Tech Tip 11: Use of Autosamplers – Part I

Modern autosamplers have become much more versatile and sophisticated than their original antecedents, so that they are not merely a device for replacing manual injection, they can now offer many options and improvements to the introduction of liquid and gaseous sample aliquots into a gas chromatograph. However, as with most analytical equipment, each function needs fine tuning to produce the best results. 

Solvents and syringe cleaning. 

A basic requirement for autosamplers is the prevention of contamination and carry-over between samples.  Most autosamplers use micro-syringes very similar to those used for manual injection.  However syringes with a side-arm arrangement have been used to provide a "flush-through" approach to sample loading and syringe cleaning.  One of the drawbacks with this system is the high sample consumption needed to load the syringe and flush through connecting tubing.

Loading the autosampler syringe usually consists of a sequence of syringe "washes" and "pumps".

  • A "WASH" consists of filling the syringe to it full volume (see below) with either solvent or sample and then dispensing the full contents into a waste container.  
  • A "PUMP" fills the syringe to full volume (see below) with sample and then dispensing it back into the sample vial.

Thus when constructing a autosampler profile, the syringe is usually "washed" with solvent(s) X times, to clean the syringe, "washed" with sample  Y times to remove excess solvent and prevent sample dilution in the syringe, and finally "pumped" Z times to fill with sample and remove air bubbles from the loaded volume prior to injection.

Where there is an option for multiple cleaning solvents, the primary cleaning solvent should match the solvent used for the samples.  This will provide sufficient solvation to dissolve and clean out residues from the syringe after injection. Where this solvent is very volatile, and/or the analytical run time is long, there is a possibility for the solvent plunger to "dry-out" between injections. This may cause the plunger to stick, and the autosampler to fault, stopping the analytical sequence.  

As a precautionary action, a second wash solvent may be used that is less volatile and more polar (methanol, iso-propanol).  This will wet the syringe plunger and barrel, maintaining lubrication until the next injection.  As part of the injection programme, the syringe can then be pre-washed with the primary solvent, to remove all traces of the secondary solvent before loading the next sample for injection.

Where there is a limited volume of sample or solvent available, (cost, rarity, limited reservoir size) modern autosamplers can be set to wash and rinse with a reduced volume, thus conserving sample and/or solvents.

To learn more about sample preparation for GC & GC-MS, attend Day 3 / Module 8 of our Complete GC & GC-MS course.

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