Mass Spectrometry Fundamental GC-MS GC-MS Interfaces Wherever you see ths symbol, t s mportant to access the on-lne course as there s nteractve materal that cannot be fully shown n ths reference manual.
Ams and Objectves Ams and Objectves Ams Explan the workng prncple of the most mportant GC/MS nterfaces currently avalable Explore the correct nterface types for use wth dfferent GC Columns and MS Instrument types Descrbe some of the more common problems affectng the nterface n GC/MS equpment Explan the prncples of Curtan Gas equpment to allow column changng wthout ventng the analyser Objectves At the end of ths Secton you should be able to: Descrbe the most common nterfaces for GC-MS drawng clear dstnctons between them and explanng the workng prncples assocated to each of them Lst the man elements present n a GC/MS nterface Choose the correct nterface for a gven nstrument confguraton Undertake troubleshootng of MS nterface problems
Content Introducton 3 Column Dameter 3 The Jet Separator 4 The Beman Concentrator 5 Drect Introducton 5 Vacuum System Consderatons 6 Couplng GC to MS Detectors Jet Separator 6 Couplng GC to MS Detectors Drect Interface 7 Interface-Column Couplng I 8 Interface-Column Couplng II 9 Interface-Column Couplng III 10 Interface-Column Couplng IV 11 References 12 Crawford Scentfc www.chromacademy.com 2
Introducton After separaton n the GC column, analyte speces have to be transported to the mass spectrometer to be onsed, mass fltered and detected. [1] The column outlet needs to be connected to the on source of the mass spectrometer and dfferent strateges had been mplemented, all of whch need to fulfl the followng condtons: Analyte must not condense n the nterface Analyte must not decompose before enterng the mass spectrometer on source The gas load (dctated by the moble phase gas flow rate) enterng the on source must be wthn the pumpng capacty of the mass spectrometer Interface GC/MS dagram Column Dameter The most common capllary GC columns have an nternal dameter below around 0.32mm and the flow rate assocated wth these columns wll not requre that a separator type nterface s used. However 0.32mm.d. columns used at hgher flow rate and most 0.53mm.s. columns wll requre specal attenton when nterfacng wth MS nstruments. The table below presents common typcal flow rates (measured at 40 o C) for selected GC- MS columns (carrer gas helum 100%). [12,13] Table 1. Typcal flow rates for selected GC-MS columns. Colum dmensons Lnear Velocty (cm/s) Flow rate (ml/mn) 60m 0.53mm 30 11 30m 0.32mm 32 2.5 60m 0.25mm 30 1.3 30m 0.25mm 30 0.7 40m 0.15mm 35 1.2 20m 0.15mm 35 0.6 25m 0.1mm 40 1.0 12.5m 0.1mm 40 0.4 Crawford Scentfc www.chromacademy.com 3
GC-MS applcatons where the flow rate to the MS detector do not exceed 2.0 ml/mn can usually be acheved by usng drect nterfaces. Hgher flow rates wll requre the use of vapour concentrator devces (lke the Beman concentrator) or jet separator nterfaces. [2,14] At flow rates above around 2mL/mn. even the most effcent two stage vacuum systems wll struggle to attan the requred level of vacuum (~10-5 to 10-6 torr) to carry out the analyss wth the requred senstvty. Further, flaments wll have much reduced lfetmes at compromsed vacuum levels. GC-MS Couplng Drect Introducton Jet separator Beman Concentrator The Jet Separator The Jet Separator nterface s used wth packed columns or wth wde bore capllary columns at hgher flow rates. In ths devce the carrer gas s removed n preference to the analyte and hence analyte enrchment occurs. Jet Separator Crawford Scentfc www.chromacademy.com 4
The Beman Concentrator The Beman concentrator devce s used wth packed columns or wth wde bore capllary columns at hgher flow rates. In ths devce the carrer gas s removed n preference to the analyte and hence analyte enrchment occurs. Drect Introducton Beman Concentrator Drect ntroducton s typcally used wth capllary GC columns and most modern nstruments can easly cope wth flow rates up to 2 ml/mn. Drect Introducton Crawford Scentfc www.chromacademy.com 5
Vacuum System Consderatons The entre MS process must be carred out at very low pressures (~10-8 atm) and n order to meet ths requrement a vacuum system s requred. It s dffcult for packed GC columns to be nterfaced to an MS detector because they have carrer gas flow rates that cannot be as successfully pumped away by normal vacuum pumps; however, capllary columns' carrer flow s 25 or 30 tmes less and therefore easer to "pump down." That sad, GC/MS nterfaces have been developed for packed column systems that allow for analyte molecules to be dynamcally extracted from the carrer gas stream at the end of a packed column. Couplng GC to MS Detectors Jet Separator Many dfferent strateges have been used to couple gas chromatography columns to mass spectrometrc detectors. Packed GC columns produce a relatvely hgh volumetrc flow of carrer gas, whch f ntroduced drectly nto the on source of the mass spectrometer may compromse the vacuum leadng to much reduced effcency of the onsaton process and potentally shortenng the lfetme of the flaments used for electron producton. Although modern MS vacuum pump equpment s hghly effcent, even the most hghly effcent systems cannot cope wth carrer flows n excess of 4 ml/mn. Jet Separator Interface. Crawford Scentfc www.chromacademy.com 6
Consequently, nterface devces used wth packed columns or where carrer flow s hgh (>2mL/mn. wth typcal equpment), employ a jet separator devce. Ths contans a very small gap between the column and the transfer lne whch s held under vacuum and strps away the lghter carrer gas molecules (they have hgher dffuson rates), whlst the analyte molecules whose hgher mass gves them hgher momentum, brdge the gap and travel towards the on source of the mass spectrometer. These devces have the dsadvantage that some analyte molecules (especally the more volatle speces) may be lost to the vacuum. Couplng GC to MS Detectors Drect Interface Perhaps the most wdely used desgn for modern capllary GC s the capllary drect nterface. The advent of capllary columns brought about a sgnfcant reducton n the volumetrc gas flow extng the column (typcally 1ml/mn or below for columns of 0.32mm d and less), and the need to splt the analyte away from the carrer gas to reduce gas load nto the on source was elmnated. In a drect nterface, the column s nserted drectly nto the mass spectrometer onsaton chamber. Ths nterface gves the hghest senstvty, however changng the GC column may be a tme consumng process unless curtan gas devces are ftted. [2] Drect Interface Crawford Scentfc www.chromacademy.com 7
It should be noted that ALL nterface desgns contan a heat source or are lagged wth a heatng jacket n order to prevent analyte condensaton wthn the transfer lne. The heat appled to the nterface must prevent condensaton but must also avod thermal decomposton of lable analytes. In general the heatng of the GC-MS system wll ncrease as analytes transton from the column through the nterface to the on source and mass analyser. Dfferent onsaton modes (Electron Impact (EI) and Chemcal Ionsaton (CI) requre dfferent nterface, on source and mass analyser temperatures for optmum operaton). Interface-Column Couplng I In GC-MS, the tradtonal couplng between the GC column and the nterface has uses a smple nut and a ferrule. It should be noted that Ferrules used for GC-MS dffer from those used for capllary GC wth other detector types. GC-MS ferrules tend to be harder and therefore less permeable to gas and are usually made from a Graphte / Vton composte materal. The dsadvantage of these ferrules s that they have a shorter usable lfe. Ths tradtonal couplng method s not longer recommended, long perods of tme are requred to properly change the GC column. [2] Interface-column couplng Crawford Scentfc www.chromacademy.com 8
The process of changng column s started wth turnng the mass spectrometer and the vacuum system off. The GC column can be replace as soon as the nterface reaches a temperature low enough to safety dsmantle t, then and only then the mass spectrometer can be turned on. [3] One of the man reasons for the long tme requred to change the GC-MS column s the need for re-establshng optmum levels of vacuum (mosture and ar) after the mass spectrometer was down. The removal of water s always the bggest problem, takng up to twelve hours n most systems. Interface-Column Couplng II Changng GC columns when usng a drect nterface can be protracted, as to prevent catastrophc vacuum loss (and possble damage to the vacuum system), the nstrument must be vented (.e. the vacuum system must be gradually slowed untl the nstrument s no longer under vacuum). Ths can be tme consumng and to vent and then pump down an nstrument (.e. re-establsh vacuum) may take between 4 and 8 hours. Varous devces exst to allow the GC-MS column to be changed wthout havng to vent the mass spectrometer. Curtan gas couplng GC-MS devce Crawford Scentfc www.chromacademy.com 9
Most common devces work on the curtan gas prncple n whch the GC column and the MS nterface are joned usng a tee-pece nto whch s also pumped an nert gas. When the column s removed for changng the nert gas s pressursed and forms an effectve barrer to the rapd ngress of ar and mosture, hence servng to retan the vacuum wthn the nstrument. The nert gas supply s turned on ONLY when the GC column needs to be changed, once the new column s ftted, then the nert gas s turned off and the new analyss can take place. In order to avod condensaton, an addtonal heatng devce may be place at the end of the GC column. Interface-Column Couplng III Other devces desgned to reduce the tme taken to change the GC column nclude restrcton devces. [7,8,9] Restrctor couplng GC-MS devce Crawford Scentfc www.chromacademy.com 10
The restrctor presents an orfce (100μm nternal dameter) specally desgned for mnmzng the amount of ar (oxygen) and mosture enterng the nterface when the GC column s removed. Wth a restrcton connector nstalled, the pressure drop across the 0.100μm transfer lne makes t necessary to ncrease the head pressure to obtan retenton tmes equal to those you obtaned wthout the connector. The hgher head-pressures used wth restrcton connectors mght change njecton port characterstcs. Interface-Column Couplng IV In order to mprove the effcency of the vent free adaptor devces, some manufacturers recommend the use of deactvated capllary tubng between the nterface and the vent free adaptor. Ths capllary tube can help to prevent mosture enterng nto the nterface and to the mass analyser f arranged n the correct fashon. [10,11] The capllary tube remans fxed and the column can be changed nto the vent free adaptor as descrbed n the prevous pages. Deactvated capllary tube connecton Crawford Scentfc www.chromacademy.com 11
References 1. A. Brathwate and F. J. Smth. Chromatographc Methods Blacke Academc & and Professonal. PP 375-379. UK 1996 2. Suart C. Hansen and Jean-Luc Truche. Flow-lmted drect GC/MS nterface Unted States Patent # 4662914, May 5, 1987. 3. Robert Stevenson. New nstruments for separaton scence at PITTCON 2000: The new age of allances? Amercan Laboratory Separaton scence. Pp 19-42. May 2000 4. SGE Chromatography Products. http://www.sge.com/ 5. MS No-Vent. Installaton & Operatng Instructons. Applcaton note SGE. 6. Alltech GC/MS Swtchng Valve. Applcaton note. Copyrght 2004 Alltech Assocates, Inc. 7. http://www.restek.com 8. EZ No-Vent GC/MS Connector for Aglent 5971/5972 and 5973 Mass Spectrometers. Applcaton note RESTEK, July 2007. 9. CLINICAL/ FORENSICS Products & Applcatons for GC & HPLC. Applcaton note RESTEK, 2006. 10. http://www.fronter-lab.com/products/vf-adaptor/ndex.html 11. Vent-free GC/MS Adapter. Fronter Laboratores Ltd. Applcaton note. 12. GC Columns. www.varannc.co 13. The Essental Chromatography Catalog Aglent Technologes. 14. Raymond P. W. Scott, Tandem Technques John Wley & Sons. Pp 165-173. USA 1997 Crawford Scentfc www.chromacademy.com 12