14
3. Heat the pressurized canisters to120 - 250°C, dependingon the type of
canister being cleaned.Donot allow the temperature of a SUMMA® can-
ister to exceed155°C, because theNupro valve it employs hasViton®O-
rings and requires greases that cannot be exposed tohigh temperatures.
Many commercial cleaning systems avoid this problemby ensuring the
valve is notwithin theheated zone.The canister below the valve is heated
but the valve receives only radiant heat. In contrast, theParkerHannifin
diaphragm valve in a SilcoCan™ canister is far less heat sensitive, allow-
ing the canister tobe cleaned at temperatures up to250°C, tohelp
remove less labile impurities.
Heat the canisters filledwithhumidified air for at least 1hour.
4. Re-evacuate the canisters to remove the desorbed impurities.Allow the
canisters to equilibrate for 1hour.
5. Determine if the canisters have been cleaned effectivelyby following the
procedure inCertifying theCanister, below.USEPAmethods recom-
mend testing every canister until a reliable procedure is developed.
Repeat steps 1-5 as necessary; thenumber of cycleswill be determinedby
howdirty the canisters are andhow easily they are cleaned.We recom-
menddeveloping a cleaningprocedure thatmatches your specific sam-
plingprocedure, by testing the canisters for cleanliness after each cycle
anddetermining thenumber of cycles necessary for proper cleaning. If
the canisters arenot heated, thenumber of cycles required to clean the
canistersmight behigher.
6. Once a canister is clean, prepare it for collecting a sample by evacuating it
to10-50mTorr. If your system is leak-tight, you cando this byusing a
roughingpump, butmany commercial systems include amolecular drag
pump to reach final vacuumquickly.
IX.Certifying theCanister
We recommendcertifyingcanisters forbothcleanliness and for analyte stabil-
ity.To certify a canister clean, pressurize the canister to14.7psigwithhumid-
ified ultra-high purity air or nitrogen after it has gone through the cleaning
cycles.Thehumid air or nitrogen streammust be certified cleanbefore it can
beused for canister certification.Analyzeanaliquot of the canister content by
GC/MS or GC/FID/ECD. US EPA Method TO-14A/15 specifies a canister
must contain less than0.2ppbvof any targetVOCcompound (Figure9);EPA
MethodTO-12 specifies less than0.02ppmC,asdetectedbyGC/FID. If acan-
isterdoesnotmeet specification, itmust be re-cleaned and re-tested for certi-
fication.
To certify a canister for analyte stability, introduce a lowworking concentra-
tionof acharacterized testmix into thecanister.Analyzeanaliquotof thecon-
tents of the canister immediately after introducing the test mixture and at
periodic intervals.We recommendmonitoring for changes for aminimumof
2 weeks, or for a timeframe similar to your anticipated holding period.
Responses shouldnot decreasemore than20%over this period.
Commerciallyavailable standardsareavailable for stability testing,butwe rec-
ommend youmake your own test mixture that is comparable to the target
compound list that the canister will hold. For example, if you are analyzing
sulfurcompoundcontent inambientair,preparea sulfur-specific testmixand
evaluate the canister's performance for sulfurs.Maintain a log sheet for each
canister, and record the test results and certification.Thiswill be apermanent
record for each canister. Some labs certify canisters for certain compounds
anduse a canister only for this specific application.
Figure8
User-designed system for
cleaning24 six-liter canisters.
