Re: LIMIT VALUES FOR DIOXINS (fwd)

[slester@essential.org]

Date: 31 Jan 96 07:30:58 EST
From: John Sadler <100335.1664@compuserve.com>
To: "INTERNET:slester@essential.org" <slester@essential.org>
Subject: Re: LIMIT VALUES FOR DIOXINS

>From my perspective of looking at dioxin data over many years, I can state with
some confidence that talking femtograms for air samples is dangerous. The best
we can do in reporting levels in air, or any other matrix, is to use a validated
method for analysis with the best limit of detection possible.

The units the lab result is expressed in may be ppt, parts per trillion, which
is nanograms per kilogram. A nanogram is 0.001ug. The next one down is ppq or
parts per quadrillion, pg/kg. Below that we get to femtograms, or phantomgrams
as they are better known, which are 0.001pg.

Meanwhile if you are still awake, back at the lab method. US EPA method 23 is
used for stack emissions and works on a 5 cu m sample. US EPA method 8290 is
used for ambient samples of 100 cu m volume collected over 8 hours. The
detection limit (DL) for either method is 50pg (TEQ as 2378TCDD). The method
blank run with the sample will normally show 5-10pg because remember there are
17 dioxins and furans which make up the TEQ. The TEQ of the method blank is
often just the sum of the DL's with factors for each of the 17 contributors to
the TEQ.

What this all means is that for a stack sample it is possible to achieve a TEQ
result of 50pg, which in a sample of 5 cu m which allows you to be confident
when you are regulating to 0.1ng/cu m ( that is 100pg/cu m). However in your
ambient sample of 100cu m the 50 pg found is equivalent to 0.5pg/cu m (that is
500 fg/cu m). The level in the method blank will be typically 5-10pg (ie
50-100fg/cu m), so it is not sensible to talk about ambient levels of less than
1000fg/cu m. You may get even more difficult and say the limit of quantitation
(measuring amounts) is only possible to 10xDL .

I hope this made sense and helped explain why we are at the front of technology
with these measurements and that we must not misrepresent the case by using
indefensible data.

Best Regards     John Sadler UK