Problem with peak tailing

[alemaggot]

Hi guys,

I must validate a GC method with HS apparatus. I must search residual solvent, such as Methanol, Ethanol and DMF in my powder drug.

I use DMSO such as solvent. The column that I've try is Zebron 624 30m x 0.32 mm x 1,80 um. With this column I've a satisfactory resolution from Methanol to Ethanol at 40°C (oven temperature). But I've a rilevant tailing for them. The Ethanol peak eluite on Methanol final tailing.

How can I resolve this problem? I can't improve resolution decrasing oven T. Split ratio is high (10:1). Carrier flow is 21 PSI.

I think that the best choice for this analysis is WAX column. What you think? But in market I found only one column with high film thickness. 30 m x 0.25 mm x 1.00 um. What do you think about it?

Thank you very much!

Ale

[Johnny Rod]

The 624 is good ofr a range of solvents, and separating methanol and ethanol should be easy. I would try running something like acetone (as analyte) that should give almost no tailing, to check out your system. If this tails, look for system problems. What levels of the analytes are you using?

[christenseng]

You can expect MeOH to have some tailing with the 624 phase but you should be able to have good resolution between MeOH and EtOH. I would suggest that you change your inlet liner to a 2 mm i.d. to help peak shape (if you don't already have this installed). Make sure your inlet temperature is high enough (not less than 200C). If this is an old column, trim about 0.1 m off the front.

A wax phase column will increase the retention of the alcohols but I think you should be able to achieve satisfactory results with the 624.

Here is a link to a good article on headspace analysis of residual solvents using a 624 column:
http://www.chromatographyonline.com/lcg ... ?id=653129

Good luck.

[alemaggot]

Hy guys.

I respound to your question:

- injector temperature: 210 °C
- I use yet 2 mm inlet liner
- I work with this STD concentration: 30mg/ml for MeOH, 50 mg/ml for EtOH, 0,88 mg/ml for DMF. This concentration are calculated for 10ul of solution into HS vial and for 100 mg of sample.

Phenomex specialist say to me that I must work with high film thickness to have less tailing problem. He sostain that 0.25 mm column for 1.00 um of flim thickness coated with wax phase don't resolve my Alcools tailing. What you think about it?

Thank you very much

Best regards

[chromatographer1]

Tailing can be a physical chemistry issue, or it can be a physical issue.

Poor placement of the column in the splitter can cause tailing. A particle within the column can cause tailing.

Too low a pressure can make a solvent plug difficult to focus at the head of the column although at a pressure of 21 psi that seems unlikely.

I never liked splitting the headspace sample and always preferred a direct injection decreasing my sample loop from 1mL to 100 microliters if a reduction in sample size was needed. This gave an added benefit of removing dead volume in the sample flow path and helped to reduce tailing for the early eluting solvents like methanol.

Your problem is either a dirty transfer line, a dirty inlet, mis-installed column, or too fast a flow rate.

Here is an example of a chromatogram using the 624 column(OV-1301)



Rod

[alemaggot]

Hi Rod!

I've seen this chromatogrham. And I've read the posted article. In this method the tailing factor of Methanol is 1.2. Similar to mine. Then is too high for that I want. In 624 column I can't separate Methanol and Ethanol without tailing peaks. In the article they've used 32°C of initial oven temperature. I can't use this becuase for my oven achieve 32°C temperature is a big problem.
Another thing that is write in this article is that if you don't obtain the reqeust resolution or tailing with 624 column, you should use Wax column.

I think that I'll buy 30m x 0,25 mm x 1.00 um wax clomun.

Wich one have another suggestion?

Best regards

[chromatographer1]

When I did my research on a generic HS method back in the 90s (Analytical Chemistry 1997 June) I used a 3 meter piece of Wax column, 0.53mm ID, 1um film,

attached ahead of the 624 column to reduce the methanol tailing and to more strongly focus the sample plug I injected (directly, using a 1mL loop) at 40C.

My article was noted as footnote 18 in the article you read and from which I borrowed the chromatogram.

You get both positive benefits without the extension of the method time using the Wax column as a 30 meter length.

The Wax column has a reduced lifetime because you are condensing water upon it when you are doing headspace. This breaks the bonding of the phase and causes tailing.

I got 90 to 180 days of continuous use before the 3 meter column needed to be replaced. But a 30 meter column will last you 2.5 to 5 years as you use 3 m pieces of it.

I don't know how long you will be able to use the Wax column if you use it alone.

Good luck in whatever you try to do. I hope it is not an expensive solution for you.

best wishes,

Rod

[alemaggot]

Mmm Good idea Rod.

I could try it. One question. If I use 624 with 0.32 mm ID, it's ok to add 0.53 mm ID column? If I add, for example, 5 meters of wax column the split ratio of my system must be calculated with 0.53 mm and 35 m of column lenght?

Thank toy

[chromatographer1]

If you are using a .32mm ID 624 column then I would also use a 0.32mm ID wax piece in front of it.

Calculate your flow then as the column was 33 meters long.

The 10:1 ratio of columns was an intentional one. With the exception of one pair of analytes, MEK and EtoAC,
the separation of most solvents was improved using the 10:1 ratio.

For your needs with methanol and ethanol, this won't be critical, but if you seek to measure ether and some
other analytes, it may be important.

best wishes,

Rod

[alemaggot]

If in laboratory I've only 0.53mm ID columns with high film thickness (1.00 um) coated with WAX phase, is a problem?

[chromatographer1]

Go ahead and use it with the proper connective device.

Calculate your split and flow as though it was not present.

best wishes,

Rod

ps: My 624 column lasted over two years and really did not need to be replaced when I did replace it, just as a precaution. I had samples
to test that each cost more than the cost of a new column, so it was better to be careful than to be sorry.