A Detachable Trap Preconcentrator with a Gas Chromatograph-Mass

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A Detachable Trap Preconcentrator with a Gas Chromatograph-Mass Spectrometer for the Analysis of Trace Halogenated Greenhouse Gases Doohyun Yoon, Jeongsoon Lee, and Jeong Sik Lim Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b04551 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 20, 2019

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Analytical Chemistry

A Detachable Trap Preconcentrator with a Gas Chromatograph-Mass Spectrometer for the Analysis of Trace Halogenated Greenhouse Gases

Doohyun Yoon, Jeongsoon Lee*, Jeong Sik Lim*

Center for gas analysis, Korea Research Institute of Standards and Science (KRISS), Gajeong-ro 267, Yuseong-gu, Daejeon, 34113, Republic of Korea Department of Measurement Science, University of Science and Technology (UST), Gajeong-ro 217, Yuseong-gu, Daejeon, 34113, Republic of Korea

ABSTRACT A detachable trap preconcentrator coupled with a gas chromatograph-mass spectrometer (GC-MS) was developed for measuring trace amounts of anthropogenic halogenated greenhouse gases such as hydrofluorocarbons (HFCs) and nitrogen trifluoride (NF3). Hayesep D cooled to -135°C was used as an adsorbent for preconcentrating the target analytes. A differential trapping method was applied to remove major interfering substances such as CO2, N2, and O2 in order to ensure sufficient sampling volume of secondary injection trap. This was accomplished without using any CO2-removal agent such as molecular sieve adsorbents. Consequently, the temperature of the primary transfer trap was set to −75°C for selective desorption of a significant amount of CO2 that could be vented out. Meantime, the major components of air, e.g., N2 and O2, were vented out before transferring the analytes to the secondary injection trap, in order to protect the gas plumbing from pressure shock induced by rapid temperature ramping over 100°C/min in the secondary trap. When the traps were heated, linear motion was operated to detach them from the copper baseplate on the freezer, thereby restricting heat transfer to the freezer and maintaining the freezer close to the background temperature of −135°C. This trap design is a key improvement to address the insufficient cooling capacity of the employed freezer, allowing sensitive detection of trace halogenated greenhouse gases in GC-MS. NF3 and various HFCs at ambient levels were quantitatively and qualitatively measured with a precision of 0.35% at rates below 45 min/cycle. In particular, the limit of detection for NF3 was evaluated to be 0.2 pmol/mol, with linear responses at ambient concentration.

* Corresponding authors: Jeongsoon Lee ([email protected]) and Jeong Sik Lim ([email protected])

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1. Introduction The international community adopted the United Nations Framework Convention for Climate Change (UNFCCC) in response to the climate change problem in 1992. Based on this convention, major agreements such as the Kyoto Protocol in 1997 and the Paris Agreement in 2015 were made. The Kyoto Protocol specifies the implementation of the greenhouse gas reduction obligation and sets up detailed reduction obligation for seven chemicals including carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), perfluorinated compounds (PFCs), hydrofluorocarbons (HFCs), sulfur hexafluoride (SF6), and nitrogen trifluoride (NF3). The atmospheric mixing ratios of NF3 and HFCs, which are frequently used as substituents for CFCs, are continually increasing.1-4 In particular, NF3 is an anthropogenic greenhouse gas that has been internationally regulated since the implementation of the Kyoto Protocol (revised since 2005) in 2013. The global warming potential of NF3 is 16,600-18,100 times that of CO2,5-7 and the background atmospheric concentration increases by approximately 0.1 parts per trillion annually.8,9 Therefore, reliable surveillance of NF3 has become important. Despite the need to monitor NF3 and HFCs, their background atmospheric concentrations are on the order of a few pmol/mol, and the sensitivity of the gas chromatographmass spectrometer (GC-MS) typically used for measuring NF3 and HFCs is very low. Miller et.al. developed the so-called Medusa preconcentrator to measure trace greenhouse gases by GC-MS, which they used continuously and successfully to conduct studies on atmospheric monitoring within the Advanced Global Atmospheric Gases Experiment (AGAGE) program.10 This system automatically samples 2 L of air in the concentration trap of Hayesep D cooled to −165°C. Then, the preconcentrated analytes are heated to −65°C for carbon tetrafluoride (CF4) to be transferred and refocused in the second trap at −165°C. The remaining airborne N2, O2, Ar, Kr, CH4, and CO2 in the CF4 stream are separated and removed by a combined precolumn with MS-4A and Hisiv-3000. Other halogenated carbons of CFCs and HFCs are sequentially transferred to the second trap after CF4 injection. By using this analytical technique, ambient CF4 and other greenhouse gases were successfully measured with a Porabond-Q porous layer open tubular (PLOT) column in a continuous stream mode for many years at numerous AGAGE sites. However, since NF3 is highly adsorptive to HiSiv-3000, it was critically removed from the analyte stream. In the following study, NF3 was measured with an Hisiv-free precolumn, and the Gaspro column was replaced with HiSiv-3000 to increase the resolving power between CF4 and NF3.11 Inspired by the Medusa preconcentration system, we developed a preconcentration system for GC-MS empowered by a detachable trap for the quantitative analysis of NF3 and other trace fluorinated gases with high precision. The cooled preconcentration trap was separated from the cold-end of the cryogenic refrigeration unit during heating for desorption of the preconcentrated analytes, thereby restricting the heat transfer to the cold-end and secondary trap. Thus, the system steadily maintains the ground-state temperature of the base plate at approximately −135°C, thereby improving the cooling rate of the heated trap compared to that in the integrated configuration. In this study, a preconcentration of NF3 in a matrix of air at −135°C is demonstrated by fully custom-built preconcentration system with high measurement precision. It will be demonstrated that ambient concentrations of NF3 can be measured with a precision of 0.35% at the rate of 35 min/cycle. The evaluated limit of detection (LOD) of airborne NF3 is 0.21 pmol/mol. Additionally, we evaluated the analytical performance against HFC-134a, HCFC-22, and HCFC-142b. Measurement of HFC-134a, HCFC-22, and HCFC-142b in air-archived samples was also attempted with enhanced precision than that achieved in our previous study on the international key comparison of the CCQMK83.12

2. Experiment 2.1. Gas line configuration ACS Paragon Plus Environment

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Analytical Chemistry The gas plumbing consists of multiposition valves (Valco Instrument Company Inc.), mass flow controller (MFC,

5850E, Brooks Instruments), flow meters, helium purifier, electric pressure controllers (EPC), two adsorptive traps, split inlet, precolumn, and main column. (Figure 1) The multiposition valves (referred to as V1, V2, V3, V4, and V5 in Figure 1), MFC, and flow meters (referred to as FM1 and FM2 in Figure 1) are controlled by a custom-built system based on a programmable logic controller (PLC; XBM-DR16S, LSIS). The operation sequences stored in a non-volatile memory of the PLC was written by the supervisory control and data acquisition (SCADA) system (Cimon). PC and PLC section communicated with the RS232 protocol. The temperature controller (TM4, Autonics) communicated with the PLC via the RS-485 protocol and used the solid-state relay (SSR, 24 VDC, LSIS) to control the temperature of the heater cartridge through the Proportional–Integral– Derivative (PID) control. The actuator control is divided into a solenoid valve control using air pressure and a multi-position valve driver control. V1, V2, V3, V4, and linear motions are controlled in two states, on and off. V5 is controlled to set three types of gas flow pathways. (Figure S1) See Figure S2 for a schematic illustration of the electronic control section. The EPC and the inlet, both of which are capable of controlling the flow rate of the analyte stream, were installed in the GC (Agilent 7890) body and controlled by the GC control program (Chemstation, Agilent) and they were operated separately from the PLC. Note that the pre-programmed analytical method for the GC-MS was triggered by the SCADA for the preconcentrator and GCMS to be synchronized in time. The temperature of the main column was controlled by the GC oven. T1 and T2 are adsorptive traps consisting of stainless steel tubes with an inner diameter of 2.18 mm and length of 100 mm. T1 (concentrating trap) and T2 (refocusing trap) were filled with 120 mg and 10 mg, respectively, of Hayesep D (100/120 mesh, Valco Instruments). The temperature of the traps could be controlled stably at designated temperature between −135°C and 0°C over ~5 min, thereby determining the type of gas desorbed. The order of the desorption of gasses according to the trap temperature tends to be proportional to the order of their freezing points; however, anomalous behavior might be observed depending on the selectivity of Hayesep D. Nevertheless, in this study, we did not observe exceptional desorption behaviors. Detailed operation sequences of the multiposition valves and the linear motions of the traps are described in the supporting information. Briefly, a differential trapping method was applied to remove decent portion of major interfering substances such as CO2, N2, and O2 in order to ensure sufficient sampling volume of T2. The gas were adsorbed in T1 that was maintained at the -135°C, and then the temperature of T1 was set to −75°C for selective desorption of a significant amount of CO2 that could be vented out. This was accomplished without using any CO2-removal agent such as the Molecular Sieve adsorbents, so which allowed for the strong retention and back-flushing of CO2. Residual CO2 was chromatographically separated in the main column of Porabond Q (length, 75 m; i.d., 530 µm; film thickness, 10 µm; Varian Inc.). Further details will be discussed in section 3.1. Meantime, the major components of air, e.g., N2 and O2, were also vented out before transferring the analytes to T2, which played a crucial role to break the gas seal from pressure shock induced by rapid release of the interfering substances when the T1 temperature ramped over 100°C/min. When the trap was heated at 125°C to enable desorption of all absorbed gases, the trap was detached from the cold-end in order to prevent heat transfer to the freezer and therefore to another trap, thus improving the stability of the temperature control and the cooling rate. (Figure 2) Helium (99.999%, Doek-yang Energen, Korea) passed through a helium purifier (HP2-220, VICI Valco Instrument) was fed as the carrier gas. The helium purifier was used to prevent the reduction in the adsorption efficiency of the traps, which could be caused by adsorbable impurities in He. The flow rate of the carrier gas was controlled using EPC1, EPC2, and the front inlet module which was controlled by the GC body. A flow meter (200 ml/min, Honeywell) was installed at the end of the vent line to monitor the flow rate. This aids in estimating the delivery time of the analytes and for detecting any abnormal operation of the system. The inlet allowed flexible adjustment of the concentration of the preconcentrated analytes by diluting the injected samples. Additionally, it automatically adjusted the pressure of the sample flow with respect to the programmed oven temperature. The MFC was installed upstream of the preconcentrator in order to load samples with precision. The flow ACS Paragon Plus Environment


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rate of the sample was 200 mL/min and 2 L of the sample was sampled over 10 min. The precolumn (stainless steel, internal diameter: 2.18 mm, length: 30 cm) was filled with 50 mg of 100/120 mesh molecular sieve 4 Å (MS 4 Å, Sigma Aldrich) and then with 200 mg of 100/120 mesh HiSiv-3000 (Hisiv, UOP). The MS-4A/HiSiv-3000 mixed precolumn was used to separate CF4 from interfering gases such as N2, O2, Ar, Kr, CH4, and CO2. In particular, an excess of CO2 was back-flushed by the aid of strong retention in MS-4A. However, in case of measuring NF3, the precolumn was bypassed from the flow path owing to the NF3 removal characteristics of HiSiv-3000. The interference by CO2 resulting from the bypass of MS-4A was solved by partial venting, the efficiency of which was determined by the transfer temperature. The details are discussed in the results and discussion. The main column was placed on the GC oven (Agilent 7890). The analyte stream separated in the main column was injected to the ionization source and then introduced into the Quadrupole mass spectrometer (Agilent 5975C) for the ionized fragments to be filtered as a function of unit mass-to-charge ratio (m/z) and the GC-MS chromatogram was recorded in the selected-ion monitoring (SIM) mode. Details regarding operation sequences and their strip chart are given in the ST 1, ST 2, ST 3, Figure S3, Figure S4, and Figure S5 of the supporting information.

2.2. Detachable trap A cold-end (PT-14, Polycold Division of Brooks Automation, Petaluma) connected with a high-performance cryogenic refrigeration unit (Cryotiger, Polycold Division of Brooks Automation, Petaluma) was used as a cooling unit for the detachable adsorption traps. The cold-end used in this study has a cooling capacity of 30–50% at −160°C compared to that of PT-16 used in Miller’s work.11 Though, in our work, the ground-state temperature of the baseplate could be down to −145°C, it was maintained at -135°C during continuous measurements. To minimize heat transfer between the trap and the freezer, the copper baseplate buffered thermal flow by being placed between the cold end and the trap. In case of that T1 and T2 shared the baseplate, the temperature of T2 was gradually increased at the rate of ~10°C/min by the T1 at −65°C. Therefore, in order to minimize the thermal exchange between T1 and T2, the baseplates of those were divided into two parts. (Figure 2) Each baseplate (dimensions, 15 × 7 × 40 mm) has two grooves for tight contact between the straight adsorptive tube and welded cartridge heater, where thermocouples are welded on the rear side of the baseplate. Nevertheless, the temperatures of two traps were affected by each other when attached simultaneously in the base plate. Since the temperature deviation induced by another trap strongly hinders to take precision control of adsorption amount of target analytes, it was essential to improve a temperature isolation of two traps. For this, the trap was physically detached by air-actuated linear motions (L-2171-1, Huntington Mechanical Labs) when if the temperature was increased higher than 0 ° C. Cooling to the background temperature or maintaining a moderate temperature, the trap is pushed toward the baseplate and attached-on. A polymer union placed between the arm of the linear motion and the trap served to block incoming and outgoing heat through the linear motion. In this way, even when the trap is heated to 200°C, the temperature of the base plate can stay at the background level of -135°C, and therefore sufficient cooling power is provided for quick cooling. As presented in the Figure S3 (~15 min), a detachment of T1 to be heated to 125°C led the baseplate temperature gradually down to the background level of -135°C. Though, the reattachment of the T1 to be cooled impulsively increased T1 temperature (~22 min), this is trivial to the measurement precision because of no gas flow between T2 and the GC-MS. (see the supporting text 1) Note that consecutive injection of NF3 followed by HFCs within one cycle is required for T1 to be maintained at −75°C during first injection-heating and cooling-down of T2 as in Miller’s study.11 This requirement might be fulfilled by ensuing higher cooling power. It is worth to note that ambient level NF3 in synthetic air gas mixtures of which composition was well controlled without CO2 and H2O were measured to provide precision less than 0.35% (1σ). (Figure S6) On the other hand, the Tim approach, where measurements were conducted in the long term with directly sampled air, achieved precision less than 2% (1σ).11 This might ACS Paragon Plus Environment

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originate from higher concentrating volume with the precision control of the temperature of T1 ensured by maintaining the corresponding base plate at the ground-state temperature at all times. Since this study did not aim to measure all measurable analytes in a single cycle, two injections were made in each complete cycle to shorten the analysis time and improve the thermal efficiency. The linear motion and preconcentration trap were connected via a polymer union made of fiberglass laminate (G10, Glass epoxy) to block the heat transfer from the chamber wall to the cold traps, the cooling efficiency of which were enhanced by the vacuum chamber. A turbo pump (Hicube 80 Eco pumping station, PFEIFFER) was used to maintain the pressure at the 10−6 mbar level.

3. Results and discussion 3.1. The impact of the trap temperature on the chromatographic resolution In order to remove CO2, which is the most interfering substance during the measurement of NF3, the temperature of T1 was set close to the sublimation point of CO2 during the transfer of analytes from T1 to T2. In Figure 3, the chromatograms recorded at the T1 temperatures of −50 and −75°C are compared. A peak at the fastest retention time is assigned to NF3, and that with a slower retention time is assigned to CO2. Since m/z = 52 (NF2+) was monitored, the CO2 peak at m/z = 52 might be due to the ambient CO2 ion that leaked through the quadrupole mass filter.9 The CO2 peak was confirmed by the chromatogram obtained at m/z = 45, based on the fact that the retention time of CO2 at m/z = 45 is exactly the same as that at m/z = 52. (Figure 3) Similarly, the CO2 peak recorded at m/z = 45 is considered to be due to 12CO2 (m/z = 44) rather than 13CO2, considering that the natural abundance of 13CO2 is only ~1.2% among all isotopomers of CO2. The area of the NF3 peak remained the same regardless of the transfer temperature ranging from −75 to −50°C. (Figure S7) However, in the case of transferring at −50°C, the chromatographic resolution decreased compared to that of −75 ° C. This phenomenon might have resulted from the considerable retention by the adsorbent Hayesep D in T1. In order to achieve sufficient refocusing power of T2, reducing the amount of the adsorbent can be considered, however, a compromise is needed to minimize the adsorption loss. Additionally, we confirmed that the transferred amount of CO2 reduced at −75°C of T1 compared to that at −50°C. This low-temperature transfer method allowed us to bypass the MS-4A/Hisiv-300 precolumn, which is needed in the case of back-flushing CO2 for removal, so that the run time can be shortened. However, when measuring samples with a higher concentration of NF3 and CO2 compared to ambient levels, sufficient resolution should be ensured to minimize the interference between NF3 and CO2. Alternatively, CO2 should be completely removed by back-flushing in the precolumn or by using a chemical trap such as lime soda before preconcentration. Step 8 in the measurement of NF3 and HFCs involves refocusing and the temperature of T2 was maintained at a high value with the closure of T2. In this step, the delayed retention time and tailing phenomena caused by the interaction between stationary and mobile phases at T1, precolumn, and T2, can be improved by a long dwelling time of Step 8. The peak shape of HCFC-22 (Figure 2 (b)) improved significantly by the increase in the T2 temperature. This is because a uniform temperature distribution from the trap surface to the core might be derived by long dwelling time at T2. As shown in Figure 3 (c), prolonging Step 8 for more than 5 min improved the peak shape to be symmetric, suggesting repeatable peak integration, namely detection response.

3.2. Limit of detection and precision of measurement The responsivity of NF3 was tested at various concentrations generated by diluting 5 pmol/mol NF3 in air gravimetric gas mixture with high-purity N2 (99.9999%). The dilution ratio was controlled by the flow rates of two MFCs (5850E, Brooks Instruments), which were calibrated by an integrating flowmeter. The diluted concentrations were 0.5, 1, 2, and 3 pmol/mol ACS Paragon Plus Environment


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and the flow-rate ratios were 1:9, 1:4, 2:3, and 3:2, respectively. The total flow rate of the diluted sample was 200 mL/min. NF3 impurity in 2 L of the dilution agent N2 was preconcentrated to be measured, resulting in no detection. Therefore, it can be said that NF3 from the dilution agent didn’t bias the limit of detection (LOD) of the preconcentration techniques attempted in this study. Then, the LOD is calculated based on the sensitivities at various concentrations (Figure 4), using the following equation.

LOD = 3 × SDy

(1)

Here, SDy is the standard deviation of the intercept of y in the flat line regression equation in which the x axis is the calculated concentration and the y axis is the sensitivity. In case of a 2-L sample, the LOD was estimated to be 0.21 pmol/mol. The relative standard deviation of each measurement tended to increase as the concentration of the test sample approached the LOD, implying the poorest precision at low concentrations. The instrumental drift was also evaluated. When 3 pmol/mol of the sample was continuously measured for 12 h, the drift, which is defined as the difference between the minimum and maximum values, turned out to be 1% of the mean of the responses. This might be a combined effect originating from the drifts in the ambient pressure (loading amount), adsorption rate (amount of preconcentrated analyte), and detection sensitivity. These results are comparable to that reported by Arnold et. al..11 Therefore, it can be stated that the same analytical capability was realized under mild temperature conditions by the aid of the detachable traps, which indicated that the run time and precision can be improved further through efficient heat transfer management using a system with higher cooling power. This may be an important factor in the continuous observation of background atmospheric samples. There was a concern regarding inefficient adsorption at −135°C, which is close to the boiling point of NF3, owing to the unstable adsorption and consequent desorption loss caused by the high flow rate of the analyte stream. This desorption loss was observed only when the amount of the analyte was greater than 10 L, suggesting that the active volume of the trap and the adsorption strength for the analytes are sufficient under our analytical conditions. The sensitivity of HFC-134a, HCFC-22, and HCFC-142b was investigated by varying the concentration time, instead of varying the analyte concentration. This approach is valid because the sensitivities are proportional to the preconcentration times, as shown in Figure S8. Based on this results, the LODs of HFC-134a, HCFC22, and HCFC-142b were estimated to be 0.25, 1.32, and 1.23 pmol/mol, respectively, in the case of dry air. The analytical precisions verified using the reference cylinders of D015147 and D985596, which were employed for an international key comparison of CCQM-K83 (Halocarbons in air), were 0.34%, 0.35%, and 0.35% for HFC-134a, HCFC-22, and HCFC-142b, respectively.8 (Figure S9) These values suggest that the KRISS measurement capabilities for these gases have been improved at least 10 times compared to that of CCQM-K83 with reported precisions of 5%, 3.9%, and 5%, respectively. Note that the measurement method for CCQM-K83 was based on a preconcentration by Carboxen 1000 cooled at −20°C coupled with GCMS.12

3.3. Ambient air measurement The total ion chromatogram of a trace HFC mixture was reconstructed from independent acquisitions of chromatograms over sixty m/z units. (Figure 5, Figure S10) For this purpose, the three independent chromatograms were recorded in SIM (selected-ion monitoring) mode at m/z values covering 10~69, 70~129, and 130~189. Measurement procedure of atmospheric HFCs and associated strip chart are given in ST 2 and Figure S4 of the supporting information. In postprocessing, chromatograms obtained at m/z = 28, 32, 44, 18, 14, 16, and 17 corresponding to N2, O2, CO2, H2O, N, O, and OH, respectively, were discarded. The last chromatograms were combined to yield the reconstructed total ion chromatogram (TIC). ACS Paragon Plus Environment

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The side-peak of the major abundant N2 corresponding to m/z = 29 was also blind-treated. Their chemisorption on the Hayesep D should be poor; however, physisorption caused by low-temperature cooling at −135°C allows a considerable amount of these components to be concentrated and then injected into the mass spectrometer along with the analytes. These physisorbed components, which account for 99.07% of ambient air, were excessively injected and remained in the source region to be ionized. In this sense, the side band of CO2 (m/z = 45) and N2 (m/z = 29) indicated that excessive amounts of parent ions with m/z = 29 and 44 leaked through the mass filter to reach the detector. Evidently, the response of the corresponding m/z in the chromatogram was consistent (>30000) for ~10 min after injection, implying continuously leaked (or unfiltered) ion current. Unless they are discarded, the baseline of the TIC is saturated to blind the peaks where various analytes are separated. Dry air collected in pressurized aluminum cylinders in August 2015 at Anmyeondo (AMY), Republic of Korea, was analyzed. By contrast to the NIST 2008 mass library, HFO-1234yf (CH2CFCF3) and HFO-1243zf (CH2CHCF3), which are increasingly used as the fourth-generation refrigerants, were newly identified in the sampled dry air. (Figure 6 and Figure S11, respectively) In the case of HFO-1234yf, the mass spectrum was verified by measurement of the commercially available raw gas (Opteon YF, Chemours company, USA) diluted with nitrogen. Qualitative assignments of the other unknown peaks will be obtained in the near future. Additionally, dry air samples archived in 2006, 2011, and 2015 at Gosan, Republic of Korea, were analyzed to compare the results with the AGAGE-reported observation values for NF3, HFC-134a, HCFC-22, and HCFC142b at Zeppelin.8 The calibration was carried out using the aforementioned gravimetric standards, 3 pmol/mol NF3 in N2 and HFC-134a, HCFC-22, and HCFC-142b in air (D015147) prepared for the CCQM-K83. As shown in Figure S12, the overall trends of the corresponding gases are very similar. The assigned concentrations are slightly higher than the values reported for the Zeppelin site,13 probably because of Korea’s industrial structure involving semiconductor manufacturing industries. Equivalence between the AGAGE scale of SIO-12 used in AGAGE is another rationale to be evaluated. Further research is needed in this regard.

4. Conclusion An ultralow-temperature preconcentrator coupled with a GC-MS system was developed for measuring various trace greenhouse gases such as NF3 and HFCs, which are rapidly increasing in the modern atmosphere. For the efficient elimination of the major components of air, the differential adsorption traps equipped with Hayesep D were cooled at -135°C before the injection of the analyte sample into the GC-MSD. The transfer temperature of the primary adsorption trap, T1, was adjusted to −75°C for efficient desorption of CO2 (one of the major interfering substances of NF3/CF4), and significant ventilation was induced to secure sufficient sampling volume in the refocusing trap, T2. This approach presents the possibility of bypassing the use of back-flushing. In particular, it is an essential process for the detection of NF3, considering that NF3 is highly adsorbed onto the Hisiv 3000. Linear motions were used to detach the traps from the freezer, thereby limiting heat transfer to the freezer to maintain the baseplate in the ground state temperature of −135°C. This strongly implies that NF3 can be detected by GCMSD under mild temperature conditions in the preconcentrator and suggests further improvement of the LOD and measurement precision. This trap design might effectively overcome the insufficient cooling capacity. We demonstrated that NF3 and HFCs at background atmospheric concentration levels can be measured with precisions of 0.35% at the rate of 35 min/cycle and 0.35% at 45 min/cycle, respectively. NF3 was measured by diluting 5 pmol/mol NF3 in air (prepared via the gravimetric method) with N2. A linear response was observed within 0.5% up to 0.51 pmol/mol, and the LOD was estimated to be 0.21 pmol/mol. Based on these results, we aim to establish a calibration scale for the measurement of gases by developing a highly precise NF3 standard using the gravimetric method. ACS Paragon Plus Environment


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ASSOCIATED CONTENTS Supporting information (ST 1) Measurement procedure for airborne NF3 (ST 2) Measurement procedure for airborne HFCs (ST 3) Measurement procedure for NF3 in synthetic air (N2/O2) (ST 4) Difference between Medusa GC-MS and this work (ST 5) Evaluation of calibration uncertainty (ST 6) Measurement procedure of CF4 (Table S1) Difference between Medusa GC-MS and this work (Figure S1) Gas flow pathways of V5 (Figure S2) Schematic illustration of the electronic control unit (Figure S3) Strip chart of the NF3 measurement procedure (Figure S4) Strip chart of HFCs measurement procedure (Figure S5) Strip chart of NF3 in CO2 free synthetic air measurement procedure (Figure S6) Precision test of NF3 (Figure S7) Comparison of NF3 transfer efficiency as a function of T1 temperature (Figure S8) Response linearities of HFCs (Figure S9) Reevaluation of gravimetric concentration of the standard gas mixtres prepared for conducting CCQM-K83 (Figure S10) Chromatograms at various m/z and NIST mass spectra libraries (Figure S11) Combined chromatogram of HFO-1234zf and corresponding NIST mass spectrum library (Figure S12) Comparison of ambient NF3 and HFCs concentrations between the data reported by AGAGE and this work (Figure S13) Chromatogram of CF4

ACKNOWLEDGMENT This work was funded by the Korea Research Institute of Standards and Science (KRISS) under the basic R&D project of establishment of measurement standards for atmospheric environment (Grant No. 19011047). The authors thank Tae-wan Kim for contributing to the improvement of the structure of the adsorptive traps and for the dry air samples in the final step of the study.

REFERENCES 1. Simmonds, P. G.; Rigby, M.; Manning, A. J.; Lunt, M. F.; O’Doherty, S.; McCulloch, A.; Fraser, P. J.; Henne, S.; Vollmer, M. K.; Mühle, J.; Weiss, R. F.; Salameh, P. K.; Young, D.; Reimann, S.; Wenger, A.; Arnold, T.; Harth, C. M.; Krummel, P. B.; Steele, L. P.; Dunse, B. L.; Miller, B. R.; Lunder, C. R.; Hermansen, O.; Schmidbauer, N.; Saito, T.; Yokouchi, Y.; Park, S.; Li, S.; Yao, B.; Zhou, L. X.; Arduini, J.; Maione, M.; Wang, R. H. J.; Ivy, D.; Prinn, R. G. Atmos. Chem. Phys. 2016, 16 (1), 365–382. 2. Simmonds, P. G.; Rigby, M.; McCulloch, A.; O’Doherty, S.; Young, D.; Mühle, J.; Krummel, P. B.; Steele, P.; Fraser, P. J.; Manning, A. J.; Weiss, R. F.; Salameh, P. K.; Harth, C. M.; Wang, R. H. J.; Prinn, R. G. Atmos. Chem. Phys. 2017, 17 (7), 4641–4655. 3. Simmonds, P. G.; Rigby, M.; McCulloch, A.; Vollmer, M. K.; Henne, S.; Mühle, J.; O’Doherty, S.; Manning, A. J.; Krummel, P. B.; Fraser, P. J.; Young, D.; Weiss, R. F.; Salameh, P. K.; Harth, C. M.; Reimann, S.; Trudinger, C. M.; Steele, L. P.; Wang, R. H. J.; Ivy, D. J.; Prinn, R. G.; Mitrevski, B.; Etheridge, D. M. Atmos. Chem. Phys. 2018, 18 (6), 4153–4169. 4. Stanley, K. M.; Grant, A.; O’Doherty, S.; Young, D.; Manning, A. J.; Stavert, A. R.; Spain, T. G.; Salameh, P. K.; Harth, C. M.; Simmonds, P. G.; Sturges, W. T.; Oram, D. E.; Derwent, R. G. Atmos. Meas. Tech. 2018, 11 (3), 1437–1458. 5. Robson J.I.; Gohar L.K.; Hurley M. D.; Shine K.P.; Wallington T.J. Geophys. Res. Lett. 2006, 33 (10), L10817. 6. Prather, M. J.; Hsu, J. Geophys. Res. Lett. 2008, 35 (12), L12810. 7. Dillon, T. J.; Vereecken, L.; Horowitz, A.; Khamaganov, V.; Crowley, J. N.; Lelieveld, J. Phys. Chem. Chem. Phys. 2011, 13 (41), 18600–18608. 8. Weiss, R. F.; Mühle, J.; Salameh, P. K.; Harth, C. M. Geophys. Res. Lett. 2008, 35 (20), L20821. ACS Paragon Plus Environment

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Analytical Chemistry 9. Arnold, T.; Harth, C. M.; Mühle, J.; Manning, A. J. Salameh, P. K.; Kim, J.; Ivy, D. J.; Steele, L. P.; Petrenko, V. V.; Severinghaus, J. P.; Baggenstos, D.; Weiss, R. F. Proc. Natl. Acad. Sci. 2013, 110 (6), 2029–2034. 10. Miller, B. R.; Weiss, R. F.; Salameh, P. K.; Tanhua, T.; Greally, B. R.; Mühle, J.; Simmonds, P. G. Anal. Chem. 2008, 80 (5), 1536–1545. 11. Arnold, T.; Mühle, J.; Salameh, P. K.; Harth, C. M.; Ivy, D. J.; Weiss, R. F. Anal. Chem. 2012, 84 (11), 4798–4804. 12. Rhoderick, G.; Guenther, F.; Duewer, D.; Lee, J.; Moon, D.;Lee, J.; Lim, J. S.; Kim, J. S. Metrologia 2014, 51, Thechnical supplement 13. AGAGE web page, https://agage.mit.edu/data/agage-data



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Figures

Figure 1. Schematic illustration of gas line configuration of the developed detachable trap preconcentrator-GC/MS system. The five multiposition valves are referred to as V1, V2, V3, V4, and V5, and the traps are referred to as T1 and T2. Default coordinate of the position states of V1, V2, V3, V4, and V5 is given underneath respective symbols to be set to (Off, Off, Off, Off, 2).Pathways of gas flows at V5 are depicted in Figure S1. The carrier gas He is refined and then allowed to flow through the purifier, and its flow rate is controlled by the electric pressure controllers (EPC1 and EPC2). The flow rate of the sample is adjusted by the MFC and is located in upstream of T1. A combined precolumn sequentially filled with MS-4A and Hisiv3000 was used to separate CF4 and analytical interfering substances before the analyte is transported to the main column at T2. For measuring NF3 the combined precolumn was bypassed in order to avoid the removal of NF3 by Hisiv-3000. CO2 was selectively desorbed in T1, whose temperature was tuned at -75°C, and then vented out before transferring T2. Flow meters are referred to as FM1 and FM2.


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Figure 2. View of the detachable preconcentration traps. (a) The baseplate is divided into two parts for blocking heat transfer from another trap that is heated. There are two grooves for tight attachment of the trap unit of adsorbent line and heat cartridge, where thermocouples are welded on the rear side of the baseplate. The trap unit is connected to the arm of the linear motion spaced by a polymer union, which serves to block incoming and outgoing heat through the linear motion (b) “On” state. When cooling or maintaining a moderate temperature, the trap is pushed toward the baseplate and attached-on. (c) “Off” state. The linear motion pulls the trap for heating to high temperature. This structure is registered in Korean Patent No. 1017078640000.



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Figure 3. (a) Comparison of the m/z = 52 (NF2+) chromatograms with respect to the transfer temperature of T1. The solid line (-) chromatogram is recorded at -75°C and dotted line (- -) is recorded at -50°C. The long tail following the NF3 peak is owing to excess CO2, that is repeatedly observed in the m/z = 45 chromatogram color coded as red. (see the text) (b) Chromatograms of HFC-134a (black), HCFC-22 (red), and HCFC-142b (blue) at ambient levels. The target m/z of HFC-134a, HCFC-22, and HCFC-142b is tuned to 83, 67, and 65, respectively. As shown in the inset (c), By prolonging Step 8, more symmetric peak shape is obtained. (black: 7 min, red: 5 min, green: 3 min, blue: 1 min).


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Figure 4. Sensitivity as a function of NF3 concentration. A gravimetric mixture of 5 pmol/mol NF3 in air was diluted with 99.9999% N2 using N2-calibrated MFCs. The gray line represents the confidence band (2σ) of normal distributions centered at each measured value. At 1 pmol/mol, the confidence band was widened by 3σ. The LOD was evaluated to be 0.51 pmol/mol. (See the text)



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Figure 5. Mass spectrum according to the retention time of the total ion chromatogram (TIC) (top) and analytes (bottom); m/z = 14, 16, 17, 18, 28, 29, 32, and 44 were blinded with a high baseline for easy identification of the peaks (see text). a: PFC218 (CF2CFCF2) and HFC-143a (CH3CF3) (Figure S10 (a), (d)), b: Hexafluoropropylene (CF3CF=CF2), c: HFC-134a (CH2FCF3), d: HCFC-22 (CHClF2), e: Combined peaks of 3,3,3-trifluoropropyne (CF3C2H) and HFC-245ca (CH2FCF2CHF2) (Figure S10 (b), (e)), f: Propene (CH3CH=CH2), g: HFO-1234yf (CH₂=CFCF), h: Chloromethane (CH3Cl), i: HFC-1243zf (CH2=C2HF3), j: CFC-12 (CCl2F2), and k: HCFC-142b (C2H3ClF2).


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Figure 6. (a) Chromatograms of HFO-1234yf (CH2CFCF3) of dry air sample peaked at 10.7 min. Chromatograms are taken at m/z = 45 (blue), 64 (orange), (69green), 75 (red), 95 (violet), 113 (dark yellow), and 114 (magenta). (b) The NIST mass spectrum library of HFO-1234yf and (c) measured mass spectrum of a diluted HFO-1234yf raw gas are also represented. C3H2F4+ (114), C3HF4+ (113), C3H2F3+ (95), C3HF2+ (75), CCF3+ (69) and C2H2F2+ (64), and CH3CF+ (45) were identified in both mass spectra.



For TOC only


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A Detachable Trap Preconcentrator with a Gas Chromatograph-Mass

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