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Forum index » Science and Technology » Chem » Analytical
internal standard responses deteriorating throughout the run
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bp.gabriel2006@gmail.com1
science forum beginner


Joined: 28 Mar 2006
Posts: 6

PostPosted: Tue Mar 28, 2006 11:18 pm    Post subject: internal standard responses deteriorating throughout the run Reply with quote

GC: HP 6890 /Agilent
MS: HP 5973 / Agilent

Hello,

Internal standard responses are deteriorating throughout the run.
It seems that we've narrowed the problem down to the GC/MS side. I've
seen this behavior before with a weak multiplier, but the one in this
system is new. Has anyone seen a mass spec (or GC) related issue
involving a steady decrease (as well as periodic 'jumps') in ISTD
response over run time?

Here's some sample data (from first injection to last injection) which
was taken over 24 hours for illustration. Each column is a single
standard. Each "row" (including the offset response to the left is one
sample:

GC/MS QA-QC Check Report

File Internal Standard Responses
=========================================================================
V00002.D 450533 590424 880709 826660

------------------------------------------------------------------------- AQ
V00003.D 453952 593140 872136 826181

------------------------------------------------------------------------- AQ
V00005.D 476864 589687 878828 838848

------------------------------------------------------------------------- AQ
V00006.D 510346 626419 917971 874959

------------------------------------------------------------------------- AQ
V00007.D 546248 668221 975288 938784

------------------------------------------------------------------------- AQ
V00008.D 582226 713665 1034404 994373

------------------------------------------------------------------------- AQ
V00009.D 492154 690306 1007237 938576

------------------------------------------------------------------------- AQ
V00010.D 451643 634588 942657 878910

------------------------------------------------------------------------- AQ
V00011.D 438573 606203 889773 847372

------------------------------------------------------------------------- AQ
V00012.D 424468 590662 861429 831345

------------------------------------------------------------------------- AQ
V00013.D 416662 573327 857250 808296

------------------------------------------------------------------------- AQ
V00014.D 395911 559216 831411 779648

------------------------------------------------------------------------- AQ
V00015.D 385257 540478 813409 773904

------------------------------------------------------------------------- AQ
V00016.D 383724 539731 799278 759136

------------------------------------------------------------------------- AQ
V00017.D 378391 534751 796121 747984

------------------------------------------------------------------------- AQ
V00018.D 378146 513961 779471 737826

------------------------------------------------------------------------- AQ
V00021.D 422795 524481 772851 734951

------------------------------------------------------------------------- AQ
V00023.D 361293 530306 777239 732041

------------------------------------------------------------------------- tAQ
V00029.D 392821 507644 762030 720414

------------------------------------------------------------------------- tAQ
V00030.D 390069 510278 762940 720017

------------------------------------------------------------------------- tAQ
V00037.D 393536 506257 759289 716239

------------------------------------------------------------------------- tAQ
V00038.D 382922 509525 764194 714252

------------------------------------------------------------------------- tAQ
V00040.D 345196 492997 733527 687487

------------------------------------------------------------------------- tAQ
V00041.D 331154 479300 717364 678737

------------------------------------------------------------------------- tAQ
V00042.D 362100 481891 717830 683572

------------------------------------------------------------------------- tAQ
V00044.D 374659 481741 718320 687620

------------------------------------------------------------------------- tAQ
V00045.D 406110 500132 739924 704909


Thanks,

BP


Keywords: GC/MS Volatile Organic Analysis Volatiles Organics
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bp.gabriel2006@gmail.com1
science forum beginner


Joined: 28 Mar 2006
Posts: 6

PostPosted: Sun Apr 09, 2006 3:36 pm    Post subject: Re: internal standard responses deteriorating throughout the run Reply with quote

Update:

Seemed to have resolved the problem. I tried running with both
filaments and the same results. Replaced the filaments anyways and the
problem was eliminated. Inspection of the old filaments showed
significant burn discoloration around all of the shielding plates
around the filament wire. Bingo. They were just wore out.

Unfortunately now there is a (possibly) seperate problem. Running
blank samples with only ISTD's and Surrogates will yield consistant
responses throughout the run. Although runs with high analyte
concentrations (ie calibration curve points of 100ppb or greater) will
correlate with higher ISTD response. This, in effect, will 'throw off'
the calibration %RSD for most compounds, particularly at the higher
concentrations. I've entertained the idea of active sites somewhere,
but also considered replacing the multiplier. Any other suggestions?

Thanks,

BP
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bp.gabriel2006@gmail.com1
science forum beginner


Joined: 28 Mar 2006
Posts: 6

PostPosted: Wed Apr 12, 2006 9:14 pm    Post subject: Re: internal standard responses deteriorating throughout the run Reply with quote

Another UPdate:

Replaced multiplier from a three month old DeTech 2300 to a brand new
ETP 14617 Electron Multiplier. Works great thus far.
No more increasing istd responses relative to the amount of analyte
concentration.


One last problem.... for now...:

Vinyl Chloride and Iodomethane both have calibration %RSD beyond EPA
limits.

I noticed vinyl chloride's plotted cal. curve shows higher relative
responses than do the larger concentration points. I also noticed that
many of the 'gas' analytes have a bit of peak tailing. Vinyl
Chloride's tail is odd, however. It has a little "plateau" (about
three times the width, and a third as high as the peak) before finally
dropping off.

Iodomethane's peak seams normal except the tailing stretches far beyond
the retention time window, and is very low near the baseline.

Do GC settings near beginning of the run need adjustment? Or methanol
interference? Thanks in advance.

-bp
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bp.gabriel2006@gmail.com1
science forum beginner


Joined: 28 Mar 2006
Posts: 6

PostPosted: Mon Apr 24, 2006 9:29 pm    Post subject: Re: internal standard responses deteriorating throughout the run Reply with quote

Actually,

I learned that methanol can interfere heavily with vinyl chloride. I
took two steps to eliminate / sidestep this interference:

1) use higher concentratrion solvent/analyte mix to spike my
calibration samples, and
2) 'equalize' methanol (MeOH) content in all samples. This means that
my highest curve cal sample was 50ul of standard, so I added the
difference in methanol for each lower cal point. So for a 10ppb (2.5
ul of cal std) i added 47.5ul methanol; for a 100ppb (25ul std) i added
25ul of MeOH, and so on for each calibration sample. This step
particularly made the calibration plot for vinyl chloride significantly
more linear.
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