Accurate dilutions of sample gases are important for obtaining quality data from any gas analysis system. Follow these simple (and some not-so-simple) rules and procedures to obtain the most reliable performance from your analyzer.
The methods most commonly used for obtaining accurate vapor samples at high concentrations are:
purchased standard gases
Methods for diluting to working concentrations are:
stopwatch and flowmeter
mass flow controller
A more thorough coverage of gas handling techniques can be found in Gary O. Nelson's book, Gas Mixtures: Preparation and Control, (Lewis Publishers, 1992). This is a good general reference for anyone who makes a living working with gases and instrument calibration.
Universally-Required Gas Handling Supplies
Some items are needed for all work with gas dilutions. Chief among these are tubing and gas sample bags. These are made in a variety of sizes and materials. It is important to choose materials that are compatible with the chemicals being handled. Compatibility involves three considerations:
Reactivity - Will the vapor react chemically with the material? Ozone, for example, will react not only with many plastics, but with the oils and processing chemicals left over from their manufacture.
Permeation - Will the vapor leak out through the material? Polyethylene, in particular, is permeable to many gases and in general should not be used.
Absorption - Will the tubing or bag absorb the vapor from the sample? Absorption losses become especially important at low concentrations. Diluting benzene vapor to the important part-per-billion concentrations, for example, requires great care and special equipment.
For most purposes, 4-liter and 40-liter sample bags of 2-mil or 4-mil Tedlar, polyethylene-lined Tygon tubing (Bev-A-Line), and polypropylene Luer-type fittings are the most generally useful materials for gas handling.
Electric gas pumps are useful for moving gases between sample bags at a relatively constant flow rate. These pumps are available from several sources, but materials compatibility is often a problem. The neoprene diaphragms used in the majority of pumps, for example, will absorb most organic solvent vapors, especially from low concentrations of benzene or chlorinated hydrocarbons.
A very useful appliance is an electric or water-driven vacuum pump for evacuating sample bags. This obviously must be done before any filling operation. Flushing the bag with clean air is also advisable when the bag has previously contained a different chemical or one at high concentration.
Making Standard, or Stock, Vapor Samples
It is not usually a good idea to move from a pure (or "neat") chemical, directly to a low concentration in the PPB or low PPM range. (Dilutions made with permeation tubes are an exception.) A small sample of vapor of intermediate concentration (100 to 10,000 PPM) may be made first, and then diluted to the working concentrations. Generally, a sample at a high concentration can be saved and used for a longer time than the same gas at lower concentrations.
The syringe method is useful for liquid chemicals that are not particularly toxic. This includes most solvents. The liquid is measured with a microliter syringe and injected into a sample bag. The bag is then inflated with a known volume of air.
1. Prepare a sample bag with a long Teflon or Bev-A-Line tube (24") on the outlet and a closure on the end of the tube.
2. Measure the calculated volume of liquid and inject it into the end of the tube. Place the closure in place immediately.
3. Pump a volume of air equal to the final total volume into the bag through the tube. This forces evaporation of the liquid and promotes thorough mixing.
The most accurate way to measure the dilution air is from a large-volume syringe (e.g., the 4-liter Model 722K from Tracor- Atlas, Inc.). The syringe can be filled to the mark from a second bag containing clean cylinder air or nitrogen; laboratory air may be drawn in through a charcoal zero filter. The measured gas is then expelled into the dilution bag through the access tube.
An alternate method is to use a stopwatch and flowmeter to measure the diluting gas. This method can be very accurate after a little practice. This is described under the next main heading, "Making Dilutions to Working Concentrations", below.
To calculate the correct amount of liquid to inject, you must know the molecular weight of the liquid, its density, the final volume of the dilution to be made, and the final concentration expected.
Problem: You want to make 40 liters of a vapor sample containing 100 PPM 1,1,1-trichloroethane (TCE). The molecular weight is 133.4 and the density is 1.349.
Solution: 100 parts per million is the same as 100 microliters of vapor per one liter of vapor sample. 40 liters of sample will therefore contain 4000 microliters (4.0x10-3 L) of TCE. One mole of TCE vapor would have a volume of 22.4 L. Therefore, 4.0.10-3 L of TCE represents
4.10-3 / 22.4 = 1.785.10-4 moles
which in turn equals 1.785.10-4 x 133.4 MW = 0.0238 grams of TCE. A volume of TCE weighing 0.0238 grams is found by dividing by the density, yielding 0.0238/1.349 = 0.0177 milliliters. In order to make 40 liters of 100 PPM TCE, therefore, you must measure 17.7 microliters of liquid TCE into the sample bag before inflating to 40 liters.
One last note: Very volatile liquids, such as methylene chloride, ether, and acetaldehyde, are very difficult to pipet accurately. Chilling the bottle and syringe may help, but this risks contamination of the reagent with condensed water. A cylinder of standard gas or a permeation tube is best for these compounds.