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The Wonder of Gas Saver Mode

Split injection is conventionally used for analyses where the sample concentration is high and the user wishes to reduce the amount of analyte reaching the capillary column by performing an 'on-instrument' dilution.

Why perform an on-instrument dilution rather than just diluting the sample further prior to injection?  Capillary GC columns are limited to the amount of each analyte which can be introduced onto the column before peak shapes begin to deteriorate.  Smaller i.d. columns and thinner stationary phase films have lower capacity, and analyte concentrations in the order of a few nanograms on column are typical, therefore, we require a reasonably dilute sample (Table 1).

Column i.d. (mm) Film Thickness (μm)
0.1 0.25 0.5 1.0
0.10 10 ng 30-40 ng 50-70 ng 100-200 ng
0.18 20-30 60-80 100-150 250-350
0.25 30-40 125-175 175-250 400-500
0.32 50-70 200-250 250-350 600-800
0.45 80-100 300-400 400-500 800-1000
0.53 100-120 400-500 500-700 1000-15000

Table 1: Typical column capacities for capillary GC columns.  Mass (ng) per analyte.

 

Performing an on-instrument dilution is preferable as it results in sharper peaks.  The split ratio is used to control the amount of analyte reaching the column, which ultimately affects peak width and sensitivity.  Typical split ratios are in the range 1:20 to 1:400.  When using thick stationary phase film (>0.5 μm) or wide bore (0.533 mm i.d.) columns the sample capacity increases and lower split ratios are typical, 1:5 to 1:20.  With very narrow GC columns (<100 μm i.d.) split ratios can be as high as 1:1000+.

High-concentration samples are analyzed using a large split ratio in order to reduce the amount of sample introduced into the column.  However, during normal GC analysis it is not necessary to maintain the large split ratio throughout the entire analysis time, as the sample is vaporized immediately after injection and then transported by the carrier gas on to the column (Figure 1).  The large split ratio used results in a high consumption of carrier gas which is ultimately costly. 

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Figure 1: Split injection.

Gas saver mode can be used can be used during split injection to change the split ratio at a specified time after sample injection in order to reduce carrier gas consumption (Figure 2).  In the example shown, gas saver mode reduces the split flow rate from 150 mL/minute to 25 mL/minute one minute after the sample is injected (at this point the required sample amount will have been transferred to the column). When an autoinjector is used, the reduced split flow rate is maintained throughout the analytical run until the next analysis.

Figure 2: Graphical representation of gas saver mode.

Gas saver mode can reduce carrier gas consumption by approximately 79% per analysis under the conditions described (Figure 3).  
Analysis time: 20 minutes
Split ratio: 100
Gas saver mode: Split ratio 15 after 1 minute
Column temperature: 100 °C
Column: 30 m x 0.25 mm, 0.25 μm

Figure 3: Carrier gas saving using gas saver mode.

 

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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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