PIDs are used in a number of activities ranging from soil head space analysis for VOCs, benzene monitoring in refineries and detection of hazardous compounds for homeland security. They are often used in conjunction with other gas detection techniques for monitoring sites of spills, leaks, and other emergencies.
Photoionisation detectors (PIDs) are capable detecting a vast number of hazardous gases and are utilised commonly in hand-held multigas detection instruments or handheld PIDs. The right PID may offer the combination of fast response, ease of use, maintenance, convenient size and ability to detect very low levels of hazardous compounds. However, the type of PID is often used for the wrong application; the following article explains what we should be using for soil headspace analysis, occupational hygiene monitoring and HAZMAT response.
Which Lamp do I use? 9.8, 10.6 or 11.7eV
The most common PID have a photon energy of about 9.8 eV or 10.6 eV, with lower-energy lamps providing the ability to discriminate against higher-energy compounds that may not be of interest in a particular application. If we were to have a 9.8 eV lamp that can detect benzene (IP = 9.25 eV), but not acetaldehyde (IP = 10.21 eV), in a mixed VOC application we would be able see the benzene only. To see a compound the lamp energy must be higher than that of the gas.
11.7eV lamps allow ionisation of gases with higher IPs such as chloromethane, but the ability of high-energy lamps to ionise larger numbers of compounds is offset by their short life, caused by the absorbtion of water on the lithium fluoride (LiF) windows, causing the chemically unstable crystal to degrade. The typical lifetime of these lamps varies from one to two months.
So unless you need to see something with a high IP, always go for a 9.8 or 10.6eV lamp, it should last up to three years if used properly.
Soil and Water Headspace Analysis
Soil headspace analysis is the method of sampling the air volume above a soil or water sample. The application requires PIDs that actively draw a sample, have a rapid response time, are resistant to moisture and easily cleaned.
Due to the nature of headspace analysis, you ae trying to capture a peak concentration value from a heavily moisture laden air sample. This means you should use a PID that does not utilise miniPID technology, as the ionising surface area of the lamp is reduced. Hence, the response time is much slower and it is significantly more susceptible to contamination; giving false readings and requiring maintenance much more regularly.
As such, we would recommend the use of the MiniRae Lite or MiniRae 3000, as the PID has a fast response time, is much less susceptible to contamination and is easily cleaned and maintained. However, DO NOT use any products that use miniPIDs, such as the MultiRae or ToxiRae Pro, they are not suitable for this application.
It is also important to consider durability, these will be field based instruments and as such, are more likely to experience trauma and extreme conditions. The build quality of the MiniRae puts itself leaps and bounds ahead of anything else on the market.
Occupational Hygiene Monitoring
For occupational hygiene monitoring we must consider the following to be of most importance:
Its portability and wear ability – they are not of use if they are not taken with the user
Ease of calibration and ability to hold calibration – regular calibration is essential when looking at occupational exposure as sensor drift poses a significant risk in this application
Adequate resolution in ppm and ppb – a lot of compounds are harmful at very low levels, we must be able to measure them to the right resolution bearing in mind limits of detection and error threshold of calibration gases
Cross sensitivity and stability of installed electrochemical sensors – a number of electrochemical sensors are effected by VOCs, we must consider this if we are going to be monitoring them simultaneously with VOCs
Convenience of replacing the water and dust filters – maintenance is essential. Filters hold contamination on them and if an instrument is calibrated with a dirty filter that is later replaced, we can significantly affect the zero point of the instrument
Ease and projected frequency of lamp cleaning – maintenance is more essential on these instruments as they are being used for a life critical application
The ability to data log – for occupational hygiene we will be looking at exposures to persons, hence we need a record of that exposure
Calculation of STELs and TWAs – the recording and alarming on a STEL or TWA is essential in this application
As such, for these applications we would recommend the use of the MultiRae or the ToxiRae Pro PID. Their wear ability means that they are well suited to this type of application and with the MultiRae providing ppb level detection, we can monitor very low level exposures. We would not recommend hand held monitors as these are more suited to surveying, identify contamination hot spots.
Increasing awareness of the long-term toxicity of the many VOCs has led to a rapid growth in use of PIDs for HazMat response, either as stand-alone instruments or in combination with the other sensors listed above. PIDs measure VOCs at low ppm levels that can be toxic but are not detectable by standard LEL sensors. Also, recent improvements in PID ruggedness, reliability and affordability have made them more accessible to local HazMat teams. PID’s are a valuable tool for making HazMat decisions including:
PID use in HazMat response requires consideration of a significant number of factors which are too great for a summary article. As such I would recommend reading: http://www.raesystems.com/sites/default/files/content/resources/Application-Note-203_PIDs-As-HazMat-Response-Tools_01-02.pdf
To select the right PID for your application we must assess the technology carefully for its performance, ease of calibration, maintenance frequency, lamp access, data logging, calculation of STELs and TWAs, to name but a few factors. All PID lamps will degrade unless cleaned, with the appropriate frequency depending upon the application and the design of the overall system. High energy lamps see a lot more compounds but have significant disadvantages. High concentrations of VOCs in dirty environments along with temperature and humidity changes often mean that miniPIDs are not suitable, but their application in occupational hygiene is essential. Be sure to compare wisely before choosing a PID instrument, they are not all suitable for every application.