Dean, F.W.H., Stockdale, M. and Witty, A., Ion Science Ltd, UK(free)
In this paper we describe volatile organic compound (VOC) detection by a submersible photoionisation detection (PID ) sensor, which delivered broad range sensitivity to most VOCs in air. The sensing components comprised a water repellent membrane overlaying a shallow photoionisation cavity, which included an electrodic means of adventitious water elimination therefrom.
The response of the sensor to various organics in potable water was compared with what might be predicted from equilibration of gas in the PID cavity and surrounding water according to Henry’s Law. Compounds such as alcohols for which the Henry’s Law constant kH is high, responded according to Henry’s Law, whereas low miscibility compounds were sensed at only a fraction of the expected equilibrated concentration.
The total, background response of the sensor to chlorinated tap water was typically equivalent to ~5-10 μg /L benzene, and was sufficiently invariant to enable the screening of water containing less than this concentration of benzene. It is thought that the background may be due in part to chloramines.
Keywords: benzene screening, PID pollution sensor, VOC volatile water
Photoionisation detection (PID) was invented in the 1960’s (Lovelock 1963 and Lovelock et al.,1982), specifically to sense volatiles eluted in gas chromatography. Since then it has evolved into an increasingly field worthy technique, by virtue of more sensitive and temperature stable amplifiers, more resilient components, more stable electrical and electrochemical configurations, condensation resistance and miniaturisation (Stockdale and Eagling 2010, Dean and Stockdale 2006). We have modified a state of the art commercial PID air sensor to enable extended immersion in water, with particular attention to removal of any condensate generated underwater by means of a patented water electrolysing electrode.
The outstanding attribute of PID used in field environments is its rapid and broad range sensitivity to most volatiles. Remarkably, clean fresh air remote from sources of pollution commonly delivers a response equivalent to one or two hundred parts per billion (ppb v/v) equivalent of a typical VOC. Thus PID easily finds use as a ‘clean air’ detector in personnel safety, process safety and integrity and various screening applications.
Figure 1 illustrates the sensing mechanism in PID . (1), analytes passively diffuse through a porous membrane into a shallow cavity, where (2), they are exposed to radiation emitted from a PID high UV lamp window opposing the porous membrane. The radiation breaks up susceptible VOC molecules in the cavity into charged molecular fragments, known as photo-ions (3a). The electric current arising from ion collection at the cathode and anode in the PID cavity (3b) is amplified measured as a photoionisiation current.
Other prospective sources of current arising in the cell, such as photoelectrons and electrolytic current flowing across the PID walls, are collected by an intervening third electrode. This also electrolyses incipient water condensation breaching the two outer electrodes, so that substantially, only a photocurrent flows and is measured, between the cathode and anode, from which the VOC concentration is calculated.
The most common PID UV source is a low pressure krypton electrodeless discharge lamp (‘PID Kr’), which releases ionising photons at 10.0 and 10.6 eV. According to their ionisation threshold energies, about 90% of common industrial VOC’s are photoionised by the 10.6 eV photons, and thus sensed. Notable is the high sensitivity to aromatics and unsaturated compounds; resolution below 1 part per billion (ppb) in a stable ambient atmosphere is achievable. Most single carbon compounds and saturated CFC’s, as well as acetylene, ethane and propane are not sensed by PID Kr.
The relative gaseous responsivities of PID Kr to specific VOC analytes are known to be invariant, and quoted as ‘sensitivity factors’ Sanalyte, relative to responsivity Rcal to a calibration gas at 298 K, enabling the gaseous concentration p of hundreds of analytes S to be determined from a PID response R on the basis of a single calibration:
p = S R / Rcal (i)
At low concentrations, a VOC chemically in equilibrium between water and a gaseous headspace establishes an equilibrium vapour pressure p (atm) according to Henry’s Law:
p = kH,cp ѳ exp[C(1/T – 1/Tѳ)] caq (ii)
where kHѳ (mol/(L.atm)) is the Henry’s Law constant at standard temperature (25 oC), T (K, oC +273.15) is the ambient temperature, C is the Henry’s Law temperature coefficient, and caq (mol/L) is the aqueous concentration of the volatile.
The PID probe in water is shown schematically in Figure 2. Water ingress into the PID containing cradle was prevented by means of an intervening hydrophobic membrane. Since volatile analyte consumption by a PID sensor is slow, and the PID cavity was very shallow, the PID in water sensor is anticipated to deliver rapid and substantially complete equilibration of a volatile between the gas sensing cavity and surrounding aqueous solution. This prospectively enables a PID in water sensor to be used to estimate the concentration of a specified VOC analyte in water on the basis of a single calibration. Combining (i) and (ii):
caq, measured = (1 / kH,cpѳ ) exp[C(1/Tѳ – 1/T)] SR / Rcal (iii)
In the discussion below we refer to percentage equilibration Ѳ, which relates the concentration of caq calculated from the sensor response and (iii) to the actual concentration:
Ѳ = caq, measured / caq (iv)
The variation of Ѳ with analyte, flow rate, temperature and aqueous concentration are all discussed below.
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