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Five Things You May Not Know About GC Cheat Sheet (DRAFT) by [deleted]

Five Things You May Not Know About GC

This is a draft cheat sheet. It is a work in progress and is not finished yet.

Introd­uction

These five tips for gas chroma­tog­raphy (GC) could improve your methods.

Hydrogen Is the Best Carrier Gas

Hydrogen can provide increased analysis speed by increasing carrier gas linear velocity without losing too much effici­ency, allowing for shorter run times and thereby increasing throug­hput. Lower temper­ature separa­tions are another benefit; at faster elution times, it may not be necessary to increase the column temper­ature ramp rate. It may even be possible to lower the maximum temper­ature needed for the analysis, which will reduce the reequi­lib­ration time too. The use of lower temper­atures leads to less column bleed, which in turn can mean a longer column life. Hydrogen is readily available through the electr­olysis of water and, with gas genera­tors, it can be generated safely on demand. Hydrogen gas is already used in the laboratory for a variety of purposes; it is the fuel gas for the most commonly used GC detector, the flame ionization detector.

GC–MS Columns for GC Methods

Analytical sensit­ivity and detection limits are ultimately a function of the signal­-to­-noise ratio (S/N). 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 baseline noise. Degrad­ation is accele­rated when excess oxygen is present and at higher temper­atures; hence the elevated baseline that is seen as the temper­ature rises to the column's upper limit. Columns designated for gas chroma­tog­rap­hy–mass spectr­ometry (GC–MS) have been designed to exhibit reduced bleed and high inertness at elevated temper­atures, which ultimately increases S/N. Many detectors are sensitive to contam­ination from bleed products and require less mainte­nance when using low-bleed columns. For GC–MS applic­ations, low-bleed stationary phases reduce column contri­bution to background noise, resulting in improved mass spectral purity and more-a­ccurate library identi­fic­ation.

The Wonder of Gas-Saver Mode

Gas-saver mode can be used during split injection to change the split ratio at a specified time after sample injection, which reduces carrier gas consum­ption. For example, if the split flow rate is reduced from 150 mL/min to 25 mL/min 1 min after sample injection (ensure all the sample and solvent vapors have been transf­erred to column prior to using the gas saver), the reduced split flow rate can be maintained throughout the analytical run until the next analysis. Under these condit­ions, that is a 79% gas saving per analysis: analysis time: 20 min; split ratio: 100:1; gas saver mode: split ratio 15 after 1 min; column temper­ature: 100 °C; column: 30 m × 0.25 mm, 0.25 µm.
 

Gas chroma­­to­g­raphy

Advantages of Highly Inert Columns

Manufa­cturers have developed new ways of deacti­vating GC columns in recent years, which provides improved sensit­ivity because they 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 and cause peak tailing that impacts sensit­ivity (via reduced peak heights relative to baseline noise) and makes integr­ation (and hence quanti­tation) more challe­nging and less reprod­ucible. The benefit of highly deacti­vated columns can only be fully realized in conjun­ction with a system that has an inert flow path (that is, inlet liner and packing, column, ion source, and so forth). Any advantages from low silanol activity in only the column will be mitigated if peak tailing occurs in the inlet. Most manufa­cturers provide highly inert consum­ables, including liners and packing, as well as deacti­vated inlets.

A Little Bit of Split

Splitless injection is used for low-co­nce­ntr­ation samples to provide optimum sensit­ivity. Sample vapor transfer from the inlet is much slower compared with split injection, resulting in band broade­ning; therefore, sample vapors must be trapped at the head of the column by using a low initial oven temper­ature. If some sensit­ivity can be sacrif­iced, it may be better to use a little bit of split, for the following reasons:
Better peak shape: the liner is cleared more quickly, which introduces the analytes onto the column in a narrower band.
No need for cryo-c­ooling: the analytes are being introduced in a narrower band; hence, the oven temper­ature does not need to be lowered to allow for focusing at the head of the column.
Shorter run times: analyses can be started at higher initial oven temper­atures, which decreases run and oven reequi­lib­ration times.
Use of isothermal methods: With no need for low initial oven temper­atures, isothermal methods that start at higher temper­atures can be used. These methods are partic­ularly good for samples that contain higher boiling point compon­ents.
A split of 1:10 is good for balancing sensit­ivity with the benefits of split injection.