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By Anonymous on Wednesday, February 25, 2004 - 06:53 am:
iam using a complex mixture for gcms analysis. all my analytes of interest are in trace quantities.hence iam injecting 1 micro litre of the sample in splitless mode.i have to analyze so many samples. but fatty acids in the mixture are not going out of the column in the run and interfering in the subseqent runs.iam not getting a neat blank.
iam using spb-624(6% cyanopropyl phenyl pdms stationay phase) and maximum temperature in my run goes upto240c. can u suggest me a solution?
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By Anonymous on Wednesday, February 25, 2004 - 07:20 am:
what phase thickness are you using?
are the fatty acids derivatized?
what is your blank consisting of?
what temperature are you using for the injector?
what is your sample?
Give us more information and you might get an answer worth reading.
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By mgoodwin on Wednesday, February 25, 2004 - 02:23 pm:
for methyl ester derivatives, I set my injection port to 310 deg. C.; same with detector. I have experienced problems with C26 fatty acid methyl derivatives with an injector temp under 300 deg C.
gas flow rate can also affect retention, is your instrument set to constant pressure, or constant flow? Keeping the flow rate up, will improve recovery and peak shape.
Hope this helps out.
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By Anonymous on Thursday, February 26, 2004 - 06:00 am:
such extreme temperatures are not necessary if one uses a solvent plug behind the sample to be injected and a moderate to slow injection plunger rate. A 無 of methylene chloride or chloroform will work wonders when placed behind a 無 of sample using a 0.5無/sec plunger rate. A temp of 250 to 275蚓 should be adequate. Detector temperature should be under maximum temperature of the column phase.
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By Ron on Thursday, February 26, 2004 - 06:48 am:
You are using a thick phase column for an analysis usually done on a thinner phase. The column you are using has a phase thickness of 1.4 microns if it is a 0.25 mm id column. For a fatty acid analysis I usually use no more than a 0.5 micron phase. Try using either a wax column or a 1301 phase. The polymer for a 1301 column is the same as that in a 624 column, but the phase thickness is less.
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By mgoodwin on Thursday, February 26, 2004 - 08:56 am:
Interesting. I always thought a slower injection would cause loss of sample in GC as the compounds condense on the needle, especially with splitless mode. Hence why autosamplers inject so swiftly. Is the solvent plug suppose to protect this?
I will try this out next time doing some FAMES as your recommended temperature zones are much less harsh on columns and septa.
Also, why have a colder detector than the column? Wouldn't you experience sample condensation in the transfer line?
btw I failed to mention above that I use a 30m HP-5ms column ID 0.25mm wall thickness 0.25um
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By Anonymous on Thursday, February 26, 2004 - 10:04 am:
When I worked for Sigma Chemical years ago, I assayed FAMEs as high as 29 carbons. These methyl esters can partially deposit as waxes in the liner as it is cooled from the evaporization of the carrier solvent, unless rinsed off the inlet liner into the column. Other non-volatile components of course reside in the liner. But a slow injection of a solvent plug benind the sample will tend to improve your sample reproducibility. The reason fast injection speeds are used are to improve reproducibility for sensitive compounds which can react with the bare metal of needles at temperature. The fast speed produces a quick burst of evaporation of the solvent into a solvent cloud. This has a danger of flashing back upstream into colder areas of the inlet and depositing out analytes, which then wait for the next or another solvent cloud to rinse them downstream, hurting reproducibility of the amount of sample reaching the column inlet from injection to injection.
A slower injection allows the cloud to begin to flow downstream before all the solvent is vaporized, resulting in a smaller flash cloud. It also helps to rinse any deposited low volatile components from the inlet surface back into the solvent cloud and onto the column.
As long as your analytes tolerate contact with metal slow injections are not a problem. There is a loss of column plate numbers due to the wider sample plug but that can usually be refocused.
The speed of the injection is not a fixed optimum. It depends upon what you are doing and what analytes you are working with.
That is why some vendors allow variable injection plunger speeds. It has real scientific value, and is not just a marketing gimic.
iam using a complex mixture for gcms analysis. all my analytes of interest are in trace quantities.hence iam injecting 1 micro litre of the sample in splitless mode.i have to analyze so many samples. but fatty acids in the mixture are not going out of the column in the run and interfering in the subseqent runs.iam not getting a neat blank.
iam using spb-624(6% cyanopropyl phenyl pdms stationay phase) and maximum temperature in my run goes upto240c. can u suggest me a solution?
-------------------------------------------------------------------------------------------------------
By Anonymous on Wednesday, February 25, 2004 - 07:20 am:
what phase thickness are you using?
are the fatty acids derivatized?
what is your blank consisting of?
what temperature are you using for the injector?
what is your sample?
Give us more information and you might get an answer worth reading.
-------------------------------------------------------------------------------------------------------
By mgoodwin on Wednesday, February 25, 2004 - 02:23 pm:
for methyl ester derivatives, I set my injection port to 310 deg. C.; same with detector. I have experienced problems with C26 fatty acid methyl derivatives with an injector temp under 300 deg C.
gas flow rate can also affect retention, is your instrument set to constant pressure, or constant flow? Keeping the flow rate up, will improve recovery and peak shape.
Hope this helps out.
-------------------------------------------------------------------------------------------------------
By Anonymous on Thursday, February 26, 2004 - 06:00 am:
such extreme temperatures are not necessary if one uses a solvent plug behind the sample to be injected and a moderate to slow injection plunger rate. A 無 of methylene chloride or chloroform will work wonders when placed behind a 無 of sample using a 0.5無/sec plunger rate. A temp of 250 to 275蚓 should be adequate. Detector temperature should be under maximum temperature of the column phase.
-------------------------------------------------------------------------------------------------------
By Ron on Thursday, February 26, 2004 - 06:48 am:
You are using a thick phase column for an analysis usually done on a thinner phase. The column you are using has a phase thickness of 1.4 microns if it is a 0.25 mm id column. For a fatty acid analysis I usually use no more than a 0.5 micron phase. Try using either a wax column or a 1301 phase. The polymer for a 1301 column is the same as that in a 624 column, but the phase thickness is less.
-------------------------------------------------------------------------------------------------------
By mgoodwin on Thursday, February 26, 2004 - 08:56 am:
Interesting. I always thought a slower injection would cause loss of sample in GC as the compounds condense on the needle, especially with splitless mode. Hence why autosamplers inject so swiftly. Is the solvent plug suppose to protect this?
I will try this out next time doing some FAMES as your recommended temperature zones are much less harsh on columns and septa.
Also, why have a colder detector than the column? Wouldn't you experience sample condensation in the transfer line?
btw I failed to mention above that I use a 30m HP-5ms column ID 0.25mm wall thickness 0.25um
-------------------------------------------------------------------------------------------------------
By Anonymous on Thursday, February 26, 2004 - 10:04 am:
When I worked for Sigma Chemical years ago, I assayed FAMEs as high as 29 carbons. These methyl esters can partially deposit as waxes in the liner as it is cooled from the evaporization of the carrier solvent, unless rinsed off the inlet liner into the column. Other non-volatile components of course reside in the liner. But a slow injection of a solvent plug benind the sample will tend to improve your sample reproducibility. The reason fast injection speeds are used are to improve reproducibility for sensitive compounds which can react with the bare metal of needles at temperature. The fast speed produces a quick burst of evaporation of the solvent into a solvent cloud. This has a danger of flashing back upstream into colder areas of the inlet and depositing out analytes, which then wait for the next or another solvent cloud to rinse them downstream, hurting reproducibility of the amount of sample reaching the column inlet from injection to injection.
A slower injection allows the cloud to begin to flow downstream before all the solvent is vaporized, resulting in a smaller flash cloud. It also helps to rinse any deposited low volatile components from the inlet surface back into the solvent cloud and onto the column.
As long as your analytes tolerate contact with metal slow injections are not a problem. There is a loss of column plate numbers due to the wider sample plug but that can usually be refocused.
The speed of the injection is not a fixed optimum. It depends upon what you are doing and what analytes you are working with.
That is why some vendors allow variable injection plunger speeds. It has real scientific value, and is not just a marketing gimic.
