Custom Sensor Solutions, Inc.Custom Sensor Solutions, Inc.
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.
Methods for making accurate vapor samples at high concentrations are:
syringe method
purchased standard gases
permeation tubes
chemical generation
Methods for diluting to working concentrations are:
stopwatch and flowmeter
dynamic flow-switching
gas syringes
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.
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 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.
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 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.
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.
Vapors of many chemicals can be ordered ready-made and certified from specialty gas suppliers such as Matheson or Scott Specialty Gases. These standard gases are generally prepared gravimetrically and their accuracy is certified. Nothing can be more convenient, but the cost is high. A cylinder of standard gas may cost $200 to $500, not counting the cost of special regulators that are often required. Also, gas cylinders cannot be shipped by air. This, together with safety considerations, restricts the number of different gases and concentrations that can be stored in a laboratory.
There are certain limitations on the availability of standard gases, which should be discussed with a supplier before ordering.
1. Some chemicals are too corrosive or too unstable to be made into standard gases.
2. There is usually a limit on the concentrations that are available. Above a certain concentration, many liquid chemicals will condense from the gas phase at the high pressures found inside the cylinder.
3. Nearly all standard gases in cylinders have a limited lifetime.
4. Read the specs. on the gas carefully. "Zero" grade compressed air can still contain up to 1 ppm of CO.
Permeation tubes are a nearly ideal way of making large volumes of vapor from certain chemicals at low concentrations. They are made by sealing volatile chemicals in tubes, usually of a fluorinated polyethylene. The chemicals will permeate (or "leak") through the walls of the tube at a rate that is controlled by the temperature.
The permeation tube is typically enclosed in a thermostated glass chamber. The chamber is flooded with a controlled flowrate of the diluting gas. The constant leakage of chemical into the constant flow of gas produces a constant concentration. Special ovens and apparatus are sold for heating and using permeation tubes. If you have an water bath with accurate temperature control and a good flowmeter, it is possible to construct an apparatus cheaply.
The concentration can be varied by changing the flowrate of the carrier gas, or by changing the temperature of the chamber. The flowrate is the "fine control", allowing variation of concentration over a two- to five-fold range. Temperature is the "coarse control"; a 10 degree change will cause a four- to ten-fold change in permeation rate, depending on the chemical and the tube material.
Several manufacturers make permeation tubes. The tubes may cost from $20 to $200 and most will last for months or years. They are usually supplied with a certificate of calibration, but they can be calibrated by any user with a five-place analytical balance. The history of the tube is kept in a logbook, so that its weight loss over a period of operation at a specific temperature can be used to confirm its calibration.
Permeation tubes are available for a wide variety of chemicals. They work best for liquids with low boiling points and for gases that are liquified at low pressures and room temperature. Hard-to-handle chemicals like nitrogen dioxide, ethyl mercaptan, and acetaldehyde are ideal for permeation tubes. It is well to remember, however, that oxygen leaks into a permeation tube as quickly as the chemical leaks out. Easily oxidized compounds like thioethers and mercaptans will eventually become contaminated with oxidation products.
Permeation tubes are a safety hazard if they are damaged or stored elsewhere than in a fume hood. The vapor continues to leak even at room temperature for most compounds, and if they are stored in a tight container, the pressure may build up as the compound permeates outward, causing a sudden release when the container is opened. Potentially, the container may burst, but we have never heard of this happening.
Some gases are more easily made as they are used, rather than stored. Examples are ozone, hydrogen chloride, and hydrogen bromide. Low concentrations of ozone can be made with devices that pass ordinary air across a powerful ultraviolet lamp. A calibrated source that we use in our laboratory is the Thermo Environmental Co. Model 565 Ozone Generator, which will produce 2 liters/min of 0.5 PPM ozone. A dial on the front panel will vary the ozone concentration continuously over the range 0 to 500 PPB.
Hydrogen chloride can be made by injecting a measured amount of concentrated sodium chloride solution into a flask containing concentrated sulfuric acid. All the generated hydrochloric acid will be converted to HCL gas, which can be swept out of the container with a stream of nitrogen. A simple way to do this is to use a 60 cc serum bottle. Put 10 mL sulfuric acid into the bottle and close it with a rubber serum cap. Heavy-gauge (16 or 18) hypodermic needles can be pushed through the serum cap and connected as inlet and outlet for the nitrogen. The hydrochloric acid is injected (slowly!) through the serum cap with a Teflon or plastic disposable syringe and another stainless steel needle. (The needle will be corroded by HCl and should be washed, cut in two and discarded immediately.) Flush the gas generated into a sample bag with a measured volume of nitrogen using the large syringe or stopwatch-and-flowmeter method described below.
Caution: Dispose of used sulfuric acid solutions properly.
We're not going to be shy about it. We think that our Model 1010 Precision Gas Diluter is the best all-around way to make dilutions of standard gases. But to be fair, we're going to cover all the major methods. You can make up your own mind.
From an accurate stock concentration, a series of dilutions can be rapidly made for preparing calibration curves. The term stock gas refers to a precisely-prepared higher concentration, and diluting gas or diluent refers to the gas used to dilute it to working concentrations. Typical diluting gases may be air or nitrogen.
For volumes of gas of five liters or more, the stopwatch and flowmeter method is convenient. It requires more attention, because it depends on careful timing, and on maintaining a constant flow rate over the time it takes to fill a sample bag.
The stock gas source is connected to a flowmeter. The gas flow is started and adjusted to a suitable value, say 1.00 liter per minute. The flow is then connected to the sample bag and the stopwatch is started. The flow rate is remeasured and the total time needed to pump the required volume of air is then calculated. At the end of this time, the bag is disconnected. The diluting gas may come from a cylinder of air or nitrogen or from an air pump. If laboratory air is used, it is a good idea to pass it through a charcoal or Purafil filter, although this does not guarantee purity. When using typical floating-ball flowmeters with needle valves, the flowrate may have to be adjusted from time to time during the filling of the sample bag.
For smaller volumes of gas, syringes may be used in some cases. A typical strategy used in our lab is to make a 10,000 PPM stock sample in a 4-liter bag. Some chemicals might be stable for as long as a week. Working dilutions of 10 PPM would be made by withdrawing 40 cc of stock, injecting it into a sample bag, and filling the bag with 40 liters of air.
Gastight syringes can be purchased at fabulous cost, but plain disposable polyethylene syringes can often be used instead. These syringes are available in volumes up to 60 cc. To avoid loss of standard gas from the open syringe by diffusion, a 12" length of thin tubing is connected to the Luer fitting on the syringe, and the end of the tubing is closed with a valve or a plug closure.
The large-volume syringes such as the Tracor-Atlas 722K four- liter model, are useful for measuring diluting gases, but they should not be used for measuring standard gases. These syringes use a thin layer of silicone oil inside the barrel to aid plunger movement. Silicone oil will efficiently absorb many chemicals from the vapor phase, reducing the final concentration.
The apparatus for the multiple flowmeter method consists of two flowmeters, preferably equipped with needle valves. One flowmeter is connected to the standard gas, and the other to the diluting gas. The outlets of both flowmeters are mixed together at a tee connection. The flow rates of both gases should be set with the needle valves and allowed to stabilize for a few minutes before the sample bag is connected to the third arm of the tee. Be sure to check the flowmeters frequently during the filling operation.
(That's us.)
Gas mixtures can be conveniently made by drawing the sample air through a three-port solenoid valve whose inlets are connected to the standard and diluting gases, respectively. The solenoid valve is switched back and forth between the two gas sources at intervals of about one second. The pulses of gas are combined in a small-volume mixing chamber before being conducted to the instrument under calibration. The percentage of time that the solenoid is drawing from the standard gas is a measure of the dilution ratio. Dilution ratios of 20:1 are easily obtained, and dilutions of 1000:1 or more can be reliably carried out using two or more systems connected in tandem.
The cycling time and duty cycle of the solenoid valve must be controlled by highly-accurate electronics, and precautions must be taken to assure that the flow rates through the two gas paths are the same.
Custom Sensor Solutions, Inc., sells an instrument for diluting gases by dynamic flow switching, the Model 1010 Precision Gas Diluter.
Mass flow controllers are the "Cadillacs" of gas mixing devices. When supplied with a gas sample at low pressure, they will provide an accurately-controlled flow rate at the outlet. They must be provided with electrical power, since they use a delicate flow sensor and control valve to maintain a constant flow. Many can be operated under computer control.
The downside of mass flow controllers is that they are not compatible with all gases. Also, they seem to spend a great deal of time in the repair shop.
Mass flow controllers are now available from many sources, but the cost begins at about $800.
As mentioned before, ozone is a special case among important gases because it is so reactive. Any apparatus that is used with ozone should be first "scrubbed" with ozone for several hours before use. If you are using the ozone to calibrate a measuring instrument, you can determine when scrubbing is finished by taking periodic measurements of the ozone concentration at the outlet, without changing the span adjustment on the instrument. When the output signal no longer increases, the apparatus is ready to be used.
For information on calibrating ozone instruments, call us at 630-548-3548 or contact us at info@customsensorsolutions.com
Nitrogen dioxide calibration is often done with standard gas from a cylinder. Under the pressures existing in the cylinder, NO2 is actually present as the dimer, N2O4, even at concentrations of 50 PPM and below. Some instruments will respond differently to nitrogen tetroxide than to nitrogen dioxide; electrochemical analyzers, for example, do not seem to respond to N2O4 at all. After making dilutions of nitrogen dioxide at atmospheric pressure, let the gas sample stand for ten to fifteen minutes to allow the nitrogen tetroxide to dissociate.
Current March 3, 1998
Contact us at info@customsensorsolutions.com