-
- Posts: 4
- Joined: Mon May 15, 2023 12:44 pm
- Location: United States
I am experiencing a very bizarre repeatability problem with my GC-MS/MS that has so far stumped a few Shimadzu engineers and am wondering if anyone has seen this. The problem is best visualized by the following chart showing the peak area of a methanol solution injected repeatedly on the instrument.
Here are my method and instrument conditions:
Shimadzu TQ-8040nx (GC-MS/MS) with HS-20 headspace autosampler
Column is SH-RXI-624Sil MS 30m x 0.25 mm ID x 1.4 um
HS equilibration temperature is 60 C
Transfer line temperature is 150 C
Sample line temperature is 150 C
HS pressuring gas pressure is 50 kPa
HS vial equilibration time is 22 min
Pressurizing time is 2 min
Split ratio is 50
Column flow 1.9 mL/min
Oven temp is constant at 50 C
Ion source is EI heated to 200 C
MS interface temp is 230 C
m/z monitored is 31 using Q3 SIM
Test sample is 1000 ppm methanol in water
1 mL is placed into a 20 mL crimp-top headspace vial
Peak shape is very good and retention time is consistent
The above data is NOT repeat injections from the same vial, it is 20 separate vials with the methanol solution in each.
I am baffled by the pattern of peak area steadily decreasing in the above chart coupled with the extremely regular up and down pattern. The methanol is being used only as a test standard for repeatability which mimics what the engineers use to test the system repeatability. The method I normally run on this system is terpene and solvent analysis by full evaporation headspace technique. This is not a one-off occurrence. I have performed a weeks worth of repeat injections, column conditioning, etc since the issue started occurring and there is no improvement. This is not a new configuration of this instrument - it has been working flawlessly for the last two years.
Here are a couple of possible clues:
My system is running a twin column. It is configured with the above mentioned column which connects the HS-20 sampler to the MS. The other column is a RXI-5MS, 30 m x 0.25 mm x 0.25 um, and connects the liquid injector to the MS. The injection repeatability of the liquid injector is GOOD - it DOES NOT exhibit this decreasing sawtooth pattern. This indicates to me that the issue is within the HS autosampler and not the detector.
Both columns have been replaced and the issue remains.
This problem manifested directly after the periodic preventative maintenance was performed by Shimadzu. Two years ago, after the annual PPM, this SAME PROBLEM appeared and mysteriously went away after 7 weeks, with no cause determined. For the two years in between the system has been working flawlessly with peak areas consistent across many hundreds of injections and several weeks at a time before needing recalibration.
Any help is greatly appreciated!!!
For good measure, here is the same injection repeatability test performed prior to installation of the new columns showing 40 + injections:
Thanks,
Tom