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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                                                             TN-4120-85-03
                                                           NOVEMBER 1985
    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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NORTHROP                                                              TN41208503
Environmental Sciences
                                           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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NORTHROP                                                             TIM-4120-85-03
Environmental Sciences
                                          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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NORTHROP                                                               TIM-4120-85-03
Environmental Sciences
                                            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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[»PRTHROP                                                                TN-4120-85-03
Environmental Sciences                                                                       U °  UO
                                            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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NORTHROP                                                                TN 4120 85 03
Environmental Sciences
    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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                                                                                 TN-4120-85-03
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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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Environmental Sciences                                                                 TN-41 20-85-03
    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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NORTHROP                                                                TN 4120 85 03
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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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NORTHROP
Environmental Sciences
                 TN-4120-85-03
                                                        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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NORTHROP
Environmental Sciences
                                                       TN-4120-85-03
    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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NURTHROP                                                                TNI 41 ?n asm
Environmental Sciences                                                                 I N-41 ^U-Hb-U3
    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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NORTHROP                                                                TN-4120-85-03
Environmental Sciences
                                             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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NORTHROP                                                                 TN-4120-85-03
Environmental Sciences
    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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NORTHROP
Environmental Sciences
TN-4120-85-03
     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.
                                                  16

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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.
                                                17

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NORTHROP                                                                TN-4120-85-03
Environmental Sciences
                                             SECTION 5

                                            REFERENCES



    Baker, B.B.  1974. Measuring trace impurities in air by infrared spectroscopy at 20 meters path and
    10 atmospheres pressure. Am. Ind. Hyg. Assoc. J. November pp. 735-740.

    Bertsch, W., R.C. Chang, and A. Zlatkis. 1974. The determination of organic volatiles in air pollution
    studies: Characterization of profiles.  J. Chromatogr. Sci.  12:175-182.

    Burch, D.E. and D.S. Goodsell.  1981. Development of an H2O monitor using differential infrared
    absorption.  U.S. EPA Final Report (EPA-600/2-81-162) on Contract 68-02-3238 to Ford Aerospace and
    Communications Corporation. Research Triangle Park, NC: U.S.  Environmental Protection Agency.

    Burns, W.F., D.T. Tingey, R.C. Evans, and E.H. Bates. 1983. Problems with a  Nafion membrane dryer
    for drying chromatographic samples. J. Chromatogr. 269:1-9.

    Cox, R.D. and R.F. Earp.   1982.  Determination  of trace level organics in ambient air by high-
    resolution gas chromatography with simultaneous photoionization and flame ionization  detection.
    Anal. Chem. 54:2265.

    Cox, R.D., M.A. McDevitt, K.W. Lee, and G.K. Tannahill.  1982.  Determination of low levels of total
    non-methane hydrocarbon content in ambient air. Environ. Sci.  Technol.  16:57-61.

    Des Marais, D.J.  1978.  Variable temperature cryogenic trap  for the separation of gas mixtures.
    Anal. Chem. 50:1405-1406.

    Farwell, S.O., S.J. Gluck, W.L Bamesberger, T.M. Schuttle, and T.F. Adams.  1979. Determination of
    sulfur  containing gases by a deactivated cryogenic enrichment and capillary gas chromatographic
    system. Anal. Chem. 51:609-615.

    Foulger, B.E. and P.G. Simmonds. 1979.  Drier for field use in the determination of trace atmospheric
    gases.  Anal. Chem.  51:1089.

    Holdren, M., S. Rust, R. Smith, and J. Koetz. 1985. Evaluation of cryogenic trapping as a means for
    collecting organic compounds in ambient air. U.S. EPA Final Report (EPA-600/4-85-002) on Contract
    68-02-3487 to Battelle Columbus Laboratories. Research Triangle Park,  NC: U.S. Environmental
    Protection Agency.

    Kertzman, J. 1973.  Continuous drying of process sample streams. Paper 73425.  Instrument Society
    of America, Analytical Instrumentation Division.

    McClenny, W.A. and J.D. Pleil.  1984. Automated calibration and analysis of VOCs with  a capillary
    column gas chromatograph equipped for reduced temperature trapping.   Paper 84-17.6.
    Proceedings of the 77th APCA Meeting, San Francisco, CA, June 24-29.

    McClenny, W.A., J.D. Pleil, M.W. Holdren, and R.N. Smith.  1984.   Automated  cryogenic
    preconcentration and gas chromatographic determination of  volatile organic  compounds in  air.
    Anal. Chem. 56:2947.

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NORTHROP
   .  	                                                               TN-4120-85-03
Environmental Sciences
    McCienny, W.A., J.D. Pleil, K.D. Oliver, and M.W. Holdren.  1985. Comparison of two volatile organic
    compounds monitors equipped with cryogenic preconcentrators   J. Air Pollut. Control Assoc.
    35(10):1053-1056.

    Pellizzari, E.D., J.E. Bunch, R.E. Berkeley, and J. McRae.   1976. Determination of trace hazardous
    organic  vapor  pollutants in ambient atmospheres  by  gas chromatography/mass
    spectrometry/computer. Anal. Chem. 48:803-806.

    Pellizzari, E.f B. Demian, and K. Krost.  1984a. Sampling  of organic compounds in the presence of
    reactive inorganic gases with TenaxGC.  Anal. Chem. 56:793-798.

    Pellizzari, E.D.,W.F.Gutknecht,S. Cooper, and D. Hardison. 1984b.  Evaluation of sampling  methods
    for gaseous atmospheric samples.  U.S. EPA Final  Report on Contract No. 68-02-2991  to  Research
    Triangle Institute. Research Triangle Park, NC: U.S.  Environmental Protection Agency.

    Perma-Pure Products, Inc.  Bulletin 106.  Farmingdale, NJ.

    Pleil, J.D. and W.A. McCienny.  1984.  Temperature-dependent collection efficiency of a cryogenic
    trap for trace-level volatile  organic compounds.   Paper  84-17.5.  Proceedings of the 77th APCA
    Meeting, San Francisco, CA, June 24-29.

    Rasmussen, R.A., D.E. Harsch, P.M. Sweany, J.P. Krasnec,  and D.R.  Cronn.  1977.  Determination of
    atmospheric halocarbons  by a temperature-programmed gas chromatographic freezeout
    concentration method. J. Air Pollut. Control Assoc. 27:579-581.

    Singh, H.B., L.J. Salas, H. Shigeishi, and E. Scribner.  1979a. Atmospheric halocarbons,  hydrocarbons,
    and SFe: Global distributions, sources, and sinks. Science 203:899.
    Singh, H.B., L.J. Salas, R. Stiles, and H. Shigeishi.  1979b.  Measurements of hazardous organic
    chemicals in the ambient atmosphere.  U.S.  EPA Final Report on Cooperative Agreement 805990 to
    SRI International. Research Triangle Park, NC: U.S. Environmental Protection Agency.

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