Troubleshooting HPLC Gradient Valve / Proportioning Valve / MCGV GPV Leaks. How to Identify Them.

HPLC pumps which utilize low-pressure mixing VALVES are known by names such as: "Ternary" (3-solvents) or "Quaternary" (4-solvents) pumps.These types of HPLC pump configurations use a single, high-pressure pump head coupled to a multi-port / proportioning valve and represent some of the most popular and versatile pump configurations offered. Featuring random access to multiple solvent bottles (more than two is always better), lower operating costs and less maintenance work provides you with one of the best platforms to develop new methods on. I highly recommend them for most, but not all, HPLC applications (vs. Dual pump, high-pressure "Binary Pumps").

• If your HPLC system utilizes a single, high-pressure pump head coupled to a multi-port valve, then please remember that in addition to pump head maintenance, regular maintenance of the multi-port / proportioning valve is also required.

A few weeks ago I was hired by well known Pharma company to solve a gradient method problem that I was told has stumped their best scientists for almost one year. The client presented me with their validated UHPLC method which suddenly developed a shift in retention time of all peaks. The shift was significant, about 10% of the previous values over a 20 minute run, and had been observed on two different, but similarly configured HPLC systems in their lab. Changing the column to a new one showed no change on either HPLC system. They were out of ideas.

• Before I reveal the cause of the trouble, let us briefly think about what types of changes can result in a small, repeatable shifts of peak retention times. Four common ones that come to mind are: 

(1) Flow Rate changes;

(2) Column Temperature changes;

(3) Column Fouling;

(4) Mobile phase composition changes. 

Start the troubleshooting by ruling out the easy causes first (#1, 2 and 3 above).  

• (1) Flow Rate: When the actual flow rate is in question, start by measuring it manually Never trust the instrument's display screen value or the software's value for flow rate. Measure it. An easy way to measure the flow rate involves timing the amount of liquid that exits the HPLC detector line after a defined period of time. For example: If your flow rate is set at 1.000 ml/minute, then using water, measure the time it takes to fill a 10mL graduated cylinder to the 5 mL line. It must take exactly 5.00 minutes (= 1.00 mL/min). Run this flow test on each pump channel.
  
• (2) Temperature: The HPLC method should be run under controlled column temperature conditions. Verify this. Retention times are a function of temperature (i.e. cooler temps usually result in longer retention times, warmer = shorter). The temperature should be stable (~ 1 or 2 degrees C).
  
• (3) Column Fouling: To prevent fouling, wash the HPLC column with a solution that is STRONGER than the mobile phase after each analysis. Use fresh, clean solutions. Verify that the samples are dissolved in the mobile phase (100% dissolved) and filtered before injection. Verify that the injection volume is less than ~3% of the column volume and the concentration of the sample is not too high (avoid saturating or overloading the column). Solubility is very important for both the sample and any additives used in the mobile phase (to prevent precipitation). Anything that "fouls" the column support will directly effect the retention times and often the peak shape too. Be aware of these causes and take action to avoid them.  *Replacing a suspect column with a new one is often an inexpensive way of troubleshooting a "peak" problem. Always have a NEW spare column on hand for testing. *Columns are consumable items.
  

• (4) Mobile Phase: Changes to the actual amounts of additives, pH or final composition of the mobile phase may impact peak retention times (sometimes, the peak shape too). After all, the final composition used was developed for the purpose of establishing a reliable and reproducible method of analysis. It must be controlled. We must take steps to insure the mobile phase preparation and delivery are accurate. Always prepare fresh solutions each day (esp. all aqueous solutions!). pH values may change after a few days (e.g. even in MeOH / acidic solutions), bacteria/mold/algae grow quickly in many solutions, even in the refrigerator, so only prepare what you need for the day. Evaporation of more volatile solvents (in pre-mixed solutions) can change their actual concentration (always protect them from heat and evaporation).
  

*There is another way that the mobile phase composition can change which often goes unseen. It can change during delivery to the column. The HPLC's low pressure proportioning valve that allows us to easily select and use different solvents can develop small internal leaks, resulting in valve cross-flow leakage. This cross-flow leakage allows liquid (or air, if the line is not connected) to be drawn out of one channel and into another, changing the actual mobile phase composition. This happens because the valve seals, esp if they have been left unused for a long time, can change shape (e.g. shrink) and begin to leak over time. Often the amount of leakage is very small (ul/min), but depending on the method, a small change may result in a significant change to the chromatography.

I reviewed the client's method parameters and concluded that the method met good chromatography fundamentals. Checking the flow rate (using a graduated cylinder) confirmed the flow rate was accurately shown. A review of their mobile phase preparation procedures and methods also appeared OK. Degassing of mobile phase and column temperature were also satisfactory. 

As I looked more closely at the two running HPLC instruments they used, I began to quickly zero in on the most likely problem. 

• A long stream of air bubbles were observed exiting the HPLC pump's gradient valve leading into the high pressure pump head, but no air bubbles were seen exiting the degasser's outlet line (IOW: The vacuum degasser may or may not be the cause, though it is critical to insure the degasser is clean and fully serviced before use. Have the degasser professionally serviced first before proceeding with troubleshooting. Using a damaged degasser will make it difficult to use the pump or run any valid tests as degassed solution is needed). This was observed on several of their HPLC systems, including the two used for this method. The fittings connecting the lines from the degasser module to the valve were correctly connected (as a loose connection would cause air to leak in and must be quickly ruled out). 

The cause was from one or more of the unused gradient valve positions leaking air into the flow path, changing the mobile phase composition. Of four possible mobile phase lines available (A,B,C,D), the client only had two lines connected to mobile phase bottles (A,B) with the remaining two lines left open to the air. The internal valve seals in the unused 'C' and 'D' valve positions had deformed, shrinking in size, sticking,leaking, allowing air to flow into the mobile phase on one of the channels. This resulted in a change of the organic composition % used in the method (due to a cross-flow leak), changing the peak retention times (as the actual mobile phase composition used in their gradient was different). I directed the HPLC pump's outlet line to waste, placed all of the solvent pickup bottle lines (A,B,C,D) in a beaker filled with IPA and allowed the pump to run pure IPA at 1 mL/min across each channel, one-at-a-time (100%), for ~ 20 minutes to re-hydrate the internal gradient valve seals. This was repeated with each valve position, then all of the lines were placed in fresh mobile phase solution, primed and flushed. The system was restarted and the method now ran showing the expected peak retention times. Instructions were provided which included regularly using all of the channels and valve positions plus flushing weekly to maintain valve operation. Use ALL of the lines and flush the valve(s) through all positions, one-at-a-time, on a regular basis. If prolonged flushing with pure IPA does not fix the leak, then it is time to replace the valve. All valves eventually wear out and must be included in maintenance inspections and checks. This is especially true when you purchase your HPLC system at an auction or from an 'equipment' reseller. Never assume that the 10+ year old HPLC valve is OK. Test it first (e.g. Acetone tracer test).

 

Acetone Tracer Test: If you suspect that a cross-flow leak exists on a gradient valve, then one method I use to check for leakage is to mix up a "Tracer" solution of pure organic (often ACN) that has 1% Acetone mixed in (for RP methods). Remove the column and replace with a restriction capillary. Place the tracer solution on the valve position you suspect may be leaking at an appropriate flow rate and set it for 0%. Run one of the other channels with 100% (pure ACN in this example) and monitor the UV (265nm) for the presence of acetone. If the acetone leaks into the channel you are using, it will be easy to observe on the UV trace. So called "bubble" tests (introducing and monitoring the position of a gas bubble into the low pressure solvent line) are not reliable leak detection methods for small leaks. Use a tracer such as acetone to find the leaking channel(s). You can read more about these types of Valve Leakage tests in this article (Click Here).

Tips and Advice for Priming your HPLC PUMP (or similar pumps, FPLC or UHPLC Pump)

The single most important component of any HPLC system is the Pump module. We often refer to it as "the heart of the HPLC system". 

• You may have the most sensitive HPLC detector, the best column, a perfect method of analysis, but none of this will matter unless the HPLC pump(s) that provide mobile phase to the system operate perfectly, all of the time. If you have a poor quality (or poorly maintained) system, then you will spend much of your time trying to establish reliable flow through the HPLC system, not running samples. 
• Before using an HPLC system, you should prime all of the lines in your HPLC pump. This is needed to purge any air from the tubing, introduce fresh mobile phase to each line and then to VERIFY that each channel delivers the reported amount of fluid to the column (measure it).
  

• NOTE: This is a LONG, detailed article with lots of information, Hints and Tips. It is available in PDF format for download, here.

The HPLC pump's ability (stability) to provide reliable operation depends on: 

(1) The Chemical, Physical and Miscibility properties of the Liquid(s) being pumped;

(2) The Amount of dissolved gas inside the liquid (must be minimized);

(3) The Temperature of the room (or HPLC) must be stable;

(4) The Position of the mobile phase bottles (relative to the pump, above or below);

(5) The Solvent Pickup Filters used (are clean and appropriate in material & porosity);

(6) The Fittings used are correctly installed & tightened;

(7) The types of Tubing used are chemically, temperature and pressure compatible (esp. the Inside Diameter of the tubing);

(8) The Selected Flow Rate(s) and Back-pressure are within the optimal range of the pump;

(9) All mobile phase solutions are Filtered, Freshly prepared and Degassed;

(10) How often the Pump is properly Inspected, Cleaned & Serviced.

 The HPLC pump is the most important part of your HPLC system. Take care of it. Neglect or abuse it, and you may lose time and money. Almost every problem you experience using an HPLC will be related in some way to the pump. Make sure you understand the flow path of the system in detail, and have the training to setup and use it properly. Take a hands-on training class (not a video or web based tutorial) to learn how to use the pump on your specific HPLC system. Learn how to run simple verification tests to check the flow rate (best done with a graduated cylinder). Never rely on the software values, check and verify everything yourself. Priming and flushing are needed any time air bubbles are present, mobile phase solutions are changed or the system has sat unused (this includes overnight). Always flush multi-channel pumps and valves (i.e. Binary, Ternary, Quaternary...) using a setting of 100% channel composition. Run one channel at a time at 100%, not 25% or 50% to flush channels (a common novice mistake). Flush ALL channels on a regular basis.

OK, so what can you do to make sure your HPLC pump is properly primed with fluid and operating to the best of its ability?

Start, by reading the operator's manual for your pump. Review the procedures for connecting it to the system, become familiar with the flow path and understand the procedures to prime the pump heads. Practice these procedures.

If an inline vacuum degasser is used, become familiar with the specifications, chemical compatibility (some are not compatible with solvents such as strong acids, strong bases, THF, chloroform, fluorinated additives and so on) and internal channel volume of each chamber used. It is useful to know what the degasser chamber volume is to figure out what the total channel priming volume is. This may be different for similar systems. Check, measure, verify, do not assume.

Priming Volume: The total volume contained in each channel's low-pressure line from the mobile phase bottle to the degasser + the degasser chamber channel volume + the total volume in the line from the degasser to the pump head (or multichannel valve) = the total minimum volume you must flush out before using the system. Because flushing just the minimum of volume (1x) of fluid through the channel is unreliable, flush 2x, 3x or more times this total volume, per channel (or as much fluid as it takes), to prime each channel. *If no degasser is present, then just calculate the volume contained in the low pressure tubing from the bottle to the pump head/valve. Set the pump to direct the flow to waste and use a high initial flow rate to speed up the priming process.

Use fresh mobile phase (prepared daily and filtered). Make sure the solvent pickup filters are clean. If possible, have the bottles placed higher than the pump's inlet (once flow has been established, this will allow natural siphoning to push liquid towards the pump head). Prime all of the lines used. The pumps run on liquid, not air so try and fill any of the lines with pure mobile phase before you connect them to the pump and/or degasser (If all of the lines are prefilled with fresh liquid, you can skip this part).  

There are two ways to PRIME EACH line (Flushing the Channels).

• *First, open any Prime/Purge or Waste Valve so the mobile phase is directed to waste, not the injector, column or detector. Our goal is to initially fill the lines with liquid, quickly, and we do not want these fluids to go through the entire HPLC system (i.e. column), just the HPLC pump.

(1) Wet Priming use a syringe fitted with a Luer-to-threaded fitting adapter (usually 1/4-28) to draw liquid through the tubing in the mobile phase bottle and into the pump's degasser and/or pump head's inlet. Be sure to have this type of syringe available (very useful). Never push fluid, only draw fluid through the tubing, just like the pump does. Connect the syringe to the mobile phase bottle lines, degasser ports and/or pump head multichannel valve or pump head inlet, as needed, to draw liquid through until all lines are filled.

(2) Dry Priming using the HPLC pump to draw the mobile phase out of the bottles, through the lines, degasser channels and to the pump head or multichannel valve. Note: "Dry" because the lines (low pressure tubing) are initially dry when we start. Always do this one channel at a time (e.g. A = 100%). This insures no miscibility or mixing problems and is standard procedure. Start with a modest flow rate to get the fluid moving through the lines, then increase the flow rate to speed up the process. The low pressure Teflon tubing is transparent so you can watch this process. Repeat with each channel. Note: Some HPLC pumps will struggle to perform this type of dry priming, as they will be unable to draw the liquid up from the bottle and/or pump the air out of the system. Pre-priming the lines using a syringe (as in #1 above) will help solve this. Running the pump with just air inside the lines may result in increased wear on the system (esp the piston seals) so if the system struggles to fill with liquid after one minute, discontinue and manually wet prime each line.

NOTES: 

• The back-pressure shown on the system readout should be very low during this initial  priming process (e.g. < 15 bars) as the HPLC system should not be plumbed with the column or detector inline, during the priming process (it should be by-passing those parts). Only the viscosity of the solution, the selected flow rate and the internal diameter of the tubing going into and out of the pump will contribute to the observed back-pressure, and this should be very low value.

• Once you have verified that liquid is exiting through the pump head waste port, you can increase the flow rate to speed up the priming process, but pay attention to the back-pressure. It should increase as the flow rate increases and drop as the flow rate drops. Continue to prime each channel in this way, one-at-a-time, until all channels are primed and flushed with liquid. You can gradually slow the flow rate down as you stop, to transition from one channel to another.
  

• If liquid has been drawn to the pump head, but the pump head still is not pumping liquid through it, it may be experiencing cavitation (air locked). If there is an outlet port on top of the pump head, the outlet fitting above the pump head can sometimes be briefly loosened with a wrench, allowing the system to push the air out (open it slightly with a wrench, then quickly close it after liquid comes out). Have a towel ready to soak up any fluid that comes out. Keep the area clean and dry. Alternatively, try drawing liquid through this port, while it is running, to gently fill the pump head chamber and remove the air.

• In some case, the inlet or especially the outlet check valve(s) can also become "stuck" open. When buffers are left in the system (they should be flushed out with water), crystals and particulate matter may deposit on the valve resulting in poor sealing, leaks or air being drawn through. Drawing liquid out of the pump head's outlet port with a syringe (or gently pushing it through the pump head) may remove the air bubble, debris and prime the valve, restoring function. Note: If needed, shut down the pump and clean/replace any contaminated or worn check valves before proceeding.
  

• In more extreme cases, you can change the mobile phase going into the pump head to a more viscous intermediate solvent to get things moving (an alcohol such as IPA might work well. If buffers have been used, then always first flush with pure water). 

• Degas all eluents / mobile phase solutions used. All of them. Degassing will help reduce the formation of bubbles inside the pump head. Failure to properly degas the solutions used may result in loss of prime, baseline and/or pressure instability. Make sure your degasser is operating properly (electronic vacuum degassers only last ~ 5 years at most. Be sure to have them professionally serviced). Sonicating fluids at the bench or using vacuum filtration to initially remove gas from the solution will only degas the solution for a short time (minutes). Gas will slowly diffuse back into the solution resulting in baseline noise, drift and pump problems (for HPLC, only use inline degassing or Helium sparging).
  

• Verify the flow rate. It may be unwise to rely on the indicated flow rate shown on the instrument screen or display. It is wise to measure the flow rate of each channel, separately, using a graduated cylinder and a timer. This is the most reliable way to determine what the actual flow rate is through the system (and is also the method we use during performance verification or qualification testing too). To check the flow, make sure the system has been primed and flushed. Install a flow restriction capillary in place of the column (to provide the required back pressure). Set the flow rate to a value which is appropriate for the pump and measure/record the volume delivered vs. time. Example: Using a flow rate of 1.000 mL/min obtain a 10 mL volume, glass laboratory grade graduated cylinder. At time zero, direct the flow from the restrictor's outlet into the graduated cylinder. Measure the volume of fluid collected in 8 minutes. *It should be 8.00 mLs.
  

If you continue to have priming problems and/or air bubbles disrupting the flow there are four more things you can check. 

1. Make sure the solvent pickup filters/frits are clean and unobstructed (these are maintenance items). If the filters are obstructed, then a vacuum may form on the line resulting in pump cavitation and loss of prime. One quick way to check if this might be a problem is to remove the suspect solvent pickup filter from the tubing, then try again. If flow is restored w/o the filter in place, then the filter may have been clogged. Install a new solvent filter as soon as possible. *Never run the HPLC without solvent filters installed. Those filters perform a very important job and protect the flow path of the system.  
2. Service the Pump Heads. Regular cleaning, inspection and replacement of worn parts must be done to maintain the function of the pump. Worn parts will result in failures, instability, lost time, plus invalid data. The pump has many mechanical parts which wear out and require replacement. Most pumps should be inspected/serviced every 6 months. Keep the pumps clean and fully serviced (replace: piston seals, pistons, frits, check valves as needed). Depending on the brand, model and applications, the types of parts needed and the frequency of repairs varies widely. *This is discussed in another article. 
3. If your HPLC system has an inline vacuum degasser (either a standalone or integrated module), it may be damaged, contaminated or broken. The typical service life of an electronic inline vacuum degasser is only five years (some models have even shorter lifespans). Degasser's with internal damage may result in contamination of the mobile phase. A failing or damaged HPLC vacuum degasser may directly contribute to analysis problems (ghost peaks, pressure instability, poor baseline stability...). Have your degasser professionally diagnostically tested and serviced often.  
4. Clean and/or replace any worn or damaged inlet or outlet pump head valves. Not flushing buffers out of the HPLC system on a regular basis or remain in contact with the solution for long periods of time can damage the valves. In some cases, cleaning is all that is needed, but in others, replacement is required to restore function. Be sure to have the system professionally serviced on a regular basis.
   

• Additional Troubleshooting Info can be found here:

Diagnosing & Troubleshooting HPLC Pressure Fluctuation Problems (Unstable Baseline)

HPLC Tubing and Fittings; An introduction to Nuts, Ferrules and Tubing Choices

INTRODUCTION:

Setting up a high pressure liquid chromatography (HPLC) system to run trouble-free takes  patience and a strong set of troubleshooting skills. The patience aspect has usually worn out with most of us, but the troubleshooting skills often come from years of tinkering and practical experience. As a consultant who works with chromatographers on a daily basis, I have found that most chromatographers share many of the same basic HPLC hardware problems. Some of these problems the result of a failure to logically troubleshoot a problem from scratch or by overlooking seemingly minor changes that have been made to the system over time. One common area that is often overlooked in the area of HPLC is that of connection fittings (nuts and ferrules) and tubing selection. Selection and installation of the correct HPLC fittings and tubing can help you avoid future problems while allowing your system to run at peak performance. Common types of high pressure chromatography fittings and tubing found in the laboratory will be discussed in this article.

Please click on this link to download the entire article in PDF format.

Modern HPLC Method Development Tips (PART I):

This is the first of two articles which will provide suggestions on how to improve the HPLC (UHPLC) method development process. - PART I

Method Development, BEFORE YOU BEGIN

1. Start with a working system: Before you start to develop a method, make sure your chromatograph is operating in top condition! Passing an OQ test and regular maintenance are not enough. You need to have the proper training and experience which will allow you to identify signs of trouble all of the time. For example: Do you see any salt or buffer crystals on the outside of one or more parts (esp the fittings or the pump head)? If you do, then you have a leak or other problem which needs to be addressed. Do you know what the normal backpressure should be when the system is operating correctly at a fixed flow rate with a single pure solvent (or defined mixture) running through? You should, as this provides a quick check that the pump is functioning properly. These types of quick checks should be done each time you use the HPLC and, if applicable, should be part of a general SOP. Is the flow rate shown on the display correct? A small graduated cylinder and a stopwatch (watch, timer) to measure the mobile phase output from the detector can answer this question in a few minutes. Are the pressure and flow rate stable (ripple <1%)? Verify by recording the pressure readings over time and measuring the S/N of the baseline with your detector to estimate the flow rate stability (noise). If they are not stable, then do not start an analysis. Find and fix the cause of the instability before running samples or standards. Some common causes of flow instability include: a sticking check valve, failure to properly degas the mobile phase, a leak, poor mixing or a miscibility problem.

2. Mobile Phase Preparation: Have you prepared enough fresh mobile phase to use during the day? Mobile phase which is mostly aqueous and contains buffer salts often experiences significant growth of microbes in just one or two days (esp. at RT, so store at 4 C). Mobile phase must be clean, filtered and freshly prepared using an established standard method (this should be described in a SOP so it is made the same each time). This applies to all mobile phase additives used too (choose the highest grade possible. “HPLC Grade” or ultra-high purity and look for an analysis sheet). Water used should be freshly prepared, high resistance (~18 Mega ohm) HPLC grade water which has been filtered through a 0.22 u filter.

3. Degassing. Mobile phase solutions should be continuously degassed before being introduced into the HPLC pump(s). This is sometimes done via low pressure Helium gas sparging, but more commonly today, via in-line, electronic vacuum degassing modules. Placing bottles in an ultrasonic bath or using vacuum filtration systems alone to degas solutions before use does a very poor job of degassing and results in the gas slowly dissolving back into the solution the very moment you stop degassing them. Baseline instability, drift and flow rate variation are the result. These problems may contribute to poor quality integration and high %RSD. To insure that your electronic vacuum degasser provides the maximum benefit, it must be purged before use each day to remove the gas which has re-dissolved back inside them. To properly flush each chamber/channel, you will need to know its volume (provided by the manufacturer). *Individual vacuum chamber volumes vary from ~ 0.50 ml to as much as 50.00 ml EACH! At a flow rate of 1.000 ml/min, it could take from 0.5 to 50 minutes just to flush one vacuum chamber (and most systems have 4 chambers!). Luckily, most modern vacuum degassers have chambers with volumes of between 0.5 and 12 mls each and HPLC pumps which can be run at either 5 or 10 ml/min maximum to make this process go faster. Remember to purge these chambers with fresh solution at the start of each day. 

Best Practice: Each channel of your HPLC system should be flushed (purged) with freshly degassed mobile phase at the start of each day, before you run any samples.

 

Before starting ANY HPLC analysis, the HPLC pump must be running and operating with no problems, achieving a stable baseline, steady flow rate with as little pulsation as possible (~ 1% ripple or less). Accuracy depends on this.

Part II of this article discusses initial method development steps.