOPs 
4 and 5 the samplers were deployed on the 200, 400, 800, and 1600 m arcs. The arc samplers 
were mounted atop plastic boxes and secured in place with bungee cords attached to metal 
fence posts. Sample inlet tubes were at about 1 m agl. In addition, bag samplers were 
deployed on 3 towers for the measurement of vertical concentration profiles. Four bag 
samplers were deployed on the 15 m (50 ft) sampling tower shown at 201 m (1, 5, 10, and 15 
m agl), 5 samplers were deployed on the 21 m (70 ft) sampling tower shown at 408 m (1, 5, 
10, 15, and 20 m agl), and 7 samplers were deployed on the 30 m (100 ft) meteorological 
and sampling tower at 499 m (1, 5, 10, 15, 20, 25, and 30 m agl). The locations of each 
bag sampler were specified by (1) latitude and longitude and (2) distance of the arc from 
the release location at the center of the arc array and an angle in degrees clockwise from 
north along each arc. In addition to these 128 primary samplers, an additional 22 samplers 
were deployed for quality control (QC) purposes.  This included 16 field duplicate, 3 
field control, and 3 field blank samplers.  The arc angle positions of these QC samplers 
are listed in table below. 

Arc and arc angle location of field duplicate, field control, and field blank samplers.

	Arc Angle Position
Arc (m)	Duplicate		Control	Blank
200	16, 31, 55, 73		
400	7, 37, 55, 82		43	31
800	16, 25, 49, 85		40	70
1600	10, 34, 52, 76		70	58
3200	13, 37, 55, 76		

Sampler Cartridge Analysis

Sample cartridges were analyzed at the Tracer Analysis Facility (TAF) in Idaho Falls, ID.  
The TAF hosts four gas chromatographs (GC), each housed within its own autosampler module 
and connected to a computer with the master data acquisition system.  The complete 
configuration with GC, autosampler, and data acquisition system is called an Automated 
Tracer Gas Analysis System (ATGAS). A dedicated small black handheld computer was used 
to set the operational parameters on each ATGAS.

Each GC housed two Supelco 60/80 Molecular Sieve-5A columns (5' x 1/4" and 2' x 1/4"), a 
10-port sample valve, and a sample loop.  These columns were maintained at 65 C inside 
their respective ovens.  Two columns (pre-column and main column) were used to reduce 
analysis time and to vent interfering species, mainly oxygen, which can damage the columns 
and detector.  After the SF6 sample was injected onto and eluted by the first 2-foot 
(610 mm) pre-column, the gas flow was switched to back-flush the pre-column while the 
sample loop was filled with the next sample.  The SF6 continued on to the main 5-foot 
(1520 mm) column where further separation occurred before being passed to the detector. 
Detection of SF6 was accomplished using a Valco Instrument Co., Inc., Model 140BN electron 
capture detector (ECD) containing 5 millicuries of Ni-63.  The ECD operating temperature 
was kept at 170 C.  The ECDs and columns were protected by a Supelco High Capacity Gas 
Purifier tube heated inside an oven to remove oxygen, water, carbon monoxide and carbon 
dioxide in the carrier gas as well as a Supelcarb HC hydrocarbon trap to remove organic 
impurities.  Ultra high purity (UHP) nitrogen served as the carrier gas and filtered 
compressed air was used as the valve actuator gas. Concentration ranges from 2 pptv to 
about 1 ppmv have been analyzed using this methodology.

The ATGAS computer software (Carter, 2003) was developed in-house and was used to analyze 
the tracer gas chromatograms, calculate concentrations, and perform quality control 
functions.  The software incorporates a history file system that records all operations 
performed on each ATGAS. 

Sampler Handling and Chain of Custody

A history file in the master ATGAS computer maintained a complete, comprehensive record 
for each sampler cartridge.  The scheme for maintaining the comprehensive history file was 
based upon unique bar coded serial numbers attached to both samplers and sample cartridges 
and the use of bar code scanners.  In addition, prior to the start of the project, each 
field sampling location was identified and tagged with a location number that consisted of 
a weatherproof bar code label.  These were affixed to the metal fence posts installed at 
each sampling location.  A file with a list of the locations was uploaded to the ATGAS 
computer in the TAF.  The bar code labels for the samplers, cartridges, and locations were 
used to automatically generate a chain of custody record for each sample. 

In preparation for each test, a sample cartridge was placed inside each sampler and then 
transported to the field.  Samplers were deployed at each location, the tubing was 
connected, clips were opened, and a sampling program downloaded into the memory of each 
sampler’s microprocessor.  The latter was accomplished with the use of a small hand-held 
computer (Videx Timewand II).  The Timewands were programmed with sample start and stop 
times for each bag prior to each test using a dedicated laptop computer in the TAF.  They 
were then used in the field to download the sampling program and acquire and record the 
location number, sampler number, and cartridge number.  The complete field download records 
were later retrieved from the Timewands and transferred into the history file on the ATGAS 
computer in the TAF prior to the start of cartridge analysis.

Personnel responsible for deploying the samplers in the field received classroom and 
hands-on training in Idaho Falls prior to the experiment.  It was also required that 
handwritten Sampler Servicing Record sheets be completed in the field for each removed or 
installed cartridge.  These records were created to provide the TAF analyst with details 
of potential problems pertaining to each cartridge and sample bag.  In combination with 
the history files, these records were invaluable as a reference for sample check-in and 
later for QC flagging of data.  The Sampler Servicing Records were given to the laboratory 
analyst after sampler collection and delivery were performed.  All record sheets were 
organized and placed in a binder for future reference.

The sample cartridges were transported back to the TAF at the completion of each IOP and 
analyzed within a few days of sampling.  They were all checked in prior to analysis using 
a bar code scan.  During this process each bag was inspected and the following flags were 
entered into the computer for each bag:
 	B   =  Too big (overfilled)
 	G   =  Good
	L   =  Low
	F   =  Flat
	D   =  Damaged clip or bag
	I   =  Improper hookup (tubes crossed, clip open, etc.)
These flags were used later for querying, sorting and generating final QC flags as well as 
for monitoring sampler performance and checking for mistakes by field personnel. Each 
cartridge was again scanned when it was attached to the ATGAS prior to analysis. This 
linked the GC identity and the acquired chromatogram and calculated concentration data to 
the computerized data previously collected in the field that specified the project 
identification, test number, grid location number, grid location coordinates, sampling 
start time, the sample time per bag, and sampling type (primary or quality control sample).
The record also included the cartridge check-in record and cleaning records.  Thus a 
complete computer-generated chain of custody is available for each bag sample as well as 
automatically linking all field, chromatogram, concentration, and quality control data 
into one comprehensive data record that could be readily reviewed.  This minimized the 
possibility of errors caused by mistakes in manually recording, copying, or entering of 
location information and provided an invaluable source of information in the event of a 
discrepancy or a question about the data.

Quality Control Procedures and Measurement Quality Objectives

The following are detailed descriptions of the quality control and quality assurance 
methods followed for the sampling, analysis, and reporting of the PSB1 time-integrated 
bag sampler tracer data.  Protocols established in the Environmental Protection Agency’s 
(EPA) Guidance for Data Quality Assessment (U.S. EPA 2000a), the general requirements for 
the competence of calibration and testing laboratories of International Standards 
Organization/IEC Guide 25 (ISO 1990), the quality systems established by the National 
Environmental Laboratory Accreditation Conference (U.S. EPA 2000b), and the Department of 
Defense Quality Systems Manual for Environmental Laboratories (U.S. DOD 2002) provided a 
basis for quality assurance and quality control procedures followed during analysis.  
Instrument and method limits of detection (ILOD/MLOD) were calculated based upon 40 CFR 
Part 136, Appendix B and the American Chemical Society (ACS) Committee on Environmental 
Improvement’s paper titled, “Principles of Environmental Analysis” (Keith et al. 1983).  
ACS principles relative to detection limit calculations in 40 CFR Part 136, Appendix B are 
documented in “Revised Assessment of Detection and Quantitation Approaches” (U.S. EPA 2004).
Although our research-based automated analysis of tracer gases has no specified method 
performance or regulatory criteria, compliance with the established quality control 
procedures stated above were followed, where applicable, to provide high quality data that 
is both accurate and reliable.

The laboratory procedures followed were designed to ensure meeting the stated Measurement 
Quality Objectives (MQO) for the project are listed below.

Measurement quality objectives (MQO) for the bag sampling Data Quality Indicators

Data Quality Indicator		Objectives (MQO)	How Determined

Instrument sensitivity		Instrument Limit of	Lab blanks and low concentration
				Detection (ILOD)<4 pptv	calibration checks

Between instrument precision	RSD(1) < 10%		Lab background checks

Low end instrument bias		< 1 pptv		Lab blanks

Instrument precision		|RPD(2)| < 5%		Lab duplicates above MLOQ
				RSD < 10%		Lab controls above MLOQ

Instrument accuracy		|RPD(3)| < 20%		Required by calibration check
							and recalibration protocol

Low end method bias(4)		< MLOQ(5)		Field blanks

Method sensitivity		Method limit of 	Calculated from field blanks,
				detection (MLOD)	low concentration field controls,
				< 12 pptv		field duplicates, or background
							samples

Method precision		|RPD(2)| < 15%		Field duplicates above MLOQ
				RSD < 15%		Field controls

Transport&storage		< MLOQ			Transport blanks
contamination

Completeness %			90%			Percentage of samples producing
							good measurements

(1) RSD is relative standard deviation: standard_deviation/average
(2) RPD is relative percent difference: for duplicates is 
    (measure_1 – measure_2)/average_of_1&2
(3) RPD is relative percent difference: for known concentrations is 
    (measure – actual)/actual
(4) “Method” is entire sampling method including sampling and analysis.
(5) Method Limit of Quantitation

Quality control issues pertaining to procedures for sample handling in the field and 
chain of custody were described in the previous section.  Pre-project and laboratory QC 
procedures are described below and consisted of the following 21 steps:
1.	Pre-project maintenance of bag samplers.
2.	Testing of all sample bags.
3.	Pre-project cleaning and analysis checks of all sample bags.
4.	Development of analysis protocols for the expected sample concentration ranges.
5.	Use of a written standard operating procedure (SOP).
6.	Pre-project calculation of instrument limit of detection (ILOD) and instrument 
        limit of quantitation (ILOQ).
7.	Holding time studies.
8.	Daily calibration of the ATGAS.
9.	Initial ATGAS Calibration Verification (ICV).
10.	Continuing ATGAS Calibration Verification (CCV) and analysis of laboratory controls.
11.	Atmospheric background checks of SF6 at the tracer analysis facility (TAF).
12.	Analysis of laboratory (instrument) blanks.
13.	Analysis of laboratory duplicates.
14.	Analysis of field blanks.
15.	Analysis of field controls.	
16.	Analysis of field duplicates.
17.	Software quality control checks.
18.	Data verification.
19.	Post-project determination of MLOD and MLOQ.
20.	Final data review.
21.	Data handling.
22. 	Summary of Data Completeness

1.   Pre-project maintenance of the bag samplers.

Prior to deployment to the field, each of the original 135 bag samplers was extensively 
tested to ensure proper operation in the field and to ensure the collection of an adequate 
sample volume.  This mainly involved checking the function of the microprocessor and pumps. Fifteen new bag samplers were built for PSB1 using Texas Instruments MPS-430 series microprocessors and similarly tested.

2.   Testing of all sample bags.

Experience has shown that almost all leaks in sample bags occurred around the fitting used 
for attachment to the sample tubing. To rectify this problem prior to PSB1, the seam 
between the fitting and the bag was permanently sealed in all sample bags using Pliobond 
30. All bags were also inspected and if there were any holes or suspected holes besides 
the fitting seam they were discarded prior to gluing. Previously bags had been checked for 
leaks using the procedure detailed in Clawson et al. (2008) but the bag sealing resulted 
in a lower failure rate than had been achieved by the leak checking procedure of the past.

3.   Pre-project cleaning and analysis checks of all sample bags.

After the bags were leak checked but prior to deployment to the field, all bags in the 
sampler cartridges were cleaned.  The bags were cleaned by repeatedly filling them with 
UHP nitrogen and then evacuating them on the cartridge cleaning apparatus. The apparatus 
consisted of a nitrogen tank and vacuum connected to a system that fills and evacuates the 
sample bags by changing valves.  Seventy-two bags in 6 cartridges were cleaned at one time. 
The computer mounted underneath the cleaning apparatus was used to create cartridge 
cleaning records.  This information was then uploaded into the ATGAS history file.  
An 8-step cleaning protocol was used to clean the bags:
	1.	Connect all tubes to the cleaning machine.
	2.	Open all clips.
	3.	Make sure the cleaning machine valves are set so that nitrogen can flow 
                into all connected cartridges.
	4.	Evacuate bags.
	5.	Fill all bags with nitrogen and then evacuate.  Repeat until all bags have 
                been evacuated 5 times.
	6.	Fill all bags with nitrogen for analysis.
	7.	Scan all cartridge bar codes with the bar code scanner and upload the data
		to the ATGAS PC.
	8.	After analysis, place the cartridges back on the cleaning machine, 
                evacuate the nitrogen, disconnect the tubes and wait 30 seconds before 
                closing clips.

This protocol was developed after significant testing to ensure that bags containing 
concentrations in the expected high range of up to 150,000 pptv or more could be cleaned 
to less than background levels.  After cleaning, the bags were filled with UHP nitrogen 
and analyzed to ensure there was no contamination from previous tests or from long-term 
storage.  Any bags with a concentration greater than 5 pptv were re-cleaned and re-analyzed.
All but 23 out of 6744 bags (562 cartridges) were successfully cleaned below 5 pptv in the 
initial cleaning and none were greater than 10 pptv.  The vast majority were below the 
instrument limit of detection and within 0.1-0.2 pptv of zero.  The 23 exceptions were 
successfully re-cleaned and analyzed.  All bags were stored evacuated until their use. 

4.   Development of analysis protocols for the expected sample concentration ranges.

Analysis protocols were developed to optimize instrument performance, accuracy and 
efficiency during the project.  In particular, each GC was configured to optimize the 
detection of the lowest possible concentrations in line with the expectation that the 
planned tracer release rates would result in mostly low to moderate concentrations and 
relatively fewer very high concentrations.  Larger volume sample loops were selected in 
anticipation of measuring mostly lower concentrations.  However, smaller volume sample 
loops were also evaluated to characterize the dynamic range available for measuring high 
concentrations on each GC in the event these were encountered.  Analysis parameters were 
adjusted to account for the magnitude of concentration ranges that were expected.  One set 
of parameters dealt with the worst case scenario carryover issue resulting from measuring 
extremely low concentration samples immediately following extremely high concentration 
samples.  Nitrogen purge and vacuum times and the number of purge-vacuum cycles of the GC 
were set to ensure no carryover of high concentrations.  Other parameters controlling the 
timing of the injection, switch to back-flush, and total length of the analysis cycle were 
set to ensure that oxygen and other contaminants were back-flushed before reaching the ECD 
to avoid any interferences.  Electron capture detector attenuation adjustments were also 
tested at different concentration levels to provide quick adjustments to the instruments 
in the case of unexpected concentration ranges. 

5.  Use of a written standard operating procedure (SOP).

A written SOP entitled, “Standard Operating Procedure for Sampling and Analysis of Sulfur 
Hexafluoride Using Programmable Integrating Gas Samplers (PIGS) and Automated Tracer Gas 
Analysis Systems (ATGAS)” was used by all personnel performing SF6 analysis so that all 
analyses were performed consistently. The SOP contained the following sections:
	1.    Scope and Application.
	2.    Summary of Method.
	3.    Health and Safety Warnings.
	4.    Interferences.
	5.    Personnel Qualifications.
	6.    Equipment and Supplies.
	7.    ATGAS Setup.
	8.    Sample Collection.
	9.    Cartridge Check-In.
	10.  Analysis Preparation.
	11.  Analysis.
	12.  Sample Handling and Holding Times.
	13.  Data Analysis and Calculations.
	14.  Quality Control and Quality Assurance.
	15.  Data and Records Management.
	16.  Trouble-shooting.
	17.  References.

6.   Pre-project calculation of instrument limit of detection (ILOD) and instrument 
limit of quantitation (ILOQ).

Prior to the start of the project, the ILOD and ILOQ were established for each ATGAS to 
provide information on instrument performance.  The ILOD is the instrument’s limit of 
detection and is defined as the lowest concentration that can be determined to be 
statistically different from zero.  It is a measure of instrument sensitivity and based 
upon the specific instrument’s ability to differentiate a low level concentration standard 
from instrument noise.  One bag filled with a low level standard was analyzed on each of 
the 12 autosampler ports on each ATGAS.  The analysis at each port was preceded by the 
analysis of a higher concentration standard of at least 10,000 pptv to evaluate any 
possible carryover effects.  The ILOD was calculated as three times the standard deviation 
of a low level standard that was analyzed twelve times.  The ILOQ is the instrument’s 
limit of quantitation and is defined as the lowest concentration that can be determined 
within 30% of the actual concentration.  The ILOQ was calculated as ten times the standard 
deviation of the same low level standard analyzed 12 times.  Since using different 
concentrations will yield different ILOD and ILOQs, the analyst selected the lowest 
concentration standard to meet as many of the following criteria as possible:
•	Has a relative standard deviation (RSD), i.e., the standard deviation divided by 
the mean multiplied by 100 of less than 15%.
•	Has a signal to noise (S/N; the mean divided by the standard deviation) between 
3 and 10 (a higher value does not invalidate the result; rather it indicates that a lower 
concentration standard can be used).
•	Has a percent recovery (analyzed value divided by the certified value multiplied 
by 100) between 90% and 110%.
 
Results for the pre- and post-project estimation of ILOD and ILOQ for each ATGAS are 
shown in table below.  All initial ILOD were less than 1 pptv and much less than the 
stated measurement quality objective (MQO) of less than 4 pptv.  All initial ILOQ were 
less than 2 pptv.  No carryover effects were observed.

ATGAS	1	2	3	4 w. outlier	4 no outlier	All
Pre-Project (3.11 pptv)
Number	12	12	12	12		
Mean	3.44	3.37	3.46	3.25		
S.D.	0.08	0.16	0.16	0.2		
RSD%	2.33	4.75	4.62	6.15		
S/N	43	21.0625	21.625	16.25		
ILOD	0.24	0.48	0.48	0.6		
ILOQ	0.8	1.6	1.6	2		
Lab Blank
Number	174	132	174	96		
Mean	0	0	0.24	0		
S.D.	0	0	1.47	0		
ILOD	0	0	4.41	0		
ILOQ	0	0	14.7	0		
Lab Control (3.11 pptv)
Number	81	75	99	57		56		311
Mean	3.02	3.03	3.21	2.74		3		3.065
S.D.	0.45	0.59	0.62	2.04		0.42		0.53
ILOD	1.35	1.77	1.86	6.12		1.26		1.59
ILOQ	4.5	5.9	6.2	20.4		4.2		5.3
Post-Project (3.11 pptv)
Number	12	12	12	12		
Mean	3.25	3.14	3.2	3.12		
S.D.	0.095	0.15	0.16	0.26		
RSD%	2.92	4.78	5.00	8.33		
S/N	34.21	20.93	20.00	12.00		
ILOD	0.285	0.45	0.48	0.78		
ILOQ	0.95	1.5	1.6	2.6
		
7.  Holding time studies.

Holding time studies are determinations of the length of time a sample can be held in its 
container before the sample concentration changes appreciably.  Holding time studies are 
conducted whenever the method or sampling container is changed in any way prior to 
commencement of a project. These studies are used to determine what effect degradation of 
the materials will have on sample results. Knowledge of the length of time the samples 
can be held will help in planning the analysis schedule for the samples in the field.  
Holding time studies on the Tedlar sample bags performed in 2004 showed no appreciable 
change in sample concentration for up to six months if stored indoors and away from 
temperature extremes.  Artifact studies on the Pliobond-sealed bag sample fittings and 
R-3603 tubing were performed in 2011 and early 2013 and showed no evidence of sample 
contamination or bag leakage.  All samples were initially analyzed within a week of 
sampling for this project.

ATGAS	Loop Volume	Calibrated Range	Number of Standards
1	1 ml		ILOD-52,600 pptv	18
2	1 ml		ILOD-75,100 pptv	19
3	500 ul		ILOD-158,200 pptv	initial 20
3	500 ul		ILOD-210,700 pptv	final 23
4	1 ml		ILOD-36,900	17

8.  Daily calibration of the ATGAS.

In order to quantify the concentration of the samples, each of the four ATGASs was 
calibrated at the beginning of each analysis day using 10 to 18 NIST-traceable SF6 
standards.  The number of standards used was dependent upon the concentration range 
available to each ATGAS as they were configured for this experiment and the expected 
range of concentrations.   Each ATGAS was configured to optimize the ability to detect 
very low concentrations, principally by choice of a sufficiently large sample loop. This 
low end optimization had the effect of restricting the ability to quantify higher 
concentrations without changing sample loops.  The analytical ranges for each ATGAS as 
configured for the experiment are shown in the table above.  Differences relate to sample 
loop size and the specific performance characteristics of each ATGAS.

The commonly used calibration standards ranged from 3.11 pptv to 36,900 pptv and covered 
most of the range of field sample concentrations encountered. There were a few exceptions 
that required the use of an additional 7 standards ranging up to 210,700 pptv to quantify 
these samples (run on GC3).  Two standards were depleted and replaced by standards with 
similar concentrations immediately prior to the start of analyses of test samples (24.8 
replaced with 19.19 pptv; 307 replaced with 301 pptv). Three other standards were depleted 
and replaced by standards with similar concentrations during analyses of the test samples 
(3110 replaced by 3140 pptv; 5220 replaced by 4980 pptv; 8300 replaced by 8270 pptv). A 
UHP nitrogen zero point was also used in the calibration since it is very difficult to 
find UHP air with undetectable amounts of SF6.  Concentrations of samples were calculated 
using a point-to-point fit calibration of the standards.  The calibration curve was 
examined for "wild fits" and an error message was displayed if such an event occurred so 
that the analyst could more closely examine the curve and decide if it was appropriate to 
use.

9.  Initial ATGAS calibration verification (ICV).

After each calibration was completed and reviewed, the curve was validated by analyzing 
the same calibration standards as if they were field samples.  This validation 
demonstrated that sample concentrations within the calibration range could be quantified 
correctly.  The recoveries were required to be within ±10% of the certified value or the 
standards were re-analyzed.  If the recoveries still did not meet the acceptance limits, 
the bags were refilled and analyzed again.  If the recoveries were still not acceptable, 
the instrument was re-calibrated and ICV was attempted again. 

10. Continuing ATGAS calibration verification (CCV) and laboratory controls.

The validity of the ATGAS instrument calibration curves were regularly checked by 
re-analyzing calibration standards as if they were field samples.  This procedure, called 
continuing calibration verification (CCV), was performed to provide evidence that 
instrument drift had not caused the calibration to be unable to correctly quantify sample 
results within the MQO acceptance level.  Standards were chosen to cover the concentration 
range of samples that had been analyzed since the last calibration verification. The 
standards were required to have a recovery of ±20% of the certified value for that section 
of the curve to be considered valid.  If any of the standards were not within the 
acceptance window, the instrument was re-calibrated and the curves were re-validated.
All data within the unacceptable concentration range, from the point of the last 
acceptable CCV, were flagged and re-analyzed.

There was a tendency for the responses of the GCs to become more stable with 
continued operation but all of them exhibited some susceptibility to drift of the 
calibration. The frequency of CCVs ranged from less than 1 to about 3 h depending on 
the GC and how long it had been in operation with a relatively stable calibration for 
any given day. In general, calibration checks were done more frequently in the first 
few hours of operation and less frequently after that if the GC was exhibiting stable 
behavior. Recalibrations were usually done if the response had drifted significantly 
(> about 6-8%) as there was a tendency that once drift had commenced it often continued 
and raised the prospect of performing analyses that would have to be redone due to 
violating the MQO requirement of ±20%. Furthermore, the intent was to keep all results 
within 10% even though the stated MQO calls for ±20%.  Following any recalibration, 
responses were often stable within ±5% for the remainder of the day.  In some cases 
it was not necessary to recalibrate after the initial calibration although it was 
common for GCs 1 and 3 to be recalibrated once a few hours into the day and then 
remain stable for an extended period of time. GCs 2 and 4 were the most susceptible 
to problematic calibration drift but even they sometimes had stable calibrations. 
Considerable time was spent in calibration and recalibration of the GCs to ensure 
achieving MQO, especially GCs 2 and 4. There was also some analysis time lost due to 
the necessity to rerun some sets of sample cartridges due to failure to achieve the 
requisite CCV recoveries.

The CCV serve as laboratory control samples and measures of instrument precision and 
instrument accuracy.  Results for the combined laboratory control samples (CCV) are
summarized in the table below.  All of the RSD were well below the 10% limit specified 
in the MQOs and indicated excellent instrument precision. The regression slope (1.007) 
and intercept (28.6) indicate no appreciable bias and the Pearson’s r correlation value 
of 0.9995 shows excellent precision.  The average recoveries are indicative of excellent 
accuracy across the full range of concentrations used and are easily within the 
100±20% requirement.

		Measured	Measured
Concentration	Average		S.D.	Recovery	RSD%	S/N	Number		
0		0.21		0.91					222
0		-0.05		4.05					223
3.11		3.13		0.36	100.6		11.5	8.7	306
10.1		10.14		0.46	100.4		4.5	22.0	277
19.19		19.48		1.02	101.5		5.2	19.1	276
35.1		34.91		1.46	99.5		4.2	23.9	268
88.7		89.5		2.99	100.9		3.3	29.9	266
301		304.2		9.33	101.1		3.1	32.6	264
504		509.9		16.81	101.2		3.3	30.3	268
818		829.6		30.4	101.4		3.7	27.3	267
1550		1583.1		71.13	102.1		4.5	22.3	265
3110		3154.8		113.99	101.4		3.6	27.7	133
3140		3257.5		123.45	103.7		3.8	26.4	127
4980		5105.3		214.01	102.5		4.2	23.9	125
5220		5302.4		195.01	101.6		3.7	27.2	135
8270		8406.4		264.22	101.6		3.1	31.8	85
8300		8346		343.8	100.6		4.1	24.3	150
9730		9920.9		424.13	102.0		4.3	23.4	217
16370		16582.7		784.08	101.3		4.7	21.1	210
21720		21928.9		727.61	101.0		3.3	30.1	185
36900		37062.9		1655.32	100.4		4.5	22.4	179
52600		53275.1		1238.15	101.3		2.3	43.0	139
75100		75652.7		1964.19	100.7		2.6	38.5	36
90100		90670.2		2581.11	100.6		2.8	35.1	14
103600		103528.4	3077.36	99.9		3.0	33.6	14
152300		153541.6	3254.72	100.8		2.1	47.2	9
179300		178952.1	3129.02	99.8		1.7	57.2	2
210700		210492.1	1562.92	99.9		0.7	134.7	2

11.   Atmospheric background checks of SF6 at the tracer analysis facility (TAF).

A background atmospheric check of SF6 in the TAF consisted of analyzing three samples 
of the room air in the TAF on each GC every analysis day.  This information was used to 
determine if there was any leakage in the analysis system when compared to the instrument 
blanks that were subsequently analyzed.  The data provided for an inter-comparison 
between GCs that were being used on the same day to check the between instrument 
precision.  The results were also used to reveal discrepancies between GCs to indicate 
a problem that otherwise might go undetected.  The results shown in the table below 
indicate that there was good precision between the 4 GCs.  The average concentration for 
all background checks was 8.3 pptv with a standard deviation of 0.74 pptv.  With the 
exception of GC3, the combined and individual RSD values are all less than the 10% MQO 
for “Between instrument precision”). GC3 was susceptible to baseline instabilities at 
very low concentrations. One consequence of that is the larger standard deviation 
associated with the measurement of room air.

Room Air	#	Mean	S.D.	RSD
GC1		48	8.3	0.62	7.5
GC2		37	8.38	0.83	9.9
GC3		48	7.96	1.69	21.2
GC4		24	7.92	0.73	9.2
All		157	8.27	0.74	8.9

12.   Laboratory (instrument) blanks.

A laboratory or instrument blank was analyzed on each ATGAS each analysis day to verify 
that there was no contamination or leaks within the analysis system as compared to the 
background checks analyzed that day, that there was no carry-over from previously 
analyzed high concentration standards, and to ensure carrier gas purity.  The blank sample 
consisted of a cartridge of 12 bags that were each filled with ultra high purity (UHP) 
nitrogen.  The concentration results of all bags were required to be less than the lowest 
calibration standard and close to a concentration of 0 pptv.  If the concentration of 
one or more of the bags was higher than the acceptable range, the bag was re-filled and 
re-analyzed.  If the concentration still was not within acceptable limits, the instrument 
was re-calibrated and re-verified or the samples were flagged and re-analyzed.  If there 
were still indications of contamination, the problem was identified and fixed before 
analysis continued.

The average results indicate no contamination or leakage problems within any of the 
ATGASs as well as no carryover issues and meet the MQO of <4 pptv.  The higher mean 
and standard deviation for ATGAS 3 reflect its sensitivity to the effect of very small 
changes in baseline on the peak integration at very low level concentrations.  This 
features also shows up in some calculations of the ILOD and ILOQ for ATGAS 3.

13.   Laboratory duplicates.	

Analyses of laboratory duplicates were performed each day to provide evidence of 
instrument precision.  Each day at least one primary field bag sampler cartridge was 
analyzed in duplicate on each ATGAS.  The sample cartridge and its duplicate were 
analyzed at least 3 hours apart in order to ensure an appropriate estimation of 
instrument precision over time.  The duplicate cartridges were selected to encompass 
as much variation and range of concentration as possible within the concentration 
range bracketed by the calibration curve for each ATGAS.  The mean of the absolute 
value of the relative percent differences (RPD),

RPD = (100*(measure#1 - measure#2)/average(#1 and #2)) 

were required to be within 5%.  Any result not within the acceptable limits was flagged 
and re-analyzed.  If the result was still not within acceptable limits, the analysis was
terminated until the ATGAS precision could be re-established.

The |RPD| laboratory duplicate results are shown in the table below. With the exception
of GC4, all are less than 5% indicating excellent instrument precision.  

Laboratory Duplicates			
		Mean %	Mean %
GC	#	RPD	|RPD|
1	235	-0.1	2.1
2	362	1.7	3.6
3	262	0.35	3.4
4	354	-1.8	6.4

14.   Field blanks.

Field (method) blanks were sampled and analyzed to indicate if there was any 
contamination or leakage introduced by any part of a bag sample’s history from sampling, 
handling, and transport through to the final analysis.  For example, isolated instances 
of high concentrations of SF6 in the field blanks can indicate holes in the sampling bag, 
clips not properly closed, wrong location number, or other operational problems.  
Consistently high concentrations would indicate a sampling method that could not measure 
null concentrations accurately.

Three field blank samplers were deployed during each IOP as described in Description of 
Bag Sampling Grid above.  A field blank consisted of a sampler containing a cartridge 
filled with ultra high purity (UHP) nitrogen. Each sampler was deployed at its designated 
location and collocated with a regular sampler with the tubes connected and clips left 
open.  Software requirements of the sampling program made it necessary for the pump on 
the first bag to turn on for one short pulse.  However, after that, all pumps were left 
off and there was no additional filling of any of the bags.  At the end of each test, 
the clips on the blank cartridges were closed and the cartridges were collected, 
transported, and stored along with all the regular sample cartridges.  With the 
exception of the special sampling program, the field blanks were treated identically 
to the regular samples.

A summary of field blank results is presented in the table below. The means and 
standard deviations for IOPs 1, 4, and 5 are all very low indicating no contamination 
or sample handling problems. The non-zero mean and larger standard deviation for IOP2 
is attributable to the use of GC3 for one cartridge of blanks. The non-zero mean and 
larger standard deviation for IOP3 is likely attributable to one cartridge that was 
deployed where it was in the plume with high concentration values for extended periods 
of time. It is likely that small amounts of tracers diffused into the sample bags 
through the open, unclipped tubing. However, even this cartridge did not have any bags 
with values greater than 10 pptv.

Test	#	Mean	S.D.	MLOQ
1	36	0	0	0
2	35	-1.46	2.76	27.6
3	36	1.95	3.33	33.3
4	36	0.01	0.05	0.5
5	36	0	0	0

The consequences of these observations are considered more fully in the determination 
of final MLOQ for the project results (step 19 of this chapter).  Briefly, the field 
blank results adversely affected some of the project MQOs (Table 4): (1) They sometimes 
indicated values for MLOD greater than 12 pptv in some cases (MQO “Method sensitivity”) 
and (2) the field blanks were often greater than the nominal MLOQ.

15.   Field controls

Three field control samplers were deployed during each IOP as described in the section 
on Description of Bag Sampling Grid above.  The cartridge for each control sampler was 
filled with NIST-certified tracer concentrations ranging from 14.79 pptv to 5170 pptv. 
 Bags 1-3 contained 5170 pptv, bags 4-6 contained 199.5 or 283.9 pptv, bags 7-9 contained 
14.79 pptv, and bags 10-12 contained 1571 pptv.  During IOP5, bag 9 was inadvertently 
filled with 1571 pptv instead of 14.79 pptv. Each sampler was deployed at its designated 
location and collocated with a regular sampler with the tubes connected and clips left 
open.  Software requirements of the sampling program made it necessary for the pump on 
the first bag to turn on for one short pulse.  However, after that, all pumps were left 
off and there was no additional filling of any of the bags.  At the end of each test, 
the clips on the control cartridges were closed and the cartridges were collected, 
transported, and stored along with all the regular sample cartridges.  With the exception 
of the special sampling program, the field controls were treated identically to the 
regular samples.

The field control samplers served two primary purposes.  First, they checked for any 
biases or inaccuracies introduced during the sampling, handling, and storage of the 
samples.  Second, recall that the standards used to calibrate the GCs (up to 210,700 
pptv) were all NIST-certified.  The tracer concentrations used to fill the control 
bags also came from NIST-certified standards but they were different from those used 
in the calibration of the ATGASs.  As a consequence, the field control samples serve 
as a semi-independent measure of quality control of the overall process, essentially 
a method audit.

The results for the field control samples expressed in terms of the individual ATGAS 
are shown in the table below.  In general there was a very good comparison between the 
NIST-certified standards used in the field controls with the NIST-certified standards 
used to develop the calibration curves for the GCs. One notable exception was the 199.5 
pptv field control standard. The measured results for that standard were consistently 
lower. The measured results for the 1571 pptv standard also had a low bias but were 
within the ±5% uncertainty of each standard and were much closer than for the 199.5 pptv 
standard. Linear regression on the combined field control samples calculated a slope of 
1.004, an intercept of 38.5, and a Pearson’s r value of 0.999 indicating that overall 
there was no significant overall bias and good precision. With the exception of 199.5 
pptv for IOP3, the mean |RPD| and the RSD MQO requirements were all satisfied and mostly 
by very comfortable margins (<20% and <15%, respectively).

Standard	IOP1	IOP2	IOP3	IOP4	IOP5	All
14.79						
#		9	9	9	9	8	44
Mean		15.10	14.71	15.62	15.81	15.33	15.31
S.D.		0.41	0.41	1.42	0.27	0.19	0.54
Avg. Recovery	1.02	0.99	1.06	1.07	1.04	1.04
Mean RPD	2.04	-0.57	5.10	6.66	3.55	3.36
Mean |RPD|	2.18	1.97	7.94	6.66	3.55	4.46
RSD		2.71	2.77	9.07	1.72	1.25	3.50
S/N		36.90	36.09	11.02	58.28	80.29	44.52
199.5						
#		9	9	9	9	3	39
Mean		178.70	171.62	163.59	177.21	173.87	173.00
S.D.		3.42	8.65	15.19	3.21	4.69	7.03
Avg. Recovery	0.90	0.86	0.82	0.89	0.87	0.87
Mean RPD	-11.01	-15.13	-20.15	-11.85	-13.75	-14.38
Mean |RPD|	11.01	15.13	20.15	11.85	13.75	14.38
RSD		1.91	5.04	9.29	1.81	2.70	4.15
S/N		52.24	19.85	10.77	55.24	37.06	35.03
283.9						
#					6	6
Mean					284.55	284.55
S.D.					4.31	4.31
Avg. Recovery					1.00	1.00
Mean RPD					0.22	0.22
Mean |RPD|					1.04	1.04
RSD					1.51	1.51
S/N					66.07	66.07
1571						
#		9	9	8	9	10	45
Mean		1497.44	1454.86	1428.45	1488.10	1446.75	1463.12
S.D.		33.35	43.97	86.39	39.12	20.97	44.76
Avg. Recovery	0.95	0.93	0.91	0.95	0.92	0.93
Mean RPD	-4.82	-7.72	-9.66	-6.06	-8.24	-7.30
Mean |RPD|	4.82	7.72	9.66	5.45	8.24	7.18
RSD		2.23	3.02	6.05	2.63	1.45	3.08
S/N		44.90	33.09	16.53	38.04	68.98	40.31
5170						
#		9	9	9	9	9	45
Mean		5265.70	5187.80	5275.19	5194.93	4941.64	5173.05
S.D.		65.02	186.84	181.54	74.37	64.76	114.51
Avg. Recovery	1.02	1.00	1.02	1.00	0.96	1.00
Mean RPD	1.83	0.29	1.96	0.17	-4.52	-0.05
Mean |RPD|	1.83	2.91	2.97	1.27	4.52	2.70
RSD		1.23	3.60	3.44	1.43	1.31	2.20
S/N		80.99	27.77	29.06	69.86	76.30	56.79

16.   Field duplicates.

Sixteen field duplicate samplers were deployed for each IOP during PSB1 as described 
in the section on Description of Bag Sampling Grid (above). The duplicate samplers 
were handled identically to the primary samplers with which they were collocated. 
They were mounted at the same height at sites a few feet away.  A summary of the 
results is provided in the table below.

Field Duplicates
IOP		#	Avg. % RPD	Avg. % |RPD|
1		185	-3.7		10.2
2		177	-0.5		12.5
3		187	4		12.8
4		182	-4.5		12.2
5		189	1.4		10.2
Combined	920	-2.3		11.6

Overall, there was good agreement between collocated samplers. The MQO mean |RPD| 
requirement was satisfied for all IOPs (<15%). This is confirmed by linear regression 
(slope=0.941, intercept=38.5, r=0.994).

17.   Software quality control checks.

Several important quality checks were built into the software to efficiently aid the TAF 
analyst in ensuring that the ATGAS instruments were functioning correctly during analysis. 
•	Since the concentration is dependent upon the temperature of the ATGAS ovens, it 
        is critical that oven temperatures do not fluctuate widely during analysis.  
        Temperature acceptance limits were set (±2 °C) and the software produced a pop-up 
        window to alert the analyst in case of unacceptable oven temperature readings. 
        All samples obtained using the incorrect oven temperatures were re-analyzed.
•	To check for instrument drift, the software alerted the analyst to validate the 
        calibration curve when more than three hours had elapsed from the last CCV.  The 
        analyst had the option of overriding the alert or checking the calibration and 
        re-starting the 3-hour clock.  This option was always exercised except on a few 
        occasions near the end of the analysis day when only 1-2 more cartridges required 
        analysis.  Even then this was only done on ATGASs that had previously been 
        exhibiting consistently stable response for extended periods of time during that 
        day.
•	In order to verify the calibration curve in the area of interest and to save time, 
        the software produced on the computer screen a record of the highest and lowest 
        concentrations measured since the last CCV.  The analyst had only to re-analyze 
        calibration samples within that range.  However, the complete calibration range 
        was routinely done to most fully evaluate the current status of instrument 
        response and performance. 
•	Several data flags were shown immediately on the computer screen to aid the 
        analyst in deciding whether the data for each bag was “good” or re-analysis was 
        necessary.  For example, the low pressure flag alerted the operator to a problem 
        with the analysis that was almost invariably due to pinched tubing restricting 
        sample flow.
•	The software kept track of which ATGAS field duplicate was analyzed on and 
        directed the analyst to use the same GC for the duplicate cartridge.  This helped 
        to quantitate the variability of the field analysis without adding the extra 
        variability of analyzing on a separate ATGAS.  However, due to limitations 
        imposed by the restricted calibration ranges of ATGASs 2 and 4, it was not 
        uncommon for the field duplicates to be done on different ATGASs.
•	The software alerted the analyst if any calibration points did not meet 
        pre-determined acceptance criteria.  The analyst could then review the 
        calibration curve to determine the acceptable course of action.

18.   Data verification.

Data verification was performed to ensure that the samples met all QC acceptance limits 
and that all samples had been analyzed for that particular test.  Transcription and 
calculation errors were reduced by automated data reduction techniques such as automated 
flagging of results outside acceptable limits, raw data summary sheets, auto-generated 
quality control sheets, auto generation of chromatogram plots including calibration curves,
and electronic transfer of data from the ATGASs to Excel spreadsheets.  The analyst and 
at least one other person familiar with the data analysis process reviewed all data 
packages.  All data packages were batch processed per run on each ATGAS.  All data 
packages included the raw data sheets, quality control sheets that summarized the results 
of all QC data generated for that batch, plots of all chromatograms and calibration curves,
a copy of the laboratory notebook pages for that analysis, and a data verification sheet
to ensure the verifier checked all QC parameters.  Software produced an Analysis Summary 
that was utilized to ensure that there was at least one acceptable result for each bag 
for each location that was downloaded for each IOP. Any samples noted by the software 
were re-analyzed and the Analysis Summary report was re-run until all samples had been 
analyzed or a justifiable reason had been determined for a missing sample.  Cartridges 
were not cleaned until all available samples had been analyzed.

19.   Post-project determination of ILOD, ILOQ, MLOD, and MLOQ.

ILOD and ILOQ were previously defined in step 6 above of the quality control procedures.
In that section a procedure was described for obtaining a preliminary pre-project estimate 
of the ILOD and ILOQ using a very low concentration calibration standard.  These results 
were reported in Table 5.  There are additional ways to estimate ILOD and ILOQ.  These 
include the use of laboratory blanks and the low level laboratory control standards 
used for calibration and CCV.  These alternative determinations together with a 
post-project repeat of the initial procedure are also shown in a table above.  With two 
exceptions, all of the various estimates for ILOD were consistently low and well below 
the stated MQO of 4 pptv.  One exception was the ILOD for the lab blank result for GC3. 
As noted earlier, this is due to the sensitivity of GC3 to the effect of very small 
changes in baseline on the peak integration at very low level concentrations. The other 
exception was for the ILOD for the 3.11 pptv lab control for GC4. In this case, the 
exclusion of a single outlier decreased the ILOD from 6.12 to 1.26 pptv.

The method limit of detection (MLOD) and method limit of quantitation (MLOQ) are 
estimates of the lowest field concentration level that can be determined with some 
degree of certainty.  Unlike ILOD and ILOQ, MLOD and MLOQ incorporate all the sources 
of variability and uncertainty introduced during each phase of the sampling, handling, 
and analysis.  The MLOD is defined as the lowest field concentration measurement that 
can be determined to be statistically different from zero.  It is based upon the method’s 
ability to differentiate a low-level concentration standard from the combined effects 
of instrument and method noise.  The MLOD and MLOQ are calculated exactly the same as 
ILOD and ILOQ except that method variability is factored into the determination by 
using results from samples that have been put through the rigors of field sampling.
The MLOD is calculated as 3 times the standard deviation of a low level standard. The 
MLOQ is defined as the lowest concentration that can be determined within 30% of the 
actual concentration.  The MLOQ is calculated as 10 times the standard deviation of 
the same low level standard.

There are several ways to attempt to estimate MLOD and MLOQ.  These include field blanks, 
low concentration field controls, and field duplicates. Ambient background samples of all 
regular field samples can also be used to estimate MLOQ. However, these samples do not 
incorporate all sources of variability observed during experiments.  Specifically, 
background samples, by definition, were not exposed to the higher level concentrations 
measured by many of the samplers that were strongly impacted by the tracer plume. 
Sampler cartridges located on parts of the grids that were heavily impacted by the 
tracer plume were seen to occasionally have their lower concentration bags affected. 
There is also the problem of setting a cutoff value separating truly background samples 
from those that were slightly influenced by the plume. For these reasons, the ambient 
background method was not calculated. Estimates of MLOQ were made using each of the 
other methods. The table below summarizes the results of the analysis for the estimate 
of MLOQ.

	Field Duplicates
	IOP1		IOP2		IOP3		IOP4		IOP5	IOP 	Average
	dup<10	dup<20	dup<10	dup<20	dup<10	dup<20	dup<10	dup<20	dup<10	dup<20	dup<10	dup<20
count	87	113	36	50	91	106	46	61	65	75		
mean	-0.04	-0.19	-0.32	-0.54	-0.30	-0.09	-0.11	0.19	-0.42	-0.51		
s.d.	0.47	3.40	0.93	2.13	1.64	1.90	0.53	1.50	1.23	1.84		
mloq	4.73	33.98	9.32	21.34	16.37	19.01	5.27	15.00	12.32	18.37	9.60	21.54
Field Blanks											Combined
count	36		35		36		36		36			
mean	0		-1.46		1.95		0.01		0		0.11	
s.d.	0		2.76		3.33		0.05		0		2.19	
mloq	0		27.61		33.30		0.50		0		21.93	

Field Controls (14.79 ppt)								Combined
	14.8		14.8		14.5		15.5		15.1			
	14.8		15.5		14.4		15.6		15.7			
	14.9		14.9		14.2		16.1		15.2			
	14.7		14.7		17.7		15.7		15.4			
	14.8		14.8		16.9		15.9		15.4			
	15.2		14.9		16.2		15.5		15.2			
	15.7		14.4		14.2		16.1		15.2			
	15.2		14.2		15.2		16.2		15.4			
	15.8		14.2		17.3		15.7					
mean	15.10		14.71		15.62		15.81		15.33		15.31	
s.d.	0.41		0.41		1.42		0.27		0.19		0.78	
mloq	4.09		4.08		14.18		2.71		1.91		7.83	

Estimates of MLOQ using the field duplicates technique provided estimates ranging by IOP 
from 4.7 to 16.4 pptv for duplicate pairs less than 10 pptv with an overall IOP average 
of 9.6 pptv. For duplicate pairs less than 20 pptv estimates of MLOQ ranged from 15 to 3
4 pptv by IOP. Estimates of MLOQ using field blanks ranged from zero to 33 pptv. The 
higher estimates for IOPs 2 and 3 are due to the use of GC3 with its baseline sensitivity 
issues at low concentration (IOP2) and one cartridge that was clearly affected by high 
plume concentrations (IOP3). The highest concentrations measured during PSB1 were 
during IOP3. Estimates of MLOQ using the low concentration field control ranged from 
1.9 to 14.2 pptv for all IOPs with only IOP3 having an MLOQ greater than 4.1 pptv. 
The MLOQ for the combined field control sample population was 7.8 pptv.
        
For reasons given earlier, it is preferable to use the lowest practicable concentrations 
for the calculation of MLOQ which would discount the estimates of MLOQ using duplicate 
pairs <20 pptv and recommend the usage of duplicate pairs <10 pptv. While there is 
considerable disparity in the remaining estimates of MLOQ, a universal value of 9 pptv 
was adopted. The overall field duplicate estimate for pairs <10 pptv was 9.6 pptv. The 
combined estimate from the low concentration field control was 7.8 pptv. With the 
already noted exceptions of IOPs 2 and 3, the MLOQ given by the field blanks was zero. 
Even for the IOP3 case no affected field blank values were >10 pptv. While arguments 
could be made for a higher, somewhat more conservative value, the weight of evidence 
suggests that a reasonable universal value for PSB1 MLOQ is 9 pptv. For this reason, 
all values less than 9 pptv have been flagged as estimates in the final database.

20.   Final data review.

All field data were verified to make sure there was a result for every location, 
cartridge, and sample bag and that all results were flagged appropriately.  The 
following examples of verification plots and summaries were chosen to illustrate the 
diligence with which each data point is reviewed.  Every quality control sheet for each 
data package was reviewed to ensure proper flagging of final data.  Bubble/dot plots 
were created and reviewed to ensure all data were reasonable and consistent with 
respect to the overall concentration pattern and the nearby neighbors of each bag 
sample.  Any suspicious data point was traced back through the analysis and deployment 
records to determine if it was indeed a valid result.  The sampler servicing records 
maintained by all field sampler deployment personnel for noting any problems, were 
used to check any outliers or anomalies in the data.  Cartridge time history plots 
as well as individual chromatograms were also reviewed to determine any suspicious data 
points.  Any suspicious data point was traced back through the analysis and deployment 
records, sometimes with the aid of the master history file, to determine if it was 
indeed a valid result.  All field QC was scrutinized.  All suspicious data were 
appropriately flagged.

The finalized data set was then analyzed using a program used to determine if all flags 
were added correctly and if the sample results could possibly be QC results. Any results 
appearing on this sheet were verified and changes to the data base were made as 
necessary.

21.   Data handling.

All results were printed on hard copy as a backup in case of loss of the data files 
and to aid in the data verification process.  The data packages were filed for future 
reference and to be readily available during the project for immediate review.  
Backup copies of the raw ATGAS data were made occasionally and at the end of the 
project to prevent total loss of data in the case of a computer failure.  All final 
QC and sample results were printed on hard copy and placed in a binder to be stored 
with any reference materials in the project archive. 

22.  Summary of Data Completeness and Contribution by GC

The table below summarizes bag sampling data completeness for each test as well as for 
the entire project.  The MQO of 90% was exceeded in every case.  ‘Field Problems’ 
incorporates the complete range of possible field problems (e..g. clips found open, 
irregular random flat bags, entire cartridges with most or all bags flat, overfilled bags). 
In the worst case of cartridges with all bags flat, this represented a failure by the 
field operator to correctly download the sampling program into the sampler or a failure 
of the sampler itself. One of the more common ‘Lab Problem’ was clips being open during 
the GC purge cycle resulting in the bags being diluted with the nitrogen purge gas thus 
invalidating the sample. The 12 samples not analyzed for IOP4 was due to the fact that 
one cartridge was used for sampling in IOP3 and then redeployed again for IOP4 sampling 
without first being analyzed and cleaned in between. The results for this cartridge were 
flagged with having a field problem.
 
The numbers in the table indicate that GCs 1 and 3 were the workhorses.  GC1 provided 
the most consistently stable operation and required the fewest calibration checks and 
recalibrations. It did have a slight tendency for temperature drift that occasionally 
required rerunning samples. GC3 had the widest analytical range available without 
resorting to sample loop changes, the shortest analytical cycle time, and usually 
provided consistently stable operation.  The lower numbers for GC4 reflect a longer 
analytical cycle time, a strong tendency toward calibration drift, and a restricted 
analytical range as configured (Table 6).  The lower numbers for GC2 mostly reflect 
the difficulties often experienced in achieving stable operation.  Regardless of GC, 
however, data had to at a minimum satisfy the MQO to be acceptable.

			IOP	
GC			1	2	3	4	5	Total
1			466	544	392	591	497	2490
2			347	263	297	298	415	1620
3			495	536	476	491	466	2464
4			228	193	371	144	157	1093
Total			1536	1536	1536	1536	1536	7680
Valid			1488	1495	1486	1467	1504	7440
Field Problems		39	19	23	65	19	165
Lab Problems		8	22	24	4	13	71
Estimate (lab prob.)	1	0	3	0	0	4
Not Analyzed		0	0	0	12	1	13
Completeness%		96.9	97.3	96.7	95.5	97.9	96.9

Final Bag Sampler Data Files and Format

The final bag sample tracer data files provided with this report contain 12 columns:

1.	test (IOP) number
2.	bag number (1-12)
3.	date (yyyymmdd)
4.	start time (hhmmss)
5.	sampling period (seconds)
6.	dist (distance from release point in meters)
7.	angle (angle in degrees along respective arc from north)
8.	agl (meters, above ground level)
9.	latitude
10.	longitude
11.	concentration (SF6 pptv)
12.	quality control flag

The files are in csv format with fixed width fields.  The data files are named 
‘PSB1_IOP#_BagSampling_Final.csv’ where ‘#’ is the number of the individual IOP test. 
The bag sampling Readme file accompanying this report summarizes the contents of this 
chapter on the bag sampling.

Final Data File Quality Control Flags

All of the data were flagged with one of six possible quality flags: These are:

0	> MLOQ; good data to be used without qualification.
1	< MLOD (4 pptv)
2	< MLOQ (9 pptv) and > MLOD (4 pptv). Treat as an estimate.
3	missing – field problem (check in was F, I, or B), also missing analyses are 
        included here; data values set equal to -999.
4	missing – lab problem; data values set equal to -999.
5	estimate because of laboratory problem (don’t use = 1 or 2) data values set 
        equal to -999.

Flag ‘1' applies primarily to anomalously low ambient samples. Ambient background 
samples were generally in the range from 6-8 pptv. Most values less than 5 pptv were 
preemptively designated as estimates (Flag ‘2’) since anything less than 5 pptv for an 
ambient sample is unlikely. However, a few samples were still flagged with ‘1’.

Flag ‘2' applies primarily to ambient background samples and those samples that were 
affected by the plume but still had concentrations below the MLOQ of 9 pptv.  Flag ‘3' 
was applied to any data that was suspect due to field-related problems.  This includes 
improperly connected bags, clips in the open position when they were checked in before 
laboratory analysis, flat bags, and overfilled bags.  Flat bags were the most common 
problem in this category.  The reasons for flat bags include the sampling program 
failed to download from the Timewand into the sampler or the sampler failed to function 
properly.  In some cases it might be attributable to operator error.  The bags remained 
flat because there was no program loaded to turn on the pumps to fill the bags.  Flags 
‘4' and ‘5’ were applied to any data that was suspect due to problems with the 
laboratory analysis.  An example of this was clips being open during the purge cycle 
of the analysis resulting in bag-filling and sample dilution.

