Environmental Protection Technology Series
EVALUATION OF A NEW MICROVOLUME 3 HSc
ELECTRON CAPTURE DETECTOR AND
ANCILLARY DATA SYSTEM FOR PESTICIDE
RESIDUE ANALYSIS
Health Effects Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development. U S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-78-010
February 1978
EVALUATION OF A NEW MICROVOLUME HSC ELECTRON CAPTURE DETECTOR
AND ANCILLARY DATA SYSTEM FOR PESTICIDE RESIDUE ANALYSIS
By
Robert C. Hanisch and Robert G. Lewis
Health Effects Research Laboratory
Environmental Toxicology Division
Analytical Chemistry Branch
Project No. 7529 and 7602
Program Element No. 1EA615
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
HEALTH EFFECTS RESEARCH LABORATORY
RESEARCH TRIANGLE PARK, N.C. 27711
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DISCLAIMER
This report has been reviewed by the Health Effects Research
Laboratory, U.S. Environmental Protection Agency, and approved for
publication. Approval does not signify that the contents necessarily
reflect the views and policies of the U.S. Environmental Protection
Agency, nor does mention of trade names or commercial products
constitute endorsement or recommendation for use.
ii
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FOREWORD
The many benefits of our modern, developing, industrial society are
accompanied by certain hazards. Careful assessment of the relative risk
of existing and new man-made environmental hazards is necessary for the
establishment of sound regulatory policy. These regulations serve to
enhance the quality of our environment in order to promote the public
health and welfare and the productive capacity of our Nation's population.
The Health Effects Research Laboratory, Research Triangle Park
conducts a coordinated environmental health research program in toxicology,
epidemiology, and clinical studies usihg human volunteer subjects.
These studies address problems in air pollution, non-ionizing radiation,
environmental carcinogenesis and the toxicology of pesticides as well as
other chemical pollutants. The Laboratory develops and revises air
quality criteria documents on pollutants for which national ambient air
quality standards exist or are proposed, provides the data for registration
of new pesticides or proposed suspension of those already in use, conducts
research on hazardous and toxic materials, and is preparing the health
basis for non-ionizing radiation standards. Direct support to the
regulatory function of the Agency is provided in the form of expert
testimony and preparation of affidavits as well as expert advice to the
Administrator to assure the adequacy of health care and surveillance of
persons having suffered imminent and substantial endangerment of their
health.
This report represents a research effort to extend and improve
analytical measurement of a variety of pesticides, polychlorinated
biphenyls and other pollutants in our environment. The emphasis is on
the evaluation of new detector systems for such environmental contam-
inants .
John H. Knelson, M.D.
Director,
Health Effects Research Laboratory
iii
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ABSTRACT
The performance of a linearized HSc electron capture detector (BCD) and
its ancillary data system was evaluated for use in the analysis of
pesticide residues. Serial dilutions of pesticide standards were used
to determine the maximum linear range and sensitivity of the detector.
This detector was found to have a significantly greater linear range for
the test compounds than a linearized Ni electron capture detector
63
evaluated. The sensitivity was only marginally better than the Ni
BCD.
iv
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CONTENTS
Page
Disclaimer ii
Foreword iii
Abstract iv
List of Figures vi
List of Tables vi
Sections
I Conclusions 1
II Introduction 2
III Experimental 4
IV Discussion 9
V References 25
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LIST OF FIGURES
No. Page
1 ATC 140A baseline current levels 14
2 Linearized Ni ECD baseline current 15
levels
3 Peak area integration on the ATC Model 24
160 Memory and Display (MAD) System
LIST OF TABLES
No. Page
1 Compounds Used for Detector Evaluation 5
2 Comparison of the Lower Limits of 9
Detection (LLD) for Two Linearized
Electron Capture Detectors
3 Maximum Linear Range with +5* K-Value 11
Variation
4 Maximum Linear Range with 4^10% K-Value 12
Variation
5 Data Collection Time as a Function of 20
Dwell Time
vi
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SECTION I
CONCLUSIONS
The micro-volume electron capture detector evaluated in this study was
found to possess an extended linear range compared to another popular EC
detector. In addition, the detector was a factor of ten more sensitive
for certain compounds than another popular EC detector tested. These
two advantages were more than nullified, however, by the susceptibility
of the detector to contamination, which severely limited its useful
life. The ancillary data system was useful in terms of data acquisition,
storage, and computation of data, but the scope of application of the
system is limited by certain design deficiencies.
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SECTION II
INTRODUCTION
Since its first application in 1960 as a gas chromatographic detection
technique/ the electron capture (EC) detector has played a major role
in the analysis of pesticides, polychlorinated biphenyls, and other
organic pollutants of environmental interest. The various types of EC
detectors that are commercially available differ in the ionizing source
used, the design of the detector cell, and the modes of electrical
stimulation and measurement. Each of these facets can have considerable
effect on the performance of an EC detector.
Three types of radioactive sources are commonly used in EC detectors:
tritiated-titanium, nickel-63, and tritiated-scandium foils. The
tritiated-titanium ( HTi) sources are characterized by high specific
activities which result in greater sensitivity and linear range than
sources employing other radioactive isotopes. In addition, the low
energy (18 keV) beta-rays emitted from such sources permit the con-
struction of a detector with minimum internal volume, a characteristic
which can be helpful in providing ionizing radiation throughout all of
the interelectrode space. HTi sources are prepared by occluding
tritium in titanium-plated stainless steel or copper foils. Since
tritium is easily lost from these foils, the maximum temperature at
which they can be used is about 225°C. However, it is often necessary
or desirable to operate an EC detector at higher temperatures; e.g., for
analysis of high-boiling compounds and for "bake-out" (rejuvenation of
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the foil by heating). The DC electrical stimulation typically employed
for these sources interposes additional obstacles in achieving optimum
detector performance. Electron capture detectors operated in the DC
mode characteristically exhibit drifting baselines, sensitivity vari-
234
ations, and limited dynamic range. ' '
Detectors utilizing nickel-63 as the radioactive source can be operated
at higher temperature (up to 375°C) but only at the expense of linear
dynamic range and sensitivity. By operating the detector in the fixed-
frequency pulsed modei the limited linear range can be extended somewhat
and the problem of baseline drift encountered with tritium detectors can
be eliminated. '
Tritiated-scandium ( HSc) sources have a maximum temperature limit of
2
325°C and source activities are available that approach one curie/cm .
This provides a higher baseline current than any other electron-capture
beta source. When this type of source is operated in the constant-
current/variable-frequency pulsed mode, the linear range of the detector
is significantly increased.
The purpose of this study was to evaluate an electron capture detector
employing a HSc source operated in the constant current/variable-
frequency pulsed mode. The ATC Model 140A detector, which is manufactured
by Analog Technology Corporation, Pasadena, California, is the only high
temperature tritium detector currently on the market. This detector is
represented as having a maximum linear range of 10 when utilized in
conjunction with an ATC Model 160A Memory and Display (MAD) system.
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SECTION III
EXPERIMENTAL
In order to test the sensitivity and linearity of the detector against
a wide range of response characteristics, a variety of compounds were
chosen from several different chemical classes. Thirteen compounds or
mixtures were selected which included chlorinated hydrocarbons, organo-
phosphates, polychlorinated biphenyls and polychlorinated naphthalenes:
e-BHC Ronnel
Hexachlorobenzene Malathion
Aldrin Ethyl Parathion
Perthane Phorate
p_,£'-DDE Aroclor 1254
p_,p_'-DDT Halowax 1014
Mir ex
All materials were primary reference standards obtained from the Pesti-
cide Repository, Health Effects Research Laboratory, Research Triangle
Park, N. C. Empirical formulae and chemical nomenclature are given for
each standard in Table 1.
The test solutions of various concentrations were prepared from each
standard in n_-hexane (pesticide quality). Each of the standards con-
sisted of a series of concentrations covering each decade from 1 pg/yl
to 1 pg/pl. This provided a total of seven different concentration
levels over which the linearity and sensitivity of the detector could be
evaluated. The standards were evaluated individually to simplify
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Table 1. COMPOUNDS USED FOR DETECTOR EVALUATION
Common Name
8-BHC
Hexachlorobenzene
Ronnel
Malathion
Perthane
Aldrin
Ethyl Parathion
Mirex
p,p'-DDE
p,p'-DDT
Aroclor 1254
Halowax 1014
Phorate
Empirical
Formula
C6H6C16
C6C16
C8H8C13°3PS
C10H19°6PS2
C18H20C12
C12H8C16
C10C112
C14H8C14
C14H9C15
C12H5C15
C10H2-5Cl5-5
Chemical Name
£ isomer of hexachlorocyclo-
hexane
Hexachlorobenzene
0,0-Dimethyl-O-(2,4,5-tri-
chlorophenyl)phosphorothioate
S-[1,2-bis(ethoxy carbonyl)
ethyl]-O,O-dimethyl phosphoro-
dithioate
1,l-Dichloro-2,2-bis(p-ethyl
phenyl)ethane
1,2,3,4,10,10-Hexachloro-
1,4,4a,5,8,8a-hexahydro-
1,4-endo-exo-5,8-dimethano-
naphthalene
0,0-Diethyl-0-p_-nitro-
phenylphosphorothioate
Dodecachlorooctahydro-1,3,4-
metheno-2H-cyclobuta[cd]-
pentalene
1,l-Dichloro-2,2-bis
(p-chlorophenyl)ethylene
1,1,l-Trichloro-2,2-bis
(pj-chlorophenyl) ethane
mixture of polychlorinated
biphenyls
mixture of polychlorinated
naphthalenes
0,0-Diethyl-5-[(ethylthio)-
methyl]phosphorodithioate
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identification and avoid potential coelution from the gas chromatograph.
The compounds were analyzed on a Tracer 560 gas chromatograph equipped
with a 183 cm x 4 mm (i.d.) glass column packed with 3% OV-1 on 80- to
100-mesh Gas Chrom Q. The column was maintained at a temperature of
200°C with a 70 ml/min flow of 5% methane/95% argon carrier gas. The
inlet ports and detector were maintained at 210°C and 240°C, respec-
tively.
The Model 160A MAD system is designed to allow the collection of data in
the form of automatically calculated peak area and peak heights, as well
as simultaneous production of gas chromatograms on a conventional strip
chart recorder. In addition, it has the capability of data retrieval
from the disk file for later generation in the form of hard (paper)
copy. Consequently, the data obtained in this study was reported from
all four outputs when possible. Peak height was used as the response
parameter for the data obtained from the strip chart chromatogram.
To ensure the statistical validity of the data, five replicate injec-
tions were made for each concentration level of each standard used in
the study. The response data obtained was then normalized to a 5.0 yl
injection volume to eliminate variation in response due to injection
technique. The normalized data were averaged and the standard
deviation and percent relative standard deviation for each
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of the data outputs were calculated using a small sample statistics
Q
table. The average response data for each of the output modes at
each concentration level for a given standard were used for the final
linearity calculations.
For a given standard, the linearity of response was calculated from
the average response for each concentration level of each test
solution. The calculation used in this analysis is:
,. , _ ,,,. Response X Attenuation Factor
Normalized Response (K) P Sample
If a detector is linear, K should be constant within +5% over the
entire range of concentration levels. For automatically-calculated
peak area and peak height data, the equation is reduced to:
_ Response
Sample Amount
since the attenuator control has no effect on these two forms of
data output.
The sensitivity of the detector was established by injecting progressively
smaller amounts of the standards into the GC system until the lower
limit of detection(LLD) was reached. The LLD was defined as that
amount of sample which would produce a signal equal to twice the noise
level.
An aldrin standard with a concentration of 20 pg/pl was selected to
serve as a reference to determine the long-term response stability of
the detector. The standard was analyzed on a daily basis under identical
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chromatographic conditions in order to detect any variation in response
over the duration of the study. Some of the late-eluting compounds
required more than one day of analysis time to complete the entire
concentration series. In these cases, the data obtained from the
analyses were adjusted in a corresponding fashion for any variations
observed in the response from the aldrin standard. All data were
corrected to the response level observed on the first day of analysis.
Since the baseline current available to an electron capture detector
critically affects the overall system sensitivity and performance,
this parameter was monitored on a daily basis. With the electrometer
offset control centered and pure carrier gas flowing through the
detector, the adjustable reference current was varied until a recorder
output of zero volts was obtained. At this point the nominal base-
line frequency was 5 kHz. The available baseline current was then
recorded and the system was operated at that current level for the
remainder of the day. Any subsequent variations in the baseline were
corrected through use of the offset control.
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SECTION IV
DISCUSSION
DETECTOR
As can be seen from the data presented in Table 2, the sensitivity of
the detector was found to be comparable to that of a commercially-
available linearized Ni electron capture detector. Although the HSc
detector was more sensitive by a factor of 10 for three of the 13
standards tested, this was not considered to be a significant advantage
over other EC detectors.
Table 2. COMPARISON OF THE LOWER LIMITS OF DETECTION (LLD)
FOR TWO LINEARIZED ELECTRON CAPTURE DETECTORS
Compound
B-BHC
HCB
Ronnel
Malathion
Perthane
Aldrin
Ethyl Parathion
Mirex
p,p'-DDE
p,p/-DDT
Aroclor 1254
Halowax 1014
Phorate
LI
ATC Model 140A
5
0.05
0.5
50
50
0.5
5
5
0.5
5
50
50
50
J) (pg) 63
Tracor Linear Ni
5
0.05
5
5
50
5
5
5
5
5
50
50
50
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The results of the linearity determinations are found in Tables 3 and 4.
The detector has been represented as having a linear range of 10 . The
results listed in Table 3 indicate that the maximum linear range achieved
4
in this laboratory was 10 with a +_5% deviation allowance. Linearity
was not measurably greater when the response factor was allowed to vary
by +10%. As can be seen from the tabulated data, the observed linear
range was highly dependent on compound type.
This, behavior has been previously reported for constant-current EC
detectors which utilize a low-frequency pulsed operation. The high-
frequency operation employed in the ATC Model 140A should have circum-
vented this difficulty but apparently did not. Despite these aberrations,
the linear range of the HSc detector was found to be significantly
greater than that of a Tracer linearized Ni EC with which it was
simultaneously compared.
Background noise from the detector was monitored on a daily basis in
order to establish the maximum usable sensitivity of the instrument.
The short-term noise characteristics of the detector when operated at
maximum sensitivity were quite good. The amplitude of the short-term
noise was usually less than 0.5% full-scale deflection (f.s.d.). With
the exception of the four-minute period immediately following injection,
the long-term noise was typically less than 2% f.s.d. when the detector
was operated at maximum sensitivity. At the most sensitive attenuation,
the chromatograms exhibited a 10% overshoot after the solvent peak which
usually required approximately four minutes of recovery time to return
10
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Table 3. MAXIMUM LINEAR RANGE WITH +5% K-VALUE
VARIATION
Compound
B-BHC
HCB
Ronnel
Malathion
Perthane
Aldrin
E. Parathion
Mirex
p,p'-DDE
p,p'-DDT
Aroclor 1254
Halowax 1014
Phorate
Linearity
MAD Peak
Area
io3
io2
<101
io1
io1
io4
io1
io1
io2
1
io1
1
io3
as calculated
MAD Peak
Height
io3
io1
io2
io1
1
io3
io1
1
io1
1
io2
1
io1
from:
MAD Recorder
Peak Height
io4
io2
io1
IO1.'
io1
io4
io2
io2
1
1
<10
io2
io1
io1
Recorder
Peak Height
io4
io2
io2
1
io2
io3
io1
1
1
1
io2
io1
io1
11
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Table 4. MAXIMUM LINEAR RANGE WITH +10%
K-VALUE VARIATION
Compound
8-BHC
HCB
Ronnel
Malathion
MAD Peak
Area
io4
2
io3
io1
,«2
MAD Peak
Height
io3
io2
io3
io1
,~1
MAD Recorder
Peak Height
IO4
io2
io3
io1
,~1
Recorder
Peak Height
io4
io2
io3
1
<10
-,,,2
Aldrin
E. Parathion
Mi rex
p, p'-DDE
p,p'-DDT
Aroclor 1254
Halowax 1014
Phorate
10
10
10
10'
10
10
10
10
10
10
10"
10"
10"
10
10
10'
10
10
10"
10
10J
10
10
10
10
10
10
10
10
10
12
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to baseline. The negative peaks reported by Olds were not observed in
this evaluation.
As previously mentioned, the zero-volt baseline is a measure of the
detector baseline current, and is therefore a useful parameter for
monitoring the condition of the detector source foil, the carrier gas
purity, air leaks into the system, and all other conditions that can
result in reduced baseline current. The stability of the detector in
terms of baseline current variations is illustrated in Figure 1. As
indicated in the graph, it was necessary to change the first foil (A)
after the sixteenth day of use. It should be noted that the actual time
from the beginning of the evaluation to the point at which the baseline
current had degenerated to an unacceptable level spanned a period of 42
days. A statistical analysis of the baseline current levels shows that
for foil A the arithmetic mean, standard deviation, and percent relative
—8 —9
standard deviation are 1.24 x 10 amps, 2.5 x 10 amps, and 20%
respectively. For foil B, the same analysis results in the respective
—8 -9
figures of 1.66 x 10 amps, 2.3 x 10 amps, and 14%. This foil was
used for a total of 25 days out of 72. In either case the variance is
unacceptable when compared to the baseline current variations of a
linearized Ni EC detector as shown in Figure 2.
It could be argued that the observed variations for the ATC 140A are the
results of factors independent of the detector itself. However, when
it is considered that the detector under study and the linearized
Ni BCD against which it was compared were both mounted on identi-
cal gas chromatographs employing identical columns, ferrules, and septa,
13
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1 I I I [
2.00
0.75
i i i i r
11 13 15
DAYS OF ACTUAL USE*
17
21
23
25
Figure 1. ATC 140 A baseline current levels. Currents monitored only on day of use. Total
time Foil A in detector ca. 6 weeks; Foil B, ca. 14 weeks.
Does not include weekends or other inoperative periods.
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9
I I. I I I I I 1 I I I I I I 7 7
I 7P:OK>OO-OK>O<>OO^
en
- 6
^x
t-
uj 5
DC
oc
3 4
C9
I I I I I I I I I I I I I I I I I I
3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41
DAYS OF ACTUAL USE*
Figure 2. Linearized 63Ni EC baseline current levels. Currents monitored only on day of use. Total time 20 weeks.
* Does not include weekends or other inoperative periods.
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and utilizing a common source of carrier gas, it is apparent that the
difference in performance must either lie in the detector or in the
possibility of an air leak in the system. The latter possibility was
eliminated through periodic inspections of all the gas lines and
connectors.
It can be concluded from the data presented in Figure 1 that the overall
lifetime of the source foil is unacceptably limited. When using most
EC detectors, it is good practice to reduce the carrier flow rate to a
minimum and add a purge flow at the end of the operating day in order
to conserve the argon-methane carrier gas. This procedure was initially
used in the evaluation of the ATC '140A with the exception of the
application of a purge flow. The design of this detector did not include
a purge line for the source cell. It was observed that the baseline
current level steadily decreased with each day that the carrier flow
rate was minimized for overnight operation. Consequently, it was
necessary to maintain the carrier flow at operating level throughout
the duration of the study.
On one occasion it was necessary to reduce the carrier flow rate
overnight to conserve argon-methane. When the carrier flow was returned
to normal operating level on the next day, it was discovered 'that the
—8 -9
baseline current had fallen from 1.3 x 10 to 4.4 x 10 amps, an
unacceptable level for operation. The flow was maintained at 70 ml/min
for a period of three days with the hopes that the baseline current
would be restored. The temperature of the detector was also increased
from 240°C to 280°C in an attempt to decontaminate the foil, but this
only resulted in further decrease of the baseline current.
16
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In another instance, a regional electric power "brownout" required that
all of the GC column ovens in the laboratory be shut down overnight.
After power had been restored the following morning, the ATC detector
required three days to return to its former baseline current despite the
fact that the detector temperature and carrier flow rate had been main-
tained at normal operating levels. The Ni EC detectors in the lab-
oratory had returned to their former baseline current levels within an
hour after power restoration.
The loss of baseline current, and consequently detector sensitivity in
all cases cited above was probably caused by the adsorption of material
eluted from the GC column onto one or both of the electrodes or by the
coating of the electrodes with "dirt" or carbonaceous material. Such
contamination can give rise to the formation of a contact potential
that may either enhance or oppose the applied potential. The fact that
the internal volume of the cell is only 180 microliters and that the
detector has no provision for purging probably makes the ATC.140A more
susceptible to contamination than other EC detectors.
The ATC Model 140A EC detector employs a scandium tritide source with
an activity of one curie. Consequently, the safety aspects of operating
and maintaining the device should be considered. The beta-particles
emitted from the source have a maximum energy of 0.018 MeV and are
absorbed by less than 1 mg/cm2 of aluminum. As a result, there should
be no detectable radiation from the tritium external to the detector
17
-------
chamber. However, there is a possibility of tritium leakage from the
cell through the exhaust port, especially at high temperature operation.
The installation kit provided with the detector includes several meters
of flexible tubing which is used to vent the effluent from the exhaust
port of the detector into a fume hood. Several of these detectors
operated in an inadequately ventilated room could constitute a potential
health hazard.
For laboratories that have in-house EC detector maintenance programs
involving replacement of foils and cleaning of detector cells, additional
safety considerations are necessary. When the foil of the ATC 140A
EC detector employed in this evaluation was changed, wipe tests were
made on various detector components -during the disassembly process.
One of the tests indicated low level contamination on the exterior
surface of the detector housing. Another demonstrated that the outside
surface of the detector cell itself was contaminated at the 0.5 yCi
level. It could not be determined whether this high contamination
resulted from tritium leakage from the source foil or from external
contamination by the manufacturer during the assembly process. In
either case, regular maintenance of this type of detector would require
stringent observation of radiation safety practices.
MEMORY .AND DISPLAY (MAD) SYSTEM
The Model 160A Memory and Display (MAD) system was evaluated in terms of
display utility, storage and recall capability, ease of integration, and
mechanical reliability. Data recorded in the MAD data memory is shown
18
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graphically on the CRT display monitor with operator control of all
display parameters including the memory segment displayed, attenuation,
baseline offset/ and linear or semilogarithmic presentation. The
maximum number of data memory channels that are available in this
system is 4096 (4K mode). The system is capable of splitting the
memory into two equally-sized segments of 2048 channels each (2K mode).
This capability offers two distinct advantages: first, when the CRT
display is operated in the split-screen format two chromatograms that
have been previously recorded in the 2K mode can be displayed simul-
taneously for comparison; secondly, a chromatogram capable of being
contained within 2K of memory will only occupy half as much disk storage
space as a chromatogram contained within 4K of memory. This latter
characteristic becomes important when it is considered that the MAD
system stores all available memory channels in the disk file whether or
not data collection was terminated prior to filling all of the channels.
Consequently, in order to conserve disk storage space, it is necessary
to use the minimum allowable memory range. The frequency with which
data points are collected can be controlled by the dwell time selector.
With this control, the sampling interval can be varied from 0.01 seconds
to 40.95 seconds in 0.01 second increments. Since the CRT is only
capable of displaying 512 data points at any given time, depending on
the selected display span and memory range, the display will show a
maximum of every fourth data channel (2K mode) or every eight data
channel (4K mode). The MAD system selects 512 equally-spaced channels
for display if the displayed memory region contains more than 512
channels.
19
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In this evaluation the display system was found to be quite useful to
the extent that a chromatogram could be manipulated or attenuated after
it was generated. The original chromatogram and any alterations thereof
could be stored in the disk file memory for later retrieval or generated
in hard copy form as a strip chart chromatogram or both. The primary
difficulty encountered with the display system was the limited capa-
bility for real-time data acquisition. Table 5 lists the maximum data
collection time as a function of dwell time for each of the memory
ranges. In theory, these collection times should be adequate for most
chromatographic applications. For example, at a dwell time of 40.95
sec/channel over a range of 4096 channels the system can collect data
for over 46 hours. However, when actual chromatograms are generated
while using the MAD system, it becomes apparent that the maximum useful
dwell times and resulting data acquisition and display times are con-
Table 5. DATA COLLECTION TIME AS A
FUNCTION OF DWELL TIME
Dwell
Time
(sec)
0.01
0.1
1.0
10
40
Maximum Data
2K
. .Mode
0.34
3.4
34
340
1365
Collection Period (min)
4K
Mode
0.68
6.8
68
680
2730
20
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siderably less than the theoretical limit. This is illustrated by the
following case in which an aldrin standard was used to generate chro-
matograms for display on the MAD system. Initially the 2K memory range
was used in conjunction with a display span of 2048 channels. Repeated
injections of the aldrin standard at dwell times varying from 0.1
seconds to 3.0 seconds produced chromatograms which exhibited several
interesting characteristics. It was found that a dwell time of 0.1
seconds was inadequate in that the display filled so rapidly there was
insufficient time for the peak of interest to completely fit on the
screen. In addition, as the dwell time was increased, the baseline
noise increased correspondingly and increasing amounts of background had
to be subtracted from the chromatogram to keep the baseline at roughly
the same level from sample to sample. As the dwell time increased, it
became more and more difficult to distinguish small sample peaks from
the background noise. Additionally, the apparent position of the aldrin
peak was shifted from right to left on the display as the dwell time was
increased. While quantification by peak area measurement was not
affected by this behavior, the peak height was found to increase pro-
portionally with dwell time as expected.
The experiment was repeated using dwell times of 0.3, 10, and 40 seconds.
Each dwell time was tested at span ranges of "2048, 1024, and 512. This
was done in order to determine whether or not the GC peak of interest
would be visible as it shifted to the extreme left side of the display
at the higher dwell times. With the exception of the sample run at the
40-second dwell time, the peak was discernable at all span ranges. At
21
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a dwell time of 40 seconds no peak was readily apparent at a span range
of 2048. Neither the span range nor the dwell time had any affect on
the peak area measurement. The measured peak height increased with
increasing dwell time, but in a non-linear fashion. Again background
noise increased appreciably as dwell time increased.
Using the results of these experiments/ it was deduced that for most GC
applications encountered in this laboratory the optimum dwell time is
0.3 seconds. This setting yields an acceptable compromise between
maximum data collection time and acceptable background noise. However,
this means that the chromatographic conditions of an experiment will
have to be adjusted so that the compounds of interest will elute in less
than 20.48 minutes, as this is the maximum allowable data collection
period at this dwell time. Somewhat longer run times can be achieved by
increasing the dwell time to a level required to collect the data while
at the same time attenuating the display to the point where background
noise is minimized. Unfortunately, this must be accomplished at the
expense of sensitivity.
The MAD system exhibited another interesting characteristic during the
course of the evaluation. When extremely dilute samples were being
analyzed, the system sensitivity was not as good as that derived from
data presented on a simultaneously-generated strip chart chromatogram.
When using the MAD system, peak parameter computations are performed by
manually defining the boundaries of a peak and allowing the system to
automatically compute the peak area, amplitude, and retention time. The
22
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peak is defined by setting movable upper and lower level cursors, which
appear on the video display with the data. The area bounded by the data
curve and the straight line joining the two cursors is automatically
shaded by the MAD system so that the defined area is clearly indicated.
An example of this can be seen in Figure 3. For single-component
chromatograms, peak parameter computations were found to be no problem.
However, in a multicomponent mixture, calculation of peak area and
amplitude was tedious as each peak had to be defined manually prior to
computation.
The storage and recall capability of the disk file is quite useful. As
mentioned previously, data can be stored for later manipulation and
calculation. During the course of the evaluation, no variations in a
recalled data set in terms of peak limits, area, amplitude, and re-
tention time were found. The major difficulty encountered with the
memory system was the poor mechanical reliability of the disk file. The
disk system had to be returned to the factory on three separate occasions
for repairs. The MAD system can be operated without the disk unit but
this severely limits the utility of the system.
23
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-
Figure 3. Peak area integration on the ATC Model 160 Memory and Display (MAD) system.
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SECTION V
REFERENCES
1. Lovelock, J. E., and S. R. Lipsky. J. Am. Chem. Soc. 82, 431
(1960).
2. Lovelock, J. E.. Nature 182, 1460 (1958).
3. Wentworth, W. E., R. S. Becker, and R. Tung. £. Phys. Chem. 71,
1652 (1967).
4. Steelhammer, J. C., and W. E. Wentworth. J_. Chem. Phys. 51, 1802
(1969)I
5. Wentworth, W. E., E. Chen, and J. E. Lovelock. £. Phys. Chem. 70,
445 (1970).
6. Lovelock, J. E., and N. L. Gregory. 3rd Int. Symp. Gas Chromatogr.,
Academic Press, New York 1962, p. 219.
7. Maggs, R. J., P. L. Joynes, A. J. Davies, and J. E. Lovelock.
Anal. Chem. 43, 1966 (1971).
8. Dean, R. B., and W. J. Dixon. Anal. Chem. 23, 636 (1951).
9. Karasek, F. W., and L. R. Field. Research/Development 28, No. 3,
42 (1977).
10. Sullivan, J. J., and C. A. Burgett. Chromatographia 8, No. 4, 176
(1975).
11. Olds, K. L. "Pesticide Monitoring Special Study 44-0107-77:
Evaluation of Linear Electron Capture Detector," U. S. Army,
Environmental Hygiene Agency, Aberdeen Proving Ground, Maryland,
May, 1976 (NTIS No. AD A030980).
25
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TECHNICAL REPORT DATA
(Mease read Instructions on the reverse before completing)
1. REPORT NO.
:PA-6QO/2-78-01Q
J:
4. TITLE AND SUBTITLE
Evaluation of A NEW MICROVOLUME 3HSc ELECTRON CAPTURE
DETECTOR AND .ANCILLARY DATA for Pesticide Residue
Analysis
3. RECIPIENT'S ACCESSION-NO.
5. REPORT DATE
Fphriiarv
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
8. PERFORMING ORGANIZATION REPORT NO.
Robert C. Hanisch and Robert G. Lewis
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Analytical Chemical Branch
Environmental Toxicology Division
Health Effects Research Laboratory
Research Triangle Park. N.C. 27711
10. PROGRAM ELEMENT NO.
1EA615
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
Health Effects Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, N.C. 27711
13. TYPE OF REPORT AND PERIOD COVERED
RTP.NC
14. SPONSORING AGENCY CODE
EPA 600/11
16. SUPPLEMENTARY NOTES
16. ABSTRACT
The performance of a linearized 3HSc electron capture detector (ECD) and its
ancillary data system was evaluated for use in the analysis of pesticide residues.
Serial dilutions of pesticide standards were used to determine the maximum linear
%.
range and sensitivity of the detector. This detector was found to have a
significantly greater linear range for the test compounds than a linearized &3Ni
electron capture detector evaluated. The sensitivity was only marginally better
than the 63Ni ECD.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COSATI Field/Group
electron capture
chemical detection
pesticides
residues
14 B
07 C, D
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (ThisReport)'
UNCLASSIFIED
21. NO. OF PAGES
32
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
26
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