Chromatography Forum

I am relatively new to GCMS techniques and am attempting to implement the EPA 525.2 method for drinking water samples. I would like to get some feedback from people on what their results are like. Particularly with the recovery of internal standards. In this method the internal standards and surrogates are added prior to extraction, and a fortification solution of terphenyl-d14 is added after extraction "to monitor the recovery of the internal standards in laboratory fortified blanks and samples". But the EPA method does not clearly explain how to perform the recovery calculations.

I can look at the ratio of area counts (IS/terphenyl-d14) for each sample, but how do you know the expected ratio to determine percent recovery?

Thanks!

I don't use tertphenyl d14 as it interferes with DDE, and it's only a recommendation in the method ( despite what some auditors say!).
I average the responses for the IS's and surrogates in my calibration. The method allows +/- 50% range from initial calibration for the samples ( or +/- 30% from the last calibration check.

Good luck

I have done quite a bit of EPA 625 in the past, which is 525 for wastewater.

I'm curious why the method has you adding internal standards prior to extraction...seems counterproductive to me. Or is that one of those new isotope-dilution methods?

Just curious...

625,

Internal standards added at the beginning of the extraction automatically corrects for any losses during the extractrion. If you lose half your analytes, it doesn't matter because (theoretically) you have also lost half your IS and the ratio stays the same. Isotope-dilution in this way as you mention, is very useful for many applications in the lab, however I am unclear why out of all the EPA methods we run, 525.2 is the only one (at least in my lab) to do it this way.

BigBear,

Guess I am still on a mission to find someone who does use the terphenyl for IS recovery. But since you do run 525.2, any thoughts on another question I posted about RRFs? I know you don't *have* to obtain RSD <30 if you use linear regression instead of average response, but since it is suggested in the method I figure I should *able* to. but I cannot because my response factors increase with increasing standard concentration. Am I doing something wrong. I get good linearity (.995) but the non consistent RF obviously makes my RSD much higher than 30 sometimes.

http://www.sepsci.com/chromforum/viewto ... highlight=

We find the recovery of the spiked internal standards to be fairly matrix dependent. We tend to see similar recoveries from the same water sources over time.

Gpronger,

How do you calculate your IS recoveries? Do you add IS before extraction and terphenyl after?

Shan820;

Exactly, the terphenyl-d14 is added after. There are solubility issues with this compound so you need to ensure that it is completely in solution when you spike it or you'll end up with it also having erratic recoveries.

The method (section 10.3.4) has specific area based recovery criteria.

Greg

I also don't use terphenyl, but the internal standards and surrogates I do use are added prior to a solid phase extraction. I've noticed my IS/Surr recoveries are much more dependent on extraction consistency. My CCV's perform consistently.

My % recovery is determined by the values in my compound table which is dictated by my calibration curve. For post-extraction terphenyl, I'd probably add 10ul to my 1ml final volume extract (1L initial vol. spiked w/ 1ml) to match the concentration of a mid-point on my calibration curve. I don't know if that makes any sense....

As a side note, have you had an issue with DDT breakdown?

It sounds like you have the instrument end largely dialed in. The SPE tends towards art (as opposed to science). I would recommend that the prep group experiment with spikes of tap water varying the parameters (Solvent volume, pressure, slightly to dial in the prep). Different manufacturers disks are slightly different and tend to need to be optimized.

Are you having common breakdown problems? How often do you do inlet maintenance?

I agree that SPE is highly variable. SPE sounds good in theory, but for every potential human error minimized, there seems to be at least two mechanical errors/reagent variabilities to takes its place.

I'm currently trying to isolate possible causes of contamination. Yesterday I extracted a spiked and non-spiked prep blank using OFW (I use tap water for my QC). A few analytes I've been having a problem with appearing in my QC are Di(2-ethylhexyl)phthalate, Di(2-ethylhexyl)adipate, and Di-N-octyl phthalate. Are these contaminants so ubiquitous that their presence is common in 525.2 analysis?

As far as the breakdown goes, my lab was recently dinged for not calculating the breakdown for 525.2. When I first calculated the breakdown yesterday I was thoroughly disappointed. Although I had just performed injector port maintenance, I had significant breakdown (55%+). I dropped the injector temp from 280C to 205C and the breakdown is now <15%. Although my DFTPP passed, I have yet to run a CCV. What injector temp do you have your instrument set at?

Thanks,
Victor E.

Victor:
I have not experienced any problems with DDT breakdown. (Fingers crossed) If I do see a small breakdown peak I prep a fresh standard and that helps. My inlet temp is 250C and I use agilent dual taper direct connect deactivated liners. They work very well for this method, but you have to be very careful when installing the column or else a peice breaks off and you just wasted $40. What liner are you using?

As for your contaminants, pthalates and adipates, yes they are very ubiquitous and will commonly show up in everything you do. For this reason it is important to avoid plastics when possible including gloves, pipette tips, rubber bulbs, plastic tubing etc. (especially avoid plastic solvent squeeze bottles!) We also rinse all of our glassware with mulitple portions of DCM and hexane, concentrate the final hexan rinse, and analyze to confirm absence of contaminants prior to extraction.

Regarding phthalates, we ended up installing a separate Millipore type system with UV in organic prep to help get the phthalates under control. The lab is an older building and PVC (fine for metals) was used through-out the DI water delivery system.

We use the Millipore system as well. Our phthalates predominantly come from handling in the field and in the lab.

And as a general point of interest the National Research Council has released an paper indicating that the EPA look at phthalates as a class and needing further research due to "demasculinizing" action for the group of compounds. For more info I have a Slashdot journal post with the links, etc (http://slashdot.org/~gpronger/journal/220241).

More than likely this will mean that we'll need to sort out how to get around their (phthalates) usual background levels.

Greg