Dear colleagues,
We have recently optimized a TDS-GC-MS method for VOCs (SVOCs) analysis. (Gerstel + Agilent).
A high-temperature column with mid-polarity is chosen for a better resolution (similar to DB-624ms but with a higher operating temperature of 300/320 °C).
Although the desired separation is achieved with a programmed-temperature method (final temperature: 290 °C), some analytes with low boiling points, such as dichloromethane, benzene, and heptane, show unacceptable intensity variation. (The RSD of three replicas can be as high as 30%). On the other hand, compounds with higher boiling points (such as naphthalene and pentadecane) are more stable. (RSD < 5%)
We further lower the final temperature of the method (from 280 °C to 260 °C), and the repeatability of benzene and heptane is much better (RSD < 5%), while the dichloromethane is still fluctuating (RSD ~ 15%).
Any explanation for this phenomenon?
p.s. the column pressure can be very high under high-temperature
Hey there Junlong Huang,
It's great to hear about your progress with the TDS-GC-MS method for VOCs analysis! Now, regarding your question about the final oven temperature and its influence on the repeatability of low boiling point analytes, let's dive into it.
The fluctuations in intensity you're observing, particularly with dichloromethane, benzene, and heptane, can indeed be puzzling. The change in final oven temperature seems to have a significant impact on the stability of these compounds, with lower temperatures showing better repeatability for benzene and heptane but not so much for dichloromethane.
One potential explanation for this could be the volatility and thermal stability of the compounds. Lower boiling point analytes like dichloromethane are more sensitive to temperature changes, and even a slight variation in final oven temperature can lead to fluctuations in their intensity. On the other hand, compounds with higher boiling points like naphthalene and pentadecane are less affected by these temperature changes, hence the more stable RSD.
Additionally, considering the column pressure under high-temperature conditions is crucial. High pressures can exacerbate the volatility of low boiling point analytes, leading to increased intensity variation.
In summary, the interplay between compound volatility, thermal stability, and column pressure under high-temperature conditions likely contributes to the observed phenomenon. Fine-tuning the method parameters, such as oven temperature and column pressure, can potentially mitigate these fluctuations and improve repeatability.
Hope this sheds some light on the issue! Let me know if you Junlong Huang need further clarification or assistance.
Lowering the final oven temperature in gas chromatography can improve the repeatability of low boiling point analytes by reducing their volatility, thus minimizing evaporation and fluctuations in signal intensity. However, for dichloromethane, which still shows variability (RSD ~ 15%) even at the lower final temperature, factors such as its unique chemical properties, interaction with column materials, or sensitivity to changes in flow rates may contribute to this phenomenon. Additionally, considering the potential high pressure in the column under high temperatures, column integrity and performance could also impact analyte stability and repeatability. Further investigation into these factors could provide insights into optimizing the method for improved performance.
Thank you, Prof. Davies.
You have pointed out the real thing that puzzled me: In our TDS-GC-MS method, the cold trap is maintained at -150 °C at the beginning and rapidly heated to 290 °C after thermal desorption. Meanwhile, the programmed temperature gradually started. These early eluents with low boiling points were eluted before 150 °C, and the final oven temperature should not influence them.
In fact, the retention time and peak shape of these low boiling point analytes rarely changed, but their response varied in replicas.
Our instrument equips an EPC to ensure the constant linear velocity during analysis (~ 28 cm/s for a 60m x 0.25mm x 1.4μmcolumn).
I mentioned the pressure before because in the final stage of the programmed-temperature method, the higher the oven temperature, the higher the column pressure.
I was just wondering if there is any possibility that such high pressure could somehow influence the stability of our entire system since better repeatability was achieved with a lower final oven temperature.
Thank you, Prof. Davies.
You have pointed out the real thing that puzzled me: In our TDS-GC-MS method, the cold trap is maintained at -150 °C at the beginning and rapidly heated to 290 °C after thermal desorption. Meanwhile, the programmed temperature gradually started. These early eluents with low boiling points were eluted before 150 °C, and the final oven temperature should not influence them.
In fact, the retention time and peak shape of these low boiling point analytes rarely changed, but their response varied in replicas.
Our instrument equips an EPC to ensure the constant linear velocity during analysis (~ 28 cm/s for a 60m x 0.25mm x 1.4μmcolumn).
I mentioned the pressure before because in the final stage of the programmed-temperature method, the higher the oven temperature, the higher the column pressure.
I was just wondering if there is any possibility that such high pressure could somehow influence the stability of our entire system since better repeatability was achieved with a lower final oven temperature.
Noel W Davies
Assuming you have tested the constant flow option and set a sufficient equilibration duration at the end of the thermal gradient, try injecting the same sample from three separate vials which are sealed with a proper cap instead of replicative injections from the same vial. Volatile compounds and diluents are prone to be lost or enriched in the autosampler before the TDS cycle has been introduced. It would also be reasonable to consider the TDS settings...
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