Over the last decade, few things have had as dramatic an impact on liquid chromatography as the availability and subsequent proliferation of mass spectrometry. Aided in part by lower overall ownership costs and greatly improved technology, mass spectrometry has seen tremendous growth, both in the number of instruments sold and in the number of analysts actively using the equipment.
A key challenge when coupling mass spectrometry instruments to liquid chromatography equipment is how to best design the fluid pathway to ensure that optimal results are achieved. In fact, one of the most critical parts of the fluid pathway is the tubing, and as such, several things must be taken into account with regards to tubing selection and implementation:
- Tubing Internal Diameter– One of the advantages of mass spectrometry is the ability to obtain a lot of information about a sample in a short amount of time. It is often assumed that the smallest possible internal diameter should be used for the flow path tubing to help reduce the time required for sample to transfer to the mass spectrometer, and to minimize band broadening and sample dilution. However, this is rarely the best option, primarily due to the relationship between internal diameter and back pressure created.Consider the following formula, which details the relationship between pressure drop and internal diameter:
The pressure drop along a given fluid pathway is inversely proportional to the diameter of the fluid pathway taken to the fourth power! In other words, even small changes to the internal diameter can have a dramatic impact on the pressure drop experienced along that pathway.
Additionally, as the tubing internal diameter reduces, the probability of a partial or complete blockage forming inside the tubing dramatically increases.
Because of these and other factors, rather than choosing the smallest internal diameter possible, it is usually a better strategy to balance the benefits of having a smaller internal diameter (e.g., lower band broadening, less delay time, etc.) with the pressure-drop increase that will occur along with the increased likelihood of a tubing blockage.
- Tubing Material– In addition to the internal diameter, another major factor that must be considered is the material from which the tubing has been manufactured. This material can impact a number of characteristics, each of which may have an impact on the success of any given analysis:
- Chemical compatibility – Is the material compatible with both the mobile phase passing through as well as with the components that comprise the sample?
- Pressure holding ability – Based upon the system and application parameters, will the tubing be able to withstand the anticipated pressure in the area of the system where it is located?
- Electrical conductivity – Is the tubing part of the source assembly, and if so, will its electrical conductivity (or lack thereof) impact the expected performance?
- Temperature stability – Can the tubing withstand the temperature to which it will be exposed during the run without having its physical properties altered dramatically?
Other things – including gas permeability, overall flexibility, dimensional tolerances, etc. – help make choosing the right tubing material critical to designing an optimal fluid pathway.
- Tubing Preparation– Once the optimal tubing internal diameter and the best tubing material are chosen, if the tubing isn’t cut properly, it can lead to less-than-optimal results.All too often, analysts don’t exercise much care in the way the flow path tubing is cut. However, poorly cut tubing can result in partially occluding the opening—even pushing the flow path opening off-center. In some cases, these problems can lead to higher pressures and even the complete blockage of a fluid pathway.The best way to cut flow path tubing is to circumscribe the tubing – that is, to rotate the cutting tool around the outer diameter of the tubing, slowly cutting through the tubing wall and terminating the cut once the blade breaches the internal diameter. Preparing the tubing this way helps to not only maintain a quality surface on the end of the tubing, but it also helps ensure the thru-hole is centered and fully open.
As can be seen in the picture below, with the tubing on the left being cut properly and the tubing on the right being prepared incorrectly, cutting the tubing can have a dramatic impact – either positive or negative – on the overall fluid pathway.
Certainly there are other aspects that should be considered when designing an optimal fluid pathway with LC-MS applications. However, because of the tremendous impact that tubing internal diameter, tubing material, and tubing preparation can have on the results achieved, investing some time and effort in this area will often lead to dramatic improvements in both using the equipment and the results obtained.
About the author: John Batts is a Technical Specialist and chromatography expert at IDEX Health & Science. He is also the author of the “All About Fittings Guide,” which you can download at idex-hs.com.