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Create High Quality VOC Gas Mixtures

Create High Quality VOC Gas Mixtures at Low to sub-PPBv Levels for GC/MS Calibration

The need to make accurate TO15 standard mixtures in the 0.2 to 2 PPB range has lead to the development of the Entech 4700 Precision Static Diluter. This instrument accurately meters standard into Silonite™ canisters from up to 5 different cylinders, then dilutes these as much as 100 times using UHP Zero Air, Nitrogen, Helium, Hydrogen, or one of several other balance gases. When starting off with a 1PPM stock, this allows the preparation of standards down to 10-100 PPB. However, the new 4700 allows this to be the new “stock” for further dilution by another 10-100 times, yielding standards in the 0.1 to the 1PPB range for ultimate testing at typical ambient concentrations. The other tremendous advantage that precision static dilution has over dynamic dilution is that almost no standard is wasted. Dynamic Dilution, by its very nature, requires an equilibrium to be established using 2 to 6 mass flow controllers prior to filling a receiving canister. During this time, the standard flows through a mixing manifold and then out to a hood. Once starting the process of filling the receiving canister, the flow and pressures that have been set up in the mixing region cannot be disturbed, so most of the flow continues to

Dynamic Dilution, by its very nature, requires an equilibrium to be established using 2 to 6 mass flow controllers prior to filling a receiving canister. During this time, the standard flows through a mixing manifold and then out to a hood. Once starting the process of filling the receiving canister, the flow and pressures that have been set up in the mixing region cannot be disturbed, so most of the flow continues to be vented to the hood. In the end, perhaps 90-95% of the standard when using Dynamic Dilution never makes it into the receiving canister. Since these cylinders are only certified for 1 year, that generally does not create a problem when making just 2-4 dilutions per month for purposes of methods validation and determination of GCMS response factors. However, recently there has been a desire to start testing individual field canisters for inertness, which mean occasionally delivering 6L of the

Since these cylinders are only certified for 1 year, that generally does not create a problem when making just 2-4 dilutions per month for purposes of methods validation and determination of GCMS response factors. However, recently there has been a desire to start testing individual field canisters for inertness, which mean occasionally delivering 6L of standard into each canister in use. This may require the filling of literally 1000 or more canisters per year. The new 4700 allows more than 10,000 6L canisters to be filled from a single 6R cylinder, which are the smallest cylinders currently in use by specialty gas companies. This is simple mathematics. With the quoted 102L of standard (at STP) in each 6R cylinder, this will create over 100,000L of gas when diluted down to 1PPB (1000x dilution of 100L = 100,000L). For accurate testing of inertness, canisters should only be filled to atmospheric pressure like they are in the field, which is done by adding 6L to a 6L canister initially under full vacuum. Although this would indicate that each cylinder would fill 100,000 / 6 >16,000 canisters, a little standard may be required for initial line flushing if the diluter has not been used for a while. In all, the cost of standards to test each canister is now drastically reduced. Finally, air laboratories can also make custom standards more easily by using neat compounds injected into a Silonite Canister or

For accurate testing of inertness, canisters should only be filled to atmospheric pressure like they are in the field, which is done by adding 6L to a 6L canister initially under full vacuum. Although this would indicate that each cylinder would fill 100,000 / 6 >16,000 canisters, a little standard may be required for initial line flushing if the diluter has not been used for a while. In all, the cost of standards to test each canister is now drastically reduced. Finally, air laboratories can also make custom standards more easily by using neat compounds injected into a Silonite Canister or Bottle-Vac and then attaching this to the 4700 to dilute this stock down to ambient levels. These would not be traceable standards but would allow for specialty compounds to be monitored as needed.