Troubleshooting of gas chromatography (G.C.)

Troubleshooting of gas chromatography (G.C.)


Troubleshooting of gas chromatography (G.C.)

Troubleshooting of split peak.

We're going to write about split peaks. Let's say we're doing some gas chromatography, and we see some split peaks. What might be causing that? If we think about split peaks, in general what's happening there is something is interfering with our sample transfer from the injection port into the column, in most circumstances. That's not proceeding as a nice, tight band. It's getting spread out over time or separated from one another. What could cause that? It could be an injection problem. Even something as simple as, our syringe isn't smoothly injecting our sample into the injection port, or it could be that our sample is not making a homogeneous vapor cloud inside the inlet before it transfers into the column. If aerosol droplets might be making their way into the column, that could be a contributing factor to split peaks. If we want to manage that a little bit better, we want to have some type of packing in our liner, or a liner with a specific geometry, to help make sure that everything moves into the vapor phase and that no aerosol droplets make their way into the column. As for the column itself, we could have some problems there, too, especially a solvent stationary phase polarity mismatch, where the solvent is unable to evenly wet the stationary phase. In order to fix that, what we really want to do is choose a solvent and a stationary phase of more similar polarity, so that they are miscible with one another. Those are some of the common causes and solutions for split peaks.

Troubleshooting tailing peak.

In this article, we’re going to write about tailing peaks, something we’ll often encounter when we’re doing gas chromatography. Now, tailing peaks can typically have one of two causes. Either we have some type of disruption to our flow path, or we have some type of reversible chemical adsorption going on. But how do we tell the difference? The first thing we want to do is take a look at our chromatogram. If most or all of our peaks are tailing, that type of indiscriminate tailing is often caused by a flow path disruption, something that can affect all of the compounds present. Conversely, if only a select few of our peaks are affected, it very well could be due to some type of chemical behavior, some type of adsorption or interaction due to a specific compound or compound class. So, now once we’ve sorted that out, what can we do to remedy those problems? Well, in the case of a flow path problem, what we want to do is make sure all of our installation distances are correct, make sure our column cuts are square, and all of our connections are leak free and ideally have as little dead volume as possible. On the cleanliness side of things, the chemical absorption side of things, we want to make sure everything is clean and inert. So, it may be time to do some maintenance – replace our liner, replace our inlet seal, maybe trim a little bit off our guard column, anything that we can do to make sure that the whole flow path is clean and well deactivated so there’s little chance of any interactions occurring. So, those are really the two approaches that we have to address the root causes that are behind most of our tailing problems.

Troubleshooting Carryover & host peak. 

  Let's write about carryover or ghost peaks today. Say we're doing some gas chromatography, and we have some extra peaks. If those extra peaks are especially broad, or even maybe like a hump in our chromatogram, there's a good chance that, that might be carryover. Material that's in our first run, but our run didn't last long enough for it to come out, so it starts to drag out and show up in subsequent chromatographic runs. How can we check for this? Probably the best technique is to do a no-injection instrument blank. We just run the instrument without making an injection, and if we see these really broad peaks show up, that's a very good indication that we have some carryover. How do we fix it? Probably something along the lines of increasing our run time, or taking our run up to a higher temperature, or maybe even both of those. Say we have some extra peaks, but they're not particularly broad. They have nice peak shape. We'd usually call those ghost peaks. Because they're nice-looking peaks, we know they've chromatographed, right? That means they've been introduced into the system towards the front. Maybe in our injection, or contamination in the injection port, or perhaps even something like contamination hanging out in our syringe, a bad rinse file, or a dry rinse file, or even something with how we've prepared our sample. That's where we want to focus our troubleshooting attention for a ghost peak problem. And if you're having an awful lot of trouble with it, just change your paradigm a little bit.

Troubleshooting of high column bleed.

We're going to write about high bleed. In gas chromatography, what is bleed, anyway? Bleed is the natural degradation process of our stationary phase. It's going to break down a little bit over time, and those components of the stationary phase are going to pass through the column and contribute to some signal. High bleed, though, is when that moves from normal behavior into a problem. The signal's really high, it starts occurring at lower temperatures, unusual circumstances. What can cause that to happen? Something is degrading your stationary phase. Most likely, it's going to be some oxygen. Oxygen is a column killer. It can come into our system from leaks or contaminated carrier gas. Alternatively, we also can damage our stationary phase with aggressive chemicals. Maybe we're injecting some privatization reagents, or some strong acids, or bases. They also can chew up a column. In that case, likely what we will want to do, take some steps to make our injection a little more mild, so that what we're putting on our column isn't as aggressive. If we get back to the oxygen problem, though, that's something that we want to always keep in our minds. We always want to be leak-checking our G.C. systems. We always want to be making sure that we are scrubbing oxygen from our carrier gas. Having some type of inline filter to remove that is really important. Those are the tools and techniques for managing bleed when you're doing your G.C. work. 

Troubleshooting of  no peak.

We’re going to do some G.C. troubleshooting. And the problem we’re going to take a look at, no peaks. So, if we think about this a little bit, what might cause us to not have peaks? Well, it could be our detector, right? We might not be generating a signal there. But it also could be that are sample isn’t making its way to the detector. It calls to mind the idea of divide and conquer. Can we sort out the difference? Is it a detector problem, or is it not getting to the detector? If we focus on the front end, how about administering our sample? Is our auto sampler working correctly? How about our syringe? Did we even prepare the sample correctly? Then, if we move on and think about the column, even something as common as a break in our column, well, that would certainly prevent material from making its way to the detector. And then, if we consider the detector itself, say we’re working with an FID. Is our flame lit? Could our jet be clogged? Do we have the correct gas flows? These are all good examples of problems that can occur at the detector that would prevent us from getting a signal. So, these are some of the things that we want to keep in mind when we’re tackling a problem like no peaks. Be sure to check out some of our other G.C. troubleshooting videos as well.

Troubleshooting of fronting peak

We're going to write about fronting peaks, one of the more common problems that we can encounter when doing gas chromatography. Fronting peaks and their distinctive shark fin peak shape really tell us that we have put more material into the column than it can handle. We can address that in two principal ways. We can make a change to the column, so that it can handle more, or we can put less material into the column. Let's walk through each of those. If we want to make a change to the column, so that it can hold more stuff, we can choose a column that has a thicker stationary phase or we can choose a column that has a stationary phase that is more compatible with the material that we're injecting. If we want to put less material into the column, we can either quite simply inject less, or if we're doing a split injection, we can increase our split ratio, which in turn is resulting in less material being put into the column. Those are the two solution paths that we have to solve fronting peaks. 

Column Conditioning:

We're going to write about fronting peaks, one of the more common problems that we can encounter when doing gas chromatography. Fronting peaks and their distinctive shark fin peak shape really tell us that we have put more material into the column than it can handle. We can address that in two principal ways. We can make a change to the column, so that it can handle more, or we can put less material into the column. Let's walk through each of those. If we want to make a change to the column, so that it can hold more stuff, we can choose a column that has a thicker stationary phase or we can choose a column that has a stationary phase that is more compatible with the material that we're injecting. If we want to put less material into the column, we can either quite simply inject less, or if we're doing a split injection, we can increase our split ratio, which in turn is resulting in less material being put into the column. Those are the two solution paths that we have to solve fronting peaks.

Trimming a GC Column to Remove Contamination:

In this article, we're going to be writing about column trimming. It's a really simple technique that we can use to remove some contamination from our G.C. columns. Very straightforward, as well. All we're going to do is uninstall our column and chop off about the first 15 centimetres or so. That's the area where most of the contamination accumulates on our column, on the inlet side of things. After we've done that trimming, we can simply re-install our column and proceed as usual. One other note, though. It is important for us to update our instrument software with our new column length. That software uses physical dimensions of the column—ID and length specifically—in order to help regulate pressure and flow. As we trim a column over time, we want to make sure that we keep updating our column length in the software. With that in mind, that's all that's involved with trimming a column to remove contamination.

The Problem with Gas Leaks:

Let’s write about the importance of leak checking our gas chromatographic system. It’s advice that you’re given all the time – to leak check, and leak check, and leak check again. But why? Why are we given this advice all the time? Well, the fact of the matter is with a high-pressure instrument and numerous seals and connections throughout, leaks can always crop up in a G.C. instrument. So, we want to be checking for them on a regular basis. But then the question is, well, so what? What’s the big deal if I do have a leak? And the truth of the matter is it is a big deal. And in a few different ways. First, if we have a leak, we’re disrupting our flow path. That can result in loss of sample, poor chromatography, or other issues. So, we certainly want to eliminate those in order to get the best chromatography possible. But there’s another problem as well. Leaks can allow materials to get introduced into the system as well, especially compounds like oxygen. Why is oxygen a big deal? Well, oxygen is a pretty effective column killer and can damage many gas chromatographic columns. So, we certainly want to have a leak-tight system to get the most lifetime out of our G.C. columns. Well, we leak check regularly. And ideally, we use an electronic leak detector. We want to avoid liquid leak detectors. Those liquid materials can get actually sucked into the instrument through those leaks and can contribute to contamination. So, that’s a little bit of why leak checking is so important. And again, the idea of making sure that we use an electronic leak detector regularly to look for those leaks throughout our instrument.
  




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