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sawsqac

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Treatment of method blanks
Nov 08^th, 2011, 9:56am

A question has come up in my lab regarding method blanks. For a few methods we encounter the situation where some samples in the same prep batch require more clean-up than others. A simple example would be some samples requiring filtration while others do not. Should the method blank undergo all the clean-up steps required of the "dirtiest" sample in the batch? My thought is yes, just to demonstrate that none of the prep procedures are introducing contamination. I have gotten the counter-argument that since reagent water is a clean matrix, it is not necessary to perform additional clean-up steps that wouldn't be required of a "clean" sample.

I would appreciate hearing how other labs handle this.

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Victor

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Re: Treatment of method blanks
Reply #1 - Nov 8^th, 2011, 2:55pm

You would be going under the assumption that a "clean" sample is always "clean" or has very little of the constituents that are being analyzed. Never treat any sample any different than all the other samples. You run blanks to ensure that not only your glassware is clean but that the procedure does not introduce contaminants along the way. No different than microbiological techniques that ensure sterility is maintained throughout the process by having excellent aseptic techniques. Never assume.

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Charles Lytle YaBB Newbies Offline I love Water and Wastewater.com Posts: 43 Portland, OR Gender: Company or Organization: City of Portland, Oregon	Re: Treatment of method blanks Reply #2 - Nov 9^th, 2011, 8:44am The simple answer is that you should use the same preparation steps for all the samples in a batch. Thus, the method blank should be filtered. However, I'm curious as to what you're actually measuring; you wouldn't want to create a total/dissolved disparity. Chuck Lytle, City of Portland (OR)
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sawsqac

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Re: Treatment of method blanks
Reply #3 - Nov 9^th, 2011, 10:41am

I think I was too vague with my examples. We are not prepping total and dissolved analytes in the same prep batch.

We do solid-phase extraction for oil and grease, and all the samples, including QC, go through the same basic prep. But particularly dirty samples need to be pre-filtered, and the dirtiest samples have to be passed through glass wool before the pre-filter. My question is: if any of the samples in a given batch have to go through one or both of these additional clean-up steps, should the method blank also?

We also analyze for dissolved hex chrome, filtering all samples through a 0.45 µm filter. However, wastewater influent samples contain a lot of particulates and have to be put through a 1.0 µm filter syringe first. The question is, should the method blank be filtered, even though it is composed of Type I water? My argument is yes, to make sure there's no contamination from the filtering steps, but the analysts contend that it is unnecessary to filter Type I water again.

As a compromise, I suppose we could run two blanks- one that goes through just the preparation steps that all samples go through, and a second one that is subjected to the same clean-up steps as the dirtiest sample.

Thanks for the responses and I hope I've clarified things a bit. I sent this question to our state accrediting authority (in Texas) yesterday and will let everyone know what their take is on it. We are not accredited for either of these tests yet and are trying to get all our ducks in a row.

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Charles Lytle

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Re: Treatment of method blanks
Reply #4 - Nov 9^th, 2011, 12:09pm

Hi again,

My take is that you'd need to do two method blanks: one for the samples that went through only the SPE disc and one for the samples that required a pre-filter. If you stack a gf filter on top of the pre-filter for one or more samples, you'd need a third MB for that.

We let ugly samples sit, decant, and then pour in the glop. And wait, and wait....and wait (direct quote from the movie Casablanca).

Chuck Lytle

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BarfingCat YaBB Newbies Offline Water and Wastewater.com is the best! Posts: 34 Independence, MO Gender: Company or Organization: Little Blue Valley Sewer Dist.	Re: Treatment of method blanks Reply #5 - Nov 9^th, 2011, 12:35pm If you do not treat the blank the same as a sample, then you only have a reagent blank instead of a method blank.
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sawsqac

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Re: Treatment of method blanks
Reply #6 - Nov 15^th, 2011, 4:00pm

Thanks to all for their comments. The response I got from our accrediting authority also recommends preparing and analyzing two method blanks for batches that contain routine and non-routine samples. One blank would be treated like the routine samples, and the second would go through all the cleanup steps.

So there we have it. Now it's time to update a few SOP's.

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