Mass Spec losing Internal standards

[Stormyrene]

Time Temp 5975
Purge 11min. Inlet Head Pressure 12.33psi
Dry Purge 1min. Inlet Temperature 250C
Desorb 3min. 255C Mode Split
Bake 10min. 270C 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.

[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.

[LindseyPyron]

The first this 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 suing. 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.