millipore vs GFA filter papers for CAMs

[JGinniver@AOL.COM]

In the UK we have traditionally used GFA filter papers for the measurement of 

airborne activity levels.  The papers are manufactured with a smooth side and 

a rough side and until the advent of card mounted papers great care had to be 

taken to ensure that they were installed correctly on sampling equipment.  

These days with the use of card mounted GFA filters which have a corner cut 

out this problem no longer exists, and the papers should always be mounted 

with the 'smooth' surface used for the collection of airborne activity.  This 

has been the traditional method used in the UK.  However recently I was 

offered Continuous Air Monitors (CAMs) that would only accept millipore 

filter papers.  As one who does not believe that "because we have always done 

it that way" is an appropriate scientific argument for not purchasing a 

different CAM to those we currently use I wondered whether anyone had done 

any research on the use of millipore filters vs GFA.



I thought I had read in the past that there was little difference in 

performance between the two types and a significant difference in price, but 

I couldn't find the original article.  Can anyone help me on this, either 

with direct experience or references to published literature (both would be 

great).



I discussed the subject recently with a manufacturer of this type of 

equipment who felt that millipore filters would give superior resolution for 

a time after a filter change, but that after a period of operation where 

there would be a build up of dust (even small levels) on the filter paper 

this improved resolution would gradually disappear.  Where some form of 

compensation is applied to the measurement for the radon daughters, the 

manufacturer felt that it would be more difficult to achieve consistent 

results with millipore filters because of the larger change that occurred in 

the shape of the spectrum over the 'lifetime' of the filter.  The 

manufacturer felt that although the GFA filter paper would result in a lower 

overall counting efficiency than the millipore filter the spectrum changed 

less and was more 'stable' over time than that of the millipore.  However I 

also recognise that every manufacturer uses the Best method of measurement in 

their equipment, and in this case the equipment offered by the manufacturer 

used card mounted sample papers.  (I was going to write Kel Su-prise, but my 

French was good enough LOL)



Warmest regards

                   Julian 

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