Getting OMNISEC ready for analysis
We’ve all been there: in the lab first thing in the morning wanting to kick off the day by starting your OMNISEC samples. However, before you click start you want to make sure the system is ready. And depending on the status of the instrument, there could be a number of things to consider.
In this post, I’ll describe my process for getting OMNISEC ready for sample injections. Hopefully, this can serve as a guide to help you find similar success with your OMNISEC!
Mobile phase and waste bottles
Safety first! Before I do anything else I check my mobile phase and waste bottle levels. I make sure the waste bottle has enough capacity to accommodate the volume of mobile phase left in the reservoir. This way, even if something goes wrong, I can prevent an accidental overflow!
This is also the perfect time to check the mobile phase, waste, and wash lines. If I’ve been recycling my mobile phase, I move the waste line to the waste bottle to avoid mobile phase contamination. I also check that the ends of the mobile phase and wash lines are as close to the bottom of the container as possible and completely submerged. This prevents air from being pulled into the system and interrupting the flow.
Start baseline monitor and activate detectors
If I’m collecting data from the UV-Vis photodiode array detector (PDA), I prefer to turn the lamp on before starting a baseline monitor. This way, the bulb has time to warm up before data collection begins.
At this point, I start a baseline monitor, usually for at least two hours. This way, I can monitor changes to the detector responses as they equilibrate. You don’t need to let the two hours finish; you can stop the baseline monitor once everything looks good.
Once the baseline monitor is in progress, I turn on the laser of the light scattering detector. I like to wait until data is being collected so I can watch the baselines shift from 40-50 mV (when the laser is off) to the operating baseline levels. This lets me know the laser is working and I can ensure it stabilizes before starting my sequence.
Temperature settings and flow rate
After the baseline monitor has started, then I check the temperature values of each zone and click Set. While the temperature values are adjusting, I set the flow to its operating rate. If you’re working with a viscous mobile phase, such as DMF, DMAc, NMP, etc. let the temperatures reach their final level before setting the flow rate. This way you will avoid exceeding the system pressure limit by pumping cooler, more viscous mobile phase.
As the temperature and flow rates settle into their set values, you will be able to observe the detectors’ responses. The refractive index (RI) and viscometer signals will be affected most significantly by temperature and flow rate variations. The IP viscometer signal responds most strongly to system flow, as shown below.
If the flow of your system was off, then your light scattering detector might be noisy to start. This should settle down as the flow picks up, as shown above. If it doesn’t, consider changing your post-column filter.
Baseline values
My main objective when getting OMNISEC ready for analysis is to obtain stable baselines. Stable baselines generally mean everything else has equilibrated and is ready. The baseline values and noise levels are key when determining if my system is ready for injection. A good noise level for each detector is <0.5 mV. However, I still feel comfortable using the system even if the noise is a bit higher on some detectors. I’ve listed typical detector baseline values in the table below.
| Detector | Typical Baseline Range |
| Refractive index | <±500 mV |
| UV-Vis PDA | <100 mV |
| Right angle light scattering | Between 50 – 300 mV |
| Low angle light scattering | Between 50 – 1,000 mV |
| Multi-angle light scattering | >200 mV |
| Viscometer DP | <±500 mV |
| Viscometer DP robust | <±25 mV |
| Viscometer IP | Between 10 – 80 mV |
A few notes: the RI baseline should be <±500 mV to ensure you have enough dynamic range to accommodate your samples. The right angle and low angle light scattering detector baseline values should be greater than 50 mV. A value less than 50 mV indicates the laser is off. The threshold for the MALS detector is 200 mV. There are two types of Viscometer DP detectors listed; your system will have one or the other, not both. The IP viscometer baseline depends on the viscosity of the mobile phase, the flow rate, and temperature of the detectors.
Detector purges and needle rinse
If the RI or viscometer baseline values do not fall into the ranges indicated above then I purge the relevant detector. I like to purge detectors at this point because the flow rate is at its operational level, meaning there will be sufficient mobile phase flushed through, and I can monitor changes on the baseline monitor.
I also like to take the time here to prime the autosampler. This ensures the injection needle and tubing connecting to the injection loop are filled with the mobile phase. Remember, an autosampler prime uses enough mobile phase to fill the entire line, so it takes a few minutes!
Zeroing detectors
If the RI and viscometer DP signals still aren’t within the expected range, then I’ll zero the detectors. With the baseline monitor, you can watch as the signals approach 0 mV during the process. It’s important to remember to purge before zeroing!
Sequence set up and start!
At this point, my baselines look good, my mobile phase and waste bottles are in good shape, and the flow rate and temperatures are stable. I populate my sample sequence, checking twice to make sure my vials are in the indicated positions. Then I click start and look forward to the results!
Final thoughts
In conclusion, I hope reading about my approach to getting OMNISEC ready for analysis is helpful. Stay tuned for a future post in which I describe how this process can be automated! Furthermore, if you have any questions, please don’t hesitate to contact us or email me directly at kyle.williams@malvernpanalytical.com.
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