S06'29 PREPARING TRACE MOISTURE STANDARDS IN REACTIVE MATRIX GASES

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S-06.29 PREPARING TRACE MOISTURE STANDARDS IN
REACTIVE MATRIX GASES

• JIM MCKINLEY
• KIN-TEK LABORATORIES, INC.
• La Marque, Texas 77568

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Introduction

• Trace moisture contamination is a serious problem
  in many industries
• Measuring and calibration techniques are well
  developed for moisture in inert gases
• Measuring techniques have also been developed for
  moisture in many reactive gases
• None of the methods is absolute - all need
  frequent calibration and data verification

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The Problem - Calibration

• Gas standards with known moisture content are
  needed
• No problem - in an inert matrix like nitrogen
• In reactive matrices, like ammonia or hydrogen
  chloride, its another story!
• But often the matrix is also an interferrent,
  so calibration must be done with a matrix-based
  standard
• Common strategies no longer work

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Common Strategies for Standards

• The dewpoint/frostpoint method
• The absolute standard method for inert matrices
• Useless if the matrix also condenses
• Fails if the matrix gas and analyte interact
• Compare to chilled-mirror hygrometer
• Fails if matrix condenses
• Fails if the matrix attacks the mirror

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Properties of Some Reactive Compounds
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An Additional Problem

• Establishing a Zero reference is also a problem
• The Dewpoint/Frostpoint method adds or removes
  water to create an absolute concentration
• Other methods simply add water to the existing
  background concentration
• An effective method must deal with a non-zero
  reference point

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Permeation Tubes
Permeation Tubes are devices that use
permeation through a membrane to control the
analyte compound (water) flow
f is measured by measuring rate of weight
loss due to permeation
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Using Permeation Tubes
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The Moisture Concentration
Where C is the concentration in ppm
(v/v), f is the flow of water vapor,
F is the flow of dilution gas.
Because f is very, very much smaller than F, this
equation reduces to
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The Analyte Emission Rate
Where f is the flow rate of analyte
emitted by the tube K(T) is the
permeability of the membrane to the analyte at
temperature T A is the membrane
area ?P is the partial pressure
difference of analyte vapor across the membrane
t is the membrane thickness
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Temperature Sensitivity
Where
are constants T is the absolute
temperature inK
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Design Considerations

• The curse(s) of working with water
• There is no Zero
• Water interacts with almost everything
• Water is pervasive in our atmosphere - preventing
  contamination is a serious problem
• The matrix gas may affect the permeation tube!
• solubility in the membrane, may change the perm
  rate
• Matrix gas permeating in may contaminate the
  water
• Matrix dissolving in the water may make
  certification impossible

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Addressing the Problems
FLOW DIAGRAM
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Three Concentrations Created

• Base Concentration - the mixture of permeated
  moisture and the carrier flowing over the tube
• Matrix gas is inert
• Concentration is fixed
• Primary Mixture - diluted base mixture
• Matrix gas is inert
• Concentration is adjustable (201 range)
• Final Mixture - portion of primary mixture
  diluted in reactive matrix

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Base Concentration
Concentration in the fixed carrier flow
Where CB is the Base Concentration
(ppmv) E is the flow rate of water vapor
from the permeation tube F2 is the
carrier flow of inert gas over the permeation
tube 1.24 converts nanograms of water to
nanoliters of vapor Note The ppm conversion
is hidden in the ratio of nanoliters/milliliters
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Primary Mixture
Diluted Base Concentration
Where CP is the moisture concentration
in the primary mixture CB is the
moisture concentration in the base mixture
C is the background moisture concentration
F2 is the fixed carrier flow F1 is
the adjustable flow of inert, primary dilution
gas
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Final Concentration
Final concentration of water in the reactive
matrix
Where C0 is the unknown background
concentration of water F3 is the main
dilution flow of reactive matrix F4 is
the split flow of primary mixture added to the
main dilution flow
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Calibration By Standard Additions

• Set the system to zero
• Observe the analyzer response R0 to the
  background concentration C0
• Switch to the span state - observe the analyzer
  response to the total concentration RT
• Calculate the sensitivity factor S ?R/?C
• The background C0 R0/S
• The total concentration is CT ?C C0

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Graphic Illustration
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Conclusions

• Traditional calibration methods for trace
  moisture often dont apply in a reactive matrix
• Key problems are
• Lack of a zero reference
• Adding a known amount of water concentration
• Accounting for stray contamination of the
  standard
• Permeation tubes can be used to add the known
  concentration
• Standard Additions can compensate for no zero and
  stray contamination