Translator for HPLC HINTS and TIPS for Chromatographers

Saturday, February 10, 2018

HPLC Baseline Stabilization Tips for Refractive Index Detectors (RI or RID)

If you use refractive index detection (RID) for your HPLC samples, then you are already familiar with the very long equilibration periods needed to stabilize the system and associated baseline drift. Initial equilibration can take several hours. In fact, re-equilibration takes far longer to achieve with this detection mode than many others (i.e. UV/VIS, FLUOR). While there is no quick cure for these delays, there are a number of things that you may be able to do to minimize or reduce these wait times. Here are a few to think about.

  • ROOM TEMPERATURE: Locate the RID in a quiet, stable location. If the room temperature in which your HPLC system with RID system is located fluctuates by even one degree C, that can effect the stabilization of the system. The ideal room to use RID will be away from any windows, drafts, doorways, direct sunlight and HVAC vents or ducts. It should be located in a quiet area away from people walking by it. All of these things can contribute to temperature instability, which is what you want to avoid.
  • INSULATION of HPLC CAPILLARY LINES: All of those stainless steel capillary lines leading from your column outlet to the RID's flow cell are loosing heat to the surrounding air (cooling). To reduce this thermal effect, insulate any metal lines with plastic tubing to reduce the heat loss. Most any type of laboratory grade, thick walled plastic tubing can be used. Pass the SS tubing through the plastic insulated tubing or use a section of split-tubing to cover it. Cover as much of the exposed tubing as possible, right up to the fittings. - Note: Sometime the HPLC system's solvent bottles may be subjected to varying temperature changes too. In these cases you can wrap the bottles with an appropriate insulating material to reduce the effects.
  • FLOW CELL TEMPERATURE: Modern RID units have a heated flow cell with thermostat to control the temperature of the flow cell. This helps stabilize the temperature inside the flow cell as well as minimize the unintended effect that the heat given off by the RID's electronics has on the temperature inside the flow cell. If the flow cell temperature does not stabilize, then the baseline will drift in response to it. For most methods, select a flow cell temperature which is at least 10 degrees C above ambient (since most of these units can heat only, not cool). Factor in any column temperature used too. If you are maintaining your column at 40C, then try to maintain your flow cell at the same temperature to minimize any differences. Feel free to experiment to find the best temperature for your flow cell. Try different temperatures (in 5 degree C intervals), wait for the system to fully equilibrate, then measure the baseline S/N ratio. You may find best results using different column and flow cell temperatures. Sometimes the room temperature effect can be countered by using an optimized flow cell temperature (higher or lower). Always factor in your mobile phase boiling point (b.p.) into your method and keep the column and flow cell temperatures well below the b.p.
  • DEGASSING / DECREASING DISSOLVED OXYGEN: Reduce and stabilize the amount of dissolved gas inside the mobile phase and you may achieve faster equilibration times with a RID. You do not need to remove all the dissolved gas (in fact, a reduction of 50% is often enough). The amount of dissolved gas inside the mobile phase effects the measured refractive index. As it changes, so does your baseline. High percentages of mobile phase dissolved gas = lower RI; Less dissolved gas = higher RI. Now water holds less dissolved gas than non-polar organic solvents (e.g. THF) so this effect is more pronounced when you are running non-aqueous GPC separations, but maintaining a stable dissolved gas level for all mobile phase types is important to reduce baseline drift. Stability is our goal. Continuous degassing of the mobile phase either through sparging with high purity helium gas (best for non-aqueous separations) OR using an inline vacuum degasser should provide you with a way to control the amount of dissolved gas in the solution and reduce drift.

These are a few of the factors which can effect the equilibration and drift times of an HPLC system equipped with a refractive index detector. Careful selection of the location, insulating the exposed capillary lines and bottles, optimizing the column and flow cell temperatures plus removing dissolved gas from the mobile phase may all contribute to more stable baselines and better quality peak integration.

Saturday, January 6, 2018

UHPLC TIP: Reducing the Column Temperature to Offset Frictional Heating Effects (Causing Poor Resolution)

HPLC column temperature is a critical variable that we adjust and optimize during method development. We use it as a variable during the method development process to improve solubility, optimize peak shape and increase resolution. Once established, it must be carefully controlled during the method analysis to provide reliable and reproducible analysis results. Change the column temperature and you may also change the results obtained. This is a fundamental method development tool and must not be forgotten.

If you are developing a new UHPLC method OR perhaps scaling an HPLC method to utilize 2.5 micron or smaller support particles, then you may observe a loss of resolution or poor peak shape in the new method. There are many reasons why this may occur, and the most common ones relate to not optimizing all of the method parameters correctly when scaling the method (e.g. dwell volume too large, flow cell volume too large, injection volume too large, sample rate too slow, flow rate not optimized, mobile phase composition changes not in scale with the gradient...). But there is another reason...

Resolution may be reduced or lost when all of the initial scaling and instrument set-up parameters are optimized. What is the most likely reason for this? In many cases the use of substantially higher flow rates (relative to linear flow rates) and the use of smaller diameter particles results in much higher backpressures (you may recall that if you halve the particle size, the backpressure increases 4x). The resulting backpressure might be 2, 3 or even 4 times higher than observed in the original method. While these higher backpressures were well within the operating parameters of the HPLC system used, the results obtained were poor. The possible cause? The much higher backpressure increased the amount of frictional heating inside the column, raising the actual analysis method temperature and changing the separation conditions. 

Pushing mobile phase (liquid) through a chromatography column generates heat and pressure. The heat generated increases the actual temperature of the column. In conventional columns (i.e. 4.6 x 150 mm, 5u) at 1.00 ml/min, this heating effect is minimal, but at much greater column pressures, > 400 bars, the frictional effects may be substantial. These types of very high pressures may be seen with methods which utilize columns containing the smallest particles (1.9 to 2.5 micron). Enough to change the temperature in the column by several degrees (e.g. >5 degrees C) and result in different method conditions. So, what can you do about this? The most direct way to address the problem is to run the same method at a lower temperature (perhaps decrease by 5 C to start with). This will slightly raise the backpressure (lower temperature equals higher viscosity), but it should cool the column and restore the original temperature conditions used. You may need to try several temperatures and this may be easiest to do if your HPLC has a column compartment with heating and COOLING capabilities.