Advancing precision and efficiency in ion chromatography with automated inline dilution

Laboratory technicians often struggle to balance the high accuracy their analytical work requires with their full workload and limited resources. To obtain accurate and reliable results, two fundamental procedures are frequently required that afford utmost precision: calibration of the analytical system and dilution of highly concentrated samples. When using ion chromatography, fully automated solutions exist to reduce this manual lab work and facilitate efficiency. Sample preparation steps such as dilution can be performed automatically with the Metrohm Inline Dilution Technique.

Introduction

Quantification in ion chromatography (IC) requires a calibration that covers the defined measuring range [1,2]. The necessary calibration standards of different concentrations are usually prepared from a concentrated standard by manual dilution. Sample dilution is also crucial in ion chromatography to protect the separation column and detector, mitigate matrix effects, and primarily to ensure that the measured concentration falls within the calibrated range. These processes are time-consuming and pose a high risk of human error and contamination. Metrohm’s fully automated intelligent inline solution for dilution and calibration overcomes these challenges (Table 1).

The Metrohm 800 Dosino is the key to maintaining the required accuracy and precision in this fully automated process. It is a precise and highly flexible dosing device. With the MagIC Net IC software, all the liquid handling and dosing tasks are controlled and entirely automated – with no need for manual intervention.

**Table 1.** Evaluation of manual and automated processes for dilution of highly concentrated samples and calibration standards.
Manual dilution, manual preparation of calibration standards and samples	Automated inline process
Manual errors	Accurate liquid handling via Dosino
Time-consuming	Runs independently
Multi-step preparation	Single-step dilution
High contamination risk	Inline rinsing procedure

Male lab techniocian standing next to an open ion chromatograph and a full autosampler.

This blog post explores the fully automated Metrohm Inline Dilution Technique (MIDT) which makes analyzing various analytes (from g/L to µg/L range) less time consuming while keeping accuracy and precision high. MIDT involves automatic calibration and the implementation of dilution and logical features that extend the measuring range and improve accuracy and reproducibility of the results. The shorter preparation time reduces the workload and also saves costs.

Metrohm Inline Dilution Technique (MIDT)

One of the oldest sample preparation techniques is dilution. Dilution of samples is often necessary not only to protect the column and improve the separation of analytes, but also to guarantee that the working range of the IC method is not exceeded. Instead of multiple manual steps, this procedure can be carried out inline using an automated system as shown in Animation 1.

Animation 1: How does Metrohm Inline Dilution work?
The only manual steps required are placing samples in the autosampler rack and entering the dilution information (dilution factor) into the sample table. The entire dilution procedure is done automatically. The Dosino transfers the exact amount of sample necessary for the dilution step into a buffer loop, which is then filled and fed with ultrapure water or another diluent agent. The measured amount of sample plus the respective amount of ultrapure water or diluent is then transferred to the dilution vessel of the Liquid Handling Station mounted onto the autosampler (Figure 1). Thorough mixing is guaranteed by a stirrer. Finally, the diluted and well-mixed sample is transferred to the injection valve of the ion chromatograph using the peristaltic pump. Parallel to the analysis, the needle is cleaned in the rinsing unit of the Liquid Handling Station (Figure 1) and the dilution vessel is entirely rinsed to minimize carryover and ensure cleanliness. Undiluted samples can also be analyzed with the same setup.

Figure 1. The Liquid Handling Station (left) is mounted onto the autosampler to facilitate automated dilution and rinsing.

The dilution factor can be freely chosen within a range of undiluted measurement to 1:100, or up to 1:2000 with the necessary additional equipment (e.g., coated steel needle for sample uptake). The dilution factor is simply added in the sample table (Figure 2).

Figure 2. MagIC Net workplace showing the sample table with specifications necessary for the analysis (e.g., method, sample type, injection volume, and dilution factor). The dilution factor (highlighted in blue) can be adapted depending on the expected analyte concentration in the respective sample. The sample type is crucial, as the calibration proceeds automatically by defined dilutions of a mixed standard solution.

Logical Inline Dilution

A further development of MIDT is «Logical Inline Dilution». In this case, the system automatically determines the ideal dilution factor based on the calibrated range and proceeds to analyze the sample (Figure 3). Even if different dilution factors are necessary for different analytes, the system will carry out the necessary steps fully automatically. This ensures highly dependable results, as they always fall within the calibration range [3].

Figure 3. Illustration of the Metrohm logical dilution concept. If the sample concentration is outside of the calibration range, it is diluted with the optimum dilution factor and analyzed again. Thus, the results always fit within the calibration range.

Automatic calibration

Of course, it is not only samples but also standards that can be diluted in this way. Consequently, Metrohm offers the possibility of automatic calibration, i.e., a single multi-ion standard is all that is needed. The system dilutes this highly concentrated standard with different dilution factors and carries out a multi-point calibration as shown in Figure 4 [2,4].

Figure 4. Example of an automatic calibration of major anions achieved by using the Metrohm Inline Dilution Technique. The calibration standards were diluted automatically from a mixed standard for anions for the anion calibration, and a mixed standard for cations for the cation calibration, respectively. The correlation coefficients in Table 2 reveal the accuracy of this automated calibration technique.

The superior quality of the automatic calibration procedure is shown by high correlation coefficients (0.9999) and recoveries for multiple injections between 98% and 101% (Table 2) [2,4].

**Table 2.** Correlation coefficients for automatic calibration of cations and anions with the Metrohm Inline Dilution Technique.
All standards were injected multiple times (n = 3) and prepared from a mixed standard stock solution for anions and cations, respectively, adapting dilution factors (DF) of 2, 5, 10, 20, and 50.
Cations	Correlation coefficient	Recovery (%) for the standard DF 5	Anions	Correlation coefficient	Recovery (%) for the standard DF 5
Lithium	1.000000	100.7	Fluoride	0.999984	99.4
Sodium	0.999998	101.1	Chloride	0.999993	100.6
Potassium	0.999999	101.0	Nitrite	0.999972	98.7
Calcium	0.999993	101.9	Bromide	0.999981	98.2
Magnesium	0.999998	102.8	Nitrate	0.999981	98.6

MIDT fields of application

There are multiple application fields ideal for using Metrohm Inline Dilution. These include drinking and tap water analysis as well as more complex matrices like hemodialysis solutions or food and beverage samples. Learn from our IC application expert about the ultimate solution to increase efficiency and enable maximum protection of the IC system and the IC columns, especially regarding more complex sample matrices.

Ask the expert – IC: Can I combine inline dialysis or inline ultrafiltration with inline dilution?

Conclusion

In short, MIDT will offer the following advantages for your analysis:

MIDT automates the dilution of samples and standards, ensuring that results consistently fall within the desired calibration range. Logical features as well as nested sample handling can further improve the efficiency. This automation minimizes human error and enhances reproducibility – and is all done by the MagIC Net software.

The Metrohm Inline Dilution Technique is highly adaptable to every Metrohm ion chromatography system with a 858 Professional Sample Processor by simply adding an accessory kit for Inline Dilution. Combination of MIDT with Inline Ultrafiltration or Inline Dialysis can further improve efficiency and protect the IC system.

MIDT is capable of handling various sample types and concentrations, and can manage diluted as well as undiluted samples. While the standard setup can perform dilutions in a range of 1:1 to 1:100, this range can be extended very easily to 1:2000 with additional parts.

Sample concentrations are automatically calculated. Every single methodological step is traceable and can be summarized by MagIC Net in a user-defined report in the same way as the results.

MIDT significantly reduces preparation time and associated costs by reducing manual intervention, making this technique ideal for high-throughput environments.

References

[1] Weiss, J.; Shpigun, O. Handbook of Ion Chromatography, 4th ed.; Wiley-VCH: Hoboken, New Jersey, USA, 2016; Vol. 3.

[2] Seubert, A.; Frenzel, W.; Schäfer, H.; Bogenschütz, G.; Läubli, M. Monograph: Sample Preparation Techniques for Ion Chromatography, 2nd ed.; Metrohm AG: Herisau, Switzerland, 2021.

[3] Metrohm AG. Metrohm Inline Sample Preparation; Metrohm AG: Herisau, Switzerland, 2014.

[4] Hartmann, T.; Czyborra, S. Straightforward Multi-Point Calibration Using a Single Standard.