Using 11.7eV PID Lamps To Accurately Measure VOCs With Ionization Energy Above 10.6eV

Transcription

1 Using 11.7eV PID Lamps To Accurately Measure VOCs With Ionization Energy Above 10.6eV Summary Honeywell RAE Systems strongly recommends using 100 ppm propane as calibration gas for instruments with 11.7eV lamps when the purpose is to measure VOCs with IE (ionization energy) above 10.6eV. The statement that the best policy is Calibration gas = Measurement gas is still valid and is preferred. Honeywell RAE Systems is well-established manufacturer of photoionization detector (PID) lamps with nominal values of 9.8eV, 10.6eV, 11.7eV and different sizes, including 1/4", 3/8, and 1/2. The most widely used is the 10.6eV lamp, with its long life and broad detection range. The 11.7eV analog is demanded by customers for its ability to measure VOC with high ionization energy (IE) up to 11.7eV. In this document, we discuss the main 11.7eV lamp properties and provide recommendations to better calibrate instruments so that measurements with this lamp type are much more accurate and reliable. Lamp Design Typical PID lamp design comprises a glass bulb, crystal window, and glue to attach crystal to glass. The volume inside the lamp is filled with one or more noble gases. Once a high electric field is applied to the lamp, the gas inside is ionized, creating plasma. Plasma emits photons with specific energy, defined by the gas. A crystal window is used so that photons with the desired energy pass through the window, reach the VOC, and ionize it. When an electric field is applied to the PID sensor electrodes, ions migrate to them and create an electric current registered by the instrument. Lamp design is shown in the figure below. Crystal Glue Glass Bulb The crystal used in 11.7eV lamp design is transparent to the photons with energy up to 11.7eV, which allows measuring VOCs with ionization energy up to 11.7eV. To provide photons with such high energy, the lamp is filled with the gas, which has emission spectral lines at 11.83eV and 11.62eV and relative intensity of 1000 and 500, respectively 1. Crystal Transmission Spectra As mentioned before, two main factors affect lamp efficiency: emission lines of the gas (their energy and intensity) and crystal window transmission spectra that allows photons pass through the crystal and reach VOC in the PID compartment. 1

2 Below is an example of modeled transmission spectra of crystal with around 0.5 mm thickness. Transmission, % Crystal transmission spectra vs. photon energy 120 Noble gas lines >T <T Photon Energy, ev The blue curve in the figure is transmission vs. photon energy. The two red vertical lines represent emission lines of the gas that fill the 11.7eV lamp. The higher energy line (11.83eV) coincides with the sharp cut off slope of transmission crystal spectra, and as a result its intensity is very sensitive to minor changes in the crystal s optical properties. The lower energy gas emission line is on the more shallow part of the crystal s spectra and less vulnerable to changes in the crystal s properties. Even though there is no quantitative relationship between VOC sensitivity from photon energy and IE of VOC, the rule of thumb is that the greater the difference between the lamp photon energy and IE of VOC, the more sensitive VOC in the instrument measurement. That is, its CF (correction factor) relative to calibration gas is lower. Note: Because the higher-energy emitted line of the gas inside the lamp is in the same range as the sharp slope of the transmission curve, the 11.7eV lamp output s spectra are very vulnerable to any crystal properties change. There is another intrinsic property of the crystal window that affects the crystal s performance. It is the ability of the transmission curve vs. temperature to shift the cut-off 2 position. Two arrows on the figure show the direction of the curve shift vs. temperature. Once temperature goes up (red arrow), the curve shifts left. That is, the transmission of the higherenergy emission line in the crystal may drop significantly, up to several times. If the temperature decreases, the curve shifts to the right (blue arrow), and in turn, transmission of crystal in the range of higher emitted gas line increases. Changing the intensity of the higher-energy gas emitted line passing through the crystal window relative to the lower one changes the lamp energy spectra and leads to modified VOC sensitivity and its CF relative to calibration gas. Note: This is the intrinsic property of crystal, and we can only mitigate it by calibrating the instrument at about the same ambient temperature as the conditions in the field. VOCs Correction Factor Drift Crystal properties change as the lamp matures. Most likely, changing lamp-emitted spectra leads to changes in the sensitivity of different VOCs to be ionized, depending on IE. For example, VOCs with low IE like isobutylene (IBE) are less sensitive to a change in lamp output from 11.7eV to 11.6eV. At the same time, VOCs with high IE, close to 11eV and above, may be significantly affected by reducing lamp output energy in the high-energy part of spectra. It means real CFs of VOCs with high IE change more significantly as an 11.7eV lamp matures relative to calibration gas with low IE like IBE. The greatest change takes place at the beginning of new a lamp s use and then gradually continues to change. The figure below includes test data from three different batches of 11.7eV lamps. Batch 1 and 2 were made at the same time using the same technological process. Batch 3 is from the newest adjusted and locked-in process of manufacturing. 2

3 Lamps were tested in MiniRAE 3000 instruments for their response to three gases: 100 ppm IBE, 100 ppm propane, and 10 ppm chlorine. Records were done in raw counts in the instruments diagnostic mode. Measurements were performed with fresh lamps, after lamps sat for 5 days on the shelf, and after 8 hours in which the lamps were burning. Raw-count data were converted to CF of Propane vs. IBE (left column), Chlorine vs. IBE (middle column), and Chlorine vs. Propane (right column). Lamp Batch Test CF Pr/IBE CF Cl2/IBE CF Cl2/Pr Batch 1 Initial data days on shelf hours burning Batch 2 Initial data days on shelf hours burning Batch 3 Initial data days on shelf hours burning As shown in the table, CFs of propane and chlorine vs. IBE significantly increase during time sitting on the shelf and in the first 8 hours burning. At the same time, CF of chlorine vs. propane stays constant (batches 1 and 2) or change just slightly (batch 3). Does the CF for chlorine vs. propane change as the lamp is used? The answer to this question is given in the figure below eV lamp, VOCs CFs change CFs CFPr/ibe CFCl2/ibe CFCl2/Pr Time, Hrs Batch 3 was tested in a MiniRAE 3000 for over 300 hours, which is equivalent to more than 30 days of lamp operation running for 8 hours per day and 7 days a week. (That is, the time exceeds lamp warranty life.) Regardless, CFs of propane and Cl 2 vs. IBE changed significantly, while the CF of Cl 2 vs. propane is nearly constant (see table below). Time, hours in test CF Pr/IBE CF Cl 2/IBE CF Cl 2/IBE

4 In addition, comparison measurements of the effect of maturing 11.7eV lamps on CFs were done for the most commonly used VOCs (see table below). Column 2 presents the data from Honeywell RAE Systems Technical Note TN-106 for the VOC CFs developed with new 11.7eV lamps, and column 3 comprises data for newly developed CFs with mature 11.7eV lamps. In both columns 2 and 3, VOCs were calibrated with IBE. Column 4 shows CF changes from mature lamps. The change is significant and varies by factors of 3.6 to 9 times, depending on the VOC. It shows that if the IE of a VOC is below 10.6eV, this change is less drastic (for example, ammonia and ethylene oxide in the table), but it is still noticeable. So, there are no benefits to using an 11.7eV lamp to measure VOCs with IE below 10.6eV. The 10.6eV lamp allows higher accuracy and more consistency. The situation with CFs variability using an 11.7eV lamp looks better when using CFs relative to propane (that is, using propane as a calibration gas). Data in column 7 of the table show that CFs vary, depending on the VOC in the range of 0.7 to 1.8. That is much narrower than if the same VOCs were calibrated to IBE and CFs were used, relative to IBE. To obtain these data, CFs from TN-106 were recalculated using the formula explained below the Table and placed in column 5. Data in column 6 are newly developed CFs of the VOCs most often used together with an 11.7eV lamp and calibrated to 100 ppm propane. So, keeping in mind everything discussed above, if VOCs with a high IE are measured with an 11.7eV lamp calibrated to propane, the existing CFs in TN-106 still could be used, if requirements for the measurement accuracies aren t high. CFs from TN-106 could simply be recalculated to propane as calibration gas, using the formula below the table. The recalculated CFs can be used in the instruments as CFs for the Custom Gases. This option exists in the instrument menu and is described in the instrument manual. For the accurate measurements, CFs developed for the most-often used VOCs in combination with 11.7eV lamps calibrated to propane can be taken from column 6 of the table below. They also can be saved in the instrument as a Custom Gas option. Accurate customer CFs can be developed on request and per the settled procedure in TN Note: Based on the data above, Honeywell RAE Systems strongly recommends using 100 ppm propane as calibration gas for instruments with 11.7eV lamps when the purpose is to measure VOCs with IE above 10.6eV. The statement that the best policy is Calibration gas = Measurement gas is still valid and is preferred. Ionization Energy, ev TN-106 New Lamp CFs to IBE Mature Mature vs. Lamp New lamp TN-106* New lamp CFs to Propane Mature Mature vs. Lamp New lamp Gas Chlorine Propane Methanol Formaldehyde ,1-Dichlorethane ,1,2-11 Trichloroethane Chloroform Ammonia Ethylene oxide

5 * CFs of VOCs relative to propane (column 5 of the table) were calculated based on the CFs of the VOCs relative to isobutylene from TN-106 data, using the formula CF Pr = CF 1 IBE /CF 2 IBE, where CF 1 is the CF of VOC relative to isobutylene and CF 2 is CF of propane relative to isobutylene. All CFs in TN-106 were developed for the freshly made 11.7eV lamps and placed into the column 2 of the table. Column 3 has newly measured CF data for mature 11.7eV lamps for the same VOCs calibrated to IBE. Columns 4 and 7 shows CFs changing of VOCs, measured for new and mature 11.7eV lamps. Column 4 comprises data for the VOCs calibrated with 100 ppm IBE (isobutylene). Column 7 comprises data for the VOCs calibrated with 100 ppm propane. Column 6 includes newly developed CFs of the VOCs calibrated with 100 ppm propane and most often used together with mature 11.7eV lamps. Lamp Maturing Behavior of 11.7eV lamp vs. time was monitored with the lamps from batch 3, the current manufacturing process. Data are plotted in the figure below. Raw counts eV lamp sensitivity drop vs. time Exposure gases concentrations normalized IBE Pr Cl Hrs in test The test was running for over 500 hours in MiniRAE 3000 instruments. This corresponds with approximately 2 months usage, 8 hours per day, running continuously. Lamps show the typical behavior when intensity of the lamp output drops significantly during the burning period and the first hours of work, and then stabilizes at some level while lamps continue running. The end of a lamp s usable life is considered when the lamp cannot be turned on or it fails calibration. Lamp Maintenance For proper lamp function, regular maintenance such as periodic lamp cleaning with IPA (isopropyl alcohol) included in the RAE Systems PID cleaning kit (p/n ) together with other actions, is a good practice. Typical procedures for maintenance are outlined in the instrument manuals and in Technical Note TN A special precaution for 11.7eV lamps is avoiding high humidity or keeping the instruments in such conditions as little as possible. Do not use acetone for lamp cleaning. Never remove depositions from the lamp with just mechanical rubbing, which created scratches on the crystal surfaces and decreases lamp efficiency and can even permanently damage the lamp. Keeping in mind the rather short life of 11.7eV lamps, turn off instruments when they are not in use. And, always store instruments in a dry area. 5

6 References 1 The PID Handbook, p.7, Honeywell RAE Systems Factors-for-Volatile-Compounds-with-RAE-Systems-Instruments_12-13.pdf 4 Moisture-Sensitivity_08-15.pdf 6