552.3 Method Development

Question

I am currently developing a method for using EPA Method 552.3 on our 8890 w/uECD detector using He carrier gas and N2 make-up gas. My predecessor settled on using the columns from Agilent's App Note (DB-35ms UI and DB-XLB). The main thing I'm trying to determine is which type of liner would be best. 
 The EPA method was developed (in 2003) using 2-mm straight quartz liners. The App Note (which used SPE for pre-concentration that I won't be doing) used 4-mm Helix double taper liners. I've tried to reference others' methods online and found one that uses a Shimadzu GC with H2 carrier gas and used a Restek 3.5-mm single taper w/wool topaz liner. 
 There are several liners already in the lab (5190-2293 UI single taper w/wool), but I don't know if these were ordered by my predecessor or just offered as samples by Agilent. From what I've read, using liners with wool would just be offering unnecessary filtering and be adding activation sites, making it better to have no wool. So I assume I shouldn't use these liners. 
 The cost difference between the various Agilent 2-mm straight quartz liners and the 4-mm helix double taper liner are fairly large (4-5x higher for the double taper). Once my method has been developed and verified, I would likely only be running samples once a month, the run be mostly calibration standards and QC samples with up to 10 actual samples (all of which would be finished drinking water). 
 Is there any reason that I should go with the extra expense of the double taper liners over the straight liners that the 20 year old EPA method used? The App Note (that I've only skimmed so far) doesn't seem to offer any explanation of why they chose these liners except to stress the importance of inertness. 
 Additionally, the three different splitless 2-mm straight liners I am looking at actually decrease in price with increased deactivation/inertness. Does that make sense? Because of that, my current thought is to go with the 5190-6168 UI splitless 2-mm straight liners, but the webstore description says "for use with HS Transferline". I would still be okay to use them, right? 
 
 Thanks in advance for any assistance.

Dr. Know · Accepted Answer

Hi scruz, 
 the UI single taper w/wool is fine for the analyses, so is the UI splitless 2-mm straight liner. It all depends on your injection volume and if the evaporated sample fits into the liner's internal volume. In headspace you already have a gas injection and the 5190-6168 is ok. The 5190-2293 has glass wool to evaporate a liquid sample better. 
 In your case, you will have aqueous samples that will turn to a lot of gas during evaporation and I would switch to a 4 mm ID single or double taper liner, otherwise you will get break-through or backflash into the gas supply lines (that's expensive to clean). 
 You can also call the US Agilent Technical Support to discuss this in-depth. 
 Regards, 
 Norbert Reuter

scruz · Answer

Hi, 
 Unfortunately, due to work situations I haven't been able to get any further with my method development, so I haven't gone through the sample preparation myself yet. 
 However, based on my understanding of the method, I believe it is assumed that all of the internal standard stays within the MTBE during the extraction process. This means that the concentration of the original IS in MTBE should be the same as the IS in the final MTBE, so the concentration of IS should stay the same. Hopefully someone with more experience will see this and be able to contribute.

Dr. Know · Answer

An internal standard is always added prior to sample preparation, so that it undergoes the same extraction/preparation steps than the analytes. This normally means that after sample preparation you also lost a little ISTD. If that would not happen you could use an external standard for calibration. 
 BTW: an internal standard therefore should be structurally and chemically similar to the analytes (textbook talk)

552.3 Method Development

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