Standard operating conditions
To begin with, the standard operating conditions should be considered, such as the eluent (mobile phase) composition, the eluent flow rate, the column oven temperature, and the detection method. These standard conditions are specific for each individual column type and correspond to the conditions that work best with the application the column is intended for. Every analytical column sold by Metrohm is delivered with a certificate of analysis (CoA) which is recorded under standard operating conditions. In the following table, you can find the standard operating conditions for three different Metrohm IC columns.
|Column||Metrosep C 6 – 150/4.0||Metrosep A Supp 17 – 150/4.0||Metrosep Carb 2 – 150/4.0|
|Eluent composition||1.7 mmol/L nitric acid
1.7 mmol/L dipicolinic acid
|5.0 mmol/L sodium carbonate
0.2 mmol/L sodium bicarbonate
|100 mmol/L sodium hydroxide
10 mmol/L sodium acetate
|Flow rate||0.9 mL/min||0.6 mL/min||0.5 mL/min|
|Oven Temperature||30 °C||25 °C||30 °C|
|Detection||Non-suppressed conductivity||Suppressed conductivity||Amperometric detection|
|Analytes||Lithium, sodium, ammonium, potassium, calcium, magnesium||Fluoride, chloride, nitrite, bromide, nitrate, sulfate, phosphate||Inositol, arabitol, sorbitol, glucose, xylose, fructose, lactose, sucrose|
Next to the standard operating conditions, the start-up parameters play an important role for the lifetime of the separation column. High mechanical and thermal stress is a frequent reason for decreased column lifetime. It is therefore recommended to slowly increase the eluent flow rate to the column and to avoid thermal and pressure shocks. For more information regarding specifics, please refer to the recommended equilibration conditions in the corresponding column leaflet.
Based on the different stationary phase designs that are required to achieve different selectivities and guarantee a wide range of IC applications, the chemical and physical properties of the ion exchangers can vary. Therefore, different operating limits are recommended for various column types and set according to stress tests performed during the development of a product. Before beginning to optimize an application, please refer to the respective column leaflet for the corresponding operating limits to achieve optimal results and guarantee a long column lifetime.
The chemistry of the ion exchanger defines the limits of operating temperature, eluent pH, and organic modifiers that can be present in the eluent. These values are valid for every dimension of a column type (e.g., the Metrosep C 6). Exceeding these limits can strongly affect column performance and, in the worst case, lead to irreversible damage. Tolerated flow rates and maximum pressure correlate with physical properties and of course column dimension—therefore these limits are set for every column dimension (e.g., Metrosep C 6 – 100/4.0 vs. Metrosep C 6 – 250/2.0).
In addition to the CoA, a lot of necessary valuable information can be found in the respective column leaflet. Instructions about equilibration, regeneration, operating limits, and much more can be found in the column leaflet which is provided for every IC column type offered by Metrohm.
Depending on the operation conditions and the properties of the ion exchanger material, different storage conditions are recommended for different column types. These conditions (storage temperature and storage eluent) are described on the respective column leaflet and should be strictly followed.
Please note that storing a new column for long time periods as well as frequently switching between storage and operation can actually be more stressful to the column than operating it 24/7! It is therefore not recommended to stock a column for a long time without using it.
Guard columns are offered in two designs: «on column» (e.g. Metrosep A Supp 17 Guard/4.0, on the left) and as a separate guard that is connected to the IC column with a capillary (e.g. Metrosep A Supp 17 S-Guard/4.0, on the right). For microbore separation columns, the corresponding microbore column guards are recommended (e.g. Metrosep A Supp 10 Guard/2.0).
With the exception of the Eco IC product line, all Metrohm ion chromatography instruments are equipped with a pulsation absorber. For the Eco IC, it is strongly recommended to add the pulsation absorber as an option. As mentioned earlier, IC columns do not like repeated mechanical stress, especially those based on polyvinyl alcohol or polymethacrylate stationary phases. Therefore, the pulsation absorber is a useful tool to protect the column from possible pressure fluctuations in the system and enhance the column lifetime.
In ion chromatography, the ratio between the volume of sample and the volume of eluent that flows through the column is very small, usually in the range of 1:1000. Therefore the quality of the chemical reagents used in the eluent preparation plays a crucial role for the column lifetime. To guarantee optimal performance, use chemicals denominated as «for IC» whenever possible as they are tested particularly on impurities (e.g. metals) that could harm the chromatographic column.
For dilution of the eluent components, ultrapure water is typically used in ion chromatography. To ensure good chromatographic results, the ultrapure water should have a specific resistance greater than 18 MΩ · cm and be free from particles. The ultrapure water is filtered through a 0.45 µm filter and treated with UV. Modern ultrapure water sources for laboratory use guarantee this level of water quality (Type I).
One of the most prominent MISP techniques is Inline Ultrafiltration (illustrated here), which efficiently removes particles from the sample in a fully automated way, before they ever reach the column. In that way, column blockage from dirty samples can be avoided without any manual effort.
All Metrosep columns offered by Metrohm are equipped with an intelligent chip containing useful information about various column operation conditions (e.g., equilibration, standard operation conditions, operation limits, and more) and tracks certain parameters (e.g., set to work, number of injections, number of working hours, and maximum working values such as pressure and flow rate) over the column lifetime. It is beneficial to attach the column chip to the chip reader as illustrated here for proper monitoring as well as support.
Column shelf life
How many injections are possible on a specific column? Unfortunately, it is not possible to deliver an exact answer to this question. This is because the column lifetime strongly depends on the sample matrix and elution conditions. Due to the multitude of different applications and samples that can be covered with a single IC column, it is not possible to predict the column life for every application and sample type.
During column development, several endurance tests are performed under standard conditions with a guard column using appropriate standards. Under these conditions, the column must withstand at least 2000 injections.