Positive temperature coefficient compensating heating for analytical

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Positive temperature coefficient compensating heating for analytical devices Ronda Gras, Jim Luong, Matthias Pursch, and Robert A. Shellie Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b01229 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 12, 2018

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Analytical Chemistry

Positive temperature coefficient compensating heating for analytical devices Ronda Gras1,2, Jim Luong1,3, Matthias Pursch4, Robert A Shellie*2,5,6 1 Dow Chemical Canada ULC, Highway 15, Fort Saskatchewan, Alberta, T8L 2P4, Canada. 2 ARC Training Centre for Portable Analytical Separation Technologies (ASTech), University of Tasmania, Private Bag 75 Hobart 7001 Australia 3 Australian Centre for Research on Separation Science (ACROSS), University of Tasmania, Private Bag 75 Hobart 7001 Australia 4 Dow Deutschland Anlagen GmbH, Analytical Sciences, 21677 Stade Germany 5 Trajan Scientific and Medical, 7 Argent Place, Ringwood 3154 Australia 6 School of Science, RMIT University, Private Bag 2476V Melbourne 3001 Australia ABSTRACT: Positive temperature coefficient thermistors acting as heating devices are quickly growing in popularity and are being adapted into critical applications in many sectors from medical to space discovery. Positive temperature coefficient heating offers substantial benefits for miniaturized and portable analytical devices in key aspects such as energy efficiency, safety in overheating, size, scalability, and in discovering new thermal management strategies. These heaters can reach 230 oC without additional requirements for regulating electronics. By incorporating positive temperature coefficient technology into a commercial diode array photometric detector, the detector is made suitable for coupling with gas chromatography. The detector cartridge flow cell is heated to a specific target temperature within the range of 70 oC to 150 oC without impacting the detector’s construction material or imparting any negative effect to the surrounding detector system electronics. Applying a temperature of 150 oC to the cell permits analysis of volatile and semi-volatile compounds with a boiling point equivalent to that of n-hexadecene (285 oC). Model compounds of alkene homologues from C8 to C16 showed a maximum peak asymmetry of 1.10 with exception of octene where peak symmetry was found to be no more than 1.28 with the heated cell design. A high degree of repeatability was observed with RSD of less than 0.01% in retention time and 3% in peak area (n = 10).

Positive Temperature Coefficient (PTC) heaters utilize unique semiconductors which are sensitive to temperature change. PTC semiconductors are polycrystalline ceramic with (typically) barium titanate and appropriate dopants to enhance their performance [1-5]. Barium titanate forms five different crystalline structures across various temperatures. All phases are ferroelectric except the cubic structure. In this state, the resistance increases sharply in the thermistors resulting in selfregulating temperature behavior. This is known as the transition, switching, or Curie temperature. The self-regulating property makes a PTC heating element a safe alternative for situations in which overheating could be dangerous or costly. A self-regulating heater automatically adjusts the power produced without sensors and/or electronics. When the temperature increases the power produced is automatically reduced, and vice-versa. Not only does the system self-regulate, it manages power input to match the specific heat output requested, all without requiring a secondary limiting system. PTC heaters typically keep a constant temperature within a reasonable tolerance, regardless of the ambient conditions, providing the heat loss is not excessive. PTC heaters reduce the power when the temperature increases and switch off the heat production above a certain temperature. In this

way PTC self-regulating heaters cannot overheat; they are self-limiting, and they do not require overheat protection [6]. PTC heaters have broad applications in critical medical devices [7], as well as domestic appliances [8-9]. PTC semiconductors provide compact, self-limiting, and self-regulating heat management without additional electronics. This type of heating is highly appropriate for incorporation into portable and miniaturized analytical devices. In the present investigation we developed a PTC-based heating strategy for a compact diode array cartridge flow cell to substantially enhance the compatibility of diode array detection (DAD) for gas chromatography (GC). Ultraviolet-Visible spectroscopic detection in GC has been well documented in the literature [10-35]. Recently we reported a novel pairing of DAD with GC [36-38]. Diode array brings ultraviolet-visible spectroscopy (190 – 640 nm) onto the capillary GC time scale, and enabling spectrophotometric detection of analytes with peak widths in the 3 to 5 s range. GC-UV was shown to be effective across an equivalent volatility range spanning C2 to C7 olefins (bp 94 oC ), although substantial peak tailing was observed above this volatility range. The detector’s primary purpose is for detection in liquid chromatography and it does not have thermostatically con-

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trolled temperature. The non-heated surface of the cell acts as a cold zone for re-condensation and peak distortion. We introduce a modification of the optical cell that substantially improves the volatility range up to n-hexadecene (bp 285 o C). The functionality of GC coupled to a DAD equipped with a PTC heated cell is discussed and detector performance is highlighted with selected model compounds. The novel incorporation of the heated cartridge cell design aids in extending the analytical capability, improving throughput and overall system performance. EXPERIMENTAL Detector cell modifications The Max-Light™ cartridge flow cell (the cartridge) (Agilent Technologies, Waldbronn, Germany) contains the optical flow cell (the cell), two metallic unions that interface the column entrance and exit, the associated tubing to connect the former, and two three-way unions to integrate the optical cell into the light path, all contained in a plastic casing. The cartridge was disassembled by unfastening the T-10 screws and gently prying apart the two halves of the clam shell housing assembly leaving the RFID tag with the housing to ensure proper identification of the cell by the detector electronics. This step was performed carefully to avoid breaking the uncoated fused silica tube as the fused silica is suspended between two end posts and spans 60 mm making it susceptible to breakage. A 10 mm hole was drilled in the front top half of the cell housing assembly. This orifice acts as a conduit for electrical wires for heating as well as a high temperature thermocouple to monitor the temperature of the cartridge during the testing phase. A high temperature thermocouple designed for internal combustion engine manifolds (Autoparts, Edmonton, Canada) capable of tracking temperature up to 500 oC was used. Digital temperature readout was accomplished by using a palm-size UT30C multi-meter with temperature resolution of 1 oC over a range from -40 oC to 1000 oC and accuracy of 1.5% (UniTrend, Kowloon, Hong Kong). PTC heaters were obtained from Veteran Electronics (Edmonton, Canada). PTC heaters have specific target temperatures. Three temperatures were tested, 70 oC (PN LG-12V70A1), 150 oC (PN LG-12V-150A1), and 230 oC (PN LG12V-230A1) and 12 VDC was selected. A layer of rigid 1 mm thick aluminum sheet with a dimension of 75 mm × 35 mm tapered to 55 mm over 45 mm was used to facilitate effective and even heat distribution across the cell surfaces (Figure 1). The PTC heater was affixed to the center of the embodiment with 0.14 mm thick high temperature aluminum adhesive tape (3M Minnesota, USA). Silica cloth was used to insulate the assembly to minimize excessive heat loss and to prevent thermal cross talk between components of the detector. The custom sized heater assembly was secured to the DAD cell cartridge using the same tape materials to provide good contact of the heater assembly to the column flow path including the optical cell. 12V DC was generated by a 12.5 Amp power supply converter (DMiotech, Shanghai, China).

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Figure 1. Sketch of PTC heater assembly and the DAD cartridge cell. GC-DAD analysis All analyses were performed using an Agilent 7890A gas chromatograph (Agilent Technologies, Wilmington, Delaware USA) equipped with flame ionization detector (FID), and multi-mode inlet (MMI) that was fitted with an Ultra-inert inlet liner (Agilent PN 5190-2294). The MMI was operated at 2150 o C in split mode at a split ratio of 10:1. The column ensemble used included a 15 m × 0.25 mm i.d. × 0.1 µm DB-1HT capillary column coupled to a 30 m × 0.25 mm i.d. × 1.4 µm DB624UI capillary column (Agilent Technologies, Middelburg, the Netherlands). The GC was temperature programmed from 40 oC (1 min) to 210 oC @ 10 oC.min−1, and maintained at 210 o C for 5 min. The carrier gas flow rate used for all analyses was 3 mL.min−1 (helium; constant flow). The FID was operated at 250 oC with hydrogen flow rate of 30 mL.min−1, air flow rate of 350 mL.min−1 and nitrogen flow rate of 25 mL.min−1. Detection was performed using an Agilent 1290 LC Infinity II Diode Array Detector (Agilent Technologies, Waldbronn, Germany) with 60 mm flow cell (Agilent G4212-60007). A wavelength of 195 nm was used. A reference wavelength of 360 nm with a bandwidth of 100 nm was selected. The detector signal was sampled at a rate of 40 Hz. The DAD was coupled to the GC by connecting the outlet of the GC column to a short length of deactivated fused silica tubing (0.6 m × 0.32 mm) using a deactivated press-fit connector (Agilent PN 5190–6979). The outlet of the fused silica tubing was connected directly to the DAD flow cell using a Valco FSR 0.5 connector assembly (Valco PN FS1R.5). Another 0.6 m × 0.32 mm length of deactivated fused silica tubing was connected to the outlet of the flow cell to the FID using Valco FSR 0.5 connector assembly. For heat tracing purpose, the aforementioned fused silica tubing was threaded inside a 0.50 m × 3.2 mm id (1/8 inch) length copper tube. An Omega silicone band heater was wrapped around the copper tube, held in place using 3M high temperature 0.14 mm thick aluminum tape (Minnesota, USA). The temperature was regulated with a RoboTemp Mini Rheostat (Ulanet, Newark, USA) at 150 oC. Samples and standards


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Unless otherwise stated, all chemicals were obtained from Sigma-Aldrich (Oakville, Canada). Gas test mixtures were obtained from Linde (Edmonton, Canada). RESULTS AND DISCUSSION Compensating isothermal heated cell cartridge performance A PTC heater weighing only 7 g was used without any addition electronics and control. 12 V DC PTC was chosen to provide flexibility in power availability in various regions worldwide, suitable for portable applications, and for additional electrical safety for the detector and personnel in case of a short circuit or improper grounding. A PTC heater provided radiant heat to the entire cell cartridge area between the GC column exit/cell entrance interface, associated tubing, and the optical flow cell. The PTC heaters were found to be repeatable over the course of six months, achieving the same temperature through normal use with multiple thermal cycles of the heater. Heat management in UV measurement is important to ensure consistent response factors; accuracy of heater temperature is desirable for this application, but repeatability is more critical. In all cases, the measured temperature was within 10% of the temperature specified by the manufacturer. Insulation has a key positive effect on the PTC to achieve and sustain a maximum temperature. The air gap between the integrated heated cell cartridge and the detector optics affords the optics to stay well within the manufacturer’s specification of 30 to 45 oC, which typically operates 10 oC above ambient temperature to minimize detector electronic noise drift (Figure S1, Supporting Information). The time required for the surface of the heater to reach the designated temperature varied as a function of the targeted temperature, with a shorter time required for lower set temperature (Figure 2). Steady-state temperature on the surface of the heater over the range from 70 to 150 oC was achieved within 3 min as shown in Figure 2. For the 230 oC PTC, 95% of the targeted temperature was reached in 3 min with final temperature attained after an additional 7 min.

an aluminum sheet and there is a finite heat capacity. The aluminum sheet was used in conjunction with the PTC heater to distribute the heat over a custom area, dissipate the heat evenly, and to lower the heat to an appropriate temperature. The aluminum sheet is in contact with the flow cell, the associated tubing, and the interfaces located in the cell housing. The heat up time of the aluminum sheet generally follows PTC heating rates but has a lower temperature.

Figure 2. PTC Surface Temperature of a 70, 150, & 230 ⁰C PTC Heater. With heating the cartridge environment, the best case scenario with the 70 oC heater, to reach the set temperature is in 30 min. However, with the 150 oC and 230 oC heaters, it took over 1 h to achieve their respective target temperatures. Heat loss to the surrounding area was minimized with silica cloth insulation material designed for continuous high temperature operation of about 982 oC . Figure 3 shows a plot of time versus temperature attained on the surface of the 230 oC PTC heater, for the aluminum sheet, and for the cell environment to reach the set temperature. The PTC itself operates at 230 oC, however, with the 1 mm aluminum sheet attached, the temperature of the aluminum sheet is reduced. As shown in this figure, the 230 oC PTC reached a steady temperature state within 10 min, the aluminum sheet in 20 min followed by the cartridge assembly temperature at around 70 min. Therefore, using the set up described for optimum performance over the range from 70 oC to 230 oC and without thermal monitoring equipment to track the temperature of the internal cartridge cell environment, we recommend the PTC heater be activated 2 h in advance prior to conducting experiments to ensure the entire flow path is in thermal equilibrium with the cell environment temperature.

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Time vs. temperature a 230 PTCheater heater affixed affixed to aluminum sheet Temperature vs. Time of of a 230 ⁰C°CPTC toaa11mm mm aluminum sheet PTC Surface

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Figure 3. Time vs. Temperature of an in-house constructed PTC Heater Assembly. Formatted: Indent: First line: 0"

A substantially longer time is required for the cartridge environment to reach the target temperature when compared to the heater surface as illustrated in Figure 3.. The environment of the cell cartridge is heated by the PTC thermistor affixed to

Chromatographic Performance Multidimensional gas chromatography was employed with the use of a planar microfluidic device for switching to provide the analytical system with enhanced capabilities such as


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additional peak capacity for target compounds in complex matrices or for backflushing to ensure overall analytical system cleanliness. Chromatograms of alkenes up to C8 at different cell temperatures are shown in Figure S2 (Supporting Information). Respectable peak symmetry was obtained for the light compounds up to C6. Peak tailing intensified beyond C8 and continued to deteriorate with an increase in carbon number. We attribute this to the cold surface of the detector cell which was originally conceived for liquid chromatography. In contrast, a series of alkene homologues up to C16 (bp 285 oC) was successfully separated, representing an increase in carbon number range of eight (Figure 4). With the exception of 1-octene, a peak symmetry factor of no more than 1.10 was obtained for the model compounds with high degree of repeatability with RSD of less than 0.01% on retention time and 3% in area counts for all compounds (n = 10). Symmetry factor measurement was executed with ChemStation software. With 1-octene a peak symmetry of 1.28 was obtained. We postulate the greater value of peak symmetry was due to insufficient stationary phase and thermal trapping. Solvent blank analysis conducted after the analysis of the sample showed an unremarkable and clean chromatographic profile. Also, no baseline increase was observed with subsequent analyses of the same sample. These facts suggested that an effective heat management is in place for the heated cell in preventing the condensation of high boiling point analytes.

Figure 4. Chromatogram of a homologous series of alkenes of C8 to C16. The heated cell design aids in extending the range of applicability of the DAD beyond volatile and into the semivolatile compounds regime where many aromatics can be characterized. As an illustration of performance of the heated cell and analytical apparatus, a mixture of chemicals of industrial and environmental significance was examined using the technique described. Figure 5 shows chromatograms obtained using two detectors place in series of a mixture of alkanes, dienes, cyclic alkyldienes and aromatics including benzene, toluene, styrene, and indene with boiling points ranging from 44 oC to 182 oC. In the upper chromatogram all compounds were detected using FID. In contrast, in the lower chromatogram, with DAD as a detector using a wavelength of 195 nm, as expected no alkanes were detected whereas compounds with delocalized π-bonds, like dienes, cyclic alkyldienes, and aromatics were detected illustrating the high degree of selectivity of the DAD. The heated cell successfully extends the range of applicability for the analyte involved as shown by the excellent peak symmetry achieved.

Figure 5. CStacked chromatograms using two detectors place in series of a mixture of alkanes, dienes, cyclic alkyldienes and aromatics. CONCLUSION We successfully developed and implemented a compensating isothermal PTC heater for thermal management of an analytical device. The heated cartridge cell design is simple to construct, self-aligned, compact, and easily interchangeable. With the incorporation of PTC technology for heating, selflimiting in heating, and self-regulating in input and output power, can be attained without any additional electronics required. The heated cell substantially expands the range of applicability of the GC-DAD hyphenated technique to compounds with boiling point equivalent to C16 (285 oC). The successful implementation of a PTC driven heating strategy for this research has the potential of inspiring new analytical technology developments where small, lightweight, low power isothermal temperature is required such as the heating of a transfer line to prevent analyte re-condensation, portable heat evaporators, and chemical reaction cells to name a few. The self-regulating and self-limiting features of PTC lends themselves to the PTC technology being employed in many fields of applications by reducing the risk of overheating to protect polymeric components from excessive temperature. The heated cartridge cell is unique in, first self-aligned and easily interchangeable, secondly, self-limiting in heating, and finally self-regulating in input and output power. The synergy of the above elements provide critical novelties to the cell design performance and ease of use.

AUTHOR INFORMATION Corresponding Author *Robert Shellie, E-mail [email protected] Telephone +61-3-9874-8577

ACKNOWLEDGMENT We dedicate this article to Professor Paul Haddad on the occasion of his 70th birthday. We are indebted to his guidance and thank him for his tireless effort in advancing separation sciences and allied technologies over the decades. Dr. Tonya Stockman, Dr. Wayde Konze, Mr. Mike de Poortere of Dow Chemical, Ms. Shanya Kane, Dr. Paul Dietrick, and Mr. Dave Judd of Agilent Technologies are acknowledged for their support and encouragement. Dr. Karsten Kraiczek of Agilent Technologies was acknowledged for the fruitful discussions and sage advice he provided on the topic of diode array detection for gas chromatography. Agilent Technologies was acknowledged for providing the Max-Light cells for modification and experimental design. This work was supported by the Australian Research Council's Industrial Transformation Training Centres Scheme


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(Project No. IC140100022: ARC Training Centre for Portable Analytical Separation Technologies).

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Positive temperature coefficient compensating heating for analytical

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