Nine tidbits to enhance the performance of the inlet on the GC-MS in running samples.

Most of this information is for individuals who are using a high throughput autosampler gas chromatograph-mass spectrometer (GC-MS).?Most of these are my personal preference and opinion and are not based on the guideline from the instrument manufacturer.?A note, here is that I am making an opinion based on my experience with Agilent and Thermo Fisher GC’s.?My references will be toward Agilent, but other manufacturers are considered.?Here are the following listed tidbits:

1.?????Syringe, in reality, is the most overlooked item on the list of inlet maintenance. In forensic work, it is rather difficult to try and fix a syringe, especially when running a sample that in forensic is unknown. The majority of the problem is in the choice of solvents.?99% of the time the choice is methanol.?The wear on the syringe plunger causes pitting of the shaft and there is no way to correct this.?Especially, if you are quantitating concentration unless you are using an internal standard.?The solution is to find a reliable supplier of syringes such as SGE, or Hamilton and purchase it as a bulk pack.?Consider, replacing it after 1000 or so injections but make a plan to check the syringe weekly.

2.?????Inlet septum.?This is one where I have found a very reliable source of septa and that is Restek.?The particular one I like is the “soylent” green one ( a note here, the movie was based on the year 2022). I picked the name from the movie, but I find it to be very easy to install low bleed and very little evidence of coring.?It can handle a large range of temperatures.

3.?????Glass liner.?Of course, it depends a lot on the work being done.?I am mainly concentrating on the use in forensic applications which mainly requires a split liner.?I highly recommend a straight glass liner (Restek) with a 4 mm ID.?Nothing fancy and it does the job with my modification below.?Replacement is based on the number of samples injected and the conditions of your instrument.

4.?????Glass wool.?Of course, the liner cannot work without some inert material to catch particulates and aid in the vaporization of your sample.?I recommend inert plain glass wool that can be obtained from a laboratory supplier or seized clandestine laboratory supply.?This is cheap and relatively easy to make.?I use a 10 mm length x 4 mm packed semi tightly by rolling it making a small twist into it.?With practice, it is easy to form a 10 mm size which I can insert into the center of the liner.?I use a scissor to trim off the excess and a clean applicator swab with a cotton bulb to push the glass wool into the liner approximately 3.5 cm from both ends.?With plain glass wool meaning non-silanized, there is a slight problem with active sites which is remedied by running a few samples of various standards to bind the active sites.?With experience, this can be easily corrected. Another issue to note is the size of the glass wool insert.?Typically, the syringe needle is wiped by the glass wool, and the glass wool aids in the volatility of the liquid sample.?Why do we limit the size of the glass wool or why bother using it??For one, it captures the particulates that can fall from the breakdown of the septum if it is cored.?Second, chemicals not volatilized are retained by the liner to prevent clogging of the column.?Third, the larger the insert takes up the volume of the liner which a 78 mm long x 4 mm giving a volume of approximately 0.972 mL of vapor.?Finally, why use un-silanized glass wool.?The reason is simple when extracting in methanol, the favorite choice of forensic chemists, certain chlorinated compounds will not give a sharp peak such as methamphetamine.?It tends to spread out and the only way to get a sharp Gaussian peak is to extract it in a base solution.

5.?????Solvent choices. The best solvent for forensic applications is methanol, hands down.?The favorite choice for the instrument and column is hexane for several reasons. One, it has greater capacity in terms of injection volume in the GC inlet. The density is less than one making it easy to use and it is great at extracting oils.?Still, methanol is the prime choice for any unknown forensic compounds whether it is acid or basic.?The major drawback is that in a typical Agilent GC-MS or similar using a 78 x 4 mm liner, the volume limit is less than 2 uL.?Here is an excellent vapor volume calculator provided by Agilent.

PowerPoint Presentation (agilent.com) (www.agilent.com/cs/library/eseminars...). Generally, you want an all-purpose solvent and it is hard to exclude methanol.?The danger of shooting 2 uL is that you may encounter ghost peaks in other sample runs.?

6.?????Ghost peaks. A perfect definition is provided by Restek.??“A?ghost peak?is a peak that appears at a position where we do not expect a peak”. It is a component that shows up in the system and it may show up everywhere in the chromatogram. Sometimes it is a sharp peak, sometimes it’s a broad peak or a “hump” and sometimes it is a rising baseline.?Most of these are introduced due to not applying good practice or in a hurry in the injection of samples.?The most common is the injecting of 2 uL or more of the sample.?This causes a problem with subsequent samples.?Another problem is the neglect of maintenance.?Some ghost peaks toward the end of the run are not due to column breakdown or bleed but are the results of bits of liner baking on top of the glass wool due to coring.?Also, the lack of changing out the split vent trap can be an issue.

7.?????Temperature settings.?We are relegated to the setting based on company manufactured guidelines.?But it doesn’t say whether you can make some changes to your instrument assuming you understand your total applications and that there is no deleterious effect on the overall performance of your samples.?In forensic work, the quality of the work product and reproducibility is of utmost importance especially in light of your testimony as an expert witness. ?A company manufacturer suggested lowering the inlet temperature to 10 degrees C below the solvent boiling point.?I have for years been running my samples at 220o C as opposed to the nominal setpoint of 250o C.?This is especially beneficial for more volatile samples such as GBL, 1,4-BD, and methamphetamine.?The suggestion is that by lowering the inlet temperature the solvent would coat the inside of the column and protect the sample as it travels into the column.?This would allow better resolution of the total ion chromatogram (peak or TIC) and result in a better mass spectrum.

8.?????Guard column.?The addition of a neutral section of the column is ideally designed for the forensic laboratory. ?The drawback is in the use of retention time (RT) or RT index unless you use an RT lock.?This will protect your expensive column and save cutting or trimming the column. Many laboratories do not use this because of issues related to installation and leaks.

9.?????Odds and Ends.?The last point of this is the application of injection volume.?As previously mentioned the key is what solvent to use.?Methanol is usually the number one choice and the first step in identifying an unknown compound.?Typically, it is important not to inject a true unknown compound onto a GC-MS, but that idea is gone by the wayside.?Many times the GC-MS is the first test and the last test used in a high production forensic laboratory.?There are many ways to control the injection volume to protect carryover or overloading the source. This is controlled by the inlet temperature and pressure which the above calculator can aid in making that determination.?The other way is to establish the best solvent that can be used that allows for the injection of a larger volume. ?Using the vapor calculations you can see that adjusting pressure and temperature is an easier way to help increase the expansion volume.?Unfortunately, Agilent ID is limited to 4 mm.?One thing to note is that even using methanol is not just the primary problem but your sample, insert, and purity of your mass spectral grade methanol greatly decrease the 0.972 mL vapor space. The true test is to run samples before and after making changes and using some samples as a control. Other inlet applications such as pulse injections are choices that can be used by the individual laboratory.