Optimizing lab efficiency with MiPT for versatile, high-precision sample handling

The most-used injection technique in ion chromatography (IC) is full-loop injection. This technique is both easy and highly reproducible. As the IC system is always calibrated to inject an identical volume, the absolute value of the injection loop volume is not important. However, in some cases it can be more convenient to inject variable volumes of samples or standards into the IC. So-called «partial-loop injection» makes IC analysis more flexible and convenient. The Metrohm intelligent Partial-Loop Injection Technique (MiPT) takes this to another level of precision with fully automated liquid handling steps performed by a Dosino.

How does the partial-loop injection technique work?

To perform partial-loop injection, a large loop (typically 250 µL) is partially filled with sample solution which is subsequently injected into the IC. The injected volume is used for the automatic calculation of the final concentration in the chromatographic software (e.g., MagIC Net).

Using the Metrohm intelligent Partial-Loop Injection Technique (MiPT), an 800 Dosino aspirates the sample solution (or standard solution) from its vial into the buffer tubing while bypassing the sample valve (Figure 1, left). After switching the sample valve to fill, the Dosino loads the loop with the specified sample volume (Figure 1, center). The partially filled loop of sample is then injected into the IC (Figure 1, right). In the case of the 250 µL loop, the injected sample volume can be between 4 µL and 200 µL.

Figure 1. Partial-loop injection process. 1) Sample (red) transfers into a buffer loop (left). 2) Injector switches to fill and loads the loop with a precisely defined sample volume by a Dosino. 3) injection of the sample volume to the ion chromatograph [1,2].

The complete MiPT process is fully automated, as shown in Animation 1. MiPT saves valuable laboratory time and equipment costs while reducing the risk of human error and carryover to an absolute minimum (<0.001%).

Animation 1: Setup of the Metrohm intelligent Partial-Loop Injection Technique (MiPT).
The 800 Dosino (center) transfers the sample into the buffer loop. Using the Dosino (precise to 0.2 μL), the sample loop is filled with the exact amount of sample or standard, which is then injected into the ion chromatograph.

By involving logical features in the MagIC Net method, the injection volume of undiluted samples can be automatically adjusted so that the measured concentration fits within the calibration range. Automation improves the overall analysis efficiency and makes logical MiPT perfect for sequential and high-throughput analyses where precision is paramount.

Streamlined instrument calibration with MiPT

Like the Metrohm Inline Dilution Technique (MIDT), MiPT simplifies the calibration process using a single multi-ion standard. Figure 2 shows the procedure for accurate and automated calibration using MiPT [3]. This streamlining further enhances the efficiency and accuracy of the analytical workflow.

Figure 2. Enhancing performance with automated calibration using MiPT. To obtain a complete calibration range corresponding to different analyte concentrations, multiple injections with different injection volumes are performed. The injection volumes can cover a range from 4 to 200 microliters.

The reproducibility, accuracy, and precision are excellent (Figure 3) – for calibration as well as for sample injection. MiPT provides users a fully automated and reliable way to analyze samples with varying concentrations, involving both an efficient single-standard calibration and the flexibility to inject samples with different volumes to fit to the calibrated range.

Figure 3. Performance of multiple injections carried out with MiPT. An overlay (with offset) is shown of multiple injections of a 500 μg/L mixed cation standard (injection volumes were six times each of 2 μL, 10 μL, 100 μL, and 200 µL). The overall precision is very high with an RSD of < 0.1% and recoveries of > 98%. The sample handling took less than two minutes.

MiPT is one of the most frequently used injection techniques offered by Metrohm. There are several fields of application – from environmental analysis of various water types to quality control in the electronics sector and even in food and beverage analysis. Read more in the knowledge take-aways section at the end of this article!

MiPT offers many advantages for IC analysis

Single-standard calibration and adjustable injection volumes if the sample concentration exceeds the calibration range (either manually or as a logical automated method) within a variable volume range of 4–200 µL (dilution factor 1:50).

MiPT can be adapted to every Metrohm IC system with the 858 Professional Sample Processor or 889 Sample Center, and it can also be used in combination with Metrohm Inline Ultrafiltration.

Analyze samples containing ppb (µg/L) and ppm (mg/L) concentrations of analytes with minimal carryover risk due to the protection of the Dosino by a buffer loop (Figure 1) and a fully automated thorough rinsing procedure (Animation 1).

Accuracy and reproducibility are guaranteed thanks to the innovative and precise Dosino device.

All steps including the calibration are fully automated, saving valuable laboratory time and making the analysis faster, more reliable, and free of human sources of error.

Conclusion

The Metrohm intelligent Partial-Loop Injection Technique as well as the Metrohm Inline Dilution Technique represent key advancements in ion chromatography (Table 1). By automating crucial steps such as calibration or sample dilution and offering the highest precision, these techniques enhance the accuracy and reliability of analyses and contribute to significant time and cost savings in the laboratory.

**Table 1.** Comparison of the Metrohm Inline Dilution Technique and the Metrohm intelligent Partial-Loop Injection Technique.
Technique:	Inline Dilution (MIDT)	Partial-Loop Injection (MiPT)
Concentration range	mg/L to g/L	µg/L to mg/L
Calibration range	1:2000	1:100
Sample concentration range	1:4,000,000	1:5000
Combinations possible	Ultrafiltration, dialysis, internal loop injection	Ultrafiltration
Dilution factor	1:2000	1:50
Automatic calibration out of one standard	Yes	Yes
Logical dilution/variable partial-loop injection	Yes	Yes
Nested handling	Yes	No
Sample preparation time	Approximately 5 minutes	Approximately 2 minutes

References

[1] Seubert, A.; Frenzel, W.; Schäfer, H.; et al. Monograph: Sample Preparation Techniques for Ion Chromatography, 2nd ed.; Metrohm AG: Herisau, Switzerland, 2021. 8.108.5070

[2] Metrohm AG. Metrohm Inline Sample Preparation; Metrohm AG: Herisau, Switzerland, 2022. 8.940.5004

[3] Hartmann, T.; Czyborra, S. Straightforward Multipoint Calibration Using a Single Standard. 8.000.6042