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Frequently Asked Questions

PID/FID
Calibration Curve
Water Injections
Guard Columns
Flow on dual column assemblies
Conditioning New Columns
Inlet activity problems
Flowmeter readings


PID/FID

Question:
I've noticed an interesting phenomenon when running a diesel standard on my tandem PID/FID; hydrocarbons eluting after C16 start to significantly decrease and also tail very badly. What causes this?

Answer:
This is a symptom of condensation in the PID. As the relative volatility of solutes decreases, the effect becomes more pronounced. No
rmally, this phenomenon can be minimized by increasing the temperature of the detector, up to the upper temperature limit (about 250C). The life of the PID lamp is greatly reduced with higher temperatures, so compounds should be restricted to the volatile range.

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Calibration Curve

Question:
EPA Method 8270 (GC/MS of semivolatile organics via capillary column techniques) requires a five-point calibration curve, typically 20, 40, 80, 120 and 160 ppm. Because the EPA recommends a 30 meter, 0.25 mm I.D. capillary column with a 1.0 m film, my chromatograms exhibit all of the symptoms of overload for the 120 and 160 ppm standards. What can I do?

Answer:
0.25 mm I.D. capillary columns with a 1.0 m film can handle approximately 125 to 175 ng1 of each individual analyte in a matrix; this means that if a 1.0 L sample mix has 100 ppm each of two components, for a total of 200 ng, the column will not overload. Faced with column overload, the analyst may select a column with greater capacity (e.g., a 0.32 mm I.D. column with comparable b). Unfortunately, not all benchtop GC/MS systems can handle the greater flow rates of 0.32 mm I.D. columns. Another approach is to position the top of the column within the injector so that a smaller amount of sample is introduced to the column. The calibration curve remains linear because the injector discrimination is constant for all concentrations. This might be the simplest solution for most analysts. Alternatively, split injection could reduce the amount of analyte on column (e.g., 25:1). Many of the late eluting and trace concentration compounds will become difficult to detect.

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Water Injections

Question:
Our lab routinely injects samples with an aqueous matrix, and we commonly have problems getting reproducible results. Can we improve this?

Answer:
Water has one of the largest vapor volumes of the common laboratory solvents. For water injections, the injector liner may be too small to accommodate the vaporized mixture, so the excess vapor will "backflash" outside of the injection port. This vapor mixture condenses on cooler surfaces resulting in a loss of sample. In the case of water, losses can be minimized by setting the injection port temperature to between 150 and 220C (lowering the expansion volume) or using a smaller injection volume.

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Guard Columns

Question:
How long should a guard column be?

Answer:
Guard columns are typically from 0.5 to 10 meters long. Although there are no definitive lengths that are good for all samples, the following guidelines can be used.

If the sample matrix is relatively "clean" (a small concentration of non-volatile compounds) and the solutes are active, the guard column should be 0.5 meter to 1 meter in length. If the sample matrix is dirty, the guard column should be longer (to collect the nonvolatile compounds). Five to ten meters help simplify system maintenance. With use a guard columns saturates and it becomes necessary to replace it. The longer guard column allows the user to simply cut off the first meter or so and reinstall it into the injector instead of replacing the entire guard column.

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Flow on dual column assemblies

Question:
I'm having a problem matching flows on my homemade dual-column assemblies and resolving some of my anlaytes. Help!

Answer:
Even though a column manufacturer specifies particular dimensions, the dimensions are not exact. This can cause problems when coupling columns in a single injection, dual-column analysis. J&W offers a column connection service, but if you want to do it yourself, try the following.

Connect the columns with the Y splitter and guard column. Verify the integrity of the connection. Heat the columns to a temperature at which you can inject a detectable, unretained compound.1 I like 150C. Note the elution time of the compound. If it elutes more than 0.1 minute apart, cut 10-15 cm from the column with the later time. Repeat the process until the nonretained compound elutes within 0.1 minute on both columns, making the column's flow rates nearly the same.

Run a standard under typical run conditions until resolution criteria are met. Pick a member of a pair of compounds that is difficult to resolve on one or both columns. Raise the column oven temperature high enough so that it will elute the compound between 5 and 10 minutes, inject it and note the elution times on both columns.

When installing new dual columns for the same analysis, repeat the steps in paragraph two, inject the chosen compound and adjust the head pressure until the retention time for both columns is within a percentage or two of the previously recorded times.

For a list of compounds for different detectors, refer to: Rood, D. A Practical Guide to the Care, Maintenance, and Trouble Shooting of Capillary Gas Chromatography Systems; Huthig, Heidelberg, 1991.


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Conditioning New Columns

Question:
I've heard conflicting opinions about conditioning new columns. Some of my coworkers say it isn't necessary, some say you should bake the thing overnight, and others say you should ramp the column slowly. So what's the deal? Is it necessary to condition a new column? If so, how?

Answer:
Condition a new capillary column at approximately twenty degrees higher than the final temperature of your oven program without exceeding the upper temperature limit of the column. If a temperature higher than the isothermal temperature limit of the column is needed for your analysis, recondition the column at that higher temperature, but, again, don't exceed the upper program limit.

When you install your column, purge it with at least three volumes of carrier gas prior to ramping it to the conditioning temperature. The total column conditioning time will depend on the type of application you're running and how much bleed is acceptable. The lower the detection limit that's needed, the longer the column will need to be conditioned. (Column bleed is closely related to the polarity and the film thickness of the stationary phase.) Polar and thick film columns bleed more and require more conditioning. For most applications, 30-60 minutes of conditioning is usually sufficient.

But how can you really determine when a column is sufficiently conditioned?

A flame ionization detector (FID) works best for monitoring the baseline during conditioning. Toward the end of the temperature ramp (i.e., 30-40C below the isothermal upper temperature limit), the baseline will rise, then come down and level off, at which time you may consider the column conditioned. There are those that report detector fouling during conditioning when using other types of detectors (e.g., ECD, MS), but it's generally considered a safe practice to condition the column while connected to these detectors.

One more thing: don't condition a column overnight. Column life expectancy is greatly reduced when the column is stored at high temperatures. If you're experiencing an excessive amount of bleed for more than two hours, bring the oven down to room temperature and locate the source of the problem (usually oxygen entering the column from loose fittings or a leaky septum). Baseline signals that mimic column bleed can also originate from residues present in the GC itself.

One more note: if the column has not been in use for a while, a mild conditioning step may be needed to drive off contamination which may have condensed inside the column during storage. Also, there is nothing to suggest a limit to the ramp rate of the oven when conditioning a column.


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Inlet activity problems

Question:
How do you have chemically deactivate injection liners? How do I know when I have an activity problem in my inlet?

Answer:
Most inlet liners are made of borosilicate glass (e.g., Pyrex). Borosilicate glass exhibits characteristics advantageous to gas chromatography. It has a low coefficient of thermal expansion and is resistant to thermal shock. Most glasses contain Lewis acid sites, and in the borosilicates, these are in the form of boron, metal oxides, and surface silanols. These sites can interact with solutes in the sample, resulting in tailing peaks. These sites may also contribute to solute degradation. Thus, when you experience tailing peaks or loss in sensitivity for chromatographically active solutes, you may be experiencing an activity problem in the inlet liner.

The deactivation process entails two basic steps: a leaching step to remove metal oxides at the glass surface and a derivatization step to deactivate surface silanols. Leaching involves soaking the inlet liner in a 25% mineral acid solution (e.g., hydrochloric, nitric, and sulfuric acids, but not chromic acid), usually overnight at room temperature. This portion of the deactivation process can be shortened to several hours if the acid solution is mildly heated (65C).

The derivatization step is more involved. After leaching, the liner is heated to remove free and bound water from the surface of the glass2), and then it is derivatized with a chemical agent to deactivate the surface silanol groups The choices for derivatizing agents are numerous, and methods are just as varied.

Although simple deactivation procedures exist, and are fairly effective (40-50%), the procedure is a very thorough deactivation procedure, which produces a more chemically inert liner than is commonly commercially available. This procedure is especially effective for very active compounds.

For more information on the properties of glass and chemical deactivation, we recommend two books by Walt Jennings: Analytical Gas Chromatography, Academic Press, and Comparisons of Fused Silica and Other Glass Columns in Gas Chromatography, Hthig. Also, Dean Rood's book, A Practical Guide to the Care, Maintenance, and Troubleshooting of Capillary Gas Chromatographic Systems, offers discussions on poor peak shape and activity phenomena.

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Flowmeter readings

Question:
I am getting a different reading on my flowmeter than I get if I inject an unretained compound and calculate the flow. The unretained compound elutes in 1.04 min on my 30 meter, 0.32 mm I.D. DB -5, which gives me a calculated 2.31 mL/min flow rate. My flowmeter says the flow rate is 4.59 mL/min. Am I doing the calculation wrong, or is my flowmeter wrong?

Answer:
Both answers are correct, but they answer different questions. The flowmeter is measuring the flow rate at the exit end of the column, whereas the calculated flow rate is a measure of the average flow rate through the column. The calculation should look something like this.

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