Challenge of Zero Head Space in VOC
As an ongoing challenge, the environmental/analytical industry struggles to sample water (waste water/drinking water/ground water/surface water) for Volatile Organic Analysis (VOA) free of head space, or more commonly, air bubbles. The theory is that, after sample collection, any head space/air bubbles in the sample vial (standard 40 mL glass vial/septum closure) allows space for off gassing of some volatile analytes. Thus, the testing of the water would be unrepresentative of the sample site. The acceptance criteria for headspace is widely accepted as bubbles not exceeding five to six millimeters in diameter (fn1) (~0.1mL).
The challenge falls on two parties: the field sampler, who takes the actual sample for analysis, and the laboratory bottle prep staff, who has to provide trip blanks for sample kits. Trip blanks are VOC vials filled with lab reagent water and travel with the sample kits as a control to monitor contaminate exposure during transit and storage and not from the sample source. The first step to ensure zero headspace is to take a full sample or full trip blank. A full positive meniscus of water must occur before the vial is capped. The capping must be done with caution so as to not spill the micro amount of water that creates the positive meniscus. The vial cap should be turned to a full tightness, such that the silicone septum in the open-top closure pushes upwards, creating a slight dome. Overtightening can create bubbled septa that can compromise septum integrity and cause instrument rejection by some auto-samplers. An undertightened cap, where the septa lies flat in the closure, has a high likelihood of forming an air bubble during storage. Samples with high carbonate, dissolved gasses or effervescing water should collected in unpreserved vials to reduce gas formation. A good technique to determine if your sample is free of headspace is to turn the vial upside down and tap it to see if an air bubble rises (air bubbles are difficult to detect when the vial is upright — they would be hidden behind the closure).
VOC water samples are to be preserved at a temperature of 6 C without freezing by method. Temperature has a substantial influence on air bubble formation. Whether from field to cooler, cooler to sample control or sample control to refrigeration, a change in temperature can result in headspace. The open top cap with the silicone septa is a moving closure system. The septum rises and lowers with temperature change that produces a water volume change. This up and down motion stresses the seal created by silicone rubber, allowing air bubbles to develop if there is insufficient compression of the septum onto the top of the glass vial. A similar stress can happen with coolers shipped via air. Low-pressure storage compartments on airlines can also influence headspace. Time is also a big factor — the longer the time between sampling and analysis, the higher risk of temperature and pressure fluctuations. EPA methods require analysis of VOA samples within seven days if unpreserved and 14 days if preserved — typically with hydrochloric acid. ESS (San Leandro, California) conducted an internal trip blank study in 2018. Seventy-two EPA vials were filled to zero headspace with medium cap tightness. After two weeks in refrigeration, only one vial had an air bubble of greater than six millimeters. Those vials were removed from refrigeration and stored at room temperature. Within 24 hours, 12 vials had developed macro bubbles (greater the six millimeters). After 48 hours, 31 had macro bubbles. The time component is more problematic for the trip blanks that are prepared before the sample. If not closely tracked, the time spread can be weeks. Commercial environmental testing labs routinely give out sample kits to their clients as part of the analytical service. These may sit with environmental consultants for a long time before environmental samples are collected.
The standard EPA vial consists of three components: the polypropylene open top closure, the septa (Teflon bonded to silicone) and the glass vial. The closure is made from a steel mold. The septum is die cut from sheet material with a steel die. The glass vials are made from glass tubing that has the bottom and threads machined by a fine flame. Between the cap mold, die cut septa and machined vials, the mold has the smallest plus or minus tolerances, followed by septum die and the machined glass vial. Subtle dips that may occur in the lip of the vial should be off-set by the compression of the silicone rubber. Cap thickness may vary by vendor — heavy- versus thin-walled. There are several variations of septa used in the market: variances of thickness of silicone and Teflon, silicone hardness (durometer) and construction (bonded versus press fit). Historically, press fit caps (septa inserted into the closure held by friction) were the standard VOA seal. In recent years, closure manufacturers introduced bonded septum as an alternative. The advantage of machine bonding (thermal or other bonding technology) is the labor savings from assembly. Also, liners would not fall out of the caps that otherwise might jeopardize the integrity of the container. A problem that occurs with bonded septum results from the diameter not being wide enough. To mechanically bond septa to a closure, they need to be cut narrower than a press fit septa. This prevents the part from catching on the cap threads during assembly. To make the best seal, silicone needs to compress both on the inside top of the cap and sides. Bonded caps relinquish some of this compression. While the level of water, cap tightness and temperature consistency are the most dominant factors, narrower diameter septa can also contribute to air bubble formation.
Proper attention to sampling technique, temperature level and consistence and the highest quality vial components will minimize headspace formation in VOC sampling.
Matt Macy — General Manager, Environmental Sampling Supply (ESS), San Leandro, California
(fn1) SW-846, Method 5030B, 1996
Teflon is a registered trade mark of DuPont