[28] What do Chromatograms tell us? First Eluting Peaks are very Broad when using Direct Injection and Water as Matrix.

2011-jaap-pasfoto4-smallChromatograms are like fingerprints. If you can “read” chromatograms well, you often can find a plausible cause. In this series, we will show a series of GC-chromatograms that are obtained from users and discuss some potential causes for the phenomena. Then we can move into some solutions for improvement.

 

 

Direct injection is often used if low levels of volatile components have to be analyzed in a higher boiling matrix. The main peak will elute later then the components, so we cannot use splitless injection. Direct injection is always a challenge as here a relative big injection-error is made.  If the conditions are not optimized, one can get a chromatogram as shown in fig.1. Here the matrix is water which complicates the injection.  Important is to minimize the band broadening during injection.

Fig 1  Note broadened early eluting peaks due to poor focusing

Fig 1 Broadened early eluting peaks due to poor focusing

Conditions for a good direct injection:

  • –    Using smallest injection amount;
  • –     Inject fast;
  • –     Use wide bore columns with high retention;
  • –     Use a uniliner (see fig.2) for most ideal gas path.
  • –     Find the best oven temperature to start the analysis;

See also: http://blog.restek.com/?p=614

 

Fig. 2  Uniliner for Direct Injection

Fig. 2 Uniliner for Direct Injection

To get sufficient focusing of the early eluting compounds one need to find a compromise between initial oven temperature and retention of the stationary phase.  Usually high retentive phases (thick films or PLOT’s) will have a good focusing effect, and result in sharp peaks.

The additional advantage is, that the starting temperature can be relative high, so there is reduced risk for the matrix to condense, which helps the chromatography.

 

Fig. 1 was using a 30m x 0.53mm Rt-Q BOND where oven temperature was 110C for 20 s, followed by a program ballistic to 130ºC, → 250ºC @ 10ºC/min

Under these conditions there is little focusing for volatiles happening which results in broadened peaks.  The later eluting peaks are focused and sharp.

By reducing the oven temperature to 80 C and using the same program, the focusing improved significantly, resulting in good peaks for early eluting compounds, see fig. 3.

Fig 3  dropping oven temp to 90C makes good focusing possible using water as matrix

Fig 3 dropping oven temp to 80C makes good focusing possible using water as matrix

By using a divinyl benzene porous polymer, the hydrophobicity also helps to make the water peak move very fast, so it has minimal impact on the retained analytes.

 

Installation of a column with the uniliner , Fig 2, remains a challenge as this connection is made inside the injection port. See for practical details, here: Installation uniliner 05-04-002

Figure 4 shows an other procedure to install a uniliner.Blog28-fig 4

 

For some systems, like Agilent, the uniliner must have a “hole” on the side of the liner, in order to make the EPC work. This “hole can be on top or on the bottom. For Direct injections, it is preferred to use the hole on the top

http://www.restek.com/catalog/view/9232 (4mm uni-liner) and

http://www.restek.com/catalog/view/9201 (1mm uni-liner)

 

More reading about Direct injections: http://www.restek.com/pdfs/59882B.pdf

 

3 Responses to “[28] What do Chromatograms tell us? First Eluting Peaks are very Broad when using Direct Injection and Water as Matrix.”

  1. Dear Jaap –

    If you have both Purged Packed Direct Inlet as well as Split/Splitless Inlet configured in Direct Inlet mode mounted in your GC, which injection port would you prefere for trace analysis?

    With kind regards –
    Lars Kurstein, Copenhagen

  2. Miron says:

    Dear Mr. Jaap De Zeeuw,

    I have a question not related to this article.
    I am developing a static head space method for determination of residual solvents in raw material of active pharmaceutical ingredient (API).
    Is it a must for sample to be completely dissolved in diluent?
    Thanks.

    Miron

  3. Hi Lars
    direct injection is all about efficient transfer of sample to column.
    I do not know the “purged packed inlet”, If the connection with the column is similar as the uniliner in a split/splitless, there will be little difference.

    regards
    jaap

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