From: s893971@minyos.xx.rmit.OZ.AU (Adrian Ugoni)
Newsgroups: sci.chem
Subject: Re: Electron Capture on GC
Date: 22 Jan 93 12:43:57 GMT
Organization: RMIT Computer Centre
NNTP-Posting-Host: minyos.xx.rmit.oz.au

ckh@varian.com (Chuck Huston) writes:


>Jerry,

>If the GC method is set up properly (i.e., to allow efficient resolution 
>between water and your analyte), you shouldn't have any significant 
>problems;  often, a pre-column ("retention gap") is used for solute 
>focusing and the column is initially held at slightly above the boiling 
>point of water to make sure that it is not retained.  In Europe, a direct 
>injection of drinking water into a GC/ECD system (normally with a cold, on-
>column inlet) is commonly used to screen for trihalomethanes. 

In a lab manual I have from an old Job..
(I worked in a water quality laboratory)

Test for THM's

Principle
{much drivel removed}

The instrument is equiped with a headspace unit, VDU and keyboard (!),
fused silica capillary column and an electron capture detector.
Hight purity nitrogen is used ad the carrier gas..
One standard is all that's needed to calibrate the method
(an internal standard)

Inside the ec detector, the radioactive foil emits beta particles with a
maximum energy. As the beta particles travel through the carrier gas,
they ionize the carrier gas and produce positive ions and secondary electrons
The negative cell voltage applied to the cathode connection causes
secondary electrons (free electrons) to be driven towards the anode connection
The free electrons are collected at the anode and produce a steady background
(standing) current.
When a sample compound with an affinity for free electrons enters the

detector, the standing current is reduced. the amount of reduction is
a function of the sample concentration and the electron affinity
(capture coefficient) of the compound. The decrease in standing current
is sensed by an electrometer through the anode connection. this signal
is then amplified to a sufficient level to operate a potentiometric
strip-chart recording device.

In general, 2.0ml of sample or THM standard along with 100uL of
internal standaard is placed into a crimped vial. The vial is the heated
to 70 degrees (C) in the headspace unit for 10 minutes. The vapour of THM
is then injected into the capillary column, which is preheated to 75
degrees C. The inert nitrogen gas carries and separates the components
of the vapour through the column. The retention time of each....
(basic stuff about Partition ration deleted)


INTERFERENCES.

Chloroform in the atmosphere causes significant positive errors.
Dirty glassware...Glassware should be heated for 2 hours at 200 deg C if
this presents a problem.
Chlorinated tap water causes significant + errors. Closed distillation
systems concentrate rather than remove THM's. Therefore, Boiled, Milli-Q
water must be used
Dilution water must be checked for THM's before making standards.

Free Cl will react with organics to form additional THM's after
sampling. To prevent this, Sod.Thiosulphate is added to the bottles
prior to sampling to remove excess Cl

Water samples need to be taken in photoprotective amber bottles.
...since algal growths may alter the organic composition of samples
upon standing...


Anyway..you get the Idea..
If you want, I could type the whole article in. 
It would be easier if you told me what image viewing programs you have at
your disposal and I can scan it for you.
(for heavens sake. It's only 19 pages..I could typ it all (yeah Right))


Adrian

-- 
_______________________________________________________________________________
Bart, you think I can ever find another girl like her??
You're asking the wrong guy, Millhouse. They all look the same to me.
Newsgroups: sci.chem