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Ion chromatography and sample filtration go hand-in-hand since this pretreatment step is advised for the majority of sample matrices. It is essential to avoid injecting particles, algae, or bacteria into the IC system, as these can jeopardize both the instrument and the separation column. Samples are often prepared manually, for example by using filtration or centrifugation to remove contaminants and interfering matrix components. A more efficient way to prepare samples for IC analysis is by using Metrohm Inline Sample Preparation (MISP) techniques to automate the process. Either Metrohm Inline Ultrafiltration (UF) or Inline Dialysis can be used to safeguard the IC system from harmful matrix components while simultaneously reducing manual labor and increasing sample throughput.
Click below to learn more about each topic.
Why should samples be filtered before analysis by ion chromatography?
Various water types (e.g., process water, surface water, wastewater) as well as beverages, extracts, and digestion solutions are liable to harm an IC system. Particles, silt, or plant residue in a sample will not only contaminate the IC system but may also accumulate within the separation column material. As a result of particle accumulation, the exchange capacity of the column (required for appropriate peak separation) decreases in combination with a fast increase in the system pressure. The peak resolution therefore suffers, and the overall lifetime of the guard column as well as the separation column is substantially reduced.
Metrohm Inline Ultrafiltration
Manual filtration is a well-known sample preparation step. The sample is first aspirated into a syringe and then passed through a suitable filter as it is injected into a sample container. This is a labor-intensive, often repetitive task requiring both time and costly consumable products. A significant amount of waste is produced as a result of the used consumables.
The Metrohm Inline Ultrafiltration technique is a fully inline and automated solution to these drawbacks. Inline Ultrafiltration is the most commonly used MISP technique. This solution allows users to analyze over 100 samples with only a single ultrafiltration membrane. Depending on the matrix, one UF membrane can even be used to filter up to 500 or 600 samples.
In the related blog post below, you can find our recommendations regarding when a UF membrane should be exchanged.
When do I have to exchange the filtration membrane with Inline Ultrafiltration?
Technically, ultrafiltration systems can be added to upgrade any Metrohm IC. The only requirement is that the autosampler needs to be equipped with a peristaltic pump. The following flow chart shows the connection principle for UF systems (Animation 1).
Animation 1. Animation of the ProfIC Vario 2 Anion setup with the UF cell as described in the following section.
Once the unfiltered samples are placed onto the sample changer and entered into the MagIC Net sample list, nothing more is required from the user. The autosampler will aspirate the sample from its vial, bringing the solution towards the UF cell (Animation 1).
The Metrohm ultrafiltration cell
The UF cell consists of two chambers with a shared spiral flow path. These chambers are connected to two separate peristaltic pumps and are separated by a membrane (Animations 1 and 2). The unique concept of Metrohm Inline Ultrafiltration is that only an aliquot from the sample is filtered through the membrane surface (Animation 2). The majority of the sample is guided at the sample side of the ultrafiltration membrane towards the waste stream. This allows a continuous flow on the sample side, discouraging particles from clinging to the filter. Particles and other undesired matrix components are flushed away continuously so that no filter cake can be formed. The «clean» filtrate is filled in the sample loop for sample injection in the ion chromatograph.
Animation 2. The principle of Inline Ultrafiltration is shown here. On the bottom, the sample continuously flows through a spiral-wound pathway and out to a waste stream. This continuous flow avoids clogging and filter cake formation on the sample side, while the filtrate crosses the membrane (middle) and is transported to the injection loop (on top). Sample and filtrate are pumped at different rates, facilitating a pressure-driven filtration process without damaging the membrane surface.
The standard UF membrane offered by Metrohm (Figure 1) is made of regenerated cellulose with a 0.20 µm pore size. The pore size is much smaller than other membrane technologies like manual filter membranes or dedicated filtration caps offered from different manufacturers. Particles that are 0.5 µm and larger can already create blockages within the column material. Because of the small pore size of the membrane, Metrohm can guarantee that no particles larger than 0.20 µm enter and affect or damage the analytical system.
Ultrafiltration system performance tests with the cellulose membrane material over the past several years have shown negligible contamination by leaching, giving users a very good and reliable experience. Moreover, the setup is flexible and can be adapted to the needs of the laboratory. Other ultrafiltration membranes available on the market can also be used. However, these require thorough testing by the consumer to exclude any analytical effects or biases before analyses are performed.
After the filtered sample has been injected onto the IC column, the complete sample path (including the UF cell) is rinsed thoroughly with one or multiple rinsing solutions in parallel to the detection. Standard rinsing is done with ultrapure water (UPW). If required, additional rinsing solutions (e.g., 30% methanol) can be used prior to the UPW to help avoid the growth of bacteria in the sample path. It is important to always use UPW as the final rinsing solution as this will be the medium in which the UF cell is stored. With this inline rinsing procedure, Inline Ultrafiltration guarantees less than 0.1% carryover between samples.
Metrohm Inline Ultrafiltration can also be combined with other MISP techniques mentioned in part 1 of this series. Adding the Metrohm intelligent Partial-Loop Injection Technique (MiPT) or Inline Dilution to the IC system can automatically prepare each sample to lie within the calibrated concentration range. These sample preparation techniques are powerful tools that offer additional system protection, prolong separation column lifetime, save significant time in the laboratory, and keep maintenance and other costs to a minimum.
Metrohm Inline Dialysis
Often, Carrez clarification or offline dialysis are used to remove colloids, proteins, oil drops, and more from sample matrices with high organic loads. Samples with this kind of matrix include fruit and vegetable juices, milk, infant formula, and other dairy products. Traditionally, fat and proteins are precipitated from the sample by consecutively adding Carrez reagent I (potassium hexacyanoferrate (II)) and Carrez reagent II (zinc acetate). After precipitation, the solution is centrifuged and filtered, and the clear supernatant solution is injected and analyzed by IC. This pretreatment involves several manual steps and a considerable amount of chemicals and equipment. In contrast, Metrohm offers an automated, inline solution for sample preparation in these situations. Metrohm Inline Dialysis is a very economical alternative solution to other time-intensive sample pretreatments and does not require the use of chemical reagents.
The Metrohm Low Volume dialysis cell (Figure 2) at the heart of this technique consists of two chambers separated by a cellulose acetate membrane, similar to the cell and membrane setup used for Inline Ultrafiltration. However, unlike the Inline Ultrafiltration principle, the stopped-flow principle is applied with Inline Dialysis. This enables analytes to pass through the membrane surface by means of diffusion while harmful matrix components such as particles, oil, fat, and larger proteins are excluded. As the matrix is constantly washed away, membrane fouling is inhibited, and the formation of a filter cake prevented as no pressure is applied.
The 930 and 940 IC families from Metrohm can be easily upgraded with a kit for use with Inline Dialysis. However, the IC and autosampler must both be equipped with peristaltic pumps. The technical standard setup is shown in Animation 3.
Animation 3. Inline Dialysis standard setup featuring the ProfIC Vario 3 Anion system. The sample solution is continuously pumped from the sample changer into the sample chamber of the dialysis cell through a spiral flow path. The sample chamber is separated from the acceptor chamber by a cellulose acetate membrane. The latter chamber is filled with an acceptor solution. Using the stop-flow principle, only ions pass through the membrane by diffusion and the acceptor solution is enriched until an equilibrium is reached. The acceptor solution is then pumped into the IC sample loop – after which it is injected on to the column for separation of the analytes and their subsequent detection (e.g., suppressed conductivity).
Comparison of Carrez clarification and Metrohm Inline Dialysis on milk samples
A thorough investigation was performed to compare the efficiency of two sample preparation methods for milk sample analysis by ion chromatography: Inline Dialysis and Carrez clarification. Overall, both IC system setups yielded comparable results over the testing period of six months where approximately 2000 milk sample injections were made.
One of the monitored parameters was the system pressure. If raised, this can indicate the presence of blockages or separation column degradation. System pressure remained very stable over the complete test series. Quality control standards were also injected at defined intervals and monitored to check the column performance. Decreasing column performance is indicated by the development of poor peak shape and trend in decreasing resolution, resulting in substandard recovery values. This parameter can be induced if fat, proteins, or particles enter the separation column. However, the test series showed that the recoveries of the quality control standards (chloride, nitrate, and nitrite) remained quite stable for both setups (Figure 3).
There was hardly any effect on the column lifetime for both IC setups. However, the extensive comparison of the traditional manual sample preparation method (Carrez clarification) to automated Metrohm Inline Dialysis proved the efficacy of the latter membrane process to properly protect the analytical system against bacteria, proteins, fat, and other contaminants while minimizing the overall laboratory work effort and chemical usage. More details about this study can be found in our White Paper «Simplified analysis of dairy products with Metrohm Inline Dialysis».
As such, Metrohm Inline Dialysis was proved to be the ideal sample preparation tool for dairy products like milk samples. It can be applied for a wide range of food samples, but also for challenging samples in other industry sectors as biochemistry, fuels, and pharmaceuticals. Several application examples for these industries are shared at the end of this article.
Conclusion
Both membrane-based inline sample preparation techniques described in this article (UF and dialysis) are powerful tools that make repetitive lab work easier. Not only that—they also help to safeguard the IC system while reducing the overall analysis costs and the amount of chemicals used. Nevertheless, the sample matrix is the final decision-maker regarding whether to choose Metrohm Inline Ultrafiltration or Inline Dialysis.
Your knowledge take-aways
Video: Basics of maintaining and working with ion chromatographs
Monograph: Sample preparation techniques for IC
Application Note: Four anions in a protein formulation using dialysis for sample preparation
Simplified analysis of dairy products with Metrohm Inline Dialysis
This White Paper compares automated Inline Ultrafiltration and Inline Dialysis to the traditional Carrez clarification procedure for the analysis of milk samples by ion chromatography (IC). A continuous test series over approximately six months proved Inline Dialysis to be a reliable and valuable alternative to treat dairy products prior to IC analysis.
