Chromatography Forum

I am still having some issues with method 524.2. Everytime I report proficiency tests, I come back high on all of the early eluters. I don't see a drop in my IS response anymore from run to run but I do still notice that the higher the concentration, the more tailing of these early eluting peaks. I'm not sure if it is due to the higher concentration or if it is because I run my calibration from high to low and by the highest cal point, I may have additional water contributing to my early peaks. I guess my biggest questions are the following: What volume of liner is best for THM's and what is best for the 21 analytes in the safe drinking water act? I am currently using a 3.5mm liner because my auditor would only settle for a minimum of 3.5 min desorb. Also, I have an initial total flow of 40 ml/min and then at 0.76 min, I have a total flow of 200ml/min and then at 3.51 min, I bring my flow back to 40 ml/min. What flow options are you using? In addition, what temperature ramp are you using? I was using 35C hold 0.5 min then 20C to 220 hold 10 but now I am using 35C hold 4 min then 20C to 220 hold 10. Could this be why my early eluters are broad and tailing as the run progresses? Any help would be greatly appreciated!

Don't use an initial hold any longer than 1 minute, especially at your starting temp of 35C.

Why are you changing your purge flow in the middle of your run? Leave it 40 mL/min. All of your analytes are desorbed within the first 0.5 minute or so - the only thing you'll do by changing your purge flow is to screw with your split rates and probably cause hysteresis in your column.

Brominated compounds are non-linear using MS detection - you should not expect linearity. Also, they are reactive in most P&T systems, so you'll see non-linearity as you get towards the bottom end of your curve.

If you're running a 40 mL purge flow into your injector you need to run 40-80 mL split flows. While I agree with BigBear that you need a large ID liner, I disagree on the packing - I have found that glass wool packing (more or less in the middle) radically improves flow conditions. Remember, you are making a gas phase injection. As non-intuitive as it may be, sometimes you'll find that increasing the split ratio actually lowers your MDL. It has to do with maintaining laminar flow around the column - you'll need to experiment. My initial impression is that you need to at least double your split flow. Some of the requirements in 524 are a bit (OK, more than a bit) strange, and hard to pull off (10% RSD, for example). But, overall, if you pay attention to the chromatography you'll manage.

I, too, prefer the 0.18 columns - the lower your flow into the MS, the less likely you'll have pressure-related issues in the MS. That said, hundreds of thousands of samples have been run using your column, so it definitely works.

I doubt that water is causing you issues - the THMs elute relatively late off that column compared to water. I think you have issues with activity in your system, low split ratios (severe band broadening) and potential overloading of your column.

Even though I'm using a 3.5 min desorb, you don't think I should hold at 35C for more than 1 min?

I'm not changing my purge flow but rather my total flow on the GC end. I was told that with the long desorb time that increasing the total helium flow to like 200 mL/min would help "dry" the column and prevent water problems.

Right now I am running a 40 split which means my total flow starts at around 40 (since my column flow is 0.95mL/min) and then after all of the analytes have reached the head of the column which is about 45 sec, I turn on the increased total flow to 200 or more mL/min.

I also forgot to add to my earlier post that I am no longer having problems with the THM's but rather the early eluters (ie. vinyl chloride, 1,1-dichloroethene, methylene chloride, and trans and cis 1, 2-dichloroethene).

How do you know if you are overloading the column? My instrument doesn't say that the detector is saturated and I'm not getting flat top peaks. Someone said that you will start to get fronting peaks if you overload the column? Is this one of the only symptoms you will see?

A long initial hold will only lead to band broadening for the front end compounds - you don't have enough phase or a low enough temperature to hold them. They're already iffy as far as chromatographic performance at these temperatures (remember, they're mostly gases), so you're actually better off starting your run and using chromatography to your advantage.

If your flow rate is 0.95 mL/min, and your initial purge flow (split, in your case) is 40 mL/min, then you are only pushing 0.95 mL/min through your column. Increasing the flow out the split does nothing to remove water from your column. You are just wasting your helium. Also, depending upon your MS, 0.95 mL/min is pushing the upper end capacity for many pumping systems. That, coupled with the expansion of water into your source, is going to give you a rough ride on the front end. You can significantly improve the performance of your pumping system by going to a 0.18 mm ID column.

Make sure you're running a dry purge cycle. If you are using silica gel in your trap you seriously need to consider using a jet separator in your system - it'll eliminate a significant portion of the water that reaches your MS.

Overloading will manifest itself as poorly-shaped peaks. You can overload your column capacity and still be well within your MS working range. Also, remember that you have both methanol and water eluting in the region where you're having issues, so you need to control the amount of both that is getting to your system, or at least take steps to ensure that the amount reaching the MS is constant.

The OI #11 trap, I believe, is the equivalent of the VOCARB, so dry purge can help remove water there (no silica gel). I personally hate losing all that time to dry purge and mostly do without it, but perhaps it will help.

With a total flow through the inlet of 20 cc/min or greater, the analytes will all be off the trap. Some regulators still insist on a longer desorb, so the trick of blasting the split flow up at 0.5 to 1 min allows you to satisfy the regulation while blowing most of the extra water and methanol out the split vent (while wasting helium). Its a silly work around, but you see it more and more these days. We get away with a 0.5 to 1 min desorb, but we do mostly contract lab work for the EPA, they have not raised a fuss (yet, you never know what will strike an auditors fancy, even after years of status quo).

OI claims a desorb preheat is not needed for thier equipment, it is just there to compete with Tekmar, but sometimes it can help tighten up the early peaks. With the #11 trap, a preheat of 230 to 235 degrees, then desorb at 240 degrees is appropriate.

We have seen the problems with dropping signal over several runs on newer Agilent equipment (5973 onward). It seems we have to replace the repellor eventually. Frequent source cleanings help for a while. I guess this is the source activation issue that BigBear is referring to. This is annoying because this behavior also arises from active plumbing in the flow path; it can be a pain to decide which issue to address first.

Mckrause, the wool in the liner is interesting, I'll have to give that a try.

Thanks folks, interesting thread. I hope things work out for the original poster.

Tim

Bingo Yama001 you got it. With the new more sensitive MS's are more prone to water. I was fat, dumb, and happy doing 524 on a 5890/5970. Cleaned the source every year or so
just cause I thought it needed it. Failed to realize that my jet sepp was my water management tool.
After changing to a 6890/5973 things looked good for a while then everything went south! Chased the problem for about a year until it was explained that my problem is/was water.
I now use a Vocarb 3000 trap and dry purge it @ 40 cc/min for 6 min. Desorb for 3 min and set my gas saver to 150 cc/min at 1.5 min ( this increase flow dries my transfer line and injection port). I only run one P&T so I extend my GC final time to just under 10 min ( time set by checking on oven cooling ) so that the oven is at ready before P&T gets to the end of dry purge.
With all these steps I still have to clean my source every 2-300 injections.