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A Little Bit of Split

Splitless injection is used for low concentration samples to provide optimum sensitivity, i.e. for trace analysis. In splitless injection the split valve is initially closed, ensuring that the entire sample passes onto the column, then at an optimized time the split line is turned on to clear the inlet of any residual vapors (Figure 1).

The transfer of the sample vapor (diluted with carrier gas) from the inlet is much slower compared with split injection. This can result in band broadening, therefore, the sample vapors must be trapped at the head of the column by using a low initial oven temperature (Figure 2).

The parameters which must be optimized with splitless injection (oven temperature, splitless time, solvent choice etc.) make it a more complex sample introduction technique than split injection.

Split injection is typically used to perform an on-instrument dilution of concentrated samples; therefore, it may not be amenable to methods with low concentration samples which have high sensitivity requirements. Typical split ratios are in the range 20:1-400:1 - as the split ratio increases the less sample is introduced onto the column (Equation 1).

Equation 1
 

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Figure 1: Splitless injection.
 

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Figure 2: Focusing techniques.

If a little bit of sensitivity can be sacrificed it may be better to use a little bit of split, for the following reasons:

  1. Better peak shape.  The liner is cleared more quickly which introduces the analytes onto the column in a narrower band.
  2. No need for cryo-cooling.  Since the analytes are being introduced in a narrower band the oven temperature does not need to be lowered to allow for focusing at the head of the column. 
  3. Shorter run time.  Analyses can be started at higher initial oven temperatures which can decrease run and oven re-equilibration times.
  4. Use of isothermal methods.  Again, there is no need for low initial oven temperatures; therefore isothermal methods which start at high temperatures can be employed.  These methods are particularly good for samples which contain high boilers.

A split of 1:10 is good for balancing sensitivity of low concentration samples, with the benefits of split injection.  The calculation shown (Figure 3) demonstrates the increased efficiency which will be gained by transferring a sample more quickly onto the column.  Instead of a peak which is 1 minute wide reaching the column in splitless injection (this will require focusing using a low oven temperature), a 10:1 split means that the analytes can be transferred in 5.45 s giving a much narrower peak (increasing the split to 20:1 would reduce this time further to 2.5 s).

 

Transfer time using a little bit of split.

1:10 split.  1 µL sample which vaporizes to 1 mL.

Transfer time for the same volume using splitless injection. 

In splitless injection the carrier gas flow rate through the injection port liner is simply the column flow rate.

Figure 3: Comparison of analyte transfer time using split and splitless injection.


Therefore, using a low split ratio, even with low concentration samples, could bring many benefits.  For those samples which still require a high level of sensitivity, the method can be optimized by using, for example, low bleed GC-MS columns (improved signal-to-noise ratio), optimizing sample preparation (solid phase extraction to remove matrix components), decreasing column diameter to give taller more efficient peaks, or the use of more selective detectors (e.g. mass spectrometers).


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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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