inaccurate SPME measurements

[ChrisT]

Dear GC/MS users,

I hope for your help! We encounter a problem with our quantifications using HS-SPME-GC/MS. The standards decrease in intensity (20%-30%) during a run (ca. 24 h). The calibration curve however looks nicely linear. Why do we have this decrease in intensity?

My thoughts were:
1. Can the detector lose sensitivity due to samples that emit many volatiles? (would seem strange to me for SPME)
2. Can the detector loses sensitivity when the SIM measurement is too long? (We have area values around 10^6.)
3. Can the fiber age so quickly?
4. Do we have to perform an automatic gain correction during a run?

Have you got ideas?


The details of the experiment are as following:

Sample type
The sample is a suspension of 0.5 g protein isolate in 5 mL of salt water (20%).
The SPME fiber samples the head space.

Sample sequence:
2xBlank
Calibration mix 1-9
2xBlank
Samples 1-3
Calibration mix 4
Blank
Sample 4-6
Calibration mix 4
Blank
(The calibration mix 4 was used to calculate the accuracy.)

Extraction
Hardware: Gerstel autosampler
Fiber: Supelco50/30um DVB/CAR/PDMS, stableflex 2 cm
Incubation time 15 plus shaking at 60 °C
Extraction time 10 min plus shaking at 60 °C
(We reduced the extraction time as the signals were too intense.)
fiber backout before extraction (5 min at 270 °C) and after injection (20 min at 270 °C)

Injection
Inlet temperature 250 °C
Split 1:25 (We introduced the split as the calibration curves lost linearity)
Straight liner (ID 2 mm)
Constant flow 1 mL/min

GC hardware:
Agilent 8890 with a G7077C mass spectrometer
Column: 30 m HP5-ultrainert MS, ID 250 um, film 0.25 um
Run time 45 min, 15 min cool down.

MS configuration:
We record the mass of the aroma components in Scan/SIM mode.
Quantification is being done using SIM recordings only: dwell time for each ion is 30 ms.
MS source 230 °C, MS Quad 150 °C, Gain factor 1,

Thanks for your help in advance!
Greetings Christian

[ChrisT]

We can try to correct the quantifications by using an internal standard. Of course, it would be ideal to use the targeted molecules (aroma molecules such as hexanal, pentanol, nonanal, octenal, pentylfurane etc.) labelled with deuterium. We will probably start with one compound that might not exist as aroma molecule.

In literature they use 2,4,6-trimethylpyridine which I consider as too different too the aroma components. It might adsorb completely different to the fiber..
Also 2-methyl-3-heptanone has been used. That seems better to me.
hexanal-d12 has also been used.
Any other suggestions?