Which is Better, Manual Injection or an Autosampler?

by | Feb 9, 2021

This Edition of HPLC solutions is analyzing the advantages of both manual and autosampler injection.

Q: I’m wondering why most of scientists prefer to use a manual injector?! I know that autosamplers are used more to improve the productivity and it is standard with many HPLC systems. I was working with both and the autosampler produced better precision. Please can you tell me in which situations one is better than the other?

JWD: You must be working in a different environment than I am, because I would have to look very hard to find a colleague using a manual injector. Not only are manual injectors less convenient to use, they tend to be less precise, as you have observed. Unless you use the injection loop volume to determine the injection volume, it is unlikely that the manual injector will compare favorably to an autosampler, and even then, you have to be very consistent about overfilling the loop with sample. (We’ll discuss loop-filling and accuracy in the next installment of HPLC Solutions #119.) The reason is quite simple. With manual injections, you are relying on the combination of your dexterity, vision, syringe performance, and injection technique to determine the repeatability of injections. Each of these items can vary to some extent. On the other hand, autosamplers rely on a syringe held firmly in a motor-driven device that is extremely reproducible in its movements, so autosampler performance almost always beats manual injection if the mechanism is working properly.

HPLC Solutions #118: Which is Better, Manual Injection or an Autosampler? Table 1

   Table 1 is from our Master Class on performance qualification, where we recommend regularly testing autosampler performance, such as every 6-12 months. The test is quite simple with your favorite C18 column. Use a mobile phase, such as 80/20 methanol/water, that will give k ˜ 5 for anthracene. Make 6 replicate injections of 10 µL of a solution of ˜2 mg/mL of anthracene with the UV detector set at 260 nm. Our acceptance criteria are that the relative standard deviation (%RSD) of peak areas should be no more than 1%. With the autosamplers in our lab, typical imprecision is ˜0.3% under these conditions. It would be interesting to compare this to manual injection.

   For your final question about when manual injection might be preferred, I can think of only three cases, although there may be more. (1) Manual injection is a better choice in some training environments, because it gives a “hands-on” experience that may aid learning. (2) Manual injectors are considerably less expensive than autosamplers, so in a laboratory where budgetary requirements are more important than system performance, a manual injector may be preferred. This might be the case, for example, in a student laboratory class in a university. (3) Sometimes the flexibility of the manual injector may be an advantage. For example, if you need to inject extremely large samples, such as for on-line concentration of pollutants out of river water, installing a large loop (e.g., 10 mL) and filling it with a syringe may be much easier than trying to reconfigure an autosampler, which typically is designed for use with no more than ˜100 µL loops.

This blog article series is produced in collaboration with John Dolan, best known as one of the world’s foremost HPLC troubleshooting authorities. He is also known for his research with Lloyd Snyder, which resulted in more than 100 technical publications and three books. If you have any questions about this article send them to TechTips@sepscience.com

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