purge and trap sparge vessel

[chloesabrina]

I have always used a fritted 5ml sparge vessel (running EPA8260B and 624's) with an EST encon. We are looking at trying to lower some reporting limits and will be purging larger volumes. Can anyone let me know their experience with the non-fritted sparge vessel and the difference when purging 10/25ml sample volumes.

[Yama001]

25 mL purge will get you better response, just not always 5X more.

Non-frtted vessels seem to perform just as well, with the added benefit of less chance to push surfactants into your plumbing.

[James_Ball]

When running 5ml samples I have seen no real difference between using 5ml or 25ml sparge tubes with or without the frits as far as response goes. The methods state a requirement of a water column of at least 5cm deep and less than 3mm bubbles of Helium, but in my experience any size works well.

What I have found that is also widely published is that going from 5ml to 25ml sample size will not give a 5x increase in response, but more like a 3x increase in response. Purge efficiency drops somewhat with increasing volume. To increase purge efficiency it is better to increase the sample temperature, which also has the down side of introducing more water to the trap, but with proper water management techniques that can be mostly negated.

The one thing going fritless will do is improve the purgeability of foaming samples. You will have far less foam problems with the fritless spargers, and for most of my wastewater samples I use a 25ml fritless sparger just because so many have foaming problems.

[chloesabrina]

If you use an unfritted sparge vessel, how do you get around the requirement of the 3mm bubble?

[James_Ball]

If you use an unfritted sparge vessel, how do you get around the requirement of the 3mm bubble?

What I have noticed is that the bubble is usually around 3mm diameter when it forms, but increases in size as it moves up through the water column. It could be an optical illusion but how can one accurately measure the bubble size. Method 5030 states:

4.6.1
The recommended purging chamber is designed to accept 5-mL samples with
a water column at least 3 cm deep. The gaseous headspace between the water column and
the trap must have a total volume of less than 15 mL. The purge gas must pass through the
water column as finely divided bubbles with a diameter of less than 3 mm at the origin. The purge
gas must be introduced no more than 5 mm from the base of the water column. The sample
purger, illustrated in Figure 1, meets these design criteria. Alternate sample purge devices may be used, provided equivalent or improved performance is demonstrated.

The two "must" in there seem to contradict the last bold statement to some extent. Also I have found that if you place the water inlet needle very near the gas orifice at the bottom of the sparger you will get smaller bubbles which should meet the 3mm requirement.

What really bothers me about these methods are a few things. One Method 5030 was last revised in 1996, so you have an almost 20 year old method and the equipment has improved quite a bit since then. Two, is the fact that when you talk to people who actually run the methods for a living you will find that some of the requirements like the bubble size really have no impact on your results. If you were barely passing enough gas through the sample to extract the analytes of interest then the higher efficiency of the smaller bubbles would come into play, but for most analytes 40ml/min for 10 minutes of helium through the sample in plenty to extract the analytes of interest no matter what the bubble size or column depth. Three, temperature of the sample is really not taken into account in the methods, but increasing the sample temperature to 40c will greatly improve the extraction of the more water soluble compounds like ketones, aldehydes, and alcohols.

The key to good results for these methods is reproducibility of the procedure. As long as you hold all the variables the same, you should get very reproducible and accurate results. What the original authors of most of these methods seem to not take into account is the need to run sometimes dozens of samples each day, any of which could foam and ruin a whole nights work all while the client and the regulatory agencies are demanding the lowest possible detection limits, which limits the dilutions you can perform. The writers of the methods will tell you to dilute the sample to prevent problems like foaming, then the permit writer calls you up and wants to know why you diluted the sample so that the detection limit goes above the allowable concentrations in the permit. If only scientists instead of lawyers were allowed to write methods and permits, life for the analyst would be so much easier

[chloesabrina]

I agree wholeheartedly!!! The problem is when you are dealing with certain states or agencies who don't understand the methodology. We are having our NELAC audit this year performed by an outside agency so who knows what they'll have printed on their piece of paper.

Thanks for the references though, I am going to order one so we can run ww's and gw's on the same instruments and have less downtime.

[James_Ball]

I agree wholeheartedly!!! The problem is when you are dealing with certain states or agencies who don't understand the methodology. We are having our NELAC audit this year performed by an outside agency so who knows what they'll have printed on their piece of paper.

Thanks for the references though, I am going to order one so we can run ww's and gw's on the same instruments and have less downtime.

What state will be doing your NELAC audit?

We currently go through Virginia and I believe they will be here again sometime in March, so we are getting ready also.

[chloesabrina]

We go through Florida - since we are in GA it is the nearest agency. Post your experience here if yours is in March. We normally have it in April/May, are you also using the outside audit agency?

[James_Ball]

We go through Florida - since we are in GA it is the nearest agency. Post your experience here if yours is in March. We normally have it in April/May, are you also using the outside audit agency?

Virginia sends down their auditors, not sure but I think they work for the state there.

We had Florida do an audit here once, it wasn't so fun. A former co-worker who is now in Savana just had the same Florida auditors there a month ago, he said it was the same experience there as it was here.

Most are thorough, but personality can make or break the experience.