Where do these peaks come from?

We are running a method to analyze assay and some impurities in a pharmaceutical material. We are running under the following conditions:

Agilent HP-1 30m x 0.53mm x 3.0 um column

200 C inlet

1:1 split

5 mg/mL (high concentration to detect impurities)

2.5 mL/min flow with temp gradient with 250C bakeoff for 20min

2 uL inj

Diluent: ACN 

Liquid inject

Liner: 4mm deactivated straight through with wool (Restek cat # 20782)

Septum: Bleed/Temp Opt. non-stick 11mm (Agilent PN 5182-3413)

 

We have peaks that look like "impurities" peaks but do not increase and decrease with same magnitude as the sample concentration and are inconsistent between injections of the same sample. They are not in the ACN blank. They increase with inlet temperature increase to 250C.

I also see them (with lesser magnitude) with an injection of just hydrogen peroxide in ACN only.

These peaks are seen at 7.8  min and 8.2 min in the attached chroms.

 

Any clues where these are coming from? Interaction with injection and something in the inlet? Is there a more inert liner or septum I should be using?

attachments.zip
Parents
  • Hi all,

    I'll contribute to this topic with a few comments.

    I'm in no way a GC expert but I have been working with a GC-MS, basically on my own, for about a year now and I have been trying to gain knowledge on how to best operate the machine.

    Please correct me if I'm wrong in any way.

     

    The first thing that caught my attention was the injection volume (2µL).

    According to a solvent expansion calculator, with the injector temp @ 200ºC and ACN as solvent, backflash happens during the injection.

     

    I used another calculator to find the head pressure according to the column dimensions and other parameters.

     

     

    So, according to this document, backflash "leads to poor reproducibility, sample loss, ghost peaks, carry-over, split peaks, tailing peaks and loss of resolution. Without doubt, backflash is the primary cause for the largest number of reported gas chromatographic anomalies."

     

    With such a low head pressure (dictated by the type of column used and method parameters), there is not much to do to lower the expansion volume of the solvent: decrease inlet temp (if possible), decrease injection volume (best option for backflash, worst for contaminant analysis), change solvent.

    There is the option of using Pulsed Pressure Splitless GC Injection if your instrument has that possibility. I'll just leave a link to a document explaining better this feature.

    The liner you are using is fine for split/splitless injection. Just don't use split liners for splitless injections.

     

    About injecting hydrogen peroxide.

    I would not be amazed if H2O2 would decompose in the injector to give water and oxygen. Two compounds not typically detected by FID (if it is the detector used here).

    As well, GC columns do not usually like oxygen when hot.

     

    So, what I would do first (here is the part that might need more tuning by experts...):

    - clean the injector,

    - change septum and liner,

    - bake injector @ 300ºC (good practice when putting a new liner) without the column attached if possible because max temp of the column is 260-280ºC,

    - bake column @ max temp,

    - if working splitless, choose starting oven temp below solvent boiling point (solvent effect),

    - choose correct linear velocity for carrier gas, @ starting oven temp, for my column (Van Deemter), this will give me the head pressure of the carrier,

    - determine the volume of the injection to avoid backflash,

    - determine column holdup time and calibrate column,

    - inject standard(s) (not H2O2) indicated in the column documentation.

Reply
  • Hi all,

    I'll contribute to this topic with a few comments.

    I'm in no way a GC expert but I have been working with a GC-MS, basically on my own, for about a year now and I have been trying to gain knowledge on how to best operate the machine.

    Please correct me if I'm wrong in any way.

     

    The first thing that caught my attention was the injection volume (2µL).

    According to a solvent expansion calculator, with the injector temp @ 200ºC and ACN as solvent, backflash happens during the injection.

     

    I used another calculator to find the head pressure according to the column dimensions and other parameters.

     

     

    So, according to this document, backflash "leads to poor reproducibility, sample loss, ghost peaks, carry-over, split peaks, tailing peaks and loss of resolution. Without doubt, backflash is the primary cause for the largest number of reported gas chromatographic anomalies."

     

    With such a low head pressure (dictated by the type of column used and method parameters), there is not much to do to lower the expansion volume of the solvent: decrease inlet temp (if possible), decrease injection volume (best option for backflash, worst for contaminant analysis), change solvent.

    There is the option of using Pulsed Pressure Splitless GC Injection if your instrument has that possibility. I'll just leave a link to a document explaining better this feature.

    The liner you are using is fine for split/splitless injection. Just don't use split liners for splitless injections.

     

    About injecting hydrogen peroxide.

    I would not be amazed if H2O2 would decompose in the injector to give water and oxygen. Two compounds not typically detected by FID (if it is the detector used here).

    As well, GC columns do not usually like oxygen when hot.

     

    So, what I would do first (here is the part that might need more tuning by experts...):

    - clean the injector,

    - change septum and liner,

    - bake injector @ 300ºC (good practice when putting a new liner) without the column attached if possible because max temp of the column is 260-280ºC,

    - bake column @ max temp,

    - if working splitless, choose starting oven temp below solvent boiling point (solvent effect),

    - choose correct linear velocity for carrier gas, @ starting oven temp, for my column (Van Deemter), this will give me the head pressure of the carrier,

    - determine the volume of the injection to avoid backflash,

    - determine column holdup time and calibrate column,

    - inject standard(s) (not H2O2) indicated in the column documentation.

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