Frank Thibodeau, Bruker Detection, Billerica, MA USA Franziska Lange, Bruker Detection, Leipzig, Germany INTRODUCTION Ion Mobility Spectrometry (IMS) is the scientific technique of measuring the drift time of ions across a uniform electric field. It is based on the principle that when different charged molecules (ions) are accelerated through an electric field (with a "drift flow" of gas slightly opposing them) they will travel at different speeds and reach the detector at different times. The advantages of IMS instruments include, relatively small footprint, fairly inexpensive, rugged, extremely fast and extremely sensitive. The purpose of this investigation was to evaluate the effectiveness of IMS technology for detection of TDI in foam plants. IMS technology has been assessed in numerous foam plants under field conditions. TDI was monitored in foam plants which produced automotive, molded and slab products. A handheld IMS instrument surveyed in and around production lines, hot and cold foam, storage tanks, transfer lines, and transfer operations. BACKGROUND All site visits were conducted with the Bruker handheld RAID-M IMS instrument. The IMS instrument was programmed with a TDI library fixed to detect and alarm at or below 1 ppb. Spectra were also collected at various locations at all foam plants for more detailed post analysis. Eight foam plants were visited in the US and Canada and two chemical terminal transfer sites which utilized TDI. Additionally, one plant with installed units for more than 20 months provided feedback. The observations, data and post spectral analyses are included the findings. The RAID-M had the highly sensitive TDI library in Library A of the instrument as shown in Table 1. Page 1 Table 1: Highly sensitive TDI library in ppb. The library above shows TDI readings in ppb, the current instrument version displays on the instrument interface in ppb readings, and records data in ppb.
BASICS OF ION MOBILITY IMS is based on the principle that when charged molecules (ions) are accelerated through an electric field (with a "drift flow" of gas slightly opposing them) they will travel at different speeds (due to different size and shape) and reach a detector at different times. All molecules are drawn through the drift region of the detector by the electric field, but larger ions with bigger cross sectional areas will interact with the drift gas longer thus arriving at the detector later than smaller ions. The analysis is carried out under atmospheric pressure which eliminates the need for large, heavy, power hunger pumps to generate a vacuum as is required for a mass spectrometer. As a result, handheld, portable ion mobility instruments are in use in many places, where they are used to detect chemical weapons, conventional weapons, drugs and industrial gases. IMS is an ionization-based time-of-flight technique performed at atmospheric pressure. The heart of IMS is the time-of-flight tube. Time-of-flight tubes come in different sizes and measure ions in single or dual polarity. Generally speaking, the longer the time-of-flight tube, the better the resolution or ability to differentiate between compounds. Other key components that comprise an IMS instrument are described in the following pages. MEMBRANE A continuous ambient air sample is drawn over a semi-permeable membrane. The membrane serves to protect the interior of the cell from particles and moisture, provide a degree of contaminant selectivity, and allow various levels of sensitivity based on ambient contamination conditions. It also enhances the analytical integrity of the instrument by controlling contamination and helping to maintain humidity at desired levels. REACTION CHAMBER The molecules of interest permeate through the membrane, and are transferred to the reaction region. There, the sample is ionized by low-level beta energy emitted by a sealed nickel-63 radiation source or other method such as high energy photoionization. Figure 1 describes the reaction region of an IMS. The sealed nickel-63 source is most commonly used in IMS because it is inexpensive and reliable. The radioactivity is extremely low and is often below threshold values in many countries for licensing and monitoring. Bruker IMS instruments are exempt by the Nuclear Regulatory Commission for licensing purposes and therefore do not require record keeping, controlled radiation areas or wipe testing. Page 2
Figure 1. IMS Reaction Region The ions are moved by an electric field to the exit of the reaction chamber. At this point, an electric voltage on a grid prevents the ions from entering into the drift tube. Figure 2. Time of Flight & the Drift Tube By reversing this voltage momentarily, ions are injected into the drift tube for a brief moment and attracted to the faraday plate detector with opposite polarity. The resultant detection parameters are than evaluated as the below figure demonstrates. Figure 2 illustrates the movement of ions in the electric field. Page 3
Figure 3. Ion Mobility Spectra with Monomer & Dimer Peak The drift times (in milliseconds) of the ions are corrected to compensate for varying temperature and pressure conditions during analysis. The resultant reduced ion mobility constant K o (in cm² V -1 s -1 ) for each ion along with peak intensity (in Pico amps) and peak area correspond to particular compounds in an IMS library. Figure 3 is a representation of the data generated from the detector in Bruker s proprietary analysis software. Some molecules form a monomer during ionization while others form dimers or even adducts. The formation of monomer or dimer peak systems is compound and concentration dependent. Presence of a monomer with dimer peak series in the positive mode provides confirmation and more confident identification of a chemical compound than the single monomer peak for that compound. With ion mobility, size and shape of the ionized compounds are important in characterization of species for not only single, but multiple compound detections. In some cases, chemical modifiers called dopants are intentionally added to the ionization region to react with the sample. Molecules from the sample source, any water vapor present, and dopant molecules may combine or react when compounds are introduced into the ionization region. The IMS technology uses different reactants ions in each polarity. These form what is called a reactant ion peak which is usually water vapor in the negative mode and a dopant like ammonia in the positive mode. The interaction of the reactant ions with the sampled air form new compounds (product-ions) measured in the time-of-flight tube. All information is vital to correct identification of the intended target species. Library development of course is the key to detecting any substance required. Known samples of analyte are introduced to an IMS instrument and the response observed. If the reactant ion peak diminishes in direct correlation to the introduced analyte peak, then the chemical of interest is detected with confidence. The chemical is then introduced at small concentrations intervals to determine the lowest limit of detection while observing the spectra. From this data, the library is then calibrated for this lone agent. With the upper and lower limits determined temperature and humidity differences are introduced with corresponding spectral reaction observed again. Common or suspected interferent Page 4
compounds could then be introduced to minimize masking effects. The resultant individual compound library is used by itself or added to another library. FINDINGS & OBSERVATIONS The beginning of any site survey started with a proof of concept usually in a lab testing the handheld IMS instrument (RAID-M manufactured by Bruker) with a TDI sample. The response was immediate with a reading from 4-16 ppb. Having established confidence that the RAID-M could detect TDI at very low levels, the site survey then commenced on the floor of the factory. Storage Storage tanks, transfer lines, valves, and joints were always surveyed with the handheld to observe any response. The instrument was held for seconds or laid underneath for minutes with no response for TDI in any plant or facility. This shows sound safety practices and monitoring by PFA member plants, but does not diminish the need for constant monitoring wherever TDI is stored or transferred as valves, hoses and fittings can wear out over time. Figure 4 is an example of the storage tank monitoring setup for Bruker s RAID-M. Figure 4. TDI Storage Tank Monitoring with IMS Page 5
Smaller amounts of TDI were also stored in drums with either pure TDI or mixed with MDI. Capped drums were surveyed on several occasions with only one response noted with an empty drum. The RAID-M alarmed after five minutes. This only occurred on one occasion. Sniffing with the cap removed on full drums of MDI/TDI mixture with 70%/30% ratio resulted in alarms in the range of 1-8 ppb. Half full or less resulted in no alarms after 1-2 minutes of exposure. Production Lines-Slab Slab foam production lines generally have barriers or side panels and may or may not be vented from the top. The reactor at the head of course has hot TDI and or MDI mixed with other chemicals as part the foam mixture. The hot TDI gases rendered a good signal or instrument response at the beginning of the line and decreased toward the end of the line as the foam formed and cooled. This produced a linear response as high as 12 ppb down to 1 ppb of TDI. Cutting a slab, depending on the temperature of the foam at the time, generally elicited a response of 1-4 ppb. An example of the instrument datalog results as shown below from a slab line foam sample are shown in Table 2. Date: 2017-10-17 ; Instrument RM_02708 No. ; Time Agent; Conc Bars; RI+; RI-; PBars; lib;state 375; 10:17:22 ; T;TDI; 6;ppb ; 4 ; 28%; 23%; 2; 2 ; A ;WORK 376; 10:17:25 ; T;TDI; 6;ppb ; 4 ; 29%; 22%; 2; 2 ; A ;WORK 377; 10:17:28 ; T;TDI; 6;ppb ; 4 ; 30%; 22%; 2; 2 ; A ;WORK 378; 10:17:32 ; T;TDI; 5;ppb ; 4 ; 26%; 23%; 3; 2 ; A ;WORK 379; 10:17:35 ; T;TDI; 5;ppb ; 4 ; 30%; 27%; 2; 2 ; A ;WORK 380; 10:17:38 ; T;TDI; 4;ppb ; 4 ; 28%; 26%; 2; 2 ; A ;WORK 381; 10:17:41 ; T;TDI; 4;ppb ; 4 ; 34%; 28%; 2; 2 ; A ;WORK 382; 10:17:44 ; T;TDI; 4;ppb ; 3 ; 33%; 29%; 2; 2 ; A ;WORK 383; 10:17:47 ; T;TDI; 4;ppb ; 3 ; 32%; 29%; 2; 2 ; A ;WORK 384; 10:17:50 ; T;TDI; 4;ppb ; 3 ; 32%; 30%; 2; 1 ; A ;WORK 385; 10:17:53 ; T;TDI; 3;ppb ; 3 ; 33%; 31%; 2; 1 ; A ;WORK Page 6
386; 10:17:56 ; T;TDI; 3;ppb ; 3 ; 33%; 31%; 2; 1 ; A ;WORK 387; 10:18:00 ; T;TDI; 3;ppb ; 3 ; 34%; 31%; 2; 1 ; A ;WORK 388; 10:18:03 ; T;TDI; 3;ppb ; 3 ; 34%; 31%; 2; 1 ; A ;WORK 389; 10:18:06 ; T;TDI; 3;ppb ; 3 ; 31%; 31%; 2; 1 ; A ;WORK 390; 10:18:09 ; T;TDI; 3;ppb ; 3 ; 32%; 29%; 2; 2 ; A ;WORK 391; 10:18:12 ; T;TDI; 3;ppb ; 3 ; 32%; 32%; 2; 1 ; A ;WORK 392; 10:18:15 ; T;TDI; 3;ppb ; 3 ; 32%; 33%; 2; 1 ; A ;WORK 393; 10:18:18 ; T;TDI; 3;ppb ; 3 ; 32%; 32%; 2; 1 ; A ;WORK 394; 10:18:21 ; T;TDI; 3;ppb ; 3 ; 32%; 32%; 2; 1 ; A ;WORK 395; 10:18:25 ; T;TDI; 2;ppb ; 3 ; 32%; 33%; 2; 1 ; A ;WORK 396; 10:18:28 ; T;TDI; 2;ppb ; 3 ; 33%; 32%; 2; 1 ; A ;WORK 397; 10:18:31 ; T;TDI; 2;ppb ; 3 ; 33%; 34%; 2; 1 ; A ;WORK 398; 10:18:34 ; T;TDI; 2;ppb ; 3 ; 33%; 34%; 2; 1 ; A ;WORK 399; 10:18:37 ; T;TDI; 2;ppb ; 3 ; 33%; 34%; 2; 1 ; A ;WORK 400; 10:18:40 ; T;TDI; 2;ppb ; 3 ; 33%; 35%; 2; 1 ; A ;WORK Table 2: Data log file of Slab Production Line The initial site visits at facilities used a library with HDI included. The were no actual sites visited that used HDI and false alarms did occur with HDI due to similar chemical vapors in the background. Because of this, the TDI only library was used in all subsequent visits. Of course, background spectral readings were taken and analyzed at all sites. The false positives here were a result of taking risks to provide an extremely sensitive library and detection capability. Adjustments in many cases have to be made and libraries customized to meet specific site requirements. In another example, while surveying the entire plant even outside TDI generating areas, the usual response from the handheld IMS unit was no response to any TDI present. In one exception, TDI at was at 1-4 ppb was observed in a general area where none was generated. This was a consistent signal throughout the day. The production of slab foam with TDI occurred behind a wall in another section of the plant. That section was vented from the ceiling when production was underway with readings up to 12 ppb in that area. Therefore the readings in the fabrication area of 1-4 ppb were either false readings or somehow after the production terminated and the fans were turned off, the TDI sank and then Page 7
migrated to the non -production area through outer doors. analysis is the only method to determine the ground truth. An air sample and subsequent GC/MS Spectral analysis of the fabrication area reveals many peaks surrounding the location where TDI would be detected. Figure 5 shows an example of one spectrum from the fabrication area. Figure 5: Spectra analysis of Fabrication Area The left spectral capture shows a normal TDI response with well-defined peak of TDI at 1.46. The right spectral capture shows many close peaks surrounding the 1.46 location. This would cause interference and therefore a likely false reading. The library used was optimized for TDI sensitivity. A higher threshold level would not have caused the response in the detector. Molded Foam Molded foam production plants have some similarities to slab or rigid foam production in the methodology of using TDI. The storage and transfer operations are much the same however, the production lines tend to be much more self-contained. Generally the TDI is contained within the production line itself. TDI at 1-4 ppb range can be detected in vented breaks in the metal housing around the line. In one case, TDI was even observed at the top of the production enclosure. Along the production line itself, sampling with the IMS handheld responded with 1-8 ppb inside the moving line during production. Almost nothing was observed during non-production times. The hot Page 8
foam exposed when the mold opened did emit TDI. The handheld IMS was positioned from inch to two feet from the foam within the production enclosure. Sometimes TDI could be detected outside the enclosure, but seldom. It usually had to be within 2 feet of the hot foam. As the foam cooled over 5-10 minutes the signal would diminish to zero. Foam at ambient temperature produced no signal or response from the IMS instrument. Hot foam produced a TDI alarm at every site but cured, cooled foam produced no response. While collecting background spectra, it was observed that there was a very strong signal in the positive mode of a chemical vapor not in the library. Figure 6 is a spectrum containing the unknown chemical in positive mode. It reacted extremely well with the positive reactant ion peak which lends itself to excellent detection capability. There are theories as to what the chemical could be, but without an air sample analyzed by GC/MS, there is not definitive way of knowing. Once this occurs and a sample provided to the lab, a library can be calibrated to include it in any IMS instrument for the customer application. Page 9 Figure 6: Spectral Capture Positive Mode Unknown This chemical does permeate the plant and is common to several flexible foam plants, but very prominent in one. It may or may not be harmful. It does however completely dominate detection in the positive mode without affecting negative mode detection of TDI. Wall Mounted vs Handheld IMS Instruments Handheld units are certainly needed for conducting plant surveys, leak detection, monitoring transfer operations, defining limits of any contaminated zone, and monitoring the thoroughness of
decontamination with its speed, sensitivity and reliability. Wall mounted IMS would be used at critical points to continuously monitor for any fugitive TDI emissions. How can they be used in tandem? In one installed base, the wall mounted IMS positioned for 24/7 monitoring near an outside door detected TDI while transfer operations from a delivery truck were ongoing. The plant personnel responded with a handheld IMS for verification and detected TDI leaking from a transfer hose. It was discovered that the delivery company was using low quality hoses. The company was directed to use the proper hoses in the future thus preventing a more serious situation. Experimental During the last site visit, we decided to experiment with a new software upgrade to the RAID-M handheld. Samples were collected with a RAID-M with upgraded software and with a standard RAID-M then the results were compared in real time to the industry standard colorimetric instrument. A flexible foam production line with large buns off a long conveyor provided ample TDI emissions at levels below 20 ppb maximum. All three instruments were detecting TDI in tandem. Figure 7 is a picture of all three instruments alarming with roughly the same detection levels. Figure 7: RAID-M standard read bar readout; RAID-M w/new prototype ppb readout; Colorimetric reading The first RAID-M reads 3 bars or 2.1 to 4.1 ppb TDI. The second RAID-M reads directly 1-8 ppb, the colorimetric instrument reads 2.5 ppb. The instruments are certainly in agreement given standard margin of error. Page 10
The two RAID-M IMS units were almost perfectly in agreement, took only seconds to respond and alarm, and display in either bars or ppb readings. The colorimetric instrument took 2-3 minutes to alarm and in many cases did not detect any TDI while both IMS instruments alarmed, cleared down, and then reengaged simultaneously. The speed of the IMS allowed operators to ascertain the real location of TDI emissions. CONCLUSIONS 1. IMS instruments provide fast, reliable, accurate, and very sensitive TDI detection in foam plants. 2. TDI can be detected with IMS in hot foam until cooled some 5-10 minutes after forming. 3. TDI can be detected by IMS some 2 ft from a hot source depending on concentration. 4. Slab production lines cause dispersals of TDI within all open areas around the conveyor line at levels observed below 8 ppb. 5. The speed and accuracy of IMS instruments provides actionable information to protect the health and safety of plant workers with minimal resources which positively affects the bottom line. RECOMMENDATIONS 1. Consider IMS technology for real time, actionable monitoring for TDI emissions 2. Continue development of customized IMS libraries to meet industry needs. 3. Provide readouts in ppb for all IMS instruments to include in the datalogger. 4. Develop external display for wall mounted IMS units. Page 11