Designing the ideal standby method

Question

Hello all

I'm looking at designing some standby methods for my 4 GC-FIDs and 3 GC-MSs as they are often sitting idle for 3/4 days a week and are happily consuming helium, hydrogen and generally sucking money away basically for no reason. Turning them off/on is not an option as we often need some of them fairly urgently, so I thought a 'Standby' method would be the best way to conserve!

I just want some advice on the key things to be looking at regulating. From scrawling this and other forums I've pieced together the following information so if anybody spots any glaring holes, please let me know

Oven temperature: Low (30-50)

Column: Carrier gases set to a minimum flow (~0.5ml?) to protect column and/or MS (we are an oils lab so use non-polar columns)

FID: Keep at around 100 to prevent moisture forming on jet. Turn off all gases except for carrier gas

MS: Keep at temperature to ensure no build up of moisture/solvent anything on source etc. Same with aux line?

My main point of concern is the inlet. We have split and splitless injectors - how much flow do we need to keep going for the split flow? Any? Low? I've seen some people say to set the method to splitless and put a low flow on it (1-2ml/min). Currently it's on Gas Saver running 20ml/min which I feel is fairly high compared to the remainder of the system.

Any advice on this will be appreciated!

csiquet_flex2000 · Answer

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james_jenkins · Answer

Hi candy, Thank you for the post and the great detail. Column damage is difficult to determine. Is it damage or natural age from use, hard to tell. Columns do have a lifespan depending on gas quality, use and proper storage and care. For a standby method, I wouldn't reduce any of my MSD heated zones and would suggest leaving them at temperature. This helps keep the source not only ready to run, but aids in keeping it cleaner. For column flow, reducing to 0.5ml/min from 1ml/min wouldn't really show a cost benefit as its such a small change. I would suggest leaving the flow at the usual operating flow of 1ml/min. Changing to 0.5ml/min will only save you about 720ml of He per day. Over the lifespan of a He tank this will give you a couple of more days of He usage at the possible sacrifice of column life. Below, some rough math that I think will help, hopefully I haven't made any errors here, its early and I havent started coffee yet : A regular, common size gas cylinder ( North America) UHP grade (0.999995% pure) filled under standard temperature and atmospheric conditions will hold approximately 291 cubic feet of helium at 2500 PSI. This equates to 8240 cubic liters of gas. A split/splitless inlet in standby only with gas saver on will have a flow of 24ml/min approx. ( 20ml/min gas saver, 3ml/min septum purge, 1 ml/min column flow). Typical inlet usage 0.024 liters X 60 minutes= 1.44 liters per hour or 34.56 liters per day for one inlet. 8240/34.56 = 238.42 Days. One inlet in standby from filled tank to depleted tank should go 238 days. In practice, you should never drain a tank below a couple of hundred PSI, so practical terms 230 days would be a good round number per inlet. Regards James

james_jenkins · Answer

Hi ryanedwardson, The idea to create a standby method is a good one. Many instrument users do this successfully, but many others create methods that shorten column life and end up costing more money in the long run. The idea of a standby method is to keep the system healthy and able to perform at a moments notice. All points that you outlined. I have been a service rep since the late 1980's and some of what I will suggest here will cause debate for those that are able to create methods that are below factory spec and seemingly have no issues. Taking your points: Oven temperature: Low (30-50) ( My suggestion, stay at the main method starting temperature ) Column: Carrier gases set to a minimum flow (~0.5ml?) to protect column and/or MS (we are an oils lab so use non-polar columns) ( Reducing column flow in some cases with reduced oven temperature can leave the column with not enough flow and head pressure to keep a column healthy and prevent air infiltration through the split vent. You are only saving 0.5ml/min typically here anyway so its not worth it ) FID: Keep at around 100 to prevent moisture forming on jet. Turn off all gases except for carrier gas ( Gases off, but keep the temperature up to eliminate any siloxane build up from the column. 150c or higher. Higher temperature costs nothing, moisture should not be an issue anyway with proper gas purity and filters ) MS: Keep at temperature to ensure no build up of moisture/solvent anything on source etc. Same with aux line?( Yes ) The Inlet should maintain proper total flow to maintain its operation, this is why Agilent/HP came up with the gas saver feature. The older instruments such as the 5890 just had a total flow knob and usually it wasn't adjusted for times when the instrument was idle and used more gas. Gas saver also kicks in at a defined time , such as 2 minutes, as high flow is really only needed at injection. This next statement may cause the most discussion. Leave the total inlet flow at 20ml/min in gas saver, you could try and reduce it to something like 15ml/min (below factory specs) , but keep in mind we want to maintain head pressure and avoid any air entering the inlet through the split vent which is detrimental to the column. In short, you have a modern instrument with saving features already there, Gas saver, EPC detector flow for FID gases can be off with the simple load of a standby method. I hope this helps. Regards James

james_jenkins · Answer

The helium make up gas can be turned off for the detector along with all the other detector support gases. 40ml/min is quite high for make up gas. Regards James

csiquet_flex2000 · Answer

Have you considered gas generators for Air, H2 and N2?

Helium could be substituted by H2 as a carrier and, from what I read, this allows for faster analysis.

The danger is H2 accumulation in oven (broken column) or MSD (stopped pump).

What do you think james_jenkins?

Would there be much saving this way?

james_jenkins · Answer

Hi csiquet_fles2000,

Generators are a viable solution as long as they are well maintained. The switch on the carrier gas side is not straight forward as there are method and column changes that will be needed in most cases. One reference is below:

Converting EI GC/MSD to Hydrogen

For anyone considering the change to H2, it is always good to reach out to your local support and review your instrument configuration to make sure the change is applicable to your hardware. For example H2 would not be appropriate with a purge and trap system or headspace. There are alternate solutions in each case. Agilent contact centers can provide information on applications as well as safety information.

Regards

James

Designing the ideal standby method

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