Tekmar Talk Blog

VOC analysis of drinking water by Purge and Trap

Posted by Amy Nutter on Wed, Apr 03, 2024 @ 12:57 PM

VOC Analysis of Drinking H2O by PandT-1If you work in a drinking water lab, you know there can be specific pain points to drinking water analysis. Here at Teledyne LABS, we design, manufacture, and distribute products that help make this analysis easier for our customers. Five of these pain points are:

  • Reproducibility
  • Carryover and contamination
  • Sample cycle time
  • Water vapor
  • Lowering detection limits

Inconsistent internal standard or surrogate concentrations or an active site within your systems can cause issues with linearity and recovery of analytes. Drinking water methods have strict carryover requirements. Therefore, being able to clean up the system between samples and after higher level concentrations is critical.

Teledyne LABS’ Purge and Trap (P&T) systems allow for hot water rinses between samples and the ability to create an efficient bake cycle, which can really help reduce or eliminate carryover between samples. If you work in a contract laboratory, you know that the more samples you can run within a shift or an analysis batch, the more profitable you can be.

Being able to create efficient purge parameters within our TekLink software, and an efficient oven profile for the Gas Chromatograph (GC), can help you run more samples per hour. Too much water being transferred to the GC can impact sensitivity and cause analytes to co-elude and have poor peak shape.

Teledyne LABS’ P&T moisture control system, combined with optimizing dry purge, desorb, and bake settings within your method, can help reduce water vapor transferred to the GC. Whether at the federal or regional/state level, detection limits are only getting lower, making the ability to clean up the system between samples and sustain high sensitivity especially important.

Mentioned above, certain method parameters can lead to a more efficient purge and cleanup. Overall, the more efficient your purge, the better the chromatography you will see and, hopefully, the easier your analysis will be. These parameters can lead to a more efficient purge:

  • Vapor pressure
  • Solubility
  • Sample temperature
  • Sample size
  • Purge volume

Typically, the chemical nature of the compounds of interest will affect the purge efficiency, like the vapor pressure of the compounds of interest. The vapor pressure is the pressure at which a liquid and its vapor are in equilibrium at a given temperature. The higher the vapor pressure, the faster a liquid evaporates. Components that have a higher vapor pressure will have a higher purge efficiency. Vinyl Chloride and the other gases have high vapor pressures.

Solubility is the maximum amount of a substance that will dissolve in a solvent at a specified temperature. If a compound is very soluble in the sample matrix, it will be harder to remove from the matrix.

There are certain chemical parameters you can change to improve the purge efficiency. A good example would be the sample temperature. We can heat samples with a sparger heater to drive compounds from the liquid or solid phase to the gaseous phase. There is a tradeoff though: the amount of water vapor that makes it onto the trap doubles for each 10°C rise in sample purge temperature.

Sample temperature was one of those parameters in U.S. Environmental Protection Agency (EPA) drinking water methods 524.3 and 524.4 that you can change to increase the purge efficiency, but you must remember, it must work for all the compounds, and they must all pass quality control (QC) requirements. If you are interested in changing your sample temperature, you could run a simple sample temperature study and see how your compounds do at a range of different temperatures.

A reason you might want to increase the sample size would be to increase sensitivity to get more of that sample trapped and then sent to the GC. But remember, an increase in sample size requires an increase in purge volume. If the purge volume is not increased, you can expect only a 70-80% increase in sensitivity for a 100% increase in sample size. Another good reason you might want to increase sample size is for achieving those low detection limits in some drinking water methods. But if you are running EPA method 524.3 or 524.4, the 5 milliliter (mL) sample size recommended in the method will work great.

We can also control the purge gas volume, so the longer the headspace is swept away in the sparger, the more volatile are the compounds that extract from the sample. When considering purge volume, we need to look at the gas flow rate, which is the amount of time we are purging and how fast. A fast flow leads to more purge gas used in a set amount of time. An aggressive flow can cause pressure issues or push compounds of interest deep into the sorbent bed of the trap.

We also want to look at purge time. A longer purge uses more gas and if the purge is too long, it can waste gas and push compounds of interest deep into the sorbent bed of the trap too. EPA methods 524.3 and 524.4 allow you to change the purge time and volume. Therefore, completing a study to find the best parameters that work for your compounds can save you time and resources.

Lastly, another way to save resources during your drinking water analysis would be to utilize EPA method 524.4, which allows for using nitrogen as the purge gas. Nitrogen can be a suitable alternative to helium because it has similar inertness, low cost for cylinders, and the ability for generators to produce it. Nitrogen often provides more than adequate purge efficiency for compounds of interest under standard EPA parameters. When nitrogen is selected as the purge gas in the TekLink software, the mass flow controller can perform leak checks and accurately calculate flow rate throughout the entire P&T process.

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Tags: VOC, Volatile Organic Compounds, drinking water, P&T, Purge and Trap

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