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With liquid samples, the main challenge is to avoid gas bubbles in the injected sample as these will cause erratic measurements. To guard against bubbles, keep the pressure of a liquid sample as high as possible, consistent with the pressure rating of the sample injector valve.

A volume of liquid contains about 300 times as many molecules as an equal volume of vapor. Therefore, to inject the same number of molecules, a liquid sample volume needs to be very small, usually less than one microliter (1 μL). In such a small volume, even the smallest bubble will displace a significant amount of the sample volume and cause low measurement values.

It's easy to visualize a microliter since it's the same size as a one‐millimeter cube (1 mm³). A volume of one thousand microliters is equal to one milliliter (1 mL) and to one cubic centimeter (1 cm³), commonly called a cc.

Another challenge with liquid samples is getting a complete and instant vaporization without making the sample too hot, lest it start to react or decompose. The small volume is helpful, and most process liquids quickly vaporize without significant decay.