TN-85-03
NOVEMBER 1985
VALUATION OF VARIOUS CONFIGURATIONS
OF NAFION DRYERS: WATER REMOVAL
FROM AIR SAMPLES PRIOR TO GAS
CHROMATOGRAPHIC ANALYSIS
Subm/tredto.
U.S. Environmental Protection Agency
Environmental Monitoring Systems Laboratory
Research Triangle Park, NC 27711
Under Contract 68-02-4035
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EVALUATION OF VARIOUS CONFIGURATIONS OF NAFION DRYERS: WATER
REMOVAL FROM AIR SAMPLES PRIOR TO GAS CHRdMATOGRAPHIC ANALYSIS
by
Joachim D. Pleil and Karen D. Oliver
Chemistry and Field Monitoring Research
Northrop Services, Inc. - Environmental Sciences
Research Triangle Park, NC 27709
Submitted to:
WHliaffiA. McClenny
Methods Development Branch
Envirortmental Monitoring Systems Laboratory
U,S, Environmental Protection Agency
Research Triafigte Park, NC 27711
Reviewed and Approved by.
N. E. Short
Program Manager
NORTHROP SERVICES, INC.
ENVIRONMENTAL SCIENCES
P.O. Box12313
Research Triangle Park, NC 27709
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DISCLAIMER
This report has been reviewed by Northrop Services, Inc. - Environmental Sciences and
approved for publication. Mention of trade names or commercial products does not constitute
endorsement or recommendation for use.
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FOREWORD
This report represents the results of work performed by Northrop Services, Inc. -
Environmental Sciences under Contract 68-02-4035 for the Methods Development Branch,
Environmental Monitoring Systems Laboratory, U.S. Environmental Protection Agency, Research
Triangle Park, NC. This work was conducted in response to Technical Directive 1.0-7 during the
period April through September 1984.
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ABSTRACT
Commercially available Nation tube dryers were tested for applications of sample preparation
prior to gas chromatographic analysis. Steady-state drying efficiency was measured for various dryer
models, configurations, and experimental parameters at 95% relative humidity. A procedure was
developed in which water removal efficiency was temporarily enhanced by a factor of 20 as
compared to the steady state. This was accomplished by heating the dryer while purging it with a
dry sample stream immediately prior to processing the gas sample of interest. The procedure was
tested on an automated gas chromatographic system equipped with a cryogenic sample
preconcentrator. Data for 15 volatile organic compounds of interest showed no effect of this
procedure on sample integrity; some improvement in run-to-run precision was observed.
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SECTION 1
INTRODUCTION
The accurate identification and quantitation of the many volatile organic compounds (VOCs)
at sub-parts per billion by volume levels in ambient air generally requires preconcentration of
analytes to enhance instrument sensitivity. Analyses are typically performed with high-resolution
chromatographs coupled to appropriate detectors (Holdren eta/. 1985; Singh eta/. 1979a,b; Cox and
Earp 1982; Pellizzari eta/. 1984b). In this laboratory, preconcentration is accomplished by passing
whole air through a cryogenic trap. The sample is then thermally desorbed into a capillary-column
gas chromatograph (GC) using flame ionization detection (FID), electron capture detection (ECD),
and/or mass selective detection (MSD). The methodology has been described elsewhere (McClenny
andPleil 1984; McClenny eta/. 1984; McClenny eta/. 1985).
Cryogenic trapping has been shown to be a preferred preconcentration technique,
particularly for the lighter VOCs, i.e., Q, Cz, and C% compounds (McClenny et a/. 1984; Pleil and
McClenny 1984; Cox eta/. 1982; Rasmussen eta/. 1977; Fan/veil eta/. 1979; Des Marais 1978). These
compounds tend to have low breakthrough volumes on the solid sorbents (such as Tenax-GC) that
are often used as preconcentration media (Singh 1979a,b; Pellizzari et al. 1984a,b; Bertsch et al.
1974; Pellizzari et al. 1976). However, the co-collection of ambient water vapor in a cryogenic trap
can cause a number of problems that are not encountered while using the hydrophobic Tenax-GC.
Ice formation during sample collection can alter sample flow or clog the trap, the variability of
ambient water vapor can cause detector baseline shifts, and co-collected water can cause blockage
upon injection onto the capillary column in separation techniques that require sub-freezing initial
GC oven temperatures. In extreme cases, the excess water when eluted can even extinguish the
hydrogen flame of an FID.
Water-related problems can be alleviated by various methods. The simplest method is to
reduce sampling volume, but this also reduces sensitivity. Other techniques involve pre-drying the
sample with various desiccants; however, these desiccants can affect sample integrity by adsorbing
or outgassing some compounds. Also, dessicants are inconvenient to use because they require
periodic replacement or reconditioning.
In this laboratory, a permeable membrane dryer commercially available from Perma-Pure
Products, Inc. (Farmingdale, NJ) is used. The permeable membrane consists of Nafion tubing (E.I.
Dupont de Nemours, Inc., Wilmington, DL), a copolymer of tetrafluoroethylene and fluorosulfonyl
monomer, that is coaxially mounted within larger tubing (Baker 1974). The sample stream is passed
through the interior of the Nafion tubing; this allows water (and other light, polar compounds) to
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permeate through the walls into a dry air purge stream flowing through the annular space between
the Nafion and the outer tubing (Kertzman 1973; Perma-Pure Products, Inc.). This is a constant
process that requires no maintenance. The Nafion dryer's water removal efficiency for particular
applications and its transmission efficiency for specific compounds have been described in the
literature (Baker 1973; Kertzman 1973; Perma-Pure Products, Inc.; Foulger and Simmonds 1979;
Burns et a/. 1983). The use of these dryers has already been validated for our sampling conditions
and compounds of interest, i.e., selected light, nonpolar hydrocarbons and halogenated
hydrocarbons of environmental concern (McClenny et a/. 1984).
This paper presents performance data for different sizes and configurations of Nafion dryers.
Experiments cover steady-state drying efficiencies and the time dependence of drying efficiency in
response to a step function in sample moisture content. A novel dryer cleanup technique is
presented that can be implemented during the non-sampling portion of a GC analysis cycle. This
technique temporarily increased the water vapor removal efficiency of the dryer by a factor of 20
over the steady-state efficiency and reduced system memory effects.
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SECTION 2
EXPERIMENTAL
EQUIPMENT
A number of different dryer configurations are available from the manufacturer for drying air
streams at flow rates of a few milliliters per minute to more than 30 L/min. The single-stranded
models that cover the flow range up to approximately 200 ml/min were chosen for our application of
GC sample drying. The manufacturer's model numbers and a physical description of all dryers used in
this work are presented in Table 1.
TABLE 1. NAFION TUBE DRYERS-AS PURCHASED FROM PERMA-PURE PRODUCTS, INC.
Model No. Physical Description
MD-125-12(T) 0.125 in. o.d.a Teflon purge tubing shell with polypropylene fittings; 12 in.
length.
Nafiontube: 0.12cm o.d.; 0.065cm i.d>.
MD-125-12(S) 0.125 in. o.d. 304 stainless steel purge tubing shell with stainless steel fittings
and Viton o-ring seals; 12 in. length.
Nafiontube: 0.12 cm o.d.; 0.065cm i.d.
MD-125-48(T) 0.125 in. o.d. Teflon purge tubing shell with polypropylene fittings; 48 in.
length.
Nafion tube: 0.12 cm o.d.; 0.065 cm i.d.
MD-125-48(S) 0.125 in. o.d. 304 stainless steel purge tubing shell with stainless steel fittings
and Viton o-ring seals; 48 in. length.
Nafiontube: 0.12 cm o.d.; 0.065cm i.d.
MD-250-12(S) 0.250 in. o.d. 304 stainless steel purge tubing shell with stainless steel fittings
and Viton o-ring seals; 12 in. length.
Nafion tube: 0.30 cm o.d.; 0.250 cm i.d.
3 o.d. = outside diameter.
b i.d. = inside diameter.
Most dryer performance data were obtained with an optical water vapor monitor. This
instrument is a prototype developed under contract for the U.S. Environmental Protection Agency by
Ford Aerospace and Communications Corporation, Newport Beach, CA (Burch and Goodsell 1981).
Water vapor concentration in a flowing gas stream is determined by differential infrared absorption
using bandpass filters centered at 2.51 and 2.59 urn. Sample cell volume is 56 cm3 and the detection
limit is 10 parts per million by volume (ppmv) at 1 atm. Calibration curves cover ranges of up to 5%
water vapor concentration. The monitor was spot checked periodically with known samples near
100% relative humidity, and measurements were compared to measurements of room air made by a
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sling psychrometer and a calibrated hygrometer. The monitor was extremely stable when sampling
zero gas and at the span settings. For the various experiments in which the water vapor monitor was
used, individual data points consisted of three concentration measurements: dry zero air(Z),
humidified sample at dryer inlet (H), and dried sample at dryer outlet (S). Percent drying
efficiency, E, was defined as E = 100-[1-(S/H)]. Because Z, the baseline response, was always two or
three orders of magnitude less than S, it was only used as a qualitative check on the water vapor
monitor's stability. Normalizing with H removed the effect of laboratory temperature drifts that
slightly affected the water vapor concentration at the dryer inlet.
In the final experiments, the analytical instrumentation consisted of a fully automated
sampling and analysis system that included an HP-5880 Level 4 GC (Hewlett-Packard, Avondale, PA)
and a modified Nutech 320-1 cryogenic trapping and desorption unit (Nutech Corp., Durham, NC).
All critical gas flows were regulated by Tylan mass flow controllers (Tylan Corp., Carson, CA). For
some tests, a quadrupole mass spectrometer (HP-5970 MSD, Hewlett-Packard) was used as a GC
detector for water.
PERFORMANCE SURVEY EXPERIMENTS
Initial experiments consisted of a survey of drying performance of five different dryer models
(see Table 1). All tests were performed at room temperature, and Tylan mass flow controllers were
used to set and maintain sample and purge flow (zero-grade air) at 50 ml/min and 250 ml/min,
respectively. Humidified sample (at 95% relative humidity) was generated by inserting a 500-ml
water impinger flask into the sample stream. Teflon tubing and fittings were used for all gas flow
plumbing.
For each dryer in the initial survey, the steady-state drying efficiency was determined by using
the method described earlier. The humidifier was then removed from the sample stream so that dry
zero air could flow through the interior of the Nafion dryer, and the dryer was heated to 100°C. The
time, Tj, needed for the sample humidity at the dryer outlet to reach a zero baseline, i.e., for any
residual water to be removed, was recorded. After the dryer had cooled to room temperature, the
zero baseline was accurately measured and the humidifier was reinserted into the sample stream.
The time, Tu/ required for the sample humidity at the dryer outlet to increase to 10% of the steady-
state value was recorded. The.following three parameters: steady-state drying efficiency, Tj, and Tu
were used to initially characterize the various models.
PURGE FLOW DEPENDENCE AND PERFORMANCE REPEATABILITY
Three sets of more detailed experiments were then performed: the series/parallel purge gas
test, the purge flow dependence test, and the drying efficiency precision test. Because these
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experiments were very time consuming, only those dryers of immediate interest for our specific
applications were tested.
Two dryers each of model numbers MD-125-48(T) and MD-125-12(S) (see Table 1 for
descriptions) were tested in pairs in either a series or parallel configuration for steady-state drying
efficiency with a 500-ml/min purge flow. Sample flow was 50 ml/min of humidified zero air. For
comparison, dryers were also tested individually using a 500-ml/min purge flow. Figure 1 shows the
flow arrangement for each case. In each configuration the same total purge flow was used. The
series configuration essentially lengthens a single dryer. The parallel purge configuration acts as two
separate dryers; the second further dries the sample output of the first. Flows were maintained with
mass flow controllers.
In the second set of tests, one M0-250-12(5) and four MD-125-48(T) dryers were used. Steady-
state drying efficiency was determined for a 50-ml/min humidified zero air sample stream as a
function of dryer purge flow; a range of 200 to 1250 ml/min was used for the 1/8 in. outside diameter
MD-125-48(T) dryers, and additional settings up to 3500 ml/min were used for the larger outside
diameter M0-250-12(5) dryer. Mass flow controllers were used to supply consistent purge flows. All
flows were periodically audited with a bubble flow meter.
The third set of experiments used four MD-125-48(T) dryers. Steady-state drying efficiency
was independently measured for each dryer at various times throughout a one-month testing
period. Room temperatures that ranged from 19 to 24°C caused variation in the absolute water
content of the sample at the dryer inlet. Humidified sample flow and dry purge flow were
maintained at 50 and 250 ml/min, respectively. Flows were periodically audited with a bubble flow
meter to ensure consistency over this extended testing period.
DRYER CLEANUP PROCEDURE EXPERIMENTS
Previous work had demonstrated that the permeable membrane of the dryers, Nafion tubing,
could be successfully cleaned by applying heat and substituting pure gas for the sample flow
(McClenny eta/. 1984). A final set of experiments was performed to determine the effects of such
treatment on dryer performance under actual sampling conditions.
Typically, an ambient air analysis m our laboratory consists of three distinct phases: sample
collection using a cryogenic trap (15 min); GC analysis using a capillary column (30 min); and
instrument resetting, report printing, etc. (15 min). This procedure is fully automated and repetitive;
run-to-run precision is determined primarily by the sample. Because sample drying is only necessary
for 15 min out of the total analysis time of 60 min, a procedure was devised to automatically clean up
the dryer during its idle time. This procedure involved flushing the dryer with dry zero-grade air
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Dryer
Vent
ToH2O
Monitor
500 ml/min
Dryer 1
250 ml/min
Dryer 1
500 ml/min
c
Dryer 2
Vent
250 ml/min
Dryer 2
Figure 1. Arrangement for measuring steady-state drying performance of various dryer
configurations: (A) single dryer, (B) series purge-gas configuration, and (C) parallel purge-gas
configuration. The flow of dry zero gas is regulated with two mass flow controllers (MFC).
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while the dryer was heated to 100°C It was then allowed to cool to room temperature between
sample collections.
The MD-125-48(T) dryer (currently in use on the instrument) was wrapped with aluminum foil,
heating tape, and insulation. The heating tape was wired through a solid state relay to a variable
transformer (used to set a maximum temperature). A solenoid gas switching valve, also wired
through a relay, was installed at the dryer inlet to select either sample or dry air. Both relays were
connected to the GC to allow computer-controlled activation. A diagram of this arrangement is
shown in Figure 2. The appropriate programming was installed to allow automated repetitive
analysis with or without the cleanup procedure. A number of experiments were then performed to
evaluate the new method.
Analytical
System
I
Computer
Heater
Relay
Relay
Control
Variable
Transformer
Dryer Purge Flow
Valve
Relay
115 VAC
T
Signals
Figure 2. Schematic diagram of automated dryer cleanup procedure. When valve relay is off, valve
is closed and sample is pulled from manifold. When valve relay is activated, valve is open and zero
air overfills dryer inlet and displaces manifold sample. Heater relay can then be activated to apply
pre-set voltage from variable transformer to heat the dryer.
In the first set of experiments, the mass spectrometer was used as the GC detector. The system
was set up to display the elution of 18 atomic mass units (amu) fragments, the parent ion for water.
A humidified zero air stream at a flow rate of 2 L/min introduced into a glass manifold was the
sample. Four consecutive analyses were then performed using the described cleanup procedure;
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these were followed by four analyses without the procedure. Before each set of four analyses the
dryer was thoroughly purged of all residual water.
A second set of experiments was performed to determine the dependence of drying efficiency
on time during such cleanup cycles. The water vapor monitor was inserted into the sample stream
between the dryer and the analytical system (refer to Figure 2). The analytical procedure used in the
first set of experiments was repeated while the water content at the dryer outlet was monitored.
In the third set of experiments, the effect of the cleanup procedure on sample integrity was
tested. The water vapor monitor and the mass spectrometer were removed from the system.
Simultaneous FID and ECD were used. A flowing calibration mixture of 15 compounds of interest, all
within the concentration range of 8 to 12 parts per billion by volume (ppbv) m humid zero air, was
established in the glass manifold at a flow rate of approximately 2 L/min. Seven consecutive analyses
were performed with the cleanup procedure, and seven were performed without the procedure.
These two sets of runs were performed on consecutive days under identical conditions. After each
set, humidified zero air was sampled to determine residual contamination.
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SECTION 3
RESULTS AND DISCUSSION
As presented in Table 2, the initial survey of various types of dryers showed consistent drying
efficiencies of about 80% for our conditions: 95% relative humidity of the sample and a flow rate of
50 ml/min; dry purge flow rate of 250 ml/min. This suggests that an equilibrium between sample and
purge flow humidity was reached at a dryer length less than 12 in. (the shortest dryer). This
corresponds to a residence time of 0.12 s. Though undocumented here, lowering the incoming
sample humidity also lowers the product humidity; however, resultant drying efficiencies can be less
than 80%. This behavior has been documented elsewhere (Baker 1974, Foulgar and Simmonds 1979;
Burns et al. 1983). All drying efficiencies presented here are based on an incoming sample stream at
about 95% relative humidity to anticipate worst-case conditions for ambient sampling.
TABLE 2. RESULTS OF INITIAL PERFORMANCE SURVEY OF VARIOUS MODELS OF DRYERSa
Model No.
MD- 125-12(T)
MD-125-12(S)
MD- 125-48(T)
M 0-125-48(5)
MD-250-12(S)
Eb
(percent)
80
82
81
80
82
Tde
(min)
80
80
70
75
170
V
(min)
2
2
23
23
11
3 Test conditions: 50-ml/min humid sample flow; 250-ml/mm dry purge gas flow; room temperature.
b Steady-state drying efficiency.
c Time to purge residual water.
d Time to reach 10% of E.
The time dependence of dryer performance varied greatly with configuration (see Table 2).
The time required to remove residual water from a dryer starting at steady-state conditions
essentially depended on the outside diameter of the Nafion tubing, not on the length. The longer
dryers had a slightly shorter purge time requirement. Thus, it appears that residual water removal
depends on the amount of Nafion tubing per length rather than on total material present. The time
required for a dryer to reach 10% of its steady-state value when starting totally dehydrated shows
that there is an initial period during which drying efficiency is greatly enhanced. Here, both an
increase in length and an increase in the Nafion tubing's outside diameter increased this time of
improved performance. These results indicate an initial condition in which water is absorbed by the
Nafion tubing without appreciable permeation into the purge stream. However, once the humidity
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gradient across the Nafion membrane becomes established, the water removal rate becomes limited
by the uptake capacity of the purge gas and the water permeation rate through the Nafion.
Although this could not be measured directly, the explanation is consistent with our experiments.
The purpose of the purge gas configuration test was to determine the effects of a dual dryer
arrangement. Total purge flow was set at 500 ml/min in all tests so that a two-dryer combination
would be comparable to previous tests. The results, shown in Table 3, show that the parallel
configuration was more efficient than a series configuration which, in turn, was more efficient than
a single dryer (see Figure 1 for configuration diagrams). The relatively small improvements in drying
efficiency, however, do not appear to warrant the addition of an extra dryer. The parallel
configuration's superior performance reinforces the earlier observation that a humidity equilibrium
is reached quickly, i.e., that added dryer length is less effective than removing accumulated water in
the purge gas. These tests were performed with samples at 95% relative humidity and at room
temperatures to anticipate worst-case ambient conditions.
TABLE 3. DRYING EFFICIENCIES FOR DRYERS UNDER VARIOUS PURGE FLOW CONFIGURATIONS*
EC
Model No. Configuration^ (percent)
MD- 125-12(5)
MD-125-48(T)
Single
Series
Parallel
Single
Series
Parallel
85.3
86.6
89.0
87.9
88.4
94.9
3 Test conditions: 50-ml/min humid sample flow; 500-ml/mm total purge flow; room temperature.
b Referto Figure 1 forflow diagrams.
c Steady-state drying efficiency.
Table 4 shows that increasing the purge gas flow resulted in an increase in drying efficiency, E.
This relationship is approximately E = 1-(SF/PF), where SF and PF denote sample flow and purge flow,
respectively. It is appropriate at purge flows at or below 500 ml/min when the sample is highly
humidified. (It was determined that 500 ml/min was a reasonable upper limit for the 1/8 in. outside
diameter dryers.) At higher purge flows, drying efficiency approached an upper limit of about 89%
for our particular conditions. There was very little difference between the MD-125-48(T) and MD-
250-1 2(5) dryers other than the option of increased purge flow.
The last test of this series was performed to determine the overall stability in performance of
these dryers. Results of 17 independent tests to measure steady-state drying efficiency showed a
mean of 80.83% and a standard deviation of 0.84. All four MD-125-48(T) dryers were treated
identically in this test even though two had been used in our laboratory for about two years and the
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other two had just been purchased. Individual dryers showed no appreciable bias in drying
efficiency. These encouraging results indicate that individual dryers of the same model can be
considered interchangeable and that prolonged use does not appear to degrade performance.
TABLE 4. DRYING EFFICIENCIES USING VARIOUS PURGE FLOWSa
Purge Flow
(ml/min)
200
250
360
500
660
1,250
2,400
3,500
Eb
(percent)
MD-125-48(T)
75.2
80.8
-
87.9
-
88.3
-
-
MD-250-12(S)
79.2
81.6
82.6
-
87.4
89.1
89.1
89.5
a. Test conditions: 50-ml/min humid sample flow; room temperature.
b Steady-state drying efficiency.
As noted earlier, a fully dehydrated dryer exhibited a temporary increase in drying efficiency
when a humidified sample was first introduced. Because our sampling requirements allowed about
45 min of down time between sample collections, we used this time to recondition the dryer. The
MD-125-48(T) dryer was chosen for this set of tests because it exhibited the longest period of
enhanced performance (see the Tu parameter in Table 2), as well as a reasonable purge time. Results
of experiments to test the automated cleanup system (see Figure 2) showed that it greatly reduced
the water introduced into the analytical system as compared to using the dryer under steady-state
conditions. Qualitative results (using the mass spectrometer as the GC detector) are presented in
Figure 3, in which the eluting water vapor peak is shown for three consecutive GC runs with and
without the cleanup cycle. The amount of eluting water was constant from run to run when the
cleanup procedure was used, but increased dramatically when the cleanup procedure was not used.
This experiment could not be used to quantify water removal improvements because steady-state
dryer use (as reached after the fourth run) allowed so much water into the analytical system that the
detector overloaded.
12
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B
Time
Time
Figure 3. Two sequences of three consecutive GC runs showing the elution of the 18 amu fragment
(parent ion of^water) as measured by the mass spectrometric detector: (A) with interim dryer
cleanup procedure and (B) without interim dryer cleanup procedure. In each case, the dryer was
initially dehydrated.
The drying improvement caused by the automated cleanup system was measured with the
water vapor monitor. The water content of the sample stream as it entered the analytical system is
presented in Figure 4 as two overlapping graphs of water monitor response versus time that are
annotated to indicate the sampling periods. These curves correspond to four complete sampling
cycles for both drying procedures, i.e., with and without interim cleanup. When the dryer is at room
temperature, the area under the curves is proportional to the total amount of water passing through
the monitor. Careful study has shown a typical 20-fold improvement in water removal for our
analytical conditions when the cleanup procedure is used. This is illustrated in Figure 4 by an
expanded view of one sample collection portion of these curves.
Results for the final experiment, in which the cleanup procedure was used on well-
characterized calibration samples, are presented in Table 5. The individual compounds in the
calibration mixture, particularly those eluting late, tended to equilibrate throughout the day. This
sample concentration drift during the tests was unavoidable because time and resource constraints
did not permit equilibration prior to each experiment. Thus, the data were compared based on
linear regressions of peak area versus run number for each compound. The calculated regression
13
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Sample
Water
Content
«steady.,._L_
[State
Figure 4. Sequence of four consecutive GC runs showing water monitor response versus time as
measured at the dryer outlet just prior to entering the GC: (A) with interim dryer cleanup procedure
and (B) without interim dryer cleanup procedure. Expanded view of the sample collection portion
of the analytical cycle depicts reduction in injected water vapor. The heavily shaded region is
proportional to the amount of water collected with the sample when the dryer is pretreated; the
lighter shaded area beneath the steady-state line is proportional to water collected without dryer
pretreatment. Sample content at dryer inlet (95% relative humidity at room temperature) would be
far off the scale in this graph.
slope is a combination of overall sample concentration drift and any run-to-run memory effects in
the system. For the FID data, the cleanup procedure exhibited a consistently lower area/run number
drift. The ECD data showed no consistent differences.
To determine the effect of the dryer cleanup on system precision, the root mean square
difference between the regression curve and the individual data points was calculated for each
compound. These values are expressed as a percentage of the mean peak area to facilitate
comparisons among compounds and among detectors and to determine whether or not the cleanup
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TABLE 5. COMPARISON STATISTICS FOR SEVEN ANALYSIS RUNS WITH AND WITHOUT THE INTERIM
DRYER CLEANUP PROCEDURES
Compound
FID
Vinyl chloride
Vinylidene chloride
1,1,2-Trichloro-1,2,2-
trifluoroethane
Chloroform
1,2-Dichloroethane
Methyl chloroform
Carbon tetrachloride
Trichloroethylene
1 ,3-Dichloropropene
-OS
-trans
1 ,2-Dibromoethane
Tetrach I oroethyl ene
Chlorobenzene
Benzyl chloride
Hexachlorobutadiene
ECD
1,1,2-Trichloro- 1,2,2-
trifluoroethane
Chloroform
Methyl chloroform
Carbon tetrachloride
Trichloroethylene
1,2-Dibromoethane
Tetrachloroethylene
Hexachlorobutadiene
Regression
(area/run
Cleanup
0.104
-0.098
0.036
0.029
0.081
0.073
-0.003
0.111
0.075
0.076
0.038
0.120
0.389
0.831
0.619
389
287
984
2,045
444
9,011
9,274
7,419
Slopeb
no.)
No Cleanup
-0.040
-0.105
-0.155
0.114
0.275
0.052
0.076
0.326
0.298
0.308
0.646
0426
1.418
2.917
1.773
-564
110
-3,799
-2,836
539
570
639
18,751
RMS Residualc
(percent)
Cleanup No
1.52
1.48
1.87
1.45
0.84
0.77
1.34
0.83
0.81
0.93
1.06
0.73
1.06
1.49
1.12
0.23
0.35
0.13
0.27
0.51
4.78
3.06
2.40
Cleanup
2.16
2.95
1.20
3.79
1.40
1.16
6.53
1.28
1.54
5.53
1.49
2.26
3.50
6.36
2.92
1.30
1.46
4.17
1.20
2.83
4.31
5.26
2.24
a Target compounds ranged from 8 to 12 ppbvin humidified zero-grade air. Samples were collected at 34 ml/min for 14mm
into a cryotrap maintained at-1 55°C.
b Least squares linear regression slope of peak area vs. run no. for seven consecutive analyses either with or without the
dryer cleanup procedure. For FID, mean areas ranged from 5 to 55; for ECD, mean areas ranged from 100,000 to 1,000,000.
c Root mean square difference between calculated linear regression curve and individual data points; normalized to a
percentage of the mean peak area for the particular compound.
procedure was used. Data from the FID showed consistently lower scatter when the cleanup
procedure was used. The ECD data showed either lower or equivalent residuals. In addition, when
the procedure was not used, one instance of temporary column blockage and another instance in
15
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NORIHROP TN 4120 85 03
Environmental Sciences
which the FID flame was extinguished were encountered. These occurrences were attributed to
excess accumulated water; this was confirmed in similar tests using the mass spectrometer.
Finally, the zero air run performed after the set of seven calibration runs showed little or no
contamination when the cleanup procedure was used. Residual peaks of about 0.1 to 0.3 ppbv each
for carbon tetrachloride, tetrachloroethylene, benzyl chloride, and hexachlorobutadiene were
observed when the interim cleanup procedure was not used.
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?. • ••••»••»*• TNI-4120-85-03
Environmental Sciences
SECTION 4
CONCLUSIONS
From the experiments presented here some general conclusions can be made about the use of
Nafion tube dryers for chromatographic sample conditioning. Dryer models should be chosen for
their physical attributes (i.e., size requirements, flow considerations). Steady-state drying
performance tends to be similar for the various single-stranded models. If time-dependent response
is important, dryer models should be chosen based on the Nafion tube configuration. Equilibration
time from a dehydrated state to steady-state performance tends to scale with length, whereas purge
time (necessary to dehydrate the dryer at 100°C) tends to scale with the amount of Nafion tubing per
length. Drying efficiency can be improved by increasing purge flow; however, the point of
diminishing returns is reached fairly quickly. In our very restricted sample set, essentially no variation
in performance among dryers of the same type and no degradation in performance of dryers used
daily over a period of two years were observed.
Drying efficiency can be greatly improved for a short time by first processing the dryer with a
dry sample flow and raising the temperature to approximately 100°C to drive out any residual water.
In our experiments, water removal during short sampling periods could be increased by a factor of
20 over steady-state operation. Such a procedure does not effect sample integrity, at least for all
15 compounds tested here, and tends to reduce memory effects from previous samples. This cleanup
procedure is particularly useful for our application, cryogenic preconcentration of VOCs from
ambient air for subsequent GC analysis, because excess accumulated water can cause trap and
column blockage and also adversely affect detector precision. In addition, the improvement in water
removal will allow sampling of much larger volumes of ambient air in the event that greater system
sensitivities to compounds of interest are required.
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SECTION 5
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