Kinesis Solutions Techtips
Choosing the right inlet supplies for trace analysis
When performing trace level analysis, particularly of active compounds such as certain pesticides, any source of activity can present major a problem. As can be seen (Figure 1), any activity in the liner can cause peaks to tail, or disappear entirely, or compounds to breakdown. This all leads to a loss of sensitivity, which is exactly what you are trying to avoid. In these circumstances some of the tips & suggestions we often make, no longer apply.
For example, we usually suggest using a liner containing wool, both to trap any non-volatile material and, in the case of the FocusLiner™, to improve reproducibility. However even if a liner has been deactivated with the wool in-situ, the wool can still be a source of activity. During analyses the wool can be disturbed, leading to the fibres fracturing. These fractured ends are then a source of activity. For this reason, it is common for people performing trace level analysis of active compounds, to use liners that do not contain any wool.
Of course the longer any active compounds spend in the liner, the worse the situation is going to be. As we have already discussed in our previous tech tip "Splitless Injection Choosing the right conditions", one of the complications of splitless injection is that it takes a lot longer for the vaporised sample to be transferred to the column and this exacerbates any problem with activity in the liner (Figure 2). So, although splitless injection is often suggested as a way of improving sensitivity, with active compounds this may not always be the case. Often, better results will be achieved by using split injection using a relatively low split ratio. The goal in this case is to balance the reduction in sample breakdown offered by split injection, against the lower amount of sample being transferred onto the column.
Another potential site of activity in the injector, is the metal seal at the base of the injector. Unfortunately when you inject a liquid sample into a hot injector, the vaporisation is not instantaneous. As the liquid leaves the tip of the needle as a spray, some of the small droplets will be swept down to the bottom of the injector, where they can hit the base seal. This hot metal surface can cause some compounds to breakdown. Using a liner with a taper at the bottom (Figure 3) means that droplets of sample are unable to come into contact with any hot metal surfaces, at the base of the injector.
Of course no matter how good the deactivation of the liner is to start with, or whether it has a taper or not, it’s going to need to be changed regularly if you a performing trace level analysis. Depending on how clean the samples are, non-volatile material can build up relatively quickly. This build up can be a source activity, & can lead to ghost peaks & a noisy baseline (Figure 4), none of which is going to help your cause.
Once you’ve addressed all of the issues regarding the liner, keep in mind that some of the same things apply to the column. The longer any active compounds spend in the column, the more chance there is of tailing, or breakdown occurring. So keeping the column as short as possible, can help improve your sensitivity.
If your still looking for better detection limits, particularly with later eluting compounds, why not ask us about the AirSharp™. This simple, but effective device addresses the problem of peak broadening, that occurs with late eluting compounds, giving a significant improvement in sensitivity.
In short, AirSharp:
- sharpens peaks and increases sensitivity without injecting an excess volume of sample into the injector
- uses air as the coolant
- is automatically controlled through the instrument’s software
- can be installed in 30 minutes
- is easy to use
