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GC-MS Columns for GC Methods

Could you gain sensitivity in your GC method by utilizing a GC-MS column?  The answer is yes.

Analytical sensitivity and detection limits are ultimately a function of the signal to noise (S/N) ratio. A decrease in noise increases sensitivity.  All GC columns bleed, although polar phases and thicker films are more prone to bleed. This normal degradation of the stationary phase polymer results in increased background noise. Degradation is accelerated when excess oxygen is present and at higher temperatures; hence the elevated baseline which is normally seen as the temperature rises to the column’s upper limit (Figure 1). GC-MS designated columns have been designed to exhibit reduced bleed and high inertness even at elevated temperatures, which ultimately increases S/N ratio.

Figure 1: Comparison of pesticide analysis on a standard GC column and a GC-MS column (GC-FID method).

 

For GC-MS applications low bleed stationary phases reduce column contribution to background noise which results in improved mass spectral purity and more accurate library identification (Figure 2). Ion trap based MS systems will greatly benefit from low bleed columns due to the unique relationship between ion storage capacity of the trap and sensitivity; less trapping of unwanted background bleed ions in the same mass range as the target analyte will increase sensitivity.

Figure 2

Many detectors are sensitive to contamination from bleed products and will require less maintenance when using low bleed columns. Bleed may appear worse when using detectors which are particularly sensitive to the siloxane bleed products, e.g. cyanopropyl phases with nitrogen phosphorus detectors and polyethylene glycol phases with electron capture detectors.    

Reduced bleed and improved inertness is achieved by altering the chemistry of the polysiloxane backbone and carrying out special surface all of which enhance the chromatographic performance of siloxane polymers. In the case of phases which contain phenyl groups an arylene moiety is included in the siloxane polymer which provides increased stability and in turn reduces degradation and bleed. For 100% PDMS columns, optimized siloxane polymers are used.

Figure 3: 5% phenyl and arylene GC stationary phases.

 

Another column type to consider for improved sensitivity of GC methods are ultra-inert columns which exhibit low bleed and low silanol activity. Decreased silanol activity is particularly pertinent when analyzing bases or polar compounds or for certain specialist applications, such as pesticide, food, environmental, or drug analyses all of which require ever decreasing detection limits. Active silanol species in the column can interact with bases or polar compounds resulting in peak tailing which impacts sensitivity and can make quantitation more challenging and less reproducible. 

However, the benefit of ultra-inert columns can only be fully realized in conjunction with a system which has an inert flow path (i.e. inlet liner and packing, column, ion source etc.). Any advantage from low silanol activity in only the column will be mitigated if peak tailing occurs prior to this in the inlet. Manufacturers provide ultra-inert consumables, including liners and packing, as well as deactivated inlets.

The chromatogram shown in Figure 4 demonstrates the advantages of ultra-inert columns by analyzing active probe species. Peak tailing or poor sensitivity for acidic analytes indicates that there are basic interaction sites within the column. Conversely poor chromatographic behavior of basic analytes confirms the presence of active acidic sites in the column. Analytes with alcohol functional groups can be used to probe for oxygen damage or exposed silanols. It can be seen that on a standard GC column several of these analyte types exhibit poor peak shapes whereas on the ultra-inert column this problem is resolved. The improved peak shape, which results in improved resolution and consistent integration, for active compounds when using ultra-inert columns improves analytical sensitivity.

Figure 4

Column: 5% - Phenyl 30 m x 0.25 mm x 0.25 μm.
Oven: 65 °C isothermal.
Carrier: H2, constant pressure, 38 cm/s.
Injection: Split, 250 °C, 1.4 mL/min, split column flow 900 mL/min, gas saver flow 75 mL/min at 2.0 min.
Detection: FID, 325 °C, air (450 mL/min): H2 (40 mL/min): N2 makeup (45 mL/min).
Liner: Deactivated single taper with glass wool.
 

 

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Dr. Dawn Watson
 

This article was written by Dr. Dawn Watson.

Dawn received her PhD in synthetic inorganic chemistry from the University of Strathclyde, Glasgow. The focus of her PhD thesis was the synthesis and application of soft scorpionate ligands. As well as synthetic skills, this work relied on the use of a wide variety of analytical techniques, such as, NMR, mass spectrometry (MS), Raman spectroscopy, infrared spectroscopy (IR), UV-visible spectroscopy, electrochemistry, and thermogravimetric analysis.

Following her PhD she spent two years as a postdoctoral research fellow at Princeton University studying the reaction kinetics of small molecule oxidation by catalysts based on Cytochrome P450. In order to monitor these reactions stopped-flow kinetics, NMR, HPLC, GC-MS, and LC-MS techniques were utilized.

Prior to joining the Crawford Scientific and CHROMacademy technical team she worked for Gilson providing sales and support for the entire product range including, HPLC (both analytical and preparative), solid phase extraction, automated liquid handling, mass spec, pipettes, and laboratory consumables.

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