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
