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A simple method for pre-calibration storage of sulphur hexafluoride permeation tubes
Animal Feed Science and Technology 166–167 (2011) 198–200
Contents lists available at ScienceDirect
Animal Feed Science and Technology journal homepage: www.elsevier.com/locate/anifeedsci
A simple method for pre-calibration storage of sulphur hexafluoride permeation tubes M.H. Deighton a,b,∗ , B.M. O’Loughlin a , F. Buckley a , T.M. Boland b a b
Animal and Grassland Research and Innovation Centre, Teagasc, Moorepark, Fermoy, Co. Cork, Ireland School of Agriculture, Food Science and Veterinary Medicine, University College, Dublin, Belfield, Dublin 4, Ireland
a r t i c l e
i n f o
Keywords: Sulphur hexafluoride Permeation Tube Frozen Storage
a b s t r a c t Long term storage of sulphur hexafluoride (SF6 ) permeation tubes, used to determine enteric emissions from ruminants using a calibrated tracer (ERUCT) technique, is desirable to increase flexibility of post-manufacture tube handling. To be effective, such storage must decouple the date of tube manufacture from the date of subsequent expiration due to gas loss. We hypothesized that release of SF6 gas could be effectively inhibited by freezing tubes at −80 ◦ C without affecting their subsequent performance. Thirty permeation tubes with mean SF6 content of 3.081 g were blocked by SF6 content and randomly allocated to one of two treatments being: immediate incubation at 39 ◦ C, or incubation at 39 ◦ C following 75 d storage at −80 ◦ C. The SF6 permeation rate at 39 ◦ C of all tubes was determined by repeated weighing over a 50 d period. Storage of permeation tubes at −80 ◦ C effectively inhibited release of SF6 but had no effect on subsequent permeation rate at 39 ◦ C. Deep frozen storage offers improved flexibility in handling of SF6 permeation tubes for the ERUCT technique. This article is part of the special issue entitled: Greenhouse Gases in Animal Agriculture – Finding a Balance between Food and Emissions, Guest Edited by T.A. McAllister, Section Guest Editors; K.A. Beauchemin, X. Hao, S. McGinn and Editor for Animal Feed Science and Technology, P.H. Robinson. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Methane emissions from ruminants using a calibrated tracer (ERUCT) technique developed by Johnson et al. (1994) is widely used to determine enteric CH4 production of ruminants by estimating expired and eructated CH4 dilution in gas sampled from free ranging animals. A gas sample is continuously collected from near the mouth and nostrils of individual animals for a 24 h period, typically replicated for five consecutive days, to obtain samples representative of daily emissions of CH4 and tracer gas (Johnson et al., 2007). An intra ruminal slow release device for consistent delivery of an inert tracer gas is central to the ERUCT technique. It is assumed that the emission of tracer gas arising from the slow release device exactly simulates CH4 emission from the rumen and that the subsequent dilution of these gases with ambient air is identical (Johnson et al., 1994). The emission rate of CH4 is determined from the measured concentration of CH4 and tracer gas and the known release rate of the tracer from the slow release device as: Q methane = Q tracer × [CH4 ]/[tracer]
Abbreviations: ERUCT, emissions from ruminants using a calibrated tracer; PTFE, polytetrafluoroethylene; SF6 , sulphur hexafluoride. ∗ Corresponding author. Tel.: +353 25 42671; fax: +353 25 42340. E-mail address: [email protected] (M.H. Deighton). 0377-8401/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.anifeedsci.2011.04.011
M.H. Deighton et al. / Animal Feed Science and Technology 166–167 (2011) 198–200
199
Fig. 1. A three dimensional view of the brass permeation tube design and component parts. The range of tare weights of assembled tubes was 56.4–57.0 g.
To date, researchers have utilised sulphur hexafluoride (SF6 ) tracer gas as it is non-toxic, has a low atmospheric abundance, is readily measured via electron capture gas chromatography and has a boiling point of −64 ◦ C. Sublimation of SF6 at this temperature enables slow release ‘permeation tubes’ to be easily filled via cooling to the temperature of liquid N (i.e.,−196 ◦ C; Lassey et al., 2001). As the permeation rate of SF6 from each prepared tube is not identical, tubes must be balanced among experimental treatments to remove potential bias (Pinares-Patino et al., 2008). Permeation tubes selected within the narrowest possible permeation range are blocked according to their individual SF6 permeation rate and randomly allocated to experimental treatments and animals. Tubes excluded from use in an experiment due to their outlier permeation rates are normally discarded and reusable parts recycled. A long term storage method would enable storage of unused permeation tubes between experiments, batch manufacture of tubes well in advance of experiments and more flexibility in transport of tubes among countries. In order to implement such a strategy, it is necessary to decouple the timing of bolus manufacture and subsequent tube expiration by developing a storage method that inhibits SF6 permeation. We hypothesized that permeation of SF6 could be effectively inhibited by freezing tubes at −80 ◦ C without affecting their subsequent performance.
2. Material and methods Thirty brass permeation tubes with 8.1 mm ID, 12.7 mm OD, 45 mm length and an internal volume of 1.92 cc (P&T Precision Engineering Limited, Naas, Ireland) were assembled (Fig. 1) and their tare weight determined to the nearest 0.1 mg. Tubes were filled with SF6 in a fume hood. The nut, frit, membrane and washer were removed and the brass tube body was 3/4 submerged in a proprietary liquid N bath until its temperature equilibrated with the cryogen (i.e.,−196 ◦ C). Care was taken to prevent liquid entry into the lumen of the tube. Tubes remained in the bath during filling, raised sides of the liquid bath trapped a layer of evaporating N gas that excluded air from the tube lumen. Each tube was filled by subliming ∼480 cc of high purity SF6 gas (99.95%, Air Products and Chemicals Incorporated., Allentown, PA, USA) into its lumen. Gas was gradually delivered into each tube using 60 cc luer lock syringes fitted with a one way stopcock and a washer to inhibit gas leakage between the stopcock and brass tube. This enabled uniform filling and minimal SF6 loss to the atmosphere. Sublimed SF6 was immediately sealed within the tubes by replacing the pre-weighed 1.5 mm thick nylon 6,6 washer (6.4 mm ID; 12 mm OD; Radionics Limited, Dublin, Ireland) and 100 m thick polytetrafluoroethylene (PTFE) membrane (12 mm OD; Enflo Canada Limited, Grand Falls, NB, Canada). The permeable PTFE membrane was supported by a 1.6 mm thick sintered 316 stainless steel frit (12.7 mm OD; Mott Corporation, Farmington, CT, USA) and brass tube fitting nut with a 9.5 mm ID aperture (Swagelok, Solon, OH, USA). This design is the same as the cattle size permeation tubes of Lassey et al. (2001) and used within our group (Wims et al., 2010). However a thinner PTFE membrane was used to achieve the high SF6 permeation rate desired for this experiment. Sealed tubes were warmed to room temperature (i.e., 20 ◦ C) while nuts were progressively tightened to 9 nm using a torque wrench. Following filling, tubes were placed in a dry incubator at 39 ◦ C overnight and weighed using a digital analytical balance with 0.1 mg resolution (Explorer Pro, Ohaus Corp., Pine Brook, NJ, USA) following verification of calibration using a standard 50 g weight (Thermo Fisher Scientific Incorporated, Waltham, MA, USA). Mean SF6 content was 3.081 ± 0.0756 g. Tubes were blocked by SF6 content and randomly allocated to treatments being: immediate incubation at 39 ◦ C or incubation at 39 ◦ C following 75 d storage in a −80 ◦ C freezer. Following storage, frozen tubes were thawed at room temperature, warmed to 39 ◦ C to remove external moisture and weighed to determine SF6 loss during storage. The SF6 permeation rate of all tubes was determined by linear regression using tube mass loss recorded three times per week for 7 weeks. Weight recording for each treatment group began 2 weeks after tubes were placed in the incubator. Data were analysed using the general linear models procedure of SAS Institute (2005).
200
M.H. Deighton et al. / Animal Feed Science and Technology 166–167 (2011) 198–200
Table 1 Effect of pre-calibration storage at −80 ◦ C upon subsequent SF6 permeation. Treatment
Immediate calibrationa
Calibration following frozen storagea
n Storage period at −80 ◦ C (days) Calibration period at 39 ◦ C (days) Initial SF6 weight (g) Post-thaw SF6 weight (g) Permeation rate at 39 ◦ C (mg d−1 ) Permeation linear goodness of fit (R2 ) Linear permeation duration (days)
15 0 50 3.08 (0.071) na 20.5 (2.02) 0.9995 144 (12)
15 75 50 3.08 (0.083) 3.07 (0.082) 20.8 (2.18) 0.9996 142 (12)
a
P
0.6199 0.6558 0.2750 0.6974
Least squares mean (standard deviation).
3. Results Storage of permeation tubes at −80 ◦ C inhibited release of SF6 . During the 75 d of frozen storage, loss of SF6 was 6.91 ± 0.896 mg), or 0.22% of pre-storage gas content. Comparison of daily SF6 permeation rates between treatments showed that the freeze/thaw process had no effect on subsequent permeation rate linearity at 39 ◦ C (Table 1). 4. Conclusions Freezing of SF6 permeation tubes at −80 ◦ C causes sublimation of SF6 gas and lowers the gas pressure within the tube thus inhibiting gas permeation across the PTFE membrane. Tubes subjected to this method of storage for a period of 75 d lost only 0.22% of their pre-storage SF6 gas. Following thawing and monitoring of SF6 permeation rates at 39 ◦ C during a 50 d period, we conclude that frozen storage has no effect upon the post-thaw SF6 permeation rate and tubes remain suitable for use in the ERUCT technique. Long term storage of permeation tubes in −80 ◦ C laboratory freezers offers the potential to purchase or manufacture tubes in large batches and to store them until required for pre-experiment calibration. Tubes not selected for specific experiments on the basis of permeation rate may be stored for future use rather than being discarded due to expiry of their gas content. This simple and effective storage method offers increased flexibility in handling of permeation tubes for the ERUCT technique. Conflict of interest None Acknowledgements The authors thank Dr Kenton Hart of Harper Adams University College and Mr Daragh Mortimer of P&T Precision Engineering Limited for their input. This study was made possible by a research stimulus grant from the Irish Department of Agriculture, Fisheries and Food, RSFP 07 517. References Johnson, K.A., Huyler, M., Westberg, H., Lamb, B., Zimmerman, P., 1994. Measurement of methane emissions from ruminant livestock using a SF6 tracer technique. Environ. Sci. Technol. 28, 359–362. Johnson, K.A., Westberg, H.H., Michal, J.J., Cossalman, M.W., 2007. The SF6 tracer technique: methane measurement from ruminants. In: Makkar, H.P.S., Vercoe, P.E. (Eds.), Measuring Methane Production From Ruminants. Springer, Dordrecht, Netherlands, pp. 33–67. Lassey, K.R., Walker, C.F., McMillan, A.M.S., Ulyatt, M.J., 2001. On the performance of SF6 permeation tubes used in determining methane emission from grazing livestock. Chemosphere: Global Change Sci. 3, 367–376. Pinares-Patino, C.A., Machmüller, A., Molano, G., Smith, A., Vlaming, B., Clark, H., 2008. The SF6 tracer technique for measurements of methane emission from cattle – effect of tracer permeation rate. Can. J. Anim. Sci. 88, 309–320. SAS Institute, 2005. SAS User’s Guide: Statistics. SAS Inst. Inc, Cary, NC, USA. Wims, C.M., Deighton, M.H., Lewis, E., O’Loughlin, B., Delaby, L., Boland, T.M., O’Donovan, M., 2010. Effect of pre-grazing herbage mass on methane production, dry matter intake and milk production of grazing dairy cows during the mid-season period. J. Dairy Sci. 93, 4976–4985.
Contents lists available at ScienceDirect
Animal Feed Science and Technology journal homepage: www.elsevier.com/locate/anifeedsci
A simple method for pre-calibration storage of sulphur hexafluoride permeation tubes M.H. Deighton a,b,∗ , B.M. O’Loughlin a , F. Buckley a , T.M. Boland b a b
Animal and Grassland Research and Innovation Centre, Teagasc, Moorepark, Fermoy, Co. Cork, Ireland School of Agriculture, Food Science and Veterinary Medicine, University College, Dublin, Belfield, Dublin 4, Ireland
a r t i c l e
i n f o
Keywords: Sulphur hexafluoride Permeation Tube Frozen Storage
a b s t r a c t Long term storage of sulphur hexafluoride (SF6 ) permeation tubes, used to determine enteric emissions from ruminants using a calibrated tracer (ERUCT) technique, is desirable to increase flexibility of post-manufacture tube handling. To be effective, such storage must decouple the date of tube manufacture from the date of subsequent expiration due to gas loss. We hypothesized that release of SF6 gas could be effectively inhibited by freezing tubes at −80 ◦ C without affecting their subsequent performance. Thirty permeation tubes with mean SF6 content of 3.081 g were blocked by SF6 content and randomly allocated to one of two treatments being: immediate incubation at 39 ◦ C, or incubation at 39 ◦ C following 75 d storage at −80 ◦ C. The SF6 permeation rate at 39 ◦ C of all tubes was determined by repeated weighing over a 50 d period. Storage of permeation tubes at −80 ◦ C effectively inhibited release of SF6 but had no effect on subsequent permeation rate at 39 ◦ C. Deep frozen storage offers improved flexibility in handling of SF6 permeation tubes for the ERUCT technique. This article is part of the special issue entitled: Greenhouse Gases in Animal Agriculture – Finding a Balance between Food and Emissions, Guest Edited by T.A. McAllister, Section Guest Editors; K.A. Beauchemin, X. Hao, S. McGinn and Editor for Animal Feed Science and Technology, P.H. Robinson. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Methane emissions from ruminants using a calibrated tracer (ERUCT) technique developed by Johnson et al. (1994) is widely used to determine enteric CH4 production of ruminants by estimating expired and eructated CH4 dilution in gas sampled from free ranging animals. A gas sample is continuously collected from near the mouth and nostrils of individual animals for a 24 h period, typically replicated for five consecutive days, to obtain samples representative of daily emissions of CH4 and tracer gas (Johnson et al., 2007). An intra ruminal slow release device for consistent delivery of an inert tracer gas is central to the ERUCT technique. It is assumed that the emission of tracer gas arising from the slow release device exactly simulates CH4 emission from the rumen and that the subsequent dilution of these gases with ambient air is identical (Johnson et al., 1994). The emission rate of CH4 is determined from the measured concentration of CH4 and tracer gas and the known release rate of the tracer from the slow release device as: Q methane = Q tracer × [CH4 ]/[tracer]
Abbreviations: ERUCT, emissions from ruminants using a calibrated tracer; PTFE, polytetrafluoroethylene; SF6 , sulphur hexafluoride. ∗ Corresponding author. Tel.: +353 25 42671; fax: +353 25 42340. E-mail address: [email protected] (M.H. Deighton). 0377-8401/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.anifeedsci.2011.04.011
M.H. Deighton et al. / Animal Feed Science and Technology 166–167 (2011) 198–200
199
Fig. 1. A three dimensional view of the brass permeation tube design and component parts. The range of tare weights of assembled tubes was 56.4–57.0 g.
To date, researchers have utilised sulphur hexafluoride (SF6 ) tracer gas as it is non-toxic, has a low atmospheric abundance, is readily measured via electron capture gas chromatography and has a boiling point of −64 ◦ C. Sublimation of SF6 at this temperature enables slow release ‘permeation tubes’ to be easily filled via cooling to the temperature of liquid N (i.e.,−196 ◦ C; Lassey et al., 2001). As the permeation rate of SF6 from each prepared tube is not identical, tubes must be balanced among experimental treatments to remove potential bias (Pinares-Patino et al., 2008). Permeation tubes selected within the narrowest possible permeation range are blocked according to their individual SF6 permeation rate and randomly allocated to experimental treatments and animals. Tubes excluded from use in an experiment due to their outlier permeation rates are normally discarded and reusable parts recycled. A long term storage method would enable storage of unused permeation tubes between experiments, batch manufacture of tubes well in advance of experiments and more flexibility in transport of tubes among countries. In order to implement such a strategy, it is necessary to decouple the timing of bolus manufacture and subsequent tube expiration by developing a storage method that inhibits SF6 permeation. We hypothesized that permeation of SF6 could be effectively inhibited by freezing tubes at −80 ◦ C without affecting their subsequent performance.
2. Material and methods Thirty brass permeation tubes with 8.1 mm ID, 12.7 mm OD, 45 mm length and an internal volume of 1.92 cc (P&T Precision Engineering Limited, Naas, Ireland) were assembled (Fig. 1) and their tare weight determined to the nearest 0.1 mg. Tubes were filled with SF6 in a fume hood. The nut, frit, membrane and washer were removed and the brass tube body was 3/4 submerged in a proprietary liquid N bath until its temperature equilibrated with the cryogen (i.e.,−196 ◦ C). Care was taken to prevent liquid entry into the lumen of the tube. Tubes remained in the bath during filling, raised sides of the liquid bath trapped a layer of evaporating N gas that excluded air from the tube lumen. Each tube was filled by subliming ∼480 cc of high purity SF6 gas (99.95%, Air Products and Chemicals Incorporated., Allentown, PA, USA) into its lumen. Gas was gradually delivered into each tube using 60 cc luer lock syringes fitted with a one way stopcock and a washer to inhibit gas leakage between the stopcock and brass tube. This enabled uniform filling and minimal SF6 loss to the atmosphere. Sublimed SF6 was immediately sealed within the tubes by replacing the pre-weighed 1.5 mm thick nylon 6,6 washer (6.4 mm ID; 12 mm OD; Radionics Limited, Dublin, Ireland) and 100 m thick polytetrafluoroethylene (PTFE) membrane (12 mm OD; Enflo Canada Limited, Grand Falls, NB, Canada). The permeable PTFE membrane was supported by a 1.6 mm thick sintered 316 stainless steel frit (12.7 mm OD; Mott Corporation, Farmington, CT, USA) and brass tube fitting nut with a 9.5 mm ID aperture (Swagelok, Solon, OH, USA). This design is the same as the cattle size permeation tubes of Lassey et al. (2001) and used within our group (Wims et al., 2010). However a thinner PTFE membrane was used to achieve the high SF6 permeation rate desired for this experiment. Sealed tubes were warmed to room temperature (i.e., 20 ◦ C) while nuts were progressively tightened to 9 nm using a torque wrench. Following filling, tubes were placed in a dry incubator at 39 ◦ C overnight and weighed using a digital analytical balance with 0.1 mg resolution (Explorer Pro, Ohaus Corp., Pine Brook, NJ, USA) following verification of calibration using a standard 50 g weight (Thermo Fisher Scientific Incorporated, Waltham, MA, USA). Mean SF6 content was 3.081 ± 0.0756 g. Tubes were blocked by SF6 content and randomly allocated to treatments being: immediate incubation at 39 ◦ C or incubation at 39 ◦ C following 75 d storage in a −80 ◦ C freezer. Following storage, frozen tubes were thawed at room temperature, warmed to 39 ◦ C to remove external moisture and weighed to determine SF6 loss during storage. The SF6 permeation rate of all tubes was determined by linear regression using tube mass loss recorded three times per week for 7 weeks. Weight recording for each treatment group began 2 weeks after tubes were placed in the incubator. Data were analysed using the general linear models procedure of SAS Institute (2005).
200
M.H. Deighton et al. / Animal Feed Science and Technology 166–167 (2011) 198–200
Table 1 Effect of pre-calibration storage at −80 ◦ C upon subsequent SF6 permeation. Treatment
Immediate calibrationa
Calibration following frozen storagea
n Storage period at −80 ◦ C (days) Calibration period at 39 ◦ C (days) Initial SF6 weight (g) Post-thaw SF6 weight (g) Permeation rate at 39 ◦ C (mg d−1 ) Permeation linear goodness of fit (R2 ) Linear permeation duration (days)
15 0 50 3.08 (0.071) na 20.5 (2.02) 0.9995 144 (12)
15 75 50 3.08 (0.083) 3.07 (0.082) 20.8 (2.18) 0.9996 142 (12)
a
P
0.6199 0.6558 0.2750 0.6974
Least squares mean (standard deviation).
3. Results Storage of permeation tubes at −80 ◦ C inhibited release of SF6 . During the 75 d of frozen storage, loss of SF6 was 6.91 ± 0.896 mg), or 0.22% of pre-storage gas content. Comparison of daily SF6 permeation rates between treatments showed that the freeze/thaw process had no effect on subsequent permeation rate linearity at 39 ◦ C (Table 1). 4. Conclusions Freezing of SF6 permeation tubes at −80 ◦ C causes sublimation of SF6 gas and lowers the gas pressure within the tube thus inhibiting gas permeation across the PTFE membrane. Tubes subjected to this method of storage for a period of 75 d lost only 0.22% of their pre-storage SF6 gas. Following thawing and monitoring of SF6 permeation rates at 39 ◦ C during a 50 d period, we conclude that frozen storage has no effect upon the post-thaw SF6 permeation rate and tubes remain suitable for use in the ERUCT technique. Long term storage of permeation tubes in −80 ◦ C laboratory freezers offers the potential to purchase or manufacture tubes in large batches and to store them until required for pre-experiment calibration. Tubes not selected for specific experiments on the basis of permeation rate may be stored for future use rather than being discarded due to expiry of their gas content. This simple and effective storage method offers increased flexibility in handling of permeation tubes for the ERUCT technique. Conflict of interest None Acknowledgements The authors thank Dr Kenton Hart of Harper Adams University College and Mr Daragh Mortimer of P&T Precision Engineering Limited for their input. This study was made possible by a research stimulus grant from the Irish Department of Agriculture, Fisheries and Food, RSFP 07 517. References Johnson, K.A., Huyler, M., Westberg, H., Lamb, B., Zimmerman, P., 1994. Measurement of methane emissions from ruminant livestock using a SF6 tracer technique. Environ. Sci. Technol. 28, 359–362. Johnson, K.A., Westberg, H.H., Michal, J.J., Cossalman, M.W., 2007. The SF6 tracer technique: methane measurement from ruminants. In: Makkar, H.P.S., Vercoe, P.E. (Eds.), Measuring Methane Production From Ruminants. Springer, Dordrecht, Netherlands, pp. 33–67. Lassey, K.R., Walker, C.F., McMillan, A.M.S., Ulyatt, M.J., 2001. On the performance of SF6 permeation tubes used in determining methane emission from grazing livestock. Chemosphere: Global Change Sci. 3, 367–376. Pinares-Patino, C.A., Machmüller, A., Molano, G., Smith, A., Vlaming, B., Clark, H., 2008. The SF6 tracer technique for measurements of methane emission from cattle – effect of tracer permeation rate. Can. J. Anim. Sci. 88, 309–320. SAS Institute, 2005. SAS User’s Guide: Statistics. SAS Inst. Inc, Cary, NC, USA. Wims, C.M., Deighton, M.H., Lewis, E., O’Loughlin, B., Delaby, L., Boland, T.M., O’Donovan, M., 2010. Effect of pre-grazing herbage mass on methane production, dry matter intake and milk production of grazing dairy cows during the mid-season period. J. Dairy Sci. 93, 4976–4985.