Mass Spec losing internal standard

[Stormyrene]

5975
Purge 11min.
Inlet Head Pressure 12.33psi
Dry Purge 1min.
Inlet Temperature 250C
Desorb 3min. 255C
Bake 10min. 270C
Mode Split
Split Ratio 65/1
Valve Oven 150C
Total Flow 42.9mL/min
Transfer Line 150C
Column RTX-VMS
20m X 0.18mm I.D.
1.0um Film Thickness
Oven Temp. Program
40C-100C at 10C/min
100-250C at 25C/min hold for 2min.
Column flow rate 0.6mL/min.
Mode Constant Flow
Gas Saver 150mL/min. @1.50min.

The problem is that as a run goes on I see less and less internal standard. Monday morning, after the mass spec hasn’t been used all weekend, is the highest the internal standard will be all week. For example, Monday morning my Fluorobenzene will be 700,000 area counts. 20 samples later, fluorobenzene will be 400,000 area counts. Tuesday morning, fluorbenzene will be 550,000 area counts, down to 350,000 at the end of the run. By Friday, if the equipment has been used all week, the starting sample will be almost half of what it was Monday morning. Also, once I have lost that much area, even after a weekend, it never goes back to where it was. After the week used in the example, I would be lucky to see 600,000 area counts on Monday morning.

The transfer line was replaced in September, trap was replaced in November.

I understand this is probably a water carryover situation, but don't know how to fix it.

[nvalentine]

Decreasing response over a series of runs is normal. This is part of why internal standards are used. As long as the loss is proportional across all compounds (or at least relative to the closest IS compound), it really should not affect the analysis. Losing almost 50% area over 20 runs does seem a little high though.

I'm guessing since you only mention fluorobenzene that you are running 524.2? I do have some tips and tricks for running this analysis, even if you are held to a long desorb time. A lot of the issues are tied to water because of the method requirements.

If you would like the information please shoot me an email at nvalentine@teledyne.com with a little more detail on your parameters for both the purge and trap and GC/MS.

[JonKlar]

Im running 8260 volatiles with a very similar set up and the same column. I am noticing dropping standards as well and would be interested in any developments in your situation. Do you use an auto sampler? If so what kind?

[Stormyrene]

I use an Archon Autosampler 40mL VOA vials

[Bigbear]

Easy fix. First I would suggest you increase your dry purge to 6 min @ 40 cc/min. If you can afford the time extend the time your oven is at 250 ( have to time a few cooldowns to see how long it takes from 250 to GC ready). Ulitmately you want the oven at max t for as long as possible to get rid of the water. In my case with an 11 min purge and 6 min dp I can hold @ 230 for just under 10 min and still have the gc ready by the time the P&T gets to desorb.
This next one is very critical. With the 5973 and 75 systems the source needs regular cleanings. In my case every 2-300 runs.
Everything else in your method tracks very closly to my parameters for 524. What trap are you using. It seems as the favorite for P&T is a VOcarb 3000.

[Stormyrene]

I am now holding at 250 for 5 minutes and dry purging for 5 minutes. No difference.

[Bigbear]

It may be time to clean your source. When was the last time you did injector maintence?

[Stormyrene]

Cleaned the source on Friday 10th. I replaced the transfer line(from Archon autosampler to GC) about 4 months ago.

[nvalentine]

What MS conditions are you using? Have you tried talking to Agilent about this issue?

I could send you our standard GC/MS parameters for 524.2 if you would like, but I would need to know the exact model of mass spec, whether it is a triple axis or not.

[Stormyrene]

5975
Purge 11min.
Inlet Head Pressure 12.33psi
Dry Purge 5min.
Inlet Temperature 250C
Desorb 1min. 255C
Bake 10min. 270C
Mode Split
Split Ratio 65/1
Valve Oven 150C
Total Flow 42.9mL/min
Transfer Line 150C
Column RTX-VMS
20m X 0.18mm I.D.
1.0um Film Thickness
Oven Temp. Program
40C-100C at 10C/min
100-250C at 25C/min hold for 5min.
Column flow rate 0.6mL/min.
Mode Constant Flow
Gas Saver 150mL/min. @1.50min.


Mass Spec is 5975 inert Mass Selective Ion Detector.
It uses a turbo pump, can this be replaced with a diffusion pump?

[Bigbear]

What inlet liner are you using? Do you always cool down your inler to < 70 when you change liners? How old is your shell weldment? If it's a few years old you may consider changing it or cleaning. I have had good luck cleaning the weldments after removing them from the GC using the alumina powder. I feel the Agilent gun brushing method is a little too agressive.

[jochem92]

Had the same problem for a long time. The solution for our lab was a relatively simeple one. We simply changed from the small diameter (3mm) drawout plate to a large aperature (6mm) drawout plate and changed to a Restek liner catalog #20713. This has worked wonders. We do use differrent method parameters than what you have documented also. Hope this helps.

[LindseyPyron]

The first thing I would try is to reduce your desorb time to 30 seconds. This is long enough to completely desorb your analytes and not transfer as much water.

I am also not a fan of the column you are using. Restek makes a new RTX 5-sil column that has lower bleed and better resolution. We have gone from the 0.18 column to a 0.25 without a loss in resolution and the 0.25 has better loading capacity and can help linearity a bit.

You can also replace your current aperture lens with a 3mm or 6mm lens. If you are STILL struggling, give our tech support team a call at 800-283-3510. They will help you and it is free.

[bvbdede]

Hi stormyrene,

did you solve the problem and could tell us what parameters were responsible causing the problems?

I have quite the same problems...

Dennis

[James_Ball]

If you are having trouble with internal standards decreasing during the run, one thing we found that helps is to switch to the ETP electron multiplier, they seem to hold up better than a standard K&M, though the newer triple axis ones seem to do well.

I have tried the Rtx-VMS columns and I don't like them as much as I do the good old Rtx-502.2, especially if you are analyzing for acetone and ketones. I run 40m x 0.18 Rtx-502.2 columns and it seems to work pretty well. You will run with a lot higher head pressure, but I am starting to think that actually helps with the water some, not sure why. I am also now starting to use the new Rxi-624SilMS column, looks ok so far but will know more later on.

Another tip I found while researching this problem was to keep your multiplier voltage as low as possible, this helps with the problem of internal standard dropping during the run, and with the 1,4-Dichlorobenzene-d4 increasing as the concentration of your standards increases. A link to that publication is here: http://www.chem.agilent.com/Library/app ... 0603EN.pdf