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Looking for maintenance and troubleshooting advice? Agilent’s experts are backed by 39 years of practical experience and they want to share it with you. So check here first for the fast tips and tricks that can help you get the results you need. |
I'm using the Deans switch with PLOT columns. What should I do to ensure I'm getting the best possible performance?
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Chun-Xiao Wang and James D. McCurry,
Senior Applications Chemists
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Porous Layer Open Tubular (PLOT) columns have been replacing the traditional packed columns used for gas analysis because of advantages in resolution, speed and selectivity. But to get the best possible performance from these columns when working with the Deans Switch, you need to use the Flow Calculator – a free tool that you can download from the Agilent website – to determine the effective (real) internal diameter (ID) of the column.
Why do I need to determine the effective internal diameter (ID) of the PLOT column when using the Deans Switch?
Normally, there is a difference between the labeled column ID value and effective ID. Although this difference is small, it is significant enough to make a difference when you determine the correct restrictor dimension by Deans Switch Calculator and balance the Deans Switch flows.
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Figure 1. Step 5: Enter the related parameters in the Column Flow Calculator.
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Figure 2. Step 6: Adjust the column ID to get the effective column ID.
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What is the fastest way to determine the effective column ID?
Follow these steps to determine your effective column ID:
- Install the column in the inlet and detector without the Deans Switch. Make sure that you know the true column length.
- Set the column pressure mode to "constant pressure".
- Set the oven temperature and column pressure to those used in the method.
- Be sure the detector gases are off, then measure and record the column flow at the detector. The column flow example used in Figure 1 is: 4.29mL/min.
- Use the Flow Calculator and enter the column length, labeled ID, pressure, oven temperature, and carrier gas type (He, N2, or H2). See Figure 1.
- In the Flow Calculator, adjust the column ID until the calculated flow is the same as the measured flow from step #4 (Example: 4.29mL/min). This is now the "effective" column ID. See Figure 2.
- Use this "effective" column ID in the Deans Switch Calculator for this column.
Learn more about Porous Layer Open Tubular (PLOT) columns.
I just installed a new column and the resolution between a critical pair of analytes dropped significantly, although my instrument parameters are set to the same values as before. Is something wrong with my new column?
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Allen K. Vickers,
Agilent Technical Support Chemist
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Here at Agilent Technologies, we've seen this problem before. A technician installs a new column on an instrument that's been running an application for a long time. She sets all the instrument parameters to the same values as before, but the resolution between a pair of critical analytes drops unacceptably, so she returns the column as defective.
If you examine the chromatograms that accompany columns like these, you will see that the retention times of the poorly resolved peaks on the new column have shifted. In the instance I mentioned above, they were over one minute earlier than in the original method. This difference indicates a change in linear velocity of the carrier gas, since the run conditions were the same.
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Figure 1. van Deemter Curves for Common Carrier Gases*
* The y-axis shows the height of a theoretical plate (H), which is inversely proportional to the number of theoretical plates per meter. The smaller the H, the greater the efficiency of a column.
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Figure 2. HP-5ms 30 m x 0.25 mm I.D. x 0.25 µm
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The Dutch chemist J.J. van Deemter established the relationship between carrier gas velocity and efficiency. As you can see in Figure 1, the number of theoretical plates – and the resultant resolving power – is greatly influenced by linear velocity. By adjusting the carrier gas velocity to elute the two compounds at the same retention time as before, you can restore the resolution to the desired level. (See Figure 2.)
But the question still remains: Why did the retention times of those compounds shift under identical run conditions?
The answer is that the run conditions are NOT identical because the columns are not truly identical. In any manufacturing process, products are built to tolerances. In the case of a GC capillary column, these tolerances include diameter and length. And even small differences in diameter and length will affect the actual velocity and flow rate through a column at a given head pressure. Gas chromatography instruments do not have a velocity and flow-measuring device built in. The linear velocity displayed is calculated from the head pressure and the column dimensions that the user inputs. If you use nominal values (30-m length, 0.25-mm diameter), the instrument automatically calculates flow and linear velocity data for a 30.00 m x 250.0-µm column. Many columns do not have these exact dimensions, which makes it imperative to measure the actual linear velocity by injecting a non-retained compound (such as methane) and using its retention time and the actual length of the column to calculate average linear velocity. (See Equation 1) The flow rate can also be calculated by incorporating the column cross-section into the equation. (See Equation 2.)
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If you want to operate under truly identical conditions, you must adjust the column head pressure to match the retention time of a non-retained compound from the previous column. Relying solely on instrument features, such as electronic pneumatics control (EPC), can lead to incorrect settings and cause problems.
To get a better feel for the magnitude of the effect that column dimensions have on the head pressure/linear velocity relationship, you can plug some numbers into the Agilent Flow Calculator.
The results may surprise you.
If you don't have an Agilent Flow Calculator, you can download it for free from the Agilent website.
Where can I go to find the information I need to transition my lab to the latest technology?
Paul Kirmayer,
Agilent Technologies
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