Translator for HPLC HINTS and TIPS for Chromatographers

Saturday, June 22, 2013

Troubleshooting HPLC Injectors (Manual and Automated)

Sample injectors are a critical component of a chromatography system. Understanding how they operate as well as the proper techniques to use and maintain them are fundamental skills needed to operated an HPLC system. Lets briefly discuss some of these fundamentals as applied to a standard manually operated HPLC injection valve and also in an automated mode as found in an autosampler. *Note: You should always refer to the specific manufacturer's product specifications, operation, servicing documentation or support personnel before servicing any injector.

MANUAL INJECTION VALVE Notes:

These valves allow you to use a high precision syringe to manually fill a fitted "loop" with a sample and then, by turning a valve handle, introduce the sample to the high pressure flow stream directed toward the column inlet. Sample loops are available in a wide range of volumes and take just minutes to install. The injector valve has very tiny openings inside which are moved between two different positions (LOAD and INJECT). The LOAD positions allow the valve to seal off the internal high pressure flow from the loop to allow it to be safely filled with sample at atmospheric pressure. The INJECT position introduces the liquid contained inside the loop to the main flow path (under high pressure). The parts must be clean and seal well to insure proper function. Leaks from all areas (except the vent) are not acceptable and indicate a problem. Here are a few tips regarding the use of manual injectors in HPLC.

  • Use the correct type of sample syringe. Usually these are high precision glass syringes with Teflon plungers. The needle tip style is the most critical item! Most injectors are designed to only work with a needle which has a squared off tip (NOT a point as is commonly used in GC!). The most common gauge used is #22. Always check with the valve manufacturer to determine the correct style and gauge of needle before use.
  • Leave the valve in the INJECT position during the entire run to flush it clean of sample and stay equilibrated with your method. Switch it back to LOAD only when you are ready to load a new sample.
  • For high reproducibility and accuracy within one HPLC system, fill the loop with at least three times the volume of the loop with sample to insure that the entire path is full of sample. This is known as the complete or over-filled loop method. *Choose your loop volume size with this in mind. Loading the same volume as the loop will often result in poor accuracy.
  • Loops often do not contain the exact volume stated on them. They can be off by ~25% so consider this when injecting partial volumes and not using the standard over-filled loop method.
  • Types of common leaks: (1) Leaks at the needle port (needle seal worn); (2) Leaks behind the valve stator (worn rotor seal, buffer crystals dried inside, over pressured, scratch on rotor); (3) Leaks at the vent (liquid should be expelled from the vent when filling the loop only. Other leaks indicate a problem). Note: Rotor seal damage can cause sample carry-over problems so valves should be inspected at regular intervals (~ 6 months).

AUTOMATED INJECTION VALVE (auto-injectors) Notes:

These valves use a high precision syringe or high pressure pump to fill a fitted "loop" with a sample and introduce the sample to the high pressure flow stream, all automatically. Most function exactly the same as described above, though some are based on true high performance liquid chromatography pumps so have no "syringe" at all (e.g. Agilent 1100/1200 designs) Here are a few tips regarding the use of automated injectors in HPLC.
  • Regular maintenance is even more critical with auto-injectors since you often can not see what is going on during the injection cycle. Leaks, if present can be harder to find so make it a habit to visually check all of the areas around the injector regularly.
  • Needle and Needle Seats are normal wear items on these instruments. As such, they require routine checking for leaks or damage and replacement when worn.
  • Vial Caps: If you make multiple injections from one vial (or large volume injections) with a tightly sealed vial cap, a vacuum can form inside the vial causing volumetric errors to occur in your samples (resulting in you injecting less sample each time). Leave the caps slightly loose to avoid this problem. Multiple injections into the same vial can also cause the needle hole to become enlarge over time allowing the sample or solvent to evaporate over time, changing the concentration of the sample (more concentrated). Replace the cap and seal with a new one if used multiple times. Always leave the cap slightly loose.
  • Loop volume: Autoinjectors often incorporate one large loop to handle a wide range of sample volumes. This is a trade off of accuracy for convienence. Accuracy is often poorest at the very low end of the range and best near the middle to high end. Always verify the reproducibility of the injector to inject a specific volume through statistical analysis of repetitive injections.
  • Types of common leaks: (1) Leaks at the needle seat (needle seat worn); (2) Leaks behind the valve stator (worn rotor seal, buffer crystals dried inside, over pressured, scratch on rotor); (3) Leaks at the vent (liquid should be expelled from the vent when filling the loop only. Other leaks indicate a problem). Note: Rotor seal damage can cause sample carry-over problems so valves should be inspected at regular intervals (~ 6 months).
These are just a few tips related to HPLC injectors. Please consult the service documentation for your specific instrument to better understand how the system works and what areas you should be monitoring. Understanding HOW these systems operate (and can fail) is one of the most important skills you can learn as a chromatographer. Take the time to understand the complete flow path of your system before using it.

No comments:

Post a Comment