Capillary GC Column Killers – Part 4

Aqueous Injections

In my final installment of Capillary GC Column Killers, I would like to discuss aqueous (water) injections. There are a lot of misconceptions about injecting water into a GC, and for good reason. Besides the general knowledge that electronics and water should not be combined, many analysts have experienced chromatography issues when injecting water. Because we get so many calls in tech service about injecting water, we even have a FAQ written on the topic (link below).

FAQ:  Can I inject water onto my column? Will it damage the column?

When it comes to aqueous (water) injections, put your mind at ease, as most fully bonded (cross-linked polymer) phase capillary columns will not be damaged by injections of water. However, water injections should be avoided if possible with non-bonded phases, partially bonded phases, or high-polarity phases.

To determine if your column is fully bonded, carefully read the product description provided by the manufacturer. Generally speaking, Restek columns that begin with Rtx-___, Stx-___ or Rxi-___ are all fully bonded. In addition, most MXT columns offered by Restek are also fully bonded. However, Restek columns that begin with Rt-___ are not fully bonded. While the general rule-of-thumb is to avoid injecting water into columns which are not fully bonded, below are a few exceptions:

1.  Porous Polymer PLOT columns. Although these columns are named Rt-Q-Bond, Rt-QS-Bond, Rt-S-Bond, and Rt-U-Bond, water will not damage these columns (or their MXT equivalents). However, never inject water into any Alumina or Molecular Sieve PLOT columns, as they will absorb the water leading to loss of compound separation and retention. (Note: if this happens, don’t be alarmed, this is a reversible process. Columns can be regenerated by heating to their maximum temperature for several hours to remove the water).

2.  If you must use a non-bonded, partially bonded, or high-polarity column for water injections, make sure the column temperature is kept high enough (greater than 80°C) to prevent water from condensing inside the column, or attach a Polar-Deactivated guard column (also known as a retention gap) prior to the analytical column. This will ensure that only water vapor reaches the analytical column.

Polar-Deactivated fused silica guard columns

As mentioned earlier, while water itself will usually not damage most bonded phase columns, non-volatile components in water-based samples, including particulates, salts, sugars, dissolved metal salts and/or suspended metals, can cause degradation of the column or adversely affect its performance. If you notice degradation in chromatography when analyzing water samples, determine if non-volatile impurities are causing the problem (inspect the injection port liner for residue) and take appropriate action to remove any impurities from the sample/extract prior to injection. This can be accomplished through a clean-up/filtration step during sample preparation. If you cannot clean/filter your sample prior to injection, make sure you use deactivated glass wool or a Carbofrit® packed injection port liner.

Everything written up to this point assumes you are injecting the water into a heated split/splitless injection port (containing a deactivated glass liner), and doing a split injection. Other injection techniques can be much more challenging when analyzing aqueous samples. On-column injections seem to be the most challenging. As a result, in tech service, we never recommend on-column injections of water samples. While some analysts have no trouble obtaining good chromatography and reproducible results doing on-column water injections, most analysts have multiple issues that seem impossible to correct. This is especially true for on-column injection ports that are only heated by the GC oven (and not heated independently).

While we are on the topic of on-column water injections, if you must do them, we strongly recommend using a Polar-deactivated guard column, even if using fully bonded stationary phases. This guard column (retention gap) will improve wettability when using non-polar stationary phases, and it will help keep non-volatile residue out of all columns, no matter the phase.

I would like to provide just a few final thoughts to those of you who need to analyze water samples via syringe injection.

  • First, backflash* is always a concern. Because water expands to more than 1400 times its liquid volume when vaporized, always inject the smallest volume possible (0.5µL or less) into the largest internal diameter injection port liner available for the make/model of your GC.
  • Second, I always recommend injecting water using split mode (with the largest split possible) rather than splitless mode (or Direct injection).
  • Finally, I usually recommend using a Gas-Tight (PTFE-tipped plunger) syringe as most customers report better reproducibility.

*If interested, try out our Solvent Expansion Calculator, which can be found in the link below.

Restek Chromatography Calculators

Hopefully you all have found my series on Capillary GC Column Killers informative. For those of you who may not have read Parts 1-3, the links are below:

Capillary GC Column Killers – Part 1

Capillary GC Column Killers – Part 2

Capillary GC Column Killers – Part 3

Although my list of capillary column killers is not all-inclusive, it does highlight several of the most common column-damaging situations. Hopefully, this information will provide valuable insight into protecting your capillary column from factors which are within the analyst’s control. Thank you.

6 Responses to “Capillary GC Column Killers – Part 4”

  1. Dear Alan –

    Thank your for your nice installments on Capillary Column Killers! Very usefull readings!

    According to water injections, its true its a challenge! However, we have very good and long experience injecting water samples using on-column injection and polar phase columns (HP-INNOWax) for trace oxygenate analysis, even without RT-Gaps or pre-columns. Split/Splitless injection ports are also a challenge when injecting water, but can also be done succesfully ….. specially if you have the time available preparing a method and tune up the GC parameters!

    With kind regards –
    Lars Kürstein, Copenhagen, Denmark

  2. Alan Sensue says:

    Hi Lars:

    I am glad to hear that you are able to perform on-column water injections successfully. I wish everyone had the same positive experience. Thank you for the valuable feedback and for reading my posts.

    Alan

  3. Charles Seager says:

    Alan

    Thank you for your time and efforts. My question involves Headspace, more specifically Full Evaporation Technique. Are there any caveats regarding water and this type of sample introduction?

    Respectfully

    Charles Seager

    St. Louis, Missouri

  4. Alan Sensue says:

    Hi Charles. Thank you for reading my post.

    Water issues, when using a headspace injection technique, seem less problematic for columns (reports of suspected damage are minimal), and seldom does back-flash seem to be a concern (most likely because split mode is used), but other issues may come into play such as condensation in the transfer line and other lines/tubing, which can lead to carry over issues (ghost peaks) and activity issues (when the lines begin to get contaminated). To prevent these issues, make sure the lines are heated and properly insulated. In addition, ask your instrument manufacturer when (how often) to clean or replace the lines. I believe the most common cleaning procedure is to rinse the lines with deionized water, followed by methanol (as long as it is not a target compound), then blow dry with clean nitrogen.

    I hope this helps. Alan

  5. Hi Alan, thanks for your efforts.
    Im newbie in GC and i have been learning GC for 3 months. Therefore, your topics here are very good and helpful for me.
    Recently, Im reading some documents related to GC-DAI, and for your explanation make me clearer about aqueous injection.
    So, could you tell me “why do we need to eliminate water (trace content) in carrier gas?” when some columns completely suffer water.
    If any visitors see and can give the answer for this post, Please take a time and help me!
    I hope to receive the respond as soon as possible.
    Thanks.

  6. Alan Sensue says:

    Hello: You have asked an excellent question. Since I am not an expert, I needed to ask others for assistance. Generally speaking, this link will contain your answer. I have also pasted it below. I hope you find it informative. Alan

    http://www.americanlaboratory.com/913-Technical-Articles/18810-Increasing-the-Stability-of-Liquid-Stationary-Phases-for-Improved-Stability-in-High-Temperature-GC/

    Impact of oxygen/water

    Oxygen is generally not a problem in GC when analysis temperatures are low. In some applications, air is used as the carrier gas at temperatures up to 130 °C. Above 130 °C, the impact of oxygen increases. When applications are run higher than 200 °C, oxygen must be removed.

    Water can also be a problem at lower temperatures and must be eliminated. Because water forms terminal silanol groups, it is the main cause of stationary phase breakdown. As the reactivity increases with higher temperatures, the high-temperature applications require ultradry conditions. Therefore, leaks must be minimized and carrier gas must be absolutely pure, preferably filtered with special filters before the GC. Gas Clean filters (Varian) are highly recommended. With the indicator in the filter, the risk of water introduction is reduced to a minimum.

Leave a Reply


Restek Domestic Customer Service

Subject

Message

Your Full Name

Your Email

Company Name

Address

Spam Block (Please leave this blank)

all fields required

Thank you

Your message has been sent. We will be in touch shortly.

Message not sent

Sorry, your message could not be sent at this time. Please try again later, or contact Restek or your local Restek representative via phone.

www.restek.com/Contact-Us