There is a solution to sequences with a very long number of samples that require many washes. This is called solvent saver. Depending on the syringe volume configured, Mass Hunter enables selection of wash volumes at 50, 40, 30, 20 and 10 percent of the syringe volume. For a 10 micro liter syringe, we can choose a wash volume between 1 and 5 micro liters. When using solvent saver mode, a PTFE tipped syringe must be used, using this mode for a fitted syringe would drastically reduce it's lifetime. This is because washing in solvent saver mode is a little different then in maximum mode.
Insuring solvent vials are clean and not contaminated is essential in preserving longer syringe lifetime. If you observe particulates in the bottom of wash vials, you can try to use a cotton swab to remove it. Or if you see a wash vial like one on the right side, don't bother to clean it out, just replace it. Trying to save on small items such as wash vials is not worth the bigger headache of having to re-run an entire sequence because your blank was contaminated from your wash vial. Another suggestion is to empty, rinse and replace solvent in existing vials, rather then topping them off.
Here are some wash solvent strategies that can help during method development. First make sure you are using a solvent or a series of solvents that makes sense for the analysis. For example, is the analyte really soluble in the solvent? Using a solvent for an analyte that is insoluble is like trying to wash oil off glassware with water. If you are using a wash solvent that is not the same as your analyte solvent, make sure they are miscible. A miscibility chart is shown here for future reference. Also, don't use acidic or alkaline solvents with syringes. They are not good for most capillary columns either.
It helps to use both A and B solvents in the method since the auto sampler will rinse from the A vial first and the B vial second, Because of this, the B vials will always be cleaner then the A vials. never use water as the second wash in the syringe which might cause rusting of the syringe plunger. Finally, your best bet is to avoid using viscous solvents and solvents with high vapor expansion values which may overload the inlet liner. We will talk about this in future slides.
Here is a chart to provide a visual list of the solvents commonly used with the GC. On the y axis we have increasing boiling point. Typically in GC the boiling point of the solvent is usually targeted as the first peak to elute. So in a standard column phase such as DB-1 or Db-5 that separates by boiling points, lower boiling points are better. On the x axis, increasing expansion volume is charted. It is important not to exceed the vapor volume capacity of the liner. Increasing the injection volume with increase the vapor expansion volume which it is another reason to inject less sample. For this example, I've input the parameters in the vapor volume expansion calculator using a 1 micro liter injection at 250 degrees C and and 1 milli liter per minute column flow. We are using a splitless, single taper liner with glass wool, which has a capacity of 900 micro liters. That means that any solvent vapor greater the 900 micro liters will overload the liner and result in flash back. The size of the ball is dictated by it's capacity. A larger ball has a larger capacity. Dealing with samples with higher viscosity can be cumbersome and should be avoided.
The red ball shown indicates that the solvent is not water miscible. The blue ball indicates that the solvent is water miscible. Here is a look at the common solvents that are not water miscible. Look how large the iso-octane ball is and how high it's boiling point is. We know this is a solvent we would like to avoid in most cases. conversely, if we are doing a head space analysis we need a high boiling solvent that does not interfere with our low boiling analytes that elute early. We can see that iso-octane might be a good choice for this application. <break time="2s"/> Adding in the water miscible solvents we can see that methanol has a lower boiling point but it's expansion volume is higher then most solvents at 725 micro liters. If we were to inject 2 micro liters our vapor expansion volume would double and we would overload our liner capacity with 1400 micro liters. <break time="2s"/> Way out there we see the injection of 1 micro liter of water, which will significantly overload our liner. This is one reason to avoid water as solvent, but if you must use it, use a very small injection volume. <break time="2s"/> It is very important to use diffusion caps with wash solvents, these help reduce volatile evaporating. They are very good alternative to using septa, which can result in coring and contamination of the wash vial. This will show up as bleed peaks in the chromatogram.
Up to now we have been talking about solvent washing which occurs during pre and post injection. Now let's talk about sample washes. Sample washing primes the syringe barrel, then discards the wash volume in the waste bottle. <break time="1s"/> Sample pumping draws the sample into the syringe but discards it into the sample vial. Sample washes are executed before sample pumps. <break time="1s"/> Different applications may require either sample washing or pumping or both, but sample washing helps improve reproducibility. <break time="1s"/> If you have a reduced sample volume, caution should be used selecting the sample washes so you don't use up all the sample in the vial. Sample pumps also improve reproducibility by reducing the amount of air bubbles in the injection. Sample pumping can be useful for volatile samples by leaving a film on the inner needle wall, improving the precision of these sample types. When using viscous samples be careful in selecting the number of sample pumps since residue buildup can occur which affects reproducibility. <break time="1s"/>One caveat on sample pumping without a wash beforehand is that if the needle contains residual solvent from a previous solvent wash, the sample can be diluted. In this case, a sample wash can help. Don't do too many sample pumps, usually 3 to 5 is adequate, excessive movement of the plunger can reduce syringe lifetime.<break time="1s"/> It is also important to avoid filling the sample above the vial shoulder. This leaves a small headspace that prevents cavitation and vacuum formation that results in better reproducibility. In a complex mixture or dirty sample would you rather sample from the top or middle of the sample if your goal is to get a representative sample? It is important to fill the vial up to the shoulder. If enough sample is not available to fill to the shoulder, use a micro vial insert, to insure proper sampling depth and preserve your sample.
Now lets discuss some of the advanced sampling parameters. You can get to the advanced parameters by clicking on the button shown here. <break time="3s"/> The first parameter is Sample Depth. For most applications, sampling from the default of 3.6 milli meters from the bottom of the vial is recommended. In this case, no change needs to be made to the input box. However, there are some applications where changing the sampling depth can be beneficial. These include ambient headspace analysis or when injecting high sediment samples, although filtering the sample would be ideal. The range for this parameter is minus 2 milli meters to 30 milli meters. Since the default depth is 3.6 milli meters, inputting a value of minus 2 milli meters will sample 1.6 milli meters from the bottom of vial. If making a change to this parameter, make sure the autosampler is properly alligned, and observe the AutoSampler to make sure it is sampling from the desired location.
Next let's talk about plunger speed and viscosity delay. Remember, it is important to inject the sample quickly to prevent discrimination. In the bottom right chromatogram you can see the differences in response between the top automated injection and the bottom manual injection. Since the manual injection is slower then the automated injection, the later eluters show a decreased response, indicating sample discrimination is a factor. The first of three parameters involving plunger speed is the draw, which is how quickly the liquid is drawn into the syringe. The goal here is to prevent air bubble formation. The next action the syringe takes is to dispense the sample, which should be completely dispensed from the syringe barrel to prevent carryover. Lastly, the sample is injected into the inlet, which in most cases should be done quickly to avoid discrimination. When the syringe volume is configured in Mass Hunter software, the plunger speed setpoints are automatically set depending on the syringe size. The default plunger setpoints is either fast or variable. Fast and variable plunger speeds have the same starting setpoints, a slow draw at 300 micro liters per minute to insure complete uptake of the sample or solvent and prevent air bubbles, and rapid dispense and inject speeds at 6000 micro liters per minute. the only difference between fast and variable setpoints is that the variable selection allows the user to change the setpoint for customization while fast is fixed. The default setpoints are reccommended for almost all hot split splitless applications. Slow pluger speeds change the speed with which the plunger injects the sample on to the column. this can be beneficial for large volume injections into MMI or PTV inlets or a cool on column injection, or even to reproduce a manual injection. Slow injection speeds are at risk for broad or split peaks in the chromatogram, which happens when more volatile compounds leave the needle, before the plunger is actually depressed. These conditions should be avoided. The viscosity delay setpoint changes the time in seconds that the plunger pauses between pump and injection. this allows addition time for viscous samples and solvents to flow into the syringe during the sample pump. It should be used for viscous samples such as iso-octane, or with highly volatile solvents such as dichloro methane to prevent cavitation. A 2 second viscosity delay is the best bet for most applications, which is why it is the setpoint in GC and GCMS OQ parameters and IDL procedure. Last there are different injection types that can help automate sample prep. These can get complex so it is best not to change these conditions unless you are experienced.
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