Introduction of Sippers That Are Effective for Multisample Continuous Measurements

1 Sipper Types and Selection

Fig.1 Measurement Using a Sipper

Fig. 1 shows the measurement of a sample solution introduced by a sipper from a test tube. When the lever is pressed on the sipper accessory, the sample is drawn in and measurements start automatically. Various sippers are available.

Sippers can be broadly divided into two types, depending on the method of drawing in the sample solution. Sipper 160 models use a peristaltic pump, while Syringe Sippers use a syringe pump. The models are further classified according to the absence or presence of thermostatic functions and the type of flow cell. Table 1 shows a list of sipper models. The standard required sample volume is the volume of sample needed to perform measurements that are unaffected by the previously measured sample.

Fig. 2 shows the appearance of Sipper Unit 160. The Sipper Unit 160 is an extremely compact unit that can be mounted inside the sample compartment. Four models are available, varying according to the flow cell used. Fig. 3 shows the shapes of the Sipper Unit 160 flow cells. The L-Type is the standard flow cell. The T-Type is a triple-pass flow cell. These flow cells have an approximately straight construction that allows a smooth sample flow during intake and discharge. The C-Type is a constant-temperature flow cell that permits measurements at constant temperature. The U-Type is the supermicro flow cell, which can measure smaller sample volumes than other flow cells.

Fig. 4 shows the appearance of a Syringe Sipper installed on a UV-1800 instrument. As a Syringe Sipper uses a syringe to draw in the sample, it achieves extremely high suction volume repeatability (repeat precision: ±0.03 mL). When choosing a Syringe Sipper, first select the N-Type (normal-temperature type) or the CN-Type (constant-temperature water circulator type). Next, select the standard required sample volume of the flow cell. Normally, a sample volume larger than the standard required sample volume is introduced for measurements. One of the features of Syringe Sipper flow cells is that they can be easily replaced. This simplifies maintenance. Table 2 shows the flow cells recommended for Syringe Sippers.

2 Chemical Resistance

The chemical resistance differs according to the type of sipper. As the Sipper Unit 160 uses a PVC tube in the peristaltic pump, the standard configuration cannot handle strong acids, strong alkalis, or some organic solvents. Therefore, the All-fluoropolymer Solenoid Valve and SWA-2 Sample Waste Unit are available as options. These options allow the analysis of diverse sample types. In contrast, the liquid-contact parts in the Syringe Sipper are made of PTFE, glass, and quartz. These offer superb chemical resistance that permits the measurement of most samples.

3 Measuring Bubble-Prone Samples

Bubbles may form when viscous samples are measured using a sipper. It is important to avoid bubble formation in the flow cell as the bubbles can affect the measured values. If bubbles do form, first reduce the suction speed in the measurement method. In addition, setting the discharge volume to the minimum value (0 mL) or extending the stabilization time can improve the situation. If the sample is highly viscous, the effects of the previously measured sample are greater if the standard required sample volume is used. It may be beneficial to increase the suction volume.

4 Automated Continuous Measurements

The sipper can be combined with an ASC-5 Auto Sample Changer to handle the automatic measurement of multiple samples. It permits the automated continuous measurement of up to 100 samples. Fig. 5 shows the appearance of the ASC-5 Auto Sample Changer. The ASC-5 comes with a test-tube rack and test tubes as standard. However, it can also handle non-standard sample containers, such as beakers. The installation footprint does not exceed 200 mm (D) × 200 mm (W) × 150 mm (H).

5 Carryover and Required Sample Volume

When continuously performing a series of measurements, a portion of the previous sample may remain in the flow cell and affect the measurement of the next sample. This is called "carryover." The suction volume that achieves carryover not exceeding 1.0 % is set as the standard required sample volume of the sipper. In practice, the required sample volume differs according to the length of the intake tube leading up to the flow cell. Therefore, when an ASC-5 Auto Sample Changer is used, the suction volume must be larger than the standard required sample volume.

We followed the method described in the instruction manual to test the carryover of a UV-1800 with Syringe Sipper N and a square flow cell (micro) with 1.0 mL standard required sample volume. Table 3 shows a list of the sipper setting conditions. The sample was an aqueous solution of potassium dichromate prepared to approximately 0.5 Abs. The absorbance at 350 nm was measured three times each for water (blank) and the sample. The sequence of measurements was: (1) Water 1, (2) Water 2, (3) Water 3, (4) Sample 1, (5) Sample 2, and (6) Sample 3.
The carryover is defined as the difference between the absorbance of (4) Sample 1 (immediately after switching from water to the sample) and the absorbance of (6) Sample 3, divided by the absorbance measured for (6) Sample 3. If no carryover occurs, the absorbance of (4) Sample 1 and (6) Sample 3 should both have the same value, within the range of measurement error.

However, if (4) Sample 1 is affected by (3) Water 3 that immediately precedes it, the absorbance measured for (4) Sample 1 will be less than the absorbance measured for (6) Sample 3.

The measured results are shown in Fig. 6. It was confirmed that almost no carryover occurred. Next, baseline correction was performed using water (blank), and the absorption spectra of (a) Sample and (b) Water were measured in sequence. The measured results are shown in Fig. 7. The spectrum for (b) Water exhibits 0.001 Abs absorption at 350 nm, which is virtually unobservable. We confirmed that accurate measurements are possible using the sipper set conditions shown in Table 3.

6. Maintenance

When operation is complete, immediately pump water or detergent to thoroughly rinse out the sipper. If detergent is used, subsequently rinse it out with water. Sample remaining in the flow cell can result in contamination. Bubbles occur more readily in a dirty flow cell. For the Sipper Unit 160, periodic replacement of the peristaltic pump tube is required in addition to rinsing the flow cell. Refer to the instruction manual for details about the replacement procedure. For a Syringe Sipper, it is adequate to thoroughly rinse the flow cell and other liquid-contact parts.

7. Summary

A sipper can continuously supply sample to save the effort required to introduce the sample solution into the cell. Care must be taken about some important differences between cells. Particular care is required with bubble formation. Syringe Sippers offer excellent suction volume repeatability, chemical resistance, and ease of maintenance, making them extremely easy-to-use accessories.